JPH11218465A - Optical fiber characteristic measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize an optical fiber characteristic measuring device wherein a mutual frequency interval |f1-f2| can be precisely and arbitrarily set and coherent detection corresponding to the frequency component of reflection light contained in return light or various scattering light is performed well. SOLUTION: Since the frequency of one light source out of two light sources is made a reference and the frequency of the other light source can be controlled in response to the fixed reference, the frequencies of optical signals emitted from a first light source 18 and a second light source 19 are automatically corrected into a frequency difference, the frequency difference of the optical signals can be precisely and arbitrarily set, and as a result, coherent detection corresponding to the frequency component of reflection light contained in return light or various scattering light can be performed well.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber characteristic measuring device for measuring various characteristics of an optical fiber.

[0002]

2. Description of the Related Art FIG. 2 is a block diagram showing a configuration of a conventional optical fiber characteristic measuring device. In this figure, a temperature control circuit 1 includes a coherent light 3 emitted from a light source 3.
In order to stabilize the frequency f1 of a, control is performed via the drive circuit 2 to keep the frequency f1 constant. The coherent light 3a emitted from the light source 3 is applied to an optical pulse generation circuit 4
After passing through the optical connector 6, the light passes through the optical directional coupler 5 and enters the optical fiber 7 to be measured via the optical connector 6. When the pulsed light 4a is incident on the measured fiber 7, a reflection or scattering phenomenon occurs according to the state of the fiber, whereby a part of the pulsed light 4a is returned from the measured fiber side as a return light 6a. Come back to 5.

On the other hand, the temperature control circuit 11
Light 13a emitted from the light source 13 through the
Control for stabilizing the frequency f2 of (local light) is performed. The optical directional coupler 15 multiplexes the coherent light 13 a and the return light 6 a from the fiber under test 7 through the optical directional coupler 5, and outputs an optical signal 15 a obtained thereby to the photodetector 16. The light detection unit 16 coherently detects the light signal 15a, converts the light signal 15a into an electric signal 16a, and outputs it. The signal processing unit 17 obtains various characteristics of the measured optical fiber 7 based on the detected electric signal 16a,
6a is processed on the time axis to create these characteristics as a distribution on the distance axis of the optical fiber.

In the optical fiber characteristic measuring device having such a configuration, the relative frequency interval | f1 between the coherent light 3a and the coherent light 13a (local light) according to the frequency components of the reflected light and the scattered light included in the return light 6a. By appropriately setting -f2 |, it is possible to detect return light such as reflected light in addition to backscattered light such as Rayleigh scattered light and Brillouin scattered light generated inside the optical fiber 7 to be measured. I have.

[0005]

In the above-described conventional optical fiber characteristic measuring apparatus, the frequency of each of the light sources 3 and 13 is controlled to detect the return light 6a, and the relative frequency interval | f1-f2 | is appropriately controlled. However, since the frequencies of the light sources 3 and 13 are individually controlled, it is not easy to accurately set the relative frequency interval between the light sources. Therefore, the error of the relative frequency interval | f1-f2 | There is a problem that the detection accuracy is affected.

Accordingly, the present invention has been made in view of such circumstances, and the relative frequency interval | f1-f2 | can be set accurately and arbitrarily, and the relative light interval | f1-f2 | It is an object of the present invention to provide an optical fiber characteristic measuring device capable of satisfactorily performing coherent detection according to a frequency component.

[0007]

In order to achieve the above object, according to the first aspect of the present invention, a first light source and a second light source are provided, and coherent light of the first light source is converted into pulsed light to be applied. An optical fiber characteristic measuring device that emits light to a measurement optical fiber, coherently detects an optical signal multiplexed with return light from the fiber to be measured and coherent light of the second light source, and measures characteristics of the optical fiber to be measured. Calculating means for calculating a relative frequency difference between the frequency component of the first light source and the frequency component of the second light source; calculating a difference between the relative frequency difference and a set value Vref2; First control means for controlling the relative frequency difference by changing the frequency of the coherent light of the first light source; calculating a difference between a frequency component of the second light source and a set value Vref1; A difference between the frequency component of the first light source and the set value Vfr is calculated, and a second control means for changing the frequency of the coherent light of the second light source in accordance with the difference is calculated. And a third control means for changing the frequency of the control signal.

