CN118501891A - Measuring device of frequency modulation continuous wave laser radar based on double optical frequency combs - Google Patents
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Abstract
The measuring device of the frequency modulation continuous wave laser radar based on the double optical frequency combs comprises a first optical frequency comb generator, a first optical comb for generating comb teeth f R and a second optical comb for detecting a target object; the second optical frequency comb generator is configured to generate a second optical comb with comb teeth f R+ΔfR, and the beam combination conversion module is used for mixing the second optical comb with the back scattered light beam to generate a beat frequency signal; and the back-end processing module is used for carrying out back-end time domain splicing data processing on the beat frequency signals, equivalently realizing the sweep frequency bandwidth Nf R, and obtaining the speed and distance information of the target object by comparing the spliced beat frequency signals obtained by the upper sweep frequency and the lower sweep frequency. The invention realizes high distance resolution and high speed resolution without sacrificing FMCW detection frame rate, realizes phase continuous splicing of time domain, does not need complex wavelength tracking and phase compensation device, only needs single PD to realize goods to auction signal reception of different channels, and has the advantages of low cost and simple structure.
Description
Technical Field
The invention relates to the field of laser radar measurement, in particular to a frequency modulation continuous wave laser radar measurement device based on a double-optical-frequency comb, which realizes expansion of sweep frequency bandwidth and improves detection resolution.
Background
Laser radars have been widely used in many fields such as autopilot, remote sensing, underwater target detection, etc. The Frequency Modulation Continuous Wave (FMCW) laser radar mixes a local reference signal with a backscatter signal to realize coherent amplification of the signal, has ultrahigh sensitivity, and can detect a weak backscatter signal with Pi Wate power by using a low-cost detector. Compared with direct detection, the FMCW laser radar system has a great technical advantage that speed and distance measurement can be simultaneously realized.
The range resolution of a conventional FMCW lidar system depends on the frequency tuning bandwidth. However, it is difficult to achieve a wide range of mode-hop free frequency tuning while ensuring low phase noise, subject to the limitations of the physical mechanism of the laser itself. For example, a vertical cavity surface emitting laser has a large frequency tuning range, but the phase noise is large, and it is difficult to use it in actual detection. Therefore, laser radar engineers adopt various technical means to improve the frequency tuning range in the FMCW laser radar system and realize high-resolution detection on the premise of ensuring the linewidth of the laser.
There are several methods currently used to increase the frequency tuning range in FMCW lidar systems:
(1) And the frequency-sweeping bandwidth multiplication is realized by combining the frequency-shifted linear frequency modulation signals for a plurality of times. However, the scheme needs to precisely control the length of the frequency shift ring, ensures that the time delay introduced by the length of the frequency shift ring is equal to the single frequency sweep time, and further needs to carry out optical amplification by the EDFA for each frequency shift, so that the implementation difficulty is great. In addition, the large bandwidth arbitrary signal generator is required to drive the large bandwidth intensity modulator to achieve chirp, so that not only is the range of chirp limited, but also the cost becomes very high. In addition, this solution does not allow a two-way sweep and does not allow simultaneous speed-distance measurements.
(2) The increase of the sweep bandwidth is achieved by four-wave mixing. According to the scheme, a sweep frequency light source is used as an initial signal, and the sweep frequency bandwidth is increased in a cascade four-wave mixing mode. However, cascading multiple four-wave mixes can complicate system variations and power fluctuations in the system can also lead to resolution degradation.
(3) And the multiplication of the sweep frequency bandwidth is realized by adopting a high-order modulation sideband injection locking technology. However, the frequency sweeping range of the method is still limited by the modulator and the radio frequency amplifier, and a larger frequency sweeping range is difficult to realize.
(4) And by a coherent diversity receiving system, double expansion of the sweep frequency bandwidth is realized by splicing positive and negative sweep frequency sidebands of intensity modulation. However, this method only doubles the frequency modulation range, and requires complex back-end data processing, and the required coherent receiving system also increases the cost of the system.
(5) The sweep frequency light source is used as the input of the electro-optical comb, the sampling periods of different channels are synchronized, after the multichannel reception is carried out, then the splicing of sweep frequency of different comb teeth in different time periods is completed on the time domain, and the increase of the sweep frequency bandwidth is realized. However, this approach requires the use of separate optical filters, detectors and analog-to-digital converters for each individual channel, which is very costly and unacceptably high once the number of swept comb teeth is increased.
