JP7627487B2 - LIDAR DEVICE AND LIDAR CONTROL METHOD - Google Patents

Description

The present invention relates to a lidar device and a lidar control method, and in particular to a lidar device and a lidar control method that utilizes an optical frequency comb.

"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 a 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.

A method for detecting the distance to an object by FMCW LIDAR will be described with reference to Figures 16 and 17. Figure 16 shows an example of the configuration of a distance measuring device 160 using a conventional FMCW LIDAR. A CW laser 1609 irradiated from a CW laser light 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 angle α, and the scattered light 1613 enters the angle sweep mechanism 1604. The scattered light 1613 incident on the angle sweep mechanism 1604 is input to the circulator as scattered light 1614 as it is and 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 the output light 1617. The 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 target object 1605.

As an example of frequency sweep of the CW laser 1609, 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 interference between the scattered light 1615 and the reference light 1616, and the frequency f _beat of the beat signal can be detected by the photodetector 1607 as shown in Fig. 17(c). The frequency of the beat signal is expressed by the following equation:

のように表されることから、対象体１６０５までの距離Ｌを算出することが出来る。ここで、ｃは光速、Ｔは図１７に示す三角波形の半周期、Δは図１７に示す周波数変化量の最大値である。

17, it is possible to calculate the distance L to the object 1605. Here, c is the speed of light, T is the half period of the triangular waveform shown in FIG. 17, and Δ is the maximum value of the frequency change amount shown in FIG.

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,

のように表され、周波数変化量Δが大きい程距離分解能は向上する。一方、ＣＷレーザ１６０９の線幅の逆数で測定距離範囲が決定する。通常ＣＷレーザ光源装置１６０１の内部の共振器長が大きくなると、ＣＷレーザ１６０９の線幅は向上するが、周波数変化量Δは小さくなる。このことから、測定距離範囲と周波数変化量Δとの間にはトレードオフの関係がある。

The larger the frequency change amount Δ, the better the distance resolution. Meanwhile, the measurement distance range is determined by the inverse of the linewidth of the CW laser 1609. Normally, when the internal resonator length of the CW laser light source device 1601 increases, the linewidth of the CW laser 1609 improves, but the frequency change amount Δ decreases. For this reason, there is a trade-off between the measurement distance range and the frequency change amount Δ.

In response to this, a method called frequency-modulated comb LiDAR (FMcomb LiDAR) that 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.

Figure 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 Figure 18 with the distance measurement device 160 shown in Figure 16, an optical frequency comb generator 1802 is connected to a CW laser light 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 the 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 nth photodetectors 18091 to 1809n.

FIG. 19 shows an example of frequency sweep 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 , the frequency of each comb mode coincides with the comb mode frequency at time t=0 of the comb mode that is one order higher than each comb mode.

The comb modes shown in Figure 19(a) are coherently combined. Here, coherently combining means combining two signals with each other, including not only their intensity but also their phase. The combined comb modes are shown in Figure 19(b). As shown in Figure 19(b), by signal processing the frequency-swept comb modes and coherently combining them, a large effective frequency range can be obtained, and thus the distance resolution can be improved.

N. Kuse, et al. , APL Photonics 4, 106105 (2019)

However, as shown in Figure 18, according to the FM comb lidar 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 the wavelength resolution element 1805, the first to nth photodetectors 18061 to 1806n, and multiple analog-digital converters.

In view of the above problems, the present invention aims 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.

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 by 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 sweep that linearly increases the comb mode frequency with time while simultaneously performing a sweep that linearly increases the frequency interval of the comb modes of the optical frequency comb with time, and 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 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 LIDAR signal for each generated comb mode so that the time for sweeping by the comb mode interval for each comb mode is 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.

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, and an optical frequency comb is generated by inputting a CW laser to the electro-optical modulator, 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 interval of the comb mode of the optical frequency comb with respect to time may be performed by controlling the RF oscillator to control the modulation signal.

In a first aspect of the present invention, the laser control unit includes a microresonator, a resonant frequency control mechanism that controls the resonant 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 the sweep that linearly increases the comb mode frequency of the optical frequency comb with respect to time and the 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 resonant frequency control mechanism to control the resonant frequency.

In the first aspect of the present invention, the resonant frequency control mechanism may be a microheater attached to the microresonator.

