Light-receiving device, light reception method, and measurement light generation device

Abstract

[Problem] To prevent light reflected by a light-emitting unit from returning to a light source and affecting the laser oscillation frequency of the light source. [Solution] This light-receiving device comprises: a measurement optical system that generates measurement light having a frequency that shifts at least discretely to two or more levels; and a light processing unit that causes prescribed oscillation light and return light of the measurement light from an object being measured to interfere with each other. The measurement optical system has a light-emitting unit that transmits the measurement light. The measurement optical system is also capable of transmitting the measurement light through laser resonance caused by at least inner-surface reflection of the light-emitting unit.

Description

Light-receiving device, light-receiving method, and measurement light generation device

[0001] The present disclosure relates to a light-receiving device, a light-receiving method, and a measurement light generation device.

[0002] When measuring the distance to a measurement object, a coherent LiDAR (Light Detection And Ranging) is known that interferes the return light with a reference light and measures the distance and speed to the measurement object based on the beat signal generated by the difference in the optical frequencies of the two (see Patent Document 1). In a coherent LiDAR, the measurement light generated by a light source is irradiated onto a measurement object through a light-emitting unit.

[0003] Japanese Patent Application Laid-Open No. 2021-092437

[0004] However, the light reflected by the light-emitting unit may return to the light source and affect the laser oscillation frequency of the light source.

[0005] To solve the above problems, according to the present disclosure, there is provided a light-receiving device including: a measurement optical system that generates measurement light whose frequency is at least discretely shifted to two or more levels; a predetermined oscillation light; and an optical processing unit that interferes the return light of the measurement light from the measurement object.

[0006] The measurement optical system may have a light-emitting unit that transmits the measurement light.

[0007] The measurement optical system may transmit the measurement light by laser resonance at least due to internal reflection of the light-emitting unit.

[0008] The measurement optical system may transmit the measurement light by laser resonance between the end face of a semiconductor optical amplifier and the inner face of the light-emitting unit.

[0009] The measurement optical system may further include a modulator that is disposed in a waveguide of the measurement optical system and is used to shift the frequency to at least two or more levels.

[0010] The modulator may be a phase modulator.

[0011] The phase modulator may be a phase shifter based on the plasma dispersion effect.

[0012] The modulator may also be an intensity modulator.

[0013] The system may further include: a signal processing unit capable of generating at least one of the distance to the object to be measured and the velocity based on the output signal of the optical processing unit; and a drive control unit that controls at least one of the modulator and the semiconductor optical amplifier.

[0014] The measurement optical system may further include a microring resonator disposed in the waveguide of the measurement optical system.

[0015] The drive control unit may control the gain current of the semiconductor optical amplifier.

[0016] The drive control unit may control at least one of the modulator and the semiconductor optical amplifier according to a time-series code.

[0017] The optical processing unit may include a 90-degree optical hybrid circuit that takes at least a portion of the predetermined oscillating light and the reflected light as inputs and converts them into four output lights, and two balanced photodetectors that convert the four output lights into a real analog current and an imaginary analog current.

[0018] The predetermined oscillating light may be at least a part of the measurement light.

[0019] The predetermined oscillating light may be emitted from a light source that is different from the light source of the measurement optical system.

[0020] The system may further include a second optical processing unit that interferes the predetermined oscillating light with the measurement light.

[0021] The system may further include a third optical processing unit that interferes the aforementioned measurement light with light obtained by delaying the measurement light.

[0022] To solve the above problems, the present disclosure provides a light receiving method comprising: a measurement light generation step that generates measurement light whose frequency is shifted at least two discrete levels or more; and a light processing step that interferes a predetermined oscillation light with the reflected light from the object to be measured.

[0023] To solve the above problems, the present disclosure provides a measurement light generation device comprising: a gain medium that spontaneously emits photons when an electric current is passed through it; and a resonator that resonates the photons between one reflective surface and the other inner reflective surface of a light-emitting unit that transmits measurement light, thereby generating measurement light whose frequency is shifted at least discretely by two levels or more.

[0024] The waveguide between the one reflective surface and the inner reflective surface may further include a modulator that changes the group refractive index, and a drive control unit that controls at least one of the current and the modulator according to a time-series sign.

[0025] A diagram schematically showing an example of a photodetector according to this disclosure. A diagram showing an example configuration of an external resonant laser. A diagram showing the optical frequency filter characteristics. A diagram showing an example of the overall configuration of the measurement optical system as a resonator. A diagram showing the relationship between the phase shift amount by the modulator and the relative optical frequency of the measurement optical system. A diagram showing the transmitted optical waveform of the measurement optical system. A diagram showing a first example configuration of the modulator. A diagram showing a second example configuration of the modulator. A diagram showing a third example configuration of the modulator. A diagram showing an example of the characteristics of a phase modulator of a comparative example. A diagram showing the optical waveforms of the returned light and the local oscillation light. A diagram showing an example configuration of the optical processing unit. A diagram showing an example configuration of a balanced photodetector and a transimpedance amplifier. A diagram showing the output of the transimpedance amplifier. A diagram showing an example of the instantaneous frequency converted by the signal processing circuit. A diagram showing an example of the instantaneous frequency of the returned light. A diagram showing an example of the cross-correlation value with the instantaneous frequency. A diagram showing an example of the result of a Fourier transform on the Doppler component generated by the signal processing circuit. A diagram showing an example of the configuration of the photodetector in an integrated circuit. A flowchart showing an example of the processing of the signal processing circuit. A schematic diagram showing an example of a photodetector according to the second embodiment. A schematic diagram showing an example of a photodetector according to the third embodiment. A diagram showing the relationship between the phase shift amount by the modulator and the relative optical frequency of the measurement optical system. A schematic diagram showing an example of a photodetector according to the fourth embodiment. A diagram showing the relationship between the gain current by the gain control unit and the relative optical frequency and number of the measurement optical system. A schematic diagram showing an example of a photodetector according to the fifth embodiment. A schematic diagram showing an example of a photodetector according to the fifth embodiment. A diagram showing the characteristics of the oscillating light and the measurement light. A schematic diagram showing an example of a photodetector according to the seventh embodiment. A diagram showing an example of the measurement light according to the eighth embodiment.

