WO2020194856A1 - Optical coherent sensor and optical coherent sensing method - Google Patents

Optical coherent sensor and optical coherent sensing method Download PDF

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WO2020194856A1
WO2020194856A1 PCT/JP2019/045776 JP2019045776W WO2020194856A1 WO 2020194856 A1 WO2020194856 A1 WO 2020194856A1 JP 2019045776 W JP2019045776 W JP 2019045776W WO 2020194856 A1 WO2020194856 A1 WO 2020194856A1
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time
light
optical
respect
phase
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French (fr)
Japanese (ja)
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直毅 山城
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沖電気工業株式会社
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J9/00—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
    • G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for

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  • the present invention relates to, for example, an optical coherent sensor and an optical coherent sensing method applicable to a distributed vibration sensor using an optical fiber.
  • This OTDR is also applied to optical sensors such as distributed measurement of vibration transmitted to optical fibers.
  • vibration When vibration is applied to the optical fiber, the phase of the backscattered light generated near the position where the vibration is applied changes. Therefore, by observing the phase change of the backscattered light obtained by the OTDR, it is possible to obtain information on the vibration applied to the optical fiber in a distributed manner.
  • a distributed vibration sensor DAS: Distributed Vibration Sensor
  • DAS Distributed Acoustic Sensor
  • the phase of the backscattered light changes due to the vibration applied to the optical fiber from the outside, but it also depends on the phase of the probe light.
  • the phase of the probe light changes due to the vibration received before reaching the backscattered light generation position (observation point). Therefore, in order to measure the vibration at each position, a means for measuring the phase difference of the backscattered light generated at two different points on the optical fiber is usually adopted. Generally, the distance between these two points is called the gauge length.
  • a vibration waveform can be obtained from the obtained time change of the phase difference.
  • the OTDR waveform obtained by using a high coherence laser as a light source of probe light is obtained as a result of interference of backward scattered light from a plurality of scattering centers generated during the propagation of an optical pulse. Therefore, the intensity of the backscattered light varies irregularly with respect to the generation position. This phenomenon is called fading and affects the vibration measurement results.
  • the present invention has been made in view of the above-mentioned problems, and an object of the present invention is an optical coherent sensor and an optical sensor that avoids deterioration of accuracy due to fading by signal processing without introducing an expensive device.
  • the purpose is to provide a coherent sensing method.
  • the optical coherent sensor of the present invention coherently detects a light source unit that generates an optical pulse as probe light and a signal light generated by a measurement object by the probe light to generate a beat signal. It is configured to include a light receiving unit and a calculation unit into which a beat signal is input.
  • the calculation unit includes optical information acquisition means, accuracy deterioration avoidance means, and phase difference information acquisition means.
  • the optical information acquisition means acquires the distribution of the intensity I (tj) and the phase P (tj) of the signal light from the beat signal with respect to the reception time tj of the signal light for each optical pulse.
  • the accuracy deterioration avoiding means sets the reference time ST.
  • the phase difference information acquisition means determines the phase difference of the signal light with respect to the light receiving time tj, the difference between the phase P (tk) with respect to the light receiving time tk and the phase P (ti) with respect to the light receiving time ti, P (tk) ⁇ P (ti). And the distribution of the phase difference with respect to the reception time of the signal light is acquired.
  • the difference between tk and ti is the reference time ST.
  • the accuracy deterioration avoiding means changes the first time ⁇ t1 with respect to the reference time ST0 in the initial state, and a plurality of lights for each first time ⁇ t1.
  • the minimum value of the intensity I (tk + ⁇ t1) in the pulse is acquired, and the maximum first time ⁇ t1max at which this minimum value is maximum is acquired.
  • the second time ⁇ t2 is changed with respect to the reference time ST0 in the initial state, and the minimum value of the intensity I (ti ⁇ t2) in each of a plurality of optical pulses is obtained for each second time ⁇ t2, and this minimum value is obtained.
  • the maximum second time ⁇ t2max at which the value becomes the maximum is acquired. After that, ST0 + ⁇ t1max + ⁇ t2max is set as the reference time ST.
  • the optical coherent sensing method of the present invention is configured to include the following processes. First, an optical pulse is generated as probe light. Next, the signal light generated in the object to be measured by the probe light is coherently detected to generate a beat signal. Next, for each optical pulse, the distribution of the intensity I (tj) and the phase P (tj) of the signal light with respect to the light reception time tj of the signal light is acquired from the beat signal. Next, the reference time ST is set. Next, the phase difference of the signal light with respect to the light receiving time tj is acquired as the difference between the phase P (tk) with respect to the light receiving time tk and the phase P (ti) with respect to the light receiving time ti, P (tk) ⁇ P (ti). Acquire the distribution of the phase difference with respect to the reception time of the signal light.
  • the first time ⁇ t1 is changed with respect to the reference time ST0 in the initial state, and each first time ⁇ t1
  • the minimum value of the intensity I (tk + ⁇ t1) in each of the plurality of optical pulses is acquired, and the maximum first time ⁇ t1max at which this minimum value is maximum is acquired.
  • the second time ⁇ t2 is changed with respect to the reference time ST0 in the initial state, and the minimum value of the intensity I (ti ⁇ t2) in each of a plurality of optical pulses is obtained for each second time ⁇ t2, and this minimum value is obtained.
  • the maximum second time ⁇ t2max at which the value becomes the maximum is acquired.
  • ST0 + ⁇ t1max + ⁇ t2max is set as the reference time ST.
  • the optical coherent sensor and the optical coherent sensing method of the present invention in obtaining the phase difference, the first time ⁇ t1 and the second time ⁇ t2 are changed so that the intensity I of the signal light from the two points becomes large.
  • the reference time ST By setting the reference time ST, it is possible to avoid deterioration of accuracy due to fading.
  • FIG. 1 is a schematic diagram for explaining the optical coherent sensor of the present invention.
  • a vibration detection optical fiber sensor will be described as a configuration example of the optical coherent sensor.
  • the vibration detection optical fiber sensor includes a light source unit 10, an optical circulator 20, an optical fiber 30, a light receiving unit 50, and a calculation unit 60. This vibration detection optical fiber sensor is used for OTDR.
  • the light source unit 10 periodically generates an optical pulse as probe light. That is, the light source unit 10 generates an optical pulse train.
  • the spatial resolution of the vibration detection optical fiber sensor depends on the width of this optical pulse. Further, the measurement distance of the vibration detection optical fiber sensor depends on the repetition frequency of the optical pulse. It takes 5 ns for the optical pulse to propagate 1 m through the optical fiber 30 which is the object to be measured. When observing the backscattered light generated by the optical fiber 30 as the signal light, it takes a round trip time of the forward propagation and the reverse propagation, so that a delay of 10 ns per 1 m occurs. For example, when the pulse width is 100 ns and the repetition frequency is 5 kHz, the spatial resolution is 10 m and the maximum measurement distance is 20 km.
  • the light source unit 10 includes, for example, a laser light source 12, a fiber coupler 13, an intensity modulator 14, a function generator 16, and an optical amplifier 18.
  • the laser light source 12 generates laser light as continuous light in the communication wavelength band.
  • the laser light source 12 it is preferable to use a so-called narrow line width laser having a line width of 10 kHz or less.
  • this vibration detection optical fiber sensor can be used for a phase sensitive OTDR.
  • the wavelength of the laser beam may be arbitrary, but it is preferable to use a standard single-mode optical fiber with a low loss of 1550 nm.
  • the laser beam generated by the laser light source 12 is sent to the fiber coupler 13.
  • the fiber coupler 13 splits the laser beam into two. One of the two branches is sent to the intensity modulator 14. The other bifurcated light is sent to the light receiving unit 50 as reference light.
  • the function generator 16 generates a rectangular electric pulse. This electric pulse is sent to the intensity modulator 14.
  • the electric pulse generated by the function generator 16 has, for example, a pulse width of 100 ns width and a repetition frequency of 5 kHz.
  • the output of the function generator 16 is also sent to the analog-to-digital (A / D) converter 56, which will be described later, and is used as a trigger signal.
  • the intensity modulator 14 converts the laser beam into an optical pulse with an electric pulse to generate an optical pulse.
  • This optical pulse is sent to the optical amplifier 18.
  • the pulse width and repetition frequency of the optical pulse generated by the intensity modulator 14 are both the same as the electric pulse generated by the function generator 16.
  • the optical pulse has a pulse width of 100 ns and a repetition frequency of 5 kHz.
  • an acousto-optic modulator (AOM: Acoustic Modulator) is used.
  • AOM Acoustic Modulator
  • the frequency of the optical pulse generated by the intensity modulator 14 changes from the frequency of the laser beam input to the intensity modulator 14 due to the optical Doppler effect. For this reason, heterodyne detection is often performed during coherent detection by the light receiving unit 50.
  • the optical pulse generated by the intensity modulator 14 receives a predetermined amplification by the optical amplifier 18. This is because the stronger the intensity of the optical pulse, the stronger the intensity of the backscattered light in the optical fiber 30.
  • the optical pulse amplified by the optical amplifier 18 is sent to the optical fiber 30 as probe light via the optical circulator 20. Although illustration and description are omitted here, a bandpass filter (BPF) is generally used after the optical amplifier 18.