In the first aspect of the present invention, the frequency change measurement unit may use an unbalanced Mach-Zehnder interferometer to measure the frequency sweep amount of the CW laser and the comb mode frequency sweep amount of the optical frequency comb.

In a first aspect of the present invention, the comb mode interval 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, the time required to sweep the comb mode frequency of the optical frequency comb by the comb mode interval from the 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.

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 time change in 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 the signal variation caused by the nonlinearity of the frequency sweep.

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 with time while simultaneously sweeping the frequency interval of the comb mode of the optical frequency comb linearly with time, measuring the sweep amount of the CW laser frequency and the sweep amount of the comb mode frequency of the optical frequency comb; calculating the time for sweeping the comb mode frequency of the optical frequency comb by the comb mode interval for each comb mode; and measuring scattered light and light due to irradiation of the optical frequency comb on a target object. The method includes 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 LIDAR signal for each generated comb mode so that the time for sweeping the comb mode interval for each comb mode is the same for all of the generated signals, and calculating the distance using a beat signal obtained by coherently combining and Fourier-transforming the corrected LIDAR signals for each comb mode.

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 a 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.

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.

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

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.

In a second aspect of the present invention, the step of calculating for each comb mode the time 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 using an RF bandpass filter, and calculating for each comb mode the time to sweep the comb mode frequency of the optical frequency comb by the comb mode interval from the extracted signal.

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.

The present invention provides 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.

FIG. 2 is a block diagram showing an example of a configuration of a LIDAR device according to an embodiment of the present invention. 4 is a block diagram showing an example of a configuration of a frequency change measuring unit according to the embodiment. FIG. FIG. 2 is a block diagram showing an example of the configuration of a rider section according to the embodiment. Graph showing (a) comb mode frequency and (b) lidar signal according to the prior art. 1 is a graph showing (a) comb mode frequency and (b) lidar signal according to an embodiment. 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. 1A-1C are graphs illustrating a method for combining LIDAR signals according to an embodiment, (a) LIDAR signal, (b) before time axis correction, and (c) after time axis correction. 1A-1B are graphs illustrating a method for combining LIDAR signals according to an embodiment, (a) a graph of the combined LIDAR signal and (b) a graph of the beat signal. Graphs illustrating a method of correction when combining LIDAR signals according to an embodiment, (a) before correction, (b) after correction. 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 reference and the frequency of the comb mode, and (b) is a diagram of the resulting signal. 4 is a block diagram showing an example of a configuration of a comb mode interval measurement unit according to the embodiment. FIG. 11 is a graph showing an interference signal according to an embodiment. 1A and 1B are graphs showing interference signals according to an embodiment, (a) k-th comb mode, (b) k-1-th comb mode. 13A and 13B are graphs of LIDAR signals from a LIDAR device according to an embodiment of the present invention, showing (a) the second to fourth comb modes, (b) the second and third comb modes, and (c) the third and fourth comb modes. 1 is a beat signal from a LIDAR device according to an embodiment of the present invention. FIG. 1 is a block diagram showing an example of the configuration of a distance measurement device using a conventional FMCW LIDAR. 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. FIG. 1 is a block diagram showing an example of the configuration of a LIDAR device according to a conventional technology. 1 is a graph showing a LIDAR signal according to the prior art.

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 component, etc., differ from the actual ones. Therefore, the specific thickness and dimensions should be determined with reference to the following explanation. In addition, it goes without saying that the drawings include parts with different dimensional relationships and ratios.

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.

(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.

The laser control unit 101 is composed 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.

The optical frequency comb generator 108 may be, for example, an electro-optical modulator or 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.

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 and controls 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.

A CW laser is input from the CW laser light source device 105 via 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 matches 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 the resonant frequency control mechanism.

An optical frequency comb using an electro-optic modulator or a microresonator has a comb mode frequency spacing of about 10 GHz-1 THz, which is wider than the comb spacing of conventional optical frequency combs such as mode-locked fiber lasers, which is about 100 MHz-1 GHz. 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 multiple CW lasers, and the comb mode frequency of each comb mode is swept. Therefore, it is desirable for the comb spacing of the optical frequency comb to be wider, and from the viewpoint of comb spacing, it is desirable to use an electro-optic modulator or a microresonator as the optical frequency comb generator 108.

