Optical interference measuring device and program used for optical interference measuring program

Abstract

To provide a technique of suppressing the speckle in an FFT signal by integrating plural FFT signals based on beat signals obtained at a plurality of frequencies, the technique reducing adverse influence of Doppler effects and suppressing the speckle more than existing techniques.SOLUTION: The ratio of the second chirp rate γ2 to the first chirp rate γ1 is set to be the same as the ratio of the second frequency f2 to the first frequency f1. Additionally, a frequency axis transformation of multiplying the frequency spectrum of a second beam by the inverse of the ratio of the second frequency f2 to the first frequency f1 is performed.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to an optical interference measurement apparatus and a program for use in the optical interference measurement apparatus. [Background technology]

[0002] Conventionally, in FMCW-LiDAR, a technique is known in which speckles in an FFT signal are suppressed by integrating multiple FFT signals based on beat signals obtained at multiple frequencies. For example, Non-Patent Document 1 discloses a technique for suppressing speckles by using two light sources with different frequencies. [Prior art documents] [Patent documents]

[0003] [Non-Patent Document 1] Japanese Patent Application Publication No. 5-296511 Summary of the Invention [Problem to be solved by the invention]

[0004] However, according to the inventor's study, since the Doppler effect for the same target differs depending on the frequency, the FFT signals obtained for each frequency are affected by the Doppler effect differently. Non-Patent Document 1 does not mention this point, and therefore the speckle suppression effect is low.

[0005] In view of the above, an object of the present invention is to reduce the adverse effects of the Doppler effect and improve the suppression effect compared to conventional techniques in a technology for suppressing speckle in an FFT signal by accumulating a plurality of FFT signals based on beat signals obtained at a plurality of frequencies. [Means for solving the problem]

[0006] According to one aspect of the present disclosure, an optical interference measurement apparatus includes: a light source (11-14) that outputs a first emitted light (Lt1) that changes with a first chirp rate (γ1) with respect to a first frequency (f1) and a second emitted light (Lt2) that changes with a second chirp rate (γ2) with respect to a second frequency (f2) different from the first frequency; an optical system (21a to 26b) that transmits the first outgoing light and the second outgoing light to a target (TR), combines reflected light (Ltr) generated by reflection of the first outgoing light and the second outgoing light at the target with a first reference light (Lr1) based on the first outgoing light, and outputs the combined light as a first interference light (Lb1), and combines a second reference light (Lr2) based on the second outgoing light with the reflected light, and outputs the combined light as a second interference light (Lb1); a photodetector (31, 32) that receives the first interference light and the second interference light output by the optical system and outputs a first beat signal (Sb1) corresponding to the first interference light and a second beat signal (Sb2) corresponding to the second interference light; a processing circuit (40) that identifies information regarding a position of the target based on a frequency distribution of the first beat signal and a frequency distribution of the second beat signal output by the photodetector, The magnitude relationship of the second chirp rate to the first chirp rate is the same as the magnitude relationship of the second frequency to the first frequency, the processing circuit performs frequency axis conversion so that a relative change in the frequency axis of the frequency spectrum of the second beat signal with respect to a frequency axis of the frequency spectrum of the first beat signal becomes inverse to a magnitude relationship of the second frequency with respect to the first frequency, The processing circuit synthesizes the frequency spectrum of the first beat signal and the frequency spectrum of the second beat signal after the frequency axis conversion, and identifies information about the position of the target based on the synthesized frequency spectrum.

[0007] According to another aspect of the present disclosure, a light source (11-14) that outputs a first emitted light (Lt1) that changes with a first chirp rate (γ1) with respect to a first frequency (f1) and a second emitted light (Lt2) that changes with a second chirp rate (γ2) with respect to a second frequency (f2) different from the first frequency; an optical system (21a to 26b) that transmits the first outgoing light and the second outgoing light to a target (TR), combines reflected light (Ltr) generated by reflection of the first outgoing light and the second outgoing light at the target with a first reference light (Lr1) based on the first outgoing light, and outputs the combined light as a first interference light (Lb1), and combines a second reference light (Lr2) based on the second outgoing light with the reflected light, and outputs the combined light as a second interference light (Lb1); a photodetector (31, 32) that receives the first interference light and the second interference light output by the optical system and outputs a first beat signal (Sb1) corresponding to the first interference light and a second beat signal (Sb2) corresponding to the second interference light; a calculation circuit (44) that specifies information regarding a position of the target based on a frequency distribution of the first beat signal and a frequency distribution of the second beat signal output by the photodetector, The magnitude relationship of the second chirp rate to the first chirp rate is the same as the magnitude relationship of the second frequency to the first frequency, The arithmetic circuit, a conversion unit (S130) that performs frequency axis conversion so that a relative change in the frequency axis of the frequency spectrum of the second beat signal with respect to the frequency axis of the frequency spectrum of the first beat signal becomes the opposite of the magnitude relationship of the second frequency with respect to the first frequency; and The frequency spectrum of the first beat signal and the frequency spectrum of the second beat signal after the frequency axis conversion are synthesized, and the signal processing unit functions as an identification unit (S140, S150) that identifies information regarding the position of the target based on the synthesized frequency spectrum.

[0008] This makes it possible to suppress the adverse effect of the Doppler effect caused by the difference between the first frequency f1 and the second frequency f2 on speckle reduction.

[0009] The reference symbols in parentheses attached to each component indicate an example of the correspondence between the component and specific components described in the embodiments described below. [Brief description of the drawings]

[0010] [Figure 1] 1 is a configuration diagram of an optical interference measurement apparatus according to a first embodiment. [Diagram 2] FIG. 2 is a configuration diagram of a processing circuit. [Diagram 3] 13 is a flowchart of a process executed by an arithmetic circuit. [Figure 4] FIG. 1 is a diagram illustrating an example of sampling and FFT. [Diagram 5] 11 is a graph showing an example of changes over time in the frequencies of transmitted light, reflected light, and reference light. [Figure 6] FIG. 1 is a diagram illustrating frequency axis conversion. [Figure 7] 1 is a graph showing the relationship between roughness and wavelength difference satisfied by two correlation coefficients at a wavelength of 1550 nm. [Figure 8] 1 is a graph showing the relationship between roughness and wavelength difference satisfied by two correlation coefficients at a wavelength of 1310 nm. [Figure 9] FIG. 11 is a configuration diagram of a processing circuit in the second embodiment. [Figure 10] FIG. 5 is a diagram showing an example of sampling and FFT in the same format as FIG. [Figure 11] 13 is a flowchart of a process executed by an arithmetic circuit. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] (First embodiment) The first embodiment will be described below. An optical interference measurement device 100 according to this embodiment is mounted on a vehicle and detects objects around the vehicle using the FMCW-LiDAR method. As shown in FIG. 1, the optical interference measurement device 100 has a first laser oscillator 11, a second laser oscillator 12, a first chirp circuit 13, and a second chirp circuit 14. These constitute a light source in the optical interference measurement device 10.

