JPH0798264A - Low-coherent reflectometer - Google Patents

Abstract

PURPOSE:To make it possible to perform balanced detection with a simple constitution in a low-coherence reflectometer for measuring the distribution of the reflection in an optical waveguide at high spatial resolution by utilizing the low-coherent optical interfrence. CONSTITUTION:In a photocoupler 20, at least three input/output ports 21-23 and at least two input/output ports 24 and 26 are so provided as to face each other. This photocoupler 20 is used. A light source 1 is optically connected to the input/output port 21. An optical waveguide 7 is provided at the input/ output port 24. A reflecting mirror 6 is provided at the input/output port 26 through a collimater lens 5. The difference in optical intensities at the input/ output ports 9 and 13 is provided.

Description

Detailed Description of the Invention

[0001]

The present invention is used in the inspection of optical waveguides. In particular, a reflectometer that measures the reflection distribution of an optical waveguide with high spatial resolution by using low coherent light interference,
Low coherence reflectometer (OLCR)
Regarding

[0002]

2. Description of the Related Art FIG. 4 is a diagram showing a basic structure of a low coherence reflectometer. In this configuration, a light source 1 having an emission spectrum width of 10 nm and an optical coupler 2 having two inputs and two outputs (2 × 2) are provided. As the optical coupler 2, for example, an optical fiber coupler is used. The light emitted from the light source 1 is incident on one port of the optical coupler 2, and a collimator lens 5 and a reflecting mirror 6 are provided on one of the two ports facing the port, that is, the input / output port 3.
The optical waveguide 7 to be tested is optically connected to the other of the two ports, that is, the input / output port 4, which faces the port on which the light emitted from the light source 1 enters. A photodetector 9 is provided at the other port 8 on the side on which the light emitted from the light source 1 enters,
It is connected to the level meter 10.

Light emitted from the light source 1 is divided into two by an optical coupler 2. One of the two halves is used as local oscillation light, passes through the input / output port 3, becomes parallel light by the collimator lens 5, is reflected by the reflecting mirror 6, and then enters the optical coupler 2 again. The other half is passed through the input / output port 4 which is a probe port and is incident on the optical waveguide 7 under test. Backscattered light and reflected light are generated by the scattering points and reflection points existing at each point in the optical waveguide 7 under test.
Hereinafter, these lights will be simply referred to as "reflected lights". This reflected light passes through the input / output port 4 and is combined with the local oscillation light at the input / output port 8. The obtained combined light is received by the photodetector 9, and the interference component thereof is measured by the level meter 10. This measurement is repeated by moving the reflecting mirror 6.
That is, the reflecting mirror 6 is moved at a constant speed, and the interference intensity with respect to each position of the reflecting mirror 6 is detected by the level meter 10.

The coherent length of the light emitted from the light source 1 is 50 μm.
Since it is about m, only the reflected light whose optical path length is matched with the local oscillation light within 50 μm interferes with the local oscillation light at each position of the reflecting mirror 6. Therefore, each position of the reflecting mirror 6 and each scattering point of the optical waveguide 7 under test have a one-to-one correspondence. At this time, the photocurrent I ₁ generated in the photodetector 9 is expressed by the following equation.

[0005]

[Equation 1] Here, I ₃ is the photocurrent of the reflected signal (proportional to the reflection intensity) generated at a specific reflection point in the optical waveguide 7 under test, I ₄ is the photocurrent of the locally oscillated light, C ₁ is a constant, and f is the reflection. The Doppler frequency displacement caused by the constant velocity movement of the mirror 6, t is time. Therefore, the interference intensity measured for that point, that is, 2C ₁ (I ₃ I ₄ ) ^1/2 cos (2π
root mean square value 2C ₁ ² I ₃ I
₄ is proportional to the reflection intensity. Therefore, level meter 1
The output of 0 represents the reflectance at that point. In this way, the distribution of scattering points and reflection points in the optical waveguide 7 under test can be measured by moving the reflecting mirror 6.

In this reflectometer, as the light source 1, a light source having a wide spectrum width which follows the same statistics as the heat radiation light is used. Therefore, the intensity noise of the light source 1 becomes extremely large as compared with the laser light, which becomes a factor that limits the sensitivity. Therefore, a so-called balanced detection method has been conventionally used for the low coherence reflectometer. Such a conventional example is shown in FIG.

The conventional example shown in FIG. 5 is greatly different from the basic configuration shown in FIG. 4 in that an optical coupler 12 having two inputs and two outputs and a branching ratio of 1: 1 is added. One input / output port 15 of the optical coupler 12 is optically coupled to the input / output port 8 of the optical coupler 2, and the input / output port 16 on the same side is connected from the input / output port 3 of the optical coupler 2 to the collimator lens 5. The retro-type reflector 11 and the lens 14 are provided so that the light emitted as a light enters the input / output port 16. The optical coupler 2 also has two input / output ports 17 and 18 on the side facing the input / output ports 8 and 16, and the two input / output ports 17 and 18 are provided with photodetectors 9 and 13, respectively. The photodetectors 9 and 13 are connected to the level meter 10 via a differential amplifier 19.

