A high-precision frequency-modulated continuous-wave laser radar system and control method thereof

Abstract

The invention relates to the technical field of laser radar, and discloses a high-precision frequency-modulated continuous wave laser radar system and a control method thereof. An FMCW light source emits a continuous frequency-modulated laser, and after being split by a first coupler, one laser beam is incident on a second coupler as a reference signal, and the other laser beam is incident on a circulator; the circulator and the second coupler are connected by an optical fiber, and the circulator and a collimating lens are connected by an optical fiber; the collimating lens emits a laser signal to a target, and collects a ranging signal reflected by the target; a balanced detector mixes the ranging signal reflected by the target with a reference signal after passing through the circulator; an LD control circuit, a data acquisition module and a signal processor are integrated into a circuit board to realize LD nonlinear correction, ranging signal synchronization and beat frequency extraction functions; the invention splices beat frequency signals of multiple continuous cycles, equivalently improves the frequency modulation bandwidth of FMCW, improves ranging accuracy, and obtains a high-precision ranging signal.

Description

Technical Field

[0001] The present invention relates to the field of laser radar technology, and more specifically to a high-precision frequency-modulated continuous-wave laser radar system and a control method thereof. Background Art

[0002] With the rapid development of LiDAR technology, the real-time positioning and perception systems of autonomous vehicles, drones, and robots urgently need high-dynamic LiDAR technology. Especially in the case of highly dynamic, unstructured and complex external environment changes, high-dynamic LiDAR technology has become particularly indispensable.

[0003] Compared with the time-of-flight (TOF) ranging method, the frequency modulated continuous wave (FMCW) coherent ranging method is an advanced technology for laser radar (LiDAR) to achieve anti-interference ranging. It is particularly suitable for high-precision measurement within a range of 1 meter around the robot, thereby realizing sub-millisecond perception capabilities of bionic robotic hands, legged robots, etc. like animals. However, FMCW requires a narrow linewidth light source with accurate linear frequency modulation, and the data acquisition (DAQ) rate is not high and the measurement rate is low. In order to obtain ideal linearity within a high modulation bandwidth range, it is usually necessary to pre-correct the light source, or use multiple light sources with different wavelengths for synchronous scanning without phase mismatch, or use optical frequency comb technology to excite multiple wavelengths simultaneously, which is technically difficult.

[0004] The ranging accuracy of FMCW laser radar is limited by the modulation bandwidth of the light source. Improving the modulation bandwidth of the laser diode LD means higher costs and more complex technology. Problems with existing technologies: 1) The ranging accuracy of traditional FMCW laser radar is limited by the modulation bandwidth of the light source. Improving the modulation bandwidth of the laser diode LD means higher costs or more complex technology; 2) Traditional FMCW ranging algorithms cannot simultaneously obtain ranging signals of multiple modulation cycles. Summary of the invention

[0005] The purpose of the present invention is to provide a high-precision frequency modulated continuous wave laser radar system and a control method thereof, which can effectively improve the frequency modulation bandwidth of FMCW by splicing beat frequency signals of multiple continuous periods, and finally improve the ranging accuracy, and obtain high-precision ranging signals with advanced algorithms and low cost.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] The present invention provides a high-precision frequency modulated continuous wave laser radar system, comprising: an FMCW light source, a first coupler, a circulator, a collimating lens, a second coupler, a balanced detector, and an integrated control device;

[0008] The FMCW light source is used to emit a continuously frequency modulated laser; the FMCW light source is connected to a first coupler via an optical fiber, the first coupler is respectively connected to the circulator and the second coupler via an optical fiber, the first coupler divides the continuously frequency modulated laser into two laser beams, wherein one laser beam is incident on the second coupler via an optical fiber as a reference signal, and the other laser beam is incident on the circulator; the circulator and the second coupler are connected via an optical fiber, and the circulator and the collimating lens are connected via an optical fiber; the collimating lens is used to emit a laser ranging signal to a target and collect the ranging signal reflected by the target;

