Airborne tunable three-mirror optical communication system based on surface acoustic wave frequency shift reflector

Abstract

The application discloses an airborne adjustable three-mirror optical communication system based on a surface acoustic wave frequency shift reflection surface, and belongs to the technical field of optical engineering and free space optical communication, and comprises a ground station and an airborne frequency shift reverse reflection terminal carried by a UAV; the airborne terminal is provided with an adjustable three-mirror assembly, a radio frequency driving module, an angle compensation module and a controller. The three reflection surfaces in the adjustable three-mirror assembly are orthogonal to each other, one of the reflection surfaces is integrated with a surface acoustic wave frequency shift reflection surface, a radio frequency driving active dynamic acousto-optic grating introduces exclusive frequency shift to incident light, and the controller controls the angle compensation module to finely adjust the common reflection surface and compensate for the light beam deviation caused by frequency shift diffraction. The radio frequency driving has three working modes, and can realize single machine identity recognition, multi-UAV frequency shift distinction and low-speed auxiliary data transmission; the airborne adjustable three-mirror optical communication system based on the surface acoustic wave frequency shift reflection surface can stably complete UAV reverse optical communication transmission.

Description

Technical Field

[0001] This invention relates to the fields of optical engineering and free-space optical communication technology, and in particular to an airborne tunable three-mirror optical communication system based on a surface acoustic wave frequency-shifting reflector. Background Technology

[0002] In traditional symmetric free-space optical communication systems, both communicating parties typically require laser emitting modules, optical receiving modules, acquisition, tracking, and aiming devices, as well as complex stabilization and control mechanisms. For small UAVs, micro-aircraft, or payload-constrained platforms, a complete optical communication terminal would impose a significant burden on size, weight, power consumption, and heat dissipation, severely limiting its deployment on lightweight platforms.

[0003] To reduce the complexity of airborne systems, inverse modulation free-space optical communication systems have been proposed in existing technologies. In this system, a ground station or master station transmits interrogation light or continuous light, and the UAV modulates the incident light using a retroreflector and modulator, returning it along the same path. This type of system can integrate complex functions such as high-power transmission, precise tracking, and high-sensitivity reception at the ground station, significantly reducing the burden on the UAV. Existing inverse modulators often employ corner reflectors, cat's-eye reflectors, micro-reflector arrays, liquid crystal modulators, microelectromechanical modulators, or electro-absorption modulators, primarily focusing on intensity modulation or spatial modulation of the returned light.

[0004] However, the existing solutions described above still have significant shortcomings in practical free-space links. First, the echo signal received by the ground station may simultaneously contain effective echoes from the UAV, ambient scattered light, ground object reflected light, and other stray light. When relying solely on intensity changes for identification and demodulation, the anti-interference capability and the ability to distinguish echo sources are very limited. Second, adding an active transmitting light source to the UAV would increase the payload burden, contradicting the original intention of the inverse modulation system to reduce the complexity of the airborne end.

[0005] To overcome the limitations of intensity modulation schemes, some researchers have attempted to introduce optical frequency shifting technology, which involves loading identifiable frequency markers into the return light, allowing ground stations to distinguish effective echoes from stray light through heterodyne detection or narrowband filtering. Surface acoustic wave (SAW) acousto-optic structures can form dynamically propagating acousto-optic gratings on the surface of piezoelectric materials, enabling reflected or diffracted light to acquire a frequency shift related to the surface acoustic wave frequency. However, if the SAW frequency shifting structure is simply placed on the surface of an ordinary mirror, the frequency-shifted diffracted light often experiences directional shift, disrupting the reverse return path. Furthermore, if all three reflecting surfaces of a trihedral reflector are modified into SAW frequency shifting surfaces, it significantly increases structural complexity, RF drive power consumption, and optical path uncertainty.

