WO2021148118A1

WO2021148118A1 - Optical routing

Info

Publication number: WO2021148118A1
Application number: PCT/EP2020/051544
Authority: WO
Inventors: Roberto Magri; Alberto Deho; Stefano Orsi; Giovanni Fiaschi
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2020-01-22
Filing date: 2020-01-22
Publication date: 2021-07-29
Also published as: EP4094384A1

Abstract

An optical routing apparatus for an optical network. The optical routing apparatus comprises an optical transmitter configured to transmit an optical signal at a selected one of a first wavelength and a second wavelength, and a controller configured to control the optical transmitter to transmit the optical signal at a selected one of the first wavelength and second wavelength. The optical routing apparatus further comprises a passive optical device comprising an input port connected to the optical transmitter and configured to receive the optical signal at the selected one of the first wavelength and second wavelength. The passive optical device comprises a first output port connected to a first optical line, and a second output port connected to a second optical line. The passive optical device is configured to connect the optical signal at the input port to one of the first output port or second output port based on the wavelength of the optical signal. The controller is configured to control the optical transmitter to transmit the optical signal at the other one of the first wavelength and second wavelength.

Description

Optical Routing

Technical Field

The disclosure relates to an apparatus and method for Optical routing. In some examples, the disclosure provides for optical protection.

Background

Optical protection schemes have been widely used in different architectures and solutions for years. A common scheme includes switches and optical splitters. For example, Figure 1 shows a 1+1 protection scheme 1. A multiplexer 2 comprises a plurality of input ports 3 and a single output port 4, outputting a plurality of optical wavelengths which are wavelength division multiplexed (WDM). The output port 4 is connected to an input port 5 of a switch 6. The switch 6 is controlled to connect the input port 5 to either a first output port 7 for transmission over a normal link 8, or to a second output port 9 for transmission over a protection link 10. Both the normal link 8 and protection link 10 terminate at a coupler/splitter 11. The coupler/splitter 11 outputs the wavelengths to a demultiplexer 12. Thus, a switch 6 is used on one side of the optical link and couplers/splitters on remote nodes. In this example, an active 1 :2 switch 6 is controlled by a control logic to connect its input port 5 to one of the output ports 7,9, e.g. in response to detecting a fault.

This known protection scheme involves the addition of splitters and switches. Both components add significant loss to the optical budget (especially splitters) reducing the allowed number of concatenated remote nodes, and the switches require active hosts to be powered. This requirement for electrical power prevents solutions which re purely passive, which are preferred in cost sensitive, high availability environments such as a Radio Access Network. Summary

The disclosure provides, in a first aspect, an optical routing apparatus for an optical network. The optical routing apparatus comprises an optical transmitter configured to transmit an optical signal at a selected one of a first wavelength and a second wavelength, and a controller configured to control the optical transmitter to transmit the optical signal at a selected one of the first wavelength and second wavelength. The optical routing apparatus further comprises a passive optical device comprising an input port connected to the optical transmitter and configured to receive the optical signal at the selected one of the first wavelength and second wavelength. The passive optical device comprises a first output port connected to a first optical line, and a second output port connected to a second optical line. The passive optical device is configured to connect the optical signal at the input port to one of the first output port or second output port based on the wavelength of the optical signal. The controller is configured to control the optical transmitter to transmit the optical signal at the other one of the first wavelength and second wavelength.

The disclosure provides, in a second aspect, an optical routing apparatus for a node an optical network. The optical routing apparatus comprises an optical transmitter configured to transmit an optical signal at a selected one of a first wavelength and a second wavelength, and a controller configured to control the optical transmitter to transmit the optical signal at a selected one of the first wavelength and second wavelength. The apparatus further comprises a plurality of filters comprising a first filter and a second filter, the first filter and the second filter configured to receive the optical signal at the selected one of the first wavelength and second wavelength. The first filter comprising a first output port connected to a first optical line, and the second filter comprising a second output port connected to a second optical line. One of the first filter and second filter is configured to connect the optical signal at the input port to one of the first output port or second output port based on the wavelength of the optical signal. The controller is configured to control the optical transmitter to transmit the optical signal at the other one of the first wavelength and second wavelength.

The disclosure provides, in a third aspect, a node comprising the optical routing apparatus as claimed in any example, wherein the node comprises radio access network equipment.

The disclosure provides, in a fourth aspect, an optical network comprising: a hub node comprising the optical routing apparatus according to any example, and at least one remote node configured to receive the optical signal from the hub node at one of the first wavelength and second wavelength. The optical network comprises a first optical link connecting the first output port of the hub node and the remote node, and a second optical link connecting the second output port of the hub node and the remote node.

The disclosure provides, in a fifth aspect, a method of optical routing at an optical routing apparatus connected to a first optical line and a second optical line. The method comprising transmitting an optical signal from the optical routing apparatus on a first wavelength on one of the first optical line and second optical line, and determining to switch the optical signal to the alternate one of the first optical line and second optical line. The method comprises controlling the transmitting of the optical signal to an alternative, second, wavelength. The second wavelength is routed by a passive optical device to the alternate one of the first optical line and second optical line.

Brief Description of the drawings

Embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings.

