Noncontact-type distance measuring device and method

Abstract

To provide a method of correcting a beat signal that functions correctly even when a beat signal corresponding to a frequency component other than a 0th order component among optical comb frequency components is corrected.SOLUTION: A noncontact-type distance measurement device includes: an optical frequency comb generator 7 that replicates incident light into light of a plurality of different frequencies; a main interferometer 20 that detects a minimum beat frequency obtained by interference between the light of a plurality of frequencies and the reference light or probe light not incident on the optical frequency comb generator 7; an auxiliary interferometer 21 for acquiring a phase X(t) due to interference between one and the other delayed by a delay device 6; and a calculation unit 11 for calculating a phase XN(τ) of frequency swept light at a delay time τ based on the phase X(t) and the influence of frequencies other than the frequency corresponding to the reference light or the probe light among a plurality of different frequencies, and for correcting the nonlinearity of the frequency swept light at the minimum beat frequency obtained by the main interferometer 20.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 technology]

[0002] Accurate distance measurement technology is an important technology for measuring the shape of large-scale structures. There are many such applications, including measuring the shape of man-made objects such as parabolic antennas and buildings, and measuring natural structures 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 large-scale structures for building health monitoring and disaster prevention purposes.

[0003] LiDAR (Light Detection And Ranging) is a non-contact distance measurement technology that uses laser light to measure the optical path length to an object to be measured. Among such technologies, frequency modulated continuous wave (FMCW) LiDAR measures distance using a single light source and is capable of detecting speed and vibration. FMCW LiDAR sweeps the optical frequency and converts the frequency difference (beat frequency, IF) that occurs due to interference with the returned light into distance. Using a light source with a 100nm sweep bandwidth, a resolution of approximately 12μm can be achieved.

[0004] For example, Patent Document 1 discloses an example of a non-contact distance measuring device that employs an FMCW LiDAR method. The non-contact distance measuring device in Patent Document 1 places an optical comb generating means in the reference light path or the probe light path, and measures the interference between the probe light and the frequency component closest to it among the optical comb frequency components to measure the distance to the target.

[0005] In addition, in the non-contact distance measuring device of Patent Document 1, the delay time τ aux The phase X(t) of the beat signal obtained by the auxiliary interferometer with N (t) is used to correct the beat signal of the main interferometer, where N is an integer.

number

[0006] As a result, in Patent Document 1, the time Nτ that exceeds the coherence time of the laser light aux This makes it possible to measure distances equivalent to [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2022-022391 [Non-patent literature]

[0008] [Non-Patent Document 1] 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 [Non-patent document 2] 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 Summary of the Invention [Problem to be solved by the invention]

[0009] However, the method disclosed in Patent Document 1 works when correcting the beat signal corresponding to the zeroth-order component of the optical comb frequency components, i.e., the frequency component of the original light source, but has the problem that it does not function properly when correcting beat signals corresponding to frequency components of other orders.

[0010] Therefore, an object of the present disclosure is to provide a beat signal correction method that functions correctly even when correcting a beat signal corresponding to a frequency component other than the zeroth-order component of the optical comb frequency components. [Means for solving the problem]

[0011] To achieve the above object, the non-contact distance measuring device and method disclosed herein employs a technique for correcting the beat signal of the main interferometer, taking into consideration the influence of frequencies other than the zeroth-order component among the optical frequency components.

