JPH075068A - Light frequency region reflection measuring device - Google Patents

Abstract

PURPOSE:To measure a relatively wide distance range with a high distance resolution by dividing a reference light into two light waves and then giving frequency shift and delay to one of them and then performing multiple wave- synthesis again. CONSTITUTION:Reference light which is branched by a light directional coupler 3 is divided in 50:50 by light directional couplers 6, 9, and 12 and then frequency shift and delay light path differences L, 2L, and 4L are given by light frequency shifters 7, 10, and 13 and light delay lines 8, 11, and 14, respectively, at each stage. Two light waves through different light paths are synthesized and divided again by the light directional couplers 9 and 12 at each time, finally are synthesized by a light directional coupler 15, and then are synthesized 5 with the reflection signal light from optical parts 20 to be measured. Therefore, by performing superposition while shifting the light frequency and the delay light path difference L of reference light, the light path time difference between the measurement points of the parts 20 is reduced to obtain an interference signal with a sufficient intensity and hence a measurement region can be distinguished since frequency differs. Therefore, a wide distance range can be measured while maintaining a high distance resolution.

Description

Detailed Description of the Invention

[0001]

The present invention is used in the inspection of optical parts. In particular, the present invention relates to an optical frequency domain reflectometer capable of measuring the intensity distribution of reflected light or backscattered light from an optical fiber or other optical component with a high distance resolution over a relatively wide range.

[0002]

2. Description of the Related Art As a distributed optical sensor capable of measuring the intensity distribution of reflected light and backscattered light from an optical fiber and other optical components with high distance resolution, coherent optical frequency domain reflectometry (C-OFDR: Coherent-Optical
Frequency Domain Reflectometry) devices are known. In C-OFDR, the intensity distribution of reflected light or backscattered light in the frequency domain can be measured by using coherent laser light whose frequency is repeatedly swept linearly with respect to time.

FIG. 6 shows the configuration of a conventional C-OFDR device. The light source 61 has a variable emission frequency, and is controlled so that the frequency changes linearly with the passage of time to obtain coherent light whose frequency is swept repeatedly. The coherent light is demultiplexed into the signal light and the reference light by the optical demultiplexer 62, and the signal light is incident on the measured optical component 64 via the optical multiplexer / demultiplexer 63. The reflected signal light from the measured optical component 64 is extracted by the optical multiplexer / demultiplexer 63. The reflected signal light includes the end surface reflected light of the measured optical component 64 and the backscattered light generated inside. This reflected signal light is combined with the reference light from the optical demultiplexer 62 using the reflecting mirror 65 and the optical multiplexer 66. The combined light wave is heterodyne detected by the heterodyne receiver 67, and the frequency difference between the reflected signal light from the measured optical component 64 and the reference light is analyzed by the spectrum analyzer 68 and the signal processing device 69. Since the frequency difference at this time is proportional to the delay time between the return light and the reference light, the light intensity distribution of the reflected signal light can be measured in the frequency domain. Further, if the reception bandwidth is set narrow, a high range resolution can be obtained, and measurement can be performed with low noise.

[0004]

However, the conventional C-O
The FDR device has a problem that an interference signal having a sufficient intensity can be obtained only with respect to the optical path time difference within the coherence range of two light waves, and the measurable range at one time is limited. This is a problem particularly when a distributed feedback laser diode (hereinafter referred to as "DFB laser") having excellent high-speed frequency sweeping property is used as a light source, and its measurable range is limited to about 10 m.

An object of the present invention is to solve such problems and to provide an optical frequency domain reflectometry apparatus having a high distance resolution in a relatively wide range.

[0006]

The optical frequency domain reflectometry device of the present invention comprises a third optical means for demultiplexing the reference light into two light waves on the optical path of the reference light, and one of the two light waves. And a fourth optical means for frequency-shifting and delaying and re-multiplexing.

An optical circuit comprising third optical means and fourth optical means is provided in multiple stages, and the frequency shift amount by the optical circuit of the nth stage (n = 1, 2, ...) Is 2 ^n-1 f, Delay optical path difference is 2
It is desirable to set it to ^n-1 L.

