JPH07260624A - Optical pulse tester - Google Patents

Abstract

PURPOSE:To provide an optical pulse tester in which the dynamic range is widened while suppressing the distortion. CONSTITUTION:A coherent light is outputted from a light source 2 with the optical frequency thereof being controlled by a frequency control circuit 1. The light is then split through an optical splitter 3 and one split light is subjected to pulse modulation through an optical switch 4 before entering into an optical fiber 7 and the backscattering light thereof is branched through an optical splitter 6. On the other hand, a coherent light is outputted from a light source 8 with the optical frequency thereof being controlled by a frequency comparing circuit 12. The light is then split through an optical splitter 9 and h an optical multiplexer 10 multiplexes the other split light from the splitter 3 and one split light from the splitter 9 before being fed to an optical detector 11. A frequency comparing circuit 12 receives the output from the detector 11 along with the output from a signal generator 13 generating a signal having the passing frequency of a band-pass filter 17 and delivers a signal for matching both output frequencies to the light source 8. An optical multiplexer 14 multiplexes the split lights from the splitters 6, 9 and an optical detector 15 detects the multiplexed light which is then amplified through an amplifier 16, subjected to bandwidth limitation by a band-pass filter 17. The data is operated at a processing section 18 and presented on a display section 19.

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

[0002]

2. Description of the Related Art Next, the structure of an optical pulse tester according to the prior art is shown in FIG. In FIG. 8, 1 is a frequency control circuit, 2 is a light source, 4 is an optical switch, 5 is a pulse generator, 6 is an optical splitter, 7 is an optical fiber, 8 is a light source, 14 is an optical multiplexer, and 15
Is a photodetector, 16 is an amplifier, 17 is a bandpass filter (hereinafter referred to as BPF), 18 is a data processing unit, 19
Is a display unit.

In FIG. 8, a frequency control circuit 1 outputs a signal to the light source 2 and the light source 8 to change the frequency continuously or stepwise. At this time, the optical frequency difference between the light source 2 and the light source 8 is preset or adjusted so as to be f _IF . The optical frequency difference f _IF is the center frequency of the pass band of the BPF 17. The light source 2 outputs light having an optical frequency according to the signal of the frequency control circuit 1.

The optical switch 4 pulse-modulates the output light of the light source 2 by the output signal of the pulse generator 5 and outputs the pulsed light. The optical splitter 6 inputs the pulsed light from the optical switch 4 into the optical fiber 7 and splits the backscattered light from the optical fiber 7.

The optical multiplexer 14 multiplexes the backscattered light from the optical splitter 6 and the output light of the light source 8. The photodetector 15 detects the light from the optical multiplexer 14, and outputs a signal of carrier frequency f _IF . The BPF 17 is the photodetector 1 amplified by the amplifier 16.
The output signal of No. 5 is band-limited to the bandwidth Δf. The data processing unit 18 performs A / D conversion, averaging processing, and the like on the signal whose band is limited by the BPF 17. The display unit 19 is the data processing unit 18.
Display the result of. 9A shows an example of the measured waveform.

[0006]

With the configuration shown in FIG. 8, the BPF is
The pass band width Δf of 17 is several MHz. However, the optical frequency stability is poor, and the relative frequency fluctuation between the light source 2 and the light source 8 is larger than Δf due to a disturbance such as a temperature change.
The signal frequency may deviate from the pass band of. Therefore, there are problems that the signal level is attenuated and the dynamic range is reduced as shown in (a) of FIG. 9, and that the measured waveform is distorted as shown in (c) of FIG.

According to the present invention, the output lights of the two light sources are combined and detected, the frequency difference between the two light sources is obtained, and the BP is calculated.
An object of the present invention is to provide an optical pulse tester with a wide dynamic range and less distortion by stabilizing the frequency difference between two light sources compared with a signal having the same frequency as the center frequency of the pass band of F.