  • BPF removes noise of spontaneous emission light (ASE: Amplified Spontaneous Emission) generated by the optical amplifier 18.
  • the probe light sent to the optical fiber 30 propagates through the optical fiber 30, and backscattered light is generated along with the propagation of the probe light.
  • This backscattered light is sent to the light receiving unit 50 as signal light via the optical circulator 20.
  • an optical amplifier and a BPF are often provided in front of the light receiving unit 50 in order to amplify the backscattered light.
  • the light receiving unit 50 coherently detects the backscattered light generated in the optical fiber 30 by the probe light and generates an electric signal.
  • the light receiving unit 50 includes a coherent receiver 52, a balanced photodiode (PD) 54, and an analog-to-digital (A / D) converter 56.
  • the coherent receiver 52 uses the reference light to perform coherent detection of backscattered light.
  • the coherent receiver 52 for example, an optical 90 ° hybrid coupler can be used.
  • the output from the coherent receiver 52 is sent to the balanced PD 54.
  • the balanced PD54 balance-detects the output from the coherent receiver 52.
  • I-phase (cos wave) and Q-phase (sine wave) beat signals having information on the intensity and phase of backscattered light are generated.
  • the I-phase and Q-phase beat signals are sent to the A / D converter 56.
  • the A / D converter 56 converts the I-phase and Q-phase beat signals into digital signals.
  • the I-phase and Q-phase beat signals converted into digital signals are input to the arithmetic unit 60.
  • the arithmetic unit 60 for example, a commercially available personal computer (PC) can be used.
  • the arithmetic unit 60 will be described as being configured to include a CPU (Central Processing Unit) 70, a RAM (Random Access Memory) 62, a ROM (Read Only Memory) 64, and a storage means 66.
  • the CPU 70 realizes each functional means described later by executing a program stored in the ROM 64.
  • the processing result of each functional means is temporarily stored in the RAM 62.
  • Functional means included in the calculation unit 60 includes optical information acquisition means 72, phase difference information acquisition means 74, and accuracy deterioration avoidance means 76.
  • the optical information acquisition means 72 sends the distribution of the intensity I (t) and the phase P (t) of the backscattered light with respect to the light receiving time t of the backscattered light from the A / D converter 56 for each light pulse. Obtained from the beat signal.
  • the optical information acquisition means 72 In acquiring the intensity I (t) and the phase P (t), the optical information acquisition means 72 first generates a complex amplitude of backscattered light from the beat signals of the I phase and the Q phase.
  • the complex amplitude of this backscattered light is also a beat signal, it is necessary to down-convert it. Therefore, next, the complex amplitude of the backscattered light is down-converted.
  • a method of this down-conversion for example, a method of integrating the complex amplitude of reverse rotation having a beat frequency with the complex amplitude of backscattered light and operating a low-pass filter (LPF) is used.
  • LPF low-pass filter
  • the distribution of the intensity I (t) of the backscattered light with respect to the reception time t of the backscattered light can be obtained.
  • the distribution of the phase P (t) of the backscattered light can also be obtained by calculating the phase of the complex amplitude.
  • the distribution of the intensity I (t) and the phase P (t) is stored in the storage means 66.
  • the storage means 66 stores the distribution of the intensity I (t) and the phase P (t) for a number of optical pulses determined according to the capacitance of the storage means 66.
  • the phase difference information acquisition means 74 sets the phase difference ⁇ P (tj) of the backward scattered light with respect to the light receiving time tj, the phase P (tk) with respect to the light receiving time tk, and the phase P (ti) with respect to the light receiving time ti. )
  • the difference P (tk) ⁇ P (ti) and the distribution of the phase difference ⁇ P (tj) with respect to the reception time tj of the signal light is acquired.
  • the difference between tk and ti is the reference time ST.
  • the phase difference information acquisition means 74 acquires vibration information at each position of the optical fiber 30 from the distribution of the phase difference ⁇ P (x) by any suitable conventionally known method.
  • the accuracy deterioration avoiding means 76 sets the reference time ST, and sends the set reference time ST to the phase difference information acquisition means 74.
  • the accuracy deterioration avoiding means 76 acquires the minimum value min ( ⁇ t1) of the intensity I (tk + ⁇ t1) in the plurality of optical pulses stored in the storage means 66 with respect to the reference time ST0 in the initial state and the first time ⁇ t1. To do. Further, by changing the first time ⁇ t1, among the minimum values min ( ⁇ t1) obtained for each first time ⁇ t1, the first time ⁇ t1 at which the minimum value min ( ⁇ t1) is the maximum is set to the maximum first time ⁇ t1max. Get as.
  • the accuracy deterioration avoiding means 76 has a minimum value min (ti ⁇ t2) of the intensity I (ti ⁇ t2) in a plurality of optical pulses stored in the storage means 66 with respect to the reference time ST0 in the initial state and the second time ⁇ t2. ⁇ t2) is acquired. Further, by changing the second time ⁇ t2, among the minimum values min ( ⁇ t2) obtained for each second time ⁇ t2, the second time ⁇ t2 at which the minimum value min ( ⁇ t2) is the maximum is set to the maximum second time ⁇ t2max. Get as.
  • the variable width of the first time ⁇ t1 and the second time ⁇ t2, that is, the variable width of the reference time ST can be set arbitrarily and preferably. For example, when the reference time ST is increased, the ability to avoid accuracy deterioration due to fading increases, but the spatial resolution decreases. On the other hand, when the reference time ST is reduced, the degree of increase in the ability to avoid accuracy deterioration due to fading is not large, but the decrease in spatial resolution can be suppressed. In the characteristic test described later, it is shown that the influence of accuracy deterioration can be reduced by making the reference time ST variable in the range of, for example, about ST0 to ST0 + ST0 / 2.
  • the accuracy deterioration avoiding means 76 has a gauge length GL0 in the initial state and a minimum value min (d1) of the intensity I (xk + d1) in a plurality of optical pulses stored in the storage means 66 with respect to the first distance d1. ) To get. Further, by changing the first distance d1, among the minimum values min (d1) obtained for each first distance d1, the first distance d1 at which the minimum value min (d1) is the maximum is set to the maximum first distance d1max. Get as.
  • the accuracy deterioration avoiding means 76 has a gauge length GL0 in the initial state and a minimum value min of the intensity I (xi ⁇ d2) in a plurality of optical pulses stored in the storage means 66 with respect to the second distance d2. Acquire d2). Further, by changing the second distance d2, among the minimum values min (d2) obtained for each second distance d2, the second distance d2 at which the minimum value min (d2) is the maximum is set to the maximum second distance d2max. Get as.
  • GL0 + d1max + d2max is set as the gauge length GL.
  • the variable width of the first distance d1 and the second distance d2, that is, the variable width of the gauge length GL can be set arbitrarily and preferably.
  • the gauge length GL may be made variable in the range of, for example, about GL0 to GL0 + GL0 / 2.
  • FIG. 2 is a diagram showing the results of a characteristic test of a vibration detection optical fiber sensor.
  • FIG. 2 (A) shows the result when the gauge length GL is fixed (comparative example)
  • FIG. 2 (B) shows the result when the gauge length GL is adapted (example).
  • the horizontal axis represents the distance [km] from the input end of the optical fiber
  • the vertical axis represents the time [ms].
  • the shading indicates the phase.
  • the length of the optical fiber 30 is set to 12 km.
  • a fiber stretcher was used to vibrate the optical fiber 30 over a length of 40 m.
  • the laser light source 12 a narrow line width laser having a line width of 3 kHz is used.
  • FIG. 2 (A) shows the result when the gauge length GL is fixed at 80 m. At a position 1 km from the input end of the optical fiber 30, the phase change due to the applied vibration can be confirmed. In FIG. 2A, a large phase change is observed near the position of 2.5 km, 2.8 km, and 4.8 km from the input end of the optical fiber 30. These phase changes are not due to the applied vibration, but due to the deterioration of accuracy due to fading.
  • FIG. 2B shows the result when the gauge length GL is changed within the range of 80 m to 120 m.
  • the phase change due to the applied vibration can be confirmed.
  • no large phase change was observed in the vicinity of the positions 2.5 km, 2.8 km, and 4.8 km from the input end of the optical fiber 30 seen in FIG. 2 (A). That is, it can be seen that the accuracy deterioration due to fading can be reduced by making the gauge length GL variable.
  • an optical coherent sensor capable of reducing accuracy deterioration due to fading has been described by taking a vibration detection optical fiber sensor as an example, but the present invention is not limited to this.
  • the present invention can be applied to any optical coherent sensor that utilizes the phase difference between two points.
  • the homodyne detection may be used.
  • an intensity modulator 16 having no frequency shift may be used, or a frequency shifter that shifts the frequency of the reference light between the fiber coupler 14 and the coherent receiver 52 may be used.

