Non-contact distance measuring device and method

Abstract

To enable an identification of a frequency that caused a beat signal from among a plurality of duplicated frequencies when detecting the beat signal.SOLUTION: A non-contact type distance measurement device irradiates a measurement object with frequency sweep light, and measures a frequency of a beat signal by an interference of probe light reflected at the measurement object with reference light so as to obtain a distance to the measurement object. The non-contact type distance measurement device includes: a light duplication part configured to duplicate one of the reference light and the probe light into light with a plurality of different frequencies. The light duplication part duplicates into light with plural kinds of light intensity at the duplication. By multiplexing the light with a plurality of frequencies duplicated at the light duplication part with the other of the reference light and the probe light, the non-contact type distance measurement device generates a plurality of beat signals, and measures frequency of at least one of the plurality of beat signals according to the plural kinds of light intensity.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This disclosure relates to a non-contact distance measuring device and method using frequency-swept light. [Background technology]

[0002] Accurate distance measurement technology is crucial for measurements such as the shape measurement of large structures. Numerous applications exist, including the measurement of shapes of artificial objects like parabolic antennas and buildings, and the measurement of natural formations such as ice sheet thickness and forest height. In particular, there is a demand for high-precision measurement of the position and shape of distant objects in measurements of large structures for purposes such as health monitoring of buildings and disaster prevention.

[0003] LiDAR (Light Detection and Ranging) is a non-contact ranging technology that uses laser light to measure the optical path length to an object being measured. In this technology, frequency-modulated continuous wave (FMCW) LiDAR measures distance using a single light source and has the capability to detect velocity and vibration. In FMCW LiDAR, the optical frequency is swept, and the frequency difference (beat frequency, IF) caused by interference with the reflected light is converted into distance. Using a light source with a sweep bandwidth of 100 nm, a resolution of approximately 12 μm can be achieved.

[0004] However, with FMCW-type LiDAR, the measurement distance is limited to several tens of meters due to the coherence length of the light source. Furthermore, measurements cannot be taken when the obtained beat frequency exceeds the reception bandwidth. This means that, as long as the reception bandwidth is constant, there is an upper limit to the product of the frequency sweep speed and the measurement distance. In other words, if the reception bandwidth is constant, there is a problem in that either the measurement distance or the frequency sweep speed must be sacrificed. Moreover, 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 these two factors, FMCW-type LiDAR has been limited exclusively to measuring relatively short distances of several tens of meters or less. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Takahiro Nagata et al., "Frequency Modulated Continuous Wave Optical Distance Meter Using Wavelength Sweep Type Optical Frequency Comb," 2022 (73rd) Joint Conference of Electrical and Information Engineering Societies, Chugoku Branch, R22-11-04. [Overview of the project] [Problems that the invention aims to solve]

[0006] A technique has been proposed to extend the measurement distance in a simple distance measuring device using a single light source without reducing the frequency sweep speed (see, for example, Non-Patent Document 1). In Non-Patent Document 1, one of the reference light or probe light is duplicated into multiple lights of different frequencies and combined with the other of the reference light or probe light. This generates beat signals of multiple frequencies, and the beat frequency that falls within the receiving band is detected.

[0007] However, in Non-Patent Document 1, multiple frequencies of light were generated, and it was not possible to identify which frequency generated the beat signal when detecting the beat signal. Therefore, the present disclosure aims to make it possible to identify which of the multiple replicated frequencies generated the beat signal when detecting the beat signal. [Means for solving the problem]

[0008] To achieve the above objective, this disclosure replicates light of one frequency of either the reference light or the probe light, and interferes the replicated light of multiple frequencies with the other of the reference light or probe light. During this replication, the light intensity is made non-uniform with respect to the light frequency.

[0009] Specifically, the non-contact distance measuring device according to this disclosure is a non-contact distance measuring device that irradiates an object to be measured with frequency-swept light and measures the frequency of a beat signal resulting from the interference between probe light and reference light reflected from the object to be measured, thereby determining the distance to the object, and performs the non-contact distance measuring method according to this disclosure.

[0010] The non-contact distance measuring device relating to this disclosure is The system includes an optical replication unit that replicates either the reference light or the probe light into light of multiple different frequencies. The aforementioned light replication unit replicates light of multiple light intensities during the replication process. By combining the multiple frequencies of light replicated in the optical replication unit with the other of the reference light or the probe light, multiple beat signals are generated. At least one of the frequencies of the plurality of beat signals is measured based on the plurality of types of light intensity.

