JPH08210945A - Optical fiber inspection device - Google Patents

Abstract

PURPOSE: To control the linear sweep of a modulation frequency of light intensity by providing a delay circuit, and controlling a modulation signal so that the change quantity of the modulation frequency per unit time is constant. CONSTITUTION: The reflected light from an optical fiber 7 to be inspected is branched by a beam splitter 5, transmitted by an optical fiber 9 having a known length which is a delay circuit, and detected by a light detector 10. The output of the detector 10 is mixed with the output of a distributor 22, or the output of a voltage control oscillator 21 by a mixer 12, and the difference of the modulation frequency changed within a delay time, or a frequency signal equal to the product of the inclination of change of the modulation frequency (modulation frequency change quantity per unit time) and the delay time is outputted, and compared with the signal of the known frequency of an oscillator 16 by a phase comparator 17 to detect the phase difference. A control circuit 18 outputs a control signal so that the frequency of the output signal of the mixer 12 is constant by the output of the comparator 17, adds 20 it to the output of a modulated signal generator 19, and outputs the result to an oscillator 21 to realize the direct sweep of the modulated frequency.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber inspection device (OFDR) using optical intensity modulation.
The present invention relates to an optical fiber inspection apparatus in which a delay circuit is provided in a main reflectmeter) to detect a deviation of a modulation frequency and linear sweep control of an intensity modulation frequency can be performed.

[0002]

2. Description of the Related Art OFDR is an optical fiber inspection apparatus that outputs light whose frequency is modulated, detects reflected light at a fault point in an optical fiber, and identifies the fault point from frequency information of the detected signal. . In OFDR, there are a method of modulating the optical frequency and a method of frequency modulating the light intensity.

[0003]

The present invention particularly relates to the latter OFDR which frequency-modulates the light intensity. However, the conventional OFDR has a drawback that it is difficult to linearly sweep the modulation frequency of the light intensity. there were.

It is an object of the present invention to realize an optical fiber inspection apparatus capable of detecting the amount of change in the modulation frequency per unit time by providing a delay circuit and performing linear sweep control of the modulation frequency.

[0005]

In order to achieve such an object, according to the present invention, an E / O converter, a drive circuit for driving the E / O converter, and an optical circuit of the E / O converter are provided. A modulation signal generator that outputs a modulation signal that modulates intensity, a distributor that branches the output of this modulation signal generator, and an output light of the E / O converter that is transmitted and output to an optical fiber to be inspected. Means for transmitting the reflected light from the optical fiber to a branch other than the E / O converter, a photodetector for detecting the output light of this means, the output of this photodetector and the distributor A mixer that mixes the output, a frequency analyzer that frequency-analyzes the output of the mixer, and a delay circuit are provided to detect the amount of change in the modulation frequency of the light intensity per unit time to detect the E /
A linear sweep control means for performing a control for linearly sweeping the modulation frequency of the light intensity of the output light of the O converter is provided.

[0006]

A part of the output light of the E / O converter or the modulation signal is delayed by the delay circuit, and the amount of change in the modulation frequency per unit time is detected. By controlling the modulation signal so that the amount of change becomes constant, linear sweep control of the modulation frequency of the light intensity becomes possible.

[0007]

The present invention will be described in detail below with reference to the drawings.
FIG. 1 is a block diagram showing an embodiment of an optical fiber inspection apparatus according to the present invention. In the figure, 1 is an electro-optical converter (hereinafter referred to as an E / O converter), 2 is a drive circuit for driving the E / O converter 1 by direct current, and 3 is a multiplexer for multiplexing a direct current signal and a high frequency signal. Reference numeral 4 is an optical isolator, 5 and 6 are beam splitters for splitting light, 7 is an optical fiber to be inspected, 8 is a photodetector for detecting reflected light from a failure point of the optical fiber 7, and 9 is a beam splitter 5. Is an optical fiber of known length for delaying the output light of.

Reference numeral 10 is a photodetector for detecting the output light of the optical fiber 9, 11 and 12 are mixers, 13 and 14 are low-pass filters for transmitting only necessary low frequency components, 1
5 is a frequency analyzer for analyzing the frequency and signal strength of the output of 13, 13 is an oscillator for outputting a known frequency signal, 1
Reference numeral 7 denotes a phase comparator which detects a frequency difference or a phase difference between the output of the low pass filter 14 and the output of the oscillator 16.
A control circuit 18 controls the modulation frequency based on the output of the phase comparator 17. Low pass filter 1
4 is preferable, but there is no problem in operation without it.

Reference numeral 19 is a modulation signal generator for outputting a modulation signal, 20 is an adder for adding the output of the control circuit 18 and the output of the modulation signal generator 19, and 21 is an output oscillation according to the output of the adder 20. A voltage-controlled oscillator whose frequency changes, 22
Is a distributor for branching the output of the voltage controlled oscillator 21.

