JP4930068B2 - Interferometric optical fiber sensor system and sensing method - Google Patents

Description

  The present invention relates to an interference type optical fiber sensor system capable of detecting various physical quantities.

  The interference type optical fiber sensor system can detect various physical quantities. In an optical fiber interferometer, a sensing fiber is used as an arm of an optical fiber interferometer, and a physical quantity to be detected is changed into a strain of the sensing fiber. The physical quantity is detected by utilizing the change of the interference light according to the strain of the sensing fiber. As an example of a conventional interference type optical fiber sensor, a sensor that detects an acoustic signal is described in Non-Patent Document 1 and Patent Document 1, and a sensor that detects a magnetic signal is Patent Document 2, and a sensor that detects acceleration. Are described in Non-Patent Document 2, respectively.

Non-Patent Document 1 also describes that a sensor array is constituted by a plurality of sensors. Non-Patent Document 1, Non-Patent Document 3, and the like describe a phase demodulation method for detecting a physical quantity included in interference light.
Japanese Patent No. 3323701 Japanese Patent No. 3107986 Ryo Sato, 3 others, "Development of optical fiber hydrophone", IEICE technical report, May 1995, OPE95-2, p.7-12 Yugo Shindo and three others, “Fiber-Optic Accelerometer”, 12th International Conference on Optical Fiber Sensors, October 1997, p. 202-205 (OWC15-1-4) Richard G. Richard G. Priest, “Analysis of Fiber Interferometer Utilizing 3 × 3 Fiber Coupler”, Quantum Electronics (IEEE JOURNAL OF QUANTUM ELECTRONICS), I Triple E (IEEE) ) October 1982, Volume QE-18, No. 10, p.1601-1603

  Although the light passing through the sensing fiber, that is, the phase change of the sensing light occurs due to the physical quantity to be detected, it also occurs due to temperature change or pressure change applied to the sensing fiber or the like. Since the frequency of temperature change and pressure change is low, it can be separated from the physical quantity by utilizing the difference in frequency. When temperature and pressure change greatly with a sensing fiber or the like, the frequency of the noise may reach the signal band of the physical quantity to be detected and become noise that cannot be separated.

  The present invention has been made in view of such problems, and an object thereof is to provide an interference type optical fiber sensor system in which noise is suppressed.

  In order to solve the above-described problems, the present invention provides an interference type including an interferometer having a sensing fiber and a reference fiber for detecting a physical quantity, and a detection unit for detecting a measurement signal corresponding to the physical quantity from the interference light from the interferometer. In the optical fiber sensor system, including an applying unit that applies a noise reduction signal for reducing a noise signal included in the interference light to the interferometer, and the detection unit outputs a measurement signal in which the noise is reduced by the noise reduction signal It is characterized by that.

  The noise reduction signal is preferably a signal for changing the frequency of the noise signal. For example, the noise reduction signal is a signal for increasing the frequency of the noise signal. At that time, the noise reduction signal is a sine wave, and the amplitude of the sine wave is such that the value of the 0th order Bessel function becomes zero when twice the amplitude is used as an argument of the 0th order Bessel function. Is preferred.

  The frequency of the noise reduction signal is preferably higher than the sum of the upper limit frequency of the measurement signal and the frequency of the noise signal. At this time, it is preferable to include a reduction means for reducing a noise reduction signal included in the measurement signal detected by the detection unit.

  Note that the noise reduction signal applied by the applying unit may be a signal for lowering the frequency of the noise signal. In this case, the application means is provided for the interferometer arm constituting the interferometer, and the means includes a material having a coefficient of thermal expansion larger than a predetermined value and a temperature control means for heating or cooling the material. be able to. Further, the material having a coefficient of thermal expansion larger than a predetermined value is preferably an aluminum alloy, and the temperature control means is preferably a Peltier element.

  According to the present invention, the noise reduction signal for reducing the noise signal included in the interference light is applied to the interference light, and the detection unit outputs the measurement signal in which the noise is reduced by the noise reduction signal. A suppressed interference type optical fiber sensor system can be provided.

  Next, an embodiment of an interference type optical fiber sensor system according to the present invention will be described in detail with reference to the accompanying drawings. First, the measurement principle of the physical quantity in the interference type optical fiber sensor system will be described with reference to FIG. 2, and an embodiment of the present invention in which the influence of noise generated in the system of FIG. 2 is suppressed will be described with reference to FIG. Hereinafter, the same reference numerals are assigned to the signal line and the signal flowing through the signal line.

