JP6763567B2 - Fiber optic sensor - Google Patents

Description

In the present invention, the optical path length is changed by an external force, or a reflection mirror is attached to an object whose position changes mechanically, and the change in the position of the object is regarded as a change in the optical path length. The present invention relates to an optical fiber sensor utilizing the characteristic that the phase of a signal changes.

The optical fiber sensor is superior to the conventional electric sensor in the place where the electromagnetic environment is bad and the temperature environment is bad because the optical sensor part can be turned off, and the low loss characteristic of the optical fiber makes it possible to use the optical fiber sensor. It can also exhibit excellent characteristics for long-distance sensing.

Japanese Patent No. 5118246

In the interference method of the conventional optical fiber sensor, the interference output (interference light) is indicated by the following equation 1 when the phase difference between the reference light and the measurement light is θ.

ここで、ｉは干渉光から変換された電気信号の値（電気信号値）、ｒは参照光から変換された電気信号値、ｓは計測光から変換された電気信号値である。

Here, i is the value of the electric signal converted from the interference light (electric signal value), r is the electric signal value converted from the reference light, and s is the electric signal value converted from the measurement light.

As can be seen from the above equation 1, the sensor signal that is truly desired is the value of θ, but since it is obtained only as i in the conventional interference method, the fluctuation error due to the intensity fluctuation of r and s in the transmission path is calculated. There was a drawback that it could not be separated.

Further, in the conventional interference method, in the phase relationship between the reference light and the measurement light, setting the center of the operating point at a phase difference of 90 degrees is the setting with the best linearity, but the optical path length difference between the reference light and the measurement light. In order to keep the temperature at 90 degrees, high machining accuracy is required, and it is necessary to maintain a constant relationship with respect to thermal expansion, which is a practical difficulty.

Further, in the conventional interference method, when the phase difference between the reference light and the measurement light is 90 degrees as the center of the operating range, the maximum measurable phase is ± 90 degrees (half wavelength), and it is a principle to exceed this. It was impossible.

To solve these problems, an optical fiber sensor as disclosed in Japanese Patent No. 5118246 (Patent Document 1) has been proposed.

The optical fiber sensor of Patent Document 1 uses an optical pulse in which the optical phase difference between the first half t1, t2 and the second half t3 of an optical pulse having a pulse width of 3 t with a period T is phase-modulated to 90 degrees as a measurement signal for measurement. The signal is demultiplexed, delayed by t time, and the optical path length is further changed by an external force as the measurement light, and the other optical signal is used as the reference light to interfere with each other. As a result, a portion (R) of only the first half t1 of the reference light, an interference portion (I1) between the first half t1 of the measurement light and the intermediate t2 of the reference light, and an interference portion (I1) between the intermediate t2 of the measurement light and the second half t3 of the reference light ( A time-divided multiplex optical signal having only a portion (S) of I2) and the latter half t3 of the measurement light is generated, and the phase difference θ1 and θ2 between the measurement light and the reference light in I1 and I2 are set as the electric signal of each interference portion. The values r, i1, i2 and s are used to obtain the values based on the following equations 2 and 3.

そして、１周期前に求めた位相差θ１ａ及びθ２ａとの差分Δθ１及びΔθ２を積算した値をセンサ出力としている。

Then, the value obtained by integrating the differences Δθ1 and Δθ2 with the phase differences θ1a and θ2a obtained one cycle before is used as the sensor output.

Patent Document 1 is accurate on the premise that the magnitudes of the optical signal, that is, the reference light (R), the first interference portion (I1), the second interference portion (I2), and the measurement light (S) can be accurately measured. The calculation result of the phase (i) can be obtained. Generally, the magnitude of an optical signal is detected by electrically converting the intensity of light with a photodiode made of a semiconductor. However, the photodiode generates an output voltage proportional to the input optical signal power at 0.01 mW or less, and the detection voltage is very small. Therefore, it is necessary to amplify and use the photodiode except in special cases. is there. Commercially available optical / electrical converters have a built-in transimpedance amplifier with high input impedance that amplifies the photodiode output voltage along with the photodiode. This transimpedance amplifier is an AC amplifier in which a capacitor is inserted in series at the input. In the measurement method in Patent Document 1, R, I1, I2 and S are measured in this order, but when I1 of the sensor changes due to an external force or the like, the subsequent detection voltages of I2 and S are affected by the voltage change of I1. Therefore, this effect becomes a measurement error. Further, the change in I2 causes a measurement error in S. In Patent Document 1, in order to eliminate or reduce this error to a negligible degree, the photodiode output voltage of the optical / electric converter is DC-amplified, or the error that I1 gives to I2 and S and the error that I2 gives to S. A troublesome process such as predicting and removing the error by calculation is required.

