JPH0130097B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention detects the point of failure in an optical fiber and detects the loss by inputting a light pulse from a laser light source into an optical fiber to be measured and detecting the backward Rayleigh scattering light that returns from the optical fiber. This invention relates to a light pulse device that performs measurements. Today, to find fault points and measure losses in optical fibers, a pulse of light is entered from one end of the fiber, and information about the amount of backward Rayleigh scattering, that is, the temporal change in the amount of light returning from the fiber, is processed. This method is considered to be the most effective. Multimode optical fiber has a core diameter of ~50 μm, which is 4 to 5 times larger than single mode optical fiber.
Optical pulse tests, that is, failure point searches and loss measurements, have been performed using semiconductor lasers. The situation is shown in Figure 1. A semiconductor laser 12 is excited by an electric pulse from a pulse generator 11, and the pulsed light from the laser 12 passes through a beam splitter 13 and is made incident on one end of an optical fiber 15 by an objective lens 14. The backward Rayleigh scattering light generated within the optical fiber 15 by the incident light pulse returns within the optical fiber 15 and returns to the beam splitter 1.
3 and enters the photodetector 16. The output signal of the photodetector 16 is amplified by an amplifier 17, averaged by a boxcar integrator 18, logarithmically converted by a logarithmic amplifier 19, and then recorded on an X-Y plotter 21. This conventional device had the disadvantage that there was a loss of 3 bB in one direction due to the beam splitter 13, and a loss of 6 dB for the entire system up to the photodetector 16.

Figure 2 shows a conventional optical pulse testing device using a YAG laser. Electric pulses from the pulse generator 11 are supplied to a Q switch driver 23 through a pulse delay device 22, and this driver 23
Drive the YAG laser 24. The pulsed light from the laser 24 passes through a polarizer 25 and a beam splitter 13.
The light is then incident on the optical fiber 15 by the objective lens 14. The light from the optical fiber 15 is incident on the photodetector 16 from the beam splitter 13 through the analyzer 26 . The processing for the output of the photodetector 16 is the same as that shown in FIG.
A polarizer 25 and an analyzer 26 are inserted to eliminate strong Fresnel reflection from the YAG laser 24 at the incident end face of the optical fiber 15. In this conventional device, the beam splitter 13 reciprocates.
The disadvantage was that the dynamic range of the system as a whole decreased by 6 dB, 3 dB for analyzer 26, and 9 dB for the entire system. These reductions in dynamic range are unavoidable as long as the beam splitter 13 is used.

In addition, when evaluating backscattered light from a long distance using conventional optical fibers, the rear amplifier is saturated by Fresnel reflection at the near end of the optical fiber and strong backward Rayley scattering light, and the effects of this saturation remain over time. The disadvantage was that the amplified output of the weak Rayleigh scattered light from the sensor was distorted, making accurate measurement impossible.

In order to solve these drawbacks, this invention uses an ultrasonic optical deflector to increase the dynamic range of the amount of backscattered light, and will be explained in detail with reference to the drawings starting from Figure 3. . FIG. 3 shows an embodiment of the present invention, in which an ultrasonic light deflection element 27 is arranged between the Nb:YAG laser 24 and the objective lens 14. This ultrasonic light deflection element 27 is driven by an ultrasonic light deflector driver 28, and the output pulse of the delay device 22 is supplied to the driver 28 after the pulse delay and pulse width are controlled by a pulse control circuit 29. The backward Rayleigh scattered light from the optical fiber 15 is deflected by a deflector 27, further reflected by a reflecting mirror 31 provided as necessary, and is incident on the photodetector 16.

