OPTICAL MEASUREMENT DEVICE, OPTICAL MEASUREMENT METHOD, AND OPTICAL MEASUREMENT PROGRAM

Abstract

An optical measurement device that splits 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 into a probe light and a reference light by an optical branching element. The optical measurement device has 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 generator that generates a first complex beat signal derived from the high-frequency subcarrier and a second complex beat signal derived from the low-frequency subcarrier from the reference light and the signal light, and a calculation device that calculates the velocity of the object using the average value of the sum of the frequencies of the first complex beat signal and the second complex beat signal.

Description

[Technical field]

[0001] The present invention relates to an optical measurement device, an optical measurement method, and an optical measurement program suitable for an environment recognition sensor used in an automobile, an autonomous robot, or the like. [Background technology]

[0002] The 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 with 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 speed from the Doppler shift. FMCW LiDAR in the millimeter wave region has been put to practical use as an on-board collision prevention sensor. If FMCW LiDAR can be used in the light wave region, it is expected that the spatial resolution and measurement accuracy will be significantly improved.

[0003] One of the challenges for FMCW lidar is to overcome the limitation of the maximum measurement distance due to the laser coherence. FMCW lidar modulates the frequency of the laser output light, mixes the scattered light from the target with the reference light that serves as the measurement standard, performs coherent detection, and generates a beat signal. In addition to frequency modulation, the laser output light is accompanied by fluctuations due to frequency noise generated by the laser itself. Therefore, the beat signal contains a mixture of components caused by frequency modulation and components caused by frequency noise. As the distance to the target increases, the time delay (delay time) of the scattered light relative to the reference light increases, and the components caused by frequency noise become relatively large compared to the components caused by frequency modulation, which reduces the signal-to-noise ratio of the beat signal and causes deterioration of the measurement accuracy.

[0004] When using FMCW lidar as an environmental recognition sensor for autonomous vehicles, 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, and 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. In FMCW lidar, the measured light travels a round trip 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 utilizes a difference frequency (difference) signal between two complex beat signals generated by a multi-frequency laser in order to overcome the limitation on the maximum measurement distance caused by laser coherence when measuring the speed of an object. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2021 / 131315 Summary of the Invention [Problem to be solved by the invention]

[0007] The FMCW lidar calculates the relative velocity of the 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 of Patent Document 1 is configured to measure the velocity of the object by using a difference frequency signal between complex beat signals generated by two measurement lights with different frequencies. A Doppler shift proportional to the frequency occurs in the two measurement lights used in the optical measurement device, but in the difference frequency signal, the Doppler shift is subtracted and a Doppler shift proportional to the frequency difference between the two measurement lights remains. For example, when the frequency difference between the two measurement lights is 50 GHz at a wavelength of 1550 nm (frequency of 193.4 THz), the magnitude of the Doppler shift is about 1 / 3870, and the detection sensitivity is low. For this reason, it is not possible to improve the measurement accuracy of 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 speed of an object with high accuracy by overcoming the distance limitation between the object and the device caused by the coherence length of the laser. [Means for solving the problem]

[0009] An optical measurement device according to one embodiment 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 symmetrically positioned with respect to the carrier frequency, a light 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 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 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, which are 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 scattered light from the object as 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 a sum frequency that is the sum of the frequency of the first complex beat signal and the frequency of the second complex beat signal, and calculates the velocity of the object using an average value of the calculated sum frequencies.

[0011] In one embodiment of the optical measurement method of the present invention, a calculation device 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, demodulating 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, demodulating 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, 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 embodiment of the present invention causes 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 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. Effect of the Invention

[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. Therefore, it is possible to remove components caused by frequency noise, thereby overcoming the distance limitation between the object and the object caused by the coherence length of the laser, and enabling the velocity of the object to be measured with high accuracy. [Brief description of the drawings]

