JP2023119469A - Contactless distance measuring device and method - Google Patents

Abstract

To realize the extension of measured distance without lowering a frequency sweep speed in a simple distance measuring device that uses a single light source.SOLUTION: The present disclosure pertains to a contactless distance measuring device that irradiates a measurement object with frequency sweep light and measures the frequency of a beat signal due to the interference of probe light reflected by the measurement object with reference light, thereby finding a distance to the measurement object. The contactless distance measuring device comprises a light replication unit that the reference light or the probe light enters and that replicates light of a plurality of different frequencies from the incident light, and main interferometer that multiplexes the light of the plurality of frequencies and the reference light or the probe light that did not enter the replication unit and detects a smallest beat frequency that is obtained by interference of the light of the plurality of frequencies with the reference light or the probe light that did not enter the light replication unit.SELECTED DRAWING: Figure 1

Description

The present disclosure relates to a non-contact ranging device and method using frequency swept light.

Accurate ranging technology is an important technique in metrology such as shape measurement of large-scale structures. There are many such applications, including shape measurement of man-made objects such as parabolic antennas and buildings, measurement of natural formations such as the thickness measurement of ice sheets and the height measurement of forests. In particular, in the measurement of large-scale structures for the purpose of health monitoring of buildings and disaster prevention, there is a demand for high-definition measurement of the position and shape of distant objects.

LiDAR (Light Detection And Ranging) is a non-contact ranging technology that uses laser light to measure the optical path length to an object. Among such technologies, frequency-modulated continuous wave (FMCW) LiDAR provides ranging with a single light source and is capable of velocity and vibration sensing. The FMCW LiDAR sweeps the optical frequency and converts the frequency difference (beat frequency, IF) caused by interference with the returned light into distance. A resolution of about 12 μm can be achieved using a light source with a swept bandwidth of 100 nm.

However, in FMCW-type LiDAR, the measurement distance is limited to several tens of meters due to the coherence length of the light source. In addition, it is not possible to measure the obtained beat frequency exceeding the reception band. This means that there is an upper limit to the product of frequency sweep speed and measurement distance as long as the reception band is constant. That is, if the reception band is constant, there is a problem that either the measurement distance or the frequency sweep speed must be sacrificed. Since the measurement repetition frequency (refresh rate) is proportional to the frequency sweep speed, sacrificing the frequency sweep speed means sacrificing the refresh rate. Due to the above two factors, FMCW-type LiDAR has been limited exclusively to relatively short distance measurement within several tens of meters.

T. Sakamoto et al. "Asymptotic formalism for ultraflat optical frequency comb generation using a Mach-Zehnder modulator," OPTICS LETTERS Vol. 32, No. 11 p. 1515 June 1, 2007 F. Ito et al. , "Long-Range Coherent OFDR with Light Source Phase Noise Compensation," JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 8, pp. 1015-1024, April 15, 2012

In order to solve the above problems, an object of the present disclosure is to extend the measurement distance without lowering the frequency sweep speed in a simple rangefinder using a single light source.

To achieve the above object, the present disclosure replicates light of multiple frequencies from one of the reference light and the probe light, and causes the replicated light of multiple frequencies to interfere with the other of the reference light and the probe light.

Specifically, the non-contact ranging device according to the present disclosure includes:
A non-contact type that determines the distance to the object by irradiating the object to be measured with frequency swept light and measuring the frequency of the beat signal resulting from the interference between the probe light and the reference light reflected by the object to be measured. In the rangefinder,
an optical duplicator that receives the reference light or the probe light and duplicates the incident light into light of a plurality of different frequencies;
combining the light of the plurality of frequencies and the reference light or the probe light that has not entered the replication section, and combining the light of the plurality of frequencies and the reference light or the probe light that has not entered the optical replication section; a main interferometer for detecting the minimum beat frequency obtained by the interference of
Prepare.

Specifically, the non-contact ranging method according to the present disclosure includes:
A non-contact type that determines the distance to the object by irradiating the object to be measured with frequency swept light and measuring the frequency of the beat signal resulting from the interference between the probe light and the reference light reflected by the object to be measured. In the ranging method,
duplicating the reference light or the probe light into light of different frequencies;
A minimum beat obtained by combining the lights of the plurality of frequencies and the non-duplicated reference light or the probe light, and by interference of the light of the plurality of frequencies and the non-duplicated reference light or probe light. detecting a frequency;
I do.

