Non-contact distance measuring device and method

Abstract

To improve a generation efficiency of a beat signal in a non-contact type distance measurement device configured to obtain a distance to a measurement object through measuring a frequency of a beat signal by an interference of probe light and reference light.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 frequencies; and an optical frequency shifter configured to lower the frequency of light duplicated at the light duplication part if the light duplicated at the light duplication part is the reference light, and enhances the frequency of the light duplicated at the light duplication part if the light duplicated at the light duplication part is the probe light. 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.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] Accurate distance measuring technology is an important technology in measurements such as the shape measurement of large-scale structures. There are many such applications, including the shape measurement of artifacts such as parabolic antennas and buildings, the measurement of natural formations such as the thickness measurement of ice sheets and the height measurement of forests, etc. In particular, in the measurement of large-scale structures for the purpose of health monitoring and disaster prevention of buildings, etc., there is a need to measure the position and shape of distant measurement objects with high definition.

[0003] LiDAR (Light Detection And Ranging) is a non-contact distance measuring technology that measures the optical path length to the object to be measured using laser light. In such a technology, frequency-modulated continuous wave (FMCW) type LiDAR performs distance measurement with a single light source and has the ability to detect speed and vibration. In FMCW type LiDAR, the optical frequency is swept, and the frequency difference (beat frequency, IF) generated by the interference with the return light is converted into distance. If a light source having a sweep band of 100 nm is used, a resolution of about 12 μm can be achieved.

[0004] However, in FMCW type LiDAR, the measurement distance is limited to several tens of meters by the coherence length of the light source. Also, measurements where the obtained beat frequency exceeds the reception band cannot be performed. This means that as long as the reception band is constant, there is an upper limit to the product of the frequency sweep speed and the measurement distance. That is, there is a problem that either the measurement distance or the frequency sweep speed must be sacrificed if the reception band is constant. 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 relatively short distance measurements within about several tens of meters.

Prior Art Documents

[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, when the signal was duplicated into multiple frequencies of light, there were frequencies that could not generate a beat signal that entered the receiving band. Therefore, this disclosure aims to improve the efficiency of beat signal generation in a non-contact distance measuring device that determines the distance to the object to be measured by measuring the frequency of the beat signal caused by the interference between probe light and reference light. [Means for solving the problem]

[0008] To achieve the above objective, the Disclosure replicates light of multiple frequencies from one of the reference light or probe light, and interferes the replicated light of multiple frequencies with the other of the reference light or probe light. During this replication, the frequencies of the replicated light are shifted so that more of the replicated light can generate a beat signal.

[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 An optical replication unit that replicates either the reference light or the probe light into light of multiple different frequencies, An optical frequency shifter that lowers the frequency of the light replicated by the optical replication unit if the light replicated by the optical replication unit is the reference light, and raises the frequency of the light replicated by the optical replication unit if the light replicated by the optical replication unit is the probe light, Equipped with, 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. The frequency of at least one of the aforementioned multiple beat signals is measured.

[0011] The non-contact distance measuring method relating to this disclosure is: The optical replication unit replicates either the reference light or the probe light into multiple lights of different frequencies, The procedure involves lowering the frequency of the light replicated by the optical replication unit if the light replicated by the optical replication unit is the reference light, and raising the frequency of the light replicated by the optical replication unit if the light replicated by the optical replication unit is the probe light. Equipped with, 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. The frequency of at least one of the aforementioned multiple beat signals is measured.

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

[0013] Further, the optical replication unit may generate an optical frequency comb having a frequency band wider than the frequency sweep width of the frequency sweep light as the light of the plurality of frequencies.

[0014] In addition, the above disclosures can be combined as much as possible.

Advantages of the Invention

[0015] According to the present disclosure, in a non-contact distance measuring device that obtains the distance to the object to be measured by measuring the frequency of the beat signal due to the interference between the probe light and the reference light, the generation efficiency of the beat signal can be improved. Therefore, although the present disclosure has a simple configuration using a single light source, it can realize a measurement distance of the km level and a resolution of several tens of μm without reducing the measurement speed, and further, the reference light or the probe light after optical replication can be efficiently used for distance measurement.