  In the interference optical fiber sensor system 10 of FIG. 2, the pulse light source 12 generates pulsed light 12a and outputs the generated pulsed light 12a to the first optical coupler 14 via the optical fiber 12b. The first optical coupler 14 to which the pulsed light 12a is input splits the pulsed light 12a into two pulsed lights. The first optical coupler 14 outputs the two pulse lights obtained by the division to the mirrors 16a and 16b via the optical fibers 14a and 14b, respectively. The optical fiber 14a is a sensing fiber, and the optical fiber 14b is a reference fiber. The mirrors 16a and 16b reflect the two pulse lights, respectively. The reflected pulsed light returns through the optical fibers 14a and 14b, respectively, and is input to the second optical coupler 18 via the first optical coupler 14 and the optical fiber 14c.

  One of the pulsed light 12a passes through the sensing fiber 14a and becomes the sensing light 20, and the other passes through the optical fiber 14b and becomes the reference light 22. When the sensing light 20 passes through the sensing fiber 14a, it is phase-modulated by a physical quantity applied to the sensing fiber 14a. For this purpose, the sensing fiber 14a is provided with means for changing the physical quantity into the strain of the sensing fiber 14a as required according to the physical quantity to be measured. In addition to the distortion caused by the physical quantity, the sensing fiber 14a also generates distortion that generates a noise signal. The distortion that generates the noise signal is caused by a temperature change of the sensing fiber 14a itself.

  The sensing light 20 is also subjected to propagation delay by the sensing fiber 14a which is longer than the optical fiber 14b. As a result, two pulses 20 and 22 are transmitted on the optical fiber 14c.

  The sensing light 20 and the reference light 22 input to the second optical coupler 18 are each divided into two by the second optical coupler 18. One of the divided pulse lights passes through the delay compensation fiber 18a connected to the second optical coupler 18 and enters the mirror 24a. The light incident on the mirror 24a is reflected by the mirror 24a, passes through the delay compensation fiber 18a, the second optical coupler 18, and the optical fiber 18c in this order, and enters the O / E (Opto / Electronics) converter 26. The other part of the divided pulse light passes through the optical fiber 18b connected to the second optical coupler 18 and enters the mirror 24b. The light incident on the mirror 24b is reflected by the mirror 24b, passes through the optical fiber 18b, the second optical coupler 18, and the optical fiber 18c in this order, and enters the O / E converter 26.

  Since the lengths of the sensing fiber 14a and the delay compensation fiber 18a are set so that the propagation delay amount is equal, the light that has passed through the sensing fiber 14a but has not passed through the delay compensation fiber 18a, and the sensing fiber 14a The light that has passed through the delay compensation fiber 18a without passing through is input to the O / E converter 26 at the same timing. As a result, the two lights interfere with each other, and the interference light 25 shown in FIG. 1 enters the O / E converter 26 via the optical fiber 18c.

  Optical fiber 12b, first optical coupler 14, optical fibers 14a and 14b, mirrors 16a and 16b, optical fiber 14c, second optical coupler 18, delay compensation fiber 18a, mirrors 24a and 24b, optical fiber 18b, and optical fiber 18c Thus, an interferometer is configured. The interference light 25 is a pulse whose level changes due to interference. The intensity of the interference light 25 is A + B cos [Ccos (ωt) + φ (t)], which will be described later. Since the pulse 27 propagating before and after the interference light 25 does not interfere on the optical fiber 18c, the level thereof is constant.

  The delay compensation fiber 18a is attached to the piezoelectric electron 28, and a sine wave voltage is applied to the piezoelectric electron 28 from the PGC signal generator 30 via the signal line 30a. Therefore, sinusoidal distortion is applied to the delay compensation fiber 18a by the piezoelectric electrons 28. As a result, PGC (Phase Generated Carrier) is generated in the interference light. The PGC method is one of optical signal modulation / demodulation methods used in an interference type optical fiber sensor system. The laser light to which sinusoidal distortion is added is added to an interferometer having an optical path difference, and the output of the interferometer is input to the O / E converter 26 to convert it into an electrical signal.

  If the intensity of the signal output from the O / E converter 26 is I (t), I (t) is expressed in the form of A + B cos θ. In this equation, θ is the phase difference between the two interfering lights. The phase difference θ includes a phase change of the interfering light. Specifically, the phase change is a measurement signal to be detected by the sensor, a PGC signal, a temperature drift (noise signal), or the like. That is, I (t) = A + Bcos [(PGC signal) + (measurement signal) + (temperature drift)]. In FIG. 2, in order to explain the measurement principle, only the measurement signal and the PGC signal are considered, and the others are not considered. Noise and the like will be described later with reference to FIG. A sinusoidal phase change such as a PGC signal can be developed by a Bessel function as follows.

  In the case of FIG. 2 in which only the measurement signal and the PGC signal are considered, I (t) is given by A + B cos [Ccos (ωt) + φ (t)] (where C is the modulation factor). ωt is the angular frequency of the sine wave output from the PGC signal generator 30 that modulates the laser light, and φ (t) is the phase difference due to the measurement signal of the light that has passed through the interferometer. Ccos (ωt) in this equation is called PGC. The demodulation process for obtaining φ (t) from I (t) is performed as follows. In the demodulation process, when cos [Ccos (ωt) + φ (t)] is expanded by a Bessel function, the following expression is used.