According to the second aspect of the present invention, the set value Vref1 and the set value Vref
2 and set value control means for setting the set value Vfr to an arbitrary value.

According to a third aspect of the present invention, the third control means includes:
It is characterized by comprising coarse adjustment means for setting the frequency of the coherent light of the light source to a predetermined value.

In the present invention, the frequency of one of the two light sources is fixed on the basis of the reference and the frequency of the other light source is controlled in accordance with the fixed reference. The frequencies of the optical signals respectively emitted from the light source and the second light source are automatically calibrated to a certain frequency difference, and the frequency difference between the optical signals can be set accurately and arbitrarily. As a result, the reflected light included in the return light And coherent detection according to the frequency components of various scattered lights.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an optical fiber characteristic measuring device according to one embodiment. In this figure, the same reference numerals are given to the same elements as those in the conventional example shown in FIG. 2, and the description thereof will be omitted. FIG.
Of the coherent light 18a (hereinafter, referred to as first coherent light 18a) emitted from the light source 18 (hereinafter, referred to as first light source) by the temperature control circuit 1 and the driving circuit 34, And optical pulse generation circuit 4
The light branched to the side is converted into pulsed light, passes through the optical directional coupler 5 and the optical connector 6 sequentially, and enters the fiber 7 to be measured. At this time, the return light 6a generated in the measured fiber 7 returns to the optical directional coupler 5.

On the other hand, a coherent 19a (hereinafter, referred to as a second coherent light 1) emitted from a light source 19 (hereinafter, referred to as a second light source) by a temperature control circuit 28 and a driving circuit 36.
9a), the light 21a branched to the optical directional coupler 15 side via the optical directional coupler 21 is multiplexed with the return light 6a, and after being coherently detected by the photodetector 16, In the signal processing section 17, the optical fiber characteristics are measured by various processes as in the conventional case.

When the optical fiber characteristics are measured in this manner, the second coherent light 19a having the frequency f2 emitted from the second light source 19 enters the optical directional coupler 22 via the optical directional coupler 21, and the optical Branch light 22 at directional coupler 22
a and 22b. The branch light 22b is an optical switch O
The light enters the terminal 2 of the SW 29. This optical switch OSW2
9, the frequency component f2 of the second coherent light 19a is converted into a signal 32b through an F / V converter 30 and an amplifier circuit 31, and then the electric switch S
The signal is input to the comparison circuit 35 via W32.

In the comparison circuit 35, the electric switch SW
32 and the set value Vr supplied from the terminal 2 side of the terminal 32
ef1 and the output signal 3 which is the comparison result.
The drive circuit 36 is operated using 5a as a drive signal, and the second
The light source 19 is controlled. Thus, the components 19,
In a processing loop including 21, 22, 29, 30, 31, 32, 35, and 36, the electric signal 32b corresponding to the frequency f2 of the second coherent light 19a is set to the set value Vref1.
, The reference point of the frequency characteristic of the F / V converter 30 is locked, and the optical switch OSW29 and the electric switch SW32 are simultaneously switched to the terminal 1 side.

On the other hand, the first coherent light 18a having the frequency f1 emitted from the first light source 18 is incident on the optical directional coupler 23 via the optical directional coupler 20. The light is split into split lights 23a and 23b.
The split light 23b enters the terminal 1 of the optical switch OSW29. At this point, as described above, the optical switch OSW
29 and the electric switch SW32 are respectively switched to the terminal 1 side, the optical signal 29a emitted from the optical switch OSW29 is transmitted to the F / V converter 30 and the amplifier circuit 3
1, the frequency component f of the first coherent light 18a
1 is converted to an electric signal 32a.

The electric signal 32a is supplied to the electric switch SW32
Is input to the comparison circuit 33 via the terminal 1 of the comparator. In the comparison circuit 33, the electric signal 32b supplied from the terminal 1 side of the electric switch SW32 is compared in magnitude with the set value Vfr, and the driving circuit 34 is operated using the output signal 33a as a driving signal as a comparison result, The first light source 18 is controlled. That is, the components 18, 20, 23, 29,
In the processing loop including 30, 31, 32, 33, and 34, control is performed so that the electric signal 32b corresponding to the frequency f1 of the first coherent light 18a matches the set value Vfr.

