LIDAR DEVICE AND LIDAR CONTROL METHOD

Abstract

To provide a lidar device of an FM comb lidar using only a single photodetector and a single analog-digital converter, and to provide a lidar control method.SOLUTION: A lidar device is configured to: irradiate an object with an optical frequency comb; detect, by a single photodetector, an interference signal of mixed light of scattered light and the optical frequency comb; and calculate a distance to the object. The lidar device simultaneously executes a sweeping to increment a comb mode frequency of the optical frequency comb in a line shape with regard to time, and a sweeping to increment a frequency interval of the comb mode of the optical frequency comb in a line shape with regard to time. Fourier transformation is used to separate a lidar signal calculated from the interference signal to a lidar signal by each comb mode. A temporal axis of the lidar signal by each comb mode is corrected so that each duration for sweeping per com mode interval of each com mode becomes the same. The corrected lidar signal by each com mode is coherent-bonded and Fourier-transformed, and an obtained beat signal is used to calculate a distance.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a LIDAR device and a LIDAR control method, and more particularly to a LIDAR device and a LIDAR control method that utilizes an optical frequency comb. [Background technology]

[0002] "Light Detection and Ranging (LiDAR)" is a technology that irradiates a target with a laser, detects the scattered light, and measures the distance to the target. One of the techniques, frequency-modulated cw LiDAR (FMCW LiDAR), is a method that detects the distance and speed to the target by irradiating the target with a frequency-modulated CW laser and measuring the frequency of the beat signal generated by the interference between the reference light and the scattered light.

[0003] A method for detecting the distance to an object by FMCW LIDAR will be described with reference to Figs. 16 and 17. Fig. 16 shows an example of the configuration of a distance measuring device 160 by a conventional FMCW LIDAR. A CW laser 1609 emitted from a CW laser source device 1601 is separated into a reference light 1610 and a reference light 1616 by an optical directional coupler 1602. The reference light 1610 enters an angle sweep mechanism 1604 via a circulator 1603, and is irradiated in the direction of an angle α by the angle sweep mechanism 1604. The irradiated reference light 1612 is scattered by an object 1605 located in the direction of the angle α, and the scattered light 1613 enters the angle sweep mechanism 1604. The scattered light 1613 entering the angle sweep mechanism 1604 is input to the circulator as scattered light 1614 as it is, and is output as scattered light 1615. The scattered light 1615 is merged with the reference light 1616 by the optical directional coupler 1606 and output as an output light 1617. An interference signal of the output light 1617 is detected by the photodetector 1607, and the detected interference signal 1618 is input to the calculation device 1608. While the angle sweeper 1604 keeps irradiating the reference light 1612 in the direction of the angle α, the CW laser source device 1601 sweeps the frequency of the CW laser 1609, thereby calculating the distance from the distance measuring device 160 to the object 1605.

[0004] As an example of frequency sweep of the CW laser 1609, let us assume that the frequency of the CW laser 1609 is changed over time with a triangular waveform as shown in Fig. 17(a). Since the scattered light 1615 propagates with a delay relative to the reference light 1616, the amount of frequency change when the frequency of the output light 1617 is swept is expressed as shown in Fig. 17(b). A beat signal is generated by the interference between the scattered light 1615 and the reference light 1616, and the frequency f of the beat signal is detected by the photodetector 1607 as shown in Fig. 17(c). beat The frequency of the beat signal is expressed by the following formula:

[0005]

number

[0006] In distance measurement by FMCW lidar, the distance resolution dL is proportional to the inverse of the frequency sweep range of the CW laser 1409,

[0007]

number

[0008] In response to this, a method called frequency-modulated comb LiDAR (FMcomb LiDAR), which uses a frequency-modulated optical frequency comb as a light source, is disclosed in Non-Patent Document 1. The method disclosed in Non-Patent Document 1 uses an optical frequency comb as a light source instead of a CW laser, thereby obtaining an effectively large frequency sweep range and thereby improving distance resolution.

[0009] FIG. 18 shows an example of the configuration of a distance measurement device 180 using an FM comb lidar according to the method disclosed in Non-Patent Document 1. Comparing the distance measurement device 180 shown in FIG. 18 with the distance measurement device 160 shown in FIG. 16, an optical frequency comb generator 1802 is connected to a CW laser source device 1801, and an optical frequency comb 1803 is generated. The optical frequency comb 1803 is divided into an optical frequency comb 1804 and an optical frequency comb 1805, and the optical frequency comb 1804 is irradiated onto the object 1605. In order to detect the interference signal of the output wave 1807, which is a combination of the optical frequency comb 1805 and the scattered light 1806 scattered by the object 1605 from the optical frequency comb 1804, for each comb mode, the output wave 1807 is input to a wavelength decomposition element 1808 and separated for each comb mode, and the separated interference signals of each comb mode are detected by the first to n-th photodetectors 18091 to 1809n.

[0010] FIG. 19 shows an example of frequency sweeping of the reference light (optical frequency comb 1803 in FIG. 18) and the frequency swept scattered light (scattered light 1805 in FIG. 18). In FIG. 19, the optical frequency comb 1803 is shown by a solid line, and the scattered light 1805 is shown by a dashed line. FIG. 19 shows only five consecutive comb modes CH1 to CH5. As shown in FIG. 19(a), the frequency of each comb mode of the optical frequency comb 1603 is swept by the interval of the comb mode so that each comb mode frequency increases in proportion to time from time t=0. At time t=T cycle In, the frequency of each comb mode corresponds to the comb mode frequency of the next higher order comb mode at time t=0.

[0011] The comb modes shown in Fig. 19(a) are coherently combined. Here, coherently combining means combining two signals with each other including not only the intensity but also the phase. The combined comb modes are shown in Fig. 19(b). As shown in Fig. 19(b), by signal processing the frequency-swept comb modes and coherently combining them, a large effective frequency range can be obtained, and therefore the distance resolution can be improved. [Prior art documents] [Non-patent literature]

[0012] [Non-Patent Document 1] N.Kuse,et al.,APL Photonics 4,106105(2019) Summary of the Invention [Problem to be solved by the invention]

[0013] However, as shown in FIG. 18, according to the FM comb rider disclosed in Non-Patent Document 1, it is necessary to detect an interference signal for each comb mode of the optical frequency comb 1803, and the system becomes complicated because many elements are required to detect the interference signal, such as a wavelength decomposition element 1805, first to nth photodetectors 18061 to 1806n, and multiple analog-digital converters.

