Non-contact distance measuring device and method

Abstract

To enable an identification of a frequency having generated 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 frequencies whose frequency interval is uneven. 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 detects two or more frequencies from among the plurality of beat signals, to obtain a distance to the measurement object.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 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 Literature 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] In a simple distance measuring device using a single light source, a technique has been proposed to extend the measurement distance without reducing the frequency sweep speed (see, for example, Non-Patent Literature 1). In Non-Patent Literature 1, one of the reference light or the probe light is replicated into lights of a plurality of different frequencies and multiplexed with the other of the reference light or the probe light. Thereby, beat signals of a plurality of frequencies are generated, and the beat frequency that falls within the reception band is detected.

[0007] However, in Non-Patent Literature 1, lights of a plurality of frequencies are generated, and it is impossible to specify which frequency among the plurality of frequencies generated the beat signal at the time of detecting the beat signal. Therefore, an object of the present disclosure is to make it possible to specify which frequency among the plurality of replicated frequencies is the frequency that generated the beat signal at the time of detecting the beat signal.

Means for Solving the Problems

[0008] To achieve the above object, the present disclosure replicates light of one frequency of the reference light or the probe light and causes the replicated lights of the plurality of frequencies to interfere with the other of the reference light or the probe light. At the time of this replication, the intervals between the replicated frequencies are made non-uniform.

[0009] Specifically, the non-contact distance measuring device according to the present disclosure irradiates the object to be measured with frequency-swept light, and measures the frequency of the beat signal due to the interference between the probe light reflected by the object to be measured and the reference light, thereby obtaining the distance to the object to be measured. In the non-contact distance measuring device, the non-contact distance measuring method according to the present disclosure is executed.

[0010] The non-contact distance measuring device according to the present disclosure includes an optical replication unit that replicates one of the reference light or the probe light into light having a plurality of non-uniform frequency intervals. By combining the lights of a plurality of frequencies replicated by the optical replication unit with the other of the reference light or the probe light, a plurality of beat signals are generated. The distance to the object to be measured is obtained by detecting two or more frequencies among the plurality of beat signals.

[0011] The non-contact distance measuring method according to the present disclosure includes a procedure in which an optical replication unit replicates one of the reference light or the probe light into light having a plurality of non-uniform frequency intervals. By combining the lights of a plurality of frequencies replicated by the optical replication unit with the other of the reference light or the probe light, a plurality of beat signals are generated. The distance to the object to be measured is obtained by detecting two or more frequencies among the plurality of beat signals.

[0012] The optical replication unit may adopt a mode of generating the lights of the plurality of frequencies by combining two types of spectra having different frequency intervals f and f + Δf. Here, f may be different from an integer multiple of Δf. Also, f + Δf may be smaller than twice the reception band of the main interferometer that detects the frequency of the beat signal. Also, f may be smaller than the reception band of the main interferometer that detects the frequency of the beat signal. The light intensity of the frequency interval f and the light intensity of the frequency interval f + Δf may be different.

[0013] An embodiment can be adopted in which the light of the multiple frequencies can be used for distance measurement by shifting the delay time or frequency of the light of the multiple frequencies and the reference light or probe light that was not incident on the light replication unit.

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

[0015] According to this disclosure, when detecting a beat signal, it is possible to identify which of the multiple replicated frequencies generated the beat signal. Therefore, despite the simple configuration using a single light source, this disclosure can achieve a measurement distance of several kilometers and a resolution of several tens of micrometers without reducing the measurement speed, and furthermore, it can mitigate the periodic ambiguity of the distance measurement results. [Brief explanation of the drawing]

