Laser radar device
The laser radar system addresses mode hopping inaccuracies by using instantaneous phase detection and control to maintain phase continuity, enhancing accuracy in FMCW-type LiDAR systems.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- DENSO CORP
- Filing Date
- 2024-11-26
- Publication Date
- 2026-06-05
Smart Images
Figure 2026092411000001_ABST
Abstract
Description
[Technical Field]
[0001] This disclosure relates to a laser radar system. [Background technology]
[0002] In recent years, development of FMCW-type LiDAR has been progressing as a laser radar system. FMCW stands for Frequency Modulated Continuous Wave. LiDAR stands for Light Detection And Ranging. FMCW-type LiDAR emits a chirp wave with a time-varying frequency as the transmitted wave, receives the reflected wave, and calculates the distance to the target object based on the frequency of the received reflected wave. In order to improve the accuracy of distance calculation with this type of LiDAR, the transmitted wave must be a linear and continuous chirp wave.
[0003] One factor that hinders the linearity and continuity of the chirp wave is the occurrence of mode hopping in the laser light source that generates the transmitted wave. The laser light source has a gain medium and a cavity, which is an optical resonator, and amplifies the light emitted from the gain medium by inducing it between the gain medium and the cavity. The laser light source oscillates at a frequency in which the peak of the gain curve of the gain medium matches the resonant frequency (i.e., the longitudinal mode) of the cavity. Mode hopping occurs, for example, when the peak of the gain curve of the gain medium straddles between multiple longitudinal modes of the cavity, and is a phenomenon in which the frequency of the light emitted during operation switches. When mode hopping occurs, errors occur in the distance calculation results in the LiDAR described above, and the accuracy decreases, so devices for detecting the occurrence of mode hopping are being considered.
[0004] An example of a mode hop detection device is the one described in Patent Document 1. The mode hop detection device described in Patent Document 1 comprises a laser light source, an optical interference section having a plurality of mirrors that guide the laser light generated by the laser light source, and a differential circuit that performs differential processing on the interference signal obtained by converting a part of the optical signal in the optical interference section into an electrical signal. The differential circuit utilizes the fact that when a mode hop occurs in the laser light source, a large discontinuous phase change occurs in the laser light with respect to time, and is used to detect a mode hop based on the discontinuity in the differential value of the interference signal based on the laser light. [Prior art documents] [Patent Documents]
[0005] [Patent Document 1] Japanese Patent Publication No. 2002-39713 [Overview of the Initiative] [Problems that the invention aims to solve]
[0006] However, when the interference signal, which is an electrical signal, contains noise, the above detection device experiences large fluctuations in the derivative value obtained by the differentiating circuit, making it difficult to accurately detect phase shifts caused by mode hopping. Furthermore, while this detection device can detect the occurrence of mode hopping in the light source, it cannot detect whether the laser oscillation has returned to the intended longitudinal mode after mode hopping has occurred.
[0007] In view of the above, this disclosure aims to provide a laser radar device that can detect the occurrence of a mode hop even when noise is present in the signal obtained by converting laser light into an electrical signal, and that can control the laser oscillation to the intended oscillation mode after the occurrence of a mode hop. [Means for solving the problem]
[0008] According to one aspect of this disclosure, a laser radar system, an FMCW type laser radar system, A laser light source (2) that emits laser light, An optical interference unit (61) that splits the laser light emitted from the laser light source into two and propagates them, An IQ signal generation unit (62) outputs beat signals of two laser beams branched at the optical interference unit, An instantaneous phase calculation unit (64) that calculates the instantaneous phase of the beat signal, A mode hop determination unit (65) detects discontinuities in the instantaneous phase calculated by the instantaneous phase calculation unit and determines whether or not a mode hop has occurred in the laser light source based on the result of the detection, The system includes a light source control unit (7) that, when the mode hop determination unit determines that a mode hop has occurred, performs feedback control of the oscillation mode of the laser light source so that the instantaneous phase change before and after the mode hop determination is less than or equal to a predetermined value.
[0009] This laser radar system uses laser light from a laser light source split into two by an optical interference unit. An IQ signal generation unit outputs a beat signal based on this beat signal, and an instantaneous phase calculation unit calculates the instantaneous phase from the beat signal. The laser radar system also includes a mode hop determination unit that determines whether a mode hop has occurred in the laser light source based on the discontinuity of the instantaneous phase calculated by the instantaneous phase calculation unit, and a light source control unit that controls the laser light source if a mode hop has been determined to have occurred. The light source control unit then performs feedback control of the oscillation mode of the laser light source so that the change in instantaneous phase before and after the occurrence of a mode hop is below a predetermined level. Because this laser radar system determines the occurrence of a mode hop based on the discontinuity of the instantaneous phase calculated from the beat signal, it can detect the occurrence of a mode hop even if the beat signal contains noise. Furthermore, because the light source control unit controls the laser light source, it is possible to control the laser to the intended longitudinal mode after a mode hop has occurred.
