Frequency scanning characteristic measurement device, LiDAR device, and frequency scanning characteristic measurement method

Abstract

An asymmetric MZI (120) of a frequency scanning characteristic measurement device (2) splits a laser beam and generates a difference of a predetermined optical path length, and then multiplexes the split light to generate multiplexed light, and a light receiver (121) receives the multiplexed light, converts the multiplexed light into a beat frequency signal, and outputs the beat frequency signal. A second signal processing unit (123) calculates the frequency scanning characteristics of the laser light output by the LD (100) and calculates a nonlinear component by performing predetermined signal processing on the beat frequency signal. At this time, the second signal processing unit (123) adjusts the waveform so that the amplitudes and phases at both ends of the beat frequency signal coincide, performs Hilbert transform on the adjusted beat frequency signal to calculate a quadrature component, and calculates the arc tangent between the quadrature component and the adjusted beat frequency signal to calculate the instantaneous phase of the beat frequency signal. And the frequency scanning characteristic of the laser is calculated according to the difference between the instantaneous phase and the optical path length.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of priority based on Japanese application No. 2022-084179 filed on May 23, 2022, the contents of which are incorporated herein by reference. Technical Field

[0003] The present disclosure relates to a frequency sweep characteristic measuring device and a frequency sweep characteristic measuring method for measuring the frequency sweep characteristic of light having swept frequencies, and also to a FMCW LiDAR device. Background Art

[0004] LiDAR (Light Detection and Ranging) is a technology that uses lasers to detect objects and measure the distance to them. LiDAR's ranging methods include TOF (Time of Flight) and FMCW (Frequency Modulated Continuous Wave). The FMCW method modulates the frequency of light into a triangular wave and measures the distance to an object and the speed of the object based on the beat signal of the reference light and the received light.

[0005] The frequency of the laser is scanned by scanning the input current into a triangular wave shape. However, due to the heat release characteristics of the laser and the change in carrier concentration accompanying the injection current, the graph of the time characteristic of the frequency (hereinafter referred to as the "frequency sweep characteristic") draws a nonlinear curve with respect to time. When such a nonlinear component is small, the frequency of the beat signal is also approximately constant, the spectrum width is narrow, and the sensitivity and resolution are also good (see Figure 8 On the other hand, when the nonlinear component is large, the frequency of the beat signal is extended and the spectrum width is widened, so the sensitivity and resolution are reduced (refer to Fig. 9 ).

[0006] In addition, the frequency characteristics of laser light vary due to the influence of temperature changes in the surrounding environment, etc. Therefore, it is necessary to constantly or periodically analyze the frequency sweep characteristics of laser light and to correct them so that the frequency sweep characteristics become linear.

[0007] In non-patent document 1, there is described a method for improving the linearity of frequency scanning characteristics as follows. First, after the electromagnetic wave passes through an asymmetric Mach-Zehnder interferometer, it is received by a photoreceiver and a beat signal is generated. Then, the beat signal is subjected to Hilbert transform to determine the orthogonal component (Q component). Next, the instantaneous phase of the beat signal is calculated by calculating the arctangent of the orthogonal component and the original beat signal (I component). Then, using an approximation based on Taylor expansion, the frequency scanning characteristics of the laser are calculated according to the instantaneous phase of the beat signal. Using the calculated frequency scanning characteristics, the input signal to the laser is updated by the Voltage Update Algorithm. Thus, the frequency scanning characteristics are corrected to be linear. Here, in order to minimize the influence of non-ideal migration, 20% of the signals at both ends of the beat frequency in the beat signal are not used for input to the Voltage Update Algorithm, and the remaining 80% of the signals are input.

[0008] Non-patent literature 1: X.Zhang, J.Pouls, and MCWu, "Laser frequency sweeplinearization by iterative learning pre-distortion for FMCW LiDAR," OpticsExpress, 27(7)9965(2019)

[0009] However, in one method of non-patent document 1, the ROI (Region of Interest; region for analyzing the frequency sweep characteristics) of the beat signal is limited to 80% of the entire region. This is for the following reasons. Non-ideal migration occurs due to the distortion (large offset and ripple) at both ends of the orthogonal signal caused by the Hilbert transform. Therefore, the distortion at both ends of the orthogonal signal is not included in the calculation of the frequency sweep characteristics. Therefore, in one method of non-patent document 1, the 20% region outside the region for analyzing the frequency sweep characteristics cannot be fully linearized, and a part of the signal cannot be used for ranging. As a result, in one method of non-patent document 1, it is believed that a decrease in detection sensitivity will be caused.

[0010] The distortion at both ends of the orthogonal signal can be improved by multiplying the window function. However, the improvement effect is limited and the measurement result will have errors. In addition, in the method using the window function, since the amplitude near the two ends of the orthogonal signal is close to zero, the error caused by other factors such as interference in the atmosphere is likely to increase.

[0011] As a method of obtaining the quadrature component of the beat signal, a method of using quadrature detection can be considered, but quadrature detection requires expensive equipment such as an acousto-optic frequency shifter, and requires additional parts. Summary of the invention

[0012] An object of the present disclosure is to provide a frequency sweep characteristic measuring device capable of measuring the frequency sweep characteristic of light with high accuracy.