First, the gate of the boxcar integrator 18 is set using a pulse generated by the pulse generator 11. Thereafter, the pulses from the pulse generator 11 are time-delayed by a pulse delayer 22, causing the YAG laser 24 to oscillate with a Q-switch, and the optical pulses are incident on the optical fiber 15. At this time, no signal from the ultrasonic optical deflector driver 28 is applied to the ultrasonic optical deflector 27, the deflector 27 is in a non-operating state, that is, an undeflected state, and the optical pulse is traveling straight. on the other hand,
The electric pulse time-delayed by the delay device 22 is guided to the pulse control circuit 29, where it is delayed by an arbitrary time from the operation time of the YAG laser 24, and ultrasonic light is deflected by a predetermined time width corresponding to a predetermined section to be measured. Vessel 2
By deflecting 7, the optical fiber 15
Only a predetermined portion of the Rayleigh scattered light from the incident light is separated from the incident optical path and extracted. After this, the light from this optical fiber 15 is transmitted to the photodetector 1 as in the conventional device.
6 and its output is amplified and further averaged. A logarithmic amplifier 19 is used to determine the loss from the slope of Rayleigh scattered light. The result is X-Y
Drawn on Protsuta 21.

When the ultrasonic optical deflector 27 is used in place of the beam splitter in this way, the approximately 9 dB drop in measurement level that was unavoidable with the device shown in FIG. 2 can be reduced by approximately 1 dB.
The reduction can be suppressed to ~2dB. That is, in the conventional method, in order to remove the strong Fresnel reflection (having an optical power 30 dB or more greater than Rayleigh scattering) from the input end of the optical fiber, a combination of crossed Nicols (the polarization plane of the polarizer 25 and the polarization plane of the analyzer 26) is used. (orthogonal), a total loss of 9 dB was unavoidable for the backward Rayleigh scattered light to be measured, including the loss at the beam splitter 13. On the other hand, in the present invention, by using an ultrasonic light deflector and operating the deflector after the Fresnel reflection passes through the deflector, the Fresnel reflection and the backward Rayleigh scattered light can be completely separated. The loss when passing through the ultrasonic optical deflector twice is about 1 to 2 dB, which is significantly reduced compared to conventional methods. Therefore, in the case of optical fiber with a loss of 1 dB/Km, the distance is 3.5 to 4 Km compared to conventional equipment.
Loss measurement (diagnosis) of distant fibers can be performed.

Furthermore, in the present invention, by selecting the drive time for the ultrasonic light deflector 27, the Rayleigh scattering waveform in the optical fiber 15 can be extracted partially at will, so that saturation of the downstream amplifier can be eliminated.
Therefore, the ultrasonic light deflector 27 is turned off for Rayleigh scattered light at a short distance, and only minute optical information at a far distance can be sufficiently amplified, thereby increasing the dynamic range. This situation is shown in FIG. Figure A shows the intensity of the backward Rayley scattered light returning to the input end of the optical fiber and the Fresnel reflected light at the far end with respect to the fiber length l, and Figure B shows the intensity of the backward Rayley scattered light returning to the input end of the optical fiber and the intensity of the Fresnel reflected light at the far end. This shows the selected light that is applied to the photodetector selected by the ultrasonic optical deflector out of the Rayleigh scattered light, and C in the same figure shows the waveform of the selected backward Rayleigh scattered light that has been detected and further amplified. This is what is shown. In this manner, the present invention allows a selected weak signal to be amplified without distortion.

Taking all the above into account, it is easily possible to increase the dynamic range by about 10 dB, so compared to the conventional method, a 1 dB/Km optical fiber can reach 5 km,
At 0.5dB/Km, loss measurement (diagnosis) can be performed up to 10km away.

FIGS. 4 and 5 show experimental results, where the horizontal axis is the fiber length and the vertical axis is the received light level expressed in decibels after logarithmically converting the backward Rayleigh scattered light incident on the photodetector 16. FIG. 4 shows the conventional device, and FIG. 5 shows the device of the present invention. From this, it was confirmed that the length that can be used for loss measurement (diagnosis) was increased from about 11.5 km with the conventional device to about 17 km with the device of the present invention, and the dynamic range increased by about 11 dB. The peak near 19.5 km on the right side of Figure 5 is the Fresnel reflection at the far end of the single mode optical fiber. Also, the ultrasonic optical deflector 27
It is operated from a point of 60 μs, that is, 6 km in terms of fiber length.