Further, the branched lights 22a and 23a branched by the optical directional couplers 22 and 23, respectively, are multiplexed by the optical directional coupler 24 and then enter the photodetector 25.
The photodetector 25 detects a difference | f1−f2 | between the frequency component f1 of the first coherent light 18a and the frequency component f2 of the second coherent light 19a, converts the difference into an electric signal 25a, and outputs the electric signal 25a. . In the comparison circuit 26, the difference |
f1-f2 | and the set value Vre
f2 and the output signal 26 which is the result of the comparison.
By adjusting the output of the adjustment circuit 27 according to a,
The temperature control circuit 1 corrects the frequency f1 of the first light source 18. That is, the frequency f1 of the first coherent light 18a
Can be matched to the frequency f2 of the second coherent light 19a according to the set value Vfr and the set value Vref2.

The frequency f1 of the first coherent light 18a is equal to the frequency f1 of the second coherent light 19a.
In order to obtain the relative frequency with respect to the coarse adjustment signal, the adjustment circuit 2
7, the frequency f1 of the first coherent light 18a is shifted according to the coarse adjustment signal via the temperature control circuit 1, and the optical directional couplers 20, 23 and the optical switch OSW2 are shifted.
9, the frequency component f1 of the first coherent light 18a is converted into an electric signal 32a by the F / V converter 30, the amplifier 31, and the terminal 1 of the electric switch SW32. The electric signal 32a is set by the comparison circuit 33 to the set value Vfr.
In conjunction with, the drive circuit 34 is operated to control the first light source 18. As a result, the center frequency of the optical signals emitted from the two light sources can be fixed to a certain relative frequency difference, and the frequency f1 of the first light source 18 can be set to an arbitrary value from the set value Vfr.

That is, according to the above-described configuration, first, the optical switch OSW29 and the electric switch SWW are used to adjust the initial frequencies of the frequency component f1 of the first coherent light 18a and the frequency component f2 of the second coherent light 19a.
32 is switched to the terminal 2 side at the same time, and of the second coherent light 19a, the split light 22b split by the optical directional coupler 21 and the optical directional coupler 22 undergoes F / V conversion and has an optical frequency component f2. After being converted into the electric signal 32b of
The frequency component f2 of the second coherent light 19a is stabilized based on the set value Vref1 by comparing the set value Vref1 and operating the drive circuit 36 according to the comparison result.

Next, when the frequency component f2 of the first coherent light 19a is stabilized, the reference point of the frequency characteristic of the F / V converter 30 is locked, and the optical switch OSW29 and the electric switch SW32 are simultaneously switched to the terminal 1 side.
Then, of the first coherent light 18a, the split light 23b split by the optical directional coupler 20 and the optical directional coupler 23 is converted into an electric signal 32a of an optical frequency component f1 through F / V conversion. The frequency f1 of the first coherent light 18a is controlled by being compared with the set value Vfr and operating the drive circuit 34 according to the result of the comparison.

Further, the first coherent light 18a passes through the optical directional couplers 21 and 22, and then passes through the second coherent light 1a.
9a passes through the optical directional couplers 20 and 23 and is multiplexed by the optical directional coupler 24. The difference | f1-f2 | of the frequency component of the multiplexed optical signal 24a is detected by the photodetector 25. ,
Through the comparison circuit 26, the adjustment circuit 27 and the temperature control circuit 1,
The frequency of the first light source 18 is adjusted. At this time, since the state of the F / V converter 30 has not changed, if the set value Vfr is made to match the set value Vref1, the first coherent light 1
That is, the frequency component f1 of 8a can be matched with the frequency component f2 of the second coherent light 19a.

