JP2629700B2 - Optical fiber sensor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical fiber sensor, and more particularly, to an optical fiber sensor that stabilizes detection of an acoustic signal applied to an optical fiber.

[Conventional technology]

2. Description of the Related Art Conventionally, various types of optical fiber sensors using optical fibers have been proposed. In particular, there are several types of optical fiber sensors that are used only for hydrophones that detect sound waves propagating in water. (For example, The Acoustical Society of Japan, Vol. 40, Nos. 2 and 3
Of these, the polarization plane system using two eigenmodes that propagate independently in the polarization-maintaining optical fiber has a simpler configuration than the homodyne system or the heterodyne system using two optical fibers. It has the characteristic of being hardly affected by changes in temperature and the like.

FIG. 9 is a configuration diagram of a conventional polarization plane method. In the polarization plane method, as shown in FIG. 9, light emitted from a light source 91 is converted into parallel light by a lens 92, enters a polarizer 93, and becomes linearly polarized light. Here, the polarizer 93 does not need to be particularly provided when the light emitted from the light source is linearly polarized light or circularly polarized light. Light emitted from the polarizer 93 is incident on the polarization-maintaining optical fiber 95 via the lens 94. Here, polarization-maintaining optical fiber
The polarizer is arranged such that the polarization axis of the light emitted from the polarizer 93 is 45 degrees with respect to the eigenpolarization axis of the polarization-maintaining optical fiber 95 so that the two eigenmodes 95 are excited with equal amplitude. The arrangement is such that a 93 is arranged, or a quarter-wave plate is inserted after the polarizer 93 to convert the light into circularly polarized light, and thereafter, the light enters the polarization-maintaining optical fiber 95.

A part of the polarization-maintaining optical fiber 95 is arranged at a position for receiving an incoming signal 96 to be detected, such as sound. The outgoing light from the polarization-maintaining optical fiber 95 is converted into parallel light by a lens 97 and is incident on an analyzer 98 composed of a polarizing beam splitter or the like. Analyzer
98 is 45 with respect to the intrinsic polarization axis of the polarization-maintaining optical fiber 95.
Arrange so that it becomes a degree.

In this case, if a polarizing beam splitter is used as the analyzer 98, two light outputs whose sum is always constant can be obtained. The signal to be detected can be reproduced by converting one of these optical outputs into an electric signal by a light receiving element. Preferably, the two optical outputs are converted into electric signals by the light receiving elements 99 (A) and (B). The difference output between the two is obtained by the differential circuit 100, and the sum of the two is obtained by the addition circuit 101, and the difference output is divided by the sum output by the division circuit 102 to obtain a detection output in which the level fluctuation of the light source is reduced. Can be.

Now, when a signal to be detected such as pressure or temperature is applied to the polarization-maintaining optical fiber 95, the propagation constants of the two eigenmodes change. The phase difference, that is, the polarization state changes.
The output voltage of the two light receiving elements is 11th with respect to the change in the phase difference.
It changes as shown in the figure. FIG. 11 is an output voltage characteristic diagram of a conventional polarization plane type light receiving element. When a change in a minute level such as an acoustic signal is to be detected, the change in phase difference between the two eigenmodes is small, so that the phase difference must be set at point A. In this case, when the temperature, pressure, and the like change, the phase difference deviates from the point A as shown by δ in FIG. 11, and the sensitivity to a small signal such as sound, which is represented by the slope with respect to the change in the phase difference of the output voltage of the light receiving element, changes. And drift compensation is performed to maintain the optimal phase difference at point A.

For example, as shown in FIG. 9, a polarization-maintaining optical fiber
Part of 95 is wound around a piezoelectric cylinder 103 made of piezoelectric material,
When the phase difference deviates from the optimal phase, the compensation signal is
In addition to 103, using the phase difference change due to elongation of the polarization-maintaining optical fiber 95 to return to the optimal phase difference, or
A method is employed in which a compensation signal is superimposed on the drive current of the light source so as to change the wavelength of light, and the phase difference is changed to return the phase difference to an optimum phase difference.