[0012] Specifically, the non-contact distance measuring device of the present disclosure includes: 1. A non-contact distance measuring device that irradiates a frequency sweep light onto an object to be measured, and measures the frequency of a beat signal generated by interference between a probe light and a reference light reflected by the object to determine a distance to the object, an optical duplication unit that receives the reference light or the probe light and duplicates the incident light into light of a plurality of different frequencies; a main interferometer that combines the light of the plurality of frequencies and the reference light or the probe light that has not been input to the optical duplication unit, and detects a minimum beat frequency obtained by interference between the light of the plurality of frequencies and the reference light or the probe light that has not been input to the optical duplication unit; a coupler that splits a part of the frequency swept light into two; One of the two portions of the frequency swept light split by the coupler is provided with a delay time τ shorter than the coherence time of the frequency swept light. aux a delayer for generating a delay of an auxiliary interferometer for obtaining a phase due to interference between the one part delayed by the delay device and the other part of the frequency swept light branched into two by the coupler; and a calculation unit that calculates the phase of the frequency swept light at a delay time τ of the probe light relative to the reference light, which corresponds to a difference in optical paths between the reference light and the probe light, based on the phase and the influence of frequencies among the plurality of different frequencies other than the frequency corresponding to the reference light or the probe light, and corrects nonlinearity of the frequency swept light at the smallest beat frequency obtained by the main interferometer.

[0013] Further, the non-contact distance measuring method of the present disclosure includes: A non-contact distance measuring method for determining a distance to an object to be measured by irradiating the object with frequency sweep light and measuring a frequency of a beat signal generated by interference between a probe light reflected by the object to be measured and a reference light, the method comprising: replicating the reference light or the probe light into light of different frequencies; combining the light of the plurality of frequencies and the unduplicated reference light or the unduplicated probe light, and detecting a minimum beat frequency obtained by interference between the light of the plurality of frequencies and the unduplicated reference light or the unduplicated probe light; splitting a part of the frequency swept light into two; One of the two split portions of the frequency swept light is given a delay time τ shorter than the coherence time of the frequency swept light. aux and causing a delay of acquiring a phase due to interference between the delayed one of the two branches of the frequency swept light and the other of the two branches of the frequency swept light; and determining a phase of the frequency swept light at a delay time τ of the probe light relative to the reference light, which corresponds to a difference in optical paths between the reference light and the probe light, based on the phase and the influence of frequencies among the plurality of different frequencies other than the frequency corresponding to the reference light or the probe light, and correcting nonlinearity of the frequency swept light at the minimum beat frequency.

[0014] Further, the calculation unit calculates the phase X(t) of the frequency sweep light at a delay time τ of the probe light relative to the reference light, which corresponds to the optical path difference between the reference light and the probe light, by using equation (5) expressed using the phase X(t) of the beat signal acquired by the auxiliary interferometer, the order M of the different frequencies, the interval Δf of the different frequencies, and an integer N. N (τ) may be calculated to correct for nonlinearity of the frequency swept light at the minimum beat frequency obtained by the main interferometer.

[0015] The delay time τ is estimated in advance, and τ≒Nτ aux The integer N may be any integer that satisfies the following formula:

[0016] The delay time is estimated in advance, and τ≒Mτ ref The order M may be used, where τ ref is the time interval between the different frequencies.

[0017] The optical duplication section may generate, as the light of the plurality of frequencies, an optical frequency comb having a frequency range wider than a frequency sweep width of the frequency sweep light.

[0018] The above disclosures can be combined as much as possible. [Effects of the Invention]

[0019] According to the present disclosure, it is possible to provide a beat signal correction method that functions correctly even when correcting a beat signal corresponding to a frequency component other than the zero-order component of the optical comb frequency components. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a diagram illustrating an example of a configuration of a non-contact distance measuring device according to an embodiment of the present disclosure. [Figure 2] 1A and 1B are diagrams illustrating the frequencies of reference light and probe light and the interference signal spectrum in a conventional FMCW LiDAR. [Figure 3]FIG. 10 is a diagram illustrating the frequencies of a reference light and a probe light in a non-contact distance measuring device according to an embodiment of the present disclosure. [Figure 4] FIG. 10 is a diagram illustrating an interference signal spectrum in a non-contact distance measuring device according to an embodiment of the present disclosure. [Figure 5] FIG. 10 is a diagram illustrating the frequencies of a reference light and a probe light in a non-contact distance measuring device according to an embodiment of the present disclosure. [Figure 6] FIG. 10 is a diagram illustrating the frequencies of a reference light and a probe light in a non-contact distance measuring device according to an embodiment of the present disclosure. [Figure 7] FIG. 10 is a diagram illustrating the frequencies of a reference light and a probe light in a non-contact distance measuring device according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0021] 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 forms with various modifications and improvements based on the knowledge of those skilled in the art. Note that components with the same reference numerals in this specification and drawings indicate the same components.