[0006] Therefore, there is an urgent need for a new type of airborne reverse optical communication structure that can provide an identifiable frequency identifier for the return light using surface acoustic wave frequency shifting technology, retain the inherent reverse reflection characteristics of the trihedral reflector, and effectively compensate for the directional shift caused by frequency shift diffraction, so as to achieve compact, identifiable, and controllable airborne frequency shift reverse reflection communication. Summary of the Invention

[0007] The purpose of this invention is to provide an airborne tunable three-mirror optical communication system based on a surface acoustic wave frequency-shifting reflector, which solves the problems in existing airborne reverse modulation optical communication systems, such as the difficulty in distinguishing effective echoes from stray reflections, the heavy active transmission burden on UAVs, the incompatibility between frequency-shifting modulation structures and trihedral echo structures, and the difficulty in compensating for frequency-shifting diffraction direction shifts.

[0008] To achieve the above objectives, the present invention provides an airborne adjustable three-mirror optical communication system based on a surface acoustic wave frequency shifting reflector, including a ground station and an airborne frequency shifting retroreflection terminal mounted on an unmanned aerial vehicle platform; The ground station includes a modulated signal light transmission module, a tracking and pointing module, a frequency-shifted echo receiving module, a demodulation processing module, and a heterodyne or coherent detection unit; The modulated signal light transmitting module generates an optical signal carrying downlink information, interrogation information, or synchronization information; the tracking and pointing module directionally transmits the modulated incident light to the airborne frequency-shifted back echo terminal; the frequency-shifted echo receiving module receives the frequency-shifted back echo and sends it to the heterodyne or coherent detection unit; the demodulation processing module recovers the information carried in the optical signal based on the frequency shift, beat frequency, amplitude change, or phase change, and identifies that the frequency-shifted back echo comes from a UAV platform with a corresponding frequency shift identifier. The airborne frequency-shift retroreflector terminal includes an adjustable three-mirror assembly, an RF drive module, an angle compensation module, and a controller; The adjustable three-mirror assembly includes three orthogonally arranged reflective surfaces that form a trihedral retroreflective optical path; one of the specific reflective surfaces is set as a surface acoustic wave frequency-shifting reflective surface, while the other two reflective surfaces are ordinary high reflective surfaces; The radio frequency driving module is electrically connected to the surface acoustic wave frequency shifting reflector and is used to drive the surface acoustic wave frequency shifting reflector to generate reflected light with a preset frequency shift. The angle compensation module is mechanically connected to the adjustable three-mirror assembly, which is used in conjunction with the controller to adjust the attitude of the reflecting surface and compensate for the frequency shift echo direction offset caused by the surface acoustic wave frequency shift reflecting surface. The controller is electrically connected to the RF drive module and the angle compensation module respectively, and is used to coordinate the RF drive and the fine adjustment of the reflector.

[0009] Preferably, the optical signal generated by the modulated signal optical transmitting module adopts intensity modulation, phase modulation, frequency modulation, or other optical communication modulation formats.

[0010] Preferably, the tracking and pointing module is used to correct the pointing of the modulated incident light during communication based on the UAV's position, attitude, echo intensity, or image tracking information.

[0011] Preferably, the surface acoustic wave frequency shifting reflector is selected as the first, second, or third reflecting surface in the optical path of the adjustable three-mirror assembly; the surface acoustic wave frequency shifting reflector is arranged at a reflection position where the beam footprint is stable, the electrode feed line is easy to lay out, and heat dissipation is convenient.

[0012] Preferably, the surface acoustic wave frequency shifting reflector includes a piezoelectric substrate, an interdigital transducer, an RF feed electrode, a reflective film, an acoustic absorption end, and an acoustic wave propagation region; A piezoelectric substrate serves as the support matrix, and interdigital transducers are fabricated on the surface of the piezoelectric substrate. Radio frequency (RF) feed electrodes are led out from both ends of the interdigital transducers for connecting to an external RF drive module. A reflective film layer is covered on the surface of the piezoelectric substrate, and incident laser light irradiates the reflective film layer region. The interdigital transducers directionally excite surface acoustic waves along the acoustic wave propagation region, and an acoustic absorption end is set at the end of the acoustic wave propagation region to absorb the surface acoustic waves propagating to the end, thereby reducing standing waves and modulation instability caused by acoustic wave reflection. The radio frequency drive module is electrically connected to the interdigital transducer through the radio frequency feed electrode. It is used to apply a radio frequency electrical signal to the interdigital transducer, which then excites surface acoustic waves on the piezoelectric substrate surface.

