JP7493772B2 - Distance measuring device and distance measuring method - Google Patents

Description

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

Conventionally, a method is known in which an intensity-modulated probe light is generated, an intensity correlation signal is obtained by calculating the intensity correlation between the return light reflected from the measurement object and an intensity-modulated reference light, and the distance to the measurement object is measured based on the obtained intensity correlation signal (for example, Patent Document 1).

JP 2019-215165 A

The above method uses probe light and reference light that are intensity modulated by a sine wave, so in order to obtain a good signal-to-noise ratio, it is necessary to increase the average optical power of the probe light and reference light to expand the modulation amplitude. However, if the power of the probe light is increased too much, especially in the case of optical fiber, stimulated Brillouin scattering, which is backscattered light, occurs, making it impossible to transmit the probe light far away. In addition, if the power of the reference light is increased too much, it can cause damage to the equipment and can also pose problems such as the need for a high-output optical amplifier in the first place. Therefore, even with the above method, which can measure the distance to multiple reflection points in the longitudinal direction, there is a limit to the number of measurement points in terms of optical power.

The present invention was made in consideration of the above problems, and its purpose is to provide a distance measuring device etc. that can improve the signal-to-noise ratio while suppressing the average optical power.

(1) The present invention relates to a distance measuring device that includes a first light generating unit and a second light generating unit that generate laser light having different wavelengths and intensity modulated at the same frequency, a signal generator that outputs a modulation signal to the first light generating unit and the second light generating unit, a multiplexer that multiplexes return light reflected by a measurement object from the laser light from the first light generating unit and the laser light from the second light generating unit, a photodetector that receives light from the multiplexer and outputs an intensity correlation signal by a two-photon absorption response, and a control unit that controls the signal generator and calculates the distance to the measurement object based on the intensity correlation signal from the photodetector, and the second light generating unit generates pulsed light as the laser light.

The present invention also relates to a distance measuring method including a light generating step of generating laser light having different wavelengths and intensity modulated at the same frequency by a first light generating unit and a second light generating unit; a signal generating step of outputting a modulation signal to the first light generating unit and the second light generating unit by a signal generator; a combining step of combining, by a combiner, return light of the laser light from the first light generating unit reflected by an object to be measured and the laser light from the second light generating unit; a light detecting step of receiving the light from the combiner by a photodetector and outputting an intensity correlation signal by a two-photon absorption response; and a control step of controlling the signal generator and calculating a distance to the object to be measured based on the intensity correlation signal from the photodetector, wherein in the light generating step, the first light generating unit and the second light generating unit generate laser light intensity modulated based on the modulation signal from the signal generator, and the second light generating unit generates pulsed light as the laser light.

According to the present invention, by using pulsed light as the reference light (laser light generated by the second light generating unit), it is possible to improve the signal-to-noise ratio while suppressing the average optical power.

(2) In the distance measurement device and distance measurement method according to the present invention, the signal generator may output a pulse signal to the second light generating unit as the modulation signal.

(3) In the distance measurement device and distance measurement method according to the present invention, the second light generating unit may include a pulsed laser light source.

FIG. 1 is a diagram showing an example of the configuration of a distance measuring device according to an embodiment of the present invention. FIG. 4 is a diagram showing a Fourier spectrum obtained in an experiment for measuring distance using the method of the present embodiment. FIG. 4 is a diagram showing another example of the configuration of the distance measuring device according to the embodiment.

The present embodiment will be described below. Note that the present embodiment described below does not unduly limit the content of the present invention described in the claims. Furthermore, not all of the configurations described in the present embodiment are necessarily essential components of the present invention.

Figure 1 is a diagram showing an example of the 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 generating unit, a laser light source 12 and an intensity modulator 13 functioning as a second light generating unit, a signal generator 20, a photodetector 30, and a control unit 40 having an arithmetic processing unit (processor) and a memory unit. In the example shown in Figure 1, a reflection point R (mirror) to be measured is placed at the tip of a 5 km optical fiber that serves as the probe light path.

