AU592668B2

AU592668B2 - A two way optical system

Info

Publication number: AU592668B2
Application number: AU78368/87A
Authority: AU
Inventors: Hans Jurgen Matt
Original assignee: Alcatel NV
Current assignee: Alcatel Lucent NV
Priority date: 1986-09-20
Filing date: 1987-09-14
Publication date: 1990-01-18
Anticipated expiration: 2007-09-14
Also published as: DE3632047A1; DE3632047C2; AU7836887A

Description

5926 68 T his dcumnent contains the amendmnts made undk~r wctioi 49 aind is corrtect f'or p~rin~tinrg.

COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952-1969 COMPLETE SPECIFICATION FOR THE INVENTION ENTITLED "A TWO WAY OPTICAL SYSTEM" The following statement is a full description oC this invention, including the best method of performing it known to us:- The present invention relates to an optical conrmunication system applicable to the bidirectional transmission of digital narrow band and digital broadband signals over a single waveguide. In one such system, all signals of the narrowband services are transmitted between a centre, also called "exchange", and each subscriber over an optical waveguide using Sbidirectional wavelength-division multiplexing in the wavelength region from 800 to 900 nm, and the signals of the broadband services, such as picture communication, in the 1,300-nm region.

From the description in the first-mentioned printed publication it can i0 be concluded that the fibre used there is a so-called two-window fibre which is optimized in the wavelength region from 900 nm and in the 1,300-nm wavelength region in terms of attenuation and bandwidth. Such optical waveguides are difficult to manufacture and expensive, and are not optimal with respect to bandwidth in either of the two regions of interest. Therefore one method proposes to use single-mode fibres and to adjust the optical transmitters and receivers to the optical long-wavelength region from 1,300 nm to 1,600 nm, to place both the wavelengths for transmitting the signals of the narrow-band services and the wavelengths for transmitting the signals of the broadband services in this optical long-wavelength I~J region.

I r It is desirable to provide a solution which differs from the prior art solutions in the choice of the optical-waveguide types and the wavelengths suitable for the signals of the narrow-band and broadband services. A preferred embodiment of the invention shows how the invention can be used in optical communication systems for the subscriber area which include a remote distribution unit common to a group of subscribers.

The invention will now be explained in more detail, by way of example, with reference to the accompanying drawings, in which: E 6 j Lrein

II

~1

I

Fig. 1 shows the basic principle or the invention as applied to a subscriber loop network in which each subscriber is connected via an optical waveguide to the centre in a star configuration; Fig. 2a shows a subscriber loop network with a remote distribution unit for a group of subscribers in which the signals from the subscribers are combined by time-division multiplexing; Fig. 2b shows a modification of th(- subscriber loop network of Fig.

2a; ij Fig. 3a shows a subscriber loop network with a remote distribution unit for a group of subscribers and with subscriber-assigned wavelengthdivision multiplex transmission; Fig. 3b shows a modification of the subscriber loop network of Fig.

3a, and Fig. 4 shows an embodiment of the wavelength-division multiplexer/demultiplexer 45 of Fig. 1.

C The system of Fig. I will be described with respect to an application to the subscriber area in which each subscriber is connected to the centre by an optical waveguide. It should be pointed out, however, that the two terminal stations of this system may also be terminals of any other commu- ?4~nication channel over which both narrow-band and broadband intelligence signals have to be transmitted.

In Fig. 1, the left half shows the devices contained in the centre for one subscriber, and the right half the depvices at the subscriber. Thie subscriber line between the centre and tho subscriber is an optical waveguide At the centre, a broadband multiplexer' 41 combines two or more, e.g., four, broadband signals from broadband channels whose bit rates are approximately 140 Mb/s (1 video signal associated audio signals stereo broadcast signals PCM-30 ISDN signals) into a signal of, 560 Mb/s (in the case of four broadband signals) by electric time-division multiplexing.

An electric-to-optical transducer 42, also called "optical transmitter",

I

N

converts this TDM signal into an optical signal with a wavelength' 1 of about 1,300 rnm.