The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to exclude the influence of the fluctuation of the intensity of the interfering light and the fluctuation of the intensity of the light after the interference on the sensor output. In addition, the phase range that can be measured as a sensor signal is allowed to exceed ± 90 degrees, and the measurement error does not increase even if an AC amplifier is used for electrical amplification after optical / electrical conversion, resulting in cost reduction. The purpose is to provide an optical fiber sensor.

An object of the present invention is to generate a measurement optical signal that is divided at predetermined time intervals and has at least one orthogonal phase section that is 90 degrees phase-modulated with respect to the phase of another section. The measurement light and the reference light, which are demultiplexing of the measurement optical signal, are delayed by an integral multiple of the predetermined time interval, and the optical path length is changed by an external force. The phase difference calculation is provided with an optical interference unit that combines reference lights to generate an interference light signal, and a phase difference calculation unit that calculates the phase difference between the measurement light and the reference light based on the interference light signal. The unit includes an electric signal 1 of the interference light signal generated by combining the measurement light in the in-phase section that is not 90-degree phase-modulated and the reference light in the in-phase section, and the measurement light in the in-phase section. And the electrical signal 2 of the interference light signal generated by the coupling of the reference light in the orthogonal phase section, and the interference generated by the coupling of the measurement light in the orthogonal phase section and the reference light in the in-phase section. This is achieved by using at least the electric signal 3 of the optical signal and calculating the phase difference from the difference between the electric signal 1 and the electric signal 2 and the difference between the electric signal 3 and the electric signal 1.

Further, an object of the present invention is that the phase difference calculation unit uses an electric signal 11 and an electric signal 12 having different sections as the electric signal 1, or the measurement optical signal generation unit is continuous. Of the four sections, the third section is the orthogonal phase section, and the other three sections are the in-phase sections to generate the measurement optical signal, and the optical interference unit is the measurement light and the reference. By providing a delay by the predetermined time interval between the lights, or in the measurement optical signal generator, the third section of the five continuous sections is the orthogonal phase section, and the other four. The measurement optical signal whose section is the in-phase section is generated, and the optical interference unit provides a delay between the measurement light and the reference light by the predetermined time interval, or is homodyne type or It is achieved more effectively by being a Machzenda type.

According to the present invention, the measured value of an electric signal obtained by detecting an optical signal with a photodiode is affected by a capacitor inserted in series in an AC amplifier inserted for the purpose of amplifying the electric signal. However, by taking the difference between the measured values, the influence is eliminated and accurate and high-performance measurement is possible. As a result, fluctuations in the level of the light source, fluctuations in the loss of the transmission line, temperature fluctuations in the optical coupler, etc. can be tolerated, making it suitable for use in places with harsh environmental conditions, and the usage conditions for individual parts can be relaxed, thus reducing manufacturing costs. It can be cheap.

According to the present invention, the phase difference θ between the two orthogonal reference lights and the measurement light can be easily obtained, and the measurement range is limited to 180 degrees (π) with a normal interferometer. The measurement range can be expanded under the condition that the measurement phase does not change by ± 90 degrees (π / 2) or more within one measurement cycle.

Further, since continuous light rays can be used instead of optical pulses, cost reduction can be achieved.