[0013] Preferably, the piezoelectric substrate is selected from any one of lithium niobate, quartz, lithium tantalate, zinc oxide film, aluminum nitride film, or other piezoelectric materials that can effectively excite surface acoustic waves; the reflective film layer adopts a metal-dielectric composite reflective film, whose working wavelength matches the wavelength of the light emitted by the ground station.

[0014] Preferably, the angle compensation module compensates for the frequency shift echo direction shift caused by the frequency shift reflecting surface of the surface acoustic wave by adjusting the dihedral angle, mounting angle or slight orientation of at least one ordinary high reflective surface in the adjustable three-mirror assembly.

[0015] Preferably, the frequency-shifted echo receiver module is used to identify and demodulate the return light with a preset frequency shift through narrowband optical filtering, heterodyne detection, coherent reception, or beat frequency detection.

[0016] Preferably, the controller is configured to output a control signal to the angle compensation module based on a preset geometric model, the received intensity fed back by the ground station, the UAV attitude information or the frequency shift echo angle error information, and use a drive actuator to perform a small angle adjustment on at least one ordinary high reflective surface in the adjustable three-mirror assembly to counteract the beam angle shift caused by the diffraction of the surface acoustic wave frequency shift reflector, so that the first-order frequency shift reflected light returns to the ground station as frequency shift return light.

[0017] Preferably, the RF driver module is configured to operate in the following modes: In mode 1, the RF drive module outputs a fixed frequency RF signal, and the surface acoustic wave frequency shift reflector introduces a fixed frequency shift for the return light. The ground station relies on this fixed frequency shift to identify valid UAV echoes. Mode 2: The RF drive module selectively outputs multiple different frequency RF signals, matching differentiated frequency shift identifiers for different UAV platforms and different communication links; In mode three, the radio frequency drive module changes the switching state, amplitude, frequency, or phase of the radio frequency signal in real time according to the airborne control information, so that the frequency shift back transmits light carrying low-speed auxiliary data information.

[0018] Therefore, the airborne tunable three-mirror optical communication system based on the surface acoustic wave frequency shifting reflector described above has the following beneficial effects: (1) The present invention integrates a surface acoustic wave frequency shifting reflector on only one side of the trihedral corner reflector, and uses conventional high reflectivity mirrors on the other two sides. Compared with the scheme of arranging surface acoustic wave devices on all three sides, the number of radio frequency driving channels is greatly reduced, the radio frequency power consumption of the airborne terminal is reduced, and the circuit structure is simplified. At the same time, the orthogonal retroreflection optical characteristics of the trihedral corner reflector are retained, and the basic structure of the original reflected light path is not destroyed. (2) By introducing a dedicated frequency shift identifier to the return light through the surface acoustic wave structure, the ground station can use heterodyne and coherent detection methods to distinguish between the effective communication echo with frequency shift and the ground object reflection and atmospheric stray light without frequency shift, which significantly improves the link signal-to-noise ratio and anti-interference capability in complex environments and solves the problems of traditional intensity modulation optical communication being susceptible to stray light interference and difficult signal identification. (3) An angle compensation module and actuator are provided to finely adjust the attitude of the ordinary reflector according to the attitude of the UAV and the feedback from the ground receiver, compensate for the beam angle shift caused by surface acoustic wave diffraction, so that the first-order frequency shift signal can return to the ground receiver field of view stably, reduce the communication interruption caused by the frequency shift beam deviating out of the field of view, and improve the reliability of the link. (4) The radio frequency drive module supports multiple working modes. It can output a fixed frequency shift to identify a single UAV, or switch multiple frequency shifts to distinguish different UAVs in the cluster. It can also carry low-speed auxiliary data by modulating radio frequency signals, taking into account the identification of multiple UAVs in the network and the synchronous transmission of auxiliary information, thus expanding the application scenarios of the system. (5) The system adopts an asymmetric communication architecture. All complex functions such as laser emission, precision tracking, and coherent demodulation are deployed on the ground station. The UAV end only retains a lightweight reverse echo terminal, which is low in size, weight, and power consumption, and is suitable for the use of small and micro UAVs with limited payload. (6) The acoustic absorption end is set on the frequency shift reflection surface of the surface acoustic wave, which can suppress the standing wave formed by the reflection at the end of the acoustic wave, reduce optical modulation jitter, and improve the stability of the frequency shift signal output; the piezoelectric substrate and the reflective film layer can be made of a variety of mature materials, which are easy to process and prepare and have strong engineering applicability.