A narrow-band multiplexer ("NB MUX" 43 combines the narrow-band digital signals to be transmitted to the subscriber, preferably signals from ISDN channels and/or digital signals for transmitting data, telemetry signals or sound broadcasts, and/or PCM-30 digital signals into a narrowband TDM signals which is converted by an optical transmitter 44 into an optical signal with a wavelength A 3 of about 810 nm. This manner of combining narrow-band signals does not preclude the possibility that, as mentioned above, the 140-Mb/s broadband signals, in turn, contain further narrow-band signals of the same kind.

A bidirectional optical wavelength-division multiplexer/demultiplexer

V

combines the optical signal containing the broadband signals (wavelength about 1,300 nm) and the optical signal containing the narrow-band TDM signal (wavelength about 810 nm) into an optical WDM signal and feeds the latt 't ter into the optical waveguide 10, which transmits it to the subscriber.

There, a bidirectional optical wavelength-division multiplexer/demultiplexer 46 separates it into two optical signals or wavelengths 1 and A 3 which are converted back into the subscriber's broadband electric TDM signal and narrow-band electric TDM signal by optical receivers 47 and 49, respectively. A broadband demultiplexer (BB DEMUX) 48 separates the broadband TDM signal into the individual 140-Mb/s broadband signals, and a narrow-band demultiplexer 50 separates the narrow-band TDM signal from the optical receiver 49 into the signals from which it was formed in the narrow-band multiplexer 43 at the transmitting end.

The narrow-band digital signals to be transmitted from the subscriber to the centre, one or more 144-kb/s ISDN signals and one or more 2.048-Mb/s PCM-30 signals, are combined into a narrow-band TDM signal by a narrow-band multiplexer 51 (NB MUX), and an optical transmitter 52 converts the narrow-band TDM signal into an optical signal with a wavelength X 4 of Sabout 870 nm. The bit rate of the narrow-band TDM signal may be up to Mb/s.

4 If the subscriber also has to transmit broadband digital signal to the centre, if he wants to use not only broadband distribution services but also broadband communication services such as video telephony, and if he has to send several 140-Mb/s signals to the centre, he has a broadband multiplexer 53 (BB MUX), which combines these broadband signals into a broadband TDM signal, and an optical transmitter 54, which converts this electric signal into an optical signal with a wavelength X 2 of about 1,500 1i 6 nm. The bidirectional optical wavelength-division multiplexer/demultiplexer 46 combines the signals to be transmitted to the Scentre, in the so-called upstream direction, into an optical WDM signal of wavelengths X2 and X 4 which is transmitted over the optical waveguide to the centre. There, the bidirectional optical wavelength-division multiplexer/demultiplexer 45 separates it into optical signals of wavei lengths X 2 and X 4 An optical receiver 55 converts the optical signal containing the broadband TDM signal into an electric signal which is separated by a broadband demultiplexer 56 (BB DEMUX) into the 140-Mb/s broadband i o v.rt T; -1 r rr. 4l 1r _11 4 hf T

I

1 i I i: i s &.LCa som w LI c t wasn ome au the stubscriber. CE, tha subscriber has S to send only a single 140-Mb/s signal to the centre, the broadband multiplexer 53 at the subscriber and the broadband demultiplexer 56 at the centre can, of course, be dispensed with.) The optical output signal of the bidirectional optical wavelengthdivision multiplexer/demultiplexer 45 containing the narrow-band TDM signal is converted by an optical receiver 57 into an electric signal which is separated by a narrow-band demultiplexer 58 (NB DEMUX) into the narrow-band signals from which it was formed at the subscriber.

The optical transmitters for the broadband TDM signals are preferably lasers, and those for the narrow-band TDM signals are preferably lightemitting diodes. Bidirectional optical wavelength-division

I

4 r 4 #4 rt #4 St4 44 multiplexer/demultiplexers are known per se from, EP-A2-0 164 652 and, therefore, need not be explained here.

The type of the optical waveguide 10 in connection with the choice of the wavelengths of the optical signals transmitted over this waveguide is important. According to the embodiment of the invention, the optical waveguide is a "single-window fibre" which is optimized in the 1,300-nm wavelength region, which is used for transmitting the broadband signals, in terms of attenuation and bandwidth, and the wavelengths used for transmitting the narrow-band signals lie outside that region.