Figure 1 illustrates a prior art optical protection switching apparatus;

Figure 2 illustrates an example of an optical system according to the present disclosure;

Figures 3a and 3b illustrate the functioning of a passive optical device of the optical routing apparatus;

Figure 4 illustrates a further example of an optical system comprising the optical routing apparatus;

Figure 5 illustrates an Optical Add/Drop node according to an example of the disclosure;

Figure 6 illustrates a further example of an optical system comprising the optical routing apparatus;

Figure 7 illustrates an Optical Add/Drop node according to a further example of the disclosure;

Figure 8 illustrates a state machine to an example of the disclosure; and

Figure 9 illustrates steps of methods according to embodiments of the disclosure;

Figures 10a and 10b illustrate an alternative architecture of the optical network according to examples of the present disclosure; and Figures 11 a and 11 b illustrate a further example of the present disclosure.

Detailed description

The present disclosure relates to an optical routing apparatus and method for an optical network. The apparatus and method avoids the need for switches, and minimize the need of splitters. The optical routing apparatus is applicable to optical network in form of a line and ring architecture. In some examples, the routing provides for optical protection. In this case, the optical routing apparatus may be considered as an optical protection apparatus.

Aspects of the disclosure provide for optical protection using an optical transmitter operable to transmit an optical signal at different wavelengths, e.g. a first wavelength and a second wavelength. The wavelengths are input into a passive optical device, e.g. an arrayed waveguide grating (AWG). The passive optical device has the characteristic of outputting the optical signal to a particular output port according to the wavelength of the optical signal. The transmitter is controlled to generate the optical signal with a wavelength dependent on whether the optical signal is to be transmitted over a first optical link (e.g. a “normal” link) or a second optical link (e.g. “alternative” or “protection” link).

Figure 2 shows an optical system 15 comprising an optical routing apparatus 100 for an optical network 20. The optical network 20 comprises a first link 22 and a second link 24. In some examples, the first link 22 and the second link 24 are provided by a single optical line or single optical fiber. For example, the single optical line is in the form of a ring, and the first link 22 and the second link 24 correspond to different directions of transmissions around the ring, e.g. clockwise and anti-clockwise. In other examples, the first link 22 and the second link 24 are provided by separate first and second optical lines or optical fibers.

The optical network 20 connects two nodes or a plurality of nodes. In some examples, both or a plurality of nodes comprise an optical routing apparatus 100. Alternatively, only one node comprises the optical routing apparatus 100. The further one or more nodes are configured to receive the optical signal over the first or second links. In some examples, the optical network 20 carries wavelength division multiplexed (WDM) signals. The example shown is a point to point architecture, with symmetric optical routing apparatus at each node, i.e. both sides of the linear optical links 22,24.

In some aspects, the optical network connects a hub node comprising a first optical routing apparatus 100a to one or more remote nodes, e.g. comprising a second optical routing apparatus 100b. In some examples, the optical network is in a radio access network. The nodes comprise radio access network equipment. For example, a hub node comprises a baseband unit for baseband processing of a radio signal. The further one or more nodes comprise radio units, e.g. remote radio units. The optical network 20 may be considered as part of a fronthaul network or a backhaul network, or a combination of a fronthaul and backhaul network.

The optical routing apparatus 100 comprises one or more optical transmitters 110. The transmitters 110 may be part of a transceiver (TRX), i.e. combined with a receiver to transmit and receive optical signals. Examples will be described with respect to transmission, for which the transmitter may be alternatively considered as a transceiver. The transmitters are configured to generate an optical signal for transmission, e.g. based on a received modulated electrical signal in order to transmit data. The receiver is configured to receive an optical signal, and convert it to a modulated electrical signal (i.e. at baseband or an intermediate frequency) for receiving the transmitted data.

The transmitters 110 are each configured to generate an optical signal at a selected wavelength. In some examples, each transmitter 110 generates an optical signal at one wavelength at a time. The transmitter 110 is operable to change the wavelength of the optical signal which is output. As such, the transmitter 110 may be considered as tunable. The transmitter 110 is configured to transmit the optical signal at a selected one of a first wavelength and a second wavelength. The different wavelengths may be generated by tuning of the transmitter to the selected wavelengths, e.g. the transmitter is a tunable laser. Each transmitter 110 is configured to transmit at a first and second wavelengths which are different to the respective first and second wavelength for the other transmitters 110 of the optical routing apparatus 100. As such, the first wavelengths of the plurality of transmitters 110 can be wavelength division multiplexed together for transmission over the optical network 20, and the second wavelengths of the plurality of transmitters 110 can be wavelength division multiplexed together for transmission over the optical network 20. The unique wavelengths provide for WDM and a selection of which link the optical signal will be transmitted on. For example, at the first routing apparatus 100a, a first transmitter 110a is configured to transmit at a selected one of the first wavelength A1 and a different wavelength, second wavelength A2. A second transmitter 110b is configured to transmit at a selected one of the first wavelength A3 and a different wavelength, second wavelength A4. As such, all of the transmitters are configured to only work at different wavelengths from each other at any given time.

