A phase-coded laser radar system based on frequency sweeping and a method for measuring distance and speed

Abstract

The present invention belongs to the technical field of laser radar ranging and speed measurement, and discloses a phase-coded laser radar system based on frequency sweeping and a ranging and speed measurement method, wherein the system comprises two lasers, two / three circulators, an unequal-arm interferometer, an optical fiber amplifier, a transmitting telescope, a receiving telescope, a frequency sweeping module, an optical isolator, a polarization beam splitter, two balanced detection modules, a differential circuit and a main control module. Compared with the prior art, the system performs phase encoding and decoding on the laser pulse signal, detects the decoded echo signal through a highly sensitive balanced detector for differential, and performs frequency sweeping on the echo signal so that the absolute value of the differential signal reaches the maximum, which 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 obtain the speed of the target through the frequency sweeping result.

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 system based on frequency sweeping and a 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 the authorization public 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 phase-coded laser radar system based on frequency scanning and a distance and speed measurement method.

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

[0006] A phase-coded laser radar system based on frequency sweeping includes lasers LD1 and LD2, circulators CIR1 and CIR2, a first beam splitter BS1 and a second beam splitter BS2, an optical fiber amplifier, a transmitting telescope, a receiving telescope, a frequency sweeping module, an optical isolator ISO, a polarizer POL, a first balanced detection module HD1, a second balanced detection module HD2, a differential circuit and a main control module.

[0007] The three ports of CIR1 are connected to LD1, the first input port of BS1, and HD1 respectively;

[0008] The three ports of CIR2 are connected to LD2, the second input port of BS1, and HD2 respectively;

[0009] The two output ports of BS1 are connected to the two input ports of BS2 through optical fibers of unequal lengths, forming an unequal-arm interferometer;

[0010] The first output port and the second output port of BS2 are connected to the input port of the optical fiber amplifier and the output port of the optical isolator respectively;

[0011] Connect the output port of the fiber amplifier to the transmitting telescope;

[0012] The receiving telescope is connected to the frequency scanning module, the polarizer POL, and the input port of the optical isolator in sequence;

[0013] The two input ports of the differential circuit are respectively connected to the first balance detection module HD1 and the second balance detection module HD2;

[0014] The main control module is respectively connected to LD1, LD2, the frequency sweep module and the differential circuit.

[0015] The present invention also provides another phase-coded laser radar system based on frequency sweeping, comprising lasers LD1, LD2, circulators CIR1, CIR2, a first beam splitter BS1, a second beam splitter BS2, an optical fiber amplifier, a transmitting telescope, a receiving telescope, CIR3, a frequency sweeping module, an optical isolator ISO, a polarization beam splitter PBS|, a first balanced detection module HD1, a second balanced detection module HD2, a differential circuit and a main control module.

[0016] The three ports of CIR1 are connected to LD1, the first input port of BS1, and HD1 respectively;

[0017] The three ports of CIR2 are connected to LD2, the second input port of BS1, and HD2 respectively;

[0018] The two output ports of BS1 are connected to the two input ports of BS2 through optical fibers of unequal lengths, forming an unequal-arm interferometer;

[0019] The first output port and the second output port of BS2 are connected to the second port of CIR3 and the output port of the optical isolator respectively;

[0020] The first port and the third port of CIR3 are connected to the reflection port of PBS and the input port of the optical fiber amplifier respectively;

[0021] The output port of the fiber amplifier is connected to the transmitting telescope;

[0022] The receiving telescope is connected to the input port of the frequency sweep module and PBS in sequence;

[0023] The transmission port of the PBS is connected to the input port of the optical isolator;

[0024] The two input ports of the differential circuit are respectively connected to the first balance detection module HD1 and the second balance detection module HD2;

[0025] The main control module is respectively connected to LD1, LD2, the frequency sweep module and the differential circuit.

[0026] Preferably, the frequency sweep module is an acousto-optic modulator (AOM).

[0027] Preferably, HD1 and HD2 are single-photon level balanced detectors.

