JP2022035564A - Distance measuring device and distance measuring method - Google Patents

Abstract

To provide a distance measuring device, etc., with which it is possible to improve a signal-to-noise ratio while suppressing average optical power.SOLUTION: A distance measuring device comprises: a first light generation unit and a second light generation unit for generating laser beams mutually different in wavelength and intensity modulated with the same frequency; a signal generator for outputting modulation signals to the first light generation unit and the second light generation unit; a multiplexer for multiplexing the return light of a laser beam from the first light generation unit and having been reflected by a measurement object and a laser beam from the second light generation unit; a photodetector for receiving light from the multiplexer and outputting an intensity correlation signal by two-photon absorption response; and a control unit for controlling the signal generator and calculating the distance to the measurement object on the basis of the intensity correlation signal from the photodetector. The second light generation unit generates pulse light as a laser beam.SELECTED DRAWING: Figure 1

Description

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

Conventionally, an intensity-modulated probe light is generated, an intensity correlation signal is acquired by taking an intensity correlation between the return light reflected by the measurement target and the intensity-modulated reference light, and the measurement target is measured based on the acquired intensity correlation signal. A method for measuring the distance to a distance is known (for example, Patent Document 1).

Japanese Unexamined Patent Publication No. 2019-215165

In the above method, probe light and reference light that have been intensity-modulated by a sine wave are used. Therefore, in order to obtain a good signal-to-noise ratio, the average optical power of the probe light and reference light is increased to expand the modulation amplitude. There is a need to. However, if the power of the probe light is increased too much, guided Brillouin scattering, which is backscattered light, occurs, especially in the case of an optical fiber, and there is a problem that the probe light cannot be transmitted to a distant place. In addition, if the power of the reference light is increased too much, the equipment may be damaged and a large output optical amplifier may be required in the first place. Therefore, even in the above method of measuring the distances to a plurality of reflection points existing in the longitudinal direction, there is a limit to the number of measurement points in terms of optical power.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a distance measuring device or the like capable of improving the signal-to-noise ratio while suppressing the average optical power. It is in.

(1) The present invention includes a first light generating unit and a second light generating unit that generate laser light having different wavelengths and intensity-modulated laser light at the same frequency, and the first light generating unit and the above. A signal generator that outputs a modulation signal to the second light generator, a return light reflected by the laser beam from the first light generator, and a laser beam from the second light generator are combined. Based on the intensity correlation signal from the light detector, which controls the combiner, the light detector that receives the light from the combiner and outputs the intensity correlation signal by the two-photon absorption response, and the signal generator. The second light generating unit includes a control unit for calculating the distance to the measurement target, and the second light generating unit relates to a distance measuring device that generates pulsed light as the laser light.

Further, according to the present invention, the light generation step of generating laser light having different wavelengths and intensity-modulated at the same frequency by the first light generator and the second light generator, and the signal generator are used. A signal generation step that outputs a modulation signal to the first light generation unit and the second light generation unit, return light reflected by the laser light from the first light generation unit on the measurement target, and the second light generation unit. A combine wave step in which the laser light of the above is combined by a combiner, a light detection step in which light from the combiner is received by an optical detector and an intensity correlation signal is output by a two-photon absorption response, and the signal generator. Including a control step of controlling and calculating the distance to the measurement target based on the intensity correlation signal from the light detector, in the light generation step, the intensity was modulated based on the modulated signal from the signal generator. The second light generating section relates to a distance measuring method in which laser light is generated by a first light generating section and a second light generating section, and pulsed light is generated as the laser light.

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

(2) Further, in the distance measuring device and the distance measuring method according to the present invention, the signal generator may output a pulse signal as the modulation signal to the second light generator.

(3) Further, in the distance measuring device and the distance measuring method according to the present invention, the second light generating unit may include a pulse laser light source.

The figure which shows an example of the structure of the distance measuring apparatus which concerns on this embodiment. The figure which shows the Fourier spectrum obtained by the experiment which measured the distance by the method of this embodiment. The figure which shows the other example of the structure of the distance measuring apparatus which concerns on this embodiment.

Hereinafter, this embodiment will be described. The embodiments described below do not unreasonably limit the content of the present invention described in the claims. Moreover, not all of the configurations described in the present embodiment are essential constituent requirements of the present invention.