Further, in order to obtain a relative frequency difference between the frequency component f1 of the first coherent light 18a and the frequency component f2 of the second coherent light 19a, a coarse adjustment signal is input to an adjustment circuit, and the first coherent light 18a Of the optical directional coupler 2 is shifted by the coarse adjustment signal value.
The frequency component f1 of the first coherent light 18a is converted into an electric signal 32a by the F / V converter 30 and the amplifier 31 via the optical switches OSW29 and OSW29. The electric circuit 32 operates the drive circuit 34 in accordance with the set value Vfr in the comparison circuit 33 to control the first light source 18. As a result, the center frequency of the optical signal emitted from each of the two light sources 18 and 19 can be fixed to a certain relative frequency difference, and the frequency f1 of the first light source 18 becomes an arbitrary value based on the set value Vfr. Can be set.

[0023]

According to the first aspect of the present invention, the coherence of the first light source is determined according to the difference between the relative frequency difference between the frequency component of the first light source and the frequency component of the second light source and the set value Vref2. While controlling the relative frequency difference by changing the frequency of light, the frequency component of the second light source and the set value Vref
The frequency component of the first light source and the set value Vf are changed while changing the frequency of the coherent light of the second light source according to the difference between the first and second light sources.
The frequency of the coherent light of the first light source is changed according to the difference between r and r. That is, of the two light sources, the frequency of one light source is fixed as a reference, and the frequency of the other light source can be controlled in accordance with the fixed reference.
The frequencies of the optical signals respectively emitted from the first light source and the second light source can be automatically calibrated to a certain frequency difference, and the frequency difference between these optical signals can be set accurately and arbitrarily. The coherent detection according to the frequency components of the reflected light and the various scattered lights included in the above can be effectively performed. According to the second aspect of the present invention, since the set value control means for setting the set value of the frequency difference between the first light source and the second light source to an arbitrary value is provided, the first and second coherent lights are provided. Can be set to an arbitrary value with high accuracy, or the frequencies of the first and second coherent lights can be set to predetermined values. Further, according to the third aspect of the present invention, the frequency of the coherent light of the first light source is set to a predetermined value, and the frequency of the coherent light of the first light source is set in accordance with a difference from the set value at the frequency of the coherent light of the first light source. Since the frequency of the coherent light of one light source is adjusted, if the value of the predetermined frequency is appropriately set, the optical frequency difference between the coherent light of the first light source and the second light source can be quickly set. It has the effect of going away.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an optical fiber characteristic measuring device according to an embodiment of the present invention.

FIG. 2 is a diagram for explaining a conventional example.

[Explanation of symbols]

1, 28 Temperature control circuit 34, 36 Drive circuit 18, 19 Light source 4 Optical pulse generation circuit 6 Optical connector 7 Optical fiber to be measured 5, 15, 20, 21, 22, 23, 24 Optical directional coupler 16, 25 Light Detection unit 17 Signal processing unit 26, 33, 35 Comparison circuit 27 Adjustment circuit 29 Optical switch 30 F / V conversion (etalon) 32 Electric switch

Claims (3)

[Claims] A first light source and a second light source, wherein coherent light of the first light source is converted into pulsed light, emitted to an optical fiber to be measured, and returned light from the fiber to be measured and the second light source. An optical fiber characteristic measuring device that coherently detects an optical signal multiplexed with coherent light of a light source and measures characteristics of the optical fiber to be measured, wherein a relative frequency between a frequency component of the first light source and a frequency component of the second light source Calculating means for calculating the difference, calculating a difference between the relative frequency difference and the set value Vref2, and changing the frequency of the coherent light of the first light source according to the difference to control the relative frequency difference. A second control unit that calculates a difference between a frequency component of the second light source and a set value Vref1, and changes a frequency of coherent light of the second light source according to the difference. And a third control means for calculating a difference between the frequency component of the first light source and the set value Vfr, and changing the frequency of the coherent light of the first light source according to the difference. Optical fiber characteristics measuring device. 2. The setting value Vref1 and the setting value Vref
2. The optical fiber characteristic measuring apparatus according to claim 1, further comprising a set value control means for setting the value 2 and the set value Vfr to an arbitrary value. 3. The optical fiber characteristic measurement according to claim 1, wherein said third control means includes a coarse adjustment means for setting a frequency of the coherent light of said first light source to a predetermined value. apparatus.