[0014] [Figure 1] 1 is a schematic diagram illustrating a basic configuration of an optical measurement device according to an embodiment of the present invention; [Diagram 2] FIG. 2 is a diagram showing an example of a specific configuration of the multi-frequency laser in FIG. [Diagram 3] 2 is a configuration diagram showing another example of a specific configuration of the multi-frequency laser in FIG. 1. [Figure 4] 2 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. [Diagram 5] 2 is a configuration diagram showing a first example of the beat signal generating section of FIG. 1. [Figure 6]6 is a configuration diagram showing a specific example of one of the quadrature detectors in FIG. 5. [Figure 7] 6 is a configuration diagram showing a specific example of the other quadrature detector in FIG. 5. FIG. [Figure 8] 1. FIG. 4 is a configuration diagram showing a second example of the beat signal generating unit in FIG. [Figure 9] FIG. 9 is a block diagram showing a specific example of one of the phase diversity detectors in FIG. 8. [Figure 10] 9 is a block diagram showing a specific example of the other phase diversity detector in FIG. 8. FIG. [Figure 11] 2 is an explanatory diagram illustrating an example of a Doppler shift that a carrier and each subcarrier undergo when an object moves relative to the optical measurement device of FIG. 1. [Figure 12] 2 is an explanatory diagram illustrating a process in which the arithmetic device of FIG. 1 calculates the velocity and distance to an object by using a sum frequency and a difference frequency based on the frequencies of two complex beat signals. [Figure 13] 1A and 1B are diagrams for explaining frequency modulation of reference light and signal light caused by high frequency subcarriers and low frequency subcarriers, as well as beat signal frequencies, sum frequencies, and difference frequencies. [Figure 14] 4 is a flowchart illustrating a schematic flow of operations in an optical measurement method according to an embodiment of the present invention. [Figure 15] This is a diagram in which two graphs of time waveforms related to frequency are associated with each other, where (a) shows the time waveform of the frequency modulation vM(t) of a high-frequency subcarrier, and (b) shows the time waveform of the frequency noise vN(t) of a multi-frequency laser. [Figure 16] This is a diagram that correlates two graphs of time waveforms related to amplitude, where (a) shows the time waveform of shot noise I+ shot(t) appearing in the in-phase component of the high-frequency subcarrier, and (b) shows the time waveform of photodetector noise I+ PD(t) appearing in the in-phase component of the high-frequency subcarrier. [Figure 17] This is a diagram showing the complex beat signals 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. [Figure 18] This figure shows the time changes of multiple frequencies, where (a) shows the time waveform of the high-frequency subcarrier frequency, (b) shows the time waveform of the low-frequency subcarrier frequency, (c) shows the time waveform of the difference frequency, and (d) shows the time waveform of the sum frequency. [Figure 19] 13 is a diagram showing the time waveform of the difference frequency when the signal light power is changed from 10 μW to 100 pW. [Figure 20] 13 is a diagram showing the time waveform of the sum frequency when the signal light power is changed from 10 μW to 100 pW. [Figure 21] 1 is a graph showing the relationship between signal light power and distance calculated from the difference frequency on a coordinate system with the horizontal axis representing signal light power and the vertical axis representing the distance to the object. [Figure 22] 1 is a graph showing the velocity calculated from the sum frequency and the velocity calculated from the difference frequency on a coordinate system with the signal light power on the horizontal axis and the velocity of the object on the vertical axis. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Embodiment 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 Fig. 1 to Fig. 22. In each figure, some of the reference symbols may be omitted in order 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 generating unit 9, and a calculation 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 located 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 obtain a two-dimensional image of the distance or speed of the object 7. Incidentally, FIG. 1 shows an example in which the optical system 5 has the optical circulator 5a and the transmitting and receiving optical system 5b, but 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, and generates and outputs a first complex beat signal 31 originating 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 originating 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 calculation device 14.

[0021] The arithmetic device 14 obtains 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 obtained 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 device 14 demodulates the phases of the two complex beat signals (the first complex beat signal 31 and the second complex beat signal 32), obtains the respective frequencies by time differentiation of the phases, and calculates the sum frequency from the obtained frequencies. The arithmetic device 14 then calculates the velocity of the object 7 from the obtained 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 has a communication unit 14a, an arithmetic processing unit 14b, and a storage unit 14c. The communication unit 14a is an interface for the arithmetic device 14 to communicate with an external device such as the beat signal generating unit 9 by wire or wirelessly. The storage unit 14c stores various information in addition to the operation program of the arithmetic processing unit 14b such as the optical measurement program 14p. The storage unit 14c is composed of a RAM (Random Access Memory) and a ROM (Read Only Memory), a PROM (Programmable ROM) such as a flash memory, an SSD (Solid State Drive), or an HDD (Hard Disk Drive). The arithmetic processing unit 14b has a speed calculation means for calculating the speed of the object 7. The arithmetic processing unit 14b may have a distance calculation means for calculating the distance to the object 7. The arithmetic processing unit 14b is composed of a CPU (Central Processing Unit) or a GPU (Graphics Processing Unit), etc. The optical measurement program 14p cooperates with the calculation processing unit 14b to realize various functions such as a speed calculation means.