[0028] The present invention also provides a phase-coded laser radar ranging and speed measurement method based on frequency sweeping, comprising the following steps:

[0029] The following steps are involved:

[0030] 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;

[0031] S2: Two driving signals drive the laser to generate light pulses and enter the unequal-arm interferometer to output phase-encoded signals;

[0032] S3: amplify the phase-coded signal, expand the beam, illuminate the target, and receive the echo signal reflected by the target;

[0033] S4: After polarization processing, the echo signal enters the unequal-arm interferometer for interference, and the interference results are detected and then differentiated;

[0034] S5: Sweep the echo signal, maximize the absolute value of the differential result through the optimization algorithm, and calculate the speed of the target;

[0035] S6: converting the detection sequence into a receiving sequence according to the positive or negative value of the detection result;

[0036] 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.

[0037] Preferably, the polarization processing in step 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.

[0038] Preferably, the polarization processing in step 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 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.

[0039] Preferably, the predetermined optimization algorithm in step S5 is a gradient descent algorithm or a PID control algorithm.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] The present invention proposes a phase-coded laser radar system based on frequency scanning and a distance and speed measurement method. The laser pulse signal is phase-encoded and decoded, and the decoded echo signal is detected by a high-sensitivity balanced detector for differentiation. The echo signal is frequency-sweeped so that the absolute value of the differential signal reaches the maximum. This can not only greatly reduce the background noise in the echo signal and improve the signal-to-noise ratio of the system distance measurement, but also obtain the speed of the target through the frequency scanning result. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 This is a process diagram of ranging and speed measurement of the phase-coded laser radar ranging and speed measurement method based on frequency sweeping of the present invention;

[0043] 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);

[0044] 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);

[0045] 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);

[0046] Figure 3 It is a schematic diagram of Embodiment 1 of the phase-coded laser radar system based on frequency sweeping of the present invention;

[0047] Figure 4 This is a schematic diagram of the second embodiment of the phase-coded laser radar system based on frequency sweeping of the present invention. DETAILED DESCRIPTION

[0048] The present invention will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present invention.

[0049] like Figure 1 As shown, a phase-coded laser radar ranging and speed measurement method based on frequency sweeping includes the following steps:

[0050] 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;

[0051] 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;

[0052] S3: The phase-coded signal is amplified to a predetermined power by a frequency sweep module and an optical amplifier module, and then irradiated to the target after beam expansion through a transmitting telescope, and an echo signal reflected from the target is received by a receiving telescope;

[0053] S4: After the echo signal is polarized, it 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 differentiated by a differential circuit to obtain a detection sequence;

[0054] S5: The main control module controls the frequency sweep module to perform frequency sweep in the range [f-Δ, f+Δ], where f is the center frequency of the laser and 2Δ is the frequency sweep range; the frequency sweep is stopped when the absolute value of the detection sequence reaches the maximum through a predetermined optimization algorithm, and the speed of the target is calculated according to the corresponding frequency shift amount;

[0055] 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;

[0056] 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.

[0057] The specific working process is as follows:

[0058] 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.

[0059] 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.

[0060] 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

[0061] ,

[0062] 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.

[0063] 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, and the target distance is L. Ignoring the same path length of the two sub-pulses in the system, the paths of the two sub-pulses that interfere in the unequal-arm interferometer are L1+2L+L2 and L2+2L+L1 respectively. The phase changes they experience 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 optical 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)*(L1-L2) / c.

[0064] 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:

[0065] ,

[0066] 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.

[0067] The main control module controls the frequency sweep module to perform frequency sweep in the range [f-Δ, f+Δ], where f is the center frequency of the laser and 2Δ is the frequency sweep range, eliminating the phase change caused by the Doppler frequency shift, so that the absolute value of the differential result reaches the maximum. Use a predetermined optimization algorithm, such as a gradient descent algorithm or a PID control algorithm, so that δ→0, that is, when (f1+fs)*n2=f0*n1, The frequency sweep stops when the maximum value is reached. At this time, the speed of the target can be calculated based on the corresponding frequency shift fs.

[0068] ,

[0069] 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. The frequency shift fs of the sweep frequency can be approximately equal to the Doppler frequency, and the target speed can be obtained as .

[0070] By sweeping the frequency to make δ→0, we can get the amplitude of the differential result when α=0 and π is A and -A respectively. Correspondingly, we can replace them with 0 and 1 respectively to get the received sequence. The first sequence is shifted bit by bit and cross-correlated with the received sequence to get 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.