In addition to transmission, wavelengths are also received by the first routing apparatus 100a, i.e. transmitted by the second routing apparatus 100b. From the point of view of the first routing apparatus 100a, the received wavelengths re-used from the transmitted wavelengths. Different wavelengths are used for transmission and receiving on the same optical link. This avoids interference. The same wavelength, e.g. A2, is used for receiving on the first optical line 22 and transmitting on the second optical line 24. Since the first optical line 22 and the second optical line 24 are used at different times (i.e. when there is “normal” operation and “protection” operation), the transmitting and receiving of the same wavelength does not occur at the same time or over the same optical line. In the example shown in Figure 2, the first transceiver 110a is configured to transmit and receive one wavelength, i.e. A2. In addition, the first transceiver 11a is configured to transmit (but not receive) a wavelength, i.e. A1 , and receive (but not transmit) a further wavelength, i.e. A3. The same applies to each transceiver 110.

The optical routing apparatus 100 comprises one or more controller 130. In some examples, the optical routing apparatus 100 comprises one controller 130 connected to each transmitter 110, in order to control the optical transmitter to transmit the optical signal at a selected one of the first wavelength and second wavelength. Alternatively, or in addition, the transmitters 110 comprise a controller configured to control the optical transmitter to transmit the optical signal at a selected one of the first wavelength and second wavelength. In some examples, the controller(s) receives or generates a signal indicating that an alternative route should be used. In some aspects, the signal is from a network entity, e.g. controlling routing, e.g. a network management entity. In some examples, the controller(s) 130 is configured to receive, detect or generate a fault signal indicating a fault with an optical link, e.g. the first optical link. The fault signal may be based on a loss of signal over the optical link. The controller is configured to change the output wavelength of the transmitter 110 in response to the fault signal. In some examples, the transmitter is configured to generate an optical signal at a selected one of a first wavelength and a second wavelength. The controller is configured to control the transmitter 110 to transmit at the other one of the first wavelength and second wavelength.

The optical routing apparatus further comprises a passive optical device 120. The passive optical device 120 may be an Arrayed Waveguide Grating (AWG). For ease of reference, the passive optical device 120 will be referred to as a AWG 120, although all references to AWG may be replaced by references to the passive optical device 120. In this example, the transceivers 110 are in the same node as the AWG 120.

The AWG 120 comprise one or a plurality of input ports 122 configured to receive optical signals from a transmitter 110a, or a plurality of the transmitters 110. The input ports 122 are configured to receive either the first or second wavelengths from each of the transmitters 110. The AWG 120 further comprises a first output port 132 connected to a first optical line 22 and a second output port 134 connected to the second optical line 24.

The AWG 120 is configured to connect the optical signal at the input port 122 to one of the first output port 132 or second output port 134 based on the optical signal having the selected one of the first wavelength and second wavelength. The routing by the AWG 120 to one of the first output port 132 or second output port 134 is determined only by the wavelength of the optical signal at the input port 122. Thus, the selection of the first optical line 22 or second optical line 24 for transmission is based only on the wavelength generated by the transmitter 110. The selection of the first optical line 22 or second optical line 24 for transmission is not based on any active control or switching carried out by the AWG 120. As such, the AWG 120 is a passive optical device. The AWG 120 has characteristics which connect an input port receiving the optical signal to a particular one of the first or second output ports 132,134 based only on the wavelength of the optical signal. The AWG 120 is configured to output an optical signal at only one of the first output port and second output port.

In some examples, the AWG 120 comprises a plurality of input ports 122, wherein the optical signals at the plurality of input ports are multiplexed to one of the first output port or second output port. As such, the AWG 120 functions as a multiplexer. The wavelengths of the optical signals transmitted by the transmitters 110, and received at the input ports 122 of the AWG 120 are different from each other, e.g. l1 , l3, l5. The AWG 120 multiplexes these wavelengths into a WDM optical signal, which is transmitted on one of the optical lines 22,24. The AWG performs the functions of both routing according to the wavelength selected by the transceiver tuning onto the first or second optical line 22,24 (i.e. switching/splitting) and multiplexing optical signals.

The AWG 120 is also configured to receive optical signals transmitted over the optical lines 22,24. In this case, the AWG 120 functions as a demultiplexer. The AWG 120 demultiplexes a WDM optical signal received on either of the first or second optical line, and outputs the optical signal to a particular receiver (e.g. as part of a transceiver 110) or a separate receiver (not shown). The AWG 120 routes the optical signal to a particular port 122 (labelled as an input port, but operating as an output port of the AWG 120) based on the wavelength of the optical signal. In a corresponding manner to the described transmission using the AWG to select the first or second optical line 22,24 based on wavelength only, the AWG 120 receiving optical signals selects a particular one of the receivers 110 based on wavelength only. For example, at the first optical routing apparatus 110a, A2 received on the first optical line 22 is demultiplexed and output to transceiver 110a, A4 is output to transceiver 110b. For example, at the first optical routing apparatus 110a, A3 received on the first optical line 22 is demultiplexed and output to transceiver 110a, A5 is output to transceiver 110b. Different wavelengths are received at the same receiver 100, dependent on which of the first or second optical line 22,24 the optical signal on. Thus, the transceivers are operable to receive on two different wavelengths, as well as transmit on two different wavelengths. The AWG 120 is functioning as a multiplexer, demultiplexer, and switch in both transmit and receive directions.