A part of the polarization-maintaining optical fiber 95 is placed at a position where an incoming signal to be detected, such as sound, is received. Since this causes a phase difference in the eigenmode, if the position where the light source or the light receiving element is provided is far from the position where the signal to be detected is located, it is easily affected by disturbance and the signal-to-noise ratio deteriorates. FIG. 10 is a configuration diagram when a lead fiber is used in a conventional polarization plane method. Tenth
As shown, a light source (not shown)
After propagating the light at 111 and passing through a polarizer 112, the light enters a polarization-maintaining optical fiber 115 as a sensing fiber, and the emitted light propagates through a lead fiber 114 after passing through an analyzer 113, and is transmitted through a lead fiber 114 to a light receiving element (not shown). If the configuration is used to convert the signal into an electric signal, the effect of disturbance applied to the lead fibers 111 and 114 can be reduced.

In addition, instead of using a polarizer or analyzer, a polarization-maintaining optical fiber is used as the lead fiber, and only the part that receives the signal to be detected is rotated by 45 degrees in the intrinsic polarization axis of the polarization-maintaining optical fiber, so that the lead before and after the lead is used. Similar effects can be obtained by fusing or connecting to the fiber.

[Problems to be solved by the invention]

In the above-mentioned polarization-type sensor using the conventional polarization-maintaining optical fiber, the sensitivity changes in proportion to the optical power reaching the light-receiving element, and the optical power of the light source fluctuates and the connection loss of the optical fiber fluctuates. However, there is a disadvantage that the reproducibility of each sensor is poor due to the influence of the transmission loss of the optical fiber.

In order to reduce the influence of the loss and the like, a method of providing an addition circuit, a subtraction circuit, a division circuit and the like after the light receiving element and dividing the subtraction result by the addition result is also used as described above. However, there is a drawback that a sufficient effect cannot be obtained from the point of dynamic range calculation accuracy when dealing with a complicated signal.

Also, when a large number of optical fiber sensors are provided to form an array, it is necessary for the polarization plane type sensor to maintain each sensor at an optimum phase difference.For example, when a piezoelectric cylinder is used, a control signal is transmitted to each sensing fiber. It is necessary to use an electro-optical composite cable having a large outer diameter and weight in order to propagate the light. Also, if an attempt is made to maintain the optimum phase difference by changing the wavelength of the light source, the number of light sources, lead fibers, light receiving elements, etc. must be provided for the number of sensors when arraying, so that the light sources and optical fibers are not effectively used. There is a disadvantage that it cannot be used.

Furthermore, when the light propagating through one light-transmitting lead fiber is branched and distributed to a plurality of sensors, the sensitivity changes in proportion to the optical power, so it is necessary to precisely control the branching ratio. However, as the number of sensors increases, the optical power that can be distributed to each sensor decreases and the sensitivity decreases,
There is a disadvantage that the number of sensors that can be arrayed is limited to at most a few.

The object of the present invention is to eliminate the above-mentioned disadvantages and detect the arrival signal at a time when the output of the light receiving element is at the same level, thereby significantly improving the reproducibility of the sensor sensitivity and achieving a significant reduction in size and weight. An object of the present invention is to provide an inexpensive optical fiber sensor that can significantly increase the number of sensing fibers that can be arrayed.

[Means for solving the problem]