[0012] The first laser oscillator 11 generates a laser beam having a predetermined first frequency f1 (e.g., wavelengths of 1500 nm and 1350 nm). The second laser oscillator 12 generates and outputs a laser beam having a predetermined second frequency f2 (e.g., wavelengths of 1500 nm and 1350 nm). The first frequency and the second frequency are different.

[0013] The first chirp circuit 13 performs frequency modulation with a predetermined modulation amplitude (for example, 0.001 nm width, 0.005 nm width, etc.) on the laser light of the first frequency f1 generated by the first laser oscillator 11. As a result, the frequency-modulated laser light output from the first laser oscillator 11 becomes the first emitted light Lp1 whose frequency increases and decreases. More specifically, the first emitted light Lp1 becomes a triangular wave whose chirp rate is a predetermined first chirp rate γ1. That is, the first emitted light Lp1 becomes a triangular wave having an up-chirp period in which the frequency increases linearly with the first chirp rate γ1, and then a down-chirp period in which the frequency decreases linearly with the first chirp rate γ1.

[0014] The second chirp circuit 14 performs frequency modulation with a predetermined modulation amplitude (for example, 0.001 nm width, 0.005 nm width, etc.) on the laser light of the second frequency f2 generated by the second laser oscillator 12. As a result, the frequency-modulated laser light output from the second laser oscillator 12 becomes the second emitted light Lp2 whose frequency increases and decreases. More specifically, the second emitted light Lp2 becomes a triangular wave whose chirp rate is a predetermined second chirp rate γ2. That is, the second emitted light Lp2 becomes a triangular wave having an up-chirp period in which the frequency increases linearly with the second chirp rate γ2, and then a down-chirp period in which the frequency decreases linearly with the second chirp rate γ2.

[0015] 1, the optical interference measurement device 100 has couplers 21a and 21b, an optical multiplexer 22, a circulator 23, a beam scanner 24, an optical splitter 25, and couplers 26a and 26b. These components constitute the optical system of the optical interference measurement device 100. The optical paths between the components in the optical system may be formed by, for example, optical fibers.

[0016] The branching coupler 21a is an optical component that outputs a portion of the first output light Lp1 output from the first laser oscillator 11 to the optical multiplexer 22, and branches another portion of the first output light Lp1 and outputs it to the coupler 26a as the first reference light Lr1.

[0017] The branching coupler 21b is an optical component that outputs a portion of the second output light Lp2 output from the second laser oscillator 12 to the optical multiplexer 22, and branches another portion of the second output light Lp2 and outputs it to the coupler 26b as the second reference light Lr2.

[0018] The optical multiplexer 22 is an optical component (e.g., a wavelength multiplexing / demultiplexing module) that multiplexes light having different frequencies, namely, the first outgoing light Lp1 output from the coupler 21a and the second outgoing light Lp2 output from the coupler 21b, and outputs the combined light to the circulator 23 as outgoing light Lp.

[0019] The circulator 23 is an optical component having three ports. The circulator 23 outputs the emitted light Lp input from the optical multiplexer 22 to a first port to the beam scanner 24 from a second port, and outputs the reflected light Ltr input from the beam scanner 24 to the second port to the optical demultiplexer 25 from a third port.

[0020] The beam scanner 24 is a scanning device capable of beam scanning a detection range including the target TG according to a predetermined scanning pattern by manipulating the direction of the emitted light Lp output from the circulator 23. The beam scanner 24 also changes the direction of reflected light Ltr generated when the emitted light Lp is reflected by the target TG, and guides the reflected light Ltr to the circulator 23. The target TG may be, for example, a preceding vehicle traveling in front of the vehicle, a following vehicle traveling behind the vehicle, people and obstacles around the vehicle, etc.

[0021] For example, the beam scanner 24 is a scanning mirror that can reflect the output light Lp from the circulator 3 and manipulate the direction of the reflected output light Lp to perform beam scanning on a detection range including the target TG. In this case, the beam scanner 24 reflects reflected light Ltr generated when the output light Lp is reflected by the target TG and guides it to the circulator 23. The beam scanner 24 may be rotatable by, for example, one or more galvanometers, may rotate a polygonal mirror, or may be realized by another configuration. Alternatively, the beam scanner 24 may be configured by a device other than a mirror.

[0022] The optical splitter 25 is an optical component (e.g., a wavelength multiplexing / splitting module) that splits the reflected light Ltr, which enters the circulator 23 from the beam scanner 24 and is output from the circulator 23, into light beams according to wavelength and outputs the split light beams to the couplers 26a and 26b. The optical splitter 25 outputs light having a first frequency f1 and a frequency nearby the first frequency f1 of the reflected light Ltr to the coupler 26a, and outputs light having a second frequency f2 and a frequency nearby the second frequency f2 of the reflected light Ltr to the coupler 26b.

[0023] The combining coupler 26a combines the reflected light Ltr from the optical splitter 25 with the first reference light Lr1 from the coupler 21a, and outputs the first interference light Lb1 obtained as a result of the combination. The combining coupler 26b combines the reflected light Ltr from the optical splitter 25 with the second reference light Lr2 from the coupler 21b, and outputs the second interference light Lb2 obtained as a result of the combination.

[0024] 1, the optical interference measurement device 100 has a first optical detector 31, a second optical detector 32, and a processing circuit 40. The first optical detector 31 is a light-receiving element that detects the first interference light Lb1 output from the coupler 26a, generates a first beat signal Sb1 as an electrical signal corresponding to the first interference light Lb1, and outputs the generated first beat signal Sb1 to the processing circuit 40. The second optical detector 32 is a light-receiving element that detects the second interference light Lb2 output from the coupler 26b, generates a second beat signal Sb2 as an electrical signal corresponding to the first interference light Lb1, and outputs the generated second beat signal Sb2 to the processing circuit 40.

