GB2595861A

GB2595861A - Optical apparatus and associated methods

Info

Publication number: GB2595861A
Application number: GB2008609.6A
Authority: GB
Inventors: James Poustie Alistair; Ashley Harrison James
Original assignee: Rushmere Tech Ltd
Current assignee: Rushmere Tech Ltd
Priority date: 2020-06-08
Filing date: 2020-06-08
Publication date: 2021-12-15
Also published as: GB202008609D0

Abstract

In a system with wavelength division multiplexors 114 and demultiplexers 116, which have ports with passbands limiting which wavelengths will transmit, the invention proposes transmitting two or more signals through each port. The two signals may have different wavelengths, both falling within the passband (Fig. 8a,c: 312a,b) and/or different polarisations (Fig. 8a,b: 312c,d). The optical signals may be from two sources 406, 408 connected via a combiner 112. The receiver may have filters 118 and detectors 122. The multiplexers may be arrayed waveguide gratings (AWG). The invention may be used to retro-fit/upgrade existing networks (e.g. passive optical networks PON). Also disclosed but not claimed, multiplexing using an AWG port’s free spectral range (FSR), and pulse amplitude modulation (PAM) in a port using two light sources.

Description

Optical apparatus and associated methods The field of the present invention is optical apparatus, in particular, but not limited to, optical apparatus for generating optical signals to increase the amount of data in an optical network.

Optical technologies are used in a variety of communications applications. Technologies range from optical fibres, optical sources such as lasers, detectors, optical modulators and other devices that provide optical functionality in integrated-optic form. The applications of optical communication technologies include different optical networks for data and telecommunications, such as core networks, metro networks and edge networks. The optical networks, in turn, serve other networks such as Fibre-To-The-Home (FTTH) and communications over radio waves (300GHz-30MHz) such as 5G networks. An example of an optical network oriented towards 56 operation is shown in figure 1A. The network topology in figure 1A is only an example representation of a network and may have additional network features or have some features not present. In figure 1A, the optical network 1 consists of an optical fibre-based core network 2A in optical communication with an optical fibre-based metro network 2B which in turn is in optical communication with an optical fibre-based edge network 2C. Each of the core 2A, metro 2B and edge 2C networks may have optical cross connects (OXC) or reconfigurable optical add/drop multiplexers (ROADMs) 3 to switch high-speed optical signals in each fibre optic network 2A, 2B, 2C. The edge network 2C is in communication with several subsidiary networks 4A, 4B.

The subsidiary network 4A is a fronthaul network that is in optical communication with an OXC 3 of the edge network 2C via Optical Transport Network (OTN) equipment 5A. This fronthaul network 4A is facilitated using single mode fibre carrying signals to base stations and cell sites 6A that in turn facilitates wireless communication between user equipment such as a user's mobile phone and the fronthaul network 4A. This fronthaul network may, for example, use standards such as Common Public Radio Interface CPRI and eCPRI to define an interface between Radio Equipment Control (REC) and Radio Equipment (RE).

An OXC 3 of the edge network may also be connected to wired or wireless equipment to transmit signals directly to base stations 6B.

An OXC 3 of the edge network may also be in optical communication with different types of access networks that connect subscribers to particular service providers. One example of an access network is a Passive Optical Network (PUN) 4B that consists of an optical line terminal (OLT) 7 at the service provider's central office (hub) connected, via optical fibre, to a number of optical network terminals (ONTs) 8A-8B. These ONTs may be, for example, businesses premises or individual homes 8B. Access networks may use different standards, such as Ethernet. Ethernet when run over a PUN may be, for example, Ethernet PUN (EPON) or 10G-EPON.

More recently, 56 networks have been deployed for applications such as mobile technology and the Internet of Things (loT). The 56 standard includes the use of millimetre waves between 30GHz and 300GHz. Use cases for 5G include enhanced Mobile Broadband (eMBB), massive Machine Type Communications (mMTC) and ultra-reliable and low-latency communications (uRLLC). To meet the demands of uRLLC in 56, optical networks need to provide more cost-effective transmission capacity. This calls for enhanced fibre transmission capacity. Several modulation schemes exist that and are proposed to increase transmission capacity. These include: intensity modulation and direct detection, Differential Phase Shift Keying (DPSK); coherent modulation and detection. To meet the bandwidth requirement of optical networks supporting services such as 5G, higher order modulation schemes can be required.

When designing fronthaul optical networks, there is typically a desire to minimise cost of the overall system, simplify the system to make it easy to install and repair and maximise the reliability of the system and its components. Furthermore, components such as active components should not be over utilised and thus are prone to failure. A network designer may therefore want to minimise the number of components to keep the system simple and the costs low but conversely include enough components to provide the desired operational characteristics.

One type of higher order scheme is Pulse Amplitude Modulation (PAM). PAM4 is an example of a multi-level amplitude modulation standard that uses four-level amplitude detection. PAM4 doubles the transmission capacity with the same bandwidth optical devices.

Figure 1B shows an example of a typical Dense Wavelength Division Multiplexing (DWDM) front haul PON 4B. Unlike the PON in figure 1A that serves ONTs such as businesses premises or individual homes 8A-8B, the PON in figure 1B serves multiple base stations 6C. The PON in figure 1B has its OLT 7 sitting at the Exchange or Central Office (CO) whereby different transmitters or transceivers, labelled TX 2.1-n in figure 1B, separately encode the incoming data from the edge network 2C into different wavelength channels that are wavelength multiplexed by an upstream Arrayed Waveguide Grating (AWG) 9A into a single Feeder Fibre (FF). The FF transports the signals to the local vicinity of the base stations 6C. A second downstream AWG 9B, typically located in a 'footbox' in the street demultiplexes the different wavelength channels, received from the FF, into corresponding separate optical fibres that forward each spatially separated wavelength channel onto one of the receivers or downstream transceivers labelled RX 2.1-n in figure 1B. The electronic signals generated by the different receivers or transceivers RX 2.1-n feed the separate base stations 6C, for example feeding separate 5G cell towers. In this system one unique wavelength is transmitted to each cell tower. Despite using multiple wavelengths to serve the multiple cell towers 6C, this set-up is still limited in its data rate to transmission to the cell towers 6C. The same data rate issues may also be found when using a similar set-up in figure 1B with other applications such as, but not limited to, sensing.

Summary

There is presented herein a number of optical apparatus, optical communication apparatus and methods. These optical apparatus and methods are described further herein with respect to different aspects. Any of the optical apparatus and methods may be adapted according to any components, features and configurations described herein. The components, features and configurations of any of the aspects may be combined with any of the other aspects. Furthermore, the teaching of any one method may be combined with any other another method, for example any of the methods of the first aspect may be combined with methods of the second or third aspect. Furthermore, the teaching of any one optical apparatus may be combined with any other another optical apparatus, for example any of the optical apparatus of the first aspect may be combined with the optical apparatus of the second or third aspect. The methods may utilise features of the optical apparatus and vice versa. When combining optical apparatus or methods, it is to be understood that the whole or portions of the method (or optical apparatus) may be combined with the whole or portions of another one or more method (or optical apparatus).

There is presented an optical communications apparatus for generating an optical signal at different amplitude modulation levels; the optical apparatus comprising: a first optical source for outputting light centred at a first wavelength; a second optical source 8 for outputting light centred at a second wavelength; the second wavelength being different to the first wavelength; wherein: at least one amplitude modulation level of the optical signal comprises light output from the first optical source; at least one amplitude modulation level of the optical signal comprises light output from the second optical source.

There is also presented in, an aspect, an optical apparatus for an optical network; the optical network comprising a wavelength demultiplexer comprising a plurality of spatially separated optical output paths; each optical output path associated with a different optical wavelength passband; the optical communication apparatus comprising one or more optical sources for generating a plurality of optical signals; the plurality of optical signals comprising at least: a first optical signal centred at a first wavelength and having a first polarisation; one or more further optical signals that comprise at least one of: a wavelength centred at a second wavelength that is different to the first wavelength; a polarisation that is different to the first polarisation; the first and one or more further optical signal wavelengths being within a common first wavelength passband of the wavelength demultiplexer.

The optical apparatus may be adapted according to any teaching herein, including, but not limited to any one or more of the following.

The optical apparatus may be configured such that the one or more further optical signals comprises: a second optical signal comprising second wavelength that is different to the first wavelength; a third optical signal comprising a polarisation that is different to the first pola risation.

The optical apparatus may be configured such that at least one of the one or more further optical signals comprises: A) a wavelength centred at a second wavelength that is different to the first wavelength; B) a polarisation that is different to the first polarisation.

The optical apparatus may be configured such that at least one of the one or more further optical signals comprises: A) a wavelength centred at a second wavelength that is different to the first wavelength; B) a polarisation that is substantially the same to the first polarisation. The optical apparatus may be configured such that optical frequency spacing between the first and second optical signals is in the range 20GHz-40GHz.

The optical apparatus may be configured such that at least one of the one or more further optical signals comprises: A) a wavelength centred at the first wavelength; B) a polarisation that is different to the first polarisation.

The optical apparatus may be configured such that the plurality of optical signals is associated with a series of different wavelengths; at least one of the further optical signals comprises one of the adjacent wavelengths, in the said series, to the first wavelength.

The optical apparatus may be configured such that the one or more optical sources comprises a plurality of optical sources comprising: a first optical source for outputting the first optical signal; a second optical source for outputting the further optical signal.

The optical apparatus may be configured such that at least one of the one or more optical sources comprises a laser.

The optical apparatus may be configured such that at least one of the one or more optical sources is wavelength tuneable.

The optical apparatus may be configured such that the plurality of optical signals comprises a third optical signal centred at a third wavelength that is: i) different to the first and second wavelengths; ii) within a second passband of the wavelength demultiplexer that is different to the first passband of the wavelength demultiplexer.

The optical apparatus may be configured such that at least one of the first and second optical signals is modulated using a multi-amplitude modulation.

The optical apparatus may be configured such that the multi-amplitude modulation is PAM4.

There is presented an optical system comprising an optical apparatus as described in the above aspect; and a wavelength multiplexer for receiving the first and further optical signals and outputting the first and second optical signals along a common optical path towards the wavelength demultiplexer.

The optical system may be configured such that any of the wavelength multiplexer or wavelength demultiplexer comprises an Arrayed Waveguide Grating, AWG.

The optical system may further comprise the wavelength demultiplexer as defined in the above aspect.

The optical system may further comprise an optical element for receiving the first one or more further optical signals and from the wavelength demultiplexer and outputting: the first optical signal in a first optical output path; at least one of the one or more further optical signals in a further optical output path that is spatially separate from the first optical output path.

The optical system wherein the optical element comprises any of: a wavelength filter; a polarisation splitter.

The optical system may further comprise one or more optical receivers for receiving the first and further optical signals and outputting corresponding electrical signals.

The optical system may comprise one or more base stations for receiving the electrical signals and transmitting radio waves associated with the said received electrical signals.

There is also presented a method of assembling an optical system; the method comprising: optically linking an optical apparatus as described in the optical apparatus presented immediately above (and optionally any modified variant of it) to a wavelength multiplexer for receiving the first and further optical signals and outputting the first and second optical signals along a common optical path towards the wavelength demultiplexer.

The method of assembling an optical system may further comprise: optically linking an optical element to the wavelength demultiplexer as defined in claim 1; the optical element for receiving the first one or more further optical signals output from the wavelength demultiplexer; and in turn, outputting: i) the first optical signal in a first optical output path; H) at least one of the one or more further optical signals in a further optical output path that is spatially separate from the first optical output path.