[0026] Embodiments of this disclosure will be described below with reference to the drawings.

[0027] (First Embodiment) Preferred embodiments of the present disclosure will be described in detail below with reference to the attached drawings. In this specification and the drawings, components having substantially the same functional configuration are denoted by the same reference numerals, and redundant explanations will be omitted.

[0028] Figure 1 is a schematic diagram showing an example of a light receiving device according to the present disclosure. The light receiving device 1 in Figure 1 is a device capable of irradiating measurement light with discretely switched frequencies, and comprises a measurement optical system 10, a drive control unit 20, a light receiving unit 30, an optical processing unit 40, a signal processing unit 50, a control unit 60, and a memory 70.

[0029] The measurement optical system 10 is an optical system that irradiates the target object (Target) with measurement light TX that discretely varies to multiple frequencies. The discrete modulation width Fs of the optical frequency of the measurement light TX is, for example, 10 MHz to 10 GHz. The measurement optical system 10 includes an external cavity laser (ECL) 12, a modulator (in this embodiment, a phase modulator PM) 14, a beam splitter 16, and a light-emitting unit 18. Details of the measurement optical system 10 will be described later.

[0030] The drive control unit 20 controls the modulation timing of the measurement optical system 10. The drive control unit 20 includes a laser drive circuit 22 and a code generator 24. The laser drive circuit 22 controls the emission of light according to the gain current of the external resonant laser 12, in accordance with the control unit 60 described later. The code generator 24 controls the group refractive index ng of the measurement optical system 10, in accordance with the control unit 60 described later. More specifically, the code generator 24 changes the group refractive index ng of the measurement optical system 10 by controlling, for example, the gain current of the external resonant laser 12, the phase of the modulator 14, and the light intensity, thereby discretely varying the frequency of the measurement light TX to multiple levels.

[0031] The light-receiving unit 30 is, for example, an optical phased array. The light-receiving unit 30 receives the reflected light RX from the object to be measured. The light-receiving unit 30 may also be, for example, an optical antenna (gratting coupler, edge coupler).

[0032] The optical processing unit 40 is, for example, a 90-degree optical hybrid coupler and a balanced photodiode (90° Hybrid + BPD), which converts the interference light between the return light RX and the local oscillator light LO (LO: Local Oscillator) into an analog current signal. Details of the optical processing unit 40 will be described later. In Figure 1, the local oscillator light LO has the same wavelength as the measurement light TX. Note that the local oscillator light LO in this embodiment corresponds to a predetermined oscillator light.

[0033] The signal processing unit 50 converts the analog current signal output by the optical processing unit 40 into a digital voltage signal and generates a distance value to the object to be measured. This signal processing unit 50 includes a transimpedance amplifier (TIA) 52, an AD conversion circuit 54, and a signal processing circuit (DSP) 56. The signal processing unit 50 also controls the entire light receiving device 1 using programs, processing parameters, characteristic tables, etc., stored in the memory 70. Details of the signal processing unit 50 will be described later.

[0034] The control unit 60 is configured to include, for example, a CPU (Central Processing Unit). The control unit 60 controls the entire light receiving device 1 using programs and processing parameters stored in the memory 70. The memory 70 stores programs, processing parameters, characteristic tables, etc., for the signal processing unit 50 and the control unit 60.

[0035] Here, the details of the measurement optical system 10 will be explained using Figures 2 to 9. Figure 2 is a diagram showing an example configuration of the external resonant laser 12. The external resonant laser 12 includes, for example, a reflective semiconductor optical amplifier (RSOA) 120, a Fabry-Perot phase shifter 122, a first ring resonator 124, and a second ring resonator 126.

[0036] The reflective semiconductor optical amplifier (RSOA) 120 operates in response to a drive signal input from the laser drive circuit 22. The left end face of the reflective semiconductor optical amplifier 120 is mirror-finished, and the right end face is connected to an external resonator made of a Si waveguide.

[0037] When current is passed through the gain medium of the SOA according to the drive signal of the laser drive circuit 22, photons are generated by spontaneous emission. The photons emitted by spontaneous emission are reflected by a mirror at the left end face of the SOA, and the light emitted from the right end face is selectively reflected (= optical frequency filtering) by an external resonator, with only specific optical frequency components being reflected.

[0038] Photons that are reflected and re-entered into the SOA are amplified by stimulated emission. By repeating this reflection, wavelength filtering, and amplification process, a highly coherent laser beam (suitable for Coherent LiDAR) with a sharp spectral peak at a specific wavelength is generated. A portion of this beam is ultimately output from the output port and directed towards the light-emitting unit 18, irradiating the object to be measured.

[0039] Figure 3 shows the optical frequency filter characteristics of the Fabry-Perot phase shifter 122, the first ring resonator 124, and the second ring resonator 126. From top to bottom, the resonance characteristics of the first ring resonator 124, the second ring resonator 126, and the Fabry-Perot phase shifter 122 are shown. The vertical axis represents transmittance, and the horizontal axis represents optical frequency.

[0040] As shown in Figure 3, the overall transmittance in the external resonators (Fabry-Perot phase shifter 122, first ring resonator 124, and second ring resonator 126) is maximized when the comb-shaped transmittance peaks of the multiple resonators align. This maximum transmittance is the optical frequency at which selective reflection is maximized, and is the oscillation optical frequency of the external resonant laser 12. The FSR (Free Spectral Range) [Hz] indicates the resonant frequency interval in the optical resonator. The resonant frequency interval FSR is a function of the resonator's physical length L, the speed of light c, and the group refractive index ng.

[0041] Thus, in the external resonance type laser 12, the Fabry-Perot phase shifter 122 and the two ring resonators 124 and 126 each have a phase shifter, and the filter characteristics can be shifted by changing the group refractive index ng according to the phase shift amount and changing the resonance frequency interval FSR.