A first allocation of the light wavelengths uses the combination.

X1 1,300 nm for broadband signals in the range of 3 to 10 Gb/s in the downstream direction from the centre to the subscriber or to a remote t distribution unit; 12 1,550 nm for broadband signals up to about 200 Mb/s in the upstream direction to the centre, and 13 810 nm and 14 870 nm for the narrow-band signals, the wavelength region around 850 nm.

This has the advantage that the optical components (lasers, couplers) for broadband transmission can have relatively wide tolerances.

A second allocation of the light wavelengths uses the combination: 1 ~t 1,300 nm for broadband signals in the range of 3 to 10 Gb/s in the downstream direction; 12*61,200 nm for broadband signals in the upstream direction, and the wavelengths 13, 14 for the narrow-band signals either in the 850-nm or 1,550-nm region or in both these regions.

The optical waveguide can be either a single-mode fibre or a gradedindex multimode fibre. Examples of such optical waveguides are described in the periodical "ntz", Vol. 139 (1986), No. 7, pp. 454-459, especially with reference to Figs. 6 and 7.

1 i 7 1 The invention is predicated on the discovery that for the small bandwidth of the narrow-band signals, which is mostly one order or magnitude smaller than that or the broadband signals, and the relatively short length of the optical-waveguide subscriber line of only a few kilometer5s, the bandwidth or the "single-window fibre" and the attenuation or this ribre outside the optimized wavelength region ('or these signals are still sufficient to ensure sufricient transmission quality.

The above-described communication system according to the invention has the following advantages: 1. For wavelengths in the 1,300-nm region and wavelengths in the region or 800 to 900 nm, proven optical transmitters and receivers are available at reasonable cost, and the optical waveguide 10 is a type obtainable at low cost.

11 2. To transmit the broadband and narrow-band signals, use can be made or equipment with different quality standards, such as particularly reliable commercial-quality equipment ror the narrow-band signals and less expensive consumer equipment ('or the broadband signals.

3. The equipment ror the broadband signals can be switched off independent or the equipment ror the narrow-band signals, to save energy during times or slack traffic, for example.

4.As the power supplies or the broadband equipment and the narrow-band equipment are independent or each other, in the event or a power railure, an emergency service with exclusive narrow-band signal transmission and very low power consumption can be maintained.

The narrow-band and broadband signals may come fromn networks with power frequencies varying relative to each other. Thus, there are no synchronization problems between transmission equipment for the narrowband and broadband sizgnals.

S6. The system can be extended step by step according to a subscriber's needs for communication services.

7 The system of Fig. 1 may include further equipment (not shown) designed to transmit additional broadband TDM signals using additional wavelengths in the 1,300-nm region, also in both directions. The embodiment of Fig. 1 shows only the transmission of a single broadband TDM signal per direction of transmission. The same applies analogously to the narrow-band signals in the wavelength region of 800 to 900 nm and to broadband signals lQ' in the region of 1,500 to 1,600 nm.

Fig. 2a shows an application of the invention to a system as disclosed in EP-A2-0 151 454, in which a group of subscribers is connected via a remote distribution unit to the centre, and in which the centre is connected to the remote distribution unit by a single optical waveguide. In that prior art system, no distinction is made between broadband signals and narrow-band signals as far as the time-division multiplexing of different signals to be transmitted from the centre to the group of subscribers or in the opposite direction is concerned. The following describes how that system can be modified to transmit narrow-band signals independent of io broadband signals.

