TEST METHOD FOR OPTICAL FIBRES AND APPARATUS FOR PERFORMING THE TEST METHOD

Abstract: The optical transmission characteristics of a length of opti­ cal fibre are determined by launching a light pulse through non­ directional coupling means into one end of the length of fibre, allowing returned light to pass through the light source produc­ ing the pulse, and detecting the amount of returned light. The light source may be a semiconductor laser which is energised to a level high enough to cause it to generate the light pulse and which is subsequently energised to a level at which it will be triggered by returned light and amplify the returned light for detection.

Description: Test Method for Optical Fibres and Apparatus for performing the test method. This invention relates to an improvement to a method of testing an optical fibre to determine the optical transmission characteristics or the break points of the optical fibre, comprising the steps of injecting an optical pulse signal at one end of the optical fibre and receiving light propagating in the opposite direction because of the reflection or the backscattering of the optical pulse signal within the optical fibre, and apparatus for performing the improved method. When an optical pulse signal is injected into one end of an optical fibre, optical energy scattered backwards in the optical fibre and Fresnel-reflection at non-uniform points or break points of the optical fibre return to the input end as a wave propagating in the opposite direction. From a return wave received at the end where the optical pulse signal is injected, it is possible to determine the optical transmission characteristics or non-uniform points or break points of the fibre. Fig. 1 shows a prior art apparatus for such a purpose. A semiconductor laser, which is used as a light source 1, is made to generate optical pulse signals by a driving circuit 2. The optical pulse signal is injected at one end of an optical fibre 5 which is being tested, via an optical directional coupler 4. The return wave generated within the test fibre 5 enters the optical direction coupler 4 from the fibre to be received by an optoelectric transducer 7 and converted into an electrical signal. The electrical signal is amplified by an amplifier 9 and displayed in a display 11 after the removal of noise by an averager 10. The optical directional coupler 4 is driven by-an ultrasonic driving circuit 12. In the type of apparatus described above, the optical directional coupler 4 interposed between the light source 1 and the test fibre 5 separates the light injected into the test fibre 5 from the returned light from the fibre 5, but inevitably introduces an energy loss, and the detection of .t, the signal scattered backwards insthe optical fibre is made more difficult because the loss further reduces an already small signal. Further, the ultrasonic driving circuit 12 which is needed for some optical directional couplers, increases the bulk and complexity of the apparatus. An object of the present invention is the provision of a method, which is an improvement over the prior art, of testing an optical fibre and apparatus for performing the improved method. A method of testing an optical fibre, in accordance with the present invention, is characterised in that light which has returned to the input end of an optical fibre is made to pass through the light source which generates the optical pulse signal giving rise to the returning light. That is, a light pulse from a light source is coupled into a length of optical fibre and return light energy in the optical fibre is made to pass through the light source, for detection. The method of testing an optical fibre may include the steps of launching the light pulse from a semiconductor laser and amplifying the return light energy by means of the semiconductor laser. The method of testing an optical fibre may include the steps of energising the semiconductor laser to a level at which it generates a light pulse, and, subsequently, energising the semiconductor laser to a level below which it generates a pulse and at which it amplifies the return light energy passing through it. Apparatus for testing an optical fibre includes a light source for launching a light pulse into a length of optical fibre, and means for detecting light energy returning towards the light source by way of the optical fibre, characterised in that, the light source is coupled to the optical fibre by way of non-directional coupling means. It is possible to construct the apparatus with a semiconductor laser as the light source, and include means for supplying the semiconductor laser with a driving current at or above its oscillation threshold current to generate the light pulse, and to supply a driving current of a level lower than that of the oscillation threshold current to amplify the light passing through the laser after the generation of the optical pulse. The apparatus of this invention may. comprise, a light source, a driving circuit arranged to pulse drive the light source to generate optical pulse signals, coupling means which couples the light source optically