[0016] [Figure 1] This figure shows an example of the configuration of a non-contact distance measuring device related to this disclosure. [Figure 2] This diagram illustrates conventional reference and probe light, where (a) shows the frequencies of the reference and probe light, and (b) shows the interference signal spectrum. [Figure 3] This diagram illustrates the frequencies of the reference light and probe light. [Figure 4] An example of a beat frequency is shown. [Figure 5] This disclosure shows an example of the configuration of the optical replication unit. [Figure 6] This is a diagram illustrating the frequency of the reference light in this disclosure. [Figure 7] This is a diagram illustrating the frequency of the reference light in this disclosure. [Figure 8] This figure illustrates the frequencies of the reference light and test light in this disclosure. [Figure 9] An example of the beat frequency of this disclosure is shown. [Figure 10] This is a diagram illustrating the frequency of the reference light in this disclosure. [Figure 11] An example of the beat frequency of this disclosure is shown. [Figure 12] Here is an example of a workflow for calculating the distance to the object being measured. [Figure 13] This is a diagram illustrating the frequency of the reference light in this disclosure. [Figure 14] An example of the beat frequency of this disclosure is shown. [Figure 15] This disclosure shows an example of the configuration of the optical replication unit. [Figure 16] This is a diagram illustrating the frequency of the reference light in this disclosure. [Figure 17] This is a diagram illustrating the frequency of the reference light in this disclosure. [Figure 18] An example of the beat frequency of this disclosure is shown. [Figure 19] Here is an example of a workflow for calculating the distance to the object being measured. [Figure 20] This disclosure shows an example of the configuration of the optical replication unit. [Figure 21] This is a diagram illustrating the frequency of the reference light in this disclosure. [Figure 22] This is a diagram illustrating the frequency of the reference light in this disclosure. [Figure 23] This is a diagram illustrating the frequency of the reference light in this disclosure. [Figure 24] An example of the beat frequency of this disclosure is shown. [Figure 25] Here is an example of a workflow for calculating the distance to the object being measured. [Modes 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] (First Embodiment) Embodiments of the present invention will be described in detail below with reference to the drawings. Figure 1 shows an embodiment of the non-contact distance measuring device of the present disclosure based on the 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 replication unit, 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 replication unit 7 replicates 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 in 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 the frequency-swept light source 1 into two, inputting one as probe light to the optical circulator 3 and the other as reference light to the optical replication unit 7. The optical circulator 3 inputs the probe light from coupler 2-1 to lens 4. The optical circulator 3 also inputs the light from lens 4 to optical 90-degree hybrid 8. Lens 4 converts the probe light from the 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 the optical circulator 3. In this disclosure, the probe light reflected from the object under test 5 may be referred to as test light.

[0022] The optical replication unit 7 replicates the reference light into multiple lights of different frequencies. The optical replication unit 7 inputs the multiple generated reference lights into 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.

[0023] 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.

[0024] 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.

[0025] Here, the interference between the reference light and probe light in a conventional FMCW-type LiDAR without an optical replication unit 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.

[0026] 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)

[0027] 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.

[0028] 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)

[0029] 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.

[0030] In this disclosure, in order to eliminate the limitation of measurement distance due to the receiving bandwidth, the aforementioned optical replication unit 7 is introduced into the reference optical path, that is, between the coupler 2-1 and the optical 90-degree hybrid 8.

[0031] Figure 3 shows an example of the reference light generated by the optical replication unit 7. In this embodiment, the optical replication unit 7 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.

[0032] The optical frequency comb according to this embodiment is just one example, and the number of frequencies is not limited thereto. Furthermore, the frequency intervals of the optical frequency comb do not have to be equal; they 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 replication unit 7; any means capable of generating sidebands for the input light can be employed.

[0033] 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.

[0034] 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, which has been 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.

[0035] If the frequency spacing of the optical frequency comb shown in Figure 3, where the frequencies are equally spaced, is f, then the minimum beat frequency will be less than or equal to f / 2. Therefore, it is desirable that the main interferometer 20 have a receiving bandwidth of at least f / 2. Narrowing the frequency spacing f of the optical frequency comb lowers the maximum value f / 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 where the frequency spacing is not equally spaced, the explanation can be the same as in the case of equally spaced frequencies if the maximum frequency spacing is taken as f.

[0036] Figure 3 only shows a portion of the reference and probe light, and it is assumed that the reference and probe light are linearly swept to even higher frequency bands. If the optical replication unit 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.

[0037] In this disclosure, the multiple beat frequencies shown in Figure 4 depend on the frequencies of the frequency-swept light generated by the optical replication unit 7. The frequency-swept light generated by the optical replication unit 7 is light that has been shifted in the frequency direction from the original frequency-swept light. Since the optical replication unit 7 generates multiple frequency-swept lights, a periodic ambiguity remains in the frequency-swept light that generated the beat frequency IF2. Therefore, in this disclosure, the optical replication unit 7 replicates the incident light into multiple lights of different frequencies so that the frequency intervals are non-uniform. The calculation unit 11 determines that the frequency of the reference light that generated the beat frequency IF2 is L based on the frequency difference between beat frequencies IF2 and IF3 and the frequency difference between beat frequencies IF3 and IF1. ref Determine that the frequency is 2.