The described switching may be considered as passive, since no active switch is required, with a power budget saving of around 5 dB. The passive architecture is suitable for the radio access network. The combination of functions provided by the AWG provides for a system with fewer components, requiring less space a lower insertion loss, e.g. approximately 5dB saving.

The architecture shown is Single Fiber Working with, in a given line, even wavelengths traveling one direction and odd wavelengths traveling the other direction: the rule being mirrored between the two lines. As such, the first and second optical lines 22,24 in this example use the same optical fiber. The wavelengths in one direction alternate with the wavelengths in the opposite direction on the same optical line. For example, odd numbered wavelengths (e.g. l1 , A3, etc) are used in one direction on the first optical line, and even numbered wavelengths (e.g. A2, A4, etc) used in the opposite direction on the first optical line. The same is applicable to the second optical line, with the directions reversed for the odd and even wavelengths. In other examples, the first and second optical lines 22,24 may use separate optical fibers, i.e. dual fiber working, The optical routing apparatus 100 provides a routing function, e.g. for providing protection in the event of a loss of signal. The switching is replaced by wavelength tuning on the transceiver 110, removing need of switches and additional couplers. The result is a lean architecture with no active elements and reduced insertion loss. The AWG 120 may be considered as a Nx2 AWG, i.e. having N ports for N wavelengths connected to the transceivers 110, and two ports for connection to the first and second optical lines for WDM signals.

The controller may comprise processing circuitry, e.g. implemented by semiconductor circuitry. In some aspects, the controller further comprises memory. The processing circuitry is configured to carry out the functions described. For example, the processing circuitry is configured to receive or determine control signaling that a different route for the optical signal should be used, or detect a loss of signal, or receive an indication of a loss of signal, on the first or second line. The processing circuitry is configured to control one or more transmitters to transmit on the alternative wavelength (i.e. change from the first wavelength to the second wavelength or change from the second wavelength to the first wavelength). As such, the processing circuitry is configured to change the operating optical line between the first and second optical lines 22, 24. Examples of the disclosure may include a computer program, or a computer program carrier comprising a computer program. The computer program may be run by the processing circuitry. The computer program is configured to cause the controller to operate according to any example, e.g. tune the wavelengths to initiate the switch to the alternative optical line.

The AWG 120 may be a Cyclic or non-cyclic AWG. Figure 3a and 3b illustrate the routing by the AWG 120 for wavelengths 30 on the N input ports and for a cyclic AWG 120 in Figure 3a and for a non-cyclic AWG 120 in Figure 3b. Each row of the matrixes of Figures 3a and 3b indicates an input port to the AWG, i.e. connected to a transmitter 110. The columns indicate the output ports of the AWG 120, i.e. connected to the first and second optical lines 22,24.

For example, Figure 3a shows that a first input port (first row) can receive A1 or K2 from the first transmitter 110. The A1 is routed by the AWG 120 to the first optical line 22 (first column) and the K2 is routed by the AWG 120 to the second optical line 24 (second column). The non-cyclic AWG uses N wavelengths for N traffic channels. Note the wavelengths in this example are labelled from 0 to N- 1. Other examples of the disclosure label the wavelengths as 1 to N.

Figure 3b shows corresponding allocation to an output port based on the input port of the non-cyclic AWG. Note the wavelengths in this example are labelled from -N+ 1 to 1. Other examples of the disclosure label the wavelengths as 1 to N.

As described above, the routing by the AWG 120 is based, or dependent on, the wavelength only of the received optical signal. In particular, the AWG 120 is a passive device and is not controlled to route the optical signals. In case of a command to change routing, or a loss of signal (LOS) detected by each transceiver (or controller) on one of the optical lines, the transceivers are configured to tune to their alternate wavelength. Thus, the optical routing apparatus couples the traffic to the surviving alternative line or fiber. The tuning to the alternative wavelength generally comprises the detuning to an adjacent wavelength (except for the last port in the Cyclic-AWG case, which tunes to a non-adjacent wavelength). Thus, the tuning change is fast, and can be achieved within a typically required protection time of 50ms.

Figure 4 shows an optical routing apparatus 100 as part of an optical network having a ring topology. The optical routing apparatus 100, as described above, is considered as a hub node. The optical network connects the hub node to one or more remote nodes 150. The optical network comprises first and second optical lines. In this example, the first and second optical lines 122,124 correspond to opposite directions around the ring, i.e. clockwise and anticlockwise. The remote nodes comprise optical add drop (OAD) filters configured with a specific wavelength mapping to match the AWG operating wavelengths in the two output ports, i.e. first and second optical lines. A single optical fiber provides the first and second optical lines 122,124. On a given fiber segment different wavelengths are used in the two propagation directions. Wavelengths are reused without overlapping.

In this example, the system comprises only one optical routing apparatus 100. The remote nodes are configured to receive the different wavelengths, on the alternative optical line. In the event that the optical routing apparatus 100 determines that switching is required, the remote nodes passively accept the alternate wavelengths.

In the case of a cyclic AWG the last wavelength is l (k-1)*2*M+1 , while in the non-cyclic AWG it is l k*2*M+1 , thus requiring one more wavelength compared to the cyclic (and conventional) case.