(6) The frequency sweep bandwidth is increased by performing time domain splicing on a plurality of frequency sweep light sources with different wave bands. However, the method often needs to adopt a predistortion method to overcome the nonlinearity of the sweep frequency, so that the Doppler shift measurement cannot be realized, and the speed measurement capability cannot be realized. And the sweep time of this scheme is typically long, making the frame rate of FMCW sounding with this scheme extremely low. Second, complex wavelength calibration devices, such as air chambers, optical frequency combs, etc., are often required to track the sweep rate and wavelength swept by the different light sources. The method has high system complexity and high price.
In general, the above schemes have the problems of high control complexity and high cost while increasing the frequency tuning range of the FMCW laser radar. Some methods have very limited increases in sweep bandwidth. Therefore, a new method is needed to greatly increase the frequency sweep bandwidth, and has the advantages of low cost, easy realization and high frame rate while improving the FMCW detection precision.
Disclosure of Invention
The invention aims to solve the problem that the FMCW laser radar has insufficient detection resolution, provides a measuring device and a measuring method of the FMCW laser radar based on a double optical frequency comb, can realize a low-cost frequency synthesis technology received by a single PD, realizes expansion of sweep frequency bandwidth, has the advantages of low complexity and high frame rate, and simultaneously realizes improvement of speed measurement resolution on the premise of not sacrificing the frame rate. The continuous wave with linear frequency modulation generated by internal modulation or external modulation is taken as seed light, and the seed light is split by a coupler and then enters a double-optical-comb system. In the optical domain, the comb teeth of the double optical combs are slightly smaller than the frequency tuning range of the seed light, and a certain heavy frequency difference exists between the two optical combs. One path of the light scattered by the target object is collected by the detection system and mixed with the other path of signal of the double optical comb to obtain beat frequency signals. Because of the tiny heavy frequency difference between the two optical combs, beat frequencies of different channels are projected into different medium frequency bands, and beat frequency signals generated by different channels can be distinguished by only one detector. The expansion of the sweep frequency bandwidth can be equivalently realized by carrying out coherence on beat frequencies in different frequency bands in the time domain, and the reduction of the detection frame rate and the sweep frequency linearity of the laser can not be brought while high-resolution detection is realized.
In order to achieve the above object, the technical solution of the present invention is as follows:
a measurement device for frequency modulated continuous wave lidar based on a dual optical frequency comb, comprising:
The laser source is configured to emit bidirectional linear sweep laser with a sweep bandwidth B and a sweep period 2T;
The beam splitting module is used for receiving the bidirectional linear sweep laser and splitting the bidirectional linear sweep laser into detection sweep laser and reference sweep laser; it is characterized in that the method comprises the steps of,
A first optical frequency comb generator is configured in the detection sweep frequency laser channel, and a first optical comb with comb teeth f R is generated and used for detecting a target object and receiving back light of the target object;
A second optical frequency comb generator is configured in the reference sweep frequency laser channel to generate a second optical comb with comb teeth f R+ΔfR, wherein Deltaf R is a fine heavy frequency difference between the double optical combs;
The beam combination conversion module is used for mixing the second optical comb and the return light beam combination to generate beat frequency signals;
And the back-end processing module is used for performing back-end time domain splicing data processing on the beat signals, so that the beat signals generated by N different comb teeth are spliced in the time domain, the equivalent realization of the sweep frequency bandwidth Nf R is realized, and the speed and distance information of the target object are obtained by comparing the spliced beat signals obtained by the upper sweep frequency and the lower sweep frequency.