[0019] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0020] Figure 1 This is a diagram illustrating the overall communication architecture of an embodiment of the present invention; Figure 2 This is a schematic diagram of the adjustable three-mirror assembly structure according to an embodiment of the present invention; Figure 3 This is a schematic diagram of the surface acoustic wave frequency shifting reflector structure according to an embodiment of the present invention; Figure 4 This is a schematic diagram illustrating the working principle of the surface acoustic wave frequency shifting reflector in an embodiment of the present invention.

[0021] Figure label: 100. Ground station; 101. Modulated signal light transmission module; 102. Tracking and pointing module; 103. Frequency-shifted echo receiving module; 104. Demodulation processing module; 105. Heterodyne or coherent detection unit; 200. UAV platform; 201. Airborne frequency-shifted back echo terminal; 204. Radio frequency drive module; 221. First reflecting surface; 222. Second reflecting surface; 223. Third reflecting surface; 224. Flexible hinge; 225. Base; 226. Drive actuator; 2211. Reflective film layer; 2212. Piezoelectric substrate; 2213. Interdigital transducer; 2214. RF feed electrode; 2215. Sound absorption end; 2216. Sound wave propagation region; 301. Modulated incident light; 302. Frequency-shifted return light; 303. Unshifted reflected light; 304. First-order frequency-shifted reflected light. Detailed Implementation

[0022] The following detailed description of embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0023] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0024] I. System Overall Structure like Figure 1 As shown, this embodiment provides an airborne tunable three-mirror optical communication system based on a surface acoustic wave frequency-shifting reflector. The system includes a ground station 100 and an airborne frequency-shifting retroreflection terminal 201 mounted on a UAV platform 200. The ground station 100 transmits modulated incident light 301 to the UAV platform 200 and receives frequency-shifted return light 302 returned by the airborne frequency-shifting retroreflection terminal 201.

[0025] Ground station 100 includes a modulated signal light transmitting module 101, a tracking and pointing module 102, a frequency-shifted echo receiving module 103, a demodulation processing module 104, and a heterodyne or coherent detection unit 105. The modulated signal light transmitting module 101 generates an optical signal carrying downlink information, interrogation information, or synchronization information. This optical signal can employ intensity modulation, phase modulation, frequency modulation, or other optical communication modulation formats. The tracking and pointing module 102 directs the modulated incident light 301 to the UAV platform 200 and performs pointing corrections based on the UAV's position, attitude, echo intensity, or image tracking information during communication. The frequency-shifted echo receiving module 103 receives the frequency-shifted return light 302 and sends it to the heterodyne or coherent detection unit 105. The frequency-shifted echo receiving module 103 can employ narrowband optical filtering, heterodyne detection, coherent reception, or beat frequency detection to identify and demodulate the return light with a preset frequency shift. The demodulation processing module 104 recovers the information carried in the optical signal based on the frequency shift, beat frequency, amplitude change or phase change, and identifies that the frequency shifted return light 302 comes from a UAV platform with a corresponding frequency shift identifier.

[0026] An airborne frequency-shifting back-echo terminal 201 is installed on the UAV platform 200. The airborne frequency-shifting back-echo terminal 201 does not need to actively generate high-power transmitted light for the main communication link; instead, it utilizes the modulated incident light 301 transmitted by the ground station 100 as the back-echoed and frequency-shifted optical carrier. Thus, the communication system forms an asymmetric free-space optical communication architecture, where the ground station undertakes the complex functions of transmission, tracking, reception, and demodulation, while the UAV terminal undertakes the frequency-shifting back-echo and necessary micro-angle adjustment functions.