The wavelength mapping rules can be stored in the transceiver or host equipment software so that the transceiver automatically knows the wavelengths associated to the two output ports. The Nx2 AWG can be realized with the same technology used with conventional AWG mux/demux and with comparable costs, e.g. a Planar Lightwave Circuit (PLC) design. The use of tunable transceivers is already commercially viable, due to spare parts savings.

Figure 5 shows an example structure of a remote node 150, i.e. comprising OAD filters 160. The remote node 150 comprises one or more (e.g. a plurality of) transceivers, for convenience a first transceiver 210a and a second transceiver 210b are shown. The remote node 150 further comprises a plurality of filters 160. A first set of filters 162 connects to the first optical line 124. A second set of filters 172 connects to the second optical line 122.

The filters 162,172 are each configured to select one wavelength. A first subset of the filters 162, add filters 163, are configured to add wavelengths to the first optical line 124 which are transmitted by one of the first or second transceiver 210a, 210b. A second sub-set of the filters 162, drop filters 164, are configured to drop (i.e. select) wavelengths from the first optical line for receiving by the first or second transceiver 210a, 210b. Correspondingly, the second set of filters 172 comprises add filters 173 and drop filters 174 for adding or dropping optical signals to the one or more transceiver.

In some examples, the filters 160 comprise a separate filter for each wavelength. In other examples, a plurality of wavelengths are selected by a filter 160.

A plurality of splitters 180 connect the transceivers, e.g. first and second transceivers 210a, 210b, with each of the filters 160. The splitters 180 are configured such that a transmitter and receiver of each of the transceivers, e.g. first transceiver 210a and second transceiver 210b, are connected to both of the first set of filters 162,172, i.e. connecting to both optical lines 122,214.

The transmitters of the transceivers 210a, 210b are tunable to a first and second wavelength. The first and second wavelength will be a different wavelength for each transmitter, as described for the examples above. The splitters 180 pass either wavelength to two filters 160. Since each filter 160 selects only one particular wavelength, only one of the filters 160 will pass the wavelength onto the optical line. The other filter 160 receiving a wavelength will block that optical signal. As such, the tuning of the transmitters of the first transceiver 210a and second transceiver 210b determines which of the first and second output line 122,124 is used. As for the optical routing apparatus 100, the tuning of the first transceiver 210a and second transceiver 210b may be controlled by one or more controller. As such, the filters and splitters may be considered as a passive optical device, and to work in a similar manner to the AWG 120 described in Figure 2.

The example node 150 is shown with a first and second transceiver 210a, 210b. Each transceiver provides an independent bidirectional service. The number of transceivers may be one, two or a plurality M of transceivers. Examples include one transceiver per channel (service), with up to M TRXs per OADM node providing M independent bidirectional services. The example describes M TRX, each able to tune to a first or a second wavelength. Each of the M transceivers uses different first and second wavelengths, i.e. shifted between transceivers, as described above. The OADM provides a shifted-wavelength in the east and west OAD filters.

The filters 162 that face eastward (i.e. on the first optical line 124) use wavelengths shifted by one place in the wavelength grid with respect to the filters 172 that face westward (i.e. on the second optical line). The K-th OAD in the ring supports M services. In a single fiber architecture, the service uses two different wavelengths in downlink and in uplink. In some aspects, the optical network comprises an optical fiber (single fiber), and the first optical line and second optical line are provided by the optical fiber. Different wavelengths are used in different directions on the optical fiber at any given time. In some examples, the wavelengths in one direction alternate with the wavelengths in the opposite direction on the same optical line.

Figure 6 shows a further network example of the disclosure using a chain topology. For example, the optical routing apparatus 100 of any example is connected to one side of the chain. The optical routing apparatus 100 of any example is connected to a first remote node comprising a OAD (OAD-1) 250. A further OAD (OAD-2) 252 is connected to the first OAD 250. Further OADs 254,256 are connected in a chain with the optical network. The optical network connecting the optical routing apparatus 100 and OADs 252 provides the first and second optical lines 222,224 using a single fiber, i.e. the architecture is single fiber working. On each fiber section, e.g. between two OADs, of the active line there are different wavelengths in the two propagation directions. Each fiber section between OADs or between OAD 250 and the optical routing apparatus may independently use either the first or second optical lines 222,224.

Figure 7 shows an example OAD unit structure 350, e.g. for a k-th OAD. The OAD 350 is connected to two other OADs (or to an OAD and the optical routing apparatus) with a first optical line 222, having a first section 222a and a second section 222b, and a second optical line 224 having a first section 224a and a second section 224b. The OAD comprises a plurality of first filters 262 on the first optical line and a plurality of second filters 272 on the second optical line. Each filter 262,272 is configured to select, e.g. for adding or dropping, a particular wavelength. The OAD 350 comprises one transceiver or a plurality of transceivers, e.g. TRX 1 to TRX M. Splitters connect each transceiver with a filter on the first optical line 222 and the second optical line 224. A separate connection, and filter, is provided for transmission and receiving by the transecivers.

The filters 272 on the second optical line 224 use shifted wavelengths with respect to the other filters 262 on the first optical line 222, in order to meet the AWG Nx2 wavelength mapping. The OAD 350 supports M services in a single fiber working solution. The OAD 350 is part of a remote node, connected to a separate node comprising the optical routing apparatus. If the node 350 is connected to an AWG which is cyclic, e.g. designed for the last wavelength of the filter on the second optical line 2 is A(k-1)*2*M+1. For connection to a non- cyclic AWG it is l k*2*M+1 , i.e. requiring one more wavelength compared to the cyclic (and conventional) case.