to one end of a fiber, and light receiving means including an optoelectric transducer which receives return light energy from the light source end of the optical fibre, characterized in that, the receiving means and the optical fibre are positioned on opposite sides of the light source and the receiving means receives the light from the optical fibre after it has passed through said coupling means and light source. The light source is preferably a laser. It is preferable that the receiving means is provided with an optical gate circuit for the light entering the optoelectric transducer and the optical gate circuit is so controlled as to be in the OFF-state at the times when the light source is generating optical pulses. The present invention provides an optical time domain reflectometer which does not include an optical directional coupler. This removes the loss caused by the optical directional coupler and consequently, expands the dynamic range of measurement by an amount equal to the loss. The apparatus according to the present invention is simpler than that using an optical directional coupler and therefore may be more compact. When a semiconductor laser is used to amplify the light passing through the laser, the dynamic measurement range may be improved. Although the amplification factor of a semiconductor laser is small, it is possible to achieve amplification to a considerably higher order, the laser wavelength and the resonance wavelength being almost identical for any device capable of performing both functions. As when a semiconductor device is used as the optical gate circuit, the available bandwidth may be as high as 2 GHz, and the length resolution in measurements of this type may be greatly enhanced. This invention is highly effective in the non-destructive testing of optical fibers in communication systems. The invention will now be described by way of example only, and with reference to the accompanying drawings, in which: Fig. 1 is a block diagram representation of a prior art apparatus. Fig. 2 is a block diagram representation of an embodiment of the apparatus according to the present invention. Fig. 3 are signal waveform representations of forward and return light energy levels in Fig. 2, Fig. 4 is a graph showing the backscattered optical signal versus fibre length resulting from a test carried out with an embodiment of the apparatus according to the present invention, and, Fig. 5 is a graph showing the measured amplification characteristics of a semiconductor laser when it is used as an optical amplifier in an embodiment of the apparatus according to the present invention. An embodiment of the present invention will now be described in more detail referring to the attached drawings, of which Fig. 2 is a block diagram of an embodiment of the apparatus according to the present invention. Referring to Fig. 2 a semiconductor laser light source 1 is supplied with driving current by a driving circuit 2. Optical pulse signals generated by the light source 1 are optically coupled with an end of a test fibre 5. Within the test fibre 5, the optical pulse signal propagates in the direction marked by the arrow (b). The backscattered optical signal and the reflected optical signal arising from the optical pulse signal propagate in the direction marked by the arrow (c) to come out of the input end the optical fibre. A feature of this invention is that the light propagating in the opposite direction to the launch direction in the test fiber 5 returns to the light source which emitted the optical pulse signal without passing through any optical directional couplers, passes through the light source and is finally received by an optoelectric transducer 7 via an isolator 15 and an optical gate circuit 13. The optical gate circuit 13 in this embodiment comprises a second semiconductor laser and the second semiconductor laser is driven by a second driving circuit 14 which is synchronized with the driving circuit 2. An isolator 15 is inserted so as to allow optical transmission only in one direction, i.e. from the light source 1 to the optical gate circuit 13 in order to prevent cross-coupling between the semiconductor laser of the light source 1 and the semi-conductor laser of the optical gate circuit 13. The isolator 15 used in this embodiment operates by Faraday rotation and is made of YIG crystal to which a magnetic field is applied. The output electric signal from the optoelectric transducer 7 is amplified by an amplifier 9, noise is removed by an averager 10 and the result is displayed on a display 11. Figs. 3 (a) through (f) are charts showing signal waveforms at the points (a) through (f) in Fig. 2. In this apparatus the semiconductor laser of the light source 1 is driven by the driving circuit 2 by driving current shown in Fig. 3(a). In other words, driving current I1 of a level high enough to cause oscillation of the semiconductor laser is supplied for a short time while driving current I2 of a level lower than the threshold current which causes laser oscillation of the semiconductor laser is supplied for a considerably longer time. During the supply of