[0027] II. Airborne Frequency Shift Reverse Echo Terminal Structure The airborne frequency-shift retroreflector terminal 201 includes an adjustable three-mirror assembly, a radio frequency drive module 204, a controller, and an angle compensation module.

[0028] like Figure 2As shown, the adjustable three-mirror assembly adopts a trihedral reflector structure, including a first reflecting surface 221, a second reflecting surface 222, and a third reflecting surface 223. In this embodiment, the second reflecting surface 222 and the third reflecting surface 223 are configured as ordinary high-reflectivity surfaces, while the first reflecting surface 221, which has a stable beam footprint, facilitates electrode feed line layout, and facilitates heat dissipation, is configured as a surface acoustic wave frequency-shifting reflecting surface. Alternatively, the second reflecting surface 222 and the third reflecting surface 223 can also be configured as surface acoustic wave frequency-shifting reflecting surfaces. When selecting and arranging the reflecting surfaces, priority is given to selecting reflection positions with stable beam footprints, convenient electrode feed line layout, and good heat dissipation conditions as surface acoustic wave frequency-shifting reflecting surfaces. The three reflecting surfaces are arranged at approximately right angles to each other, so that the incident light returns approximately along the incident direction after undergoing multiple reflections within them.

[0029] Unlike schemes that set all three reflecting surfaces as frequency-shifting reflecting surfaces, this embodiment sets a surface acoustic wave (SAW) frequency-shifting reflecting surface on only one specific reflecting surface. This allows the SAW structure to introduce a preset frequency shift into the returned light while preserving the stabilizing support of the trihedral return geometry from the other ordinary high-reflectivity surfaces, thus reducing the complexity of RF driving and the risk of acoustic crosstalk.

[0030] The adjustable three-mirror assembly also includes a base 225, a flexible hinge 224, and a drive actuator 226. The base 225 is fixedly mounted on the UAV platform 200, providing a supporting foundation for the entire adjustable three-mirror assembly. The second reflective surface 222 is connected to the base 225 via the flexible hinge 224, which allows the second reflective surface 222 to deflect at a small angle relative to the base 225 about a predetermined axis. One end of the drive actuator 226 is connected to the base 225, and the other end is connected to the back or side of the second reflective surface 222. Under the control of the controller, the drive actuator 226 generates displacement, pushing the second reflective surface 222 to rotate about the axis of the flexible hinge 224, thereby achieving precise adjustment of the dihedral angle of the reflective surface. The adjustment method of the third reflective surface 223 is the same as that of the second reflective surface 222. The first reflective surface 221 (i.e., the surface acoustic wave frequency shift reflector) is fixedly mounted on the base 225 and does not participate in angle adjustment. By using the above adjustment methods, while ensuring a compact structure, it is possible to effectively compensate for the frequency shift echo direction offset caused by the frequency shift reflection surface of surface acoustic waves.

[0031] The adjustable three-mirror assembly is used to form a retroreflection path, the surface acoustic wave frequency-shifting reflector is used to frequency-shift the beam passing through it, the radio frequency drive module 204 is used to drive the surface acoustic wave frequency-shifting reflector, the controller is used to coordinate the radio frequency drive and the fine adjustment of the reflector, and the angle compensation module is used to compensate for the echo direction shift caused by frequency shift diffraction by adjusting the dihedral angle, mounting angle or small attitude of the second reflector 222 and the third reflector 223 in the adjustable three-mirror assembly.

[0032] III. Specific Structure of the Surface Acoustic Wave Frequency Shift Reflector like Figure 3 As shown, the surface acoustic wave frequency shifting reflector includes a piezoelectric substrate 2212, an interdigital transducer 2213, an RF feed electrode 2214, a reflective film layer 2211, an acoustic absorption end 2215, and an acoustic wave propagation region 2216. The interdigital transducer 2213 is formed on the surface of the piezoelectric substrate 2212 and is connected to the RF drive module 204 through the RF feed electrode 2214.