Further, the processing of the back-end time domain spliced data specifically includes the following steps:
S1, dividing the beat frequency signal into an upward sweep frequency part and a downward sweep frequency part according to the sweep frequency direction;
s2, dividing the beat frequency signal spectrum by the length of comb teeth delta f R through Fourier transformation, namely the range of the ith frequency band is [ (i-1) delta f R,iΔfR ];
s3, moving the frequency spectrums of the beat frequency signals corresponding to all the obtained sub-frequency bands into a baseband [0, deltaf R ];
s4, performing inverse Fourier transform on each moved sub-band;
S5, reserving BT/f R parts for each sub-frequency band obtained in the step S2, and performing head-tail splicing on the two parts of upward frequency sweeping and downward frequency sweeping respectively in a time domain to obtain an upper frequency sweeping splicing signal and a lower frequency sweeping splicing signal, wherein T is a frequency sweeping period;
S6, carrying out Fourier transformation on the upper sweep frequency splicing signal and the lower sweep frequency splicing signal respectively to equivalently realize Nf R sweep frequency bandwidth, wherein N is the number of comb teeth;
s7, comparing the upper sweep frequency spliced signal and the lower sweep frequency spliced signal after Fourier transformation to obtain speed and distance information of the target object.
Preferably, in the step S5, the end-to-end splicing is specifically:
Upward swept signal portion: the corresponding beat frequency signals at the low frequency are in front, and the corresponding beat frequency signals at the high frequency are in back for time domain signal splicing, so that an upper sweep frequency spliced signal is obtained;
Sweep down signal portion: and the corresponding beat frequency signals at the high frequency are in front, and the corresponding beat frequency signals at the low frequency are in back for time domain signal splicing, so that a lower sweep frequency spliced signal is obtained.
Preferably, the bi-directional chirped light source with the swept bandwidth B adopts a distributed feedback laser (distributed feedback laser, DFB), a distributed bragg reflector laser (distributed Bragg reflector, DBR), a vertical cavity surface emitting laser (VERTICAL CAVITY surface EMITTING LASER, VCSEL) or an External Cavity Laser (ECL).
Preferably, the linear frequency modulation signal is generated by adopting methods of internal modulation predistortion correction, external modulation of an electro-optical modulator, injection locking and the like, has the characteristic of high frequency sweep linearity, and has known frequency sweep rate and frequency sweep period of a bidirectional frequency sweep signal.
Preferably, the optical comb generator is an electro-optical comb (electronic optics frequency comb, EOFC) or a Kerr optical comb. Two optical combs with known fixed weight frequency differences are generated by outputting single-tone drive signals with frequencies fr+Δfr and fR from a dual-channel high-speed radio frequency source.
Preferably, the distance between the optical comb teeth is slightly smaller than the bandwidth of the seed optical sweep, i.e. f R < B.
Compared with the prior art, the invention has the following advantages:
1. Compared with a scheme adopting frequency shift ring spread spectrum, the method is easier to realize. And the scheme can adopt bidirectional linear sweep frequency, so that the speed measuring capability is realized.
2. Compared with the scheme of increasing the sweep frequency bandwidth by using a high-order modulation sideband injection locking technology and four-wave mixing, the invention has the advantages of better system stability and lower complexity.
3. Compared with the scheme of splicing the positive and negative sweep frequency sidebands after coherent reception, the method does not need a complex coherent receiving device, and has greater potential in the aspect of improving the sweep frequency bandwidth.
4. Compared with the scheme of splicing a plurality of lasers, the invention does not need a complex wavelength tracking device, and simultaneously, the invention does not need a resampling method to overcome the problem of nonlinearity of sweep frequency, thereby ensuring the speed measuring capability of the system.
5. Compared with the sweep frequency single optical comb scheme, the invention can realize the receiving of beat frequency signals generated by different channels only by a single PD, and the PD bandwidth is far smaller than the PD bandwidth in the sweep frequency single optical comb. Meanwhile, the scheme does not need to synchronize beat signal measurement of different channels, and has lower control complexity.
6. The invention realizes the improvement of the speed resolution under the condition of keeping the frame rate not to be degraded.
7. The invention can realize high distance resolution and high speed resolution without sacrificing FMCW detection frame rate. By utilizing the fixed phase relation between the optical frequency comb teeth, the continuous phase splicing of the time domain is realized, a complex wavelength tracking and phase compensating device is not needed, and goods to auction signal receiving of different channels can be realized by only a single PD, so that the optical frequency comb has the advantages of low cost and simple structure.
Drawings
Fig. 1 is a schematic structural diagram of an embodiment of a measurement device of an FMCW lidar based on a dual-optical-frequency comb of the present invention.
