A laser radar system and a velocity measurement method based on an unequal arm interferometer

Abstract

The application belongs to the technical field of laser radar, and discloses a laser radar system and a velocity measurement method based on an unequal-arm interferometer, which comprises a laser, a light splitting module, an optical amplification module, a transmitting light path, a receiving light path, a filtering module, an unequal-arm interferometer, two-way detection modules and a main control module; the main control module is used for controlling the system timing, collecting first to fourth electric signals for data processing, calculating a first phase according to the first and second electric signals, calculating a second phase according to the third and fourth electric signals, and finally obtaining the velocity information of the target according to the difference between the first and second phases. Compared with the prior art, the application utilizes the unequal-arm interferometer to make the echo signal delay self-interference, can obtain the Doppler shift information of the target, and thus obtains the velocity of the target. The scheme does not need to transmit double-pulse signals, does not need to perform coherent detection and Fourier analysis, or sweep the echo signal, and thus reduces the complexity of the system.

Description

Technical Field

[0001] This invention relates to the field of lidar technology, and in particular to a lidar system and speed measurement method based on an unequal-arm interferometer. Background Technology

[0002] Radar plays a vital role in military, civil aviation, and autonomous driving fields. LiDAR, using short-wavelength lasers as its information carrier, boasts high resolution and can be used for target detection, tracking, identification, ranging, velocity measurement, and imaging.

[0003] For ranging and velocity measurement, the commonly used method is to coherently detect the echo signal after frequency shifting of the local oscillator light and performing continuous frequency modulation or random amplitude or phase modulation, and then obtain the target velocity information by measuring the Doppler frequency through Fourier analysis. Coherent lidar has a natural filtering effect; using the local oscillator light to coherently detect the real signal can not only filter out incoherent background noise but also provide a certain gain. However, at long distances, the decoherence effect of the echo signal is significant, making coherent detection difficult. The increased influence of background noise under sunlight results in a low signal-to-noise ratio. Patent CN118011415B proposes a two-pulse relative phase encoding method to solve the decoherence problem caused by atmospheric turbulence and other factors. However, this scheme cannot measure the velocity of the target, and when the target's speed is high, the Doppler frequency shift causes a change in the echo signal frequency, resulting in a change in the phase difference between the two pulses, which in turn increases the bit error rate of the interferometric decoding, affecting the ranging performance. Summary of the Invention

[0004] To address the aforementioned shortcomings of existing technologies, this invention proposes a lidar system based on an unequal-arm interferometer, comprising:

[0005] A laser is used to generate laser pulse signals.

[0006] The beam splitter module is used to split the laser pulse signal into a first optical pulse and a second optical pulse.

[0007] An optical amplification module is used to amplify the first optical pulse to a predetermined power;

[0008] The transmitting optical path is used to expand the amplified first optical pulse and illuminate the target;

[0009] The receiving optical path is used to receive the echo signal reflected from the target;

[0010] The filtering module is used to filter out background stray light noise in the echo signal;

[0011] The unequal-arm interferometer comprises two input ports and two output ports, is used for making a second light pulse incident from one input port thereof, performing delay self-interference, and generating a first interference result and a second interference result respectively emitted from two output ports thereof; and is used for making a return signal incident from the other input port thereof, performing delay self-interference, and generating a third interference result and a fourth interference result respectively emitted from two output ports thereof; wherein the second light pulse and the return signal enter the unequal-arm interferometer in a time-division multiplexing manner;

[0012] The first detection module is used for detecting the first interference result and the third interference result, and respectively corresponds to generating a first electric signal and a third electric signal; and the second detection module is used for detecting the second interference result and the fourth interference result, and respectively corresponds to generating a second electric signal and a fourth electric signal.

[0013] The main control module is used for controlling system timing, collecting the first electric signal to the fourth electric signal for data processing, calculating a first phase according to the first electric signal and the second electric signal, calculating a second phase according to the third electric signal and the fourth electric signal, and finally obtaining speed information of the target according to a difference between the first phase and the second phase.

[0014] Preferably, the unequal-arm interferometer has a phase difference between long and short arms which is adjustable and maintained at π / 2 through feedback control.