[0033] After the radio frequency drive module 204 applies a radio frequency electrical signal to the interdigital transducer 2213, the interdigital transducer 2213 excites surface acoustic waves on the surface of the piezoelectric substrate 2212. The surface acoustic waves propagate along the acoustic wave propagation region 2216 and form time-varying periodic surface perturbations or refractive index perturbations in the region where the reflective film layer 2211 is located. For a light beam incident on this region, this region is equivalent to a propagating reflective acousto-optic grating.

[0034] A reflective film layer 2211 covers the surface of the piezoelectric substrate 2212. In this embodiment, the reflective film layer 2211 is a metal-dielectric composite reflective film, and its operating wavelength matches the wavelength of the light emitted by the ground station. The piezoelectric substrate 2212 can be made of lithium niobate, quartz, lithium tantalate, zinc oxide film, aluminum nitride film, or other piezoelectric materials that can effectively excite surface acoustic waves. The sound absorption end 2215 is disposed at the end of the sound wave propagation region 2216 to absorb the surface acoustic waves propagating to the end, thereby reducing standing waves and modulation instability caused by sound wave reflection.

[0035] When the modulated incident light 301 is incident on the surface acoustic wave frequency-shifting reflector in the three-mirror back-reflection path, the light beam forms a zero-order, weak unshifted reflected light 303 and a first-order frequency-shifted reflected light 304 under the action of the dynamic acousto-optic grating. The first-order frequency-shifted reflected light 304 obtains a preset frequency shift related to the surface acoustic wave frequency.

[0036] like Figure 4 As shown, the modulated incident light 301 emitted by the ground station 100 enters the airborne frequency-shifting retroreflection terminal 201 and is reflected sequentially on the reflective surface of the adjustable three-mirror assembly. When the beam is incident on the surface acoustic wave frequency-shifting reflective surface, the beam forms unshifted reflected light 303 and first-order frequency-shifted reflected light 304 under the action of the dynamic acousto-optic grating.

[0037] Since the first-order frequency-shifted reflected light 304 may experience angular shift while gaining frequency shift, it may deviate from the original reverse reflection path of the trihedral corner reflector if no compensation is performed. To address this, the controller, based on a preset geometric model, the received intensity fed back from the ground station, the UAV attitude information, or the frequency-shifted echo angle error information, controls the angle compensation module to drive the actuator 226, causing at least one ordinary reflecting surface (e.g., the second reflecting surface 222) to undergo a slight angular adjustment.

[0038] Through the above fine-tuning, the frequency shift diffraction deflection caused by the frequency shift reflection surface of the surface acoustic wave can be compensated by the geometric adjustment of the adjustable three-mirror assembly, so that the first-order frequency shift reflected light 304 can still return to the ground station 100 as the frequency shift return light 302 in an approximately original direction.

[0039] IV. Operating Modes of the RF Driver Module In this embodiment, the radio frequency driver module 204 has multiple operating modes to adapt to different communication needs.

[0040] In the first working mode, the radio frequency drive module 204 outputs a fixed frequency radio frequency signal, and the surface acoustic wave frequency shift reflector introduces a fixed frequency shift for the return light. The ground station 100 identifies the valid echo of the UAV through this fixed frequency shift.

[0041] In the second operating mode, the RF drive module 204 outputs RF signals of different frequencies, so that different UAV platforms or different link states correspond to different frequency shifts. Specifically, when there are multiple UAV platforms in the system, different RF frequencies can be assigned to different platforms, and each platform returns return light with its own frequency shift identifier. The ground station 100 distinguishes different platforms by measuring the frequency shift of the received echo.

[0042] In the third operating mode, the RF drive module 204 changes the switching state, amplitude, frequency, or phase of the RF signal according to the airborne control information, so that the frequency-shifted backhaul light 302 carries low-speed auxiliary data information. For example, when it is necessary to report UAV status information to the ground station, the controller encodes the data to be transmitted into a control sequence, and realizes the uplink transmission of auxiliary data by controlling the switching or frequency change of the RF drive module 204.

[0043] V. System Workflow The airborne adjustable three-mirror optical communication system based on surface acoustic wave frequency shifting reflector in this embodiment has the following working process.

