Optical measurement device, optical measurement method, and optical measurement program

Abstract

This optical measurement device divides output light into probe light and reference light using an optical branching element, the output light being from a multi-frequency laser that generates a high-frequency subcarrier that was frequency modulated and a low-frequency subcarrier that was frequency modulated, said subcarriers being in mutually symmetrical positions with respect to carrier frequency. The optical measurement device comprises: an optical system that radiates probe light onto an object and outputs, as signal light, the light scattered from the object; a beat signal generation unit that generates, from the reference light and the signal light, a first complex beat signal derived from the high-frequency subcarrier and a second complex beat signal derived from the low-frequency subcarrier; and an arithmetic device that calculates the speed of the object using the average value of the sum of the frequency of the first complex beat signal and the frequency of the second complex beat signal.

Description

Optical measurement device, optical measurement method, and optical measurement program

[0001] The present invention relates to an optical measurement device, an optical measurement method, and an optical measurement program suitable for environment recognition sensors used in automobiles, autonomous robots, and the like.

[0002] Development of LiDAR (Light Detection and Ranging) is progressing for applications such as environmental recognition sensors mounted on automobiles and autonomous robots, and shape measurement at construction and civil engineering sites. Compared to ToF (Time of Flight) LiDAR, which is being put to practical use, FMCW (Frequency Modulated Continuous Wave) LiDAR (hereinafter referred to as FMCW LiDAR) is capable of high-sensitivity detection and can measure not only the distance to an object but also its relative velocity from the Doppler shift. FMCW LiDAR in the millimeter wave region has been put to practical use as an in-vehicle collision prevention sensor. If FMCW LiDAR can be used in the light wave region, significant improvements in spatial resolution and measurement accuracy can be expected.

[0003] One of the challenges of FMCW lidar is overcoming the limitation on the maximum measurement distance imposed by laser coherence. FMCW lidar modulates the frequency of a laser output beam, mixes scattered light from a target with a reference beam serving as a measurement standard, and performs coherent detection to generate a beat signal. In addition to frequency modulation, the laser output beam also fluctuates due to frequency noise emitted by the laser itself. Therefore, the beat signal contains a mixture of components due to frequency modulation and frequency noise. As the distance to the target increases, the time delay (delay time) of the scattered light relative to the reference beam increases, and the component due to frequency noise becomes larger relative to the component due to frequency modulation. This reduces the signal-to-noise ratio of the beat signal, resulting in a degradation of measurement accuracy.

[0004] When an FMCW lidar is used as an environmental recognition sensor for an autonomous vehicle, it is necessary to detect objects at a distance of 200 m or more. The coherence length of the laser output light is a measure of the maximum measurement distance. The coherence length is defined as the distance at which the value of the coherence function becomes 1 / e. The factor that determines the coherence length is the frequency noise of the laser output light; the greater the noise, the shorter the coherence length. The coherence length of a typical semiconductor laser for optical communications is 100 m or less. With an FMCW lidar, the light to be measured travels back and forth to the target, so the maximum measurement distance is limited to 50 m or less.

[0005] For example, Patent Document 1 discloses a technology that uses a difference frequency (difference) signal between two complex beat signals generated by a multi-frequency laser to overcome the limitation on the maximum measurement distance caused by laser coherence when measuring the velocity of an object.

[0006] International Publication No. 2021 / 131315

[0007] FMCW lidar calculates the relative velocity of an object from the magnitude of the Doppler shift. The magnitude of the Doppler shift is proportional to the relative velocity and the frequency of the light. The optical measurement device described in Patent Document 1 is configured to measure the velocity of an object using a difference frequency signal between complex beat signals generated by two measurement beams with different frequencies. The two measurement beams used in the optical measurement device generate a Doppler shift proportional to the frequency. However, the Doppler shift is subtracted from the difference frequency signal, leaving a Doppler shift proportional to the frequency difference between the two measurement beams. For example, if the frequency difference between the two measurement beams is 50 GHz at a wavelength of 1550 nm (frequency of 193.4 THz), the magnitude of the Doppler shift is approximately 1 / 3870, resulting in low detection sensitivity. This makes it difficult to improve the accuracy of measuring the velocity of the object.

[0008] The present invention has been made to solve the above-mentioned problems, and aims to provide an optical measurement device, an optical measurement method, and an optical measurement program that measure the velocity of an object with high accuracy by overcoming the distance limitation between the object and the device due to the coherence length of the laser.

[0009] An optical measurement device according to one aspect of the present invention includes: a multi-frequency laser that generates a frequency-modulated high-frequency subcarrier and a frequency-modulated low-frequency subcarrier that are positioned symmetrically with respect to the carrier frequency; an optical branching element that splits the output light of the multi-frequency laser into a probe light and a reference light; an optical system that irradiates an object with the probe light and outputs scattered light from the object as signal light; a beat signal generation unit that receives the reference light and the signal light and generates and outputs a first complex beat signal derived from the high-frequency subcarrier and including an in-phase component and a quadrature component, and a second complex beat signal derived from the low-frequency subcarrier and including an in-phase component and a quadrature component; and a calculation unit that calculates a sum frequency that is the sum of the frequencies of the first complex beat signal and the second complex beat signal, and calculates the velocity of the object using an average value of the calculated sum frequencies.

[0010] An optical measurement method according to one aspect of the present invention includes: a subcarrier generation step in which a multi-frequency laser simultaneously generates a frequency-modulated high-frequency subcarrier and a frequency-modulated low-frequency subcarrier, the subcarriers being symmetrically positioned with respect to each other with respect to the carrier frequency; a light branching step in which a light branching element divides the output light of the multi-frequency laser into a probe light and a reference light; an irradiation processing step in which an optical system irradiates an object with the probe light input from the light branching element and outputs light scattered from the object as signal light; a complex beat signal generation step in which a beat signal generation unit receives the reference light and the signal light and generates and outputs a first complex beat signal derived from the high-frequency subcarrier and including an in-phase component and a quadrature component, and a second complex beat signal derived from the low-frequency subcarrier and including an in-phase component and a quadrature component; and a velocity calculation step in which a calculation device calculates a sum frequency, which is the sum of the frequencies of the first complex beat signal and the second complex beat signal, and calculates the velocity of the object using an average value of the calculated sum frequencies.

[0011] In one aspect of the present invention, an optical measurement method determines the velocity of an object based on output light from a multi-frequency laser that generates a frequency-modulated high-frequency subcarrier and a frequency-modulated low-frequency subcarrier that are symmetrically positioned with respect to the carrier frequency, the method comprising: demodulating a first complex beat signal derived from the high-frequency subcarrier and including an in-phase component and a quadrature component to determine a first frequency that is the frequency of the first complex beat signal; demodulating a second complex beat signal derived from the low-frequency subcarrier and including an in-phase component and a quadrature component to determine a second frequency that is the frequency of the second complex beat signal; adding the first frequency and the second frequency to determine a sum frequency; and determining the velocity of the object using an average value of the sum frequency.

[0012] An optical measurement program according to one aspect of the present invention causes a computer included in an arithmetic device that determines the velocity of an object based on output light from a multi-frequency laser that generates a frequency-modulated high-frequency subcarrier and a frequency-modulated low-frequency subcarrier that are positioned symmetrically with respect to the carrier frequency to function as velocity calculation means that demodulates a first complex beat signal that is derived from the high-frequency subcarrier and includes an in-phase component and a quadrature component to determine a first frequency that is the frequency of the first complex beat signal, demodulates a second complex beat signal that is derived from the low-frequency subcarrier and includes an in-phase component and a quadrature component to determine a second frequency that is the frequency of the second complex beat signal, adds the first frequency and the second frequency to determine a sum frequency, and determines the velocity of the object using an average value of the sum frequency.

[0013] The present invention calculates the velocity of an object using the average value of the sum of the frequency of a first complex beat signal derived from a frequency-modulated high-frequency subcarrier and the frequency of a second complex beat signal derived from a frequency-modulated low-frequency subcarrier. This makes it possible to remove components caused by frequency noise, thereby overcoming the distance limitation to the object caused by the coherence length of the laser and enabling the velocity of the object to be measured with high accuracy.

