A polarization-insensitive laser radar and ranging method

Abstract

The present invention belongs to the field of laser radar ranging technology, and discloses a polarization-insensitive laser radar and ranging method, including a laser, a first circulator, two integrated optical chips, an optical switch, an optical amplifier module, a transmitting optical path, a receiving optical path, a second circulator, two detection modules, a differential circuit and a main control module, the first integrated optical quantum chip includes at least one unequal arm interferometer, and the second integrated optical quantum chip has the same structure as the first integrated optical chip. Compared with the prior art, the present invention uses a polarization-insensitive integrated optical chip to encode and decode laser pulse signals, and detects the decoded echo signal through a highly sensitive balanced detector and performs differential, which can receive echo signals of arbitrary polarization, and greatly reduce the background noise in the echo signal, thereby improving the signal-to-noise ratio of the system. In addition, the integrated optical chips used for transmitting and receiving are exactly the same, and are easy to produce in large quantities.

Description

Technical Field

[0001] The present invention relates to the technical field of laser radar ranging, and in particular to a polarization-insensitive laser radar and a ranging method. Background Art

[0002] Radar plays a vital role in military, civil aviation, autonomous driving, and other fields. LiDAR, which uses shorter-wavelength lasers as information carriers, has higher resolution and can be used for target detection, tracking, and identification, as well as distance measurement and imaging.

[0003] Signal-to-noise ratio (SNR) is a crucial metric in LiDAR detection, directly impacting system performance. Prior art LiDARs using photon counting can achieve a relatively long ranging range. This is due to the high sensitivity of single-photon detectors. However, these detectors require cooling to below -30°C to reduce dark counts, resulting in high cost and complexity, making them difficult to miniaturize. Daylight background noise can only be filtered through filters, which in turn results in some loss of the true signal. Consequently, the SNR is low during daylight hours, making proper operation difficult. A Chinese invention patent, authorized with publication number CN118011415B, proposes a method for achieving superior noise reduction by phase encoding and then interfering the echo signals and performing differential analysis. However, this approach requires the use of two lasers, and when the target depolarizes the optical signal, the echo signal requires polarization processing. For example, polarization filtering can result in echo signal loss, reducing the SNR. Summary of the Invention

[0004] In view of the above defects in the prior art, the present invention proposes a polarization-insensitive laser radar and ranging method.

[0005] The technical solution of the present invention is achieved as follows:

[0006] A polarization-insensitive laser radar includes a laser LD, a first circulator CIR1, a first integrated optical chip, an optical switch OS, an optical amplifier module, a transmitting optical path, a receiving optical path, a second circulator CIR2, a second integrated optical chip, a first detection module, a second detection module, a differential circuit, and a main control module.

[0007] The laser LD is used to generate a horizontally polarized laser pulse signal with a narrow pulse width;

[0008] The first integrated optical chip includes a reflective unequal-arm interferometer for converting a laser pulse signal incident on one port thereof into a first optical signal with two sub-pulses having a phase difference of 0 and a second optical signal with a phase difference of π, which are output from two ports thereof respectively;

[0009] The first circulator CIR1 is used to transmit the laser pulse signal emitted by the laser LD to a port of the first integrated optical chip, and to transmit the first optical signal transmitted from the first integrated optical chip to an input port of the optical switch OS;

[0010] The optical switch OS is used to randomly cause the first optical signal or the second optical signal incident on another input port thereof to be emitted from its output port, thereby generating a coded optical signal;

[0011] The optical amplification module is used to amplify the coded optical signal to a predetermined power;

[0012] The transmitting optical path is used to expand the amplified coded optical signal and irradiate the signal to the target;

[0013] The receiving optical path is used to receive the echo signal reflected from the target;

[0014] The second integrated optical chip has the same structure as the first integrated optical chip and is used to interfere the two sub-pulse signals in the echo signal to generate a first interference result and a second interference result;

[0015] The second integrated optical chip is further configured to cause the background noise signal in the echo signal to be emitted from its two ports with equal amplitude;

[0016] The second circulator CIR2 is used to transmit the echo signal to an input port of the second integrated optical chip, and transmit the first interference result to the first detection module;

[0017] The first detection module and the second detection module are respectively used to detect the amplitudes of the first interference result and the second interference result correspondingly;

[0018] The differential circuit is used to differentiate the output electrical signals of the first detection module and the second detection module to eliminate background noise signals;

[0019] The main control module is used to control the optical switch to perform encoding to obtain a coding sequence, collect the signal output by the differential circuit to generate a decoding sequence, and perform correlation operation on the coding sequence and the decoding sequence to obtain the distance information of the target.