[0044] Step 1: Ground station 100 generates modulated incident light 301 through modulated signal light transmitting module 101, and the tracking and pointing module 102 points it to UAV platform 200. Modulated incident light 301 carries downlink information, interrogation information, or synchronization information.

[0045] Step 2: The airborne frequency-shifting retroreflection terminal 201 receives the modulated incident light 301 and directs it into the adjustable three-mirror assembly.

[0046] Step 3: The radio frequency drive module 204 applies a radio frequency signal to the interdigital transducer 2213 to form a surface acoustic wave that propagates along the reflective surface on the surface acoustic wave frequency shift reflection surface.

[0047] Step 4: The modulated incident light 301 is incident on the surface acoustic wave frequency-shifting reflector in the three-mirror backscattering path, and obtains a preset frequency shift during the reflection diffraction process to form a first-order frequency-shifted reflected light 304.

[0048] Step 5: The angle compensation module drives the actuator 226 based on the feedback from the ground station, the attitude of the UAV, or a preset compensation model, so that at least one reflective surface of the adjustable three-mirror assembly undergoes a slight angle adjustment to compensate for the directional shift of the first-order frequency-shifted reflected light 304.

[0049] Step six: The frequency-shifted backhaul optical 302 returns to the ground station 100. The ground station 100 detects the backhaul optical 302 through the frequency-shifted backhaul receiving module 103 and the heterodyne or coherent detection unit 105. Then, the demodulation processing module 104 completes signal recovery, echo identification, and link status judgment.

[0050] Therefore, the present invention employs the above-mentioned airborne adjustable three-mirror optical communication system based on the frequency shift reflector of surface acoustic waves, which can effectively distinguish between valid echo and stray light, while retaining the inherent retroreflection characteristics of the trihedral corner reflector, and using the angle compensation module to eliminate the directional shift caused by frequency shift diffraction, thus realizing compact, identifiable and adjustable airborne frequency shift retroreflection communication.

[0051] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. An airborne tunable three-mirror optical communication system based on a surface acoustic wave frequency-shifting reflector, characterized in that, This includes ground stations and airborne frequency-shift back-echo terminals mounted on UAV platforms; The ground station includes a modulated signal light transmission module, a tracking and pointing module, a frequency-shifted echo receiving module, a demodulation processing module, and a heterodyne or coherent detection unit; The modulated signal light transmission module is used to generate an optical signal carrying downlink information, interrogation information, or synchronization information; the tracking and pointing module directs the modulated incident light to the airborne frequency-shifted back-emission terminal. The frequency-shifted echo receiving module receives the frequency-shifted echo light and sends it to the heterodyne or coherent detection unit; the demodulation processing module recovers the information carried in the optical signal based on the frequency shift, beat frequency, amplitude change or phase change, and identifies that the frequency-shifted echo light comes from a UAV platform with a corresponding frequency shift identifier. The airborne frequency-shift retroreflector terminal includes an adjustable three-mirror assembly, an RF drive module, an angle compensation module, and a controller; The adjustable three-mirror assembly includes three orthogonally arranged reflective surfaces that form a trihedral retroreflective optical path; one of the specific reflective surfaces is set as a surface acoustic wave frequency-shifting reflective surface, while the other two reflective surfaces are ordinary high reflective surfaces; The radio frequency driving module is electrically connected to the surface acoustic wave frequency shifting reflector and is used to drive the surface acoustic wave frequency shifting reflector to generate reflected light with a preset frequency shift. The angle compensation module is mechanically connected to the adjustable three-mirror assembly, which is used in conjunction with the controller to adjust the attitude of the reflecting surface and compensate for the frequency shift echo direction offset caused by the surface acoustic wave frequency shift reflecting surface. The controller is electrically connected to the RF drive module and the angle compensation module respectively, and is used to coordinate the RF drive and the fine adjustment of the reflector.

2. The airborne tunable three-mirror optical communication system based on a surface acoustic wave frequency-shifting reflector according to claim 1, characterized in that, The optical signal generated by the modulated signal optical emission module is modulated by intensity, phase, or frequency.