It is a block diagram which shows the structural example (first embodiment) of the optical fiber sensor which concerns on this invention. It is a time chart which shows the operation example (1st Embodiment) of the optical fiber sensor which concerns on this invention. It is a time chart which shows the other operation example of 1st Embodiment. It is a block diagram which shows the structural example (second embodiment) of the optical fiber sensor which concerns on this invention. It is a time chart which shows the operation example (second embodiment) of the optical fiber sensor which concerns on this invention. It is a block diagram which shows the structural example (third embodiment) of the optical interference part of the optical fiber sensor which concerns on this invention. It is a block diagram which shows the structural example (4th Embodiment) of the optical interference part of the optical fiber sensor which concerns on this invention. It is a block diagram which shows the structural example (fifth embodiment) of the optical interference part of the optical fiber sensor which concerns on this invention. It is a block diagram which shows the configuration example which mounted the reflection mirror directly on the surface of the distance measuring object in 4th Embodiment. It is a block diagram which shows the configuration example which used the Faraday rotator mirror instead of the reflection mirror in 4th Embodiment.

The optical fiber sensor according to the present invention uses a highly stable laser oscillator as a light source, provides measurement sections t1, t2, t3 and t4 having a period T, for example, a time τ during the period, and the relative phase of the t3 section. Is changed by 90 degrees (either advance or lag) with respect to the t1, t2 and t4 sections to create a laser beam. The t3 section is a quadrature phase section, and the t1, t2, and t4 sections are in-phase sections. The phase of light can be changed at high speed by a phase modulator or the like using Lithium Niobate.

Hereinafter, the principle of the present invention will be described by taking a homodyne type (also referred to as Michelson type) interferometer as an example. The principle is the same for the Machzenda type.

The above laser light for measurement (optical signal for measurement) is demultiplexed into two waves by an optical coupler, the first demultiplexing is delayed by an optical delay fiber or the like for τ / 2 hours, and then an external force that becomes a sensor signal, etc. Therefore, the optical path length is changed, the return light totally reflected by the mirror is delayed again by an optical delay fiber or the like for τ / 2 hours, and the light returned to the above optical coupler is returned in the order of t1, t2, t3, t4. , Measurement light (φ1a, φ2a, φ3a, φ4a). The return light obtained by totally reflecting the second demultiplexing by the mirror is used as the reference light (φ1b, φ2b, φ3b, φ4b), and in the return output of the above optical coupler, the measurement sections t1, t2, t3 and t4 within the period T are used. Corresponding to, the interference portion (Michaelson interference portion) (I1) between the reference light t2 (φ2b) and the measurement light t1 (φ1a), the reference light t3 (φ3b) and the measurement light t2 (φ2a). A time-divided multiplex optical signal (interference light signal) having an interference portion (I2) of the above and an interference portion (I3) between t4 (φ4b) of the reference light and t3 (φ3a) of the measurement light is output.

The above time-divided multiplex optical signals (I1, I2, I3) are converted into electric signals by photodiode detection, and the individual electric signal values i1, i2 and i3 are obtained, and at the time of interference corresponding to each electric signal value. Assuming that the phase difference between the reference light and the measurement light is θ1, θ2, and θ3, the electric signal values i1, i2, and i3 can be represented by using the electric signal value r of the reference light and the electric signal value s of the measurement light. Since the diode detection is a square detection, the following equations 4, 5, and 6 are obtained.

ここで、干渉結果の電気信号間の差分ｉ１−ｉ２及びｉ３１−ｉ１を演算すると、下記数７及び数８となる。

Here, when the differences i1-i2 and i31-i1 between the electric signals of the interference result are calculated, the following equations 7 and 8 are obtained.

数７を数８で除算すると、下記数９となる。

Dividing the number 7 by the number 8 gives the following number 9.

よって、計測される電気信号値ｉ１、ｉ２、ｉ３から計算される（ｉ１−ｉ２）及び（ｉ３−ｉ１）の逆正接計算から、参照光と計測光の位相差θ１を求めることが出来る。

Therefore, the phase difference θ1 between the reference light and the measurement light can be obtained from the inverse tangent calculation of (i1-i2) and (i3-i1) calculated from the measured electrical signal values i1, i2, and i3.

The inverse tangent function can only express ± 90 degrees (π / 2), but under the condition that the measured value does not change beyond the range of ± 90 degrees during the measurement cycle T, this time from the previous measurement. It is easy to determine if the measured value exceeds +90 degrees and if the measured value falls below −90 degrees during the measurement of. As a result, the optical fiber sensor according to the present invention enables a wide range of interference measurement corresponding to a very large phase change exceeding ± 90 degrees.