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Abstract

Without incorporating an expensive device, this invention uses signal processing to prevent fading from causing accuracy degradation. This invention comprises: a light source unit that produces optical pulses as probe light, a light reception unit that produces a beat signal through coherent detection of signal light produced by an object of measurement as a result of the probe light, and a computation unit that receives the beat signal. The computation unit comprises a light information acquisition means, accuracy degradation prevention means, and phase difference information acquisition means. For each optical pulse, the light information acquisition means acquires, from the beat signal, the distribution of the intensity I(tj) and phase P(tj) of the signal light in relation to the light reception time tj of the signal light. The accuracy degradation prevention means sets a reference time ST. The phase difference information acquisition means acquires a phase difference for the light reception time tj of the signal light as the phase difference P(tk) - P(ti) between the light reception time tk, where tk > tj > ti and the difference between tk and ti is the reference time ST, and the light reception time ti and acquires the distribution of the phase difference in relation to the light reception time of the signal light.

Description

光コヒーレントセンサ及び光コヒーレントセンシング方法Optical coherent sensor and optical coherent sensing method
　この発明は、例えば、光ファイバを用いた分布型振動センサに適用可能な、光コヒーレントセンサ及び光コヒーレントセンシング方法に関する。 The present invention relates to, for example, an optical coherent sensor and an optical coherent sensing method applicable to a distributed vibration sensor using an optical fiber.
　光ファイバにプローブ光として光パルスを入射すると、光パルスの伝搬に伴って後方散乱光が発生していく。光ファイバの長手方向の各位置において発生した後方散乱光は、光が光ファイバの入力端から後方散乱光の発生位置までの往復に要する時間だけ遅れて観測される。例えば、光ファイバに破断点がある場合、破断点に対応する時刻において後方散乱光の強度が変化する。この原理は、通信用光ファイバの破断点の検知に利用され、時間領域反射測定（ＯＴＤＲ：Ｏｐｔｉｃａｌ　Ｔｉｍｅ　Ｄｏｍａｉｎ　Ｒｅｆｌｅｃｔｒｏｍｅｔｒｙ）として知られている。 When an optical pulse is incident on an optical fiber as probe light, backscattered light is generated as the optical pulse propagates. The backscattered light generated at each position in the longitudinal direction of the optical fiber is observed with a delay of the time required for the light to reciprocate from the input end of the optical fiber to the position where the backscattered light is generated. For example, when the optical fiber has a breaking point, the intensity of the backscattered light changes at the time corresponding to the breaking point. This principle is used to detect a break point of an optical fiber for communication, and is known as a time domain reflection measurement (OTDR) (Optical Time Domain Reflectometry).
　このＯＴＤＲは、光ファイバに伝わる振動の分布的な測定など、光センサにも応用されている。光ファイバに振動が加わると、振動が加わった位置近傍で発生した後方散乱光の位相が変化する。このため、ＯＴＤＲによって得られた後方散乱光の位相の変化を観測することにより、分布的に光ファイバに加えられた振動の情報を得ることができる。このような、光ファイバに加えられた振動を分布的に測定する光ファイバセンサは、分布型振動センサ（ＤＶＳ：Ｄｉｓｔｒｉｂｕｔｅｄ　Ｖｉｂｒａｔｉｏｎ　Ｓｅｎｓｏｒ）や、分布型音響センサ（ＤＡＳ：Ｄｉｓｔｒｉｂｕｔｅｄ　Ａｃｏｕｓｔｉｃ　Ｓｅｎｓｏｒ）と呼ばれる。 This OTDR is also applied to optical sensors such as distributed measurement of vibration transmitted to optical fibers. When vibration is applied to the optical fiber, the phase of the backscattered light generated near the position where the vibration is applied changes. Therefore, by observing the phase change of the backscattered light obtained by the OTDR, it is possible to obtain information on the vibration applied to the optical fiber in a distributed manner. Such an optical fiber sensor that measures the vibration applied to the optical fiber in a distributed manner is called a distributed vibration sensor (DVS: Distributed Vibration Sensor) or a distributed acoustic sensor (DAS: Distributed Acoustic Sensor).
　後方散乱光の位相は、光ファイバに外部から加えられた振動によって変化するが、プローブ光の位相にも依存する。プローブ光の位相は、後方散乱光の発生位置（観測点）に到達するまでに受けた振動によって変化する。このため、通常、各位置の振動を測定するために、光ファイバ上の異なる２点で発生した後方散乱光の位相差を測定する手段が採られる。一般に、この２点間の距離は、ゲージ長と呼ばれる。 The phase of the backscattered light changes due to the vibration applied to the optical fiber from the outside, but it also depends on the phase of the probe light. The phase of the probe light changes due to the vibration received before reaching the backscattered light generation position (observation point). Therefore, in order to measure the vibration at each position, a means for measuring the phase difference of the backscattered light generated at two different points on the optical fiber is usually adopted. Generally, the distance between these two points is called the gauge length.
　時間経過に伴って入射される複数の光パルス（以下、光パルス列とも称する。）に対して同様の測定を行うと、求めた位相差の時間変化から振動の波形が得られる。 When the same measurement is performed on a plurality of optical pulses (hereinafter, also referred to as optical pulse trains) that are incident with the passage of time, a vibration waveform can be obtained from the obtained time change of the phase difference.
　一般に、プローブ光の光源として高コヒーレンスなレーザを利用して得られるＯＴＤＲの波形は、光パルスが伝搬する間に発生する複数の散乱中心からの後方散乱光が干渉した結果として得られる。このため、後方散乱光の強度は、発生位置に対して不規則に異なる。この現象は、フェーディングと呼ばれ、振動の測定結果に影響を与える。 Generally, the OTDR waveform obtained by using a high coherence laser as a light source of probe light is obtained as a result of interference of backward scattered light from a plurality of scattering centers generated during the propagation of an optical pulse. Therefore, the intensity of the backscattered light varies irregularly with respect to the generation position. This phenomenon is called fading and affects the vibration measurement results.
　ＤＡＳにおいて、位相差の算出に利用する２点からの後方散乱光のうち少なくとも一方の強度が、フェーディングにより微弱な場合、この点を利用して求めた位相情報の精度が著しく劣化する。フェーディングによる精度の劣化を回避する技術として、プローブ光の波長の多重化（例えば、Y. Lu, X. Zhang, C. Liang, M. Chen, J. Wang, and Z. Meng, "Fading noise reduction in distributed vibration measurements utilizing multi-wavelength based Φ-OTDR," in 26th International Conference on Optical Fiber Sensors, OSA Technical Digest (Optical Society of America, 2018),paper TuE21（非特許文献１）参照。）や、強度が微弱になる位置を補完する位相変調などの技術がある。 In DAS, when the intensity of at least one of the backscattered light from two points used for calculating the phase difference is weak due to fading, the accuracy of the phase information obtained by using this point is significantly deteriorated. As a technology to avoid deterioration of accuracy due to fading, multiplexing of the wavelength of probe light (for example, Y. Lu, X. Zhang, C. Liang, M. Chen, J. Wang, and Z. Meng, "Fading noise" reduction in distributed vibration measurements utilizing multi-wavelength based Φ-OTDR, "in 26th International Conference on Optical Fiber Sensors, OSA Technical Digest (Optical Society of America, 2018), paper TuE21 (Non-Patent Document 1). There are technologies such as phase modulation that complement the position where is weak.
　しかしながら、上述の波長の多重化や位相変調の技術では、高価な装置を導入しなければならず、光コヒーレントセンサの入手コストを増大させる課題がある。 However, in the above-mentioned wavelength multiplexing and phase modulation technology, an expensive device must be introduced, and there is a problem of increasing the acquisition cost of the optical coherent sensor.
　この発明は、上述の問題点に鑑みてなされたものであり、この発明の目的は、高価な装置を導入することなく、信号処理によってフェーディングによる精度の劣化を回避する、光コヒーレントセンサ及び光コヒーレントセンシング方法を提供することにある。 The present invention has been made in view of the above-mentioned problems, and an object of the present invention is an optical coherent sensor and an optical sensor that avoids deterioration of accuracy due to fading by signal processing without introducing an expensive device. The purpose is to provide a coherent sensing method.
　上述した目的を達成するために、この発明の光コヒーレントセンサは、プローブ光として光パルスを生成する光源部と、プローブ光によって測定対象物で発生する信号光をコヒーレント検波してビート信号を生成する受光部と、ビート信号が入力される演算部とを備えて構成される。演算部は、光情報取得手段と、精度劣化回避手段と、位相差情報取得手段とを備える。 In order to achieve the above-mentioned object, the optical coherent sensor of the present invention coherently detects a light source unit that generates an optical pulse as probe light and a signal light generated by a measurement object by the probe light to generate a beat signal. It is configured to include a light receiving unit and a calculation unit into which a beat signal is input. The calculation unit includes optical information acquisition means, accuracy deterioration avoidance means, and phase difference information acquisition means.
　光情報取得手段は、光パルスごとに、信号光の受光時刻ｔｊに対する、信号光の強度Ｉ（ｔｊ）及び位相Ｐ（ｔｊ）の分布を、ビート信号から取得する。精度劣化回避手段は、基準時間ＳＴを設定する。位相差情報取得手段は、信号光の受光時刻ｔｊに対する位相差を、受光時刻ｔｋに対する位相Ｐ（ｔｋ）と受光時刻ｔｉに対する位相Ｐ（ｔｉ）との差、Ｐ（ｔｋ）－Ｐ（ｔｉ）として取得し、信号光の受光時刻に対する位相差の分布を取得する。ここで、ｔｋ＞ｔｊ＞ｔｉであり、ｔｋとｔｉとの差が基準時間ＳＴである。 The optical information acquisition means acquires the distribution of the intensity I (tj) and the phase P (tj) of the signal light from the beat signal with respect to the reception time tj of the signal light for each optical pulse. The accuracy deterioration avoiding means sets the reference time ST. The phase difference information acquisition means determines the phase difference of the signal light with respect to the light receiving time tj, the difference between the phase P (tk) with respect to the light receiving time tk and the phase P (ti) with respect to the light receiving time ti, P (tk) −P (ti). And the distribution of the phase difference with respect to the reception time of the signal light is acquired. Here, tk> tj> ti, and the difference between tk and ti is the reference time ST.
