A method of laser radar ranging and speed measurement based on phase scanning

Abstract

The present invention belongs to the technical field of laser radar ranging and speed measurement, and discloses a laser radar ranging and speed measurement method based on phase scanning, wherein two bit complementary driving signals respectively drive the laser to generate pulses entering the first unequal arm interferometer, outputting phase coded signals and amplifying and expanding the beam to illuminate the target, and receiving the echo signal reflected by the target; the echo signal enters another unequal arm interferometer for interference, and the interference results are detected and differentiated respectively; the echo signal is phase modulated to calculate the speed of the target; the detection sequence is converted into a receiving sequence according to the positive and negative of the detection result; and the distance information of the target is obtained according to the cross-correlation peak. Compared with the prior art, the present invention can not only greatly reduce the background noise in the echo signal and improve the signal-to-noise ratio of the system ranging by performing phase encoding and decoding on the laser pulse signal and performing differentiation on the echo signal after detection and decoding, but also obtain the speed of the target through phase scanning.

Description

Technical Field

[0001] The present invention relates to the technical field of laser radar ranging and speed measurement, and in particular to a laser radar ranging and speed measurement method based on phase scanning. Background Art

[0002] Radar plays a very important role in the fields of military, civil aviation, and autonomous driving. LiDAR uses a shorter wavelength laser as the information carrier, so it has a higher resolution and can be used to detect, track, and identify targets, as well as to measure distance, speed, and imaging.

[0003] For ranging and speed measurement, the commonly used method is to perform coherent detection on the echo signal after frequency shifting the local oscillator light and continuously frequency modulated or randomly amplitude or phase modulated, measure speed by measuring the Doppler frequency, and perform cross-correlation ranging after demodulating the echo signal. Coherent laser radar has a natural filtering effect. Using the local oscillator light and the real signal for coherent detection can not only filter out irrelevant background noise, but also have a certain gain. However, at long distances, the decoherence effect of the echo signal is more significant, making coherent detection difficult, and the influence of background noise under sunlight increases, making the signal-to-noise ratio of the system low. The Chinese patent with the authorization public number CN118011415B proposes a two-pulse relative phase encoding method to solve the decoherence problem caused by atmospheric turbulence and other influences. However, this scheme cannot measure the speed of the target, and when the target moves at a high speed, the Doppler frequency shift will cause the echo signal frequency to change, thereby causing the phase difference between the two pulses to change, which in turn increases the bit error of the interference decoding and affects the ranging performance. Summary of the invention

[0004] In view of the above defects in the prior art, the present invention proposes a laser radar ranging and speed measurement method based on phase scanning.

[0005] The technical solution of the present invention is achieved in this way:

[0006] A laser radar ranging and speed measurement method based on phase scanning comprises the following steps:

[0007] S1: The main control module generates a first sequence and a second sequence with complementary bits, which constitute a first driving signal and a second driving signal respectively;

[0008] S2: Two driving signals drive the laser to generate pulses that enter the first unequal-arm interferometer and output phase-coded signals;

[0009] S3: amplify and expand the phase-coded signal to illuminate the target, and receive the echo signal reflected by the target;

[0010] S4: The echo signal enters the second unequal-arm interferometer for interference, and the interference results are detected and then differentiated;

[0011] S5: Phase the echo signal, maximize the absolute value of the differential result through an optimization algorithm, and calculate the speed of the target;

[0012] S6: converting the detection sequence into a receiving sequence according to the positive or negative value of the detection result;

[0013] S7: Perform shift cross-correlation operation on the first sequence and the received sequence, and obtain the distance information of the target according to the cross-correlation peak value.

[0014] Preferably, in S1, the main control module generates a first sequence in which 0 and 1 bits appear randomly, and inverts the 0 and 1 bits in the first sequence to obtain a second sequence, and generates a first driving signal and a second driving signal for triggering the corresponding laser according to the first sequence and the second sequence, wherein the 0 bit corresponds to a low level and does not trigger the laser to emit a laser pulse; and the 1 bit corresponds to a high level and triggers the laser to emit a laser pulse;

[0015] S2: the first driving signal triggers the laser to emit a laser pulse signal, which enters the first input port of the first unequal-arm interferometer to generate a first twin pulse signal with a phase difference of 0; the second driving signal triggers the laser to emit a laser pulse signal, which enters the second input port of the first unequal-arm interferometer to generate a second twin pulse signal with a phase difference of π; the first twin pulse signal and the second twin pulse signal are randomly emitted from the output port of the first unequal-arm interferometer to form a phase coded signal;

[0016] S3: the phase-coded signal is amplified to a predetermined power by an optical amplifier module, and then irradiated to the target after beam expansion by a transmitting telescope, and an echo signal reflected from the target is received by a receiving telescope;