FIG. 1 is a diagram showing an example of a configuration of a distance measuring device according to a first embodiment. The distance measuring device 1 includes a laser light source 10 and an intensity modulator 11 that function as a first light generator, a laser light source 12 and an intensity modulator 13 that function as a second light generator, a signal generator 20, and a photodetector. A device 30 and a control unit 40 having an arithmetic processing unit (processor) and a storage unit are included. In the example shown in FIG. 1, a reflection point R (mirror) to be measured is arranged at the tip of an optical fiber of 5 km which is a probe optical path.

The intensity modulator 11 intensity-modulates the laser light from the laser light source 10 with a sine wave having a modulation frequency _fm to generate intensity-modulated laser light (probe light) at the modulation frequency _fm , and the intensity modulator 13 generates the intensity-modulated laser light. _Intensifies the laser light from the laser light source 12 with a pulse wave having a modulation frequency of fm to generate pulsed laser light (reference light) whose intensity is modulated at the modulation frequency _fm . In the measurement using the two-photon absorption response of the light receiving element, the optical electric field interference becomes noise. Therefore, as the laser light sources 10 and 12, single-mode laser light sources having slightly different wavelengths are used. 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 where the first and second light generators are configured by a laser light source and an intensity modulator will be described, but the modulation signal is output to the laser light sources 10 and 12 to directly modulate the intensity of the laser light. A 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) may be adopted.

The signal generator 20 outputs a sine wave modulation signal having a modulation frequency _fm to the intensity modulator 11 and a pulse wave modulation signal having a modulation frequency _fm to the intensity modulator 13 based on the control signal from the control unit 40. (Pulse signal) is output. Further, the signal generator 20 outputs a reference signal having a frequency of _fm to the lock-in amplifier 70. The intensity modulator 11 intensity-modulates the laser light from the laser light source 10 based on the modulated signal from the signal generator 20 to generate probe light, and the intensity modulator 13 is based on the modulated signal from the signal generator 20. The intensity of the laser light from the laser light source 12 is modulated to generate reference light.

The probe light emitted from the first light generator (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, is combined with the reference light by the optical coupler 61 (combiner), is condensed by the lens 62, and is incident on the photodetector 30. do. On the other hand, the reference light emitted from the second light generator (laser light source 12, intensity modulator 13) is amplified by the optical amplifier 50 (EDFA) (the average optical power is adjusted) and then by the optical coupler 61. It is combined with the probe light, condensed by the lens 62, and incident on the light detector 30. A half mirror may be used instead of the optical circulator 60 and the optical coupler 61.

The photodetector 30 receives the light from the optical coupler 61 (the probe light (return light) and the reference light are combined). The high cutoff frequency of the photodetector 30 is lower than the modulation frequency _fm , and the photodetector 30 detects only the DC component of the optical signal. Here, Si-APD (Avalanche Photo Diode) is used as the light receiving element of the photodetector 30. When high-intensity light with a wavelength of 1.5 μm is incident on Si-APD, the current (two-photon absorption current) proportional to the squared average of the incident light intensity (sum of the intensity of the return light and the reference light) is generated by the two-photon absorption response. Since it is generated, an intensity correlation signal can be obtained at high speed by using this. The signal from the photodetector 30 is locked-in detected (synchronous detection) at a frequency _fm by the lock-in amplifier 70. 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 the distance to the reflection point R (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. do. More specifically, the control unit 40 controls the signal generator 20 to sweep the modulation frequency _{fm discretely} at regular frequency intervals, and the intensity correlation that changes periodically when the modulation frequency fm is swept. The distance to the reflection point R is calculated based on the period of the signal (output signal).

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

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, I _p is a coefficient on the light receiving surface of the optical detector 30 of the probe light, _Ir is a coefficient on the light receiving surface of the light detector 30 of the reference light, t is time, and n is the propagation of light. It is the refractive index of the medium to be used, and ΔL is the propagation distance difference between the probe light and the reference light reflected at the reflection point R (the propagation light path length difference between the probe optical path and the reference optical path), that is, the round-trip distance to the reflection point R. c is the optical path and δ is the delta function. In the example shown in FIG. 1, since 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, nΔL = 15 km.

When _Ir >> _Ip , the two-photon absorption current _iTPA 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 the two-photon absorption current is locked in at a frequency _fm when _IS << _Ir , the detected signal i is expressed by the following equation (4). However, the term of _2aIS ² cos [ _2πfm (tnΔL / c)] was 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. That is, the peak position in the spectrum obtained by Fourier transforming the output signal corresponds to nΔL. Further, from the equation (4), it can be seen that the signal is multiplied by _Ir . Further, 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 signal amplification of T / τ times the average optical power is possible.