[0010] The reference numerals in parentheses attached to each component indicate an example of the correspondence between that component and the specific components described in the embodiments described later. [Brief explanation of the drawing]
[0011] [Figure 1] It is an explanatory diagram of distance calculation by the laser radar device of the first embodiment mounted on a vehicle. [Figure 2] It is a block diagram showing the configuration of the laser radar device. [Figure 3] It is a diagram showing a configuration example of a laser light source. [Figure 4] It is an explanatory diagram of laser oscillation in a laser light source. [Figure 5] It is an explanatory diagram of each parameter in the distance calculation formula by the laser radar device. [Figure 6] It is a diagram showing an example when mode hopping occurs in a laser light source. [Figure 7] It is a beat signal when mode hopping occurs, and it is a diagram showing an example when there is no noise. [Figure 8] It is a diagram showing a signal obtained by processing the beat signal of FIG. 7 by a differential circuit of a comparative example. [Figure 9] It is a diagram showing the results of calculating the instantaneous phase of the beat signal of FIG. 7 and the instantaneous phase of the beat signal when mode hopping does not occur, respectively. [Figure 10] It is an enlarged view of the X region of FIG. 9. [Figure 11] It is a beat signal when mode hopping occurs, and it is a diagram showing an example when there is noise. [Figure 12] It is a diagram showing a signal obtained by processing the beat signal of FIG. 11 by a differential circuit of a comparative example. [Figure 13] It is a diagram showing the results of calculating the instantaneous phase of the beat signal of FIG. 11 and the instantaneous phase of the beat signal when mode hopping does not occur, respectively. [Figure 14] It is an enlarged view of the XIV region of FIG. 13. [Figure 15] It is an explanatory diagram of the transmission characteristics of two ring filters, the gain characteristics of a gain medium, and laser oscillation. [Figure 16]It shows the changes in the phase amount and longitudinal mode of the phase adjuster in the laser light source, and is an explanatory diagram for the case where no mode hop occurs. [Figure 17] It shows the changes in the phase amount and longitudinal mode of the phase adjuster in the laser light source, and is an explanatory diagram for the case where a mode hop occurs. [Figure 18] It is an explanatory diagram for the correction control of mode hop by the light source control unit. [Figure 19] It is an explanatory diagram for the calculation region of the instantaneous phase. [Figure 20] It is a beat signal when a mode hop occurs, and is a diagram showing an example when there is no noise. [Figure 21] It is a diagram showing the time change of the difference in the instantaneous phase calculated using the beat signal of FIG. 20. [Figure 22] It is a beat signal when a mode hop occurs, and is a diagram showing an example when there is large noise. [Figure 23] It is a diagram showing the time change of the difference in the instantaneous phase calculated using the beat signal of FIG. 22. [Figure 24] It is an explanatory diagram for mode hop determination using the cross-correlation of the instantaneous phase in the laser radar device of the second embodiment.
Mode for Carrying Out the Invention
[0012] Hereinafter, embodiments of the present disclosure will be described based on the drawings. In the following embodiments, parts that are the same or equivalent to each other will be described with the same reference numerals.
[0013] (First Embodiment) A laser radar device 1 of the first embodiment will be described. The laser radar device 1 of this embodiment is an FMCW type LiDAR, and is preferably applied to in-vehicle applications mounted on a moving object such as a passenger car V, as shown in Figure 1, for example, and performs detection of surrounding targets, i.e., targets TG, and calculation of their distance. In this specification, the case in which the laser radar device 1 is mounted on a passenger car V will be described as a representative example, but it is not limited to this.
[0014] The laser radar device 1 comprises, for example, a laser light source 2, an optical amplifier 3, a scanner 4, a distance measurement calculation unit 5, a mode hop detection unit 6, and a light source control unit 7, as shown in Figure 2. The laser radar device 1 transmits laser light as a transmitted wave within a predetermined range as shown by the dashed line in Figure 1, receives the reflected wave reflected by the target TG, etc., from the transmitted wave, and calculates the distance to the target TG.
[0015] Laser light source 2 is a light source that emits laser light and is capable of FMCW frequency modulation. Laser light source 2 is, for example, an ECL type light source that utilizes the Vernier effect by a ring filter. ECL is an abbreviation for external cavity laser. Laser light source 2 comprises a gain medium 20 and an optical resonator 21, as shown in Figure 3, for example. In laser light source 2, light generated in the gain medium 20 repeatedly travels back and forth within the optical resonator 21, and each time the light passes through the gain medium 20, it is amplified by stimulated emission before being emitted to the outside as laser light. Laser light source 2 oscillates at a frequency where the peak P in the gain curve of the gain medium 20 matches the resonance frequency of the optical resonator 21, i.e., the longitudinal mode, as shown in Figure 4, for example. In the example shown in Figure 4, multiple resonance frequencies of the optical resonator 21 are set from the smallest frequency side to f -3 ,f -2 ,f -1 With f0, f1, f2, and f3 as the longitudinal modes, the laser light source 2 oscillates at a frequency f0 that matches the peak P in the gain curve of the gain medium 20. For the sake of explanation, the light emitted from the laser light source 2 may be referred to as "chirp light" below.
[0016] The gain medium 20 is composed of, for example, a III-V semiconductor structure and serves as a light source that amplifies light by stimulated emission internally, while also having a mirror surface 20a that reflects light towards the optical resonator 21. The gain medium 20 is integrated with the optical resonator 21, for example, by having its other side 20b, opposite to the mirror surface 20a, attached to one end face 21a of the optical resonator 21. The gain medium 20 is connected to one end face 21a of the optical resonator 21, which will be described later, and is configured to optically couple with the first waveguide 221. The gain medium 20 may also be referred to as a semiconductor optical amplifier (SOA).
[0017] The optical resonator 21 is formed, for example, using a semiconductor substrate and a known semiconductor process. The optical resonator 21 comprises, for example, a plurality of waveguides 22, a first ring filter 23, a first heater 24, a second ring filter 25, a second heater 26, a phase adjuster 27, and an optical coupling unit 28. The optical resonator 21 is also referred to as, for example, a "cavity".