上記展開式の１次の成分と２次の成分を、第１のAM復調器34aと第２のAM復調器34bにより、以下のように抽出する。

The first-order component and the second-order component of the expansion formula are extracted by the first AM demodulator 34a and the second AM demodulator 34b as follows.

The first AM demodulator 34a is the primary signal: 2J ₁ (C) · sinφ (t) · cosωt, and the second AM demodulator 34b is the secondary signal: 2J ₂ (C) · cosφ (t). Cos2ωt is synchronously detected to extract primary signals: J ₁ (C) · sinφ (t) and secondary signals: J ₂ (C) · cosφ (t), respectively. The first AM demodulator 34a and the second AM demodulator 34b receive a sine wave voltage from the PGC signal generator 30 via the signal line 30b for synchronous detection.

There are various methods for extracting the signal phase φ (t) from the outputs of the first AM demodulator 34a and the second AM demodulator 34b, but J ₁ (C) · sinφ (t ) Divided by J ₂ (C) · cosφ (t), then multiplied by J ₂ (C) / J ₁ (C), and tan ⁻¹ (arc tan) is taken to obtain the signal phase φ (t) To extract.

Specifically, the above processing will be described. An electrical signal corresponding to the interference light I (t) is output from the O / E converter 26 to the A / D converter 32 via the signal line 26a. The signal A / D converted by the A / D converter 32 is sent to the first AM demodulator 34a and the second AM demodulator 34b via the signal line 32a. The first AM demodulator 34a and the second AM demodulator 34b receive the electrical signal output from the A / D converter 32 and the sine wave voltage from the PGC signal generator 30, respectively. Signal: J ₁ (C) · sinφ (t) and secondary signal: J ₂ (C) · cosφ (t) are extracted. That is, the first AM demodulator 34a and the second AM demodulator 34b synchronized with the PGC signal generator 30 are used to determine the sine of the phase of the interference light, which is the odd-order amplitude of the PGC, and the even-order amplitude of the PGC. The cosine of the phase of the interference light is extracted.

  There are various specific methods for configuring the first AM demodulator 34a and the second AM demodulator 34b. For example, the AM demodulator may be configured by a multiplier and a low-pass filter (LPF). That is, the first AM demodulator 34a multiplies the electrical signal output from the A / D converter 32 by the sine wave voltage from the PGC signal generator 30, and processes the multiplication result with the LPF. The second AM demodulator 34b multiplies the electrical signal output from the A / D converter 32 by the frequency obtained by doubling the sine wave voltage from the PGC signal generator 30, and the multiplication result is LPF. Process with. The reason for doubling the sinusoidal voltage is that, as described above, the first AM demodulator 34a demodulates the primary signal including cosωt, and the second AM demodulator 34b includes the secondary signal including cos2ωt. This is for demodulating the signal.

The first AM demodulator 34a and the second AM demodulator 34b are respectively connected to a primary signal: J ₁ (C) · sinφ (t) and a secondary signal: J ₂ (C) via signal lines 36a and 36b. Cosφ (t) is output to the arc tangent calculator 38. The arc tangent calculator 38 calculates the phase φ (t) by arc tangent as described above, and outputs the phase φ (t) to the unwrap processor 40 via the signal line 38a.

  The unwrap processor 40 performs unwrap processing for connecting discontinuous points of arctangents. The reason why the unwrap process is necessary is as follows. In general, the result of the arc tangent calculation is output in the range of −π to π even if there are two arguments. Therefore, for example, when the input signal φ changes from 3.0 → 3.1 → 3.2 → 3.3, the calculation result by the arctangent calculator 38 becomes 3.0 → 3.1 → −3.08 (= 3.2−2π) → −2.98. The unwrap processor 40 performs processing for correcting a shift of 2πn (n is an integer) with reference to past values. The unwrap processor 40 outputs the obtained phase φ (t) to the output terminal 42 via the signal line 40a. In this way, the signal included in the phase of the interference light is demodulated. The A / D converter 32, the first AM demodulator 34a, the second AM demodulator 34b, the arc tangent calculator 38, and the unwrap processor 40 constitute a PGC demodulator 44. The PGC demodulator 44 detects a measurement signal corresponding to the physical quantity from the interference light from the interferometer.

  As described above, the phase change of the sensing light occurs due to the physical quantity to be detected, but it also occurs due to temperature change and pressure change. Since the frequency of temperature change and pressure change is low, it can be separated from the physical quantity by utilizing the difference in frequency.