Further, in the optical fiber sensor according to the present invention, r and s representing the electric output level of the optical signal branched from the laser light source are irrelevant to the calculation result, and when r and s fluctuate due to temperature change, time change, or the like. However, there is an advantage that this fluctuation does not affect the measurement result.

Since the output voltage of the photodiode detector that performs optical / electrical conversion is very small, it is necessary to amplify the output voltage with a transimpedance amplifier or the like before use. Since such an amplifier is generally required to have a high amplification factor even at a high frequency, an AC dedicated amplifier in which a capacitor is inserted between amplification elements is used. Therefore, it is often difficult to actually obtain accurate measured values of i1, i2, and i3. However, the optical fiber sensor according to the present invention measures the three measured values i1, i2 and i3 as described above, calculates the difference between them, and obtains the angle between the reference light and the measured light by the inverse tangent calculation. Therefore, the actually required values are not the measured values of i1, i2 and i3, but the differences between the measured values i1-i2 and i3-i1. Therefore, by using an AC dedicated amplifier with a capacitor inserted in the series in the middle, even if the measured values of i1, i2 and i3 do not accurately represent the amplitude, the difference is accurate and the calculation result is also accurate. Is.

If the capacitor capacitance is very small and i1, i2, and i3 in the period T are affected by the AC amplifier, measurement sections t1 to t5 having, for example, time τ are provided in the period T, and the relative of the t3 sections. A laser beam whose phase is changed by 90 degrees with respect to the t1, t2, t4 and t5 sections is created. In this case, the t3 section is a quadrature phase section, and the t1, t2, t4 and t5 sections are in-phase sections. Then, the first demultiplexing is the measurement light (φ1a, φ2a, φ3a, φ4a, φ5a), and the return light obtained by fully reflecting the second demultiplexing by the mirror is the reference light (φ1b, φ2b, φ3b, φ4b, φ5b). ), In the return output of the optical coupler, the interference portion (I1) between the reference light t2 (φ2b) and the measurement light t1 (φ1a) corresponding to the measurement sections t2, t3, t4 and t5 in the period T. , Interference portion (I2) between t3 (φ3b) of reference light and t2 (φ2a) of measurement light, interference portion (I3) between t4 (φ4b) of reference light and t3 (φ3a) of measurement light, and reference light. A time-divided multiplex optical signal (I1, I2, I3, I4) having an interference portion (I4) between t5 (φ5b) of the above and t4 (φ4a) of the measurement light is generated. From the signals i1 to i4 converted into the electric signals of I1 to I4, the phase difference θ1 between the reference light and the measurement light is obtained by using the following equation tens.

これにより、キャパシタ容量が非常に小さく、計測される電気信号の大きさが計測区間の時間内で変動する状態においても、隣り合う電気信号間の差分は、原計測値に正確に比例し、その比例定数はキャパシタの容量により一義的に決定されるので、計測結果がキャパシタ容量の大小による影響を受けない光ファイバセンサを提供することが可能である。

As a result, even when the capacitor capacity is very small and the magnitude of the measured electric signal fluctuates within the time of the measurement section, the difference between adjacent electric signals is exactly proportional to the original measured value. Since the proportionality constant is uniquely determined by the capacitance of the capacitor, it is possible to provide an optical fiber sensor whose measurement result is not affected by the magnitude of the capacitor capacitance.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a configuration example (first embodiment) of the optical fiber sensor according to the present invention in the case of the homodyne type and the interference portions are I1 to I3.

With respect to the narrow line width optical signal Sr generated by the laser light source 101, the optical phase modulator 102 changes the phase of only the t3 section during the period T by 90 degrees as compared with the other t1, t2, and t4 sections. Let me. The timing for changing the phase and the time τ for changing the phase are controlled by the pulse generator 113. The optical signal (measurement optical signal) Sra1 to which the phase change is intermittently given is supplied to the optical coupler 103 and branched into two. The optical coupler 103 is composed of a fiber coupler, a beam splitter, and the like.