There is also presented a method for transmitting optical signals onto an optical network, the method comprising: outputting a first optical signal 312a centred at a first wavelength and having a first polarisation; outputting one or more further optical signals 312b that comprise at least one of: a wavelength centred at a second wavelength that is different to the first wavelength; a polarisation that is different to the first polarisation; inputting the first and one or more further optical signals into a first of a plurality of input channels of a wavelength multiplexer; the first and one or more further optical signal wavelengths being within a common first wavelength passband 305 of the wavelength multiplexer; wavelength multiplexing, with the wavelength multiplexer, the: first and one or more further optical signals; with, other optical signals from a second optical passband of the wavelength multiplexer associated with a second input channel, into a common optical output for transmission onto the optical network.

There is also presented a method for generating electrical signals for a communications network; the method comprising: receiving, by an optical wavelength demultiplexer: a first optical signal centred at a first wavelength and having a first polarisation, one or more further optical signals that comprise at least one of: a wavelength centred at a second wavelength that is different to the first wavelength; a polarisation that is different to the first polarisation; outputting, to one or more optical receivers, the first and one or more further optical signals in the same optical output channel of the optical wavelength demultiplexer; receiving the first and one or more further optical signals using one or more optical receivers; generating electrical signals, using the one or more optical receivers.

The method may further comprise: receiving, from the optical wavelength demultiplexer, the first and one or more further optical signals, with an optical element, and: i) outputting the first optical signal to a first optical receiver of the one or more optical receivers; and, H) outputting the one or more further optical signals to one or more further optical receivers of the one or more optical receivers.

There is also presented an optical apparatus for an optical network; the optical network comprising a wavelength demultiplexer comprising a plurality of spatially separated optical output paths and having a Free Spectral Range, FSR; each optical output path associated with a plurality of different optical wavelength passbands separated by one or more of the FSR; the optical apparatus comprising one or more optical sources for generating a plurality of optical signals; the plurality of optical signals comprising at least: a first optical signal centred at a first wavelength, the first wavelength in a first optical wavelength passband of a first optical output path; one or more further optical signals centred at a second wavelength, the second wavelength in a second optical wavelength passband of the first optical output path; the first optical wavelength passband being different to the second optical wavelength passband.

There is also presented a method of assembling an optical system. The method comprising: optically connecting the optical apparatus presented immediately above (and optionally any modified variant of it) to a wavelength multiplexer. The wavelength multiplexer for receiving the first and further optical signals and outputting the first and second optical signals along a common optical path towards the wavelength demultiplexer.

There is also presented a method for transmitting optical signals onto an optical network, the method comprising: outputting a first optical signal 312a centred at a first wavelength; outputting one or more further optical signals 312b at a wavelength centred at a second wavelength that is different to the first wavelength; inputting the first and one or more further optical signals into a first of a plurality of input channels of a wavelength multiplexer; the wavelength multiplexer having a Free Spectral Range, FSR; the first wavelength being within a first optical wavelength passband associated with the first input channel; the second wavelength being within a second optical wavelength passband associated with the first input channel; the first and second optical wavelength passbands being separated by one or more of the FSR; wavelength multiplexing, with the wavelength multiplexer, the: first and one or more further optical signals; with, other optical signals from a second input channel of the wavelength multiplexer; into a common optical output for transmission onto the optical network.

There is also presented a method for generating electrical signals for a communications network; the method comprising: receiving, by an optical wavelength demultiplexer, a first optical signal centred at a first wavelength, one or more further optical signals centred at a second wavelength that is different to the first wavelength; the wavelength multiplexer having a Free Spectral Range, FSR; the first wavelength being within a first optical wavelength passband; the second wavelength being within a second optical wavelength passband; the first and second optical wavelength passbands being separated by one or more of the FSR; outputting, to one or more optical receivers, the first and one or more further optical signals in the same optical output channel of the optical wavelength demultiplexer; the first and second optical wavelength passbands associated with the optical output channel; receiving the first and one or more further optical signals using one or more optical receivers; generating electrical signals, using the one or more optical receivers.

Brief description of the drawings

Figure 1A shows an example of a network using optical technologies; Figure 1B shows an example of an PUN; Figure 2 shows a schematic example of an optical apparatus for generating an optical signal at different amplitude modulation levels; Figures 3a-3c show different examples of how different amplitude modulation levels may be generated with the optical apparatus shown in figure 2; Figure 4 shows an example of an optical network comprising a plurality of the optical apparatus exemplified in figure 2; Figure 5 shows another example of an optical network comprising a plurality of the optical apparatus exemplified in figure 2; Figure 6A shows an example of an AWG; Figure 6B shows expanded versions of the star couplers of figure 6A; Figure 7 shows a schematic example of an optical apparatus for generating optical signals; Figure 8a-8c show schematic examples of optical signals output from optical apparatus according to a second aspect; Figure 9a shows an example of optical apparatus of the second aspect used in a network; Figure 9b shows another example of the optical apparatus of the second aspect at the exchange of a network; Figure 10 shows a schematic example of optical apparatus according to a third aspect; Figure 11 shows an example of downstream portion of a network using an optical apparatus of the third aspect; Figure 12 shows an example of an optical apparatus according to the first, second and third aspects.

Detailed description

The present application presents various optical apparatus that generate optical signals.

These optical signals may be used in optical networks such as a PUN. The apparatus may be used in optical networks other than PONs such as, but not limited to, any of the telecommunication and data networks described above in the background section. The apparatus is not limited to use in communication networks and may be used in other technology fields such as, but not limited to, sensors that require the use of optical signals such as optical sensors that detect external stimuli and send optical signals to a receiver to be processed.

The optical apparatus described herein are presented as different main aspects wherein examples of each aspect are further discussed. It is to be appreciated that the components and/or configurations of the different aspects (including any one or more of the examples) may be combined with another aspect.

The examples presented may often refer to being used in a network such as a PON, however it should be appreciated that each example may not be limited as such.

In developing the aspects of the application, the inventors have realised that certain optical systems, such as optical networks may need to be upgraded to have better data rates. This may be required when, for example, an end user requires an increased data rate or that a new communications standard is being deployed. Instead of replacing all of the network components, the present inventors have realised that it may be desirable to replace only certain components and utilise existing components already in the network. It may be desirable to re-utilise existing components for a variety of reasons including any of: the existing components being of good quality and not wanting to waste equipment; the components being difficult to access and replace (for example being in a location that has access difficulties); replacement components being expensive and or expensive to deploy.

These issues are not limited to networks but may equally be applicable to other optical based systems such as a network of distributed optical sensors. The aspects presented herein may be included not just in retrofitting or upgrading optical systems, but also in completely new deployed systems as well.

First aspect In a first aspect there is presented an optical apparatus that may be used for communications although in principle the apparatus may have applications that are not communications oriented. As such, the first aspect may be referred to hereinafter as the optical communications apparatus' or the 'apparatus'. A schematic diagram exemplifying the apparatus is shown in figure 2. The optical communications apparatus 10 is for generating an optical signal 12 at different amplitude modulation levels 14a, 14b. The optical communication apparatus comprising: a first optical source 16 for outputting light centred at a first wavelength (X1); and, a second optical source 18 for outputting light centred at a second wavelength (X2). The second wavelength being different to the first wavelength. At least one amplitude modulation level of the signal comprises light output from the first optical source. At least one amplitude modulation level of the signal comprises light output from the second optical source. The optical sources may be referred to as 'sources' herein.

Multi-level amplitude modulated signals (e.g. PAM4) may be used to increase the bit rate of optical communications signals, whilst keeping the electrical bandwidth to half of the bit rate. The standard way of implementing these multi-level amplitude modulated signals is by using a single wavelength laser source and amplitude modulating to several discrete levels (e.g. 4 levels for PAM4). This becomes an issue in that the laser source needs to be very linear to maintain the open eye for e.g. a 4-level signal.

By utilising at least two optical sources to generate the signal levels, the optical apparatus may share the light generating function for generating signals between the two sources.

This may reduce the demand on each individual source. The first and second optical sources are at different wavelengths so that there is significantly no interference between the optical signals from the said sources as they combine and propagate through the communication system they are input into. Although the first aspect describes first and second optical sources 16, 18, the apparatus 10 may in principle use a single optical source that generates multiple wavelengths and use an optical arrangement (for example a filter) to create multiple different wavelength signals from the same optical source and amplitude modulate them accordingly using two different optical modulators.

The optical apparatus (and associated methods) of this aspect may therefore use a combination of multiple wavelength laser sources to achieve a multi-amplitude level optical signal. Each laser source may generate a two-level binary signal (0 and 1). The laser sources are at different wavelengths so that there is no interference (or substantially no interference) between the optical signals. The optical sources may be closely spaced in wavelength so that the combined multi-wavelength, multi-amplitude signals can be passed through a single channel of an optical filter or multiplexer (e.g. AWG).

The optical signals are combined in amplitude and time such that the integrated optical intensity is the same as a single source multi-amplitude modulated waveform (such as PAM4). One advantage is that each optical source 16, 18 may be driven at half the data rate of the multi-level signal and that several multi-wavelength sources can be combined to form more complex multi-level amplitude data signals. With this approach, a standard receiver (for example a PAM4 receiver) with a single detector is used to detect the multi-level optical signal and decode the data. The system may need to be calibrated such that any bit skew between the two wavelengths after transmission through the fibre is compensated.

Alternatively, if there are two filtered receivers, then the data stream can be encoded onto the two wavelengths at half the baud rate such that the synchronised received data at each wavelength corresponds to the two-level digital symbol. In this case, the amplitudes of wavelength 1 binary signal and wavelength 2 binary signal do not need to have different amplitude levels.

The optical apparatus 10 may be used as an optical source for an optical system or an optical network including, but not limited to, a network using Ethernet (E), GbE, 400GbE. A plurality of optical apparatus 10 may be used for outputting a plurality of multi-level amplitude channels. Such channels may be distinguished between each other by wavelength. For example a first optical apparatus uses first and second sources with wavelengths Al and 12 whereas a second optical apparatus uses third and fourth sources with wavelengths 13 and 14 wherein X1 and 12 are closely spaced in wavelength so that both are filterable into a single wavelength channel. Similarly, wavelengths 13 and 14 are closely spaced in wavelength so that both are filterable into a different single wavelength channel. For example, the optical sources 16, 18 are closely spaced in wavelength so that the combined multi-wavelength, multi-amplitude signals can be passed through a single channel of an optical filter or multiplexer (e.g. an AWG).

The amplitude modulation levels are optional levels of the output optical signal that the optical apparatus 10 generates and outputs: i.e. the amplitude level output by the apparatus 10 depends upon an input signal to the optical sources. In some examples certain light levels are formed from light output from both the first and second optical sources 16, 18. For these amplitude modulation levels the output amplitude is a linear addition of the output light amplitudes from both sources 16, 18. The optical apparatus 10 may not be limited to a first 16 and second 18 optical source but may include a plurality of sources that may be three or more optical sources.

Figures 3a-3c show different examples of different amplitude modulation levels wherein the optical signal 12 shows pulse amplitudes of each level where increasing levels of light amplitude are shown vertically. It is assumed here that different levels are output in the same time window.

Figure 3a shows a first example of different pulse levels output by the optical apparatus 10.

In this example the optical apparatus 10 only outputs two amplitude modulation levels which are: a first level 14a which contains light only from the first source 16; and, a second level 14b which contains light only from the second source 18.

The light output from the first source 16 has a light amplitude that is less than that of the light output from the second source 18.

Figure 3b shows a second example of different pulse levels output by the optical apparatus 10. In this example the optical apparatus 10 outputs four amplitude modulation levels which are: a zero level 14d which contains significantly no light from either the first source 16 or the second source 18; a first level 14a which contains light only from the first source 16; and, a second level 14b which contains light only from the second source 18; and, a third level 14c which contains light from the first source 16 and the second source 18.