A broadband multiplexer 61 (BB MUX) combines all broadband digital intelligence signals to be transmitted from the centre to the group of subscribers, Tin- through Tlnl0. These are several 140-Mb/s signals, for example, which may also contain narrow-band signals. The broadband TDM signal BS delivered by the broadband multiplexer 61 is converted by an optical transmitter 62 into an optical signal of wavelength xi 1,300 nm. A narrow-band multiplexer 63 combines the narrow-band digital signals to be transmitted to the group of subscribers, 12 144-kb/s ISDN signals, into a narrow-band TDM signal SS with a bit rate of 2.048 Mb/s, which corresponds to the lowest level of the POM hierarchy (PCM-30). An optical

;?I

s

L,

I-;

i 1.% transmitter 64 converts this signal into an optical signal of wavelength X 3 810 n. A bidirectional optical wavelength-division multiplexer/demultiplexer 65 combines the optical signal containing the broadband TDM signal BS and the optical signal containing the narrow-band TDM signal SS into an optical WDM signal and feeds the latter into the optical waveguide 20 for transmission to the remote distribution unit. The optical waveguide 20 is a "single-window fibre" with the properties explained in connection with Fig. 1.

A bidirectional optical wavelength-division multiplexer/demultiplexer 66 separates the optical WDM signal into the two optic 1 signals of wavelengths A1 and X 3 and optical receivers 67 and 69 convert these two optical signals back into the broadband electric TDM signal BS and the narrow-band electric TDM signal SS, respectively. The remote distribution unit contains broadband multiplexer/demultiplexers 68, which separate the broadband TDM signal into subscriber-assigned broadband TDM signals and transmit the latter over subscriber-assigned broadband electric lines KH% to KH 10 preferably coaxial lines, to the subscribers Tln to Tlnl 0 Similarly, narrow-band demultiplexers 70 in the remote distribution unit separate the narrow-band TDM signal SS into subscriber-assigned ISDN 2Q signals which are transmitted over the subscriber-assigned narrow-band electric lines in the form of conventional copper pairs D through D 10 to the subscribers Tln 1 through Tlnl 0 In the opposite direction from the subscribers to the centre, in the upstream direction, the subscribers Tln 1 to Tlnl0 transmit their broadband TDM signals over subscriber-assigned broadband electric lines KR 1 to KR 10 preferably coaxial lines, to the broadband multiplexer/demultiplexers 68, which combine these signals into a broadband TDM signal containing all broadband signals to be transmitted from the subscribers of the group to the centre. An optical transmitter 71 converts this signal into an optical signal of wavelength X2 1,200 nm and transit

I

mits it over an optical waveguide to the bidirectional optical wavelength- V division multiplexer/demultiplexer 66.

SSimilarly, the subscribers TrI through Tinl 0 transmit their narrowband ISDN signals over the subscriber-assigned narrow-band electric lines D1 through D0 to the narrow-band multiplexer/demultiplexers 70. The mode 1 10 I of operation on these lines corresponds to the well-known bidirectional mode on the subscriber lines of the ISDN network. Since, in the embodiment shown, 12 ISDN channels are provided for only 10 subscribers, at least one 1 of the subscribers has more than one ISDN channel at his disposal.

The narrow-band multiplexer/demultiplexers 70 combine the subscriberassigned digital signals to be transmitted to the centre into a 2.048-Mb/s narrow-band TDM signal. An optical transmitter 72 converts this signal into an optical signal of wavelength 7 870 nm and transmits it over an 4 4 optical waveguide to the bidirectional optical wavelength-division multiplexer/demultiplexer 66. The latter combines the two optical signals to be transmitted in the upstream direction into a WDM signal of wavelengths X and X and feeds this signal into the optical waveguide 20 for 2 4 transmission to the centre. There, the two optical signals are separated into broadband electric TDM signals and narrow-band electric TDM signals, i 2G and finally separated by suitable demultiplexers into the broadband and S narrow-band digital signals suitable for switching and transmission.

In this system, too, the narrow-band signals, as described, are transmitted in a wavelength region of a "single-window fibre" which lies outside the wavelength region used for signal transmission. This applies to both directions of transmission. The example just described again shows only one wavelength for broadband transmission and one wavelength for narrowband transmission. If required, however, further devices may be provided for transmitting additional broadband and narrow-band signals on additional wavelengths in corresponding regions.

ri tl

I'

I.

i- Es In a preferred embodiment of the system of Fig. 2a, the subscriberassigned broadband and narrow-band signals are transmitted as optical signals between the remote distribution unit and the subscribers Tln1 to Tlnl0 over subscriber-assigned optical waveguides using bidirectional wavelength-division multiplexing, as is shown in Fig. 2b.