[0015] Preferably, the unequal-arm interferometer comprises a first beam splitter BS1, a second beam splitter BS2 and a first phase shifter PS1, two output ports of the first beam splitter BS1 are connected to two input ports of the second beam splitter BS2 through optical fibers with different lengths, thereby constituting long and short arms of the unequal-arm interferometer; and the first phase shifter PS1 is arranged on the long arm.

[0016] Two input ports of the first beam splitter BS1 are used as two input ports of the unequal-arm interferometer; and two output ports of the second beam splitter BS2 are used as two output ports of the unequal-arm interferometer.

[0017] The first phase shifter PS1 is an optical fiber stretcher.

[0018] Preferably, the unequal-arm interferometer is an unequal-arm interferometer chip, comprising a first multi-mode interference coupler MMI1, a second multi-mode interference coupler MMI2, a second phase shifter PS2 and a waveguide delay line DL,

[0019] Two output ports of the first multi-mode interference coupler MMI1 are connected to two input ports of the second multi-mode interference coupler MMI2 through waveguides, wherein a waveguide delay line DL and a second phase shifter PS2 are arranged on a section of the waveguide, thereby constituting a long arm of the unequal-arm interferometer; and another section of the waveguide constitutes a short arm of the unequal-arm interferometer.

[0020] The two input ports of the first multimode interference coupler (MMI1) are respectively used as the two input ports of the unequal-arm interferometer chip; the two output ports of the second multimode interference coupler (MMI2) are respectively used as the two output ports of the unequal-arm interferometer chip.

[0021] The second phase shifter (PS2) is a thermal phase shifter.

[0022] Preferably, the time-domain width of the laser pulse signal is 50 ns; and the delay difference between the long arm and the short arm of the unequal-arm interferometer is 5 ns.

[0023] Preferably, the first and second detection modules are first and second photodetectors (PD1 and PD2).

[0024] Preferably, the first and second detection modules are first and second single-photon detectors (SPD1 and SPD2).

[0025] Preferably, the emission optical path and the receiving optical path are respectively an emission telescope and a receiving telescope, and the filtering module is a narrow-band optical filter.

[0026] Preferably, the splitting module is a third beam splitter (BS3).

[0027] The application further discloses a laser radar speed measurement method based on an unequal-arm interferometer.

[0028] S1: a master control module drives a laser to emit a laser pulse signal, which is split into a first light pulse and a second light pulse by a splitting module;

[0029] S2: the first light pulse is amplified to a predetermined power by an optical amplification module, and is expanded by an emission optical path before being irradiated to a target, and a receiving optical path is used to receive a return signal reflected from the target;

[0030] S3: the second light pulse enters an unequal-arm interferometer to perform delay self-interference, to generate a first interference result and a second interference result, which are respectively input into first and second detection modules for detection, to obtain a first electric signal and a second electric signal ; by adjusting the phase difference of the unequal-arm interferometer, the first electric signal and the second electric signal are kept equal;

[0031] S4: the return signal and the second light pulse enter the unequal-arm interferometer to perform delay self-interference in a time-division multiplexing manner, to generate a third interference result and a fourth interference result, which are respectively input into the first and second detection modules for detection, to obtain a third electric signal and a fourth electric signal .

[0032] S5: the master module collects the first to fourth electric signals for data processing, calculates a first phase according to the first and second electric signals , and calculates a second phase according to the third and fourth electric signals , and finally obtains the speed of the target according to the difference between the first and second phases .

[0033] Compared with the prior art, the present application has the following beneficial effects:

[0034] The present application provides a laser radar system and a speed measurement method based on an unequal arm interferometer. The unequal arm interferometer is used to make the echo signal delay self-interference, so that the phase difference carried by the echo signal can be obtained. The phase difference between the long and short arms of the unequal arm interferometer obtained from the interference result of the local pulse is subtracted, so that the Doppler shift information of the target can be obtained, and thus the speed of the target can be obtained. The present application does not need to transmit double pulse signals, does not need to perform coherent detection and Fourier analysis, or sweep the echo signal, and thus the complexity of the system is reduced. By setting the arm length difference of the unequal arm interferometer, the present application can be applied to different target speed scenes, and has a wide application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 The present application is based on the principle diagram of the laser radar system based on the unequal arm interferometer;

[0036] Figure 2 The present application is based on the principle diagram of the laser radar system based on the unequal arm interferometer;

[0037] Figure 3 The present application is based on the principle diagram of the laser radar system based on the unequal arm interferometer. DETAILED DESCRIPTION

[0038] The present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application.