The light output from the first source 16 has a light amplitude that is less than that of the light output from the second source 18. Therefore the first level 14a and second level have different amplitudes as described above for figure 3a, however the third level 14c has a higher magnitude of light amplitude than the second level 14b because the third level 14c combines light from level 14a and level 14b. Therefore, as described above, the following conditions may apply: A) the optical communications apparatus may be configured such that the amplitude modulation levels comprise at least a first amplitude level and a second amplitude level, the first amplitude level comprising light of a different amplitude than light of a second amplitude level.

B) the optical communications apparatus may be configured such that: a first amplitude level comprises light from the first optical source; and, a second amplitude level comprises light from the second optical source.

C) An optical communications apparatus as claimed in any preceding claim wherein at least one amplitude modulation level comprises light output from the first optical source and the second optical source.

Any of these above conditions relating to Figure 3 may apply to other examples of optical apparatus described herein.

The amplitude modulation levels may therefore have a plurality of non-zero amplitudes levels. For purposes of this discussion a non-zero amplitude level may be defined as one which a signal is sent to at least one of the light sources of the optical apparatus to output an optical signal. A non-zero amplitude level is typically a level of light that is detectable by an optical detector. Preferably the non-zero level has an amplitude level of light that is above the average level of any optical noise emitted by the optical apparatus, preferably above the average level of any optical noise in the optical system in which the optical signal is input into.

In figure 3b each of the first source 16 and the second source 18 are either outputting significantly no light or outputting light at a single amplitude level for that particular source, i.e. the sources 16, 18 may each have bi-state operation. These states maybe be outputting a pulse or no pulse or outputting two levels of light pulses (but not ever returning to zero when outputting light signals). In other words, therefore, any of the following conditions may apply: A) the optical communications apparatus may be configured such that: the first optical source 16 comprises a first optical output state and a second output state; the second optical source 18 comprises a first optical output state and a second output state; the different amplitude modulation levels being formed from: I) at least one optical output state of the first optical source; and II) at least one optical output state of the second optical source.

B) the optical communications apparatus may be configured such that the different amplitude modulation levels Na, 14b; 14c, 14d of the optical signal may comprise: i) a first level Na formed from a) the second state of the first optical source; and, b) the first state of the second optical source; ii) a second level 14b formed from a) the first state of the first optical source; and, b) the second state of the second optical source; Hi) a third level 14c formed from a) the second state of the first optical source; and, b) the second state of the second optical source; Hi) a fourth level 14d formed from a) the first state of the first optical source; and, b) the first state of the second optical source.

C) the optical communications apparatus may be configured such that: i) the first state of the first optical source outputs significantly no light from the first optical source; ii) the first state of the second optical source outputs significantly no light from the second optical source; Hi) the first, second and third levels having different amplitudes to each other.

In figure 3b, the first source 16 may be driven to output light in its second state at around half of the light amplitude of the second state of the second source 18. For example, using unitless amplitude values for discussion purposes, the second output state of the first source is 0.25 and the second output state of the second source is 0.5. If both of the first output states are 0, then the following table (Table 1) represents the output levels of the optical signal modulation which shows a linearly increasing succession of amplitude levels.

Table 1

First source Second source Combined output 0 0 0 0.25 0 0.25 0 0.5 0.5 0.25 0.5 0.75 Therefore, in figure 3b any of the following conditions may apply: A) I) at least one of the said at least one amplitude level that comprises light output from the first optical source comprises a first optical amplitude of light from the first optical source; and, II)at least one of the said at the at least one amplitude level that comprises light output from the second optical source comprises a second optical amplitude of light from the second optical source.

B) The first optical amplitude may be different to the second optical amplitude.

In principle however the first optical amplitude is substantially the same as the second optical amplitude.

In figure 3c the optical apparatus 10 uses a bi state for both sources 16, 18 but only operates them as both 'on' (i.e. outputting light) or both off (i.e. not outputting light). This gives rise to only two optical output modulations levels '0' and '1'. Other different modulation level systems may be formed using one or more states of each source, for example each source may be operated to have three or more states.

The first optical source may output light pulses for propagating along an optical path whilst the second optical source outputs light pulses for propagating along the optical path. The optical communication apparatus may be configured such that the first and second optical pulses overlap in time along and/or at the end of the optical path.

The optical system may be formed that comprises an optical filter in optical communication with the optical apparatus. The filter may be for receiving the optical signal from the optical apparatus and spatially splitting the light from the first optical source to the second optical source. The optical system may further comprise a first optical detector for receiving light from the first optical source from the filter; and, a second optical detector for receiving light from the first optical source from the filter.

Figure 4 shows an example implementation of the first aspect in an optical network 100.

Different sets of electrical signals 102 from the edge network are input into different upstream electronic apparatus 104. Each set of electrical signals 102 represents data to be sent to a different SG cell tower 126. Figure 4 shows different electrical signals 102 being sent via spatially separate electrical transmission paths to a plurality of upstream electronic apparatus 104, each associated with different optical apparatus 110a-n.

The upstream electronic apparatus 104 may be electronic controllers and may comprise an electronic processor. A plurality of such upstream electronic apparatus is shown in figure 4, one for each optical apparatus 110a-n, however one or more upstream electronic apparatus may be used to send electrical signals to all the optical apparatus 110a-n. Each upstream electronic apparatus 104 receives the electrical signals, processes the electrical signals and outputs further electrical signals associated with the input signal 102 in a plurality of electrical channels to the corresponding optical apparatus 110a of the optical network. For example, the top-most upstream electronics apparatus 104 shown in figure 4 outputs two electrical signals to the first optical apparatus 110a. The two electrical signals provide the separate driving current for the optical sources 106, 108 to enable them to be driven in accordance with the optical modulation schemes shown in figures 2, 3a-3c and discussed elsewhere herein. In this example each of the optical apparatus 110a-n outputs a PAM4 signal wherein a single PAM4 modulation channel is composed of light output from two sources (the first optical source 106 and the second optical source 108).

In this example, the optical network 100 has, at the exchange, a plurality of optical apparatus 110a-n, including a first optical apparatus 110a, a second optical apparatus 110b and further optical apparatus up to 110n. Multiple sets of electrical signals are sent into corresponding electronic apparatus 104 which in turn feed electrical signals to the separate optical apparatus 110a-n at the exchange. The electrical signals are carried over any suitable electronic transmission components including electronic wires, coaxial cables, electronic waveguides, etc. Using the first optical apparatus 110a as an example, the optical apparatus 110a comprises two optical sources. In this example the optical sources in the first apparatus 110a are a first optical source 106 and a second optical source 108. The electronic apparatus output two separate electrical signals, one to the first optical source 106 and the other to the second optical source 108. These electrical signals are used to drive the optical sources 106, 108 such that the combined outputs of the first and second optical sources 106, 108 represents the data to be transmitted as a single PAM4 modulation channel. The frequency spacing between the first and second optical sources 106, 108 is 5GHz however any frequency spacing may be used that allows both wavelengths to be wavelength multiplexed and demultiplexed into/from the same common passband as each other, as described below. For example, frequency spacings between 1GHz-30GHz may be used. The optical sources 106, 108 in figure 4 are labelled TXX1-n.

The optical outputs of the first and second optical sources 106, 108 are optical signals that are input into an optical combiner 112a. The optical combiner receives both the separate optical signals that are received on spatially separate optical channels (for example separate optical fibres) and combines them into a single optical output channel that is, in turn fed, to a multiplexing (mux) AWG 114. The optical combiner may be a simple 2 to 1 optical combiner whereby half of the optical power from each of the optical source outputs is lost as the optical signals are combined by the optical combiner 112a. The optical combiner may be other types of optical combine including an interferometer such as a Mach-Zehnder interferometer, that wavelength multiplexes the optical signals output by the optical sources 106, 108 into a single output optical channel.

The AWG 114 multiplexes each of the sets of optical signals from the first, second and further optical apparatus 110a-n into a single output channel. The AWG may be of any design as discussed elsewhere in the present application.

The optical output of the AWG is a single spatial optical channel that is connected to an optical fibre 112 that extends and optically connects to a demultiplexing (demux) AWG 116. This optical fibre 112 propagates all the wavelengths from all the sources 106, 108 in all the optical apparatus 110a-n at the exchange, shown in this example.

The length of optical fibre 112 between the wavelength multiplexing AWG 114 and the demultiplexing AWG 116 may be formed of a span of a plurality of optically connected optical fibres. In this span of optical fibres there may be other optical components such as optical filters, optical amplifiers, and optical generators. The optical fibre 112 may be in principle any type of optical fibre, for example, a single mode optical fibre.

The demultiplexing AWG receives the wavelength multiplexed optical signals from the span of optical fibre 112 and wavelength demultiplexes them into spatially separated optical output channels where in each optical channel is associated with a different wavelength passband. The wavelength pass bands of the demultiplexing AWG 116 substantially correspond to the multiplexing pass bands of the multiplexing AWG 114 such that the wavelengths of light output by the first optical source 106 and second optical source 108 in the first optical apparatus 110a is wavelength-routed into the same optical output channel of the demultiplexing AWG 116. Similarly, the wavelengths from the plurality of optical sources in the further optical apparatus 110b-n are wavelength routed into other different optical output channels of the demultiplexing AWG 116.

Each optical output of the demultiplexing AWG is optically connected to an optical filter 118a-n. A plurality of these optical filters are associated with the multiplexing AWG 116 such that each of the optical outputs of the demultiplexing AWG 116 (hence each AWG output passband) is input into a separate different optical filter 118a-n. The optical filters may be any particular optical filter that receives light from the demultiplexing AWG 116 and outputs at least two spatially separated optical output channels wherein one of the output channels propagates light from the first optical source 106 and the other optical output channel outputs light from the second optical source 108. The optical filters 118a-n may be any type of optical filter including, but not limited to, an unbalanced Mach-Zehnder interferometer or a thin film filter.

The optical output channels from the optical filters 118a-n, in figure 4, are two optical output channels. These are input into separate optical receivers 122a, 122b that each receive optical signals and output corresponding electrical signals. The optical receivers 122a, 122b in figure 4 are labelled RXX1-n.

The set of receivers associated with each of the separate optical filters 118a-n may be termed receiver assemblies 120a-n. These receiver assemblies may be co-located in a common device or on a common platform and/or in a common package. Alternatively, the receivers in the receiver assemblies 120a-n may be separately packaged. In figure 4, for example, light from the first optical source 106 and the second optical source 108 of the first optical apparatus 110a are both transmitted to the receiver assembly 120a. The first receiver 122a of this receiver assembly 120a receives the light from the first optical source 106. The second receiver 122b of this receiver assembly 120a receives light from the second optical source 108.

The electrical signals output from the first and second optical receivers 122a/b are inputs into a downstream electronic apparatus 124a that receives the electrical signals from the receivers 122a/b of the receiver assembly 120a and performs electronic processing to determine the PAM4 modulation level represented by both of the first and second optical signals from the respective first and second optical sources 106, 108. The determined electrical signal which may represent the same or a similar electrical signal input into the first optical apparatus 110a, is sent to a Sc cell tower 126.

A plurality of downstream electronic apparatus 124a-n is shown in figure 4, one for each 5G cell tower 126 and receiver assembly 120a-n, however one or more downstream electronic apparatus may be used to send electrical signals to all the 56 cell towers 126.

In a similar manner, light from the second optical source and first optical source of the second optical assembly 110b at the exchange and passes through an optical combiner 112b which is then input into another optical input channel of the multiplexing AWG 114 (that is different to the channel used for receiving light from the first optical apparatus 110a) and transmitted along the optical fibre 112 to the demultiplexing AWG 116. At the demultiplexing AWG 116, light from the second optical apparatus 110b is then spatially demultiplexed and optically outputted into another of the output optical output channels of this AWG that is input into another receiver assembly 120b. This AWG optical output channel has a different optical passband to that of the optical channel feeding the receiver assembly 120a. A further optical filter 110b spatially separates the light from the first and second optical sources of the second optical apparatus 110b into corresponding first and second receivers of the second receiver assembly 120b. The output of these two optical receivers in the second receiver assembly 120b is an electrical signal output whereby both of the electrical outputs are input into another downstream electrical apparatus 124b.