The CW laser 113 irradiated from the CW laser light source device 105 is separated into the CW laser 114 and the CW laser 115 by the first optical directional coupler 110. The CW laser 115 is input to the frequency change measurement unit 102, where the frequency sweep amount of the CW laser 115 is measured. The CW laser 114 is input to the optical frequency comb generator 108, where the optical frequency comb 116 is generated. The optical frequency comb 116 is separated into the optical frequency comb 118 and the optical frequency comb 119 by the second optical directional coupler 117. The optical frequency comb 119 is input to the frequency change measurement unit 102, where the frequency sweep amount of the optical frequency comb 119 is measured. The optical frequency comb 118 is separated into the optical frequency comb 121 and the optical frequency comb 122 by the third optical directional coupler 120. The optical frequency comb 122 is input to the comb mode interval measurement unit 103, which measures the time swept by the comb mode interval 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.

In this embodiment, since each comb mode of the optical frequency comb 116 is used as an optical carrier for 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. Also, as described later, it is necessary to measure the frequency sweep amount of the CW laser 115 in order to measure the comb mode spacing. The frequency sweep amount of the CW laser 115 is also measured using the frequency change measurement unit 102.

The method for measuring the frequency sweep amount 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 amount 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 amount of all comb modes of the optical frequency comb 116 can be calculated by measuring the frequency sweep amount of the Nth and Mth comb modes of the optical frequency comb 116 (where N ≠ M).

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.

In this embodiment, the optical frequency comb generator 108 is composed of an electro-optical modulator 111 and an RF oscillator 112. A method for calculating the frequency sweep amount 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 Nth comb mode of the optical frequency comb 116 when the optical frequency comb generator 108 is composed of the electro-optical 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.

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 the CW laser 216 and through the second waveguide 212 as the CW laser 217, and is merged at the fifth optical directional coupler 214, outputting the output light 218, and the 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, the occurrence of interference is observed.

When the first control device 107 controls the current controller 106 and sweeps 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. By measuring the amount of change δφ _CW in the phase of this interference waveform before and after changing the frequency of the CW laser 113, the amount of change δν _CW in the frequency of the CW laser 113 can be calculated from the following equation.

ここで、上式中のτは２波長アンバランスマッハツェンダ―干渉計２０１の第２導波路２１２を伝搬した光の遅延時間であり、既知の値である。

Here, τ in the above formula is the delay time of the light propagated through the second waveguide 212 of the two-wavelength unbalanced Mach-Zehnder interferometer 201, and is a known value.

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 output light 221 is output. The optical bandpass filter 202 is set to transmit only the Nth 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 Nth comb mode is detected by the second photodetector 204.

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. If the amount of change in the phase of the interference signal of the Nth comb mode is δφ _N , the comb mode frequency sweep amount δν _N of the Nth comb mode can be calculated by the following formula.

Here, the relationship between the amount of change δν _CW in the frequency of the CW laser 113 and the amount of comb mode frequency sweep δν _N of the Nth comb mode is expressed by the following equation.

ここで、δν_ＥＯは、ＲＦ発振器１１２から電気光学変調器１１１へ入力される変調信号の周波数の変化量である。上式より、任意のコムモード周波数の掃引量δν_Ｋは、

Here, δν _EO is the amount of change in the frequency of the modulated signal input from the RF oscillator 112 to the electro-optic modulator 111. From the above formula, the sweep amount δν _K of an arbitrary comb mode frequency is given by

より算出することが出来る。

It can be calculated from the above.

When the optical frequency comb generator 108 is composed of a microresonator, an optical bandpass filter and a photodetector set to transmit only the Mth comb mode of the output light 221 are connected to the fourth optical directional coupler 213 of the frequency change measurement unit 102 shown in Figure 2 in order to measure the comb mode frequency sweep amount δν _M of the Mth comb mode. The interference signal of the Mth comb mode is detected in a manner similar to the method for detecting the interference signal of the Nth comb mode. If the amount of change in phase of the interference signal of the Mth comb mode accompanying the sweep of the comb mode frequency of the optical frequency comb 116 is δφ _M , the comb mode frequency sweep amount δν _M of the Mth comb mode can be calculated by the following formula.

任意のコムモード周波数の掃引量δν_Ｋは、

The sweep amount δν _K of an arbitrary comb mode frequency is given by

より算出することが出来る。

It can be calculated from the above.