The intensity modulator 11 intensity-modulates the laser light from the laser light source 10 with a sine wave of a modulation frequency f _m to generate a laser light (probe light) intensity-modulated with the modulation frequency f _m , and the intensity modulator 13 intensity-modulates the laser light from the laser light source 12 with a pulse wave of a modulation frequency f _m to generate a pulse laser light (reference light) intensity-modulated with the modulation frequency f _m . In a measurement using the two-photon absorption response of a light receiving element, optical field interference becomes noise, so single mode laser light sources having slightly different wavelengths are used as the laser light sources 10 and 12. Here, the wavelength of the laser light source 10 is set to 1550 nm, and the wavelength of the laser light source 12 is set to 1552 nm. In the example shown in FIG. 1, a case will be described in which the first and second light generating units are configured with a laser light source and an intensity modulator, but a direct modulation method (a method in which the output intensity of the laser light source is directly modulated by intensity-modulating the drive current based on the modulation signal) in which a modulation signal is output to the laser light sources 10 and 12 to intensity-modulate the laser light may also be adopted.

Based on a control signal from the control unit 40, the signal generator 20 outputs a sine wave modulation signal with a modulation frequency _fm to the intensity modulator 11, and outputs a pulse wave modulation signal (pulse signal) with a modulation frequency _fm to the intensity modulator 13. The signal generator 20 also outputs a reference signal with a frequency _fm to the lock-in amplifier 70. The intensity modulator 11 generates probe light by intensity modulating the laser light from the laser light source 10 based on the modulation signal from the signal generator 20, and the intensity modulator 13 generates reference light by intensity modulating the laser light from the laser light source 12 based on the modulation signal from the signal generator 20.

The probe light emitted from the first light generating unit (laser light source 10, intensity modulator 11) passes through the optical circulator 60 and reaches the reflection point R. The probe light (return light) reflected at the reflection point R passes through the optical circulator 60 and is multiplexed with the reference light by the optical coupler 61 (multiplexer), and is collected by the lens 62 and enters the photodetector 30. On the other hand, the reference light emitted from the second light generating unit (laser light source 12, intensity modulator 13) is amplified (average optical power is adjusted) by the optical amplifier 50 (EDFA), and then is multiplexed with the probe light by the optical coupler 61, and is collected by the lens 62 and 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 light (probe light (return light) and reference light combined) from the optical coupler 61. Here , an Si-APD (Avalanche Photo Diode) is used as the light receiving element of the photodetector 30. When high-intensity light in the 1.5 μm wavelength band is incident on the Si-APD, a current (two-photon absorption current) proportional to the root mean square of the incident light intensity (the sum of the intensities of the return light and the reference light) is generated by two-photon absorption response, and by utilizing this, an intensity correlation signal can be obtained at high speed. The signal from the photodetector 30 is subjected to lock-in detection (synchronous detection) at a frequency f _m by the lock-in amplifier 70. The output signal of the lock-in amplifier 70 is converted to 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 the distance to the reflection point R (the difference in propagation distance between the probe light reflected at the reflection point R and the reference light) based on the output signal of the lock-in amplifier 70. More specifically, the control unit 40 controls the signal generator 20 to discretely sweep the modulation frequency _fm at constant frequency intervals, and calculates the distance to the reflection point _R based on the period of the intensity correlation signal (output signal) that changes periodically when the modulation frequency fm is swept.

Here, the intensity p of the probe light on the light receiving surface of the photodetector 30 is expressed by the following formula (1), and the intensity r of the reference light on the light receiving surface of the photodetector 30 is expressed by the following formula (2).

ここで、Ｉ_ｐはプローブ光の光検出器３０の受光面における係数であり、Ｉ_ｒは参照光の光検出器３０の受光面における係数であり、ｔは時間であり、ｎは光が伝搬する媒質の屈折率であり、ΔＬは反射点Ｒで反射したプローブ光と参照光の伝搬距離差（プローブ光路と参照光路の伝搬光路長差）、すなわち、反射点Ｒまでの往復距離であり、ｃは光速であり、δはデルタ関数である。図１に示す例では、プローブ光路となる光ファイバ長が５ｋｍ（往復１０ｋｍ）、光ファイバの屈折率ｎが１．５であるから、ｎΔＬ＝１５ｋｍとなる。

Here, _Ip is a coefficient of the probe light on the light receiving surface of the photodetector 30, _Ir is a coefficient of the reference light on the light receiving surface of the photodetector 30, t is time, n is a refractive index of the medium through which the light propagates, ΔL is a propagation distance difference between the probe light reflected at the reflection point R and the reference light (propagation optical path length difference between the probe optical path and the reference optical path), i.e., a round trip distance to the reflection point R, c is the speed of light, and δ is a delta function. In the example shown in Fig. 1, the length of the optical fiber serving as the probe optical path is 5 km (round trip 10 km), and the refractive index n of the optical fiber is 1.5, so that nΔL = 15 km.