　この発明の光コヒーレントセンサの好適実施例によれば、精度劣化回避手段は、初期状態の基準時間ＳＴ０に対して、第１時間Δｔ１を変化させて、各第１時間Δｔ１について、それぞれ複数の光パルスにおける強度Ｉ（ｔｋ＋Δｔ１）の最小値を取得し、この最小値が最大となる最大第１時間Δｔ１ｍａｘを取得する。また、初期状態の基準時間ＳＴ０に対して、第２時間Δｔ２を変化させて、各第２時間Δｔ２について、それぞれ複数の光パルスにおける強度Ｉ（ｔｉ－Δｔ２）の最小値を取得し、この最小値が最大となる最大第２時間Δｔ２ｍａｘを取得する。その後、ＳＴ０＋Δｔ１ｍａｘ＋Δｔ２ｍａｘを基準時間ＳＴとする。 According to a preferred embodiment of the optical coherent sensor of the present invention, the accuracy deterioration avoiding means changes the first time Δt1 with respect to the reference time ST0 in the initial state, and a plurality of lights for each first time Δt1. The minimum value of the intensity I (tk + Δt1) in the pulse is acquired, and the maximum first time Δt1max at which this minimum value is maximum is acquired. Further, the second time Δt2 is changed with respect to the reference time ST0 in the initial state, and the minimum value of the intensity I (ti−Δt2) in each of a plurality of optical pulses is obtained for each second time Δt2, and this minimum value is obtained. The maximum second time Δt2max at which the value becomes the maximum is acquired. After that, ST0 + Δt1max + Δt2max is set as the reference time ST.
　また、この発明の光コヒーレントセンシング方法は、以下の過程を備えて構成される。先ず、プローブ光として光パルスを生成する。次に、プローブ光によって測定対象物で発生する信号光をコヒーレント検波してビート信号を生成する。次に、光パルスごとに、信号光の受光時刻ｔｊに対する、信号光の強度Ｉ（ｔｊ）及び位相Ｐ（ｔｊ）の分布を、ビート信号から取得する。次に、基準時間ＳＴを設定する。次に、信号光の受光時刻ｔｊに対する位相差を、受光時刻ｔｋに対する位相Ｐ（ｔｋ）と受光時刻ｔｉに対する位相Ｐ（ｔｉ）との差、Ｐ（ｔｋ）－Ｐ（ｔｉ）として取得し、信号光の受光時刻に対する位相差の分布を取得する。 Further, the optical coherent sensing method of the present invention is configured to include the following processes. First, an optical pulse is generated as probe light. Next, the signal light generated in the object to be measured by the probe light is coherently detected to generate a beat signal. Next, for each optical pulse, the distribution of the intensity I (tj) and the phase P (tj) of the signal light with respect to the light reception time tj of the signal light is acquired from the beat signal. Next, the reference time ST is set. Next, the phase difference of the signal light with respect to the light receiving time tj is acquired as the difference between the phase P (tk) with respect to the light receiving time tk and the phase P (ti) with respect to the light receiving time ti, P (tk) −P (ti). Acquire the distribution of the phase difference with respect to the reception time of the signal light.
　また、この発明の光コヒーレントセンシング方法の好適実施例によれば、基準時間ＳＴを設定する過程では、初期状態の基準時間ＳＴ０に対して、第１時間Δｔ１を変化させて、各第１時間Δｔ１について、それぞれ複数の光パルスにおける強度Ｉ（ｔｋ＋Δｔ１）の最小値を取得し、この最小値が最大となる最大第１時間Δｔ１ｍａｘを取得する。また、初期状態の基準時間ＳＴ０に対して、第２時間Δｔ２を変化させて、各第２時間Δｔ２について、それぞれ複数の光パルスにおける強度Ｉ（ｔｉ－Δｔ２）の最小値を取得し、この最小値が最大となる最大第２時間Δｔ２ｍａｘを取得する。その後、ＳＴ０＋Δｔ１ｍａｘ＋Δｔ２ｍａｘを基準時間ＳＴとする。 Further, according to a preferred embodiment of the optical coherent sensing method of the present invention, in the process of setting the reference time ST, the first time Δt1 is changed with respect to the reference time ST0 in the initial state, and each first time Δt1 The minimum value of the intensity I (tk + Δt1) in each of the plurality of optical pulses is acquired, and the maximum first time Δt1max at which this minimum value is maximum is acquired. Further, the second time Δt2 is changed with respect to the reference time ST0 in the initial state, and the minimum value of the intensity I (ti−Δt2) in each of a plurality of optical pulses is obtained for each second time Δt2, and this minimum value is obtained. The maximum second time Δt2max at which the value becomes the maximum is acquired. After that, ST0 + Δt1max + Δt2max is set as the reference time ST.
　この発明の光コヒーレントセンサ及び光コヒーレントセンシング方法によれば、位相差を求めるにあたり、第１時間Δｔ１及び第２時間Δｔ２を変化させて、２点からの信号光の強度Ｉがともに大きくなるように基準時間ＳＴを設定することにより、フェーディングによる精度劣化を回避することができる。 According to the optical coherent sensor and the optical coherent sensing method of the present invention, in obtaining the phase difference, the first time Δt1 and the second time Δt2 are changed so that the intensity I of the signal light from the two points becomes large. By setting the reference time ST, it is possible to avoid deterioration of accuracy due to fading.
この発明の光コヒーレントセンサを説明するための模式図である。It is a schematic diagram for demonstrating the optical coherent sensor of this invention. 振動検知光ファイバセンサの特性試験の結果を示す図である。It is a figure which shows the result of the characteristic test of the vibration detection optical fiber sensor.
　以下、図を参照して、この発明の実施の形態について説明するが、この発明が理解できる程度に概略的に示したものに過ぎない。また、以下、この発明の好適な構成例につき説明するが、単なる好適例にすぎない。従って、この発明は以下の実施の形態に限定されるものではなく、この発明の構成の範囲を逸脱せずにこの発明の効果を達成できる多くの変更又は変形を行うことができる。 Hereinafter, embodiments of the present invention will be described with reference to the drawings, but they are merely schematic to the extent that the present invention can be understood. Further, although a preferable configuration example of the present invention will be described below, it is merely a suitable example. Therefore, the present invention is not limited to the following embodiments, and many modifications or modifications can be made that can achieve the effects of the present invention without departing from the scope of the constitution of the present invention.
　図１を参照して、この発明の一実施形態に係る光コヒーレントセンサを説明する。図１は、この発明の光コヒーレントセンサを説明するための模式図である。ここでは、光コヒーレントセンサの構成例として、振動検知光ファイバセンサを説明する。 An optical coherent sensor according to an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a schematic diagram for explaining the optical coherent sensor of the present invention. Here, a vibration detection optical fiber sensor will be described as a configuration example of the optical coherent sensor.
　振動検知光ファイバセンサは、光源部１０、光サーキュレータ２０、光ファイバ３０、受光部５０、演算部６０を備えて構成される。この振動検知光ファイバセンサは、ＯＴＤＲに用いられる。 The vibration detection optical fiber sensor includes a light source unit 10, an optical circulator 20, an optical fiber 30, a light receiving unit 50, and a calculation unit 60. This vibration detection optical fiber sensor is used for OTDR.
　光源部１０は、プローブ光として、周期的に光パルスを生成する。すなわち、光源部１０は光パルス列を生成する。振動検知光ファイバセンサの空間分解能は、この光パルスの幅に依存する。また、振動検知光ファイバセンサの測定距離は、光パルスの繰り返し周波数に依存する。光パルスは、測定対象物である光ファイバ３０を１ｍ伝搬するのに５ｎｓの時間を要する。信号光として、光ファイバ３０で発生する後方散乱光を観測する場合は，順方向の伝搬と，逆方向の伝搬の往復の時間を要するので、１ｍあたり１０ｎｓの遅延が発生する。例えば、パルス幅を１００ｎｓ、繰り返し周波数を５ｋＨｚとしたとき、空間分解能は１０ｍとなり、最大の測定距離は２０ｋｍとなる。 The light source unit 10 periodically generates an optical pulse as probe light. That is, the light source unit 10 generates an optical pulse train. The spatial resolution of the vibration detection optical fiber sensor depends on the width of this optical pulse. Further, the measurement distance of the vibration detection optical fiber sensor depends on the repetition frequency of the optical pulse. It takes 5 ns for the optical pulse to propagate 1 m through the optical fiber 30 which is the object to be measured. When observing the backscattered light generated by the optical fiber 30 as the signal light, it takes a round trip time of the forward propagation and the reverse propagation, so that a delay of 10 ns per 1 m occurs. For example, when the pulse width is 100 ns and the repetition frequency is 5 kHz, the spatial resolution is 10 m and the maximum measurement distance is 20 km.