Advantageous Effects of Invention According to the present disclosure, in a simple distance measuring device using a single light source, it is possible to extend the measurement distance without reducing the frequency sweep speed.

1 is a diagram showing an example of a configuration of a non-contact rangefinder according to Embodiment 1; FIG. FIG. 4 is a diagram for explaining frequencies of reference light and probe light and interference signal spectra in a conventional FMCW LiDAR; 4 is a diagram for explaining frequencies of reference light and probe light in the non-contact rangefinder according to Embodiment 1. FIG. FIG. 4 is a diagram for explaining an interference signal spectrum in the non-contact ranging device according to Embodiment 1; 4 is a diagram for explaining frequencies of reference light and probe light in the non-contact rangefinder according to Embodiment 1. FIG. 4 is a diagram for explaining frequencies of reference light and probe light in the non-contact rangefinder according to Embodiment 1. FIG. FIG. 10 is a diagram showing an example of the configuration of a non-contact rangefinder according to Embodiment 2;

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the embodiments shown below. These implementation examples are merely illustrative, and the present disclosure can be implemented in various modified and improved forms based on the knowledge of those skilled in the art. In addition, in this specification and the drawings, constituent elements having the same reference numerals are the same as each other.

(Embodiment 1)
BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 shows an embodiment of the present disclosure based on the FWCW LiDAR scheme. 1 is a frequency sweep light source, 2 is a coupler for demultiplexing or combining light, 3 is an optical circulator, 4 is a lens, 5 is an object to be measured, 6 is a delay device, 7 is an optical frequency comb generator, and 8 is light 90. 9 is a balanced photodetector; 10 is an AD converter; 11 is an arithmetic unit such as a computer; and 12 is an RF synthesizer.

The optical frequency comb generator 7 functions as the optical replicator of the present disclosure. The optical 90-degree hybrid 8, the balanced photodetectors 9-1 and 9-2, and the AD converter 10-1 constitute a main interferometer 20 functioning as a photodetector of the present disclosure. Hereinafter, the "receiving band of the photodetector" is abbreviated as "receiving band".

A frequency swept light source 1 oscillates laser light whose frequency is linearly modulated. The frequency is swept over a frequency sweep width ΔF at a constant sweep rate γ [Hz/s] and a constant frequency sweep time ΔT. In the present embodiment, the light output from the frequency swept light source 1 is described as laser light, but it is not limited to this as long as it is coherent light.

The coupler 2-1 splits the light input from the frequency sweep light source 1 into two, and inputs one of them into the optical circulator 3 as probe light and the other into the optical frequency comb generator 7 as reference light. The optical circulator 3 inputs the probe light from the coupler 2-1 to the lens 4. FIG. Also, the optical circulator 3 inputs the light from the lens 4 to the optical 90-degree hybrid 8 . A lens 4 converts the probe light from the frequency sweep light source 1 into a plane wave. Also, the lens 4 collects the probe light reflected from the object 5 to be measured and inputs it to the optical circulator 3 .

The optical frequency comb generator 7 generates high-order modulation sidebands for the input light. A specific configuration of the optical frequency comb generator 7 is described in Non-Patent Document 1, for example. In this embodiment, as an example, a signal from the RF synthesizer 12 via the amplifier 13 is input to the optical frequency comb generator 7, and the optical frequency comb generator 7 generates optical frequencies at frequency intervals according to the signal from the RF synthesizer 12. Here is an example of generating a comb.

The optical frequency comb generator 7 generates an optical frequency comb composed of a plurality of different frequencies including the frequency of the reference light input from the coupler 2-1. The optical frequency comb generator 7 inputs the generated optical frequency comb to the optical 90-degree hybrid 8 as reference light for the main interferometer 20 . The probe light reflected by the object 5 interferes with the laser light (reference light) at the optical 90-degree hybrid 8 (primary interferometer). The main interferometer 20 can measure the delay time of the probe light reflected from the object 5 to the reference light. The calculation unit 11 performs distance measurement using this delay time. Although the calculation unit 11 is included in the main interferometer 20 in FIG. 1, the main interferometer 20 and the calculation unit 11 may be separate.