An example of the configuration of the LIDAR section 104 is shown in FIG. 3. The LIDAR section 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.

The optical frequency comb 121 is separated into an optical frequency comb 307 and an optical frequency comb 308 in 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.

The LIDAR device 10 of this embodiment is a LIDAR device using an FM comb LIDAR, and similar to the method disclosed in Non-Patent Document 1, the LIDAR device 10 measures the distance between the LIDAR device 10 and the target object 123 by sweeping the comb mode frequency of the optical frequency comb 121 and measuring the beat frequency in the LIDAR section 104. 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.

When each comb mode frequency of the optical frequency comb 116 is changed at a constant rate a _n with respect to time, the lidar signal 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 superimposed sine waves, that is, the following equation is given:

で表すことが出来る。ここで、上式のω_ｎは割合ａ_ｎに依存する値であり、ｎはコムモードの次数である。

Here, ω _n in the above equation is a value that depends on the ratio a _n , and n is the order of the comb mode.

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 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, _ωn in the above equation for each comb mode becomes the same value, and therefore, as shown in Fig. 4(b), the LIDAR signals of each comb mode cannot be distinguished from each other.

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 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, ω _n in the above equation for each comb mode becomes a different value, and therefore, as shown in FIG. 5(b), it becomes possible to distinguish the LIDAR signals of each comb mode from each other.

The 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 Figures 6, 7, 8, and 9. In addition to the change in which all comb mode frequencies of the optical frequency comb 116 increase linearly with time, the frequency intervals of all comb modes of the optical frequency comb 116 increase linearly with time. In this case, as shown in Figures 5(b) and 6(a), the lidar signals of each comb mode are represented by sine waves with different periods. By adding the sine waves shown in Figures 5(b) and 6(a) and then Fourier transforming them, a discrete signal in the frequency domain can be obtained as shown in Figure 6(b). Note that the frequency of these signals is the beat frequency. Here, although details will be described later, when the superposition of the sine waves shown in Figure 6(a) is Fourier transformed to obtain a discrete signal in the frequency domain as shown in Figure 6(b), a correction is made for the signal variation caused by the nonlinearity of the frequency sweep.

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

As shown in FIG. 7(a), for all signals of CH2 to CH5, the lidar signal is acquired in the same procedure as when the lidar signal of only CH1 shown in FIG. 6(e) is acquired. For all signals of CH1 to CH5 shown in FIG. 7(a), the time T _n,cycle swept by the comb mode interval is obtained. The method of obtaining the time T _n,cycle swept by the comb mode interval will be described later. FIG. 7(b) shows the time T _n,cycle obtained for all signals of CH1 to CH5. The time at which the line P in FIG. 7(b) intersects with the time axis of each comb mode is the time T _n,cycle in each comb mode. For all signals of QCH1 to CH5 of the lidar signal, the time axis is corrected so that T _n,cycle = T _cycle as shown in FIG. 7(c). The line Q in FIG. 7(c) indicates that the time is T _cycle .

As shown in Fig. 8(a), the LIDAR signals of each comb mode are coherently combined at T _cycle . 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).

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 and the modulation signal to the input signal that controls the CW laser 115 and 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. As an example, FIG. 9(a) shows 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.

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, a time axis correction is performed on the lidar signal before Fourier transform so that the nonlinear change over time in the comb mode frequency of the optical frequency comb 116 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.

A method for obtaining the time T _n,cycle swept by the comb mode interval will be described below with reference to FIG. 10. As shown in FIG. 10(a), a CW laser having a frequency ν _ref is made to interfere with the comb mode, and an interference signal is detected. The comb mode of the optical frequency comb is frequency swept, and an interference signal is detected when the comb mode frequency and the reference frequency match, and a signal such as that shown in FIG. 10(b) is obtained. According to FIG. 10(b), at time t ₁ , the comb mode frequency of the kth comb mode matches the frequency of the CW laser, and at time t ₂ , the comb mode frequency of the (k-1)th comb mode matches the frequency of the CW laser.

The frequency v _ref satisfies the following two equations, which respectively indicate the condition that the comb mode frequency of the k-th 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 .

