A parallel FMCW lidar transmitting device and method in the mid- and far-infrared bands

Abstract

The present invention relates to the technical field of lidar, and more specifically, to a parallel FMCW lidar transmitting device and method in the mid- and far-infrared wavelength bands. The lidar transmitting device includes a laser, an electro-optic modulator, an arbitrary function generator, a chalcogenide chip, a diffraction grating, and an optical fiber link; the output port of the laser is connected to the light source input port of the electro-optic modulator, the waveform output port of the arbitrary function generator is connected to the microwave signal input port of the electro-optic modulator, and both ends of the chalcogenide chip are respectively connected to the optical signal output port of the electro-optic modulator and the diffraction grating through lens optical fibers. The present invention utilizes the cascaded four-wave mixing effect of the chalcogenide microcavity to convert a single FMCW into a mid- and far-infrared frequency comb light source, reducing the complexity of the linear frequency modulation narrow linewidth laser technology and greatly improving the transmission rate.

Description

Technical Field

[0001] The present invention relates to the technical field of lidar, and more specifically, to a parallel FMCW lidar transmitting device and method in the mid- and far-infrared wavelength bands. Background Art

[0002] Most lidars on the market currently use the "time-of-flight - TOF" technology, that is, discrete optical pulses are emitted, and a photodetector is used to detect the returned optical power to calculate the distance. The advantages of TOF technology are very obvious, with mature technology, short development cycle, and low cost. However, this direct detection method has problems such as poor anti-interference ability and short detection distance, and it is difficult to meet the requirements of automotive-grade lidars. Lidars based on the frequency-modulated continuous wave (FMCW) scheme can achieve coherent detection. For example, Patent CN111239754A, on June 5, 2020, discloses a lidar system based on tunable frequency continuous wave and its imaging method, which can effectively overcome the problems existing in TOF. However, due to the complexity of the current precise linear frequency modulation technology, it is difficult for FMCW lidars to transmit in parallel. The principle of FMCW lidar: Due to the Doppler effect, there is a frequency difference between the transmitted chirp signal (green) and the reflected chirp signal (blue), and the speed and distance information are related to the frequency difference. Coherent detection can be used to obtain the frequency difference information, thereby obtaining the distance and speed of each pixel point. This coherent detection based on FMCW lidar has many inherent advantages, such as enhanced range resolution, direct speed detection using the Doppler effect, and avoidance of sunlight glare and interference. However, the measurement accuracy is very sensitive to the linearity of the chirp slope, and coherent detection requires a high coherence of the light source. This technology for precisely controlling linear frequency modulation narrow linewidth lasers is very complex, which causes great difficulties in the realization of parallel measurement for FMCW lidars. Summary of the Invention

[0003] In order to overcome at least one of the above-mentioned defects in the prior art, the present invention provides a parallel FMCW lidar transmitting device and method in the mid- and far-infrared wavelength bands, which reduces the complexity of the device, effectively improves the laser emission rate, and helps to improve the measurement rate of the lidar.

[0004] To solve the above technical problems, the technical solution adopted by the present invention is: A parallel FMCW lidar transmitting device in the mid- and far-infrared wavelength bands, comprising a laser, an electro-optic modulator, an arbitrary function generator, a chalcogenide chip, a diffraction grating, and an optical fiber link; the output port of the laser is connected to the light source input port of the electro-optic modulator, the waveform output port of the arbitrary function generator is connected to the microwave signal input port of the electro-optic modulator, and both ends of the chalcogenide chip are respectively connected to the optical signal output port of the electro-optic modulator and the diffraction grating through lens optical fibers.

[0005] In one embodiment, the chalcogenide chip includes a substrate, a chalcogenide microring resonator, and a bus straight waveguide. The chalcogenide microring resonator and the bus straight waveguide are both disposed on the top of the substrate, and the chalcogenide microring resonator is coupled to the bus straight waveguide. Due to the dual balance of dispersion, nonlinearity, cavity pumping, and loss in the chalcogenide chip, continuous-wave laser is converted into a stable optical pulse sequence, generating a stable mid-infrared and far-infrared band soliton frequency comb.

[0006] In one embodiment, the microring radius value of the chalcogenide microring resonator is 50um to 200um, the microring thickness is 0.7um to 1um, the width of the microring is 1.9um to 2.5um, and the free spectral range (FSR) of the cavity is 130GHz to 520GHz.

[0007] In one embodiment, both ends of the chalcogenide chip are coupled to lens optical fibers through inverse tapered waveguides respectively.