[0013] One method of the present disclosure is a frequency scanning characteristic measuring device, in which the frequency scanning characteristic measuring device for measuring the frequency scanning characteristic of a laser whose frequency is scanned comprises: an asymmetric Mach-Zehnder interferometer, which combines the divided light to generate combined light after branching the laser and generating a predetermined difference in optical path length; a photoreceiver, which receives the combined light output from the asymmetric Mach-Zehnder interferometer, converts the combined light into a beat signal as an electrical signal, and outputs the combined light; a waveform adjustment unit, which adjusts the waveform so that the amplitude and phase of the beat signal at both ends are consistent; a Hilbert transform unit, which performs a Hilbert transform on the beat signal after the waveform adjustment to calculate the orthogonal component; an instantaneous phase calculation unit, which calculates the instantaneous phase of the beat signal by calculating the inverse tangent of the orthogonal component calculated by the Hilbert transform and the beat signal after the waveform adjustment; and a frequency calculation unit, which calculates the frequency scanning characteristic of the laser according to the instantaneous phase and the difference in the optical path length.

[0014] In the frequency scanning characteristic measuring device disclosed in the present invention, the waveform adjusting unit may also adjust the waveform so that the amplitude and the phase at both ends of the beat signal are consistent by respectively extracting two adjacent zero crossing points near the two ends of the beat signal, calculating the differential values ​​of the beat signal at the extracted four zero crossing points, extracting two points with the same signs of the calculated differential values, and deleting the measuring points on the outside of the two extracted points from the beat signal.

[0015] In the frequency scanning characteristic measuring device disclosed in the present invention, the asymmetric Mach-Zehnder interferometer may be configured such that the difference in the optical path length is variable, and the waveform adjustment unit adjusts the waveform by controlling the difference in the optical path length so that the amplitude and the phase at both ends of the beat signal are consistent.

[0016] In the frequency scanning characteristic measuring device disclosed in the present invention, the asymmetric Mach-Zehnder interferometer may also include an optical path switching switch, the optical path switching switch switches a plurality of optical paths having different optical path lengths, the asymmetric Mach-Zehnder interferometer is configured to switch the optical path using the optical path switching switch so that the difference in the optical path lengths is variable, and the waveform adjustment unit adjusts the waveform by controlling the selection of the optical path in the optical path switching switch so that the amplitude and the phase at the two ends of the beat signal are consistent.

[0017] In addition, another embodiment of the present invention is a LiDAR device, which is an FMCW-type LiDAR device having a laser diode that radiates laser light and the above-mentioned frequency scanning characteristic measuring device, and has a drive signal control unit. The above-mentioned drive signal control unit corrects the drive signal of the above-mentioned laser diode based on the frequency scanning characteristic of the above-mentioned laser calculated by the above-mentioned frequency calculation unit to reduce the above-mentioned nonlinear component of the above-mentioned laser.

[0018] In addition, another embodiment of the present invention is a frequency scanning characteristic measuring method. In the frequency scanning characteristic measuring method for measuring the frequency scanning characteristic of a laser whose frequency is scanned, after the laser is branched and a difference in a specified optical path length is generated, the branched light is combined to generate a combined light; the combined light is received and converted into a beat signal as an electrical signal; the waveform is adjusted so that the amplitude and phase of the two ends of the beat signal are consistent; the beat signal after the waveform adjustment is subjected to Hilbert transform to calculate the orthogonal component; the instantaneous phase of the beat signal is calculated by calculating the inverse tangent of the orthogonal component calculated by the Hilbert transform and the beat signal after the waveform adjustment; and the frequency scanning characteristic of the laser is calculated based on the instantaneous phase and the difference in the optical path length.

[0019] According to the technology provided in the present disclosure, it is possible to measure the frequency sweep characteristics of light with high accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The above-mentioned object and other objects, features and advantages of the present disclosure will become more apparent through the following detailed description with reference to the accompanying drawings.

[0021] Figure 1 It is a diagram showing the configuration of an FMCW-LiDAR device including the frequency sweep characteristic measuring device according to the first embodiment.

[0022] Figure 2 Graphs showing the analysis results of the frequency sweep characteristics and the analysis results of the nonlinear component of the instantaneous frequency when the waveforms at both ends of the beat signal are discontinuous.

[0023] Figure 3Graphs showing the analysis results of the frequency sweep characteristics and the analysis results of the nonlinear component of the instantaneous frequency when the waveforms at both ends of the beat signal are continuous.

[0024] Figure 4A 4C is a diagram showing a method of performing waveform adjustment so that the waveform is continuous at both ends of the beat signal.

[0025] Figure 5 It is a diagram showing the configuration of an FMCW-LiDAR device including a frequency sweep characteristic measuring device according to a second embodiment.

[0026] Figure 6 FIG. 1 is a diagram showing the structure of an asymmetric MZI having a control mechanism for the difference in optical path length.

[0027] Figure 7 : is a graph showing the relationship between the difference in optical path length (delay fiber length) ΔL and the maximum value of the nonlinear component νnl of the instantaneous frequency.

[0028] Figure 8 This is a diagram for explaining the case where the nonlinear component of the frequency sweep characteristic is small.