時刻ｔ_０は周波数掃引を開始する最初の時刻、Δ_ＣＷ（ｔ）は時刻ｔにおけるＣＷレーザの周波数の変化量、Δ_ｒｅｐ（ｔ）は時刻ｔにおけるコムモード間隔周波数の変化量、ｆ_ｒｅｐ（ｔ）は時刻ｔにおけるコムモード間隔周波数、Ｋはコムモードの次数である。上２式より、時刻ｔ_０におけるコムモード間隔周波数ｆ_ｒｅｐ（ｔ_０）は以下の式から求めることが出来る。

Time _t0 is the first time when frequency sweeping starts, _ΔCW (t) is the change in the CW laser frequency at time t, _Δrep (t) is the change in the comb mode spacing frequency at time t, _frep (t) is the comb mode spacing frequency at time t, and K is the order of the comb mode. From the above two equations, the comb mode spacing frequency _frep ( _t0 ) at time _t0 can be calculated from the following equation.

The time T _n,cycle swept over the comb mode interval satisfies the following formula:

図１に示すコムモード間隔測定部１０３によって測定される。図１１に、本実施形態に係るコムモード間隔測定部１０３の構成の一例を示す。図１１に示すコムモード間隔測定部１０３は、単一周波数発振ＣＷレーザ光源装置１１０１、光方向性結合器１１０２、光検出器１１０３、ＲＦバンドパスフィルタ（ＲＦＢＰＦ）１１０４、アナログ―デジタルコンバータ１１０５、第２計算装置１１０６から構成される。

The comb mode spacing is measured by the comb mode spacing measurement unit 103 shown in Fig. 1. Fig. 11 shows an example of the configuration of the comb mode spacing measurement unit 103 according to this embodiment. The comb mode spacing measurement unit 103 shown in Fig. 11 includes a single frequency oscillation CW laser light source device 1101, an optical directional coupler 1102, a photodetector 1103, an RF bandpass filter (RFBPF) 1104, an analog-digital converter 1105, and a second calculation unit 1106.

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

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 the time range (C) where the frequency difference is 10 MHz, and the interference signal disappears at the point (B) where the frequency of the kth comb mode and the CW laser 1107 match. Similarly, for the k-1th comb mode, the point (E) where the frequency of the k-1th comb mode and the CW laser 1107 match can be estimated, and the comb mode interval frequency f _rep (t ₀ ) at the start of the frequency sweep can be calculated from the points (B) and (E). Then, using the calculated comb mode interval frequency, the time T _n,cycle swept by the comb mode interval can be calculated from (Equation 13). The process of calculating the time T n, _cycle swept by the comb mode interval from points (B) and (E) is performed in the second calculation unit 1106.

The actual frequency sweep method of the optical frequency comb 116 is described below. In this embodiment, a sweep is performed to linearly increase the frequency of all comb modes of the optical frequency comb 116 with respect to time, while at the same time, a sweep is performed to linearly increase the frequency interval of all comb modes of the optical frequency comb 116 with respect to time.

The comb mode frequency of the Pth comb mode of the optical frequency comb 116, counting from zero frequency, can be written as follows:

また、ＣＷレーザ１１３から数えてＱ番目のコムモードのコムモード周波数は次式のように書ける。

Further, the comb mode frequency of the Qth comb mode counted from the CW laser 113 can be written as follows:

ここで、ｆ_ＣＷ＝ｆ_ＣＥＯ＋Ｒｆ_ｒｅｐである（ただし、Ｑ＝Ｐ－Ｒ）。

Here, f _CW =f _CEO +Rf _rep , where Q=P-R.

When an electro-optical modulator is used as the optical frequency comb generator 108, the change in f _CW is realized by only changing f _CEO . Changing the frequency of the modulation signal input from the RF oscillator 112 to the electro-optical modulator 111 corresponds to changing 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 f _CEO and f _rep simultaneously. 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.

When an electro-optical 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-optical 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-optical modulator 111 so that it increases at the same rate with respect to time.

Since the sweep of the oscillation frequency of the RF oscillator 112 and the sweep of the frequency of the CW laser 113 must be performed simultaneously, the controls by the first control device 107 and the second control device 109 are performed in synchronization with each other.