[0018] The multiple waveguides 22 are composed of, for example, a first waveguide 221, a second waveguide 222, and a third waveguide 223. The first waveguide 221 is optically coupled at one end to a gain medium 20 and at the other end to a first ring filter 23, forming a light propagation path between the gain medium 20 and the first ring filter 23. Light from the third waveguide 223 propagates through the optical coupling section 28 of the first waveguide 221 and propagates to the gain medium 20. The second waveguide 222 is optically coupled at one end to a first ring filter 23 and at the other end to a second ring filter 25, forming a light propagation path between them. The third waveguide 223 is configured such that, for example, one end 223a is optically coupled to the second ring filter 25, and the other end 223b is in communication with the outside at the other end face 21b of the optical resonator 21, opposite to the one end face 21a, and the other end 223b is the laser beam exit port. The third waveguide 223 is equipped with a phase adjuster 27, which allows for adjustment of the phase of the light passing through the third waveguide 223. Part of the third waveguide 223, on its way from the phase adjuster 27 to the exit port, forms an optical coupling section 28 facing the first waveguide 221 with a predetermined gap, and a portion of the light that has passed through the phase adjuster 27 is propagated to the first waveguide 221. In other words, the third waveguide 223 constitutes a light propagation path through which light that has passed through the second ring filter 25 propagates to the first waveguide 221 or the exit port at the other end 223b via the phase adjuster 27.
[0019] The first ring filter 23 and the second ring filter 25 are resonators that, upon input of light, generate transmission spectra each having a predetermined free spectral width (FSR). The ring filters 23 and 25 are, for example, composed of different perimeters and each generates a transmission spectrum with a different FSR. The ring filters 23 and 25 may also be referred to as "optical filters" or "ring resonators." The FSR of the transmission spectra of the ring filters 23 and 25 can be intentionally changed by temperature control using the first heater 24 or the second heater 26, respectively. This allows the laser light source 2 to be controlled to a desired longitudinal mode.
[0020] The first ring filter 23 is positioned between the first waveguide 221 and the second waveguide 222, at a predetermined distance from them, and is optically coupled to them. The first ring filter 23 allows light from the first waveguide 221 to propagate, and also propagates the light propagated within the first ring filter 23 to the second waveguide 222. The first ring filter 23 has a configuration in which, for example, a support substrate, an underclad layer, a core layer, and an overclad layer are stacked in this order, and the first heater 24 is positioned on the overclad layer.
[0021] The second ring filter 25 is positioned between the second waveguide 222 and the third waveguide 223, at a predetermined distance from them, and is optically coupled to them. The second ring filter 25 allows light from the second waveguide 222 to propagate, and also propagates the light propagated within the second ring filter 25 to the third waveguide 223. The second ring filter 25 has the same configuration as the first ring filter 23, with a second heater 26 positioned on the overcladding layer.
[0022] The phase adjuster 27 uses, for example, a known phase modulator that utilizes thermo-optic effects, carrier-plasma effects, or electro-optic effects to adjust the phase of light propagating through the third waveguide 223. Note that the phase adjuster 27 is not limited to the third waveguide 223, but may also be provided in the first waveguide 221 or the second waveguide 222.
[0023] The optical amplifier 3 is, for example, an optical amplifier that receives a portion of the laser light emitted from the laser light source 2 and amplifies the incident laser light by stimulated emission. The light amplified by the optical amplifier 3 is then incident on the scanner 4.
[0024] Scanner 4 transmits laser light amplified by optical amplifier 3 to the outside in a predetermined scan pattern and receives the light reflected by target TG from the transmitted laser light. Scanner 4 includes, for example, a transmitting antenna (not shown) that transmits the laser light wave to the outside, and a plurality of receiving antennas (not shown) that receive the reflected wave from the target TG. Scanner 4 includes a control unit, for example, a microcomputer (not shown) having a processor and memory, which performs various controls such as the scan pattern of the transmitted wave. Scanner 4 outputs the signal of the received reflected wave to distance measurement calculation unit 5.
[0025] The distance measurement calculation unit 5 receives a portion of the chirp light from the laser light source 2 as a reference signal, and the reflected wave signal output from the scanner 4 as a received signal, and calculates the distance to the target TG based on these signals. The distance measurement calculation unit 5 performs various processes such as generating an optical beat signal, converting the optical signal to an electrical signal, and performing calculations using the converted signal. The distance measurement calculation unit 5 includes, for example, a signal generation unit 51, a signal conversion unit 52, a signal processing unit 53, and a distance calculation unit 54.
[0026] The signal generation unit 51 generates a beat signal by taking the difference between a reference signal from the laser light source 2 and a received signal from the scanner 4. For example, a Balanced Photo Detector is used for the signal generation unit 51. The signal conversion unit 52 is, for example, a TIA, which converts the beat signal generated by the signal generation unit 51 from an optical signal to an electrical signal. TIA stands for Transimpedance Amplifier. The electrical signal output by the TIA is converted from an analog signal to a digital signal by an ADC. ADC stands for Analog to Digital Converter. The signal processing unit 53 performs a Fourier transform on the beat signal converted by the signal conversion unit 52 and performs frequency analysis. The distance calculation unit 54 calculates the distance R using the frequency of the beat signal analyzed by the signal processing unit 53 according to equation (1) below.
[0027]
number
[0028] The signal processing unit 53 and the distance calculation unit 54 are configured to include, for example, a processor such as a CPU, memory such as ROM or RAM, and logic circuits such as an FPGA. CPU, ROM, RAM, and FPGA are abbreviations for Central Processing Unit, Read Only Memory, Random Access Memory, and Field Programmable Gate Array, respectively.