Brief Description of the Drawings

[0016] [Figure 1] It is a diagram showing an example of the configuration of a non-contact distance measuring device according to Embodiment 1. [Figure 2] It is a diagram for explaining conventional reference light and probe light, (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 frequency of conventional reference light. [Figure 4] It is a diagram for explaining the frequency of the reference light of the present disclosure. [Figure 5] It is a diagram for explaining the frequencies of the reference light and the test light of the present disclosure. [Figure 6] It is a diagram for explaining the interference signal spectrum of the present disclosure.

Embodiments for Carrying Out the Invention

[0017] Embodiments of this disclosure will be described in detail below with reference to the drawings. However, this disclosure is not limited to the embodiments shown below. These examples are illustrative, and this disclosure can be implemented in various modified and improved forms based on the knowledge of those skilled in the art. In this specification and in the drawings, components with the same reference numerals refer to the same components.

[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, 12 is an RF synthesizer, and 14 is an optical frequency shifter.

[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] The optical frequency comb generator 7 generates new frequencies on both sides of the original frequency-swept light. As a result, half of these frequencies are unusable and wasted. Therefore, this disclosure provides an optical frequency shifter 14 downstream of the optical frequency comb generator 7. The optical frequency shifter 14 provides a negative optical frequency shift. Figure 4 shows an example of the reference and test light frequencies when the distance between the measuring device and the object under test 5 is zero. Thus, this disclosure shifts the frequency of the reference light so that all teeth are below the frequency of the probe light, as shown in Figure 4.

[0035] 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.

[0036] As shown in Figure 5, the test light, which is 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. In this disclosure, the smallest frequency of the multiple beat signals is measured. For example, as shown in Figure 6, the beat frequency at which the receiving bandwidth of balanced photodetectors 9-1 and 9-2 is observed is measured using the test light and the reference light L with the closest frequency. ref Only the beat frequency IF4, obtained through interference with 4, should be used.

[0037] Here, if we let Δf be the frequency spacing of the optical frequency comb shown in Figure 3, where the frequencies are equally spaced, 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. As shown in Figure 3, 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 the main interferometer 20 having 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.

[0038] Figures 3 and 5 only show portions of the reference and probe light, and it is assumed that the reference and probe light are linearly swept to higher frequency bands as well. If the optical frequency comb generator 7 generates an optical frequency comb with a frequency band sufficiently wider than the frequency sweep width ΔF of the frequency sweep light source 1, interference with any of the optical frequency comb components can be observed even when the object under measurement 5 is far away, without extending the receiving bandwidth of balanced photodetectors 9-1 and 9-2. In this case, it is not possible to know which frequency component interfered, so a periodic ambiguity remains in the distance measurement result, but this can be eliminated by rough measurement beforehand.

[0039] In the above configuration, the optical frequency comb generator 7 and the optical frequency shifter 14 may be placed in the probe light path before the main interferometer 20, i.e., 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. The optical frequency shifter 14 then applies a positive optical frequency shift. This allows the present disclosure to adjust the delay of the reference light so that all teeth are below the probe light. Meanwhile, the reference light becomes a single beam of light. In this case, by measuring the frequency of the beat signal resulting from the interference between the light with the frequency closest to the reference light frequency in the optical frequency comb based on the probe light and the reference light, the same effect as the configuration shown in Figure 1 can be obtained.

[0040] 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. By introducing the optical frequency shifter 14 and inserting it into the reference path (or test light path), all teeth of the comb can be used for distance measurement.

[0041] 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.

[0042] 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]

[0043] The non-contact distance measuring device and method described herein can be applied to the information and communication industry. [Explanation of symbols]

[0044] 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 14: Frequency Shifter 20: Main interferometer

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. A light generation unit that generates a plurality of reference lights having a constant frequency interval with the aforementioned reference light, An optical frequency shifter that lowers the frequencies of all of the aforementioned multiple reference lights to below the frequency of the probe light, Equipped with, By combining the multiple reference beams from the optical frequency shifter with the test beam, multiple beat signals are generated. The frequency of at least one of the aforementioned plurality of beat signals is measured. Non-contact distance measuring device.

2. The lowest frequency among the aforementioned multiple beat signals is 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. A procedure for a light generation unit to generate a plurality of reference lights having a constant frequency interval with respect to the aforementioned reference light, The optical frequency shifter is used to lower the frequencies of all of the multiple reference lights to below the frequency of the probe light. Equipped with, By combining the multiple reference beams from the optical frequency shifter with the test beam, multiple beat signals are generated. The frequency of at least one of the aforementioned plurality of beat signals is measured. A non-contact distance measuring method comprising the following features.

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