[0008]

The reference light is demultiplexed into two light waves, one of which is frequency-shifted and delayed to be recombined. If the optical frequency of the reference light before the frequency shift is f ₀ , the frequency shift is f, and the delayed optical path difference that gives the delay is L, then after the delay time of the delayed optical path length difference L from the reference light of the optical frequency f ₀ , ,
Reference light of optical frequency f ₀ + f is obtained. Using this reference light, measurement is performed in a region separated by the optical path length L from the range measured by the reference light having the optical frequency f ₀ . Since the optical frequencies are different, the regions can be distinguished from each other for measurement, and the reference light is delayed, so that the optical path time difference between the reference light and the measurement point of the optical component to be measured can be reduced and an interference signal of sufficient intensity can be obtained. Therefore, it is possible to perform measurement over a relatively wide distance range while maintaining high distance resolution.

By branching, frequency shifting, delaying, and combining the reference light in multiple stages, the measurable distance range can be further expanded.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the configuration of an optical frequency domain reflectometer according to a first embodiment of the present invention.

This optical frequency domain reflectometer comprises a laser light source 1 and an external control circuit 2 as light emitting means for generating coherent light whose optical frequency changes with time, and outputs the output light of this laser light source 1 as signal light. And an optical directional coupler 3 as a first optical means for demultiplexing into the reference light,
The optical directional couplers 4 and 5 are provided as second optical means for entering the signal light into the measured optical component 20 and combining the reflected signal light from the measured optical component 20 with the reference light. A heterodyne receiver 16, a spectrum analyzer 17, and a signal processing device 18 are provided as detection analysis means for converting the converted light wave into an electric signal for analysis.

The feature of this embodiment is that an optical directional coupler 6 is provided on the optical path of the reference light as a third optical means for demultiplexing the reference light into two light waves. The optical frequency shifter 7, the optical delay line 8 and the optical directional coupler 9 are provided as the fourth optical means for giving a frequency shift and a delay to one of them and multiplexing them again. Further, this embodiment includes three stages of optical circuits each including an optical directional coupler, an optical frequency shifter, an optical delay line and an optical directional coupler. That is, an optical circuit including the optical directional coupler 9, the optical frequency shifter 10, the optical delay line 11, and the optical directional coupler 12, which share the optical directional coupler 9 with the first-stage optical circuit, and the optical direction. The optical circuit includes the optical directional coupler 12, the optical frequency shifter 13, the optical delay line 14, and the optical directional coupler 15 that share the directional coupler 12 with the second-stage optical circuit. The optical frequency shifters 7, 10 and 13 have frequency shift amounts of f and 2, respectively.
f, 4f, and the optical delay lines 8, 11, and 14 have delay optical path differences L, 2L, and 4L of the two demultiplexed light waves, respectively.
Is set to be

The laser light source 1 is capable of repeating frequency sweeping, and under the control of the external control circuit 2, generates a coherent laser light whose frequency is swept linearly with respect to time. This laser light is demultiplexed into a signal light and a reference light by the optical directional coupler 3. The signal light enters the optical component 20 to be measured through the optical directional coupler 4, and is reflected or backward Rayleigh scattered. The reflected signal light is taken out by the optical directional coupler 4 and is combined by the optical directional coupler 5 with the reference light demultiplexed in advance. The combined light is received by the heterodyne receiver 16, and the frequency difference between the reflected signal light and the reference light is measured as the frequency of the heterodyne beat signal. At this time, the frequency difference is proportional to the optical path length difference (delay time difference) between the reflected signal light and the reference light. Therefore, by analyzing the frequency of the output electric signal by the spectrum analyzer 17 and the signal processing device 18, the reflected signal light intensity distribution can be obtained. It can be measured in the frequency domain.

However, since the heterodyne beat signal can be obtained only when the difference in optical path length between the reflected signal light and the reference light is short due to the coherence length, the conventional optical frequency domain reflectometry device can measure the measurable range. It was limited to a range shorter than the interference distance. For example, when a DFB laser with a line width of 1 MHz is used, the strength of the heterodyne beat signal decreases due to a decrease in coherence when the optical path length difference increases, and when the optical path length difference is 20 m, the optical path length difference is zero compared to the case where the optical path length difference is zero. About 3d
B decreases.

Therefore, in the present embodiment, the reference light is simultaneously multiplexed with respect to the frequency and the delay time difference,
It compensates for the decrease in the heterodyne beat signal and greatly expands the measurement range. Specifically, frequency multiplexing and delay time multiplexing are realized by connecting an optical circuit including an optical directional coupler, an optical frequency shifter, and an optical delay line in multiple stages to pass the reference light.

