Long-distance laser ranging method and device for adaptive compensation of pneumatic optical effects

Abstract

The present invention discloses a method for adaptively compensating the aero-optical effect of a transceiver-separated multi-channel laser coherent ranging radar, belonging to the field of wavefront aberration compensation for airborne lidar. It includes a transceiver-separated lidar transmitting optical antenna, a variable-focus Moiré lens, a deformable mirror, a wavefront detector, a wavefront correction controller, a beam splitter, an infrared beam analyzer, etc. The lidar transmitting optical antenna is a transceiver-separated multi-channel optical antenna, including a high-power transmitting channel and three weak echo receiving channels, located at the forefront of the entire system. The variable-focus Moiré lens is arranged in front of the receiving collimator to perform adaptive focusing correction on the negative lens defocus effect in the echo. The wavefront detector, deformable mirror, and wavefront correction controller constitute an adaptive optical closed-loop subsystem module within the entire radar system to achieve the correction of wavefront aberration. This method is applicable to long-distance laser coherent radar ranging systems that require the emission of high-power lasers.

Description

Technical Field

[0001] The present invention belongs to the field of lidar coherent ranging, and relates to a long-distance laser ranging method and device for adaptive compensation of aerodynamic optical effects, which can adaptively compensate for the wavefront distortion caused by aerodynamic optical effects in the weak signal wavefront of long-distance echoes. Background Art

[0002] Traditional laser ranging mainly has two systems: single-photon direct detection and coherent detection. The laser ranging technology of the single-photon direct detection system is relatively mature and has been widely used in application scenarios such as short-range driverless vehicle radars and long-range lunar-earth ranging. However, the detector is easily interfered by background stray light noise such as sunlight and cannot work all day and all night. In comparison, the main working band of the coherent laser ranging system is 1550nm. The principle of coherent mixing makes it immune to background noise interference and can work all day and all night. This advantage can be fully exploited especially in the scenario of long-distance ranging on airborne platforms. Since the echo signal needs to be processed by coherent mixing with the local signal, a high requirement for the wavefront coherence of the echo is imposed. The airborne platform will face the aerodynamic optical effects generated by high-speed movement. The resulting wavefront distortion and even the defocus phenomenon caused by the flow field leading to negative lens effects will greatly reduce the coherence efficiency of the ranging system. Therefore, wavefront adaptive compensation is required.

[0003] In the patent of Hu Yuan et al., an airborne laser communication system environment defocus adaptive compensation method was proposed (see patent application number CN104765128A). This method proposed a defocus adaptive environment compensation scheme based on a liquid lens for the airborne laser communication system. The system is based on a transceiver coaxial optical antenna, where the emission and reception of laser are on the same optical axis. Before beam expansion and emission, as well as for the received beam, a variable-focus liquid lens is used for defocus correction. Since laser communication is a two-way working mode, the communication terminal receives the laser beam sent by the other party. After passing through the aerodynamic boundary layer, the beam enters the communication optical system. Its defocus detection and wavefront detection modules can directly obtain the additional distortion information generated by the aerodynamic layer. The wavefront correction compensation scheme through the above defocus compensation method is suitable for the laser communication system, but not for the lidar system with separated transceiver and weak signal of the received echo being the reflected echo from a long-distance target, because the light received by the lidar is emitted by itself, and the emitted laser wavefront is a standard Gaussian wavefront, and pre-conjugate correction setting of the aerodynamic boundary layer cannot be carried out in the emission channel. In addition, the transmission power of the long-distance ranging lidar is very high, and the liquid lens in the coaxial optical system will be damaged under the continuous irradiation of high-power laser. For the lidar, although it also faces wavefront distortion and even the defocus effect caused by aerodynamic optics under a high-speed airborne platform, its wavefront compensation correction requires a new working mode such as separated transceiver to be redesigned, and only a new type of variable-focus optical device is adopted in the receiving channel. Summary of the Invention

[0004] To overcome the deficiencies of the above prior art, the present invention provides a long-distance laser ranging method and device for adaptive compensation of aerodynamic optical effects, which solves long-distance laser ranging, reduces the background noise interference at the detection end while increasing the laser emission power, and improves the detection efficiency.