[0014] In view of the above problems, an object of the present invention is to provide a LIDAR device and a LIDAR control method using an FM comb LIDAR that does not require a wavelength resolution element and uses only a single photodetector and a single analog-to-digital converter. [Means for solving the problem]

[0015] A first aspect of the present invention is a LIDAR device comprising: a laser control unit that generates an optical frequency comb from a CW laser oscillated from a CW laser light source device and sweeps the comb mode frequency of the optical frequency comb by a comb mode interval; a frequency change measurement unit that measures the sweep amount of the CW laser frequency and the sweep amount of the comb mode frequency of the optical frequency comb; a comb mode interval measurement unit that calculates the time for sweeping the comb mode frequency of the optical frequency comb by the comb mode interval for each comb mode; and a LIDAR unit that detects an interference signal of a mixture of scattered light and the optical frequency comb caused by irradiating the optical frequency comb on an object using a single photodetector and calculates a distance to the object. The laser control unit detects the comb of the optical frequency comb. The LIDAR unit performs a Fourier transform on the LIDAR signal calculated from the interference signal to obtain discrete signals in the frequency domain, and then performs a Fourier transform on each of the discrete signals individually to generate a LIDAR signal for each comb mode. The time axis of the LIDAR signal generated for each comb mode is corrected so that the time for sweeping the comb mode interval for each comb mode is all the same, and the corrected LIDAR signals for each comb mode are coherently combined and Fourier transformed to obtain a beat signal to calculate the distance.

[0016] In a first aspect of the present invention, the laser control unit includes an electro-optical modulator, an RF oscillator that inputs a modulation signal to the electro-optical modulator, and a current controller connected to a CW laser light source device, wherein a CW laser is input to the electro-optical modulator to generate an optical frequency comb, and a sweep that linearly increases the comb mode frequency of the optical frequency comb with respect to time is performed by controlling the current controller to control the frequency of the CW laser, and a sweep that linearly increases the frequency spacing of the comb modes of the optical frequency comb with respect to time may be performed by controlling the RF oscillator to control the modulation signal.

[0017] In a first aspect of the present invention, the laser control unit includes a microresonator, a resonance frequency control mechanism that controls the resonance frequency of the microresonator, and a current controller connected to a CW laser light source device, and a CW laser is input to the microresonator to generate an optical frequency comb, and a sweep that linearly increases the comb mode frequency of the optical frequency comb with respect to time and a sweep that linearly increases the frequency interval of the comb mode of the optical frequency comb with respect to time may be performed by controlling the current controller to control the frequency of the CW laser and controlling the resonance frequency control mechanism to control the resonance frequency.

[0018] In the first embodiment of the present invention, the resonant frequency control mechanism may be a microheater attached to the microresonator.

[0019] In the first aspect of the present invention, the frequency change measuring unit may measure the frequency sweep amount of the CW laser and the comb mode frequency sweep amount of the optical frequency comb using an unbalanced Mach-Zehnder interferometer.

[0020] In a first aspect of the present invention, the comb mode spacing measurement unit may include a single frequency oscillation CW laser light source device, a second photodetector, and an RF bandpass filter, and may detect an interference signal between the second CW laser irradiated from the single frequency oscillation CW laser light source device and the optical frequency comb using the second photodetector, and calculate, for each comb mode, a time required to sweep the comb mode frequency of the optical frequency comb by the comb mode spacing from a signal when the comb mode frequency of the optical frequency comb extracted by the RF bandpass filter matches the frequency of the second CW laser.

[0021] In the first aspect of the present invention, when the LIDAR unit performs a Fourier transform on the LIDAR signal calculated from the interference signal to obtain a discrete signal in the frequency domain, the time axis of the LIDAR signal calculated from the interference signal may be corrected using the change over time of the sweep amount of the comb mode frequency of the optical frequency comb measured by the frequency change measurement unit, and a correction may be performed for signal variations caused by the nonlinearity of the frequency sweep.

[0022] A second aspect of the present invention is a lidar control method, comprising the steps of: generating an optical frequency comb from a CW laser oscillated from a CW laser light source device; sweeping the comb mode frequency of the optical frequency comb linearly increasing with time while simultaneously sweeping the frequency interval of the comb modes of the optical frequency comb linearly increasing with time, the steps of measuring the sweep amount of the CW laser frequency and the sweep amount of the comb mode frequency of the optical frequency comb; calculating, for each comb mode, a time for sweeping the comb mode frequency of the optical frequency comb by the comb mode interval; The method comprises the steps of detecting an interference signal of the mixed light with the frequency comb using a single photodetector, Fourier transforming a LIDAR signal calculated from the interference signal to obtain discrete signals in the frequency domain, Fourier transforming each of the discrete signals individually to generate a LIDAR signal for each comb mode, correcting the time axis of the generated LIDAR signal for each comb mode so that the time for sweeping the comb mode interval for each comb mode is all the same, and calculating the distance using a beat signal obtained by coherently combining and Fourier transforming the corrected LIDAR signals for each comb mode.

[0023] In a second aspect of the present invention, the step of generating an optical frequency comb includes a step of inputting a CW laser to an electro-optical modulator to generate an optical frequency comb, and the step of sweeping by the comb mode spacing may include a step of simultaneously performing a sweep to linearly increase the comb mode frequency of the optical frequency comb with respect to time by controlling a current controller connected to the CW laser light source device to control the frequency of the CW laser, and a sweep to linearly increase the frequency spacing of the comb modes of the optical frequency comb with respect to time by controlling an RF oscillator that inputs a modulation signal to the electro-optical modulator to control the modulation signal.

[0024] In a second aspect of the present invention, the step of generating an optical frequency comb includes a step of inputting a CW laser into a microresonator to generate an optical frequency comb, and the step of sweeping by the comb mode spacing may include a step of controlling a current controller connected to a CW laser light source device to control the frequency of the CW laser, and simultaneously performing a sweep that linearly increases the comb mode frequency of the optical frequency comb with respect to time by controlling the resonant frequency of the microresonator, and a sweep that linearly increases the frequency spacing of the comb modes of the optical frequency comb with respect to time.

[0025] In a second embodiment of the present invention, control of the resonant frequency of the microresonator may be performed by a microheater attached to the microresonator.

