OPTICAL TIME DOMAIN REFLECTOMETRY

Abstract: Apparatus and a method for increasing the optical measurement range in optical time domain reflectometers in which a fibre laser amplifier is used to amplify the optical time domain reflectometer pulses launched into a waveguide. This apparatus and method have particular usefulness for branched local networks. The signal returning to the optical time domain reflectometer is amplified as well as the outbound signal.

Description: OPTICAL TIME DOMAIN REFLECTOMETRY This invention relates to Optical Time Domain Reflectometry (OTDR), and especially to enhancing the dynamic range of OTDR equipment. In OTDR.an optical pulse is launched into an optical fibre (or more generally into a waveguide; in this specification the expression 'fibre 1 is used to include the more general case of a waveguide as well as optical fibres) and backscattered signal returning to the launch end is monitored. In the event that there are discontinuities such E faults or splices in the fibre, the amount of backscattering generally increases and this change is detected in the monitored return signal. Backscattering and reflection also occur from elements such as couplers and so the monitored signal is usually compared with a reference record, new peaks and other cnanges in the monitored signal level or plot being indicative of changes in the fibre path, normally indicating a fault. The time between pulse launch and receipt of the backscattered pulse is proportional to the distance along the fibre to the source of the backscattering, and so OTDR is a useful technique for fault location. However there are presently distance limitations on how far it is possible to see along a fibre. One of the limitations is the dynamic range of the OTDR receiver which has to be sufficiently sensitive to detect the low level of backscattered light returning from the more distant part of the fibre, but also capable of receiving, without damage, much greater intensity reflection and backscattering from fibre locations close to the launch end. In practice OTDR receivers have about a 30dB optical one-way measurement range, that is a 60dB optical dynamic range, and this requires the electronics to have a twofold dynamic range of 120dB resulting from the optical to electrical conversion. It is desirable to be able to detect faults along fibre of a length exceeding that equivalent to 30dB, there already being lengths equivalent to 40dB in existence and further increases being probable. At present it is necessary to utilise OTDR measurements from both ends of a 40dB span in order to test the full span, and this arrangement is not always convenient, and in any event requires more time and movement of, or additional, equipment. Additionally, in branched networks, where fibre spans are usually shorter but where there is much effective loss of power resulting from splitting or distribution of signals, short duration pulses are required for high resolution. The necessary short duration pulses contain little power and this, rather than receiver electrical dynamic range limits the effective range of the OTDR in such circumstances. It would be useful if significantly higher pulse powers could be achieved, without increasing pulse width and hence losing resolution. Because OTDR equipment (optical time domain reflectometers) needs to be portable and hence relatively compact, the optical sources chosen for use in such equipment are almost invariably semiconductor laser diodes. The optical source in an OTDR needs to be capable of providing short-duration, well-defined optical pulses if the OTDR is to provide satisfactory resolution. Moreover, satisfactory resolution requires the use of a narrow linewidth source, since the dispersion properties of commonly used optical fibres result in pulse spreading if other than narrow linewidth sources are used. Unfortunately, narrow linewidth semiconductor diode lasers naturally have very limited power outputs. Thus it is not feasible to increase the useful range of an OTDR simply by replacing the source laser with a higher power semiconductor diode. For experimental purposes, however, bulky and expensive non-semiconductor lasers have occasionally been used as sources in OTDR experiments. The bulk of the replacement laser sources precluding them from fitment within the reflectometer itself. Increases in OTDR range have in general come about from continual and gradual improvements to the signal processing, software, electronics, and the laser diodes. So far it has not been thought to be feasible to amplify optically the pulses output from the OTDR source. In part this s perhaps due to a perception that the recovery times of laser amplifiers, and in particular semiconductor laser amplifiers, are too short to be useful with pulse lengths in the range normally used in OTDR reflectometry. Other perceived problems or disadvantages nave been complexity, polarisation sensitivity, excess spontaneous and stimulated noise, and relatively low gains. Thus it is that no-one, as far as we are aware, has reported an OTDR or OTDR system in which an optical amplifier is used to amplify pulses from the source for launching into a fibre under test. According to a first aspect the present invention provides an optical time domain reflectometer comprising a source of optical pulses, means for launching the optical pulses into a waveguide and means for optically pumping the waveguide to produce gain at the optical pulse wavelength prior to launching the amplified pulses into a waveguide to be tested. A further aspect of the invention provides a method of increasing the measurement range of an optical time domain reflectometer, the method comprising launching an optical pulse into a waveguide and optically pumping the waveguide to induce gam of ne pulse prior to launching the amplified pulses into a waveguide to be tested. The invention is now described by way of example with reference to the accompanying drawings in which: Figure I is a schematic diagram of an embodiment of the invention; and Figures 2a and 2b show OTDR traces respectively without and with the invention; and Figure 3 is a plot cf forward spontaneous emission spectrum of an erbium doped fibre