[0029] To improve the accuracy of the distance calculation R in the distance calculation unit 54, the chirp light (reference signal) must be linear and continuous, as shown in Figure 5. One factor that hinders the linearity and continuity of the chirp light is mode hopping in the laser light source 2. Mode hopping occurs, for example, when the peak P of the gain curve of the gain medium 20 spans multiple longitudinal modes of the optical resonator 21 (Figure 6), or when the amount of shift of the gain curve in the frequency axis does not match the amount of shift of the longitudinal mode. When mode hopping occurs, laser light of a longitudinal mode different from the target longitudinal mode is emitted, resulting in errors in distance calculation. To improve the accuracy of distance calculation in the distance calculation unit 54, the laser radar device 1 includes a mode hop detection unit 6 that determines whether or not mode hopping has occurred, and a light source control unit 7 that controls the oscillation mode of the laser light source 2 when mode hopping occurs.
[0030] The mode hop detection unit 6 detects the occurrence of a mode hop in the laser light source 2 based on a portion of the chirp light from the laser light source 2. The mode hop detection unit 6 comprises, for example, an optical interference unit 61, an IQ signal generation unit 62, a signal conversion unit 63, an instantaneous phase calculation unit 64, and a mode hop determination unit 65. The mode hop detection unit 6 is configured to include, for example, an optical system constituting the optical interference unit 61, a detector constituting part of the IQ signal generation unit 62, and a microcomputer composed of a CPU, ROM, RAM, etc., which performs various signal processing and arithmetic processing.
[0031] The optical interference unit 61 receives, for example, a portion of the chirp light from the laser light source 2, splits the input chirp light into two by a coupler (not shown), and outputs it to the IQ signal generation unit 62. As the optical interference unit 61, for example, a self-delayed heterodyne interference system is used, but it is not limited to this, and other known optical interference systems may be used.
[0032] The IQ signal generation unit 62 generates a beat signal by taking the difference between the two chirp light signals split by the optical interference unit 61, and also generates an IQ signal from the beat signal. The IQ signal generation unit 62 is configured to include, for example, an IQ detector and a Balanced Photo Detector. The signal conversion unit 63 is, for example, a TIA, which converts the IQ signal generated by the IQ signal generation unit 62 from an optical signal to an electrical signal and outputs it to the instantaneous phase calculation unit 64. The electrical signal output by the TIA is then converted from an analog signal to a digital signal by an ADC.
[0033] The instantaneous phase calculation unit 64 calculates the instantaneous phase from the IQ signal of the beat signal at predetermined timing intervals. The instantaneous phase calculation unit 64 determines the phase of the beat signal based on the IQ signal, i.e., the In phase and Quadrature components, and calculates the instantaneous phase of the beat signal at a certain time. The instantaneous phase calculated by the instantaneous phase calculation unit 64 is used by the mode hop determination unit 65 to determine the occurrence of mode hopping in the laser light source 2.
[0034] The mode hop determination unit 65 determines whether or not a mode hop is occurring in the laser light source 2 (hereinafter referred to as "mode hop determination") based on the change in instantaneous phase calculated by the instantaneous phase calculation unit 64. The mode hop determination unit 65 performs mode hop determination by utilizing, for example, the fact that when a mode hop occurs in the laser light source 2, the instantaneous phase fluctuates, and a discontinuity in the instantaneous phase occurs before and after the mode hop occurs. Details of the mode hop determination will be described later. The mode hop determination unit 65 outputs a signal to the light source control unit 7 according to the result of the mode hop determination, for example.
[0035] When the mode hop detection unit 6 detects the occurrence of a mode hop, the light source control unit 7 controls the laser light source 2 to return an unintended oscillation mode to the desired oscillation mode. For example, the light source control unit 7 controls the laser light source 2 to reduce the change in oscillation mode due to mode hops, based on the instantaneous phase or instantaneous frequency change calculated by the instantaneous phase calculation unit 64. The control of the laser light source 2 by the light source control unit 7 will be described later.
[0036] The above describes the basic configuration of the laser radar device 1 of this embodiment.
[0037] [Mode Hop Determination] Next, the effect of mode hop detection by the mode hop detection unit 6 will be explained. The inventors generated a signal corresponding to the beat signal when a mode hop occurs using simulation software and investigated mode hop detection using differential processing and mode hop detection using instantaneous phase.
[0038] The beat signal shown in Figure 7 was generated by simulation software and assumes the absence of noise. The beat signal in Figure 7 has a beat amplitude of 1.0, a frequency of 5 MHz, a sampling frequency of 250 MHz, a chirp time of 10 μs, and a mode-hop phase shift of π rad. Furthermore, this beat signal is approximately 0.6 × 10⁻¹⁶. -5This assumes a phase shift due to mode hopping occurring within seconds. For example, if the longitudinal mode of laser light source 2 before the phase shift occurs is mode i, then the longitudinal mode after the phase shift occurs is mode i+1, which is different from mode i.
[0039] First, as a comparative example of mode-hop detection, the beat signal shown in Figure 7 was subjected to differential processing, and the discontinuity in the signal intensity obtained by the differential processing was confirmed, as shown in Figure 8. The signal intensity is a continuous sinusoidal waveform before and after the phase shift caused by mode-hop, but approximately 0.6 × 10⁻⁶ -5 A discontinuity occurred at the second mark. Although this comparative example method can detect the occurrence of a mode hop, it can only observe a single point of discontinuity in the beat signal. If this single point cannot be reliably captured, the mode hop will be missed. Furthermore, even if the discontinuity point is captured, the comparative example method only reveals that a mode hop has occurred. In other words, even if a mode hop is detected using the comparative example method, the amount of phase shift caused by the mode hop is unknown, making it impossible to control the subsequent longitudinal mode of the laser light source to the desired longitudinal mode.