[0026] In a second aspect of the present invention, the step of measuring the frequency sweep of the CW laser and the comb mode frequency sweep of the optical frequency comb may be performed using an unbalanced Mach-Zehnder interferometer.

[0027] In a second aspect of the present invention, the step of calculating, for each comb mode, the time required to sweep the comb mode frequency of the optical frequency comb by the comb mode interval may include the steps of detecting an interference signal between the second CW laser and the optical frequency comb, extracting a signal when the comb mode frequency of the optical frequency comb and the frequency of the second CW laser match by an RF bandpass filter, and calculating, for each comb mode, the time required to sweep the comb mode frequency of the optical frequency comb by the comb mode interval from the extracted signal.

[0028] In a second aspect of the present invention, the step of Fourier transforming the LIDAR signal calculated from the interference signal to obtain a discrete signal in the frequency domain may include a step of correcting the time axis of the LIDAR signal calculated from the interference signal and correcting for signal variations caused by nonlinearity of the frequency sweep. Effect of the Invention

[0029] According to the present invention, it is possible to provide a LIDAR device and a LIDAR control method using an FM comb LIDAR that does not require a wavelength resolution element and uses only a single photodetector and a single analog-to-digital converter. [Brief description of the drawings]

[0030] [Figure 1] FIG. 2 is a block diagram showing an example of a configuration of a LIDAR device according to an embodiment of the present invention. [Diagram 2] 4 is a block diagram showing an example of a configuration of a frequency change measuring unit according to the embodiment. FIG. [Diagram 3] FIG. 2 is a block diagram showing an example of the configuration of a rider section according to the embodiment. [Figure 4] Graph showing (a) comb mode frequency and (b) lidar signal according to the prior art. [Diagram 5] 1 is a graph showing (a) comb mode frequency and (b) lidar signal according to an embodiment. [Figure 6] 1A-1D are graphs illustrating a method for combining LIDAR signals according to an embodiment, where (a) LIDAR signal, (b) beat signal, (c) beat signal, and (d) LIDAR signal. [Figure 7] 1A-1C are graphs illustrating a method for combining LIDAR signals according to an embodiment, where (a) is a LIDAR signal, (b) is a graph before time-base correction, and (c) is a graph after time-base correction. [Figure 8] 1A-1B are graphs illustrating a method for combining LIDAR signals according to an embodiment, where (a) is a graph of the combined LIDAR signal and (b) is a graph of the beat signal. [Figure 9] Graphs illustrating a method of correction when combining LIDAR signals according to an embodiment, (a) before correction and (b) after correction. [Figure 10] 1A and 1B are diagrams for explaining a method for determining the time swept by a comb mode interval according to an embodiment, in which (a) is a relationship between the frequency of the reference and the comb mode, and (b) is a diagram of the resulting signal. [Figure 11] 4 is a block diagram showing an example of a configuration of a comb mode interval measurement unit according to the embodiment. FIG. [Figure 12] 11 is a graph showing an interference signal according to an embodiment. [Figure 13] 1A and 1B are graphs showing interference signals according to an embodiment, (a) the kth comb mode, and (b) the k-1th comb mode. [Figure 14] 13A and 13B are graphs showing LIDAR signals from a LIDAR device according to an embodiment of the present invention, in which (a) the second to fourth comb modes, (b) the second and third comb modes, and (c) the third and fourth comb modes. [Figure 15] 1 is a beat signal from a LIDAR device according to an embodiment of the present invention. [Figure 16] FIG. 1 is a block diagram showing an example of the configuration of a distance measurement device using a conventional FMCW LIDAR. [Figure 17] 1 is a graph showing the amount of change in reference light, scattered light, and beat frequency by a conventional FMCW lidar, where (a) is a graph of the reference light, (b) is a graph of the reference light and scattered light, and (c) is a graph of the beat frequency. [Figure 18] FIG. 1 is a block diagram showing an example of the configuration of a LIDAR device according to a conventional technology. [Figure 19] 1 is a graph showing a LIDAR signal according to the prior art. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] Next, an embodiment of the present invention will be described with reference to the drawings. In the description of the drawings relating to the embodiment, the same or similar parts are given the same or similar reference numerals. However, it should be noted that the drawings are schematic, and the relationship between thickness and planar dimensions, the thickness ratio of each member, etc. are different from the actual ones. Therefore, the specific thickness and dimensions should be determined with reference to the following description. In addition, it goes without saying that the drawings include parts with different dimensional relationships and ratios.

[0032] Furthermore, the embodiments are merely examples of devices and methods for embodying the technical idea of ​​the present invention, and the technical idea of ​​the present invention does not specify the configuration, arrangement, layout, etc. of each component to those described below. The technical idea of ​​the present invention can be modified in various ways within the technical scope defined by the claims.

[0033] (Embodiment) A LIDAR device and a LIDAR control method according to an embodiment of the present invention will be described with reference to Fig. 1. As shown in Fig. 1, a LIDAR device 10 according to this embodiment includes a laser control unit 101, a frequency change measurement unit 102, a comb mode spacing measurement unit 103, and a LIDAR unit 104.

[0034] The laser control unit 101 is made up of a CW laser light source device 105 , a current controller 106 , a first control device 107 , an optical frequency comb generator 108 , a second control device 109 , and a first optical directional coupler 110 .

[0035] Examples of the optical frequency comb generator 108 include an electro-optical modulator and a microresonator. When an electro-optical modulator is used as the optical frequency comb generator 108, a CW laser is input from the CW laser source device 105 to the electro-optical modulator and modulated to generate a sideband, i.e., an optical frequency comb. In this embodiment, as an example, the optical frequency comb generator 108 is composed of an electro-optical modulator 111 and an RF oscillator 112. The CW laser is input from the CW laser source device 105 to the electro-optical modulator 111 via the first optical directional coupler 110, and a modulation signal is input from the RF oscillator 112 to the electro-optical modulator 111, whereby the optical frequency comb generator 108 generates an optical frequency comb 116.