the high level driving current I1, the light source 1 supplies an optical pulse signal P1 shown in Fig. 3(b) to the test fibre 5. During the period when the optical pulse signal P1 propagates through the test fibre 5 and a wave P3 propagating in the opposite direction occurs, the light source 1 is supplied with the low level driving current I2 and the semiconductor laser of the light source 1 functions as an optical amplifier. The optical signal which passes through the light source 1, therefore, becomes an amplified signal shown in Fig. 3 (d). This optical signal then passes through the isolator 15 and through the optical circuit 13. The semiconductor laser of the optical gate circuit 13 is driven in a manner shown in Fig. 3 (e). In other words, as the driving circuit 14 is synchronized with the driving circuit 2, during the time tl when the driving circuit 2 provides high level driving current, the circuit 14 does not provide any driving current. Conversely, during the period when the driving circuit 2 provides low level driving current,the circuit 14 provides a driving current 12' of a level lower than the threshold current which causes oscillation of the semiconductor laser. Therefore, the optical gate circuit 13 shuts off the optical pulse signal which is emitted from the rear of the light source 1 and high level reflected light P2 which occurs at the input end of the test fiber 5, and only the weak light from the test fibre propagating later in the opposite direction through the light source is allowed to pass. The output light of the optical gate circuit 13 is shown in Fig. 3 (f). The light is received by an optoelectric transducer 7. Semiconductor lasers suitable for use as the light source 1 and the optical gate circuit 13 have characteristics such that, if they are provided with a high level of driving current, they oscillate but if provided with a suitably low level of driving current, they will amplify light reaching them. These are known characteristics usually applied to optical switching. For example, a semiconductor laser starts amplification when fed with driving current which is almost 90% or higher but less than 100% of the threshold current which causes laser oscillation. Fig. 4 shows the test result for the above embodiment of Fig. 2. In this test, a single-mode optical fibre 20.8 km long was used as the optical fibre 5 and the relative value of the backscattered light power and the injected light power were measured consecutively. The remote end of the test fibre 5 was immersed in a refractive index matching oil to remove Fresnel reflection. The optical pulse peak power of the light source was 3 dBm and the optical width was 1 um. A semiconductor laser of InGaAsP BH (buried heterojunction) structure and oscillation wavelength of 1.3 um (as described, for example, by Toukada in J. Appl. Phys. Vol. 45 No. 11) was used as the light source 1. This semiconductor laser has a laser oscillation threshold of about 56 mA. The high level driving current mentioned above was set at 100 mA while the low level driving current was set at 53.2 mA. In this test, light is generated on the side of the laser away from the optical fibre, and a short burst of light, and reflected light, will come from the rear end of the fibre (distance zero km.) immediately after the emission of the light pulse from the laser, but these effects are omitted from Fig. 4 Fig. 4 shows that backscattered light is measured all along the length of the optical fibre 5 and the power ratio of backscattered light is measurable to -27dB, making the dynamic range of this apparatus about 27dB. The dynamic range of the prior art apparatus shown in Fig. l is about 18 dB. It will be evident from the description of the prior art apparatus and the apparatus of the present invention that the apparatus of the present invention is not only simpler than that of the prior art but also has improved dynamic range. The optical amplification characteristics and the level linearity of the semiconductor laser, referred to above, are shown in Fig. 5. In deriving Fig. 5, a low level driving current of 53.2 mA was supplied to the semiconductor laser apparatus, an optical signal of between -27d Bm and -48 dBm was then injected into the semiconductor laser via a short length of optical fibre and the amplification was measured. The polarisation of the injected light was made coincident with the TE mode of the semiconductor laser. Fig. 5 shows that a gain of about 19 dB with excellent linearity is obtained in the range of the aforementioned input signal level from the semiconductor laser when it is used as an optical amplifier by being driven at a low current level. Not shown is the case when the polarisation of the injected light varies freely, when the gain is, on average, identical to that for 450 polarisation, the gain being then about 16 dB, that is, lower by 3 dB, but still with excellent linearity. The performance of the apparatus, in accordance with the present invention, may be improved when a laser is included and used both as a light source and an amplifier. Improvements of some 10 dB may be possible. For