[0011] The non-contact distance measuring method relating to this disclosure is: The optical replication unit includes a procedure for replicating either the reference light or the probe light into multiple lights of different frequencies. By combining the multiple frequencies of light replicated in the optical replication unit with the other of the reference light or the probe light, multiple beat signals are generated. At least one of the frequencies of the plurality of beat signals is measured based on the plurality of types of light intensity.

[0012] The non-contact distance measuring device according to this disclosure may measure the minimum frequency and the next minimum frequency among the plurality of beat signals.

[0013] The optical replication unit may also generate an optical frequency comb with a frequency band wider than the frequency sweep width of the frequency-swept light, as the light of the multiple frequencies.

[0014] Furthermore, the above disclosures can be combined as much as possible. [Effects of the Invention]

[0015] According to the present disclosure, it is possible to specify which frequency among a plurality of replicated frequencies is the frequency at which a beat signal is generated when the beat signal is detected. Therefore, the present disclosure can realize a measurement distance of km level and a resolution of several tens of μm without reducing the measurement speed, even with a simple configuration using a single light source. Furthermore, it is possible to alleviate the periodic ambiguity of the distance measurement result. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] [Figure 1] It is a diagram showing an example of the configuration of a non-contact distance measurement device according to Embodiment 1. [Figure 2] It is a diagram for explaining conventional reference light and probe light, where (a) shows the frequencies of the reference light and the probe light, and (b) shows the interference signal spectrum. [Figure 3] It is a diagram for explaining the frequencies of the reference light and the probe light. [Figure 4] An example of a beat signal is shown. [Figure 5] It is a diagram for explaining the frequencies of the reference light and the probe light of the present disclosure. [Figure 6] An example of a beat signal of the present disclosure is shown. [Figure 7] A configuration example of the optical frequency comb generator of the present disclosure is shown. DETAILED DESCRIPTION OF THE INVENTION

[0017] 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 examples are merely illustrative, and the present disclosure can be implemented in various modified forms based on the knowledge of those skilled in the art. In the present specification and drawings, components having the same reference numerals are assumed to be the same as each other.

[0018] Embodiments of the present invention will be described in detail below with reference to the drawings. Figure 1 shows an embodiment of the present disclosure based on a FWCW type LiDAR system. 1 is a frequency-swept light source, 2 is a coupler for splitting or combining light, 3 is an optical circulator, 4 is a lens, 5 is the object to be measured, 6 is a delay unit, 7 is an optical frequency comb generator, 8 is an optical 90-degree hybrid, 9 is a balanced photodetector, 10 is an AD converter, 11 is a computing unit such as a computer, and 12 is an RF synthesizer.

[0019] The optical frequency comb generator 7 functions as the optical replication unit of this disclosure, replicating the reference light into light of multiple different frequencies. The optical 90-degree hybrid 8, balanced photodetectors 9-1 and 9-2, and AD converter 10-1 constitute the main interferometer 20, which functions as the photodetector of this disclosure. Hereinafter, the "receiving band of the photodetector" will be abbreviated as "receiving band".

[0020] The frequency-swept light source 1 emits laser light whose frequency is linearly modulated. Its frequency is swept at a constant sweep speed γ [Hz / s] over a constant frequency sweep time ΔT and over a frequency sweep width ΔF. In this 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.

[0021] Coupler 2-1 splits the light input from frequency-swept light source 1 into two, inputting one as probe light to optical circulator 3 and the other as reference light to optical frequency comb generator 7. Optical circulator 3 inputs the probe light from coupler 2-1 to lens 4. Optical circulator 3 also inputs the light from lens 4 to optical 90-degree hybrid 8. Lens 4 converts the probe light from frequency-swept light source 1 into a plane wave. Lens 4 also focuses the probe light reflected from the object under test 5 and inputs it to optical circulator 3.

[0022] The optical frequency comb generator 7 generates higher-order modulation sidebands in response to the input light. A specific configuration of the optical frequency comb generator 7 is described, for example, in Non-Patent Document 1. In this embodiment, as an example, the 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 frequency combs at frequency intervals corresponding to the signal from the RF synthesizer 12.

