Frequency-modulated continuous wave laser ranging device based on frequency-shift double optical combs

Abstract

The invention provides a frequency-modulated continuous wave laser ranging device based on frequency shift double optical combs, which integrates frequency-modulated continuous wave and double optical comb technologies and comprises an FMCW light source, a beam splitter, a detection and reference light channel, a beam combiner, a photoelectric detector and a processing module. The FMCW light source outputs linear frequency sweeping laser which is divided into two paths through the beam splitter, and the detection light channel detects a target through the first optical comb generator and the circulator. The frequency fFS of the reference optical channel is increased through a frequency shifter, optical combs with different comb tooth intervals are generated by a second optical comb generator, and the fFS and the repetition frequency difference delta fR are in a non-integer multiple relation, so that interference is effectively avoided. After being combined by the beam combiner, the two paths of signals are converted into beat frequency signals by the photoelectric detector, and then time domain signals are spliced by the processing module by using coherence, so that the distance measurement precision is improved. According to the invention, the frequency shift double-optical comb technology improves the precision and anti-interference performance; an efficient data processing algorithm ensures rapid measurement and a wide range of application.

Description

Technical Field

[0001] The present invention relates to the field of laser ranging, and in particular to a frequency modulated continuous wave laser ranging device based on a frequency shifted dual optical comb, which can fully utilize the spectrum of the optical frequency comb and improve the ranging accuracy. Background Art

[0002] Laser ranging uses the high resolution of lasers to achieve high-precision ranging of target objects, and has wide application value in the fields of autonomous driving and remote sensing. By combining frequency-modulated continuous wave (FMCW) technology with dual optical comb technology, millimeter-level measurement accuracy can be obtained with low system complexity. The previously applied patent CN202410559087.X discloses a measurement device for frequency-modulated continuous wave lidar based on dual optical frequency combs, which uses the coherence of the signal spectrum to improve the equivalent sweep bandwidth through coherent splicing, thereby improving the ranging accuracy. However, in this scheme, in order to avoid the mirror beat interference introduced by the symmetrical frequency comb spectrum during the coherent detection of the dual optical comb, one of the optical combs of the dual optical comb is filtered out by half of the spectrum. Although this method alleviates the interference problem to a certain extent, it inevitably sacrifices the effective detection power, which has an adverse effect on the ranging accuracy.

[0003] In view of the above limitations, the industry urgently needs a more efficient and innovative solution to make more comprehensive use of the comb line power of the optical comb, thereby significantly improving the sensitivity of the ranging system without affecting the ranging accuracy. This will not only promote the development of laser ranging technology towards higher accuracy and higher efficiency, but also lay a solid foundation for the further development of autonomous driving, remote sensing detection and other fields. Summary of the invention

[0004] The problem to be solved by the present invention is that the existing FMCW combined with dual optical comb laser ranging solution needs to sacrifice half of the optical frequency comb spectrum, resulting in reduced sensitivity. An FMCW laser ranging device and method based on frequency-shifted dual optical frequency comb are provided, which can more effectively utilize the optical comb spectrum power and improve the ranging sensitivity.

[0005] In order to achieve the above objectives, the technical solutions of the present invention are as follows:

[0006] A frequency modulated continuous wave laser ranging device based on a frequency shifted dual optical comb, comprising:

[0007] FMCW light source, outputting bidirectional linear frequency sweeping laser with frequency sweeping bandwidth B and frequency sweeping period 2T;

[0008] A beam splitter receives the FMCW light source and splits it into a detection light and a reference light;

[0009] In the detection optical channel, the optical signal passes through the first optical comb generator and the circulator in sequence. The first optical comb generator generates a comb tooth interval f R The first optical comb is used to detect the target object; the first optical comb enters the first port of the circulator, and is emitted to the target object from the second port of the circulator, and the echo light of the target object enters the second port of the circulator, and is output from the third port of the circulator;

[0010] In the reference optical channel, the optical signal passes through the frequency shifter and the second optical comb generator in sequence. The frequency shifter increases the frequency of the optical signal by f FS The second optical comb generator generates a comb tooth spacing f R +Δf R The second optical comb, where Δf R is the repetition frequency difference between the two optical combs; the frequency shift frequency f FS It cannot be the repetition frequency difference Δf R An integer multiple of ;

[0011] A beam combiner combines the two signals of the detection optical channel and the reference optical channel and sends them into a photodetector to generate a beat frequency signal;

[0012] The processing module performs back-end data processing on the beat frequency signal, utilizes the coherence between the beat frequency signals generated by different comb teeth, realizes time domain signal splicing, equivalently improves the sweep bandwidth, and thus improves the ranging accuracy.