An experiment was conducted to measure nΔL in the example shown in FIG. In this experiment, the modulation frequency _fm was swept at 2.5 kHz intervals over the range of 500 kHz to 1500 kHz. This sweep range provides a spatial resolution of 300 m. Further, the pulse time width of the modulated signal of the pulse wave 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.

FIG. 2 shows the Fourier spectrum obtained in this experiment. As shown in FIG. 2, a peak appears in the vicinity of 15 km, indicating that the distance measurement corresponding to the optical fiber can be performed by the method of the present embodiment.

According to the method of the present embodiment, by using the 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 light power. Can be done. By suppressing the average optical power, the burden of the driving power of the light source can be suppressed and the burden of the light source power can be reduced, and there is also an effect that there is no risk of damage to the optical equipment due to the high optical power.

In the above example, the case where the reference light is used as pulse light by supplying the pulse wave modulation signal to the intensity modulator 13 of the second light generation unit has been described, but the light source of the second light generation unit is a pulse laser light source. By doing so, the reference light may be made into pulsed light. In the example shown in FIG. 3, a mode-synchronized fiber ring type pulse laser light source is used as the light source of the second light generation unit. The mode-synchronized fiber ring type pulse laser light source includes a fiber ring 14, an intensity modulator 13, and an optical amplifier 15. A modulation signal having a modulation frequency fm is supplied to the intensity modulator 13, and the mode-synchronized fiber ring type pulse laser light source generates pulse laser light (reference light) intensity-modulated at the modulation frequency _fm . In this configuration, the pulse time width can be set to about 1 ns, and if the conditions of the modulation frequency _fm are the same as those in the above experiment, the peak power of the reference light should be 650 times or more the average light power. Can be done.

The distance measuring device according to the present invention can be applied to a multi-point FBG sensor using an optical fiber diffraction grating (FBG: Fiber Bragg Grating). The multi-point FBG sensor is used for diagnosing the soundness of a structure, but the number of measurement points is generally about 10. By using the reference light having a high peak power in the method of the present invention, it is possible to increase the measurable score by an order of magnitude.

Further, 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 divided into N cores and the return light is returned to the same optical fiber, so that the optical power is attenuated to at least 1 / ^N2 . By using the method of the present invention, the influence can be suppressed, and multipoint simultaneous bending sensing can be realized without using an optical switch or the like to control the optical input to the core.

The present invention is not limited to the above-described embodiment, and various modifications can be made. The present invention includes substantially the same configurations as those described in the embodiments (eg, configurations with the same function, method and result, or configurations with the same purpose and effect). The present invention also includes a configuration in which a non-essential part of the configuration described in the embodiment is replaced. Further, the present invention includes a configuration having the same action and effect as the configuration described in the embodiment or a configuration capable of achieving the same object. Further, the present invention includes a configuration in which a known technique is added to the configuration 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 ... Optical detector, 40 ... Control unit, 50 ... Optical amplifier, 60 ... Optical circulator, 61 ... Optical coupler, 62 ... Lens, 70 ... Lock-in amplifier, R ... Reflection point (measurement target)

Claims (4)

A first light generator and a second light generator that generate laser beams having different wavelengths and intensity-modulated at the same frequency.
A signal generator that outputs a modulated signal to the first light generator and the second light generator,
A combiner that combines the return light reflected by the laser beam from the first light generating section and the laser beam from the second light generating section with the measurement target.
A photodetector that receives light from the combiner and outputs an intensity correlation signal by a two-photon absorption response.
It includes a control unit that controls the signal generator and calculates the distance to the measurement target based on the intensity correlation signal from the photodetector.
The second light generator is
A distance measuring device that generates pulsed light as the laser light. In claim 1,
The signal generator is
A distance measuring device that outputs a pulse signal as the modulation signal to the second light generation unit. In claim 1,
The second light generator is
A distance measuring device equipped with a pulsed laser light source. A light generation step in which the first light generation unit and the second light generation unit generate laser light having different wavelengths and intensity-modulated at the same frequency.
A signal generation step of outputting a modulated signal to the first light generator and the second light generator by a signal generator, and
A combined wave step in which the return light reflected by the laser beam from the first light generating section and the laser beam from the second light generating section are combined by a combiner.
A photodetection step in which the light from the combiner is received by a photodetector and an intensity correlation signal is output by a two-photon absorption response.
It includes a control step of controlling the signal generator and calculating the distance to the measurement target based on the intensity correlation signal from the photodetector.
In the light generation step,
A laser beam whose intensity is modulated based on the modulation signal from the signal generator is generated by the first light generator and the second light generator.
The second light generator is
A distance measuring method that generates pulsed light as the laser light.

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