JP3019530B2 - Optical pulse tester using heterodyne light reception - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pulse tester for measuring the loss and break point of an optical fiber by heterodyne light reception.

[0002]

2. Description of the Related Art Next, the configuration of a conventional optical pulse tester will be described with reference to FIG. 5 is a coherent light source, 2, 4, and 9 are multiplexer / demultiplexers, 3 is an A / O switch, 5 is an oscillator, 6 is an amplifier, 8 is a switch, 10 is an optical fiber to be measured, and 11 is a photodetector. , 12 is an amplifier, 17 is a detector, 15 is an A / D converter, and 16 is a signal processing device.

The coherent light 1A of the optical frequency fa output from the light source 1 is divided into light 2A and light 2B by the multiplexer / demultiplexer 2, and
A is input to the A / O switch 3, and the light 2B is input to the multiplexer / demultiplexer 9 as the local light 2B. The output 6A obtained by amplifying the output 5A of the oscillator 5 with the amplifier 6 through the switch 8 controlled by the control signal 16A output from the signal processing device 16 is given to the A / O switch 3, and the output of the A / O switch 3 is output. Is a pulse light 3A having the same shape as the control signal 16A.

When the pulse light 3A passes through the A / O switch 3, the frequency is shifted by fb, and the light frequency fa +
fb. The pulse light 3A passes through the multiplexer / demultiplexer 4 and enters the optical fiber 10 as pulse light 4A. Pulse light 4
When A propagates through the optical fiber 10, backscattered light 10A is generated and propagates to the multiplexer / demultiplexer 4. This backscattered light 10A
The loss characteristics and break point of the optical fiber 10 can be known by measuring how the size of the optical fiber 10 changes with time.

The backscattered light 10A passes through the multiplexer / demultiplexer 4 and enters the multiplexer / demultiplexer 9 as signal light 4B. The multiplexer / demultiplexer 9 multiplexes the signal light 4B and the local light 2B and outputs the multiplexed light to the light receiver 11. The optical receiver 11 heterodyne-receives these two mixed lights, sets the frequency difference fb between them as an intermediate frequency,
The current is converted into a current 11A having the loss characteristic of 0 as an envelope.
The current 11A is converted into a voltage by the amplifier 12 and amplified.
Output to the detector 17.

The detector 17 is for converting an envelope signal of the frequency fb into a baseband signal, and an envelope detector, a square detector, or the like is used. The baseband signal 17A obtained by the detector 17 is converted into a digital value by the A / D converter 15 and input to the signal processing device 16. The backscattered light 10A is extremely small, and is at most 35 dB smaller than the pulsed light 4A. Therefore, the S / N ratio is poor, and a measurement result to be obtained cannot be obtained unless a large number of averaging is performed. For this reason, the signal processing device 16 outputs the control signal 16A at a predetermined cycle, and performs averaging and logarithmic conversion of the input measurement data and displays the result.

FIG. 6 shows the waveform of each part in FIG. 5 in time, and the numbers in FIG. 6 correspond to the same numbers in FIG. The control signal 16A is a digital value indicating the magnitude of the backscattered light 10A obtained by the addition in the signal processing device 16. In FIG. 6, the horizontal axis represents time, and the vertical axis represents voltage at 16A and 8A, optical power at 4A and 10A, and 11A at 11A.
Is a current and 17A is a digital value.

[0008]

In FIG. 5, since the detector 17 is a linear approximation of the nonlinearity of the forward voltage-forward current characteristic of the diode, the actual diode characteristic deviates from the polygonal line characteristic. At the time of inputting a small signal that is affected by the presence, the detection efficiency is reduced and the dynamic range cannot be widened. For example, when an envelope detector is used as the detector, the dynamic range is slightly less than 20 dB in one-way conversion, and when the square detector is used, the dynamic range is smaller.

As a detection method used when a wide dynamic range is required, there is a synchronous detection method. In the synchronous detection method, an intermediate frequency of a signal to be detected and a local oscillation frequency (hereinafter, referred to as a local oscillation) having the same frequency and phase. )is necessary. In an optical pulse tester using heterodyne light reception, the refractive index and birefringence of the optical fiber 10 fluctuate due to a change in the state of the optical fiber 10, for example, a pressure applied to the optical fiber 10 or a temperature change. As a result, the phase of the light that is the carrier of the backscattered signal 10A that must be measured fluctuates to an unpredictable degree.