[0042] FIG. 4 is a diagram showing an example of the overall configuration as a resonator of the measurement optical system 10. Each embodiment of the present application can be simplified as shown in FIG. 4 regarding the operation as the external resonance type laser 12.

[0043] That is, when the antenna which is an example of the light emitting part 18 has internal reflection (for example, 10%), the measurement optical system 10 constitutes a Fabry-Perot resonator having the length L from the left end face of the external resonance type laser 12 to the light emitting part 18 via the modulator 14 as the resonator length. That is, the measurement optical system 10 resonates by laser between the end face of the reflective semiconductor optical amplifier 120 and the inner face of the light emitting part 18, and transmits measurement light from the light emitting part 18. Thus, the measurement optical system 10 according to the present embodiment resonates by laser at least due to the internal reflection of the light emitting part 18, and transmits measurement light from the light emitting part 18.

[0044] The measurement optical system 10 according to the present embodiment has a configuration including the external resonance type laser 12, the modulator (PM: Phase Modulator in the present embodiment) 14, and the light emitting part 18, but is not limited thereto. For example, the measurement optical system 10 may be configured to be able to irradiate a measurement object (Target) with measurement light TX that discretely varies at a plurality of frequencies. The external resonance type laser 12 may be a distributed Bragg reflector (DBR) provided with a diffraction grating in addition to the ring resonator. Alternatively, the external resonance type laser 12 may be provided with a loop mirror (sagnac loop).

[0045] FIG. 5 is a diagram showing the relationship between the phase shift amount by the modulator 14 and the relative optical frequency of the measurement optical system 10. The vertical axis is the relative optical frequency [MHz] of the measurement optical system 10, and the horizontal axis is the phase shift amount of the modulator 14. The modulator 14 is, for example, a phase modulator (PM).

[0046] As shown in FIG. 5, the modulator 14 is discretely controlled in terms of phase between PH1 and PH2 by the control signal S(t) of the code generator 24. That is, if the waveguide refractive index is changed by the modulator 14, the group refractive index ng also changes, and the resonance frequency interval FSR changes slightly. Thereby, the optical frequency of the measurement optical system 10 changes between Fr1 and Fr2. Let the difference between Fr1 and Fr2 be the modulation width Fs of the optical frequency.

[0047] The absolute value of the optical frequency is approximately 200 THz at a wavelength of 1500 nm. Therefore, even a minute change in the group refractive index such as 0.001% due to the phase change of the modulator 14 can be converted into a change of several GHz in terms of the optical frequency. Thus, it changes in accordance with the phase shift within a width of about 2000 MHz (slightly) with respect to the optical frequency (e.g., 200 THz).

[0048] FIG. 6 is a diagram showing the transmitted optical waveform of the measurement optical system 10. The horizontal axis represents time, and the vertical axis represents the optical frequency. Here, an example of discrete coding between the high-level optical frequency Fr1 and the low-level optical frequency Fr2 is shown. As shown in FIG. 6, the measurement light of the measurement optical system 10 is discretely coded between two levels of the high-level optical frequency Fr1 and the low-level optical frequency Fr2 by the control signal S(t) of the code generator 24. For example, the time width of the narrowest bit in FIG. 6 is, for example, 1 nsec (example of a clock frequency of 1 GHz). Thus, in this embodiment, an FSK (Frequency Shift Keying) transmitted optical waveform that discretely modulates the frequency is generated.

[0049] A configuration example of the modulator 14 will be described using FIGS. 7 to 9. The modulator 14 is a phase modulator and is a phase shifter based on the Plasma Dispersion Effect.

[0050] Figure 7 shows a first configuration example of the modulator 14. The modulator 14 according to the first configuration example is called a carrier accumulation type, and consists of layers of silicon, silicon dioxide, p-type silicon (p-type Si), insulating film, and n-type silicon (n-type Si). A transformer 24a controlled by the control signal S(t) of the code generator 24 applies voltage to the p-type silicon (p-type Si) and n-type silicon (n-type Si). The clock frequency of the transformer 24a is, for example, in the range of 10 MHz to 10 GHz. As a result, the waveguide refractive index in the stacking direction is also changed in the range of 10 MHz to 10 GHz. The modulation width Fs of the optical frequency at this time is also in the range of 10 MHz to 10 GHz. In one example, the modulation width Fs = 120 MHz.

[0051] Furthermore, the optical path length L of the measurement optical system 10 is typically several tens of micrometers to several millimeters. As a result, the light travels back and forth through the measurement optical system 10, which acts as a resonator, on the order of psec. After several round trips, the optical frequency change of the measurement optical system 10 stabilizes, so the modulation of the modulation width Fs becomes on the order of several psec to 100 psec, and the modulation of the measurement optical system 10 becomes steeper with respect to the time width of the narrowest bit (1 nsec).

[0052] Figure 8 shows a second configuration example of the modulator 14. The modulator 14 according to the second configuration example is called a carrier injection type, and silicon, silicon dioxide, and silicon semiconductor layers are stacked. A transformer 24a controlled by the control signal S(t) of the code generator 24 applies a voltage to both ends of the silicon semiconductor layer. The clock frequency of the transformer 24a is, for example, in the range of 10 MHz to 10 GHz. As a result, the waveguide refractive index in the stacking direction is also changed in the range of 10 MHz to 10 GHz. Similarly, the modulation width Fs of the optical frequency at this time is also in the range of 10 MHz to 10 GHz. In one example, the modulation width Fs = 120 MHz.

[0053] Figure 9 shows a third configuration example of the modulator 14. The modulator 14 according to the third configuration example is called a carrier depletion type, and consists of stacked silicon, silicon dioxide, and p-type and n-type silicon semiconductor layers. A transformer 24a, controlled by the control signal S(t) of the code generator 24, applies voltage to both ends of the silicon semiconductor layer. The clock frequency of the transformer 24a is, for example, in the range of 10 MHz to 10 GHz. As a result, the waveguide refractive index in the stacking direction is also changed in the range of 10 MHz to 10 GHz. The modulation width Fs of the optical frequency at this time is also in the range of 10 MHz to 10 GHz. In one example, the modulation width Fs = 120 MHz. Thus, the modulator 14 according to this embodiment can be integrated on a semiconductor substrate using Si photonics.