[0029] Fig. 9 This is a diagram for explaining the case where the nonlinear component of the frequency sweep characteristic is large. DETAILED DESCRIPTION

[0030] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0031] (First Embodiment)

[0032] Figure 1 1 is a diagram showing the configuration of the FMCW-LiDAR device according to the first embodiment. The FMCW-LiDAR device according to the first embodiment can measure the frequency sweep characteristic in real time and correct the frequency sweep characteristic in the device to be linear.

[0033] like Figure 1 As shown, the FMCW-LiDAR device of the first embodiment includes a LiDAR device 1 and a frequency sweep characteristic measuring device 2. The LiDAR device 1 includes an LD (laser diode) 100, optical couplers 101 to 103, an optical transmission antenna 104, an optical reception antenna 105, a light receiver 106, an AD converter 107, a first signal processing unit 108, a DA converter 109, and an LD driving circuit 110. In addition, the frequency sweep characteristic measuring device 2 includes an asymmetric MZI (asymmetric Mach-Zehnder interferometer) 120, a light receiver 121, an AD converter 122, and a second signal processing unit 123.

[0034] (Structure of LiDAR Device 1)

[0035] First, each structure of the LiDAR device 1 will be described.

[0036] LD100 is a variable frequency laser diode that emits laser light. The frequency is controlled by the LD drive circuit 110, and the time characteristic of the frequency (frequency sweep characteristic) is controlled so that a triangular waveform is repeated periodically. In other words, the frequency is controlled so that a period in which the frequency increases linearly with respect to time and a period in which the frequency decreases linearly with respect to time are repeated. The wavelength band of the laser light is arbitrary, for example, an infrared wavelength band.

[0037] The optical coupler 101 is an optical device that splits the laser light from the LD 100 into two lights. One of the split laser lights is input to the optical coupler 102 , and the other is input to the asymmetric MZI 120 .

[0038] The optical coupler 102 is an optical device similar to the optical coupler 101 , and is an optical device that splits the laser light from the optical coupler 101 into transmission light and reference light. The transmission light is input to the optical transmission antenna 104 , and the reference light is input to the optical coupler 103 .

[0039] The optical transmission antenna 104 is an optical device that irradiates the transmission light from the optical coupler 102 to an object.

[0040] The light receiving antenna 105 is an optical device that receives the transmission light (received light) reflected by the object. The received light is input to the optical coupler 103 .

[0041] The optical coupler 103 is an optical device that multiplexes the reference light from the optical coupler 102 and the received light from the optical receiving antenna 105 and outputs the multiplexed light.

[0042] The light receiver 106 is a device that receives the combined light from the optical coupler 103 and converts it into an electrical signal, generates a beat (beat frequency signal) caused by the interference between the reference light and the received light, and outputs it. The light receiver 106 is, for example, a Ge photodiode. The light receiver 106 is preferably a differential balanced detector. Alternatively, a filter that cuts off the DC component is preferably provided between the light receiver 106 and the AD converter 107.

[0043] The AD converter 107 is a device that converts the beat signal from the light receiver 106 from an analog signal into a digital signal and outputs the digital signal.

[0044] The first signal processing unit 108 calculates the distance to the object and the relative speed of the object by performing a predetermined signal processing on the beat signal from the AD converter 107. In addition, the first signal processing unit 108 generates an LD drive signal for driving the LD 100. The first signal processing unit 108 has a Fourier transform unit 111, a peak detection unit 112, a distance / speed calculation unit 113, and an LD drive reference signal generation unit 114. The functions / actions of these components will be described later. In addition, the first signal processing unit 108 is a signal processing device that includes at least one or more processors, one or more memories, and one or more communication I / Fs, and is configured so that the processor, memory, and communication I / F can communicate with each other via a bus. In the case of such a structure, the first signal processing unit 108 can, for example, read out and execute a program stored in a memory of a recording medium equivalent to a non-transitional entity through a processor to realize one or more of the above functions. That is, the program includes a processor executable instruction set for realizing the functions of the Fourier transform unit 111, the peak detection unit 112, the distance / speed calculation unit 113, and the LD drive reference signal generation unit 114. In addition, the first signal processing unit 108 may also be a signal processing device composed of one or more integrated circuits (ASIC: Application Specific Integrated Circuit). Furthermore, the first signal processing unit 108 may also be composed of a signal processing device that includes at least one or more processors, one or more memories, and one or more communication I / Fs and a signal processing device that is composed of one or more integrated circuits.

[0045] The DA converter 109 is a device that converts the LD driving signal from the second signal processing unit 123 from digital to analog and outputs the analog signal.

[0046] The LD driving circuit 110 is a circuit that controls the frequency of the laser light radiated from the LD 100 based on the LD driving signal from the DA converter 109. Specifically, the frequency is controlled by controlling the input current to the LD 100.

[0047] (Structure of Frequency Sweep Characteristics Measurement Device 2)

[0048] Next, each structure of the frequency sweep characteristic measuring device 2 will be described.

[0049] The asymmetric MZI 120 is an optical device that splits the laser light from the optical coupler 101 into two lights, generates a predetermined optical path length difference ΔL, and then combines the two split lights.

[0050] The optical receiver 121 is a device that receives the combined light from the asymmetric MZI 120 and converts it into an electrical signal, generates a beat signal generated by the interference of two lights having a predetermined optical path length difference ΔL, and outputs it. The optical receiver 121 is, for example, a Ge photodiode. The optical receiver 121 is preferably a differential balanced detector. Alternatively, a filter that cuts off the DC component is preferably provided between the optical receiver 121 and the AD converter 122.