Similar to downstream electrical apparatus 124a associated with the first receiver assembly 120a, the further downstream electrical apparatus 124b generates the electrical signals associated with those received at the corresponding the upstream electrical apparatus 104 from the Edge network feeding the second optical apparatus 110b. This the further downstream electrical apparatus 124b transmits the electrical signals to a second 5G cell tower 126.

It is to be appreciated that the optical sources 106/108 of any of the optical apparatus 110a-n may be optical transmitters such as lasers or any other optical source described herein. The optical sources may be parts of components that both transmit signals downstream towards the 5G cell towers 126 and also receive optical signals from a downstream direction, such as a transceiver. Similarly, the receiver assemblies 120a-n may be able to both receive optical signals from an upstream direction (i.e. from the communication direction of the edge network) and transmit optical signals upstream towards the communication direction of the edge network, such as a transceiver. The transmitter of the transceiver may be the one or more optical sources 106, 108 of the optical apparatus 110a.

The number of optical channels that the multiplexing AWG 114 may multiplex into the optical fibre 112 may be any number of channels. The same consideration applies for the demultiplexing AWG 116 in reverse. Preferably the multiplexing 114 and demultiplexing 116 AWGs are matched in optical passbands.

The example of figure 4 may be adapted according to any feature configuration disclosed herein. For example, the optical sources may comprise the example optical source as described underneath. The polarisations of the optical pulses output by the sources may be any polarisation state but are preferably TE or TM, wherein the output signals from both sources 106, 108 of the same optical apparatus 110a-n are preferably the same polarisation.

The receiver assemblies may comprise a plurality or three or more receivers. The optical filters 118a-n may each be a plurality of associated optical filters for wavelength separating light from the AWG 116 into three or more spatially separate output channels. The optical apparatus 110a-n may each have three or more optical sources. The optical combiners may combine light from three or more optical sources into a single optical communication channel that is input into the multiplexing AWG 114.

Figure 5 shows another example implementation of the first aspect.

The example network 200 in figure 5 is similar to that shown in figure 4 wherein like numerals represent like components. In this example the receiver side of the network is different to that of figure 4. Instead of having optical filters receive the separate passband outputs of the demultiplexing AWG 116 and output separate optical signals to multiple receivers of a receiver assembly, each separate AWG 116 passband output is input into a single receiver 222a-n, which in turn outputs electrical signals to the respective electrically connected 5G cell tower. Thus, in this example each receiver 222a-222n receives light from the plurality of optical sources 106, 108 of the corresponding optical apparatus 110a-n. Hence, first optical apparatus 110a uses light from sources 106, 108 to create a composite PAM4 modulation set of optical data signals that are routed through the network 200 and are input into first optical receiver 222a. The mux. AWG 114 input channel that receives the light from the first optical apparatus 110a is passband-matched to the demux. AWG 116 output channel that is optically linked to the receiver 222a. The second optical apparatus 110b uses light from sources 106, 108 to create a composite PAM4 modulation set of optical data signals that are routed through the network 200 and are input into second optical receiver 222b. The mux. AWG 114 input channel that receives the light from the second optical apparatus 110a is passband-matched to the demux. AWG 116 output channel that is optically linked to the receiver 222a. This relationship similarly applies to other AWG passband channels and optical apparatus/receiver pairings in the network 200.

The examples in figures 4 and 5 may be adapted according to any teaching herein, for example, but not limited to: using a different wavelength multiplexer and demultiplexer; using more or fewer optical sources; having different optical sources with less or more features; having a different optical transport medium between the multiplexer and demultiplexer; have different configurations of components at the optical outputs of the demultiplexer; having more or fewer end terminals; having different end terminals than the 5G transmitters.

There is also presented a method of assembling an optical system; the method comprising: optically connecting an optical apparatus, as described in any of the first aspect, to a wavelength multiplexer for receiving the first and second optical signals and outputting the first and second optical signals along a common optical path towards the wavelength demultiplexer.

This method may be adapted with any step or feature described herein, particularly, but not limited to features describes in the first aspect. The method may relate to the assembly of the whole or at least part of the network of figures 4 or 5 as described above.

This method of assembling may therefore be used to create part of a new optical network. Alternatively, this method may be used to adapt an existing optical network by adding in or replacing optical sources and/or adding in and replacing the wavelength multiplexer. The method may include other steps such as optically/electrically connecting any of the other optical/electrical components as described above in the first aspect.

The method may also comprise, in any order: optically connecting an optical filter to an output channel of a wavelength demultiplexer; the optical filter for separating the first and second wavelengths into separate optical outputs the wavelength demultiplexer for receiving the first and second optical signals and routing them into the output channel; and optically connecting a first optical receiver to a first optical output of the optical filter; optically connecting a second optical receiver to a second optical output of the optical filter.

The method may further comprise: electrically connecting an electronic apparatus to the first and second optical receivers; the electronic apparatus for: receiving electrical signals from the first and second optical receivers that correspond to the first and second optical signals; and, processing the electronic signals to determine electrical data signals for outputting.

The method may further comprise: electrically connecting the electronic apparatus to an electronic terminal; the electronic terminal for wirelessly outputting radio signals associated with the first and second optical signals.

There is also presented a method for generating an optical signal 12 at different amplitude modulation levels 14a, 14b. The method comprising: I) outputting light centred at a first wavelength; II) outputting light centred at a second wavelength; the second wavelength being different to the first wavelength; and, Ill) combining the light from the first wavelength and second wavelength into a composite signal having different amplitude modulation levels 14a, 14b; wherein: at least one amplitude modulation level of the composite signal comprises light of the first wavelength; and, at least one amplitude modulation level of the composite signal comprises light of the second wavelength.

This method may be adapted with any step or feature described herein, particularly, but not limited to features describes in the first aspect. The method may relate to the use of the network of figures 4 or 5 as described above.

The method may further comprise a first optical source 16 for outputting light centred at a first wavelength; and a second optical source 18 for outputting light centred at a second wavelength.

The method may further comprise outputting the composite signal into an input channel of a multiplexing AWG. The multiplexing AWG may form part of an optical network such as any of the networks described herein.

The method may further comprise routing the composite signal to a receiver assembly.

The receiver assembly may comprise a receiver for receiving the light from the first and second wavelengths.

There is further presented a method for generating an electrical signal for a communications network. The method comprising: I) receiving light centred at a first wavelength, using one or more optical receivers; and, II) receiving light centred at a second wavelength, using the one or more optical receivers; the second wavelength being different to the first wavelength; light of the first and second wavelengths being associated with the same multi-amplitude level modulation signal; the one or more optical receivers outputting a corresponding first set of one or more electrical signals associated with the receiving of the light at the first and second wavelengths. The method further comprising: Ill) receiving, using electronic apparatus, the first set of electrical signals; and IV) generating, using the electronic apparatus, a further electrical signal associated with a multi-amplitude level modulation signal.

The method may further comprise I) receiving the light from the first wavelength and second wavelength by an optical wavelength demultiplexer; and, II) outputting, to the one or more optical receivers, the light of the first wavelength and second wavelength in the same optical output channel of the optical wavelength demultiplexer.

The optical sources may be any optical source outputting any suitable wavelength of light. The light sources may comprise one or more lasers or other light sources such as LEDs. The light sources may be directly modulated light devices such as directly modulated Distributed FeedBack (DFB) laser or combinations of a light source and an optical modulator optical coupled to the light source and configured to receive light from the source and output modulated output light pulses. The laser may be a wavelength tuneable laser.

Modulators may include, but are not limited to: electro-absorption modulators or MachZehnder -based modulators.

The wavelength of operation of the optical apparatus 10 may be any wavelength, for example between 700-1700 nm. For telecommunications and other applications this may be in any one or more of the following bands: the 0-band (original band: 1260-1360 nm); the C-band (conventional band: 1530-1565 nm), the L-band (long-wavelength band: 1565-1625 nm); the 5-band (short-wavelength band: 1460-1530 nm); the E-band (extended-wavelength band: 1360-1460 nm). The optical source may be wavelength tuneable.

The pulse repetition frequency may be any frequency including any of the following ranges: 100MHz to 25GHz. The pulse time widths of the sources may be any width, including but not limited to any of the following: 1Ons to 25ps. The pulses may overlap in time at the detector such that the pulses are detectable in the same bit interval for each source.

An example optical source that may be used with the optical apparatus is described below. The optional operating characteristics provided about may apply to the example optical source.

Example optical source An example of an optical source that may be used in the optical apparatus is now described. It should be appreciated that other optical sources may be used in addition or in replacement of the optical source described below. The optical source may be substantially similar to that described in any of EP3028352 and/or W02019122877, the entire contents of both are incorporated by reference herein. The summary of an example of such an optical source is set out below.

The example optical source comprises a laser having a laser cavity. The laser cavity is disposed between a first optical reflector and a second optical reflector. The laser cavity may include an optical gain section and an optical phase control section in optical communication with the gain section. The optical phase control section is configured to be able to change the longitudinal mode frequency of the laser. This is typically done by controllably changing the amount of electrical drive current input into the phase control section. The optical source may also utilise a laser without a multi-section design.

The optical source further comprises an optical filter (for example a thin film transmission filter) that is external to the laser cavity and is configured to receive and filter light that is output from the laser cavity. The optical filter may comprise a passband filter response. At least one of the optical reflectors is a partial optical reflector configured to receive filtered light from the filter and to input filtered light back into the laser. The optical source is configured to change the central wavelength of the passband response of the optical filter by changing the angle of incidence that the output laser light subtends with the optical filter.

The optical source may also comprise at least one base member and a temperature control element thermally connected to the base member. The base member/s may be used to mount the above optical source components upon. The base member/s may be a substrate formed of extremely low expansion glass ceramic such as lithium-aluminosilicate glass-ceramic material. The extremely low expansion glass ceramic which may have a coefficient of linear thermal expansion (CTE) of 0 ± 0.007 x 10<6>/°K in the temperature range 0°C to 50°C. It is possible for other materials to be used for the base member. For example, the base member may comprise a substrate having a different thermal expansion coefficient, for example a Nickel-iron alloy such as Fe-33Ni-4.5Co or FeNi36. Such materials may have a CTE of 0.55 x 10<6>/°C in the temperature range 20°C to 100°C.

The optical apparatus may combine the outputs of the optical sources using any suitable means including, but not limited to any of the following: optical fibre combiners, integrated optic combiners; beam splitter/combiners.

The optical apparatus 10 may be an integrated optic apparatus. Any optical components described herein that may form the optical apparatus may be integrated together. This integration may take the form of monolithic integration, hybrid integration or both. For example, certain components of the optical apparatus 10 may be monolithically integrated together wherein the monolithic device may be further hybrid integrated with other portions of the optical apparatus 10.

The processes used to create and assemble different portions of the optical apparatus 10 may use those processes that are standard with integrated optic device manufacture and assembly including material deposition, etching and packaging processes. These processes may be similar to those used for CMOS.

The optical pathways optically linking different components together in the apparatus may be any appropriate optical component or configuration including, but not limited to, any one or more of: free space optical pathways; bulk optical components; integrated optic waveguides; optical fibres.

It is appreciated that any of the components described herein that require an electrical signal to operate, such as a laser or detector, may be electrically coupled to appropriate driving electronics including electrical lines and electrical apparatus outputting electrical signals via the electrical lines to the said components.