[0071] like Figure 3 As shown, the first embodiment of the laser radar system of the present invention:

[0072] A phase-coded laser radar system based on frequency sweeping includes lasers LD1 and LD2, circulators CIR1 and CIR2, a first beam splitter BS1 and a second beam splitter BS2, an optical fiber amplifier, a transmitting telescope, a receiving telescope, a frequency sweeping module, an optical isolator ISO, a polarizer POL, a first balanced detection module HD1, a second balanced detection module HD2, a differential circuit and a main control module.

[0073] The three ports of CIR1 are connected to LD1, the first input port of BS1, and HD1 respectively;

[0074] The three ports of CIR2 are connected to LD2, the second input port of BS1, and HD2 respectively;

[0075] The two output ports of BS1 are connected to the two input ports of BS2 through optical fibers of unequal lengths, forming an unequal-arm interferometer;

[0076] The first output port and the second output port of BS2 are connected to the input port of the optical fiber amplifier and the output port of the optical isolator respectively;

[0077] Connect the output port of the fiber amplifier to the transmitting telescope;

[0078] The receiving telescope is connected to the frequency scanning module, the polarizer POL, and the input port of the optical isolator in sequence;

[0079] The two input ports of the differential circuit are respectively connected to the first balance detection module HD1 and the second balance detection module HD2;

[0080] The main control module is respectively connected to LD1, LD2, the frequency sweep module and the differential circuit.

[0081] The specific working process of embodiment 1 is as follows:

[0082] 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.

[0083] The laser pulse signal generated by the first driving signal triggers LD1 to enter the unequal arm interferometer from one input port of BS1 through CIR1, 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 unequal arm interferometer from another input port of BS1 through CIR2, 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 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 BS2 is 0 or π, thereby obtaining a phase coded signal. It is then amplified to a predetermined power by the optical amplifier module, expanded by the transmitting telescope and irradiated to the target. The coded double pulse optical signal emitted from the second output port of BS2 is isolated by ISO and will not affect the subsequent process.

[0084] After being reflected by the target, the phase-coded signal enters the receiving telescope and becomes an echo signal, which then passes through the polarizer POL and enters the unequal-arm interferometer for interference decoding. Since the target surface is generally rough, the linearly polarized light pulse signal will become randomly polarized after reflection, and the echo signal will have a 3dB loss after being polarized by POL. After polarization, the echo signal passes through ISO and enters the unequal-arm interferometer for interference. The interference results emitted from its two input ports can be written as

[0085] ,

[0086] 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.

[0087] 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 2c Both 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, and the target distance is L. Ignoring the same path length of the two sub-pulses in the system, the paths of the two sub-pulses that interfere in the unequal-arm interferometer are L1+2L+L2 and L2+2L+L1 respectively. The phase changes they experience 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 optical 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)*(L1-L2) / c.

[0088] 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:

[0089] ,

[0090] 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.

[0091] The main control module controls the frequency sweep module to perform frequency sweep in the range [f-Δ, f+Δ], where f is the center frequency of the laser and 2Δ is the frequency sweep range, eliminating the phase change caused by the Doppler frequency shift, so that the absolute value of the differential result reaches the maximum. Use a predetermined optimization algorithm, such as a gradient descent algorithm or a PID control algorithm, so that δ→0, that is, when (f1+fs)*n2=f0*n1, The frequency sweep stops when the maximum value is reached. At this time, the speed of the target can be calculated based on the corresponding frequency shift fs.

[0092] ,

[0093] 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. The frequency shift fs of the sweep frequency can be approximately equal to the Doppler frequency, and the target speed can be obtained as .

[0094] By sweeping the frequency to make δ→0, we can get the amplitude of the differential result when α=0 and π is A and -A respectively. Correspondingly, we can replace them with 0 and 1 respectively to get the received sequence. The first sequence is shifted bit by bit and cross-correlated with the received sequence to get 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.

[0095] like Figure 4 As shown, the second embodiment of the laser radar system of the present invention:

[0096] A phase-coded laser radar system based on frequency sweeping includes lasers LD1 and LD2, circulators CIR1 and CIR2, a first beam splitter BS1 and a second beam splitter BS2, an optical fiber amplifier, a transmitting telescope, a receiving telescope, CIR3, a frequency sweeping module, an optical isolator ISO, a polarization beam splitter PBS|, a first balanced detection module HD1, a second balanced detection module HD2, a differential circuit and a main control module.

