GB2182223A - Optical fibre reflectometer - Google Patents

Abstract

In a coherent light optical time domain reflectometer which has a line-narrowed semiconductor laser source (1), optical fibre beam splitters (2, 8 and 30) are arranged in such away as to enable the extraction and mixing (12) of local oscillator (LO) and backscatter (BS) signals in a way that does not provide a loop path through the beam splitters which is liable to disturb the operation of the laser. <IMAGE>

Description

SPECIFICATION Optical fibre reflectometer This invention relates two a coherent light optical time domain reflectometer (OTDR) for making measure mention optical fibres.

Heterodyne detection in OTDR using a gas laser has been described by P.Healeyand D.J.Malyon in an article entitled "OTDR" in Single-Mode Fibre at 1.5um Using Heterodyne Detection', Electronics Let ters 30th September Vol. 18 No.20 pp 862-3 (Reference 1). Asimilar layoutof optical components is described in a later paper by S.Wright et al entitled 'Practical Coherent OTDR at 1.3 um' given at the 'Second International Conference on Optical Fibre Sensor', held in 1984 at Stuttgart (Conference publication pp 347-350), (Reference 2) where mention is made of the use of Nd:YAG lasers and line-narrowed semiconductor lasers as alternative forms of light source.In these examples of coherent OTDR the layout of optical components has been such as to provide a loop path through one or more optical fibre beam-splitters by which a proportion ofthe light from the laser is fed back into the source. Particularly in the case of reflectometers using semiconductor laser sources, it has been found that the presence of such a loop is liable to give rise to undesirable instabilities of laser operation which degrade the performance ofthe instrument.

According to the present invention there is provided a coherent light optical time domain reflectometer (OTDR) which reflectometer includes a laser light source and plurality of optical beam-splitters for extracting and mixing local oscillator and backscatter signals, in which reflectometerthe arrangement of the beam-splitters is such that no loop path is prov ided through them by which light from the laser source is fed back into that source.

There follows a description of two designs of reflectometer embodying the present invention in preferred form. This description is prefaced with a description of the reflectometer of Reference 2 together with an explanation of some of the observed problems associated with that design. The description refers to the accompanying drawings in which: Figure lisa schematic diagram of a prior art (Reference 2) reflectometer, Figure2is a diagram of a backscattertrace provided by the reflectometer of Figure 1, Figures 3 and 4are schematic diagrams of reflectometers embodying the present invention in preferred forms.

In the reflectometer of Figure 1 lightfrom a laser 1 is directed through a first single mode optical fibre beam splitter 2 and beam expanding lens 3 to an acousto-optic Bragg optical frequency shifting modulator4. This Bragg modulator4 is pulsed with power from a frequency offset signal generator 5 which is typically designed to operate at a frequency of 40MHz. The frequency offset optical output signal from the Bragg modulator is collected buy a lens 6 and directed through more single mode fibre 7 to a second single mode optical fibre beam splitter 8. This is a 3dB coupler. From there the light is launched via a suitable coupler 9 into the length of fibre 10 under test.

The first beam splitter 2 is a 6dB couplerthat dir ects most of the light from the laser 1 to the Bragg modulator 5, but a small proportion is directed down single mode fibre 11 to form a local oscillator(LO) signal for mixing with the backscattered (BS) signal returning from the test fibre 10. These signals are mixed by the 3dB cou pler 8 wh ich directs half the re- sulting power on to a photodetector 12. The electrical output from the detector 12 is then fed through a filter 13 tuned to the offsetfrequency.

In one example of this apparatus the laser 1 was formed by a semiconductor laser diode la coupled to a length 1 b of about 1 km of single mode fibre which causes line-narrowing of the laser emission by virtue of the effects of its Rayleigh backscattering prop erties. In operation of this apparatus itwas foundthat there were two sources of unwanted feedback into the laser which co-operated in a way to detract from the usefulness ofthe instrument. One of these sources of feedback was provided by Fresnel reflections at the expanded beam termination lenses 3 and 6 as indicated by arrows 14. The other was prov ided by light travelling in either direction around the loop path comprising the 6dB coupler 2, the Bragg modulator4,andthe3dBcoupler8.Figure2schema- tically depicts the result of these feedback effects upon a typical backscattertrace 20 of the instrument.