Also, the level of the backscattered signal 10A is extremely weak, about 35 dB or less than that of the emitted pulse light, as described above, and the duty ratio is low and the noise is sometimes larger than the signal. Further, it is not possible to generate a local oscillation by extracting an intermediate frequency, and the synchronous detection method cannot be applied.

In the present invention, a synchronous detector is employed in place of the detector 17 shown in FIG. 5, and the output of the drive signal oscillator 5 of the A / O switch 3 is used as a local oscillator, and the output of the synchronous detector is used. An object of the present invention is to provide an optical pulse tester having a wide dynamic range by averaging after squaring.

[0012]

In order to achieve this object, according to the present invention, a light source 1 for emitting continuous coherent light, and a multiplexer / demultiplexer for dividing the light emitted from the light source 1 into continuous light 2A and local light 2B 2 and the continuous light 2A are pulse-modulated by the output of the switch 8 to give a frequency shift, and the pulse light 3A
A / O switch 3 for converting the light into incident light 3
A enters the optical fiber 10 to be measured, and the optical fiber 1
Multiplexer / demultiplexer 4 for splitting backscattered light 10A from zero as signal light 4B, and oscillator 5 for generating a continuous electric signal
An amplifier 6 for amplifying the output of the oscillator 5, an amplifier 7 for amplifying the output of the oscillator 5, and an amplifier 6 for a time set by a control signal pulse 16A from the signal processing device 16.
, A switch 8 for adding the output of the A / O switch 3 to the A / O switch 3, a multiplexer / demultiplexer 9 for multiplexing the local light 2B signal light 4B, and a multiplexer / demultiplexer 9
A photodetector 11 for heterodyne receiving the light combined in
The amplifier 12 converts the output current 11A of the optical receiver 11 into a voltage and amplifies the voltage.
, A squaring circuit 14 for squaring the output of the synchronous detector 13, and a squaring circuit 1
An A / D converter 15 for A / D converting the output of
And a signal processing device 16 for performing an averaging / logarithmic conversion of the output of the converter 15 and generating operation timing.

[0013]

Next, the configuration of the optical pulse tester according to the present invention will be described with reference to FIGS. 1 is an amplifier, 13 is a synchronous detector, 14 is a squaring circuit, and the others are the same as those in FIG. The synchronous detector 13 includes a mixer 131 and a filter 132. FIG. 2 shows the operation of FIG. 1 in time, and the numbers in FIG. 2 correspond to the same numbers in FIG. 1, respectively. In FIG. 2, the horizontal axis represents time, and the vertical axis represents voltage at 16A, 8A, 7A, and 13A, optical power at 4A and 10A, current at 11A, and digital value at 15A.

Similarly to FIG. 5, a current 11A heterodyne-received by the photodetector 11 is converted into a voltage by the amplifier 12, and is amplified to a voltage 12A. The voltage 12A is mixed by the mixer 131 with the local light 7A obtained by amplifying the high frequency output 5A of the oscillator 5 by the amplifier 7. The filter 132 extracts only the baseband component 13A from the output of the mixer 131. The baseband component 13A has a phase difference between the local light 7A and the signal light 4B as an amplitude component. Since the phase difference varies variously depending on environmental conditions of the optical fiber 10, the amplitude of the baseband component 13A also As shown in FIG. 2, the inside of the same envelope as 10A varies variously. 13A and 15A in FIG. 2 show the above-mentioned envelopes by broken lines. This fluctuation is random, and fluctuates faster than the backscattered light 10A in time. The baseband component 13A is converted into a digital value by the A / D converter 15, averaged by the signal processing device 16, and displayed, whereby the state of the backscattered light 10A can be measured. FIG. 3 shows this state.

In the above description, the output of the squaring circuit 14 is A /
Although the analog output is processed up to the output of the squaring circuit 14 as an input to the D converter 15, the position of the A / D converter 15 can be changed within the range permitted by the processing capability of the digital processing circuit. Filter 15 and filter 132 and squaring circuit 14
, Between the mixer 131 and the filter 132, between the amplifier 12 and the mixer 131, between the optical receiver 11 and the amplifier 12
May be arranged between them.

[0016]

Next, an embodiment of the present invention will be described with reference to FIG. In FIG. 4, the frequency-stabilized laser diode (L
D) Light sources, 2, 4, and 9 are fiber couplers, 5 is a 100 MHz sine wave crystal oscillator, 8 is a relay switch, 11 is a photodiode (PD), and 131 is a double balance mixer (DBM).