[0097] The three ports of CIR1 are connected to LD1, the first input port of BS1, and HD1 respectively;

[0098] The three ports of CIR2 are connected to LD2, the second input port of BS1, and HD2 respectively;

[0099] The two output ports of BS1 are connected to the two input ports of BS2 through optical fibers of unequal lengths, forming an unequal-arm interferometer;

[0100] The first output port and the second output port of BS2 are connected to the second port of CIR3 and the output port of the optical isolator respectively;

[0101] The first port and the third port of CIR3 are connected to the reflection port of PBS and the input port of the optical fiber amplifier respectively;

[0102] The output port of the fiber amplifier is connected to the transmitting telescope;

[0103] The receiving telescope is connected to the input port of the frequency sweep module and PBS in sequence;

[0104] The transmission port of the PBS is connected to the input port of the optical isolator;

[0105] The two input ports of the differential circuit are respectively connected to the first balance detection module HD1 and the second balance detection module HD2;

[0106] The main control module is respectively connected to LD1, LD2, the frequency sweep module and the differential circuit.

[0107] The specific working process of the second embodiment is as follows:

[0108] 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.

[0109] The laser pulse signal generated by the first driving signal triggers LD1 to enter the unequal arm interferometer from one input port of BS1 through CIR1, 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 unequal arm interferometer from another input port of BS1 through CIR2, 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 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 BS2 is 0 or π, thereby obtaining a phase coded signal. It is then amplified to a predetermined power by the optical amplifier module, expanded by the transmitting telescope and irradiated to the target. The coded double pulse optical signal emitted from the second output port of BS2 is isolated by ISO and will not affect the subsequent process.

[0110] After being reflected by the target, the phase-coded signal enters the receiving telescope and becomes an echo signal, which is then polarized and split into a horizontal polarization component and a vertical polarization component by the polarization beam splitter PBS. Since the target surface is generally rough, the linearly polarized light pulse signal will become randomly polarized after reflection, so the amplitudes of the horizontal polarization component and the vertical polarization component are consistent, and there will be no 3dB loss. Among them, the horizontal polarization component enters the unequal-arm interferometer through ISO for interference decoding, and the vertical polarization component enters the unequal-arm interferometer through CIR3 for interference decoding. By setting the delay between PBS and CIR3, the time when the horizontal polarization component and the vertical polarization component enter the unequal-arm interferometer is staggered, so that the interference process of the two has no mutual influence. The interference results emitted from its two input ports can be written as

[0111] ,

[0112] 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.

[0113] 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 2c Both 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, and the target distance is L. Ignoring the same path length of the two sub-pulses in the system, the paths of the two sub-pulses that interfere in the unequal-arm interferometer are L1+2L+L2 and L2+2L+L1 respectively. The phase changes they experience 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 optical 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)*(L1-L2) / c.

[0114] 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:

[0115] ,

[0116] 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.

[0117] The main control module controls the frequency sweep module to perform frequency sweep in the range [f-Δ, f+Δ], where f is the center frequency of the laser and 2Δ is the frequency sweep range, eliminating the phase change caused by the Doppler frequency shift, so that the absolute value of the differential result reaches the maximum. Use a predetermined optimization algorithm, such as a gradient descent algorithm or a PID control algorithm, so that δ→0, that is, when (f1+fs)*n2=f0*n1, The frequency sweep stops when the maximum value is reached. At this time, the speed of the target can be calculated based on the corresponding frequency shift fs.

[0118] ,

[0119] 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. The frequency shift fs of the sweep frequency can be approximately equal to the Doppler frequency, and the target speed can be obtained as .

[0120] By sweeping the frequency to make δ→0, we can get the amplitude of the differential result when α=0 and π is A and -A respectively. Correspondingly, we can replace them with 0 and 1 respectively to get the received sequence. The first sequence is shifted bit by bit and cross-correlated with the received sequence to get 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.