The optical apparatus 10 and any one or more components associated with the apparatus 10 may be part of an optical system. Any one or more components of the optical system may be integrated into an integrated optical device. Such associated components may be, but not limited to: optical filters, the optical sources, optical modulators, one or more optical detectors to detect any of the light output from the apparatus; one or more optical fibres to input light into the apparatus 10 or receive light output from the apparatus 10; any electrical apparatus to drive any of the components requiring or outputting electrical signals.

Multiplexers and demultiplexers In the above examples, and indeed in any of the optical apparatus or optical systems described herein according to any of the aspects, one or more AWG's may be used. An AWG is also commonly referred to as a Waveguide Grating Router (WGR). An AWG performs the function of a diffraction grating by using an array of waveguides. In a wavelength de-multiplexing operation, the AWG inputs light containing a plurality of wavelengths in the same spatial optical channel, for example a single waveguide. The AWG splits the light from the optical input channel into separate output channels, for example a plurality of waveguides wherein each output waveguide carries a different one or more light wavelengths than the other output channels. In a wavelength multiplexing mode, light is input back through the AWG in the opposite direction to the light travelling in a demultiplexing operation, i.e. light is input back into the output waveguides described above for the demultiplexing operation. This principle of operation may also be applicable for other demultiplexing and multiplexing.

The AWG used in the presentation application may be replaced by other types of diffraction gratings including any transmission or reflection diffraction gratings. The AWG is a type of transmission diffraction grating whilst an Echelle grating is an example of a type of reflection diffraction grating.

An example of an AWG 1000 is shown in figure 6A. The AWG 1000 comprises one or more input waveguides 1002, a first slab coupler 1004, a plurality of grating waveguides 1006 (otherwise known as arrayed waveguides), a second slab coupler 1008 and a plurality of output waveguides 1010. The grating waveguides are essentially an array of waveguides each having different lengths to each other that vary by a constant length difference, AL, between sequential waveguides in the array. Figure 6A is shown for discussion and explanation purposes only and shows three input 1002 waveguides, three output waveguides 1010 and six grating waveguides 1006, however any number of these waveguides may be used. An expanded view of the two slab couplers 1004, 1008 is shown in figure 6B. The slab couplers 1004, 1008 are shown as star couplers but other couplers such as Multi-Mode Interference (MMI) couplers may be used. The waveguide interfaces into and out of the first and second slab couplers 1004, 1008 are shown to have tapers however the waveguides may not have tapers. The waveguides used may be any type of waveguide as described elsewhere herein, for example a buried integrated optic waveguide.

The configuration of the AWG including the number of output waveguides, the number of grating waveguides and the slab coupler design, directly affects the number of wavelengths that the input light is demultiplexed into and the wavelength spacing of those output wavelengths. Generally, a higher resolution (or narrower output channel passband) is obtained by increasing the number of grating waveguides used.

A summary of the operation of an AWG now follows. Light enters the first slab coupler 1004 from an input waveguide 1002 (assuming it's the centre one for discussion purposes) and is Fourier transformed by the first slab coupler 1004 to a far field image at the opposite end of the slab coupler. The array of grating waveguides 1006 samples this far field image. Each grating waveguide carries light from the first slab coupler 1004 to the second slab coupler 1008. Light is input into the second slab coupler 1008 from the plurality of grating waveguide 1006 via an array of input apertures to the second slab coupler 1008. The second slab coupler 1008 Fourier transforms each input aperture to an image at the opposite end of the second coupler 1008 where the output waveguides 1010 sample the said images. The different lengths of the grating waveguides entail that the focal point along the array of output waveguides array is wavelength dependent. Constructive interference occurs at wavelengths that are an integer multiple of the optical path length difference between adjacent grating waveguides. The condition for constructive interference for the mth grating order can be written as follows in Equation 1: [Equation 1] The major contributor is the fixed path length difference, neAL, where ne is the waveguide effective index. The slab effective index is ns. The propagation through each slab waveguide contributes nsd sine to the optical path length difference. The grating waveguide pitch is 'd' at the slab couplers 1004, 1008. The input 1002 and output 1010 waveguide array pitches are di and do. The values Bijo are the respective input or output waveguide angle as shown in figure 6B.

The Free Spectral Range (FSR) of the AWG is the spacing in optical frequency or wavelength spacing between two successive transmitted optical intensity maxima of the same output.

The spectral response of each physical output channel of the AWG in a demultiplexing configuration therefore has a number of optical wavelength passbands, the peaks of which are separated by the FSR. Figure 6A shows, in an inset box, example transmission spectra for the different AWG output channels 1010A, 1010B, 1010C. The FSR of the AWG is shown as the wavelength spacing between successive different wavelengths that constructively interfere and are output by the same output waveguide.

The AWG may have any of: 4 or more output channels, 8 or more output channels, 16 or more output channels, 32 or more output channels, 64 or more output channels, 128 or more output channels. The AWG may have any output channel frequency spacing (i.e. the frequency difference between the centre frequency of one output channel and the closest centre frequency of the next adjacent output channel between) including between 3.125GHz-400GHz, for example any of: 6.256Hz; 12.56Hz; 256Hz; 506Hz; 1006Hz; 1506Hz; 200GHz; 4006Hz. The AWG channel spacing may be: any of: less than or equal to 6.256Hz; less than or equal to 106Hz; less than or equal to 12.56Hz; less than or equal to 156Hz; less than or equal to 206Hz; less than or equal to 256Hz; less than or equal to 306Hz; less than or equal to 356Hz; less than or equal to 40GHz; less than or equal to 45GHz; less than or equal to 506Hz, less than or equal to 100GHz, less than or equal to 200GHz, less than or equal to 4006Hz; and/or, any of: more than or equal to 6.25GHz; more than or equal to 106Hz; more than or equal to 12.56Hz; more than or equal to 156Hz; more than or equal to 206Hz; more than or equal to 256Hz; more than or equal to 30GHz; more than or equal to 356Hz; more than or equal to 40GHz; more than or equal to 45GHz; more than or equal to 50GHz, more than or equal to 100GHz, more than or equal to 200GHz, more than or equal to 4006Hz.

The FSR of the AWG may be any FSR. For example, the AWG FSR may be 40nm For example, using the above optional channel spacings, an AWG channel spacing may be more than 10GHz; or it may be between less than or equal to 30GHz, or it may be between and include 6.25GHz and 30GHz.

When forming an AWG or other similar device, each of these frequencies may vary, for example by any of 0.5%, 1%, 2%, 3%, 4%, 5%.

The -1dB passband may be any passband width, for example: any one or more of: 0.1nm or above, 0.2nm or above, 0.3nm or above; 0.4nm or above, 0.5nm or above, 0.6nm or above, 0.7nm or above, 0.8nm or above, 0.9nm or above, mm or above: and/or, any one or more of 0.2nm or below, 0.3nm or below; 0.4nm or below, 0.5nm or below, 0.6nm or below, 0.7nm or below, 0.8nm or below, 0.9nm or below, mm or below.

The AWG output passbands may by Gaussian shaped, flat topped or other type of passband responses.

Second aspect In a second aspect there is presented an optical apparatus that may be used for communications although in principle the apparatus may have applications that are not communications oriented. As such, the second aspect may be referred to hereinafter as the optical communications apparatus' or the 'optical apparatus'. A schematic diagram exemplifying the apparatus is shown in figure 7. The optical apparatus 310 is suitable for (but not limited to) an optical network 300. The optical network 300 comprising a wavelength demultiplexer 302 comprising a plurality of spatially separated optical output paths 304; each optical output path associated with a different optical wavelength passband 305. The optical apparatus 302 comprises one or more optical sources 306, 308 for generating a plurality of optical signals 312. The plurality of optical signals comprises at least: I) a first optical signal 312a centred at a first wavelength and having a first polarisation; II) one or more further optical signals 312b that comprise at least one of: A) a wavelength centred at a second wavelength that is different to the first wavelength; B) a polarisation that is different to the first polarisation. The first and one or more further optical signal wavelengths being within a common first wavelength passband 305 of the wavelength demultiplexer. Methods associated with the second aspect are described below.

Standard optical networks use a single signal per demultiplexer output passband. Typically, this demultiplexer is an AWG. An AWG will be referenced regarding describing the second aspect however other types of demultiplexer may be used and may be applicable to the apparatus 310 of the second aspect. The signal of the standard network set-up is centred on one wavelength having one polarisation and is generated by a single laser source.

The optical apparatus of the second aspect provides multiple signals, associated with different data streams, within the same demultiplexer output passband. This means that at least one output channel of the AWG (which supports a different set of passbands to the other output channels of the AWG) has two optical signals being transmitted through it towards the end terminal, such as a 56 cell tower. These signals within the same passband are differentiated by wavelength, polarisation, or both. Having a plurality of optical signals propagating out of the same AWG output channel at the downstream end of the network mean the end user terminal may receive a higher data rate than a standard network. This extra data may be used by the same end terminal or may allow for further end terminals to be served by the same AWG output channel. In some examples, the same data rate as a standard network may be output to the same end terminal, however if multiple optical sources are used to generate the first and second signals, each of the sources are driven at a reduced rate. This means that the sources may have greater life.

The optical apparatus in the second aspect may therefore use several closely spaced wavelength sources within one DWDM grid channel width to increase the data capacity per DWDM grid channel. Using a typical AWG to mux/demux DWDM channels may provide a 1dB channel width of 0.4nm (506Hz) so that closely spaced wavelength sources can be spaced by <306Hz. As an example, two wavelength sources may be used with a PAM4 12.5Gbaud/s modulation to achieve 25Gb/s bit rate per source and 50Gb/s bit rate for both sources together (within the same channel). One or two (or more) other polarisation multiplexed optical sources can also be combined into the one DWDM passband channel to achieve 75Gb/s or 100Gb/s total bit-rate respectively.

Other modulation formats can also be used on each of the closely spaced wavelength sources as described. As a further example, three or more closely spaced wavelength sources can also be used. Using a multi-amplitude modulation format such as PAM4 modulation minimises the spectral width of each optical source, allowing them to be spaced by small frequency spacing such as between >25GHz and <50GHz.

The receiver for these DWDM channels may need to separate the two closely spaced wavelength sources, requiring an optical filter with, for example, <30GHz resolution. This can be achieved in different ways. For example, a steep edge optical filter can spatially separate each source. In this case, the thin film bandpass filter can be designed to be athermal, or to be a thermally tunable device mounted on a temperature control element to adjust the absolute optical wavelength of the filter edge. Additionally, or alternatively, an optical waveguide-based filter (e.g. asymmetric Mach Zehnder) can be designed to separate the two sources, as described elsewhere herein. This configuration could be fabricated in silica with separate InP photodiodes, or silicon photonics so that the photodiodes could also be part of a silicon photonics device. If polarisation multiplexing is also used, then a polarisation splitter is included before either of the detection schemes to detect the orthogonal sources.

As described above, the second signal may differ from the first signal in either or both wavelength or polarisation. There may also be more than two signals propagated within the common AWG passband, for example three, four or more optical signals.

For example, the one or more further optical signals may comprise: a second optical signal comprising second wavelength that is different to the first wavelength; and, a third optical signal comprising a polarisation that is different to the first polarisation. Figure 8a shows an example of this where, within the common passband of 305a of the AWG, the first signal 312a has a different wavelength to the second signal 312b. The third signal 312c has the same wavelength as the first signal 312a, but has a different polarisation (for example an orthogonal polarisation).

In another example as shown in figure 8b, at least one of the one or more further optical signals comprises: a wavelength centred at a second wavelength that is different to the first wavelength; a polarisation that is different to the first polarisation. Thus, the first signal 312a in figure 8b has a different polarisation and wavelength to the further signal 312d.

