WO2004036794A1

WO2004036794A1 - Optical modulation devices

Info

Publication number: WO2004036794A1
Application number: PCT/EP2003/050719
Authority: WO
Inventors: Robin Jeremy Richard Morris
Original assignee: Bookham Technolgy Plc
Priority date: 2002-10-16
Filing date: 2003-10-15
Publication date: 2004-04-29
Also published as: AU2003301486A1; GB0224055D0

Abstract

An optical modulation device in the form of a reflection transceiver 1 integrated an a chip 2 is provided for transmitting data signals to a plurality of remote locations along a single optical fibre. The device comprises a demultiplexer 3, a plurality of channel waveguides 4 incorporating VOAs 5, and a multiplexer 6. The transceiver 1 receives a multi-channel input signal at its input 7 by way of an optical fibre, the input signal being supplied from a broadband light source 8 by way of a SOA 9 and an optional modulator 10 for adding data/services for all subscribers to the signal to be transmitted by the transceiver. The multi-channel input signal is split into a plurality of channel signals by the demultiplexer 3 which are supplied to the channel waveguides 4. Each VOA 5 can be used to modulate the channel signal so as to apply local content relevant to the individual channel. The multiplexer 6 combines together the data modulated channel signals to produce a multiplexed output signal at the output 12 which is supplied to the subscriber network and incorporates both broadcast and local content. The signals reflected back to the transceiver 1 are demultiplexed by the multiplexes 6 so that the reflected channel signals are supplied to the channel waveguides 4, and proportions of these signals are supplied to respective photodetectors 20 by way of tap couplers 21. Such an arrangement gives advantages of small size and reduced manufacturing costs, whilst enabling independent data modulation on a large number (for example 40) of wavelength channels.

Description

"Optical Modulation Devices"

This invention relates to optical modulation devices and is concerned more particularly, but not exclusively, with optical communication networks of the type comprising a central optical transceiver, a plurality of local optical transceivers and an optical fibre network interconnecting the central optical transceiver and the local optical transceivers.

One example of an optical communication system is an optical subscriber system for a telephone network which comprises a central optical transceiver installed in a central location and a plurality of local optical transceivers installed in subscriber houses and connected to the central transceiver by optical fibre transmission cable. It is known to transmit telephone signals using multi-wavelength transceivers which transmit and receive signals along a bidirectional optical transmission path but with transmission and reception being effected using different wavelengths. Single wavelength optical transceivers utilising transmission and reception along a bidirectional optical transmission path are also known. Such devices comprise a light source and light receiver which are connected to the bidirectional optical transmission path by means of a power splitter, such as a Y junction or evanescent coupler. In such an arrangement both the light source and the light receiver are permanently connected to the bidirectional light transmission path so that a compromise must be reached in the design of the splitter between the transmitted output power and the responsivity of the receiver.

WO 00/55994 discloses a single wavelength optical transceiver for transmitting and receiving optical signals of the same wavelength utilising a bidirectional optical transmission path. In this case the transceiver comprises a light source, a light receiver and input-output means for receiving light from and transmitting light to the bidirectional optical transmission path. An optical switching arrangement, which preferably utilises a Mach-Zehnder interferometer comprising one or more p-i-n diode phase modulators is provided to selectively effect optical communication between the light source and the input-output means and between the input-output means and the light receiver to provide low-loss paths therebetween. With a reflector acting as the light source, the optical switching arrangement may be used to modulate the output of the transceiver. Such an arrangement has the advantage that the optical losses associated with the known passive coupling arrangements are reduced or eliminated. The speed of a solid state or monolithic optical switch also offers an important advantage over mechanical switching in that it provides the capability of supporting greater data rates. Furthermore it is possible to dispense with the need to provide a separate laser source at each subscriber location.

It is an object of the invention to provide an optical modulation device which enables low-loss bidirectional communication utilising dense wavelength division multiplexing (DWDM) for example.