[0022] (Basic configuration) An embodiment of the present disclosure based on an FMCW LiDAR system is shown in Figure 1. Reference numeral 1 denotes a frequency swept light source, 2 denotes a coupler that multiplexes or splits light, 3 denotes an optical circulator, 4 denotes a lens, 5 denotes an object to be measured, 6 denotes a delay device, 7 denotes an optical frequency comb generator, 8 denotes an optical 90-degree hybrid, 9 denotes a balanced photodetector, 10 denotes an AD converter, 11 denotes an arithmetic unit such as a computer, 12 denotes an RF synthesizer, and 13 denotes an amplifier.

[0023] The optical frequency comb generator 7 functions as an optical duplication unit in this disclosure. The optical 90-degree hybrid 8, balanced photodetectors 9-1 and 9-2, and AD converter 10-1 constitute a main interferometer 20 that functions as an optical detection unit in this disclosure. In the following description, the reception band of the optical detection unit, i.e., the main interferometer 20, will be abbreviated as "reception band."

[0024] The frequency sweep light source 1 oscillates a laser beam whose frequency is linearly modulated. The frequency is swept over a frequency sweep width ΔF at a constant sweep speed γ [Hz / s] for a constant frequency sweep time ΔT. In this embodiment, the light output from the frequency sweep light source 1 is described as laser beam, but is not limited to this as long as it is coherent light.

[0025] Coupler 2-1 splits the light input from frequency sweep light source 1 into two, and inputs one of the beams to optical circulator 3 as probe light and the other to optical frequency comb generator 7 as reference light. Optical circulator 3 inputs the light from lens 4 to optical 90-degree hybrid 8. Lens 4 converts the probe light from frequency sweep light source 1 into a plane wave. Lens 4 also focuses the probe light reflected by device under test 5 and inputs it to optical circulator 3.

[0026] The optical frequency comb generator 7 generates high-order modulation sidebands from the input light. A specific configuration of the optical frequency comb generator 7 is described, for example, in Non-Patent Document 1. As an example of a specific configuration of the optical frequency comb generator 7, FIG. 1 shows an example in which a signal from an RF synthesizer 12 via an amplifier 13 is input to the optical frequency comb generator 7, and the optical frequency comb generator 7 generates an optical frequency comb at a frequency interval corresponding to the signal from the RF synthesizer 12.

[0027] 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 the 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 from 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 performs distance measurement using this delay time. In FIG. 1, the calculation unit 11 is configured to be included in the main interferometer 20 and / or the auxiliary interferometer 21, but the calculation unit 11 may be provided separately from the main interferometer 20 and the auxiliary interferometer 21.

[0028] Specifically, the optical 90-degree hybrid 8 generates an in-phase component I of a beat signal by combining the reference light and the light reflected from the object under test 5, and inputs this to a balanced photodetector 9-1. The optical 90-degree hybrid 8 also generates a quadrature component Q of a beat signal by combining the reference light, which has been phase-shifted by 90 degrees, and the light reflected from the object under test 5, and inputs this to a balanced photodetector 9-2.

[0029] The balanced photodetector 9-1 acquires 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 acquires 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 photodetector 9-1 and the analog electrical signal of the quadrature component Q of the beat signal input from the photodetector 9-2 into digital signals and inputs them to the calculation unit 11.