One of the branched light signals of the optical coupler 103 travels in space by the collimator 106, is reflected by the reflection mirror 108, and returns to the optical coupler 103 as reference light SrR1 again through the collimator 106. On the other hand, the other branched optical signal branched by the optical coupler 103 is guided to the fiber sensor 105 after being given a delay of τ / 2 with respect to the reference optical SrR1 by a delay line 104 such as an optical delay fiber. The fiber sensor 105 is a sensor in which the refractive index or effective length of the fiber changes depending on the external pressure applied to the optical fiber. The branched light signal that has passed through the fiber sensor 105 propagates in the air by the collimator 107, is reflected by the reflection mirror 109, passes through the collimator 107 and the fiber sensor 105, and is measured again with a delay of τ / 2 on the delay line 104. It is supplied to the optical coupler 103 as optical SrM1. The delay line can be inserted on the reference optical side, and the same optical fiber sensor can be configured in that case as well.

The reference light SrR1 from the collimator 106 and the measurement light SrM1 from the delay line 104 are both added to the optical coupler 103, and the combined light output thereof is supplied to the light / electric (O / E) converter 114 as an interference light signal Li1. , The electric signal Ea1 corresponding to the interference light signal Li1 is obtained. The optical / electrical converter 114 is a diode detector that detects the square of an optical signal. The electric signal Ea1 output from the optical / electric converter 114 is amplified to a required size by the RF amplifier 115, and then converted from the analog signal to the electric signal Ed1 of the digital signal by the A / D converter 116. The electric signal Ed1 is supplied to the registers 117, 118 and 119, and according to the timing signal sent from the pulse generator 113, the electric signal values i1, i2 and i3 of the interference portions I1, I2 and I3 in the time chart described later. Is obtained on each register. With the electric signal values i1 to i3 stored in the registers 117 to 119, the difference between the electric signals indicating the interference phase, that is, i1-i2 and i3-i1, is calculated by the next difference calculator 121. The ATAN calculator 122 performs inverse tangent calculation of (i1-i2) / (i3-i1) from the i1-i2 and i3-i1 obtained by the difference calculator 121, and measures the reference light SrR1 from the relationship of several tens. The phase difference θ1 of the optical SrM1 is calculated. The output of the ATAN calculator 122 is the final result required.

In the configuration example shown in FIG. 1, the laser light source 101 and the optical phase modulator 102 form a measurement optical signal generator, and the optical coupler 103, the delay line 104, the fiber sensor 105, the collimators 106 and 107, and the reflection mirror 108. And 109 constitute an optical interference unit, and an optical / electric converter 114, an RF amplifier 115, an A / D converter 116, registers 117 to 119, a difference calculator 121, and an ATAN calculator 122 form a phase difference calculator. ing.

In such a configuration, an operation example thereof will be described with reference to the time chart of FIG.

FIG. 2 is a time chart showing an operation example of the first embodiment. a1 indicates the phase of the reference light SrR1 in the period T, a2 indicates the phase of the measurement light SrM1, and a3 indicates the phase of the interference portions (I1 to I3) of the interference light signal Li1 obtained as an interference result. "0" represents a reference phase whose phase has not been changed, "90" represents a phase whose phase has been changed by 90 degrees (hereinafter referred to as "90 degree phase"), and "A" represents an arbitrary phase. Represents.

As shown in a1, the reference light SrR1 is created so that the phase of the t1, t2 and t4 sections in the period T is the reference phase and the phase of the t3 section is the 90 degree phase. Further, the measurement light SrM1 shown in a2 is obtained by delaying the reference light SrR1 by τ, which is the time interval of each section. a3 shows the state of the interference light signal Li1 in which the reference light SrR1 shown in a1 and the measurement light SrM1 shown in a2 are interfered with each other. Of the interference light signal Li1, the result of interference between the position of t2 of the reference light SrR1 and the position of t1 of the measurement light SrM1 is the interference portion I1, and similarly, the result of interference between t3 and t2 is the interference portions I2 and t4. The result of the interference of t3 is the interference portion I3. From the electric signal values i1, i2, and i3 of these interference portions, the phase difference θ1 between the reference light SrR1 and the measurement light SrM1 can be obtained.