[0040] In contrast, the mode hop determination by the mode hop determination unit 65 is performed using the instantaneous phase calculated by the instantaneous phase calculation unit 64, as shown in Figure 9, for example. The instantaneous phase in Figure 9 is calculated based on the beat signal in Figure 7.
[0041] When a mode hop occurs, a phase shift occurs, so the instantaneous phase changes at the time of the mode hop, as shown in Figure 10, and becomes discontinuous before and after the mode hop change. When no mode hop occurs, the instantaneous phase changes with respect to time at a predetermined slope, as shown by the dashed line in Figure 9, and is linear and continuous. Furthermore, when a mode hop occurs, the instantaneous phase is discontinuous not only at the time of the mode hop but also in the subsequent phase compared to the phase before the mode hop.
[0042] The mode hop determination unit 65 determines that a mode hop has occurred when it detects the discontinuity in the instantaneous phase described above. As described above, the instantaneous phase remains discontinuous even after the mode hop has occurred. This result suggests that by using instantaneous phase based on the beat signal, even if the phase shift at the moment of mode hop occurrence cannot be captured, the discontinuity in the instantaneous phase before and after the mode hop can be detected, making it easy to determine the occurrence of a mode hop.
[0043] Furthermore, the laser radar device 1 may pre-calculate the instantaneous phase calculation result for each of the multiple longitudinal modes of the laser light source 2 when no mode hop has occurred (the dashed-dotted line portion in the example of Figure 9) as a design value and record it on a recording medium (not shown) of the mode hop detection unit 6. In this case, for example, the mode hop determination unit 65 calculates the difference between the instantaneous phase calculated by the instantaneous phase calculation unit 64 and the pre-recorded design value (i.e., expected value), and monitors the time elapsed of this difference. Then, for example, the mode hop determination unit 65 can determine that a mode hop has occurred if the difference between the calculated instantaneous phase and the design value exceeds a predetermined threshold.
[0044] Next, we will explain the case where the beat signal contains noise. The beat signal shown in Figure 11 was generated by simulation software and assumes the presence of noise. The beat signal in Figure 11 has the same beat signal amplitude, frequency, sampling frequency, chirp time, and phase shift due to mode hop as the beat signal in Figure 7, but the noise amplitude is 0.25 and the noise is included in a normal distribution (Gaussian distribution).
[0045] First, as a comparative example, Figure 12 shows the signal intensity results obtained by differentiating the beat signal in Figure 11. In this case, as shown in Figure 12, the signal intensity contains many discontinuities due to noise, making it difficult to distinguish discontinuities caused by mode hopping. Therefore, the mode hop detection method in the comparative example cannot perform mode hop detection when the beat signal contains noise.
[0046] On the other hand, Figure 13 shows the instantaneous phase change calculated using the beat signal in Figure 11. In Figure 13, the thick solid lines represent calculations using the beat signal in Figure 11, while the thin solid lines represent calculations using a beat signal in which no mode hop occurred. As shown in Figure 13, when the beat signal contains noise, the instantaneous phase, regardless of the presence or absence of mode hop, exhibits small fluctuations due to the noise, but generally follows a predetermined linear pattern with respect to time. Furthermore, as shown in Figures 13 and 14, the phase of the instantaneous phase changes discontinuously when a mode hop occurs, and the phase after a mode hop is clearly different from the phase in the case without a mode hop. This result suggests that by using instantaneous phase based on the beat signal, it is possible to capture the phase shift caused by mode hop even when the beat signal contains noise, and the occurrence of mode hop can be easily determined.
[0047] Furthermore, by monitoring the instantaneous phase based on the beat signal, it is possible to understand the amount of phase shift before and after mode hopping. Based on this amount of shift, it becomes possible to control the oscillation mode that has shifted due to mode hopping back to the original intended oscillation mode.
[0048] [Light source control during mode hopping] Next, we will explain the control of the oscillation mode by the light source control unit 7 when it is determined that a mode hop has occurred.
[0049] First, regarding an example of controlling the longitudinal modes in the laser light source 2, we will explain a typical case where the laser light source 2 is an ECL type light source that utilizes the Vernier effect by a ring filter. In this case, as shown in Figure 15, for example, the gain curve of the gain medium 20 of the laser light source 2 is filtered by the first ring filter 23 and the second ring filter 25. The frequency at which the gain of the gain medium 20 is maximum is located at the frequency where the peak positions of the transmission characteristics of the first ring filter 23 and the second ring filter 25 overlap. At this time, the laser light source 2 oscillates at the frequency of the single longitudinal mode that is closest to the maximum gain of the gain medium 20 among the multiple longitudinal modes of the optical resonator 21, i.e., single-mode oscillation.
[0050] Furthermore, in the laser light source 2, the longitudinal mode of the optical resonator 21 is controlled by the phase adjuster 27, and frequency modulation using the FMCW method is performed by frequency sweeping the longitudinal mode and making the gain of the gain medium 20 follow. The transmission characteristics of the ring filters 23 and 25 are controlled by temperature adjustment using heaters 24 and 26 placed on the waveguide. This utilizes the property that the refractive index of the waveguides constituting the ring filters 23 and 25 depends on temperature. In the laser light source 2, when no mode hopping occurs (normal operation), the phase amount is adjusted by the phase adjuster 27, for example, as shown in Figure 16, and the frequency at which the gain of the gain medium 20 is maximized changes according to the phase amount, and the frequency of the oscillating longitudinal mode changes. In Figure 16, two states with different phase amounts are conceptually shown separated into upper and lower sections, and the parts where the peaks of the transmission characteristics, i.e., the transmission spectra of the two ring filters overlap are circled. The same applies to Figure 17.