Specifically, the optical 90-degree hybrid 8 generates the in-phase component I of the beat signal by combining the reference light and the reflected light from the object 5, and inputs it to the balanced photodetector 9-1. In addition, the optical 90-degree hybrid 8 generates the quadrature component Q of the beat signal by combining the reference light phase-shifted by 90 degrees and the reflected light from the object to be measured, and inputs it to the balanced photodetector 9-2. .

The balanced photodetector 9-1 obtains an analog electrical signal of the in-phase component I of the beat signal based on the input from the optical 90-degree hybrid 8, and inputs it to the AD converter 10-1. The balanced photodetector 9-2 obtains an analog electrical signal of the quadrature component Q of the beat signal based on the input from the optical 90-degree hybrid 8, and inputs it to the AD converter 10-1. The AD converter 10-1 converts the analog electrical signal of the in-phase component I of the beat signal input from the coupler 9-1 and the analog electrical signal of the quadrature component Q of the beat signal input from the coupler 9-2 into digital signals. , is input to the calculation unit 11 .

Here, the interference between the reference light and the probe light in a conventional FMCW LiDAR without the optical frequency comb generator 7 will be described with reference to FIG. That is, the conventional FMCW LiDAR has a configuration in which the reference light from the coupler 2-1 is directly input to the optical 90-degree hybrid 8 in the main interferometer of FIG. In the conventional FMCW type LiDAR, the reference light from the coupler 2-1 and the probe light delayed by the round trip distance to the device under test 5 with respect to the reference light arrive, and the frequency corresponding to the frequency difference A beat signal with Hereinafter, the frequency of the beat signal will be referred to as beat frequency IF. Since this beat frequency IF is proportional to the delay time of the probe light reflected from the object 5 to the reference light, distance measurement can be performed.

Specifically, the beat signal is represented by the complex number expression (1) using the I-phase component and the Q-phase component of the beat signal output from the optical 90-degree hybrid 8 .
(Number 1)
I+jQ=exp(jγτt) (1)

The calculation unit 11 obtains the phase of the beat signal from equation (1) based on the in-phase component I and the quadrature component Q of the hybrid signal input from the AD converter 10-1. Here, τ is the delay time of the probe light with respect to the reference light corresponding to the optical path difference between the reference light and the probe light, and γτ is the beat frequency IF.

In general, the range resolution Δz of FMCW-type LiDAR is expressed as follows using the frequency sweep width ΔF.
(Number 2)
Δz=c/(2ΔF) (2)

That is, in order to improve the distance resolution Δz, it is necessary to increase the frequency sweep width ΔF. Moreover, the beat frequency IF is proportional to the delay time τ and the sweep speed γ, which vary depending on the distance to the device under test 5 . Therefore, if the reception band is constant, there will be a limit to the product of the sweep speed γ and the distance. Specifically, as shown in FIG. 2B, the conventional FMCW LiDAR cannot detect a beat signal whose beat frequency IF is outside the reception band, and the measurement distance is limited by the reception band. Since the repetition frequency (refresh rate) is proportional to the sweep speed γ, if the sweep speed γ is limited, the refresh rate will also be limited.

In the present disclosure, in order to eliminate the limitation of the measurement distance due to this reception band, the optical frequency comb generator 7 described above is introduced in the reference optical path, that is, between the coupler 2-1 and the optical 90-degree hybrid 8. .

FIG. 3 shows an example of the optical frequency comb generated by the optical frequency comb generator 7. As shown in FIG. The optical frequency comb generator 7 according to this embodiment generates an optical frequency comb composed of a plurality of different frequencies including the frequency of the reference light input from the coupler 2-1. In the optical frequency comb shown in FIG. 3, for ease of understanding, the reference light L _ref 1 corresponds to the laser light whose frequency is swept by the frequency sweep light source 1, and at each time, the reference light L _ref 1 is the center. An example composed of a total of five frequencies including two equally spaced frequencies on each side is shown.