[0054] Figure 10 shows an example of the characteristics of a phase modulator in a comparative example. The horizontal axis represents voltage, and the vertical axis represents the phase shift amount. In conventional continuous frequency modulation LiDARs (PM RMCW LiDARs), the phase shift amount required for the modulator to ensure sufficient SNR is typically large, around 1π (BPSK). To obtain a shift amount of 1π, for example, the modulator length would need to be 5 mm or more, and the voltage 8 V or more would need to be 8 V or more.

[0055] In a phase modulator, the longer the required phase shift amount [rad], the longer the element length and the greater the parasitic capacitance of the element. Furthermore, the increased applied voltage leads to higher driving power. This increases the time constant, making high-speed operation difficult. In addition, the longer the element length, the greater the loss of light transmitted through the modulator (insertion loss). This optical loss during transmission and reception reduces LiDAR performance (SNR and power consumption). In contrast, the measurement optical system 10 according to this embodiment can obtain Fs = 120 MHz with a shift amount of 0.1 rad = 0.03π. As can be seen, the modulator 14 according to this embodiment is smaller, capable of high-speed operation, and has lower power consumption compared to the phase modulator of the comparative example.

[0056] Figure 11 shows the optical waveforms of the reflected light RX and the local oscillator light LO. The horizontal axis represents time, and the vertical axis represents frequency. Referring again to Figure 1, the measurement light TX (see Figure 4) is irradiated onto the target via the light-emitting unit 18. The reflected light RX, reflected by the target, is incident on the optical processing unit 40. The light-emitting unit 18 is, for example, an optical antenna (Grating Coupler Edge Coupler). Note that the emission of light via the light-emitting unit 18 may be referred to as transmission or irradiation.

[0057] Time ToF is the time from when the measurement light TX is irradiated onto the target until it is incident on the optical processing unit 40 as the return light RX. If the target is moving, a Doppler shift frequency Fd is generated as the frequency difference due to the Doppler shift in the difference between the optical frequency of the measurement light TX and the optical frequency of the return light RX.

[0058] Figure 12 shows an example of the configuration of the optical processing unit 40. The optical processing unit 40 according to this embodiment includes, for example, a 90-degree optical hybrid circuit 40a and balanced photodetectors 40b and 40c. The 90-degree optical hybrid circuit 40a has a delay time in one of the two paths of the local oscillator light LO, and is set so that the optical phase difference between the two paths is π / 2. The 90-degree optical hybrid circuit 40a outputs a pair of optical signals E1 to E2 to the balanced photodetector 40b. Similarly, the 90-degree optical hybrid circuit 40a outputs a pair of optical signals E3 to E4 to the balanced photodetector 40c. The balanced photodetector 40b converts the pair of optical signals E1 to E2 into a real part analog current signal Ii. The balanced photodetector 40c converts the pair of optical signals E3 to E4 into an imaginary part analog current signal Iq.

[0059] Figure 13 shows an example configuration of balanced photodetectors 40b and 40c and a transimpedance amplifier (TIA) 52. The transimpedance amplifier 52 is provided between each of the balanced photodetectors 40b and 40c and the AD conversion circuit 54 (see Figure 1). In other words, the transimpedance amplifier 52 is connected to each of the balanced photodetectors 40b and 40c and the AD conversion circuit 54. The analog current signal Ii output by the balanced photodetector 40b is converted to an analog voltage signal Vi (In-phase). The analog current signal Iq output by the balanced photodetector 40c is converted to an analog voltage signal Vq (Quadrature).

[0060] Figure 14 shows the output of the transimpedance amplifier 52. The horizontal axis represents time, and the vertical axis represents voltage. Next, the AD conversion circuit 54 (see Figure 1) converts the analog voltage signal Vi and the analog voltage signal Vq into digital voltage signals. The signal processing circuit (DSP) 56 (see Figure 1) converts the digital voltage signal Vid and the digital voltage signal Vqd into instantaneous frequency signals.

[0061] Figure 15 shows an example of the instantaneous frequency Sω(t) converted by the signal processing circuit 56. The horizontal axis represents time, and the vertical axis represents the instantaneous frequency. Since it is an interference signal (complex received beat signal) between the local oscillator light LO (measurement light TX) and the return light RX, the frequency has a frequency range of ± modulation width Fs.

[0062] More specifically, the signal processing circuit 56 generates a real part signal Re(t) from the voltage signal Vid and an imaginary part signal Im(t) from the voltage signal Vqd at time t. Subsequently, the signal processing circuit 56 calculates θ(t) = arctan(Im(t) / Re(t)) as the instantaneous phase. Then, the signal processing circuit 56 generates the time change of the instantaneous phase Sω(t) = θ(t) - θ(t - Δt) as the instantaneous frequency, where Δt is the sampling interval.

[0063] The signal processing circuit 56 further calculates the instantaneous frequency of the local oscillator light LO. More specifically, it calculates the instantaneous frequency LOω(t) of the local oscillator light LO using the control signal S(t) of the code generator 24 and the information from the characteristic diagram (see Figure 5). At this time, the signal processing circuit 56 generates the difference between Fr1 and Fr2 as the modulation width Fs of the optical frequency.

[0064] More specifically, the FSK modulation effect of the LO light (which has the same frequency characteristics as the TX light) is removed by multiplying Sω(t) by LOω(t) = k1 × exp(-i * 2πFs * s(t)) using the transmitted code sequence s(t). k1 is a constant for normalization and is a device-specific constant. In this way, the signal processing circuit 56 removes the instantaneous frequency LOω(t) of the local oscillator light LO from the instantaneous frequency Sω(t) in Figure 15 to generate the instantaneous frequency Rtw(t) of the return light.