[0025] The processing circuit 40 is an electronic circuit that receives the first beat signal Sb1 and the second beat signal Sb2 output from the first photodetector 31 and the second photodetector 32. The processing circuit 40 calculates the behavior of the target TG based on the received first beat signal Sb1 and second beat signal Sb2. The behavior of the target TG includes the distance from the vehicle to the target TG (hereinafter simply referred to as the distance of the target TG), the relative speed of the target TG with respect to the vehicle (hereinafter simply referred to as the speed of the target TG), etc.

[0026] 2, the processing circuit 40 has A / D converters 41 and 42, a storage device 43, and an arithmetic circuit 44. The A / D converter 41 is a circuit that samples the first beat signal Sb1 as an analog signal output from the first photodetector 31 at a predetermined sampling frequency and converts it into digital data. The A / D converter 42 is a circuit that samples the second beat signal Sb2 as an analog signal output from the second photodetector 32 at a predetermined sampling frequency and converts it into digital data. The A / D converter 41 corresponds to the first A / D converter, and the A / D converter 42 corresponds to the second A / D converter.

[0027] The storage device 43 has a volatile memory such as a RAM and a non-volatile memory such as a flash memory and a ROM. The non-volatile memory stores programs and the like to be read and executed by the arithmetic circuit 44. Furthermore, the non-volatile memory stores the values ​​of the first frequency f1, the second frequency f2, the first chirp rate γ1, and the second chirp rate γ2 as fixed values ​​in advance.

[0028] The arithmetic circuit 44 reads out and executes a predetermined program from the nonvolatile memory of the storage device 43, and uses the volatile memory as a working area during the execution to realize various functions of the processing circuit. Specifically, the arithmetic circuit 44 acquires the first beat signal Sb1 and the second beat signal Sb2 converted into digital data by the A / D converters 41 and 42, and calculates behaviors such as the position and speed of the target TR based on the first beat signal Sb1 and the second beat signal Sb2.

[0029] The information on these behaviors is output to other devices in the vehicle and is used in the other devices to notify users such as occupants, control the behavior of the vehicle, etc. Alternatively, the processing circuit 40 may notify users such as occupants using an alarm device based on the information on these behaviors, and drive, brake, and steer the vehicle using actuators such as a drive device, a braking device, and a steering device. The arithmetic circuit 44 may not be a general-purpose circuit that executes a program, but may be a dedicated circuit configured to realize predetermined processing content.

[0030] Here, the relationship between the first chirp rate γ1 and the second chirp rate γ2 will be described. The first chirp rate γ1 and the second chirp rate γ2 are as follows: γ2=(f2 / f1)×γ1 (1) Specifically, the first chirp rate γ1 and the second chirp rate γ2 that satisfy this relationship are stored in the non-volatile memory of the storage device 43.

[0031] Furthermore, the first chirp circuit 13 and the second chirp circuit 14 may be configured in advance to realize the first chirp rate γ1 and the second chirp rate γ2, respectively, that are in this relationship.

[0032] Alternatively, the first chirp circuit 13 and the second chirp circuit 14 may be controlled by the processing circuit 40 so that the first chirp rate γ1 and the second chirp rate γ2 having this relationship are realized. In this case, the arithmetic circuit 44 reads a predetermined control program from the nonvolatile memory and executes it to output signals instructing the first chirp circuit 13 and the second chirp circuit 14 to have the first chirp rate γ1 and the second chirp rate γ2 having this relationship. At this time, the arithmetic circuit 44 may refer to values ​​stored in the nonvolatile memory as the first chirp rate γ1 and the second chirp rate γ2. Alternatively, the arithmetic circuit 44 may calculate the second chirp rate γ2 from the first frequency f1, the second frequency f2, and the first chirp rate γ1 stored in the nonvolatile memory by the above formula (1).

[0033] The operation of the optical interference measuring device 100 configured as described above will be described below. When the vehicle IG is turned on, the optical interference measuring device 100 starts operating. Then, the first laser oscillator 11 emits a first output light Lp1 having a first frequency f1 as a triangular wave with a first chirp rate γ1. Then, a part of the first output light Lp1 is output to the target TG via the coupler 21a, the optical multiplexer 22, the circulator 23, and the beam scanner 24 in this order. Also, another part of the first output light Lp1 is branched by the coupler 21a as a first reference light Lr1 and is guided to the coupler 26a.

[0034] Furthermore, of the reflected light Ltr generated when the first output light Lp1 is reflected by the target TG, the light of the first frequency f1 and its neighboring frequencies passes through the beam scanner 24, the circulator 23, and the optical splitter 25 in this order, and is then combined with the first reference light Lr1 by the coupler 26a. Then, the first interference light Lb1 generated by the combination is output from the coupler 26a to the first photodetector 31. The first photodetector 31 receives the first interference light Lb1 and outputs a first beat signal Sb1 corresponding to the first interference light Lb1 to the processing circuit 40.

[0035] Similarly, a second output light Lp2 having a start frequency of a second frequency f2 and a triangular wave with a second chirp rate γ2 is emitted from the second laser oscillator 12. Then, a part of the second output light Lp2 is output to the target TG via the coupler 21b, the optical multiplexer 22, the circulator 23, and the beam scanner 24 in this order. In addition, another part of the second output light Lp2 is branched by the coupler 21b as a second reference light Lr2 and guided to the coupler 26b.

[0036] Furthermore, of the reflected light Ltr generated when the second output light Lp2 is reflected by the target TG, the light of the second frequency f2 and its neighboring frequencies passes through the beam scanner 24, the circulator 23, and the optical splitter 25 in this order, and is then combined with the second reference light Lr2 by the coupler 26b. Then, the second interference light Lb2 generated by the combination is output from the coupler 26b to the second photodetector 32. The second photodetector 32 receives the second interference light Lb2 and outputs a second beat signal Sb2 corresponding to the second interference light Lb2 to the processing circuit 40.

[0037] While the light source, the optical system, the first photodetector 31, and the second photodetector 32 are operating, the A / D converter 41 converts the first beat signal Sb1 into digital data, and the A / D converter 42 converts the second beat signal Sb2 into digital data in the processing circuit 40. The sampling frequencies of the A / D converters 41 and 42 are the same.

[0038] At this time, the arithmetic circuit 44 executes the program from the nonvolatile memory described above to execute the processing shown in Fig. 3. First, in step S110, the arithmetic circuit 44 acquires the first beat signal Sb1 and the second beat signal Sb2, which have been digitized by the A / D converters 41 and 42, for the up-chirp period and down-chirp period of one triangular wave.

