GB2167261A

GB2167261A - Optical fibres

Info

Publication number: GB2167261A
Application number: GB8429078A
Authority: GB
Inventors: George James Cannell; David James Struthers
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1984-11-16
Filing date: 1984-11-16
Publication date: 1986-05-21
Also published as: GB2167261B; GB8429078D0

Abstract

The optical time domain reflectrometry (OTDR)/backscatter trace of an optical fibre is obtained used a light emitting diode (LED) (19) as the source in place of the more conventional laser diode. The LED 19 is driven to produce a pulsed output which is applied to one end of the optical fibre 20 via a 3dB fibre coupler 21. Photons output from the fibre are coupled to a detector 22 such as an avalanche photodiode operating in a photon counting mode. The detector output is integrated (13, Fig. 1) and used to provide an OTDR/backscatter trace on an oscilloscope or chart (18, Fig. 1). For a 1.3 mu m LED, a germanium avalanche photodiode detector cooled to temperature of the order of liquid nitrogen temperatures is required, but at shorter wavelengths an uncooled silicon photodiode suffices. The apparatus may be used as a fault finder for use in the field. It is simple to operate and safer and cheaper than one employing a laser diode as the source. <IMAGE>

Description

SPECIFICATION Optical fibres This invention relates to optical fibres and, in particular, to apparatus for and a method of obtaining the OTDR (Optical Time Domain Reflectometry) or backscatter trace of a fibre in order, for example, to provide information on loss uniformity and for fault location.

According to one aspect of the present invention there is provided an apparatus for obtaning an optical time domain reflectometry (OTDR)/backscatter trace of an optical fibre, including a light-emitting diode (LED), drive means to produce a pulsed output from said LED, means to couple the pulsed output of the LED to one end of the optical fibre, means to detect photons output from said one fibre end, means to integrate the detector output and means to display the integrated output as an OTDR/backscatter trace.

According to another aspect of the present invention there is provided a method of obtaining an optical time domain reflectometry (OTDR)/backscatter trace of an optical fibre, including the stages of driving an LED whereby to produce a pulsed output therefrom, coupling the pulsed output to one end of the optical fibre, detecting photons output from said one fibre end, integrating the detector output and displaying the integrated output as an OTDR/backscatter trace.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 illustrates schematically a known OTDR apparatus; Figure 2 illustrates schematically the launch and receive arrangement used in the apparatus of the present invention, and Figure 3 illustrates a backscatter trace obtained with an LED source.

It has previously been proposed to employ OTDR for fault location in optical fibres or to examine the Rayleigh backscattered signal. In the article "OTDR in monomode fibres at 1.3 iim using a semiconductor laser" by P. Healey, Electronics Letters 22nd January 1981 Vol. 17. No. 2 p. 62-64, there is described a fault locator which for portability purposes employs a semiconductor source such as an lnGaAsP laser diode, rather than an Nd:YAG laser, together with a photon counting receiver comprised of an avalanche photodiode, typi cally a germanium avalanche photodiode. An avalanche photodiode may be used as a very sensitive photon detector by biasing it above the breakdown voltage. Breakdown can then be initiated by a single carrier entering the space charge region.The avalanche photodiode has to be cooled to liquid nitrogen temperature for operation, which somewhat restricts the use of the apparatus in the field for fault location etc., since a supply of liquid nitrogen must always be available.

Whereas the OTDR apparatus described in the above-mentioned article was aimed at fault location and the circuitry specifically designed for optimisation of fault location, in a subsequent article "Multichannel photon-counting backscatter measurements in monomode fibre" by P. Healey, Electronics Letters 1st October 1981 Vol. 17 No. 20-p. 751-752 there is described an apparatus which also can be used to examine the continuous Rayleigh backscattered signal. The apparatus employs a 256 channel signal integrator rather than the two channel approach of the apparatus employed for fault location and described in the first-mentioned Electronics Letters article.

This known apparatus is illustrated schematically in Fig. 1. The source 1 is an InGaAsP laser diode driven by a 1,us current pulse to launch an optical pulse into a test fibre 2 via optics indicated by lenses 4 and 5 and a cladding mode stripper 6. The detector 7 comprised by a germanium avalanche photodiode 8 cooled by liquid nitrogen and operating in the photo-counting mode receives photons reflected back along the test fibre and directed thereto via optics including lens 9. The photodiode output pulses are amplified at 10 and made TTL compatible by a following discriminator 11. The receiving system is a digital correlator 12 followed by a multichannel linear digital integrator comprised by summer 13 and TTL memory 14 under the control of program controller 15, which also controls the correlator 12 and a pulser 16 producing the drive pulse for laser 1.Multichannel averaging makes best use of the available information content of the signal returning between successive operation of the laser and is therefore very time efficient. In each cycle of operation the 256 memory locations are updated with the new integrated signal level. The real-time summation is converted to an anaiogue signal by digital-to-analogue converter 17 to drive an oscilloscope 18. Alternatively the memory may be read later at a slower rate to enable hard copy generation on a chart recorder. The launch power was approximately -15 dBm (peak) with the arrangement of apparatus described with a range of 20dB or 37 km. However, it was suggested that 10dB more power might be launched via a fibre Y coupler, yielding an improvement in range of about 5dB.

