GB2227623A

GB2227623A - Optical fibre network

Info

Publication number: GB2227623A
Application number: GB8901931A
Authority: GB
Inventors: John Stuart Heeks
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1989-01-28
Filing date: 1989-01-28
Publication date: 1990-08-01
Also published as: GB8901931D0

Abstract

An ultra high capacity optical fibre network comprising a plurality of passive star coupler devices (C) interconnected with one another so that there is only one network path between any two coupler devices, each coupler device being additionally connected to a number of subscribers terminal equipments (T), all the connections and interconnections being formed with effective separate go and return channels (G, R), each terminal equipment having allocated thereto a unique optical frequency and each go channel to a coupler device including signal amplification means to balance losses introduced by the coupler and transmission losses. <IMAGE>

Description

Ultra High Capacity Optical Network This invention relates to an ultra high capacity optical network for telecommunications.

Modern telecommunications systems provide an ever increasing range of services and facilities for customers, both individual and corporate. This expansion of services is considerably helped by the advent of optical fibre networks which have an enormous capacity in terms of bandwidth. Hence it is possible not only to provide large numbers of channels but at the same time make those channels of wide bandwidth so that they can carry a variety of complex facilities. In addition to ordinary telephone traffic other, more demanding services are now being offered, such as high speed data links. However, there are still some services which are technically feasible in isolated dedicated systems but which still pose problems when considered in the context of a public network.A typical example is the so-called "viewphone" in which ordinary telephone calls are supplemented by television pictures of the call participants. The Viewphone, however, utilising as it does television technology, requires a very wide bandwidth. Even with advanced data compression techniques a bandwidth in excess of 1MHz is required, although it is foreseeable that technology will continue to improve to reduce bandwidth requirements. Currently so-called "slow-scan" television is achieved with 2MHz and, provided the frame rate is reduced sufficiently pictures, albeit a slow series of still pictures, could be sent using 1MHz.

To all intents and purposes a single mode optical fibre operated in a coherent mode can be considered as having infinite bandwidth. A more useful way of looking at this is to assume that the real practical limitation to communication capacity derives from the fibre power capability (approximately 100mW).

Thus a fibre optics network covering, say, the United Kingdom can provide a large number of users with individual channels of megaherz, or greater, capacity.

According to the present invention there is provided an ultra high capacity optical fibre network comprising a plurality of passive star coupler devices interconnected with one another so that there is only one network path between any two coupler devices, some at least of said coupler devices being additionally connected to a number of subscribers terminal equipments, all the connections and interconnections being formed with effective separate go and return channels, each terminal equipment having allocated thereto a unique optical frequency and each go channel to a coupler device including signal amplification means.

An embodiment of the invention will now be described with reference to the accompanying drawing which is a schematic illustration of an optical network.

In the network a plurality of passive optical star couplers C are connected together in an exclusive star configuration where each coupler is connected either directly or indirectly to every other coupler by an optical connection such that there is only one network pathway between any two couplers. Each coupler may also be connected by optical connections directly to a number of subscriber terminals T such that each terminal is connected to every other terminal via one or more couplers and there is only one network pathway between any two terminals. Each terminal/coupler and coupler/coupler pathway consist of separate go and return channels G, R. Each go channel G to a coupler, whether from a terminal or another coupler, incorporates an amplifier, the power of which is sufficient only to balance the splitting losses introduced at the coupler and nominal system transmission losses.Thus any optical signal emitted from a subscriber terminal T will be broadcast to all the terminals T in the network.

Each terminal is allocated a unique optical frequency which is centred in for, example, a 1MHz channel. Each terminal includes means for transmitting in a unique 1MHz channel and means for receiving in another unique 1MHz channel. There are two options. Each terminal can transmit at its own frequency and can select another terminal's frequency in its receiver, or it can transmit at another terminal's frequency and receive at only its own frequency The critically difficult items in the terminal equipments are those required for the generation of optical frequency markers, requiring better than 1MHz stability and sideband noise, and (if the receiver frequency is fixed) optical VCO's tuneable over the system bandwidth For a national system it is convenient to envisage a total of 20 million subscribers, with 2 million subscribers active at one time.If the individual channels are of 1MHz bandwidth the total system will be 20,000 GHz, which represents 78 of the optical bandwidth at lum.

One method of providing the optical frequency markers is to arrange for a comb of standard optical references to be distributed over the network from a single marker terminal. Interpolation to the required subscriber channel frequencies for transmission and reception are then effected via an internally generated offset tone in each terminal using, for example, a single sideband modulation technique. The optical markers can be spaced at up to a 10 to 100GHz spectrum interval, the offset frequency itself being derived from a high stability standard. In addition, the receiver front end filter would almost certainly require some optical preselection. This can be based on an optical superheterodyne mechanism and/or passive optical filter. The latter could be implemented as a voltage tunable integrated optical structure, e.g. in a lithium niobate device.

Another key element in the system is the amplifier required in each go channel. Ideally, considering that the network is an optical network, optical amplifiers are required. Consider for a moment a possible power budget. Taking a received power figure of -65dBm for a BER of 10 9 in 1MHz and 10% instantaneous occupancy, the lower limit to the power in the fibre is just under 10mW. Clearly a passive network would require each transmitter to put 10mW into the system, with a resulting total fibre power of 20,000 optical watts. However, with the star network as described above the signal is amplified at every coupler sufficient only to balance the coupler losses and nominal system transmission losses so it ensures that the system power limit is held down to the lower of the two figures given above An alternative to the use of optical amplifiers is based on classical IF amplifiers with an electronic IF gain block employing, for example, an FET chain giving a bandwidth of around 10GHz, i.e. 10,000 channels per unit.

Conveniently the exclusive star network ensures that there is no multipath situation, provided that there are no back reflections in the system and that there are separate go and return paths. The latter can be achieved in an optical fibre system by using either orthogonal polarisations in a single fibre or separate go and return fibres.

In one embodiment of the invention it is expedient to multiplex the subscribers into, say, 100MHz blocks prior to entering the optical net in order to reduce the requirements on the number of optical amplifiers, etc, or on the optical local oscillator performance in each terminal.

Claims

1. An ultra high capacity optical fibre network comprising a plurality of passive star coupler devices interconnected with one another so that there is only one network path between any t-o coupler devices, some at least of said coupler devices being additionally connected to a number of subscribers terminal equipments, all the connections and interconnections being formed with effective separate go and return channels, each terminal equipment having allocated thereto a unique optical frequency and each go channel to a coupler device including signal amplification means.

2. A network according to claim 1 wherein the amplification is sufficient only to balance the losses introduced by the coupler and transmission losses.

3. A network according to claim 1 or 2 including means for coupling into the network a comb of standard optical reference frequencies, each terminal equipment including means for interpolating from said standard frequencies to a required unique optical frequency allocated to a terminal.

4. A network according to claim 3 wherein each terminal includes single sideband modulation means for generating a tone offset from a reference frequency to effect the required unique optical frequency.

5. A network according to any preceding claim wherein said separate go and return channels are provided by separate optical fibres.

6. A network according to any preceding claim wherein said separate go and return channels are provided by orthogonal polarisations of optical signals in a single optical fibre.

7. A network according to any preceding claim wherein groups of subscriber's terminal equipments are multiplexed onto a common go and return channel path to a passive star coupler device.

8. An ultra high capacity optical fibre network substantially as described with reference to the accompanying drawing.