The 90-degree phase section in the period T is not limited to the t3 section as shown in FIG. 2, and in the interference light signal, "0-0" and "90-" are used as a combination of the phases of the reference light and the measurement light. If three patterns of "0" and "0-90" are formed, the position of the 90-degree phase section may be changed or the number of the same section may be increased. For example, as shown in FIG. 3A, the phase of the t2 section may be 90 degree phase. Alternatively, as shown in FIG. 3B, the phase of the t3 section is 90 degrees, the phase of the t5 section adjacent to the t4 section is the reference phase, the delay of the measurement light is 2τ, and the interference light signal is included. The three patterns of "0-0", "90-0" and "0-90" may be formed.

Other embodiments of the present invention will be described.

FIG. 4 is a configuration example (second embodiment) of the optical fiber sensor according to the present invention in the case of the homodyne type and the interference portions are I1 to I4.

For the narrow line width optical signal Sr generated by the laser light source 101, the optical phase modulator 202 compares the phase of only the t3 section during the period T with the other t1, t2, t4, and t5 sections by 90. Change the degree. The timing for changing the phase and the time τ for changing the phase are controlled by the pulse generator 113. The optical signal (measurement optical signal) Sra2 to which the phase change is intermittently given is supplied to the optical coupler 103 and branched into two.

One of the branched light signals of the optical coupler 103 travels in space by the collimator 106, is reflected by the reflection mirror 108, and returns to the optical coupler 103 as reference light SrR2 again through the collimator 106. On the other hand, the other branched optical signal branched by the optical coupler 103 is guided to the fiber sensor 105 after being given a delay of τ / 2 with respect to the reference optical SrR2 by the delay line 104. The branched light signal that has passed through the fiber sensor 105 propagates in the air by the collimator 107, is reflected by the reflection mirror 109, passes through the collimator 107 and the fiber sensor 105, and is measured again with a delay of τ / 2 on the delay line 104. It is supplied to the optical coupler 103 as optical SrM2.

The reference light SrR2 from the collimator 106 and the measurement light SrM2 from the delay line 104 are both added to the optical coupler 103, and the combined light output thereof is supplied to the optical / electric converter 114 as the interference light signal Li2, and the interference light signal Li2 The electric signal Ea2 corresponding to the above is obtained. The electric signal Ea2 output from the optical / electric converter 114 is amplified to a required size by the RF amplifier 115, and then converted from the analog signal to the electric signal Ed2 of the digital signal by the A / D converter 116. The electric signal Ed2 is supplied to the registers 117, 118, 119 and 120, and according to the timing signal sent from the pulse generator 113, the electric signal values i1 of the interference portions I1, I2, I3 and I4 in the time chart described later. , I2, i3 and i4 are obtained on their respective registers. Based on the electric signal values i1 to i4 stored in the registers 117 to 120, the next difference calculator 221 calculates the difference between the electric signals indicating the interference phase, that is, i1-i2 and i3-i4. The ATAN calculator 122 performs inverse tangent calculation of (i1-i2) / (i3-i4) from i1-i2 and i3-i4 obtained by the difference calculator 221, and the reference light SrR2 and the measurement light are measured from the relationship of the number 11. The phase difference θ1 of SrM2 is calculated. The output of the ATAN calculator 122 is the final result required.

In such a configuration, an operation example thereof will be described with reference to the time chart of FIG.

FIG. 5 is a time chart showing an operation example of the second embodiment. b1 indicates the phase of the reference light SrR2 in the period T, b2 indicates the phase of the measurement light SrM2, and b3 indicates the phase of the interference portions (I1 to I4) of the interference light signal Li2 obtained as an interference result. The meanings of "0", "90" and "A" are the same as in the case of FIG.

As shown in b1, the reference light SrR2 is created so that the phase of the t1, t2, t4 and t5 sections in the period T is the reference phase and the phase of the t3 section is the 90 degree phase. Further, the measurement light SrM2 shown in b2 is obtained by delaying the reference light SrR2 by τ, which is the time interval of each section. b3 shows the state of the interference light signal Li2 in which the reference light SrR2 shown in b1 and the measurement light SrM2 shown in b2 interfere with each other. Of the interference light signal Li2, the result of interference between the t2 position of the reference light SrR2 and the t1 position of the measurement light SrM2 is the interference portion I1, and similarly, the result of the interference between t3 and t2 is the interference portions I2, t4 and t3. The result of the interference is the interference portion I3, and the result of the interference between t5 and t4 is the interference portion I4. From the electric signal values i1, i2, i3 and i4 of these interference portions, the phase difference θ1 between the reference light SrR2 and the measurement light SrM2 can be obtained.