example, a frequency selective filter which passes only signal light frequencies may be inserted in the path of the light which has passed through the light source 1 either before or after the isolator 15, to remove noise outside the laser light band, and the signal to noise ratio of the optoelectric transducer 7 will be greatly improved. As a light source, besides the above mentioned semiconductor laser, a solid state laser such as a YAG laser or a Ruby laser, a gas laser such as a He-Ne laser, an Argon laser, or a krypton laser or other optical fibre lasers may be used. The light source does not need to be capable of amplification. A light source is suitable so long as it lets the light through with small loss. Alternatives to the optical switch mentioned above are an acoustooptic deflector, a switch using electrooptic effects, a switch using magnetooptic effects, or a mechanical optical switch. It may be possible to put the present invetion into effect without the use of an optical gate circuit at all if the optoelectric transducer which receives the return light energy is the type highly responsive to larger optical level changes. Moreover, a switch circuit or a limiter circuit may be used in order to suppress signals of high levels in the electric output from the optoelectric transducer. The optical isolater mentioned above is not always required in this invention depending on the characteristics of the light source and the optical gate circuit. It will be appreciated that the test method may be used in setting up a communication system which includes one or more optical fibres, and, subsequently, for assessing the performance of the system. The apparatus, in accordance with the present invention, may therefore be included within a communication system that relies, at least partly, on optical fibres.

Claims: What is Claimed is: 1. A test method for optical fibers comprising the steps of injecting an optical pulse signal into an end of a fibre, and receiving light due to reflection or the backscattering of the said optical pulse signal within the said fibre, the method being characterized in that the said received light from the fibre is made to pass through the light source which generated the said optical pulse signal. 2. A test method for optical fibers, as claimed in Claim 1, which includes launching the light pulse from a semiconductor laser and amplifying returned light energy by means of the semiconductor laser. 3. A test method for optical fibres, as claimed in claim 1 or claim 2, which includes launching the light pulse by energising a semiconductor laser to a level at which it generates a light pulse, and, after the light pulse is generated, energising the semiconductor laser to a level below which it generates a pulse and at which it amplifies the returned light energy passing through it. 4. A test apparatus for optical fibers comprising a light source, a circuit for driving the light source to generate optical pulse signals, optical coupling means for coupling output light from the said light source with one end of the said fibre, and receiving means including an optoelectric for receiving light returned, in the fibre, towards the light source and leaving the fibre at the light source end, the apparatus being characterized in that the said receiving means is disposed at the side of the light source remote from the fibre so as to receive light after it has passed through the said optical coupling means and the said light source. 5. A test apparatus for optical fibers, as claimed in Claim 4, wherein the light source comprises a semiconductor laser, the light source driving circuit is so arranged as to provide driving current of a level at which the semiconductor laser generates a light pulse, and, subsequently, to provide driving current at a level at which the semiconductor laser amplifies the returned light energy passing through it. 6. A test apparatus for optical fibers as claimed in Claim 4, wherein the receiving means includes an optical gate circuit which is so controlled as to be in an OFF-state at the times that the light source generates optical pulse signals, and to be in an ON-state for returned light energy. 7. A test apparatus for optical fibers, as claimed in Claim 6, wherein the optical gate circuit comprises a semiconductor laser, and a means is provided in order to supply driving current at a level which makes the said semiconductor laser amplify light passing through it at the times when returned light is passing through the optical gate circuit. 8. A test apparatus for optical fibers, as claimed in Claim 4, wherein a filter is provided between the light source and the receiving means to selectively pass the output light wavelength of the light source. 9. A system including apparatus for testing optical fibres as claimed in any one of claims -4 to 8. 10. A communication system including apparatus for testing optical fibres as claimed in any one of claims 4 to 8.

Inventors: HORIGUCHI, TSUNEO, TOKUDA, MASAMITSU
Applicants: NIPPON TELEGRAPH AND TELEPHONE CORPORATION
Application year: 1984
Main classification: G01M11
IPC codes: G02B6, G01M11