The two sets of filters 262,272 (i.e. filter chains) in the OAD 350 are the same used for the ring topology, but arranged in parallel on the two optical lines instead of Figure 5 which shows the filters 160 arranged in series on the optical lines. Therefore, the OAD 350 can alternatively be used in a ring topology as well by patching the two chains.

In case of detecting a command to change routes, or detecting a loss of signal on a fiber section 222a, 222b, 224a, 224b in the chain, the transceivers connected to the following filters on the first optical line and the corresponding transceivers of the first optical routing apparatus (i.e. connected on the AWG), will tune to the other wavelength. A passive optical device (filters 262,272) connects the re-tuned transceiver to a different optical line. As for Figure 5, each of the filters 262,272 select only one wavelength, which is different for the filters connected to the same transmitter and different on different optical lines. Thus, the filters select which optical line is used for transmission of the optical signal, based on the selected wavelength. Any example from any other embodiment may be used in combination with this or any other embodiment. The routing scheme operates on a per-wavelength basis.

Figure 8 shows an example of a control state machine 400, according to which the optical routing apparatus or OADs may operate. The control state machine is described with respect to a loss of signal (LOS). Alternatively, the same control machine is applicable with references to LOS replaced by a command or control signaling to route the optical signal on a different route through the optical network. The transceivers of the optical routing apparatus and OADs, e.g. at both ends of the link, operate according to the control state machine. Each transceiver is initialized at one operating wavelength corresponding to one of the two output ports, e.g. output ports 132,134. When a loss of signal alarm is detected, the transceivers tune to the other wavelength corresponding to the other of the two output ports 132,134. In some examples, the change in wavelength is after a hold-off time has elapsed, to assure that the transceivers at both ends of the optical lines have detected (or been informed of) the loss of signal, and have tuned to the other wavelength so that the transceivers are ready to respond to the loss of signal condition.

In this example, the routing or protection is considered as Non-Revertive since no monitoring is available on the standby port, i.e. the alternative optical line. In case of a double fault, i.e. a loss of signal on both lines, the state machine will continue to tune back and forth after each hold-off cycle until one of the faults is repaired. This is not a limitation since there are no mechanical parts involved in the process (contrary to switch-based schemes). In some examples, a mechanism halts the alternating of use of the optical lines after some number of consecutive switches. The control state machine 400 may apply to any example.

After initialization 402, the transmitter of the optical routing apparatus tunes 403 to a first wavelength which is output by the AWG 120 at the first output port 132, i.e. onto the first optical line 22. Whilst no loss of signal is detected on the first optical line 22, or the hold off timer is not expired, the transmitter continues to operate 404 at the first wavelength. In the event that a loss of signal is detected, and the hold over timer has expired, the transmitter is tuned 406 to the alternative (second) wavelength. The optical signal is then output 407 by the AWG 120 to the second output port, and transmitted on the second optical line 24. The hold off timer is started. The change in tuning is applied to all the transmitters of the optical routing apparatus. A corresponding change in tuning is applied to all transmitters in the system, e.g. at the OADs, for protection purposes.

At 408, the transmitter continues to transmit on the second wavelength whilst there is no loss of signal detected, or the hold off timer has not elapsed. In the event that a loss of signal is detected, and the hold over timer has expired, the transmitter is tuned 410 to the first wavelength. The optical signal is then output by the AWG 120 to the first output port, and transmitted on the first optical line 22. The hold off timer is started. The change in tuning is applied to all the transmitters of the optical routing apparatus. A corresponding change in tuning is applied to all transmitters in the system, e.g. at the OADs, for protection purposes.

In some examples, the transceivers or systems have automatic self-tuning mechanism to find the right wavelength to be tuned, avoiding time consuming manual settings. The scheme disclosure is able to interwork with such protocols, using the scheme as described or with the following modifications. In a first option, the self-tuning protocol replaces the state machine described with respect to Figure 8. It is the self-tuning that finds the right wavelength and reacts to the loss of signal by restarting the self-tuning process until the alternative wavelength is found. As such, the self-tuning protocol is carrying out the change in the wavelength. This results in a change to the operating second optical link (or back to the first optical link). The self-tuning process is not aware that (i.e. independent of) the change in wavelength results in a change in optical line 22,24 used when the self-tuning process change the wavelength. This option may introduce longer switching times, depending on the algorithms used in the self-tuning process.

In a second option, the self-tuning protocol is modified to store the values of the two operating wavelengths, i.e. first and second wavelengths. For example, two registers in the transceiver EEPROM store the two operating wavelengths. At the initialization 402, the registers are empty, and so the usual self-tuning protocol is used to find the first wavelength. This value is stored in one of the registers. After a loss of signal, the available wavelengths (i.e. the adjacent wavelength channels) are tried first by the self-tuning protocol. The alternative wavelength will result in the second optical line being used, and so determined as correct by the self-tuning protocol. The correct alternative wavelength is stored in the second register. At this point, the transceiver has stored the two operating wavelengths in the routing scheme. In future instances, the selftuning protocol does not need to repeat the whole self-tuning process, and can jump directly to the stored alternative wavelength values. If the transceivers or modules are extracted from the optical routing apparatus, or in case of consecutive failures of the tuning, the registers can be cleared, and the selftuning protocol started again.