[0051] On the other hand, as shown in Figure 17, for example, in the laser light source 2, the shift amount of the maximum gain of the gain medium 20 deviates significantly with respect to the change in phase amount, and when the maximum gain and the longitudinal mode do not match, mode hopping occurs. This shift amount deviation can occur, for example, when the heat obtained changes due to degradation over time, even if the power of the heaters 24 and 26 of the ring filters 23 and 25 is the same, causing a deviation from the target amount. Examples of degradation over time include changes in the thermal conductivity of the waveguides and cladding materials that make up the ring filters, and changes in the resistance of the heaters.
[0052] Then, if the mode hop determination unit 65 determines that a mode hop has occurred, the light source control unit 7 performs power control of the heaters 24 and 26 to reduce the instantaneous phase discontinuity detected by the mode hop determination unit 65, for example, based on the instantaneous phase change. In other words, the light source control unit 7 performs feedback control by temperature control of the ring filters 23 and 25 to correct the mode hop.
[0053] Specifically, for example, the mode hop determination unit 65 calculates the difference between the instantaneous phase calculated by the instantaneous phase calculation unit 64 and the target value (design value) of the instantaneous phase when no mode hop occurs (hereinafter referred to as the "instantaneous phase difference value"), as shown in Figure 18. Note that in Figure 18, the vertical axis is omitted for clarity, but the vertical axis represents the instantaneous phase difference value described above.
[0054] For example, as shown in Figure 18, if Δφ0 is the time average of the instantaneous phase difference before a mode hop occurs, then after a mode hop occurs, the time average of the instantaneous phase difference becomes Δφhop, which is different from Δφ0. Δφhop is, for example, the time average of the instantaneous phase difference at Δt_hop, where Δt_hop is the predetermined time from the moment a mode hop occurs until the light source control unit 7 performs control. At this time, the light source control unit 7 performs power control of heaters 24 and 26 so as to satisfy one of the following conditions: the absolute value of the difference between Δφ0 and Δφhop, i.e., |Δφhop-Δφ0|, is zero, minimized, or below a predetermined threshold. As a result, the time average of the instantaneous phase difference becomes Δφ1, which is less than or equal to the predetermined difference from Δφ0, as shown in Figure 18, for example, and the discontinuity in the instantaneous phase is resolved. The resolution of the discontinuity in the instantaneous phase means that the longitudinal mode, which had deviated from the design value due to some factor, returns to the original target longitudinal mode, and the mode hop is resolved.
[0055] In the above example, the calculation of instantaneous phase and difference values, and the determination of mode hop occurrence are performed in a single frequency sweep of the laser light source 2. However, the method is not limited to this, and the calculation of instantaneous phase, etc., may be performed through multiple frequency sweeps. Furthermore, if a mode hop is determined to have occurred, the power control values of the heaters 24 and 26 by the light source control unit 7 may be recorded, for example, on a recording medium (not shown) within the light source control unit 7 and used as the initial value of the heater power when the laser light source 2 is started up again.
[0056] Furthermore, it is preferable that the mode hop determination unit 65 performs mode hop determination in a predetermined time domain where the instantaneous phase error is small. Specifically, for example, as shown in Figure 19, if the boundary between the frequency increasing region and the frequency decreasing region in the frequency sweep profile of the laser light source 2 is defined as the discontinuity point DP, the calculated instantaneous phase will have an error E due to the discontinuity point DP. It is preferable that the mode hop determination unit 65 calculates the difference value of the instantaneous phase and performs mode hop determination in a time domain different from the time domain in which this error E occurs, for example, in the time domain t1 to tn in which the instantaneous phase changes linearly. This eliminates the influence of the error E and improves the accuracy of mode hop determination.
[0057] The laser radar device 1 of this embodiment includes an IQ signal generation unit 62 that outputs a beat signal based on laser light from an optical interference unit 61, and an instantaneous phase calculation unit 64 that calculates the instantaneous phase from the beat signal. The laser radar device 1 also includes a mode hop determination unit 65 that performs a mode hop determination of the laser light source 2 based on the discontinuity of the instantaneous phase calculated by the instantaneous phase calculation unit 64, and a light source control unit 7 that executes control of the laser light source 2 when it is determined that a mode hop has occurred. The light source control unit 7 performs feedback control of the oscillation mode of the laser light source 2 so that the change in instantaneous phase before and after the occurrence of a mode hop is below a predetermined level. The instantaneous phase calculated from the beat signal is less affected by noise compared to when differential processing is performed on the beat signal. Therefore, the laser radar device 1 can perform a mode hop determination even when the beat signal contains noise, and can control the laser oscillation to the intended oscillation mode after the occurrence of a mode hop.
[0058] (modified version) (1) In the first embodiment described above, an example was given in which the mode hop determination unit 65 performs mode hop determination based on the discontinuity of instantaneous phase, but it is not limited to this. For example, the mode hop determination unit 65 may perform mode hop determination based on the discontinuity of instantaneous frequency or the discontinuity of the integral of the difference value of instantaneous phase. In other words, the mode hop determination unit 65 only needs to be able to detect a discontinuity in the instantaneous phase when a mode hop occurs, or in other parameters that can be calculated using the instantaneous phase, and perform mode hop determination. Furthermore, the instantaneous frequency, the difference value of instantaneous phase, and the integral value of said difference value may be calculated by the instantaneous phase calculation unit 64 instead of the mode hop determination unit 65.