3. The airborne tunable three-mirror optical communication system based on a surface acoustic wave frequency-shifting reflector according to claim 1, characterized in that, The tracking and pointing module is used to correct the pointing of the modulated incident light during communication based on the UAV's position, attitude, echo intensity, or image tracking information.

4. The airborne tunable three-mirror optical communication system based on a surface acoustic wave frequency-shifting reflector according to claim 1, characterized in that, The surface acoustic wave frequency shifting reflector is selected as the first, second, or third reflecting surface in the optical path of the adjustable three-mirror assembly; the surface acoustic wave frequency shifting reflector is arranged at a reflection position where the beam footprint is stable, the electrode feed line is easy to lay out, and heat dissipation is convenient.

5. The airborne tunable three-mirror optical communication system based on a surface acoustic wave frequency-shifting reflector according to claim 4, characterized in that, The surface acoustic wave frequency shifting reflector includes a piezoelectric substrate, an interdigital transducer, an RF feed electrode, a reflective film, an acoustic absorption end, and an acoustic wave propagation region; A piezoelectric substrate is used as the carrier substrate, and an interdigital transducer is fabricated on the surface of the piezoelectric substrate. Radio frequency feed electrodes are led out from both ends of the interdigital transducer for connecting an external radio frequency drive module. A reflective film is covered on the surface of a piezoelectric substrate, and an incident laser irradiates the reflective film area. An interdigital transducer directionally excites surface acoustic waves along the sound wave propagation region, and an acoustic absorption end is set at the end of the sound wave propagation region to absorb the surface acoustic waves propagating to the end. The radio frequency drive module is electrically connected to the interdigital transducer through the radio frequency feed electrode. It is used to apply a radio frequency electrical signal to the interdigital transducer, which then excites surface acoustic waves on the piezoelectric substrate surface.

6. The airborne tunable three-mirror optical communication system based on a surface acoustic wave frequency-shifting reflector according to claim 5, characterized in that, The piezoelectric substrate is made of lithium niobate, quartz, lithium tantalate, zinc oxide film or aluminum nitride film; the reflective film layer is a metal-dielectric composite reflective film, whose working wavelength matches the wavelength of the light emitted by the ground station.

7. The airborne tunable three-mirror optical communication system based on a surface acoustic wave frequency-shifting reflector according to claim 1, characterized in that, The angle compensation module compensates for the frequency shift echo direction shift caused by the frequency shift reflecting surface of surface acoustic waves by adjusting the dihedral angle, mounting angle or slight orientation of at least one ordinary high reflective surface in the adjustable three-mirror assembly.

8. The airborne tunable three-mirror optical communication system based on a surface acoustic wave frequency-shifting reflector according to claim 1, characterized in that, The frequency-shifted echo receiver module is used to identify and demodulate the return light with a preset frequency shift through narrowband optical filtering, heterodyne detection, coherent reception, or beat frequency detection.

9. The airborne tunable three-mirror optical communication system based on a surface acoustic wave frequency-shifting reflector according to claim 1, characterized in that, The controller is configured to output control signals to the angle compensation module based on the preset geometric model, the received intensity fed back by the ground station, the UAV attitude information or the frequency shift echo angle error information, and use the drive actuator to perform a small angle adjustment on at least one ordinary high reflective surface in the adjustable three-mirror assembly to counteract the beam angle shift caused by the diffraction of the surface acoustic wave frequency shift reflector, so that the first-order frequency shift reflected light returns to the ground station as frequency shift return light.

10. The airborne tunable three-mirror optical communication system based on a surface acoustic wave frequency-shifting reflector according to claim 1, characterized in that, The RF driver module is configured to operate in the following modes: In mode 1, the RF drive module outputs a fixed frequency RF signal, and the surface acoustic wave frequency shift reflector introduces a fixed frequency shift for the return light. The ground station relies on this fixed frequency shift to identify valid UAV echoes. Mode 2: The RF drive module selectively outputs multiple different frequency RF signals, matching differentiated frequency shift identifiers for different UAV platforms and different communication links; In mode three, the radio frequency drive module changes the switching state, amplitude, frequency, or phase of the radio frequency signal in real time according to the airborne control information, so that the frequency shift back transmits light carrying low-speed auxiliary data information.

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