JPH06249747A - Optical pulse tester for reducing distortion of measuring waveform due to phase difference change - Google Patents

Abstract

PURPOSE:To provide an optical pulse tester in which a phase deviation at the time of multiplying is always set to 180 degrees and a distortion of a measuring waveform due to phase difference change can be reduced by multiplying two signals in which only phases of carrier frequencies are inverted. CONSTITUTION:When a light source 1 emits a continuous coherent light, an optical demultiplexer 2 demultiplexes it to a signal light 13 and a local light 14, and the demultiplexed light 13 is pulse-modulated with a signal from a pulse generator 5 by an ultrasonic modulator 3. An optical demultiplexer 4 permeates a pulse light 15 pulse-modulated by an ultrasonic modulator 3, a back scattered light from an optical fiber 6 is demultiplexed, and the back scattered light 16 is output. The light 16 and the light 14 are multiplexed.demultiplexed at 1:1 of multiplexing.demultiplexing by an optical demultiplexer 7, the demultiplexed two lights are detected by photodetectors 8, 9, an output signal 17 of the photodetector 8 is multiplied by an output signal 18 of the photodetector 9, the output of a multiplier 10 is band-limited by a low-pass filter 11, output, and displayed on a display unit by a signal processor 12.

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 detecting loss and break position of an optical fiber by using optical heterodyne light reception.

[0002]

2. Description of the Related Art Next, FIG. 6 shows the structure of a conventional optical pulse tester. In FIG. 6, 1 is a light source for emitting continuous coherent light, 2 is an optical splitter, 3 is an ultrasonic modulator (hereinafter referred to as A / O modulator), 4 is an optical splitter, 5 is a pulse generator, 6 is an optical fiber, 8 is a photodetector, 10 is a multiplier, 11 is a low-pass filter (hereinafter referred to as LPF), 12 is a signal processing unit, 21 is an optical multiplexer, 22 is a local oscillator, and 23 is 2 It is a passenger.

In FIG. 6, the light source 1 continuously emits coherent light. The optical splitter 2 splits the light emitted from the light source 1 into a signal light 13 and a local light 14. The A / O modulator 3 pulse-modulates the signal light 13 branched by the optical branching device 2 with a signal from the pulse generator 5, and emits pulse light 15. The optical splitter 4 transmits the pulsed light 15 and splits the backscattered light 16 from the optical fiber 6 generated by the pulsed light 15.

The optical multiplexer 21 has local light 14 and backscattered light 1
Combine 6 The photodetector 8 detects the light from the optical multiplexer 21. The local oscillator 22 is a stable oscillator. The multiplier 10 multiplies the signal 25 from the photodetector 8 by the output 24 from the local oscillator 22. The LPF 11 limits the high frequency side band of the output 26 of the multiplier 10. The squarer 23 is LPF1
The output 27 of 1 is squared. The signal processor 12 is a squarer 23.
Is subjected to analog / digital conversion, addition / averaging processing, logarithmic conversion, etc., and displayed on a display screen (not shown). A similar technique is described in Japanese Patent Application No. 3-246440.

[0005]

Next, the waveform of the signal 25 from the photodetector 8 is shown in FIG. In FIG. 7, the frequency shifted by the A / O modulator 3 is output as the carrier frequency, but the frequency shifted by the A / O modulator 3 has a large phase noise in its periphery, and for example, , The phase is not constant as shown in FIG.

Next, the waveform of the signal 22 from the local oscillator 22 is shown in FIG. In FIG. 8, the local oscillator 22 outputs a signal having a constant phase. Next, the waveform of the signal 26 of the multiplier 10 is shown in FIG. The multiplier 10 multiplies the waveform of FIG. 7 by the waveform of FIG. 8, but the signals of FIG. 7 do not match in phase because the phases change randomly, and the multiplied signals are as shown in FIG. Change.

Next, a waveform in which the high frequency component of the multiplier output 26 is limited by the LPF 11 is shown in FIG. 10, and a waveform obtained by squaring the waveform of FIG. 10 by the squarer 23 is shown in FIG. The waveform of FIG. 11 is the square of the waveform of FIG. 10, and the measured waveform is distorted. The waveform in which the distortion randomly changes, which is obtained by performing the same measurement, is finally processed in the display unit of the optical pulse tester by performing a process such as an averaging process in the signal processing unit 12 in FIG. Display the measured waveform, but multiply 1
There is a problem that the measured waveform is distorted by the influence of the output signal 26 of 0, that is, the change of the phase difference.

The present invention multiplies two signals in which only the phase of the carrier frequency is inverted so that the phase shift during multiplication is always 180 degrees and the distortion of the measured waveform due to the change in phase difference is reduced. For the purpose of providing.

[0009]

To achieve this object, in the present invention, a light source 1 for emitting continuous coherent light, and an optical branch for branching the coherent light of the light source 1 into a signal light 13 and a local light 14 are provided. Device 2, an ultrasonic modulator 3 for pulse-modulating the signal light 13 branched by the optical branching device 2 with a signal from the pulse generator 5, and a pulsed light 15 pulse-modulated by the ultrasonic modulator 3 are transmitted, An optical splitter 4 that splits the backscattered light from the fiber 6 and outputs a backscattered light 16, a backscattered light 16 split by the optical splitter 4, and a local light 1
Optical splitter 7 with a multiplexing / branching ratio of 1: 1 for multiplexing / branching 4
And a photodetector 8 and a photodetector 9 respectively detecting the two lights branched by the optical branching device 7, and an output signal 17 of the photodetector 8.
And a multiplier 10 for multiplying the output signal 18 of the photodetector 9,
The output signal 19 of the multiplier 10 is input, the low pass filter 11 which band-limits, and the output signal 20 of the low pass filter 11 are input, and the signal processing part 12 which performs the process displayed on a display part is provided.