[0008]

In order to achieve this object, the present invention relates to a frequency control circuit 1 for generating a signal for controlling an optical frequency, and an output signal of the frequency control circuit 1 for changing the optical frequency as an input. A light source 2 for outputting coherent light, an optical branching device 3 for branching the output light of the light source 2, an optical switch 4 for pulse-modulating one of the branched lights of the optical branching device 3 with a signal from a pulse generator 5, The optical switch 4 transmits the pulse-modulated pulsed light, enters the optical fiber 7 and branches the backscattered light from the optical fiber 7, and the output signal of the frequency comparison circuit 12 as an input to determine the optical frequency. A light source 8 for changing and outputting coherent light;
The optical splitter 9 that splits the output light of the light source 8, the optical multiplexer 10 that multiplexes the other branched light of the optical splitter 3 and the one branched light of the optical splitter 9, and the optical multiplexer 10. A photodetector 11 that detects the combined light, a frequency comparison circuit 12 that uses the output signal of the photodetector 11 as a first input, and an output signal of the signal generator 13 as a second input signal, and a bandpass filter. A signal generator 13 that outputs the same frequency as the pass frequency of the filter 17,
An optical multiplexer 14 for combining the light branched by the optical branching device 6 and a light branched by the optical branching device 9, a photodetector 15 for detecting the light combined by the optical multiplexer 14, and a photodetector 15. An amplifier 16 that amplifies the output, a bandpass filter 17 that band-limits the output of the amplifier 16, a data processing unit 18 that calculates the output of the bandpass filter 17, and a display that displays the data processed by the data processing unit 18. The frequency comparison circuit 12 includes a unit 19 and outputs the frequency of the output signal of the photodetector 11 and the frequency of the output signal of the signal generator 13 to the light source 8 so as to match each other. Further, the output of the pulse generator 5 is used as an input, the frequency control circuit 20 that controls the frequency by using the output of the pulse generator 5 as a trigger, and the output of the frequency control circuit 20 is used as an input, and the frequency is controlled according to the output signal of the frequency control circuit 20. Is provided with a voltage controlled oscillator 21 for outputting an output signal, and the frequency comparison circuit 12 inputs the output of the voltage controlled oscillator 21 and the output of the photodetector 11 to determine the frequency and voltage of the output signal of the photodetector 11. It outputs to the 2nd light source 8 so that the frequency of the output signal of the control oscillator 21 may correspond.

[0009]

The structure of the optical pulse tester according to the present invention is shown in FIG. In FIG. 1, 3 is an optical branching device provided between the light source 2 and the optical switch 4, 9 is an optical branching device provided between the light source 8 and the optical multiplexer 14, and 10 is an optical branching device 3 and the optical branching device 9. An optical multiplexer for multiplexing the branched lights, 11 is a photodetector, 12 is a frequency comparison circuit, 13 is a signal generator, and others are the same as in FIG.

Next, the operation of the optical pulse tester according to the present invention will be described with reference to FIG. The frequency control circuit 1 outputs a signal to the light source 2 so as to change the frequency continuously or stepwise. The light source 2 outputs coherent light having an optical frequency according to the signal from the frequency control circuit 1. The optical splitter 3 splits the output light of the light source 2. The optical switch 4 pulse-modulates one light of the optical branching device 3 according to the signal from the pulse generator 5, and outputs pulsed light. The optical branching device 6 inputs the pulsed light from the optical switch 4 into the optical fiber 7 and branches the backscattered light from the optical fiber 7.

The light source 8 outputs coherent light having an optical frequency according to the signal from the frequency comparison circuit 12. The optical splitter 9 splits the output light of the light source 8. The optical multiplexer 10 multiplexes the other light split by the optical splitter 3 and the one light split by the optical splitter 9. The photodetector 11 detects the light multiplexed by the optical multiplexer 10 and outputs a signal according to the frequency difference between the light from the light source 2 and the light from the light source 8.

The signal generator 13 outputs the same frequency as the center frequency f _IF of the pass band of the BPF 17, and the light source 2 and the light source 8
Is the optical frequency difference of f _IF . The frequency comparison circuit 12 compares the frequencies of the output signals of the photodetector 11 and the signal generator 13 to reduce the frequency difference between the output signals of the photodetector 11 and the signal generator 13 of the light source 8. A signal that changes the optical frequency in such a direction is output.