According to one aspect of the present invention there is provided an optical transceiver for transmitting data signals to a plurality of remote locations along a single optical fibre and for receiving return data signals transmitted in the opposite direction along the same optical fibre, comprising demultiplexing means for demultiplexing a broadband optical input signal, a plurality of channel waveguides for guiding channel signals of different wavelengths received from the demultiplexing means, a respective optical modulator associated with each channel waveguide for receiving an electrical data signal and for optically modulating the channel signal supplied to the channel waveguide to produce a respective data modulated channel signal, multiplexing means for multiplexing the data modulated channel signals from the channel waveguides and for outputting a multi-channel optical transmission signal incorporating the data modulated channel signals to the optical fibre, and optical detection means for detecting a respective return channel signal supplied to each channel waveguide by the multiplexing means demultiplexing a multi-channel return optical transmission signal received along the optical fibre.

Such a device may be integrated on a chip utilising silicon-on-insulator (SOI) technology, and preferably using fast electronic variable optical attenuators (EVOAs) as the optical modulators. The combination of these components on a monolithic chip give advantages of small size and reduced manufacturing cost, whilst providing the ability to independently modulate data onto many wavelength channels (typically 40 channels). Furthermore the device may have only two ports, namely an input port for connection to a broadband light source by means of an input optical fibre, and an output port for connection to an output optical fibre. Each optical modulator may be used to apply local content to the associated channel, and a certain part of the frequency spectrum may be reserved for content to be applied to all channels. Accordingly such a device offers the ability to apply broadcast and local content in a bidirectional communication system.

In general the invention can serve to reduce the number of lasers that are needed in an access network application. This both improves the reliability and reduces the cost of the transceiver components. In addition the invention can reduce the number of different types of customer-end components that are needed by making a generic customer-end transceiver respond to whichever of a number of input wavelengths is applied. It is possible to reduce manufacture, stocking, set-up and maintenance costs as compared with a conventional arrangement in which a laser of a different wavelength is required for each of the return wavelength channels.

According to another aspect of the invention there is provided an optical communication network comprising (i) a central optical transceiver comprising demultiplexing means for demultiplexing a broadband optical input signal, a plurality of channel waveguides for guiding channel signals of different wavelengths received from the demultiplexing means, a respective optical modulator associated with each channel waveguide for receiving an electrical data signal and for optically modulating the channel signal supplied to the channel waveguide to produce a respective data modulated channel signal, multiplexing means for multiplexing the data modulated channel signals from the channel waveguides and for outputting a multi-channel optical transmission signal incorporating the data modulated channel signals, and optical detection means for detecting a respective return channel signal supplied to each channel waveguide by the multiplexing means demultiplexing a multi-channel return optical transmission signal received along the optical fibre, (ii) an optical fibre for transmitting the multi-channel optical transmission signal and for transmitting the multi- channel return optical transmission signal in the opposite direction, and (iii) at least one local optical transceiver which is adapted to receive a respective data modulated channel signal from the optical fibre and to transmit a return data modulated channel signal to the optical fibre.

According to a further aspect of the present invention there is provided an optical communication network comprising a central optical transceiver for transmitting optical signals to, and receiving optical signals from, a remote location along a single optical fibre and incorporating (i) first optical modulating means for receiving an electrical data signal at a predetermined data rate and for optically modulating an optical signal to be transmitted in first time slots at a rate of at least twice the data rate, (ii) first photodetecting means for detecting a reflected optical signal; and a remote optical transceiver at the remote location for receiving optical signals from, and transmitting optical signals to, the central optical transceiver along the optical fibre and incorporating (a) second photodetecting means for detecting the optical signal received along the optical fibre from the central optical transceiver, (b) reflecting means for reflecting the optical signal, and (c) second optical modulating means for receiving an electrical data signal at said predetermined data rate and for optically modulating the reflected optical signal in second time slots, which alternate with said first time slots, at a rate of at least twice the data rate.

