OPTICAL COMMUNICATIONS SYSTEM

Abstract: An optical network includes optically pumped waveguide amplifiers (23). A supervisory signal uses the amplifier's pump wavelength (23), e.g. APPROX 1480 nm if EDFAs are used. Substandard pump diodes may be used to generate the supervisory signal. A repeater station for the network includes an optically pumped waveguide amplifier (23) and a bypass route (27, 28, 29, 30, 31) for supervisory signals. In a described example the bypass route comprises an optical supervisory signalling means (31), an optical receiver (29), a first wavelength division multiplexer (27) for coupling a supervisory signal from a transmission medium and applying it to the optical receiver, processing means (30) for processing supervisory signal data received by the receiving means and controlling the optical supervisory signalling means to generate a further supervisory signal, and a second wavelength division multiplexer (28) for coupling the further supervisory signal into the transmission medium, wherein the first and second wavelength division multiplexers are arranged one on either side of the waveguide amplifier.

Description: Optical Communications System Field of the Invention The present invention relates to an optical communications system including an optically pumped waveguide amplifier for amplifying message signals and an optical supervisory signalling system. Background In optical communications networks, for instance telephone networks, it is desirable to be able to monitor the performance and operation of the elements of the network. It has been suggested that this may be achieved by using an optical supervisory signal whose wavelength is outside of the band used for message signals. The supervisory signal is coupled into the transmission medium and then back out at certain points in the network. The signals coupled back out can be processed to derive information about the state of the network. The supervisory signal may comprise, for example, data reporting the performance of repeaters or other network components. Whilst the use of an out of band supervisory signal was known to be desirable, considerable difficulty has been found in selecting a suitable wavelength. Various wavelengths have been suggested, for example 1 530nm and 1620nm. Summary of the Invention According to the present invention, there is provided an optical communications system including at least one optically pumped waveguide amplifier for amplifying message signals and an optical supervisory signalling system, wherein the supervisory signalling system uses a wavelength approximately equal to a pump wavelength of the amplifier for signalling and the supervisory signal is produced by a source other than the pump of a waveguide amplifier. Preferably, the system further comprises optical message signalling means for generating optical message signals in a predetermined wavelength band, wherein the supervisory signal has a wavelength outside said predetermined band. Hitherto, supervisory signals have not been provided by a source independent of the amplifier's pump (or indeed independent of all of an amplifier's pumps where, for example, the amplifier is pumped at a plurality of wavelengths by more than one pump source). It is believed that one of the reasons for this is that significant levels of light at these wavelengths are present in networks, as a side-effect of the use of optically pumped waveguide amplifiers, such as erbium-doped fibre amplifiers (EDFA), to amplify message signals, leading to the assumption that there would be beating with a supervisory signal, producing undesirable results. However, the present inventors have discovered that these fears were unfounded and, in particular, that the use of a pump wavelength of amplifiers in the system for a supervisory signal is practical. The use of approximately 1480nm as the wavelength of the supervisory signal for an EDFA is advantageous because it makes use of readily available technology, including standard pump laser diodes and standard 1480nm/1 550nm wavelength division multiplexer (WDM) couplers. Conveniently, substandard pump laser diodes may be used. Preferably, the optical output power of the supervisory signal source is in the range -30dBm to 20dBm. Preferably, a system includes first and second optically pumped waveguide amplifiers connected by an optical waveguide, wherein the waveguide is at least 30km long. More preferably, the waveguide is at least 80km long. In a preferred embodiment, the waveguide is at least 1 20km long. Substandard pump diodes, that is diodes manufactured to be used to pump fibre amplifiers but generating insufficient output power for that purpose, may be used to generate the supervisory signal. Surprisingly, it has been found that such diodes may be easily modulated at the rate required for supervisory signals (e.g. of the order of 2Mbιts/s) and the signals produced transmitted over lengths of optical fibre corresponding to typical amplifier separations (e.g. 40km). Preferably, a system according to embodiments of the present