When I _r >>I _p , the two-photon absorption current i _TPA output from the photodetector 30 is expressed by the following equation (3).

Ｉ_Ｓ＜＜Ｉ_ｒのとき、二光子吸収電流を周波数ｆ_ｍでロックイン検出すると、検出される信号ｉは、以下の式（４）で表される。ただし、２ａＩ_Ｓ ^２ｃｏｓ［２πｆ_ｍ（ｔ－ｎΔＬ／ｃ）］の項は無視した。

When I _S <<I _r , if the two-photon absorption current is locked-in detected at a frequency f _m , the detected signal i is expressed by the following equation (4), where the term 2aI _S ² cos[2πf _m (t−nΔL/c)] is ignored.

式（４）より、変調周波数ｆ_ｍを掃引して得られた出力信号（余弦波状に変化する波形）のフーリエスペクトルピークからｎΔＬが求められることがわかる。すなわち、当該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ピーク位置がｎΔＬに対応する。また、式（４）より、信号がＩ_ｒ倍になることがわかる。また、平均光パワーはτＩ_ｒ／Ｔ（τはパルス時間幅、Ｔはパルス繰り返し周期）であり、平均光パワーのＴ／τ倍の信号増幅が可能なことがわかる。

From equation (4), it can be seen that nΔL can be obtained from the Fourier spectrum peak of the output signal (waveform that changes like a cosine wave) obtained by sweeping the modulation frequency _fm . In other words, the peak position in the spectrum obtained by Fourier transforming the output signal corresponds to nΔL. Furthermore, from equation (4), it can be seen that the signal is _Ir times stronger. Furthermore, the average optical power is _τIr /T (τ is the pulse time width, T is the pulse repetition period), and it can be seen that the signal can be amplified by T/τ times the average optical power.

An experiment was carried out to measure nΔL in the example shown in FIG. 1. In this experiment, the modulation frequency f _m was swept over the range of 500 kHz to 1500 kHz at 2.5 kHz intervals. This sweep range gives a spatial resolution of 300 m. In addition, the pulse time width of the pulse wave modulation signal supplied to the intensity modulator 13 was set to 25.5 ns. Therefore, the peak power of the reference light is 26 times or more the average light power.

Figure 2 shows the Fourier spectrum obtained in this experiment. As shown in Figure 2, a peak appears near 15 km, demonstrating that the method of this embodiment can measure distances corresponding to optical fibers.

According to the method of this embodiment, by using pulsed light as the reference light, the signal can be amplified by the peak intensity of the pulsed light, so that the signal-to-noise ratio can be improved while suppressing the average optical power. By suppressing the average optical power, the burden on the driving power of the light source can be suppressed, and the burden on the light source power can be reduced, and there is also the effect of eliminating the risk of damage to optical equipment due to high optical power.

In the above example, the reference light is pulsed light by supplying a pulse wave modulation signal to the intensity modulator 13 of the second light generating unit. However, the reference light may be pulsed light by using a pulse laser light source as the light source of the second light generating unit. In the example shown in FIG. 3, a mode-locked fiber ring type pulse laser light source is used as the light source of the second light generating unit. The mode-locked fiber ring type pulse laser light source includes a fiber ring 14, an intensity modulator 13, and an optical amplifier 15. A modulation signal with a modulation frequency fm is supplied to the intensity modulator 13, and the mode-locked fiber ring type pulse laser light source generates a pulse laser light (reference light) intensity-modulated with the modulation frequency _fm . In this configuration, the pulse time width can be set to about 1 ns, and if the condition of the modulation frequency _fm is the same as in the above experiment, the peak power of the reference light can be 650 times or more the average optical power.