[0009] The second coupler is connected to the balanced detector via an optical fiber, and the balanced detector is connected to the integrated control device via a signal cable; the second coupler is used to couple the reference signal and the ranging signal to obtain an optical mixed signal, and the balanced detector is used to perform frequency mixing to generate a beat frequency signal carrying target ranging information;

[0010] The integrated control device includes an LD control circuit, a data acquisition module and a signal processing chip integrated on the same circuit board, wherein the LD control circuit is used to send out an LD current modulation signal, and the data acquisition module is a data acquisition card with two channels, one channel is connected to a balanced detector to obtain a beat frequency signal, and the other channel is connected to the LD control circuit to obtain an LD current modulation signal; the signal processing chip is connected to the data acquisition card and is used for synchronizing the beat frequency signal with the LD injection current modulation signal, wherein time domain signals of multiple consecutive cycles are spliced, and the spliced ​​time domain signals are subjected to discrete Fourier transform to obtain a frequency domain ranging beat frequency signal.

[0011] Furthermore, the coupling ratio of the first coupler is 99:1, and the coupling ratio of the second coupler is 50:50.

[0012] Furthermore, the balanced detector is a Thorlabs PDB450C-AC, which includes a photodetection diode and a transimpedance amplifier.

[0013] The present invention also provides a control method for a high-precision frequency modulated continuous wave laser radar system, which is applied to the above-mentioned high-precision frequency modulated continuous wave laser radar system and comprises the following steps:

[0014] Each laser frequency modulation cycle T The transmitted laser signal in is expressed as:

[0015] (1)

[0016] in, is the electric field strength of the emitted laser signal; is the initial laser frequency, kis the laser frequency modulation slope; is the initial phase of the emitted laser signal; T is the laser frequency modulation period;

[0017] After being reflected by the target, the collected ranging signal is expressed as:

[0018] (2)

[0019] in, is the electric field strength of the collected ranging signal; the delay between the two optical signals is the time τ for the laser to propagate in the air, R is the distance the laser travels in air, R=cτ / 2n , where the refractive index of air is n , the speed of light is c ;

[0020] The collected ranging signal reflected by the target and the reference signal are mixed in the balanced detector to generate a beat frequency signal carrying the target ranging information. The high-order components are ignored and the intensity of each component is normalized to obtain:

[0021] (3)

[0022] Thus, the ranging beat frequency signal is obtained:

[0023] (4)

[0024] in, B represents the laser frequency modulation bandwidth;

[0025] Ranging results R The relationship with the ranging beat frequency signal is as follows:

[0026] (5)

[0027] After discrete Fourier transform, the resolution of the beat frequency signal is ,in and N are the sampling frequency and the number of sampling points respectively; the ranging accuracy is obtained:

[0028] (6)

[0029] By splicing the time domain signals of M consecutive cycles, the beat frequency signal in formula (3) becomes:

[0030] (7)

[0031] Thus we get:

[0032] (8)

[0033] Then, the ranging accuracy after splicing is deduced:

[0034] (9)

[0035] It can be seen that after splicing the signals of M cycles, the ranging accuracy is improved by M times, and the equivalent frequency modulation bandwidth is improved by M times.

[0036] Furthermore, the laser modulation bandwidth is 130G, the modulation frequency is 20kHz, and 16 microseconds in the rising / falling process of each cycle are selected for splicing.

[0037] Furthermore, the modulation frequency of the continuous frequency modulated laser emitted by the FMCW light source is expressed as:

[0038] .

[0039] It can be seen from the above technical solution that, compared with the prior art, the beneficial effects of the present invention are:

[0040] The present invention improves the time domain ranging signal acquisition rate by synchronizing the ranging signal with the modulation signal of the laser injection current. Furthermore, multiple periods of time domain signals (4 periods, 8 periods, 12 periods, 24 periods, etc.) are collected simultaneously, and the multi-period signals are spliced ​​into one signal in the time domain. This technology is equivalent to improving the FMCW frequency modulation bandwidth. Online real-time Fourier transform can obtain a stable high-precision ranging beat frequency signal, which can significantly improve the ranging accuracy. The present invention is the first to realize a high-precision FMCW laser radar with a low-cost system combined with an advanced real-time ranging algorithm. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0042] The following is a further description of a high-precision frequency modulated continuous wave laser radar system and method of the present invention in conjunction with the accompanying drawings;