[0039] As shown in the drawings, Figure 1 a laser radar system based on an unequal arm interferometer includes a laser, a light splitting module, an optical amplification module, a transmitting light path, a receiving light path, a filtering module, a first detection module, a second detection module, and a master module,

[0040] The laser is used to generate a laser pulse signal.

[0041] The light splitting module is used to split the laser pulse signal into a first light pulse and a second light pulse.

[0042] The optical amplification module is used to amplify the first light pulse to a predetermined power.

[0043] The emitting light path is used for expanding and irradiating the amplified first light pulse to the target;

[0044] The receiving light path is used for receiving the echo signal reflected from the target;

[0045] The filtering module is used for filtering out the background stray light noise in the echo signal;

[0046] The unequal-arm interferometer comprises two input ports and two output ports, is used for making the second light pulse incident from one input port thereof, carrying out delay self-interference, generating a first interference result and a second interference result respectively emitted from two output ports thereof, and is used for making the echo signal incident from the other input port thereof, carrying out delay self-interference, generating a third interference result and a fourth interference result respectively emitted from two output ports thereof; wherein the second light pulse and the echo signal enter the unequal-arm interferometer in a time-division multiplexing manner;

[0047] The first detection module is used for detecting the first interference result and the third interference result, respectively corresponding to generating a first electric signal and a third electric signal; and the second detection module is used for detecting the second interference result and the fourth interference result, respectively corresponding to generating a second electric signal and a fourth electric signal;

[0048] The main control module is used for controlling system timing, collecting the first electric signal to the fourth electric signal for data processing, calculating a first phase according to the first electric signal and the second electric signal, calculating a second phase according to the third electric signal and the fourth electric signal, and finally obtaining the speed information of the target according to the difference between the first phase and the second phase.

[0049] The specific working process is as follows:

[0050] The main control module drives the laser to emit a laser pulse signal, which is split into a first light pulse and a second light pulse by the beam splitting module; the second light pulse enters the unequal-arm interferometer to carry out delay self-interference, generating a first interference result and a second interference result, which can be respectively written as

[0051] ,

[0052] Among them, is the phase difference between the light pulses entering the long arm and the short arm of the unequal-arm interferometer, and A is the amplitude of the second light pulse entering the unequal-arm interferometer. According to can be calculated as is

[0053] ,

[0054] Since the difference of the arm length of the unequal arm interferometer is ΔL, the delay between the light pulse components respectively walking the long arm and the short arm is Δt = n*ΔL / c, where n is the refractive index of the light pulse in the medium of the unequal arm interferometer, and c is the speed of light in vacuum. The frequency of the second light pulse is Therefore, we can get

[0055] ,

[0056] The first light pulse is amplified to a predetermined power by the optical amplification module, expanded by the emission optical path, and then irradiated to the target. After being reflected by the target, it enters the receiving optical path to become a return signal, and then enters the unequal arm interferometer for interference through the filtering module. The third interference result and the fourth interference result respectively emitted from the two input ports of the unequal arm interferometer can be written as

[0057] ,

[0058] where, is the phase difference between the light pulses entering the long and short arms of the unequal arm interferometer, and A is the amplitude of the return signal entering the unequal arm interferometer. According to we can calculate is

[0059] ,

[0060] Suppose the Doppler effect of the target motion causes the frequency of the return signal to change from to , that is, the Doppler shift is We can get

[0061] ,

[0062] From the relationship between and , we can get

[0063] ,

[0064] Since the relationship between the speed v and the Doppler shift is

[0065] ,

[0066] where, is the wavelength of the laser pulse signal. Finally, we can calculate the speed of the target as

[0067] ,

[0068] Therefore, the host module can calculate the speed of the target by collecting the electric signals of the first and second detection modules and performing data processing according to the above relationship.

[0069] As Figure 2 shown in the first embodiment,

[0070] The phase difference of the long and short arms of the unequal-arm interferometer is adjustable and is maintained near pi / 2 through feedback control.

