JPH0961299A - Low coherence reflectometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To allow components caused by residual reflection to be removed and obtain a highly sensitive reflectometer by providing a modulation optic system in an optical delay circuit and also providing a wave detection system corresponding to the modulation in a signal processing system. SOLUTION: A phase modulator 21 whose refractive index periodically varies according to frequency (f) from a transmitter 22 is provided between a lens 13 and a reflecting mirror 15 of an optically variable delay circuit 7. Although a light beam 15 is subjected to phase modulation at a predetermined frequency f1 while it goes to and back from the modulator 21, residual reflection light may not be phase- modulated as it does not pass through the modulator 21. Therefore an interference signal between reference light reflected on the reflecting mirror 15 and back Rayleigh scattered light of a light waveguide 6 which has been phase-modulated at frequency (f) of a transmitter 12 includes a component which varies at frequency of f+f1 , but an interference component of residual reflection light and reflection light varies only at frequency (f). Accordingly, by setting wave detection frequency f+f1 in a signal processing system 11 and adjusting a wave detection band width so that the frequency (f) component may not be mixed, the output caused by residual reflection can be removed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low coherence refractometer for measuring backward Rayleigh scattering distribution from an optical waveguide with high spatial resolution and high sensitivity.

[0002]

2. Description of the Related Art FIG. 5 shows an example of a low coherence reflectometer. That is, this reflectometer
The loss distribution of the optical waveguide itself is measured by measuring the backward Rayleigh scattered light generated in the optical waveguide 6 with a spatial resolution of micrometers. In FIG. 5, the emitted light from the super luminescent diode 1 enters the fiber type 3db coupler 3 from the fiber port 2 and is divided into two. One of the two halves propagates through the fiber port 4 and enters the test optical waveguide 6. The other half is propagated through the fiber port 5 and enters the optical variable delay circuit 7. The return light from the delay circuit 7 propagates again as a reference light through the fiber port 5, and is then combined with the reflected light from the optical waveguide 6 by the fiber type 3 dB coupler 3. The combined light propagates through the fiber port 9 and is received by the photodetector 10, and the signal processing system 11 detects the power of the interference component. As a detection method, a voltage of a ramp waveform of f = 100 Hz is applied by an oscillator 12 to a phase modulator 8 having a structure in which a part of the optical fiber of the fiber port 4 is wound around a cylindrical electrostrictive oscillator. Since the component of frequency f is generated in the interference component between the reflected light from the optical waveguide and the reference light, this is detected by the signal processing system 11.

Reflected light from the optical waveguide 6 includes Fresnel reflected light on both end surfaces and backward Rayleigh scattered light caused by the refractive index randomly varying in the longitudinal direction with a submicron correlation length. It is mainly used for low coherence reflectometer to detect the latter Rayleigh backscattered light. Since the coherence length of the light emitted from the light source 1 is as short as several tens of microns, only the backward Rayleigh scattered light whose optical path length matches the optical path length of the reference light within the coherence length can interfere with the reference light. . That is, a specific point of the optical waveguide 6 is opposed to the optical path length Z given by the delay circuit 7, and the backward Rayleigh scattered light generated in the area of the optical waveguide of the micrometer length which can be made the coherence length around this point is generated. It is detected by this device. The signal S (Z) detected for each optical path length Z is obtained by using the backward Rayleigh scattering coefficient P (Z) generated at that point and the transmittance T (Z) from the waveguide entrance end to that point S (Z). Z) = CP (Z)
It is given by [T (Z)] ² . Here, C is a constant and P (Z) is considered to be constant irrespective of the location of the waveguide. Therefore, the transmittance T (Z) from the detection signal S (Z) to each point Z is obtained. be able to. Here, the propagation loss in dB display is −10lo by taking the common logarithm of T (Z).
It is given by g [T (Z)].