[0020] Preferably, the first integrated optical chip comprises a first polarization beam splitter PBS1, a second polarization beam splitter PBS2, a third polarization beam splitter PBS3, a first half-wave plate HWP1 and a second half-wave plate HWP2,

[0021] The two input ports of the first polarization beam splitter PBS1 serve as two ports of the first integrated optical chip respectively;

[0022] The two output ports of the first polarization beam splitter PBS1 are connected to the input port of the second polarization beam splitter PBS2 and the input port of the third polarization beam splitter PBS3 through a first optical waveguide and a second optical waveguide of equal length, respectively;

[0023] The first half-wave plate HWP1 and the second half-wave plate HWP2 are arranged to vertically penetrate the first optical waveguide and the second optical waveguide respectively;

[0024] The angles between the main axis directions of the first half-wave plate HWP1 and the second half-wave plate HWP2 and the polarization direction of the optical waveguide TE mode are both 22.5°;

[0025] The two output ports of the second polarization beam splitter PBS2 and the two output ports of the third polarization beam splitter PBS3 are respectively connected through optical waveguides of unequal lengths to form an unequal-arm interferometer.

[0026] Preferably, the first integrated optical chip comprises a first polarization beam splitter PBS1, a fourth polarization beam splitter PBS4 and a third half-wave plate HWP3.

[0027] The two input ports of the first polarization beam splitter PBS1 serve as two ports of the first integrated optical chip respectively;

[0028] The two output ports of the first polarization beam splitter PBS1 are connected to an input port and an output port of the fourth polarization beam splitter PBS4 through a first optical waveguide and a second optical waveguide of equal length respectively;

[0029] The third half-wave plate HWP3 vertically penetrates the first optical waveguide and the second optical waveguide at the same time;

[0030] The angle between the main axis direction of the third half-wave plate HWP3 and the polarization direction of the optical waveguide TE mode is 22.5°;

[0031] Another input port and another output port of the fourth polarization beam splitter PBS4 are directly connected through an optical waveguide to form a long arm of an unequal-arm interferometer.

[0032] Preferably, it further comprises a thermally adjustable phase shifter PS, which is arranged on the long arm of the unequal-arm interferometer of the first integrated optical chip and is used to compensate for the phase drift between the unequal-arm interferometers of the first integrated optical chip and the second integrated optical chip.

[0033] Preferably, the time domain width of the laser pulse signal is no greater than 100 ps, ​​and the time interval between the sub-pulse signals is no greater than 500 ps.

[0034] Preferably, the optical amplification module is an erbium-doped fiber amplifier with a gain of not less than 30 dB.

[0035] Preferably, the transmitting optical path and the receiving optical path are a transmitting telescope and a receiving telescope respectively.

[0036] Preferably, both the first detection module and the second detection module are single-photon balanced detectors, namely a first balanced detector HD1 and a second balanced detector HD2 .

[0037] The present invention also discloses a laser radar ranging method, comprising the following steps:

[0038] S1: The main control module triggers the laser LD to emit a laser pulse signal. After passing through the first integrated optical chip, a driving signal is generated by preparing a random sequence to control the optical switch to randomly switch the optical path for encoding, generating two sub-pulse signals with a random phase difference of 0 or π, forming a coding sequence;

[0039] S2: The encoded laser pulse signal is amplified to a predetermined power by the optical amplifier module, and then irradiated to the target through the transmitting optical path after beam expansion, and the echo signal reflected from the target is received by the receiving optical path;

[0040] S3: The echo signal enters the second integrated optical chip for interference. The two generated interference signals enter the first detection module and the second detection module for detection respectively. The two detection results are differentiated using a differential circuit to obtain a detection sequence. The detection sequence is converted into a decoding sequence based on the positive or negative sign of the detection result. The result greater than 0 is set to 0, the result less than 0 is set to 1, and the result equal to 0 is randomly assigned to 0 or 1.

[0041] S4: performing a shift cross-correlation operation on the encoding sequence and the decoding sequence, and obtaining the distance information of the target according to the position corresponding to the peak value of the cross-correlation operation result.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] This invention proposes a polarization-insensitive lidar and ranging method. This method uses a polarization-insensitive integrated optical chip to encode and decode laser pulse signals. A highly sensitive balanced detector detects and differentiates the decoded echo signals. This method can receive echo signals of any polarization, significantly reducing background noise in the echo signals and improving the system's signal-to-noise ratio. Furthermore, the integrated optical chips used for both transmission and reception are identical, making them easy to mass-produce. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is a principle block diagram of the polarization-insensitive laser radar of the present invention;

[0045] Figure 2 This is a principle block diagram of Example 1 of the polarization-insensitive laser radar of the present invention;

[0046] Figure 3This is a principle block diagram of Example 2 of the polarization-insensitive laser radar of the present invention. DETAILED DESCRIPTION

[0047] The present invention will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present invention.