The optical multiplexer 14 multiplexes the backscattered light from the optical splitter 6 and the other one of the lights of the optical splitter 9. Photo detector 1
Reference numeral 5 detects the light from the optical multiplexer 14, and outputs a signal having the optical frequency difference between the light source 2 and the light source 8 as the carrier frequency. BPF1
Reference numeral 7 limits the output signal of the photodetector 15 amplified by the amplifier 16 to a bandwidth Δf. The data processing unit 18 is BPF1.
The signal band-limited in 7 is subjected to A / D conversion and averaging processing. The display unit 19 displays the result of the data processing unit 18.

Next, FIG. 2 shows the relationship between the optical frequencies of the light source 2 and the light source 8 of FIG. 2A shows a waveform when the optical frequencies of the light source 2 and the light source 8 are continuously changed by the frequency control circuit 1, and a is a waveform when the optical frequency of the light source 2 is changed stepwise. is there. Δt1 in FIG. 2A is the measurement time of the optical fiber 7 per pulse, or may be the measurement time of several pulses. Further, the relationship between the optical frequencies may be either A or A in FIG.

Next, the structure according to claim 2 is shown in FIG.
The operation will be described. In FIG. 3, the signal generator 13 of FIG. 1 is replaced with a voltage controlled oscillator 21 (hereinafter referred to as VCO), and a frequency control circuit 20 is added. The frequency control circuit 20 inputs the output signal of the pulse generator 5 and changes the output signal stepwise by using the signal from the pulse generator 5 as a trigger. The VCO 21 outputs a frequency according to the output signal of the frequency control circuit 20. The frequency of the output signal of the VCO 21 is a frequency that generates an optical frequency of the output light of the light source 8 that differs by f _IF from the optical frequency of the light source 2 at the time when the optical switch 4 performs pulse modulation.

Next, the relationship between the optical frequency of the light source 2 and the optical frequency of the light source 8 shown in FIG. 3 will be described with reference to FIG. 4A is the optical frequency of the light source 2, b is the optical frequency of the light source 8, c is the optical frequency of the backscattered light from the optical fiber 7, and Δt2 is the measurement time of the optical fiber 7 per pulse. As shown in FIG. 3, by holding the optical frequency of the light source 8 for a time Δt2, it is possible to maintain the difference between the backscattered light of the optical fiber 7 and the optical frequency of the light source 8 at f _IF .

[0017]

FIG. 5 shows the construction of an embodiment of the frequency control circuit 1 of FIG. 1, and FIG. 6 shows the construction of the frequency comparison circuit 12. Further, FIG. 7 shows a configuration of an embodiment of the frequency control circuit 20 of FIG. In FIG. 5, the light source 2 shows a semiconductor laser whose temperature is controlled, but other than this, a semiconductor laser having an external resonator, a solid-state laser, or the like may be used. Also,
In FIG. 5, the frequency control circuit 1 is a circuit that controls the current flowing through the Peltier device in order to change the temperature of the light source 2. However, if the light source 2 uses an external resonator, the external resonator length or diffraction A circuit that changes the filter characteristics such as a lattice may be used.

FIG. 6 shows an example in which a photodetector 11 using a photodiode, a frequency comparison circuit 12 combining an amplifier, a mixer and a low pass filter (LPF), and a signal generator 13 use a synthesizer. A phase comparator may be used instead of the mixer. In addition, in FIG.
An example of the operation shown in FIG.

[0019]

According to the present invention, in an optical pulse tester for measuring loss and break position of an optical fiber by using optical heterodyne light reception, output lights of two light sources are combined and detected. Then, the frequency difference between the two light sources is obtained, frequency comparison is performed with a signal having the same frequency as the center frequency of the pass band of the BPF, and the frequency difference between the two light sources is stabilized to obtain the optical frequencies of the light sources 2 and 8. The difference is f _IF , that is, the center frequency of the pass band of BPF 17, and
The frequency of the signal does not deviate from the pass band width Δf of 17, and a wide dynamic range and distortion can be reduced.