FIG. 6 shows an optical variable delay circuit 7 shown in FIG.
Is shown together with the fiber port 5, and the light emitted from the fiber port 5 becomes a parallel beam 14 which is collimated by the lens 13 and propagates in space. This light beam is reflected by the reflecting mirror 15 and enters the fiber port 5 again by the lens 13. The reflecting mirror 15 is mounted on the stage 16 and can move in a direction parallel to the beam 14.
Therefore, by moving the stage, it is possible to continuously change the optical path length of the reference light that is emitted from the fiber port 5 and is incident on the fiber port 5 again. As a result, the light reflected by the reflecting mirror 15 is combined with the backward Rayleigh scattered light from the optical waveguide 6 as the reference light by the 3 dB coupler 3 and interferes.

FIG. 7 shows another example of the delay circuit 7, which includes reflectors 17 and 18 for changing the direction of a light beam by 180 degrees.
It is composed of reflecting mirrors 19 and 20, and the reflector 17 is fixed on the stage 16. The parallel beam 14 is incident on the reflector 17 by the reflecting mirror 19. The incident light is reflected by the reflector 17 twice, and is 1 against the original incident light.
It is turned by 80 degrees and enters the opposing reflector 18. The incident light, after being reflected twice, changes its direction by 180 degrees and enters the reflector 17. After that, the reflector 17
Heading toward the center of the reflector while undergoing 180 degree turns at 18 and 18, respectively. The light beam directed to the center is reflected by the reflecting mirror 20 installed near the reflector 18, propagates backward through the optical path propagated so far, is reflected by the reflectors 17 and 18 many times, and is then reflected again. Mirror 19
Is reflected by the lens 13 and is incident on the fiber port 5 again by the lens 13. Since the reflector 17 moves on the stage 16 in the optical game direction, the optical path length of the reference light changes. The feature of this delay circuit is that the change amount of the optical path length with respect to the movement of the stage 16 is M = 10 times larger than that of the configuration shown in FIG. Here, M is the number of times the light beam at the moving reflector 17 turns 180 degrees. Therefore, the use of this delay circuit has an advantage that the measurement distance range is expanded ten times as compared with that in FIG.

[0006]

However, in the examples of FIGS. 6 and 7 described above, the light reflected by the exit end face of the fiber port 5 and the lens 13 (hereinafter referred to as residual reflected light).
Of the light reflected on the fiber port 4 and the optical waveguide 6 shown in FIG. 5 that has the same optical path length as the residual reflected light (that point is designated as Z ₀ ). In the signal processing circuit 1
The residual output S ₀ occurs during the output from 1. in this case,
Since the fiber port 5 and the lens 13 do not move even if the stage 16 shown in FIG. 6 moves, the residual output S ₀ is constant and becomes the bias of the output from the signal processing circuit 11.

In this state, the point Z where the waveguide is located₁from
Signal S₁(Z₁) Is very small, S (Z₁)
Is S₀There is a case where you cannot measure because it is hidden in the. example
For example, the residual reflected light and a point Z on the fiber port 4₀Departs at
Considering the case where the generated backward Rayleigh scattered light interferes,
The optical power loss of the reference light in the delay circuit shown in FIG.
B, if the return loss of the residual reflected light is 40 dB,
Z₀From the optical waveguide Z ₁The round trip propagation loss to the point
When it is larger than 37 dB, the signal S (Z₁) <Residual output
Force S₀And the signal S (Z₁) Cannot be measured
The situation occurs. In this case, the rear lari
I Scattering coefficient P (Z) is at point Z₀And Z₁And assume the same
Was.

Further, as shown in FIG. 7, the reference in the delay circuit is made.
A similar problem occurs when the light is attenuated too much. Toes
In order to repeat many reflections between the reflectors, this
The optical power loss of the reference light in the delay circuit is about 20 dB.
On the other hand, the residual reflection still remains unchanged at -40 dB.
Therefore, some point Z on the fiber port 4₀To Z ₁
This point when the round-trip propagation loss up to is more than 20 dB
Z₁Backward Rayleigh scattered signal S from the optical waveguide at
(Z₁) Is a problem that cannot be measured. sand
That is, with the current technology, only one-way propagation loss of 10 dB is possible.
The distribution cannot be measured.