invention comprises a repeater including said waveguide amplifier, said optical supervisory signalling means, an optical receiver, a first wavelength division multiplexer for coupling a supervisory signal from the transmission medium and applying it to the optical receiver, processing means for processing supervisory signal data received by the receiving means and controlling the optical supervisory signalling means to generate a further supervisory signal, and a second wavelength division multiplexer for coupling the further supervisory signal into the transmission medium, wherein the first and second wavelength division multiplexers are arranged one on either side of the waveguide amplifier. More preferably, there will be a plurality of such repeaters. The present invention also provides an optical repeater station comprising an optically pumped waveguide amplifier and an optical supervisory signalling means other than the amplifier pump, wherein the wavelength of the supervisory signal produced by the signalling means is approximately equal to that of the waveguide amplifier's pump. Preferably, the optical supervisory signalling means is included in a bypass route comprising also an optical receiver, a first wavelength division multiplexer for coupling a supervisory signal from a transmission medium and applying it to the optical receiver, processing means for processing supervisory signal data received by the receiving means and controlling the optical supervisory signalling means to generate a further supervisory signal, and a second wavelength division multiplexer for coupling the further supervisory signal into the transmission medium, wherein the first and second wavelength division multiplexers are arranged one on either side of the waveguide amplifier Brief Description of the Drawings An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 shows an experimental arrangement used to confirm the usability of 1480nm for a supervisory signal; Figure 2 is a graph of bit error rates for the arrangement of Figure 1 under different conditions; Figure 3 is a graph illustrating typical receiver sensitivities against bit rate; Figure 4 is a simplified block diagram of a portion of an illustrative example of a network according to the present invention; and Figure 5 is a simplified block diagram of another illustrative example of a network according to the present invention. Detailed Description Referring to Figure 1 , an experimental arrangement comprises a first pump laser diode 1 arranged to output a continuous wave (CW) signal for simulating the remnant pump light from EDFAs in a network. The output of the first laser diode 1 is applied via an optical fibre to a first isolator 2 and then to a first variable attenuator 3. The output of the first attenuator 3 is applied to a polarisation controller 4. A second pump laser diode 5 is modulated to produce a light signal simulating a supervisory signal. The output of the second laser diode 5 is applied to a second isolator 6. The laser diodes 1 and 5 are substandard pump diodes, operating in the region of 1480nm. The outputs of the polarisation controller 4 and the second isolator 6 are fed to a coupler 7 which combines them and supplies them to a 40km length of optical fibre 8, representing a portion of a network. A second attenuator 9 is arranged to receive light from the optical fibre 8. The output of the second attenuator 9 is applied to a splitter 10, the outputs of which are coupled respectively to a receiver 1 1 and a power monitor (not shown). The first laser diode 1 was supplied with a bias current of 196mA and the second laser diode 5 was supplied with a 55mA bias current. Both lasers diodes 1 ,5 were temperature tuned so that their peak outputs coincided at 1470nm and so that the longitudinal cavity modes were aligned in order to maximise any detrimental mode beating effects. The polarisation controller 4 was adjusted so that both signals applied to the coupler 7 had the same polarisation. A 3V modulation at 100Mbits/s was imposed on the second laser diode 5 and the first attenuator 3 was set so that the average powers from the two laser diodes 1 , 5 were equal after the coupler 7. Bit error rate (BER) measurements, using a 2 23 - 1 pseudo random binary sequence bit pattern, were made with and without the output of the first laser diode and with and without the 40km length of optical fibre 8. The results of these measurements are shown in Figure 2. The BER results for the arrangement, described above and which represents a worst case scenario, show that little penalty (<0.2dB) can be expected due to the residual pump light even with closely matched diode sources. Obviously the penalty will increase as the sources tend towards being exactly matched but, since in practice this will not be the case, it is not a serious consideration. Some simple calculations on power budget for the 1480nm region result in the following span losses for an optical fibre waveguide with a 0.4dB/km attenuation figure (more pessimistic than the value of ~-0.3dB/km set out in ITU-T (or