[0017] S4: the echo signal enters the second unequal-arm interferometer for interference, and the two interference signals generated enter the first detection module and the second detection module for detection respectively, and the two detection results are differentiated by using a differential circuit to obtain a detection sequence;

[0018] S5: the main control module controls the phase modulation module on the long arm of the second unequal-arm interferometer to perform phase scanning, stops the phase scanning when the absolute value of the detection sequence reaches the maximum through the optimization algorithm, and calculates the speed of the target according to the corresponding scanning phase;

[0019] S6: converting the detection sequence into a receiving sequence according to the positive or negative value of the detection result at this time, wherein the result greater than 0 is 0, the result less than 0 is 1, and the result equal to 0 is randomly assigned to 0 or 1;

[0020] S7: performing a shift cross-correlation operation on the first sequence and the received sequence, and obtaining the distance information of the target according to the position corresponding to the peak value of the cross-correlation operation result.

[0021] Preferably, the predetermined optimization algorithm in step S5 is a gradient descent algorithm.

[0022] Preferably, the predetermined optimization algorithm in step S5 is a PID control algorithm.

[0023] Preferably, the phase modulation module is a phase modulator PM.

[0024] Preferably, the first detection module and the second detection module are single-photon balanced detectors.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] The present invention proposes a laser radar ranging and speed measurement method based on phase scanning. The laser pulse signal is phase-encoded and decoded, the decoded echo signal is detected by a high-sensitivity balanced detector for differentiation, and the receiving unequal-arm interferometer is phase-scanned to maximize the absolute value of the differential signal. This method can not only greatly reduce the background noise in the echo signal and improve the signal-to-noise ratio of the system ranging, but also obtain the speed of the target through phase scanning. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a process diagram of ranging and speed measurement of the laser radar ranging and speed measurement method based on phase scanning of the present invention;

[0028] Figure 2a is the frequency diagram of the two sub-pulses when they fail to reach the target in the method of the present invention (both are f0);

[0029] Figure 2b The frequency diagram of the two sub-pulses in the method of the present invention is that the first sub-pulse is reflected by the target, while the second sub-pulse does not reach the target (the first sub-pulse is f1, and the second sub-pulse is f0);

[0030] Figure 2c The two sub-pulses in the method of the present invention are both reflected by the target, and the frequency diagram of the two (both are f1). DETAILED DESCRIPTION

[0031] The present invention will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present invention.

[0032] like Figure 1 As shown, a laser radar ranging and speed measurement method based on phase scanning includes the following steps:

[0033] S1: The main control module generates a first sequence of 0 and 1 bits that appear randomly, and inverts the 0 and 1 bits in the first sequence to obtain a second sequence, and generates a first drive signal and a second drive signal for triggering LD1 and LD2 according to the first sequence and the second sequence, wherein the 0 bit corresponds to a low level, and does not trigger LD1 and LD2 to emit laser pulses; the 1 bit corresponds to a high level, and triggers LD1 and LD2 to emit laser pulses;

[0034] S2: The first driving signal triggers LD1 to emit a laser pulse signal, which enters the first input port of the first unequal-arm interferometer to generate a first twin pulse signal with a phase difference of 0; the second driving signal triggers LD2 to emit a laser pulse signal, which enters the second input port of the first unequal-arm interferometer to generate a second twin pulse signal with a phase difference of π; the first twin pulse signal and the second twin pulse signal are randomly emitted from the output port of the first unequal-arm interferometer to form a phase coded signal;

[0035] S3: The phase-coded signal is amplified to a predetermined power by an optical amplifier module, and then irradiated to the target after beam expansion by a transmitting telescope, and an echo signal reflected from the target is received by a receiving telescope;

[0036] S4: the echo signal enters the second unequal-arm interferometer for interference, and the two interference signals generated enter the first detection module and the second detection module for detection respectively, and the two detection results are differentiated by a differential circuit to obtain a detection sequence;

[0037] S5: The main control module controls the phase modulation module on the long arm of the second unequal-arm interferometer to perform phase scanning, stops the phase scanning when the absolute value of the detection sequence reaches the maximum through the optimization algorithm, and calculates the speed of the target according to the corresponding scanning phase;

[0038] S6: convert the detection sequence into a receiving sequence according to the positive or negative value of the detection result at this time, wherein the result greater than 0 is 0, the result less than 0 is 1, and the result equal to 0 is randomly assigned to 0 or 1;

[0039] S7: Perform a shift cross-correlation operation on the first sequence and the received sequence, and obtain the distance information of the target according to the position corresponding to the peak value of the cross-correlation operation result.