[0065] Figure 16 shows an example of the instantaneous frequency Rtw(t) of the reflected light. The horizontal axis represents time, and the vertical axis represents instantaneous frequency.

[0066] Next, the signal processing circuit 56 calculates the cross-correlation value between the instantaneous frequency Rtw(t) shown in Figure 16 and the transmitted code sequence s(t). T is the delay time of Rtw(t). Figure 17 shows an example of the cross-correlation value between the instantaneous frequency Rtw(t) and k1 × exp(-i * 2πFs * s(t)). The horizontal axis is the delay time T, and the vertical axis is the cross-correlation value. The signal processing circuit 56 generates the maximum value of the cross-correlation value as, for example, the round-trip time of light to the target (ToF).

[0067] Next, the signal processing circuit 56 separates and generates the Doppler component by multiplying the instantaneous frequency Rtw(t) of the returned light by k2 × exp(i * 2πFs * s(t - ToF)). k2 is a normalization constant and is a device-specific constant. k2 × exp(i * 2πFs * s(t - ToF)) corresponds to the time ToF delay component of the instantaneous frequency LOω(t).

[0068] Figure 18 shows an example of the results of a Fourier transform on the Doppler component generated by the signal processing circuit 56. The horizontal axis represents frequency, and the vertical axis shows an example of the instantaneous frequency Rtw(t) of the reflected light. The horizontal axis represents time, and the vertical axis represents frequency components. The signal processing circuit 56 sets the maximum value of the frequency components as the Doppler frequency Fd. Then, the signal processing circuit 56 calculates the target's moving speed based on the Doppler frequency Fd.

[0069] Figure 19 shows an example of the configuration of the light receiving device 1 in an integrated circuit. The light receiving device 1 has a photonic integrated circuit (PIC) 2 made of silicon (Si) photonics and an electronic integrated circuit (EIC) 3. The photonic integrated circuit 2 has a modulator 14, a beam splitter 16, a light-emitting unit 18, a light-receiving unit 30, and an optical processing unit 40. On the other hand, the electronic integrated circuit 3 has a transimpedance amplifier (TIA) 52, an AD conversion circuit 54, a signal processing circuit (DSP) 56, a control unit 60, and a memory 70. In this way, the light receiving device 1 according to this embodiment can be made smaller and thinner through integration.

[0070] Figure 20 is a flowchart showing an example of the processing of the signal processing circuit 56. As shown in Figure 20, first, the signal processing circuit 56 obtains the phase shifts pH1 and pH2 from the parameter information stored in the memory 70 (step S100). In this case, the temperature of the light receiving device 1 may also be measured.

[0071] Next, the signal processing circuit 56 generates a modulation width Fs using a table (see Figure 5) showing the relationship between the phase shift amount and the optical frequency stored in the memory 70, and stores it in the memory 70 (step S102). In this case, it is also possible to generate a modulation width Fs according to the temperature of the photodetector 1 using, for example, a table showing the relationship between the phase shift amount and the optical frequency for different temperatures.

[0072] Next, the signal processing circuit 56 acquires the transmission code sequence s(t) stored in the memory 70 (step S104). Subsequently, the signal processing circuit 56 acquires the digital voltage signal Vid(t) and the digital voltage signal Vqd(t) stored in the memory 70 and converts them into an instantaneous frequency signal Sω(t) (step S106).

[0073] Next, the signal processing circuit 56 uses the modulation width Fs and the transmitted code sequence s(t) stored in the memory 70 to generate the instantaneous frequency LOω(t) of the local oscillator light LO, removes it from the instantaneous frequency signal Sω(t), and stores it in the memory 70 as the instantaneous frequency Rtw(t) of the return light. (Step S108).

[0074] Next, the signal processing circuit 56 calculates the cross-correlation between the instantaneous frequency Rtw(t) of the reflected light and the instantaneous frequency LOω(t) of the local oscillator light LO, and detects the peak value (step S110). Subsequently, the signal processing circuit 56 stores the time corresponding to the peak value as ToF (Time of Flight) in the memory 70 (step S112). Then, the signal processing circuit 56 generates the distance to the object to be measured using ToF and stores it in the memory 70 (step S114).

[0075] Next, the signal processing circuit 56 removes the time ToF component of the instantaneous frequency LOω(t) from the instantaneous frequency Rtw(t) of the returned light to generate a Doppler component, which is stored in the memory 70 (step S116). Subsequently, the signal processing circuit 56 performs a Fourier transform on the Doppler component, detects a peak by spectral analysis (FFT, etc.), and stores it in the memory 70 as the Doppler frequency Fd (step S118). Then, the signal processing circuit 56 calculates the moving speed of the object to be measured based on the Doppler frequency Fd (step S120). In this way, by irradiating the object to be measured (Target) with measurement light TX that discretely fluctuates to multiple frequencies, the distance to the object and the moving speed can be generated without being affected by internal reflection of the light-emitting unit 18.

[0076] As described above, the measurement optical system 10 according to this embodiment is configured to have a Fabry-Perot resonator that resonates between the left end face of the semiconductor optical amplifier and the reflective surface of the light-emitting unit 18. In this way, since the reflection at the reflective surface of the light-emitting unit 18 is included in the resonance condition, the reflection at the reflective surface of the light-emitting unit 18 can suppress unintended adverse effects on the laser oscillation frequency of the measurement optical system 10. As a result, the measurement of the distance to the object to be measured and the moving speed using the measurement optical system 10 according to this embodiment can be performed with higher accuracy.

[0077] (Second Embodiment) The light receiving device 1 according to the second embodiment differs from the light receiving device 1 according to the first embodiment in that it performs the transmission and reception of the measurement light TX and the return light RX using a single light transmitting and receiving unit 18a. The differences from the light receiving device 1 according to the first embodiment will be described below.