[0014] 1 is a schematic diagram illustrating the basic configuration of an optical measurement device according to an embodiment of the present invention. It is a configuration diagram illustrating an example of a specific configuration of the multi-frequency laser of FIG. 1. It is a configuration diagram illustrating another example of the specific configuration of the multi-frequency laser of FIG. 1. It is an explanatory diagram illustrating the spectrum of output light from the multi-frequency laser of FIG. 1, and frequency modulation and frequency noise superimposed on each subcarrier. It is a configuration diagram illustrating a first example of the beat signal generation unit of FIG. 1. It is a configuration diagram illustrating a specific example of one quadrature detector of FIG. 5. It is a configuration diagram illustrating a specific example of the other quadrature detector of FIG. 5. It is a configuration diagram illustrating a second example of the beat signal generation unit of FIG. 1. It is a configuration diagram illustrating a specific example of one phase diversity detector of FIG. 8. It is a configuration diagram illustrating a specific example of the other phase diversity detector of FIG. 8. It is an explanatory diagram illustrating the Doppler shift that a carrier and each subcarrier undergo when an object is moving relative to the optical measurement device of FIG. 1. It is an explanatory diagram illustrating a process in which the arithmetic unit of FIG. 1 calculates the velocity and the distance to the object using a sum frequency and a difference frequency based on the frequencies of each of two complex beat signals. 1 is a diagram illustrating frequency modulation of reference light and signal light, a beat signal frequency, a sum frequency, and a difference frequency caused by a high-frequency subcarrier and a low-frequency subcarrier. 2 is a flowchart illustrating a flow of operations in an optical measurement method according to an embodiment of the present invention. 3 is a diagram illustrating correspondence between graphs of two time waveforms related to frequency, in which (a) shows frequency modulation ν of a high-frequency subcarrier. Ｍ (b) shows the frequency noise ν Ｎ The graph shows the time waveform of (t). Two graphs of time waveforms related to amplitude are associated with each other. (a) shows the shot noise I appearing in the in-phase component of the high-frequency subcarrier. ＋ ｓｈｏｔ (b) shows the photodetector noise I appearing in the in-phase component of the high-frequency subcarrier. ＋ ＰＤ1 shows the time waveform of (t). It is a diagram showing the complex beat signals of each of two subcarriers on the IQ plane (complex plane), where (a) corresponds to the high-frequency subcarrier and (b) corresponds to the low-frequency subcarrier. It is a diagram showing the time changes of multiple frequencies, where (a) shows the time waveform of the frequency of the high-frequency subcarrier, (b) shows the time waveform of the frequency of the low-frequency subcarrier, (c) shows the time waveform of the difference frequency, and (d) shows the time waveform of the sum frequency. It is a diagram showing the time waveform of the difference frequency when the signal light power is changed from 10 μW to 100 pW. It is a diagram showing the time waveform of the sum frequency when the signal light power is changed from 10 μW to 100 pW. It is a graph showing the relationship between the signal light power and the distance calculated from the difference frequency on a coordinate system where the horizontal axis represents the signal light power and the vertical axis represents the distance to the target. 1 is a graph showing the velocity calculated from the sum frequency and the velocity calculated from the difference frequency on a coordinate system in which the horizontal axis represents the signal light power and the vertical axis represents the velocity of the object.

[0015] An optical measurement device, an optical measurement method, and an optical measurement program according to an embodiment of the present invention will be described with reference to Figures 1 to 22. In each figure, some reference numerals may be omitted to avoid cluttering the drawings.

[0016] First, an example of the overall configuration of an optical measurement device 100 will be described with reference to Fig. 1. The optical measurement device 100 illustrated in Fig. 1 includes a multi-frequency laser 1, an optical branching element 2a, an optical system 5, a beat signal generator 9, and a computing device 14. The multi-frequency laser 1 simultaneously generates a frequency-modulated high-frequency subcarrier 16 and a frequency-modulated low-frequency subcarrier 17, which are positioned symmetrically with respect to the carrier frequency. The high-frequency subcarrier 16 is a subcarrier obtained by frequency-modulating the carrier 15 to the high frequency side, and the low-frequency subcarrier 17 is a subcarrier obtained by frequency-modulating the carrier 15 to the low frequency side.

[0017] The optical branching element 2 a splits the output light of the multi-frequency laser 1 into a probe light 3 and a reference light 4 , and outputs the probe light 3 to an optical circulator 5 a and the reference light 4 to a beat signal generating unit 9 .

[0018] The optical system 5 irradiates the probe light 3 output from the optical branching element 2a onto the object 7 and outputs the scattered light from the object 7 as signal light 8. The optical system 5 in this embodiment is composed of an optical circulator 5a and a transmitting / receiving optical system 5b. The optical circulator 5a irradiates the probe light 3 output from the optical branching element 2a onto the object 7 via the transmitting / receiving optical system 5b and outputs the scattered light from the object 7 to the beat signal generating unit 9 as signal light 8.

[0019] The transmitting and receiving optical system 5b is composed of a lens such as a collimator lens. The transmitting and receiving optical system 5b may be provided with a mechanism for spatially scanning the probe light 3 as necessary. This allows the transmitting and receiving optical system 5b to acquire a two-dimensional image of the distance or speed of the object 7. Incidentally, while FIG. 1 shows an example in which the optical system 5 includes the optical circulator 5a and the transmitting and receiving optical system 5b, the present invention is not limited to this. For example, the optical system 5 may be composed of only the optical circulator 5a.

[0020] The beat signal generating unit 9 receives the reference light 4 and the signal light 8 as input, and generates and outputs a first complex beat signal 31 derived from the high-frequency subcarrier 16 and including an in-phase component 10 and a quadrature component 11, and a second complex beat signal 32 derived from the low-frequency subcarrier 17 and including an in-phase component 12 and a quadrature component 13. The beat signal generating unit 9 of this embodiment is configured to generate the in-phase component 10 and the quadrature component 11 of the first complex beat signal 31 and the in-phase component 12 and the quadrature component 13 of the second complex beat signal 32 based on the reference light 4 output from the optical branching element 2a and the signal light 8 output from the optical circulator 5a, and output these to the computing device 14.

[0021] The arithmetic unit 14 determines a sum frequency, which is the sum of the frequency of the first complex beat signal 31 and the frequency of the second complex beat signal 32, and calculates the velocity of the object 7 from the determined sum frequency. The sum frequency is the sum of the frequency of the complex beat signal generated from the high-frequency subcarrier 16 and the frequency of the complex beat signal generated from the low-frequency subcarrier 17. That is, the arithmetic unit 14 demodulates the phases of the two complex beat signals (the first complex beat signal 31 and the second complex beat signal 32), determines their respective frequencies by time-differentiating the phases, and calculates the sum frequency from the determined frequencies. The arithmetic unit 14 then calculates the velocity of the object 7 from the determined sum frequency.

[0022] The arithmetic unit 14 may have a function of determining a difference frequency, which is the difference between the frequency of the first complex beat signal 31 and the frequency of the second complex beat signal 32, and calculating the distance to the target object 7 from the determined difference frequency. In this case, the arithmetic unit 14 demodulates the phases of the two complex beat signals, determines their respective frequencies by time-differentiating the phases, and calculates the difference frequency from the determined frequencies.

[0023] More specifically, the arithmetic device 14 includes a communication unit 14a, a calculation processing unit 14b, and a storage unit 14c. The communication unit 14a is an interface for wired or wireless communication between the arithmetic device 14 and external devices such as the beat signal generating unit 9. The storage unit 14c stores various information, including operation programs of the calculation processing unit 14b, such as the optical measurement program 14p. The storage unit 14c includes a random access memory (RAM), a read-only memory (ROM), a programmable read-only memory (PROM) such as a flash memory, a solid state drive (SSD), or a hard disk drive (HDD). The calculation processing unit 14b includes a velocity calculation unit for calculating the velocity of the object 7. The calculation processing unit 14b may also include a distance calculation unit for calculating the distance to the object 7. The calculation processing unit 14b includes a central processing unit (CPU), a graphics processing unit (GPU), or the like. The optical measurement program 14p cooperates with the calculation processing unit 14b to realize various functions such as a speed calculation means.

[0024] Next, specific configuration examples of the multi-frequency laser 1 will be described with reference to Figures 2 and 3. Figure 2 shows a first example of a specific configuration of the multi-frequency laser 1. The multi-frequency laser 1 illustrated in Figure 2 includes a single-frequency laser 18, an optical modulator 19, a modulation signal generator 20, and a subcarrier generation signal generator 21. The single-frequency laser 18 is a laser that outputs only light of a single frequency. The optical modulator 19 modulates the output light of the single-frequency laser 18. The optical modulator 19 is composed of an intensity modulator, a phase modulator, or the like, and has the function of generating sidebands through modulation.

[0025] The subcarrier generation signal generator 21 drives the optical modulator 19 to generate two subcarriers in the output light of the single-frequency laser 18. The subcarrier generation signal generator 21 has a function of modulating the frequency by an external input signal. The subcarrier generation signal generator 21 of this embodiment generates two subcarriers whose frequencies are symmetrical to each other with respect to the frequency of the carrier 15 in the single-frequency laser 18. Here, the center frequency of the subcarrier generation signal generator 21 is f ＳＣ The center frequency f of the subcarrier generation signal generator 21 is ＳＣ is set to a frequency that matches the characteristics of the beat signal generating unit 9. The center frequency of the carrier 15 of the output light of the multi-frequency laser 1 is set to ν ０ Then, the center frequency of the high frequency subcarrier 16 is v ０ +f ＳＣ and the center frequency of the low frequency subcarrier 17 is v ０ -f ＳＣ This becomes:

[0026] The modulation signal generator 20 is a signal source that applies frequency modulation to the subcarriers. The modulation signal generator 20 of this embodiment is configured to apply frequency modulation of opposite phases to each of the two subcarriers generated by the subcarrier generation signal generator 21. In other words, the multi-frequency laser 1 of Figure 2 generates a high-frequency subcarrier 16 and a low-frequency subcarrier 17 in the output light of the optical modulator 19 by the modulation signal generator 20 and the subcarrier generation signal generator 21.