[0071] The unequal-arm interferometer comprises a first beam splitter BS1, a second beam splitter BS2 and a first phase shifter PS1, two output ports of the BS1 are connected to two input ports of the BS2 through optical fibers with different lengths, thereby forming long and short arms of the unequal-arm interferometer; the PS1 is arranged on the long arm;

[0072] Two input ports of the BS1 are used as two input ports of the unequal-arm interferometer; two output ports of the BS2 are used as two output ports of the unequal-arm interferometer.

[0073] The PS1 is an optical fiber stretcher.

[0074] The first and second detection modules are photoelectric detectors PD1 and PD2.

[0075] The transmitting and receiving light paths are a transmitting telescope and a receiving telescope respectively, and the filtering module is a narrow-band optical filter.

[0076] The light splitting module is a third beam splitter BS3.

[0077] The specific working process of the first embodiment is as follows:

[0078] The host module drives the laser to emit a laser pulse signal, which is split into a first light pulse and a second light pulse by the BS3; the second light pulse enters the unequal-arm interferometer composed of the BS1, BS2 and PS1 to perform delay self-interference, thereby generating a first interference result and a second interference result, which can be written as

[0079] ,

[0080] wherein, is the phase difference between the light pulses entering the long and short arms of the unequal-arm interferometer, and A is the amplitude of the second light pulse entering the unequal-arm interferometer. can be calculated as is

[0081] ,

[0082] When the PS1 is adjusted to make , it can be known that at this time.

[0083] Since the difference of the arm length of the unequal arm interferometer is ΔL, the delay between the light pulse components respectively walking the long arm and the short arm is Δt = n*ΔL / c, where n is the refractive index of the light pulse in the medium of the unequal arm interferometer, and c is the speed of light in vacuum. The frequency of the second light pulse is Therefore, we can get

[0084] ,

[0085] The first light pulse is amplified to a predetermined power by the optical fiber amplifier, expanded by the emission telescope, and then irradiated to the target. After being reflected by the target, it enters the receiving telescope and becomes a return signal. Then, it enters the unequal arm interferometer for interference. The third interference result and the fourth interference result respectively emitted from the two input ports of the unequal arm interferometer can be written as

[0086] ,

[0087] where, is the phase difference between the light pulses entering the long arm and the short arm of the unequal arm interferometer, and A is the amplitude of the return signal entering the unequal arm interferometer. According to we can calculate is

[0088] ,

[0089] Suppose the Doppler effect caused by the target motion causes the frequency of the return signal to change from to , that is, the Doppler shift is We can get

[0090] ,

[0091] From the relationship between and , we can get

[0092] ,

[0093] Since the relationship between the velocity v and the Doppler shift is

[0094] ,

[0095] where, is the wavelength of the laser pulse signal. Finally, we can calculate the velocity of the target as

[0096] ,

[0097] Therefore, the host module can calculate the speed of the target by collecting the electrical signals of the first detection module and the second detection module and performing data processing according to the above relationship.

[0098] As Figure 3 shown in the following embodiment two:

[0099] The phase difference of the long arm and the short arm of the unequal-arm interferometer is adjustable and is maintained near pi / 2 through feedback control.

[0100] The unequal-arm interferometer is an unequal-arm interferometer chip, comprising a first multimode interference coupler (MMI1), a second multimode interference coupler (MMI2), a second phase shifter (PS2) and a waveguide delay line (DL),

[0101] Two output ports of the MMI1 are connected to two input ports of the MMI2 through waveguides, wherein a waveguide delay line (DL) and a second phase shifter (PS2) are arranged on a section of the waveguide, constituting a long arm of the unequal-arm interferometer; the other section of the waveguide constitutes a short arm of the unequal-arm interferometer.

[0102] The two input ports of the MMI1 are used as two input ports of the unequal-arm interferometer chip; the two output ports of the MMI2 are used as two output ports of the unequal-arm interferometer chip.

[0103] The PS2 is a thermal phase shifter.

[0104] The first detection module and the second detection module are single-photon detectors (SPD1 and SPD2).

[0105] The transmitting optical path and the receiving optical path are a transmitting telescope and a receiving telescope respectively, and the filtering module is a narrow-band optical filter.