[0043] Figure 1 The figure is a schematic diagram of the optical path and experimental device structure of the high-precision frequency-modulated continuous-wave laser radar system of the present invention, wherein TIA: trans-impedance amplifier; DAQ: data acquisition; LD: laser diode;

[0044] Figure 2Schematic diagram of the principle of the high-precision frequency modulated continuous wave laser radar system of the present invention, wherein (a) represents the time domain signal of the FMCW ranging beat frequency formation principle, wherein the thick solid line represents the transmitted laser signal, and the thin solid line represents the ranging signal collected after being reflected by the target; (b) represents the FMCW ranging beat frequency signal extracted from (a); (c) represents the time domain measurement signal after mixing obtained at the receiving end;

[0045] Figure 3 A schematic diagram of a splicing process of an FMCW ranging signal provided by an embodiment of the present invention;

[0046] Figure 4 The nonlinear characteristics of the DFB laser source of the present invention, wherein (a) represents the wavelength tuning measured when the injection current is increased from 20 mA to 120 mA; (b) represents the nonlinear relationship between the laser frequency and the injection current;

[0047] Figure 5 1 is an embodiment of the high-precision laser radar ranging of the present invention, wherein (a) represents the time domain ranging signal of 12 consecutive cycles initially obtained, (b) represents the time domain signal obtained after splicing the 12 consecutive cycles, (c) represents the discrete Fourier transform (DFT) of the time domain signal to obtain the beat frequency signal: the result of 12-cycle splicing and a single cycle, and (d) represents the high-precision ranging result within the range of 10 cm to 50 cm;

[0048] Figure 6 This is a comparative example of the high-precision distance measurement after multi-cycle splicing and the low-precision distance measurement after single cycle of the present invention.

[0049] Figure 7 This is a comparative example of the statistical distribution analysis of high-precision distance measurement and low-precision distance measurement of the present invention;

[0050] Explanation of the accompanying drawings: 1. FMCW light source; 2. First coupler; 3. Circulator; 4. Collimating lens; 5. Second coupler; 6. Balanced detector; 7. Integrated control device. DETAILED DESCRIPTION

[0051] The specific implementation of the present invention is further described in detail below in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0052] The purpose of the present invention is to provide a method and system for realizing high-precision FMCW LiDAR, so as to obtain high-precision ranging signals with advanced algorithms and low cost.

[0053] In order to better understand the purpose, structure and function of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings.

[0054] like Figure 1 As shown, the embodiment of the present invention discloses a high-precision frequency modulated continuous wave laser radar system, including: an FMCW light source 1, a first coupler 2, a circulator 3, a collimating lens 4, a second coupler 5, a balanced detector 6, and an integrated control device 7;

[0055] The FMCW light source 1 is used to emit a continuously frequency modulated laser; the FMCW light source 1 is connected to the first coupler 2 through an optical fiber, the first coupler 2 is respectively connected to the circulator 3 and the second coupler 5 through an optical fiber, the first coupler 2 divides the continuously frequency modulated laser into two laser beams, wherein one laser beam is incident on the second coupler 5 through an optical fiber as a reference signal, and the other laser beam is incident on the circulator 3; the circulator 3 and the second coupler 5 are connected through an optical fiber, and the circulator 3 and the collimating lens 4 are connected through an optical fiber; the collimating lens 4 is used to emit a laser ranging signal to a target and collect the ranging signal reflected by the target;

[0056] The second coupler 5 is connected to the balanced detector 6 via an optical fiber, and the balanced detector 6 is connected to the integrated control device 7 via a signal cable; the second coupler 5 is used to couple the reference signal and the ranging signal to obtain an optical mixed signal, and the balanced detector 6 is used to perform frequency mixing to generate a beat frequency signal carrying target ranging information;