[0027] FIG. 3 shows a second example of a specific configuration of the multi-frequency laser 1. The multi-frequency laser 1 shown in FIG. 3 includes a semiconductor laser 22, a modulation signal generator 20, and a subcarrier generation signal generator 21. The configurations and processing contents of the modulation signal generator 20 and the subcarrier generation signal generator 21 are the same as those of the first example. That is, the subcarrier generation signal generator 21 generates two subcarriers in the output light of the semiconductor laser 22. The modulation signal generator 20 applies frequency modulation with mutually opposite phases to each of the two subcarriers generated by the subcarrier generation signal generator 21. That is, the multi-frequency laser 1 shown in FIG. 3 generates a high-frequency subcarrier 16 and a low-frequency subcarrier 17 in the output light of the semiconductor laser 22 by the modulation signal generator 20 and the subcarrier generation signal generator 21.

[0028] The semiconductor laser 22 is configured, for example, by a distributed feedback laser (DFB laser) or a modulator-integrated semiconductor laser (EML). An EML is a semiconductor laser integrated with a modulator. When the semiconductor laser 22 is a DFB laser, the subcarrier generation signal generator 21 outputs a generation signal for generating subcarriers to the injection current of the DFB laser to generate two subcarriers. When the semiconductor laser 22 is an EML, the subcarrier generation signal generator 21 outputs a generation signal to a modulator in the EML to generate two subcarriers.

[0029] The multi-frequency laser 1 modulates the carrier 15 to generate high-frequency subcarriers 16 and low-frequency subcarriers 17, which are ±1-order subcarriers, so that the high-frequency subcarriers 16 and low-frequency subcarriers 17 have frequency noise in phase with the carrier 15. Furthermore, in the multi-frequency laser 1, the high-frequency subcarriers 16 and low-frequency subcarriers 17 are subjected to frequency modulation and the frequency noise of the subcarrier generation signal generator 21.

[0030] Next, an example of the configuration of the output light of the multi-frequency laser 1 will be described with reference to Fig. 4. Fig. 4(a) illustrates the spectrum of the output light and the frequency modulation superimposed on two subcarriers, and Fig. 4(b) illustrates the spectrum of the output light and the frequency noise superimposed on the carrier 15 and the two subcarriers. That is, the output light of the multi-frequency laser 1 has a center frequency of ν ０ Carrier 15 and center frequency ν ０ +f ＳＣ and a high frequency subcarrier 16 having a center frequency of ν ０ -f ＳＣ and a low frequency subcarrier 17 which is a frequency modulation signal. The high frequency subcarrier 16 and the low frequency subcarrier 17 are superimposed with frequency modulation and frequency noise.

[0031] As shown in Fig. 4(a), the frequency modulation is in antiphase between the high-frequency subcarrier 16 and the low-frequency subcarrier 17. Also, as shown in Fig. 4(b), the high-frequency subcarrier 16 and the low-frequency subcarrier 17 are phase-synchronized with the carrier 15 and have frequency noise in phase with the carrier 15. In this embodiment, an example is shown in which two subcarriers of ±1 orders (high-frequency subcarrier 16 and low-frequency subcarrier 17) are used. Note that, for convenience, Fig. 4 shows only the ±1 order subcarriers, but the multi-frequency laser 1 may also include subcarriers of higher orders.

[0032] Carrier 15 frequency ν ０ and the frequency v of the high frequency subcarrier 16 and the low frequency subcarrier 17. ０ ±f ＳＣ It is preferable that the center frequency f of the subcarrier generation signal generator 21 is separated from the center frequency f of the subcarrier generation signal generator 21 by a sufficient distance so that the center frequency f of the subcarrier generation signal generator 21 can be optically separated from the center frequency f of the subcarrier generation signal generator 21. ＳＣ The center frequency f is preferably within a range that can be responded to by a modulator or direct modulation of a laser. ＳＣ and frequency ν ０ can be selected arbitrarily, and in principle there is no particular limit. However, in reality, the center frequency f ＳＣ and frequency ν ０ Each setting is limited by the specifications of the available components.ＳＣ The minimum value of is, for example, the frequency v ０ The center frequency f is determined by the range in which there is a frequency difference (about 5 GHz) between the center frequency f and the frequency f that can be separated by an optical filter. ＳＣ The maximum value of the frequency v of the carrier 15 is, for example, approximately 50 GHz, and depends greatly on the modulation performance of the modulation signal generator 20. ０ All frequencies available for single frequency lasers are available. That is, the frequency v ０ can be set in all ranges from the ultraviolet range to the visible range to the infrared range, for example.

[0033] Even in the following cases, the center frequency is ν ０ The carrier 15 is generated from a center frequency of ν ０ ±f ＳＣ However, the high frequency subcarriers 16 and the low frequency subcarriers 17 are not limited to being subcarriers of ±1 order relative to the carrier 15. The optical measurement device 100 may be configured so that the multi-frequency laser 1 generates higher order subcarriers, i.e., subcarriers of ±2 order or higher, as the high frequency subcarriers 16 and the low frequency subcarriers 17, and these may be used to perform calculations of velocity, etc.

[0034] Next, specific configuration examples of the beat signal generating unit 9 will be described with reference to Fig. 5 to Fig. 10. First, a first example of the beat signal generating unit 9 will be described with reference to Fig. 5 to Fig. 7. Fig. 5 is a configuration diagram showing the first example of the beat signal generating unit 9. Fig. 6 is a configuration diagram showing a specific example of the quadrature detector 33a in Fig. 5. Fig. 7 is a configuration diagram showing a specific example of the quadrature detector 33b in Fig. 5.

[0035] 5 has a heterodyne interferometer configuration including a photodetector and a quadrature detector. More specifically, the beat signal generating unit 9 has an optical frequency shifter 26, an optical frequency shifter drive signal source 27, an optical coupling element 2b, an optical demultiplexer 29a, photodetectors 30a and 30b, and quadrature detectors 33a and 33b.

[0036] The optical frequency shifter 26 shifts the frequency of the reference light 4 and outputs the shifted light. The optical frequency shifter drive signal source 27 outputs a drive signal to the optical frequency shifter 26 to drive the optical frequency shifter 26. Furthermore, the optical frequency shifter drive signal source 27 outputs a reference signal 28, which separates the complex beat signal into an in-phase component and a quadrature component, to the quadrature detectors 33 a and 33 b.

[0037] The optical coupling element 2b is, for example, a beam splitter, which combines the light output from the optical frequency shifter 26 with the signal light 8 output from the optical circulator 5a and outputs the combined light to the optical demultiplexer 29a. The optical demultiplexer 29a separates the light output from the optical coupling element 2b into a component derived from the high-frequency subcarrier 16 and a component derived from the low-frequency subcarrier 17, and outputs one of the separated components to the photodetector 30a and the other to the photodetector 30b.

[0038] The photodetector 30a performs photoelectric conversion on the component derived from the high-frequency subcarrier 16 output from the optical demultiplexer 29a to generate a first complex beat signal 31, which is output to the quadrature detector 33a. The photodetector 30b performs photoelectric conversion on the component derived from the low-frequency subcarrier 17 output from the optical demultiplexer 29a to generate a second complex beat signal 32, which is output to the quadrature detector 33b. The quadrature detector 33a applies a reference signal 28 to the first complex beat signal 31 to generate an in-phase component 10 of the first complex beat signal 31 and a quadrature component 11 of the first complex beat signal 31, and outputs the generated in-phase component 10 and quadrature component 11 to the calculation device 14. The quadrature detector 33 b applies the reference signal 28 to the second complex beat signal 32 to generate an in-phase component 12 of the second complex beat signal 32 and a quadrature component 13 of the second complex beat signal 32, and outputs the generated in-phase component 12 and quadrature component 13 to the calculation device 14.

[0039] 5 shifts the frequency of the reference light 4 using the optical frequency shifter 26, then combines it with the signal light 8 using the optical coupling element 2b and inputs it to the optical splitter 29a. The optical splitter 29a separates the input light into a component derived from the high-frequency subcarrier 16 and a component derived from the low-frequency subcarrier 17, and outputs them. The components separated by the optical splitter 29a are received by the photodetectors 30a and 30b, respectively. The photodetector 30a outputs a first complex beat signal 31 to the quadrature detector 33a, and the photodetector 30b outputs a second complex beat signal 32 to the quadrature detector 33b. The quadrature detector 33a outputs the in-phase component 10 and the quadrature component 11 of the input first complex beat signal 31 to the calculation device 14. The quadrature detector 33 b outputs the in-phase component 12 and the quadrature component 13 of the input second complex beat signal 32 to the arithmetic unit 14 .

[0040] As shown in Fig. 6 and Fig. 7, the quadrature detector 33a and the quadrature detector 33b each include a π / 2 electrical phase shifter 34, frequency mixers 35a and 35b, and low-pass filters 36a and 36b. As shown in Fig. 6, the quadrature detector 33a is configured to split the first complex beat signal 31 in half, and input one of the two signals to the frequency mixer 35a and the other to the frequency mixer 35b. As shown in Fig. 7, the quadrature detector 33b is configured to split the second complex beat signal 32 in half, and input one of the two signals to the frequency mixer 35a and the other to the frequency mixer 35b.