[0036] When a microresonator is used as the optical frequency comb generator 108, the optical frequency comb generator 108 is composed of a microresonator and a resonant frequency control mechanism. The resonant frequency control mechanism is a mechanism attached to the microresonator to control the resonant frequency, such as a microheater attached to the microresonator and a current controller to flow to the microheater, or a piezoelectric element attached to the microresonator and a voltage controller to apply to the piezoelectric element. When the resonant frequency control mechanism is composed of a microheater and a current controller to flow to the microheater, the temperature of the microresonator is controlled by controlling the current to flow to the microheater with the current controller, thereby controlling the resonant frequency of the microresonator. When the resonant frequency control mechanism is composed of a piezoelectric element and a voltage controller to apply to the piezoelectric element, the resonant length of the microresonator is controlled by controlling the voltage to be applied to the piezoelectric element with the voltage controller, thereby controlling the resonant frequency of the microresonator.

[0037] A CW laser is input from a CW laser light source device 105 through a waveguide to a microresonator optically coupled to the waveguide to generate an optical frequency comb. One of the comb mode frequencies of the generated optical frequency comb coincides with the resonant frequency of the microresonator. In addition, the frequency of the CW laser input to the microresonator must deviate from one of the comb mode frequencies by a certain amount. Therefore, when changing the comb mode frequency of the optical frequency comb generated in the microresonator, the frequency of the CW laser input to the microresonator is changed and at the same time the resonant frequency of the microresonator is changed by a resonant frequency control mechanism.

[0038] An optical frequency comb using an electro-optic modulator or a microresonator has a comb mode frequency interval of about 10 GHz to 1 THz, which is wider than the comb interval of about 100 MHz to 1 GHz of conventional optical frequency combs such as mode-locked fiber lasers. In this embodiment, as described below, each comb mode of the optical frequency comb is separated from the others, and each comb mode is used as a plurality of CW lasers, and the comb mode frequency of each comb mode is swept. Therefore, it is desirable for the comb interval of the optical frequency comb to be wider, and from the viewpoint of the comb interval, it is desirable to use an electro-optic modulator or a microresonator as the optical frequency comb generator 108.

[0039] CW laser 113 emitted from CW laser light source device 105 is separated into CW laser 114 and CW laser 115 by first optical directional coupler 110. CW laser 115 is input to frequency change measurement unit 102, where the frequency sweep amount of CW laser 115 is measured. CW laser 114 is input to optical frequency comb generator 108, where optical frequency comb 116 is generated. Optical frequency comb 116 is separated into optical frequency comb 118 and optical frequency comb 119 by second optical directional coupler 117. Optical frequency comb 119 is input to frequency change measurement unit 102, where the frequency sweep amount of optical frequency comb 119 is measured. Optical frequency comb 118 is separated into optical frequency comb 121 and optical frequency comb 122 by third optical directional coupler 120. The optical frequency comb 122 is input to the comb mode spacing measurement unit 103, which measures the time swept by the comb mode spacing of the optical frequency comb 122. The optical frequency comb 121 is input to the LIDAR unit 104, which measures the distance between the LIDAR device 10 and the target object 123.

[0040] In this embodiment, since each comb mode of the optical frequency comb 116 is used as an optical carrier of the FM comb rider, it is necessary to simultaneously measure the frequency sweep amount of all comb modes that are frequency swept. The frequency sweep amount of the optical frequency comb 116 is measured using the frequency change measurement unit 102. In addition, as described later, it is necessary to measure the frequency sweep amount of the CW laser 115 in order to measure the comb mode interval. The frequency sweep amount of the CW laser 115 is also measured using the frequency change measurement unit 102.

[0041] The method of measuring the frequency sweep amounts of all comb modes swept in the frequency of the optical frequency comb 116 differs depending on the generation method of the optical frequency comb 116. When the optical frequency comb generator 108 is composed of an electro-optic modulator 111 and an RF oscillator 112, the frequency sweep amounts of all comb modes of the optical frequency comb 116 can be calculated by measuring the frequency sweep amount of the CW laser 113 and the frequency sweep amount of the Nth comb mode of the optical frequency comb 116. When the optical frequency comb generator 108 is composed of a microresonator, the frequency sweep amounts of all comb modes of the optical frequency comb 116 can be calculated by measuring the frequency sweep amounts of the Nth and Mth comb modes of the optical frequency comb 116 (where N≠M).

[0042] The frequency sweep amount of the swept comb mode can be measured, for example, by using an unbalanced Mach-Zehnder interferometer or by causing interference between the CW laser and the comb mode and measuring the frequency difference with a photodetector. In this embodiment, the frequency sweep amount of the optical frequency comb 116 and the CW laser 115 is measured using an unbalanced Mach-Zehnder interferometer.

[0043] In this embodiment, the optical frequency comb generator 108 is composed of an electro-optic modulator 111 and an RF oscillator 112. A method for calculating the frequency sweep amounts of all comb modes of the optical frequency comb 116 from the frequency sweep amount of the CW laser 113 and the frequency sweep amount of the N-th comb mode of the optical frequency comb 116 when the optical frequency comb generator 108 is composed of the electro-optic modulator 111 and the RF oscillator 112 will be described. The frequency change measurement unit 102 according to this embodiment is shown in FIG. 2. The frequency change measurement unit 102 shown in FIG. 2 is composed of a two-wavelength unbalanced Mach-Zehnder interferometer 201, an optical bandpass filter (BPF) 202, first and second photodetectors 203 and 204, first and second analog-to-digital converters (ADCs) 205 and 206, and a signal processing device 210. The two-wavelength unbalanced Mach-Zehnder interferometer 201 is composed of a first waveguide 211, a second waveguide 212, a fourth optical directional coupler 213, and a fifth optical directional coupler 214, each of which has a different optical path length.

[0044] The CW laser 115 output from the CW laser light source device 105 is input to the fourth optical directional coupler 213 of the two-wavelength unbalanced Mach-Zehnder interferometer 201. The CW laser 115 propagates through the first waveguide 211 as a CW laser 216 and through the second waveguide 212 as a CW laser 217, and is merged at the fifth optical directional coupler 214, outputting an output light 218, and an interference signal of the output light 218 is detected by the first optical detector 203. The CW laser 217 is a delayed light compared to the CW laser 216, and therefore, when the time change of the interference signal of the output light 218 detected by the first optical detector 203 is graphed, it is possible to observe the occurrence of interference.