[0057] The integrated control device 7 includes an LD control circuit, a data acquisition module and a signal processing chip integrated on the same circuit board, wherein the LD control circuit is used to send out an LD current modulation signal, and the data acquisition module is a data acquisition card with two channels, one channel is connected to a balanced detector to obtain a beat frequency signal, and the other channel is connected to the LD control circuit to obtain an LD current modulation signal; the signal processing chip is connected to the data acquisition card and is used for synchronizing the beat frequency signal with the LD injection current modulation signal, wherein time domain signals of multiple consecutive cycles are spliced, and the spliced ​​time domain signals are discrete Fourier transformed to obtain a frequency domain ranging beat frequency signal.

[0058] For example, the coupling ratio of the first coupler 2 is 99:1, and the coupling ratio of the second coupler 5 is 50:50. The balanced detector 6 is a Thorlabs PDB450C-AC, which includes a photodetection diode and a transimpedance amplifier.

[0059] Specifically, 4-cycle, 8-cycle, 12-cycle, and 24-cycle time domain signals may be collected simultaneously, and the multi-cycle signals may be spliced ​​into one signal in the time domain.

[0060] The high-precision FMCW laser radar system proposed in the present invention is based on a frequency modulated continuous wave FMCW laser ranging system. The LD control circuit, data acquisition module (DAQ) and signal processing are integrated into a circuit board to realize LD nonlinear correction, ranging signal synchronization, multi-cycle ranging signal splicing, and beat frequency extraction functions. Optical frequency modulation is achieved by modulating the injection current of a compact DFB laser diode. The modulation waveform of the laser injection current is obtained from the result of nonlinear correction optimization. At the same time, a synchronization voltage signal with the same current modulation waveform is provided to the DAQ to trigger the synchronization between the measurement signal and the injection current signal. The measurement signal is emitted to the target through a collimating lens (CL) and reflected back along the original path. The reflected signal is collected by the CL and coupled with the reference signal in a 2×2 coupler (50:50). Then, the optical mixed signal is guided to a balanced photodetector (BPD). A data acquisition card (DAQ) with two channels is used to simultaneously record the measurement signal and the synchronization signal at a rate of 100MS / s with an accuracy of 14 bits. Finally, the measured time domain signal is transformed into the frequency domain spectrum of the beat frequency through discrete Fourier transform (DFT), and the ranging result is output.

[0061] The frequency of the FMCW laser ranging system is generally modulated by a symmetrical triangle wave or sawtooth wave, such as Figure 2 As shown, the basic interference optical path is a Mach-Zehnder interferometer (MZI).

[0062] The modulation frequency of the continuous frequency modulated laser emitted by the FMCW light source can be expressed as:

[0063] .

[0064] The present invention provides a control method for a high-precision frequency-modulated continuous-wave laser radar system, which is applied to the above-mentioned high-precision frequency-modulated continuous-wave laser radar system, and comprises the following steps:

[0065] Each laser frequency modulation cycle T The transmitted laser signal in is expressed as:

[0066] (1)

[0067] in, is the electric field strength of the emitted laser signal; is the initial laser frequency, k is the laser frequency modulation slope; is the initial phase of the emitted laser signal; T is the laser frequency modulation period;

[0068] After being reflected by the target, the collected ranging signal is expressed as:

[0069] (2)

[0070] in, is the electric field strength of the collected ranging signal; the delay between the two optical signals is the time τ for the laser to propagate in the air, R is the distance the laser travels in air, R=cτ / 2n , where the refractive index of air is n , the speed of light is c ;

[0071] The collected ranging signal reflected by the target and the reference signal are mixed in the balanced detector to generate a beat frequency signal carrying the target ranging information. The high-order components are ignored and the intensity of each component is normalized to obtain:

[0072] (3)

[0073] Thus, the ranging beat frequency signal is obtained:

[0074] (4)

[0075] in, B represents the laser frequency modulation bandwidth;

[0076] Ranging results R The relationship with the ranging beat frequency signal is as follows:

[0077] (5)

[0078] After discrete Fourier transform, the resolution of the beat frequency signal is ,in and N are the sampling frequency and the number of sampling points respectively; the ranging accuracy is obtained:

[0079] (6)

[0080] By splicing the time domain signals of M consecutive cycles, the beat frequency signal in formula (3) becomes:

[0081] (7)

[0082] Thus we get:

[0083] (8)

[0084] Then, the ranging accuracy after splicing is deduced:

[0085] (9)

[0086] It can be seen that after splicing the signals of M cycles, the ranging accuracy is improved by M times, and the equivalent frequency modulation bandwidth is improved by M times.