[0008] In one embodiment, the length value of the inverse tapered waveguide is 200um to 500um, and the tip width value is 100nm to 150nm, achieving efficient coupling of the fiber waveguide.

[0009] In one embodiment, the laser output by the laser is a narrow linewidth light source in the mid-infrared and far-infrared band, with a central wavelength of 9.5um to 11um. Compared with traditional 905nm and 1550nm laser light sources, its laser transmittance is increased by more than 1 time in rainy, foggy and other weather conditions.

[0010] In one embodiment, the waveform generated by the arbitrary function generator is a linearly frequency-modulated signal of a triangular wave.

[0011] In one embodiment, the bandwidth of the linearly frequency-modulated signal of the triangular wave is 1GHz - 5GHz, and the modulation rate is 100KHz - 10MHz.

[0012] In one embodiment, the diffraction grating has 80 - 120 grooves per millimeter, and performs spectral diffraction on the mid-infrared frequency comb light source.

[0013] The present invention also provides a parallel FMCW lidar emission method in the mid-infrared and far-infrared band, using the above-mentioned parallel FMCW lidar emission device in the mid-infrared and far-infrared band, which specifically includes the following steps:

[0014] The laser emits pump light into the electro-optic modulator, and at the same time, the arbitrary function generator generates a specific waveform and enters the electro-optic modulator to perform frequency chirp modulation on the pump light;

[0015] The modulated light passes through the chalcogenide chip, and the cascaded four-wave mixing effect of the chalcogenide chip is used to generate a soliton frequency comb, transmit the chirped laser to all generated comb teeth without distortion, and finally generate a stable optical pulse sequence required for lidar.

[0016] The modulated light is diffracted to various places in space through a diffraction grating to realize object detection.

[0017] Compared with the prior art, the beneficial effects are as follows: A parallel FMCW lidar transmitting device and method in the mid-infrared and far-infrared bands provided by the present invention utilize the cascaded four-wave mixing effect of the chalcogenide microcavity to convert a single FMCW into a mid-infrared and far-infrared frequency comb light source (parallel FMCW), reducing the complexity of the linear frequency modulation narrow linewidth laser technology and greatly improving the transmission rate. At the same time, the light source adopted is in the mid-infrared and far-infrared bands. Compared with the traditional lidar light sources of 905 nm and 1550 nm, the attenuation is small in rainy, snowy and other weather conditions, which helps to improve the detection distance and safety performance of the lidar. Description of the Drawings

[0018] Figure 1 It is a schematic diagram of the connection relationship of the lidar transmitting device of the present invention.

[0019] Figure 2 It is a schematic diagram of the ranging and velocity measurement principle of the FMCW lidar.

[0020] Figure 3 It is a simulation diagram of the light transmittance of the light source of the present invention (taking foggy weather as an example).

[0021] Figure 4 It is a schematic diagram of the preparation process of the chalcogenide chip in the embodiment of the present invention.

[0022] Figure 5 It is a schematic diagram of the soliton frequency comb generated by the chalcogenide chip of the present invention.

[0023] Figure 6 It is a top view schematic diagram of the chalcogenide chip of the present invention.

[0024] Figure 7 It is a SEM structure diagram of the chalcogenide chip of the present invention. Detailed Embodiments

[0025] The drawings are only for illustrative purposes and cannot be construed as limitations on the present invention. For better illustration of this embodiment, some components in the drawings will be omitted, enlarged or reduced, which do not represent the dimensions of the actual product. For those skilled in the art, it is understandable that some well-known structures and their descriptions in the drawings may be omitted. The positional relationships described in the drawings are only for illustrative purposes and cannot be construed as limitations on the present invention.

[0026] As Figure 1As shown in the figure, a parallel FMCW lidar transmitting device in the mid- and far-infrared band includes a laser 1, an electro-optic modulator 2, an arbitrary function generator 3, a chalcogenide chip 4, a diffraction grating 5, and an optical fiber link; the output port of the laser 1 is connected to the light source input port of the electro-optic modulator 2, the waveform output port of the arbitrary function generator 3 is connected to the microwave signal input port of the electro-optic modulator 2, and both ends of the chalcogenide chip 4 are connected to the optical signal output port of the electro-optic modulator 2 and the diffraction grating 5 through lens optical fibers respectively.