[0024] Next, a specific configuration example of the multi-frequency laser 1 will be described with reference to Fig. 2 and Fig. 3. Fig. 2 is a first example of a specific configuration of the multi-frequency laser 1. The multi-frequency laser 1 illustrated in Fig. 2 has 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 performs modulation processing on the output light of the single-frequency laser 18. The optical modulator 19 is composed of an intensity modulator, a phase modulator, etc., and has a function of generating sidebands by modulation processing.

[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 symmetrically positioned 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 defined as f SC The center frequency f of the subcarrier generating signal generator 21 is SC is set to a frequency that matches the characteristics of the beat signal generating unit 9. If the center frequency of the carrier 15 of the output light of the multi-frequency laser 1 is ν0, the center frequency of the high-frequency subcarrier 16 is ν0+f SC and the center frequency of the low-frequency subcarrier 17 is ν0-f SC It 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 with mutually opposite phases to each of the two subcarriers generated by the subcarrier generation signal generator 21. In other words, the multi-frequency laser 1 in Fig. 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 is a second example of a specific configuration of the multi-frequency laser 1. The multi-frequency laser 1 illustrated 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 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 by, for example, a distributed feedback laser (DFB laser) or a modulator integrated semiconductor laser (EML). An EML is a semiconductor laser with an integrated 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] Since the multi-frequency laser 1 modulates the carrier 15 to generate high-frequency subcarrier 16 and low-frequency subcarrier 17, which are ±1-order subcarriers, the high-frequency subcarrier 16 and low-frequency subcarrier 17 have frequency noise in phase with the carrier 15. Furthermore, in the multi-frequency laser 1, the high-frequency subcarrier 16 and low-frequency subcarrier 17 are subjected to frequency modulation and frequency noise of the subcarrier generation signal generator 21.

[0030] Next, a configuration example 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 carrier 15 and frequency noise superimposed on the two subcarriers. That is, the output light of the multi-frequency laser 1 is composed of a carrier 15 with a center frequency of ν0 and a frequency noise with a center frequency of ν0+f SC and a high frequency subcarrier 16 with a center frequency of ν0-f SC and a low frequency subcarrier 17 which is a high frequency subcarrier 16. Frequency modulation and frequency noise are superimposed on the high frequency subcarrier 16 and the low frequency subcarrier 17.

[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, only the ±1 order subcarriers are shown in FIG. 4, but the multi-frequency laser 1 may include subcarriers of higher orders.

[0032] The frequency ν0 of the carrier 15 and the frequencies ν0±f of the high frequency subcarrier 16 and the low frequency subcarrier 17 SC It is preferable that the center frequency f of the subcarrier generation signal generator 21 is sufficiently separated from the center frequency f of the subcarrier generation signal generator 21. SC The center frequency f is preferably within the range that the modulator or the direct modulation of the laser can respond to. SC and frequency ν0 can be chosen arbitrarily, and in principle there are no particular limitations. However, in reality, the center frequency f SC The setting values ​​of the center frequency f and frequency ν are limited by the specifications of the available components. SCThe minimum value of is determined, for example, by the range in which there is a frequency difference (about 5 GHz) between the frequency ν0 and the frequency ν0 that can be separated by an optical filter. SC The maximum value of depends largely on the modulation performance of the modulation signal generator 20, and is, for example, about 50 GHz. The frequency v0 of the carrier 15 can be any frequency at which a single-frequency laser exists. In other words, the frequency v0 of the carrier 15 can be set in any region, for example, from the ultraviolet region to the visible region to the infrared region.

[0033] In the following, the center frequency is generated from the carrier 15 having a center frequency of ν0. SC An example will be described in which ±1-order subcarriers of the carrier 15 are used, but the high frequency subcarrier 16 and the low frequency subcarrier 17 are not limited to ±1-order subcarriers with respect to the carrier 15. The optical measurement device 100 may be configured so that the multi-frequency laser 1 generates higher-order subcarriers, that is, ±2-order or higher subcarriers, as the high frequency subcarrier 16 and the low frequency subcarrier 17, and these may be used to perform calculations of speed, etc.

[0034] Next, specific configuration examples of the beat signal generating section 9 will be described with reference to Figs. 5 to 10. First, a first example of the beat signal generating section 9 will be described with reference to Figs. 5 to 7. Fig. 5 is a configuration diagram showing the first example of the beat signal generating section 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 driving signal source 27, an optical coupling element 2b, an optical splitter 29a, an optical detector 30a, an optical detector 30b, a quadrature detector 33a, and a quadrature detector 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 detector 33a and the quadrature detector 33b.