[0005] The principle of the present invention is as follows:

[0006] Coherent laser ranging has technical advantages in anti-noise interference. Currently, test results show that after adopting a high-sensitivity detector with near-quantum limit, the ranging distance of coherent ranging based on multi-channel receiving and incoherent superposition can reach dozens or even nearly a hundred kilometers. When the system is placed on an airborne platform, the aerodynamic optical effect generated by the high-speed flow field movement on the outer surface of the optical window will damage the wavefront, thereby reducing the coherence efficiency of the ranging system and the ranging power. It is necessary to compensate and correct the wavefront distortion. For wavefront distortion, an adaptive closed-loop system is used for correction. For the defocus caused by the negative lens effect, it can be compensated by a Moiré lens. The Moiré lens is voltage-controlled and can seamlessly switch between different curvatures within a few milliseconds. It does not contain moving mechanical components and can resist vibration, shock, and overall wear. It is an ideal component for the focusing system. Since the ranging radar is based on multi-channel receiving, the transmitting channel and each receiving channel are separated from each other. To detect distant targets, the laser power emitted by the transmitting channel is as high as dozens of watts or even hundreds of watts, while the power of the weak signal of the echo reflected by the distant small target received by the receiving channel is in the order of picowatts or even femtowatts. The difference in power levels and the characteristics of multi-channels determine that it is impossible to use a single variable-focus Moiré lens for compensation and correction alone.

[0007] The technical solution of the present invention is as follows:

[0008] A long-distance laser ranging method for adaptive compensation of aerodynamic optical effects, based on a transceiver-separated multi-channel lidar optical antenna, includes:

[0009] Using the optical antenna to expand the laser emitted by the high-power laser and emit it towards a distant target, and after reducing the beam of the target echo reflected from the distant target, it enters N channels;

[0010] In each channel, a variable-focus Moiré lens composed of at least diffraction optical elements is arranged along the optical axis direction. The variable-focus Moiré lens is controlled by a focusing controller to change the rotation angles of the two diffraction optical elements;

[0011] After the target echo passes through the variable-focus Moiré lens, it is divided into a first transmitted light and a first reflected light by a first beam splitter; the first transmitted light enters an infrared beam analyzer through a defocus detection lens. The infrared beam analyzer analyzes the focused spots of each receiving channel and outputs a feedback signal to the focusing controller. The focusing controller adjusts the variable-focus Moiré lens of each receiving channel in the positive and negative directions according to the feedback signal until the spot on the infrared beam analyzer approaches the theoretical Airy disk, completing the axial correction of defocus;

[0012] The first reflected light is incident on the deformable mirror. After being reflected by the deformable mirror, it is divided into a second reflected light and a second transmitted light by the second beam splitter. The second reflected light enters the wavefront detector. The wavefront detector detects the target echo wavefront of each channel and calculates the wavefront aberration, and transmits the aberration value to the deformable mirror controller. The deformable mirror controller obtains the actuation voltage of the deformable mirror unit lens through calculation and processing, and uses this actuation voltage to drive the deformable mirror to perform conjugate correction on the real-time wavefront;

[0013] The second transmitted light is focused on the fiber end face by the receiving collimating mirror array and then enters the photodetector for coherent mixing processing with the local signal to obtain the long-distance ranging signals of each channel;

[0014] The non-coherent superposition of the electrical signals of the long-distance ranging signals of each channel is performed.

[0015] The transmittance ratio of the first beam splitter is 1:(9 - 99), and the transmittance ratio of the second beam splitter is (9 - 99):1.

[0016] A long-distance laser ranging device for adaptive compensation of pneumatic optical effects, based on a transceiver-separated multi-channel lidar optical antenna 1, includes: a high-power laser 2, at least two variable-focus Moiré lenses 3, a focusing controller 4, a first beam splitter 5, a defocus detection lens 6, an infrared beam analyzer 7, a deformable mirror 8, a deformable mirror controller 9, a wavefront detector 10, a second beam splitter 11, a receiving collimating mirror array 12, and a coherent detection and processing module 13;

[0017] The high-power laser emitted by the high-power laser 2 is expanded by the optical antenna 1 and then emitted towards a long-distance target. The optical antenna 1 receives the reflected echo of the long-distance target. After being focused by the optical antenna 1, the echo enters three receiving channels: the three receiving channels respectively pass through three variable-focus Moiré lenses 3, and then are divided into transmitted light and reflected light by the first beam splitter 5. The transmitted light passes through the defocus detection lens 6 and enters the infrared beam analyzer 7. The infrared beam analyzer 7 analyzes the focused light spot of each receiving channel and outputs a feedback signal. This feedback signal is input to the focusing controller 4. The focusing controller 4 performs focusing in the positive and negative directions on the variable-focus Moiré lenses 3 of each receiving channel until the light spot on the infrared beam analyzer 7 approaches the theoretical Airy disk, completing the axial correction of defocus;

[0018] The reflected light of the first beam splitter 5 is incident on the deformable mirror 8. The reflected light from the deformable mirror 8 is split into reflected light and transmitted light by the second beam splitter 11. The reflected light enters the wavefront detector 10, which detects the echo wavefront of each channel and calculates the wavefront aberration, and transmits the aberration value to the deformable mirror controller 9. The deformable mirror controller 9 obtains the control actuation voltage of the deformable mirror unit lens through calculation and processing, and uses this actuation voltage to drive the deformable mirror 8 to perform conjugate correction on the real-time wavefront.