[0023] The optical frequency comb generator 7 generates an optical frequency comb consisting of multiple different frequencies, including the frequency of the reference light input from coupler 2-1. The optical frequency comb generator 7 inputs the generated optical frequency comb to the optical 90-degree hybrid 8 as the reference light for the main interferometer 20. The probe light reflected by the object under test 5 interferes with the laser light (reference light) in the optical 90-degree hybrid 8 (main interferometer). The main interferometer 20 can measure the delay time of the probe light reflected from the object under test 5 relative to the reference light. The calculation unit 11 uses this delay time to measure the distance. In Figure 1, the calculation unit 11 is included in the main interferometer 20, but the main interferometer 20 and the calculation unit 11 may be separate.

[0024] Specifically, the 90-degree optical hybrid 8 generates the in-phase component I of the beat signal obtained by combining the reference light and the reflected light from the object under test 5, and inputs it to the balanced photodetector 9-1. The 90-degree optical hybrid 8 also generates the orthogonal component Q of the beat signal obtained by combining the reference light, which has been phase-shifted by 90 degrees, and the reflected light from the object under test, and inputs it to the balanced photodetector 9-2.

[0025] The balanced photodetector 9-1 acquires an analog electrical signal of the common-mode component I of the beat signal based on the input from the 90-degree optical hybrid 8 and inputs it to the AD converter 10-1. The balanced photodetector 9-2 acquires an analog electrical signal of the quadrature component Q of the beat signal based on the input from the 90-degree optical hybrid 8 and inputs it to the AD converter 10-1. The AD converter 10-1 converts the analog electrical signal of the common-mode component I of the beat signal input from the balanced photodetector 9-1 and the analog electrical signal of the quadrature component Q of the beat signal input from the balanced photodetector 9-2 into digital signals and inputs them to the calculation unit 11.

[0026] Here, the interference between the reference light and probe light in a conventional FMCW-type LiDAR without an optical frequency comb generator 7 will be explained using Figure 2. That is, in a conventional FMCW-type LiDAR, the reference light from coupler 2-1 is directly input to the optical 90-degree hybrid 8 in the main interferometer shown in Figure 1. In a conventional FMCW-type LiDAR, the reference light from coupler 2-1 and the probe light, which is delayed by the distance traveled to the object under test 5 relative to the reference light, arrive, and a beat signal with a frequency corresponding to the frequency difference between them is generated. Hereafter, the frequency of the beat signal will be referred to as the beat frequency IF. Since this beat frequency IF is proportional to the delay time of the probe light reflected from the object under test 5 relative to the reference light, distance measurement is possible.

[0027] Specifically, the beat signal is expressed in complex number form as shown in equation (1), using the I-phase and Q-phase components of the beat signal output from the optical 90-degree hybrid 8. (Math 1) I + jQ = exp(jγτt) (1)

[0028] The calculation unit 11 determines the phase of the beat signal from equation (1) based on the common-mode 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 relative to the reference light, which corresponds to the optical path difference between the reference light and the probe light, and γτ is the beat frequency IF.

[0029] Generally, the distance resolution Δz of an FMCW-type LiDAR is expressed as follows using the frequency sweep width ΔF. (Math 2) Δz = c / (2ΔF) (2)

[0030] In other words, to improve the distance resolution Δz, it is necessary to increase the frequency sweep width ΔF. Furthermore, the beat frequency IF is proportional to the delay time τ, which varies depending on the distance to the object under measurement 5, and the sweep speed γ. Therefore, if the receiving bandwidth is constant, there will be a limit to the product of the sweep speed γ and the distance. Specifically, in conventional FMCW type LiDAR, as shown in Figure 2(B), beat signals with a beat frequency IF outside the receiving bandwidth cannot be detected, resulting in a limitation of the measurement distance due to the receiving bandwidth. Note that the repetition frequency (refresh rate) is proportional to the sweep speed γ, so if the sweep speed γ is limited, the refresh rate will also be limited.

[0031] In this disclosure, in order to eliminate the limitation of measurement distance due to the receiving bandwidth, the aforementioned optical frequency comb generator 7 is introduced into the reference optical path, that is, between the coupler 2-1 and the optical 90-degree hybrid 8.

[0032] Figure 3 shows an example of an optical frequency comb generated by the optical frequency comb generator 7. The optical frequency comb generator 7 according to this embodiment includes the frequency of the reference light input from the coupler 2-1 and generates an optical frequency comb composed of multiple different frequencies. In the optical frequency comb shown in Figure 3, the reference light L is used for ease of understanding. ref 1 corresponds to a frequency-swept light source 1, which is a laser beam whose frequency is swept, and at each time step, the reference light L ref This example shows a total of seven frequencies, with two equally spaced frequencies on each side of frequency 1.