[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 splitter 29a. The optical splitter 29a separates the light output from the optical coupling element 2b into a component originating from the high frequency subcarrier 16 and a component originating 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 splitter 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 splitter 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 33b 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 by the optical frequency shifter 26, then combines it with the signal light 8 by the optical coupling element 2b and inputs it to the optical splitter 29a. The optical splitter 29a separates the input light into a component originating from the high frequency subcarrier 16 and a component originating from the low frequency subcarrier 17 and outputs them. The components separated by the optical splitter 29a are received by the photodetector 30a and the photodetector 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 33b outputs the in-phase component 12 and the quadrature component 13 of the input second complex beat signal 32 to the arithmetic unit .

[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, a frequency mixer 35a, a frequency mixer 35b, a low-pass filter 36a, and a low-pass filter 36b. As shown in Fig. 6, the quadrature detector 33a is configured to divide the first complex beat signal 31 into two, input one of the two into the frequency mixer 35a, and input the other into the frequency mixer 35b. As shown in Fig. 7, the quadrature detector 33b is configured to divide the second complex beat signal 32 into two, input one of the two into the frequency mixer 35a, and input the other into the frequency mixer 35b.

[0041] The quadrature detector 33a and the quadrature detector 33b each divide the reference signal 28 output from the optical frequency shifter driving signal source 27 in half, outputting one of the two to the frequency mixer 35a and outputting the other to the π / 2 electrical phase shifter 34. The frequency mixer 35a inputs one of the two divided signals 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 by the input local oscillation signal, and outputs the converted signal to the low-pass filter 36a. The frequency mixer 35a has a function of multiplying two input signals and outputting a sum frequency signal and a difference frequency signal of both signals.

[0042] The π / 2 electrical phase shifter 34 of the quadrature detector 33a and the quadrature detector 33b shifts the phase of the other signal divided by the reference signal 28 by π / 2 and outputs the shifted 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 by the input local oscillation signal and outputs the converted signal to the low-pass filter 36b. The frequency mixer 35b has a function of multiplying 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 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 driving signal source 27 as a local oscillation signal for the frequency mixer 35a. On the other hand, the quadrature detector 33a and the quadrature detector 33b each shift the phase of the reference signal 28 output from the optical frequency shifter driving 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 in 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 in 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 section 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 section 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 a configuration of a homodyne interferometer including a phase diversity detector. More specifically, the beat signal generating unit 9 has an optical splitter 29b, an optical splitter 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 reference light 37 derived from the high frequency subcarrier 16 and 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 signal 8 is input to the optical demultiplexer 29c. 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 separates and outputs the components derived from the high frequency subcarrier 16 and the low frequency subcarrier 17. In other words, the beat signal generating unit 9 inputs the reference light 37 derived from the high frequency subcarrier 16 output from the optical splitter 29b and the signal light 39 derived from the high frequency subcarrier 16 output from the optical splitter 29c to a 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 splitter 29b and the signal light 40 derived from the low-frequency subcarrier 17 output from the optical splitter 29c to a phase diversity detector 41b, and outputs the in-phase component 12 and the 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 the 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 reference lights 37 split by the beam splitter 2c via a total reflection mirror 43a and a beam splitter 2e, and also receives one of the signal lights 39 split by the beam splitter 2d via the beam splitter 2e, and generates and outputs an in-phase component 10 of the first complex beat signal 31 based on these input lights. The balanced photodetector 44b of the phase diversity detector 41a receives the other of the reference lights 37 split by the beam splitter 2c via a π / 2 optical phase shifter 42, a total reflection mirror 43b, and a beam splitter 2f, and also receives the other of the signal lights 39 split by the beam splitter 2d via the beam splitter 2f, and generates and outputs an orthogonal component 11 of the first complex beat signal 31 based on these input lights.

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

[0054] That is, the phase diversity detector 41a is configured to split the reference light 37 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 to split the reference light 38 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 in-phase component 10 of the first complex beat signal 31 is output from the balanced photodetector 44a of the phase diversity detector 41a, and the in-phase component 12 of the second complex beat signal 32 is output from the balanced photodetector 44a of the phase diversity detector 41b. The quadrature component 11 of the first complex beat signal 31 is output from the balanced photodetector 44b of the phase diversity detector 41a, and the quadrature component 13 of the second complex beat signal 32 is output from the balanced photodetector 44b of the phase diversity detector 41b.