[0048] like Figure 1 As shown, a polarization-insensitive laser radar includes a laser LD, a first circulator CIR1, a first integrated optical chip, an optical switch OS, an optical amplifier module, a transmitting optical path, a receiving optical path, a second circulator CIR2, a second integrated optical chip, a first detection module, a second detection module, a differential circuit, and a main control module.

[0049] The laser LD is used to generate a horizontally polarized laser pulse signal with a narrow pulse width;

[0050] The first integrated optical chip includes a reflective unequal-arm interferometer for converting a laser pulse signal incident on one port thereof into a first optical signal with two sub-pulses having a phase difference of 0 and a second optical signal with a phase difference of π, which are output from two ports thereof respectively;

[0051] The first circulator CIR1 is used to transmit the laser pulse signal emitted by the laser LD to a port of the first integrated optical chip, and to transmit the first optical signal to an input port of the optical switch OS;

[0052] The optical switch OS is used to randomly cause the first optical signal or the second optical signal incident on another input port thereof to be emitted from its output port, thereby generating a coded optical signal;

[0053] The optical amplification module is used to amplify the coded optical signal to a predetermined power;

[0054] The transmitting optical path is used to expand the amplified laser pulse signal and irradiate the target;

[0055] The receiving optical path is used to receive the echo signal reflected from the target;

[0056] The second integrated optical chip is the same as the first integrated optical chip, and is used to interfere the two sub-pulse signals in the echo signal to generate a first interference result and a second interference result;

[0057] The second integrated optical chip is further configured to cause the background noise signal in the echo signal to be emitted from its two ports with equal amplitude;

[0058] The second circulator CIR2 is used to transmit the echo signal to an input port of the second integrated optical chip, and transmit the first interference result to the first detection module;

[0059] The first detection module and the second detection module are respectively used to detect the amplitudes of the first interference result and the second interference result correspondingly;

[0060] The differential circuit is used to differentiate the output electrical signals of the first detection module and the second detection module to eliminate background noise signals;

[0061] The main control module is used to control the optical switch to perform encoding to obtain a coding sequence, collect the signal output by the differential circuit to generate a decoding sequence, and perform correlation operation on the coding sequence and the decoding sequence to obtain the distance information of the target.

[0062] The specific working process is as follows:

[0063] The main control module triggers the laser LD to generate a horizontally polarized laser pulse signal with a narrow temporal width. This pulse signal enters the first integrated optical chip through the first circulator CIR1, and outputs a first optical signal with a phase difference of 0 and a second optical signal with a phase difference of π from its two ports, respectively. The first optical signal passes through the first circulator CIR1 and enters one input port of the optical switch OS, while the second optical signal enters the other input port of the optical switch OS. The main control module generates a random drive signal to control the optical switch OS to randomly switch the optical path, causing the first or second optical signal to randomly exit from its output port, generating a coded optical signal and obtaining a coded sequence. This signal is then amplified to a predetermined power by the optical amplifier module, expanded through the transmit optical path, and irradiated toward the target.

[0064] After being reflected by the target, the coded optical signal enters the receiving optical path and becomes an echo signal. It then passes through the second circulator CIR2 and enters the second integrated optical chip for decoding. Assuming that the phase difference between the two sub-pulse signals of the coded optical signal is θ, since the time interval between the two is no more than 500ps, the corresponding distance difference in space transmission is no more than 15cm. Turbulence has the same effect on the phase of the two sub-pulses, so the phase difference remains unchanged when they reach the second integrated optical chip after atmospheric transmission and reflection. The two sub-pulse signals in the echo signal of any polarization state interfere with each other in the second integrated optical chip, and the first interference result and the second interference result emitted from its two ports can be written as follows:

[0065] ,

[0066] Where A is the amplitude of the sub-pulse entering the second integrated optical chip. The background noise signals accompanying the two sub-pulse signals in the echo signal are random in phase and do not interfere with each other. Therefore, the noise amplitude emitted from the two output ports of the second integrated optical chip is the same, that is, .

[0067] When the phase difference between the two sub-pulses is 0, the interference results emitted from the two ports of the second integrated optical chip are and ; When the phase difference between the two is π, the interference results are and .

[0068] The first and second detection modules detect the amplitudes of the first and second interference results, respectively. The two detection results are subtracted using a differential circuit. When the phase difference between the two sub-pulses is 0, the differential result is A; when the phase difference is π, the differential result is -A. This shows that the differential result has eliminated background noise and contains only the true reflected signal. Replacing A and -A in the differential result sequence with 0 and 1, respectively, yields the decoded sequence.