[0019] A large proportion of the transmitted light of the second beam splitter 11 is focused on the fiber end face through the receiving collimating mirror array 12, and then enters the coherent detection processing module 13 to perform coherent mixing processing with the local signal, obtaining long-distance ranging signals for three channels.

[0020] Compared with the prior art, the technical effects of the present invention are as follows:

[0021] 1) Multiple variable-focus Moiré lenses are adopted in multiple receiving channels of the airborne radar system, which can compensate for the defocus phenomenon of the negative lens effect of the high-speed flow field, but do not make prior defocus compensation for the outgoing wavefront of the emitted laser, and maintain the parallelism of the emitted wavefront.

[0022] 2) An adaptive optical subsystem composed of a deformable mirror, a deformable mirror controller, and a wavefront detector is adopted to perform adaptive compensation for the atmospheric wavefront aberration.

[0023] 3) The present invention can improve the coherent mixing efficiency of each receiving channel, enhance the multi-channel incoherent superposition effect, and improve the ranging power of the radar system. Description of the Drawings

[0024] Figure 1 is a schematic diagram of the long-distance laser ranging device for adaptive compensation of the aero-optical effect of the present invention. Detailed Embodiments

[0025] The technical solution of the present invention will be further described in conjunction with the drawings and embodiments, but the protection scope of the present invention should not be limited thereby.

[0026] The method for adaptive compensation of the aero-optical effect of the transceiver-separated multi-channel laser coherent ranging radar includes a transceiver-separated multi-channel lidar optical antenna 1, a high-power laser 2, a variable-focus Moiré lens 3, a focusing controller 4, a beam splitter 5, a defocus detection lens 6, an infrared beam analyzer 7, a deformable mirror 8, a deformable mirror controller 9, a wavefront detector 10, a beam splitter 11, a receiving collimating mirror array 12, and a coherent detection processing module 13, as Figure 1 shown. In this embodiment, three variable-focus Moiré lenses 3 and three channels are taken as examples.

[0027] The high-power laser emitted by the high-power laser 2 is expanded by the optical antenna 1 and then transmitted towards a distant target. The optical antenna 1 receives the echo reflected by the distant target. The target echo is focused by the optical antenna and then enters the corresponding receiving channel. The echo of each channel first passes through the variable-focus Moiré lens 3 in its channel, and then is split into transmitted light and reflected light by the beam splitter 5, with the transmission ratio designed to be 1:9 or 1:99. The small proportion of transmitted light passes through the defocus detection lens 6 and enters the infrared beam analyzer 7. The built-in software of the infrared beam analyzer 7 analyzes the focused spot on the detection surface, compares its spot diameter with the theoretical Airy disk of the defocus detection lens in the receiving optical path, and combines parameters such as the lens focal length to calculate the defocus amount and defocus direction introduced by the aerodynamic effect. The analysis data is input into the focus adjustment controller 4. The focus adjustment controller 4 processes according to the defocus control algorithm to obtain the rotation angles of the two diffractive optical elements built into the variable-focus Moiré lens 3 in the receiving channel, enabling it to achieve focus adjustment in the positive and negative directions until the spot on the infrared beam analyzer 7 approaches the theoretical Airy disk, completing the axial correction of defocus.

[0028] The large proportion of reflected light from the first beam splitter 5 is incident on the deformable mirror 8. After being reflected by the deformable mirror 8, it is split by the second beam splitter 11, with the transmission ratio designed to be 9:1. The small proportion of reflected light from the beam splitter 11 enters the wavefront detector 10. The wavefront detector 10 detects the wavefront of the echo of each channel and calculates the wavefront aberration, and transmits the aberration value to the deformable mirror controller 9. The deformable mirror controller 9 calculates and processes to obtain the control actuation voltage of the deformable mirror unit lens, driving the deformable mirror 8 to perform conjugate correction on the real-time wavefront. The large proportion of transmitted light from the second beam splitter 11 passes through the receiving collimating mirror array 12 and is focused on the fiber end face, and then enters the coherent detection processing module 13 to perform coherent mixing processing with the local signal to obtain a long-distance ranging signal.