[0027] In one embodiment, as Figure 6 and Figure 7 shown, the chalcogenide chip 4 includes a substrate 41, a chalcogenide micro-ring resonator 42, and a bus straight waveguide 43. The chalcogenide micro-ring resonator 42 and the bus straight waveguide 43 are both disposed on the top of the substrate 41, and the chalcogenide micro-ring resonator 42 is coupled with the bus straight waveguide 43. The core of the lidar transmitting device provided by the present invention is to utilize the Kerr dissipation effect of the chalcogenide chip 4 to transmit the frequency chirp characteristic of the single-frequency triangular wave frequency-modulated continuous wave to each generated comb tooth without distortion. As Figure 5 shown, under the condition of double balance of dispersion, nonlinearity, cavity pumping, and loss, the continuous-wave laser in the time domain is converted into a stable optical pulse sequence, thereby generating a stable mid- and far-infrared band soliton frequency comb. As Figure 4 shown, the preparation process of the chalcogenide chip 4 includes the following steps:

[0028] 1. Deposition: Deposit a chalcogenide thin film on the silicon oxide undercladding. The deposition method is thermal evaporation, electron beam evaporation, or magnetron sputtering. The deposition speed does not exceed 5 nm / minute, and the deposition thickness is 800 nm;

[0029] 2. Glue coating: Spin-coat an electron beam resist on the high-nonlinearity and low-loss chalcogenide thin film. The polymer electron resist is any one of polymethyl methacrylate PMMA, ARP, ZEP, and NR-9. The thickness of the electron resist is 1.5 μm;

[0030] 3. Lithography: Expose the pattern layer required for the electron beam resist, and after the exposure is completed, put the sample into the developer to develop and remove the electron beam resist in the exposed area to form the required electron beam resist pattern layer;

[0031] 4. Etching: Put the sample into a reactive ion etching machine, perform ion bombardment and ion reaction etching on the sample, and transfer the electron beam resist pattern layer to the chalcogenide thin film to form the chalcogenide micro-ring resonator 42 and the residual electron beam resist pattern;

[0032] 5. Residual glue removal: Remove the polymer electron resist through a remover to obtain the chalcogenide micro-ring resonator 42. The remover is acetone or 1165 remover;

[0033] 6. Thermal reflux: The sample is sealed and placed in an annealing furnace, which has a thermal reflux effect on the sidewall of the chalcogenide micro-ring resonator 42, making the sidewall smooth, reducing waveguide loss, and thus improving the quality factor.

[0034] In one embodiment, the micro-ring radius value of the chalcogenide micro-ring resonator 42 is 50 um to 200 um, the micro-ring thickness is 0.7 um to 1 um, the width of the micro-ring is 1.9 um to 2.5 um, the cavity free spectral range FSR is 130 GHz to 520 GHz, and the Q value reaches 10 to the 6th power.

[0035] In one embodiment, both ends of the chalcogenide chip 4 are respectively coupled with lens optical fibers through inverse tapered waveguides; the length value of the inverse tapered waveguide is 200 um to 500 um, and the tip width value is 100 nm to 150 nm, realizing efficient coupling of the fiber waveguide.

[0036] In one embodiment, the laser output by the laser 1 is a narrow linewidth light source, in the mid-infrared to far-infrared band, and the central wavelength is 9.5 um to 11 um. Compared with traditional 905 nm and 1550 nm laser light sources, its laser transmittance increases by more than 1 time in rainy, foggy and other weather conditions. The traditional lidar uses single-frequency light sources in the bands of 905 nm and 1550 nm, and the transmittance of this light source drops significantly in rainy, snowy, foggy and other weather conditions. As Figure 3 shown (PcModwin 3.7 simulation), at a distance of 200 meters from the ground, the transmittance is only 20%, extremely deteriorating the applicability of the lidar. Using the 10.5 um mid-infrared to far-infrared band, the transmittance can reach more than 60% under the same conditions, which is more than 3 times that of the traditional lidar light source, helping to improve the detection distance and safety performance of the lidar.

[0037] In one embodiment, the waveform generated by the arbitrary function generator 3 is a linearly frequency-modulated signal of a triangular wave, with a bandwidth of 1 GHz - 5 GHz and a modulation rate of 100 KHz - 10 MHz.

[0038] In one embodiment, the diffraction grating 5 has 80 - 120 grooves per millimeter, and performs spectral diffraction on the mid-infrared to far-infrared frequency comb light source.