[0041] The quadrature detectors 33a and 33b each divide the reference signal 28 output from the optical frequency shifter drive signal source 27 in half, outputting one of the divided parts to the frequency mixer 35a and the other to the π / 2 electrical phase shifter 34. The frequency mixer 35a receives one of the divided parts of the reference signal 28 as a local oscillation signal. That is, the frequency mixer 35a converts the frequency of the first complex beat signal 31 using the input local oscillation signal and outputs the converted signal to the low-pass filter 36a. The frequency mixer 35a has the function of multiplying the two input signals and outputting a sum frequency signal and a difference frequency signal of both signals.

[0042] The π / 2 electrical phase shifters 34 of the quadrature detectors 33a and 33b shift the phase of the other half of the reference signal 28 by π / 2 and output the resulting signal to the frequency mixer 35b. The frequency mixer 35b receives the signal output from the π / 2 electrical phase shifter 34 as a local oscillation signal. That is, the frequency mixer 35b converts the frequency of the first complex beat signal 31 using the input local oscillation signal and outputs the converted signal to the low-pass filter 36b. The frequency mixer 35b has the function of multiplying the two input signals and outputting a sum frequency signal and a difference frequency signal of both signals.

[0043] That is, the quadrature detector 33a and the quadrature detector 33b each divide the first complex beat signal 31 or the second complex beat signal 32 in half, and input one of the two signals to the frequency mixer 35a and the other to the frequency mixer 35b. The quadrature detector 33a and the quadrature detector 33b each input the reference signal 28 output from the optical frequency shifter drive signal source 27 as a local oscillation signal for the frequency mixer 35a. Meanwhile, the quadrature detector 33a and the quadrature detector 33b each shift the phase of the reference signal 28 output from the optical frequency shifter drive signal source 27 by π / 2 using the π / 2 electrical phase shifter 34, and input the result as a local oscillation signal for the frequency mixer 35b.

[0044] The quadrature detector 33a inputs the output of the frequency mixer 35a to a low-pass filter 36a and inputs the output of the frequency mixer 35b to a low-pass filter 36b, removes the sum frequency signal using each low-pass filter (36a, 36b), and outputs an in-phase component 10 and a quadrature component 11 of the first complex beat signal 31. The quadrature detector 33b inputs the output of the frequency mixer 35a to a low-pass filter 36a and inputs the output of the frequency mixer 35b to a low-pass filter 36b, removes the sum frequency signal using each low-pass filter (36a, 36b), and outputs an in-phase component 12 and a quadrature component 13 of the second complex beat signal 32.

[0045] Next, a second example of the beat signal generating unit 9 will be described with reference to Fig. 8 to Fig. 10. Fig. 8 is a configuration diagram showing the second example of the beat signal generating unit 9, Fig. 9 is a configuration diagram showing a specific example of the phase diversity detector 41a in Fig. 8, and Fig. 10 is a configuration diagram showing a specific example of the phase diversity detector 41b in Fig. 8.

[0046] 8 has the configuration of a homodyne interferometer including a phase diversity detector. More specifically, the beat signal generating unit 9 has an optical demultiplexer 29b, an optical demultiplexer 29c, a phase diversity detector 41a, and a phase diversity detector 41b.

[0047] The reference light 4 is input to the optical splitter 29b. Based on the reference light 4, the optical splitter 29b generates a reference light 37 derived from the high frequency subcarrier 16 and a reference light 38 derived from the low frequency subcarrier 17. The optical splitter 29b outputs the reference light 37 to the phase diversity detector 41a and outputs the reference light 38 to the phase diversity detector 41b.

[0048] The optical demultiplexer 29c receives the optical signal 8. Based on the optical signal 8, the optical demultiplexer 29c generates an optical signal 39 derived from the high frequency subcarrier 16 and an optical signal 40 derived from the low frequency subcarrier 17. The optical demultiplexer 29c outputs the optical signal 39 to a phase diversity detector 41a and outputs the optical signal 40 to a phase diversity detector 41b.

[0049] The phase diversity detector 41a generates and outputs an in-phase component 10 and a quadrature component 11 of a first complex beat signal 31 based on the reference light 37 and the signal light 39. The phase diversity detector 41b generates and outputs an in-phase component 12 and a quadrature component 13 of a second complex beat signal 32 based on the reference light 38 and the signal light 40.

[0050] 8 is configured to input the reference light 4 to the optical splitter 29b and the signal light 8 to the optical splitter 29c, and separate these into a component derived from the high-frequency subcarrier 16 and a component derived from the low-frequency subcarrier 17, and output them. In other words, the beat signal generation unit 9 inputs the reference light 37 derived from the high-frequency subcarrier 16 and output from the optical splitter 29b, and the signal light 39 derived from the high-frequency subcarrier 16 and output from the optical splitter 29c, to the phase diversity detector 41a, and outputs the in-phase component 10 and the quadrature component 11 of the first complex beat signal 31 to the calculation device 14. Similarly, the beat signal generating unit 9 inputs the reference light 38 derived from the low-frequency subcarrier 17 output from the optical demultiplexer 29 b and the signal light 40 derived from the low-frequency subcarrier 17 output from the optical demultiplexer 29 c to a phase diversity detector 41 b, and outputs the in-phase component 12 and quadrature component 13 of the second complex beat signal 32 to the computing device 14.

[0051] 9 and 10, the phase diversity detector 41a and the phase diversity detector 41b each include a beam splitter 2c, a beam splitter 2d, a beam splitter 2e, a beam splitter 2f, a π / 2 optical phase shifter 42, a total reflection mirror 43a, a total reflection mirror 43b, a balanced photodetector 44a, and a balanced photodetector 44b. The π / 2 optical phase shifter 42 changes the phase of input light by π / 2 (shifts the phase by π / 2) and outputs the light.

[0052] The balanced photodetector 44a of the phase diversity detector 41a receives one of the two beams of reference light 37 split by beam splitter 2c via a total reflection mirror 43a and beam splitter 2e, and also receives one of the two beams of signal light 39 split by beam splitter 2d via beam splitter 2e, and generates and outputs an in-phase component 10 of the first complex beat signal 31 based on these input beams. The balanced photodetector 44b of the phase diversity detector 41a receives the other of the two beams of reference light 37 split by beam splitter 2c via a π / 2 optical phase shifter 42, a total reflection mirror 43b, and beam splitter 2f, and also receives the other of the two beams of signal light 39 split by beam splitter 2d via beam splitter 2f, and also generates and outputs an orthogonal component 11 of the first complex beat signal 31 based on these input beams.

[0053] The balanced photodetector 44a of the phase diversity detector 41b receives one of the two beams of reference light 38 split by beam splitter 2c via a total reflection mirror 43a and beam splitter 2e, and also receives one of the two beams of signal light 40 split by beam splitter 2d via beam splitter 2e, and generates and outputs an in-phase component 12 of the second complex beat signal 32 based on these input beams. The balanced photodetector 44b of the phase diversity detector 41a receives the other of the two beams of reference light 38 split by beam splitter 2c via a π / 2 optical phase shifter 42, a total reflection mirror 43b, and beam splitter 2f, and also receives the other of the two beams of signal light 40 split by beam splitter 2d via beam splitter 2f, and also generates and outputs an orthogonal component 13 of the second complex beat signal 32 based on these input beams.

[0054] That is, the phase diversity detector 41a is configured so that the reference light 37 is split into two by the beam splitter 2c, one of which is guided to the balanced photodetector 44a via the total reflection mirror 43a and the beam splitter 2e, and the other of which is guided to the balanced photodetector 44b via the π / 2 optical phase shifter 42, the total reflection mirror 43b, and the beam splitter 2f. The phase diversity detector 41b is configured so that the reference light 38 is split into two by the beam splitter 2c, one of which is guided to the balanced photodetector 44a via the total reflection mirror 43a and the beam splitter 2e, and the other of which is guided to the balanced photodetector 44b via the π / 2 optical phase shifter 42, the total reflection mirror 43b, and the beam splitter 2f. The balanced photodetector 44a of the phase diversity detector 41a outputs the in-phase component 10 of the first complex beat signal 31, and the balanced photodetector 44a of the phase diversity detector 41b outputs the in-phase component 12 of the second complex beat signal 32. The balanced photodetector 44b of the phase diversity detector 41a outputs the quadrature component 11 of the first complex beat signal 31, and the balanced photodetector 44b of the phase diversity detector 41b outputs the quadrature component 13 of the second complex beat signal 32.

[0055] 11 to 13, the details of the calculation process by the calculation device 14 will be specifically described. The probe light 3 split from the output light of the multi-frequency laser 1 by the optical branching element 2a has a carrier and two subcarriers.

[0056] First, the Doppler shifts that the carrier and two subcarriers undergo will be described with reference to Fig. 11. Fig. 11 is a diagram illustrating the Doppler shifts that the probe light 3 undergoes when the object 7 moves relative to the optical measurement device 100. That is, in the following description, the Doppler shift refers to the Doppler shift caused by the relative movement of the object 7.

[0057] 11 illustrates an example of the Doppler shift when the object 7 is approaching the optical measurement device 100. In this case, the carrier 15 and the two subcarriers (16, 17) both shift to higher frequencies. On the other hand, when the object 7 is moving away from the optical measurement device 100, the carrier 15 and the two subcarriers (16, 17) both shift to lower frequencies (not shown).