[0040] The specific working process is as follows:

[0041] The main control module generates a first sequence of 0 and 1 bits that appear randomly, and inverts the 0 and 1 bits in the first sequence to obtain a second sequence, that is, the two are complementary. According to the first sequence and the second sequence, two first drive signals and second drive signals for triggering LD1 and LD2 are generated, wherein the 0 bit corresponds to a low level, and does not trigger LD1 and LD2 to emit laser pulses; the 1 bit corresponds to a high level, triggering LD1 and LD2 to generate a narrow pulse width laser pulse signal respectively. Since the levels that trigger the two lasers are exactly opposite, when LD1 emits light, LD2 does not emit light; conversely, when LD1 does not emit light, LD2 emits light.

[0042] The first driving signal triggers the laser pulse signal generated by LD1 to enter the first input port of the first unequal arm interferometer, generating a first twin pulse signal with a phase difference of 0; the second driving signal triggers LD2 to send a laser pulse signal, which enters the second input port of the first unequal arm interferometer, generating a second twin pulse signal with a phase difference of π; the first twin pulse signal and the second twin pulse signal are randomly emitted from the output port of the first unequal arm interferometer to form a phase coded signal. Therefore, the phase difference between the two sub-pulse signals emitted from the output port of the first unequal arm interferometer is 0 or π, thereby obtaining a phase coded signal. It is then amplified to a predetermined power by the optical amplifier module, expanded through the transmitting telescope and irradiated to the target.

[0043] After being reflected by the target, the phase-coded signal enters the receiving telescope and becomes an echo signal, and then enters the second unequal-arm interferometer for interference. The interference results emitted from its two input ports can be written as

[0044] ,

[0045] Among them, θ is the phase difference between the two sub-pulse signals, and A is the sub-pulse amplitude entering the unequal-arm interferometer. The background noise signal accompanying the two sub-pulse signals in the echo signal has a random phase and does not interfere, so the noise amplitude emitted from the two input ports is the same, that is, , the influence of noise can be eliminated by differentiating the two interference results.

[0046] Assume that the Doppler effect of the target motion causes the frequency of the echo signal to change from f0 to f1. Since the two sub-pulses of the phase-coded signal arrive at the target at different times, the time of their frequency change is also different, resulting in a change in the phase difference between the two. The specific process is as follows: Figure 2a , 2b , as shown in 2c. Among them, Figure 2a When the two sub-pulses do not reach the target, the frequencies of both are f0; Figure 2b The first sub-pulse is reflected by the target, and the frequency becomes f1, while the second sub-pulse does not reach the target, and the frequency is still f0; Figure 2cBoth sub-pulses are reflected by the target, and their frequencies become f1. The two unequal-arm interferometers are exactly the same, with the lengths of the long arm and the short arm being L1 and L2 respectively, and the target distance being L. Ignoring the same path lengths taken by the two sub-pulses in the system, the paths taken by the two sub-pulses interfering in the unequal-arm interferometers are L1+2L+L2 and L2+2L+L1 respectively. The phase changes experienced by the two are φ1=2π*[f0*(n1*L1+L)+f1*(L+n2*L2)] / c and φ2=2π*[f0*(n1*L2+L)+f1*(L+n2*L1)] / c, respectively, where n1 and n2 are the refractive indices of the light pulses with frequencies f0 and f1 in the optical fiber, respectively, and c is the speed of light. The phase difference between the two is δ=φ1-φ2=2π* (f1*n2-f0*n1)*ΔL / c, and ΔL=L1-L2 is the arm length difference of the unequal-arm interferometer.

[0047] Since the laser pulses generated by LD1 and LD2 are incident from the two input ports of the first unequal-arm interferometer respectively, the phase difference between the two sub-pulses generated has a phase difference of π, that is, 0 or π phase encoding is achieved. Therefore, when LD1 is lit, the phase difference between the two sub-pulses when interfering is θ=δ; when LD2 is lit, the phase difference between the two sub-pulses when interfering is θ=δ+π. The two interference results enter the first detection module and the second detection module for detection respectively, and the two detection results are differentiated using a differential circuit, and the differential result can be obtained as follows:

[0048] ,

[0049] Where α=0 or π is the encoding phase. When α=0, ; When α=π, Whether α=0 or π, the absolute value of the difference result remains unchanged, that is, , which can be used as a feedback signal.

[0050] The main control module controls the phase modulation module to perform phase scanning in the range [-π / 2, π / 2]. Assuming that the phase modulated by the phase modulation module is ξ, the differential result can be written as

[0051] ,

[0052] By changing ξ to offset the phase change δ caused by Doppler frequency shift, the absolute value of the differential result can be maximized. Use a predetermined optimization algorithm, such as a gradient descent algorithm or a PID control algorithm, so that -ξ→δ, that is, 2π* (f1*n2-f0*n1)*(L1-L2) / c = -ξ, The phase scan is stopped when the maximum value is reached. Since the Doppler frequency shift is very small relative to the frequency of the laser pulse itself, the difference between n1 and n2 is also very small. Let n1=n2=n, and the Doppler shift frequency Δf=f1-f0=-cξ / (2πnΔL) can be obtained, so the speed of the target can be calculated.