[0045] When the first control device 107 controls the current controller 106 to sweep the frequency of the CW laser 113 output from the CW laser light source device 105, the phase of the waveform of the interference signal detected by the first photodetector 203 changes in response to the change in the frequency of the CW laser 113. The amount of phase change δφ of this interference waveform before and after changing the frequency of the CW laser 113 is CWBy measuring the frequency change δν CW It is possible to calculate

[0046]

number

[0047] The optical frequency comb 119 input from the laser control unit 101 to the frequency change measurement unit 102 via the second optical directional coupler 117 is input to the fifth optical directional coupler 214 of the two-wavelength unbalanced Mach-Zehnder interferometer 201. The optical frequency comb 119 propagates through the first waveguide 211 as the optical frequency comb 219 and through the second waveguide 212 as the optical frequency comb 220, and is merged at the fourth optical directional coupler 213, and the output light 221 is output. The optical bandpass filter 202 is set to transmit only the N-th comb mode of the output light 221. The output light 221 is input to the optical bandpass filter 202, and an interference signal of the N-th comb mode is detected by the second photodetector 204.

[0048] When the comb mode frequency of the optical frequency comb 116 output from the laser control unit 101 is swept by the first control device 107 and the second control device 109, the phase of the interference signal of the Nth comb mode detected by the photodetector 205 changes. The amount of change in the phase of the interference signal of the Nth comb mode is expressed as Δφ N Then, the comb mode frequency sweep amount δν N can be calculated from the following formula:

[0049]

number

[0050] Here, the change in frequency of the CW laser 113, δν CW and the comb mode frequency sweep amount δν of the Nth comb mode NThe relationship is expressed as follows:

[0051]

number

[0052]

number

[0053] When the optical frequency comb generator 108 is composed of a microresonator, the comb mode frequency sweep amount δν M In order to measure the M-th comb mode, an optical bandpass filter and a photodetector are connected that are set to transmit only the M-th comb mode of the output light 221. The interference signal of the M-th comb mode is detected in a manner similar to that for detecting the interference signal of the N-th comb mode. The amount of change in phase of the interference signal of the M-th comb mode accompanying the sweep of the comb mode frequency of the optical frequency comb 116 is defined as δφ M Then, the comb mode frequency sweep amount δν M can be calculated from the following formula:

[0054]

number

[0055]

number

[0056] An example of the configuration of the LIDAR unit 104 is shown in Fig. 3. The LIDAR unit 104 shown in Fig. 3 is composed of a sixth optical directional coupler 301, a circulator 302, a seventh optical directional coupler 303, a photodetector 304, an analog-to-digital converter (ADC) 305, and a computing device 306.

[0057] The optical frequency comb 121 is split into an optical frequency comb 307 and an optical frequency comb 308 by the sixth optical directional coupler 301. The optical frequency comb 307 is irradiated to the object 123 via the circulator 302. The optical frequency comb 307 is scattered by the object 123, and the scattered light 309 is input to the circulator 302. The scattered light 309 is combined with the optical frequency comb 308 by the circulator 302 and the seventh optical directional coupler 303, and is output as the output light 310. The interference signal of the output light 310 is detected by the photodetector 304, digitized by the analog-to-digital converter 305, and input to the computing device 306.

[0058] The LIDAR device 10 of this embodiment is a LIDAR device using an FM comb LIDAR, and similarly to the method disclosed in Non-Patent Document 1, the LIDAR device 10 sweeps the comb mode frequency of the optical frequency comb 121 and measures the beat frequency in the LIDAR section 104 to measure the distance between the LIDAR device 10 and the target object 123. The first control device 107 controls the current controller 106 to sweep the frequency of the CW laser 113 input from the CW laser light source device 105 to the optical frequency comb generator 108, thereby sweeping the comb mode frequency of the optical frequency comb 116 generated in the optical frequency comb generator 108 by the comb mode interval.

[0059] Each comb mode frequency of the optical frequency comb 116 is changed at a constant rate a n When the LIDAR signal is changed by , it can be expressed as a sine wave. Therefore, when multiple LIDAR signals are detected by a single photodetector, the multiple LIDAR signals detected by the single photodetector are superpositions of sine waves, that is,

[0060]

number

[0061] According to the method disclosed in Non-Patent Document 1, all comb mode frequencies of the optical frequency comb 116 are changed linearly with respect to time. In this case, each comb mode frequency and the lidar signal are shown in FIG. 4. As shown in FIG. 4(a), when each comb mode frequency is changed at the same rate with respect to time, ω n will be the same, and therefore the lidar signals of each comb mode cannot be distinguished from each other, as shown in Figure 4(b).

[0062] In this embodiment, in addition to changing all comb mode frequencies of the optical frequency comb 116 to increase linearly with respect to time, the frequency intervals of all comb modes of the optical frequency comb 116 are increased linearly with respect to time. Each comb mode frequency and the lidar signal in this case are shown in FIG. 5. FIG. 5 shows five comb modes CH1 to CH5 as an example. As shown in FIG. 5(a), when each comb mode frequency is changed at the same rate with respect to time and each comb mode frequency interval is changed at the same rate with respect to time, the slopes of each comb mode frequency are the same in FIG. 4(a), but in FIG. 5(a), the slopes of each comb mode frequency become larger as the order of the comb mode increases. Therefore, the ω in the above equation for each comb mode n are different from each other, and therefore it becomes possible to distinguish the lidar signals of each comb mode from each other, as shown in Figure 5(b).

[0063] A procedure for detecting multiple lidar signals by a single photodetector and coherently combining the lidar signals of each comb mode will be described with reference to Figs. 6, 7, 8, and 9. All comb mode frequencies of the optical frequency comb 116 are changed to increase linearly with time, and the frequency intervals of all comb modes of the optical frequency comb 116 are increased linearly with time. In this case, as shown in Figs. 5(b) and 6(a), the lidar signals of each comb mode are represented by sine waves with different periods. When the sine waves shown in Figs. 5(b) and 6(a) are added together and then Fourier transformed, discrete signals in the frequency domain can be obtained as shown in Fig. 6(b). The frequencies of these signals are beat frequencies. Here, although details will be described later, when the superposition of the sine waves shown in Fig. 6(a) is Fourier transformed to obtain discrete signals in the frequency domain as shown in Fig. 6(b), correction is made for signal variations caused by the nonlinearity of the frequency sweep.

[0064] As shown in Fig. 6(c), one signal is extracted from the signals CH1 to CH5 shown in Fig. 6(b). Fig. 6(c) shows the signal CH1 as an example. The signal CH1 shown in Fig. 6(d) is Fourier transformed to obtain the lidar signal of only CH1, as shown in Fig. 6(d).

