JP2019215165A - Distance measuring device and distance measuring method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a distance measuring device and the like capable of shortening measurement time. A distance measuring device generates a laser beam whose intensity is modulated by a modulation signal and obtains an intensity correlation signal by taking an intensity correlation between a return light reflected by a measurement target and a reference signal based on the modulation signal. The distance to the measurement target is measured based on the acquired intensity correlation signal. Based on the period of the intensity correlation signal that changes periodically when the modulation frequency is swept over a frequency interval wider than the Nyquist interval of the intensity correlation signal that changes periodically when the modulation frequency is swept at a sufficiently small frequency interval , Calculate the round trip distance to the measurement target. [Selection diagram] Figure 1

Description

  The present invention relates to a distance measuring device and a distance measuring method.

  Conventionally, as a method (light intensity correlation method) of measuring a distance by emitting an intensity-modulated laser beam toward a measurement target and calculating an intensity correlation between the return light reflected by the measurement target and a reference signal (light intensity correlation method). (Patent Literature 1) for obtaining the intensity correlation between the light and the reference optical signal using the nonlinear response of the light receiving element, or the method for obtaining the intensity correlation with the return light by disposing an intensity modulator on the light receiving side (Patent Document 1) 2) is known. In these methods, when there are a plurality of translucent reflection points on the propagation path of the laser beam, the distance to each reflection point can be measured simultaneously.

JP 2007-205949 A JP 2018-59789 A

  In the above method, when the reflection points are distributed over a short distance to a long distance, when sweeping the modulation frequency, it is necessary to perform a wide frequency sweep at a fine frequency interval (frequency sweep step). As a result, there is a problem that it takes time to acquire data, and the measurement time becomes long.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a distance measuring device or the like that can reduce a measurement time.

を満たす周期Ｆ_ｒの１／２よりも広い周波数間隔Ｆ_ｓで前記変調周波数を掃引したときに周期的に変化する強度相関信号の周期Ｆ_ｇに基づいて、次式

により、前記測定対象までの往復距離Ｌを算出する、距離測定装置に関する。但し、ｍは
、Ｆ_ｒ／２＜Ｆ_ｓ＜ｍＦ_ｒ又はｍＦ_ｒ＜Ｆ_ｓを満たし、且つ、｜１／Ｆ_ｒ−ｍ／Ｆ_ｓ｜を最小値とする自然数である。 (1) In the present invention, a laser light intensity-modulated by a modulation signal is generated, and an intensity correlation signal is obtained by obtaining an intensity correlation between return light reflected by a measurement object and a reference signal based on the modulation signal. A distance measuring device for measuring a distance to the object to be measured based on the intensity correlation signal obtained, wherein c is the speed of light, n is the refractive index, L is the reciprocating distance to the object to be measured, and the modulation of the modulation signal is performed. when the period of periodically varying intensity correlation signal is a F _r when sweeping the frequency, the following equation

The based on the period F _g of periodically varying intensity correlation signal when the sweeping the modulation frequency at a period F wider frequency interval than half of _r F _s which satisfies the following equation

To calculate the reciprocating distance L to the object to be measured. Here, m is a natural number that satisfies F _r / 2 <F _s <mF _r or mF _r <F _s and minimizes | 1 / F _r −m / F _s |.

を満たす周期Ｆ_ｒの１／２よりも広い周波数間隔Ｆ_ｓで前記変調周波数を掃引したときに周期的に変化する強度相関信号の周期Ｆ_ｇに基づいて、次式

により、前記測定対象までの往復距離Ｌを算出する、距離測定方法に関する。但し、ｍは、Ｆ_ｒ／２＜Ｆ_ｓ＜ｍＦ_ｒ又はｍＦ_ｒ＜Ｆ_ｓを満たし、且つ、｜１／Ｆ_ｒ−ｍ／Ｆ_ｓ｜を最小値とする自然数である。 Further, the present invention generates a laser light intensity-modulated by the modulation signal, obtains an intensity correlation signal by taking an intensity correlation between the return light reflected by the measurement target and a reference signal based on the modulation signal, and acquires the intensity correlation signal. A distance measuring method for measuring a distance to the object to be measured based on an intensity correlation signal, wherein c is a light speed, n is a refractive index, L is a reciprocating distance to the object to be measured, and a modulation frequency of the modulation signal is when the period of periodically varying intensity correlation signal is a F _r when sweeping the following formula

The based on the period F _g of periodically varying intensity correlation signal when the sweeping the modulation frequency at a period F wider frequency interval than half of _r F _s which satisfies the following equation

To calculate the reciprocating distance L to the object to be measured. Here, m is a natural number that satisfies F _r / 2 <F _s <mF _r or mF _r <F _s and minimizes | 1 / F _r −m / F _s |.