[0038] Figure 5 shows an example of the configuration of the optical replication unit 7. In this embodiment, two modulators 71A and 71B are used to generate reference light modulated at two different modulation frequencies, and these are combined by a coupler 72B. The reference light, which has been split 1:1 by the coupler 72A, is incident on modulators 71A and 71B. Modulation signals from synthesizer 12A are input to modulator 71A, and modulation signals from synthesizer 12B are input to modulator 71B. As a result, reference light with frequency interval f and reference light with frequency interval f+Δf are generated.

[0039] Here, f is a value different from an integer multiple of Δf, and f+Δf is less than twice the receiving bandwidth BW. That is, the following relationship holds. f + Δf < 2 × BW (3)

[0040] Figures 6 and 7 show an example of the reference light generated by the optical replication unit 7. In the figures, the dashed line indicates the reference light RA from the modulator 71A, and the dashed-dotted line indicates the reference light RB from the modulator 71B. The reference light RA from the modulator 71A appears on both the long wavelength side and the short wavelength side of the frequency f0 of the frequency sweep light source 1, and the reference light RB from the modulator 71B appears on both sides thereof. The frequency interval between the reference lights RA and RB increases as it moves away from the frequency f0, such as Δf, 2Δf, 3Δf, 4Δf, ··· nΔf. The frequency interval between the reference lights RB and RA decreases as it moves away from the frequency f0, such as f - Δf, f - 2Δf, f - 3Δf, f - 4Δf, ···, f - nΔf.

[0041] Figures 8(a) and 8(b) show an example of the relationship between the test light and the reference light. When the distance to the measurement target 5 is zero, as shown in Figure 8(a), the test light is made to overlap with one of the reference lights RB. If they do not overlap, the frequency difference f b between the test light and the reference light RB may be corrected during distance calculation. As the measurement target 5 is farther away, as shown in Figure 8(b), the time τ of the test light shifts to the right.

[0042] When the frequency sweep speed of the frequency sweep light source 1 is γ (Hz / s), the time τ is expressed as follows. τ = f b / γ (4)

[0043] The distance z (m) to the measurement target can be obtained using the following equation. z = cf b / 2γ (5) However, c is the speed of light (m / s).

[0044] Figure 9 shows an example of the measured beat frequency. When the reception bandwidth of the beat frequency is BW, f + Δf is made smaller than twice the reception bandwidth BW. That is, the following is satisfied. (f + Δf) / 2 < BW (6)

[0045] Figures 10 and 11 show examples of beat frequency measurement. Figures 11(1) to (4) show the beat frequency patterns when the test light has the frequency shift amounts shown in Figures 10(1) to (4). Light with a frequency shift amount greater than frequency nf is removed by a filter. Here, n is an integer that satisfies the following equation. n <f / Δf+1 (7)

[0046] In this embodiment, one or two beat frequencies ν1 or ν2 are measured according to the beat frequency of the test light, as shown in Figures 11(1) to 11(4). For example, the measured beat frequencies are defined as ν1 and ν2 in ascending order. There are four possible relationships between the measured beat frequencies. In this embodiment, the distance z of the object to be measured is calculated based on the flow shown in Figure 12.

[0047] The system determines if there is only one beat frequency (S101), and if there is, it determines that it is pattern 1 as shown in Figure 11(1). Note that even if the device is at the very edge of the measurement range, only one beat frequency will be measured, but in this case, it can be treated as being outside the measurement range.

[0048] When two beat frequencies are measured, the frequency difference Δν between them is calculated. When Δν = f, it is determined to be pattern 2 as shown in Figure 11(2). When Δν is an integer multiple of Δf, it is determined to be pattern 4 as shown in Figure 11(4). When Δν is not an integer multiple of Δf, it is determined to be pattern 3 as shown in Figure 11(3).