[0078] Figure 21 is a schematic diagram showing an example of a light receiving device 1 according to the second embodiment. The light receiving device 1 according to the second embodiment differs from the light receiving device 1 according to the first embodiment in that the measurement optical system 10 includes an external cavity laser (ECL) 12, a modulator (PM in this embodiment) modulator, a 2x2 coupler 16a, and a light transmitting and receiving unit 18a.

[0079] The 2x2 coupler 16a guides one of the output beams from the modulator 14 as the measurement beam TX to the transmitting / receiving unit 18a, and the other beam as the local oscillation beam LO to the optical processing unit 40. In other words, the transmitting / receiving unit 18a irradiates the object to be measured with one of the output beams from the modulator 14 as the measurement beam TX.

[0080] Furthermore, the 2x2 coupler 16a guides one of the return light RX from the object to be measured, which has entered the light transmitting / receiving unit 18a, to the optical processing unit 40. The 2x2 coupler suppresses light loss by stabilizing the branching ratio. The processing after guiding the light to the optical processing unit 40 is the same as that of the light receiving device 1 according to the first embodiment.

[0081] As explained above, since the transmission and reception of the measurement light TX and the return light RX are performed by a single light transmitting and receiving unit 18a, further miniaturization is possible.

[0082] (Third Embodiment) The light receiving device 1 according to the third embodiment differs from the light receiving device 1 according to the first embodiment in that the modulator 14 is composed of an intensity modulator. The differences from the light receiving device 1 according to the first embodiment will be explained below.

[0083] Figure 22 schematically shows an example of a light receiving device 1 according to the third embodiment. The light receiving device 1 according to the third embodiment differs from the light receiving device 1 according to the first embodiment in that the modulator 14 is composed of an intensity modulator.

[0084] Figure 23 shows the relationship between the phase shift amount by modulator 14a and the relative optical frequency of the measurement optical system 10. The vertical axis represents the relative optical frequency [MHz] of the measurement optical system 10, and the horizontal axis represents the intensity shift amount of modulator 14a. Modulator 14a is, for example, an intensity modulator (AM).

[0085] As shown in Figure 23, the modulator 14a is discretely controlled between intensities Aa1 and Aa2 by the control signal S(t) of the code generator 24. That is, when the light intensity changes due to the modulator 14a, the transmittance and full width at half maximum of the optical frequency filter also change, and the resonant frequency interval FSR changes slightly. As a result, the optical frequency of the measurement optical system 10 changes between Fra1 and Fra2. The difference between Fra1 and Fra2 is taken as the modulation width Fs of the optical frequency. In addition, the characteristic values ​​shown in Figure 23 are stored as a table in the memory 70. The processing after the modulation of the optical frequency by the modulator 14a is the same as that of the light receiving device 1 according to the first embodiment.

[0086] As described above, the measurement optical system 10 is configured as a Fabry-Perot resonator that resonates between the left end face of the semiconductor optical amplifier and the reflective surface of the light-emitting unit 18, and the modulator 14a is configured as an intensity modulator (AM). As a result, since the reflection at the reflective surface of the light-emitting unit 18 is included in the resonance condition, the frequency of the measurement light can be discretely changed while suppressing the influence of the reflection at the reflective surface of the light-emitting unit 18 on the laser oscillation frequency of the measurement optical system 10.

[0087] (Fourth Embodiment) The light receiving device 1 according to the fourth embodiment differs from the light receiving device 1 according to the first embodiment in that it changes the frequency modulation of the measurement optical system 10 by controlling the gain current of the reflective semiconductor optical amplifier (RSOA) 120. The differences from the light receiving device 1 according to the first embodiment will be explained below.

[0088] Figure 24 is a schematic diagram showing an example of a light-receiving device 1 according to the fourth embodiment. The light-receiving device 1 according to the fifth embodiment differs from the light-receiving device 1 according to the first embodiment in that it has a gain control unit (Bias Tee) 23 that controls the gain current of the reflective semiconductor optical amplifier (RSOA) 120 (see Figure 3).

[0089] Figure 25 shows the relationship between the gain current from the gain control unit 23 and the relative optical frequency of the measurement optical system 10. The vertical axis represents the relative optical frequency [MHz] of the measurement optical system 10 and the gain current from the gain control unit 23.

[0090] As shown in Figure 25, the gain control unit 23 discretely controls the gain current between Ihb1 and Ihb2 by the control signal S(t) of the code generator 24. That is, if the gain current changes due to the gain control unit 23, the group refractive index ng also changes, and the resonant frequency interval FSR changes slightly. Thus, the resonator length L and group refractive index ng shown in Figure 2 include the optical path length and group refractive index ng inside the reflective semiconductor optical amplifier (RSOA) 120, so the oscillation optical frequency changes by changing the gain current. As a result, the optical frequency of the measurement optical system 10 changes between Frb1 and Frb2. The difference between Frb1 and Frb2 is defined as the modulation width Fs of the optical frequency. In addition, the characteristic values ​​shown in Figure 25 are stored as a table in the memory 70.

[0091] As described above, the measurement optical system 10 is configured as a Fabry-Perot resonator that resonates between the left end face of the semiconductor optical amplifier and the reflective surface of the light-emitting unit 18, and this constitutes the gain control unit 23. As a result, since the reflection at the reflective surface of the light-emitting unit 18 is included in the resonance condition, the frequency of the measurement light can be discretely changed while suppressing the influence of the reflection at the reflective surface of the light-emitting unit 18 on the laser oscillation frequency of the measurement optical system 10.

[0092] (Fifth Embodiment) The light receiving device 1 according to the fifth embodiment differs from the light receiving device 1 according to the first embodiment in that the optical processing unit 40 is composed of a single coupler and a single photodiode (PD). The differences from the light receiving device 1 according to the first embodiment will be described below.

[0093] Figure 26 is a schematic diagram showing an example of a light receiving device 1 according to the fifth embodiment. The light receiving device 1 according to the fifth embodiment differs from the light receiving device 1 according to the first embodiment in that the optical processing unit 40 is composed of a single coupler and a single photodiode (PD).