[0058] The magnitude of the Doppler shift is proportional to the relative velocity of the object 7 with respect to the optical measurement device 100 and the frequency of the light. Therefore, the Doppler shifts experienced by the carrier 15 and the two subcarriers (16, 17) are different in magnitude. Therefore, the Doppler shift of the carrier 15 is expressed as "Δν Ｄ " and the Doppler shift of the high frequency subcarrier 16 is expressed as "Δν Ｄ＋ " and the Doppler shift of the low frequency subcarrier 17 is expressed as "Δν Ｄ― " will be expressed as ".

[0059] As described above, the arithmetic unit 14 can calculate the sum frequency and the difference frequency based on the frequencies of the first complex beat signal 31 and the second complex beat signal 32. The arithmetic unit 14 then calculates the velocity of the object 7 by arithmetic processing using the sum frequency based on the respective frequencies of the two complex beat signals. The arithmetic unit 14 also calculates the distance to the object 7 by arithmetic processing using the difference frequency based on the respective frequencies of the two complex beat signals.

[0060] In the calculation of the sum frequency by the calculation device 14, the opposite-phase frequency modulations superimposed on the two subcarriers (16, 17) are cancelled out, and in-phase frequency noise is added. On the other hand, in the calculation of the difference frequency by the calculation device 14, the in-phase frequency noises superimposed on the two subcarriers (16, 17) are cancelled out, and in-phase frequency modulations are added. In the calculation of the difference frequency, the frequency noise of the multi-frequency laser 1 is completely removed, making it possible to measure distances that are not limited by the coherence length.

[0061] The Doppler shifts of the high frequency subcarrier 16 and the low frequency subcarrier 17 are subtracted at the difference frequency and added at the sum frequency, which reduces sensitivity to Doppler shift at the difference frequency. On the other hand, the Doppler shift that appears at the sum frequency is roughly doubled before being added, allowing for highly sensitive velocity measurement.

[0062] Here, with reference to FIG. 12 , the process of calculating the velocity of the object 7 and the distance to the object 7 from the first complex beat signal 31 and the second complex beat signal 32 will be described. FIG. 12 is an explanatory diagram illustrating the process by which the arithmetic device 14 calculates the velocity of the object 7 and the distance to the object 7 using a sum frequency and a difference frequency based on the frequencies of the two complex beat signals. In the following description, a superscript "+" is added to physical quantities relative to the high-frequency subcarrier 16, and a superscript "-" is added to physical quantities relative to the low-frequency subcarrier 17. Here, the description will focus on the optical measurement device 100 equipped with the beat signal generating unit 9 of the first example in FIG. 5 .

[0063] The first complex beat signal 31 and the second complex beat signal 32 are expressed by the following formula (1). Formula (1) is a complex representation of the beat signal. In formula (1), I ± (t) is the in-phase component of the complex beat signal, and Q ± (t) is the quadrature component of the complex beat signal. The same applies to the following equations.

[0064]

[0065] In the first example of the beat signal generating unit 9 shown in Fig. 5, a complex beat signal expressed by the following equation (2) is output. In the second example of the beat signal generating unit 9 shown in Fig. 8, the in-phase component I ± (t) and the quadrature component Q, which is the imaginary part ± (t) are output simultaneously.

[0066]

[0067] In formula (2), f Ｓis the amount of frequency shift by the optical frequency shifter 26. By performing the processing performed by the quadrature detectors 33a and 33b on the complex beat signal expressed by equation (2), the in-phase component I ± (t) and the quadrature component Q ± (t) can be separated and detected.

[0068] The in-phase components for the high frequency subcarrier 16 and the low frequency subcarrier 17 can be expressed by the following equation (3), and the quadrature components for the high frequency subcarrier 16 and the low frequency subcarrier 17 can be expressed by the following equation (4). ± represents the net amplitude, and Φ ± (t) represents the phase.

[0069]

[0070]

[0071] From the in-phase component expressed by equation (3) and the quadrature component expressed by equation (4), the phase Φ ± (t) can be obtained.

[0072]

[0073] Here, Unwrap represents phase unwrapping. Since the arctangent in equation (5) calculates a value in the range of -π to +π, phases exceeding ±π are folded by subtracting an integer multiple of ±2π. Phase unwrapping is a process that detects and corrects the phase subtracted from the discontinuous point of the phase change to find the true phase.

[0074] Next, the sum frequency and the difference frequency will be described. ＋ (t) is the phase Φ ＋ The time differential of (t) can be expressed by the following equation (6).

[0075]

[0076] On the rightmost side of equation (6), the first term is a component due to the frequency noise of the multi-frequency laser 1, the second term is a component due to the Doppler shift, the third term is a component due to the frequency modulation, and the fourth term is a component due to the frequency noise of the subcarrier generation signal generator 21. In equation (6), v Ｎ (t) is the frequency noise of the multi-frequency laser 1, and τ ｄ is the round trip time of light to the object 7, and v ０ is the frequency of the carrier, V is the relative velocity between the object 7, and v Ｍ (t) is the frequency modulation of the subcarrier, and c is the speed of light. ＳＣ is the center frequency of the subcarrier generating signal generator 21, and v ＲＦ (t) is the frequency noise of the subcarrier generation signal generator 21 .

[0077] Here, the magnitude of the Doppler shift is proportional to the frequency of the high frequency subcarrier 16. When light with a wavelength of 1550 nm is used, the frequency of the high frequency subcarrier 16 is approximately 193.4 THz, which causes a large Doppler shift.

[0078] Similarly, the frequency f of the complex beat signal generated from the low-frequency subcarrier 17 is － (t) can be expressed by the following formula (7).

[0079]

[0080] Then, from equations (6) and (7), the sum frequency f ｓｕｍ (t) can be calculated using the following formula (8).

[0081]

[0082] In the rightmost side of equation (8) showing the sum frequency, the first term is a component due to the frequency noise of the multi-frequency laser 1, and the second term is a component due to the Doppler shift. More specifically, the component due to the Doppler shift in the second term is the sum of the Doppler shift of the high frequency subcarrier 16 and the Doppler shift of the low frequency subcarrier 17. ＋ (t) and frequency f －By taking the sum of (t) and (t), the components due to frequency modulation and the components due to frequency noise of the subcarrier generation signal generator 21 are completely removed from the sum frequency. The component due to frequency noise in the first term on the right-hand side of equation (8) is an AC signal, so it can be removed by performing time averaging, and the Doppler shift can be calculated from the sum frequency. In other words, by calculating the average value of the sum frequency, the sum of the Doppler shift of the high-frequency subcarrier 16 and the Doppler shift of the low-frequency subcarrier 17 can be calculated. The magnitude of the Doppler shift is the sum of the Doppler shifts for the high-frequency subcarrier 16 and the low-frequency subcarrier 17, and the sensitivity is increased by approximately two times.

[0083] Furthermore, from equations (6) and (7), the difference frequency f ｄｉｆｆ (t) can be calculated as in the following equation (9).

[0084]

[0085] In the rightmost side of equation (9) showing the difference frequency, the first term is a component due to Doppler shift, the second term is a component due to frequency modulation, and the third term is a component due to frequency noise of the subcarrier generation signal generator 21. At the difference frequency, the component due to frequency noise of the multi-frequency laser 1 is completely eliminated. Frequency noise is a factor that determines the coherence length; the greater the noise, the shorter the coherence length. As reported in Patent Document 1, by using a difference frequency from which frequency noise has been eliminated, it is possible to overcome the limitation on the maximum measurement distance due to the coherence length.

[0086] In the difference frequency of equation (9), the magnitude of the Doppler shift is the difference between the Doppler shifts for the high-frequency subcarrier 16 and the low-frequency subcarrier 17. If the frequency difference between the subcarriers is 50 GHz, this is approximately 1 / 3870 of the Doppler shift of the high-frequency subcarrier 16 and the low-frequency subcarrier 17, which is approximately 193.4 THz. For this reason, when the difference frequency is used, the velocity of the object 7 cannot be determined with high accuracy.

[0087] Next, the sum frequency f ｓｕｍ (t) to calculate the velocity of the object 7, and the difference frequency f ｄｉｆｆ The process of calculating the distance from (t) to the object 7 will be described below. Here, a case where a triangular wave is used as the output of the modulation signal generator 20 will be described.

[0088] 13 is a diagram illustrating the frequency modulation of the reference light and the signal light caused by the high frequency subcarrier 16 and the low frequency subcarrier 17, the beat signal frequency, the sum frequency, and the difference frequency. ｍ 13(a) corresponds to the high frequency subcarrier 16, and FIG. 13(b) corresponds to the low frequency subcarrier 17, where ω represents the modulation period of the triangular wave, and Δν represents the chirp bandwidth.

[0089] The frequency modulation by the triangular wave causes the reference light and the signal light to alternate between up-chirp and down-chirp. Because a time delay occurs in the signal light relative to the reference light, the graphs in Figures 13(a) and 13(b) alternate between time domains where the beat signal frequency is constant and time domains where it transitions from positive to negative or from negative to positive.