An optical fiber sensor according to the present invention includes a light source in which a wavelength change of light causes a phase change of at least π / 2 radians or more by a variable voltage applied from a control terminal, and the variable voltage increases in one direction at a predetermined repetition period. Alternatively, a light source driving circuit that generates a repetitive voltage including a falling voltage and outputs the generated voltage to the control terminal, a light transmitting optical fiber that transmits light having at least two or more polarization axes emitted by the light source, Of the light output from the optical fiber for use, the light having two specific polarization axes, and the light having different propagation constants in the fiber is transmitted and the two propagation constants are transmitted by an external signal such as sound to be detected. A sensing fiber that is a polarization-maintaining optical fiber that causes a phase shift by being changed; a return lead fiber that receives and transmits light from the sensing fiber; A beam splitter that receives the light from the lead fiber for a turn and separates the two light beams having the unique polarization axes into light beams having respective unique polarization axes; and the light beams having the two unique polarization axes output from the beam splitter change with time. Two light receiving elements for converting a changing light component into first and second electric signals, respectively;
An equivalence detection circuit that outputs a detection voltage having the same value at a certain time at a change voltage of the electric signal, a repetition voltage from the light source driving circuit and a detection voltage from the equivalence detection circuit are input, and the repetition voltage A sampling circuit that outputs the detection voltage having the same value existing in a plurality of one-way increasing or decreasing voltages, and a detection voltage that receives the same value by inputting a plurality of output voltages of the sampling circuit and outputs the same value as the sound or the like. And a demodulation circuit for detecting a voltage at a time when the phase shift is caused by the external arriving signal.

〔Example〕

 Next, the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing a first embodiment of the present invention.
The light source 11 is a semiconductor laser or the like whose wavelength can be controlled from the outside, and the light transmission lead fiber 12 propagates light emitted from the light source to the sensing fiber 13. In the sensing fiber 13, a phase difference between two eigenmodes propagating in two eigenpolarization axes, that is, a polarization plane, changes depending on a signal to be detected.
The light emitted from the sensing fiber 13 is propagated through the return lead fiber 14, separated into two eigenmodes by the polarization beam splitter 20, and converted into electric signals by the light receiving elements 15 (A) and 15 (B). Further, the light receiving elements 15 (A), 15
The output of (B) is detected by the equivalence detection circuit 16 at a time when the amplitude becomes equal, and is supplied to the sampling circuit 17. The sampling circuit 17 samples the level of the control signal of the light source driving circuit 18 for controlling the light source 11.

The timing of this sampling is a cross point indicated by a point A in the example of FIG. 11 described above. This sampling value clearly has a level corresponding to the arriving signal applied to the sensing fiber 13, and the demodulation circuit 19 restores the arriving signal applied to the sensing fiber based on these sampling values, and is thus reproduced. A reproduction foreign signal is output from the demodulation circuit 19. The above-described processing will be described later in detail. FIG. 2 is a block diagram showing a second embodiment of the present invention. In the second embodiment, a circuit after the sensing fiber of the first embodiment in FIG. 1 is configured as a plurality of streams, and light is branched and supplied from a light transmitting lead fiber to each stream, and an array is provided. Optical fiber sensor.
FIG. 2 shows an example of an array configuration using two series of optical fiber sensors.

In the embodiment of FIG. 2, at least one coupler 31 as an optical branching element is interposed in the middle of the light transmitting lead fiber 22 in order to constitute an optical fiber sensor in which two sensing fibers are provided and arranged in an array. At the same time, a part of the propagating light is branched and each sensing fiber 23 (A), 23
(B). As the coupler 31, a polarization-maintaining optical fiber coupler may be used, or a beam splitter or the like may be used. The light emitted from both sensing fibers is the first
The return lead fibers 24 (A) and 24 (B) are connected to the polarization beam splitter 30 (A),
After being propagated to 30 (B) and converted into electric signals by the light receiving elements 25 (A), 25 (B) and 32 (A), 32 (B), the equivalence detection circuits 26 (A), 26 (B) Detects the time at which the outputs of the light receiving elements become equal, and the sampling circuits 27 (A) and 27
After sampling the control signal of the light source drive circuit 28 by (B), the signals detected by each sensing fiber are reproduced by the demodulation circuits 29 (A) and 29 (B). However, as long as the object to be sampled has the same generation timing as the control signal, one having a different level may be used.