According to a still further aspect of the present invention there is provided a local optical transceiver comprising input/output means for receiving an input data modulated optical channel signal from a central optical receiver along an optical fibre and for supplying an output data modulated optical channel signal to the central optical receiver along the same optical fibre, photodetector means for detecting the input optical channel signal and for supplying an electrical input data signal, reflecting means for reflecting the input optical channel signal, and an optical modulator for receiving an electrical output data signal and for optically modulating the optical channel signal reflected by the reflecting means to supply the output data modulated optical channel signal to the central optical receiver. In order that the invention may be more fully understood, embodiments in accordance with invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a diagram of a first embodiment of the invention;

Figure 2 is a diagram of a second embodiment of the invention;

Figure 3 diagrammatically shows a distribution network usable in such embodiments;

Figures 4 and 5 are diagrams of two alternative customer-end arrangements usable in such embodiments; and

Figure 6 shows a possible implementation of one embodiment of the invention.

The following description will be given with reference to telephone subscriber networks, although it should be understood that the invention is not limited to such an application, but may be used in a wide variety of other applications, for example in local area networks, video transmission systems and the like.

Figure 1 shows an optical modulation device in the form of a reflection transceiver 1 integrated on a SOI chip 2 and comprising a demultiplexer 3, a plurality of channel waveguides 4 incorporating VOAs 5, and a multiplexer 6. The transceiver 1 receives a multi-channel input signal at its input 7 by way of an optical fibre, the input signal being supplied from a broadband light source 8 by way of a semiconductor optical amplifier (SOA) 9 or erbium-doped fibre amplifier (EDFA) and an optional modulator 10 for adding dataservices for all subscribers to the signal to be transmitted by the transceiver. An appropriately modulated signal is shown at 11 in a graph of magnitude against frequency f. In operation the multi-channel input signal supplied to the input 7 of the transceiver 1 is split into a plurality of channel signals by the demultiplexer 3 which are supplied to the channel waveguides 4. Each VOA 5, which is preferably in the form of a high-speed EVOA, can be used to modulate the channel signal so as to apply local content relevant to the individual channel. To this end an appropriate electrical data signal is supplied to each VOA 5 for optically modulating the associated channel signal to produce a respective data modulated channel signal which is supplied to the multiplexer 6. The multiplexer 6 combines together the data modulated channel signals to produce a multiplexed output signal at the output 12 of the transceiver 1. This optical output signal is supplied to the subscriber network and incorporates both broadcast and local content, as well as providing for bidirectional communication as will be described in more detail below.

The graph at 14 shows two channel signals of different frequencies, as well as the optional broadcast signal provided within a reserved part of the frequency spectrum for data/services to be transmitted to all subscribers. In accordance with standard procedure optical drop filters 15 are provided at the individual subscriber locations to supply only the required signal frequencies to the relevant subscriber. A respective local reflection transceiver 16 is provided at each subscriber location and comprises a fast optical switch 17 for switching between a receiver in the form of a photodetector 18 and a transmitter in the form of a mirror 19 which can be used to reflect back the received optical signal and to modulate the reflected signal in accordance with the required voice/data to be transmitted back to the central transceiver 1. It will be appreciated that signals reflected back to the transceiver 1 are demultiplexed by the multiplexer 6 so that the reflected channel signals are supplied to the channel waveguides 4, and proportions of these signals are supplied to respective photodetectors 20 by way of tap couplers 21.

The provision of the VOAs 5, the demultiplexer 3 and the multiplexer 6 on a monolithic chip gives advantages of small size and reduced manufacturing costs, whilst enabling independent data modulation on a large number (for example 40) of wavelength channels. Furthermore the local transceiver 16 provided in the subscriber's home is a generic component and does not require accurate wavelength control of the signal to be transmitted on the return path since the received signal is reflected by the mirror 19 with the applied data signal being determined by the switch 17. Since there is no requirement for a separate laser to be provided in the local transceiver, this decreases the cost of the hardware to be installed in the subscriber's home. The local transceiver 16 preferably comprises a monolithic SOI chip on which an optical fibre interface, the optical switch 17, the photodetector 18 and the mirror 19 are integrated.