[0090] The sum frequency contains a component due to the frequency noise of the multi-frequency laser 1 and a component due to the Doppler shift. The component due to the frequency noise of the multi-frequency laser 1 is noise with an average value of 0, and can be removed by time averaging processing expressed by the following equation (10). In other words, the average value of the sum frequency corresponds to the Doppler shift due to the relative movement of the object 7, i.e., the sum of the Doppler shift of the high-frequency subcarrier 16 and the Doppler shift of the low-frequency subcarrier 17.

[0091]

[0092] The interval for which time averaging is performed can be set arbitrarily, but here it is set to one modulation period corresponding to one distance or velocity measurement. Figure 13(c) shows the sum frequency after time averaging. Using the time average value of the sum frequency, the velocity of the object 7 can be calculated using the following equation (11).

[0093]

[0094] In formula (11), v ０ is the frequency of the carrier 15, and when a carrier 15 with a frequency of 193.4 THz is used, the sensitivity of the sum frequency to the speed is 9.29 [MHz / km / h].

[0095] By using the relationships of equations (6), (7), and (8) in equation (10), the time average of the sum frequency can be expressed by the following equation (12).

[0096]

[0097] Equation (12) indicates that the time average of the sum frequency can be calculated from the phases of the high-frequency subcarriers 16 and the low-frequency subcarriers 17. That is, according to equation (12), the calculation of the time average of the sum frequency does not require the time differentiation of equations (6) and (7) or the averaging process of equation (10). In other words, in the first example of the beat signal generator 9 shown in FIG. 5 , the phase of the complex beat signal is demodulated to determine the sum frequency, so the velocity can be calculated only by calculating the phase without converting the demodulated phase to a frequency.

[0098] 13(d) shows the difference frequency, in which a component due to frequency modulation and a component due to Doppler shift remain. In calculating the distance to the target 7, the time T Ｕ１ From time T Ｕ２ The interval ΔT corresponding to the up-chirp up to Ｕ and time T Ｄ１ From time T Ｄ２ The interval ΔT corresponding to the down chirp up to Ｄ The distance to the object 7 is calculated by avoiding the time region where the beat signal frequency transitions from positive to negative or from negative to positive. Ｕ and interval ΔT Ｄ It is desirable to set the lengths of the lines equal and as long as possible.

[0099] The time domain corresponding to the up-chirp is the interval ΔT Ｕ Then, the difference frequency can be expressed by the following equation (13).

[0100]

[0101] On the right side of equation (13), the first term is the Doppler shift, the second term is the component due to frequency modulation of the subcarrier, and the third term is the component due to frequency noise of the subcarrier generation signal generator 21. The component due to frequency noise of the subcarrier generation signal generator 21 is noise with an average value of 0, and can be removed by time averaging processing expressed by the following equation (14).

[0102]

[0103] The interval ΔT is the time domain corresponding to the down chirp Ｄ Then, the difference frequency can be expressed by the following equation (15).

[0104]

[0105] As in the case of up-chirp, time averaging is performed to remove components due to frequency noise of the subcarrier generation signal generator 21, and the following equation (16) is obtained.

[0106]

[0107] Section ΔT Ｕ and interval ΔT Ｄ From the difference frequency obtained in (1), the distance to the object 7 can be calculated by the following equation (17).

[0108]

[0109] Section ΔT Ｕ and interval ΔT Ｄ When the relative velocity of the object 7 is calculated from the difference frequency calculated in (1), the following equation (18) is used.

[0110]

[0111] In formula (18), f ＳＣ is the center frequency of the subcarrier generation signal generator 21, and f ＳＣ When the frequency is 25 GHz, the sensitivity of the difference frequency to the speed is 2.40 [kHz / km / h]. This sensitivity value is approximately 1 / 3870 of that obtained from the sum frequency.

[0112] As in the case of the sum frequency, the time average of the difference frequency can be replaced with the phases of the high-frequency subcarrier 16 and the low-frequency subcarrier 17. That is, in the first example of the beat signal generator 9 shown in Fig. 5, the phase of the complex beat signal is determined by demodulation, and therefore the distance to the target 7 can be calculated simply by calculating the phase, without converting the demodulated phase to a frequency.

[0113] The following formula (19) is an arithmetic formula for calculating the average value of the difference frequency in the up-chart region only by calculating the phase, and the following formula (20) is an arithmetic formula for calculating the average value of the difference frequency in the down-chart region only by calculating the phase.

[0114]

[0115]

[0116] The process of calculating the velocity of the object 7 by the calculation device 14 can be summarized as follows. Specifically, the calculation device 14 demodulates the first complex beat signal 31 to obtain a first frequency, which is the frequency of the first complex beat signal 31, and demodulates the second complex beat signal 32 to obtain a second frequency, which is the frequency of the second complex beat signal 32. The demodulation performed by the calculation device 14 on the two complex beat signals corresponds to the process of equation (5) above. Specifically, the calculation device 14 calculates the arctangent (inverse tangent) of the value obtained by dividing the quadrature component with respect to the high-frequency subcarrier 16 by the in-phase component with respect to the high-frequency subcarrier 16, and performs phase unwrapping on the calculated value to obtain the first frequency. The calculation device 14 also calculates the arctangent (inverse tangent) of the value obtained by dividing the quadrature component with respect to the low-frequency subcarrier 17 by the in-phase component with respect to the low-frequency subcarrier 17, and performs phase unwrapping on the calculated value to obtain the second frequency. Then, the calculation device 14 calculates the velocity of the object 7 using the average value of the sum frequency, which is the sum of the first frequency and the second frequency, by processing based on the above equations (8), (11), and (12).

[0117] Next, the optical measurement method of this embodiment will be described with reference to the flowchart of Fig. 14. Here, the process of calculating the velocity using the sum frequency in the optical measurement method of this embodiment will be briefly described.

[0118] First, the optical measurement device 100 uses the multi-frequency laser 1 to simultaneously generate a frequency-modulated high-frequency subcarrier 16 and a frequency-modulated low-frequency subcarrier 17, which are positioned symmetrically with respect to the frequency of the carrier 15, and outputs light containing the two subcarriers (16, 17) to the optical branching element 2a (step S101: subcarrier generation step). Next, the optical measurement device 100 splits the output light of the multi-frequency laser 1 into a probe light 3 and a reference light 4 using the optical branching element 2a. The optical branching element 2a outputs the probe light 3 to the optical system 5 and the reference light 4 to the beat signal generating unit 9 (step S102: optical branching step).

[0119] Next, the optical measurement device 100 irradiates the object 7 with the probe light 3 via the optical system 5 and outputs scattered light from the object 7 as signal light 8 to the beat signal generator 9 (step S103: irradiation process). Subsequently, the optical measurement device 100 generates, via the beat signal generator 9 to which the reference light 4 and the signal light 8 are input, a first complex beat signal 31 derived from the high-frequency subcarrier 16 and including an in-phase component 10 and a quadrature component 11, and a second complex beat signal 32 derived from the low-frequency subcarrier 17 and including an in-phase component 12 and a quadrature component 13. The beat signal generator 9 outputs the in-phase component 10 and the quadrature component 11 of the first complex beat signal 31 and the in-phase component 12 and the quadrature component 13 of the second complex beat signal 32 to the calculation device 14 (step S104: complex beat signal generation process).

[0120] The optical measurement device 100 then uses the arithmetic unit 14 to determine a sum frequency, which is the sum of the frequencies of the first complex beat signal 31 and the second complex beat signal 32, and calculates the velocity of the object 7 from the determined sum frequency (step S105: velocity calculation step). More specifically, the arithmetic unit 14 demodulates the first complex beat signal 31 to determine the first frequency and demodulates the second complex beat signal 32 to determine the second frequency (step S201), adds the first frequency and the second frequency to determine the sum frequency (step S202), and determines the velocity of the object 7 using the average value of the sum frequency (step S203).

[0121] The optical measurement program 14p described above causes the computer provided in the calculation device 14, which determines the velocity of the object 7 based on the output light of the multi-frequency laser 1 that generates a frequency-modulated high-frequency subcarrier 16 and a frequency-modulated low-frequency subcarrier 17 that are positioned symmetrically with respect to the frequency of the carrier 15, to function as a velocity calculation means that demodulates a first complex beat signal 31 that originates from the high-frequency subcarrier 16 and includes an in-phase component 10 and a quadrature component 11 to determine a first frequency, demodulates a second complex beat signal 32 that originates from the low-frequency subcarrier 17 and includes an in-phase component 12 and a quadrature component 13 to determine a second frequency, adds the first frequency and the second frequency to determine a sum frequency, and uses the average value of the sum frequency to determine the velocity of the object 7.

[0122] [Evaluation of Measurement Accuracy] A simulation was performed to evaluate the measurement accuracy of the velocity of the object 7 using the optical measurement device 100 according to this embodiment. In this simulation, two complex beat signals containing noise were generated using the second example of the beat signal generator 9 shown in FIG. 8 , and the distance and velocity were calculated according to the process shown in FIG. 12 , and the accuracy was then evaluated. The noise considered included the frequency noise of the multi-frequency laser 1, as well as the shot noise and detector noise generated in the balanced photodetectors 44 a and 44 b. Shot noise is noise inherent in the laser light incident on the balanced photodetector. Photodetector noise is noise generated even in the absence of incident light, and includes dark current noise of the balanced photodetector and thermal noise of the amplifier located downstream. Compared to the frequency noise of the multi-frequency laser 1, the frequency noise of the subcarrier generation signal generator 21 is small and therefore ignored.