[0053] ,

[0054] By phase scanning, the amplitude of the differential result is A and -A when α=0 and π respectively, and replaced with 0 and 1 respectively, the received sequence can be obtained. The first sequence is shifted bit by bit and cross-correlated with the received sequence to obtain the cross-correlation value at each shift. When the cross-correlation value reaches the peak, it indicates that the transmit and receive sequences correspond one to one. The corresponding target distance can be obtained by the number of bits shifted, realizing the ranging function.

[0055] The present invention proposes a laser radar ranging and speed measurement method based on phase scanning. The laser pulse signal is phase-encoded and decoded, the decoded echo signal is detected by a high-sensitivity balanced detector for differentiation, and the receiving unequal-arm interferometer is phase-scanned to maximize the absolute value of the differential signal. This method can not only greatly reduce the background noise in the echo signal and improve the signal-to-noise ratio of the system ranging, but also obtain the speed of the target through phase scanning.

Claims

1. A laser radar ranging and speed measurement method based on phase scanning, characterized in that: The following steps are involved: S1: The main control module generates a first sequence and a second sequence with complementary bits, which constitute a first driving signal and a second driving signal respectively; S2: Two driving signals drive the laser to generate pulses that enter the first unequal-arm interferometer and output phase-coded signals; S3: amplify the phase-coded signal, expand the beam, illuminate the target, and receive the echo signal reflected by the target; S4: The echo signal enters the second unequal-arm interferometer for interference, and the interference results are detected and then differentiated; S5: Phase the echo signal, maximize the absolute value of the differential result through an optimization algorithm, and calculate the speed of the target; S6: converting the detection sequence into a receiving sequence according to the positive or negative value of the detection result; S7: Perform shift cross-correlation operation on the first sequence and the received sequence, and obtain the distance information of the target according to the cross-correlation peak value. S1: The main control module generates a first sequence of bits 0 and 1 that appear randomly, and simultaneously inverts the bits 0 and 1 in the first sequence to obtain a second sequence, and generates a first drive signal and a second drive signal for triggering the corresponding laser according to the first sequence and the second sequence, wherein the bit 0 corresponds to a low level and does not trigger the laser to emit a laser pulse; the bit 1 corresponds to a high level and triggers the laser to emit a laser pulse; S2: the first driving signal triggers the laser to emit a laser pulse signal, which enters the first input port of the first unequal-arm interferometer to generate a first twin pulse signal with a phase difference of 0; the second driving signal triggers the laser to emit a laser pulse signal, which enters the second input port of the first unequal-arm interferometer to generate a second twin pulse signal with a phase difference of π; The first twin pulse signal and the second twin pulse signal are randomly emitted from the output port of the first unequal-arm interferometer to form a phase-coded signal; S3: the phase-coded signal is amplified to a predetermined power by an optical amplifier module, and then irradiated to the target after beam expansion by a transmitting telescope, and an echo signal reflected from the target is received by a receiving telescope; S4: the echo signal enters the second unequal-arm interferometer for interference, and the two interference signals generated enter the two detection modules for detection respectively, and the two detection results are differentiated by using a differential circuit to obtain a detection sequence; S5: the main control module controls the phase modulation module on the long arm of the second unequal-arm interferometer to perform phase scanning, stops the phase scanning when the absolute value of the detection sequence reaches the maximum through the optimization algorithm, and calculates the speed of the target according to the corresponding scanning phase; S6: converting the detection sequence into a receiving sequence according to the positive or negative value of the detection result at this time, wherein the result greater than 0 is 0, the result less than 0 is 1, and the result equal to 0 is randomly assigned to 0 or 1; S7: performing a shift cross-correlation operation on the first sequence and the received sequence, and obtaining the distance information of the target according to the position corresponding to the peak value of the cross-correlation operation result.

2. The laser radar ranging and speed measurement method based on phase scanning according to claim 1 is characterized in that: The predetermined optimization algorithm in step S5 is a gradient descent algorithm.

3. The phase scanning-based laser radar ranging and speed measurement method according to claim 1, characterized in that: The predetermined optimization algorithm in step S5 is a PID control algorithm.

4. The laser radar ranging and speed measurement method based on phase scanning according to claim 1 is characterized in that: The phase modulation module is a phase modulator PM.

5. The laser radar ranging and speed measurement method based on phase scanning according to claim 1 is characterized in that: The detection modules are all single-photon-level balanced detectors.

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