The reference light demultiplexed by the optical directional coupler 3 is demultiplexed into 50:50 by the first stage optical directional coupler 6. In one of the optical paths, an optical frequency shifter 7 in the first stage which gives a frequency shift f and an optical delay line 8 which gives a delay optical path difference L are inserted. The frequency shift amount f and the delay optical path difference L are hereinafter referred to as "unit frequency shift amount" and "unit optical path difference". Two light waves that have passed through different optical paths are combined and re-demultiplexed by the second-stage optical directional coupler 9 having a branching ratio of 50:50. In the one optical path that was re-demultiplexed,
Second-stage optical frequency shifter 10 that gives a frequency shift 2f
And the optical delay line 11 of the second stage which gives the delay optical path difference 2L is inserted. Furthermore, two light waves that have passed through different optical paths are combined into a third-stage optical directional coupler 1 having a demultiplexing ratio of 50:50.
It is multiplexed and re-demultiplexed by 2. The third optical frequency shifter 13 for providing the frequency shift 4f and the third optical delay line 14 for providing the delay optical path length difference 4L are inserted in the one optical path re-demultiplexed here. The two light waves that have passed through different optical paths are combined by the optical directional coupler 15 and are combined by the optical directional coupler 5 with the reflected signal light from the measured optical component 20.

FIG. 2 shows an optical directional coupler, an optical frequency shifter,
An example of a configuration in which an optical circuit including an optical delay line and an optical directional coupler is further connected in multiple stages is shown. An optical directional coupler having a branching ratio of 50:50 is used, and the optical directional coupler on the input side of each optical circuit is shared with the preceding optical directional coupler.
Further, the frequency shift amount of the frequency shifter in the optical circuit of the nth stage (n = 1, 2, ...) Is set to 2 ^n-1 f and the delay optical path difference due to the optical delay line is set to 2 ^n-1 L. This way,
2 ^n- fold reference light frequency multiplexing and delay optical path difference multiplexing are possible. FIG. 3 shows the relationship between the frequency difference of the reference light and the delay optical path length difference.

FIG. 4 shows an example of the intensity distribution measurement waveform of the backscattered light. The frequency sweep rate of the laser light source 1 is adjusted so that the unit delay optical path difference L corresponds to about twice the coherence length of the laser light, and the unit frequency shift amount f corresponds to the beat frequency difference corresponding to the delay optical path difference L. Set. In this way, each interference signal deteriorates due to an increase in the delay optical path length difference, but at the point where the delay optical path length difference is reduced to about half, it overlaps with the interference signal due to the reference light in which the delay optical path length difference deviates from each other by L or −L. Since they are matched, it is possible to avoid a decrease in the strength of the entire interference signal. Therefore, even if a light source having a relatively short coherence length L is used, the measurable range can be expanded to the range of 2 ⁿ L by passing the reference light through the n-stage composite optical circuit.

The embodiment shown in FIG. 1 will be described more specifically. The laser light source 1 has a line width of 1 MHz, for example.
A DFB laser with a line width of (coherence length 25 m) is used.
The optical frequency of the DFB laser can be controlled by changing the injection current. The optical frequency shifters 7, 10, and 13 have, for example, frequency shift amounts of 50 MHz and 100 MH, respectively.
An acousto-optic frequency shifter of z, 200 MHz is used. The lengths of the optical delay lines 8, 11, 14 are 50, 100, respectively.
It will be 200 m. In this way, the optical directional coupler 1
At the output end of 5, the frequency shift amount is 0, 50, 100,
Eight optical frequencies of 150, 200, 250, 300 and 350 MHz are obtained. The delay optical path differences of the respective reference light components are 0, 50, 100, 150, 20 respectively.
It becomes 0, 250, 300m. Each of the reference light components interferes with the reflected signal light that is reflected and backscattered at different portions in the measured optical component 20, and produces a heterodyne signal.