[0013] Furthermore, the frequency shift frequency f generated by the frequency shifter is FS The value should ensure that the signals generated by the beat frequencies of the left half (i.e., the part smaller than the center frequency) and the right half (i.e., the part larger than the center frequency) of the optical comb are not interlaced in the spectrum, so as to facilitate spectrum segmentation.

[0014] Furthermore, the back-end time domain splicing data processing specifically includes the following steps:

[0015] S1. The beat frequency signal is divided into two parts, an upward sweep and a downward sweep, according to the sweep direction, and processed separately, first processing the upward sweep part;

[0016] S2. Through Fourier transform with comb teeth spacing Δf R The beat frequency signal spectrum is divided by the length of R ,iΔf R ];

[0017] S3. Set [(i-1)Δf R ,(i-0.5)Δf R ] sub-band spectrum is moved to baseband [0,0.5Δf R ], [(i-0.5)ΔfR ,iΔf R ] The sub-band spectrum is flipped and moved to the baseband [0,0.5Δf R ] (i.e. iΔf R Move to baseband 0 frequency, (i-0.5)Δf R Move to baseband 0.5Δf R ); the spectrum generated by the beat frequency of the right half of the optical comb (i.e., the part greater than the center frequency) corresponds to [(i-1)Δf R ,(i-0.5)Δf R ] sub-band, the spectrum generated by the beat frequency of the left half of the optical comb (i.e., the part smaller than the center frequency) corresponds to [(i-0.5)Δf R ,iΔf R ] sub-band;

[0018] S4. Repeat S3 to move N frequency bands, a total of 2N [0,0.5Δf R ], and then perform inverse Fourier transform on all 2N sub-bands;

[0019] S5. Perform phase continuity processing on all signals in the time domain, that is, the phase of the end time of the previous signal is equal to the phase of the start time of the next signal, and perform amplitude compensation processing, and perform time domain signal splicing;

[0020] S6. Repeat S2-S5 to perform the same processing on the signal of the downward sweep portion;

[0021] S7. Compare the upper frequency sweep stitching signal and the lower frequency sweep stitching signal after Fourier transformation to obtain the speed and distance information of the target object.

[0022] Preferably, the FMCW light source adopts a distributed feedback DFB laser, a distributed Bragg reflector DBR laser, a vertical cavity surface emitting VCSEL laser or an external cavity laser ECL.

[0023] Preferably, the optical comb generator is an electro-optical comb or a Kerr optical comb.

[0024] Preferably, the comb teeth spacing of the optical comb is slightly smaller than the sweep bandwidth of the seed light, that is, f R <B。

[0025] Preferably, the frequency shifter is an acousto-optic frequency shifter or a frequency shifter based on a single-sideband electro-optic modulator.

[0026] Preferably, the frequency shift frequency f generated by the frequency shifter is FS The value range is in the repetition frequency difference Δf R 10% to 40% of the total.

[0027] The present invention applies a frequency shift to the local reference optical comb, so that the image interference in the beat frequency between the reference optical comb and the measurement optical comb disappears, and all the comb teeth of all optical combs can provide ranging information. At the same time, by reasonably selecting the frequency shift frequency f FS and the repetition frequency difference Δf R The value of makes it possible to distinguish the signals obtained by the beat frequency of the left and right halves of the optical comb in the spectrum. Then the number of comb teeth that can be used for time domain splicing is greatly increased, and the ranging accuracy can be significantly improved.

[0028] Compared with the prior art, the present invention has the following advantages:

[0029] Compared with the previously applied patent CN202410559087.X, the present invention does not need to filter out half of the spectrum of one optical comb, which doubles the number of comb teeth that can be used for splicing and also doubles the ranging accuracy.

[0030] Compared with the traditional frequency modulated continuous wave (FMCW) scheme, the present invention uses dual optical combs to achieve multi-channel parallel measurement, and the ranging accuracy is significantly improved.

[0031] Compared with the dual-comb ranging solution based on mode-locked laser, the system structure of the present invention is simpler and more reliable.