Figure 9 describes a method 500 for providing routing in an optical network.

In 501 , one or more transmitters of an optical routing apparatus transmit on a first wavelength. The first wavelength is different for each transmitter. An optical signal is transmitted on the first wavelength to one or more remote nodes on a first optical line.

In 502, a determination to switch to the alternative line is made. For example, the determination is made based on detection of a loss of signal, or a signal indicating loss of signal is detected. In other examples, the controller or a another network entity, e.g. a network management system provides a signal indicating that the used optical line should be changed.

In 503, the controller (e.g. processing circuitry) controls the one or more transmitter to tune to an alternative, second, wavelength.

In 504, optical signal on the second wavelength is routed to a second optical line. The second wavelength is received by a passive optical device, e.g. AWG 120, or filters 160 as described in Figures 5 and 7 which passively routes the different second wavelength to a second output port connected to the second optical line.

The transceivers 110 have been described as tunable to generate the two different wavelengths. In some examples, the transceivers are tunable across a whole band or only tunable in a small wavelength range, e.g. tunable between only two wavelengths. Alternatively, the transceivers may comprise two transmitters, each generating a fixed wavelength. A switch (not shown) selects one of the fixed wavelengths. As such, the transceivers 110 may generate one of two wavelengths by any suitable method.

Figure 10 shows an example of routing in an optical network 600 between a first node A 650 and a second node B 652. The first node A 650 and the second node B 652 comprise an optical switching apparatus according to any example. The first and second nodes 650,652 are connected via a first intermediate node 654 and a second intermediate node 656. An optical link 622a connects the first node 650 and the first intermediate node 654, and a further optical link 622b connects the first intermediate node 654 to the second node 652. An optical link 624a connects the first node 650 and the second intermediate node 656, and a further optical link 624b connects the second intermediate node 656 to the second node 652. The optical links may operate in a single direction or bi-directionally. The first and second intermediate node 654, 656 may comprise an optical switching apparatus according to any example, or may be configured with a fixed optical switch, or comprise an optical switch according to any example. The optical network 600 is merely an example, and it will be appreciated that the optical network 600 may comprise additional nodes. In Figure 10a, the optical switching apparatus in the first and second nodes 650,652 are configured to communicate over the optical links 622a, 622b, i.e. via the intermediate node 654. For example, the optical switching apparatus 100 in the first node 650 is configured to select this route based on a tuning of one or more transmitters or transceivers 110. The passive optical device 120, connected to the optical links 622a, 624b passively routes the optical signal 610 based only on the optical wavelength of the optical signal. The alternative route via the second intermediate node 656 is shown in a dotted line.

In Figure 10b, the optical switching apparatus 100 in the first node 650 is configured to select a different route, i.e. to communicate over the optical links 624a, 624b, i.e. via the second intermediate node 656. The routing is based on a different tuning of one or more transmitters or transceivers 110. The passive optical device 120, connected to the optical links 624a, 624b passively routes the optical signal 610based only on the optical wavelength of the optical signal. The alternative route via the first intermediate node 654 is shown in a dotted line.

Figures 11a and 11b show a further example of routing in an optical network 700. The network 700 comprises, for example, a first node 750, a second node 752, a third node 754, a fourth node 756 and a central node 760. It will be appreciated that the optical network 700 may comprises different numbers of nodes. The first node 750, second node 752, third node 754 and fourth node 756 are each connected to the central node 760 respectively by a first optical link 722a, second optical link 722b, third optical link 722c and fourth optical link 722d.

In some aspects, the optical switching apparatus is distributed across a plurality of nodes. For example, the first node 750, second node 752, third node 754 and fourth node 756 comprise the transceivers 110 and controller. The central node 760 comprises the passive optical device, e.g. AWG 120. The first node 750, second node 752, third node 754 and fourth node 756 are controlled to transmit at one or more specific wavelength according to the destination of the optical signals. In Figure 11a, the first node 750 and second node 752 are controlled to transmit the optical signals 710a, 71 Ob at particular wavelengths. The passive optical device 120 in the central node routes the optical signals 710a, 71 Ob to the third node 754 and fourth node respectively, based only on the wavelengths of the optical signals 710a, 710b.

In Figure 11b, the first node 750 and third node 754 are controlled to transmit the optical signals 710c,710d at particular wavelengths. The passive optical device 120 in the central node routes the optical signals 710c, 71 Od to the third node 754 and fourth node respectively, based only on the wavelengths of the optical signals 710a, 710b.

Thus, in the examples described, the optical switching apparatus is distributed, e.g. across a first node and a second node. The tuning and transmission of a wavelength is carried out at one node, and the routing by the passive optical device is carried out at a different node. A plurality of nodes may be connected to the central node for routing based on the selected wavelength of the optical signal.