[0087] It can be seen that the present invention proposes a cost-effective method for improving the FMCW ranging accuracy, by splicing beat frequency signals of multiple consecutive cycles, which equivalently increases the frequency modulation bandwidth of FMCW and ultimately improves the ranging accuracy.

[0088] An example of a splicing process of an FMCW ranging signal provided by the present invention is shown in FIG. Figure 3 As shown, the laser modulation bandwidth is 130G, the modulation frequency is 20kHz, and 16 microseconds in the rising / falling process of each cycle are selected for splicing.

[0089] The nonlinear characteristics of DFB laser sources are as follows: Figure 4 shown. Figure 4 In (a), the laser spectrum shown superimposed is measured by a spectrum analyzer. When the injection current increases from 20 mA to 120 mA and the temperature is fixed at 25 ° C, the laser wavelength is tuned from 1490.188 nm to 1491.188 nm, covering a 135 GHz frequency continuous sweep range. The nonlinear relationship between the laser frequency and the laser diode injection current is a numerical fit of the experimental data, as shown in Figure 4 As shown in (b) in .

[0090] Figure 5 In the figure, (a) shows 12 cycles of ranging signals acquired continuously in the time domain, and each cycle takes a 16-microsecond signal during the frequency reduction process. After splicing the data of these 12 cycles according to formulas (7)-(9), the following is obtained: Figure 5 As shown in (b). Figure 5 In (c), the time domain signals in (a)-(b) are subjected to discrete Fourier transform (DFT) to obtain the beat signal. It can be seen that after 12 cycles of splicing, the half-height width of the beat signal is about 1mm, which is much smaller than the half-height width of the beat signal of a single cycle of 20mm. Figure 5 In the figure, (d) shows the relationship between the measured beat frequency and distance for high-precision measurement of targets ranging from 10 cm to 50 cm. According to formula (5), the experimental data is linearly fitted, and the goodness of fit reaches 0.9993, proving that the beat frequency and distance maintain an ideal linear relationship.

[0091] In the present invention, 100 consecutive measurement results are provided for high-precision and low-precision laser radars for comparison. Figure 6 Corresponding to 100 of the data obtained, it can be seen that the data deviation of the high-precision measurement results is significantly lower than that of the low-precision measurement.

[0092] Figure 7The statistical distribution analysis of 100 high-precision and low-precision measurement results is given in the figure. The low-precision measurement has an average value of 468.57 mm and a standard deviation of 9.8 mm, while the high-precision measurement has an average value of 459.95 mm and a standard deviation of 0.07 mm.

[0093] The present invention synchronizes the ranging signal with the laser diode LD modulation signal at the receiving end, splices the time domain signals of multiple continuous frequency modulation cycles (rising or falling), equivalently improves the FMCW frequency modulation bandwidth, and performs online real-time Fourier transform to obtain a stable high-precision ranging beat frequency signal.

[0094] In summary, based on the method provided by the present invention, a high-precision FMCW laser radar can be obtained at a low cost; and an advanced ranging algorithm is used to process the obtained ranging signal in real time, and a high-precision measurement result is obtained by splicing continuous multi-cycle time domain signals. This technology is highly practical and has a high reference value for technical personnel in the industry.