　光源部１０は、例えば、レーザ光源１２、ファイバカプラ１３、強度変調器１４、関数発生器１６及び光増幅器１８を備えて構成される。 The light source unit 10 includes, for example, a laser light source 12, a fiber coupler 13, an intensity modulator 14, a function generator 16, and an optical amplifier 18.
　レーザ光源１２は、通信波長帯の連続光として、レーザ光を生成する。レーザ光源１２として、線幅が１０ｋＨｚ以下のいわゆる狭線幅レーザを用いるのが良い。レーザ光源１２として狭線幅レーザを用いると、この振動検知光ファイバセンサは、位相感応ＯＴＤＲに用いることができる。レーザ光の波長は、任意で良いが、標準単一モード光ファイバで低損失の１５５０ｎｍにするのが良い。レーザ光源１２で生成されたレーザ光は、ファイバカプラ１３に送られる。 The laser light source 12 generates laser light as continuous light in the communication wavelength band. As the laser light source 12, it is preferable to use a so-called narrow line width laser having a line width of 10 kHz or less. When a narrow line width laser is used as the laser light source 12, this vibration detection optical fiber sensor can be used for a phase sensitive OTDR. The wavelength of the laser beam may be arbitrary, but it is preferable to use a standard single-mode optical fiber with a low loss of 1550 nm. The laser beam generated by the laser light source 12 is sent to the fiber coupler 13.
　ファイバカプラ１３は、レーザ光を２分岐する。２分岐された一方は、強度変調器１４に送られる。また、２分岐された他方は、参照光として受光部５０に送られる。 The fiber coupler 13 splits the laser beam into two. One of the two branches is sent to the intensity modulator 14. The other bifurcated light is sent to the light receiving unit 50 as reference light.
　関数発生器１６は、矩形状の電気パルスを生成する。この電気パルスは、強度変調器１４に送られる。関数発生器１６が生成する電気パルスは、例えば、パルス幅が１００ｎｓ幅で、繰り返し周波数が５ｋＨｚである。また、関数発生器１６の出力は、後述するアナログ－ディジタル（Ａ／Ｄ）変換器５６にも送られ、トリガー信号として用いられる。 The function generator 16 generates a rectangular electric pulse. This electric pulse is sent to the intensity modulator 14. The electric pulse generated by the function generator 16 has, for example, a pulse width of 100 ns width and a repetition frequency of 5 kHz. The output of the function generator 16 is also sent to the analog-to-digital (A / D) converter 56, which will be described later, and is used as a trigger signal.
　強度変調器１４は、レーザ光を電気パルスで光パルス化して、光パルスを生成する。この光パルスは、光増幅器１８に送られる。強度変調器１４が生成する光パルスのパルス幅と繰り返し周波数は、共に関数発生器１６が生成する電気パルスと同じである。この例では、光パルスは、パルス幅が１００ｎｓで、繰り返し周波数が５ｋＨｚである。 The intensity modulator 14 converts the laser beam into an optical pulse with an electric pulse to generate an optical pulse. This optical pulse is sent to the optical amplifier 18. The pulse width and repetition frequency of the optical pulse generated by the intensity modulator 14 are both the same as the electric pulse generated by the function generator 16. In this example, the optical pulse has a pulse width of 100 ns and a repetition frequency of 5 kHz.
　強度変調器１４として、例えば、音響光学変調器（ＡＯＭ：Ａｃｏｕｓｔｉｃ　Ｏｐｔｉｃ　Ｍｏｄｕｌａｔｏｒ）が用いられる。ＡＯＭを用いる場合、光ドップラー効果により、強度変調器１４で生成される光パルスの周波数は、強度変調器１４に入力されるレーザ光の周波数から変化する。このため、受光部５０でのコヒーレント検波の際には、ヘテロダイン検波となることが多い。 As the intensity modulator 14, for example, an acousto-optic modulator (AOM: Acoustic Modulator) is used. When AOM is used, the frequency of the optical pulse generated by the intensity modulator 14 changes from the frequency of the laser beam input to the intensity modulator 14 due to the optical Doppler effect. For this reason, heterodyne detection is often performed during coherent detection by the light receiving unit 50.
　強度変調器１４で生成された光パルスは、光増幅器１８で所定の増幅を受ける。これは、光パルスの強度が強いほど、光ファイバ３０での後方散乱光の強度が強くなるためである。光増幅器１８で増幅された光パルスは、プローブ光として、光サーキュレータ２０を経て光ファイバ３０に送られる。なお、ここでは図示及び説明を省略するが、一般的には、光増幅器１８の後段に、バンドパスフィルタ（ＢＰＦ）が用いられる。ＢＰＦは、光増幅器１８で発生する自然放出光（ＡＳＥ：Ａｍｐｒｉｆｉｅｄ　Ｓｐｏｎｔａｎｅｏｕｓ　Ｅｍｉｓｓｉｏｎ）のノイズを取り除く。 The optical pulse generated by the intensity modulator 14 receives a predetermined amplification by the optical amplifier 18. This is because the stronger the intensity of the optical pulse, the stronger the intensity of the backscattered light in the optical fiber 30. The optical pulse amplified by the optical amplifier 18 is sent to the optical fiber 30 as probe light via the optical circulator 20. Although illustration and description are omitted here, a bandpass filter (BPF) is generally used after the optical amplifier 18. The BPF removes noise of spontaneous emission light (ASE: Amplified Spontaneous Emission) generated by the optical amplifier 18.
　光ファイバ３０に送られたプローブ光は、光ファイバ３０を伝播し、プローブ光の伝播に伴って後方散乱光が発生する。この後方散乱光は、信号光として光サーキュレータ２０を経て受光部５０に送られる。なお、ここでは図示及び説明を省略するが、後方散乱光を増幅するために、受光部５０の前段に光増幅器とＢＰＦが設けられることが多い。 The probe light sent to the optical fiber 30 propagates through the optical fiber 30, and backscattered light is generated along with the propagation of the probe light. This backscattered light is sent to the light receiving unit 50 as signal light via the optical circulator 20. Although illustration and description are omitted here, an optical amplifier and a BPF are often provided in front of the light receiving unit 50 in order to amplify the backscattered light.
　受光部５０は、プローブ光によって、光ファイバ３０で発生する後方散乱光をコヒーレント検波して電気信号を生成する。 The light receiving unit 50 coherently detects the backscattered light generated in the optical fiber 30 by the probe light and generates an electric signal.
　受光部５０は、コヒーレントレシーバ５２、バランス型フォトダイオード（ＰＤ）５４、アナログ・ディジタル（Ａ／Ｄ）変換器５６を備えて構成される。 The light receiving unit 50 includes a coherent receiver 52, a balanced photodiode (PD) 54, and an analog-to-digital (A / D) converter 56.
　コヒーレントレシーバ５２は、参照光を用いて、後方散乱光のコヒーレント検波を行う。コヒーレントレシーバ５２として、例えば、光９０°ハイブリッドカプラを用いることができる。コヒーレントレシーバ５２からの出力は、バランス型ＰＤ５４に送られる。 The coherent receiver 52 uses the reference light to perform coherent detection of backscattered light. As the coherent receiver 52, for example, an optical 90 ° hybrid coupler can be used. The output from the coherent receiver 52 is sent to the balanced PD 54.
　バランス型ＰＤ５４は、コヒーレントレシーバ５２からの出力をバランス検波する。これにより、後方散乱光の強度と位相の情報を持つ、Ｉ相（ｃｏｓ波）とＱ相（ｓｉｎ波）のビート信号が生成される。このＩ相及びＱ相のビート信号は、Ａ／Ｄ変換器５６に送られる。 The balanced PD54 balance-detects the output from the coherent receiver 52. As a result, I-phase (cos wave) and Q-phase (sine wave) beat signals having information on the intensity and phase of backscattered light are generated. The I-phase and Q-phase beat signals are sent to the A / D converter 56.
　Ａ／Ｄ変換器５６は、Ｉ相及びＱ相のビート信号をディジタル信号に変換する。ディジタル信号に変換された、Ｉ相及びＱ相のビート信号は、演算部６０に入力される。 The A / D converter 56 converts the I-phase and Q-phase beat signals into digital signals. The I-phase and Q-phase beat signals converted into digital signals are input to the arithmetic unit 60.
　演算部６０としては、例えば、市販のパーソナルコンピュータ（ＰＣ）を利用できる。ここでは、一例として、演算部６０が、ＣＰＵ（Ｃｅｎｔｒａｌ　Ｐｒｏｃｅｓｓｉｎｇ　Ｕｎｉｔ）７０、ＲＡＭ（Ｒａｎｄｏｍ　Ａｃｃｅｓｓ　Ｍｅｍｏｒｙ）６２、ＲＯＭ（Ｒｅａｄ　Ｏｎｌｙ　Ｍｅｍｏｒｙ）６４及び記憶手段６６を備えて構成されるものとして説明する。ＣＰＵ７０は、ＲＯＭ６４に格納されているプログラムを実行することにより、後述する各機能手段を実現する。各機能手段での処理結果は、一時的にＲＡＭ６２に格納される。 As the calculation unit 60, for example, a commercially available personal computer (PC) can be used. Here, as an example, the arithmetic unit 60 will be described as being configured to include a CPU (Central Processing Unit) 70, a RAM (Random Access Memory) 62, a ROM (Read Only Memory) 64, and a storage means 66. The CPU 70 realizes each functional means described later by executing a program stored in the ROM 64. The processing result of each functional means is temporarily stored in the RAM 62.