[0121] From the various embodiments of the present invention, it can be seen that the present invention proposes a phase-coded lidar system based on frequency scanning and a ranging and speed measurement method, which performs phase encoding and decoding on the laser pulse signal, detects the decoded echo signal with a high-sensitivity balanced detector, and performs frequency scanning on the echo signal so that the absolute value of the differential signal reaches the maximum. This 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 the frequency scanning result.

Claims

1. A phase-coded laser radar system based on frequency sweeping, characterized in that: It includes a first laser LD1, a second laser LD2, a first circulator CIR1, a second circulator CIR2, an unequal-arm interferometer, an optical fiber amplifier, a transmitting telescope, a receiving telescope, a frequency sweeping module, an optical isolator ISO, a polarizer POL, a first balanced detection module HD1, a second balanced detection module HD2, a differential circuit and a main control module. The three ports of CIR1 are connected to LD1, the first input port of the unequal-arm interferometer, and HD1 respectively; The three ports of CIR2 are connected to LD2, the second input port of the unequal-arm interferometer, and HD2 respectively; The first output port and the second output port of the unequal-arm interferometer are respectively connected to the input port of the optical fiber amplifier and the output port of the optical isolator; Connect the output port of the fiber amplifier to the transmitting telescope; The receiving telescope is connected to the input ports of the sweep module, POL, and ISO in sequence; The two input ports of the differential circuit are connected to HD1 and HD2 respectively; The main control module is respectively connected to LD1, LD2, the frequency sweep module and the differential circuit.

2. The phase-coded laser radar system based on frequency sweeping according to claim 1, characterized in that: The frequency sweep module is an acousto-optic modulator AOM.

3. The phase-coded laser radar system based on frequency sweeping according to claim 1, characterized in that: The HD1 and HD2 are single-photon level balanced detectors.

4. A phase-coded laser radar system based on frequency sweeping, characterized in that: It includes a first laser LD1, a second laser LD2, a first circulator CIR1, a second circulator CIR2, an unequal-arm interferometer, an optical fiber amplifier, a transmitting telescope, a receiving telescope, a third circulator CIR3, a frequency sweep module, an optical isolator ISO, a polarization beam splitter PBS, a first balanced detection module HD1, a second balanced detection module HD2, a differential circuit and a main control module. The three ports of CIR1 are connected to LD1, the first input port of the unequal-arm interferometer, and HD1 respectively; The three ports of CIR2 are connected to LD2, the second input port of the unequal-arm interferometer, and HD2 respectively; The first output port and the second output port of the unequal-arm interferometer are connected to the second port of CIR3 and the output port of ISO respectively; The first port and the third port of CIR3 are connected to the reflection port of PBS and the input port of the optical fiber amplifier respectively; The output port of the fiber amplifier is connected to the transmitting telescope; The receiving telescope is connected to the input port of the frequency sweep module and PBS in sequence; The transmission port of the PBS is connected to the input port of the ISO; The two input ports of the differential circuit are connected to HD1 and HD2 respectively; The main control module is connected to LD1, LD2, the frequency sweep module and the differential circuit respectively.

5. The phase-coded laser radar system based on frequency sweeping according to claim 4, characterized in that: The frequency sweep module is an acousto-optic modulator AOM.

6. The phase-coded laser radar system based on frequency sweeping according to claim 4, characterized in that: HD1 and HD2 are single-photon level balanced detectors.

7. A method for ranging and measuring speed based on phase-coded laser radar with frequency sweeping, implemented by the laser radar system according to any one of claims 1 to 6, 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: Sweep the echo signal, maximize the absolute value of the differential result through the 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.

8. The method for measuring distance and speed using phase-coded laser radar based on frequency sweeping according to claim 7, characterized in that: The polarization processing in step 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 HD1 and HD2 for detection, and a differential circuit is used to differentiate the two detection results to obtain a detection sequence.

9. The method for measuring distance and speed using phase-coded laser radar based on frequency sweeping according to claim 7, characterized in that: The polarization processing in step 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 HD1 and HD2 for detection, and a differential circuit is used to differentiate the two detection results to obtain a detection sequence.

10. The method for measuring distance and speed using phase-coded laser radar based on frequency sweeping according to claim 7, characterized in that: The predetermined optimization algorithm in step S5 is a gradient descent algorithm or a PID control algorithm.

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