  However, when there is an error in demodulation, high frequency noise is generated. For example, when the output amplitudes of the first AM demodulator 34a and the second AM demodulator 34b are a1 and a2, respectively, and the output amplitudes a1 and a2 do not match, an error occurs in the arctangent calculation. This will be described below. Assuming that the true value of the phase is φ and the error is Δφ, Δφ is expressed by the following equation.

ここで、Δφ<<1radとして近似した。かりに、温度変化による位相ドリフトがα[rad/sec]で一定のとき、ドリフトで位相が１周するごとに正弦波状の出力が２周期現われて雑音となる。以下、これを、単に雑音と記す。次式に示すように、雑音の振幅が復調誤差の最大値と一致し、周波数はα/π[Hz]で表される。

Here, it approximated as (DELTA) phi << 1rad. Incidentally, when the phase drift due to temperature change is constant at α [rad / sec], a sinusoidal output appears for two cycles each time the phase makes one round due to drift, resulting in noise. Hereinafter, this is simply referred to as noise. As shown in the following equation, the amplitude of noise coincides with the maximum value of the demodulation error, and the frequency is represented by α / π [Hz].

位相ドリフトの速度が変化すると、それに応じて雑音の周波数が変化する。PGC信号発生器30の波形歪み、圧電子28で発生する高調波でもAM復調器34a、34bの出力の振幅誤差の原因となり、同様に雑音が発生する。センシングファイバ14aと遅延補償ファイバ18aにおいて、温度及び圧力が大きく変化する場合、雑音の周波数が信号帯域に達して分離できない雑音となる。

As the phase drift speed changes, the noise frequency changes accordingly. Even the waveform distortion of the PGC signal generator 30 and the harmonics generated by the piezoelectric 28 cause the amplitude error of the outputs of the AM demodulators 34a and 34b, and noise is generated similarly. In the sensing fiber 14a and the delay compensation fiber 18a, when the temperature and pressure greatly change, the noise frequency reaches the signal band and becomes noise that cannot be separated.

  An embodiment of the present invention for solving this problem will be described with reference to FIG. In the following, the same parts as those in FIG. 2 are denoted by the same reference numerals, and the description thereof is omitted. FIG. 1 shows a configuration of a first embodiment of an interference type optical fiber sensor system 50 according to the present invention. Referring to FIG. 1, this embodiment is characterized in that noise due to phase drift is suppressed by adding a noise reduction signal.

  For this purpose, in this embodiment, a noise reduction signal generator 46 is provided, and the generator 46 generates a noise reduction signal. The noise reduction signal generator 46 applies a noise reduction signal for reducing the noise signal included in the interference light to the interferometer. The generated noise reduction signal is output to the adder 48 via the signal line 46a.

  The adder 48 also receives a sine wave voltage from the PGC signal generator 30 via the signal line 30b. The adder 48 adds the noise reduction signal and the sine wave voltage, and outputs the added signal to the piezoelectric 28 via the signal line 48a. As a result, a signal in which the signal to be measured, the noise signal, the PGC described above, and the noise reduction signal are added is generated in the delay compensation fiber 18a. Since the purpose is to superimpose two sine waves, the adder 48 may be a subtractor. However, in the case of a sine wave, since multiplication is performed by superimposing two sine waves, a multiplier can also be used in this embodiment.

  When the noise reduction signal is added, the aforementioned I (t) = A + Bcos [(PGC signal) + (measurement signal) + (temperature drift) is I (t) = A + Bcos [(PGC signal) + (Measurement signal) + (temperature drift) + (noise reduction signal)]. When the noise reduction signal has a sine wave shape, a Bessel function resulting from the noise reduction signal is added to the Bessel function described above resulting from the PGC signal.

Here, the frequency f _shift of the noise reduction signal is set so as to satisfy the following equation.

ここで、f_SHは、測定すべき信号帯域の上限周波数、f_NHは、想定される雑音信号の最高周波数である。雑音低減信号の波形は正弦波とすることが望ましく、雑音低減信号の振幅は復調器出力において、

Here, f _SH is the upper limit frequency of the signal band to be measured, and f _NH is the highest frequency of the assumed noise signal. The waveform of the noise reduction signal is preferably a sine wave, and the amplitude of the noise reduction signal is at the demodulator output,

とすることが望ましい。ここで、J₀は０次の第１種ベッセル関数、A_shiftは雑音低減信号の振幅である。たとえば、

Is desirable. Here, J ₀ is the zeroth-order first-type Bessel function, and A _shift is the amplitude of the noise reduction signal. For example,

で(4)式を満たす。

(4) is satisfied.

In this embodiment, an LPF 54 is provided at the output stage of the demodulator 52 in accordance with the addition of the noise reduction signal. The cutoff frequency f _LPF of the LPF 54 is set so as to satisfy the following equation.