[0029] Finally, the ranging signals of the three channels are non-coherently superimposed electrically, further improving the signal-to-noise ratio and ranging power of the coherent mixing signal.

[0030] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A long-distance laser ranging method for adaptive compensation of pneumatic optical effects, based on a transceiver-separated multi-channel lidar optical antenna, characterized in that, it includes: Using the optical antenna to expand the laser emitted by the high-power laser and emit it towards a long-distance target, and after reducing the beam of the target echo reflected back from the long-distance target, it enters N channels; In each channel, a variable-focus Moiré lens composed of at least two diffractive optical elements is arranged along the optical axis direction. The variable-focus Moiré lens is controlled by a focusing controller to change the rotation angles of the two diffractive optical elements; After the target echo passes through the variable-focus Moiré lens, it is divided into a first transmitted light and a first reflected light by a first beam splitter; the first transmitted light enters an infrared beam analyzer through a defocus detection lens. The infrared beam analyzer analyzes the focused spots of each receiving channel and outputs a feedback signal to the focusing controller. The focusing controller performs focusing in the positive and negative directions on the variable-focus Moiré lens of each receiving channel until the spot on the infrared beam analyzer approaches the theoretical Airy disk, completing the axial correction of defocus; The first reflected light is incident on a deformable mirror. After being reflected by the deformable mirror, it is divided into a second reflected light and a second transmitted light by a second beam splitter. The second reflected light enters a wavefront detector. The wavefront detector detects the wavefront of the target echo of each channel and calculates the wavefront aberration, and transmits the aberration value to the deformable mirror controller. The deformable mirror controller calculates and processes to obtain the actuation voltage of the deformable mirror unit lens, and uses the actuation voltage to drive the deformable mirror to perform conjugate correction on the real-time wavefront; The second transmitted light is focused on the fiber end face through a receiving collimator array and then enters a photodetector to perform coherent mixing processing with a local signal to obtain long-distance ranging signals for each channel; Incoherently superimpose the long-distance ranging signals of each channel into electrical signals.

2. The long-distance laser ranging method for adaptive compensation of pneumatic optical effects according to claim 1, characterized in that, The transmittance ratio of the first beam splitter is 1:(9-99), and the transmittance ratio of the second beam splitter is (9-99):

1.

3. A long-distance laser ranging device for adaptive compensation of pneumatic optical effects, based on a transceiver-separated multi-channel lidar optical antenna (1), characterized in that, it includes: A high-power laser (2), at least two variable-focus Moiré lenses (3), a focusing controller (4), a first beam splitter (5), a defocus detection lens (6), an infrared beam analyzer (7), a deformable mirror (8), a deformable mirror controller (9), a wavefront detector (10), a second beam splitter (11), a receiving collimator array (12) and a coherent detection processing module (13); The high-power laser emitted by the high-power laser (2) is expanded by the optical antenna (1) and then emitted towards a long-distance target. The optical antenna (1) receives the reflected echo of the long-distance target. After being focused by the optical antenna (1), the echo enters three receiving channels: the three receiving channels respectively pass through three variable-focus Moiré lenses (3), and then are divided into transmitted light and reflected light by the first beam splitter (5). The transmitted light passes through the defocus detection lens (6) and enters the infrared beam analyzer (7). The infrared beam analyzer (7) analyzes the focused spots of each receiving channel and outputs a feedback signal. The feedback signal is input to the focus adjustment controller (4). The focus adjustment controller (4) adjusts the variable-focus Moiré lenses (3) of each receiving channel in the positive and negative directions until the spot on the infrared beam analyzer (7) approaches the theoretical Airy disk, completing the axial correction of defocus: The reflected light of the first beam splitter (5) is incident on the deformable mirror (8). The reflected light of the deformable mirror (8) is divided into reflected light and transmitted light by the second beam splitter (11). The reflected light enters the wavefront detector (10). The wavefront detector (10) detects the wavefront of the echo of each channel and calculates the wavefront aberration, and transmits the aberration value to the deformable mirror controller (9). The deformable mirror controller (9) obtains the control actuation voltage of the deformable mirror unit lens through calculation and processing, and uses the actuation voltage to drive the deformable mirror (8) to perform conjugate correction on the real-time wavefront; A large proportion of the transmitted light of the second beam splitter (11) is focused on the fiber end face by the receiving collimator array (12), and then enters the coherent detection processing module (13) to perform coherent mixing processing with the local signal, obtaining long-distance ranging signals of three channels.

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