　演算部６０が備える機能手段として、光情報取得手段７２、位相差情報取得手段７４、精度劣化回避手段７６がある。 Functional means included in the calculation unit 60 includes optical information acquisition means 72, phase difference information acquisition means 74, and accuracy deterioration avoidance means 76.
　光情報取得手段７２は、光パルスごとに、後方散乱光の受光時刻ｔに対する、後方散乱光の強度Ｉ（ｔ）及び位相Ｐ（ｔ）の分布を、Ａ／Ｄ変換器５６から送られたビート信号から取得する。 The optical information acquisition means 72 sends the distribution of the intensity I (t) and the phase P (t) of the backscattered light with respect to the light receiving time t of the backscattered light from the A / D converter 56 for each light pulse. Obtained from the beat signal.
　強度Ｉ（ｔ）及び位相Ｐ（ｔ）を取得するにあたり、光情報取得手段７２は、先ず、Ｉ相及びＱ相のビート信号から、後方散乱光の複素振幅を生成する。 In acquiring the intensity I (t) and the phase P (t), the optical information acquisition means 72 first generates a complex amplitude of backscattered light from the beat signals of the I phase and the Q phase.
　この後方散乱光の複素振幅もビート信号であるので、ダウンコンバートする必要がある。そこで、次に、後方散乱光の複素振幅をダウンコンバートする。このダウンコンバートする方法として、例えば、ビート周波数を持つ逆回転の複素振幅を、後方散乱光の複素振幅に積算し、ローパスフィルター（ＬＰＦ）を作用させる方法が用いられる。 Since the complex amplitude of this backscattered light is also a beat signal, it is necessary to down-convert it. Therefore, next, the complex amplitude of the backscattered light is down-converted. As a method of this down-conversion, for example, a method of integrating the complex amplitude of reverse rotation having a beat frequency with the complex amplitude of backscattered light and operating a low-pass filter (LPF) is used.
　その後、ダウンコンバートして得られた複素振幅の絶対値を計算することで、後方散乱光の受光時刻ｔに対する、後方散乱光の強度Ｉ（ｔ）の分布が得られる。また、このとき、複素振幅の位相を計算することで、後方散乱光の位相Ｐ（ｔ）の分布も得られる。強度Ｉ（ｔ）及び位相Ｐ（ｔ）の分布は、記憶手段６６に格納される。記憶手段６６には、記憶手段６６の容量に応じて定められる数の光パルスについて、強度Ｉ（ｔ）及び位相Ｐ（ｔ）の分布が格納される。 After that, by calculating the absolute value of the complex amplitude obtained by down-converting, the distribution of the intensity I (t) of the backscattered light with respect to the reception time t of the backscattered light can be obtained. At this time, the distribution of the phase P (t) of the backscattered light can also be obtained by calculating the phase of the complex amplitude. The distribution of the intensity I (t) and the phase P (t) is stored in the storage means 66. The storage means 66 stores the distribution of the intensity I (t) and the phase P (t) for a number of optical pulses determined according to the capacitance of the storage means 66.
　位相差情報取得手段７４は、基準時間をＳＴとしたとき、後方散乱光の受光時刻ｔｊに対する位相差ΔＰ（ｔｊ）を、受光時刻ｔｋに対する位相Ｐ（ｔｋ）と受光時刻ｔｉに対する位相Ｐ（ｔｉ）との差Ｐ（ｔｋ）－Ｐ（ｔｉ）として取得し、信号光の受光時刻ｔｊに対する位相差ΔＰ（ｔｊ）の分布を取得する。ここで、ｔｋ＞ｔｊ＞ｔｉであり、ｔｋとｔｉとの差が基準時間ＳＴである。 When the reference time is ST, the phase difference information acquisition means 74 sets the phase difference ΔP (tj) of the backward scattered light with respect to the light receiving time tj, the phase P (tk) with respect to the light receiving time tk, and the phase P (ti) with respect to the light receiving time ti. ) As the difference P (tk) −P (ti), and the distribution of the phase difference ΔP (tj) with respect to the reception time tj of the signal light is acquired. Here, tk> tj> ti, and the difference between tk and ti is the reference time ST.
　なお、受光時刻ｔｊは、後方散乱光が発生した、光ファイバ３０の長手方向の位置ｘｊに対応する。従って、位相差ΔＰを、位置ｘｊの関数として、ゲージ長ＧＬだけ離れたｋ点及びｉ点の位置ｘｋ及びｘｉを用いると、ΔＰ（ｘｊ）＝Ｐ（ｘｋ）－Ｐ（ｘｉ）と表すことができる。ここでｘｋ＞ｘｊ＞ｘｉである。位相差情報取得手段７４は、位相差ΔＰ（ｘ）の分布から、光ファイバ３０の各位置での振動情報を、任意好適な従来公知の方法で取得する。 The light receiving time tj corresponds to the position xj in the longitudinal direction of the optical fiber 30 in which the backscattered light is generated. Therefore, when the phase difference ΔP is used as a function of the position xj and the positions xx and xi of the k point and the i point separated by the gauge length GL are used, it is expressed as ΔP (xj) = P (xx) −P (xi). Can be done. Here, xk> xj> xi. The phase difference information acquisition means 74 acquires vibration information at each position of the optical fiber 30 from the distribution of the phase difference ΔP (x) by any suitable conventionally known method.
　ここで、信号光の受光時刻ｔｊに対する位相差ΔＰ（ｔｊ）を取得するために用いる、ｔｋ＞ｔｊ＞ｔｉである信号光の受光時刻ｔｋに対する強度Ｉ（ｔｋ）及び信号光の受光時刻ｔｉに対する強度Ｉ（ｔｉ）の少なくとも一方が微弱になると、位相Ｐの精度が劣化してしまう。このため、精度劣化回避手段７６が、基準時間ＳＴを設定し、設定した基準時間ＳＴを位相差情報取得手段７４に送る。信号光の受光時刻ｔｋと受光時刻ｔｉとの差が基準時間ＳＴである２点の強度Ｉ（ｔｋ）及びＩ（ｔｉ）がともに大きくなるように、基準時間ＳＴを設定することにより、フェーディングによる精度劣化を回避する。 Here, with respect to the intensity I (tk) with respect to the light receiving time tk of the signal light and the light receiving time ti of the signal light, which is used for acquiring the phase difference ΔP (tj) with respect to the light receiving time tj of the signal light, tk> tj> ti. If at least one of the intensities I (ti) becomes weak, the accuracy of the phase P deteriorates. Therefore, the accuracy deterioration avoiding means 76 sets the reference time ST, and sends the set reference time ST to the phase difference information acquisition means 74. Fading by setting the reference time ST so that the intensity I (tk) and I (ti) of the two points where the difference between the light reception time tk and the light reception time ti of the signal light is the reference time ST becomes large. Avoid deterioration of accuracy due to.
　基準時間ＳＴの設定方法を説明する。 Explain how to set the reference time ST.
　精度劣化回避手段７６は、初期状態の基準時間ＳＴ０と、第１時間Δｔ１に対して、記憶手段６６に格納されている複数の光パルスにおける強度Ｉ（ｔｋ＋Δｔ１）の最小値ｍｉｎ（Δｔ１）を取得する。さらに、第１時間Δｔ１を変化させて、各第１時間Δｔ１について得られた最小値ｍｉｎ（Δｔ１）の中で、最小値ｍｉｎ（Δｔ１）が最大となる第１時間Δｔ１を最大第１時間Δｔ１ｍａｘとして取得する。 The accuracy deterioration avoiding means 76 acquires the minimum value min (Δt1) of the intensity I (tk + Δt1) in the plurality of optical pulses stored in the storage means 66 with respect to the reference time ST0 in the initial state and the first time Δt1. To do. Further, by changing the first time Δt1, among the minimum values min (Δt1) obtained for each first time Δt1, the first time Δt1 at which the minimum value min (Δt1) is the maximum is set to the maximum first time Δt1max. Get as.
　また、精度劣化回避手段７６は、初期状態の基準時間ＳＴ０と、第２時間Δｔ２に対して、記憶手段６６に格納されている複数の光パルスにおける強度Ｉ（ｔｉ－Δｔ２）の最小値ｍｉｎ（Δｔ２）を取得する。さらに、第２時間Δｔ２を変化させて、各第２時間Δｔ２について得られた最小値ｍｉｎ（Δｔ２）の中で、最小値ｍｉｎ（Δｔ２）が最大となる第２時間Δｔ２を最大第２時間Δｔ２ｍａｘとして取得する。 Further, the accuracy deterioration avoiding means 76 has a minimum value min (ti−Δt2) of the intensity I (ti−Δt2) in a plurality of optical pulses stored in the storage means 66 with respect to the reference time ST0 in the initial state and the second time Δt2. Δt2) is acquired. Further, by changing the second time Δt2, among the minimum values min (Δt2) obtained for each second time Δt2, the second time Δt2 at which the minimum value min (Δt2) is the maximum is set to the maximum second time Δt2max. Get as.