Figure 2 shows a further, preferred embodiment of the invention comprising a central transceiver 1 in the form of a SOI chip 2 having an input 7 and an output 12, and having a demultiplexer 3, channel waveguides 4 incorporating VOAs 5 and associated photodetectors 20 and tap-off couplers 21, and a multiplexer 6 integrated on the chip 2. However, in this embodiment, the input optical signal supplied to the transceiver 1 by way of the input 7 is supplied by a 1310 nm window broadband light source 30 so that the multiplexed channel signals supplied to an output optical fibre 31 are all within the 1310 nm window, and are combined by means of a simple 1310/1550 nm WDM coupler (not shown) with a 1550 nm broadcast signal supplied by a combination of a 1550 nm DFB laser 32, a SOA 33 and an optical modulator 34 for adding data/services for subscribers. Such an embodiment is particularly applicable to CATV applications in which case the components 32, 33 and 34 are provided within the CATV transmitter.

The combination of the broadcast signal and the multiplexed output signal from the central transceiver 1 is then supplied to a distribution network, as shown diagrammatically in Figure 3. As in the previously described embodiment optical drop filters 15 are provided at each subscriber location to transmit the required channel signal to the local transceiver 16 incorporating an optical switch 17, a photodetector 18 and a mirror 19, as well as the broadcast signal which is transmitted to a 1550 nm RF receiver 35. Since a separate received path is provided for the 1550 nm broadband signal, a continuous downstream connection is provided for this signal, and the particular 1310 nm channel signal is diverted to the local transceiver which may incorporate a photodetector 18 in the form of a band-gap modified photodiode providing excellent rejection of the 1550 nm signal. The photodetectors 20 provided in the central transceiver 1 may also be band-gap modified (for negligible additional cost) to provide rejection of any back-reflected 1550 nm light. The sensitivity of such a transceiver will depend inter alia on the optical cross-talk of the chip 2, and various techniques may be applied to reduce the cross-talk, including doping of the substrate and shielding by metallisation.

Figures 4 and 5 show alternative forms of local transceiver which may be used in place of the local transceiver 16 described above with reference to Figures 1 and 2. In the case of the embodiment of Figure 4 the receiver 16' comprises on, on a SOI chip, a monolithic optical detector for detecting the incoming voice/data signal, an EVOA, a mirror 41 for reflecting the signal, and a EVOA 42 for modulating the signal to provide the required modulated return signal to be sent back to the central transceiver 1.

In the case of the embodiment of Figure 5 the local transceiver 16" comprises, on a SOI chip, a monolithic detector 50 for detecting the input signal, an optical mirror 51, a fast optical switch 52 and a beam dump 53. In this case the optical switch 52 serves to modulate the return signal from the mirror 51, and the beam dump 53 serves for disposal of that part of the incoming signal which is not reflected back by the mirror 51.

Figure 6 diagrammatically shows one possible encoding scheme which might be used for bi-directional communication in a telephone system. In the following description of this encoding scheme it will be assumed that the signal transmitted to the local transceiver from the central transceiver is produced by a single-wavelength transceiver component, and that the local transceiver incorporates an optical switch as in the transceiver 16 of Figure 1 in place of a laser diode. Furthermore it is assumed that a single laser is used to transmit information (by direct modulation) from the central location to the local transceiver, and that some of the light from the laser is reflected and modulated to send a return signal along the reverse path. This encoding scheme utilises time slots that are one bit long, in order to share the available bandwidth effectively between the downstream and upstream transmissions. Referring to Figure 6, bits A, B and C are to be transmitted downstream, as shown at 60 in the figure. They are encoded at double the basic data rate, and are represented by odd-numbered bits, as shown at 61 in the figure. The continuous bit stream is received at the local transceiver where it is divided by two and the even bits discarded (leaving just the bits representative of A, B and C). The local transceiver will synchronise with the received bit sequence so as to be able to recognise the even- numbered and odd-numbered bits.

Furthermore the even-numbered bits transmitted from the central transceiver are always set to be "1" and, at the local transceiver, this "1" setting of even-numbered bits provides a light signal that can be modulated according to the position of the optical switch 17. When the switch 17 is connected to the photodetector 18, very little of the light will be reflected back up the optical fibre. Instead the signal will be simply collected by the photodetector 8 which will provide an electrical output signal indicative of the received signal. The transceiver is synchronised to recover information from the even-numbered bits, but the signal to noise ratio will be 3 to 5 dB worse than for a simple point-to-point application because of the doubling of the basic data rate and the system may be limited by optical cross-talk.