[0059] Furthermore, if f(t) is the instantaneous frequency at a certain time t, then f(t) can be calculated, for example, by the following equation (2).
[0060]
number
[0061] (2) The laser radar device 1 may, in the mode hop detection unit 6, remove or reduce noise contained in the beat signal by any method such as filtering or time integration in multiple frequency sweeps, and then perform calculation of instantaneous phase, etc. This can further improve the accuracy of mode hop determination and feedback control by the light source control unit 7.
[0062] (3) The IQ signal generation unit 62 may generate the IQ signal of the beat signal by signal processing using, for example, a Hilbert transform. This eliminates the need for an IQ detector and makes it possible to further miniaturize the laser radar device 1.
[0063] (Second Embodiment) A laser radar device 1 of the second embodiment will be described.
[0064] The laser radar apparatus 1 of the second embodiment differs from the first embodiment in that the mode-hop determination unit 65 calculates a function of instantaneous phase cross-correlation, performs mode-hop determination based on the calculated cross-correlation, and the light source control unit 7 performs feedback control using the cross-correlation. This embodiment will mainly explain this difference.
[0065] For example, in this embodiment, the mode-hop determination unit 65 calculates the instantaneous phase cross-correlation function. The instantaneous phase cross-correlation function is a function of the cross-correlation between the instantaneous phase (calculated value) calculated by the instantaneous phase calculation unit 64 and the expected value (design value) of the instantaneous phase when no mode hop occurs under the same driving conditions of the laser light source 2. Hereafter, for the sake of simplicity, the above cross-correlation will be simply referred to as "instantaneous phase cross-correlation," and its function will be referred to as the "instantaneous phase cross-correlation function." This expected value of the instantaneous phase is internally recorded, for example, on a recording medium (not shown) of the mode-hop detection unit 6, and can be read as needed. As a result of simulation studies by the inventors, it was found that the instantaneous phase cross-correlation function is not affected by noise contained in the beat signal and can be used for mode-hop determination.
[0066] Specifically, as shown in Figure 20, for example, a signal equivalent to a beat signal was generated by simulation, and the instantaneous phase difference value was calculated, resulting in the results shown in Figure 21. When the beat signal does not contain noise, as shown in Figure 21, a phase shift caused by mode hopping can be observed in the instantaneous phase difference value.
[0067] On the other hand, for example, as shown in FIG. 22, assuming that the beat signal contains noise greater than or equal to a predetermined level, a signal corresponding to the beat signal was generated by simulation, and the difference value of the instantaneous phase was calculated. As a result, the results shown in FIG. 23 were obtained. The signals shown in FIGS. 20 and 22 have a beat signal amplitude of 1.0, a frequency of 5 MHz, a sampling frequency of 250 MHz, a chirp time of 10 μs, and a phase shift due to mode hopping of π / 4 rad. Also, the signals shown in FIGS. 20 and 22 are assumed to have a phase deviation due to mode hopping occurring in about 0.6×10 -5 seconds. Further, the signal shown in FIG. 22 has a noise amplitude of 0.36 and includes noise in a normal distribution (Gaussian distribution). When the beat signal contains noise greater than or equal to a predetermined level, as shown in FIG. 23, the difference value of the instantaneous phase has large fluctuations due to noise, and it is difficult to determine the phase shift, that is, the discontinuity, due to mode hopping.
[0068] The mode hop determination unit 65 calculates, for example, as shown in FIG. 24, the cross-correlation of the instantaneous phase. The vertical axis in FIG. 24 is the cross-correlation of the cross-correlation function, and the horizontal axis is the time delay of the cross-correlation function. Also, in FIG. 24, three situations are shown: when no mode hop occurs, when a mode hop occurs and there is no noise in the beat signal, and when a mode hop occurs and there is noise in the beat signal. "No mode hop" in FIG. 24 corresponds to the case where no mode hop occurs, and "Mode hop (no noise)" corresponds to the case where a mode hop occurs and there is no noise in the beat signal. "Mode hop (with noise)" in FIG. 24 corresponds to the case where a mode hop occurs and there is noise in the beat signal. Further, in FIG. 24, for the sake of clarity, "Mode hop (no noise)" and "Mode hop (with noise)" are shown slightly shifted from each other, but in reality, they overlap.
[0069] The cross-correlation of the instantaneous phase becomes maximum 1 when the time delay is 0 in the case of no mode hop, and the cross-correlation decreases linearly in proportion to the absolute value of the time delay.
[0070] Here, the cross-correlation function can be considered as a measure of the similarity between two signals. When a time delay is applied to one signal, a high cross-correlation function as a function of this time delay indicates a high degree of similarity between the two signals. For example, when calculating the cross-correlation (autocorrelation) of two identical signals, the cross-correlation is maximized at zero time delay. That is, when the two signals are identical, the cross-correlation is maximized at zero time delay. In the absence of mode hopping, the calculated instantaneous phase and the expected instantaneous phase essentially coincide, so the cross-correlation is maximized at zero time delay.
[0071] On the other hand, when mode hopping is involved, the calculated instantaneous phase is offset from the expected instantaneous phase, so the two signals do not match. Therefore, in this case, the maximum value of the cross-correlation is not at zero time delay, but at the time delay corresponding to the offset of the instantaneous phase.
[0072] The instantaneous phase cross-correlation, when mode hopping is enabled (no noise), was at its maximum value of 1 when the time delay was +1, and decreased linearly as the time delay moved away from +1. Furthermore, the instantaneous phase cross-correlation was identical to that when mode hopping is enabled (with noise).