[0049] When calculating the distance z (m) to the object to be measured, the frequency difference f b Set it as follows: When pattern 1 or 2 is used, the measured beat frequency ν1 is f b Set to (S111). In pattern 4, use the following equation to determine f b Set (S112). f b =(Δν / Δf)·f-ν1(7) In pattern 3, use the following equation to determine f b Set (S113). f b ={(f-Δν) / Δf+1}·f-ν²(8)

[0050] Figures 13 and 14 show specific calculation examples when f = 15 MHz, Δf = 2 MHz, and BW = 10 MHz. In this embodiment, n·f = 105, and light with a frequency shift amount greater than this is removed by a filter. When the frequency shift amount of the test light is at position (1) shown in Figure 13, it corresponds to pattern 1, so step S111 is performed, f b Set it. When the frequency shift amount of the test light is at position (2) shown in Figure 13, Δν = 4 = 2Δf, which corresponds to pattern 4. Therefore, step S112 is performed, f b Set =30-ν1 When the frequency shift amount of the test light is at position (3) shown in Figure 13, Δν = 7, which corresponds to pattern 3, so step S113 is performed, f b Set =75-ν2

[0051] The limit at which two beat frequencies can be seen is 10² + 10 = 112 MHz. At this point, the maximum distance that can be measured is: c / 2γ×112×10 6 = 56c / γ × 10 6 (m) (9) Therefore, if ΔT = 2 ms and ΔF = 5 GHz, the resolution will be 3 cm and the measurement distance will be 6720 m.

[0052] (Second embodiment) Figure 15 shows an example of the configuration of the optical replication unit 7 in this embodiment. In this embodiment, a frequency shifter 75 is provided after the coupler 72B.

[0053] Figure 16 shows an example of the relationship between the reference light and the test light. As shown in Figure 16(a), the test light is approximately the same as the reference light from frequency-swept light source 1 when the distance to the object being measured is zero, and shifts to the right as the distance increases. Therefore, the reference light above the test light cannot be used and is wasted.

[0054] In this embodiment, as shown in Figure 16(b), the frequency of the entire reference light is shifted downward by the frequency shifter 75, making the entire reference light usable for distance measurement. Alternatively, this can be achieved by shifting the frequency of the test light upward.

[0055] Figures 17 and 18 show examples of beat frequency measurement. In this embodiment, f is smaller than the receiving bandwidth BW of the main interferometer 20. In this case, two or more beat frequencies are measured depending on the beat frequency of the test light, as shown in Figures 18(1) and 18(2). The measured beat frequencies are defined as ν1, ν2, and ν3 in ascending order. There are five possible relationships between the measured beat frequencies. (1) Only two beat frequencies are measured, and the difference between them is f. (2) Three or more beat frequencies are measured, and Δν1 / Δf=a(integer), and (f-Δν2) / Δf=a-1 holds true. (3) Three or more beat frequencies are measured, and Δν1 / Δf=a (an integer), and (f-Δν2) / Δf=a holds true. (4) Three or more beat frequencies are measured, and Δν² / Δf = a (integer), and (f-Δν1) / Δf = a-1 holds true. (5) Three or more beat frequencies are measured, and Δν² / Δf=a (an integer), and (f-Δν1) / Δf=a holds true. Therefore, in this embodiment, the distance z to the object to be measured is calculated based on the flow shown in Figure 19. When calculating the distance z (m) to the object to be measured, the frequency difference f b Set it as follows:

[0056] The system determines if there are two beat frequencies (S201), and if there are, it determines that it is pattern 1 as shown in Figure 18(1). Furthermore, the intensities I1 and I2 at the two beat frequencies ν1 and ν2 are compared (S204). When intensity I2 is greater than intensity I1 (Yes in S204), use the following equation to calculate f b Set (S211). f b =nf-ν2(21) When intensity I1 is greater than intensity I2 (No in S204), use the following equation to calculate f b Set (S212). f b =nf-ν1(22)

[0057] If there are three or more beat frequencies (No in S201), calculate the difference Δν1 between beat frequencies ν1 and ν2, and determine if Δν1 / Δf is an integer (S202). If it is an integer, calculate the difference Δν2 between beat frequencies ν2 and ν3, and determine whether (f-Δν2) / Δf=a-1 holds true using an integer a (S205). If the answer in step S205 is Yes, it is determined to be pattern 2, and if the answer in step S205 is No, it is determined to be pattern 3. In pattern 2, use the following equation to determine f b Set (S213). f b =(n-Δν1 / Δf)f-ν1(23) In pattern 3, use the following equation to determine f b Set (S214). f b =(n+Δν1 / Δf)f-ν1(24)