[0106] The light splitting module is a third beam splitter (BS3).

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

[0108] The host module drives the laser to emit a laser pulse signal, which is split into a first optical pulse and a second optical pulse by the BS3; the second optical pulse enters the unequal-arm interferometer chip for delay self-interference, generating a first interference result and a second interference result, which are detected by the SPD1 and the SPD2, and the counts obtained after detection can be written as

[0109] ,

[0110] wherein, is the phase difference between the optical pulses entering the long arm and the short arm of the unequal-arm interferometer chip, and A is the amplitude of the second optical pulse entering the unequal-arm interferometer chip. According to , the speed of the target can be calculated as ​

[0111] ,

[0112] When adjusting PS2 so that , it can be seen that .

[0113] Since the arm length difference of the unequal arm interferometer chip is ΔL, the delay between the light pulse components respectively walking the long arm and the short arm is Δt = n * ΔL / c, where n is the refractive index of the light pulse in the medium of the unequal arm interferometer chip, and c is the speed of light in vacuum. The frequency of the second light pulse is Therefore, it can be obtained that

[0114] ,

[0115] The first light pulse is amplified to a predetermined power by the optical fiber amplifier, expanded by the emission telescope, and then irradiated to the target. After being reflected by the target, it enters the receiving telescope to become a return signal, and then enters the unequal arm interferometer chip for interference. The third interference result and the fourth interference result respectively exit from the two input ports of the unequal arm interferometer chip, and then enter SPD1 and SPD2 for detection. The counts of the two can be respectively written as

[0116] ,

[0117] wherein, is the phase difference between the light pulses entering the long and short arms of the unequal arm interferometer chip, and A is the amplitude of the return signal entering the unequal arm interferometer chip. According to , it can be calculated that is

[0118] ,

[0119] Suppose that the Doppler effect caused by the target motion causes the frequency of the return signal to change from to , that is, the Doppler shift is , it can be obtained that

[0120] ,

[0121] From the relationship between and , it can be obtained that

[0122] ,

[0123] Again, since the relationship between the velocity v and the Doppler shift is

[0124] ,

[0125] wherein, is the wavelength of the laser pulse signal. Finally, the speed of the target can be calculated as

[0126] ,

[0127] Therefore, the master control module can calculate the speed of the target by collecting the electrical signals of the first detection module and the second detection module and performing data processing according to the above relationship.

[0128] In summary, the present application proposes a laser radar system and a speed measurement method based on an unequal-arm interferometer. The unequal-arm interferometer is used to delay the self-interference of the echo signal, and the phase difference carried thereby can be obtained. The phase difference of the long and short arms of the unequal-arm interferometer obtained from the interference result of the local pulse is subtracted, and the Doppler shift information of the target can be obtained, thereby obtaining the speed of the target. The present application does not need to emit double-pulse signals, does not need to perform coherent detection and Fourier analysis, or sweep the echo signal, thereby reducing the complexity of the system. By setting the arm length difference of the unequal-arm interferometer, the present application can be applied to different target speed scenarios and has a wide application prospect.

Claims

1. A lidar system based on an unequal-arm interferometer, characterized in that, include: A laser is used to generate laser pulse signals. The beam splitter module is used to split the laser pulse signal into a first optical pulse and a second optical pulse. An optical amplification module is used to amplify the first optical pulse to a predetermined power; The transmitting optical path is used to expand the amplified first optical pulse and illuminate the target; The receiving optical path is used to receive the echo signal reflected from the target; The filtering module is used to filter out background stray light noise in the echo signal; The unequal-arm interferometer includes two input ports and two output ports. One input port receives a second optical pulse, which undergoes delayed self-interference to produce a first and a second interference result emitted from its two output ports. The other input port receives an echo signal, which undergoes delayed self-interference to produce a third and a fourth interference result emitted from its two output ports. The second optical pulse and the echo signal enter the unequal-arm interferometer via time-division multiplexing. A first detection module and a second detection module are used to detect a first interference result and a third interference result, and generate a first electrical signal and a third electrical signal respectively; the second detection module is used to detect a second interference result and a fourth interference result, and generate a second electrical signal and a fourth electrical signal respectively. The main control module is used to control the system timing, collect the first to fourth electrical signals for data processing, calculate the first phase based on the first and second electrical signals, calculate the second phase based on the third and fourth electrical signals, and finally obtain the target's speed information based on the difference between the first and second phases.