The 90-degree phase section in the period T is not limited to the t3 section as shown in FIG. 5, and in the interference light signal, "0-0" and "90-" are used as a combination of the phases of the reference light and the measurement light. If the four patterns of "0", "0-90", and "0-0" are formed, the position of the 90-degree phase section may be changed or the number of the sections may be increased.

The first embodiment and the second embodiment are examples of the homodyne type configuration, but the interferometer can be configured by the Machzenda type in addition to the homodyne type. The homodyne type may not be able to sufficiently separate the input and output, but the Machzenda type uses a different optical coupler for the input and output, so that the input and output can be easily separated.

FIG. 6 is a schematic configuration example (third embodiment) of the optical interference portion in the Machzenda type sensor. The intermittently phase-changed optical signal (measurement optical signal) Sra supplied to the optical coupler 103 is branched into two by the optical coupler 103, and one of the branched optical signals is referred to by the next optical coupler 110. It is supplied as SrR. The other branched optical signal is supplied to the fiber sensor 105 via a delay line 204 having a delay of τ. The fiber sensor 105 is a sensor in which the refractive index or the effective length of the fiber changes depending on the external pressure applied to the optical fiber, and the optical path length changes depending on the measurement item to be detected. The output optical signal of the fiber sensor 105 is also supplied to the optical coupler 110 as the measurement optical SrM, and the interference optical signal Li phase-modulated by the fiber sensor 105 is obtained as the output of the optical coupler 110.

The above-described embodiment (first to third embodiment) is an example of detecting an event to be measured by various strains received by the fiber. On the other hand, the configuration example shown in FIG. 7 is a configuration example (fourth embodiment) of the optical interference unit that detects the relative position fluctuation of the object by the homodyne type optical fiber sensor according to the present invention.

The optical signal Sra applied to the optical coupler 103 is branched into two by the same optical coupler, and one of the branched optical signals is propagated in the air through the collimator 106, reflected by the reflection mirror 108, and propagated in the air again to be propagated in the air again. It returns to the optical coupler 103 as the reference light SrR via. The other branched light signal is propagated in the air by the collimator 107 via a delay line 104 having a delay of τ / 2. The optical signal propagating in the air is reflected by an optical prism or a corner cube 111 fixed to an object whose relative distance variation is to be measured in the same direction as the optical signal incident on the optical prism or the like. The light signal reflected by the optical prism 111 is further reflected by the reflection mirror 109. The reflected wave is returned to the optical coupler 103 as the measurement light SrM again through the optical prism 111, the collimator 107, and the delay line 104. The optical coupler 103 causes both the reference light SrR and the measurement light SrM to interfere with each other, and outputs the interference light signal Li from the output terminal.

FIG. 8 is a configuration example (fifth embodiment) of the optical interference unit in which the homodyne type optical fiber sensor for distance measurement shown in FIG. 7 is changed to the Machzenda type. One of the optical signals branched into two by the optical coupler 103 is supplied to the next optical coupler 110 as reference optical SrR. The other optical signal bifurcated by the optical coupler 103 is propagated in the air by the collimator 107 via a delay line 204 having a delay of τ. The optical signal propagating in the air is reflected by an optical prism or a corner cube 111 fixed to an object whose relative distance variation is to be measured in the same direction as the optical signal incident on the optical prism or the like. The optical signal reflected by the optical prism 111 is received by another collimator 112, and the optical signal is supplied to the optical coupler 110 as measurement light SrM. The optical coupler 110 causes both the reference light SrR and the measurement light SrM to interfere with each other, and outputs the interference signal Li from the output terminal.

In the configuration example shown in FIG. 7, the measurement light emitted from the collimator 107 into space is reflected by an optical prism or a corner cube 111 fixed to an object whose relative distance variation should be measured. As shown in FIG. It is also possible to remove the optical prism or the corner cube 111 and mount the reflection mirror 109 directly on the surface of the distance measuring object.