The optical frequency comb according to this embodiment is an example, and the number of frequencies is not limited to this. Also, the frequency intervals of the optical frequency comb may not be equal but may be different. Note that the optical duplicator that generates an optical frequency comb including a plurality of frequencies shown in FIG. Any available means can be employed.

In this embodiment, since the reference light input from the coupler 2-1 is linearly swept by the frequency sweep light source 1, the optical frequency comb is also linearly swept. As a result, the reference light from the coupler 2-1 in the present disclosure becomes the frequency-swept optical frequency comb shown in FIG.

FIG. 4 shows an example of beat frequencies detected by the main interferometer 20 of this embodiment. Beat frequencies IF1 to IF5 of the reference lights L _ref 1 to L _ref 5 and the probe light L _p forming the optical frequency comb at time τ are shown. The beat frequency IF1 is the beat frequency between the probe light _Lp and the reference light _Lref1 . The beat frequency IF2 is the beat frequency between the probe light _Lp and the reference light _Lref2 . The beat frequency IF3 is the beat frequency between the probe light _Lp and the reference light _Lref3 . The beat frequency IF4 is the beat frequency between the probe light _Lp and the reference light _Lref4 . The beat frequency IF5 is the beat frequency between the probe light _Lp and the reference light _Lref5 .

As shown in FIG. 4, the probe light delayed by propagation to the device under test 5 interferes with all the reference lights forming the optical frequency comb to generate beat signals with beat frequencies IF1 to IF5. In the present disclosure, the receiving bands such as balanced photodetectors 9-1 and 9-2 set the observed beat frequency to the beat frequency obtained by the interference of the probe light with its closest frequency reference light L _ref 2. IF2 only.

Here, if the frequency interval of the optical frequency comb shown in FIG. 5 in which the frequencies are equally spaced is Δf, the minimum beat frequency is Δf/2 or less. Therefore, it is desirable that the main interferometer 20 has a reception band of at least Δf/2 or less. As shown in FIG. 5, when the frequency interval Δf of the optical frequency comb is narrowed, the maximum value Δf/2 of the minimum beat frequency becomes low, so even the main interferometer 20 with a narrow reception band can detect the minimum beat frequency. . In the case of an optical frequency comb whose frequency intervals are not equal, the same explanation as in the case of equal intervals can be made by setting the maximum frequency interval to Δf.

FIG. 5 shows only a part of the reference light and probe light, and it is assumed that the reference light and probe light are linearly swept to the higher frequency band side as well. If the optical frequency comb generator 7 generates an optical frequency comb in a frequency band sufficiently wider than the frequency sweep width ΔF of the frequency sweep light source 1, even if the object 5 is far away, the balanced photodetector 9-1 and 9-2, interference with either optical frequency comb component can be observed without extending the reception band. At this time, since it is not known which frequency component interfered, periodic ambiguity remains in the distance measurement result, but this can be eliminated by preliminary rough measurement.

In the above configuration, the optical frequency comb generator 7 may be arranged in the probe optical path before the main interferometer 20 , that is, between the optical circulator 3 and the optical 90-degree hybrid 8 . In this case, the probe light reflected by the device under test 5 is input to the optical frequency comb generator 7 . Therefore, as shown in FIG. 6, the optical frequency comb generator 7 generates an optical frequency comb composed of a plurality of different frequencies including the frequency of the probe light reflected by the device under test 5 . On the other hand, the reference light becomes one light. In this case, of the optical frequency comb based on the probe light, by measuring the frequency of the beat signal due to the interference between the light of the frequency closest to the frequency of the reference light and the reference light, the effect similar to that of the configuration shown in FIG. can be obtained.

As described above, by introducing the optical frequency comb generator 7, the limit of the measurement distance due to the reception band can be eliminated without lowering the frequency sweep speed.

(Embodiment 2)
FIG. 7 shows an embodiment of the present disclosure based on the FWCW LiDAR scheme. In general, a frequency swept light source has a certain level of imperfection (nonlinearity) in its frequency sweep trace, which is not perfectly linear. Since this nonlinearity means fluctuations in the sweep speed γ, the beat frequency from a fixed distance fluctuates, and the theoretical distance resolution given by equation (2) cannot be obtained.