[0058] In step S203, determine whether (f-Δν1) / Δf=a-1 holds true using an integer a. If the answer is Yes in step S203, determine that it is pattern 4; if the answer is No in step S203, determine that it is pattern 5. In pattern 4, use the following equation to determine f b Set (S215). f b=(n-Δν1 / Δf)f-ν2(25) When pattern 5, use the following equation to determine f b Set (S216). f b =(n+Δν1 / Δf)f-ν2(26)

[0059] Therefore, in this embodiment, based on the relationship between the five beat frequencies, the frequency difference f b By setting the appropriate parameters, the distance z to the object to be measured can be calculated.

[0060] (Third embodiment) Figure 20 shows an example of the configuration of the optical replication unit 7 in this embodiment. In this embodiment, a delay line 76 is provided after the coupler 72B.

[0061] Figure 21 shows an example of the relationship between the reference light and the test light. As shown in Figure 21(a), when the distance to the object to be measured is zero, the test light is almost identical to the reference light from the frequency-swept light source 1, and shifts to the right as the distance increases. Therefore, the reference light above the test light cannot be used and is wasted. In this embodiment, as shown in Figure 21(b), the entire reference light is delayed by the delay line 76, so that the entire reference light can be used for distance measurement.

[0062] The beat frequency measurement example in this embodiment is similar to that in the second embodiment, based on the relationship between five beat frequencies, and the frequency difference f b By setting the appropriate parameters, the distance z to the object to be measured can be calculated.

[0063] (Fourth embodiment) In the above embodiment, an example was shown in which the coupling ratio of reference light RB and RA is equal in coupler 72B, but these coupling ratios may be different. In this embodiment, an example is shown in which the intensity ratio of reference light RA and RB is 60% to 40%.

[0064] Fig. 22 shows an example of the reference light generated by the optical replication unit 7. In the figure, the dashed line indicates the reference light RA from the modulator 71A, and the one-dot chain line indicates the reference light RB from the modulator 71B. In this embodiment, the light intensity at the frequency interval f is different from the light intensity at the frequency interval f + Δf, and the intensity of the reference light RA is higher than the intensity of the reference light RB. Therefore, based on the intensity, the reference lights RA and RB can be discriminated.

[0065] Figs. 23 and 24 show measurement examples of the beat frequency. The horizontal axes shown in Fig. 24 are all beat frequencies. The measured beat frequencies are defined as ν1 and ν2 in ascending order, and their intensities are I1 and I2. The relationship of the measured beat frequencies is in 7 patterns. Therefore, in this embodiment, based on the flow shown in Fig. 25, the distance z of the measurement target is calculated.

[0066] It is determined whether the beat frequency is one (S301). If it is one, it is determined that it is pattern 1 shown in Fig. 24(1). When two beat frequencies are measured (No in S301), their intensities are compared (S302). When I1 < I2 (Yes in S302), it is determined whether the frequency difference Δν between ν1 and ν2 is f (S303). When Δν = f (Yes in S303), it is determined that it is pattern 2 shown in Fig. 24(2). When Δν ≠ f (No in S303), it is determined whether Δν is an integer multiple of Δf (S304). When Δν is an integer multiple of Δf (Yes in S304), it is determined that it is pattern 4 shown in Fig. 24(4). When Δν is not an integer multiple of Δf (No in S304), it is determined that it is pattern 5 shown in Fig. 24(5).

[0067] When I1 > I2 (No in S302), it is determined whether the frequency difference Δν between ν1 and ν2 is f (S305). When Δν = f (Yes in S305), it is determined that it is pattern 3 shown in Fig. 24(3). When Δν≠f (No in S305), we determine whether Δν is an integer multiple of Δf (S306). When Δν is an integer multiple of Δf (Yes in S306), it is determined to be pattern 7 as shown in Figure 24(7). When Δν is not an integer multiple of Δf (No in S306), it is determined to be pattern 6 as shown in Figure 24(6).