Since the distance between the remote distribution unit and the subscriber is very small (typically <500 very low-priced parts can be used for the components 101 to 110, and a very low-priced optical waveguide for the optical waveguides L to L 0. In this embodiment, too, broadband and narrow-band signals are transmitted in separate wavelength regions, as is described in connection with Fig. 1, the wavelengths 1 2 and X1 1 1 2 12 1 '22 1 0 1 102 of the broadband signals lying in the fibre's optimized wavelength window at 1,300 nm, and the wavelengths of the narrow-band signals A 3 4 and 1 13 14 23 24 103 A1 04 in the 850-nm or 1,550-nm region.

Fig. 2b shows the optical components contained in the remote distribution unit for the subscriber Tlnl in the components at that subscriber, and the optical waveguide L 1 which connects the subscriber Tln 1 to the remote distribution unit. In the remote distribution unit, the optical transmitters for the broadband and narrow-band signals are designated 101 (wavelength X 11 and 109 (A 13 respectively. The corresponding optical receivers at the subscriber are designated 105 and 107. At the subscriber Tlni, the optical transmitters for the broadband and narrow-band signals are designated 106 (wavelength X 12 and 108 (wavelength X 14 respectively.

The corresponding optical receivers in the remote distribution unit are designated 102 and 110. The reference numerals 103 and 104 denote bidirectional optical wavelength-division multiplexer/demultiplexers in the remote distribution unit and at the subscriber, respectively. Corresponding devices are present for the other subscribers, Tln 1 to Tlnl 0 which is indicated for the subscriber Tlnl 0 Between the broadband and narrow-band ii a- 4 multiplexer/demultiplexers 68, 70 and the centre, the transmission system is the same as that shown in Fig. 2a.

The systems of Figs. 2a and 2b differ from the prior art system disclosed in EP-A2-0 151 454 in that the narrow-band signals are transmitted between the centre and the remote distribution unit at wavelengths other than those used for the broadband signals, and between the remote distribution unit and the subscribers on subscriber-assigned lines or at wavelengths other than, those used for the broadband signals. It should be noted that this concept of Figs. 2a and 2b, aside from the feature of -the above-described choice of the optical-waveguide type and the wavelength re- ,~gions for the transmission of the broadband and narrow-band signals, must ~be considered a separately patentable modification of the prior art system.

The advantages mentioned in connection with the system of Fig. 1 also csapply without reservation to the systems 2a and 2b.

Another system for the subscriber area which contains a remote disti tribution unit for each group of subscribers, too, and in which optical 1 tsignals are transmitted between the centre and the remote distribution unit using bidirectional wavelength-division multiplexing, and between the remote distribution unit and the subscribers over subscriber-assigned optical waveguides Is the system shown in Fig. 3a. The basic concept of this system Is known from EP-A2-0 164 652. According to the invention, this system is modified with respect to the transmission of the narrow-band signals.

At the centre, shown in the left part of Fig. 3a, each of a plurality of subscriber-assigned multiplexers 15a to 15n combines the digital broadband signals to be transmitted to a subscriber of the group of subscribers 3a to 3n, four 140-Mb/s signals, into a broadband TDM signal Sa to Sn. These broadband TDM signals ar'e converted by subscriber-assigned optical transmitters 12a to 12n into optical signals having subscriberassigned wavelengths, and are transmitted via a bidirectional optical wavelength-division xnultiplexer/demultiplexer 11, an optical waveguide a bidirectional optical wavelength-division multiplexer/demultiplexer 21 in the remote distribution unit, and subscriber-assigned optical waveguides 'through 5n to the subscribers 3a through 3n.

For the transmission in the upstream direction, use is made or subscriber-assigned optical transmitters 32a to 32n, the same subscriberassigned optical waveguides, the same wavelength-division multiplexer/demultiplexers, and optical receivers 16a to 16n, which feed subscriber-assigned demultiplexers 17a to 17n, which distribute the broadband electric TDM signals to subscriber-assigned broadband channels.