According to the present invention, when sweeping the modulation frequency, the distance without finer frequency interval, the period F _g of intensity correlation signal when sweeping the modulation frequency in a wide frequency interval F _s to the measurement target measurement Therefore, the measurement time can be shortened.

  (2) Further, in the distance measuring device and the distance measuring method according to the present invention, the first light generating unit and the second light generating unit that generate laser lights having different wavelengths and intensity-modulated at the same modulation frequency. A signal generator that outputs a modulation signal to the first light generation unit and the second light generation unit; a return light in which laser light from the first light generation unit is reflected by the measurement target; A multiplexer for multiplexing the laser light from the two-light generator, a photodetector for receiving the light from the multiplexer and outputting an intensity correlation signal by a two-photon absorption response, and controlling the signal generator. A control unit for calculating a distance to the measurement target based on an intensity correlation signal from the photodetector.

  (3) In the distance measuring device and the distance measuring method according to the present invention, the light generating unit that generates the intensity-modulated laser light, and the return light from which the laser light from the light generating unit is reflected by the measurement target include: An intensity modulator that intensity-modulates at the same modulation frequency as the modulation frequency of the laser light, a signal generator that outputs a modulation signal to the light generator and the intensity modulator, and light that is intensity-modulated by the intensity modulator. And a control unit for controlling the signal generator and calculating a distance to the object to be measured based on the intensity correlation signal from the photodetector.

  (4) In the distance measuring device and the distance measuring method according to the present invention, a photodetector having a high-frequency cutoff frequency lower than the modulation frequency of the modulation signal may be used as the photodetector.

  According to the present invention, it is not necessary to separately perform signal processing on an output signal of a photodetector, and a DC component can be detected by removing a high-frequency component of an optical signal with a simple configuration.

FIG. 1 is a diagram schematically illustrating a configuration of a distance measuring device according to a first embodiment. An intensity correlation signal that changes periodically when the modulation frequency is swept at a sufficiently small frequency interval, and an intensity correlation that changes periodically when the modulation frequency is swept at a frequency interval wider than the Nyquist interval of the intensity correlation signal The figure which shows a signal. The figure which shows a measurement result. The figure which shows typically the structure of the distance measuring device which concerns on 2nd Embodiment.

  Hereinafter, this embodiment will be described. In addition, this embodiment demonstrated below does not unduly limit the content of this invention described in the claim. In addition, all the configurations described in the present embodiment are not necessarily essential configuration requirements of the present invention.

(1st Embodiment)
FIG. 1 is a diagram schematically illustrating a configuration of a distance measuring device according to the first embodiment. The distance measuring device 1 includes a laser light source 10 and an intensity modulator 11 functioning as a first light generation unit, a laser light source 12 and an intensity modulator 13 functioning as a second light generation unit, a signal generator 20, And a control unit 40 having an arithmetic processing unit (processor) and a storage unit. In the example shown in FIG. 1, a plurality of optical fiber diffraction gratings (FBG: Fiber Bragg Grating) are arranged on the probe optical path as a plurality of translucent reflection points R (R _{1 to} R ₅ ) to be measured. . It is assumed that the optical fiber diffraction grating has a low reflectance, and the effect of light that is multiple-reflected in the probe optical path can be ignored. A reflection point _{R 2} between the reflection point _{R 3} are separated by more than 100 m, the reflection point _R 1, _{R 2,} reflection point short range (near field), the reflection point _R 3 to R _5, long range (far Distance). An optical fiber attenuator AT (attenuator) is provided at the end of the optical fiber.

Intensity modulator 11 generates a laser beam from the laser light source 10 intensity modulation on the modulation frequency f _m with an intensity-modulated laser beam (probe beam), the intensity modulator 13, laser light from the laser light source 12 the intensity modulation to generate a modulation frequency f _m with an intensity modulated laser beam (reference light). Laser light sources having slightly different wavelengths are used as the laser light sources 10 and 12 in order to prevent interference. For example, the wavelength of the laser light source 10 is 1550 nm, and the wavelength of the laser light source 12 is 1552 nm. Here, a description will be given of a case where the light generating unit is configured by a laser light source and an intensity modulator. However, a direct modulation method of outputting a modulation signal to the laser light sources 10 and 12 and modulating the laser light may be employed. Incidentally, the probe light is further intensity modulated at a lock-in frequency f _l for synchronous detection. Lock-in frequency f _l, rather than the high frequency cut-off frequency of the optical detector 30 is sufficiently low, for example, the lock-in frequency f _l and 20 kHz.

Signal generator 20, based on the control signal from the control unit 40, and outputs a modulated signal having the same modulation frequency f _m to the intensity modulator 11 and 13.