CCITT) Recommendation G.957; Optical interfaces for equipment and systems relating to synchronous digital hierarchy). Span length (km) Loss @ 1480nm (dB) 80 32 1 20 48 Referring to Figure 3, the following receiver sensitivities are to be expected: Sensitivity (dBm) Commercial Low Cost Pinfet Pinfet Bit rate (Mb/s) 2 -59.5 -54.5 10 -53.5 -47.5 20 -51 .0 -44.0 50 -47.5 -40.0 1 55 -43.5 -34.0 Given the bit rates of 2, 10, 20, 50 and 1 55 Mb/s and additional incurred path losses of: optical path penalty 1 .OdB coupler losses for insertion/extraction 0.5dB from transmission path the following powers are needed: Using Commercial Receivers Spanlength (km) Transmitter powers @ 1480nm required (dBm) 2Mb/s 10Mb/s 20Mb/s 50Mb/s 155Mb/s 80 -26.0 -20.0 -1 7.5 -14.0 -10.0 120 -10.0 -4.0 -1 .5 2.0 6.0 Low Cost Receivers Spanlength (km) Transmitter powers @ 1480nm required (dBm) 2Mb/s 10Mb/s 20Mb/s 50Mb/s 1 55Mb/s 80 -21 .0 -14.0 -10.5 -6.5 -0.5 120 -5.0 2.0 5.5 9.5 1 5.5 These figures show that, surprisingly, 1480nm is quite satisfactory for use as a supervisory signal wavelength in practical optical communications networks which incorporate optical amplifiers, pumped at 1480nm. Calculations for a 0.25 bit slot dispersion limit indicate that a 3.5nm 3dB spectral bandwidth of a pump diode transmitter allows the following transmission lengths at the various bit rates: Bit rate (Mb/s) Max length (km) 2 1667 10 333 20 167 50 67 1 55 22 Using Single-Mode Fibre (SMF), bit rates of up to 50Mb/s are practical. Mode partition noise is not a major factor below bit rates of approximately 70Mb/s. Using Dispersion Shifted Fibre (DSF), dispersion is even less of a problem and bit rates of the order of 1 55Mb/s are practical. Referring to Figure 4, a first node of a network 20 is connected to a second node 21 by a fibre optic transmission path 22. The transmission path contains repeaters (only one shown) comprising EDFAs 23. Each EDFA 23 comprises a length of erbium-doped optical fibre 24, a pump laser diode 25 producing light at 1480nm wavelength to pump the erbium-doped fibre and a WDM 26 for coupling the pumping light into the transmission path. In the illustrative example, light signals will be described travelling unidirectionally from the first node 20 to the second node 21 for the sake of clarity only. Further four port WDMs 27, 28 are located on either side of the EDFA 23. The WDM 27 is coupled by an optical fibre to an optical receiver 29. The output of the optical receiver 29 is fed to a supervisory signal control unit 30. An output of the control unit 30 is connected to an optical transmitter 31 . The control unit 30 is also coupled to the EDFA 23 for controlling and monitoπng the performance of the EDFA 23. Light generated by the optical transmitter 31 is coupled into the transmission path 22 by WDM 28. WDM 28 will also have the effect providing some isolation at the pumping wavelength between the EDFA 23 and the waveguide to the second node 21 . This isolation is >10dB and a figure of 20dB would be reasonable. Supervisory signals at 1480nm are coupled out of the transmission path 22 by WDM 27 and are detected by the receiver 29 The receiver 29 outputs an electronic signal, dependent on the received 1480nm optical signal, to the control unit 30 for analysis. A new supervisory signal is generated by the transmitter 31 under the control of the control unit 30 and introduced into the transmission path 22. Thus, each repeater is provided with a bypass for supervisory signals, comprising a WDM 27, a receiver 29, a control unit 30, a transmitter 31 and another WDM 28. The supervisory signal generated by one transmitter 31 will be processed by the control unit 30 at the next repeater. Referring to Figure 5, in another embodiment of a repeater according to the present invention, no separate WDM 26 is provided for injecting pump light into the erbium-doped optical fibre 24 Instead, two pump laser diodes 25a, 25b are provided. The outputs of the laser diodes 25a, 25b are coupled into the erbium- doped optical fibre 24 by the four port WDMs 27, 28 that are used for extracting and inserting supervisory signals. One of the laser diodes 25a, 25b may be omitted if sufficient light intensity can be generated by one alone. The WDMs 27, 28 may be packaged bulk devices such as fused fibre devices L2SWM1480/1 550B from Sifam Ltd, Torquay, Devon, UK or fibre pigtailed devices from the WD141 5 Series from JDS Fitel, Nepean, Ontario, Canada. The skilled person will appreciate that many modifications could be made to the embodiments described above. For example, supervisory signals may be transmitted in either direction along the optical fibre transmission line or in both directions. Also, it is envisaged that the same supervisory signalling principle will apply to other types of amplifier, for example praseodymium-doped fibre amplifiers operating at 1 300nm and pumped at between about 950nm to 1070nm or thulium-doped amplifiers operating at about 1500nm, with the supervisory signal being arranged substantially at the, or at least one of the, pump wavelength(s).