[0069] The encoding sequence is shifted bit by bit and cross-correlated with the decoding sequence, yielding the cross-correlation value for each shift. When the cross-correlation value reaches a peak, it indicates a one-to-one correspondence between the transmit and receive sequences. The number of shifted bits allows the corresponding target distance to be determined, achieving ranging functionality.

[0070] like Figure 2 As shown, embodiment 1 of the present invention:

[0071] The first integrated optical chip includes a first polarization beam splitter PBS1, a second polarization beam splitter PBS2, a third polarization beam splitter PBS3, a first half-wave plate HWP1 and a second half-wave plate HWP2.

[0072] The two input ports of the first polarization beam splitter PBS1 serve as two ports of the first integrated optical chip respectively;

[0073] The two output ports of the first polarization beam splitter PBS1 are connected to the input port of the second polarization beam splitter PBS2 and the input port of the third polarization beam splitter PBS3 through a first optical waveguide and a second optical waveguide of equal length, respectively;

[0074] The first half-wave plate HWP1 and the second half-wave plate HWP2 are arranged to vertically penetrate the first optical waveguide and the second optical waveguide respectively;

[0075] The angles between the main axis directions of the first half-wave plate HWP1 and the second half-wave plate HWP2 and the polarization direction of the optical waveguide TE mode are both 22.5°;

[0076] The two output ports of the second polarization beam splitter PBS2 and the two output ports of the third polarization beam splitter PBS3 are respectively connected through optical waveguides of unequal lengths to form an unequal-arm interferometer.

[0077] A thermally adjustable phase shifter PS is provided on the long arm of the unequal-arm interferometer of the first integrated optical chip, for compensating for the phase drift between the unequal-arm interferometers of the first integrated optical chip and the second integrated optical chip.

[0078] The time domain width of the laser pulse signal is no greater than 100 ps, ​​and the time interval between the sub-pulse signals is no greater than 500 ps.

[0079] The optical amplification module is an erbium-doped fiber amplifier with a gain of no less than 30dB.

[0080] The transmitting optical path and the receiving optical path are respectively a transmitting telescope and a receiving telescope.

[0081] The first detection module and the second detection module are single-photon balanced detectors.

[0082] The specific working process of embodiment 1 is as follows:

[0083] The master control module triggers the laser LD to generate a horizontally polarized laser pulse signal with a narrow temporal width. This pulse passes through the first circulator CIR1 and enters an input port of the first integrated optical chip PBS1, where it is transmitted. After passing through HWP1, it undergoes a 45° polarization rotation and then polarization splitting upon entering PBS2. This generates two sub-pulses of equal amplitude. These pulses propagate along the long and short arms of the unequal-arm interferometer, respectively, before entering PBS3 and exiting from its input port. The first sub-pulse is in TE mode, while the second sub-pulse is in TM mode. Both pulses undergo a 45° polarization rotation upon passing through HWP2, reaching an output port of PBS1 and undergoing polarization splitting. A first optical signal with a phase difference of 0 and a second optical signal with a phase difference of π are output from these two ports, respectively. The first optical signal passes through the first circulator CIR1 and enters one input port of the optical switch OS, while the second optical signal enters the other input port of the optical switch OS. The master control module generates a random drive signal to control the optical switch OS to randomly switch the optical path, causing either the first or second optical signal to randomly exit from its output port, generating a coded optical signal and obtaining a coded sequence. It is then amplified to a predetermined power by an optical amplifier module, expanded through the emission light path, and irradiated toward the target.

[0084] After being reflected by the target, the coded optical signal enters the receiving optical path, becoming an echo signal. It then passes through the second circulator CIR2 and enters the second integrated optical chip for decoding. Assuming the phase difference between the two sub-pulses of the coded optical signal is θ, since the time interval between them is no more than 500 ps, ​​the corresponding distance difference in space is no more than 15 cm. Turbulence has the same effect on the phases of the two sub-pulses, so after atmospheric transmission and reflection, the phase difference remains unchanged when they reach the second integrated optical chip. The two sub-pulses of the echo signal of any polarization state enter PBS1 in the second integrated optical chip and are first polarization-split into TE mode and TM mode.

[0085] The two sub-pulses in the TE mode are polarized and rotated 45° after passing through HWP1, and are polarization-split by PBS2, generating components that propagate along the long and short arms of the unequal-arm interferometer respectively. The first sub-pulse component propagating along the long arm and the second sub-pulse component propagating along the short arm arrive at PBS3 simultaneously for polarization combination, and interfere after arriving at HWP2, generating two interference components, namely the TE mode and the TM mode, which are then polarization-split by PBS1 to generate the first TE interference result and the second TE interference result.