[0051] The asymmetric MZI 120 and the photoreceiver 121 may be mounted together as one optical integrated circuit chip. This simplifies the device structure and reduces costs. The optical waveguide in the optical integrated circuit is Si, SiN, etc., and the photoreceiver 121 is a Ge photodiode. A portion from the asymmetric MZI 120 to the photoreceiver 121 may also be optically integrated.

[0052] The AD converter 122 is a device that converts the beat signal from the light receiver 121 from an analog signal to a digital signal and outputs the digital signal.

[0053] The second signal processing unit 123 calculates the frequency sweep characteristics of the laser output by the LD100 and calculates the nonlinear component by performing a predetermined signal processing on the beat signal from the AD converter 122. The second signal processing unit 123 includes a waveform adjustment unit 124, a Hilbert transform unit 125, an instantaneous phase calculation unit 126, a frequency calculation unit 127, a nonlinear component calculation unit 128, and an LD drive signal control unit 129. The functions / actions of these components are described later. In addition, the second signal processing unit 123 is a signal processing device that includes at least one or more processors, one or more memories, and one or more communication I / Fs, and is configured so that the processor, memory, and communication I / F can communicate with each other via a bus. In the case of such a structure, the second signal processing unit 123 can, for example, read out and execute a program stored in a memory equivalent to a non-transitional physical recording medium through a processor, and can realize one or more of the above functions. That is, the program includes a processor executable instruction set for realizing the functions of the waveform adjustment unit 124, the Hilbert transform unit 125, the instantaneous phase calculation unit 126, the frequency calculation unit 127, the nonlinear component calculation unit 128, and the LD drive signal control unit 129. In addition, the second signal processing unit 123 can also be a signal processing device composed of one or more integrated circuits. In addition, the second signal processing unit 123 can also be composed of a signal processing device that at least includes one or more processors, one or more memories, and one or more communication I / Fs and a signal processing device composed of one or more integrated circuits.

[0054] (Operation of LiDAR Device 1)

[0055] Next, the operation of the FMCW-LiDAR device according to the first embodiment will be described. First, the operation of distance measurement by the LiDAR device 1 will be described.

[0056] The LD driving circuit 110 drives the LD 100 based on the LD driving signal from the second signal processing unit 123, and controls the frequency scanning characteristics of the laser light radiated by the LD 100 to be periodically repeated in a triangular waveform. Here, the LD driving signal is a signal obtained by correcting the LD driving reference signal generated by the LD driving reference signal generating unit 114 by the LD driving signal control unit 129 of the second signal processing unit 123. In addition, the LD driving reference signal is a signal whose voltage value repeatedly changes in a triangular shape.

[0057] Next, the optical coupler 101 splits the laser light from the LD 100 into two lights, and the optical coupler 102 splits it into reference light and transmission light. The transmission light is irradiated to an external object from the optical transmission antenna 104. The transmission light is reflected by the object, and the reception light as the reflected light is received by the optical reception antenna 105. Furthermore, the optical coupler 103 combines the reference light split by the optical coupler 102 and the reception light from the optical reception antenna 105, and the optical receiver 106 converts the combined light into an electrical signal to generate a beat signal.

[0058] Next, the AD converter 107 converts the beat signal from an analog signal to a digital signal. Furthermore, the first signal processing unit 108 performs signal processing on the input signal (beat signal) from the AD converter 107 to calculate the distance to the object and the relative speed of the object. The signal processing performed by the first signal processing unit 108 is specifically as follows. In addition, the signal processing of each functional unit implemented by executing a program by a processor of a signal processing device equivalent to the first signal processing unit 108 in this embodiment is described below.

[0059] First, the Fourier transform unit 111 performs Fourier transform on the input beat signal and calculates the spectrum of the beat signal (step 11). Next, the peak detection unit 112 detects the peak frequency in the calculated spectrum (step 12). Thus, the first signal processing unit 108 obtains the beat frequency in the interval where the frequency increases linearly and the beat frequency in the interval where the frequency decreases linearly (detects the frequencies of the two beat signals). Furthermore, the distance / speed calculation unit 113 calculates the distance to the object and the relative speed of the object based on the frequencies of the two detected beat signals (step 13).

[0060] (Operation of Frequency Sweep Characteristics Measurement Device 2)

[0061] Next, the operation of the frequency sweep characteristic measuring device 2 will be described.

[0062] One of the laser beams branched into two beams by the optical coupler 101 passes through the asymmetric MZI 120. Afterwards, the photoreceiver 121 receives the laser beam that has passed through the asymmetric MZLI 120 and generates a beat signal. Then, the AD converter 122 converts the generated beat signal from an analog signal to a digital signal. Moreover, the second signal processing unit 123 performs signal processing on the input signal (beat signal) from the AD converter 122, calculates the frequency scanning characteristics of the laser beam, and calculates the nonlinear component. The signal processing performed by the second signal processing unit 123 is specifically described as follows. In addition, the signal processing of each functional unit implemented by executing a program by a processor of a signal processing device equivalent to the second signal processing unit 123 in this embodiment is described below.

[0063] First, the waveform adjustment unit 124 performs waveform adjustment so that the waveforms of the input beat signal are continuous at both ends (step 21). That is, the waveform adjustment unit 124 performs waveform adjustment so that the amplitude and phase at both ends of the beat signal are consistent. The specific waveform adjustment method is as follows.