[0010]

The structure of the optical pulse tester according to the present invention is shown in FIG. In FIG. 1, 7 is an optical branching device having a multiplexing / branching ratio of 1: 1 and 9 is a photodetector. The squarer 23 of FIG. 6 is deleted and the optical branching device 7 is used instead of the optical multiplexer 21. In addition, the photodetector 9 for detecting the branched light from the optical branching device 7 is provided instead of the local oscillator 22, and the other parts are the same as those in FIG. The optical branching device 7 can use a half mirror, a fiber coupler, or the like.

In FIG. 1, when a half mirror is used for the optical branching device 7, the backscattered light 16 branched by the optical branching device 4 is used.
Is combined with the local light 14 branched by the optical branching device 2. At this time, the backscattered light 16 and the light that has passed through the optical branching device 7 are
The light reflected at 0 degrees is branched at a ratio of 1: 1. At the same time, the local light 14 is also split into transmitted light and reflected light at a ratio of 1: 1 by the optical splitter 7, and the transmitted light of the backscattered light 16 and the reflected light of the local light 14 are combined to form the reflected light of the backscattered light 16. The transmitted light of the local light 14 is multiplexed.

When the respective combined lights are detected by the photodetector 8 and the photodetector 9, a signal is detected by using the frequency shifted by the A / O modulator 3 as a carrier frequency, and the multiplier 1
0 multiplies the outputs of the photodetectors 8 and 9 as inputs.

Next, the operation of FIG. 1 will be described with reference to FIGS. FIG. 2 shows the waveform of the signal 17 of the photodetector 8,
It is the same as FIG. 7. FIG. 3 shows the waveform of the signal 18 of the photodetector 9, and the phase is 18 due to the optical branching device 7 as compared with FIG.
The waveform is shifted by 0 degree.

FIG. 4 is a waveform when the signal 17 and the signal 18 are multiplied by the multiplier 10. In FIG. 4, since the phases in FIGS. 2 and 3 are only 180 degrees out of phase, the detection efficiency does not change due to the phase deviation when multiplied. Figure 5 shows LP
It is the waveform of the signal 20 from which the high frequency components have been removed in F11.
A waveform with less distortion is obtained by band-limiting the signal 19 of the multiplier with an LPF, and the signal processor performs averaging processing or the like to display the waveform with less distortion on the display of the optical pulse tester.

[0015]

According to the present invention, the local light and the backscattered light are split into 1: 1 by the optical splitter, and the signals having the phase difference of 180 degrees are input to the two photodetectors to be multiplied. It is possible to obtain a waveform with less distortion without being affected by the phase noise of the signal, and it is possible to improve the accuracy of measurement of the loss and break point position of the optical fiber.

[Brief description of drawings]

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

FIG. 2 is an example of an output signal 17 of a photodetector 8 in FIG.

3 is an example of an output signal 18 of the photodetector 9 of FIG.

4 is an example of an output signal 19 of the multiplier 10 of FIG.

5 is an example of an output signal 20 of the LPF of FIG.

FIG. 6 is a block diagram of a conventional optical pulse tester.

7 is an example of an output signal 25 of the photodetector 8 of FIG.

8 is an example of an output signal 24 of the local oscillator 22 of FIG.

9 is an example of an output signal 19 of the multiplier 10 of FIG.

10 is an example of an output signal 27 of the LPF 11 of FIG.

11 is an example of an output signal 28 of the squarer 23 of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 light source 2 optical branching device 3 ultrasonic modulator 4 optical branching device 5 pulse generator 6 optical fiber 7 optical branching device 8 ・ 9 photodetector 10 multiplier 11 low-pass filter 12 signal processing unit 21 optical multiplexer 22 local unit Oscillator 23 squarer

Claims (1)

[Claims] 1. A light source for emitting continuous coherent light
(1), the coherent light from the light source (1) is branched into the signal light (13) and the local light (14) by the first optical branching device (2) and the first optical branching device (2) Ultrasonic modulator that pulse-modulates the signal light (13) with the signal from the pulse generator (5)
(3) and the pulsed light (15) pulse-modulated by the ultrasonic modulator (3) are transmitted, the backscattered light from the optical fiber (6) is branched, and the backscattered light (16) is output. Optical splitter (4), and the backscattered light (16) split by the second optical splitter (4) and local light
A third optical branching device (7) with a multiplexing / branching ratio of 1: 1 that enters (14) and combines and splits, and two lights branched by the third optical branching device (7), respectively. Photodetector (8) and photodetector (9) to detect, output signal (17) of photodetector (8) and output signal of photodetector (9)
(18) as an input, the multiplier (10) for multiplication, and the output signal (19) of the multiplier (10) as an input, and the low-pass filter (11) for limiting the band and the low-pass filter (11 An optical pulse tester for reducing distortion of a measured waveform due to a change in phase difference, the signal processor having an output signal (20) of (4) as an input and a signal processing unit (12) for displaying on a display unit.