The operation in such a configuration will be described below. The output of the drive circuit 2 drives the E / O converter 1. The light output from the E / O converter 1 passes through the optical isolator 4 and the beam splitters 5 and 6 and is output to the outside.
The output frequency of the voltage controlled oscillator 21 is the modulation signal generator 1
Ramp wave modulation is performed by the output of 9. Voltage controlled oscillator 2
The output of 1 is branched into three by the distributor 22 and input to the multiplexer 3 and the mixers 11 and 12.

When the output of the distributor 22, that is, the output of the voltage controlled oscillator 21 is applied to the multiplexer 3, the modulation frequency of the output light intensity of the E / O converter 1 is ramp wave modulation a ( Δf / Δt) The light output to the outside is incident on the optical fiber 7 to be inspected, is reflected at the obstacle point in the optical fiber and returns to the beam splitter 6, and the photodetector 8
Detected in. The output of the photodetector 8 is mixed with the output of the distributor 22 by the mixer 11.

When the distance to the fault point of the optical fiber 7 is L, between the output of the distributor 22 and the output of the photodetector 8, Δt = 2 · n · L / c ₀ n: optical fiber 7 Refractive index c ₀ : There is a time difference in the speed of light in vacuum. Therefore, the output of the mixer 11 includes the signal of the frequency Δf = a · Δt = 2 · a · n · L / c ₀ which is changed during the time difference Δt.

The low-pass filter 13 removes unnecessary frequency components from the output of the mixer 11, and the frequency analyzer 1
At 5, the frequency and signal strength are analyzed. If the modulation gradient a is constant, the analyzed frequency is proportional to the distance L to the fault point, so the distance L can be calculated from the frequency and the reflection amount can be obtained from the signal intensity.

The light split by the beam splitter 5 passes through an optical fiber 9 having a known length L _c and is detected by a photodetector 10. The output of the photodetector 10 is mixed with the output of the distributor 22 by the mixer 12. Since there is a constant time difference t = n · L _c / c ₀ between the output of the photodetector 10 delayed by the optical fiber 9 and the output of the distributor 22,
The product of the difference in the modulation frequency changed within the delay time, that is, the slope of the change in the modulation frequency (modulation frequency change amount per unit time) and the delay time by the optical fiber 9 f = a · t = a · n · L _c A signal having a frequency equal to / c ₀ is output from the mixer 12 (FIG. 2).

The low-pass filter 14 removes unnecessary frequency components from the output of the mixer 12. If the slope of the change in the modulation frequency is constant (the line is swept), the frequency of the signal output from the low pass filter 14 is constant. The signal output from the low pass filter 14 is compared in frequency and phase with the signal of known frequency output from the oscillator 16 by the phase comparator 17. The control circuit 18 outputs a control signal so that the frequency of the signal output from the mixer 12 is constant due to the output of the phase comparator 17. The control signal output from the control circuit 18 is added to the output of the signal generator 19 by the adder 20 and enters the voltage controlled oscillator 21.

In this way, the frequency of the signal output from the mixer 12 becomes constant under the control of the control circuit 18, and the linear sweep of the modulation frequency can be realized.

In the present invention, a portion provided with a delay circuit to detect the amount of change in the modulation frequency of the light intensity per unit time and perform the linear sweep control of the modulation frequency is called the linear sweep control means. The linear sweep control means is not limited to the above embodiment, but various configurations are allowed. For example, as shown in FIG. 3, the output of the phase comparator 17 may be stored in the storage device 23 and the data may be input to the control circuit 18. In this case, the control circuit 18 repeats the linear sweep control of the modulation frequency based on the data stored in the storage device 23. In this case, the modulation signal generator 19 outputs a signal having a ramp-wave-modulated frequency. The same applies to the following examples.

FIG. 4 shows an example of a configuration in which the output of the distributor 22 is given to the modulator 24 to control the modulation frequency of the light intensity of the output of the E / O converter 1. The linear sweep control of the modulation frequency of is possible.

In FIG. 5, the linear sweep control of the modulation frequency of the light intensity is carried out by utilizing the delay of the drive signal instead of utilizing the delay of the optical signal. Is input to the phase comparator 17 and is mixed with the other distributed signal by the mixer 12 and input to the phase comparator 17. Although the distributors 22 and 25 are separately provided in the embodiment, one distributor is sufficient if a multi-output distributor is used.

FIG. 6 is an example of a configuration in which the output of the phase comparator 17 in FIG. 5 is stored in the storage device 23.

FIG. 7 shows an example of a configuration in which the output of the distributor 22 in FIG. 5 is input to the modulator 24 which modulates the output of the E / O converter 1 to perform linear sweep control of the modulation frequency of the light intensity. Is.

[0022]

As described above, according to the present invention, by providing the delay circuit, the variation amount of the modulation frequency per unit time can be controlled to be constant, so that the linear sweep of the modulation frequency of the light intensity can be performed. This is possible, and has the effect of improving the distance resolution and distance accuracy to the obstacle point.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of an optical fiber inspection apparatus according to the present invention.

FIG. 2 is a diagram for explaining the operation.

FIG. 3 is a configuration diagram showing another embodiment of the present invention.

FIG. 4 is a configuration diagram showing still another embodiment of the present invention.