[0030] (About optical frequency combs) Next, we will explain the interference between the reference light and the probe light in a conventional FMCW LiDAR that does not have an optical frequency comb generator 7, using Fig. 2. Specifically, in a conventional FMCW LiDAR, the reference light from the coupler is directly input to the optical 90-degree hybrid in the main interferometer.

[0031] Generally, as shown in FIG. 2(A), in an FMCW LiDAR, the reference light from the coupler 2-1 interferes with the probe light delayed by the distance it travels to and from the object under test 5, generating a beat signal with a frequency corresponding to the frequency difference. Hereinafter, the frequency of the beat signal will be referred to as the beat frequency IF. The 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, enabling distance measurement.

[0032] Specifically, the beat signal is expressed as the following equation (2) in complex numbers using the I-phase component and Q-phase component of the beat signal output from the optical 90-degree hybrid 8. (Number 2) I+jQ=exp(jγτt) (2)

[0033] The calculation unit 11 calculates the phase of the beat signal from equation (1) based on the in-phase component I and quadrature component Q of the digital signal input from the AD converter 10-1, where 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.

[0034] In general, the distance resolution Δz of an FMCW LiDAR is expressed as follows using the frequency sweep width ΔF: (Number 3) Δz=c / (2ΔF) (3)

[0035] In other words, to improve the distance resolution Δz, the frequency sweep width ΔF must be increased. Meanwhile, the beat frequency IF is proportional to the delay time τ, which varies depending on the distance to the object 5, and the sweep rate γ. Therefore, if the reception band is constant, there will be a limit to the product of the sweep rate γ and the distance. Specifically, as shown in Figure 2(B), conventional FMCW LiDAR cannot detect beat signals whose beat frequency IF is outside the reception band, and the reception band imposes a limit on the measurement distance. Note that the repetition frequency (refresh rate) is proportional to the sweep rate γ, so if the sweep rate γ is limited, the refresh rate will also be limited.

[0036] In this disclosure, in order to eliminate the measurement distance limitation due to the receiving band, the above-mentioned optical frequency comb generator 7 is introduced into the reference light path, that is, between the coupler 2-1 and the optical 90-degree hybrid 8.

[0037] FIG. 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 generates an optical frequency comb consisting of a plurality of different frequencies, including the frequency of the reference light input from the coupler 2-1. For ease of understanding, FIG. 3 shows the frequency sweep light source 1 swept from the laser light, and the frequency sweep light source L ref 1 corresponds to the reference light L ref This shows an example of an optical frequency comb consisting of five frequencies, including two equally spaced frequencies on either side of the center 1. In other words, the zeroth-order (M=0) reference light L ref The frequencies of order -2, -1, 1, and 2 are arranged at equal intervals with 1 as the center, where M is the order of the frequency component.

[0038] The optical frequency comb according to this embodiment is merely an example, and the number of frequencies is not limited to this. Furthermore, the frequency intervals of the optical frequency comb may be different, rather than being equal. The optical duplication unit that generates an optical frequency comb containing multiple frequencies, such as those shown in FIG. 3, is not limited to the optical frequency comb generator 7, and any means capable of generating sidebands from input light may be used.

[0039] In this embodiment, the reference light input from the coupler 2-1 is linearly swept by the frequency sweep light source 1, and therefore the optical frequency comb is also linearly swept. As a result, the reference light from the coupler 2-1 in this disclosure becomes the frequency-swept optical frequency comb shown in FIG. 3 and enters the optical 90-degree hybrid 8.

[0040] 4 shows an example of the beat frequency detected by the main interferometer 20 of this embodiment. ref 1 to L ref 5 and probe light L p The beat frequency IF1 is the frequency of the probe light L p and reference beam L ref The beat frequency IF2 is the beat frequency between the probe light L p and reference beam L ref The beat frequency IF3 is the beat frequency between the probe light L p and reference beam L ref The beat frequency IF4 is the beat frequency between the probe light L p and reference beam L ref The beat frequency IF5 is the beat frequency between the probe light L p and reference beam L ref The beat frequency is between 0 and 5.