FIG. 2 is a schematic diagram of the beat frequencies of optical combs OFC1 and OFC2 in accordance with the present invention.
Fig. 3 is a time-frequency diagram of the swept optical comb OFC 1.
Fig. 4 shows time domain signals of different channels obtained by performing spectrum cutting, frequency shift and inverse fourier transform on waveforms in the ROI.
FIG. 5 is a flow chart of a method for implementing comb-teeth phase stitching to improve speed resolution and distance resolution in the present invention.
Detailed Description
The invention will be further described with reference to the drawings and examples, which should not be construed as limiting the scope of the invention. Embodiments of the present invention include, but are not limited to, the following examples.
Referring to fig. 1, fig. 1 is a schematic structural diagram of an embodiment of a measurement device of an FMCW lidar based on a dual-optical-frequency comb according to the present invention, which at least includes a bi-directional chirped light source with a frequency sweep bandwidth of B and a frequency sweep period of 2T, a1×2 optical coupler, a first optical-frequency comb generator with a comb teeth interval of f R, a second optical-frequency comb generator with a comb teeth interval of f R+ΔfR, a circulator, a collimator, and a2×1 optical coupler. Along the propagation direction of the signal light (detection swept laser) is in turn: the comb teeth are an optical frequency comb 1 with f R, a circulator, a collimator, a2 multiplied by 1 optical coupler and an optical detector. Along the propagation direction of the reference swept laser, the following steps are sequentially: 2 x1 optocoupler photodetector. And coherent detection with Nf R sweep frequency bandwidth is realized through back-end algorithm processing, wherein N is the number of spliced comb tooth modes.
The bi-directional linear frequency modulation light source with the sweep bandwidth of B is used as seed light, and after being split by a1 multiplied by 2 optical coupler, the seed light enters a first optical frequency comb generator OFCG1 and a second optical frequency comb generator OFCG2 respectively to generate two optical combs with comb teeth of f R and comb teeth of f R+ΔfR. The optical frequency comb with the comb teeth of f R generated by the OFCG1 is emitted by the collimator after passing through the circulator, light scattered back by the target object is collected by the collimator, and the light is combined with the optical frequency comb with the comb teeth of f R+ΔfR after passing through the 2X 1 optical coupler, so that beat signals are generated at the photoelectric detector.
The bi-directional chirped light source with the sweep bandwidth of B adopts a distributed feedback laser (distributed feedback laser, DFB), a distributed bragg reflector laser (distributed Bragg reflector, DBR), a vertical cavity surface emitting laser (VERTICAL CAVITY surface EMITTING LASER, VCSEL) or an External Cavity Laser (ECL).
The linear frequency modulation signal is generated by adopting methods of internal modulation predistortion correction, external modulation of an electro-optical modulator, injection locking and the like, has the characteristic of high frequency sweep linearity, and has known frequency sweep rate and frequency sweep period of a bidirectional frequency sweep signal.
The 1 x 2 optical coupler and the 2x 1 optical coupler are fiber couplers or polarizing beam splitters.
The optical comb generator is an electro-optical comb (electronic optics frequency comb, EOFC) or a Kerr optical comb. Two optical combs with known fixed frequency differences are generated by outputting single-tone drive signals at frequencies f R+ΔfR and f R from a dual-channel high-speed radio frequency source.
The space between the optical comb teeth is slightly smaller than the bandwidth of the seed optical sweep frequency, namely f R < B.
The 1 XN collimator array adopts a Gradient Refractive Index (GRIN) optical fiber collimator, a self-focusing lens (SELFOC) collimator, a micro lens array or a plano-convex lens, a planar biconvex lens, an aspheric plano-convex lens or an aspheric biconvex lens, and is used for expanding and collimating light emitted by the output coupler.
The 1X 2 optical coupler, the circulator and the 2X 1 optical coupler are operated in a single-mode, and a polarization-maintaining single-mode fiber is adopted. The end faces of the collimators are plated with antireflection films matched with the working wave bands so as to avoid unnecessary interference caused by back scattering.
The space between the optical comb teeth is slightly smaller than the bandwidth of the seed optical sweep frequency, namely f R < B.