[0065] As shown in FIG. 7(a), for all signals from CH2 to CH5, the lidar signal is acquired in the same manner as when the lidar signal from only CH1 was acquired as shown in FIG. 6(e). For all signals from CH1 to CH5 shown in FIG. 7(a), the time T swept by the comb mode interval is n,cycle The time T swept over the comb mode interval is calculated. n,cycle The method for calculating the time T n,cycle The time when the line P in FIG. 7(b) intersects with the time axis of each comb mode is the time T n,cycle For all the lidar signals, QCH1 to QCH5, as shown in Figure 7(c), T n,cycle =T cycleThe time axis is corrected so that the line Q in FIG. 7(c) is the time T cycle This indicates that.

[0066] As shown in FIG. 8(a), T cycle The LIDAR signals of each comb mode are coherently combined. The obtained LIDAR signals are Fourier transformed to obtain a beat signal, as shown in FIG. 8(b). The distance L to the target object 123 can be calculated from (Equation 1).

[0067] The following describes the correction of signal variations caused by the nonlinearity of frequency sweep when the superposition of sine waves shown in FIG. 6(a) is Fourier-transformed to obtain the signal shown in FIG. 6(b). When the CW laser 115 output from the CW laser light source device 105 or the modulation signal input from the RF oscillator 112 to the electro-optical modulator 111 is controlled to linearly increase the comb mode frequency of the optical frequency comb 116 with respect to time, the response of the CW laser 115 or the modulation signal to the input signal that controls the CW laser 115 or the modulation signal is ideally linear with respect to the input signal, but in reality it is not linear but nonlinear, so such signal variations occur. FIG. 9(a) shows, as an example, a signal actually obtained by Fourier-transforming a lidar signal. The signal shown in FIG. 9(a) shows three comb mode signals, but there is variation in the signal, and the three comb modes cannot be separated from each other.

[0068] Correction for this is performed as follows. When sweeping the comb mode frequency of the optical frequency comb 116, the frequency change measurement unit 102 measures the change in the comb mode frequency of the optical frequency comb 116 over time. Using this measurement result, the time axis of the LIDAR signal before Fourier transformation is corrected so that the nonlinear change in the comb mode frequency of the optical frequency comb 116 over time becomes a linear change. Figure 9(b) shows the signal obtained by performing this correction on the LIDAR signal that was the source of the signal shown in Figure 9(a) and then performing a Fourier transform. Three comb mode signals (H, I, J) are shown, and the three comb modes are separated from each other.

[0069] The time T swept over the comb mode interval is shown below. n,cycle A method for calculating the frequency v ref The comb mode is interfered with by a CW laser having a frequency of 100 kHz and the comb mode is detected, and an interference signal is detected. The comb mode of the optical frequency comb is frequency swept, and when the comb mode frequency and the reference frequency match, an interference signal is detected, and a signal like that shown in Figure 10(b) is obtained. According to Figure 10(b), at time t1, the comb mode frequency of the kth comb mode matches the frequency of the CW laser, and at time t2, the comb mode frequency of the (k-1)th comb mode matches the frequency of the CW laser.

[0070] Frequency ν ref satisfies the following two equations, which respectively represent the condition that the comb mode frequency of the kth comb mode coincides with the frequency of the CW laser at time t1 and the condition that the comb mode frequency of the (k-1)th comb mode coincides with the frequency of the CW laser at time t2.

[0071]

number

[0072]

number

[0073]

number

[0074] Time swept by the comb mode interval T n,cycle satisfies the following formula,

[0075]

number

[0076] A reference CW laser 1107 irradiated from a single frequency oscillation CW laser light source device 1101 is superimposed on the optical frequency comb 122 in an optical directional coupler 1102, and an interference signal is detected by a photodetector 1103. Furthermore, an RF bandpass filter 1104, for example of 10 MHz, detects only the interference signal when the frequency difference between the CW laser 1107 and the optical frequency comb 122 is within 10 MHz.

[0077] FIG. 12 shows interference signals corresponding to the kth and k-1th comb modes detected by the RF bandpass filter 1104. FIG. 13 shows an enlarged view of the interference signals corresponding to the kth and k-1th comb modes shown in FIG. 12. Referring to FIG. 13(a), the 10 MHz RF bandpass filter 1104 causes a relatively strong interference signal to appear in the time range (A) where the frequency difference between the kth comb mode and the CW laser 1107 is -10 MHz and in the time range (C) where the frequency difference is 10 MHz, and the interference signal disappears at the point (B) where the frequencies of the kth comb mode and the CW laser 1107 match. Similarly, for the k-1th comb mode, the point (E) where the frequencies of the k-1th comb mode and the CW laser 1107 match is estimated, and the comb mode interval frequency f at the start of the frequency sweep is calculated from the points (B) and (E). repThen, the time T swept by the comb mode interval from (Equation 13) can be calculated using the calculated comb mode interval frequency. n,cycle The time T swept from point (B) and point (E) by the comb mode interval can be calculated. n,cycle The process of calculating is performed in the second calculation device 1106.

[0078] The following describes an actual frequency sweep method for the optical frequency comb 116. In this embodiment, all comb mode frequencies of the optical frequency comb 116 are swept to increase linearly with time, and at the same time, the frequency intervals of all comb modes of the optical frequency comb 116 are swept to increase linearly with time.

[0079] The comb mode frequency of the P-th comb mode of the optical frequency comb 116, counting from zero frequency, can be written as follows:

[0080]

number

[0081]

number

[0082] When an electro-optic modulator is used as the optical frequency comb generator 108, f CW The change in f CEO Changing the frequency of the modulation signal input from the RF oscillator 112 to the electro-optic modulator 111 is realized by changing only f rep On the other hand, when a microresonator is used as the optical frequency comb generator 108, changing the frequency of the CW laser incident on the microresonator in accordance with the change in the resonant frequency caused by a microheater, a piezoelectric element, or the like attached to the microresonator corresponds to changing fCEO and f rep That is, when the frequency of the CW laser is swept, a frequency sweep that linearly increases all comb mode frequencies of the optical frequency comb 116 with respect to time and a frequency sweep that linearly increases the frequency intervals of all comb modes of the optical frequency comb 116 with respect to time are simultaneously performed.