[0041] As shown in Figure 4, the probe light delayed during propagation to the DUT 5 interferes with all of the reference lights constituting the optical frequency comb, generating beat signals with beat frequencies IF1 to IF5. In this disclosure, since the reception bands of the balanced photodetectors 9-1 and 9-2 are limited, the observed beat frequency is determined by the frequency of the probe light and the reference light L with the closest frequency. ref The only frequency that remains is the beat frequency IF2, which is obtained by interference with 2.

[0042] Here, if the frequency spacing of the optical frequency comb shown in Figure 5, where the frequencies are evenly spaced, is Δf, then the minimum beat frequency will be Δf / 2 or less. Therefore, it is desirable for the main interferometer 20 to have a reception band 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, making it possible to detect the minimum beat frequency even with a main interferometer 20 that has a narrow reception band. Note that in the case of an optical frequency comb where the frequency spacing is not even, the explanation can be similar to that for the case where the frequency spacing is even, provided that the maximum frequency spacing is Δf.

[0043] 5 only shows a portion of the reference and probe beams, and assumes that the reference and probe beams are linearly swept to even higher frequency bands. 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 without expanding the reception bands of the balanced photodetectors 9-1 and 9-2, even when the DUT 5 is far away. Since it is not clear which frequency component is interfering, periodic ambiguity remains in the ranging results, but this can be eliminated by performing a rough measurement in advance.

[0044] In the above configuration, the optical frequency comb generator 7 may be located 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 probe light reflected by the DUT 5 is input to the optical frequency comb generator 7. As a result, as shown in FIG. 6, the optical frequency comb generator 7 generates an optical frequency comb consisting of multiple different frequencies, including the frequency of the probe light reflected by the DUT 5. Meanwhile, the reference light is a single light. In this case, the same effect as the configuration shown in FIG. 1 can be achieved by measuring the frequency of the beat signal generated by interference between the reference light and the light with the frequency closest to the frequency of the reference light in the optical frequency comb based on the probe light.

[0045] As described above, by introducing the optical frequency comb generator 7, it is possible to eliminate the limit on the measurement distance imposed by the receiving band without reducing the frequency sweep speed.

[0046] (Beat signal correction) Generally, in a frequency swept light source, the trace of the frequency sweep is not perfectly linear but has a certain level of imperfection (nonlinearity). This nonlinearity means fluctuations in the sweep rate γ, which causes fluctuations in the beat frequency from a certain distance, making it impossible to obtain the theoretical distance resolution given by equation (3).

[0047] To solve this problem, the non-contact distance measuring device according to this embodiment further includes an auxiliary interferometer 21, as shown in FIG. 1. The basic configuration of the auxiliary interferometer 21 may be the same as that of the auxiliary interferometer according to Patent Document 1. Specifically, the auxiliary interferometer 21 includes a coupler 2-4, a balanced photodetector 9-3, an AD converter 10-2, and a calculation unit 11. The auxiliary interferometer 21 uses light without delay and a delay time τ aux The auxiliary interferometer 21 is an interferometer for obtaining the phase of a beat signal due to interference with light delayed by a delay time τ aux This is the same as the beat signal based on the probe light reflected from the object to be measured 5 located at a distance corresponding to

[0048] The coupler 2-2 splits the laser light from the frequency sweep light source 1 into two, and inputs one 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. This allows a portion of the frequency sweep light to be extracted.

[0049] In the auxiliary interferometer 21, the coupler 2-3 splits the light input from the coupler 2-2 into two, and inputs one of the two to the coupler 2-4 and the other to the delay device 6. The delay device 6 adds a delay time τ auxThe light is delayed and input to the coupler 2-4. The coupler 2-4 combines the light input directly from the coupler 2-3 and the light input from the delay device 6 to generate a beat signal, and then splits the beat signal into two signals which are input to the balanced photodetector 9-3. The balanced photodetector 9-3 acquires the beat signal from the coupler 2-4 as an analog electrical signal and inputs it to the AD converter 10-2. The AD converter 10-2 converts the beat signal input from the balanced photodetector 9-3 into a digital signal and inputs it to the calculation unit 11.