The 1 XN collimator array adopts a Gradient Refractive Index (GRIN) optical fiber collimator, a self-focusing lens (SELFOC) collimator, a micro lens array or a plano-convex lens, a planar biconvex lens, an aspheric plano-convex lens or an aspheric biconvex lens, and is used for expanding and collimating light emitted by the output coupler.
FIG. 2 is a schematic diagram of the beat frequencies of optical combs OFC1 and OFC2 in accordance with the present invention. The upper graph shows the intensity distribution of the double optical comb teeth over the optical domain, OFC1 and OFC2 having comb teeth f R and f R+ΔfR, respectively. The comb teeth of the OFC1 and the OFC2 simultaneously perform lower graph representation on intermediate frequency signals after the beat frequency of the OFC1 and the OFC2 at the same sweep rate. Because of the difference in the repetition frequency Δf R of the two optical combs, the signal of the ith channel is projected onto the spectrum with center frequency iΔf R-fb after the beat frequency according to vernier effect, where f b is the beat frequency carrying the distance and velocity information. The structure of the double optical comb enables beat frequencies of different channels to be projected on different frequency bands of the intermediate frequency, so that the beat frequencies can be distinguished. The beat frequency of the different channels is shifted to the baseband frequency, so that the operation of inverse Fourier transformation and sequential splicing in the time domain can be performed.
Fig. 3 is a time-frequency diagram of the swept optical comb OFC1, with a channel spacing f R. The time-frequency diagram of the sweep optical comb OFC2 is similar to that of the sweep optical comb OFC, but the channel interval is f R+ΔfR. Because the sweep bandwidth B > f R, the ROI area shown in the shadow is selected, the length of the ROI is equal to f R T/B, and the signal data processing in the selected ROI can ensure the phase continuity between different channels during splicing.
Fig. 4 shows time domain signals of different channels obtained by performing spectrum cutting, frequency shift and inverse fourier transform on waveforms in the ROI. And connecting the time domain signals obtained by the 4 different channels in sequence from head to tail to obtain the spliced beat frequency signals. And the Fourier transform is carried out on the signal to obtain the beat frequency signal with the frequency sweep range expanded by 4 times, so that the resolution is improved by 4 times.
Fig. 5 is a flowchart of a method for implementing comb teeth phase stitching to improve speed resolution and distance resolution, and as shown in the figure, the back-end time domain stitching data processing specifically includes the following steps:
1) The obtained beat frequency signals are divided into an upward sweep frequency part and a downward sweep frequency part according to different sweep frequency directions, and the following treatment is carried out respectively.
2) And carrying out Fourier transformation operation on the beat frequency signal, and equally dividing the frequency domain by the length of Deltaf R, wherein the range of the ith frequency band is [ (i-1) Deltaf R,iΔfR ].
3) And (3) moving the frequency spectrums of the beat frequency signals corresponding to all the sub-frequency bands into a baseband [0, deltaf R ], and performing inverse Fourier transform operation on the moved signals.
4) And (3) reserving BT/f R parts for the signals obtained in the step (2), and carrying out end-to-end connection on the time domain. The upper sweep part is formed by performing time domain signal splicing on the front of the corresponding beat frequency signal at the low frequency and the rear of the corresponding goods to auction signal at the high frequency. The down-sweep part is that the corresponding beat frequency signal at the high frequency is in front, and the corresponding goods to auction signal at the low frequency is in back for time domain signal splicing.
5) And (3) carrying out Fourier transform operation on the spliced signals obtained in the step (4). By splicing the N comb teeth, the Nf R sweep frequency bandwidth can be equivalently realized.
6) The speed and distance information of the target object can be obtained by comparing the spliced beat frequency signals obtained by the upper sweep frequency and the lower sweep frequency.
The invention can distinguish the beat frequencies generated by different channels by only receiving with a single PD, and can realize N times improvement of the distance resolution by splicing the beat frequency signals generated by N different channels in the time domain. Meanwhile, the splicing in the time domain brings about the increase of the detection time window length, and the speed resolution is improved. The invention can realize N times of improvement of the speed resolution while keeping high frame rate.