[0083] When an electro-optic modulator is used as the optical frequency comb generator 108 as shown in FIG. 1, a frequency sweep that linearly increases all comb mode frequencies of the optical frequency comb 116 with respect to time is performed as follows. This is performed by controlling the current controller 106 by the first control device 107 to sweep the frequency of the CW laser input to the electro-optic modulator 111 from the CW laser light source device 105 connected to the current controller 106. A frequency sweep that linearly increases the frequency intervals of all comb modes of the optical frequency comb 116 with respect to time is performed as follows. The second control device 109 controls the frequency of the modulation signal input from the RF oscillator 112 to the electro-optic modulator 111 so as to increase it at the same rate with respect to time.

[0084] Since the sweeping of the oscillation frequency of RF oscillator 112 and the sweeping of the frequency of CW laser 113 must be performed simultaneously, the controls by first controller 107 and second controller 109 are executed in synchronization with each other.

[0085] When a microresonator is used as the optical frequency comb generator 108, the second control device 109 controls the resonant frequency control mechanism to control the resonant frequency of the microresonator using a microheater, a piezoelectric element, or the like, and the first control device 107 controls the current controller 106 in synchronization with the control of the resonant frequency of the microresonator to control the frequency of the CW laser, thereby sweeping the frequency of the optical frequency comb. CW f for the change in CEO +nf rep The correspondence relationship of the change amount in all comb modes is measured in advance. Based on this correspondence, CEO +nfrep The frequency f of the CW laser is set so that increases linearly with time and the rate of increase is different for all comb modes. CW Based on this correspondence, we calculate T for each comb mode lidar signal. n,cycle =T cycle The time axis is corrected so that the LIDAR signals of all the comb modes whose time axes have been corrected are coherently combined to calculate a beat signal, thereby calculating the distance to the target object 123.

[0086] The data obtained by measuring the distance as described above is shown in FIG. 14. The lidar signal shown in FIG. 14 is obtained when an optical frequency comb generated by an electro-optical modulator is used as the optical frequency comb 116. FIG. 14(a) shows a lidar signal obtained by coherently combining lidar signals from the second, third, and fourth comb modes. FIG. 14(b) shows the coherent combining point (F) of the lidar signals from the second and third comb modes, and FIG. 14(c) shows the coherent combining point (G) of the lidar signals from the third and fourth comb modes. FIG. 15 shows the beat frequency delay in distance measurement by the FM comb lidar according to the present embodiment, and in distance measurement by a conventional FMCW lidar for comparison. As shown in FIG. 14(b) and FIG. 14(c), the lidar signals are combined between the modes for both phase and intensity. Also, as shown in FIG. 15, when comparing the half-width of the beat frequency delay data by the FM comb lidar and the beat frequency delay data by the FMCW lidar, the half-width by the FM comb lidar is about one-third of the half-width by the FMCW lidar. As shown in Figure 15, distance measurement using FM comb lidar achieves a resolution about three times higher than that achieved using conventional FMCW lidar.

[0087] As described above, the present invention naturally includes various embodiments not described here. Therefore, the technical scope of the present invention is defined only by the invention-specifying matters related to the scope of the claims appropriate from the above description. [Explanation of symbols]

[0088] 10 Lidar equipment 101 Laser control unit 102 Frequency change measurement section 103 Comb mode interval measurement unit 104 Rider Club 105, 1601, 1801 CW laser light source device 106 Current Controller 107 First control device 108 Optical Frequency Comb Generator 109 Second control device 110 1st optical directional coupler 111 Electro-optical modulator 112 RF Oscillator 113~115, 215, 216, 1107, 1609 CW laser 116, 118, 119, 121, 122, 219, 220, 307, 308, 1803, 1804, 1805 Optical frequency comb 117 Second optical directional coupler 120 Third optical directional coupler 123, 1605 Target 201 Two-wavelength unbalanced Mach-Zehnder interferometer 202 Optical Bandpass Filter (BPF) 203, 204 First and second photodetectors 205, 206 First and second analog-to-digital converters (ADCs) 210 Signal Processing Device 211 First Waveguide 212 Second Waveguide 213 4th optical directional coupler 214 5th optical directional coupler 217, 221, 310, 1617, 1807 Output light 301 6th optical directional coupler 302, 1603 Circulator 303 7th optical directional coupler 304, 1103 Photodetector 305 Analog-to-Digital Converter (ADC) 306, 1408 Computing equipment 309, 1613~1615, 1806 Scattered light 1101 Single frequency oscillation CW laser light source device 1102, 1606 Optical directional coupler 1104 RF Bandpass Filter 1105 Analog-to-Digital Converter 1106 Second computing device 1602, 1606 Optical directional coupler 1604 Angle sweep mechanism 1608 Computing equipment 1610, 1612, 1616 reference light 1618 Interference Signal 180 Distance Measuring Device 1802 Optical Frequency Comb Generator 1805 Wavelength resolution element 18061~1806n 1st to nth photodetectors

Claims

1. a laser control unit that generates an optical frequency comb from a CW laser oscillated from a CW laser light source device and sweeps a comb mode frequency of the optical frequency comb by a frequency interval of the comb mode at a time when a frequency sweep is started; a frequency change measurement unit that measures a sweep amount of the frequency of the CW laser and a sweep amount of a comb mode frequency of the optical frequency comb; a comb mode interval measurement unit that calculates, for each comb mode, a time required to sweep the comb mode frequency of the optical frequency comb by a frequency interval of the comb mode at a time when the frequency sweep is started; a lidar unit that detects an interference signal between scattered light resulting from irradiation of an object with the optical frequency comb and mixed light of the optical frequency comb using a single photodetector, and calculates a distance to the object; the laser control unit performs a sweep that linearly increases the comb mode frequency of the optical frequency comb with respect to time and simultaneously performs a sweep that linearly increases the frequency interval of the comb mode of the optical frequency comb with respect to time; the LIDAR unit performs a Fourier transform on a LIDAR signal calculated from the interference signal to obtain discrete signals in the frequency domain, performs a Fourier transform on each of the discrete signals individually to generate a LIDAR signal for each comb mode, corrects the time axis of the generated LIDAR signal for each comb mode so that the times for sweeping by the frequency interval of the comb mode at the time when the frequency sweep for each comb mode is started are all the same, and calculates the distance using a beat signal obtained by coherently combining and Fourier transforming the corrected LIDAR signals for each comb mode.