Y, 0 To separate the two directions or transmission, the centre contains directional couplers 14la to 1 1 4n, and the subscriber racilities 3a to 3n contain directional couplers 33a to 33n. The optical receivers at the subscribers are designated 31a to 31n, and the optical signals transmitted *over the subscriber-assigned optical waveguides 5a to 5n are designated Pa to Pn. This network structure requires no rurther explanation because it *does not dirrer rrom that disclosed in EP-A2-0 164 652.

The dirrerence lies in the ract that the hitherto described transmission equipment is used exclusively ror the broadband intelligence signals to be transmitted between the subscribers or a group and the centre 2G in both directions, and that the narrow-band intelligence signals to be transmitted between the subscribers or the group and the centre in both directions are transmitted over paths other than those used ror the broadband signals.

Like in the system or Fig. 2a, the centre in the system or Fig. 3a contains a narrow-band multiplexer 18 (NB MUX), which is common to the group or subscribers 3a to 3n and combines 12 ISDN signals into a 2.048 kb/s narrow-band multiplex signal, and an optical transmitter 19, which converts this electric signal into an optical signal or wavelength X 3 =810 nm. This optical signal Ps is transmitted via the bidirectional optical wavelength-division multiplexer/demultiplexer 11 and the optical waveguide *1 to the remote distribution unit, where the bidirectional optical wavelength-division multiplexer/demultiplexer 21 separates it from the optical signals containing the broadband signals.

In the "emote distribution unit, an optical receiver 22 converts the Soptical signal Ps into the narrow-band electric TDM signal SS. A narrowband multiplexer/demultiplexer 23 separates the narrow-band TDM signal SS into the subscriber-assigned ISDN signals, wliich are transmitted over subscriber-assigned narrow-band electric lines 6a through 6n to the subscribers 3a through 3n. The transmission of the subscriber-assigned 1 narrow-band signals, the ISDN signals, from the subscribers via the Sremote distribution unit to the centre is exactly as described with the aid j of Fig. 2a and, therefore, need not be explained again. Here, too, the 1 wavelength 4 for the upstream direction is 870 nm. To separate the two directions of transmission of the optical signals, the remote distribution unit and the centre each contain a wavelength-selective directional coupler 24, The optical waveguide 30 between the centre and the remote distribution unit is again a "single-window fibre" having the properties described I in connection with Fig. 1. The subscriber-assigned wavelengths for transi 2 mitting the subscriber-assigned broadband TDM signals over the optical S waveguide 30 by bidirectional wavelength-division multiplex are chosen to lie in the 1,300-nm wavelength region, which is optimized in terms of at- S tenuation and bandwidth, and the wavelengths for transmitting the narrowband TDM signals lie outside that optimized wavelength region of the optical waveguide. The resulting advantages are the same as those described above for the system of Fig. 1.

In a modification of the system of Fig. 3a, the subscriber-assigned narrow-band signals between the remote distribution unit and the subscribers 3a to 3n are transmitted as optical signals, too, as is shown in Fig.

3b.

14 In that case, the optical wavelengths a Xa 2 to Anl, n2 for transmitting the broadband signals lie in the optimized wavelength window of the fibre at 1,300 nm, and the wavelengthsA 3 and A,3 Xa4 A b3 b4 i b 3, n4 for transmitting the narrow-band signals in the 850-nm or 1550-nm wavelength region.

Fig. 3b shows the optical components contained for the subscriber 3a in the remote distribution unit, whose constituents 21 to 24 are unchanged from Fig. 3a, and the optical components at this subscriber, where the optical components 31a, 32a, 33a for transmitting the broadband signals and the subscriber-assigned optical waveguide 5a are the same as those shown in Fig. 3a. In the remote distribution unit, the optical transmitter for the narrow-band signal is designated 34a (wavelength Xa 3 and the corresponding optical receiver at the subscriber is designated 35a. The optical transmitter ofP the subscriber 3a for the narrow-band signal is designated 36a (wavelength a4), and the corresponding optical receiver in the remote distribution unit is designated 37a. Coupling into and out of the subscriber-assigned optical waveguide 5a takes place via wavelengthselective directional couplers, as shown. Corresponding optical components are provided for the other subscribers of the group 3a to 3n (Fig. 3a).