[0068] In pattern 1, use the following equation to determine f b Set (S311). f b =nf-ν1(31) In pattern 2, use the following equation to determine f b Set (S312). f b =nf-ν2(32) In pattern 3, use the following equation to determine f b Set (S313). f b =nf-ν1(33) In pattern 4, use the following equation to determine f b Set (S314). f b =(n-Δν / Δf)f-ν²(34) When pattern 5, use the following equation to determine f b Set (S315). f b ={n+(f-Δν) / Δf+1)f-ν²(35) When pattern 6, use the following equation to determine f b Set (S316). f b ={n-(f-Δν) / Δf-1)f-ν1(36) When pattern 7, use the following equation to determine f b Set (S317). f b =(n-Δν / Δf)f-ν1(37)

[0069] Therefore, in this embodiment, based on the relationship between the seven beat frequencies, the frequency difference f b By setting the appropriate parameters, the distance z to the object to be measured can be calculated.

[0070] In all of the embodiments described above, the optical replication unit 7 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 replication unit 7 receives the probe light reflected from the object under test 5. Therefore, the optical replication unit 7 generates an optical frequency comb consisting of multiple different frequencies, including the frequency of the probe light reflected from the object under test 5. On the other hand, the reference light becomes a single 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.

[0071] As described above, the introduction of the optical replication unit 7 eliminates the limitation of measurement distance due to the receiving bandwidth without reducing the frequency sweep speed. Furthermore, by replicating the incident light into light of multiple different frequencies so that the light and intensity are non-uniform with respect to frequency, the periodic ambiguity of the distance measurement results can be mitigated.

[0072] Furthermore, although the main interferometer 20 in this embodiment is configured to process using a 90-degree optical hybrid 8 in hardware, it may also be processed in software.

[0073] 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]

[0074] The non-contact distance measuring device and method described herein can be applied to the information and communication industry. [Explanation of Symbols]

[0075] 1: Frequency-swept light source 2: Coupler 3: Light Circulator 4: Lens 5: Object to be measured 6: Delay device 7: Optical replication department 8: 90-degree hybrid light 9: Balanced Photo Detector 10: AD Converter 11: Arithmetic section 12, 12A, 12B: RF Synthesizer 13, 13A, 13B: Amplifier 20: Main interferometer 71A, 71B: Modulator 72A, 72B: Coupler 75: Frequency Shifter 76: Delay Line

Claims

1. In a non-contact distance measuring device that determines the distance to an object by irradiating the object with frequency-swept light and measuring the frequency of the beat signal resulting from the interference between the probe light reflected by the object and the reference light, The light generation unit generates a first optical frequency comb with frequency interval f and a second optical frequency comb with frequency interval f + Δf from either the reference light or the probe light, and generates multiple optical frequencies with non-uniform frequency intervals by combining the first and second optical frequency combs. If the aforementioned f is different from an integer multiple of the aforementioned Δf, The light intensity of the first optical frequency comb and the light intensity of the second optical frequency comb are different. By combining the light of multiple frequencies generated in the light generation unit with the other of the reference light or the probe light, multiple beat signals are generated. The distance to the object to be measured is determined by detecting two or more frequencies among the plurality of beat signals. Non-contact distance measuring device.

2. The above f + Δf is less than twice the receiving bandwidth of the main interferometer that detects the frequency of the beat signal. The non-contact distance measuring device according to feature 1.

3. The above f is smaller than the receiving bandwidth of the main interferometer that detects the frequency of the beat signal. The non-contact distance measuring device according to feature 1.

4. By shifting the delay time or frequency of the aforementioned multiple frequencies of light, the aforementioned multiple frequencies of light can be used for distance measurement. The non-contact distance measuring device according to feature 1.

5. In a non-contact distance measurement method that determines the distance to an object by irradiating the object with frequency-swept light and measuring the frequency of the beat signal resulting from the interference between the probe light reflected by the object and the reference light, The light generation unit generates a first optical frequency comb with frequency interval f and a second optical frequency comb with frequency interval f + Δf from either the reference light or the probe light, and combines the first optical frequency comb and the second optical frequency comb to generate light of multiple frequencies with non-uniform frequency intervals. If the aforementioned f is different from an integer multiple of the aforementioned Δf, The light intensity of the first optical frequency comb and the light intensity of the second optical frequency comb are different. By combining the light of multiple frequencies generated in the light generation unit with the other of the reference light or the probe light, multiple beat signals are generated. The distance to the object to be measured is determined by detecting two or more frequencies among the plurality of beat signals. Non-contact ranging method.

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