[0039] In another embodiment, a method for transmitting a mid-infrared to far-infrared band parallel FMCW lidar is also provided. Using the above-mentioned parallel FMCW lidar transmitting device in the mid-infrared to far-infrared band, the specific implementation steps include:

[0040] The laser 1 emits pump light with a wavelength of 10.5 μm into the electro-optic modulator 2. At the same time, an arbitrary function generator 3 generates a triangular waveform frequency modulation signal and enters the electro-optic modulator 2 to perform frequency chirp modulation on the pump light. After modulation, the pump light becomes a triangular wave frequency modulation continuous wave with a bandwidth of 1.5 GHz and a modulation rate of 100 kHz. Then, the modulated light enters the chalcogenide chip 4 through a lens optical fiber, and a soliton frequency comb (with 30 teeth within its 3 dB bandwidth) is generated by using the Kerr dissipation effect. Its frequency chirp characteristic is transmitted to all excited frequency comb teeth without distortion. Finally, through a diffraction grating 5, the multi-channel FCMW light source is distributed and diffracted to various places in space to perform parallel detection of the object distance and speed. Theoretically calculated, the emission pulse rate is increased by an order of magnitude.

[0041] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limitations of the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

[0042] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention and are not limitations on the implementation manners of the present invention. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation manners here. Any modifications, equivalent substitutions, and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the claims of the present invention.

Claims

1. A parallel FMCW lidar emission method in the mid- and far-infrared band, characterized in that, Parallel FMCW lidar transmitter using the mid-infrared band. The lidar transmitter includes: a laser (1), an electro-optic modulator (2), an arbitrary function generator (3), a chalcogenide chip (4), a diffraction grating (5), and an optical fiber link; the output port of the laser (1) is connected to the light source input port of the electro-optic modulator (2), the waveform output port of the arbitrary function generator (3) is connected to the microwave signal input port of the electro-optic modulator (2), and both ends of the chalcogenide chip (4) are respectively connected to the optical signal output port of the electro-optic modulator (2) and the diffraction grating (5) through lens optical fibers; the chalcogenide chip (4) includes a substrate (41), a chalcogenide micro-ring resonator (42), and a bus straight waveguide (43), the chalcogenide micro-ring resonator (42) and the bus straight waveguide (43) are both disposed on the top of the substrate (41), and the chalcogenide micro-ring resonator (42) is coupled to the bus straight waveguide (43); both ends of the chalcogenide chip (4) are respectively coupled to the lens optical fibers through inverse tapered waveguides; the waveform generated by the arbitrary function generator (3) is a linearly chirped signal of a triangular wave. The parallel FMCW lidar transmitting method in the mid-infrared band includes the following steps: The laser (1) emits pump light into the electro-optic modulator (2), and at the same time, the arbitrary function generator (3) generates a specific waveform and enters the electro-optic modulator (2) to perform frequency chirp modulation on the pump light. The modulated light passes through the chalcogenide chip (4), and uses the cascaded four-wave mixing effect of the chalcogenide chip (4) to generate a soliton frequency comb, transmits the chirped laser to all generated comb teeth without distortion, and finally generates a stable optical pulse sequence required by the lidar. Diffract the modulated light to various places in space through the diffraction grating (5).

2. The parallel FMCW lidar emission method in the mid- and far-infrared band according to claim 1, characterized in that, The micro-ring radius value of the chalcogenide micro-ring resonator (42) is 50um to 200um, the micro-ring thickness is 0.7um to 1um, the width of the micro-ring is 1.9um to 2.5um, and the free spectral range FSR of the cavity is 130GHz to 520GHz.

3. The parallel FMCW lidar emission method in the mid- and far-infrared band according to claim 1, characterized in that, The length value of the inverse tapered waveguide is 200um to 500um, and the tip width value is 100nm to 150nm.

4. The parallel FMCW lidar emission method in the mid- and far-infrared band according to any one of claims 1 to 3, characterized in that, The laser output by the laser (1) is a narrow linewidth light source, in the mid-infrared band, and the central wavelength is 9.5um to 11um.

5. The parallel FMCW lidar emission method in the mid- and far-infrared band according to claim 1, characterized in that, The bandwidth of the linearly chirped signal of the triangular wave is 1GHz - 5GHz, and the modulation rate is 100KHz - 10MHz.

6. The parallel FMCW lidar emission method in the mid- and far-infrared band according to claim 1, characterized in that, The diffraction grating (5) has 80 to 120 grooves per millimeter, and performs spectral diffraction on the mid-infrared frequency comb light source.

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