The probe light emitted from the first light generating unit (laser light source 10, intensity modulator 11) is amplified by the optical amplifier 50 (EDFA), passes through the optical circulator 60, and reaches each reflection point R. The probe light (return light) reflected at each reflection point R passes through an optical circulator 60, is multiplexed with reference light (an example of a reference signal) by an optical coupler 61 (combiner), and is amplified by an optical amplifier 51. After that, the light is condensed by the lens 62 and enters the photodetector 30. On the other hand, the reference light emitted from the second light generation unit (laser light source 12 and intensity modulator 13) is multiplexed with the probe light by the optical coupler 61, amplified by the optical amplifier 51, and condensed by the lens 62. Then, the light enters the photodetector 30. Note that a half mirror may be used instead of the optical circulator 60 and the optical coupler 61.

The photodetector 30 receives the light (the probe light (return light) and the reference light are multiplexed) from the optical coupler 61 and receives a two-photon absorption current signal (reflected at each reflection point R) by a two-photon absorption response. (A superposition of the intensity correlation signals by the probe light). As the photodetector 30, a light receiving element such as an Si-APD (Avalanche Photo Diode) can be used. High frequency cut-off frequency of the optical detector 30 is lower than the modulation frequency f _m, the photodetector 30 detects only the DC component of the optical signal. Signal from the photodetector 30 is lock-in detection by a lock-in amplifier 70 by the lock-in frequency f _l. The output signal of the lock-in amplifier 70 is converted into digital data by an AD converter (not shown) and output to the control unit 40.

The control unit 40 controls the signal generator 20 and calculates a distance to the reflection point R (a difference in propagation distance between the probe light and the reference light reflected at the reflection point R) based on the output signal of the lock-in amplifier 70. I do. More specifically, the control unit 40, the modulation frequency f _m discretely swept at a constant frequency interval controls the signal generator 20, periodically varying intensity correlation when sweeping the modulation frequency f _m The distance to the reflection point R is calculated based on the signal period.

Here, the propagation distance of the probe light reflected at the _j- th (j = 1 to N) reflection point R _j is L _{p, j} , and the actual electric field amplitude of the probe light on the light receiving surface of the photodetector 30 is Ep _{, j,} the propagation distance of the reference light L _r, the actual electric field amplitude at the light receiving surface of the photodetector 30 of the reference beam when the E _r, the electric field e _p of the probe light on the light-receiving surface of the photodetector 30 has the following formula represented by (1), the electric field e _r of the reference light on the light-receiving surface of the photodetector 30 is represented by the formula (2).

ここで、νは光周波数であり、ｆは変調周波数であり、φは変調度であり、ｔは時間であり、ｎは光が伝搬する媒質の屈折率であり、ｃは光速であり、θは位相であり、Ｎは反射点Ｒの個数である。また、添え字付き記号の添え字のｐはプローブ光を示し、ｒは参照光を示し、ｌはロックインアンプ７０に入力される変調信号（参照信号）を示し、ｍは強度変調器１１，１３に入力される変調信号を示す。

Where ν is the optical frequency, f is the modulation frequency, φ is the modulation factor, t is time, n is the refractive index of the medium through which the light propagates, c is the speed of light, θ Is the phase, and N is the number of reflection points R. Also, the subscript p of the subscripted symbol indicates probe light, r indicates reference light, l indicates a modulation signal (reference signal) input to the lock-in amplifier 70, and m indicates the intensity modulator 11, FIG.

  The two-photon absorption current i output from the photodetector 30 is proportional to the root-mean-square of the incident light intensity and is expressed by the following equation (3).

ここで、Ａは比例定数である。Ｉは、Ｉ＝Ｅ^２で与えられ、光強度に比例する。ｉ_ｄは暗電流である。また、ΔＬ_ｊはｊ番目の反射点Ｒ_ｊで反射するプローブ光の伝搬距離Ｌ_ｐ，ｊと参照光の伝搬距離Ｌ_ｒの差（Ｌ_ｐ，ｊ−Ｌ_ｒ）、すなわち、反射点Ｒ_ｊまでの往復距離である。

Here, A is a proportionality constant. I is given by I = ^{E 2,} proportional to the light intensity. _id is the dark current. ΔL _j is a difference (L _{p, j} −L _r ) between the propagation distance L _{p, j of} the probe light reflected at the j-th reflection point R _j and the propagation distance L _r of the reference light, that is, the reflection point R _j It is the round trip distance to

Since the high frequency cut-off frequency of the optical detector 30 is lower than the modulation frequency f _m, the average in Equation (3), terms which vibrates at a high frequency cut-off higher modulation frequencies than the frequency f _m of the photodetector 30 is time Becomes 0. _{Also, n (L p, N -L} p, 1) / c is sufficiently small, the difference of the phase 2πf _l nL _p, j / c of the modulation signal for synchronous detection for the probe beam reflected by the different reflection point R Can be ignored. At this time, when lock-in detection of the two-photon absorption current signal by the lock-in frequency f _l, the current i _LIA output signal is expressed by the following equation (4).