[0094] For example, the velocity range of the object being measured (target) may be known, and the Doppler shift frequency Fd may be a positive value and subject to the constraint Fd > Fs. In such cases, the received signal will only contain positive frequency components. Therefore, under these limited conditions, even if only a real signal is obtained as shown in Figure 17, the imaginary part of the signal can be estimated by Hilbert transform of the real signal, etc., and a complex signal can be obtained. As a result, even with a simplified optical processing unit 40, distance measurement and velocity measurement equivalent to that of the light receiving device 1 according to the first embodiment can be obtained.

[0095] (Sixth Embodiment) The light receiving device 1 according to the sixth embodiment differs from the light receiving device 1 according to the first embodiment in that it further comprises a local light source 80 that emits a local oscillator light LO that is not frequency modulated. The differences from the light receiving device 1 according to the first embodiment will be described below.

[0096] Figure 27 is a schematic diagram showing an example of a light receiving device 1 according to the sixth embodiment. The light receiving device 1 according to the sixth embodiment further includes a local light source 80 and a drive control circuit 25 for the local light source 80. It also has a beam splitter 90 for splitting the oscillating light LO of the local light source 80. Furthermore, it has an optical processing unit 42 used for monitoring the measurement light TX.

[0097] The local light source 80 emits laser light independently at a different constant wavelength from the external resonant laser 12. That is, the drive control circuit 25 causes the local light source 80 to oscillate so that its optical frequency is a constant frequency different from the measurement light TX. Since the local light source 80 has no direct connection to the modulator 14 and the light-emitting unit 18 in the optical path, it is not affected by optical frequency modulation due to reflected light.

[0098] The optical processing unit 40 optically processes the interference light between the return light RX and the oscillation light LO of the local light source 80. Since the LO light is not modulated, a beat signal similar to that in Figure 16 is obtained. Therefore, there is no need to remove the modulation effect of the oscillation light LO, which further reduces the processing cost of the signal processing circuit 56.

[0099] The optical processing unit 42 optically processes the interference light between the measurement light TX and the oscillation light LO of the local light source 80. Since the LO light is not modulated, the processing cost of the signal processing circuit 56 can be further reduced.

[0100] Figure 28 shows the characteristics of the oscillating light LO and the measuring light TX. The horizontal axis represents time, and the vertical axis represents optical frequency. The oscillating light LO has a constant frequency. As a result, the control unit 60 can perform feedback control to each laser driver circuit so that the difference in optical frequency between the oscillating light LO and the measuring light TX is kept constant, according to the calculated value of the signal processing circuit 56. Even if the frequency of each laser fluctuates due to the influence of the surrounding environment, it is possible to stabilize it.

[0101] (Seventh Embodiment) The light receiving device 1 according to the seventh embodiment differs from the light receiving device 1 according to the first embodiment in that it includes an optical processing unit 44 that provides a delay difference in the light guide path to cause interference. The differences from the light receiving device 1 according to the first embodiment will be explained below.

[0102] Figure 29 is a schematic diagram showing an example of a light receiving device 1 according to the seventh embodiment. The light receiving device 1 according to the seventh embodiment differs from the light receiving device 1 according to the first embodiment in that it includes an optical processing unit 44 that causes interference by providing a delay difference in the light guide path.

[0103] As a result, even when the modulation width Fs becomes unstable and fluctuates due to changes in the surrounding environment, high LiDAR performance can be achieved without degradation of SNR by performing received signal processing using the measured transmitted optical modulation information obtained from the optical processing unit 44.

[0104] (Eighth Embodiment) The drive control unit 20 of the light receiving device 1 according to the eighth embodiment differs from the light receiving device 1 according to the first to eighth embodiments in that it discretely changes the frequency level of the measurement light TX in three or more steps.

[0105] Figure 30 shows an example of the measurement light TX according to the eighth embodiment. The horizontal axis represents time, and the vertical axis represents optical frequency. In this way, by discretely changing the frequency level of the measurement light TX in three or more steps, it becomes possible to obtain cross-correlation measurements with higher accuracy.

[0106] At least a part of the light receiving device 1 described in the above-described embodiment may be made of hardware or software. If it is made of software, a program that realizes at least a part of the functions of the light receiving device 1 may be stored on a recording medium such as a flexible disk or CD-ROM, and loaded into a computer and executed. The recording medium is not limited to removable ones such as magnetic disks or optical disks, but may also be a fixed recording medium such as a hard disk drive or memory.

[0107] Furthermore, a program that implements at least some of the functions of the light receiving device 1 may be distributed via a communication line such as the Internet (including wireless communication). In addition, the program may be encrypted, modulated, or compressed and distributed via a wired or wireless line such as the Internet, or stored on a recording medium.

[0108] Furthermore, this technology can take the following configuration.

[0109] (1) A light receiving device comprising: a measuring optical system that generates measuring light whose frequency is shifted at least two discrete levels or more; and an optical processing unit that interferes a predetermined oscillating light with the reflected light from the object to be measured.

[0110] (2) The light receiving device according to (1), wherein the measuring optical system has a light-emitting unit that transmits the measuring light.

[0111] (3) The light receiving device according to (2), wherein the measuring optical system resonates with the laser at least by internal reflection of the light-emitting part and transmits the measuring light.

[0112] (4) The light receiving device according to (3), wherein the measuring optical system transmits the measurement light by laser resonance between the end face of the semiconductor optical amplifier and the inner surface of the light-emitting part.

[0113] (5) The light receiving device according to (4), further comprising a modulator disposed in the waveguide of the measuring optical system and used to shift the frequency by at least two levels or more.

[0114] (6) The photodetector described in (5), wherein the modulator is a phase modulator.

[0115] (7) The photodetector described in (6), wherein the phase modulator is a phase shifter based on the plasma dispersion effect.

[0116] (8) The light receiving device described in (5), wherein the modulator is an intensity modulator.

[0117] (9) The light receiving device according to (5), further comprising: a signal processing unit capable of generating at least one of the distance to the object to be measured and the velocity based on the output signal of the optical processing unit; and a drive control unit that controls at least one of the modulator and the semiconductor optical amplifier.