Next, the above operation will be described in more detail. In the case of FIG. 1, the wavelength-tunable light source 11 uses a distributed feedback semiconductor laser having a wavelength control region, a distributed black reflection type semiconductor laser having a wavelength control region, or the like, and sends a control current to the wavelength control region. The wavelength of light changes in proportion to an externally applied electric signal by controlling the wavelength or by changing the drive current of a normal distributed feedback semiconductor laser or distributed black reflection laser to control the wavelength. A light source that can The light source driving circuit applies a sawtooth wave having a repetition frequency at least twice as high as the frequency of the acoustic signal to be detected to the light source 11 as a control signal, so that light having a wavelength change such as the light source wavelength shown in FIG. Can be emitted. FIG. 3 is a wavelength characteristic diagram of the light source. Light from a light source having such wavelength characteristics is converted into linearly polarized light through an isolator, a polarizer, and the like, and then is incident on a light transmitting lead fiber using a lens or the like. The light-transmitting lead fiber is a polarization-maintaining optical fiber, and light is incident so as to coincide with one of the intrinsic polarization axes of the lead fiber.

The light propagating through the light-sending lead fiber 12 reaches the emission end while the polarization plane is kept in the unique polarization axis. The sensing fiber 13 is a polarization-maintaining optical fiber, and is fused or connected to the light-transmitting lead fiber 12 with the intrinsic polarization axis rotated by 45 degrees. In some cases, a normal single mode optical fiber may be used as the light transmission lead fiber, and a polarizer inclined at 45 degrees with respect to the intrinsic polarization axis of the sensing fiber 13 may be inserted into the joint with the sensing fiber 13.

In the sensing fiber 13, the two eigenmodes are excited with the same phase and the same amplitude at the input end, but a steady phase difference occurs between the two eigenmodes at the output end due to the difference in propagation constant.

When the wavelength of the light source 11 is changed, the phase difference at the output end of the sensing fiber 13 changes in proportion to the wavelength change. This is because the propagation constants of the two eigenmodes are different, and even if the same wavelength change is applied, each eigenmode changes. This is because the amount of change in the phase of the mode is different.

This phase difference is applied to the intrinsic polarization axis of the sensing fiber 13.
At 45 degrees, the sensing fiber 13
The phase difference between the two eigenmodes can be converted into amplitude information of the two eigenmodes of the return lead fiber 24 and propagated to the light receiving element.

At the output end of the return lead fiber 24, the output light is converted into parallel light by a lens or the like, and then the polarization beam splitter 20 is made to coincide with the intrinsic polarization axis of the return lead fiber 14.
To separate the components of each unique polarization axis,
(A) and 15 (B) convert to an electric signal. FIG. 4 is an output voltage characteristic diagram of the light receiving element that changes according to the phase difference between two eigenmodes at the emission end of the sensing fiber. The output voltage of the light receiving element changes in accordance with the phase difference, and the phase difference (n
+ J / 2) π (n = 1, 2, 3,..., J = 1, 3, 5,...), The levels are the same.

Here, the wavelength of the light source 11 is changed to the two light receiving elements 15 (A) and 15 (A).
When the output voltage of (B) is changed so as to have the same value at least once, and is changed so as to have a saw-tooth shape having a repetition frequency at least twice as high as the highest frequency of the signal to be detected, The phase difference between the two eigenmodes of the sensing fiber 13 changes as the output voltage of the two light receiving elements 15 (A) and 15 (B) as shown in FIG. FIG. 5 is a graph showing the output voltage characteristics of the light receiving element when the wavelength of the light source is controlled by a sawtooth wave. The time when the output voltages of the two light receiving elements 15 (A) and 15 (B) first match, that is, the sensing fiber 13
Is the phase difference between the two eigenmodes Are always constant intervals unless the propagation constants of the two eigenmodes of the sensing fiber 13 change. However, when it is applied to the sensing fiber 13 such as an acoustic signal, the phase difference of the emitted light changes, and the light receiving elements 15 (A) and 15 (A)
The output of (B) changes as shown in FIG. FIG. 6 is a characteristic diagram of the output voltage of the light receiving element when a detection signal is applied. As is apparent from FIG. 6, the time when the output voltages of the two light receiving elements coincide with each other changes depending on an acoustic signal or the like.