LPF 54は、信号線54aを介して、雑音が低減された信号を出力端子42に出力する。LPF 54は、復調器52が出力する測定信号に含まれる雑音低減信号を低減する。復調器52は、雑音低減信号により雑音が低減された測定信号を出力する。なお、PGCの周波数の設定においては、雑音低減信号も考慮して、雑音低減信号がPGC周波数から折り返す現象で問題にならないように、PGC周波数を設定する。

The LPF 54 outputs a signal with reduced noise to the output terminal 42 via the signal line 54a. The LPF 54 reduces the noise reduction signal included in the measurement signal output from the demodulator 52. The demodulator 52 outputs a measurement signal with noise reduced by the noise reduction signal. In setting the PGC frequency, the PGC frequency is set in consideration of the noise reduction signal so as not to cause a problem due to the phenomenon that the noise reduction signal turns back from the PGC frequency.

  Next, the operation of the interference type optical fiber sensor system 50 configured as described above will be described. The difference from the system 10 of FIG. 2 is that by adding a noise reduction signal, the frequency of the noise signal is shifted to a frequency higher than the signal band and is blocked by the LPF 54. When a sinusoidal noise reduction signal is added, Δφ in the equation (1) is expressed by the following equation.

この式は、次のように、ベッセル関数で展開できる。

This equation can be expanded by a Bessel function as follows.

(6)式の[ ]内の最初の項は、図１に示す例でも発生する項であり、本実施例では、A_shiftがゼロでなくなるため、J₀(2A_shift)が１未満の係数となる。これにより、高周波にシフトしないで残る成分が小さくなり、雑音が抑制されることが分かる。[ ]内の２番目以降の項は、J₁(2A_shift)、J₂(2A_shift)がゼロではなくなることで、f_shiftの整数倍の周波数帯に、雑音成分がシフトすることを表している。さらに、(4)式の条件を満たす場合、(6)式の[ ]内の最初の項がほぼゼロとなり、雑音成分の内、高周波にシフトしないで残る成分が最小になる。

The first term in [] in equation (6) is a term that also occurs in the example shown in FIG. 1. In this embodiment, A _shift is not zero, so that J ₀ (2A _shift ) is a coefficient less than 1. It becomes. As a result, it can be seen that the component remaining without shifting to high frequency is reduced, and noise is suppressed. The second and subsequent terms in [] indicate that the noise component shifts to a frequency band that is an integral multiple of f _shift when J ₁ (2A _shift ) and J ₂ (2A _shift ) are no longer zero. Yes. Furthermore, when the condition of the expression (4) is satisfied, the first term in [] of the expression (6) is almost zero, and the remaining noise component without shifting to a high frequency is minimized.

  Since the expression (3) is satisfied, the phenomenon that the folding from the second term in [] of the expression (6) overlaps the signal band can be prevented. Further, the filter 54 satisfying the expression (5) can block the second and subsequent terms in [] of the expression (6), that is, noise shifted to a high frequency.

  According to the present embodiment, it is possible to provide an interference type optical fiber sensor system in which noise is suppressed.

  Next, a second embodiment of the interference type optical fiber sensor system of the present invention will be described. FIG. 3 shows a configuration of an interference type optical fiber sensor system 60 according to the second embodiment. The difference from the first embodiment is that a demodulation system using PGC is changed to a system using a multi-port optical coupler. Instead of the optical coupler 18 of FIG. 1, a three-port optical coupler 56 is provided. As in FIG. 1, the pulse lights reflected by the mirrors 16a and 16b return to the optical fibers 14a and 14b, respectively, pass through the first optical coupler 14 and the optical fiber 14c, and the second optical coupler 56. Is input.

  The sensing light 20 and the reference light 22 input to the optical coupler 56 are divided into two by the optical coupler 56, respectively. One of the divided pulse lights passes through the delay compensation fiber 18a connected to the optical coupler 56 and enters the mirror 24a. The light is reflected by the mirror 24a, passes through the delay compensation fiber 18a, the optical coupler 56, and the optical fibers 56a and 56b in order, and is input to the first O / E converter 58a and the second O / E converter 58b. The other of the divided pulsed light passes through the optical fiber 18b connected to the optical coupler 56 and enters the mirror 24b. The light is reflected by the mirror 24b, passes through the optical fiber 18b, the optical coupler 56, and the optical fibers 56a and 56b in order, and is input to the first O / E converter 58a and the second O / E converter 58b.

  The light passing through the sensing fiber 14a but not passing through the delay compensation fiber 18a and the light passing through the delay compensation fiber 18a without passing through the sensing fiber 14a are at the same timing as the first O / E converter 58a. Input to both of the second O / E converters 58b. As a result, the two lights interfere with each other, and the interference light 25 shown in FIG. 3 is input to the first O / E converter 58a and the second O / E converter 58b.