[0064] First, the waveform adjustment unit 124 removes high-frequency noise of the beat signal by moving average (step 21-1). Then, the waveform adjustment unit 124 extracts two adjacent zero-crossing points (points where the amplitude is zero) near both ends of the beat signal (step 21-2) (see Figure 4A ) The extraction of the zero-crossing point can be easily performed by using a balanced detector as the light receiver 121 or by cutting off the DC component using a DC block filter.

[0065] Next, the waveform adjustment unit 124 calculates the differential values ​​of the beat signal at the four extracted zero crossing points (step 21-3). Then, the waveform adjustment unit 124 selects two points whose differential values ​​have the same sign among the four zero crossing points (step 21-4) (see Figure 4B ). Both the two points with the symbol + and the two points with the symbol - are acceptable. Figure 4B In the example shown, 1 and 3 or 2 and 4 are selected.

[0066] Next, the waveform adjustment unit 124 deletes the measurement points outside the selected two points from the beat signal (step 21-5) (see Figure 4C ).exist Figure 4C In the example shown, two points 1 and 3 are selected, and the measurement points outside them are deleted. By doing so, the waveform can be adjusted so that the amplitude and phase of the two ends of the beat signal are consistent. In addition, the region of the waveform of the cutoff beat signal can be suppressed to a minimum.

[0067] The method for adjusting the waveform of the beat signal is not limited to the above method. The method for adjusting the waveform of the beat signal can be performed by extracting points where the amplitude and phase are consistent near the two ends of the beat signal by any method, and deleting the measurement points outside the extracted points from the beat signal. According to the above method of the present embodiment, it is possible to easily extract points where the amplitude and phase are consistent near the two ends of the beat signal. In addition, it is not necessary to make the amplitude and phase completely consistent, and an error can be present within the range that can achieve the purpose of the present disclosure (the range that achieves the effect of the present disclosure). For example, the absolute value of the amplitude difference at the two ends of the beat signal is less than 1% relative to the maximum value of the amplitude and the absolute value of the phase difference is less than 0.01π.

[0068] Next, the Hilbert transform unit 125 performs Hilbert transform on the waveform-adjusted beat signal to calculate the orthogonal component (Q component) (step 22). Here, since the waveform adjustment is performed before the Hilbert transform so that the waveform at both ends of the beat signal is continuous, the distortion at both ends of the Q component and the distortion at the boundary between the rising and falling frequency intervals are reduced.

[0069] Next, the instantaneous phase calculation unit 126 calculates the instantaneous phase φb(t) of the beat signal (step 23). The instantaneous phase φb(t) of the beat signal is obtained by calculating the arc tangent of the I component of the beat signal after waveform adjustment and the orthogonal component (Q component of the beat signal) calculated by Hilbert transform. That is, the instantaneous phase φb(t) of the beat signal is obtained by calculating φb(t)=arctan(Q / I). In addition, the instantaneous phase of the beat signal obtained in this way becomes a waveform that is folded back at ±π. Therefore, the instantaneous phase calculation unit 126 performs unwrapping processing.

[0070] Next, the frequency calculation unit 127 calculates the instantaneous frequency ν(t) of the beat signal based on the instantaneous phase φb(t) of the beat signal (step 24). That is, the frequency calculation unit 127 calculates the frequency sweep characteristic of the laser. The instantaneous frequency ν(t) is obtained by differentiating the instantaneous phase φb(t). Specifically, 2πν(t)=d(φb(t)) / dt. Here, a first-order approximation is performed by Taylor expansion of φb(t-τ) using τ, resulting in ν(t)=φb(t) / (2πτ). τ is the propagation delay time in the asymmetric MZI 120, τ=ΔL / c. It should be noted that ΔL is the difference in the prescribed optical path length, and c is the speed of light. τ is a value in the range of 1ns to 100ns, and its value is sufficiently small. Therefore, the above differential expression can be approximated as described above. The frequency calculation unit 127 uses the above approximation to calculate the instantaneous frequency ν(t), that is, the frequency sweep characteristic of the laser.

[0071] Next, the nonlinear component calculation unit 128 calculates the nonlinear component of the calculated instantaneous frequency ν(t) (step 25). ν(t) is represented by ν(t)=γt+νnl(t). Here, γ is the frequency change rate, νnl(t) is the nonlinear component of the instantaneous frequency ν(t), and γ is a known value. Therefore, the nonlinear component calculation unit 128 calculates the nonlinear component νnl(t) of the instantaneous frequency ν(t) according to the above formula.

[0072] Next, the LD drive signal control unit 129 corrects the LD drive reference signal input from the first signal processing unit 108 (LD drive reference signal generation unit 114) based on the calculated nonlinear component νnl(t) to generate the LD drive signal (step 26). Specifically, the LD drive signal control unit 129 adds distortion corresponding to the nonlinear component νnl(t) to the LD drive reference signal to generate the LD drive signal. Thus, the LD drive signal control unit 129 pre-subtracts the nonlinear component in the frequency sweep characteristic of the laser radiated by LD100. The LD drive circuit 110 drives LD100 based on the LD drive signal corrected in this way. Thus, the nonlinear component in the frequency sweep characteristic of the laser can be reduced.