This time is detected by the equivalence detection circuit 16, and the level of the sawtooth wave changing the light source is sampled by the sampling circuit 17 with the (n + j / 2) π cross point as the sampling timing, as shown in FIG. A pulse having an output level proportional to the amount of change in the acoustic signal or the like is obtained. FIG. 7 is an output voltage characteristic diagram of the sampling circuit. As shown in FIG. 7, this sampling is performed at the repetition period of the sawtooth wave for controlling the light source, and the repetition frequency is at least twice the frequency of the acoustic signal to be detected in consideration of the Nyquist rate. As described above, the demodulation circuit 19 can reproduce an audio signal or the like to be detected as shown in the demodulation circuit output voltage characteristic diagram of FIG.

If the phase difference between the two modes propagating in the return lead fiber 24 becomes a problem, the fusion or connector connection should be performed by rotating the intrinsic polarization axis by 90 degrees in the middle of the return lead fiber 24. Thus, the phase difference between the modes can be removed. If the time when the outputs of the two light receiving elements become the same value changes across the rising portion of the sawtooth wave, the wavelength of the light source is shifted as a whole or a bias voltage is applied to the output of one of the light receiving elements. do it,
The outputs of the two light receiving elements are set to have the same value at a position apart from the rising portion of the sawtooth wave.

FIG. 2 shows a second embodiment of the present invention, in which light propagating in a light transmitting lead fiber 22 is branched by a coupler 31 and supplied to each sensing fiber 23 (A), 23 (B). In each sensing fiber, a change in phase difference occurs between the two eigenmodes due to an acoustic signal or the like as in the first embodiment.

This output is transmitted to the light-receiving element by return lead fibers 24 (A) and 24 (B) in which the intrinsic polarization axis is inclined by 45 degrees with respect to the intrinsic polarization axis of the sensing fiber, as in the first embodiment. Polarization beam splitter 30 matched to return fiber's intrinsic polarization axis
In (A) and (B), the component of each unique polarization axis is separated, and each is converted into an electric signal by a light receiving element.

A light receiving element 25 is provided by the equivalence detection circuits 26 (A) and 26 (B).
(A), 32 (A) and 25 (A), 32 (B) are detected at the time when the outputs have the same amplitude, and the electric signals for controlling the light source 21 are sampled by the sampling circuits 27 (A), 27 (B). After sampling, if the signals are reproduced by the demodulation circuits 29 (A) and 29 (B), the acoustic signals and the like applied to the sensing fibers 23 (A) and 23 (B) can be obtained individually.

〔The invention's effect〕

As described above, according to the present invention, the time at which the outputs of the two light receiving elements have the same level corresponding to the phase difference between the two eigenmodes of the sensing fiber has information of the signal to be detected. By detecting the incoming signal by paying attention to the above, as long as the phase difference change that occurs when the signal to be detected is applied to the sensing fiber is equal for each sensing fiber, the optical power fluctuation of the light source, the connection loss of the optical fiber, or the optical fiber There is an effect that the reproducibility of the sensitivity is extremely good because it is not affected by the change in the transmission loss of the fiber.

In addition, even when a plurality of sensing fibers are provided and arrayed, there is no need to provide a piezoelectric cylinder or the like for maintaining each sensing fiber at an optimal phase difference, and it is not necessary to transmit a control signal to the sensing fiber. There is an effect that each of the fiber and the transmission cable can be reduced in size and weight. Also, since the sensitivity of each sensor does not depend on the optical power, it is not necessary to accurately control the level of the optical power branched in the middle of the light transmission lead fiber, and the number of sensing fibers that can be arrayed is increased as compared with the conventional one. There is an effect that it becomes possible.