  The outputs of the first O / E converter 58a and the second O / E converter 58b are input to the first A / D converter 64a and the second A / D converter 64b in the PGC demodulator 59. Is done. The branching ratio of the optical coupler 56 is set to 1: 1: 1, and the interference light output from the first O / E converter 58a via the signal line 62a is matched with the output timing of the first By sampling in the A / D converter 64a,

という信号が得られる。ここで、I₁は、第１のA/D変換器64aが出力する干渉光の強度である。I₁は、雑音信号が無く、かつ雑音低減信号が印加されていないとしたときの干渉光の強度である。(7)式において、π／３の項がある理由は、分岐比が１：１：１の場合、３ポートの光カプラの３個の出力の間には、２π／３の位相差が生じ、２π／３の位相差を、便宜的に(7)式と、後述する(8)式にπ／３ずつ割り当てて表現したためである。

Is obtained. Here, I ₁ is the intensity of the interference light output from the first A / D converter 64a. I ₁ is the intensity of the interference light when there is no noise signal and no noise reduction signal is applied. In equation (7), the reason for the term π / 3 is that when the branching ratio is 1: 1: 1, a phase difference of 2π / 3 occurs between the three outputs of the 3-port optical coupler. This is because the phase difference of 2π / 3 is expressed by assigning π / 3 to Equation (7) and Equation (8) described later for convenience.

  By sampling the interference light output from the second O / E converter 58b via the signal line 62b in the second A / D converter 64b in accordance with the output timing thereof,

という信号が得られる。ここで、I₂は、第２のA/D変換器64bが出力する干渉光の強度である。I₂は、雑音信号が無く、かつ雑音低減信号が印加されていないとしたときの干渉光の強度である。分岐比が１：１：１の３ポートの光カプラを用いることにより、(7)、(8)式に示すように、互いに２π／３の位相差がある干渉光を得ることができる。

Is obtained. Here, I ₂ is the intensity of the interference light output from the second A / D converter 64b. I ₂ is the intensity of the interference light when there is no noise signal and no noise reduction signal is applied. By using a three-port optical coupler with a branching ratio of 1: 1: 1, interference light having a phase difference of 2π / 3 can be obtained as shown in equations (7) and (8).

The first A / D converter 64a and the second A / D converter 64b output the sampling results to the first computing unit 68a and the second computing unit 68b via signal lines 66a and 66b, respectively. To do. The first computing unit 68a multiplies the difference between I ₁ and I _{2 by} a constant to obtain an output proportional to sinφ, as shown in the following equation.

第２の演算器68bは、次式に示すように、I₁とI₂の和から直流成分2Aを減算し、さらに定数を掛けて、cosφに比例した出力を得る。

The second computing unit 68b subtracts the DC component 2A from the sum of I ₁ and I ₂ as shown in the following equation, and further multiplies the constant to obtain an output proportional to cos φ.

このようにして、第１の実施例の第１のAM復調器34aと第２のAM復調器34bと同様の信号が得られる。第１の演算器68aと第２の演算器68bは、信号線70a、70bを介して2Bsinφ(t)および2Bcosφ(t)を逆正接演算器38に出力する。ここで、(9)式の２Bが(1)式のa₁、(10)式の2Bが(10)式のa₂に相当する。雑音信号があり、かつ雑音低減信号が印加されているときの第１の演算器68aと第２の演算器68bの出力は、第１の実施例と同様であり、雑音低減信号を用いることにより、雑音を低減することができる。

In this way, signals similar to those of the first AM demodulator 34a and the second AM demodulator 34b of the first embodiment are obtained. The first computing unit 68a and the second computing unit 68b output 2Bsinφ (t) and 2Bcosφ (t) to the arctangent computing unit 38 via the signal lines 70a and 70b. Here, 2B in equation (9) corresponds to a ₁ in equation (1), and 2B in equation (10) corresponds to a ₂ in equation (10). The outputs of the first computing unit 68a and the second computing unit 68b when there is a noise signal and the noise reducing signal is applied are the same as in the first embodiment, and by using the noise reducing signal, , Noise can be reduced.

  The first and second A / D converters 64a and 64b, the first and second AM demodulators 64a and 64b, the arctangent calculator 38, the unwrap processor 40, and the LPF 54 constitute a demodulator 59. The demodulator 59 detects a measurement signal corresponding to the physical quantity from the interference light from the interferometer.

The interference-type optical fiber sensor system 60 of this embodiment operates in the same manner as the first embodiment except that demodulation is performed using a multi-port optical coupler 56. Ideally, a ₁ = a ₂ , but the error is due to the branching ratio error of the 3-port optical coupler, the difference in conversion gain between the first O / E converter 58a and the second O / E converter 58b, etc. This is a cause of noise expressed by equation (1). However, noise in the signal band is suppressed by the noise reduction signal. As described above, even when the demodulation is performed by the method using the multi-port optical coupler, the same effect as the first embodiment can be obtained.