The locally oscillated light that has passed through the input / output port 3 of the optical coupler 2 is collimated by the collimating lens 5 and then reflected by the reflector 11, and is reflected by the lens 14 to the input / output port 16 of the second optical coupler 12. Incident. on the other hand,
The reflected light from the optical waveguide 7 under test is reflected by the first optical coupler 4
After being incident on the input / output port 15 of the second optical coupler 12 from the input / output port 8, the local oscillation light propagated to the input / output port 16 is multiplexed. This combined light is input / output port 1
It is emitted from 7 and 18 and detected by photodetectors 9 and 13, respectively. The photocurrents I ₁ and I ₂ generated in the photodetectors 9 and 13 are input to the differential amplifier 19 and the difference I ₁
-I ₂ is sought. Here, the photocurrents I ₁ and I ₂ are expressed as follows.

[0009]

[Equation 2] Is expressed as Here, C ₂ is a constant. In this equation, the first and second terms on the right side, which are the causes of the intensity noises of the photocurrents I ₁ and I ₂ , are both C ₂ (I ₃ + I ₄ ) and are common, but they are interference components to be detected. The signs of the third term on the right side are opposite to each other. Therefore, the differential amplifier 1
By calculating the difference between the photocurrents I ₁ and I _{2 using} 9, the output is as follows.

[0010]

[Equation 3] Therefore, the interference component can be detected by the level meter 10 and the intensity noise of the light source can be canceled.

[0011]

However, in the conventional example shown in FIG. 5, there is a problem that the structure becomes complicated due to the additional optical coupler and the stability is lowered. Also, since the measurement range is limited to the moving distance of the reflector and is usually about several tens of cm, the length of the port of each optical coupler is set so that the reflected light at the measurement point and the local oscillation light can interfere with the movement of the reflector. There was a problem that two optical couplers had to be connected while adjusting.

An object of the present invention is to provide a low coherence reflectometer capable of solving such problems and performing balanced detection with a simple structure.

[0013]

A low coherence reflectometer according to the present invention splits a light source and light emitted from the light source, and makes one of the split light beams incident on an optical waveguide under test. In a low coherence reflectometer comprising optical means for combining the reflected light generated inside into another branched light, and light detecting means for detecting the intensity of the light combined by this optical means, at least the optical means An optical coupler provided with three input / output ports and at least two input / output ports facing each other, and a light source is optically connected to a first port of the at least three input / output ports, and at least two input / output ports. An optical waveguide under test is optically connected to the first port of the ports, and light reflecting means is optically connected to the second port, and the light detecting means is at least three input / output ports. Characterized in that it comprises a means for determining the difference in light intensity in the second port and the third port.

[0014]

The two-input, two-output light conventionally used in the low coherence reflectometer for measuring the reflection distribution in the optical waveguide under test with a high spatial resolution by using a light source having a wide spectral width and an interferometer. M instead of coupler
An optical coupler with input N output (M ≧ 3, N ≧ 2) is used. This makes it possible to realize a reflectometer capable of balanced detection with one optical coupler.

[0015]

1 is a block diagram showing a low coherence reflectometer according to an embodiment of the present invention. In this embodiment, the light source 1 and the light emitted from the light source 1 are branched, one of the branched light beams is incident on the optical waveguide 7 under test, and the reflected light generated in the optical waveguide 7 under test is separated. An optical coupler 20, a collimator lens 5, and a reflecting mirror 6 are provided as optical means for combining the branched light, and photodetectors 9, 13 are provided as light detecting means for detecting the intensity of the light combined by the optical coupler 20. . A feature of this embodiment is that the optical coupler 20 has at least three input / output ports 21, 22, 23 and at least two, and in this embodiment, three input / output ports 24, 25, 26. Are provided so as to face each other, the light source 1 is optically connected to the input / output port 21, the optical waveguide 7 to be tested is provided at the input / output port 24, and the collimator lens 5 and the reflecting mirror as light reflecting means are provided at the input / output port 26. 6 is optically connected, and a differential amplifier 19 and a level meter 10 are provided as means for obtaining a difference in light intensity at the input / output ports 22 and 23.