[0032] The dual-optical comb technology effectively improves the accuracy of ranging by introducing repetition frequency difference and frequency shift. The dual-optical comb technology can fully utilize the high spectral resolution and pulse rate advantages of the optical frequency comb to achieve high-precision measurement. The back-end data processing module performs coherent processing on the beat frequency signal to achieve time domain signal splicing, which effectively increases the sweep bandwidth and further improves the ranging accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a structural schematic diagram of an embodiment of a frequency modulated continuous wave laser ranging device based on a frequency shifted dual optical comb of the present invention.

[0034] 1-FMCW light source, 2-beam splitter, 3-first optical comb generator, 4-circulator, 5-target, 6-frequency shifter, 7-second optical comb generator, 8-beam combiner, 9-photodetector, 10-processing module

[0035] Figure 2 It is a schematic diagram of the beat frequency of the measurement optical comb (first optical comb) and the reference optical comb (second optical comb) in the present invention.

[0036] The frequency shift f FS =25MHz, repetition frequency difference Δf R =100MHz. DETAILED DESCRIPTION

[0037] The present invention will be further described below in conjunction with the accompanying drawings and examples, but the protection scope of the present invention shall not be limited thereto. The embodiments of the present invention include but are not limited to the following examples.

[0038] See also Figure 1 , Figure 1 1 is a schematic diagram of a structure of an embodiment of a frequency modulated continuous wave laser ranging device based on a frequency shifted dual optical comb according to the present invention. As shown in the figure, a frequency modulated continuous wave laser ranging device based on a frequency shifted dual optical comb comprises:

[0039] The FMCW light source 1 outputs bidirectional linear frequency sweeping laser with a specific frequency sweeping bandwidth B and a frequency sweeping period 2T. In this embodiment, the FMCW light source can be a distributed feedback DFB laser, a distributed Bragg reflector DBR laser, a vertical cavity surface emitting VCSEL laser or an external cavity laser ECL.

[0040] The beam splitter 2 splits the laser beam output by the FMCW light source into two beams: detection light and reference light.

[0041] In the detection optical channel, the optical signal passes through the first optical comb generator 3 and the circulator 4 in sequence. The first optical comb generator 3 generates a comb tooth interval f R The first optical comb is used to detect the target object; the circulator 4 enables the detection light to be transmitted unidirectionally to the target object and receives the echo light of the target object. The first optical comb enters the first port of the circulator and is emitted to the target object from the second port of the circulator. The echo light of the target object enters the second port of the circulator and is output from the third port of the circulator. The first optical comb generator is generated by an electro-optical comb or a Kerr optical comb. The comb teeth spacing of the optical comb is slightly smaller than the frequency sweep bandwidth of the seed light, that is, f R <B。

[0042] In the reference optical channel, the optical signal passes through the frequency shifter 6 and the second optical comb generator 7 in sequence. The frequency shifter 6 increases the frequency of the optical signal by f FS The second optical comb generator generates a comb tooth spacing f R +Δf R The second optical comb, where Δf R is the repetition frequency difference between the two optical combs; the frequency shift frequency f FS It cannot be the repetition frequency difference Δf R The second optical comb generator is generated by an electro-optical comb or a Kerr optical comb, and the frequency shifter adopts an acousto-optic frequency shifter or a frequency shifter based on a single-sideband electro-optic modulator. The frequency shifter generates a frequency shift frequency f FS The value range is in the repetition frequency difference Δf R 10% to 40% to ensure the elimination of interference and effective distinction of signals.

[0043] The beam combiner 8 combines the two signals of the detection optical channel and the reference optical channel.

[0044] Photoelectric detector 9: converts the combined optical signal into an electrical signal, namely a beat signal, which contains the distance information of the target object.

[0045] The processing module 10 performs back-end data processing on the beat frequency signal, and utilizes the coherence between the beat frequency signals generated by different comb teeth to realize time domain signal splicing, which is equivalent to improving the sweep bandwidth, thereby improving the ranging accuracy.

[0046] When the detection light hits the target object, part of the light will be reflected back to form echo light. The echo light is combined with the reference light at the beam combiner and enters the photodetector. The photodetector converts the combined optical signal into an electrical signal, namely a beat signal. The frequency of the beat signal is related to the distance of the target object.

