A phase-coded laser radar distance and speed measurement method

Abstract

This invention belongs to the field of lidar ranging and velocity measurement technology, and discloses a phase-coded lidar ranging and velocity measurement method. Two complementary driving signals drive light pulses to be generated and fed into an unequal-arm interferometer, outputting a phase-coded signal to illuminate the target. The echo signal reflected from the target is received. The echo signal is polarized and then interfered with, and the interference result is differentially divided. The detection sequence is converted into a receiving sequence according to the positive or negative sign of the detection result. The first sequence and the receiving sequence are cross-correlated by shifting, and the target distance information is obtained from the cross-correlation peak value. The phase change caused by the Doppler effect is calculated based on the coded phase and the detection result, and then the target velocity is calculated. Compared with the prior art, this invention can not only significantly reduce the background noise in the echo signal and improve the signal-to-noise ratio of the system ranging, but also directly calculate the phase change caused by the Doppler frequency shift of the target motion from the interference result, thereby obtaining the target velocity.

Description

Technical Field

[0001] The present invention relates to the technical field of laser radar ranging and speed measurement, and in particular to a phase-coded laser radar ranging and speed measurement method. 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 local oscillator light and real signals 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. The influence of background noise increases under sunlight, making the signal-to-noise ratio of the system relatively low. The Chinese invention patent with announcement number CN118191861B proposes a two-pulse relative phase encoding method to solve the decoherence problem caused by atmospheric turbulence and other influences. It only requires an unequal-arm interferometer. No phase drift will occur during the flight time of the optical pulse signal, which can make the echo signal interfere stably without the need for additional devices and algorithms for phase stabilization. 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, resulting in a change in the phase between the two pulses, 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

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

[0006] A phase-coded laser radar ranging and speed measurement method 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 light pulses and enter the unequal-arm interferometer to output phase-encoded signals;

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

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

[0011] S5: converting the detection sequence into a receiving sequence according to the positive or negative value of the detection result;

[0012] S6: 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;

[0013] S7: Calculate the phase change caused by the Doppler effect according to the coding phase of the phase coding signal and the detection result, and then calculate the target speed.

[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 corresponding laser to emit a laser pulse signal, which enters the first input port of the unequal-arm interferometer to generate a first twin pulse signal with a phase difference of 0; the second driving signal triggers the corresponding laser to emit a laser pulse signal, which enters the second input port of the 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 first output port of the 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: After the echo signal is polarized, it enters the unequal-arm interferometer for interference, and the two interference signals generated enter the corresponding detection modules for detection, and the two detection results are detected using a differential circuit. Performing differentiation to obtain a detection sequence;

[0018] S5: According to the detection results The positive or negative value of the detection sequence is converted into a receiving sequence, where 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;

[0019] S6: 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

[0020] S7: According to the coding phase α and the detection result , calculate the phase change due to the Doppler effect , and then calculate the target speed as , where λ is the central wavelength of the laser pulse, c is the speed of light, n is the refractive index of the fiber, and ΔL is the arm length difference of the unequal-arm interferometer.

[0021] Preferably, the polarization processing in S4 is to polarize the echo signal through a horizontally polarized polarizer; the polarized echo signal passes through an optical isolator in the forward direction and enters from the second output port of the unequal-arm interferometer for interference, and the two interference signals generated each enter the first detection module and the second detection module through a circulator for detection, and a differential circuit is used to differentiate the two detection results to obtain a detection sequence.