The distance measuring device according to the present invention can be applied to a multi-point FBG sensor that uses an optical fiber diffraction grating (FBG: Fiber Bragg Grating). Multi-point FBG sensors are used to diagnose the soundness of structures, and typically measure around 10 points. By using a reference light with high peak power in the method of the present invention, it is possible to increase the number of measurable points by an order of magnitude.

The distance measuring device according to the present invention can be applied to a bending sensor using a multi-core optical fiber and an FBG. In this sensor, the probe light is split into N cores and the return light is returned to the same optical fiber, so the optical power is attenuated to at least 1/ ^N2 . By using the method of the present invention, the influence can be suppressed, and multi-point simultaneous bending sensing can be realized without using an optical switch or the like to control the optical input to the cores.

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 those described in the embodiment (for example, configurations with the same functions, methods, and results, or configurations with the same purpose and effect). The present invention also includes configurations in which non-essential parts of the configurations described in the embodiment are replaced. The present invention also includes configurations that achieve the same effects as the configurations described in the embodiment, or that can achieve the same purpose. The present invention also includes configurations in which publicly known technology is added to the configurations described in the embodiment.

1...distance measuring device, 10, 12...laser light source, 11, 13...intensity modulator, 14...fiber ring, 15...optical amplifier, 20...signal generator, 30...photodetector, 40...controller, 50...optical amplifier, 60...optical circulator, 61...optical coupler, 62...lens, 70...lock-in amplifier, R...reflection point (measurement object)

Claims (4)

a first light generating unit and a second light generating unit for generating laser beams having different wavelengths and intensity-modulated based on a modulation signal having the same frequency fm _;
a signal generator that outputs a modulation signal having a frequency _fm to the first light generating unit and the second light generating unit;
a multiplexer that multiplexes return light resulting from reflection of the laser light from the first light generating unit on a measurement target and the laser light from the second light generating unit;
a photodetector that receives light from the multiplexer and outputs a two-photon absorption current generated by a two-photon absorption response;
a lock-in amplifier that performs lock-in detection of the two-photon absorption current from the photodetector at a frequency fm _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 lock-in amplifier ,
The signal generator includes:
A sine wave modulation signal having a frequency _fm is output to the first light generating unit ,
The second light generating unit is
generating, as the laser light, a pulsed light intensity-modulated based on a modulation signal having a frequency _fm ;
The control unit is
A distance measuring device that controls the signal generator to sweep a frequency _fm , and calculates a distance to the object based on a period of the intensity correlation signal that changes periodically when the frequency _fm is swept. In claim 1,
The signal generator includes:
a pulse signal _{having a frequency fm} outputted to the second light generating section as the modulation signal; In claim 1,
The second light generating unit is
A distance measuring device comprising a pulsed laser light source that generates a pulsed laser light whose intensity is modulated based on a modulation signal having a frequency _fm . a light generating step of generating laser light having different wavelengths and intensity modulated based on a modulation signal having _{the same frequency fm} by a first light generating unit and a second light generating unit;
a signal generating step of outputting a modulated signal having a frequency _fm to the first light generating unit and the second light generating unit by a signal generator;
a combining step of combining, by a combiner, return light resulting from reflection of the laser light from the first light generating unit on a measurement target and the laser light from the second light generating unit;
a photodetection step of receiving the light from the multiplexer with a photodetector and outputting a two-photon absorption current generated by a two-photon absorption response;
a lock-in detection step of detecting the two-photon absorption current from the photodetector by a lock-in amplifier at a frequency fm _and outputting an intensity correlation signal;
a control step of controlling the signal generator and calculating a distance to the measurement target based on an intensity correlation signal from the lock-in amplifier ;
The signal generator includes:
A sine wave modulation signal having a frequency _fm is output to the first light generating unit ,
The second light generating unit is
generating, as the laser light, a pulsed light intensity-modulated based on a modulation signal having a frequency _fm ;
In the control step,
A distance measuring method comprising the steps of: controlling the signal generator to sweep a frequency _fm ; and calculating a distance to the object based on a period of the intensity correlation signal which changes periodically when the frequency fm is swept _.

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