In the explanation so far, the influence of the polarization fluctuation by the optical fiber on the measurement result has not been described, but the polarization fluctuation by the optical fiber affects the measured value, and as a result, an error may occur. In order to eliminate or reduce the measurement error due to polarization fluctuation, a polarization-maintaining type optical fiber may be used. Alternatively, in the configuration example shown in FIG. 7, as shown in FIG. 10, by using Faraday rotator mirrors 208 and 209 instead of the reflection mirrors 108 and 109, the polarization-retaining fiber is not used, that is, single. Using mode fiber, measurement with less error is possible.

In the above-described embodiment (first to fifth embodiments), the case where the change of the measurement target (that is, the change of the optical path length) is given only to the measurement light has been described as an example, but the reference light side and the measurement light side are mutually It is possible to double the sensitivity of the sensor by giving both the reference light and the measurement light a change that is in opposite phase, for example, a change in the front and back of an object whose position changes. ..

Further, in the above-described embodiment, continuous light rays are used as the laser beam for measurement, but it is also possible to construct a system by using optical pulses corresponding to t1 to t4 sections (or t1 to t5 sections). Is. In this case, the effect of cost reduction is reduced, but if a plurality of t1 to t4 sections (or t1 to t5 sections) of optical pulses are accommodated in the period T, a time-division multiplex measurement is performed by one system. Is possible.

Further, in the above-described embodiment, an example of a sensor for detecting a change in the optical path length of the fiber and a sensor for detecting a relative fluctuation of an object (rangefinder or vibration sensor) has been shown, but the present invention includes other temperature sensors and the like. It is applicable to many sensors.

The present invention is suitable for a sensor system installed in a remote place such as a submarine earthquake observation, a place where power cannot be supplied, a place susceptible to electromagnetic noise, a place where the temperature environment is harsh, a place where there is a possibility of ignition, and the like.

101 Laser light source 102 Optical phase modulator 103, 110 Optical coupler 104, 204 Delay line 105 Fiber sensor 106, 107, 112 Collimator 108, 109 Reflective mirror 111 Optical prism 113 Pulse generator 114 Optical / electric converter 115 RF amplifier 116 A / D converter 117, 118, 119, 120 Register 121, 221 Difference calculator 122 ATAN calculator 208, 209 Collimator mirror

Claims (5)

A measurement optical signal generator that generates a measurement optical signal having at least one quadrature phase section that is separated by a predetermined time interval and is 90 degrees phase-modulated with respect to the phase of the other section.
A delay is provided between the measurement light and the reference light, which are demultiplexes of the measurement optical signal, by an integral multiple of the predetermined time interval, and the measurement light and the reference light whose optical path length is changed by an external force are separated from each other. An optical interference unit that combines to generate an interference optical signal,
A phase difference calculation unit for calculating the phase difference between the measurement light and the reference light based on the interference light signal is provided.
The phase difference calculation unit includes an electric signal 1 of the interference light signal generated by combining the measurement light in the in-phase section that is not 90-degree phase-modulated and the reference light in the in-phase section, and the in-phase section. The electrical signal 2 of the interference light signal generated by the combination of the measurement light and the reference light in the orthogonal phase section in the above, and the measurement light in the orthogonal phase section and the reference light in the in-phase section. An optical fiber sensor that calculates the phase difference from the difference between the electric signal 1 and the electric signal 2 and the difference between the electric signal 3 and the electric signal 1 by using at least the electric signal 3 of the interference light signal. The optical fiber sensor according to claim 1, wherein the phase difference calculation unit uses an electric signal 11 and an electric signal 12 having different sections as the electric signal 1. The measurement optical signal generation unit generates the measurement optical signal in which the third section is the quadrature phase section and the other three sections are the same phase sections among the four consecutive sections.
The optical fiber sensor according to claim 1, wherein the optical interference unit provides a delay by the predetermined time interval between the measurement light and the reference light. The measurement optical signal generation unit generates the measurement optical signal in which the third section is the quadrature phase section and the other four sections are the same phase sections among the five continuous sections.
The optical fiber sensor according to claim 2 , wherein the optical interference unit provides a delay by the predetermined time interval between the measurement light and the reference light. The optical fiber sensor according to any one of claims 1 to 4, which is a homodyne type or a Machzenda type.

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