Claims (7)
1. A measurement device for frequency modulated continuous wave lidar based on a dual optical frequency comb, comprising:
The laser source is configured to emit bidirectional linear sweep laser with a sweep bandwidth B and a sweep period 2T;
the beam splitting module is used for receiving the bidirectional linear sweep laser and splitting the bidirectional linear sweep laser into detection sweep laser and reference sweep laser; it is characterized in that the method comprises the steps of,
A first optical frequency comb generator is configured in the detection sweep frequency laser channel, and a first optical comb with comb teeth f R is generated and used for detecting a target object and receiving back light of the target object;
A second optical frequency comb generator is configured in the reference sweep frequency laser channel to generate a second optical comb with comb teeth f R+ΔfR, wherein Deltaf R is a fine repetition frequency between the double optical combs;
The beam combination conversion module is used for mixing the second optical comb and the return light beam combination to generate beat frequency signals;
And the back-end processing module is used for performing back-end time domain splicing data processing on the beat signals, so that the beat signals generated by N different comb teeth are spliced in the time domain, the equivalent realization of the sweep frequency bandwidth Nf R is realized, and the speed and distance information of the target object are obtained by comparing the spliced beat signals obtained by the upper sweep frequency and the lower sweep frequency.
2. The measurement device for a frequency modulated continuous wave laser radar based on double optical frequency combs according to claim 1, wherein the back-end time domain stitching data processing specifically comprises the following steps:
S1, dividing the beat frequency signal into an upward sweep frequency part and a downward sweep frequency part according to the sweep frequency direction;
s2, dividing the beat frequency signal spectrum by the length of comb teeth delta f R through Fourier transformation, namely the range of the ith frequency band is [ (i-1) delta f R,iΔfR ];
s3, moving the frequency spectrums of the beat frequency signals corresponding to all the obtained sub-frequency bands into a baseband [0, deltaf R ];
s4, performing inverse Fourier transform on each moved sub-band;
S5, reserving BT/f R parts for each sub-frequency band obtained in the step S2, and performing head-tail splicing on the two parts of upward frequency sweeping and downward frequency sweeping respectively in a time domain to obtain an upper frequency sweeping splicing signal and a lower frequency sweeping splicing signal, wherein T is a frequency sweeping period;
S6, carrying out Fourier transformation on the upper sweep frequency splicing signal and the lower sweep frequency splicing signal respectively to equivalently realize Nf R sweep frequency bandwidth, wherein N is the number of comb teeth;
s7, comparing the upper sweep frequency spliced signal and the lower sweep frequency spliced signal after Fourier transformation to obtain speed and distance information of the target object.
3. The measurement device for a frequency modulated continuous wave laser radar based on double optical frequency combs according to claim 1, wherein in step S5, the end-to-end splicing is specifically:
Upward swept signal portion: the corresponding beat frequency signals at the low frequency are in front, and the corresponding beat frequency signals at the high frequency are in back for time domain signal splicing, so that an upper sweep frequency spliced signal is obtained;
Sweep down signal portion: and the corresponding beat frequency signals at the high frequency are in front, and the corresponding beat frequency signals at the low frequency are in back for time domain signal splicing, so that a lower sweep frequency spliced signal is obtained.
4. The measurement device for a frequency modulated continuous wave laser radar based on a double optical frequency comb according to claim 1, wherein the beam splitting module is an optical fiber coupler or a polarizing beam splitter.
5. The apparatus according to claim 1, wherein the first optical frequency comb generator and the second optical frequency comb generator are generated by an electro-optical comb (electronic optics frequency comb, EOFC) or a Kerr optical comb, and the two optical combs having a known fixed weight difference are generated by outputting a single-tone driving signal having frequencies f R+ΔfR and f R by a dual-channel high-speed radio frequency source.
6. The device for measuring the frequency modulated continuous wave laser radar based on the double optical frequency combs according to claim 1, wherein the laser light source adopts internal modulation predistortion correction, external modulation of an electro-optical modulator and injection locking generation, and generates a bidirectional linear frequency modulation signal with fixed frequency sweep rate and frequency sweep period.
7. The device for measuring the frequency modulated continuous wave laser radar based on the double optical frequency combs according to claim 1, wherein the distance between the optical comb teeth is slightly smaller than the frequency sweep bandwidth of the seed light, namely f R < B.
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