[0039] Next, in step S120, the arithmetic circuit 44 performs an FFT operation in the up chirp period and an FFT operation in the down chirp period on the acquired first beat signal Sb1. These FFT operations then acquire the frequency spectrum of the first beat signal Sb1 in the up chirp period and the down chirp period. Similarly, an FFT operation in the up chirp period and an FFT operation in the down chirp period are performed on the acquired second beat signal Sb2 to acquire the frequency spectrum of the second beat signal Sb2 in the up chirp period and the down chirp period.

[0040] Here, the relationship between the FFT calculation for the first beat signal Sb1 and the FFT calculation for the second beat signal Sb2 will be described with reference to FIG. 4. Hereinafter, the sampling frequency in the A / D converter 41 is defined as f s1 The sampling time and the number of samples in the FFT calculation for the first beat signal Sb1 are T s1 , N1. Between these, N1=f s1 ×T s1 In addition, in the FFT calculation using these FFT parameters, the bin width f of the obtained frequency spectrum is Bin1 f Bin1 =1 / T s1 =f s1 / N1. Also, the highest frequency of the resulting frequency spectrum (i.e., the Nyquist frequency) is f s1 / 2, and the number of frequency bins is N1 / 2.

[0041] The sampling frequency of the A / D converter 42 is f s2The sampling time and the number of samples in the FFT calculation for the second beat signal Sb2 are T s2 , N2. Between these, N2=f s2 ×T s2 In addition, in the FFT calculation using these FFT parameters, the bin width f of the obtained frequency spectrum is Bin2 f Bin2 =1 / T s2 =f s2 / N2. The highest frequency in the resulting frequency spectrum (i.e., the Nyquist frequency) is f s2 / 2, and the number of frequency bins is N2 / 2.

[0042] In this embodiment, the number of samplings N2 and the sampling time T s2 teeth, N2 = (f1 / f2) × N1 T s2 =(f1 / f2)×T s1 As a result, the bin width f Bin1 and the bin width f Bin2 f Bin2= (f2 / f1)×f Bin1 The following relationship arises.

[0043] For example, as shown in Fig. 4, when the wavelength λ1 corresponding to the first frequency f1 is 1500 nm and the wavelength λ2 corresponding to the second frequency f2 is 1350 nm, f1 / f2 = 0.9, and γ2 = (f2 / f1) × γ1 = 1.11 × γ1. s1 =f s2 = 10MHz, T s1 = 100μs, N1 = 1000. In this case, f Bin1 = 1 / 1000μs = 10kHz. Note that f1×λ1 = f2×λ2 = c, where c is the speed of light.

[0044] As described above, the number of samples N2 and the sampling time T s2 teeth, N2 = (f1 / f2) × N1 = 900 Ts2 =(f1 / f2)×T s1 = 90μs As a result, the bin width f Bin2 f Bin2= (f2 / f1)×f Bin1 = 11.1kHz, and the bin width f Bin1 does not match.

[0045] In this figure, N1 rectangles Tb1 lined up horizontally at the top represent a set of sampling timings of the first beat signal Sb1 used in one FFT calculation. The order N of each sampling timing is shown below the N1 rectangles Tb1. Further below that, the time T of each sampling timing is shown.

[0046] In addition, in this figure, the multiple rectangles Fb1 arranged in a row second from the top represent a set of frequency bins resulting from FFT calculation of the first beat signal Sb1. Below the multiple rectangles Tb1, the frequency Δf1 of each frequency bin is shown.

[0047] In this figure, the N2 rectangles Tb2 arranged in a row third from the top represent a set of sampling timings of the second beat signal Sb2 used in one FFT calculation. The order N of each sampling timing is shown below the N2 rectangles Tb2. Further below that, the time T of each sampling timing is shown.

[0048] In addition, in this figure, the multiple rectangles Fb2 lined up horizontally fourth from the top represent a set of frequency bins resulting from FFT calculation of the second beat signal Sb2. Below the multiple rectangles Tb2, the frequency Δf2 of each frequency bin before frequency axis conversion is shown. Furthermore, at the bottom, the frequency Δf2' of each frequency bin after frequency axis conversion is shown. In addition, in this figure, the channel that acquires and processes the first beat signal Sb1 is called the f1 channel side, and the channel that acquires and processes the second beat signal Sb2 is called the f2 channel side.

[0049] Next, in step S130, the arithmetic circuit 44 performs a similar frequency axis conversion on the frequency spectra of the second beat signal Sb2 in the up-chirp period and the down-chirp period. Specifically, for a variable Δf2 on the frequency axis in the frequency spectrum of the second beat signal Sb2, Δf2'=(f1 / f2)×Δf2 (2) The frequency spectrum obtained as a result of this coordinate transformation is called the corrected frequency spectrum.

[0050] With this frequency transformation, the corrected frequency spectrum has a bin width of f Bin2 Since the bin width f Bin1 In the example in Figure 4, the bin width f Bin2 is 11.1 bin width f Bin2 × 0.9 = 10 kHz, and the bin width f Bin1 It becomes the same as:

[0051] Next, in step S140, the arithmetic circuit 44 integrates the frequency spectrum of the first beat signal Sb1 and the corrected frequency spectrum of the second beat signal Sb2 during the up-chirp period, and integrates the frequency spectrum of the first beat signal Sb1 and the corrected frequency spectrum during the down-chirp period.

[0052] In this case, the integration involves adding up the intensities in bins that cover the same frequency range. As mentioned above, the bin width f Bin1 , f Bin2 This is possible because the values ​​of are the same. Then, the arithmetic circuit 44 identifies the peak frequencies in the up-chirp period and the down-chirp period in the frequency spectrum after the integration as the beat frequencies.

[0053] Next, in step S150, the arithmetic circuit 44 calculates the position, velocity, etc. of the target TR based on the beat frequencies in the identified up-chirp period and down-chirp period. After step S150, the process returns to step S110.

[0054] Here, the significance of setting the relationship between the first chirp rate γ1 and the second chirp rate γ2 as in the above formula (1) and of performing frequency axis conversion as in the above formula (2) in step S130 in Fig. 3 will be explained. Conventionally, a technique is known for suppressing speckles in an FFT signal by integrating multiple FFT signals based on beat signals obtained at multiple different frequencies. However, when the Doppler effect of the target TR is taken into consideration, the amount of Doppler shift differs between multiple different frequencies.