[0073] The calibration of the LD driving reference signal may be performed at all times or at any time.

[0074] As described above, in the frequency sweep characteristic measuring device 2 of the first embodiment, the waveform adjustment unit 124 performs waveform adjustment so that the waveform at both ends of the beat signal is continuous. Thereafter, the frequency calculation unit 127 calculates the instantaneous frequency of the beat signal, that is, the frequency sweep characteristic of the laser, based on the instantaneous phase of the beat signal after waveform adjustment. Thus, the frequency sweep characteristic measuring device 2 of the first embodiment can measure the frequency sweep characteristic of the laser with high accuracy. Therefore, the FMCW-LiDAR device of the first embodiment can improve the linearity of the laser sweep frequency and improve the distance measurement performance by correcting the sweep frequency of the laser using the measured frequency sweep characteristic. In addition, the FMCW-LiDAR device of the first embodiment has a wider ROI (region for analyzing the frequency sweep characteristic) of the beat signal than the conventional method, so the range of the laser can be corrected is wider, and the sensitivity and resolution (resolution of the distance) of the FMCW-LiDAR device can be improved. In addition, the FMCW-LiDAR device of the first embodiment can measure the frequency sweep characteristic of the laser without using expensive equipment such as a frequency shifter, so it can achieve low cost. In addition, the FMCW-LiDAR device of the present disclosure is, for example, mounted on a vehicle and used when measuring the distance between an object existing around the vehicle and the vehicle. As a result, the vehicle can perform various driving assistance / running assistance such as avoiding / reducing collision with the object based on the distance to the object. At this time, the FMCW-LiDAR device outputs the measurement result of the distance between the object and the vehicle to the ECU (Electronic Control Unit: Electronic Control Unit) mounted on the vehicle. The ECU is composed of peripheral devices such as a computer including a processor and a memory and a communication module. Therefore, the ECU is configured to be able to communicate with the FMCW-LiDAR device. The ECU generates a control signal for driving assistance / running assistance based on the measurement result input from the FMCW-LiDAR device, and outputs a control signal to various control devices that control the driving of the vehicle. For example, in the case of a driving assistance structure such as collision reduction and collision avoidance, the ECU outputs a control signal to a control device that controls a brake actuator and a control device that controls a steering wheel. As a result, various control devices control the drive of the control object device according to the input control signal. As a result, the vehicle can perform driving control such as braking control and steering control for collision reduction and collision avoidance. In this way, the technology disclosed in the present invention contributes to driving assistance / driving assistance of the vehicle. In addition, the technology disclosed in the present invention improves the sensitivity and resolution of the FMCW-LiDAR device, so that driving assistance / driving assistance of the vehicle based on high-precision measurement results can be achieved.

[0075] Next, analysis results of the beat signal according to the first embodiment will be described.

[0076] The frequency change rate γ = 1.0 GHz / 125 μs, the nonlinear component of the instantaneous frequency νnl(t) = 9 × 10 5 ·sin[2π·4×10 3 t], and analyze the frequency sweep characteristics of the laser. Here, by adjusting τ, a beat signal with discontinuous waveforms at both ends and a beat signal with continuous waveforms are generated. The basic oscillation wavelength of the laser is 1550nm. Figure 2 , Figure 3 is a graph showing the analysis results of the frequency sweep characteristics of the laser and the analysis results of the nonlinear component of the instantaneous frequency. Figure 2 This is the case where the waveform at both ends of the beat frequency signal is discontinuous. Figure 3 This is the case where the waveforms at both ends of the beat frequency signal are continuous.

[0077] like Figure 2 As shown in the figure, when the waveform at both ends of the beat signal is discontinuous, a steep offset occurs at the boundary b between the frequency rising interval up and the falling interval dn and at both ends a and c of the frequency, and ripples are continuously generated before and after the offset. In other words, it is known that the frequency sweep characteristics of the laser cannot be measured with high accuracy at the boundary b between the frequency rising interval up and the falling interval dn and at both ends a and c. As a result, it is known that the interval of offset and ripples cannot be corrected so that the frequency sweep characteristics of the laser become linear.

[0078] On the other hand, Figure 3 As shown, when the waveforms at both ends of the beat signal are continuous, no offset or ripple is observed. Therefore, according to the structure of the first embodiment, it is confirmed that the frequency sweep characteristics of the laser can be measured with high accuracy. It is confirmed that the frequency sweep characteristics of the laser can be corrected to be linear in a wide range.

[0079] (Second Embodiment)

[0080] The FMCW-LiDAR device of the second embodiment is modified in that the mechanism for making the waveform continuous at both ends of the beat signal in the FMCW-LiDAR device of the first embodiment is modified. Figure 5 As shown, the FMCW-LiDAR device of the second embodiment replaces the asymmetric MZI 120 and the waveform adjustment unit 124 of the frequency sweep characteristic measurement device 2 of the FMCW-LiDAR device of the first embodiment with an asymmetric MZI 220 and a waveform adjustment unit 224. The other structures are the same as those of the first embodiment.

[0081] The asymmetric MZI 220 is an asymmetric MZI that can control the difference ΔL in the optical path length. Figure 6 FIG. 2 is a diagram showing an example of a structure of an asymmetric MZI 220 having a control mechanism for a difference in optical path length. Figure 6 As shown, the asymmetric MZI 220 has optical couplers 201 - 203 and an optical path switch 204 .