ここで、Ｂは比例定数であり、ｉ_ｎはノイズ電流である。式（４）から、変調周波数ｆ_ｍを一定の周波数間隔で掃引すると、各反射点Ｒからの反射光による信号（強度相関信号）がそれぞれ正弦波（余弦波）状に（周期的に）変化することが分かる。

Here, B is a proportional constant, i _n is the noise current. From equation (4), when sweeping the modulation frequency f _m at constant frequency intervals, the signal (intensity correlation signal) (periodically) each sine wave (cosine wave) shape changes by the reflected light from the reflection point R You can see that

Here, the period f _period of the intensity correlation signal by j th probe light reflected by the reflection point _{R, j} is the frequency sweep step (frequency interval) of the modulation frequency f _m which discretely swept is sufficiently small, the following And is inversely proportional to the distance difference ΔL _j .

このとき、出力信号をフーリエ変換して、出力信号に含まれる各強度相関信号（各周波数成分）の周期ｆ_{ｐｅｒｉｏｄ，ｊ}を算出することで、各反射点Ｒの距離差ΔＬ_ｊを同時に求めることができる。すなわち、出力信号をフーリエ変換して得られるスペクトルにお
ける各ピーク位置が各反射点Ｒの距離差ΔＬ_ｊに対応する。

At this time, the output signal is Fourier-transformed, and the period f _{period, j} of each intensity correlation signal (each frequency component) included in the output signal is calculated, thereby simultaneously obtaining the distance difference ΔL _j between the reflection points R. Can be. In other words, each peak position in the spectrum obtained by the output signal by Fourier transform corresponds to the distance difference [Delta] L _j of each reflection point R.

Here, in order to accurately obtain the distance difference ΔL _j itself, it is necessary to perform frequency sweeping at a very small frequency interval in order to obtain the distance difference between the long-distance reflection points from Expression (5). I understand. On the other hand, when simultaneously calculating the distance difference between the short-range reflection points, the frequency sweep range must be widened. Therefore, when short-range reflection points and long-range reflection points coexist, it is necessary to perform a wide frequency sweep at very small frequency intervals, and the measurement time becomes long.

In Figure 2 (a) is a diagram showing a periodically varying intensity correlation signal when sweeping the modulation frequency f _m at sufficiently small frequency interval. The horizontal axis of the graph shown in FIG. 2 shows the modulation frequency f _m, the vertical axis represents the intensity of the intensity correlation signal. In the graph shown in FIG. 2, the interval between the black dots indicates the frequency sweeping step. Intensity correlation signal shown in FIG. 2 (a), the period f _period of the intensity correlation _signal, obtained when sweeping the modulation frequency f _m at a frequency spacing of less than 1/2 of the _j (hereinafter Nyquist interval of the intensity correlation signal) Can be

Here, as shown in FIG. 2 (b), (wider than the Nyquist interval of the intensity correlation signal) period _{f period,} wider than half the _j of the intensity correlation signal sweeping the modulation frequency _{f m} in the frequency interval Then, an intensity correlation signal having a period longer than the actual period f _{period, j} is observed. As a result, long-distance reflection point (e.g., the reflection point R ₃ to R 5 Figure ₁₎ even, by increasing the frequency sweep step, the reflection spectrum obtained by intensity correlation signal by Fourier transform The peak position corresponding to the point is apparently like a nearby reflection point.

When the frequency sweep step is sufficiently small, if the intensity correlation signal produced by the reflected light from the j-th reflection point R _j are periodically changes at a period F _r, a propagation distance of the probe light reflected at the reflection point R _j The distance difference ΔL from the propagation distance of the reference light is represented by the following equation (6). Also, the period of the intensity correlation signal obtained when sweeping the modulation frequency _{f m} in the cycle _F wider frequency interval than half of _{r F s (F s> F} r / 2) When the _{F g,} the The apparent distance difference ΔL _g calculated from the intensity correlation signal is represented by the following equation (7).

ここで、周期Ｆ_ｇは、以下の式（８）で表される。

Here, the period _Fg is represented by the following equation (8).

ここで、ｍは、Ｆ_ｒ／２＜Ｆ_ｓ＜ｍＦ_ｒ又はｍＦ_ｒ＜Ｆ_ｓを満たし、且つ、｜１／Ｆ_ｒ−ｍ／Ｆ_ｓ｜を最小値とする自然数である。周波数間隔Ｆ_ｓが周期Ｆ_ｒよりも小さいか大きいかによって周期Ｆ_ｇの値は異なるが、いずれの場合もΔＬ＞ΔＬ_ｇとなる。式（６）、式（８）より、距離差ΔＬは、周波数間隔Ｆ_ｓと周期Ｆ_ｇを用いて以下の式（９）で表される。

Here, m is a natural number that satisfies F _r / 2 <F _s <mF _r or mF _r <F _s and minimizes | 1 / F _r −m / F _s |. The frequency spacing F _s is the value of the period F _g depending on whether less than the period F _r large different, either the even [Delta] L> [Delta] L _g For. Equation (6), the equation (8), the distance difference ΔL is expressed by the following equation using the frequency spacing _{F s} and the period _{F g} (9).