[0050] By "resampling" the beat signal of the main interferometer 20 at the frequency of the beat signal of the auxiliary interferometer 21 using the beat signal of the auxiliary interferometer 21, the nonlinearity of the frequency sweep of the light source can be removed.

[0051] (About the phase noise compensation algorithm) However, the above method does not eliminate the delay time τ aux In other words, the phase of the laser light from the frequency swept light source 1 is only correlated within a time period specific to each frequency swept light source 1, called the coherence time, and once this time has elapsed, the phase correlation disappears. This is because the distance (delay time) to the object 5 to be measured is equal to the delay time τ aux If the difference between the auxiliary interferometer 21 and the main interferometer 20 exceeds the coherence time, the beat frequency of the auxiliary interferometer 21 and the beat frequency of the main interferometer 20 will no longer be correlated and will no longer function.

[0052] Therefore, the calculation unit 11 according to this embodiment has a built-in phase noise compensation algorithm. An example of 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 acquires the beat signal input from the AD converter 10-2 in FIG. 1 and calculates the delay time τ aux The calculation unit 11 obtains the phase X(t) of the beat signal of the auxiliary interferometer 21 for each time period. The calculation unit 11 calculates the phase X(t) of the beat signal of the auxiliary interferometer 21 using the phase X(t) of the beat signal of the auxiliary interferometer 21, as shown in the following equation (4):N Calculate (τ).

number

[0053] where N is a positive integer. τ ≒ Nτ aux The delay time τ may be longer than the coherence time of the laser beam. aux is shorter than the coherence time of the laser light, the delay time τ is aux The phase of the beat signal of the main interferometer 20 at the delay time τ can be calculated by equation (4). N If the signal of the main interferometer 20 is resampled every time (τ) increases by a certain value, the time Nτ that exceeds the coherence time of the laser light aux It is possible to measure the distance corresponding to the nearby delay time τ. As a result, the problem of the finiteness of the coherence time mentioned above can be overcome. Note that the calculation unit 11 may estimate the delay time τ in advance and perform processing using an integer N that satisfies equation (4) or the following equation (5).

[0054] Furthermore, as described in Non-Patent Document 2, the phase X N (τ) also corresponds to the phase of the laser light at the delay time τ. Therefore, by using equation (4), the nonlinearity of the frequency sweep of the laser light can be corrected, and the resolution of equation (3) can also be realized. As mentioned above, the delay time τ of the auxiliary interferometer aux must be set shorter than the coherence time of the frequency sweep light source 1 used.

[0055] However, the phase noise compensation algorithm based on the above equation (4) works correctly when correcting the beat signal corresponding to the 0th order (M=0) component of the optical comb frequency component, i.e., the frequency component of the original frequency sweep light source 1, but does not work correctly when correcting beat signals corresponding to frequency components of other orders. As an example, Figure 7 shows the case where correction is performed on a beat signal corresponding to a frequency component of M=2. This is because the delay time τ is defined based on the 0th order of the reference light, and equation (3) does not take into account the influence of other orders.