[0086] The two sub-pulses in the TM mode are polarized by 45° after passing through HWP2 and are polarization-split by PBS3, generating components that propagate along the long and short arms of the unequal-arm interferometer respectively. The first sub-pulse component propagating along the long arm and the second sub-pulse component propagating along the equal arm arrive at PBS2 simultaneously for polarization combination. After arriving at HWP1, they interfere, generating two interference components, one in the TE mode and the other in the TM mode. These are then polarization-split by PBS1, generating the first TM interference result and the second TM interference result.

[0087] Since the elapsed time is the same, the first TE interference result and the first TM interference result are emitted from one input port of PBS1 at the same time, and polarization synthesis is performed to produce the first interference result; the second TE interference result and the second TM interference result are emitted from the other input port of PBS1 at the same time, and polarization synthesis is performed to produce the second interference result. Since the second integrated optical chip performs polarization splitting on the echo light signal of any polarization state, interferes with it separately, and then performs polarization synthesis, the interference result produced is independent of polarization. Therefore, the first interference result and the second interference result can be written as

[0088] ,

[0089] Where A is the amplitude of the sub-pulse entering the fourth unequal-arm interferometer. The background noise signals accompanying the two sub-pulse signals in the echo signal have random phases and do not interfere with each other. Therefore, the noise amplitude emitted from the two output ports of the second integrated optical chip is the same, that is, A thermally tuned phase shifter PS is provided on the long arm of the unequal-arm interferometer to compensate for the phase drift between the two unequal-arm interferometers.

[0090] When the phase difference between the two sub-pulses is 0, the amplitudes of the optical signals emitted from the first output port and the second output port of the fourth unequal-arm interferometer are respectively and When the phase difference between the two is π, the amplitudes of the optical signals emitted from the first output port and the second output port of the second integrated optical chip are respectively and .

[0091] The first detection module and the second detection module are respectively the first balanced detector HD1 and the second balanced detector HD2, which respectively detect the first optical signal and the second optical signal emitted from the first output port and the second output port of the fourth unequal-arm interferometer. Among them, when the phase difference between the two sub-pulses is 0, HD1 performs balanced detection on the first optical signal to obtain the I component of the true echo. and Q component , where φ is the phase difference between the signal and the local oscillator light in HD1. After adding the noise, the amplitude of the first optical signal can be obtained as ; HD2 detects the second optical signal and obtains the noise amplitude as , using a differential circuit to subtract the two detection results, the difference result is A. Similarly, when the phase difference between the two is π, the difference result is -A. As can be seen, the difference result has eliminated background noise and contains only the true reflected signal. Replacing A and -A in the difference result sequence with 0 and 1, respectively, yields the decoded sequence.

[0092] The encoding sequence is shifted bit by bit and cross-correlated with the decoding sequence, yielding the cross-correlation value for each shift. When the cross-correlation value reaches a peak, it indicates a one-to-one correspondence between the transmit and receive sequences. The number of shifted bits allows the corresponding target distance to be determined, achieving ranging functionality.

[0093] like Figure 3 As shown, the second embodiment of the present invention:

[0094] The first integrated optical chip includes a first polarization beam splitter PBS1, a fourth polarization beam splitter PBS4 and a third half-wave plate HWP3.

[0095] The two input ports of the first polarization beam splitter PBS1 serve as two ports of the first integrated optical chip respectively;

[0096] The two output ports of the first polarization beam splitter PBS1 are connected to an input port and an output port of the fourth polarization beam splitter PBS4 through a first optical waveguide and a second optical waveguide of equal length respectively;

[0097] The third half-wave plate HWP3 is arranged to vertically penetrate the first optical waveguide and the second optical waveguide;

[0098] The angle between the main axis direction of the third half-wave plate HWP3 and the polarization direction of the optical waveguide TE mode is 22.5°;

[0099] Another input port and another output port of the fourth polarization beam splitter PBS4 are directly connected through an optical waveguide to form a long arm of an unequal-arm interferometer.

[0100] A thermally adjustable phase shifter PS is provided on the long arm of the unequal-arm interferometer of the first integrated optical chip, for compensating for the phase drift between the unequal-arm interferometers of the first integrated optical chip and the second integrated optical chip.

[0101] The time domain width of the laser pulse signal is no greater than 100 ps, ​​and the time interval between the sub-pulse signals is no greater than 500 ps.

[0102] The optical amplification module is an erbium-doped fiber amplifier with a gain of no less than 30dB.