The frequency allocation amount in the optical frequency domain is 50 MHz / 50 m (= 1 MHz / 1 m = 0.1).
(MHz / 0.1 m) so that the frequency difference corresponding to the reflected signal light from the same point is set to one even if the reference light components that generate the interference signal are different. In this way, the frequency difference F (x) [H
z] is represented by F (x) = Cx + D. Here, C is a proportional coefficient, and in this example, C = 1 MHz / 1 m. D is a constant. Also, the intensity I (x) of the frequency domain signal corresponding to the backscattered light distribution
Is I (x) = aP _ref P _sig (x) Σ exp [-4π (Δf / 2V _g ) | x
-JL |]. Where, a proportional coefficient in photoelectric conversion, P
_ref and P _sig (x) are the intensities of the reference light and the reflected signal light, Δf is the line width of the light source, V _g is the group velocity of the light, and L is the unit delay optical path difference. Σ represents the total sum of j = 0 to 2 ² −1.
If this is left as it is, the exponential change in intensity will overlap the measured waveform. When this is corrected, the signal intensity I ′ (x) in the wave number domain is I ′ (x) = aP _ref P _sig (x) Σ exp [−4π (Δf / 2V _g ) |
x−jL |] ÷ Σ exp [−4π (Δf / 2V _g ) | x−jL |]. Σ in this equation also represents the sum of j = 0 to 2 ² −1.

FIG. 5 shows the construction of the optical frequency domain reflectometry apparatus of the second embodiment of the present invention. In the first embodiment shown in FIG. 1, the optical frequency shifters 7, 10, 13 and the optical delay line 8,
It has been described that the losses due to 11 and 14 can be ignored. If this loss cannot be ignored, the optical attenuators 51, 52, and 53 are inserted in the other optical path, and the light passing through the two optical paths is adjusted so as to have equal intensities. The two output ports of each of the optical directional couplers 9 and 12 are
The light waves that have passed through the two paths are output at an intensity ratio of 1: 1.

In the above description, an example in which an optical directional coupler is used for multiplexing and demultiplexing light has been shown, but the present invention can be similarly implemented by using other optical elements.

[0023]

As described above, the optical frequency domain reflectometry apparatus of the present invention shifts and superimposes the optical frequency of the reference light and the delay optical path difference so that the optical frequency domain reflection measurement apparatus and the measurement point of the measured optical component are overlapped. The optical path time difference can be reduced to obtain an interference signal of sufficient intensity, and the measurement areas can be distinguished by using different optical frequencies. Therefore, it is possible to perform measurement over a relatively wide distance range while maintaining high distance resolution.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an optical frequency domain reflectometer according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a configuration example in which an optical circuit including an optical directional coupler, an optical frequency shifter, an optical delay line, and an optical directional coupler is connected in multiple stages.

FIG. 3 is a diagram showing a relationship between a frequency difference of reference light and a delay optical path length difference.

FIG. 4 is a diagram showing an example of an intensity distribution measurement waveform of backscattered light.

FIG. 5 is a diagram showing a configuration of an optical frequency domain reflectometer according to a second embodiment of the present invention.

FIG. 6 is a diagram showing a configuration of a conventional example.

[Explanation of symbols]

 1 laser light source 2 external control circuit 3, 4, 5, 6, 9, 12, 15 optical directional coupler 7, 10, 13 optical frequency shifter 8, 11, 14 optical delay line 16, 67 heterodyne receiver 17, 68 Spectrum analyzer 18, 69 Signal processing device 20, 64 Optical component under test 51, 52, 53 Optical attenuator 61 Light source 62 Optical demultiplexer 63 Optical multiplexer / demultiplexer 65 Reflector 66 Optical multiplexer

Claims (2)

[Claims] 1. A light emitting means for generating coherent light whose optical frequency changes with time; a first optical means for demultiplexing the output light of the light emitting means into a signal light and a reference light; and the signal light. And a second optical means for coupling the reflected signal light from the measured optical component with the reference light and for converting the combined light wave into an electric signal for analysis. In the optical frequency domain reflectometry device including means, a third optical means for demultiplexing the reference light into two light waves on the optical path of the reference light, and a frequency shift and a delay for one of the two light waves. An optical frequency domain reflectometry apparatus comprising: a fourth optical means for giving and multiplexing again. 2. An optical circuit comprising the third optical means and the fourth optical means is provided in multiple stages, and the frequency shift amount by the optical circuit of the nth stage (n = 1, 2, ...) Is 2 ^n. The optical frequency domain reflectometry device according to claim 1, wherein the optical path difference is set to ^-1 f and the delay optical path difference is set to 2 ^n-1 L.