[0082] The optical coupler 201 is an optical device that splits the laser light from the optical coupler 101 into two lights. One of the split laser lights is input to the optical path switch 204 , and the other is input to the optical coupler 203 .

[0083] The optical path switching switch 204 is an optical device that selects one of a plurality of optical paths having different optical path lengths and outputs the laser light from the optical coupler 201 to the selected optical path. The selection of the optical path is controlled by the waveform adjustment unit 224. The optical path switching switch 204 is, for example, a switch that combines a multi-mode interference coupler (MMI Coupler: Multi-Mode Interference Coupler) and a phase shifter, or a switch that utilizes the resonance of a ring resonator. The laser light output from the optical path selected by the optical path switching switch 204 is input to the optical coupler 202.

[0084] The optical coupler 202 is an optical device connected to a plurality of optical paths of the optical path switching switch 204 and outputs laser light input from those optical paths to one optical path. The optical coupler 202 is, for example, a multimode interference coupler or a star coupler. The laser light output from the optical coupler 202 is input to the optical coupler 203.

[0085] The optical coupler 203 is an optical device that combines the laser light from the optical coupler 201 and the laser light from the optical coupler 202 to output combined light. The combined light is input to the optical receiver 121 .

[0086] The waveform adjustment unit 224 generates and outputs a switch control signal for controlling the selection of an optical path in the optical path switching switch 204. The waveform adjustment unit 224 outputs the switch control signal to the optical path switching switch 204, controls the selection of an optical path in the optical path switching switch 204, and performs waveform adjustment so that the waveforms at both ends of the beat frequency signal are continuous. Specifically, the waveform adjustment unit 224 generates the switch control signal of the optical path switching switch 204 as follows, and controls the selection of an optical path in the optical path switching switch 204 through the control signal.

[0087] like Figure 6 As shown, in the second embodiment, there are N+1 optical paths selected by the optical path switching switch 204, and the numbers for identifying the selected optical paths (optical path numbers) are 0, 1, ..., i, ..., N in order from the shortest optical path length to the longest optical path length. Furthermore, the waveform adjustment unit 124 outputs a switch control signal indicating that the i-th optical path is selected to the optical path switching switch 204, and the optical path switching switch 204 selects the i-th optical path based on the input switch control signal.

[0088] Here, when the range of the amplitude of the difference in optical path length without generating offset and ripple (that is, the amplitude of the difference in optical path length with the minimum maximum value of the nonlinear component νnl(t) of the instantaneous frequency) is set to r, the amplitude of the increase in optical path length (extension amplitude: the difference between the (i)th optical path length and the (i+1)th optical path length) can be less than r / 2.

[0089] First, the waveform adjustment unit 224 removes high-frequency noise of the beat signal input from the AD converter 122 by moving average. Then, the waveform adjustment unit 224 detects an error δ (a difference in amplitude at both ends) at both ends of the beat signal.

[0090] Next, the waveform adjustment unit 224 determines whether the absolute value of the error δ is less than a predetermined value. The error δ is set so that the waveform is continuous at both ends of the beat signal. For example, the absolute value of the amplitude difference at both ends of the beat signal is set to a value less than 1% of the maximum amplitude.

[0091] When it is determined that the absolute value of the error δ is equal to or smaller than the predetermined value, the waveform adjustment unit 224 does not update the switch control signal, and outputs the beat signal to the Hilbert transform unit 125 and the instantaneous phase calculation unit 126 .

[0092] On the other hand, when it is determined that the absolute value of the error δ is greater than the predetermined value, the waveform adjustment unit 224 calculates differential values ​​a1 and a2 at both ends of the beat signal. The waveform adjustment unit 224 calculates a value represented by sgn{δ×sgn(a1·a2)}.

[0093] Next, the waveform adjustment unit 224 updates the switch control signal indicating that the i-th optical path is selected as i=i+sgn{δ×sgn(a1·a2)}. The waveform adjustment unit 224 repeatedly updates the switch control signal until it is determined that the absolute value of the error δ is equal to or less than a predetermined value.

[0094] The updating method of the switch control signal is not limited to the above method. For example, the updating method of the switch control signal may simply increase i one by one and repeat until the absolute value of the error δ at both ends of the beat signal becomes equal to or less than a predetermined value.

[0095] As described above, in the frequency sweep characteristic measuring device 2 of the second embodiment, the asymmetric MZI 220 capable of controlling the difference in optical path length is used, and the waveform adjustment unit 224 controls the difference in optical path length to perform waveform adjustment so that the waveforms at both ends of the beat signal are continuous.

[0096] Next, analysis results of the beat signal according to the second embodiment will be described.

[0097] The frequency change rate γ = 1.0 GHz / 125 μs, the nonlinear component of the instantaneous frequency νnl(t) = 9 × 10 5 ·sin[2π·4×10 3 t], and scans the difference ΔL in the optical path length in the asymmetric MZI 220 controlled by the waveform adjustment unit 224, and calculates the maximum value of the nonlinear component νnl of the instantaneous frequency.