[0056] Therefore, in this embodiment, we provide a phase noise compensation algorithm based on the following equation (5) that takes into account frequency components of orders other than 0. Specifically, equation (5) is a correction signal when the order of the optical comb frequency components used is M and the frequency interval between the frequency components of the optical comb is Δf.

number

[0057] That is, the non-contact distance measuring device according to the present disclosure is A non-contact distance measuring device that irradiates a frequency sweep light onto an object to be measured, and measures the frequency of a beat signal generated by interference between a probe light reflected by the object to be measured and a reference light, thereby determining a distance to the object to be measured, an optical frequency comb generator 7 that receives reference light or probe light and replicates the incident light into light of multiple different frequencies; a main interferometer (20) that combines light of multiple frequencies and the reference light or probe light that has not been input to the optical frequency comb generator (7) and detects the minimum beat frequency obtained by interference between the light of multiple frequencies and the reference light or probe light that has not been input to the optical frequency comb generator (7); a coupler 2-2 that splits a part of the frequency swept light into two; One of the two parts of the frequency swept light split by coupler 2-2 has a delay time τ shorter than the coherence time of the frequency swept light. aux a delay unit 6 for generating a delay of an auxiliary interferometer 21 for obtaining a phase X(t) resulting from interference between one part delayed by the delay device 6 and the other part of the frequency swept light branched into two by the coupler 2-2; Based on the phase X(t) and the influence of frequencies other than the frequency corresponding to the reference light or the probe light among a plurality of different frequencies, the phase X of the frequency swept light at a delay time τ of the probe light relative to the reference light, which corresponds to the optical path difference between the reference light and the probe light, is calculated. N a calculation unit 11 for calculating (τ) and correcting the nonlinearity of the frequency swept light at the minimum beat frequency obtained by the main interferometer 20; Equipped with.

[0058] More specifically, the calculation unit 11 The phase X(t) of the beat signal acquired by the auxiliary interferometer 21, the order M of the different frequencies, the interval Δf of the different frequencies, and the phase X of the frequency sweep light at the delay time τ of the probe light relative to the reference light, which corresponds to the optical path length between the reference light and the probe light, are calculated using equation (C1) expressed using integers. N (τ) is calculated and the nonlinearity of the frequency swept light at the minimum beat frequency obtained by the main interferometer 20 is corrected.

[0059] The order M of the frequency component of the optical comb frequency component that is closest to the probe light can be determined in advance by determining the approximate distance using a known method such as the pulse method, as in Patent Document 1.

[0060] Specifically, the present invention discloses a method for applying this phase noise compensation algorithm to distance measurement by the following procedure: First, the distance to the object 5 is calculated by the delay τ aux Generally, the coherence length of the frequency sweep light source 1 is about several tens of meters, and the delay τ aux This estimation is easy because the distance is on the same order of magnitude. For example, it is sufficient to know the approximate distance in advance using a known method such as the pulse method.

[0061] Specifically, assuming that the delay time of the probe light reflected from the object under test 5 at the estimated distance L relative to the reference light is τ, an integer M (order M) that satisfies equation (5) is found. (Number 6) τ ≒ Mτ ref (6)

[0062] where τ ref corresponds to the time interval of the optical frequency comb, as shown in Figure 7. Furthermore, the phase X M (τ) is calculated and used to correct the nonlinearity of the frequency sweep of the laser light. That is, X N (τ) is calculated. This makes it possible to achieve the theoretical resolution of equation (3) in principle. Note that if the optical frequency comb is not evenly spaced, the average time interval is calculated as τ ref It may also be used as.

[0063] Although the main interferometer 20 according to this embodiment is configured to perform hardware-based processing using the optical 90-degree hybrid 8, software-based processing using the Hilbert transform may also be used. Furthermore, the auxiliary interferometer 21 according to this embodiment is configured to perform software-based processing using the Hilbert transform, but hardware-based processing using the optical 90-degree hybrid 8 may also be used.

