JP2674660B2 - Method and device for measuring backscattered light - Google Patents

Description

TECHNICAL FIELD The present invention relates to a backscattered light measuring method and apparatus.
More specifically, using an optical fiber as a sensor, the OTDR
In a distributed optical fiber sensor that detects changes in the environment in which the optical fiber is placed based on changes in the backscattered light level detected by (Optical Time Domain Reflectometer), the backscattered light of the optical fiber in the sensor section is detected. The present invention relates to a backscattered light measuring method and apparatus for detecting a level change at high speed.

2. Description of the Related Art Today, in public communication lines, single-mode fibers have been used for the purpose of large-capacity transmission and diversification of services, and introduction of optical fibers into subscriber systems is under consideration.

On the other hand, a sensor system that uses an optical fiber as a distributed sensor is being developed, and for example, a technique that captures a temperature change of an environment in which the optical fiber is placed as a loss change of the optical fiber.

OTDR is a technique for measuring the loss in the longitudinal direction of a fiber by injecting a search pulse light into the optical fiber and detecting the reflected light level thereof. This OTDR is used for the purpose of monitoring the condition of the line in the former case where an optical fiber is used as a communication line, and for the purpose of measuring the temperature change etc. at the point due to loss change in the distributed optical fiber sensor. It is an essential technology associated with fiber.

The structure of a conventional OTDR device is shown in FIG. In FIG. 3, the semiconductor laser 2 is driven by the driver 1 and emits pulsed light. This pulsed light enters the fiber 7 through the lenses 3 and 6. The return light generated by scattering at each point in the longitudinal direction of the optical fiber 7 is the lens 6 and the beam splitter 4.
It is incident on the AO deflector 5 via. The AO deflector 5 masks Fresnel reflection at the incident end with respect to the return light,
It outputs to the photodetector 8. The signal output from the photodetector 8 is
The clock signal generator 11 whose level is adjusted by the amplifier 9
The clock pulse generated by
A / D conversion is performed by the D converter 10 and output to the memory 12.

When the backscattered light is measured by the above device, the time t required for the returned light from each point of the fiber 7 to reach the photodetector 8 is given by t = 2L · n / c. Where L is the distance from the fiber entrance to the scattering point c is the speed of light n is the refractive index of the fiber core. Therefore, the return light from each point in the longitudinal direction of the fiber 7 is temporally separated and reaches the photodetector 8. An example of the backscattered light level measured by the conventional method using the above apparatus is shown in FIG.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention When using the above-mentioned conventional device to obtain the loss variation in the longitudinal direction of the optical fiber by the conventional method, the backscattered light level serving as the reference is stored in advance in the memory, The amount of change was calculated by subtracting the measured backscattered light level. The measurement by this conventional method and apparatus has the following problems.

Since the CPU subtracts the reference backscattered light level from the newly measured backscattered light level, the amount of change cannot be grasped in real time.

Separate data processing is required to detect the characteristics of the change in the backscattered light level change data.

Therefore, an object of the present invention is to provide a backscattered light measuring method and apparatus capable of performing real-time processing, which solves the above-mentioned problems of the prior art.

Means for Solving the Problems According to the present invention, a backscattered light measuring method in which pulsed light is injected into an optical fiber and return light due to scattering of the pulsed light at each point inside the optical fiber is time-dispersively detected. , The return light from the optical fiber is divided into two, one of the branched return light is as it is, and the other return light is converted into an electric signal after a certain delay, and the difference between the two signals. Is obtained to provide a backscattered light measuring method characterized by detecting a step difference in backscattered light level. In the method of the present invention, it is preferable that an optical fiber delay line is used to delay the one of the branched return lights.

Further, in the present invention, as a device for realizing the above-mentioned method of the present invention, a pulsed light generator for injecting pulsed light into an optical fiber, and return light generated by scattering the pulsed light at each point inside the optical fiber In a backscattered light measuring device equipped with a photodetector for time-resolved detection, a branching unit that divides the return light in two, and a delay unit that gives a constant delay to one return light branched by the branching unit. A first photodetector having the other return light branched by the branching device as an input and a second photodetector having the return light delayed by the delaying device as an input; A differential amplifier having the output of the second photodetector as an input, and detecting a step difference in the backscattered light level by obtaining the difference between the signals output by the first and second photodetectors. Backward scattering characterized by Optical measurement device is provided. In the device of the present invention, it is preferable that the delay means is an optical fiber delay line.

In the backscattered light measuring method of the present invention realized using the device of the present invention, in order to obtain the difference between the observed backscattered light waveform and the same backscattered light waveform with a constant delay time, The obtained waveform is a derivative of the original waveform.
Therefore, when an increase in loss due to bending occurs at one location of the optical fiber to be measured, the waveform obtained by the device of the present invention becomes a pulse waveform as shown in FIG. 2 (b). At this time, by setting a threshold level in advance, it can be estimated that a certain increase in loss has occurred when the signal exceeds this threshold level. Further, the position can be located by using the clock pulse as in the conventional OTDR.
Further, in the present invention, it is preferable that the delay is provided by an optical fiber delay line. By providing the above-mentioned delay with an optical fiber delay line, the structure of the device is simplified, and the delay amount can be finely adjusted by simply changing the length of the optical fiber. Furthermore, in the device of the present invention,
Since the processing for giving the above delay is optically performed and the arithmetic processing is performed by the differential amplifier, real-time processing is possible.