However, if the switch is connected to the mirror 19, the light will be reflected back up the optical fibre, and a transmission controller 62 is provided to control the switch 17 so as to modulate the reflected light to transmit the bits X, Y and Z in the even-numbered bits of the return signal, as shown at 63 in the figure. Finally the even- numbered bits are detected at the central transceiver and multiplied by 2 to obtain the bits X, Y and Z as shown at 64 in the figure.

Figure 7 diagrammatically shows an alternative encoding scheme which may be used in place of the encoding scheme described above with reference to Figure 6. This alternative encoding scheme utilises bi-directional sub-carrier multiplexing (SCM). In this case the local transceiver 70 comprises an optical coupler 71, a photodetector 72 and a mirror 73 and associated EVOA 74 which is controlled by a transmitter driver 75 to modulate the signal reflected by the mirror 73. The electrical output of the photodetector 72 is supplied to a mixer 76 by way of a bandpass filter 77 so as to retrieve the received data signal under the control of a controller 78. The transmitted (upstream) and received (downstream) signals of the local transceiver are shown against frequency at 79 in Figure 7. The signal to noise ratio can be improved by ensuring that the transmitted signal has a narrower frequency range than the received signal. Furthermore the coupler 71 can be adjusted to balance the split ratio between the reception and transmission requirements. Although the presence of the coupler 71 will reduce the sensitivity and the power of the return signal will be uncertain, the electrical filtering can be reused to reduce the effect of optical cross-talk.

In a modification of the above-described encoding scheme a dual wavelength solution is used for optimum performance, the wavelength of 1310 nm being used for downstream transmission from the central transceiver and the wavelength of 1550 nm being used for upstream transmission from the local transceivers, as shown in Figure 8. In this case the central transceiver 80 comprises a laser diode 81 for transmitting 1310 nm light for the purposes of downstream data transmission and a second laser diode 82 for transmitting 1550 nm light for the purpose of return data transmission. Furthermore a Mach-Zehnder interferometer (MZI) 83 is provided for optically coupling the outputs of the two laser diodes 81, 82 to the input of a demultiplexer 84. The corresponding local transceiver 85 at the subscriber's home comprises a Mach- Zehnder interferometer (MZI) 86, a photodetector 87 and a mirror and modulator unit 88. As shown the modulator may be either a fast optical switch or an EVOA. In this case the 1310 nm light received by the interferometer 86 is passed to the photodetector 87 to enable the received data signal to be retrieved, whereas the 1550 nm light is passed by the interferometer to the mirror and modulator unit 88 which serves to produce the modulated return signal for transmission back to the central transceiver 80. In this case the photodetector 87 may be a wavelength-modified photodiode (type C) to offer very high isolation and rejection of 1550 nm light. The above-described embodiments of the invention have the particular advantage of reducing the number of lasers that are needed in an access network application, with a view to improving reliability and reducing the cost of the transceiver components. In particular the invention reduces the number of different types of customer-end components that are needed. By producing a generic customer-end transceiver which is responsive to a number of different input wavelengths, it is possible to reduce costs associated with manufacture, stocking, set-up and maintenance, as compared with a conventional implementation where a different type of customer-end transceiver is required for each of the return wavelength channels. Furthermore, by reflecting the received light at the customer-end back up the network, these embodiments dispense with the need for a local light source which is advantageous in terms of improved reliability and reduced cost.