Therefore, in order to solve this problem, the non-contact rangefinder according to this embodiment further includes an auxiliary interferometer 21 . The auxiliary interferometer 21 includes a coupler 2-4, a balanced photodetector 9-3, an AD converter 10-2 and a computing section 11. FIG. The auxiliary interferometer 21 is an interferometer that acquires the phase of the beat signal by interference between the light without delay and the light delayed by the delay time τ _aux by the delay device 6 . The beat signal at the auxiliary interferometer 21 is the same as the beat signal based on the probe light reflected from the object 5 at a distance corresponding to the delay time τ _aux .

The coupler 2-2 splits the laser light from the frequency sweep light source 1 into two, one of which is input to the coupler 2-1 as light for the main interferometer 20, and the other to the coupler 2-3 for the auxiliary interferometer 21. input. Thereby, part of the frequency swept light is extracted. Note that the coupler 2-1 performs the same operation as in the first embodiment with respect to the light input from the coupler 2-2.

In the auxiliary interferometer 21 , the coupler 2 - 3 splits the light input from the coupler 2 - 2 into two, inputs one to the coupler 2 - 4 , and inputs to the delay device 6 . The delay device 6 delays the light input from the coupler 2-3 by a delay time τ _aux and inputs it to the coupler 2-4. The coupler 2-4 multiplexes the light directly input from the coupler 2-3 and the light input from the delay device 6 to generate a beat signal, and splits the beat signal into two to produce a balanced photodetector. Enter in 9-3. The balanced photodetector 9-3 acquires the beat signal from the coupler 2-4 as an analog electric signal and inputs it to the AD converter 10-2. The AD converter 10 - 2 converts the beat signal input from the coupler 9 - 3 into a digital signal and inputs it to the calculation section 11 .

By "resampling" the beat signal of the main interferometer 20 with the beat signal of the auxiliary interferometer 21 at the frequency of the beat signal of the auxiliary interferometer 21, the non-linearity of the frequency sweep of the light source can be eliminated. .

However, this method works only near the measurement distance corresponding to the delay time τ _aux of the auxiliary interferometer. That is, the phase of the laser light from the frequency swept light source 1 is correlative only in a time called coherence time, which is unique to each frequency swept light source 1, and the phase correlation disappears after that time. This means that if the distance (delay time) to the DUT 5 differs from the delay time τ _aux of the auxiliary interferometer by more than the coherence time, the beat frequency of the auxiliary interferometer 21 and the beat frequency of the main interferometer 20 are no longer correlated and cease to function.

Therefore, as a second point of the present disclosure, the calculation unit 11 incorporates a phase noise compensation algorithm. The principle of this phase noise compensation algorithm is described in Non-Patent Document 2. The calculation unit 11 may follow the method disclosed in Non-Patent Document 2. That is, the calculation unit 11 obtains the beat signal input from the AD converter 10-2 in FIG. 7, and obtains the phase X(t) of the beat signal of the auxiliary interferometer 21 for each delay time τ _aux . The calculator 11 uses the phase X(t) of the beat signal of the auxiliary interferometer 21 to calculate the phase X _N (τ) shown in Equation (3).

where N is a positive integer. τ is the delay time described above. τ≈Nτ _aux holds. Also, the delay time τ may exceed the coherence time of the laser light. As described in Non-Patent Document 2, when the time τ _aux is shorter than the coherence time of the laser light, the delay time τ is an integer N times the delay time τ _aux , and the main interferometer 20 at the delay time τ can be calculated by equation (3). Therefore, by resampling the signal of the main interferometer 20 each time the phase X _N (τ) increases by a constant value, it is possible to measure the distance corresponding to the delay time τ in the vicinity of the time Nτ _aux exceeding the coherence time of the laser light. becomes. As a result, the aforementioned finite coherence time problem can be overcome. Further, as described in Non-Patent Document 2, the phase X _N (τ) in Equation (3) also corresponds to the phase of the laser light at the delay time τ. Therefore, by using the formula (3), the nonlinearity of the frequency sweep of the laser light can be corrected, and the resolution of the formula (2) can also be realized. As described above, the delay time τ _aux of the auxiliary interferometer must be set shorter than the coherence time of the frequency sweep light source 1 used.