[0118] (10) The light receiving device according to (9), wherein the measuring optical system further comprises a microring resonator disposed in the waveguide of the measuring optical system.

[0119] (11) The light receiving device according to (10), wherein the drive control unit controls the gain current of the semiconductor optical amplifier.

[0120] (12) The light receiving device according to (11), wherein the drive control unit controls at least one of the modulator and the semiconductor optical amplifier according to a time-series code.

[0121] (13) The light receiving device according to (12), wherein the optical processing unit includes a 90-degree optical hybrid circuit that takes at least a portion of the predetermined oscillating light and the returned light as inputs and converts them into four output lights, and two balanced photodetectors that convert the four output lights into a real analog current and an imaginary analog current.

[0122] (14) The light receiving device according to (13), wherein the predetermined oscillating light is at least a part of the measuring light.

[0123] (15) The light receiving device according to (13), wherein the predetermined oscillating light is emitted from an independent light source different from the light source of the measuring optical system.

[0124] (16) The light receiving device according to (15), further comprising a second light processing unit that interferes the predetermined oscillating light with the measurement light.

[0125] (17) The light receiving device according to (1), further comprising a third light processing unit that interferes the measurement light with light obtained by delaying the measurement light.

[0126] (18) A light receiving method comprising: a measurement light generation step of generating measurement light whose frequency is shifted at least discretely by two levels or more; and a light processing step of interfering a predetermined oscillating light with the reflected light from the object to be measured of the measurement light.

[0127] (19) A measuring light generation device comprising: a gain medium that spontaneously emits photons when an electric current is passed through it; and a resonator that resonates the photons between one reflective surface and the other inner reflective surface of a light-emitting part that transmits measuring light, thereby generating measuring light whose frequency is shifted at least discretely by two levels or more.

[0128] (20) The measuring light generation apparatus according to (19), further comprising: a modulator that changes the group refractive index in the waveguide between the reflecting surface and the internal reflecting surface; and a drive control unit that controls the current and at least one of the modulator according to a time-series sign.

[0129] The aspects of this disclosure are not limited to the individual embodiments described above, but include various modifications that a person skilled in the art could conceive, and the effects of this disclosure are not limited to those described above. In other words, various additions, modifications, and partial deletions are possible, as long as they do not depart from the conceptual idea and spirit of this disclosure derived from the claims and their equivalents.

[0130] 1: Photodetector, 10: Measurement optical system, 12: External resonant laser, 14, 14a: Modulator, 18: Light-emitting unit, 20: Drive control unit, 40, 42, 44: Optical processing unit, 40b, 40c: Balanced photodetector, 50: Signal processing unit, 120: Reflective semiconductor optical amplifier, 124: First ring resonator, 126: Second ring resonator

Claims

1. A light receiving device comprising: a measuring optical system that generates measuring light whose frequency is shifted at least two discrete levels or more; and an optical processing unit that interferes a predetermined oscillating light with the reflected light from the object to be measured.

2. The light receiving device according to claim 1, wherein the measuring optical system has a light-emitting unit that transmits the measuring light.

3. The light receiving device according to claim 2, wherein the measuring optical system resonates with the laser at least by internal reflection of the light-emitting part and transmits the measurement light.

4. The light receiving device according to claim 3, wherein the measurement optical system transmits the measurement light by laser resonance between the end face of the semiconductor optical amplifier and the inner surface of the light-emitting part.

5. The light receiving device according to claim 4, further comprising a modulator disposed in the waveguide of the measuring optical system and used to shift the frequency by at least two levels or more.

6. The light receiving device according to claim 5, wherein the modulator is a phase modulator.

7. The photodetector according to claim 6, wherein the phase modulator is a phase shifter based on the plasma dispersion effect.

8. The light receiving device according to claim 5, wherein the modulator is an intensity modulator.

9. The light receiving device according to claim 5, further comprising: a signal processing unit capable of generating at least one of the distance to the object to be measured and the velocity based on the output signal of the optical processing unit; and a drive control unit that controls at least one of the modulator and the semiconductor optical amplifier.

10. The light receiving device according to claim 9, wherein the measuring optical system further comprises a microring resonator disposed in the waveguide of the measuring optical system.

11. The light receiving device according to claim 10, wherein the drive control unit controls the gain current of the semiconductor optical amplifier.

12. The light receiving device according to claim 11, wherein the drive control unit controls at least one of the modulator and the semiconductor optical amplifier according to a time-series code.

13. The light receiving device according to claim 12, wherein the optical processing unit comprises a 90-degree optical hybrid circuit that takes at least a portion of the predetermined oscillating light and the returned light as inputs and converts them into four output lights, and two balanced photodetectors that convert the four output lights into a real analog current and an imaginary analog current.

14. The light receiving device according to claim 13, wherein the predetermined oscillating light is at least a part of the measuring light.

15. The light receiving device according to claim 13, wherein the predetermined oscillating light is emitted from an independent light source different from the light source of the measuring optical system.

16. The light receiving device according to claim 15, further comprising a second light processing unit that interferes the predetermined oscillating light with the measurement light.

17. The light receiving device according to claim 1, further comprising a third light processing unit that interferes the measurement light with light obtained by delaying the measurement light.

18. A light receiving method comprising: a measurement light generation step that generates measurement light whose frequency is shifted at least two discrete levels or more; and a light processing step that interferes a predetermined oscillating light with the reflected light from the object to be measured.

19. A measurement light generation device comprising: a gain medium that spontaneously emits photons when an electric current is passed through it; and a resonator that resonates the photons between one reflective surface and the other inner reflective surface of a light-emitting part that transmits measurement light, thereby generating measurement light whose frequency is shifted at least discretely by two levels or more.

20. The measuring light generation apparatus according to claim 19, further comprising: a modulator that changes the group refractive index in the waveguide between the reflective surface and the internal reflective surface; and a drive control unit that controls at least one of the current and the modulator according to a time-series sign.

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