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

[0056] First, the Doppler shift that the carrier and two subcarriers undergo will be described with reference to Fig. 11. Fig. 11 is a diagram for explaining the Doppler shift 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 approaches relatively to the optical measurement device 100. In this case, the carrier 15 and the two subcarriers (16, 17) both shift to the higher frequency side. On the other hand, when the object 7 moves relatively away from the optical measurement device 100, the carrier 15 and the two subcarriers (16, 17) both shift to the lower frequency side (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 frequency of the first complex beat signal 31 and the frequency of 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 opposite-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 shifts 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 measurements.

[0062] Here, a 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 with reference to Fig. 12. Fig. 12 is an explanatory diagram illustrating a process in which the arithmetic device 14 calculates the velocity of the object 7 and the distance to the object 7 by using a sum frequency and a difference frequency based on the frequencies of the two complex beat signals. In the following description, a superscript symbol "+" is added to a physical quantity with respect to the high frequency subcarrier 16, and a superscript symbol "-" is added to a physical quantity with respect to the low frequency subcarrier 17. Here, the description will be centered on an 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]

number

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

[0066]

number

[0067] In formula (2), f S is the amount of frequency shift by the optical frequency shifter 26. By subjecting the complex beat signal represented by equation (2) to the processing performed by the quadrature detector 33a and the quadrature detector 33b, 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 formula (3), and the quadrature components for the high frequency subcarrier 16 and the low frequency subcarrier 17 can be expressed by the following formula (4). ± represents the net amplitude, and Φ ± (t) represents the phase.

[0069]

number

[0070]

number

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

[0072]

number

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

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

[0075]

number

[0076] On the right-most side of equation (6), the first term is a component caused by the frequency noise of the multi-frequency laser 1, the second term is a component caused by the Doppler shift, the third term is a component caused by the frequency modulation, and the fourth term is a component caused by the frequency noise of the subcarrier generation signal generator 21. In equation (6), ν 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, ν0 is the carrier frequency, V is the relative velocity between the object 7, and ν M (t) is the frequency modulation of the subcarrier, and c is the speed of light. In equation (6), f SC is the center frequency of the subcarrier generating signal generator 21, and v RF (t) is the frequency noise of the subcarrier generating 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 equation (7).

[0079]

number

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

[0081]

number

[0082] In the rightmost side of equation (8) showing the sum frequency, the first term is a component caused by the frequency noise of the multi-frequency laser 1, and the second term is a component caused by the Doppler shift. More specifically, the component caused by 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 with (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 found from the sum frequency. In other words, by finding 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 found. 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] Moreover, from equations (6) and (7), the difference frequency f diff(t) can be calculated using the following formula (9).

[0084]

number

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

[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 obtained 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 A process for calculating the distance to the target object 7 from (t) will be described. 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 for explaining 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. In FIG. mrepresents the modulation period of the triangular wave, and Δν represents the chirp bandwidth. FIG. 13(a) corresponds to the high-frequency subcarrier 16, and FIG. 13(b) corresponds to the low-frequency subcarrier 17.

[0089] Due to frequency modulation by the triangular wave, the reference light and the signal light 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 (b) show time domains where the beat signal frequency is a constant value and time domains where it transitions from positive to negative or from negative to positive.

[0090] The sum frequency contains a component caused by the frequency noise of the multi-frequency laser 1 and a component caused by the Doppler shift. The component caused by the frequency noise of the multi-frequency laser 1 is noise with an average value of 0, and can be removed by the time averaging process represented by the following formula (10). In other words, the average value of the sum frequency corresponds to the Doppler shift caused by the relative movement of the target 7, that is, the sum of the Doppler shift of the high-frequency subcarrier 16 and the Doppler shift of the low-frequency subcarrier 17.

[0091]

number

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

[0093]

number

[0094] In equation (11), ν0 is the frequency of carrier 15, and when carrier 15 with a frequency of 193.4 THz is used, the sensitivity of the sum frequency to the velocity is 9.29 [MHz / km / h].

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

[0096]

number

[0097] Equation (12) shows that the time average of the sum frequency can be calculated from the phases of high-frequency subcarrier 16 and low-frequency subcarrier 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) and the averaging process of equation (10). In other words, in the first example of beat signal generator 9 shown in Fig. 5, the phase of the complex beat signal is demodulated to find the sum frequency, so that the velocity can be calculated only by calculating the phase without converting the demodulated phase to a frequency.

[0098] FIG. 13(d) shows the difference frequency, in which the component due to frequency modulation and the component due to Doppler shift remain. In order to calculate the distance to the target 7, the time T U1 From time T U2 The section 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 target object 7 is calculated by avoiding the time region where the beat signal frequency transitions from positive to negative or from negative to positive, and by using the time region within the interval ΔT U and interval ΔT D It is desirable to set the lengths of the sigma-based ...