[0033] The optical frequency comb according to this embodiment is just one example, and the number of frequencies is not limited thereto. Furthermore, the frequency spacing of the optical frequency comb is not equal; it may be different. Note that the optical replication unit that generates an optical frequency comb containing multiple frequencies, such as the number shown in Figure 3, is not limited to the optical frequency comb generator 7; any means capable of generating sidebands for the input light can be employed.

[0034] In this embodiment, the reference light input from coupler 2-1 is linearly swept by the frequency-swept light source 1, and therefore the optical frequency comb is also linearly swept. As a result, the reference light from coupler 2-1 in this disclosure becomes a frequency-swept optical frequency comb as shown in Figure 3 and is incident on the optical 90-degree hybrid 8.

[0035] Figure 4 shows an example of the beat frequency detected by the main interferometer 20 of this embodiment. The reference light L that constitutes the optical frequency comb at time τ ref 1~L ref 5 and probe light L p The beat frequencies IF1 to IF5 are shown below. As shown in Figure 4, the probe light, delayed by propagation to the object under test 5, interferes with all of the reference light constituting the optical frequency comb, generating multiple beat signals with beat frequencies IF1 to IF5. In this disclosure, the smallest frequency and the next smallest frequency among the multiple beat signals are measured. For example, as shown in Figure 6, the beat frequencies at which the receiving bandwidth of balanced photodetectors 9-1 and 9-2 is observed are measured by the test light and the beat frequencies IF2 and IF3 of the frequency closest to it.

[0036] If the frequency spacing of the optical frequency comb shown in Figure 3, where the frequencies are equally spaced, is Δf, then the minimum beat frequency will be Δf / 2 or less. Therefore, it is desirable that the main interferometer 20 have a receiving bandwidth of at least Δf / 2 or less. Narrowing the frequency spacing Δf of the optical frequency comb lowers the maximum value Δf / 2 of the minimum beat frequency, so that the minimum beat frequency can be detected even with a main interferometer 20 that has a narrow receiving bandwidth. Note that in the case of optical frequency combs with non-equally spaced frequencies, the explanation can be the same as in the case of equally spaced frequencies if the maximum frequency spacing is taken as Δf.

[0037] In FIG. 3, only a part of the reference light and the probe light is shown, and it is assumed that the reference light and the probe light are linearly swept further toward the higher frequency band side. If an optical frequency comb with a frequency band sufficiently wider than the frequency sweep width ΔF of the frequency sweep light source 1 is generated in the optical frequency comb generator 7, even when the object to be measured 5 is far away, interference with any optical frequency comb component can be observed without expanding the reception band such as the balanced photodetectors 9-1 and 9-2.

[0038] Here, in the present disclosure, the plurality of beat frequencies shown in FIG. 4 depend on the frequency of the frequency-swept light generated by the generated optical frequency comb generator 7. The frequency-swept light generated by the generated optical frequency comb generator 7 is light obtained by shifting the original frequency-swept light in the frequency direction. Since a plurality of frequency-swept lights are generated in the optical frequency comb generator 7, periodic ambiguity remains in the frequency-swept light that generates the beat frequency IF2.

[0039] Therefore, in the present disclosure, the intensity of the teeth of the Comb generated by the generated optical frequency comb generator 7 is made non-uniform. Thereby, the present disclosure alleviates periodic ambiguity. For example, the configuration of the optical frequency comb generator 7 is devised to make the intensity of the teeth non-uniform.

[0040] FIG. 5 shows an example of the optical intensity of the frequency-swept light. The solid line indicates the frequency-swept light generated by the optical frequency comb generator 7, and the broken line indicates the test light L p1 ,L p2 ,L p3 returning after being reflected from the object to be measured 5. For the sake of explanation in the figure, three test lights L p1 ,L p2 ,L p3 are shown, but actually only one of these is generated. For example, as shown in FIG. 5, the optical intensity of the frequency-swept light generated by the optical frequency comb generator 7 is set to twice the intensity with a period three times the frequency interval Δf.