本実施形態では、制御部４０は、式（６）を満たす周期Ｆ_ｒの１／２よりも広い周波数間隔Ｆ_ｓで変調周波数ｆ_ｍを掃引しながら取得した変調周波数ｆ_ｍ毎の出力信号をフーリエ変換して、出力信号に含まれる各強度相関信号（各反射点Ｒからの反射光によって生じる各強度相関信号）の周期Ｆ_ｇを算出し、算出した周期Ｆ_ｇと周波数間隔Ｆ_ｓとを式（９）に代入して距離差ΔＬ（反射点Ｒ_ｊまでの往復距離Ｌ_ｊ）を算出する。これにより、測定対象に長距離の反射点が含まれる場合であっても、周波数掃引ステップを細かくすることなく、広い周波数間隔Ｆ_ｓで変調周波数ｆ_ｍを掃引して各反射点までの距離（変位）を測定することができるため、測定で取得するデータ数が少なくなり、測定時間を短縮することができる。

In the present embodiment, the control unit 40, an output signal of each modulation frequency f _m obtained while sweeping the modulation frequency f _m in a wide frequency interval F _s than 1/2 of the period F _r that satisfies Equation (6) Fourier transform, and a period F _g is calculated, the calculated period F _g and the frequency spacing F _s of each intensity correlation signal included in the output signal (the intensity correlation signal produced by the reflected light from the reflection point R) calculating the (round trip distance _{L j} to the reflection point _{R j)} distance difference ΔL into equation (9). Thus, even if it contains a long distance of the reflection point on the measurement object, without fine frequency sweep step, the distance to the reflection point by sweeping the modulation frequency f _m in a wide frequency interval F _s ( Displacement) can be measured, so that the number of data acquired in the measurement is reduced, and the measurement time can be shortened.

Reciprocating distance (frequency sweep steps approximate to the reflection point is either less than half of the period _{F r} that satisfies Equation (6), or greater than 1/2 of the period _{F r,} greater than the period mF _r If either) is known, the calculated period into equation (6) calculates the distance difference ΔL for reflection points frequency sweep step is 1/2 or less of the period F _r, the frequency sweep step cycle F _r 1/2 smaller than larger period mF _r than in _{(F r / 2 <F s} <mF r) the period and the frequency sweep step calculated for the reflection point (frequency interval) and the formula (9) calculating a distance difference ΔL is substituted in the upper, the lower the frequency sweep step is greater than the period _{mF r (mF r <F s} ) wherein the period and the frequency sweep step calculated for the reflection point (9) To calculate the distance difference ΔL

On the other hand, when the approximate reciprocating distance to the reflection point is not known, the measurement is performed a plurality of times by changing the frequency sweep step. For example, in the second measurement, the frequency sweep step is set to a value twice as large as the frequency sweep step in the first measurement. When the cycle of the intensity correlation signal obtained in the first measurement is equal to the cycle of the intensity correlation signal obtained in the second measurement (the frequency sweeping step is performed in both the first measurement and the second measurement). in the case where a half or less) of the period F _r, the distance difference ΔL calculated the resulting cycle is substituted into equation (6) with the measurement result. Further, the distance difference ΔL calculated by substituting the period of the intensity correlation signal obtained in the first measurement into the equation (6), the period of the intensity correlation signal obtained in the second measurement, and the second measurement Is the same as the distance difference ΔL calculated by substituting the frequency sweeping step in the equation (9) in the upper stage of the equation (9) (in the first measurement, the frequency sweeping step is 1 in the period _Fr ). / 2 or less and becomes, in a case where the frequency sweep step is smaller than the larger cycle F _r than 1/2 of the period F _r) in the second measurement, the measurement result of the distance difference [Delta] L. Also, doubling the _{F s, | 1 / F r} -m / F s | natural number and minimum value m may be 2 times the value. Noting this, an equation obtained by substituting the cycle of the intensity correlation signal obtained in the first measurement and the frequency sweeping step in the first measurement into each of the upper and lower rows of equation (9), The cycle of the intensity correlation signal obtained in the second measurement and the frequency sweeping step in the second measurement are compared with the equations obtained by substituting the upper and lower rows of equation (9). In the latter two equations, m is replaced by 2m. When there is a natural number m such that one of the former two formulas and one of the latter two formulas have the same value, the distance difference ΔL obtained using the m is used as the measurement result. .

  In the method of the present embodiment, cosine wave signals (superposition of cosine wave signals having different periods) are sampled at regular intervals. For simplicity, consider a single cosine signal. The cosine wave signal f (t) is represented by the following equation (10). Note that t is not limited to time, but is a modulation frequency in the present embodiment.