In another example shown in figure Sc, at least one of the one or more further optical signals comprises: a wavelength centred at a second wavelength that is different to the first wavelength; a polarisation that is substantially the same to the first polarisation. Thus, the first signal 312a in figure 8b has a different polarisation and wavelength to the further signal 312d.

It is to be appreciated that at any plurality of the signals 312a-312d shown in figures 8a-8c may be transmitted in the common passband 305a. For example, at least one of the one or more further optical signals comprises: a wavelength centred at the first wavelength; a polarisation that is different to the first polarisation. Further wavelength and polarisation states may be used, for example three or more wavelength and/or three or more polarisation states.

For examples where a second signal 312b is used that has a different optical wavelength to the first signal 312a, the optical frequency spacing between the first and second optical signals is less than the frequency width of the optical wavelength passband at 10dB amplitude loss from the centre of the passband. However, in principle any optical frequency spacing may be used. It is to be understood that the frequency spacing between different signals may also be termed as a wavelength spacing.

In an optical system or network multiple AWG outputs may be used (each output associated with at least one optical passband) and a series of optical signals at different wavelengths are transmitted through the AWG. Thus, a plurality of optical signals may be associated with a series of different wavelengths; wherein at least one of the further optical signals comprises one of the adjacent wavelengths, in the said series, to the first wavelength.

For example, in figure 8a, the first and second wavelength signals 312a, 312b are wavelength adjacent to each other with respect to the series of wavelengths. The other adjacent wavelength to the first wavelength 312a in the series may fall in a different adjacent passband that is supported by a different AWG channel.

For example, where the plurality of optical signals may comprise a third optical signal centred at a third wavelength. The third wavelength may be a) different to the first and second wavelengths; and b) within a second wavelength passband of the wavelength demultiplexer that is different to the first wavelength passband of the wavelength demultiplexer. This second wavelength passband may be supported in the same or different physical output channel as the first passband. The third aspect described elsewhere herein describes an apparatus where the second optical signal is in a different optical passband of the AWG but is output via the same physical optical output waveguide (channel) as the first passband -hence taking advantage of the cyclic nature of the AWG structure (or any grating device).

The optical apparatus of the second aspect is described above to comprise one or more optical sources. This may be, for example, a single optical source that the different signals are created from. This may be a wide wavelength light source that is amplitude split and then separately sampled and data modulated by an external optical modulator. Another example of the apparatus uses a plurality of optical sources comprising: a first optical source for outputting the first optical signal; an additional optical source(s) for outputting the further optical signal(s).

The one or more optical sources may comprise a laser or any other source described herein such as a DFB laser. Any of the one or more optical sources may be wavelength tuneable such as a tuneable laser or the example optical source described elsewhere herein.

The optical signals may be of any type including a multi-amplitude modulation.

Using multi-amplitude modulation, minimises the spectral width of each optical source, allowing them to be closely spaced in wavelength and thus located in the same optical passband, for example spaced by >256Hz and <506Hz. The multi-amplitude modulation may be PAM4.

The optical apparatus may be part of an optical system or an optical network.

Figure 9a shows an example implementation of the optical apparatus of the second aspect in an optical network 400. The optical network 400 is similar to that shown in figures 4 and 5 of the first aspect. In figure 9, like numerals to figures 4 and 5 represent like components.

Methods associated with the assembly of the network and the use of the network are described below.

In this example the exchange has a plurality of optical apparatus 410a-n. Electrical signals 402 from the edge network are sent to electronic apparatus 404. Each optical apparatus 410a-n is associated with a separate electronic apparatus 404 that receives electrical signals 402 from the edge network and sends a first portion of the electrical signals to a first optical source of the optical apparatus and a second portion of the electrical signals to a second optical source of the optical apparatus. For purposes of discussing figure 9a, the first optical apparatus 410a is described, however the other optical apparatus 410b-n in the optical network 400 may work equivalently.

The first optical source 406 and second optical source 408 use the input electrical signals to modulate their output light pulses to encode data associated with the electrical signals. In this example the first optical source 406 and second optical source 408 have the same polarisation but have different wavelengths that have a frequency separation of 30GHz.

Other frequency separations may also be used. Furthermore, other configurations of optical sources in the optical apparatus 410a may be used as described above, including for figures 8a-8c.

The output of the optical sources are combined in optical combiner 112a. Because both the wavelengths of the optical sources 406, 408 are fall within the supported passband of the optical channel of the AWG 114 they are input into, both wavelengths get routed through the optical network in a similar way to that described above for figure 4. The two complementary receivers 122a and 122b of the receiver assembly receive the respective first and second signals. In an alternative arrangement where the first and second sources 406, 408 have a substantially similar or identical wavelength but a different polarisation, the filter 118a may be replaced by a polarisation splitter. In arrangements where three or more optical signals within the same common passband are differentiated by wavelength and polarisation then both one or more polarisation splitters and one or more wavelength filters may be used to split the signals and optically route them to separate receivers (typically one receiver per signal).

The optical system may therefore comprise at least one optical apparatus, preferably a plurality of optical apparatus, and a wavelength multiplexer for receiving the first and further optical signals of each optical apparatus, and outputting the first and second optical signals along a common optical path towards the wavelength demultiplexer. Any of the wavelength multiplexer or wavelength demultiplexer comprises an Arrayed Waveguide Grating, AWG. The optical system may further comprise the wavelength demultiplexer.

The optical system may therefore further comprise an optical element for receiving the one or more optical signals from the wavelength demultiplexer and outputting: i) the first optical signal in a first optical output path; and, ii) at least one of the one or more further optical signals in a further optical output path that is spatially separate from the first optical output path. The further optical path being distinct from the first output optical path. The optical element may comprise any one or more of: a wavelength filter; a polarisation splitter.

As described above in figure 9a, the optical system further comprising one or more optical receivers for receiving the first and further optical signals and outputting corresponding electrical signals. The optical system may comprise one or more base stations for receiving the electrical signals and transmitting radio waves associated with the said received electrical signals. The optical network may be a Passive Optical Network, PON, although other types of optical network may be application. Furthermore, the optical apparatus of the second aspect may be used in other non-network applications as described elsewhere herein.

Figure 9b shows an alternative example of the optical apparatus wherein the first and second optical signals input into the common AWG input channel have both a different wavelength and a different polarisation, similar to the schematic example shown in figure 8b. The components of figure 9b are similar to those of figure 9a with like numerals representing like components apart from the modification described as follows.

In figure 9b, the optical output of the second source 408 is additionally input into a polarisation rotator 411, which may alter the polarisation of light, for example to make it orthogonal to the light output from the first source 406. Item 413, replacing combiner 112a of figure 9a, may therefore be an optical intensity combiner, a wavelength combiner or a polarisation combiner. Instead of requiring component 411, the second optical source may be configured to output light at this different polarisation. The first 406 and second 408 optical sources may also alternatively generate light of substantially the same wavelength, in which case the element 413 may be an optical intensity combiner or a polarisation combiner. Where further optical sources are used (i.e. three or more sources in total per optical apparatus 410a), for example to create the set of optical signals in figure 8a, multiple optical combining elements may be used to combine the sources including one or more of any of the combiners described above.

There is further presented a method of assembling an optical system. The method comprising: optically connecting the optical apparatus to a wavelength multiplexer. The wavelength multiplexer for receiving the first and further optical signals and outputting the first and second optical signals along a common optical path towards the wavelength demultiplexer.

This method may be adapted with any step or feature described herein, particularly, but not limited to features describes in the first and/or second aspect. The method may relate to the assembly of the whole or at least part of the network of figures 9a or 9b as described above.

This method of assembling may therefore be used to create part of a new optical network. Alternatively, this method may be used to adapt an existing optical network by adding in or replacing optical sources and/or adding in and replacing the wavelength multiplexer. The method may include other steps such as optically/electrically connecting any of the other optical/electrical components as described above in the first and second aspects.

There is also presented a method for outputting optical signals onto an optical network, the method comprising: I) outputting a first optical signal 312a centred at a first wavelength and having a first polarisation; II) outputting one or more further optical signals 312b that comprise at least one of: A) a wavelength centred at a second wavelength that is different to the first wavelength; B) a polarisation that is different to the first polarisation; Ill) inputting the first and one or more further optical signals into a first of a plurality of input channels of a wavelength multiplexer; the first and one or more further optical signal wavelengths being within a common first wavelength passband 305 of the wavelength multiplexer; IV) wavelength multiplexing, with the wavelength multiplexer, the: first and one or more further optical signals; with, other optical signals from a second optical passband of the wavelength multiplexer associated with a second input channel, into a common optical output for transmission onto the optical network.

This method may be adapted with any step or feature described herein, particularly, but not limited to features describes in the first and/or second aspect. The method may relate to the operation of the whole or at least part of the network of figures 9a or 9b as described above.

There is also presented a method for generating electrical signals for a communications network, for examples the network described and shown in figures 9a or 9b. The method comprising: I) receiving, by an optical wavelength demultiplexer, i) a first optical signal centred at a first wavelength and having a first polarisation, ii) one or more further optical signals that comprise at least one of: A) a wavelength centred at a second wavelength that is different to the first wavelength; B) a polarisation that is different to the first polarisation; ii) outputting, to one or more optical receivers, the first and one or more further optical signals in the same optical output channel of the optical wavelength demultiplexer; iii) receiving the first and one or more further optical signals using one or more optical receivers; iv) generating electrical signals, using the one or more optical receivers. These optical signals may be output to an end terminal such as a cell tower as shown in figure 9a.

This method may be adapted with any step or feature described herein, particularly, but not limited to features describes in the first and/or second aspect. This method may relate to the network shown in figures 9a and 9b wherein features of those examples may be used with this method.

The method may further provide for: receiving the first and one or more further optical signals with an optical element and: i) outputting the first optical signal to a first optical receiver of the one or more optical receivers; and, ii) outputting the one or more further optical signals to one or more further optical receivers of the one or more optical receivers.

The optical element may be any one or more of an optical wavelength filter, an optical polarisation splitter.

It is to be appreciated that features described elsewhere in other aspects herein may be used with the second aspect. Such features may be, for example, but not limited to: signal types, components, modulation schemes/formats, optical sources, the number of optical sources used to create a multiple modulated signal, the receiver configurations, any other optical component used in the network such as filters and electronic apparatus.

Further examples associated with the second aspect are now described. These examples may be adapted with any step or feature described herein, particularly, but not limited to features describes in the first and/or second aspect. These examples may relate to the network shown in figures 9a and 9b wherein features of those examples may be used with these further examples.

There is also presented an optical apparatus comprising a wavelength demultiplexer; the wavelength demultiplexer configured to: I) receive a plurality of optical signals comprising: i) a first optical signal centred at a first wavelength; ii) a second optical signal centred at a second wavelength that is different to the first wavelength; the second optical signal being wavelength adjacent to the first optical signal; iii) a third optical signal centred at a third wavelength that is different to the first and second wavelengths; the third optical signal being wavelength adjacent to the first optical signal; II) output light into a plurality of spatially separated optical bandpass channels such that: the first and second optical signals are output in a first of the optical bandpass channels; the third optical signal is output in a second of the optical bandpass channels.

There is also presented an optical communication apparatus comprising an optical communications channel for carrying a plurality of optical signals. The plurality of optical signals each having different wavelengths to each other and comprising a first optical signal that is: i) wavelength adjacent, by a first frequency spacing, to a second optical signal ii) wavelength adjacent, by a second frequency spacing, to a third optical signal; the first frequency spacing being different to the second frequency spacing.