[0103] The transmitting optical path and the receiving optical path are respectively a transmitting telescope and a receiving telescope.

[0104] The first detection module and the second detection module are single-photon balanced detectors.

[0105] The specific working process of embodiment 1 is as follows:

[0106] The main control module triggers the laser LD to generate a horizontally polarized laser pulse signal with a narrow temporal width. This pulse passes through the first circulator CIR1 and enters an input port of the first integrated optical chip PBS1, where it is transmitted. After passing through HWP3, it undergoes a 45° polarization rotation and enters an input port of PBS4, where it undergoes polarization splitting. This generates two sub-pulses of equal amplitude. One sub-pulse is directly transmitted through one of its output ports, while the other sub-pulse propagates along the long arm of the unequal-arm interferometer, enters the other output port of PBS4, and exits from one of its output ports. The first sub-pulse is in TE mode, while the second sub-pulse is in TM mode. Both sub-pulses undergo a 45° polarization rotation through HWP3 and arrive at an output port of PBS1, where they undergo polarization splitting. A first optical signal with a phase difference of 0 and a second optical signal with a phase difference of π are output from these two ports, respectively. The first optical signal passes through the first circulator CIR1 and enters one input port of the optical switch OS, while the second optical signal enters the other input port of the optical switch OS. The main control module generates a random drive signal to control the optical switch (OS) to randomly switch the optical path, causing the first or second optical signal to randomly exit from its output port, generating a coded optical signal and obtaining a coded sequence. This signal is then amplified to a predetermined power by the optical amplifier module, expanded through the transmitting optical path, and irradiated toward the target.

[0107] After being reflected by the target, the coded optical signal enters the receiving optical path, becoming an echo signal. It then passes through the second circulator CIR2 and enters the second integrated optical chip for decoding. Assuming the phase difference between the two sub-pulses of the coded optical signal is θ, since the time interval between them is no more than 500 ps, ​​the corresponding distance difference in space is no more than 15 cm. Turbulence has the same effect on the phases of the two sub-pulses, so after atmospheric transmission and reflection, the phase difference remains unchanged when they reach the second integrated optical chip. The two sub-pulses of the echo signal of any polarization state enter PBS1 in the second integrated optical chip and are first polarization-split into TE mode and TM mode.

[0108] The two TE-mode sub-pulses are polarized 45° after passing through HWP3 and then polarized-split into one input port of PBS4. One component is transmitted directly, while the other propagates along the long arm of the unequal-arm interferometer. The first sub-pulse component that propagates along the long arm and the second sub-pulse component that directly transmits from PBS4 are simultaneously emitted from PBS4, undergoing polarization combination. After reaching HWP3, they interfere with each other, generating two interference components, one in TE mode and the other in TM mode. These are then polarized-split by PBS1, producing the first TE interference result and the second TE interference result.

[0109] After passing through HWP3, the two TM-mode sub-pulses undergo a 45° polarization rotation and enter an output port of PBS4 for polarization splitting. One component is transmitted directly, while the other propagates in the opposite direction along the long arm of the unequal-arm interferometer. The first sub-pulse component that propagated along the long arm and the second sub-pulse component that directly transmitted from PBS4 are simultaneously emitted from PBS4, undergoing polarization beam combining. Upon reaching HWP3, they interfere with each other, generating two interference components, one in the TE mode and the other in the TM mode. These are then polarization-split by PBS1, producing the first and second TM interference results.

[0110] Since the elapsed time is the same, the first TE interference result and the first TM interference result are emitted from one input port of PBS1 at the same time, and polarization synthesis is performed to produce the first interference result; the second TE interference result and the second TM interference result are emitted from the other input port of PBS1 at the same time, and polarization synthesis is performed to produce the second interference result. Since the second integrated optical chip performs polarization splitting on the echo light signal of any polarization state, interferes with it separately, and then performs polarization synthesis, the interference result produced is independent of polarization. Therefore, the first interference result and the second interference result can be written as

[0111] ,

[0112] Where A is the amplitude of the sub-pulse entering the fourth unequal-arm interferometer. The background noise signals accompanying the two sub-pulse signals in the echo signal have random phases and do not interfere with each other. Therefore, the noise amplitude emitted from the two output ports of the second integrated optical chip is the same, that is, A thermally tuned phase shifter PS is provided on the long arm of the unequal-arm interferometer to compensate for the phase drift between the two unequal-arm interferometers.

[0113] When the phase difference between the two sub-pulses is 0, the amplitudes of the optical signals emitted from the first output port and the second output port of the fourth unequal-arm interferometer are respectively and When the phase difference between the two is π, the amplitudes of the optical signals emitted from the first output port and the second output port of the second integrated optical chip are respectively and .