The output light 1A of the frequency stabilized LD light source 1 is divided into light 2A and light 2B by the fiber coupler 2, and the light 2A is
The light is input to the O switch 3 and the light 2B is input to the fiber coupler 9 as local light. The output of the crystal oscillator 5 is supplied to the A / O switch 3 through the relay switch 8 controlled by the control signal 16A output from the signal processing device 16 to the amplifier 6
The output amplified by the A / O switch 3 is given.
A is an optical pulse having the same shape as the control signal 16A. This light pulse is incident on the optical fiber 10 through the fiber coupler 4 and the backscattered light 10A returning from the optical fiber 10
Through the fiber coupler 4 to the fiber coupler 9 as the signal light 4B.

The fiber coupler 9 has a local light 2B and a signal light 4B.
B is mixed and incident on the PD 11. PD11 is these two
The two lights are heterodyne-received and amplified by the amplifier 12.
The output 11A of the amplifier 12 is a voltage signal with the loss characteristic of the optical fiber 10 as an envelope and 100 MHz as a carrier. The DBM 131 includes an output 7A having a frequency of 100 MHz obtained by amplifying the output of the crystal oscillator 5 and an amplifier 1 having a carrier of 100 MHz.
The two outputs 12A are mixed. The low-pass filter 132 is a DB
The baseband component 13A is extracted from the output of M131. The extracted baseband component 13A is used as a squaring circuit 1
After squaring by 4, the signal is converted into a digital value by the A / D converter 15 and the signal processor 16 performs averaging and display.

[0019]

According to the present invention, since the synchronous detector is adopted as the detector, an optical pulse tester using heterodyne light reception having a wide dynamic range can be realized.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical pulse tester according to the present invention.

FIG. 2 is a diagram showing the operation of FIG. 1 in time.

FIG. 3 is a diagram showing a state of backscattered light 10A.

FIG. 4 is a diagram showing an embodiment of the present invention.

FIG. 5 is a configuration diagram of a conventional optical pulse tester.

FIG. 6 is a time chart showing waveforms of respective parts in FIG. 5;

[Explanation of symbols]

 Reference Signs List 1 light source 2 multiplexer / demultiplexer 3 A / O switch 4 multiplexer / demultiplexer 5 oscillator 6 amplifier 7 amplifier 8 switch 9 multiplexer / demultiplexer 10 optical fiber 11 optical receiver 12 amplifier 13 synchronous detector 14 square circuit 15 A / D Converter 16 signal processing device

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-3-13835 (JP, A) JP-A-64-46627 (JP, A) JP-A-62-123327 (JP, A) JP-A 64-64 75932 (JP, A) JP-A-1-153923 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01M 11/00-11/02

Claims (1)

(57) [Claims] 1. A light source for emitting continuous coherent light
(1), a first multiplexer / demultiplexer (2) that divides the light emitted from the light source (1) into continuous light (2A) and local light (2B), and outputs the continuous light (2A) from the switch (8). An A / O switch (3) that performs pulse modulation to give a frequency shift and converts it to pulse light (3A), and the incident pulse light (3A) is incident on an optical fiber (10) to be measured, A second multiplexer / demultiplexer (4) that branches the backscattered light (10A) from (10) as signal light (4B), an oscillator (5) that generates a continuous electric signal, and an oscillator (5). The time set by the first amplifier (6) for amplifying the output, the second amplifier (7) for amplifying the output of the oscillator (5), and the control signal pulse (16A) from the signal processor (16) A switch (8) for adding the output of the first amplifier (6) to the A / O switch (3), and a third multiplexer / demultiplexer (9) for multiplexing the local light (2B) and the signal light (4B). )
And a photodetector (11) for heterodyne receiving the light multiplexed by the third multiplexer / demultiplexer (9); and a third amplifier for converting an output current (11A) of the photodetector (11) into a voltage and amplifying the voltage. The output of the second amplifier (12) and the output of the second amplifier (7) are locally generated, and the output of the third amplifier (12) is
A synchronous detector (13) that detects the output voltage (12A), a square circuit (14) that squares the output of the synchronous detector (13), and A / D conversion of the output of the square circuit (14) A / D converter to do (15)
An optical pulse tester using heterodyne light reception, comprising: a signal processor (16) that performs averaging and logarithmic conversion on the output of the A / D converter (15) and generates operation timing.