[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]

number

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

[0102]

number

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

[0104]

number

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

[0106]

number

[0107] Section ΔT U and interval ΔT D From the difference frequency obtained in step 2, the distance to the object 7 can be calculated by the following formula (17).

[0108]

number

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

[0110]

number

[0111] In equation (18), f SC is the center frequency of the subcarrier generating 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 phase of high frequency subcarrier 16 and low frequency subcarrier 17. That is, in the first example of beat signal generator 9 shown in Fig. 5, the phase of the complex beat signal is obtained by demodulation, so that the distance to the target object 7 can be calculated only by calculating the phase without converting the demodulated phase to a frequency.

[0113] Note that the following formula (19) is a 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 a formula for calculating the average value of the difference frequency in the down-chart region only by calculating the phase.

[0114]

number

[0115]

number

[0116] Here, the calculation process of the velocity of the object 7 by the calculation device 14 can be summarized as follows. That is, 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 the above formula (5). That is, the calculation device 14 obtains the arctangent (inverse tangent) of a value obtained by dividing the orthogonal 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 obtained value to obtain the first frequency. Also, the calculation device 14 obtains the arctangent (inverse tangent) of a value obtained by dividing the orthogonal 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 obtained value to obtain the second frequency. Then, the calculation device 14 obtains the velocity of the object 7 by using the average value of the sum frequency, which is the sum of the first frequency and the second frequency, through the processing based on the above formulas (8), (11), and (12).

[0117] Next, the optical measurement method according to this embodiment will be described with reference to the flowchart in Fig. 14. Here, the process of calculating the velocity using the sum frequency in the optical measurement method according to 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 that are symmetrical to each other with respect to the frequency of the carrier 15, and outputs light including the two subcarriers (16, 17) to the optical branching element 2a (step S101: subcarrier generating 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 by the optical branching element 2a. The optical branching element 2a outputs the probe light 3 to the optical system 5 and outputs 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 probe light 3 onto the object 7 through the optical system 5, and outputs the scattered light from the object 7 to the beat signal generating unit 9 as the signal light 8 (step S103: irradiation process). Next, the optical measurement device 100 generates 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, through the beat signal generating unit 9 to which the reference light 4 and the signal light 8 are input. The beat signal generating unit 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 generating process).

[0120] Then, the optical measurement device 100, by the arithmetic unit 14, obtains 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 obtained sum frequency (step S105: velocity calculation step). More specifically, the arithmetic unit 14 demodulates the first complex beat signal 31 to obtain the first frequency, and demodulates the second complex beat signal 32 to obtain the second frequency (step S201), adds the first frequency and the second frequency to obtain the sum frequency (step S202), and obtains the velocity of the object 7 using the average value of the sum frequencies (step S203).

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

[0122] [Evaluation of measurement accuracy] A simulation was performed to evaluate the measurement accuracy of the speed of the object 7 for the optical measurement device 100 in this embodiment. In the simulation, two complex beat signals including noise were generated using the second example of the beat signal generating unit 9 shown in FIG. 8, and the distance and speed were calculated according to the process of FIG. 12, and the accuracy was evaluated. As noise, the frequency noise of the multi-frequency laser 1 and the shot noise and detector noise generated in the balanced photodetector 44a and the balanced photodetector 44b were considered. The shot noise is noise that the laser light incident on the balanced photodetector has. The photodetector noise is noise that occurs even when there is no incident light, and includes the dark current noise of the balanced photodetector and the thermal noise of the amplifier arranged in the subsequent stage. Compared to the frequency noise of the multi-frequency laser 1, the frequency noise of the subcarrier generation signal generator 21 is small and is therefore ignored.

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

[0124]

number

[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 motion of the target 7, and the third term represents the component due to the frequency modulation of the subcarrier. d was set to 335 ns, which corresponds to a distance of 100.43 m, and the motion speed of the object 7 was set to V = 50.0 [km / h].

[0126] FIG. 15(a) shows the frequency modulation v M FIG. 15(a) shows the 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 N 15(b) shows a time waveform generated by a numerical calculation when white noise equivalent 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 have an 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]

number

[0129]

number

[0130] In formula (22), I ±shot (t) and I ± PD (t) are the shot noise and photodetector noise that appear in the in-phase component of the complex beat signal, respectively. In equation (23), Q ± shot (t) and Q ± PD (t) are the shot noise and photodetector noise that appear in the quadrature components of the complex beat signal, respectively. All of these noises are white noises that are uncorrelated with each other. Also, A ± is the 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]

number

[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 optical power, P sig is the signal light power.