[0098] Figure 7 : is a graph showing the relationship between the difference in optical path length (delay fiber length) ΔL and the maximum value of the nonlinear component νnl of the instantaneous frequency. Figure 7 As shown, the peak value of the maximum value of the nonlinear component νnl decreases every about 20 cm, and the amplitude of the maximum value of the nonlinear component νnl decreases by about 8 mm. If the amplitude deviates from the amplitude of the maximum value of the nonlinear component νnl decreases, the maximum value of the nonlinear component νnl increases by more than 1 digit. This is because the waveform at both ends of the beat signal is discontinuous and a large offset occurs, resulting in a measurement error. In addition, in this case, it is found that the amplitude of the increase in the optical path length in the asymmetric MZI 220 is preferably 4 mm or less.

[0099] If the effective refractive index of the optical fiber is set to 1.46, the time required for light to pass through the 8 mm length of the optical fiber is about 39 ps. Therefore, it was confirmed that if the propagation delay time is controlled to converge within the range of 39 ps in the asymmetric MZI 220, the waveform is made continuous at both ends of the beat signal, which can reduce the measurement error.

[0100] (Variation Example)

[0101] In addition, in each of the above embodiments, a configuration example of applying the frequency sweep characteristic measuring device of the present invention to an FMCW-LiDAR device is shown, but the frequency sweep characteristic measuring device of the present invention can be applied to any device that requires the measurement of frequency sweep characteristics. For example, the frequency sweep characteristic measuring device of the present invention can be applied to a spectroscopic device. In addition, in each of the above embodiments, the frequency sweep is performed linearly, but the technology of the present invention can also be applied to situations other than the frequency sweep is performed linearly.

[0102] The present disclosure is not limited to the above-mentioned embodiments, and various changes can be made within the scope of the main purpose. That is, the present disclosure is described based on the embodiments, but it is understood that the present disclosure is not limited to the embodiments, structures, etc. The present disclosure also includes various modifications and variations within the equivalent scope. In addition, various combinations and methods, further including only one of the elements, more than one, or less than one other combinations and methods are also included in the scope and thought of the present disclosure.

Claims

1. A frequency sweep characteristic measuring device for measuring the frequency sweep characteristic of a laser beam having a frequency swept, comprising: An asymmetric Mach-Zehnder interferometer that splits the laser light to produce a predetermined optical path length difference, and then combines the split light to generate a combined light. A light receiver receives the combined light output from the asymmetric Mach-Zehnder interferometer, converts the combined light into a beat signal as an electrical signal, and outputs the beat signal; A waveform adjustment unit, which adjusts the waveform so that the amplitude and phase of the two ends of the beat signal are consistent; A Hilbert transform unit, performing Hilbert transform on the beat frequency signal after the waveform adjustment to calculate the orthogonal component; an instantaneous phase calculation unit that calculates the instantaneous phase of the beat signal by calculating the arc tangent of the orthogonal component calculated by the Hilbert transform and the beat signal after the waveform adjustment; and The frequency calculation unit calculates the frequency sweep characteristic of the laser light based on the difference between the instantaneous phase and the optical path length.

2. The frequency sweep characteristic measuring device according to claim 1, wherein: The waveform adjustment unit adjusts the waveform so that the amplitude and the phase at the two ends of the beat signal are consistent by respectively extracting two adjacent zero crossing points near the two ends of the beat signal, calculating the differential values ​​of the beat signal at the extracted four zero crossing points, extracting two points with the same signs of the calculated differential values, and deleting the measurement points on the outside of the two extracted points from the beat signal.

3. The frequency sweep characteristic measuring device according to claim 1, wherein: The asymmetric Mach-Zehnder interferometer is configured so that the difference in the optical path length is variable. The waveform adjustment unit adjusts the waveform by controlling the difference in the optical path length so that the amplitude and the phase at the two ends of the beat signal coincide with each other.

4. The frequency sweep characteristic measuring device according to claim 3, wherein: The asymmetric Mach-Zehnder interferometer includes an optical path switching switch, the optical path switching switch switches between the plurality of optical paths having different optical path lengths, and the asymmetric Mach-Zehnder interferometer is configured such that the difference in the optical path lengths can be varied by switching the optical paths using the optical path switching switch. The waveform adjustment unit adjusts the waveform so that the amplitude and the phase at the two ends of the beat signal coincide with each other by controlling selection of the optical path in the optical path switching switch.

5. A LiDAR device, comprising a laser diode for emitting laser light and a FMCW LiDAR device according to any one of claims 1 to 4, wherein: A drive signal control unit is provided, and the drive signal control unit corrects the drive signal of the laser diode based on the frequency sweep characteristic of the laser light calculated by the frequency calculation unit so as to reduce a nonlinear component of the laser light.

6. A frequency sweep characteristic measuring method, wherein the frequency sweep characteristic of a laser beam having a frequency swept is measured. After the laser light is branched to generate a predetermined optical path length difference, the branched light is combined to generate combined light; receiving the combined light and converting it into a beat frequency signal as an electrical signal; Adjusting the waveform so that the amplitude and phase of the beat frequency signal at both ends are consistent; Performing Hilbert transform on the beat frequency signal after the waveform adjustment to calculate the orthogonal component; Calculating the instantaneous phase of the beat signal by calculating the arc tangent of the orthogonal component calculated by the Hilbert transform and the beat signal after the waveform adjustment; and The frequency scanning characteristic of the laser light is calculated based on the difference between the instantaneous phase and the optical path length.

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