[0022] Preferably, the polarization processing in S4 is to perform polarization splitting on the echo signal; one echo signal component after polarization splitting passes through an optical isolator in the forward direction and enters from the second output port of the unequal-arm interferometer for interference; the other echo signal component passes through a circulator and enters from the first output port of the unequal-arm interferometer for interference; the two interference signals generated each pass through a circulator into the first detection module and the second detection module for detection, and a differential circuit is used to differentiate the two detection results to obtain a detection sequence.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] The present invention proposes a phase-coded laser radar ranging and speed measurement method, which performs phase encoding and decoding on laser pulse signals and detects the decoded echo signals with a high-sensitivity balanced detector for differentiation. This method can not only significantly reduce the background noise in the echo signals and improve the signal-to-noise ratio of the ranging system, but also directly calculate the phase change caused by the Doppler frequency shift of the target motion through the interference results, thereby obtaining the target's speed. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a process diagram of ranging and speed measurement of the phase-coded laser radar ranging and speed measurement method of the present invention;

[0026] 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);

[0027] 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);

[0028] Figure 2cThe 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

[0029] The present invention will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present invention.

[0030] like Figure 1 As shown, a phase-coded laser radar ranging and speed measurement method comprises the following steps:

[0031] 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;

[0032] S2: Two driving signals drive the laser to generate light pulses and enter the unequal-arm interferometer to output phase-encoded signals;

[0033] S3: amplify the phase-coded signal, expand the beam, illuminate the target, and receive the echo signal reflected by the target;

[0034] S4: After polarization processing, the echo signal enters the unequal-arm interferometer for interference, and the interference results are detected and then differentiated;

[0035] S5: converting the detection sequence into a receiving sequence according to the positive or negative value of the detection result;

[0036] S6: 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;

[0037] S7: Calculate the phase change caused by the Doppler effect according to the coding phase of the phase coding signal and the detection result, and then calculate the target speed.

[0038] The present invention provides a phase-coded laser radar ranging and speed measurement method, which specifically comprises the following steps:

[0039] 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 first laser LD1 and the second laser 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;

[0040] S2: The first driving signal triggers LD1 to emit a laser pulse signal, which enters the first input port of the 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 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 first output port of the unequal-arm interferometer to form a phase coded signal;

[0041] 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;

[0042] S4: After polarization processing, the echo signal enters the unequal-arm interferometer for interference. The two interference signals generated enter the first detection module and the second detection module for detection respectively, and the two detection results are combined using a differential circuit. Performing differentiation to obtain a detection sequence;

[0043] S5: According to the detection results The positive or negative value of the detection sequence is converted into a receiving sequence, where 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;

[0044] S6: Perform 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.

[0045] S7: According to the coding phase α and the detection result , calculate the phase change due to the Doppler effect , and then calculate the target speed as , where λ is the central wavelength of the laser pulse, c is the speed of light, n is the refractive index of the fiber, and ΔL is the arm length difference of the unequal-arm interferometer.

[0046] The specific working process is as follows:

[0047] 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 the first laser LD1 and the second laser 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, and triggers LD1 and LD2 to generate a narrow pulse width laser pulse signal respectively. Since the levels for triggering 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.

[0048] The first driving signal triggers the laser pulse signal generated by LD1 to enter the unequal arm interferometer and enter the first input port of the 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 unequal arm interferometer and generates 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 first output port of the unequal arm interferometer to form a phase coded signal. Therefore, the phase difference between the two sub-pulse signals emitted from the first output port of the 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.

[0049] After being reflected by the target, the phase-coded signal enters the receiving telescope and becomes an echo signal. After polarization processing, it enters the unequal-arm interferometer for interference. The interference results emitted from its two input ports can be written as

[0050] ,

[0051] 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.

[0052] 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 lengths of the long arm and short arm of the unequal-arm interferometer are L1 and L2 respectively, the target distance is L, and the same path length of the two sub-pulses in the system is ignored. Then, the paths of the two sub-pulses interfering in the unequal-arm interferometer 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, 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.

[0053] Since the laser pulses generated by LD1 and LD2 are incident from the two input ports of the 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:

[0054] ,

[0055] Where α=0 or π is the encoding phase. When α=0, ; When α=π, .