Furthermore, only one light source and one lead fiber for light transmission are required, and the electronic circuit after the light receiving element can be integrated into an integrated circuit, so the cost per sensing fiber is significantly higher than that of the conventional array. There is an effect that it can be reduced.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first embodiment of the present invention, FIG. 2 is a block diagram showing a second embodiment of the present invention, FIG. 3 is a wavelength characteristic diagram of a light source, and FIG. FIG. 5 is a graph showing an output voltage characteristic of the light receiving element when the wavelength of the light source is controlled by a sawtooth wave, FIG. 6 is a graph showing an output voltage characteristic of the light receiving element when a detection signal is applied, and FIG. 8 is a demodulation circuit output voltage characteristic diagram, FIG. 9 is a configuration diagram of a conventional polarization plane system, FIG. 10 is a configuration diagram of a conventional polarization plane system using a lead fiber, and FIG. FIG. 3 is a diagram showing a conventional polarization plane type light receiving element output voltage characteristic. 11 light source, 12 lead fiber for light transmission, 13 sensing fiber, 15 (A), 15 (B) light receiving element, 16
... Equivalent value detection circuit, 17 ... Sampling circuit, 18 ... Light source drive circuit, 19 ... Demodulation circuit, 21 ... Light source, 22 ... Lead fiber for light transmission, 23 (A), 23 (B) ... Sensing fiber, 24 (A), 24 (B) ... Lead fiber for return, 25 (A), 25 (B) ... Light receiving element, 26 (A), 26
(B) ... equivalence detection circuit, 27 (A), 27 (B) ... sampling circuit, 28 ... light source drive circuit, 29 (A), 29
(B): demodulation circuit, 30 (A), 30 (B): polarizing beam splitter, 31: coupler, 91: light source, 92: lens, 93: polarizer, 94: lens, 95 … Polarization-maintaining optical fiber, 96… incoming signal, 97… lens, 98… analyzer, 99 (A), 99 (B)… light receiving element, 100… differential circuit, 101… Addition circuit, 102: division circuit, 103: piezoelectric cylinder, 111: lead fiber, 112: polarizer, 113 ...
... Analyzer, 114 ... Lead fiber, 115 ... Polarization-maintaining optical fiber.

Claims (2)

(57) [Claims] 1. A light source in which a wavelength change of light causes a phase change of at least π / 2 radians or more by a variable voltage applied from a control terminal, and an increase or decrease voltage in one direction at a predetermined repetition period of the variable voltage. A light source driving circuit for generating a repetitive voltage including the following and outputting the same to the control terminal; a light transmitting optical fiber transmitting light having at least two or more polarization axes emitted by the light source; and the light transmitting optical fiber. Light having two intrinsic polarization axes of the light output from
And a sensing fiber that is a polarization-maintaining optical fiber that transmits light having different propagation constants in the fiber and changes the two propagation constants by an external arriving signal such as sound to be detected to generate a phase shift. A return lead fiber for receiving and transmitting the light of the sensing fiber, a beam splitter for receiving the light of the return lead fiber and separating the light of the two unique polarization axes into light of the respective unique polarization axes; Two light receiving elements for converting light components of the two eigenpolarization axes, which are output from the splitter and change with time, into first and second electric signals, respectively, and changes in the first and second electric signals An equivalence detection circuit that outputs a detection voltage having the same value at a certain time, a repetitive voltage from the light source driving circuit, and a detection voltage from the equivalence detection circuit. A sampling circuit which outputs a detection voltage to be the same value that was input and voltage present in the plurality of one-way increase or decrease the voltage in said repetitive voltage,
A demodulation circuit for inputting a plurality of output voltages of the sampling circuit and detecting a voltage at a time when the detection voltage having the same value undergoes a phase shift by an external signal such as the sound. Optical fiber sensor. A plurality of sensing fibers, a return lead fiber, a polarization beam splitter, two light receiving elements, an equivalent detection circuit, a sampling circuit, and a demodulation circuit; 2. The optical fiber sensor according to claim 1, wherein the light transmitted from the lead fiber is branched and supplied.