The present invention discloses a method of applying this phase noise compensation algorithm to ranging by the following procedure. First, the distance to the object under test 5 is estimated with higher accuracy than the delay τ _aux of the auxiliary interferometer. In general, the coherence length of a frequency swept light source is about several tens of meters, and the delay τ _aux is also on the same order, so this estimation is easy. For example, the approximate distance may be grasped in advance by a known method such as the pulse method.

Next, assuming that the delay time τ of the probe light reflected from the object 5 at the estimated distance L with respect to the reference light, an integer M that satisfies the equation (4) is obtained.
(Number 4)
τ≈Mτ _ref (4)

Furthermore, the phase X _M (τ) is calculated using Equation (3) and used to correct the nonlinearity of the frequency sweep of the laser light. This makes it possible to achieve the theoretical resolution of equation (2) in principle.

In the main interferometer 20 according to the present embodiment, the optical 90-degree hybrid 8 is used for hardware-based processing, but the Hilbert transform may be used for software-based processing. Further, although the auxiliary interferometer 20 according to the present embodiment is configured to perform software-based processing using the Hilbert transform, the optical 90-degree hybrid 8 may be used for hardware-based processing.

The computing unit 11 according to this embodiment can also be implemented by a computer and a program, and the program can be recorded on a recording medium or provided through a network.

The non-contact ranging device and method according to the present disclosure can be applied to the information and communication industry.

1: Frequency sweep light source 2: Coupler 3: Optical circulator 4: Lens 5: DUT 6: Delay device 7: Optical frequency comb generator 8: Optical 90-degree hybrid 9: Balanced photodetector 10: AD converter 11: Calculation Part 12: RF synthesizer 13: Amplifier 20: Main interferometer 21: Auxiliary interferometer

Claims (5)

A non-contact type that determines the distance to the object by irradiating the object to be measured with frequency swept light and measuring the frequency of the beat signal resulting from the interference between the probe light and the reference light reflected by the object to be measured. In the rangefinder,
an optical duplicator that receives the reference light or the probe light and duplicates the incident light into light of a plurality of different frequencies;
combining the light of the plurality of frequencies and the reference light or the probe light that has not been incident on the optical replication section, and combining the light of the plurality of frequencies and the reference light or the probe that has not been incident on the optical replication section; a main interferometer for detecting the minimum beat frequency obtained by light interference;
A non-contact rangefinder with a coupler that splits a portion of the frequency sweep light into two;
a delay device for generating a delay of a delay time τ _aux shorter than a coherence time of the frequency swept light in one of the parts of the frequency swept light split by the coupler;
an auxiliary interferometer that acquires the phase X(t) of the beat signal due to interference between the one delayed by the delay device and the other of the part of the frequency swept light split into two by the coupler,
The reference of the probe light corresponding to the optical path difference between the reference light and the probe light using the phase X(t) of the beat signal obtained by the auxiliary interferometer, the integer N and equation (C1) 4. The phase X _N (τ) of the frequency swept light at the delay time τ with respect to the light is obtained, and the nonlinearity of the frequency swept light at the minimum beat frequency obtained by the main interferometer is corrected. 2. The non-contact rangefinder according to 1.

3. The non-contact distance measuring device according to claim 2, wherein the delay time τ is estimated in advance and the integer N that satisfies equation (C2) is used.
(number C2)
τ≈Nτ _aux (C2) 4. The optical replication unit according to any one of claims 1 to 3, wherein the optical replicator generates an optical frequency comb having a frequency range wider than a frequency sweep width of the frequency swept light as the light of the plurality of frequencies. Non-contact rangefinder. A non-contact type that determines the distance to the object by irradiating the object to be measured with frequency swept light and measuring the frequency of the beat signal resulting from the interference between the probe light and the reference light reflected by the object to be measured. In the ranging method,
duplicating the reference light or the probe light into light of different frequencies;
A minimum beat obtained by combining the lights of the plurality of frequencies and the non-duplicated reference light or the probe light, and by interference of the light of the plurality of frequencies and the non-duplicated reference light or probe light. detecting a frequency;
non-contact ranging method.

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