  Next, a description will be given of a third embodiment of the interference type optical fiber sensor system of the present invention. FIG. 4 shows the configuration of an interference type optical fiber sensor system 80 according to the third embodiment. In the present embodiment, a Peltier element 72 and a high thermal expansion material 74, which are means for canceling the phase drift and delaying the phase drift, are provided on the optical fiber 18a serving as the interferometer arm. The Peltier element 72 and the high thermal expansion material 74 are means for applying a noise reduction signal for lowering the frequency of the noise signal to the interferometer.

  3 differs from that shown in FIG. 3 in that a Peltier element 72, a high thermal expansion material 74 having a high coefficient of thermal expansion, and a band pass filter (BPF) 78 are provided instead of having the noise reduction signal generator 46. Yes. The delay compensation fiber 18a is bonded to the high thermal expansion material 74. The output of the unwrap processor 40 in the demodulator 76 is sent to the band pass filter 78 via the signal line 40a. The frequency of the band pass filter 78 is set so as to pass signals in the frequency band of phase drift but not signals in the frequency band of the measurement signal. The band pass filter 78 outputs a signal whose band is limited to the amplifier 82 via the signal line 78a. The amplifier 82 amplifies the input signal and outputs the amplified signal to the Peltier element 72 via the signal line 82a. The Peltier element 72 is driven by the amplifier 82. The Peltier element 72 changes the temperature of the high thermal expansion material 74 by heating or cooling the high thermal expansion material 74 to which the delay compensation fiber 18a is bonded in accordance with the input signal.

  The direction of heating or cooling by the Peltier element 72 is set to cancel the phase drift occurring in the sensing fiber 14a and the delay compensation fiber 18a. The low cut-off frequency of the BPF 78 is set so that the amplifier 82 that drives the Peltier element 72 is not saturated and the Peltier element 72 is not damaged by extreme heating or cooling. The reason why the low frequency is cut off by the BPF 78 is that when the low frequency passes, the DC component may accumulate and become large, and at this time, the amplifier 82 is saturated or extreme overheating or cooling occurs. It is. Therefore, it is necessary to slowly lower the output of the amplifier 82 so that the Peltier element 72 approaches the ambient temperature. The high cutoff frequency of BPF 78 is set lower than the lower limit of the signal band.

  As the high thermal expansion material 74, an aluminum alloy having a thermal expansion larger than that of glass and a quick response speed is suitable. According to the present embodiment, the speed of the phase drift is reduced, the noise is shifted to a lower frequency, and the noise can be suppressed.

  In this embodiment, the reason why the PGC signal generator is unnecessary is because a 3-port optical coupler is used. The reason why the noise reduction signal generator is not required is different from that in the second embodiment. This is because noise is suppressed. In the second embodiment, the noise is shifted to a frequency higher than the signal band. In the third embodiment, the noise is shifted to a frequency lower than the signal band.

  In all of the embodiments, the phase Φ (t) is calculated by the arc tangent calculation of sinφ (t) and cosφ (t) using the arc tangent calculator and the unwrapping calculator. However, the demodulation circuit disclosed in Non-Patent Document 1 can also be used. That is, the time derivative of φ (t) may be calculated by differential cross multiplication and subtraction, and this may be integrated to obtain an amount proportional to φ (t) shown in Equation (3) of Non-Patent Document 1.

  In the second embodiment, a three-port coupler having a branching ratio of 1: 1: 1 is used. However, even if an optical coupler having a different branching ratio or the number of ports is used, the calculation by the calculator is performed according to the optical coupler. By changing the phase, the phase can be demodulated.

  In all the embodiments, the example of configuring the Michelson interferometer using the optical coupler and the mirror has been described. However, the present invention can also use other interferometers such as a Mach-Zehnder interferometer. Even when the type of the interferometer is changed, the noise is shifted to a frequency higher than the signal band by the arm of the interferometer as in the first or second embodiment, or the noise is changed in the same manner as in the third embodiment. Can be shifted to a lower frequency.

  In this invention, the sensor array which consists of a some sensor shown by the nonpatent literature 1 can also be used. When the third embodiment is applied to the sensor array, it is desirable to pass signals output from all the sensors constituting the sensor array to the BPF and input an average value of these BPF outputs to the Peltier element. However, if the phase drift of all sensors is close to each other, only one typical phase drift may be used.

  In the first and second embodiments, piezoelectrons are used. However, instead of piezoelectrons, the delay compensation fiber may be distorted by an electromagnetic actuator. The phase of light may be changed by other means such as frequency modulation of 12 frequencies.

  Moreover, although the high thermal expansion material and the Peltier element are used in the third embodiment, the phase drift may be suppressed instead of other methods such as a heater, a piezoelectric electron, an electromagnetic actuator, and the like. Further, the phase drift can be suppressed by other means such as changing the frequency of the laser light source 12. Further, without using a high thermal expansion material, the temperature of the optical fiber can be controlled by the temperature control means, and the phase drift can be suppressed by the refractive index change of the optical fiber due to the temperature change of the optical fiber itself.