[Brief description of drawings]

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

FIG. 2 is a diagram showing a relationship between optical frequencies of a light source 2 and a light source 8 in the configuration of FIG.

FIG. 3 is a configuration diagram of an optical pulse tester according to a second invention.

FIG. 4 is a diagram showing a relationship between optical frequencies of a light source 2 and a light source 8 in the configuration of FIG.

FIG. 5 is an embodiment of the frequency control circuit 1 and the light source 2 of the optical pulse tester according to the first invention.

FIG. 6 is a photodetector 1 of the optical pulse tester according to the first invention.
1 is an embodiment of the frequency comparison circuit 12 and the signal generator 13.

FIG. 7 is an embodiment of the frequency control circuit 20 of the optical pulse tester according to the second invention.

FIG. 8 is a block diagram of a conventional optical pulse tester.

FIG. 9 is an example of a measured waveform according to the related art.

[Explanation of symbols]

 1 frequency control circuit 2 light source 3 optical branching device 4 optical switch 5 pulse generator 6 optical branching device 7 optical fiber 8 light source 9 optical branching device 10 optical multiplexer 11 photodetector 12 frequency comparison circuit 13 signal generator 14 optical multiplexer 15 Photodetector 16 Amplifier 17 BPF 18 Data Processing Section 19 Display Section 20 Frequency Control Circuit 21 VCO

Claims (2)

[Claims] 1. A frequency control circuit (1) for generating a signal for controlling an optical frequency, and a first light source (1) for outputting coherent light by changing an optical frequency with an output signal of the frequency control circuit (1) as an input. 2)
And a first optical branching device (3) for branching the output light of the first light source (2)
And one of the branched lights of the first optical branching device (3) to the pulse generator (5)
The optical switch (4) that pulse-modulates with the signal from and the pulsed light that is pulse-modulated by the optical switch (4) is transmitted.
A second optical branching device (6) that splits the backscattered light from the optical fiber (7) into the optical fiber (7) and the output signal of the frequency comparison circuit (12) are used as input to change the optical frequency. , A second light source that outputs coherent light (8)
And a third optical splitter (9) for splitting the output light of the second light source (8)
And a first optical multiplexer (10) for multiplexing the other branched light of the first optical branching device (3) and the one branched light of the third optical branching device (9)
A first photodetector (11) for detecting the light multiplexed by the first optical multiplexer (10), and an output signal of the first photodetector (11) as a first input, A frequency comparator circuit (12) that uses the output signal of the generator (13) as the second input signal, a signal generator (13) that outputs the same frequency as the pass frequency of the bandpass filter (17), and a second Light split by the optical splitter (6) and the third optical splitter (9)
A second optical multiplexer (14) for combining the light branched by the second optical detector, a second photodetector (15) for detecting the light combined by the second optical multiplexer (14), and a second optical detector (15) An amplifier (16) that amplifies the output of the photodetector (15) and a bandpass filter (1 that limits the output of the amplifier (16).
7), a data processing unit (18) that calculates the output of the band pass filter (17), and a display unit (1) that displays the data processed by the data processing unit (18).
9), and the frequency comparison circuit (12) is arranged so that the frequency of the output signal of the first photodetector (11) matches the frequency of the output signal of the signal generator (13). ) Is output to the optical pulse tester. 2. A frequency control circuit (20) for controlling the frequency by using the output of the pulse generator (5) as an input and the output of the pulse generator (5) as a trigger, and the output of the frequency control circuit (20) as an input And frequency control circuit
A frequency controlled oscillator (21) is provided which outputs an output signal whose frequency is changed according to the output signal of (20).
The output of the photodetector (11) is input to the first photodetector (11)
The optical pulse tester according to claim 1, wherein the optical pulse tester outputs the signal to the second light source (8) so that the frequency of the output signal of the voltage control oscillator and the frequency of the output signal of the voltage controlled oscillator (21) match.