The optical apparatus may therefore transmit down the same channel, such as an integrated optic waveguide or an optical fibre, different signal wavelengths with unequal spacings. This may be useful when demultiplexing the signals into sets of wavelengths. The term 'wavelength adjacent' is intended to mean the closest (in wavelength) next optical signal to the first optical signal. Since a wavelength adjacent optical signal may have a smaller or larger wavelength than the first optical signal, the first optical signal may have two wavelength adjacent signals in the same said plurality of optical signals. For example, the wavelength adjacent second optical signal has a lower wavelength than the first optical signal is closely spaced in wavelength to the first optical signal, for example by a frequency spacing of 30GHz. Conversely, the wavelength adjacent third optical signal has a higher wavelength than the first optical signal but is more widely spaced away from the first signal than that of the second signal, for example by a frequency spacing of 40GHz.

By having different frequency spacings between different wavelength adjacent optical signals, an optical system receiving the three signals (or more) may apply a wavelength filter that is optimised to take advantage of the wider frequency spacing between the first and third optical signals. The larger the frequency spacing between different wavelength signals, the simpler the filter design is and the more forgiving the system is to filter wavelength-edge variances.

The optical demultiplexer comprises an Arrayed Waveguide Grating. Any one or more of the first second or third optical signals may be modulated with a multilevel amplitude modulation scheme such as PAM4.

Third aspect In a third aspect there is presented an optical apparatus that may be used for communications although in principle the apparatus may have applications that are not communications oriented. As such, the third aspect may be referred to hereinafter as the 'optical communications apparatus' or the 'optical apparatus'. A schematic diagram exemplifying the optical apparatus is shown in figure 10.

The optical apparatus 510 may be for an optical network 500. The optical network 500 comprising a wavelength demultiplexer 502 comprising a plurality of spatially separated optical output paths 500, 504a, 504b and having a Free Spectral Range, FSR; each optical output path 504 is associated with a plurality of different optical wavelength passbands 505a, 505b separated by one or more of the FSR. The optical wavelength passbands for a demultiplexing AWG are described elsewhere herein. The optical apparatus 510 comprises one or more optical sources 506, 508 for generating a plurality of optical signals 512. Figure 10 shows two optical sources 506, 508 (one for each signal) however there may be one optical source or three or more optical sources for generating the plurality of signals as discussed in other aspects herein.

The plurality of optical signals 512 may comprise at least a first optical signal 512a centred at a first wavelength, the first wavelength being in a first optical wavelength passband 505a of a first optical output path 504a. The plurality of optical signals 512 comprising at least one or more further optical signals 512b centred at a second wavelength, the second wavelength in a second optical wavelength passband 505b of the first optical output path; the first optical wavelength passband 505a being different to the second optical wavelength passband 505b.

There is also presented a method of assembling an optical system. The method comprising: optically connecting the optical apparatus, as described above, to a wavelength multiplexer.

The wavelength multiplexer for receiving the first and further optical signals and outputting the first and second optical signals along a common optical path towards the wavelength demultiplexer.

This method may be adapted with any step or feature described herein, particularly, but not limited to features describes in the first and/or second aspect. The method may relate to the assembly of the whole or at least part of the network of figures 9a or 9b as described above in the second aspect but adapted as described below.

This method of assembling may therefore be used to create part of a new optical network.

Alternatively, this method may be used to adapt an existing optical network by adding in or replacing optical sources and/or adding in and replacing the wavelength multiplexer. The method may include other steps such as optically/electrically connecting any of the other optical/electrical components as described above in the first or second or third aspects.

The other optical output paths 504 of the wavelength demultiplexer 502 also have multiple optical passbands such as passbands 505c and 505d shown in figure 10.

Other optional featured and configurations described in other aspects, for example the second aspect may apply to the third aspect. For example; any of: the input of the electrical signal from the edge network; the number of optical sources per optical apparatus, the number of optical apparatus used in the network; the types of components such as optical sources, combiners, filters, multiplexer, demultiplexer, optical fibres, component-component optical connections, how components are assembled or integrated, the number and type of end terminals, the electronic apparatus used and the variations thereof.

In figure 10, the insets in dotted lines at the outputs 504a/b of the demultiplexer 502 are schematic representation of the optical wavelength response 505 of that optical output channel. In figure 10 there is shown two optical wavelength passbands 505a-d for each channel, namely 505a and 505a for channel 504a and 505c and 505d for output channel 504b however each optical output channel may support more than two other optical passbands. The optical passbands of the different channels are different to the other output channels. This allows for each optical output channel 504 to carry multiple wavelength signals in different optical passbands that are different, in wavelength, to any other signal propagating through the demultiplexer, hence, through the optical network the demultiplexer is part of. As in other aspects, this aspect is described in relation to AWGs however other grating devices may be used.

This aspect may therefore use the optical properties of the AWG to add additional wavelength channels so that the data capacity per fibre-connected antenna is increased. The aspect may therefore use the cyclic nature of the AWG design to add wavelength channels at different free-spectral ranges of the AWG so that the additional channels emerge from the same fibre output of the AWG.

In comparison to the standard network in figure 1B, the aspect exemplified in figure 10, additional one or more wavelength channels can be sent to an end terminal, such as an antenna, down the same optical output 504 from the demultiplexing AWG 502. The optical apparatus is not limited to using two optical passbands per AWG optical output channel, the apparatus may provide three or more wavelengths per channel.

An example of a network encompassing the apparatus of the second aspect may be similar to that shown in figure 9a of the second aspect. The description of figure 9a equally applies to this aspect apart from the following differences.

At the exchange, the transmitters (or transceivers) 406, 408 of each optical apparatus output wavelengths within different optical wavelength passbands associated with the same input channel of the multiplexing AWG 114. Thus, both wavelengths with the different optical passbands are routed through the optical fibre 112 and into the demultiplexing AWG 116. The optical combiners 112a may be optical wavelength combiners as described elsewhere herein or may be simple power combiners wherein around half the optical power from each optical signal is lost upon combining both signals into a single multiplexed physical channel.

The demux AWG 116 routes both wavelength signals from sources 406/408 of the same optical apparatus into a common physical output channel. The demux and mux AWG 116, 114 are sufficiently similar such that the demux AWG 116 output channels have optical passbands that closely match the those of the mux AWG 114 input channels.

The filters 118a for the third aspect have spectral responses that spatially route the different wavelengths to different optical receivers 122a, 122b. Similar to other examples described herein the receivers may be set-up in different ways.

Each receiver 122a/b (or transceiver if light signal are required to be sent upstream in the network) may receive its associated wavelength light signals, generate corresponding electrical signals and send those signals to a separate end terminal such as a 5G cell tower or other antenna. Thus, the electronic apparatus 124a in figure 9a may not be required or may be bypassed.

Each receiver may be part of a receiver assembly 120a having a plurality of receivers 122a, 122b or it may be separate to the other receivers linked to the demux AWG 116.

The receivers 122a/b of a receiver assembly may route both its electrical signals to the same end terminal. This may be done by routing them into an electronic apparatus 124a. The electronic apparatus may perform any processing on the electrical signals such as, but not limited to: cleaning the signals, perform error correction, combine the electrical signals into a composite signal, split off the electrical signals, store the electrical signals.

There is also presented a method for transmitting optical signals onto an optical network. This method relates to the example shown in figure 10 and the example described above for the third aspect that relates to similar equipment used in figure 9a. This method may be adapted with any step or feature described herein, particularly, but not limited to features describes in the first and/or second and/or third aspects.

The method comprises: outputting a first optical signal 512a centred at a first wavelength; and also, outputting one or more further optical signals 512b at a wavelength centred at a second wavelength that is different to the first wavelength. The method further comprises inputting the first and one or more further optical signals into a first of a plurality of input channels of a wavelength multiplexer. The wavelength multiplexer has a Free Spectral Range, FSR. The first wavelength being within a first optical wavelength passband associated with the first input channel. The second wavelength being within a second optical wavelength passband associated with the first input channel. The first and second optical wavelength passbands are separated by one or more of the FSR. The method the further provides wavelength multiplexing, with the wavelength multiplexer, the: first and one or more further optical signals; with, other optical signals from a second input channel of the wavelength multiplexer. These are multiplexed into a common optical output for transmission onto the optical network.

The method may therefore be associated with the processes happening at the exchange as described above.

Similarly, there now follows a further method associated with the processes occurring downstream from the above method, that create electrical signals to send to end terminals such as 5G cell towers and other end terminals as described elsewhere herein. This method relates to the example shown in figure 10 and the example described above for the third aspect that relates to similar equipment used in figure 9a. This method may be adapted with any step or feature described herein, particularly, but not limited to features describes in the first and/or second and/or third aspects.

There is also presented method for generating electrical signals for a communications network. The method comprises: receiving, by an optical wavelength demultiplexer, i) a first optical signal centred at a first wavelength; and, ii) one or more further optical signals centred at a second wavelength that is different to the first wavelength. The wavelength multiplexer having a Free Spectral Range, FSR. The first wavelength being within a first optical wavelength passband. The second wavelength being within a second optical wavelength passband. The first and second optical wavelength passbands being separated by one or more of the FSR. The method further comprises outputting, to one or more optical receivers, the first and one or more further optical signals in the same optical output channel of the optical wavelength demultiplexer; the first and second optical wavelength passbands associated with the optical output channel. The method further comprises receiving the first and one or more further optical signals using one or more optical receivers. The method further comprises generating electrical signals, using the one or more optical receivers.

The method may also provide for: receiving, from the optical wavelength demultiplexer, the first and one or more further optical signals, with an optical element. The method may further provide i) outputting the first optical signal to a first optical receiver of the one or more optical receivers; and, ii) outputting the one or more further optical signals to one or more further optical receivers of the one or more optical receivers.

Therefore, the different signals on the different wavelengths may be associated with different data streams intended for different end terminals. The optical element splits these signals from a comp physical output channel of the AWG into separate physical channels that are routed onwards downstream to the end terminals. If the signals differ in another variable such as polarisation, they may be split using a polarisation splitter as an alternative.

Each end terminal may have its own associated transceiver or other transmitter/receiver apparatus combinations. The transceiver may connect antenna on a DWDM-PON. For example, the network (hence apparatus) may contains three separate laser sources (at 2c1, X'1 and X"1) and 3 receivers (at the corresponding wavelengths) for each end terminal. The downstream transceivers contain optical filters to separate the three wavelength channels.

Since the three wavelengths are widely spaced (FSR typically 50-70nm), then the filters are simple to fabricate, small and low cost.

Any one or more of these optional functionalities may be application to other examples of other aspects described herein.

Consider figure 11 which shows an example where an optical fibre, for example around a 20km length, carries light that, when demultiplexed by the AWG creates a single optical output with three (or more) different wavelengths ( X1, X'1 and X"1), each associated with a different wavelength optical passband of that physical output channel. One or more optical wavelength filters can be used to divide the different wavelengths to different end terminals, such as antenna. Such a filter may have multiple optical outputs that are used, in turn to route light to the correct antenna. Alternatively, there may be a serial cascade of optical filters optically linked to each other wherein each optical filter routes light of one of the wavelengths to an end terminal whilst routing all the other wavelengths to the next filter in the cascade which picks out another wavelength for routing to a further different antenna whilst sending the rest of the wavelengths to the next filter, until the last filter which receives two wavelengths are routes each to two separate terminals.

There may therefore be a transceiver for an antenna on the DWDM-PON that contains 3 separate laser sources (at X1, 2/1 and 2J"1) and 3 receivers (at the corresponding wavelengths). The transceivers may contain optical filters to separate the three wavelength channels. Since the three wavelengths may be widely spaced (FSR typically 50-70nm), then the filters are simple to fabricate, small and low cost.