[0114] The first detection module and the second detection module are respectively the first balanced detector HD1 and the second balanced detector HD2, which respectively detect the first optical signal and the second optical signal emitted from the first output port and the second output port of the fourth unequal-arm interferometer. Among them, when the phase difference between the two sub-pulses is 0, HD1 performs balanced detection on the first optical signal to obtain the I component of the true echo. and Q component , where φ is the phase difference between the signal and the local oscillator light in HD1. After adding the noise, the amplitude of the first optical signal can be obtained as ; HD2 detects the second optical signal and obtains the noise amplitude as , using a differential circuit to subtract the two detection results, the difference result is A. Similarly, when the phase difference between the two is π, the difference result is -A. As can be seen, the difference result has eliminated background noise and contains only the true reflected signal. Replacing A and -A in the difference result sequence with 0 and 1, respectively, yields the decoded sequence.

[0115] The encoding sequence is shifted bit by bit and cross-correlated with the decoding sequence, yielding the cross-correlation value for each shift. When the cross-correlation value reaches a peak, it indicates a one-to-one correspondence between the transmit and receive sequences. The number of shifted bits allows the corresponding target distance to be determined, achieving ranging functionality.

[0116] A comprehensive review of the various embodiments of the present invention demonstrates a polarization-insensitive lidar and ranging method. This method uses a polarization-insensitive integrated optical chip to encode and decode laser pulse signals. A highly sensitive balanced detector detects and differentiates the decoded echo signals. This allows for the reception of echo signals of any polarization, significantly reduces background noise in the echo signals, and improves the system's signal-to-noise ratio. Furthermore, the integrated optical chips used for both transmission and reception are identical, making them easy to mass-produce.

Claims

1. A polarization-insensitive laser radar, characterized in that: include: A laser, used for generating a horizontally polarized laser pulse signal with a narrow pulse width; The first integrated optical chip includes a reflective unequal-arm interferometer for converting a laser pulse signal incident to one port thereof into a first optical signal with two sub-pulses having a phase difference of 0 and a second optical signal with two sub-pulses having a phase difference of π, which are output from two ports thereof respectively; A first circulator, used to transmit the laser pulse signal emitted by the laser to a port of the first integrated optical chip, and to transmit the first optical signal transmitted from the first integrated optical chip to an input port of the optical switch; An optical switch, used for randomly causing the first optical signal or the second optical signal incident on another input port thereof to be emitted from an output port thereof to generate a coded optical signal; An optical amplifier module, used to amplify the coded optical signal to a predetermined power; The transmitting optical path is used to expand the amplified coded optical signal and irradiate it to the target; A receiving optical path, used for receiving an echo signal reflected from a target; The second integrated optical chip has the same structure as the first integrated optical chip and is used to interfere the two sub-pulse signals in the echo signal to generate a first interference result and a second interference result; the second integrated optical chip is also used to make the background noise signal in the echo signal emit from its two ports with equal amplitude; a second circulator, configured to transmit the echo signal to an input port of the second integrated optical chip, and transmit the first interference result to the first detection module; The first detection module and the second detection module are respectively used to detect the amplitudes of the first interference result and the second interference result correspondingly; A differential circuit, used for differentiating the output electrical signals of the two detection modules to eliminate background noise signals; The main control module is used to control the optical switch to encode and obtain a coding sequence, collect the signal output by the differential circuit to generate a decoding sequence, and perform correlation operation on the coding sequence and the decoding sequence to obtain the distance information of the target. The integrated optical chip comprises a first polarization beam splitter, a second polarization beam splitter, a third polarization beam splitter, a first half-wave plate and a second half-wave plate, The two input ports of the first polarization beam splitter are respectively used as two ports of the first integrated optical chip; The two output ports of the first polarization beam splitter are connected to the input port of the second polarization beam splitter and the input port of the third polarization beam splitter through a first optical waveguide and a second optical waveguide of equal length, respectively; The first half-wave plate and the second half-wave plate are arranged to correspond to and vertically penetrate the first optical waveguide and the second optical waveguide respectively; The angles between the main axis directions of the first half-wave plate and the second half-wave plate and the polarization direction of the TE mode of the optical waveguide are both 22.5°; The two output ports of the second polarization beam splitter and the two output ports of the third polarization beam splitter are respectively connected through optical waveguides of unequal lengths to form an unequal-arm interferometer.

2. The polarization-insensitive laser radar according to claim 1, characterized in that: It also includes a thermally adjustable phase shifter, which is arranged on the long arm of the unequal-arm interferometer of the first integrated optical chip and is used to compensate for the phase drift between the unequal-arm interferometers of the first integrated optical chip and the second integrated optical chip.