2. The lidar system based on an unequal-arm interferometer as described in claim 1, characterized in that, The phase difference between the long and short arms of the unequal-arm interferometer is adjustable and maintained at π / 2 through feedback control.

3. The lidar system based on an unequal-arm interferometer as described in claim 1, characterized in that, The unequal-arm interferometer includes a first beam splitter BS1, a second beam splitter BS2, and a first phase shifter PS1. The two output ports of the first beam splitter BS1 are respectively connected to the two input ports of the second beam splitter BS2 through optical fibers of unequal length, forming the long arm and short arm of the unequal-arm interferometer; the first phase shifter PS1 is disposed on the long arm. The two input ports of the first beam splitter BS1 serve as the two input ports of the unequal-arm interferometer. The two output ports of the second beam splitter BS2 serve as the two output ports of the unequal-arm interferometer. The first phase shifter, PS1, is an optical fiber stretcher.

4. The lidar system based on an unequal-arm interferometer as described in claim 1, characterized in that, The unequal-arm interferometer is an unequal-arm interferometer chip, including a first multimode interference coupler MMI1, a second multimode interference coupler MMI2, a second phase shifter PS2, and a waveguide delay line DL. The two output ports of the first multimode interference coupler MMI1 are connected to the two input ports of the second multimode interference coupler MMI2 through waveguides. One waveguide has a waveguide delay line DL and a second phase shifter PS2, forming the long arm of the unequal-arm interferometer; the other waveguide forms the short arm of the unequal-arm interferometer. The two input ports of the first multimode interference coupler MMI1 serve as the two input ports of the unequal-arm interferometer chip; the two output ports of the second multimode interference coupler MMI2 serve as the two output ports of the unequal-arm interferometer chip. The second phase shifter, PS2, is a thermally adjustable phase shifter.

5. The lidar system based on an unequal-arm interferometer as described in claim 1, characterized in that, The time domain width of the laser pulse signal is 50 ns; the delay difference between the long and short arms of the unequal-arm interferometer is 5 ns.

6. The lidar system based on an unequal-arm interferometer as described in claim 1, characterized in that, The first detection module and the second detection module are the first photodetector PD1 and the second photodetector PD2.

7. The lidar system based on an unequal-arm interferometer as described in claim 1, characterized in that, The first detection module and the second detection module are the first single-photon detector SPD1 and the second single-photon detector SPD2.

8. The lidar system based on an unequal-arm interferometer as described in claim 1, characterized in that, The transmitting optical path and the receiving optical path are respectively a transmitting telescope and a receiving telescope, and the filtering module is a narrowband filter.

9. The lidar system based on an unequal-arm interferometer as described in claim 1, characterized in that, The beam splitting module is the third beam splitter BS3.

10. A lidar velocity measurement method based on an unequal-arm interferometer, characterized in that, The method of using any one of the lidar systems of claims 1 to 9 includes the following steps: S1: The main control module drives the laser to emit laser pulse signals, which are then split into the first optical pulse and the second optical pulse by the beam splitting module; S2: The first light pulse is amplified to a predetermined power by the optical amplification module, and then irradiated towards the target after being expanded by the transmitting optical path. The echo signal reflected from the target is received by the receiving optical path. S3: The second optical pulse enters the unequal-arm interferometer for delayed self-interference, generating a first interference result and a second interference result, which are then detected by the first and second detection modules respectively to obtain the first electrical signal. Second electrical signal By adjusting the phase difference of the unequal-arm interferometer, the first and second electrical signals are kept equal. S4: The echo signal and the second optical pulse are time-division multiplexed into the unequal-arm interferometer for delayed self-interference, generating a third and fourth interference result, which are then detected by the first and second detection modules respectively to obtain the third electrical signal. and the fourth electrical signal ; S5: The main control module acquires the first to fourth electrical signals for data processing, and calculates the first phase based on the first and second electrical signals. The second phase is calculated based on the third and fourth electrical signals. Finally, the target's velocity is obtained based on the difference between the first phase and the second phase. .

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