Therefore, instead of or in addition to increasing the data capacity to a given antenna, the apparatus can also be used to increase the deployment of new antennae from the existing deployed antennae. Instead of terminating the added wavelengths of the AWG FSRs at a single antenna, one or more wavelength filters can be used (for example) placed next to the installed antenna, so that the new optical signal is routed to a new antenna that is newly deployed. This provides a more scalable and simpler way to increase the number of antennae without installing new feeder fibre.

The features of any of the above examples in any of the above aspects may be combined together. For example, figure 12 shows an example of an optical apparatus encompassing elements of the first, second and third aspects. The example shown in figure 12 is only of the components used at the exchange and is similar to that of the similar features shown and described in figure 9A, wherein like numerals represent like components. A corresponding receiver set-up would be used downstream at the end terminals.

The optical apparatus 610a comprises a plurality of optical sub-assemblies 630a-d. The sub-assemblies 630a-d may be separate devices or may be integrated with one or more other of the assemblies 630a-d. Each optical sub assembly 630a-d comprising a plurality of optical sources similar to the optical apparatus 110a-n shown and described for figure 4. Each source, having a beginning portion of its label as ITXX'. Each sub assembly 630a-d therefore, by itself, outputs optical a plurality of optical signals that form part of a composite signal using a multi-amplitude modulation scheme such as those depicted in figures 3a-3c or described elsewhere herein. As described in the first aspect, there may be just one source that the plurality of different signals are derived from.

Electrical signals 602 from the edge network are distributed to the electronic apparatus 604 associated with each sub assembly 630a-d. In this example a single electrical communication channel is provided to the respective electrical apparatus 604 wherein the electrical apparatus 604 transmits corresponding electrical control signals to each of the sources to drive the source. As described in the first aspect, more than two sources may be used.

In figure 12, the topmost electrical signals 602 are sent to the topmost electrical apparatus 604 which in turn sends separate control signals to TXX1 and TXX2 of the first optical sub assembly 630a. In this example the optical sub-assemblies 630a-d each outs a PAM4 optical signal data stream wherein each signal in the multi amplitude modulation may comprise light from both TX2c1 and TXX,2. Other optical sub-assemblies 630b-d work in a similar way. The output from each sub assembly 630a-d is input in its own respective 2-1 optical combiner 632a-d similar that described in the first aspect. In this example the optical combiner is an optical power coupler that reduces the optical power by 3dB from each optical signal, however a wavelength multiplexing combiner may be used.

The wavelengths of TXX.1 and TXX2 are wavelength separated by 0.5-5GHz in this example, but may be any other range or value as described herein. The wavelengths of TXX3 and TX2L4 are wavelength separated by 0.5-5GHz in this example, but may be any other range or value as described herein. The wavelengths of the two sources of the other sub-assemblies 630c/d are separated similarly.

All the optical sub-assemblies 630a-d have their light eventually routed into the same physical input channel 635 of the mux AWG 114. A first plurality 630a, 630b of the optical sub-assemblies transmits light having wavelengths within a first optical wavelength passband of the AWG input channel 635 whilst a second plurality 630c, 630d of the optical sub-assemblies transmits light having wavelengths within a second different optical wavelength passband of the same AWG input channel 635. Thus, similarly to the second aspect, multiple wavelengths associated with different optical signal data are input the same optical passband of the input channel 635. Additionally, or alternatively, different polarisations sources may be used as described in the second aspect. If different polarisation sources are used then the appropriate polarisation combiners are employed in the combining of the optical signals into the same input AWG channel 635.

In figure 12 the sources of the second plurality are labelled TXX1 + FSR, TXX1 + FSR, TXX2 + FSR, TXX4 + FSR to denote that they have wavelengths of the first plurality of sub assembly sources plus the FSR.

In each of the first plurality of sub-assemblies 630a/b the first sub assembly sources TX2L1 and TXX.2 are wavelength (frequency) separated by 20-35GHz to the wavelengths of TX2L3 and TXX.4. This separation may be from nearest wavelength of each subassembly to the adjacent sub-assemblies nearest wavelength. Alternatively, the combined PAM4 modulation aggregate wavelength may be the wavelength used to determine the separation. The outputs of the first and second sub-assemblies that are combined by optical combiners 632a and 632b input into a second optical combiner 634a which combines the light into a single optical output channel.

The same arrangement happens for the sources of the second plurality of sub-assemblies 630c/d using second optical combiner 634b. These second optical combiners 634a, 634b may be power combiners or other combiners as described herein. In this example they are unbalanced Mach-Zehnder interferometers.

The optical outputs from: combiner 634a, which correspond to the first plurality of sub-assemblies 630a/b having optical source wavelengths in the first passband; and, combiner 634b, which correspond to the second plurality of sub-assemblies 630c/d having optical source wavelengths in the second passband; are input to a third optical combiner 112a similar to that of figure 9a.

In this case this is another unbalanced Mach-Zehnder optical interferometer. The output of that combiner is input into the AWG input channel 635. The optical connections between each of the sources and combiners may be any type of connections including, integrated optic, free-space and optical fibre.

In the example shown in figure 12, all of the light from all the optical sources in the optical apparatus 610a are input into the same AWG input channel 635. Other similar optical apparatus 601b similarly feed light through combiners into other input channels of the AWG. As per other examples herein, the AWG 114 multiplexes all the light from all the optical apparatus 610a, 610b (and possibly other apparatus) into the same optical output channel 112 for sending down to the demultiplexing AWG which may have complementary combinations of splitters and receivers.

The optional features and adaptions of the first, second and third aspects apply to the example. Furthermore, some parts of this example may not be required, for example: the apparatus 610a may use multiple sources to create PAM4 signals for the same data and use multiple AWG passbands to have different data streams in the same AWG input channel but both use multiple data stream signals in each passband. (i.e. combining the first and third aspect). Other combinations are possible, for example using configurations from the first aspect and second aspect; the second aspect and the third aspect.

Claims

Claims 1. An optical apparatus for an optical network; the optical network comprising a wavelength demultiplexer comprising a plurality of spatially separated optical output paths; each optical output path associated with a different optical wavelength passband; the optical communication apparatus comprising one or more optical sources for generating a plurality of optical signals; the plurality of optical signals comprising at least: I) a first optical signal centred at a first wavelength and having a first polarisation; II) one or more further optical signals that comprise at least one of: A) a wavelength centred at a second wavelength that is different to the first wavelength; B) a polarisation that is different to the first polarisation the first and one or more further optical signal wavelengths being within a common first wavelength passband of the wavelength demultiplexer.
2. An optical apparatus as claimed in claim 1 wherein the one or more further optical signals comprises: a second optical signal comprising second wavelength that is different to the first wavelength; a third optical signal comprising a polarisation that is different to the first polarisation.
3. An optical apparatus as claimed in any preceding claim wherein at least one of the one or more further optical signals comprises: A) a wavelength centred at a second wavelength that is different to the first wavelength; B) a polarisation that is different to the first polarisation.
4. An optical apparatus as claimed in any preceding claim wherein at least one of the one or more further optical signals comprises: A) a wavelength centred at a second wavelength that is different to the first wavelength; B) a polarisation that is substantially the same to the first polarisation.
5. An optical apparatus as claimed in claim 4 wherein the optical frequency spacing between the first and second optical signals is in the range 20GHz-40GHz.
6. An optical apparatus as claimed in any preceding claim wherein at least one of the one or more further optical signals comprises: A) a wavelength centred at the first wavelength; B) a polarisation that is different to the first polarisation.
7. An optical apparatus as claimed in any preceding claim wherein the plurality of optical signals is associated with a series of different wavelengths; at least one of the further optical signals comprises one of the adjacent wavelengths, in the said series, to the first 15 wavelength.
8. An optical apparatus as claimed in any preceding claim wherein the one or more optical sources comprises a plurality of optical sources comprising: a first optical source for outputting the first optical signal; a second optical source for outputting the further optical signal.
9. An optical apparatus as claimed in any preceding claim wherein at least one of the one or more optical sources comprises a laser.
10. An optical apparatus as claimed in any preceding claim wherein at least one of the one or more optical sources is wavelength tuneable.
11. An optical apparatus as claimed in any preceding claim wherein the plurality of optical signals comprises a third optical signal centred at a third wavelength that is: i) different to the first and second wavelengths; ii) within a second passband of the wavelength demultiplexer that is different to the first passband of the wavelength demultiplexer.
12. An optical apparatus as claimed in any preceding claim wherein at least one of the first and second optical signals is modulated using a multi-amplitude modulation.
13. An optical apparatus as claimed in any claim 12 wherein the multi-amplitude modulation is PAM4.
14. An optical system comprising an optical apparatus as claimed in any preceding claim; and a wavelength multiplexer for receiving the first and further optical signals and outputting the first and second optical signals along a common optical path towards the wavelength demultiplexer.
15. An optical system of claim 14 wherein any of the wavelength multiplexer or wavelength demultiplexer comprises an Arrayed Waveguide Grating, AWG.
16. An optical system of claims 14 or 15 further comprising the wavelength demultiplexer as defined in claim 1.
17. An optical system of any of claims 14-16 further comprising an optical element for receiving the first one or more further optical signals and from the wavelength demultiplexer and outputting: i) the first optical signal in a first optical output path; ii) at least one of the one or more further optical signals in a further optical output path that is spatially separate from the first optical output path.
18. An optical system of claim 17 wherein the optical element comprises any of: a wavelength filter; a polarisation splitter.
19. An optical system of any of claims 14-18 further comprising one or more optical receivers for receiving the first and further optical signals and outputting corresponding electrical signals.
20. An optical system of any of claims 14-19 comprising one or more base stations for receiving the electrical signals and transmitting radio waves associated with the said received electrical signals.
21. A method of assembling an optical system; the method comprising: optically linking an optical apparatus, as claimed in any of claims 1-13, to a wavelength multiplexer for receiving the first and further optical signals and outputting the first and second optical signals along a common optical path towards a wavelength demultiplexer.
22. A method of assembling an optical system as claimed in claim 21 further comprising: optically linking an optical element to the wavelength demultiplexer as defined in claim 1; the optical element for receiving the first one or more further optical signals output from the wavelength demultiplexer; and in turn, outputting: iii) the first optical signal in a first optical output path; iv) at least one of the one or more further optical signals in a further optical output path that is spatially separate from the first optical output path.
23. A method for transmitting optical signals onto an optical network, the method comprising: I) outputting a first optical signal 312a centred at a first wavelength and having a first polarisation; II) outputting one or more further optical signals 312b that comprise at least one of: A) a wavelength centred at a second wavelength that is different to the first wavelength; B) a polarisation that is different to the first polarisation; III) inputting the first and one or more further optical signals into a first of a plurality of input channels of a wavelength multiplexer; the first and one or more further optical signal wavelengths being within a common first wavelength passband 305 of the wavelength multiplexer; IV) wavelength multiplexing, with the wavelength multiplexer, the: first and one or more further optical signals; with, other optical signals from a second optical passband of the wavelength multiplexer associated with a second input channel, into a common optical output for transmission onto the optical network.
24. A method for generating electrical signals for a communications network; the method comprising: I) receiving, by an optical wavelength demultiplexer: i) a first optical signal centred at a first wavelength and having a first polarisation, ii) one or more further optical signals that comprise at least one of: A) a wavelength centred at a second wavelength that is different to the first wavelength; B) a polarisation that is different to the first polarisation; ii) outputting, to one or more optical receivers, the first and one or more further optical signals in the same optical output channel of the optical wavelength demultiplexer; receiving the first and one or more further optical signals using one or more optical receivers; iv) generating electrical signals, using the one or more optical receivers.
25. The method of claim 24 further comprising: receiving, from the optical wavelength demultiplexer, the first and one or more further optical signals, with an optical element, and: iii) outputting the first optical signal to a first optical receiver of the one or more optical receivers; and, iv) outputting the one or more further optical signals to one or more further optical receivers of the one or more optical receivers.