An optical pulse returning to the laser diode that is 40 MHz shifted from the centre frequency of the laser emission causes a burst of amplitude noise due to the beats between the returned pulse and the laser intrinsic light. The amplitude modulation is detected as a burst of noise lasting for the duration of the pulse. This noise falls within the receiver bandwidth which is centred at40 MHz; and thus the noise will produce a spike 21 at a one way range of the 1 km delay introduced by the length of line narrowing fibre 1 b. The emitted pulse was then observed to bounce back and forth in the system thereby producing the series of succeeding pulses 22 at regular intervals along the reflectometertrace.

In the reflectometer of Figure 3 these feedback effects are avoided by modifying the arrangement of Figure 1 to include an additional 3dB optical fibre beam splitter 30. In this instance therefore the 3dB beam splitter 8 is not used to mix the BS and LO signals, but instead directs the BS signal launched into single modefibre31 for mixing with the LO signal in beam splitter 30.

This reflectometer of Figure 3 also has the advantage that it has two output ports available for the detector. This enables a balanced receiver technique (not shown) to be used which adds the two signal components and subtracts the excess amplitude noise components. This is of particular use if the reflectometer lasersource is one which operates with a large amount of excess amplitude noise (noise in ex cessofquantum limited noise level).

Anotherfeature of thins configuration is that its beam splitter 8 has a choice of two portsto which the testfibre 10 may be connected. Conveniently these two output ports are provided with different types of termination to give the reflectometer added ver satility.

It should be clearly understood that it is not necessary to resort two the use ofthree beam splitters in orderto avoid the feedback loop problems referred to above. Two beam splitters are sufficient for the configuration of the reflectometer of Figure 4. In this reflectometerthe BS signal launched into fibre 31 is taken backto the same side of beam splitter 2 asthat into which light is launched directfrom the laser 1. In this instance therefore beam splitter2 not only provides the LO signal but also mixes it with the BS signal.

Although Figure 4 indicates that beam splitter2 is a 6dB splitter, a higher split ratio may be used so that a greater proportion of the BS signal is received at det ectorl2.

This reflectometer of Figure 4 can similarly be used with a balanced receiver configuration to eliminate excess amplitude noise on the LO signal. In this case use is made of the fact that the zero deflection condition ofthe Bragg cell 21 is able to pass lightto a second detector 1 2a.

Claims (10)

1. A coherent light optical time domain reflectometer (OTDR) which reflectometer includes a laser light source and plurality of optical beam-splitters for extracting and mixing local oscillator and backscattersignals, in which reflectometerthe arrangement of the beam-splitters is such that no loop path is provided through them by which light from the laser source is fed back into that source.

2. An OTDR as claimed in claim 1 wherein the laser light source is a semiconductor laser.

3. An OTDR as claimed in claim 1 wherein the laser light source is a semiconductor laser provided with a length of laser emission line narrowing optical fibre.

4. An OTDR as claimed in any preceding claim wherein a first optical path is provided from the light source via a first four-port beam splitter, a frequency shifting modulator, and a second four-port beam splitterto an optical output port of the reflectometer provided for the connection thereto of a length of optical fibrefortesting, wherein a second optical path is provided from the light source via said first beam splitter and a third four-port beam splitter to a (first) photodetector, and wherein athird optical path is provided from said optical output port of the reflecto metervia said second and third beam splitters to said photodetector.

5. An OTDR as claimed in claim 4 and incorporat ing a balanced detector arrangement employing a second photodetector optically coupled with a different port of said third beam splitter than thatto which the first photodetector is optically coupled.

6. An OTDR as claimed in any claim of claims 1 to 3 wherein a first optical path is provided from the light source via a first four-port beam splitter, a frequ encyshifting modulator,andasecondfour-port beam splitter to an optical output port of the reflecto meter provided forthe connection thereto of a length ofoptical fibre for testing, wherein a second optical path is provided from the light source via said first beam splitterto a (first) photodetector, and wherein a third optical path is provided from said output port of the reflectometervia said second and first beamsplitters to said photodetector.

7. An OTDR as claimed in claim 6 which reflectometer includes a second photodetector in balanced detector configuration with said first photodetector.

8. An OTDR as claimed in claim 4,5,6 or7, wherein the frequency shifting modulator is an acousto-optic Bragg modulator.

9. An OTDR as claimed in any preceding claim wherein one or more of the beam splitters is provided by an optical fibre directional coupler.

10. An OTDR substantially as hereinbefore described with reference to Figure 3 or Figure 4 ofthe accompanying drawings.