余弦波信号のフーリエ変換Ｆ（ω）は、デルタ関数を用いて、

となり、ω軸上で±ω_０に線スペクトルをもつことが分かる。サンプリング間隔Ｔ_ｓが１／２ｆ_ｏ（ナイキスト間隔）以下であれば、信号スペクトルのうち最小の｜ω｜をもつ成分は±ω_０であり、これをとりだすことで元の波形を再現できる。一方、サンプリング間隔Ｔ_ｓが、

であると、信号スペクトルのうち最小の｜ω｜をもつ成分は、｜ω_０−ｍω_ｓ｜の最小値で与えられる｜ω｜をもつ線スペクトルの成分である。但し、ｍは、｜ω_０−ｍω_ｓ｜を最小値とする自然数である。また、ω_ｓ＝２π／Ｔ_ｓである。

The Fourier transform F (ω) of the cosine wave signal is calculated using a delta function,

And it can be seen that there is a line spectrum at ± ω ₀ on the ω axis. If the sampling interval T _s is 1 / 2f o _(Nyquist interval) or less, the minimum of the signal spectrum | omega | component with is ± omega _0, can reproduce the original waveform by taking out this. On the other hand, the sampling interval _{T s} is,

, The component having the minimum | ω | of the signal spectrum is the component of the line spectrum having | ω | given by the minimum value of | ω ₀ −mω _s |. Here, m is a natural number that minimizes | ω ₀ −mω _s |. Also, ω _s = 2π / T _s .

であれば、信号スペクトルのうち最小の｜ω｜をもつ成分は、ω_０−ω_ｓ及び−ω_０＋ω_ｓの線スペクトルとなる。図１に示す例のように反射点Ｒの凡その位置が予め分かっていれば、サンプリング間隔Ｔ_ｓを式（１３）の条件が満たされるように適切に選ぶことができる。また、より一般的に式（１２）の場合であっても、反射点Ｒの凡その位置が予め分かっていれば、｜ω_０−ｍω_ｓ｜を最小値とする自然数ｍは分かるため、ω_０を求めることは可能である。 Here, in particular, the sampling interval _{T s} is,

Then, the components having the minimum | ω | in the signal spectrum are the line spectra of ω ₀ −ω _s and −ω ₀ + ω _s . If the position of the approximate reflection point R as in the example shown in FIG. 1 is long known in advance, the sampling interval T _s can be chosen so that appropriate conditions of formula (13) is satisfied. Also, more generally, even in the case of Expression (12), if the approximate position of the reflection point R is known in advance, the natural number m that minimizes | ω ₀ −mω _s | It is possible to find ₀ .

An experiment was performed to measure the reciprocating distance (distance difference) between the reflection points R _{1 to} R ₅ using the method of the present embodiment. FIG. 3 shows the measurement results. The horizontal axis of the graph (spectrum obtained by Fourier transforming the output signal) shown in FIG. 3 indicates a value obtained by converting the frequency into a distance difference by the equation (6). The graph shown in FIG. 3, the peak position has appeared corresponding to the distance difference of the reflection point R ₁ to R _5. For the short-range reflection points R ₁ and R ₂ , the peak position corresponds to the actual distance difference ΔL. On the other hand, for the long-distance reflection points R _{3 to} R ₅ , the frequency sweep step is wider than _周期 of the period Fr that satisfies the expression (6), so that the peak position is not the actual distance difference ΔL. This corresponds to the apparent distance difference ΔL _g . For the reflection points R _{3 to} R ₅ , the actual distance difference ΔL can be obtained by substituting the period of the intensity correlation signal (the reciprocal of the frequency of the peak position) and the frequency sweeping step into Expression (9).

In this experiment, it was swept frequency sweep step of 0.69MHz the modulation frequency _{f m} in the range of 2 GHz. The measurement time was about 90 minutes in the conventional method in which the frequency sweep step was set to 0.15 MHz, whereas the measurement time was about 30 minutes in this experiment. The measurement time can be reduced to about 1/3 by the method of the present embodiment.

The distance resolution is determined depending on a range of sweeping the modulation frequency f _m, the resolution does not change even if the sweep frequency step becomes wider. Also in this experiment, the distance resolution did not change between the conventional method and the method of the present embodiment. Regarding the measurement accuracy, it is considered that the shortening of the measurement time reduces the influence of disturbance which causes an error, and improves the measurement accuracy in principle.

  As shown in the present embodiment, the distance measuring device according to the present invention can be applied to a multipoint FBG sensor using an optical fiber diffraction grating. The multipoint FBG sensor is expected to be used for health monitoring of structures (bridges, buildings, and the like) and has been put to practical use. In general, only the local distortion at the point where the FBGs are arranged can be measured. However, according to the distance measuring device according to the present embodiment, the overall distortion between FBGs can also be measured. In structural health monitoring, measuring global changes is as important as finding local anomalies. With the distance measuring device according to the present embodiment, the measurement can be performed in a short time, so that the efficiency and accuracy of structural health monitoring can be improved.