[0064] The calculation unit 11 according to this embodiment can also be realized by a computer and a program, and the program can be recorded on a recording medium or provided via a network. [Industrial Applicability]

[0065] The non-contact distance measuring device and method according to the present disclosure can be applied to the information and communication industry. [Explanation of symbols]

[0066] 1: Frequency swept light source 2: Coupler 3: Optical circulator 4: Lens 5: Object to be measured 6: Delay 7: Optical frequency comb generator 8: Optical 90-degree hybrid 9:Balanced photodetector 10: AD converter 11: Arithmetic section 12: RF synthesizer 13: Amplifier 20: Main interferometer 21: Auxiliary interferometer

Claims

1. 1. A non-contact distance measuring device that irradiates a frequency sweep light onto an object to be measured, and measures the frequency of a beat signal generated by interference between a probe light and a reference light reflected by the object to determine a distance to the object, an optical duplication unit that receives the reference light or the probe light and duplicates the incident light into light of a plurality of different frequencies; a main interferometer that combines the light of the plurality of frequencies and the reference light or the probe light that has not been input to the optical duplication unit, and detects a minimum beat frequency obtained by interference between the light of the plurality of frequencies and the reference light or the probe light that has not been input to the optical duplication unit; a coupler that splits a part of the frequency swept light into two; One of the two portions of the frequency swept light split by the coupler is provided with a delay time τ shorter than the coherence time of the frequency swept light. ａｕｘ a delayer for generating a delay of an auxiliary interferometer for obtaining a phase due to interference between the one part delayed by the delay device and the other part of the frequency swept light branched into two by the coupler; a calculation unit that calculates a phase of the frequency swept light at a delay time τ of the probe light relative to the reference light, which corresponds to a difference in optical paths between the reference light and the probe light, based on the phase and an influence of a frequency other than the frequency corresponding to the reference light or the probe light among the plurality of different frequencies, and corrects nonlinearity of the frequency swept light at a minimum beat frequency obtained by the main interferometer; A non-contact distance measuring device comprising:

2. The calculation unit calculates the phase X(t) of the frequency sweep light at a delay time τ of the probe light relative to the reference light, which corresponds to the optical path difference between the reference light and the probe light, by using equation (C1) expressed using the phase X(t) of the beat signal acquired by the auxiliary interferometer, the order M of the different frequencies, the interval Δf of the different frequencies, and an integer N. Ｎ (τ) and correct the nonlinearity of the frequency swept light at the minimum beat frequency obtained by the main interferometer.

2. The non-contact distance measuring device according to claim 1. [Math C1]

3. The delay time τ is estimated in advance, and the integer N that satisfies the formula (C2) is used.

3. The non-contact distance measuring device according to claim 2. (Number C2) τ≒Nτ ａｕｘ (C2)

4. 3. The non-contact distance measuring device according to claim 2, wherein the delay time is estimated in advance, and the order M that satisfies formula (C3) is used. (Number C3) τ≒Mτ ｒｅｆ (C3) However, τ ｒｅｆ is the time interval between the different frequencies.

5. The optical duplication unit generates an optical frequency comb having a frequency range wider than the frequency sweep width of the frequency sweep light as the light of the plurality of frequencies.

5. A non-contact distance measuring device according to claim 1.

6. 1. A non-contact distance measuring method for determining a distance to an object to be measured by irradiating the object with frequency sweep light and measuring a frequency of a beat signal generated by interference between a probe light reflected by the object and a reference light, comprising: replicating the reference light or the probe light into light of different frequencies; combining the light of the plurality of frequencies and the unduplicated reference light or the unduplicated probe light, and detecting a minimum beat frequency obtained by interference between the light of the plurality of frequencies and the unduplicated reference light or the unduplicated probe light; splitting a part of the frequency swept light into two; One of the two branched portions of the frequency swept light is provided with a delay time τ shorter than the coherence time of the frequency swept light. ａｕｘ and causing a delay of obtaining a phase due to interference between the delayed one of the two branches of the frequency swept light and the other of the two branches of the frequency swept light; determining a phase of the frequency swept light at a delay time τ of the probe light relative to the reference light, the delay time τ corresponding to an optical path difference between the reference light and the probe light, based on the phase and an influence of a frequency other than the frequency corresponding to the reference light or the probe light among the plurality of different frequencies, and correcting nonlinearity of the frequency swept light at the minimum beat frequency; A non-contact distance measurement method.

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