Light emitted from the light source 1 enters the 3 × 3 optical coupler 20 from the input / output port 21, is divided into three equal powers, and is output from the input / output ports 24 to 26, respectively. Of these ports, the input / output port 24 is connected to the optical waveguide 7 under test as a probe port, like the port 4 in the basic configuration shown in FIG. The input / output port 26 is used as a port for local oscillation light, similarly to the port 3 in the basic configuration shown in FIG. The input / output port 25 is unused. Here, in this apparatus, since the reflection of the portion beyond the probe port is measured, the length of the input / output port 25 is made shorter than that of the input / output port 24, and the end face of the input / output port 25 is coated with a non-reflective coating. Every time, the reflected light from the input / output port 25 (the backward Rayleigh scattered light in this port and the end face reflection at the emitting end) is prevented from mixing with the signal (reflected light) from the optical waveguide 7 under test. The reflected light from the optical waveguide 7 under test and the local oscillation light are combined by the optical coupler 20, and the combined light passes through the input / output ports 22 and 23 and is received by the two photodetectors 9 and 13. .

The reflected light and the locally oscillated light are combined by the 3 × 3 optical coupler 20 which divides the optical power into three equal parts and the input / output port 2
When exiting from Nos. 2 and 23, the phase difference deviates from each other by 2π / 3. Therefore, the photocurrent output from the photodetectors 9 and 13 is expressed by the following equation.

[0018]

[Equation 4] Is expressed as Here, C ₃ is a constant. Ie 3x
In the case of using the optical coupler 20 of three configurations, in I ₁ and I ₂ , the first term and the second term on the right-hand side, which are the causes of the intensity noise, are both C ₃ (I ₃ + I ₄ ) but common. , The phases of the interference components of the third term on the right side are shifted from each other by 2π / 3. Therefore, the output from the differential amplifier 19 is expressed by the following equation.

[0019]

[Equation 5] Therefore, the interference component can be detected by the level meter 10, and the intensity noise of the light source 1 is canceled.

FIGS. 2 and 3 show the results of measuring the reflection distribution of the silica type optical waveguide using the embodiment shown in FIG.
FIG. 2 shows a signal when the output is detected by only one photodetector 9, and FIG. 3 shows a signal when the balance detection using the two photodetectors 9 and 13 is performed. The length of the optical waveguide under test is 10 cm, and the peaks at 0 cm and 10 cm represent Fresnel reflection at both ends of the waveguide. As shown in FIG. 2, when the number of photodetectors is only one, the reflection distribution cannot be measured due to the intensity noise of the light source, but as shown in FIG. It was possible to clearly observe the reflection distribution in the waveguide at a point up to 10 cm, and it was confirmed that balanced detection was effectively performed.

In the above embodiments, an example using a 3 × 3 optical coupler is shown, but the present invention may be an optical coupler having an M × N structure for an integer M of 3 or more and an integer N of 2 or more. Can be implemented in the same way. However, only three of the M ports and two of the N ports are used, and the remaining ports of the N ports have a length shorter than that of the probe port and their end faces. Is given a non-reflective coat,
It is necessary to prevent the backward Rayleigh scattering and the end-face reflected light generated at these ports from mixing in the signal.

[0022]

As described above, the low coherence reflectometer of the present invention enables balanced detection with one optical coupler, so that the structure is simplified and stability is increased as compared with the conventional one. Since there is no connection work between the optical couplers and the adjustment of the length of the optical fiber in the optical couplers is simplified and the manufacturing process is simplified, there is an effect that a highly stable device can be manufactured at a lower cost.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a low coherence reflectometer according to an embodiment of the present invention.

FIG. 2 is a diagram showing a measurement example of a reflection distribution, and a diagram showing a result of measurement with only one photodetector.

FIG. 3 is a diagram showing an example of measurement of a reflection distribution, showing a result of balance detection using two photodetectors.

FIG. 4 is a diagram showing a basic configuration of a low coherence reflectometer.

FIG. 5 is a diagram showing a configuration of a conventional example that performs balanced detection.

[Explanation of symbols]

 1 Light source 2, 12, 20 Optical coupler 3, 4, 8, 15-18, 21-26 Input / output port 5 Collimating lens 6 Reflector 7 Optical waveguide under test 9, 13 Photodetector 10 Level meter 14 Lens 19 Differential amplifier

Claims (1)

[Claims] 1. A light source and light emitted from this light source are branched, one of the branched lights is incident on an optical waveguide under test, and the reflected light generated in the optical waveguide under test is combined with another branched light. A low coherence reflectometer comprising optical means for oscillating and light detecting means for detecting the intensity of light combined by the optical means, wherein the optical means comprises at least three input / output ports and at least two input / output ports. And an optical coupler provided to face each other, the light source is optically connected to the first port of the at least three input / output ports, and the first port of the at least two input / output ports is connected to the first port. Light reflecting means is optically connected to the test optical waveguide and the second port, respectively, and the light detecting means is connected to the second port and the third port of the at least three input / output ports. A low coherence reflectometer characterized by including means for obtaining a difference in light intensity.