FIG. 5 is a configuration diagram showing still another embodiment of the present invention.

FIG. 6 is a configuration diagram showing still another embodiment of the present invention.

FIG. 7 is a configuration diagram showing still another embodiment of the present invention.

[Explanation of symbols]

 1 E / O converter 2 Drive circuit 3 Multiplexer 4 Optical isolator 5, 6 Beam splitter 7 Optical fiber 8 Photodetector 9 Optical fiber 10 Photodetector 11, 12 Mixer 13, 14 Low pass filter 15 Frequency analysis 16 oscillator 17 phase comparator 18 control circuit 19 modulation signal generator 20 adder 21 voltage controlled oscillator 22, 25 distributor 23 storage device 24 modulator 26 delay circuit

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Mamoru Arihara 2-3-9 Nakamachi, Musashino-shi, Tokyo Yokogawa Electric Co., Ltd.

Claims (7)

[Claims] 1. An E / O converter, a drive circuit for driving the E / O converter, a modulation signal generator for outputting a modulation signal for modulating the light intensity of the E / O converter, and this modulation. The distributor for branching the output of the signal generator and the output light of the E / O converter are transmitted to the optical fiber to be inspected and the reflected light from the optical fiber is referred to as the E / O converter. Means for transmitting to another branch, a photodetector for detecting the output light of this means, a mixer for mixing the output of this photodetector and the output of the distributor, and a frequency for frequency analysis of the output of this mixer A linear sweep control in which an analyzer and a delay circuit are provided to detect the amount of change in the modulation frequency of the light intensity per unit time and perform control for linearly sweeping the modulation frequency of the light intensity of the output light of the E / O converter. Optical fiber inspection device characterized by comprising means . 2. The linear sweep control means includes an optical distributor for branching the output light from the E / O converter, an optical waveguide for delaying the output light of the optical distributor, and an optical waveguide for the optical waveguide. A photodetector that detects output light, a mixer that mixes the output of the photodetector and the output of the distributor, an oscillator that outputs a known frequency signal, and an output of the mixer and an output of the oscillator. 2. The optical fiber inspection apparatus according to claim 1, further comprising a phase comparator that detects a frequency difference or a phase difference, and a control circuit that linearly sweeps the modulation frequency according to the output of the phase comparator. 3. The linear sweep control means, an optical distributor for branching the output light from the E / O converter, an optical waveguide for delaying the output light of the optical distributor, and an optical waveguide for the optical waveguide. A photodetector that detects output light, a mixer that mixes the output of the photodetector and the output of the distributor, an oscillator that outputs a known frequency signal, and an output of the mixer and an output of the oscillator. A phase comparator that detects a frequency difference or a phase difference, a storage device that stores the output of the phase comparator, and a correction of the output of the modulation signal generator so that the modulation frequency is linearly swept from the data of the storage device. The optical fiber inspection apparatus according to claim 1, further comprising a control circuit for controlling the optical fiber. 4. The linear sweep control means is a modulator for intensity-modulating the output light from the E / O converter, and a modulation signal generator for driving the modulator to output a modulation signal for modulating the light intensity. , A distributor for branching the output of the modulation signal generator, an optical distributor for branching the output light of the modulator, an optical waveguide for delaying the output light of the optical distributor, and an optical waveguide A photodetector for detecting the output light of, and a mixer for mixing the output of the photodetector and the output of the distributor,
An oscillator that outputs a known frequency signal, a phase comparator that detects a frequency difference or a phase difference between the output of the mixer and the output of the oscillator, and a control circuit that linearly controls the modulation frequency by the output of the phase comparator The optical fiber inspection apparatus according to claim 1, further comprising: 5. The linear sweep control means is a modulator for intensity-modulating the output light from the E / O converter, and a modulation signal generator for driving the modulator to output a modulation signal for modulating the light intensity. And a divider for branching the output of the modulation signal generator, a delay circuit for delaying one branch output from the divider, and a mixer for mixing the output of the delay circuit and the branch output. , An oscillator that outputs a known frequency signal, a phase comparator that detects a frequency difference or a phase difference between the output of the mixer and the output of the oscillator, and control that linearly controls the modulation frequency by the output of the phase comparator The optical fiber inspection apparatus according to claim 1, further comprising a circuit. 6. The linear sweep control means includes a modulation signal generator for outputting a modulation signal for modulating the light intensity of the E / O converter, and a distributor for branching the output of the modulation signal generator.
A delay circuit for delaying one branch output from the distributor, a mixer for mixing the output of the delay circuit and the branch output, an oscillator for outputting a known frequency signal, an output of the mixer and the The optical fiber inspection apparatus according to claim 1, further comprising a phase comparator that detects a frequency difference or a phase difference from the output of the oscillator, and a control circuit that linearly sweeps and controls the modulation frequency by the output of the phase comparator. . 7. The linear sweep control means includes a storage device for storing the output of the phase comparator, and the control circuit generates the modulation signal so that the modulation frequency is linearly swept based on the data of the storage device. 7. The optical fiber inspection apparatus according to claim 6, wherein the output of the container is corrected.