We have now found that the OTDR or backscatter trace of a fibre can be obtained without the use of a laser. Instead a light-emitting diode (LED) is used in conjunction with a photon-counting detector as illustrated schematically in Fig. 2. A pulse from an LED 19 is applied to a multimode test fibre 20 via coupling means such as a 23dB multimode coupler 21 and the reflected signal detected by a photon-counting detector 22. The electronics used to control the LED operation and convert the detector output pulses to a readible (chart or oscilloscope) form may be described with reference to the known apparatus Fig. 1.

The system arrangement of the present invention has been found to be sensitive enough to provide a useful backscatter trace on short lengths of multimode fibre. Fig. 3 illustrates a typical OTDR/backscatter trace obtained using a 1 .3m LED source and photon counting on a 4km length of multimode fibre (50/125-core diameter to cladding diameter in m), with an integration time of one minute. The launch power was 5X 10-6W (-23dBm) and the received power, with the end of the fibre in an index matching liquid, was 3X10-12W (-85dBm). The trace clearly shows exponential decay of backscatter and a sudden drop in signal level at the index matched end of the fibre. Similar drops in signal level are obtained from faults and splices in the fibre.

The elimination of the need for a laser diode removes all safety related problems and costs, however at the long wavelength employed (1.3,us) a germanium avalanche photodiode detector is required which must be cooled to temperatures of the order of liquid nitrogen temperatures. This, as mentioned before, somewhat restricts the portability of the apparatus. If the liquid nitrogen order of temperatures were obtained by a closed circuit cooler rather than relying on the use of liquid nitrogen itself then the portability would be greatly improved, resulting in a relatively simple and safe OTDR instrument for fault location, and possibly loss assessment, working at 1 .3ism for example in local area networks or cable TV.

The cooling probiem may be overcome by alternatively using an 850nm LED source, when an uncooled silicon detector will suffice.

Claims

1. An apparatus for obtaining an optical time domain reflectrometry (OTDR)/backscatter trace of an optical fibre, including a light-emitting diode (LED), drive means to produce a pulsed output from said LED, means to couple the pulsed output of the LED to one end of the optical fibre, means to detect photons output from said one fibre end, means to integrate the detector output and means to display the integrated output as an OTDR/backscatter trace.

2. Apparatus as claimed in claim 1, wherein the LED is a 1 .3m LED and the detecting means is a germanium avalanche photodiode which in use of the apparatus is cooled to temperatures of the order of liquid nitrogen temperatures and operated in the photon counting mode.

3. Apparatus as claimed in claim 1, wherein the LED is a 1.3 ,um LED and the detecting means is a germanium avalanche photodiode together with a closed circuit cooler whereby in use of the apparatus the photodiode is cooled to temperatures of the order of liquid nitrogen temperatures and operated in the photon counting mode.

4. Apparatus as claimed in claim 1, wherein the LED is an 850nm LED and the detecting means is a silicon avalanche photodiode.

5. Apparatus as claimed in any one of the preceding claims wherein said coupling means comprises a 3dB fibre coupler which also serves to couple the photon output to the detecting means

6. A method of obtaining an optical time domain reflectometry (OTDR)/backscatter trace of an optical fibre, including the stages of driving an LED whereby to produce a pulsed output therefrom, coupling the pulsed output to one end of the optical fibre, detecting photons output from said one fibre end, integrating the detector output and displaying the integrated output as an OTDR/backscatter trace.

7. A method as claimed in claim 6, wherein the LED is a 1.3Am LED and the photons output from said fibre are detected by a germanium avalanche photodiode cooled to temperatures of the order of liquid nitrogen temperatures.

8. A method as claimed in claim 6, wherein the LED is an 850nm LED and the photons output from said fibre are detected by a silicon avalanche photodiode.

9. A method as claimed in any one of claims 6 to 8 wherein the pulsed output is coupled to the optical fibre by a 3dB fibre coupler which also serves to couple the photons output from said fibre to means employed for detecting them.

10. Apparatus for obtaining an OT DR/backscatter trace of an optical fibre substantially as herein described with reference to Figs. 2 and 3 of the accompanying drawings.

11. A method of obtaining an OTDR/backscatter trace of an optical fibre substantially as herein described with reference to Figs. 2 and 3 of the accompanying drawings.