[0123] The flow of the simulation will be explained below. First, the frequency f of the complex beat signal generated from the high frequency subcarrier 16 and the low frequency subcarrier 17 is calculated. ± (t) is generated.

[0124]

[0125] On the right side of equation (21), the first term represents the frequency noise of the multi-frequency laser 1, the second term represents the Doppler shift due to the relative movement of the object 7, and the third term represents the component due to the frequency modulation of the subcarrier. ｄ The time was set to 335 ns, which corresponds to a distance of 100.43 m, and the movement speed of the object 7 was set to V=50.0 [km / h].

[0126] FIG. 15(a) shows the frequency modulation ν Ｍ 15(a) is a time waveform of the frequency modulation v Ｍ In (t), the modulation waveform is a triangular wave, the modulation frequency is 10 kHz, and the chirp bandwidth is 1 GHz. The frequency modulation of the low-frequency subcarrier 17 is an inverse phase -ν Ｍ (t). FIG. 15(b) shows the frequency noise v Ｎ15(b) shows the time waveform generated by numerical calculation when white noise corresponding to a spectral linewidth of 3 MHz is set as the frequency noise of the single-frequency laser 18 or the semiconductor laser 22. The frequencies of the high-frequency subcarrier 16 and the low-frequency subcarrier 17 output from the optical modulator 19 contain in-phase frequency noise v Ｎ (t) appears.

[0127] Next, the frequency f of the complex beat signal expressed by equation (21) ± By substituting (t) into the following equations (22) and (23), the in-phase and quadrature components of two complex beat signals are generated.

[0128]

[0129]

[0130] In formula (22), I ± ｓｈｏｔ (t) and I ± ＰＤ (t) are the shot noise and photodetector noise appearing in the in-phase component of the complex beat signal, respectively. ± ｓｈｏｔ (t) and Q ± ＰＤ (t) are the shot noise and photodetector noise appearing in the quadrature components of the complex beat signal, respectively. All of these noises are white noises with no correlation with each other. ± is an amplitude determined by the power of the reference light and the signal light incident on the balanced photodetector, and can be expressed by the following equation (24).

[0131]

[0132] In equation (24), η is the quantum efficiency of the balanced photodetector, G is the gain of the amplifier placed after the balanced photodetector, and P ｒｅｆ is the reference light power, P ｓｉｇ is the signal light power.

[0133] FIG. 16(a) shows the shot noise I appearing in the in-phase component of the high-frequency subcarrier 16. ＋ ｓｈｏｔ1 is a diagram showing the time waveform of (t) and the standard deviation σ of the shot noise. ｓｈｏｔ can be expressed by the following formula (25).

[0134]

[0135] In equation (25), e is the charge of an electron, f ＰＤ is the bandwidth of the balanced photodetector. The time waveform in FIG. 16(a) has G = 39 [kV / A], η = 0.9 [A / W], and P ｒｅｆ = 100 [μW], f ＰＤ 16(a) shows white noise obtained by numerical calculation using the values ​​of 1 / 2 Hz = 100 [MHz]. For the other components of equation (21), white noise uncorrelated with the time waveform in FIG. 16(a) was generated using the same numerical values.

[0136] FIG. 16(b) shows the photodetector noise I appearing in the in-phase component of the high-frequency subcarrier 16. ＋ ＰＤ 10 is a diagram showing the time waveform of (t) and the standard deviation σ of the photodetector noise. ＰＤ can be expressed by the following formula (26).

[0137]

[0138] In formula (26), N ＰＤ is the noise equivalent power of the balanced photodetector. The time waveform in FIG. 16(b) is G=39 [kV / A], N ＰＤ = 8 [pA / Hz １／２ ], f ＰＤ 16B. The white noise obtained by numerical calculation using the values ​​of 1 / 2 Hz = 100 [MHz] is shown. For the other components of equations (22) and (23), the same numerical values ​​were used to generate white noise that is uncorrelated with the time waveform in FIG. 16B.

[0139] Figure 17(a) shows the complex beat signal for the high-frequency subcarrier 16, and Figure 17(b) shows the complex beat signal for the low-frequency subcarrier 17, displayed on the IQ plane. The results are calculated using equations (22) and (23) with a reference light power of 100 μW and a signal light power of 100 nW. The in-phase and quadrature components of the complex beat signal are out of phase with each other by 90°, so points are arranged on a circle on the IQ plane. The spread of the circle corresponds to shot noise and photodetector noise.

[0140] For the complex beat signal of equation (21), the phase Φ ± After determining (t), the frequency was calculated according to equations (6), (7), (8), and (9). Figure 18(a) shows the frequency of the complex beat signal of the high-frequency subcarrier 16, and Figure 18(b) shows the time waveform of the frequency of the complex beat signal of the low-frequency subcarrier 17. The reference light power was 100 μW, and the signal light power was 100 nW.

[0141] In Figure 18, the first half of the time waveform corresponds to a down-chirp, and the second half corresponds to an up-chirp. Since the frequency modulation of the high-frequency subcarrier 16 and the low-frequency subcarrier 17 is in opposite phase, the frequencies of the complex beat signals are also in opposite phase. The noise appearing in the time waveforms of Figures 18(a) and 18(b) is primarily due to the frequency noise of the multi-frequency laser 1, but shot noise and photodetector noise also have an effect. Furthermore, due to the Doppler shift, the frequency of each waveform is shifted overall to the lower frequency side.

[0142] Figure 18(c) shows the time waveform of the difference frequency. In the difference frequency, the frequency noise of the multi-frequency laser 1 is canceled out, and the frequency difference corresponding to the down-chirp and up-chirp appears more clearly. The residual noise in the difference frequency is caused by shot noise and photodetector noise. In addition, the frequency shift caused by the Doppler shift is "-9.27 kHz," which cannot be distinguished in the time waveform.

[0143] 18(d) is a diagram showing the time waveform of the sum frequency. In the sum frequency, the frequency modulation is canceled out, and the frequency noise of the multi-frequency laser 1, shot noise, and photodetector noise appear. The frequency change due to the Doppler shift is "-35.8 MHz," and the entire frequency is shifted to the lower frequency side. The Doppler shift can be obtained by calculating the time average of the sum frequency.

[0144] Figure 19 shows the time waveform of the difference frequency when the signal light power is changed from 10 μW to 100 pW. When the signal light power is 1 μW or higher, the component due to frequency modulation is dominant, and the frequency difference corresponding to down-chirp and up-chirp is clearly visible. As the signal light power decreases, noise due to shot noise and photodetector noise increases, and when the signal light power is 100 pW, the component due to frequency modulation cannot be identified.

[0145] 20 is a diagram showing the time waveform of the sum frequency when the signal light power is changed from 10 μW to 100 pW. In the sum frequency, the frequency modulation is canceled out, and the frequency noise of the multi-frequency laser 1, shot noise, and photodetector noise appear. When the signal light power is 10 nW or more, the magnitude of the noise is roughly constant, and the frequency noise of the multi-frequency laser 1 is dominant. In the region where the signal light power is 1 nW or less, the noise caused by the shot noise and photodetector noise increases as the signal light power decreases.

[0146] 21 shows the relationship between signal light power and distance calculated from the difference frequency. For a set distance of 100.43 m, the distance measurement error is less than 1 cm when the signal light power is 400 pW or higher. When the signal light power is 400 pW or lower, the error increases as the signal light power decreases, reaching 6.4 m when the signal light power is 100 pW.

[0147] FIG. 22 shows the changes in the velocity calculated from the sum frequency and the velocity calculated from the difference frequency when the signal light power is changed. When the velocity is calculated from the difference frequency, an error of 0.7 km / h occurs even when the signal light power is 1 μW. The error is thought to be caused by a decrease in sensitivity to the Doppler shift of the difference frequency. While the error is almost constant when the signal light power is between 1 μW and 400 pW, it increases rapidly below 400 pW, reaching unrealistic values. On the other hand, when the velocity is calculated from the sum frequency, the increase in error with decreasing signal light power is gradual, remaining below 10% even when the signal light power is 100 pW. This shows that using the sum frequency enables highly accurate velocity measurement of the object 7.

[0148] As described above, in the optical measurement device 100 of this embodiment, the multi-frequency laser 1 generates the frequency-modulated high-frequency subcarrier 16 and the frequency-modulated low-frequency subcarrier 17, which are positioned symmetrically with respect to each other with respect to the carrier frequency, and the optical branching element 2a splits the output light of the multi-frequency laser into the probe light 3 and the reference light 4. Next, the optical measurement device 100 irradiates the object 7 with the probe light 3 using the optical system 5, outputs scattered light from the object 7 as the signal light 8, and generates the first complex beat signal 31 derived from the high-frequency subcarrier 16 and the second complex beat signal 32 derived from the low-frequency subcarrier 17 from the reference light 4 and the signal light 8 using the beat signal generator 9. The calculation device 14 then calculates the velocity of the object 7 using the average value of the sum frequency, which is the sum of the frequency (first frequency) of the first complex beat signal 31 and the frequency (second frequency) of the second complex beat signal 32. In this way, the optical measurement device 100 calculates the velocity of the object 7 using the average value of the sum frequency, and therefore can remove components caused by frequency noise. This makes it possible to overcome the distance limitation to the object 7 caused by the coherence length of the laser, and to measure the velocity of the object 7 with high accuracy.