3. A polarization-insensitive laser radar, characterized in that: include: A laser, used for generating a horizontally polarized laser pulse signal with a narrow pulse width; The first integrated optical chip includes a reflective unequal-arm interferometer for converting a laser pulse signal incident to one port thereof into a first optical signal with two sub-pulses having a phase difference of 0 and a second optical signal with two sub-pulses having a phase difference of π, which are output from two ports thereof respectively; A first circulator, used to transmit the laser pulse signal emitted by the laser to a port of the first integrated optical chip, and to transmit the first optical signal transmitted from the first integrated optical chip to an input port of the optical switch; An optical switch, used for randomly causing the first optical signal or the second optical signal incident on another input port thereof to be emitted from an output port thereof to generate a coded optical signal; An optical amplifier module, used to amplify the coded optical signal to a predetermined power; The transmitting optical path is used to expand the amplified coded optical signal and irradiate it to the target; A receiving optical path, used for receiving an echo signal reflected from a target; The second integrated optical chip has the same structure as the first integrated optical chip and is used to interfere the two sub-pulse signals in the echo signal to generate a first interference result and a second interference result; the second integrated optical chip is also used to make the background noise signal in the echo signal emit from its two ports with equal amplitude; a second circulator, configured to transmit the echo signal to an input port of the second integrated optical chip, and transmit the first interference result to the first detection module; The first detection module and the second detection module are respectively used to detect the amplitudes of the first interference result and the second interference result correspondingly; A differential circuit, used for differentiating the output electrical signals of the two detection modules to eliminate background noise signals; The main control module is used to control the optical switch to encode and obtain a coding sequence, collect the signal output by the differential circuit to generate a decoding sequence, and perform correlation operation on the coding sequence and the decoding sequence to obtain the distance information of the target. The first integrated optical chip comprises a first polarization beam splitter, a fourth polarization beam splitter and a third half-wave plate, The two input ports of the first polarization beam splitter are respectively used as two ports of the first integrated optical chip; The two output ports of the first polarization beam splitter are connected to an input port and an output port of a fourth polarization beam splitter through a first optical waveguide and a second optical waveguide of equal length, respectively; The third half-wave plate vertically penetrates the first optical waveguide and the second optical waveguide simultaneously; The angle between the main axis direction of the third half-wave plate and the polarization direction of the TE mode of the optical waveguide is 22.5°; Another input port and another output port of the fourth polarization beam splitter are directly connected through an optical waveguide to form a long arm of the unequal-arm interferometer.

4. The polarization-insensitive laser radar according to claim 3, characterized in that: It also includes a thermally adjustable phase shifter, which is arranged on the long arm of the unequal-arm interferometer of the first integrated optical chip and is used to compensate for the phase drift between the unequal-arm interferometers of the first integrated optical chip and the second integrated optical chip.

5. The polarization-insensitive laser radar according to claim 3, characterized in that: The time domain width of the laser pulse signal is no greater than 100 ps, ​​and the time interval between the sub-pulse signals is no greater than 500 ps.

6. The polarization-insensitive laser radar according to claim 3, characterized in that: The optical amplifier module is an erbium-doped fiber amplifier with a gain of no less than 30 dB.

7. The polarization-insensitive laser radar according to claim 3, characterized in that: The transmitting optical path and the receiving optical path are respectively a transmitting telescope and a receiving telescope.

8. The polarization-insensitive laser radar according to claim 3, characterized in that: The first detection module and the second detection module are both single-photon-level balanced detectors, namely, a first balanced detector and a second balanced detector.

9. A laser radar ranging method based on the polarization-insensitive laser radar according to any one of claims 1 to 8, characterized in that: The following steps are involved: S1: The main control module triggers the laser to emit a laser pulse signal. After passing through the first integrated optical chip, a driving signal is generated by preparing a random sequence to control the optical switch to randomly switch the optical path for encoding, generating two sub-pulse signals with a random phase difference of 0 or π to form a coding sequence; S2: The encoded laser pulse signal is amplified to a predetermined power by an optical amplifier module, and then irradiated to the target after beam expansion through the transmitting optical path, and the echo signal reflected from the target is received using the receiving optical path; S3: The echo signal enters the second integrated optical chip for interference, and the two interference signals generated enter the first detection module and the second detection module for detection respectively. The two detection results are differentiated by a differential circuit to obtain a detection sequence. The detection sequence is converted into a decoding sequence according to the positive or negative value of the detection result, 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; S4: performing a shift cross-correlation operation on the coding sequence and the decoding sequence, and obtaining the distance information of the target according to the position corresponding to the peak value of the cross-correlation operation result.

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