A comparative example of the use of another ultrasonic optical deflector is shown in FIG. In this case, the light pulse is transmitted through the optical fiber 1.
The ultrasonic light deflector 27 is operated only when the light is incident on the laser beam 5, and the ultrasonic light deflector 27 is turned off when backward Rayleigh scattering light is received. All other systems are 3rd
Same as in the figure. Although FIG. 6 has the advantage that the operating time of the ultrasonic light deflector 27 is short, it also has the problem that it is difficult to completely eliminate Fresnel reflection from the input end face of the optical fiber. The same increase in dynamic range is obtained as in the tester shown in FIG. Note that as the ultrasonic light deflection element 27 used in this invention, an element using TeO ₂ or PbMoO ₄ is useful. This is because in the tester shown in Figure 3, the backward Rayleigh scattered light is generally polarized, so it is a good idea to select an element whose deflection efficiency is small in polarization dependence as the ultrasonic light deflection element 27. be. Since the test device shown in FIG. 3 uses an ultrasonic light deflection element as a deflection element, even if the backward Rayleigh scattered light is polarized, it does not generate noise. This problem is particularly acute when the fiber under test is a single mode fiber. In the backward Rayleigh scattered light from the single mode optical fiber, two degenerate modes (HE ^x ₁₁ and HE ^y ₁₁ ) cause interference, the polarization state is changed, and the light returns to the input end. When this signal light is received by the conventional method shown in FIG. 2, the change in polarization state is converted into a change in intensity in the analyzer 26, and as shown in FIG. 8A, the output signal of the photodetector 16 has a large Amplitude fluctuations occur.
This is called polarization noise. In the conventional method, this noise made it impossible to stably measure optical loss and search for failure points, which was a big problem. However, in the present invention, since the polarization dependence of the ultrasonic light deflection element is small (approximately 0.05 dB or less), a stable backscattered signal as shown in FIG. 8B can be obtained at the output of the photodetector 16. Signal fluctuation is less than ±0.02dB.

As explained above, an optical pulse tester using an ultrasonic optical deflector can increase the dynamic range by more than 10 dB compared to conventional equipment, so it has the advantage of dramatically increasing the diagnostic distance. There is.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing a conventional optical pulse testing device using a semiconductor laser, Fig. 2 is a block diagram showing a YAG laser optical pulse testing device using a conventionally used polarizer and analyzer system, and Fig. 3 is a block diagram showing a conventional optical pulse testing device using a polarizer and analyzer system. A block diagram showing an embodiment of the optical pulse testing device according to the invention, FIG. 4 is a diagram showing the measurement results by the conventional device shown in FIG. 2, FIG. 5 is a diagram showing the measurement results by the device of the invention, and FIG. 6 7 is a block diagram showing a comparative example of the use of an ultrasonic optical deflector, FIG. 7 is a diagram showing the intensity of backward Rayleigh scattered light with respect to the optical fiber length, and FIG. 8 is a diagram showing the output waveform of the photodetector. 11...Pulse generator, 12...Semiconductor laser, 13...Beam splitter, 14...Objective lens, 15...Optical fiber, 16...Photodetector, 17...Amplifier, 18...Boxcar integrator , 19... Logarithmic amplifier, 21... X-Y plotter, 22... Pulse delay device, 23... Q-switch driver, 24... Nd ³⁺ :YAG laser,
25...Polarizer, 26...Analyzer, 27...Ultrasonic light deflection element, 28...Ultrasonic light deflector driver, 29...Pulse control circuit.

Claims (1)

[Claims] 1. A light pulse from a laser light source is made incident on an optical fiber to be measured, and a photodetector detects the backward Rayleigh scattering light that returns from the optical fiber, and the detected signal is logarithmically converted and displayed. In the optical pulse test device for measuring the loss of the optical fiber to be measured, there is a gap between the laser light source and the optical fiber.
An ultrasonic light deflector made of TeO ₂ or PbMoO ₄ is inserted, and arranged so that the laser light source and the optical fiber are coupled in the non-operating state of the ultrasonic light deflector, and the ultrasonic light deflector The photodetector is disposed on the optical path on which the backward Rayleigh scattered light returning from the optical fiber is deflected in the deflection operation state, and the ultrasonic light is arranged so as to extract only a predetermined portion of the backward Rayleigh scattered light. An optical pulse test device characterized in that a deflection operation time of a deflector is set.