In view of the above-mentioned problems, the present invention eliminates the bias component in order to remove the detection limit of the backward Rayleigh scattered signal due to the bias component generated in the detection signal due to the residual reflection based on the optical component. The purpose of the present invention is to provide a low coherence reflectometer.

[0010]

The present invention which achieves the above object is characterized by the following constitution. (1) The light from the light source is divided toward the optical waveguide and the optical variable delay circuit, and the light returned from the optical waveguide and the optical variable delay circuit is incident on the signal processing system through an interferometer, In the low-coherence reflectometer that measures the reflection distribution of the, the optical variable delay circuit has a modulation optical system that modulates the phase of the incident light at a constant frequency, in addition to the input / output combined port and the reflection optical system. The signal processing system is provided with a detection system for detecting the constant frequency component or a frequency component which is a transition from the constant frequency component by a predetermined frequency. (2) In (1), the modulation optical system comprises a reflection optical system and a driving system thereof.

A refractometer for measuring a reflection distribution of an optical waveguide with a high spatial resolution by using a light source having a wide spectrum width, an interferometer, an optical variable delay circuit, and a signal processing system. A reflection optical system for reflecting the light beam to be incident on the interferometer again, and a moving system for moving the reflection optical system in the direction of the light beam. An optical modulation system for phase modulation, and the phase modulation frequency component in the signal processing system, or
A mechanism is provided for detecting a component having a certain number of transitions from the phase modulation frequency. As the optical modulation system, a drive system for vibrating the reflection optical system or a part thereof in the delay circuit at a constant frequency is provided. In addition,
In order to set the spatial resolution to δL (<10 m), the light source has a full width at half maximum of the spectrum of 0.31λ with respect to the center wavelength λ.
It must be ² / (nδL) or more. Here, n is the refractive index of the core of the measured optical waveguide (including the optical fiber). In this way, the bias can be removed, and
The backward Rayleigh scattered signal of the waveguide having a propagation loss of B or more can be measured, and a highly sensitive low coherence reflectometer is realized.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Here, embodiments of the present invention will be described with reference to FIGS. FIG. 1 is an example of an optically variable delay circuit. In this example, the phase modulator 21 and the oscillator 22 are interposed between the fiber port 5 and the lens 13, the reflecting mirror 15 and the stage 16. That is,
A phase modulator 21 using liquid crystal was installed between the lens 13 and the reflecting mirror 15. Since the refractive index of this phase modulator changes periodically according to the sinusoidal voltage of frequency f = 70 Hz from the oscillator 22, the phase modulation of frequency f ₁ occurs while the light beam 14 reciprocates through the liquid crystal. Receive. Here, in order to prevent the reflected light from the liquid crystal surface from entering the fiber port 5, the liquid crystal surface is slightly inclined with respect to the light beam. On the other hand, the residual reflected light generated at the exit end of the fiber port 5 and the lens 13 does not pass through the phase modulator 21, and therefore is not subjected to the phase modulation of the frequency f ₁ . Therefore, the interference signal between the reference light reflected by the reflecting mirror 15 and phase-modulated at the frequency f ₁ and the backward Rayleigh scattered light from the optical waveguide 6 shown in FIG. Since it undergoes phase modulation by f, f
Although the component that changes at the frequency of + f ₁ is included, the interference component between the residual reflected light and the reflected light generated at the fiber port 4 or the optical waveguide 6 changes only at the frequency f. Therefore, in the signal processing system 11, the detection frequency is set to f + f ₁ and the detection bandwidth is adjusted so that the frequency f component is not mixed, so that the output S ₀ generated by the residual reflection can be removed. FIG. 2 shows the results of measuring the reflection distribution from the entrance end of a 10 cm long silica waveguide to 4 cm to 8 cm. In (a), the conventional example is used, and it is 6 cm due to the S ₀ component generated by residual reflection.
It was observed that the signal was almost constant from the point. (B)
Is the result of measuring the reflection distribution using the configuration shown in FIG. 1 of this example, and it was found that the signal (in dB) was attenuated linearly with distance even after 6 cm, and the loss factor of the silica-based waveguide was Was confirmed to be constant in the longitudinal direction.