When a microresonator is used as the optical frequency comb generator 108, the resonant frequency of the microresonator is controlled by a microheater, a piezoelectric element, or the like by controlling the resonant frequency control mechanism by the second control device 109, and the frequency of the CW laser is controlled by controlling the current controller 106 by the first control device 107 in synchronization with the control of the resonant frequency of the microresonator, thereby performing frequency sweeping of the optical frequency comb. The correspondence relationship between the change in the frequency f _CW of the CW laser and the change in f _CEO +nf _rep in all comb modes is measured in advance. Based on this correspondence, the frequency f _CW of the CW laser is swept so that f _CEO +nf _rep increases linearly with time for all comb modes, and the increase rate is different for all comb modes, and the lidar signal of each comb mode is obtained. Based on this correspondence, the time axis is corrected so that T _n,cycle =T _cycle for the lidar signal of each comb mode. The lidar signals of all the time-axis-corrected comb modes are coherently combined and a beat signal is calculated to calculate the distance to the target object 123.

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 delays in distance measurement by the FM comb lidar according to the present embodiment and 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 in terms of both phase and intensity. Also, as shown in Figure 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 that by the FMCW lidar. As shown in Figure 15, the distance measurement by the FM comb lidar has about three times the resolution compared to the distance measurement by the conventional FMCW lidar.

The present invention naturally includes various embodiments not described here. Therefore, the technical scope of the present invention is determined only by the invention-specific matters related to the scope of the claims that are appropriate from the above explanation.

10 LIDAR device 101 Laser control unit 102 Frequency change measurement unit 103 Comb mode spacing measurement unit 104 LIDAR unit 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 First optical directional coupler 111 Electro-optical modulator 112 RF oscillator 113 to 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 Object 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 Fourth optical directional coupler 214 Fifth optical directional coupler 217, 221, 310, 1617, 1807 Output light 301 Sixth optical directional coupler 302, 1603 Circulator 303 Seventh optical directional coupler 304, 1103 Photodetector 305 Analog-to-digital converter (ADC)
306, 1408 Calculation device 309, 1613 to 1615, 1806 Scattered light 1101 Single frequency oscillation CW laser light source device 1102, 1606 Optical directional coupler 1104 RF band pass filter 1105 Analog-digital converter 1106 Second calculation device 1602, 1606 Optical directional coupler 1604 Angle sweep mechanism 1608 Calculation device 1610, 1612, 1616 Reference light 1618 Interference signal 180 Distance measurement device 1802 Optical frequency comb generator 1805 Wavelength resolution elements 18061 to 1806n 1st to nth photodetectors

Claims (14)

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 ;
and a lidar unit that detects an interference signal between scattered light resulting from irradiation of the optical frequency comb on an object and mixed light of the optical frequency comb using a single photodetector, and calculates a distance to the object, wherein 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. 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. the laser control unit includes a microresonator, a resonance frequency control mechanism that controls a resonance frequency of the microresonator, and a current controller 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. The lidar device according to claim 3, characterized in that the resonant frequency control mechanism is a microheater attached to the microresonator. The lidar device according to any one of claims 1 to 4, characterized in that the frequency change measurement unit uses an unbalanced Mach-Zehnder interferometer to measure the frequency sweep amount of the CW laser and the comb mode frequency sweep amount of the optical frequency comb. 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. 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. 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;
and calculating a distance using a beat signal obtained by coherently combining and Fourier transforming the corrected LIDAR signals for each comb mode. 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;
and a sweep in which the frequency spacing of the comb modes of the optical frequency comb increases linearly with time by controlling an RF oscillator that inputs a modulation signal to the electro-optical modulator to control the modulation signal. 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:
The lidar control method according to claim 8, further comprising the steps of: controlling a current controller connected to the CW laser light source device to control a frequency of the CW laser; and simultaneously performing a sweep to linearly increase the comb mode frequency of the optical frequency comb with respect to time by controlling a resonant frequency of the microresonator; and a sweep to linearly increase a frequency interval of the comb modes of the optical frequency comb with respect to time. The lidar control method according to claim 10, characterized in that the control of the resonant frequency of the microresonator is performed by a microheater attached to the microresonator. The lidar control method according to any one of claims 8 to 11, characterized in that the step of measuring the frequency sweep amount of the CW laser and the comb mode frequency sweep amount of the optical frequency comb is performed using an unbalanced Mach-Zehnder interferometer. 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;
13. A lidar control method according to any one of claims 8 to 12, further comprising a step of calculating, for each comb mode, from the extracted signal, 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. 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.

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