Furthermore the head-end transceiver is suitable for applications where two or more wavelength channels are required. Rather than making use of the separate light source for each of the wavelength channels, these embodiments utilise a single broadband light source producing light which can be split into several wavelength bands with each band having its own modulation applied to add information specific to that channel. Furthermore a return path is provided to receive modulated light reflected at the customer-end. The whole device can be realised monolithically on one chip, and only one output optical fibre is required to carry a multi-channel by directional signal. Accurate wavelength control by control of temperature or other means is not required. Since the same output wavelength channel is returned directly to the multi-channel transceiver, the reciprocal path will match the returned signal. This should apply to both widely spaced wavelength channels, such as coarse WDM and more closely spaced dense wavelength division multiplexing (DWDM) versions. The wavelength must still be held within the window of the corresponding drop-off optical filters (or within the passband of the remote demultiplexer). However this is not so costly to achieve as it is to retain DWDM channel wavelengths over temperature changes and device lifetime for example. The data service for each wavelength channel can be tailored without the need to implement an individual laser or external modulator for each channel. The cost of implementing the integrated optical modulator for each channel can be relatively low, of high yield and with negligible insertion loss or PDL penalty.

In a possible development, several wideband sources are arranged on the same substrate as a multi-channel transmitter, and there outputs are combined to form a flatter and more powerful broadband light source. The advantages of using a single substrate can again be realised, and additionally an integrated optical amplifier or linear optical amplifier can be included to boost the input and output signals.

With the various encoding schemes proposed, the example in which alternating bits are used to convey information transmitted downstream and to provide the return path for upstream information is most appropriate for application to a point-to-point network with symmetric operation. Other possible implementations make use of sub- carrier signals to separate downstream and return paths (best implemented with a fast EVOA modulator) and more complicated arrangements in which multiple downstream bits can be combined and used to modulate a slower return signal. A further sub-carrier may also be used to allow modulation of a common signal onto all channels. This may be implemented with a fast external modulator, such as a GaAs or InP modulator, and could provide for broadcast services to be overlaid over all of the channels specific data connections, for example.

A further development allows an additional wavelength channel to be added for carrying services such as broadcast television, in which case the additional wavelength channel can be added and removed from the distribution network outside the multichannel transceiver.

Claims

CLAIMS:

1. An optical transceiver for transmitting data signals to a plurality of remote locations along a single optical fibre and for receiving return data signals transmitted in the opposite direction along the same optical fibre, comprising demultiplexing means for demultiplexing a broadband optical input signal, a plurality of channel waveguides for guiding channel signals of different wavelengths received from the demultiplexing means, a respective optical modulator associated with each channel waveguide for receiving an electrical data signal and for optically modulating the channel signal supplied to the channel waveguide to produce a respective data modulated channel signal, multiplexing means for multiplexing the data modulated channel signals from the channel waveguides and for outputting a multi-channel optical transmission signal incorporating the data modulated channel signals to the optical fibre, and optical detection means for detecting a respective return channel signal supplied to each channel waveguide by the multiplexing means demultiplexing a multi-channel return optical transmission signal received along the optical fibre.

2. An optical modulation device according to claim 1, wherein the optical modulator associated with each channel waveguide is a variable optical attenuator (VOA).

3. An optical modulation device according to claim 2, wherein the VOA is a highspeed electronic variable optical attenuator (EVOA).

4. An optical modulation device according to claim 1, 2 or 3, wherein the optical detection means comprises a respective photodetector associated with each of the channel waveguides for detecting a respective return channel signal supplied to each channel waveguide and for producing an electrical data signal indicative of the return channel signal.

5. An optical modulation device according to any preceding claim, wherein the optical detection means comprises a respective tap-off coupler for tapping off a proportion of each return channel signal and a respective photodiode for detecting said proportion of each return channel signal.

6. An optical modulation device according to any preceding claim, wherein the optical detection means comprises a monolithic photodetector arrangement.

7. An optical modulation device according to any preceding claim, incorporating a broadband optical source for supplying a broadband optical input signal within a required wavelength range.

8. An optical modulation device according to claim 7, wherein the broadband optical source comprises two individual optical emitters supplying output signals which are combined to produce the broadband optical input signal.

9. An optical modulation device according to any preceding claim, wherein a wavelength dependent modulator is provided for supplying a broadcast optical signal of a wavelength outside the wavelength range of the broadband optical input signal to the optical fibre for transmission along the optical fibre to the remote locations.

10. An optical modulation device according to any preceding claim, incorporating a broadcast optical source for supplying a broadcast optical signal of a wavelength outside the wavelength range of the broadband optical input signal to the optical fibre.