amplifier. Referring firstly to Figure 1, in a preferred emDodiment a modified Hewlett Packard HP8145A OTDR 1 was connected to one input arm 2 of a wavelength division multiplexer (WDM) coupler 3, which was in turn connected to a single-mode fibre under test 4. The Hewlett Packard OTDR was fitted with a suitable narrow-linewidth DFB laser, λs=1.536 micron, in place of the standard non-DFB source. Between the OTDR and the coupler, a 0.8nm bandwidth bandpass interference filter, with a centre wavelength of 1536nm, was inserted. The bandwidth was chosen to be narrow enough to reduce the amount of spontaneous amplifier noise reaching the OTDR receiver but sufficiently broad to easily allow the laser emission, which occurs over a few hundred MHz, to pass with low loss. The second input arm 5 of the WDM coupler was connected to a 1485nm pump laser 6, which could be any suitable laser such as a 1.485 micron semiconductor laser module, for example a GRINSCH M W laser. For the sake of portability, a semiconductor diode laser is preferred. Suitable pump lasers are available from OKI of Tokyo, Japan, these particular type are of VIF3 structure and are supplied in.a module with an integral WDM coupling arrangement. The OKI integral coupler serves as coupler 3. Initially the pump power was 65mW. An erbium fibre amplifier, to which standard single-mode fibre tails had been attached by fusion splicing, was spliced between the fibre under test 4 and the output arm of the coupler 3. The -ibre amplifier consisted of a 10 metre length of small core erbium fibre, having a core diameter of 5.Sam, refractive index difference Δn =-= 0.013, and a HOOppm erbium doping level in the core, further details of which can be found in the paper by Atkins et al, in Electronics Letters, 1989, Vol. 25, pp 910-911, and the references therein. Figure 3 shows the amplifier forward spontaneous emission spectrum, including the residual pump power at the amplifier output, when pumped with about 65mW at 1.485χm. At a signal wavelength of 1.53 a peak gain of 24dB was obtained, while at the OTDR wavelength of 1.536«m the amplifier gam was 20dB. Most preferably, in all embodiments of the invention, amplification of both outbound OTDR pulse and returning backscattered signals occurs in the fibre laser amplifier. Less preferably, there is only amplification of the optical pulses to be launched into the fibre under test, and not of the backscattered signals. Pulsed pump power could be employed, in conjunction with suitable - ~ - single-pulse OTDR equipment instead of the HP8145A. A Hewlett Packard HP8145A OTDR was selected for the embodiment because it automatically subtracts the mean received light level from the measurements, and thus it is able to cope with residual pump and spontaneous amplifier noise signals that reach the OTDR. Other types of OTDRs may be employed if the filtering is adequate. Figure 2a is a typical OTDR trace on a 12.4Km high attenuation test fibre 1.3dB/Km) with the power falling to a substantially steady background level given by the OTDR receiver noise floor after about 7Km. When the pump is turned on the trace modifies to that shown in Figure 2b so that instead of the trace falling to the noise floor after 7Km one is able to 'see' the entire fibre length. With the optical amplifier in place, the test fibre was measured in two stages, allowing the use of reduced optical amplifier gam on the initial fibre span, to avoid saturating the OTDR receiver.lt should be noted that both the outbound and returning signals are amplified rather than ]ust outbound signals, and thus the signals reach and can be detected from further along the fibre than would otherwise be the case. Figure 2a and the second measurement of Figure 2b were obtained with identical settings on the vertical offsets. The measurement noise floor is approximately the same in both cases, thus it can be expected that the erbium fibre gain of 18dB (20dB minus ldB for each of two fusion splices) should lead to an OTDR range increase of l8dB one way. The benefits of the invention may be utilised to aid fault detection in branched networks, where the total span length is less but division of the signal occurs. Branch line identification signals may be superimposed on the returning signals. As mentioned previously, for shorter span branched networks higher OTDR resolution is required and so the OTDR pulses used are narrower and the lower power injection means that a range of 20dB is the present upper limit. Increasing this to 30dB utilising fibre amplification is thus of benefit and is within the ability of available electrical dynamic range. The system is preferably not merely used as a power amplifier, the returning signals preferably also pass through the amplifying fibre. A particular advantage of using a fibre amplifier is that pulses of any duration can be amplified, for example from microsecond to picosecond lengths, which is more versatile than amplification provided by semiconductor laser amplifiers which at present cannot provide sufficient power amplification for pulses over IOC picoseconds. Although pump powers of 65mW have been used here, lower pump powers of approximately lOmW can give the same level of amplification, as shown by R. S. Vodhanel et al m Electronic Letters, 1989, Volume 25, page 1386. For OTDR measurements carried out in the 1500nm window, erbium is the preferred dopant for the fibre amplifier. High power semiconductor laser pumps are available with wavelengths in the range 1460-1480nm, for example the previously mentioned GRINSCH MQW lasers. It has been shown by Atkins et al that erbium fibre amplifiers can usefully be operated at wavelengths out to 1580nm. For use in the 1300nm window, neodymium can be used as the dopant for the fibre amplifier with an appropriate pump. Currently the power available fron Nd fibre amplifiers is much lower than that available from erbium fibre amplifiers, but nevertheless it is feasible to obtain usefully increased launched pulse power. Further details on neodymium doped fibre amplifiers can be found in Y. Miyajima et al's paper published in the proceedings of the Optical Fiber Communications (OFC 90) conference held in San Francisco in January 1990.