(Second Embodiment)
FIG. 4 is a diagram schematically illustrating a configuration of the distance measuring device according to the present embodiment. The distance measuring device 2 includes a laser light source 110 functioning as a light generation unit, an intensity modulator 120, a signal generator 130, a photodetector 140, and a control unit 150 having an arithmetic processing unit (processor) and a storage unit. including.

Laser source 110 generates an intensity modulated laser beam at the modulation frequency f _m. Here, a case will be described in which a direct modulation method of outputting a modulation signal to the laser light source 110 and modulating the laser light is employed. However, the light generation unit includes a laser light source and a modulator, and the modulation signal is transmitted to the modulator. An external modulation method of outputting and modulating a laser beam may be adopted. The laser light emitted from the laser light source 110 is converted into parallel light by the lens 111, passes through the half mirror 112, and reaches the measurement target (a plurality of translucent reflection points R). Return light reflected by the measurement target T (reflected light from the measurement target T) is reflected by the half mirror 112 and enters the intensity modulator 120.

Intensity modulator 120, the return light reflected by the measurement target T, based on the modulation signal from the signal generator 130 (another example of a reference signal), same as the modulation frequency of the modulation frequency f _{m (laser} light source 110 (Modulation frequency).

Signal generator 130 based on the control signal from the control unit 150, and outputs the modulated signal having the same modulation frequency f _m to the laser light source 110 and the intensity modulator 120.

The photodetector 140 receives the return light intensity-modulated by the intensity modulator 120 and outputs an intensity correlation signal (a superposition of the intensity correlation signals based on the intensity-modulated return light reflected at each reflection point R). The signal from the photodetector 140 is converted into digital data by an AD converter (not shown) and output to the control unit 150. Here, the cutoff frequency of the optical detector 140 is lower than the modulation frequency f _m, the photodetector 140 detects only the DC component of the intensity-modulated return light.

The control unit 150 controls the signal generator 130 and calculates the distance to the reflection point R based on the intensity correlation signal (output signal of the AD converter) from the photodetector 140. More specifically, the control unit 150, the modulation frequency f _m discretely swept at a constant frequency interval controls the signal generator 130, periodically varying intensity correlation when sweeping the modulation frequency f _m The distance to the reflection point R is calculated based on the signal period.

  Here, the intensity S of the intensity correlation signal from the photodetector 140 is represented by the following equation (14).

ここで、α、β、γ、ａ、ｂは定数であり、Ｌは、反射点Ｒで反射するレーザー光の伝搬距離（反射点Ｒまでの往復距離）である。式（１４）から、変調周波数ｆ_ｍを一定の周波数間隔で掃引すると、反射点Ｒからの反射光による強度相関信号が正弦波（余弦波）状に（周期的に）変化することが分かる。

Here, α, β, γ, a, and b are constants, and L is the propagation distance of the laser light reflected at the reflection point R (the reciprocating distance to the reflection point R). From equation (14), when sweeping the modulation frequency f _m at a constant frequency interval, the intensity correlation signal by the reflected light from the reflection point R is (periodically) to form a sine wave (cosine wave) can be seen to vary.

Also in the present embodiment, the distance to the reflection point R is calculated by the same method as in the first embodiment. However, the distance difference ΔL in the first embodiment is to be read as the distance L (reciprocating distance). That is, the control unit 150, an output signal of each modulation frequency f _m obtained while sweeping the modulation frequency f _m in a wide frequency interval F _s than 1/2 of the period F _r that satisfies Equation (6) Fourier transform Te to calculate the period F _g of each intensity correlation signal included in the output signal (the intensity correlation signal produced by the reflected light from the reflection point R), the calculated period F _g and the frequency spacing F _s and the formula (9 ) To calculate the distance L. In the actual measurement, first, the laser light from the laser light source 110 is directly incident on the intensity modulator 120 (without the optical path of the reciprocating movement from the half mirror 112). ₁ and L ₁ as a reference value. On top of that, it was measured in a state where the half mirror 112 is the optical path of the previous round-trip, the difference between the distance calculated at this time and L _2, and L ₂ and the reference value L ₁ (L ₂ -L ₁₎ Is determined as the distance L to the reflection point R.

Also according to this embodiment, it is possible to measure the distance (displacement) to each reflection point by sweeping the modulation frequency f _m with no wide frequency interval F _s to finer frequency sweep step, shortening the measurement time can do.