[0133] FIG. 16(a) shows the shot noise I + shot (t) is a time waveform of the shot noise standard deviation σ shot can be expressed by the following equation (25).

[0134]

number

[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) is G = 39 [kV / A], η = 0.9 [A / W], P ref = 100 [μW], f PD= 100 [MHz]. For the other components of equation (21), the same numerical values ​​were used to generate white noise that is uncorrelated with the time waveform in Figure 16(a).

[0136] FIG. 16(b) shows the photodetector noise I + PD FIG. 1 shows the time waveform of (t). PD can be expressed by the following equation (26).

[0137]

number

[0138] In equation (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 = 100 [MHz]. 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 Figure 16(b).

[0139] Figure 17(a) shows the complex beat signal on the IQ plane for the high frequency subcarrier 16, and Figure 17(b) shows the complex beat signal on the IQ plane for the low frequency subcarrier 17. 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 each point is placed on a circle on the IQ plane. The spread of the circle corresponds to the shot noise and photodetector noise.

[0140] For the complex beat signal of equation (21), the phase Φ ±After (t) was obtained, 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 is 100 μW, and the signal light power is 100 nW.

[0141] In Figure 18, the first half of the time waveform corresponds to a down-chirp, and the second half of the time waveform corresponds to an up-chirp. Since the frequency modulation of the high-frequency subcarrier 16 and the low-frequency subcarrier 17 are in opposite phase, the frequencies of the complex beat signals are also in opposite phase to each other. The noise appearing in the time waveforms of Figures 18(a) and 18(b) is mainly caused by the frequency noise of the multi-frequency laser 1, but shot noise and photodetector noise also have an effect. In addition, 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 cancelled out, and the difference in frequency 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] Fig. 18(d) shows the time waveform of the sum frequency. In the sum frequency, the frequency modulation is cancelled out, and the frequency noise of the multi-frequency laser 1, the shot noise, and the photodetector noise appear. The frequency change caused by the Doppler shift is "-35.8 MHz", and the whole frequency is shifted to the lower frequency side. The Doppler shift can be found 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 more, 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, the 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] Fig. 20 shows 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 cancelled out, and the frequency noise of the multi-frequency laser 1, the shot noise, and the photodetector noise appear. When the signal light power is 10 nW or more, the noise level 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 the photodetector noise increases as the signal light power decreases.

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

[0147] FIG. 22 shows the change 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. When the signal light power is in the range of 1 μW to 400 pW, the error is almost constant, but when the signal light power is 400 pW or less, the error increases rapidly and becomes an unrealistic value. On the other hand, when the velocity is calculated from the sum frequency, the increase in the error with respect to the decrease in the signal light power is gradual, and even when the signal light power is 100 pW, it remains below 10%. From this, it can be said that the use of the sum frequency makes it possible to measure the velocity of the target 7 with high accuracy.

[0148] As described above, the optical measurement device 100 in this embodiment generates the frequency-modulated high-frequency subcarrier 16 and the frequency-modulated low-frequency subcarrier 17, which are located symmetrically with respect to the carrier frequency, by the multi-frequency laser 1, and splits the output light of the multi-frequency laser into the probe light 3 and the reference light 4 by the optical branching element 2a. Next, the optical measurement device 100 irradiates the probe light 3 to the object 7 by the optical system 5, outputs the 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 by the beat signal generating unit 9. 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 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 between 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 frequency and the second frequency to obtain a sum frequency. These demodulations are so-called phase demodulations, and correspond to the process of the above formula (5), for example. Such processes can simplify and speed up the arithmetic process.

[0150] Various configurations capable of realizing the above functions can be adopted for the multi-frequency laser 1. For example, 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, as shown in Fig. 2, and 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. The multi-frequency laser 1 may also be configured to have a semiconductor laser 22, a modulation signal generator 20, and a subcarrier generation signal generator 21, as shown in Fig. 3, and 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] Here, the above-mentioned embodiment is merely an example of the optical measurement device, the optical measurement method, and the 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 so that 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 are output to the arithmetic device 14 as they are. In this case, it is preferable that the arithmetic device 14 is configured to perform the processes performed by the quadrature detector 33a and the quadrature detector 33b in the configuration example of Fig. 5, that is, a series of processes for separating two complex beat signals into an in-phase component and a quadrature component and outputting them. In other words, the arithmetic device 14 may have a function of extracting the in-phase component 10 and the quadrature component 11 from the first complex beat signal 31 and extracting the in-phase component 12 and the quadrature component 13 from the second complex beat signal 32. [Industrial Applicability]