[0055] This is because the Doppler effect of the light reflected from the target TR moving at a speed v changes the frequency f of the reflected light to a frequency fx according to the following equation: fx = f × β (3) β={1-(v / c) 2} 1 / 2 / {1-(v / c)cosθ} (4) Here, c is the speed of light, and θ is the angle between the line of sight from the observer to the target TR and the direction of relative movement of the target TR.

[0056] Therefore, in principle, the peak frequencies are the same between the FFT spectra of multiple beat signals obtained using multiple light sources when there is no Doppler shift, but in principle, they are different when there is the Doppler effect. Therefore, if the FFT spectra of multiple beat signals are integrated without any measures when there is a Doppler shift, spectra that are not the same peak will be integrated, and the speckle reduction effect may be insufficient. To address this, the relationship between the first chirp rate γ1 and the second chirp rate γ2 is set as shown in the above formula (1), and frequency axis conversion is performed in step S130 of Figure 3 as shown in the above formula (2).

[0057] In this way, even if multiple reflected lights with different frequencies are subjected to different Doppler effects, a peak will ultimately be obtained at the same position on the frequency axis in the frequency spectrum in principle. Therefore, by integrating the signals in step S140, the speckle reduction effect can be improved.

[0058] The reason why such an effect is obtained will be described below. First, the beat frequencies Δf1 and Δf2 obtained by FMCW distance measurement using the first output light Lp1 and the second output light Lp2 whose start frequencies are the first frequency f1 and the second frequency f2, respectively, are given by Δf1=γ1×Δt+β×f1 (5) Δf2=γ2×Δt+β×f2 (6) The following relationship holds. Here, β follows the above-mentioned formula (3). Also, Δt is the time length from the start of emission of the first outgoing light Lp1 and the second outgoing light Lp2 to the start of arrival of the reflected light Ltr (i.e., the detection distance) as shown in FIG. 5.

[0059] Here, if the first chirp rate γ1 and the second chirp rate γ2 are set as in equation (1), equation (6) becomes: Δf2=(f2 / f1)×γ1×Δt+β×f2 (7) Furthermore, when the frequency axis transformation such as that in equation (2) is applied to equation (7), Δf2'=(f1 / f2)×γ1×{(f2 / f1)×γ1×Δt+β×f2} =γ1×Δt+β×Δf1 (8) Since the right-hand side of equation (5) is the same as the right-hand side of equation (8), in principle, both equations can produce beat signals with the same beat frequency.

[0060] At this time, the frequency axis conversion has the effect of bringing the peak frequency Δf2 of the second beat signal Sb2 closer to the peak frequency Δf2' of the first beat signal Sb1, as shown in FIG.

[0061] The roughness of the target surface is generally expressed as σ hAssuming ~30um, as shown in Fig. 7, in the wavelength band of 1550nm, if a wavelength difference of Δλ~40nm is provided, and as shown in Fig. 8, in the 1310nm band, if a wavelength difference of Δλ~30nm is provided, the correlation of the signals can be made almost zero. That is, the speckle reduction effect is maximized. Note that Δλ is the absolute value of the difference between the wavelength λ1 corresponding to the first frequency f1 and the wavelength λ2 corresponding to the second frequency f2. Fig. 7 shows the relationship between Δλ and roughness σ when the correlation coefficient is zero and becomes 1 / e 2 at that time. h Fig. 8 shows the relationship between the correlation coefficient being zero and becoming 1 / e 2 at that time, and the relationship between Δλ and roughness σ h is shown.

[0062] To reduce speckle, it is desirable to lower the correlation coefficient. For this purpose, it is desired to increase Δλ, but if so, the influence of the Doppler effect becomes large. In this embodiment, since the influence of the Doppler effect can be suppressed, the demerit of increasing Δλ is reduced.

[0063] As described above, the magnitude relationship between the second chirp rate γ2 and the first chirp rate γ1 is the same as the magnitude relationship between the second frequency f2 and the first frequency f1. That is, if f1 < f2, then γ1 < γ2, and if f1 > f2, then γ1 > γ2.

[0064] Also, frequency axis conversion is performed so that the change in the frequency axis of the frequency spectrum of the second beat signal Sb2 with respect to the frequency axis of the frequency spectrum of the first beat signal Sb1 is opposite to the magnitude relationship between the second frequency f2 and the first frequency f1. That is, if f1 < f2, the frequency axis of the frequency spectrum of the second beat signal Sb2 is frequency axis converted with a positive coefficient less than 1, and if f1 > f2, the frequency axis of the frequency spectrum of the second beat signal Sb2 is frequency axis converted with a coefficient greater than 1.

[0065] By being configured in this way, the adverse influence of the Doppler effect caused by the difference between the first frequency f1 and the second frequency f2 on speckle reduction can be suppressed.

[0066] [2] More specifically, the ratio of the second chirp rate γ2 to the first chirp rate γ1 (i.e., γ2 / γ1) is the same as the ratio of the second frequency f2 to the first frequency f1 (i.e., f2 / f1). In addition, a frequency axis conversion is performed on the frequency spectrum of the second beat signal Sb2 by multiplying it by the inverse of the ratio of the second frequency f2 to the first frequency f1 (i.e., f1 / f2). In this way, the adverse effect of the Doppler effect on speckle reduction can be suppressed with high efficiency.

[0067] In addition, "the ratio of the second chirp rate γ2 to the first chirp rate γ1 is the same as the ratio of the second frequency f2 to the first frequency f1" includes not only strictly the same, but also substantially the same in light of common technical knowledge, for example, if the ratio of the latter to the former is within ±5%.

[0068] In addition, "multiplying by the reciprocal of the ratio of the second frequency f2 to the first frequency f1" includes not only the exact reciprocal, but also a number that is not exactly the same but is substantially the reciprocal in light of common technical knowledge. For example, if it is within ±5% of the exact reciprocal, it can be said to be substantially the reciprocal.

[0069] [3] Furthermore, the arithmetic circuit 44 calculates the frequency spectrum of the first beat signal Sb1 and the frequency spectrum of the second beat signal by performing FFT. Then, it performs frequency axis conversion on the calculated frequency spectrum of the second beat signal Sb2. In this way, it is possible to perform frequency axis conversion as a numerical calculation process.