In the distributed optical fiber sensor of the method and apparatus of the present invention, the positions where the loss variation occurs are discretely distributed, and it is sufficient to grasp the variation amount relatively coarsely (for example, at two levels). In particular, it is particularly effective.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the following disclosure is merely an example of the present invention, and does not limit the technical scope of the present invention.

Example FIG. 1 shows a block diagram of an example of the backscattered light measuring apparatus of the present invention. In the apparatus shown in FIG. 1, the driver 1 causes the semiconductor laser 2 to emit search pulse light, and the pulse light is efficiently guided to the optical fiber 7 by the lenses 3 and 6. The return light generated by scattering the pulsed light at each part of the optical fiber 7 is incident on the AO deflector 5 through the beam splitter 4, and after the Fresnel reflection mask at the incident end is performed, the return light is divided by 2 minutes by the beam splitter 13. To be done. Beam splitter 13
One of the return lights divided into two is incident on the photodetector 8 and converted into an electric signal. The other split return light passes through the delay line of the optical fiber 15 and then the photodetector.
It is incident on 16 and converted into an electrical signal. The signals output from the photodetectors 8 and 16 are supplied to amplifiers 9 and 17 respectively.
Is input to the pair of input terminals of the differential amplifier 18 after being adjusted in level. The output of the differential amplifier 18 is A /
A clock pulse generated by the clock signal generator 11 is A / D converted by the D converter 10 as a sampling pulse and output to the memory 12.

When the backscattered light of the optical fiber causing the loss increase at one location is measured in the above-mentioned device of the present invention, the signals output from the amplifiers 9 and 17 are as shown in the graph of FIG. And the differential amplifier that inputs them
The signal output from 18 is as shown in the graph of FIG.

Using the apparatus of the present invention described above, by the method of the present invention,
Backscattered light measurements were performed. As the searching pulsed light, a pulsed light having a pulse width of 50 ns and a wavelength of 0.85 μm was emitted from the semiconductor laser 2, and a 20 m long GI fiber was used as the optical fiber delay line 15. The optical fiber 7 is a silica-based GI fiber with a length of 2 km, and bends at one point in the middle to give a transmission loss.
Giving 3dB. As a result, a pulse waveform as shown in Fig. 2 (b) was observed, and the threshold value was set to 50%.
It was confirmed that the location where the loss occurred can be reliably determined by setting to.

EFFECTS OF THE INVENTION By performing backscattered light measurement by the method of the present invention using the backscattered light measuring apparatus of the present invention, it is possible to locate the loss increase point of the optical fiber in real time.

The method of the present invention is particularly effective in the distributed optical fiber sensor when the loss increase point is discrete and it is sufficient to detect only that a loss increase of a certain level or more occurs. By applying the method and apparatus for measuring backscattered light according to the present invention, for example, when a heavy object (for example, an automobile) passes through, a microbend or a macrobend is generated, which causes a loss increase above a certain level. By laying the optical fiber described above, it is possible to realize a system for observing the passage of the heavy object including its position in real time.

[Brief description of the drawings]

FIG. 1 is an example of a configuration diagram of the backscattered light measuring apparatus of the present invention, and FIGS. 2A and 2B are waveforms observed by the method of the present invention using the apparatus of FIG. FIG. 3 is an example of a configuration diagram of a conventional backscattered light measurement device, and FIG. 4 is a graph of an observed waveform by the conventional backscattered light measurement device. [Main reference numbers] 1 ... Driver, 2 ... Semiconductor laser, 3,6 ... Lens, 4,13 ... Beam splitter, 5 ... AO deflector, 7,15 ... Optical fiber, 8,16 …… Photo detector, 9,17 …… Amplifier, 10 …… A / D converter, 11 …… Clock signal generator, 12 …… Memory, 18 …… Differential amplifier

Continuation of front page (56) Reference JP-A-56-14738 (JP, A) JP-A-57-122340 (JP, A) JP-A-2-88936 (JP, A)

Claims (4)

(57) [Claims] 1. A backscattered light measuring method for injecting pulsed light into an optical fiber and time-resolving and detecting return light due to scattering of the pulsed light at each point inside the optical fiber,
The return light from the optical fiber is divided into two, one of the branched return lights remains unchanged, and the other return light is given a certain delay and then converted into an electric signal to obtain the difference between the two. The backscattered light measuring method is characterized by detecting the level difference of the backscattered light. 2. The backscattered light measuring method according to claim 1, wherein the one branching return light is delayed by using an optical fiber delay line. 3. A pulse light generator for injecting pulsed light into an optical fiber, and a photodetector for time-resolving and detecting return light generated by scattering the pulsed light at each point inside the optical fiber. In the backscattered light measuring device, a branching unit that divides the return light into two, a delay unit that gives a certain delay to one return light that is branched by the branching unit, and another return light that is branched by the branching unit. And a second photodetector having the return light delayed by the delay means as an input, and outputs of the first and second photodetectors as an input A backscattered light measuring device comprising a differential amplifier, and detecting a step difference in backscattered light level by obtaining a difference between signals output from the first and second photodetectors. 4. The backscattered light measuring device according to claim 3, wherein the delay means is an optical fiber delay line.