  Note that the present invention is not limited to the above-described embodiment, and various modifications are possible. The present invention includes configurations that are substantially the same as the configurations described in the embodiments (for example, configurations that have the same functions, methods, and results, or configurations that have the same objects and effects). In addition, the invention includes a configuration in which a non-essential part of the configuration described in the embodiment is replaced. In addition, the present invention includes a configuration that achieves the same effect as the configuration described in the embodiment or a configuration that can achieve the same object. Further, the invention includes a configuration in which a known technique is added to the configuration described in the embodiment.

  For example, in the above embodiment, as an example of a configuration for obtaining an intensity correlation signal by obtaining an intensity correlation between return light reflected by a measurement target and a reference signal, return light and a reference light (an example of a reference signal) are combined. A configuration for acquiring an intensity correlation signal using a two-photon absorption response of a photodetector that receives light (first embodiment), and a modulation signal of the same modulation frequency for return light (another example of a reference signal) Although the configuration (second embodiment) for obtaining the intensity correlation signal by further modulating the intensity has been described in the above, the present invention is not limited to this. For example, an intensity correlation signal is obtained by further modulating the intensity of an output signal (or a signal obtained by digitizing the output signal) of a photodetector that receives return light with a modulation signal having the same modulation frequency. Is also good.

In the above-described embodiment, the configuration has been described in which the high-frequency components of the return light are removed (only the DC component is detected) by using the photodetectors 30 and 140 having a narrow band, but the output signal of the photodetector is filtered. (For example, a low-pass filter) to remove high-frequency components of the return light. The control units 40 and 150 may be configured to remove the high-frequency component of the return light by performing signal processing on a signal obtained by digitizing the output signal of the photodetector.

1, 2, distance measuring device, 10, 12, laser light source, 11, 13 intensity modulator, 20 signal generator, 30 photodetector, 40 control unit, 50, 51 optical amplifier, 60 light Circulator, 61 optical coupler, 62 lens, 70 lock-in amplifier, 110 laser light source, 111 lens, 112 half mirror, 120 intensity modulator, 130 signal generator, 140 photodetector, 150 ... Control unit, R ... Reflection point (measurement target)

Claims (5)

Generate a laser light intensity-modulated by the modulation signal, obtain an intensity correlation signal by taking an intensity correlation between the return light reflected by the measurement target and a reference signal based on the modulation signal, based on the acquired intensity correlation signal A distance measuring device that measures a distance to the measurement target,
The speed of light and c, and the refractive index is n, the reciprocating distance of the to the measurement target is L, when the period of periodically varying intensity correlation signal when sweeping the modulation frequency of the modulated signal is a F _r, Next formula

The based on the period F _g of periodically varying intensity correlation signal when the sweeping the modulation frequency at a period F wider frequency interval than half of _r F _s which satisfies the following equation

A distance measuring device for calculating a reciprocating distance L to the object to be measured.
Here, m is a natural number that satisfies F _r / 2 <F _s <mF _r or mF _r <F _s and minimizes | 1 / F _r −m / F _s |. In claim 1,
A first light generation unit and a second light generation unit that generate laser lights of different wavelengths and intensity-modulated at the same modulation frequency;
A signal generator that outputs a modulation signal to the first light generation unit and the second light generation unit;
Return light in which the laser light from the first light generation unit is reflected by the measurement target, and a multiplexer that multiplexes the laser light from the second light generation unit;
A photodetector that receives light from the multiplexer and outputs an intensity correlation signal by a two-photon absorption response;
A control unit that controls the signal generator and calculates a distance to the measurement target based on an intensity correlation signal from the photodetector. In claim 1,
A light generating unit that generates an intensity-modulated laser beam,
Return light reflected by the measurement target from the laser light from the light generating unit, intensity modulator that intensity-modulates the same modulation frequency as the modulation frequency of the laser light,
A signal generator that outputs a modulation signal to the light generator and the intensity modulator,
A photodetector that receives the light intensity-modulated by the intensity modulator and outputs an intensity correlation signal;
A control unit that controls the signal generator and calculates a distance to the measurement target based on an intensity correlation signal from the photodetector. In claim 2 or 3,
The distance measuring device, wherein a high-frequency cutoff frequency of the photodetector is lower than a modulation frequency of the modulation signal. Generate a laser light intensity-modulated by the modulation signal, obtain an intensity correlation signal by taking an intensity correlation between the return light reflected by the measurement target and a reference signal based on the modulation signal, based on the acquired intensity correlation signal A distance measuring method for measuring a distance to the measurement target,
The speed of light and c, and the refractive index is n, the reciprocating distance of the to the measurement target is L, when the period of periodically varying intensity correlation signal when sweeping the modulation frequency of the modulated signal is a F _r, Next formula

The based on the period F _g of periodically varying intensity correlation signal when the sweeping the modulation frequency at a period F wider frequency interval than half of _r F _s which satisfies the following equation

Calculating the reciprocating distance L to the object to be measured.
Here, m is a natural number that satisfies F _r / 2 <F _s <mF _r or mF _r <F _s and minimizes | 1 / F _r −m / F _s |.

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