[0070] [4] In addition, the FFT parameters in the calculation circuit 44 are N2 = (f1 / f2) × N1, T s2 =(f1 / f2)×T s1 By doing this, f Bin1 and f Bin2 Since the frequency bins are the same, the correspondence between the frequency bins becomes self-evident, and the synthesis process becomes simple.

[0071] [5] In addition, when the absolute value of the difference between the wavelength λ1 and the wavelength λ2 is 30 nm or more, the speckle reduction effect is high, as described above. In fact, in the example of Figure 4, the absolute value of the difference between the wavelength λ1 and the wavelength λ2 is 150 nm.

[0072] In this embodiment, the arithmetic circuit functions as a conversion unit by executing step S130, and functions as a specification unit by executing steps S140 and S150.

[0073] Second embodiment Next, a second embodiment will be described with reference to Fig. 9 to Fig. 11. In the first embodiment, the second beat signal Sb2 is subjected to an FFT operation, and then the frequency axis conversion shown in equation (2) is performed. In contrast, in this embodiment, the frequency axis conversion shown in equation (2) is performed at the stage where the second beat signal Sb2 is digitized in the A / D converter 42.

[0074] Specifically, in this embodiment, the sampling frequency f s1 and the sampling frequency f s2 The sampling frequency f s1 corresponds to the first sampling frequency, and the sampling frequency f s2 corresponds to the second sampling frequency.

[0075] The A / D converters 41 and 42 each have a sampling frequency f s1 , f s2 Alternatively, the sampling frequency f may be set according to the control of the calculation circuit 44. s1 , f s2 In the latter case, the storage device 43 may be adapted to operate at a sampling frequency f s1 , f s2 The calculation circuit 44 then receives the sampling frequency f s1 , f s2and controls A / D converters 41 and 42 so as to realize the read values.

[0076] The other configurations are the same as those of the first embodiment. For example, the relationship between the first chirp rate γ1 and the second chirp rate γ2 is also the same as that of the first embodiment. The operation of this embodiment is the same as that of the first embodiment from the time when the first output light Lp1 and the second output light Lp2 are emitted, until the first beat signal Sb1 is input to the A / D converter 41 and the second beat signal Sb2 is input to the A / D converter 42.

[0077] The A / D converter 41 has a sampling frequency f s1 The A / D converter 42 outputs the first beat signal Sb1 as digital data sampled at a sampling frequency f s2 The second beat signal Sb2 is output as digital data sampled at step S2.

[0078] For example, as shown in FIG. 10, the wavelength λ1 corresponding to the first frequency f1 is 1500 nm, the wavelength λ2 corresponding to the second frequency f2 is 1350 nm, and the sampling frequency f s1 is 10MHz, sampling frequency f s2 may be 11.1 MHz.

[0079] Moreover, the arithmetic circuit 44 executes a program from the nonvolatile memory described above, thereby executing the process shown in Fig. 11 instead of the process shown in Fig. 3. In this process, in step S110, the arithmetic circuit 44 acquires the first beat signal Sb1 and the second beat signal Sb2, which have been digitized by the A / D converters 41 and 42, for the up-chirp period and the down-chirp period of one triangular wave.

[0080] Next, in step S120, the arithmetic circuit 44 performs an FFT operation in the up chirp period and an FFT operation in the down chirp period on the acquired first beat signal Sb1. These FFT operations then acquire the frequency spectrum of the first beat signal Sb1 in the up chirp period and the down chirp period. Similarly, an FFT operation in the up chirp period and an FFT operation in the down chirp period are performed on the acquired second beat signal Sb2 to acquire the frequency spectrum of the second beat signal Sb2 in the up chirp period and the down chirp period.

[0081] Here, the relationship between the FFT calculation for the first beat signal Sb1 and the FFT calculation for the second beat signal Sb2 will be described. In this embodiment, the number of samplings N2 and the sampling time T s2 teeth, N2=N1 T s2 =(f1 / f2)×T s1 As a result, the bin width f Bin1 and the bin width f Bin2 is physically f Bin2 =(f2 / f1)×f Bin1 However, in this embodiment, when the FFT operation is performed on the second beat signal Sb2, the input second beat signal Sb2 is sampled at a frequency f s1 It is assumed that the data was sampled at

[0082] For example, suppose that only sampled data is input to the FFT calculation, and no information on the sampling frequency is input. In such a case, the FFT calculation result is unconscious of the actual sampling frequency of the input data, in other words, the sampling frequency of the input data is fixed (for example, always 10 MHz) regardless of the actual frequency. When the FFT calculation is performed in this way, the second beat signal Sb2 is generated at the sampling frequency f s1Deeming the first beat signal Sb1 and the second beat signal Sb2 to have been sampled at 1000 Hz means that the results of the FFT calculation are used as is for both the first beat signal Sb1 and the second beat signal Sb2.

[0083] For example, in the example of Figure 10, f Bin1 = 10kHz, f Bin2= (f2 / f1)×f Bin1 = 11.2 kHz. However, the second beat signal Sb2 has a sampling frequency f s1 By considering it as sampled at f Bin2 = 10kHz. That is, f Bin1 =f Bin2 It becomes.

[0084] In this embodiment, N1=N2, T s2 =(f1 / f2)×T s1 In other words, this is equivalent to the frequency axis transformation shown in equation (2) being performed as shown in the dashed line frame in FIG.

[0085] In this figure, N1 rectangles Tb1 lined up horizontally at the top represent a set of sampling timings of the first beat signal Sb1 used in one FFT calculation. The order N of each sampling timing is shown below the N1 rectangles Tb1. Further below that, the time T of each sampling timing is shown.

[0086] In addition, in this figure, the multiple rectangles Fb1 arranged in a row second from the top represent a set of frequency bins resulting from FFT calculation of the first beat signal Sb1. Below the multiple rectangles Tb1, the frequency Δf1 of each frequency bin is shown.

[0087] In this figure, the N2 rectangles Tb2 arranged in a row third from the top represent a set of sampling timings of the second beat signal Sb2 used in one FFT calculation. The order N of each sampling timing is shown below the N2 rectangles Tb2. Further below that, the time T of each sampling timing is shown.

[0088] In addition, in this figure, the multiple rectangles Fb2 arranged in a row fourth from the top represent a set of frequency bins resulting from FFT calculation performed on the second beat signal Sb2. Below the multiple rectangles Tb2, the actual physical frequency Δf2 (i.e., not handled by the arithmetic circuit 44) of each frequency bin is shown. Furthermore, at the bottom, the numerical frequency Δf2 (i.e., handled by the arithmetic circuit 44) of each frequency bin is shown. In addition, in this figure, the channel that acquires and processes the first beat signal Sb1 is referred to as the f1 channel side, and the channel that acquires and processes the second beat signal Sb2 is referred to as the f2 channel side.