　その後、ＳＴ０＋Δｔ１ｍａｘ＋Δｔ２ｍａｘを基準時間ＳＴとして設定する。第１時間Δｔ１及び第２時間Δｔ２の可変幅、すなわち、基準時間ＳＴの可変幅は任意好適に設定することができる。例えば、基準時間ＳＴを大きくすると、フェーディングによる精度劣化の回避能力は高まるが、空間分解能は低下する。一方、基準時間ＳＴを小さくすると、フェーディングによる精度劣化の回避能力の高まりの程度は大きくはないが、空間分解能の低下を抑制できる。後述する特性試験では、基準時間ＳＴを、例えば、ＳＴ０からＳＴ０＋ＳＴ０／２程度の範囲で可変にすることで、精度劣化の影響を低減できることが示されている。 After that, ST0 + Δt1max + Δt2max is set as the reference time ST. The variable width of the first time Δt1 and the second time Δt2, that is, the variable width of the reference time ST can be set arbitrarily and preferably. For example, when the reference time ST is increased, the ability to avoid accuracy deterioration due to fading increases, but the spatial resolution decreases. On the other hand, when the reference time ST is reduced, the degree of increase in the ability to avoid accuracy deterioration due to fading is not large, but the decrease in spatial resolution can be suppressed. In the characteristic test described later, it is shown that the influence of accuracy deterioration can be reduced by making the reference time ST variable in the range of, for example, about ST0 to ST0 + ST0 / 2.
　ここでは、基準時間ＳＴの設定方法を説明したが、ゲージ長ＧＬを設定する場合も同様に行うことができる。 Here, the method of setting the reference time ST has been explained, but the same can be performed when setting the gauge length GL.
　この場合、精度劣化回避手段７６は、初期状態のゲージ長ＧＬ０と、第１距離ｄ１に対して、記憶手段６６に格納されている複数の光パルスにおける強度Ｉ（ｘｋ＋ｄ１）の最小値ｍｉｎ（ｄ１）を取得する。さらに、第１距離ｄ１を変化させて、各第１距離ｄ１について得られた最小値ｍｉｎ（ｄ１）の中で、最小値ｍｉｎ（ｄ１）が最大となる第１距離ｄ１を最大第１距離ｄ１ｍａｘとして取得する。 In this case, the accuracy deterioration avoiding means 76 has a gauge length GL0 in the initial state and a minimum value min (d1) of the intensity I (xk + d1) in a plurality of optical pulses stored in the storage means 66 with respect to the first distance d1. ) To get. Further, by changing the first distance d1, among the minimum values min (d1) obtained for each first distance d1, the first distance d1 at which the minimum value min (d1) is the maximum is set to the maximum first distance d1max. Get as.
　また、精度劣化回避手段７６は、初期状態のゲージ長ＧＬ０と、第２距離ｄ２に対して、記憶手段６６に格納されている複数の光パルスにおける強度Ｉ（ｘｉ－ｄ２）の最小値ｍｉｎ（ｄ２）を取得する。さらに、第２距離ｄ２を変化させて、各第２距離ｄ２について得られた最小値ｍｉｎ（ｄ２）の中で、最小値ｍｉｎ（ｄ２）が最大となる第２距離ｄ２を最大第２距離ｄ２ｍａｘとして取得する。 Further, the accuracy deterioration avoiding means 76 has a gauge length GL0 in the initial state and a minimum value min of the intensity I (xi−d2) in a plurality of optical pulses stored in the storage means 66 with respect to the second distance d2. Acquire d2). Further, by changing the second distance d2, among the minimum values min (d2) obtained for each second distance d2, the second distance d2 at which the minimum value min (d2) is the maximum is set to the maximum second distance d2max. Get as.
　その後、ＧＬ０＋ｄ１ｍａｘ＋ｄ２ｍａｘをゲージ長ＧＬとして設定する。第１距離ｄ１及び第２距離ｄ２の可変幅、すなわち、ゲージ長ＧＬの可変幅は任意好適に設定することができる。ゲージ長ＧＬを、例えば、ＧＬ０からＧＬ０＋ＧＬ０／２程度の範囲で可変にすればよい。 After that, GL0 + d1max + d2max is set as the gauge length GL. The variable width of the first distance d1 and the second distance d2, that is, the variable width of the gauge length GL can be set arbitrarily and preferably. The gauge length GL may be made variable in the range of, for example, about GL0 to GL0 + GL0 / 2.
　（特性試験）
　図２を参照して、振動検知光ファイバセンサの特性試験について説明する。図２は、振動検知光ファイバセンサの特性試験の結果を示す図である。図２（Ａ）は、ゲージ長ＧＬを固定した場合（比較例）の結果を示し、図２（Ｂ）は、ゲージ長ＧＬを適応させた場合（実施例）の結果を示している。図２（Ａ）及び図２（Ｂ）では、横軸に光ファイバの入力端からの距離［ｋｍ］を取って示し、縦軸に時間［ｍｓ］を取って示している。また、図２（Ａ）及び図２（Ｂ）では、濃淡が位相を示している。 (Characteristic test)
The characteristic test of the vibration detection optical fiber sensor will be described with reference to FIG. FIG. 2 is a diagram showing the results of a characteristic test of a vibration detection optical fiber sensor. FIG. 2 (A) shows the result when the gauge length GL is fixed (comparative example), and FIG. 2 (B) shows the result when the gauge length GL is adapted (example). In FIGS. 2 (A) and 2 (B), the horizontal axis represents the distance [km] from the input end of the optical fiber, and the vertical axis represents the time [ms]. Further, in FIGS. 2 (A) and 2 (B), the shading indicates the phase.
　ここでは、光ファイバ３０の長さを１２ｋｍとした。光ファイバ３０の入力端から１ｋｍの位置において、ファイバストレッチャーを用いて、光ファイバ３０を４０ｍの長さにわたって振動させた。レーザ光源１２として、線幅３ｋＨｚの狭線幅レーザを用いている。 Here, the length of the optical fiber 30 is set to 12 km. At a position 1 km from the input end of the optical fiber 30, a fiber stretcher was used to vibrate the optical fiber 30 over a length of 40 m. As the laser light source 12, a narrow line width laser having a line width of 3 kHz is used.
　図２（Ａ）は、ゲージ長ＧＬを８０ｍに固定した場合の結果を示している。光ファイバ３０の入力端から１ｋｍの位置において、印加された振動による位相変化が確認できる。なお、図２（Ａ）では、光ファイバ３０の入力端から２．５ｋｍ、２．８ｋｍ、４．８ｋｍの位置付近において、大きな位相変化が観測されている。これらの位相変化は、印加された振動によるものではなく、フェーディングによる精度劣化によるものである。 FIG. 2 (A) shows the result when the gauge length GL is fixed at 80 m. At a position 1 km from the input end of the optical fiber 30, the phase change due to the applied vibration can be confirmed. In FIG. 2A, a large phase change is observed near the position of 2.5 km, 2.8 km, and 4.8 km from the input end of the optical fiber 30. These phase changes are not due to the applied vibration, but due to the deterioration of accuracy due to fading.
　図２（Ｂ）は、ゲージ長ＧＬを８０ｍ～１２０ｍの範囲内で可変にした場合の結果を示している。光ファイバ３０の入力端から１ｋｍの位置において、印加された振動による位相変化が確認できる。なお、図２（Ｂ）では、図２（Ａ）で見られた光ファイバ３０の入力端から２．５ｋｍ、２．８ｋｍ、４．８ｋｍの位置付近における大きな位相変化が観測されていない。すなわち、ゲージ長ＧＬを可変にすることで、フェーディングによる精度劣化を低減できていることがわかる。 FIG. 2B shows the result when the gauge length GL is changed within the range of 80 m to 120 m. At a position 1 km from the input end of the optical fiber 30, the phase change due to the applied vibration can be confirmed. In FIG. 2B, no large phase change was observed in the vicinity of the positions 2.5 km, 2.8 km, and 4.8 km from the input end of the optical fiber 30 seen in FIG. 2 (A). That is, it can be seen that the accuracy deterioration due to fading can be reduced by making the gauge length GL variable.
　（他の実施形態）
　ここでは、振動検知光ファイバセンサを例にとり、フェーディングによる精度劣化を低減できる光コヒーレントセンサを説明したが、これに限定されない。２点間における位相の差を利用する光コヒーレントセンサであれば、この発明を適用できる。 (Other embodiments)
Here, an optical coherent sensor capable of reducing accuracy deterioration due to fading has been described by taking a vibration detection optical fiber sensor as an example, but the present invention is not limited to this. The present invention can be applied to any optical coherent sensor that utilizes the phase difference between two points.