[0047] The key to back-end data processing is to distinguish and process the signals obtained by the beat frequency of the left and right halves of the optical comb. The specific steps are as follows:

[0048] S1. Frequency division processing: The received beat frequency signal is divided into two parts, upward sweep and downward sweep, according to the sweep direction, and processed separately; the upward sweep part is processed first

[0049] S2. Spectrum segmentation: Through Fourier transform, the spectrum of the beat frequency signal is divided into two parts with a comb tooth interval of Δf. R The length of the frequency band is divided into [(i-1)Δf R ,iΔf R ];

[0050] S3. Spectrum shifting and flipping: The ith frequency band [(i-1)Δf R ,(i-0.5)Δf R ] sub-band spectrum is moved to baseband [0,0.5Δf R ], [(i-0.5)Δf R ,iΔf R ] The sub-band spectrum is flipped and moved to the baseband [0,0.5Δf R ] (i.e. iΔf R Move to baseband 0 frequency, (i-0.5)Δf R Move to baseband 0.5Δf R ); the spectrum generated by the beat frequency of the right half of the optical comb (i.e., the part greater than the center frequency) corresponds to [(i-1)Δf R ,(i-0.5)Δf R ] sub-band, the spectrum generated by the beat frequency of the left half of the optical comb (i.e., the part smaller than the center frequency) corresponds to [(i-0.5)Δf R ,iΔfR ] sub-band;

[0051] S4. Inverse Fourier transform: Repeat S3 and move N frequency bands, a total of 2N [0,0.5Δf R ], and then perform inverse Fourier transform on all 2N sub-bands to convert them from frequency domain back to time domain;

[0052] S5. Time domain splicing: In the time domain, all signals are processed for phase continuity to ensure that the phase at the end time of the previous signal is equal to the phase at the start time of the next signal. At the same time, amplitude compensation is performed to eliminate amplitude distortion caused by segmentation and movement. Finally, these processed time domain signals are spliced ​​to form a complete upward sweep signal.

[0053] S6. Repeat steps S2 to S5 for the signal of the downward frequency sweep portion to obtain a complete downward frequency sweep spliced ​​signal.

[0054] S7. Compare the Fourier transformed up-sweep stitching signal and the down-sweep stitching signal to obtain the speed and distance information of the target object. That is, by measuring the phase difference and time delay between the two stitching signals, the relative speed and absolute distance of the target object are calculated.

[0055] In this embodiment, a frequency shift is applied to the local reference optical comb to eliminate the image interference in the beat frequency between the reference optical comb and the measurement optical comb. At the same time, by reasonably selecting the frequency shift frequency f FS and the repetition frequency difference Δf R The value of makes it possible to distinguish the signals obtained by the beat frequency of the left and right halves of the optical comb in the spectrum. In this way, the number of comb teeth that can be used for time domain splicing is greatly increased, thereby significantly improving the ranging accuracy.

[0056] Figure 2 It is a schematic diagram of the beat frequency of the measurement optical comb (first optical comb) and the reference optical comb (second optical comb) in the present invention.

[0057] The frequency shift f FS =25MHz, repetition frequency difference Δf R=100MHz. When there is no target, the beat signals of the two optical combs are a series of spectral lines such as 25MHz, 75MHz, 125MHz, and 175MHz, with a frequency band interval of 100MHz. There are two spectral lines of +25MHz and +75MHz in each 100MHz frequency band. When there is a target, the +25MHz spectral line shifts to the right (for example, to +26MHz), and the +75MHz spectral line shifts to the left (for example, to +74MHz), and the frequency shift amount is equal (for example, both are 1MHz). Therefore, taking the first frequency band 0-100MHz as an example, the sub-band of 50MHz-100MHz is mirror-inverted with 50MHz as the center. At this time, the original 74MHz spectral line becomes 26MHz, which can be coherently spliced ​​with the original 26MHz spectral line in the 0-50MHz sub-band. Similarly, the second frequency band 100MHz-200MHz can also be first shifted to 0-100MHz, and then divided into two 50MHz-wide sub-bands for similar processing. Finally Figure 2 The 0~300MHz band contains three 100MHz frequency bands or six 50MHz sub-bands. After the above-mentioned frequency shifting and mirror inversion processing, a total of six spectral lines at 26MHz are obtained, which can realize phase difference splicing, thereby equivalently increasing the measurement time and achieving higher ranging accuracy.