[0089] After step S120, the arithmetic circuit 44 proceeds to step S140. The contents of steps S140 and S150 are the same as those of steps S140 and S150 in FIG.

[0090] [1] As mentioned above, the sampling frequency f s1 Sampling frequency f s2 The magnitude relationship between the first frequency f1 and the second frequency f2 is the same as that between the first frequency f1 and the second frequency f2. <f2ならfs1<fs2であり、f1> f2 is f s1 >f s2 In this way, by adjusting the sampling frequency before FFT, the frequency axis of the frequency spectrum of the second beat signal Sb2 is adjusted relative to the frequency axis of the frequency spectrum of the first beat signal Sb1, i.e., frequency conversion is substantially realized. By adjusting the frequency axis with the sampling frequency, the bin width f Bin1 and the bin width f Bin2 will automatically be the same.

[0091] More specifically, the sampling frequency f s1 Sampling frequency f s2The ratio of the first frequency f1 to the second frequency f2 is the same as the ratio of the second frequency f2 to the first frequency f1. However, this does not only mean that they are strictly the same, but also includes the case where they are not strictly the same but are substantially the same in light of common technical knowledge. For example, if the ratio of the latter to the former is within ±5%, it can be said that the two are substantially the same. Note that the same effects can be obtained from the same configurations and operations in this embodiment and the first embodiment.

[0092] (Other embodiments) The present invention is not limited to the above-described embodiment, and can be modified as appropriate. The above-described embodiments are not unrelated to each other, and can be combined as appropriate, except when the combination is clearly impossible. In the above-described embodiments, the elements constituting the embodiment are not necessarily essential, except when it is specifically stated that they are essential or when it is clearly considered to be essential in principle. In the above-described embodiments, when the numbers, values, amounts, ranges, etc. of the components of the embodiment are mentioned, they are not limited to the specific numbers, except when it is specifically stated that they are essential or when it is clearly limited to a specific number in principle. In the above-described embodiments, when it is described that the external environment information of the vehicle (e.g., humidity outside the vehicle) is acquired from a sensor, it is also possible to eliminate the sensor and receive the external environment information from a server or cloud outside the vehicle. Alternatively, it is also possible to eliminate the sensor, acquire related information related to the external environment information from a server or cloud outside the vehicle, and estimate the external environment information from the acquired related information. In particular, when multiple values ​​are exemplified for a certain quantity, it is also possible to adopt a value between the multiple values, except when it is specifically stated otherwise or when it is clearly impossible in principle. In addition, in each of the above embodiments, when referring to the shape, positional relationship, etc. of components, etc., the shape, positional relationship, etc. are not limited to those shapes, positional relationships, etc., except when specifically stated or when they are fundamentally limited to a specific shape, positional relationship, etc., the present invention also allows the following modifications and modifications within an equivalent range to each of the above embodiments. Note that the following modifications can be independently applied or not applied to the above embodiments. In other words, any combination of the following modifications can be applied to the above embodiment.

[0093] (Variation 1) In the above embodiment, the ratio of the second chirp rate γ2 to the first chirp rate γ1 (i.e., γ2 / γ1) is substantially the same as the ratio of the second frequency f2 to the first frequency f1. However, this is not necessarily the case. In other words, it is sufficient that the magnitude relationship of the second chirp rate γ2 to the first chirp rate γ1 (i.e., whether the latter is large or small) is the same as the magnitude relationship of the second frequency f2 to the first frequency f1. In this way, the influence of the Doppler effect on speckle reduction can be suppressed to some extent.

[0094] (Variation 2) In the above embodiment, a frequency axis conversion is performed by multiplying the frequency spectrum of the second beat signal Sb2 by a substantial inverse of the ratio of the second frequency f2 to the first frequency f1 (i.e., f1 / f2). However, it is not necessary to multiply by a substantial inverse. In other words, it is sufficient that the change in the relative frequency axis of the frequency spectrum of the second beat signal Sb2 with respect to the frequency axis of the frequency spectrum of the first beat signal Sb1 is opposite to the magnitude relationship of the second frequency f2 with respect to the first frequency f1. In this way, the influence of the Doppler effect on speckle reduction can be suppressed to some extent.

[0095] (Variation 3) In the second embodiment, the sampling frequency f s1 Sampling frequency f s2 The magnitude relationship between the sampling frequency f1 and the second frequency f2 is substantially the same as the magnitude relationship between the sampling frequency f1 and the second frequency f2. However, this is not necessarily the case. s1 Sampling frequency f s2 (i.e., whether the latter is large or small) and the relationship between the first frequency f1 and the second frequency f2 should be the same. In this way, the influence of the Doppler effect on speckle reduction can be suppressed to some extent.

[0096] (Variation 4) In the above embodiment, the frequency axis conversion is performed on the frequency spectrum of the second beat signal Sb2. However, the frequency axis conversion may be performed on the frequency spectrum of the first beat signal Sb1. In this case, the arithmetic circuit 44 performs frequency axis conversion by multiplying the frequency spectrum of the first beat signal by the ratio of the second frequency f2 to the first frequency f1 (i.e., f2 / f1). Even in this case, the conversion is performed so that the relative frequency axis change of the frequency spectrum of the second beat signal Sb2 with respect to the frequency axis of the frequency spectrum of the first beat signal Sb1 becomes the opposite of the magnitude relationship of the second frequency with respect to the first frequency. Furthermore, if frequency axis conversion with such a relationship can be realized, the frequency axis conversion may be performed on both the frequency spectrum of the first beat signal Sb1 and the frequency spectrum of the second beat signal Sb2.

[0097] (Variation 5) In the above embodiment, the number of channels used for transmission and reception and whose frequency spectra are combined is two, the first frequency f1 and the second frequency f2. However, the number of channels used for transmission and reception and whose frequency spectra are combined may be three or more. In that case, regardless of which two channels are selected from among the three or more channels, the relationship of the chirp rate and frequency axis conversion between the two channels is determined according to the difference in frequency between the two channels, similar to the relationship between the first frequency f1 and the second frequency f2.