[0095] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A high-precision frequency modulated continuous wave laser radar system, characterized in that: include: FMCW light source, a first coupler, a circulator, a collimating lens, a second coupler, a balanced detector, and an integrated control device; The FMCW light source is used to emit a continuously frequency modulated laser; the FMCW light source is connected to a first coupler via an optical fiber, the first coupler is respectively connected to the circulator and the second coupler via an optical fiber, the first coupler divides the continuously frequency modulated laser into two laser beams, wherein one laser beam is incident on the second coupler via an optical fiber as a reference signal, and the other laser beam is incident on the circulator; the circulator and the second coupler are connected via an optical fiber, and the circulator and the collimating lens are connected via an optical fiber; the collimating lens is used to emit a laser ranging signal to a target and collect the ranging signal reflected by the target; The second coupler is connected to the balanced detector via an optical fiber, and the balanced detector is connected to the integrated control device via a signal cable; the second coupler is used to couple the reference signal and the ranging signal to obtain an optical mixed signal, and the balanced detector is used to perform frequency mixing to generate a beat frequency signal carrying target ranging information; The integrated control device includes an LD control circuit, a data acquisition module and a signal processing chip integrated on the same circuit board, wherein the LD control circuit is used to send out an LD current modulation signal, and the data acquisition module is a data acquisition card with two channels, one channel is connected to a balanced detector to obtain a beat frequency signal, and the other channel is connected to the LD control circuit to obtain an LD current modulation signal; the signal processing chip is connected to the data acquisition card and is used for synchronizing the beat frequency signal with the LD injection current modulation signal, wherein time domain signals of multiple consecutive cycles are spliced, and the spliced ​​time domain signals are subjected to discrete Fourier transform to obtain a frequency domain ranging beat frequency signal.

2. The high-precision frequency modulated continuous wave laser radar system according to claim 1, characterized in that: The coupling ratio of the first coupler is 99:1, and the coupling ratio of the second coupler is 50:

50.

3. The high-precision frequency modulated continuous wave laser radar system according to claim 1, characterized in that: The balanced detector includes a photodetection diode and a transimpedance amplifier.

4. A control method for a high-precision frequency-modulated continuous-wave laser radar system, characterized in that: The high-precision frequency modulated continuous wave laser radar system applied to any one of claims 1 to 3 comprises the following steps: The emitted laser signal in each laser frequency modulation period T is expressed as: Wherein, E0 is the electric field strength of the emitted laser signal; f0 is the initial laser frequency, and k is the laser frequency modulation slope; is the initial phase of the emitted laser signal; T is the laser frequency modulation period; After being reflected by the target, the collected ranging signal is expressed as: Where E1 is the electric field strength of the collected ranging signal; the delay between the two optical signals is the time τ for the laser to propagate in the air, and R is the distance the laser propagates in the air, R = cτ / 2n, where the refractive index of air is n and the speed of light is c; The collected ranging signal reflected by the target is mixed with the reference signal in the balanced detector to generate a beat frequency signal carrying the target ranging information. The high-order components are ignored and the intensity of each component is normalized to obtain: Thus, the ranging beat frequency signal is obtained: f b =kτ=4nBR / cT 0<t<T (4) Wherein, B represents the laser frequency modulation bandwidth; The relationship between the ranging result R and the ranging beat frequency signal is as follows: R=cτ / 2n=cf b / 2nk=cTf b / 4nB (5) After discrete Fourier transform, the resolution of the beat frequency signal is b =f s / N=2 / T, where f s and N are the sampling frequency and the number of sampling points respectively; the ranging accuracy is obtained: δR=c / 2nB (6) By splicing the time domain signals of M consecutive cycles, the beat frequency signal in formula (3) becomes: Thus we get: Then, the ranging accuracy after splicing is deduced: It can be seen that after splicing the signals of M cycles, the ranging accuracy is improved by M times, and the equivalent frequency modulation bandwidth is improved by M times.

5. The control method of the high-precision FMCW laser radar system according to claim 4, characterized in that: The laser modulation bandwidth is 130G, the modulation frequency is 20kHz, and 16 microseconds in the rising / falling process of each cycle are selected for splicing.

6. The control method of the high-precision FMCW laser radar system according to claim 4, characterized in that: The modulation frequency of the continuously frequency modulated laser emitted by the FMCW light source is expressed as: f(t)=f0+kt+G(t) 0 <t<T。

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