[0041] In FIG. 6, the test light L p1 ,L p2 ,L p3An example of the beat frequency during measurement is shown in Figure 6(a). p1 The beat frequency is shown in Figure 6(b) with the test light L p2 The beat frequency is shown, and Figure 6(c) shows the test light L p3 The beat frequencies are shown. As shown in Figure 6, they can be distinguished because their intensities in the beat frequency spectrum are different.

[0042] The calculation unit 11 calculates the test light L based on the intensity and frequency of the two generated beat signals. p1 ,L p2 ,L p3 Determine which of the following applies. For example, if a strong signal and a weak signal are measured simultaneously, and the strong signal is on the negative frequency side, then test light L p1 It is determined that this is the case. Note that the sign of the frequency will vary depending on whether the test light or the reference light is used as the reference.

[0043] For example, the optical frequency comb generator 7 is realized in a tandem configuration of Mach-Zehnder type intensity modulators 7 as shown in Figure 7. A modulation signal of frequency 3Δf is input to the first modulator 71 and a modulation signal of frequency Δf is input to the second modulator 72, and by making the amplitudes of these modulation signals different, the desired reference light is obtained.

[0044] In the above configuration, the optical frequency comb generator 7 may be placed in the probe light path before the main interferometer 20, that is, between the optical circulator 3 and the optical 90-degree hybrid 8. In this case, the optical frequency comb generator 7 receives the probe light reflected from the object under test 5. Therefore, the optical frequency comb generator 7 generates an optical frequency comb that includes the frequency of the probe light reflected from the object under test 5 and is composed of multiple different frequencies. On the other hand, the reference light is a single beam of light. In this case, by measuring the frequency of the beat signal resulting from the interference between the optical frequency comb based on the probe light and the light with a frequency close to the frequency of the reference light, the same effect as the configuration shown in Figure 1 can be obtained.

[0045] As described above, the introduction of the optical frequency comb generator 7 eliminates the limitation of measurement distance due to the receiving bandwidth without reducing the frequency sweep speed. Furthermore, by duplicating the incident light into light of multiple different frequencies so that the light and intensity are non-uniform with respect to the optical frequency, the periodic ambiguity of the distance measurement results can be mitigated.

[0046] In this embodiment, the main interferometer 20 is configured to process data in hardware using the 90-degree optical hybrid 8, but it may also be processed in software using the Hilbert transform.

[0047] The arithmetic 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 via a network. [Industrial applicability]

[0048] The non-contact distance measuring device and method described herein can be applied to the information and communication industry. [Explanation of Symbols]

[0049] 1: Frequency-swept light source 2: Coupler 3: Light Circulator 4: Lens 5: Object to be measured 6: Delay device 7: Optical frequency comb generator 8: 90-degree hybrid light 9: Balanced Photo Detector 10: AD Converter 11: Arithmetic section 12: RF Synthesizer 13: Amplifier 20: Main interferometer 71, 72: Modulator

Claims

1. In a non-contact distance measuring device that splits frequency-swept light into probe light and reference light, irradiates the object to be measured with the probe light, and measures the frequency of the beat signal resulting from the interference between the test light reflected by the object to be measured and the reference light, the distance to the object to be measured is determined. The unit comprises a light generation unit that generates a plurality of reference lights having a constant frequency interval with the aforementioned reference light, The light generation unit outputs the multiple reference lights at multiple types of light intensities. By combining the aforementioned multiple reference lights with the test light, multiple beat signals are generated. At least one of the frequencies of the plurality of beat signals is measured based on the plurality of types of light intensity. Non-contact distance measuring device.

2. The minimum frequency and the next minimum frequency among the plurality of beat signals are measured. The non-contact distance measuring device according to claim 1.

3. The light generation unit generates optical frequency combs as the plurality of reference lights, with a frequency band wider than the frequency sweep width of the frequency sweep light. The non-contact distance measuring device according to claim 1.

4. In a non-contact distance measurement method that splits a frequency-swept beam into a probe beam and a reference beam, irradiates the object to be measured with the probe beam, and measures the frequency of the beat signal resulting from the interference between the test beam reflected by the object and the reference beam, the distance to the object to be measured is determined. The photogenerator includes a procedure for generating a plurality of reference lights having a constant frequency interval with respect to the reference light, The light generation unit outputs the multiple reference lights at multiple types of light intensities. By combining the aforementioned multiple reference lights with the test light, multiple beat signals are generated. At least one of the frequencies of the plurality of beat signals is measured based on the plurality of types of light intensity. Non-contact ranging method.

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