Claims: CLAIMS 1. An optical time domain reflectometer comprising a source of optical pulses, means for launching the optical pulses into a waveguide and means for optically pumping the waveguide to produce gain at the optical pulse wavelength prior to launching the amplified pulses into a waveguide to be tested. 2. An optical time domain reflectometer according to claim 1 in which the optical pulses experience gain when travelling in the outward direction and in the return direction along the waveguide. 3. An optical time domain reflectometer as claimed in claim 1 or claim 2 wherein the waveguide within which said gain occurs is located within the reflectometer. 4. An optical time domain reflectometer according to any preceding claim in which the waveguide comprises a fibre having a core diameter in the range 5 to 7win. 5. An optical time domain reflectometer according to any preceding claim in which the waveguide comprises a length of optical fibre doped with a rare-earth element. 6. An optical time domain reflectometer according to claim 5 in which the optical fibre is doped with erbium. 7. An optical time domain reflectometer according to any preceding claim in which the pumping power is at least lOmW. 8. An optical time domain reflectometer according to any preceding claim in which the pumping power is at least 65mW. 9. An optical time domain reflectometer according to any preceding claim in which the pumping power is at least lOOmW. 10. An optical time domain reflectometer according to any preceding claim in which the means for optically pumping comprises a semiconductor laser. 11. An optical time domain reflectometer according to any preceding claim in which the pulse source comprises a semiconductor laser. 12. An optical time domain reflectometer as claimed in any one of the preceding claims wherein an optical filter is included in the optical path between the pulse source and the waveguide in which gain occurs, the filter having a passband of between 0.5nm and l.On and a centre wavelength substantially the same as the operating wavelength of the pulse source. 13. An optical time domain reflectometer as claimed in claim 12 as dependent on claim 11, wherein the optical pulse source comprises a distributed feedback laser. 14. A method of increasing the measurement range of an optical time domain reflectometer, the method comprising launching an optical pulse into a waveguide and optically pumping the waveguide to induce gain of the pulses prior to launching the amplified pulses into a waveguide to be tested. 15. A method according to claim 14 in which the optical pulses experience gain in the outward direction and the return direction along the waveguide. 16. A method according to claim 14, wherein the waveguide within which said gain occurs is located within the reflectometer. 17. A method according to claim 14, claim 15 or claim 16, wherein the waveguide comprises a fibre having a core diameter in the range 5 to 7 microns. 18. A method according to claim 14, 15, 16 or 17, wherein the waveguide comprises a length of optical fibre doped with a rare-earth element. 19. A method according to claim 18 in which the rare-earth element is erbium. 20. A method according to any one of claims 14 to 19 in which the pumping power is at least lOmW. 21. A method according to any one of claims 14 to 19 in which the pumping power is at least 65mW. 22. A method according to any one of claims 14 to 19 in which the pumping power is at least lOOmW. 23. A method according to any one of claims 14 to 22 in which the pulse source comprises a semiconductor laser. 24. A method according to any one of claims 14 to 23 in which the means for optically pumping comprises a semiconductor laser.

Inventors: BLANK LUTZ CHRISTIAN, SPIRIT DAVID MICHAEL, BLANK, LUTZ, CHRISTIAN, SPIRIT, DAVID, MICHAEL
Applicants: BRITISH TELECOMM, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1990
Main classification: G01M11
IPC codes: G01M11