[0098] (Variation 6) In the above embodiment, a plurality of laser oscillators with different wavelengths, the first laser oscillator 11 and the second laser oscillator 12, are used. However, instead of the first laser oscillator 11 and the second laser oscillator 12, a single laser oscillator capable of switching wavelengths may be used. In this case, the single laser oscillator may generate a laser beam with a first frequency f1 and perform a measurement, and then generate a laser beam with a second frequency f2 and perform a measurement again, and the speckle may be reduced by integrating two waveforms that are independent in time. In this case, the same chirp rate setting is performed in the first measurement and the next measurement, and the same frequency conversion is performed on the measurement results, thereby obtaining the same effect as in the above embodiment.

[0099] (Variation 7) In the above embodiment, the optical system uses optical fibers, but the optical system may be a spatial optical system having a collimator, a polarizing beam splitter, a wave plate, and the like. [Explanation of symbols]

[0100] 11~14 Light source 21a~26b Optical system 31, 32 Photodetector 40 Processing circuit 44 Arithmetic circuit

Claims

1. a light source (11-14) that outputs a first emitted light (Lt1) that changes with a first chirp rate (γ1) with respect to a first frequency (f1) and a second emitted light (Lt2) that changes with a second chirp rate (γ2) with respect to a second frequency (f2) different from the first frequency; an optical system (21a to 26b) that transmits the first outgoing light and the second outgoing light to a target (TR), combines reflected light (Ltr) generated by reflection of the first outgoing light and the second outgoing light at the target with a first reference light (Lr1) based on the first outgoing light, and outputs the combined light as a first interference light (Lb1), and combines a second reference light (Lr2) based on the second outgoing light with the reflected light, and outputs the combined light as a second interference light (Lb1); a photodetector (31, 32) that receives the first interference light and the second interference light outputted from the optical system and outputs a first beat signal (Sb1) corresponding to the first interference light and a second beat signal (Sb2) corresponding to the second interference light; a processing circuit (40) that identifies information regarding a position of the target based on a frequency distribution of the first beat signal and a frequency distribution of the second beat signal output by the photodetector, A magnitude relationship of the second chirp rate to the first chirp rate is the same as a magnitude relationship of the second frequency to the first frequency, the processing circuit performs frequency axis conversion so that a relative change in the frequency axis of the frequency spectrum of the second beat signal with respect to a frequency axis of the frequency spectrum of the first beat signal becomes inverse to a magnitude relationship of the second frequency with respect to the first frequency, the processing circuit synthesizes the frequency spectrum of the first beat signal and the frequency spectrum of the second beat signal after the frequency axis conversion, and identifies information about the position of the target based on the synthesized frequency spectrum.

2. a ratio of the second chirp rate to the first chirp rate is the same as a ratio of the second frequency to the first frequency; 2. The optical interference measuring device according to claim 1, wherein the processing circuit performs the frequency axis conversion by multiplying the frequency spectrum of the second beat signal by an inverse number of a ratio of the second frequency to the first frequency.

3. 3. The optical interference measurement device according to claim 1 or 2, wherein the processing circuit calculates a frequency spectrum of the first beat signal by performing a Fourier transform on the first beat signal, calculates a frequency spectrum of the second beat signal by performing a Fourier transform on the second beat signal, and performs the frequency axis transformation such that a change in the frequency axis of the calculated frequency spectrum of the second beat signal with respect to the frequency axis of the calculated frequency spectrum of the first beat signal is reversed to a magnitude relationship of the second frequency with respect to the first frequency.

4. The processing circuitry converts the first beat signal into a first sampling frequency (f ｓ１ a first A / D converter (41) for sampling the second beat signal at a second sampling frequency (f ｓ２ a second A / D converter that samples a first beat signal at a frequency of 100 kHz, and a calculation circuit (44) that calculates a frequency spectrum of the first beat signal by performing a Fourier transform on the first beat signal sampled by the first A / D converter, calculates a frequency spectrum of the second beat signal by performing a Fourier transform on the second beat signal sampled by the second A / D converter, synthesizes the calculated frequency spectrum of the first beat signal and the calculated frequency spectrum after the frequency axis conversion of the second beat signal, and specifies information regarding the position of the target based on the synthesized frequency spectrum, 3. The optical interference measurement device according to claim 1 or 2, wherein a magnitude relationship between the first sampling frequency and the second sampling frequency is the same as a magnitude relationship between the second frequency and the first frequency, so that a change in the frequency axis of the frequency spectrum of the second beat signal with respect to the frequency axis of the frequency spectrum of the first beat signal is opposite to a magnitude relationship between the second frequency and the first frequency.

5. 3. The optical interference measurement device according to claim 1, wherein an absolute value of a difference between the wavelength corresponding to the first frequency and the wavelength corresponding to the second frequency is 30 nm or more.

6. a light source (11-14) that outputs a first emitted light (Lt1) that changes with a first chirp rate (γ1) with respect to a first frequency (f1) and a second emitted light (Lt2) that changes with a second chirp rate (γ2) with respect to a second frequency (f2) different from the first frequency; an optical system (21a to 26b) that transmits the first outgoing light and the second outgoing light to a target (TR), combines reflected light (Ltr) generated by reflection of the first outgoing light and the second outgoing light at the target with a first reference light (Lr1) based on the first outgoing light, and outputs the combined light as a first interference light (Lb1), and combines a second reference light (Lr2) based on the second outgoing light with the reflected light, and outputs the combined light as a second interference light (Lb1); a photodetector (31, 32) that receives the first interference light and the second interference light outputted from the optical system and outputs a first beat signal (Sb1) corresponding to the first interference light and a second beat signal (Sb2) corresponding to the second interference light; a calculation circuit (44) that specifies information regarding a position of the target based on a frequency distribution of the first beat signal and a frequency distribution of the second beat signal output by the photodetector, A magnitude relationship of the second chirp rate to the first chirp rate is the same as a magnitude relationship of the second frequency to the first frequency, The arithmetic circuit, a conversion unit (S130) that performs frequency axis conversion so that a relative change in the frequency axis of the frequency spectrum of the second beat signal with respect to the frequency axis of the frequency spectrum of the first beat signal becomes the opposite of the magnitude relationship of the second frequency with respect to the first frequency; and a program causing the program to function as an identification unit (S140, S150) that synthesizes the frequency spectrum of the first beat signal and the frequency spectrum of the second beat signal after the frequency axis conversion, and identifies information regarding the position of the target based on the synthesized frequency spectrum.

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