　また、ここでは、受光部５０でのコヒーレント検波の際には、ヘテロダイン検波となる場合を説明したが、ホモダイン検波であってもよい。ホモダイン検波にする場合は、強度変調器１６として周波数シフトのないものを用いるか、あるいは、ファイバカプラ１４とコヒーレントレシーバ５２の間に参照光の周波数をシフトさせる周波数シフタを用いればよい。ホモダイン検波の場合は、Ａ／Ｄ変換器５６として広帯域のものを用いる必要がない。 Further, although the case where the coherent detection by the light receiving unit 50 is performed by the heterodyne detection is described here, the homodyne detection may be used. In the case of homodyne detection, an intensity modulator 16 having no frequency shift may be used, or a frequency shifter that shifts the frequency of the reference light between the fiber coupler 14 and the coherent receiver 52 may be used. In the case of homodyne detection, it is not necessary to use a wide band A / D converter 56.
　日本出願特願２０１９－０６０６６１号の開示はその全体が参照により本明細書に取り込まれる。 The entire disclosure of Japanese application Japanese Patent Application No. 2019-060661 is incorporated herein by reference in its entirety.
　本明細書に記載された全ての文献、特許出願、および技術規格は、個々の文献、特許出願、および技術規格が参照により取り込まれることが具体的かつ個々に記された場合と同程度に、本明細書中に参照により取り込まれる。 All documents, patent applications, and technical standards described herein are to the same extent as if the individual documents, patent applications, and technical standards were specifically and individually stated to be incorporated by reference. Incorporated herein by reference.
　１０　　光源部
　１２　　レーザ光源
　１３　　ファイバカプラ
　１４　　強度変調器
　１６　　関数発生器
　１８　　光増幅器
　２０　　光サーキュレータ　　
　３０　　光ファイバ
　５０　　受光部
　５２　　コヒーレントレシーバ
　５４　　バランス型フォトダイオード（ＰＤ）
　５６　　アナログ・ディジタル（Ａ／Ｄ）変換器
　６０　　演算部
　６２　　ＲＡＭ
　６４　　ＲＯＭ
　６６　　記憶手段
　７０　　ＣＰＵ
　７２　　光情報取得手段
　７４　　位相差情報取得手段
　７６　　精度劣化回避手段 10 Light source 12 Laser light source 13 Fiber coupler 14 Intensity modulator 16 Function generator 18 Optical amplifier 20 Optical circulator
30 Optical fiber 50 Light receiving part 52 Coherent receiver 54 Balanced photodiode (PD)
56 Analog-to-digital (A / D) converter 60 Arithmetic unit 62 RAM
64 ROM
66 Storage means 70 CPU
72 Optical information acquisition means 74 Phase difference information acquisition means 76 Accuracy deterioration avoidance means

Claims (4)

  1. 　プローブ光として光パルスを生成する光源部と、   
    　前記プローブ光によって測定対象物で発生する信号光をコヒーレント検波してビート信号を生成する受光部と、
    　前記ビート信号が入力される演算部と
    を備え、
    　前記演算部は、
    　前記光パルスごとに、前記信号光の受光時刻ｔｊに対する、前記信号光の強度Ｉ（ｔｊ）及び位相Ｐ（ｔｊ）の分布を、前記ビート信号から取得する光情報取得手段と、
    　基準時間ＳＴを設定する精度劣化回避手段と
    　前記信号光の受光時刻ｔｊに対する位相差を、ｔｋ＞ｔｊ＞ｔｉ、且つ、ｔｋとｔｉとの差が前記基準時間ＳＴである受光時刻ｔｋに対する位相Ｐ（ｔｋ）と受光時刻ｔｉに対する位相Ｐ（ｔｉ）との差、Ｐ（ｔｋ）－Ｐ（ｔｉ）として取得し、前記信号光の受光時刻に対する前記位相差の分布を取得する位相差情報取得手段と、を備える
    光コヒーレントセンサ。     A light source that generates an optical pulse as probe light,
    A light receiving unit that coherently detects the signal light generated by the object to be measured by the probe light and generates a beat signal.
    A calculation unit for inputting the beat signal is provided.
    The calculation unit
    An optical information acquisition means for acquiring the distribution of the intensity I (tj) and the phase P (tj) of the signal light from the beat signal with respect to the reception time tj of the signal light for each optical pulse.
    The phase difference between the accuracy deterioration avoiding means for setting the reference time ST and the light receiving time tj of the signal light is tk>tj> ti, and the phase P with respect to the light receiving time tk where the difference between tk and ti is the reference time ST. Phase difference information acquisition means for acquiring the difference between (tk) and the phase P (ti) with respect to the light receiving time ti as P (tk) −P (ti) and acquiring the distribution of the phase difference with respect to the light receiving time of the signal light. And with an optical coherent sensor.
  2. 　前記精度劣化回避手段は、
    　初期状態の基準時間ＳＴ０に対して、第１時間Δｔ１を変化させて、各第１時間Δｔ１について、それぞれ複数の光パルスにおける強度Ｉ（ｔｋ＋Δｔ１）の最小値を取得し、該最小値が最大となる最大第１時間Δｔ１ｍａｘを取得し、
    　初期状態の基準時間ＳＴ０に対して、第２時間Δｔ２を変化させて、各第２時間Δｔ２について、それぞれ複数の光パルスにおける強度Ｉ（ｔｉ－Δｔ２）の最小値を取得し、該最小値が最大となる最大第２時間Δｔ２ｍａｘを取得し、
    　ＳＴ０＋Δｔ１ｍａｘ＋Δｔ２ｍａｘを基準時間ＳＴとする請求項１に記載の光コヒーレントセンサ。     The accuracy deterioration avoiding means is
    The first time Δt1 is changed with respect to the reference time ST0 in the initial state, and the minimum value of the intensity I (tk + Δt1) in each of a plurality of optical pulses is obtained for each first time Δt1, and the minimum value is set to the maximum. The maximum first time Δt1max is obtained,
    The second time Δt2 is changed with respect to the reference time ST0 in the initial state, and the minimum value of the intensity I (ti−Δt2) in each of a plurality of optical pulses is obtained for each second time Δt2, and the minimum value is the minimum value. Obtain the maximum second time Δt2max, which is the maximum,
    The optical coherent sensor according to claim 1, wherein ST0 + Δt1max + Δt2max is set as a reference time ST.
  3. 　プローブ光として光パルスを生成する過程と、     
    　前記プローブ光によって測定対象物で発生する信号光をコヒーレント検波してビート信号を生成する過程と、
    　前記光パルスごとに、前記信号光の受光時刻ｔｊに対する、前記信号光の強度Ｉ（ｔｊ）及び位相Ｐ（ｔｊ）の分布を、前記ビート信号から取得する過程と、
    　基準時間ＳＴを設定する過程と、
    　前記信号光の受光時刻ｔｊに対する位相差を、ｔｋ＞ｔｊ＞ｔｉ、且つ、ｔｋとｔｉとの差が前記基準時間ＳＴである受光時刻ｔｋに対する位相Ｐ（ｔｋ）と受光時刻ｔｉに対する位相Ｐ（ｔｉ）との差、Ｐ（ｔｋ）－Ｐ（ｔｉ）として取得し、前記信号光の受光時刻に対する前記位相差の分布を取得する過程と、
    を備える
    光コヒーレントセンシング方法。     The process of generating an optical pulse as probe light,
    The process of coherently detecting the signal light generated in the object to be measured by the probe light to generate a beat signal,
    The process of acquiring the distribution of the intensity I (tj) and the phase P (tj) of the signal light from the beat signal with respect to the reception time tj of the signal light for each light pulse.
    The process of setting the reference time ST and
    The phase difference of the signal light with respect to the light receiving time tj is tk>tj> ti, and the phase P (tk) with respect to the light receiving time tk and the phase P with respect to the light receiving time ti (the difference between tk and ti is the reference time ST). The process of acquiring the difference from ti) as P (tk) -P (ti) and acquiring the distribution of the phase difference with respect to the reception time of the signal light, and
    Optical coherent sensing method with.
  4. 　前記基準時間ＳＴを設定する過程では、
    　初期状態の基準時間ＳＴ０に対して、第１時間Δｔ１を変化させて、各第１時間Δｔ１について、それぞれ複数の光パルスにおける強度Ｉ（ｔｋ＋Δｔ１）の最小値を取得し、該最小値が最大となる最大第１時間Δｔ１ｍａｘを取得し、
    　初期状態の基準時間ＳＴ０に対して、第２時間Δｔ２を変化させて、各第２時間Δｔ２について、それぞれ複数の光パルスにおける強度Ｉ（ｔｉ－Δｔ２）の最小値を取得し、該最小値が最大となる最大第２時間Δｔ２ｍａｘを取得し、
    　ＳＴ０＋Δｔ１ｍａｘ＋Δｔ２ｍａｘを基準時間ＳＴとする
    請求項３に記載の光コヒーレントセンシング方法。     In the process of setting the reference time ST,
    The first time Δt1 is changed with respect to the reference time ST0 in the initial state, and the minimum value of the intensity I (tk + Δt1) in each of a plurality of optical pulses is obtained for each first time Δt1, and the minimum value is set to the maximum. The maximum first time Δt1max is obtained,
    The second time Δt2 is changed with respect to the reference time ST0 in the initial state, and the minimum value of the intensity I (ti−Δt2) in each of a plurality of optical pulses is obtained for each second time Δt2, and the minimum value is the minimum value. Obtain the maximum second time Δt2max, which is the maximum,
    The optical coherent sensing method according to claim 3, wherein ST0 + Δt1max + Δt2max is set as a reference time ST.
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