[0152] According to the optical measurement device, the optical measurement method, and the optical measurement program of the present invention, the Doppler shift caused by the relative motion of the object can be detected with high sensitivity, so that the speed of the object can be measured with high accuracy. Furthermore, since the optical measurement device, the optical measurement method, and the optical measurement program of the present invention do not require complicated devices or processing, they are industrially useful as a small and low-cost FMCW lidar system. Furthermore, the optical measurement device, the optical measurement method, and the optical measurement program of the present invention can be used in consumer devices, including as an environment recognition sensor for automobiles, autonomous robots, and the like. [Explanation of symbols]

[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 / receiving optical system, 7 target object, 8 signal light, 9 beat signal generating section, 10, 12 in-phase component, 11, 13 quadrature component, 14 arithmetic unit, 14a communication section, 14b arithmetic processing section, 14c memory section, 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 splitter, 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 measuring device.

Claims

1. a multi-frequency laser generating a frequency modulated high frequency subcarrier and a frequency modulated low frequency subcarrier, the frequency modulated subcarrier being symmetrical to each other with respect to the carrier frequency; a light 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 generating 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; a calculation device that performs demodulation processing on the first complex beat signal and the second complex beat signal to obtain their respective phases, that obtains a sum frequency that is the sum of a first frequency obtained by time-differentiating the phase of the first complex beat signal and a second frequency obtained by time-differentiating the phase of the second complex beat signal, and that calculates a velocity of the object using a value obtained by time-averaging the obtained sum frequency.

2. The multi-frequency laser includes: A single frequency laser; an optical modulator for modulating 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; a modulation signal generator that applies frequency modulation having opposite phases to each of the two subcarriers generated by the subcarrier generation signal generator; 2. The optical measurement device according to claim 1, wherein the high frequency subcarrier and the low frequency subcarrier are generated in the output light of the optical modulator by the modulation signal generator and the subcarrier generation signal generator.

3. The multi-frequency laser includes: A semiconductor laser; a subcarrier generation signal generator for generating two subcarriers in the output light of the semiconductor laser; a modulation signal generator that applies frequency modulation having mutually opposite phases to each of the two subcarriers generated by the subcarrier generation signal generator; 2. The optical measurement device according to claim 1, 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. An optical measurement method using an optical measurement device equipped with a multi-frequency laser, comprising: a subcarrier generating step of simultaneously generating a frequency modulated high frequency subcarrier and a frequency modulated low frequency subcarrier, the frequency modulated high frequency subcarrier and the frequency modulated low frequency subcarrier being symmetrical to each other with respect to the carrier frequency, by the multi-frequency laser; a light branching step of splitting the output light of the multi-frequency laser into a probe light and a reference light by a light branching element; an irradiation process step of irradiating the probe light onto an object by an optical system and outputting scattered light from the object as signal light; a complex beat signal generating step of generating and outputting 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, by a beat signal generating unit to which the reference light and the signal light are input; and a velocity calculation step of, by a calculation device, demodulating the first complex beat signal and the second complex beat signal to obtain their respective phases, calculating a sum frequency which is the sum of a first frequency obtained by time-differentiating the phase of the first complex beat signal and a second frequency obtained by time-differentiating the phase of the second complex beat signal, and calculating the velocity of the object using a value obtained by time-averaging the calculated sum frequency.

5. A calculation device for determining the velocity of an object based on output light of a multi-frequency laser generating a frequency-modulated high-frequency subcarrier and a frequency-modulated low-frequency subcarrier, the subcarriers being symmetrical to each other with respect to the carrier frequency, demodulating 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 to obtain a phase of each of the first and second complex beat signals; determining a sum frequency that is the sum of a first frequency obtained by time-differentiating the phase of the first complex beat signal and a second frequency obtained by time-differentiating the phase of the second complex beat signal; The optical measurement method further comprises the steps of: determining the velocity of the object using a value obtained by performing a time averaging process on the determined sum frequency.

6. A computer provided in a calculation device that calculates 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, an optical measurement program for causing the program to function as a velocity calculation means that performs demodulation processing on 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 to obtain their respective phases, determines a sum frequency that is the sum of a first frequency obtained by time-differentiating the phase of the first complex beat signal and a second frequency obtained by time-differentiating the phase of the second complex beat signal, and determines the velocity of the object using a value obtained by time-averaging the determined sum frequency.

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