[0056] When -π / 2<δ<π / 2, The sign of remains unchanged. According to the positive or negative of the detection result at this time, the detection sequence is converted into the receiving sequence, where 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. The first sequence is shifted bit by bit and cross-correlated with the receiving sequence to obtain the cross-correlation value at each shift. When the cross-correlation value reaches the peak value, it indicates that the sending and receiving sequences correspond one to one. The corresponding target distance can be obtained by the number of bits shifted, realizing the ranging function.

[0057] After the transmit and receive sequences are matched one by one, the coding phase α=0 or π corresponding to each interference result can be determined, and then the phase change caused by the Doppler effect can be calculated based on the interference result.

[0058] ,

[0059] 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, so n1=n2=n can be set, and the target speed can be finally obtained as .

[0060] When the arm length difference of the unequal-arm interferometer is L1-L2=1m, The range corresponds to a speed range of ±38.75 m / s, where n1=1.5 is the refractive index of the 1.55 um light pulse in the optical fiber, and c is the speed of light.

[0061] The present invention proposes a phase-coded laser radar ranging and speed measurement method, which performs phase encoding and decoding on laser pulse signals and detects the decoded echo signals with a high-sensitivity balanced detector for differentiation. This method can not only significantly reduce the background noise in the echo signals and improve the signal-to-noise ratio of the ranging system, but also directly calculate the phase change caused by the Doppler frequency shift of the target motion through the interference results, thereby obtaining the target's speed.

Claims

1. A phase-coded laser radar ranging and speed measurement method, 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 light pulses and enter the unequal-arm interferometer to output phase-encoded signals; S3: amplify the phase-coded signal, expand the beam, illuminate the target, and receive the echo signal reflected by the target; S4: After polarization processing, the echo signal enters the unequal-arm interferometer for interference, and the interference results are detected and then differentiated; S5: converting the detection sequence into a receiving sequence according to the positive or negative value of the detection result; S6: 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; S7: Calculate the phase change caused by the Doppler effect based on the coding phase of the phase coding signal and the detection result, and then calculate the target speed. 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 corresponding laser to emit a laser pulse signal, which enters the first input port of the unequal-arm interferometer to generate a first twin pulse signal with a phase difference of 0; the second driving signal triggers the corresponding laser to emit a laser pulse signal, which enters the second input port of the 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 first output port of the 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: After the echo signal is polarized, it enters the unequal-arm interferometer for interference, and the two interference signals generated enter the corresponding detection modules for detection, and the two detection results are detected using a differential circuit. Perform differential analysis to obtain a detection sequence; S5: According to the detection results The positive or negative value of the detection sequence is converted into a receiving sequence, where 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; S6: 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 S7: According to the coding phase α and the detection result , calculate the phase change due to the Doppler effect , and then calculate the target speed as , where λ is the central wavelength of the laser pulse, c is the speed of light, n is the refractive index of the fiber, and ΔL is the arm length difference of the unequal-arm interferometer.

2. The phase-coded laser radar ranging and speed measurement method according to claim 1, characterized in that: The polarization processing in S4 is to polarize the echo signal through a horizontally polarized polarizer; the polarized echo signal passes through an optical isolator in the forward direction and enters from the second output port of the unequal-arm interferometer for interference, and the two interference signals generated each pass through a circulator to enter the first detection module and the second detection module for detection, and a differential circuit is used to differentiate the two detection results to obtain a detection sequence.

3. The phase-coded laser radar ranging and speed measurement method according to claim 1, characterized in that: The polarization processing in S4 is to perform polarization splitting on the echo signal; one echo signal component after polarization splitting passes through an optical isolator in the forward direction and enters from the second output port of the unequal-arm interferometer for interference; the other echo signal component passes through a circulator and enters from the first output port of the unequal-arm interferometer for interference; the two generated interference signals each pass through a circulator to enter the first detection module and the second detection module for detection, and a differential circuit is used to differentiate the two detection results to obtain a detection sequence.

Images