Claims: Claims 1 . An optical communications system including at least one optically pumped waveguide amplifier (23) for amplifying message signals and an optical supervisory signalling system, wherein the supervisory signalling system uses a wavelength approximately equal to a pump wavelength of the amplifier for signalling and the supervisory signal is produced by a source other than the pump of a waveguide amplifier. 2. A system according to claim 1 , comprising optical message signalling means for generating optical message signals in a predetermined wavelength band, wherein the supervisory signal has a wavelength outside said predetermined band. 3. A system according to claim 1 or 2, wherein the waveguide amplifier is an erbium-doped fibre amplifier (23). 4. A system according to claim 3, wherein the wavelength of the supervisory signal is about 1480nm. 5. A system according to any preceding claim, wherein the optical supervisory signalling system comprises a substandard pump laser diode (5) for generating supervisory signals. 6. A system according to any preceding claim, wherein the optical output power of the supervisory signal source is in the range -30dBm to 20dBm. 7. A system according to any preceding claim, including first and second optically pumped waveguide amplifiers (23) connected by an optical waveguide (8), wherein said waveguide is at least 30km long. 8. A system according to claim 7, wherein said waveguide (8) is at least 80km long. 9. A system according to claim 8, wherein said waveguide (8) is at least 1 20km long. 10. A system according to claim 2, comprising a repeater including said waveguide amplifier (23), said optical supervisory signalling means (31 ), an optical receiver (29), a first wavelength division multiplexer (27) for coupling a supervisory signal from the transmission medium and applying it to the optical receiver, processing means (30) for processing supervisory signal data received by the receiving means and controlling the optical supervisory signalling means to generate a further supervisory signal, and a second wavelength division multiplexer (28) for coupling the further supervisory signal into the transmission medium, wherein the first and second wavelength division multiplexers are arranged one on either side of the waveguide amplifier. 1 1 . A system according to claim 10, wherein pump light for the waveguide amplifier (23) is applied thereto via at least one of said multiplexers (27, 28). 1 2. A system according to claim 1 0, including a further repeater comprising a waveguide amplifier (23), an optical supervisory signalling means (31 ), an optical receiver (29), a first wavelength division multiplexer (27) for coupling a supervisory signal from the transmission medium and applying it to the optical receiver, processing means (30) for processing supervisory signal data received by the receiving means and controlling the optical supervisory signalling means to generate a further supervisory signal, and a second wavelength division multiplexer (28) for coupling the further supervisory signal into the transmission medium, wherein the first and second wavelength division multiplexers are arranged one on either side of the waveguide amplifier. 1 3. A system according to claim 1 2, wherein pump light for the waveguide amplifier of the further repeater is applied thereto via at least one of said multiplexers (27, 28) of the further repeater. 14. An optical repeater station comprising an optically pumped waveguide amplifier (23) and an optical supervisory signalling means (31 ) other than the amplifier pump, wherein the wavelength of the supervisory signal produced by the signalling means is approximately equal to that of the waveguide amplifier's pump. 1 5. A station according to claim 14, wherein the optical supervisory signalling means (31 ) is included in a bypass route (27, 28, 29, 30, 31 ) comprising also an optical receiver (29), a first wavelength division multiplexer (27) for coupling a supervisory signal from a transmission medium and applying it to the optical receiver, processing means (30) for processing supervisory signal data received by the receiving means and controlling the optical supervisory signalling means to generate a further supervisory signal, and a second wavelength division multiplexer (28) for coupling the further supervisory signal into the transmission medium, wherein the first and second wavelength division multiplexers are arranged one on either side of the waveguide amplifier. 16. A station according to claim 14 or 1 5, wherein the waveguide amplifier is an erbium-doped fibre amplifier (23). 1 7. A station according to claim 1 5, wherein the wavelength of the supervisory signal is about 1 480nm 18. A station according to any one of claims 14 to 17, wherein the optical supervisory signalling means comprises a substandard pump laser diode (5). 1 9. A system according to any one of claims 14 to 18, wherein the optical output power of the supervisory signalling means is in the range -30dBm to 20dBm.

Inventors: CAMPBELL ROBERT JOHN, MASSICOTT JENNIFER FRANCES, WYATT RICHARD, CAMPBELL, ROBERT, JOHN, MASSICOTT, JENNIFER, FRANCES, WYATT, RICHARD
Applicants: BRITISH TELECOMM, CAMPBELL ROBERT JOHN, MASSICOTT JENNIFER FRANCES, WYATT RICHARD, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY, CAMPBELL, ROBERT, JOHN, MASSICOTT, JENNIFER, FRANCES, WYATT, RICHARD
Application year: 1996
Main classification: H04B10
IPC codes: H04B10