FIG. 3 shows another example, in which 23 is an electrostrictive oscillator and 24 is an oscillator. In this example, the reflecting mirror 15
Is fixed to the electrostrictive oscillator 23, and is set to vibrate in the direction of the light beam at a frequency of f ₁ = 70 Hz according to the voltage from the oscillator 24. Due to the vibration of the reflecting mirror, the position where the light beam is reflected periodically changes, so the optical path length through which the light beam propagates changes, and as a result, phase modulation is applied to the reference light. Signal processing system 1
1 was set to detect the frequency component of f + f ₁ and the reflection distribution of the silica-based waveguide was measured. As a result, similar to the first embodiment, S ₀ was successfully removed by residual reflection, It was possible to obtain the same reflection distribution as in (b) of 2.

FIG. 4 shows a third example corresponding to FIG.
In this example, the reflector 17 was fixed to the electrostrictive oscillator 23 shown in the above-mentioned example, and the reflector 17 was vibrated at a frequency f ₁ = 70 Hz in the light beam direction. As a result of the experiment, as in the first and second examples of the present invention, the S ₀ component due to the residual reflection could be removed. In this example, the reflector 17 is vibrated, but the same effect can be obtained even if the reflecting mirror 19 or 20 is vibrated.

Although the examples up to now have been described on the premise of the phase modulator 8 of the electrostrictive oscillator based on FIG. 5, in the present invention, since it suffices to detect the f ₁ component from the interference light, phase modulation is eliminated by the frequency f. However, residual reflection can be removed.

[0016]

As described above, according to the present invention, a component due to a residual component can be removed and a high-sensitivity reflectometer can be realized. Therefore, an optical waveguide with a large loss such as a silica-based waveguide having a length of 10 m or more can be used. It is effective for diagnosis.

[Brief description of drawings]

FIG. 1 is a configuration diagram according to an example of the present invention.

FIG. 2 is a characteristic diagram showing a measurement result of reflection of a silica-based waveguide.

FIG. 3 is a configuration diagram of another example of the present invention.

FIG. 4 is a configuration diagram of still another example of the present invention.

FIG. 5 is a configuration diagram of an example of a low coherence reflectometer.

FIG. 6 is a block diagram of an example of a conventional optical delay circuit.

FIG. 7 is a configuration diagram of another example of a conventional optical delay circuit.

[Explanation of symbols]

 1 Super Luminescent Diode 2, 4, 5, 9 Fiber Port 3 Fiber Type 3 dB Coupler 6 Optical Waveguide for Testing 7 Optical Variable Delay Circuit 8 Fiber Type Phase Modulator 10 Photodetector 11 Signal Processing System 13 Lens 14 Parallel Beam 15 Reflector 16 Stage 17, 18 Reflector 19, 20 Reflector 21 Phase modulator 22 Oscillator 23 Electrostrictive oscillator 24 Oscillator

Claims (2)

[Claims] 1. The light from a light source is divided toward an optical waveguide and an optically variable delay circuit, and the light returned from the optical waveguide and the optically variable delay circuit is incident on a signal processing system via an interferometer.
In the low coherence reflectometer for measuring the reflection distribution of the optical waveguide, the optical variable delay circuit is a modulation optical system that modulates the phase of incident light at a constant frequency outside the entrance / exit combined port and the reflection optical system. The low coherence reflectometer, wherein the signal processing system includes a detection system that detects the constant frequency component or a frequency component that is a transition of a predetermined frequency from the constant frequency component. 2. The low coherence reflectometer according to claim 1, wherein the modulation optical system comprises a reflection optical system and a driving system thereof.