  Although the BPF is used in the third embodiment, if the response speed of the phase change including the temperature change of the high thermal expansion material is slow enough not to affect the signal band, the function of blocking the high frequency from the BPF is provided. The removed high-pass filter (HPF) may be used.

  In all the embodiments, the interference light is sampled by the A / D converter after the O / E conversion. However, a gate circuit for cutting out a pulse, a sample hold circuit, or the like can be used.

  In the first embodiment and the second embodiment, the noise reduction signal is generated by the delay compensation fiber, but the noise reduction signal can also be generated in the other part which becomes the arm of the interferometer. Further, in the third embodiment, the phase drift is suppressed by the delay compensation fiber, but the phase drift can also be suppressed by the other part serving as the arm of the interferometer.

It is a block diagram which shows the structure of the 1st Example of the interference type optical fiber sensor system by this invention. It is a block diagram for demonstrating the measurement principle of an interference type optical fiber sensor system. It is a block diagram which shows the structure of the 2nd Example of the interference type optical fiber sensor system by this invention. It is a block diagram which shows the structure of the 3rd Example of the interference type optical fiber sensor system by this invention.

Explanation of symbols

10, 50, 60, 80 Interferometric optical fiber sensor system
14a Sensing fiber
14b Reference fiber
18a Delay compensation fiber
26 O / E converter
28 Piezoelectric
30 PGC signal generator
44, 52 PGC demodulator
46 Noise reduction signal generator
59, 76 Demodulator
72 Peltier element
74 High thermal expansion material
78 BPF

Claims (9)

An interference type optical fiber sensor system including an interferometer having a sensing fiber and a reference fiber for detecting a physical quantity, and a detection unit for detecting a measurement signal corresponding to the physical quantity from the interference light from the interferometer.
Applying means for applying a noise reduction signal for reducing a noise signal included in the interference light to the interferometer;
The detection unit outputs a measurement signal in which noise is reduced by the noise reduction signal,
The interference type optical fiber sensor system, wherein the noise reduction signal is a signal for changing a frequency of the noise signal. 2. The interference type optical fiber sensor system according to claim 1 , wherein the noise reduction signal is a signal for increasing a frequency of the noise signal. 3. The interference type optical fiber sensor system according to claim 2 , wherein the noise reduction signal is a sine wave, and the amplitude of the sine wave is 0 when an amplitude of the 0th order Bessel function is set to twice the amplitude. An interference type optical fiber sensor system characterized in that the value of the next Bessel function is zero. 2. The interference type optical fiber sensor system according to claim 1 , wherein the noise reduction signal applied by the applying unit is a signal for lowering the frequency of the noise signal. . 5. The interference type optical fiber sensor system according to claim 4 , wherein the applying means is provided to an interferometer arm constituting an interferometer, and the applying means includes a material having a thermal expansion coefficient larger than a predetermined value, And a temperature control means for heating or cooling the material. 6. The interference type optical fiber sensor system according to claim 5 , wherein the material is an aluminum alloy, and the temperature control means is a Peltier element. An interference type optical fiber sensor system including an interferometer having a sensing fiber and a reference fiber for detecting a physical quantity, and a detection unit for detecting a measurement signal corresponding to the physical quantity from the interference light from the interferometer.
Applying means for applying a noise reduction signal for reducing a noise signal included in the interference light to the interferometer;
The detection unit outputs a measurement signal in which noise is reduced by the noise reduction signal,
The interference-type optical fiber sensor system, wherein the frequency of the noise reduction signal is higher than the sum of the upper limit frequency of the measurement signal and the frequency of the noise signal. An interference type optical fiber sensor system including an interferometer having a sensing fiber and a reference fiber for detecting a physical quantity, and a detection unit for detecting a measurement signal corresponding to the physical quantity from the interference light from the interferometer.
Applying means for applying a noise reduction signal for reducing a noise signal included in the interference light to the interferometer;
The detection unit outputs a measurement signal in which noise is reduced by the noise reduction signal,
The system further includes a reduction unit that reduces the noise reduction signal included in the measurement signal detected by the detection unit. In the interferometric optical fiber sensing method of detecting a physical quantity with an interferometer having a sensing fiber and a reference fiber, and detecting a measurement signal corresponding to the physical quantity from the interference light from the interferometer, the method includes:
Applying a noise reduction signal for reducing a noise signal included in the interference light to the interferometer;
Outputting a measurement signal in which noise is reduced by the noise reduction signal ;
It said noise reduction signal, the interference type optical fiber sensing wherein the signal der Rukoto for changing the frequency of the noise signal.

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