[0073] In other words, when a mode hop occurs, the cross-correlation of instantaneous phases shifts from the point where the cross-correlation is maximum, i.e., the peak position, which is the position when there is no mode hop, but the shape of the graph does not change. Furthermore, when the same mode hop occurs, the cross-correlation of instantaneous phases remains the same regardless of the presence or absence of noise. This result suggests that mode hop detection is possible based on the shift in the peak position with respect to the time delay of the cross-correlation of instantaneous phases, and that the noise influence of the beat signal can be reduced by using the cross-correlation of instantaneous phases in this determination. Note that the cross-correlation of instantaneous phases may be calculated by the instantaneous phase calculation unit 64 instead of the mode hop determination unit 65.
[0074] In this embodiment, the light source control unit 7 controls the power of the heaters 24 and 26 of the laser light source 2 to satisfy one of the following conditions: make the absolute value of the shift in peak position with respect to the time delay of the cross-correlation of instantaneous phases zero, minimize it, or make it below a predetermined threshold. This makes it possible to accurately return the oscillation mode that has shifted when a mode hop occurs in the laser light source 2 back to the original target oscillation mode, even if the beat signal contains noise above a predetermined level.
[0075] According to this embodiment, in addition to the effects of the first embodiment described above, the laser radar device 1 further reduces the noise influence included in the beat signal and further improves the accuracy of mode hop determination and feedback control of the laser light source 2.
[0076] (Other embodiments) This disclosure is described in accordance with the embodiments, but it is understood that this disclosure is not limited to such embodiments or structures. This disclosure also includes various modifications and variations within the equivalence range. In addition, various combinations and forms, as well as other combinations and forms including one, more, or less of those elements, fall within the scope and concept of this disclosure.
[0077] It goes without saying that, in each of the above embodiments, the elements constituting the embodiment are not necessarily essential unless explicitly stated to be particularly essential or unless they are clearly considered essential in principle. Furthermore, in each of the above embodiments, when numerical values such as the number, numerical values, quantities, or ranges of the components of the embodiment are mentioned, the embodiment is not limited to those specific numbers unless explicitly stated to be particularly essential or unless it is clearly limited to a specific number in principle. Furthermore, in each of the above embodiments, when the shape, positional relationship, etc., of the components are mentioned, the embodiment is not limited to those shapes, positional relationships, etc., unless explicitly stated or unless it is clearly limited to a specific shape, positional relationship, etc., in principle. [Explanation of Symbols]
[0078] 2. Laser light source 20 Gain medium 23, 25 Light filters 27 Phase Adjuster 61 Optical Interferometer 62 IQ signal generation unit 64 Instantaneous phase calculation section 65 Mode Hop Determination Unit 7. Light source control unit
Claims
1. A laser radar system using the FMCW method, A laser light source (2) that emits laser light, An optical interference unit (61) that splits the laser light emitted from the laser light source into two and propagates them, The IQ signal generation unit (62) outputs beat signals of the two laser beams that have been branched in the optical interference unit, An instantaneous phase calculation unit (64) that calculates the instantaneous phase of the beat signal, A mode hop determination unit (65) detects the discontinuity of the instantaneous phase calculated by the instantaneous phase calculation unit and determines whether or not a mode hop has occurred in the laser light source based on the result of the detection, A laser radar device comprising: a light source control unit (7) that, when the mode hop determination unit determines that a mode hop has occurred, performs feedback control of the oscillation mode of the laser light source so that the instantaneous phase change before and after the determination of the occurrence of a mode hop is less than or equal to a predetermined value.
2. The laser radar apparatus according to claim 1, wherein the laser light source comprises a plurality of optical filters (23, 25) and a phase adjuster (27), and performs frequency sweeping.
3. The laser radar apparatus according to claim 2, wherein the mode hop determination unit calculates the cross-correlation between the calculated value of the instantaneous phase and the expected value of the instantaneous phase, and detects the discontinuity of the instantaneous phase based on the cross-correlation.
4. The laser radar apparatus according to claim 3, wherein the mode hop determination unit detects the instantaneous phase discontinuity based on the shift of the time delay in the function of the cross-correlation.
5. The laser radar apparatus according to claim 3, wherein the light source control unit performs the feedback control by controlling the transmission characteristics of the optical filter to satisfy one of the following conditions: making the absolute value of the time delay of the cross-correlation function zero, minimizing it, or making it below a predetermined threshold.
6. The laser radar apparatus according to claim 2, wherein the mode hop determination unit calculates the difference between the calculated value of the instantaneous phase and the expected value of the instantaneous phase, and detects the discontinuity of the instantaneous phase based on the difference value.
7. The laser radar apparatus according to claim 6, wherein the light source control unit performs the feedback control by controlling the transmission characteristics of the optical filter to satisfy one of the following conditions: making the absolute value of the difference value zero, making it the minimum value, or making it below a predetermined threshold.
8. The laser radar apparatus according to claim 2, wherein the light source control unit performs the feedback control by controlling the temperature of the optical filter constituting the laser light source.
9. The laser radar apparatus according to claim 2, wherein the optical filter is composed of a ring resonator.
10. The laser radar apparatus according to claim 1, wherein the IQ signal generation unit outputs the IQ signal of the beat signal.
11. The laser radar apparatus according to claim 1, wherein the optical interference section is composed of a self-delayed heterodyne interference system.
12. The laser radar apparatus according to claim 1, wherein the mode hop determination unit performs the determination using the time average value of the instantaneous phase calculated in multiple frequency sweeps.