Claims

1. An optical routing apparatus for an optical network, wherein the optical routing apparatus comprises: an optical transmitter configured to transmit an optical signal at a selected one of a first wavelength and a second wavelength, a controller configured to control the optical transmitter to transmit the optical signal at a selected one of the first wavelength and second wavelength; and a passive optical device comprising: an input port connected to the optical transmitter and configured to receive the optical signal at the selected one of the first wavelength and second wavelength, and a first output port connected to a first optical line, and a second output port connected to a second optical line; wherein the passive optical device is configured to connect the optical signal at the input port to one of the first output port or second output port based on the wavelength of the optical signal; wherein the controller is configured to control the optical transmitter to transmit the optical signal at the other one of the first wavelength and second wavelength.

2. The optical routing apparatus as claimed in claim 1 , wherein the optical transmitter comprises a tunable optical transmitter, wherein the tunable optical transmitter is tunable to one of the first wavelength and the second wavelength.

3. The optical routing apparatus as claimed in claim 1 or 2, wherein the passive optical device is an Arrayed Waveguide Grating.

4. The optical routing apparatus as claimed in any one of the preceding claims, wherein the passive optical device comprises a plurality of input ports, wherein the optical signals at the plurality of input ports are multiplexed to one of the first output port and second output port.

5. The optical routing apparatus as claimed in any one of the preceding claims, wherein the controller is configured to operate a self-tuning protocol, wherein on detection of a loss of signal, the self-tuning protocol is configured to control the optical transmitter to transmit the optical signal at the other one of the first wavelength and second wavelength.

6. The optical routing apparatus as claimed in any one of the preceding claims wherein the optical routing apparatus is distributed across a first node and a second node.

7. The optical routing apparatus as claimed in any one of the preceding claims wherein the optical network comprises an optical fiber, and the first optical line and second optical line are provided by the optical fiber; wherein different wavelengths are used in different directions on the optical fiber.

8. The optical routing apparatus as claimed in any one of the preceding claims wherein the wavelengths in one direction alternate with the wavelengths in the opposite direction on the same optical line.

9. The optical routing apparatus as claimed in any one of the preceding claims wherein the optical routing apparatus comprises a plurality of optical transmitters.

10. An optical routing apparatus for a node an optical network, wherein the optical routing apparatus comprises: an optical transmitter configured to transmit an optical signal at a selected one of a first wavelength and a second wavelength, a controller configured to control the optical transmitter to transmit the optical signal at a selected one of the first wavelength and second wavelength; and a plurality of filters comprising a first filter and a second filter: the first filter and the second filter configured to receive the optical signal at the selected one of the first wavelength and second wavelength, and the first filter comprising a first output port connected to a first optical line, and the second filter comprising a second output port connected to a second optical line; wherein one of the first filter and second filter is configured to connect the optical signal at the input port to one of the first output port or second output port based on the wavelength of the optical signal; wherein the controller is configured to control the optical transmitter to transmit the optical signal at the other one of the first wavelength and second wavelength.

11. The optical routing apparatus as claimed in claim 10, wherein the optical transmitter comprises a tunable optical transmitter, wherein the tunable optical transmitter is tunable to one of the first wavelength and the second wavelength.

12. The optical routing apparatus as claimed in claim 10 or 11 , wherein the optical routing apparatus comprises one or more splitters configured to split the optical signal from the transmitter to a filter on the first optical line and a filter on the second optical line.

13. The optical routing apparatus as claimed in claim 10, 11 or 12 wherein the optical routing apparatus comprises a plurality of optical transmitters.

14. The optical routing apparatus as claimed in any one of the preceding claims wherein the optical routing apparatus is configured to determine a loss of signal, or receive an indication of a loss of signal, for one of the first optical line and second optical line, and is configured to control the optical transmitter to transmit the optical signal at the other one of the first wavelength and second wavelength to transmit the optical signal on the alternate one of the first optical line and second optical line.

15. A node comprising the optical routing apparatus as claimed in any of the preceding claims, wherein the node comprises radio access network equipment.

16. An optical network comprising: a hub node comprising the optical routing apparatus as claimed in any of claims 1 to 14, at least one remote node configured to receive the optical signal from the hub node at one of the first wavelength and second wavelength; a first optical link connecting the first output port of the hub node and the remote node, and a second optical link connecting the second output port of the hub node and the remote node.

17. The optical network as claimed in claim 16 wherein the at least one remote node comprises the optical routing apparatus as claimed in any one of claims 1 to 14.

18. A method of optical routing at an optical routing apparatus connected to a first optical line and a second optical line, the method comprising: transmitting an optical signal from the optical routing apparatus on a first wavelength on one of the first optical line and second optical line; determining to switch the optical signal to the alternate one of the first optical line and second optical line; controlling the transmitting of the optical signal to an alternative, second, wavelength. wherein the second wavelength is routed by a passive optical device to the alternate one of the first optical line and second optical line.

19. The method as claimed in claim 18, comprising tuning the transmitter from the first wavelength to the second wavelength.

20. The method as claimed in claim 18 or 19, comprising multiplexing optical signals from a plurality of transmitters.

21 . The method as claimed in claim 18, 19 or 20 comprising determining a loss of signal, or receiving an indication of a loss of signal, for one of the first optical line and second optical line, and controlling the optical transmitter to transmit the optical signal at the other one of the first wavelength and second wavelength to transmit the optical signal on the alternate one of the first optical line and second optical line.