Claims

1. A frequency modulated continuous wave laser ranging device based on frequency shifted dual optical comb, characterized in that: include: The FMCW light source is used to output a bidirectional linear frequency-sweeping laser with a frequency-sweeping bandwidth B and a frequency-sweeping period 2T; A beam splitter, used for receiving the laser output by the FMCW light source and splitting it into a detection light and a reference light; The detection optical channel comprises a first optical comb generator and a circulator, wherein the first optical comb generator is used to generate a comb tooth interval of f R a first optical comb to detect a target object, wherein the circulator is used to transmit the first optical comb to the target object and receive its echo light; The reference optical channel comprises a frequency shifter and a second optical comb generator, wherein the frequency shifter is used to increase the frequency of the reference optical signal by f FS The second optical comb generator is used to generate a comb tooth spacing of f R +Δf R The second optical comb, where Δf R is the repetition frequency difference between the first optical comb and the second optical comb, and f FS modΔf R ≠0; A beam combiner is used to combine the two signals of the detection optical channel and the reference optical channel; A photoelectric detector, used for receiving the combined light signal output by the beam combiner and converting it into a beat frequency signal; The processing module is used to perform back-end data processing on the beat frequency signal, and utilize the coherence between the beat frequency signals generated by different comb teeth to realize time domain signal splicing, so as to equivalently increase the sweep bandwidth B, thereby improving the ranging accuracy.

2. The frequency modulated continuous wave laser ranging device based on frequency shifted dual optical comb according to claim 1, characterized in that: The processing module includes a Fourier transform unit, a spectrum segmentation unit, a spectrum shift and flip unit, an inverse Fourier transform unit, a phase continuity processing unit and an amplitude compensation processing unit, which are used to perform fine processing on the beat frequency signal to achieve time domain signal splicing and improve ranging accuracy.

3. The laser distance measuring device according to claim 2, characterized in that: The processing module further includes a signal comparison unit for comparing the upper frequency sweep splicing signal and the lower frequency sweep splicing signal to obtain speed and distance information of the target object.

4. The frequency modulated continuous wave laser ranging device based on frequency shifted dual optical comb according to claim 1, characterized in that: The FMCW light source adopts a distributed feedback DFB laser, a distributed Bragg reflector DBR laser, a vertical cavity surface emitting VCSEL laser or an external cavity laser ECL.

5. The frequency modulated continuous wave laser ranging device based on frequency shifted dual optical comb according to claim 4, characterized in that: The sweeping period 2T of the bidirectional linear sweeping laser output by the FMCW light source is a preset value to ensure that the echo light of the target object can be received within the sweeping period.

6. The frequency modulated continuous wave laser ranging device based on frequency shifted dual optical comb according to claim 1, characterized in that: The optical combs generated by the first optical comb generator and the second optical comb generator have stable comb tooth spacing and phase relationship to ensure the coherence of the beat frequency signal.

7. The frequency modulated continuous wave laser ranging device based on frequency shifted dual optical comb according to claim 6, characterized in that: The first optical comb generator (3) and the second optical comb generator (7) are both electro-optical combs or Kerr optical combs, and f R <B。 8. The frequency modulated continuous wave laser ranging device based on frequency shifted dual optical comb according to claim 1, characterized in that: The frequency shifter increases the frequency of the optical signal through electro-optic effect, acousto-optic effect or thermo-optic effect.

9. The frequency modulated continuous wave laser ranging device based on frequency shifted dual optical comb according to claim 8, characterized in that: The frequency shifter generates a frequency shift frequency f FS The repetition frequency difference Δf between the two optical combs R 10% to 40%.

10. The frequency modulated continuous wave laser ranging device based on frequency shifted dual optical comb according to claim 1, characterized in that: The back-end data processing includes: S1. Frequency division processing: the received beat frequency signal is divided into two parts, an upward sweep and a downward sweep, according to the sweep direction, and processed separately; S2. Spectrum segmentation: Through Fourier transform, the spectrum of the beat frequency signal is divided into two parts with a comb tooth interval of Δf. R The length of the split; S3. Spectrum shifting and flipping: The divided spectrum sub-bands are shifted and flipped so that the spectrum generated by the beat frequency of the left and right half of the optical comb is shifted to the baseband [0,0.5Δf R ]Inside; S4. Inverse Fourier transform: perform inverse Fourier transform on all processed sub-bands to convert them from frequency domain back to time domain; S5. Time domain splicing: In the time domain, all signals are processed for phase continuity and amplitude compensation, and then the processed time domain signals are spliced ​​to form complete upward sweeping signals and downward sweeping signals respectively; S6. Contrast processing: Compare the upward frequency sweeping stitching signal and the downward frequency sweeping stitching signal after Fourier transformation to obtain the speed and distance information of the target object.

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