With respect to the transmission between the remote distribution unit and the centre, there is no difference between the system of Fig. 3b and the system of Fig. 3a.

It should be pointed out that, aside from the specific choice of the type of the optical waveguide 30 and the wavelengths chosen for the transmission of the narrow-band signals with regard to the transmission characteristics of the optical waveguide 30, the principle explained above in connection with Figs. 3a and 3b must be considered a separately patentable modification of the prior art system forming the basis of the present invention, in as much as the narrow-band signals between the centre and the remote distribution unit are transmitted over optical channels other than r i:L

I

;S

I

r 3 r ji those used for the broadband signals, and those between the remote distribution unit and the subscribers of the group over electric transmission paths or at wavelengths other than those used for the broadband signals.

The bidirectional optical wavelength-division multiplexer/demultiplexer 45 of Fig. 1 will now be described with the aid of Fig. 4. The optical components 46 (Fig. 65, 66 (Fig. 2a), 103, 104 (Fig. 2b), and 11, 21 (Fig. 3a) are constructed correspondingly.

If single-mode fibres are used, the bidirectional optical wavelengthdivision multiplexer/demultiplexers will preferably be built with wavelength-selective optical fibre fusion couplers. A fibre coupler 453 separates the long-wave region X1 X 2 from the shortwave region 4 another coupler 451 provides separation between A 1 and A 2 (or only 3-dB light-power coupling), and a third coupler 452 separates the wavelengths X3 and b or works as a 3-dB power coupler.

Er Ib(I r, ft 1 t II r L f( glt i'

Claims

1. An optical communication system comprising a first terminal and at least one second terminal, each second terminal being connected to the first terminal via a respective optical waveguide having a transmission window of a first range of wavelengths at which transmission over the waveguide is optimized, wherein bidirectional wavelength division digital transmission between the first terminal and each second terminal over each respective waveguide comprises, in each direction, broadband signals having a bit rate in excess of 100 m bits/sec and narrow-band signals having a bit rate less than about 20 m bit/sec, the broadband signals having wavelengths within the first range of wavelengths, the narrow-band signals having wave- lengths within a second range of wavelengths different from the first range, the first terminal and each second terminal including a first op- tical transmitter to transmit broadband signals; a second optical transmit- t er to transmit narrow-band signals; means to combine the outputs of the first and second transmitters and to apply them to the waveguide; means to separate received broadband and narrow-band optical signals.

2. A system as claimed in claim 1 wherein the wavelength of the broadband signals transmitted by each second terminalJ. is different from the wavelength of the broadband signals transmitted by the first terminal over the respective waveguide.

3. A system as claimed in claim 1 or claim 2 wherein the wavelength of the narrow-band signals transmitted by each second terminal is different from the wavelength of the narrow-band signals transmitted by the first terminal over the respective waveguide.

4. A system as claimed in any one of claims 1 to 3 wherein the first terminal is a central station and each second terminal is a remote distrib- ution unit assigned to a group of subscribers.

A system as claimed in claim 4 wherein the first and each second S 1 includes first and second opto-electric transducer means to con- Tic 17 B W r I 7 I vert received broadband and narrow-band signals respectively to electric signals, electric transmission lines being provided to connect each sub- scriber to the distribution unit.

6. A system as claimed in claim 4 or claim 5 wherein the remote dis- tribution unit includes means to recognize received subscriber-assigned signals intended for specific subscribers and to transmit the subscriber- assigned signals to the appropriate subscriber.

7. A system as claimed in claim 6 wherein the distribution unit trans- mits the subscriber-assigned signals as TDM signals.

8. A system as claimed in any one of claims 4 to 7 wherein the central station transmits subscriber-assigned signals as wavelength division signals.

9. A system as claimed in any one of claims 4 to 7 wherein the central station transmits both TDM and wavelength division signals.

10. An optical comnunication system as herein described with reference to the accompanying drawings. C 94 -9 9 9 99 C 9* 9 4 9L 9 9 99 909I 9 9 9 99 99999 DATED THIS SECOND DAY OF NOVEMBER 1989 ALCATEL N.V.