2. the laser control unit includes an electro-optic modulator, an RF oscillator that inputs a modulation signal to the electro-optic modulator, and a current controller connected to the CW laser light source device; inputting the CW laser into the electro-optic modulator to generate the optical frequency comb; Sweeping the comb mode frequency of the optical frequency comb linearly with respect to time is performed by controlling the current controller to control the frequency of the CW laser; 2. The lidar device according to claim 1, wherein the sweep that linearly increases the frequency spacing of the comb modes of the optical frequency comb with respect to time is performed by controlling the RF oscillator to control the modulation signal.

3. the laser control unit includes a microresonator, a resonance frequency control mechanism that controls a resonance frequency of the microresonator, and a current controller that is connected to the CW laser light source device; inputting the CW laser into the microresonator to generate the optical frequency comb; 2. The lidar device of claim 1, wherein the sweeping of the comb mode frequencies of the optical frequency comb linearly increasing with time and the sweeping of the frequency spacing of the comb modes of the optical frequency comb linearly increasing with time are performed by controlling the current controller to control the frequency of the CW laser and by controlling the resonant frequency control mechanism to control the resonant frequency.

4. 4. The LIDAR device according to claim 3, wherein the resonant frequency control mechanism is a microheater attached to the microresonator.

5. The lidar device according to any one of claims 1 to 4, characterized in that the frequency change measuring unit measures the frequency sweep amount of the CW laser and the comb mode frequency sweep amount of the optical frequency comb using an unbalanced Mach-Zehnder interferometer.

6. the comb mode spacing measurement unit includes a single frequency oscillation CW laser light source device, a second photodetector, and an RF bandpass filter; A lidar device according to any one of claims 1 to 5, characterized in that an interference signal between a second CW laser and the optical frequency comb from the single frequency oscillation CW laser light source device is detected by the second photodetector, and a time for sweeping the comb mode frequency of the optical frequency comb by the frequency interval of the comb mode at the time the frequency sweep is started is calculated for each comb mode from a signal when the comb mode frequency of the optical frequency comb extracted by the RF bandpass filter matches the frequency of the second CW laser.

7. Using the time change in the sweep amount of the comb mode frequency of the optical frequency comb measured by the frequency change measurement unit, A LIDAR device as described in any one of claims 1 to 6, characterized in that when the LIDAR unit performs a Fourier transform on the LIDAR signal calculated from the interference signal to obtain the discrete signal in the frequency domain, the LIDAR unit corrects the time axis of the LIDAR signal calculated from the interference signal and performs correction for signal variations caused by nonlinearity of frequency sweep.

8. generating an optical frequency comb from a CW laser oscillated from a CW laser source device; a step of sweeping a comb mode frequency of the optical frequency comb in a linearly increasing manner with time and simultaneously sweeping a frequency interval of the comb mode of the optical frequency comb in a linearly increasing manner with time by a frequency interval of the comb mode at a time when a frequency sweep is to be started; measuring a frequency sweep of the CW laser and a comb mode frequency sweep of the optical frequency comb; calculating, for each comb mode, a time for sweeping the comb mode frequency of the optical frequency comb by a frequency interval of the comb mode at a time when the frequency sweep is started; Detecting an interference signal between scattered light resulting from irradiating an object with the optical frequency comb and the optical frequency comb using a single photodetector; performing a Fourier transform on a lidar signal calculated from the interference signal to obtain a discrete signal in the frequency domain; individually Fourier transforming each of the discrete signals to generate a lidar signal for each comb mode; correcting a time axis of the generated LIDAR signal for each comb mode so that the time for sweeping the frequency interval of the comb mode at the time when the frequency sweep for each comb mode is started is the same for all the comb modes; calculating a distance using a beat signal obtained by coherently combining and Fourier transforming the corrected LIDAR signals for each comb mode; A rider control method comprising:

9. generating the optical frequency comb includes inputting the CW laser into an electro-optic modulator to generate the optical frequency comb; The step of sweeping the frequency by a frequency interval of a comb mode at a time when the frequency sweep is started includes: a sweep in which the comb mode frequency of the optical frequency comb increases linearly with time by controlling a current controller connected to the CW laser light source device to control a frequency of the CW laser; a sweep in which a frequency interval of the comb modes of the optical frequency comb is linearly increased with respect to time by controlling an RF oscillator that inputs a modulation signal to the electro-optic modulator and controlling the modulation signal; 9. The method of claim 8, further comprising the step of simultaneously executing:

10. generating the optical frequency comb includes inputting the CW laser into a microresonator to generate the optical frequency comb; The step of sweeping the frequency by a frequency interval of a comb mode at a time when the frequency sweep is started includes: Controlling a current controller connected to the CW laser light source device to control a frequency of the CW laser; and sweeping the comb mode frequency of the optical frequency comb to increase linearly with time by controlling a resonant frequency of the microresonator. a linearly increasing sweep of the frequency spacing of the comb modes of the optical frequency comb with respect to time; 9. The method of claim 8, further comprising the step of simultaneously executing:

11. 11. The method of claim 10, wherein the control of the resonant frequency of the microresonator is performed by a microheater attached to the microresonator.

12. The lidar control method according to any one of claims 8 to 11, characterized in that the step of measuring the sweep amount of the frequency of the CW laser and the sweep amount of the comb mode frequency of the optical frequency comb is performed using an unbalanced Mach-Zehnder interferometer.

13. The step of calculating, for each comb mode, a time for sweeping the comb mode frequency of the optical frequency comb by a frequency interval of the comb mode at a time when the frequency sweep is started, detecting an interference signal between a second CW laser and the optical frequency comb; extracting a signal when a comb mode frequency of the optical frequency comb and a frequency of the second CW laser coincide with each other by using an RF bandpass filter; calculating, for each comb mode, a time for sweeping the comb mode frequency of the optical frequency comb by a frequency interval of the comb mode at a time when the frequency sweep is started, from the extracted signal; 13. The method for controlling a rider according to claim 8, further comprising:

14. A lidar control method according to any one of claims 8 to 13, characterized in that the step of performing a Fourier transform on the lidar signal calculated from the interference signal to obtain a discrete signal in the frequency domain includes a step of correcting the time axis of the lidar signal calculated from the interference signal and correcting for signal variations caused by nonlinearity of the frequency sweep.

Images