[0149] The arithmetic unit 14 in this embodiment is configured to demodulate the first complex beat signal 31 to obtain a first frequency, demodulate the second complex beat signal 32 to obtain a second frequency, and add the first and second frequencies to obtain a sum frequency. These demodulations are so-called phase demodulations, and correspond to the process of the above equation (5), for example. This process simplifies and speeds up the calculation process.

[0150] Various configurations capable of realizing the above functions can be adopted for the multi-frequency laser 1. For example, as shown in Fig. 2, the multi-frequency laser 1 may be configured to have a single-frequency laser 18, an optical modulator 19, a modulation signal generator 20, and a subcarrier generation signal generator 21, and to generate a high-frequency subcarrier 16 and a low-frequency subcarrier 17 in the output light of the optical modulator 19 by the modulation signal generator 20 and the subcarrier generation signal generator 21. Alternatively, as shown in Fig. 3, the multi-frequency laser 1 may be configured to have a semiconductor laser 22, a modulation signal generator 20, and a subcarrier generation signal generator 21, and to generate a high-frequency subcarrier 16 and a low-frequency subcarrier 17 in the output light of the semiconductor laser 22 by the modulation signal generator 20 and the subcarrier generation signal generator 21.

[0151] The above-described embodiments are merely examples of the optical measurement device, optical measurement method, and optical measurement program, and the technical scope of the present invention is not limited to these embodiments. For example, the optical measurement device 100 may be configured to output the first complex beat signal 31 shown in FIGS. 5 and 6 and the second complex beat signal 32 shown in FIGS. 5 and 7 directly to the arithmetic unit 14. In this case, the arithmetic unit 14 may be configured to perform the processes performed by the quadrature detectors 33a and 33b in the configuration example shown in FIG. 5, i.e., the series of processes for separating and outputting the two complex beat signals into in-phase and quadrature components. In other words, the arithmetic unit 14 may have the function of extracting the in-phase component 10 and the quadrature component 11 from the first complex beat signal 31 and the in-phase component 12 and the quadrature component 13 from the second complex beat signal 32.

[0152] The optical measurement device, optical measurement method, and optical measurement program of the present invention can detect Doppler shifts caused by the relative motion of an object with high sensitivity, thereby enabling accurate measurement of the object's velocity. Furthermore, the optical measurement device, optical measurement method, and optical measurement program of the present invention are industrially useful as small, low-cost FMCW lidar systems because they do not require complex devices or processing. Furthermore, the optical measurement device, optical measurement method, and optical measurement program of the present invention can be used in consumer devices, including as environmental recognition sensors for automobiles, autonomous robots, and the like.

[0153] 1 Multi-frequency laser, 2a Optical branching element, 2b Optical coupling element, 2c, 2d, 2e, 2f Beam splitter, 3 Probe light, 4 Reference light, 5 Optical system, 5a Optical circulator, 5b Transmitting and receiving optical system, 7 Object, 8 Signal light, 9 Beat signal generating unit, 10, 12 In-phase component, 11, 13 Quadrature component, 14 Arithmetic unit, 14a Communication unit, 14b Arithmetic processing unit, 14c Memory unit, 14p Optical measurement program, 15 Carrier, 16 High frequency subcarrier, 17 Low frequency subcarrier, 18 Single frequency laser, 19 Optical modulator, 20 Modulation signal generator, 21 Subcarrier generation signal generator, 22 Semiconductor laser, 26 Optical frequency shifter, 27 Optical frequency shifter drive signal source, 28 Reference signal, 29a, 29b, 29c Optical demultiplexer, 30a, 30b Photodetector, 31: First complex beat signal, 32: Second complex beat signal, 33a, 33b: Quadrature detector, 34: π / 2 electrical phase shifter, 35a, 35b: Frequency mixer, 36a, 36b: Low-pass filter, 37, 38: Reference light, 39, 40: Signal light, 41a, 41b: Phase diversity detector, 42: π / 2 optical phase shifter, 43a, 43b: Total reflection mirror, 44a, 44b: Balanced photodetector, 100: Optical measurement device.

Claims

1. An optical measurement device comprising: a multi-frequency laser that generates a frequency-modulated high-frequency subcarrier and a frequency-modulated low-frequency subcarrier that are symmetrically positioned with respect to each other with respect to the carrier frequency; an optical branching element that splits the output light of the multi-frequency laser into a probe light and a reference light; an optical system that irradiates an object with the probe light and outputs scattered light from the object as a signal light; a beat signal generating unit that inputs the reference light and the signal light and generates and outputs a first complex beat signal derived from the high-frequency subcarrier and including an in-phase component and a quadrature component, and a second complex beat signal derived from the low-frequency subcarrier and including an in-phase component and a quadrature component; and a calculation unit that calculates the velocity of the object using the average value of the sum frequency, which is the sum of the frequency of the first complex beat signal and the frequency of the second complex beat signal.

2. The optical measurement device of claim 1, wherein the multi-frequency laser comprises: a single-frequency laser; an optical modulator that performs a modulation process on the output light of the single-frequency laser; a subcarrier generation signal generator that drives the optical modulator to generate two subcarriers in the output light of the single-frequency laser; and a modulation signal generator that applies frequency modulation of opposite phases to each of the two subcarriers generated by the subcarrier generation signal generator, wherein the modulation signal generator and the subcarrier generation signal generator generate the high-frequency subcarrier and the low-frequency subcarrier in the output light of the optical modulator.

3. The optical measurement device of claim 1, wherein the multi-frequency laser comprises: a semiconductor laser; a subcarrier generation signal generator that generates two subcarriers in the output light of the semiconductor laser; and a modulation signal generator that imparts frequency modulation of opposite phases to each of the two subcarriers generated by the subcarrier generation signal generator, wherein the high-frequency subcarrier and the low-frequency subcarrier are generated in the output light of the semiconductor laser by the modulation signal generator and the subcarrier generation signal generator.

4. The optical measurement device according to any one of claims 1 to 3, wherein the arithmetic unit demodulates the first complex beat signal to obtain a first frequency which is a frequency of the first complex beat signal, demodulates the second complex beat signal to obtain a second frequency which is a frequency of the second complex beat signal, and adds the first frequency and the second frequency to obtain the sum frequency.

5. An optical measurement method using an optical measurement device equipped with a multi-frequency laser, comprising: a subcarrier generation step in which a frequency-modulated high-frequency subcarrier and a frequency-modulated low-frequency subcarrier, which are positioned symmetrically with respect to each other with respect to the carrier frequency, are generated simultaneously by the multi-frequency laser; an optical branching step in which an output light of the multi-frequency laser is split into a probe light and a reference light by an optical branching element; an irradiation processing step in which the probe light is irradiated onto an object by an optical system and scattered light from the object is output as a signal light; a complex beat signal generation step in which a beat signal generation unit to which the reference light and the signal light are input generates and outputs a first complex beat signal derived from the high-frequency subcarrier and including an in-phase component and a quadrature component, and a second complex beat signal derived from the low-frequency subcarrier and including an in-phase component and a quadrature component; and a velocity calculation step in which a calculation device calculates the velocity of the object using the average value of a sum frequency, which is the sum of the frequency of the first complex beat signal and the frequency of the second complex beat signal.

6. The optical measurement method according to claim 5, wherein in said velocity calculation step, said calculation device demodulates said first complex beat signal to obtain a first frequency which is the frequency of said first complex beat signal, demodulates said second complex beat signal to obtain a second frequency which is the frequency of said second complex beat signal, and adds said first frequency and said second frequency to obtain said sum frequency.

7. An optical measurement method in which a calculation device that determines the velocity of an object based on the output light of a multi-frequency laser that generates a frequency-modulated high-frequency subcarrier and a frequency-modulated low-frequency subcarrier that are symmetrically positioned with respect to the carrier frequency, demodulates a first complex beat signal that is derived from the high-frequency subcarrier and includes an in-phase component and an orthogonal component to determine a first frequency that is the frequency of the first complex beat signal, and demodulates a second complex beat signal that is derived from the low-frequency subcarrier and includes an in-phase component and an orthogonal component to determine a second frequency that is the frequency of the second complex beat signal, adds together the first frequency and the second frequency to determine a sum frequency, and determines the velocity of the object using an average value of the sum frequency.

8. An optical measurement program for causing a computer provided in a calculation device that determines the velocity of an object based on the output light of a multi-frequency laser that generates a frequency-modulated high-frequency subcarrier and a frequency-modulated low-frequency subcarrier that are symmetrically positioned with respect to the carrier frequency to function as a velocity calculation means that demodulates a first complex beat signal that is derived from the high-frequency subcarrier and includes an in-phase component and an orthogonal component to determine a first frequency that is the frequency of the first complex beat signal, demodulates a second complex beat signal that is derived from the low-frequency subcarrier and includes an in-phase component and an orthogonal component to determine a second frequency that is the frequency of the second complex beat signal, adds the first frequency and the second frequency to determine a sum frequency, and determines the velocity of the object using the average value of the sum frequency.

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