11. An optical communication network comprising (i) a central optical transceiver comprising demultiplexing means for demultiplexing a broadband optical input signal, a plurality of channel waveguides for guiding channel signals of different wavelengths received from the demultiplexing means, a respective optical modulator associated with each channel waveguide for receiving an electrical data signal and for optically modulating the channel signal supplied to the channel waveguide to produce a respective data modulated channel signal, multiplexing means for multiplexing the data modulated channel signals from the channel waveguides and for outputting a multichannel optical transmission signal incorporating the data modulated channel signals, and optical detection means for detecting a respective return channel signal supplied to each channel waveguide by the multiplexing means demultiplexing a multi-channel return optical transmission signal received along the optical fibre, (ii) an optical fibre for transmitting the multi-channel optical transmission signal and for transmitting the multichannel return optical transmission signal in the opposite direction, and (iii) at least one local optical transceiver which is adapted to receive a respective data modulated channel signal from the optical fibre and to transmit a return data modulated channel signal to the optical fibre.

12. A network according to claim 11, wherein the local optical transceiver incorporates reflecting means for reflecting the data modulated channel signal received from the optical fibre.

13. A network according to claim 12, wherein the local optical transceiver incorporates an optical modulator for receiving an electrical data signal and for optically modulating the channel signal reflected by the reflecting means to produce an optical return signal for transmission along the optical fibre back to the central optical transceiver.

14. A network according to claim 11, 12 or 13, wherein the optical modulator associated with each channel waveguide is a high-speed electronic variable optical attenuator (EVOA).

15. A network according to claim 12, wherein the local optical transceiver incorporates an optical switch for receiving an electrical data signal and for optically switching the optical fibre between a receiving section of the local optical transceiver and a transmitting section of the local optical transceiver to transmit an optical return signal along the optical fibre back to the central optical transceiver.

16. A network according to any one of claims 11 to 15, wherein the optical detection means comprises a respective photodetector associated with each of the channel waveguides for detecting a respective return channel signal supplied to each channel waveguide and for producing an electrical data signal indicative of the return channel signal.

17. A network according to any one of claims 11 to 16, wherein the optical detection means comprises a monolithic photodetector arrangement.

18. An optical communication network comprising a central optical transceiver for transmitting optical signals to, and receiving optical signals from, a remote location along a single optical fibre and incorporating (i) first optical modulating means for receiving an electrical data signal at a predetermined data rate and for optically modulating an optical signal to be transmitted in first time slots at a rate of at least twice the data rate, (ii) first photodetecting means for detecting a reflected optical signal; and a remote optical transceiver at the remote location for receiving optical signals from, and transmitting optical signals to, the central optical transceiver along the optical fibre and incorporating (a) second photodetecting means for detecting the optical signal received along the optical fibre from the central optical transceiver, (b) reflecting means for reflecting the optical signal, and (c) second optical modulating means for receiving an electrical data signal at said predetermined data rate and for optically modulating the reflected optical signal in second time slots, which alternate with said first time slots, at a rate of at least twice the data rate.

19. A network according to claim 18, wherein the second optical modulating means comprises an optical switch for directing the received optical signal alternately to the second photodetecting means and to the reflecting means, and for modulating the reflected optical signal within said second time slots at said rate of at least twice the data rate. _l

20. A local optical transceiver comprising input/output means for receiving an input data modulated optical channel signal from a central optical receiver along an optical fibre and for supplying an output data modulated optical channel signal to the central optical receiver along the same optical fibre, photodetector means for detecting the input optical channel signal and for supplying an electrical input data signal, reflecting means for reflecting the input optical channel signal, and an optical modulator for receiving an electrical output data signal and for optically modulating the optical channel signal reflected by the reflecting means to supply the output data modulated optical channel signal to the central optical receiver.

21. A transceiver according to claim 20, wherein the input/output means comprises an optical fibre interface for coupling to an optical fibre for transmission of the input and output data modulated optical channel signals.

22. A transceiver according to claim 20 or 21, wherein optical switching means is provided for selectively optically coupling the input/output means to the photodetector means and to the reflecting means.