Underwater wide-range search and tracking duplex laser communication device and method

Abstract

The application relates to an underwater laser communication device and method, in particular to an underwater wide-range search tracking duplex laser communication device and method, which solves the technical problem that it is difficult to quickly orient and establish an optical communication link between underwater random motion platforms. The application adopts a two-dimensional rotary table to cooperate with a photoelectric detector and an image detector to respectively perform primary wide-range scanning and secondary accurate tracking scanning, which ensures a large scanning range and has the characteristics of high speed and high accuracy, can quickly reduce the uncertain area for the underwater non-cooperative target scene, complete initial pointing, and then position and track the laser spot centroid position, realize the discovery and capture of the underwater random motion platform, can quickly establish an optical communication link, effectively improve the communication distance and the communication rate, and the overall layout of the equipment is compact and miniaturized, and is easy to be applied in engineering.

Description

Technical Field

[0001] This invention relates to underwater laser communication devices and methods, specifically to an underwater wide-range search and tracking duplex laser communication device and method. Background Technology

[0002] Compared to traditional underwater acoustic communication, underwater wireless optical communication offers advantages such as larger bandwidth, higher speed, lower power consumption, stronger anti-interference capabilities, and better security, attracting widespread attention in recent years. In underwater wireless optical communication, using a laser light source can effectively improve communication speed and distance compared to LED-based optical communication devices. However, due to the narrow beam and strong directivity of lasers, achieving precise alignment of the underwater optical path and establishing and maintaining a stable communication link is extremely challenging.

[0003] Acquiring, aiming, and tracking (APT) the laser communication beam is a prerequisite for underwater laser communication. Firstly, while APT systems for space laser communication are mature, the optical equipment used in space has a large aperture and doesn't account for the immense pressure and sealing requirements of the underwater environment, making direct application difficult in underwater laser communication. Currently, underwater laser communication often uses a fixed frame for ballistic beam alignment and underwater dynamically sealed gimbals or tilting mirrors for target search. However, fixed frames are insufficient for complex practical applications, underwater dynamically sealed gimbals have slow response times, low accuracy, large size and weight, and limited mounting options; while tilting mirrors have a limited rotation range, making it difficult to achieve wide-area target search.

[0004] Secondly, common space laser communication equipment uses a platform as its cooperative target. For example, a satellite platform can calculate its initial position through ephemeris data and quickly establish an optical communication link within a small, uncertain area. In contrast, underwater laser communication equipment is generally mounted on mobile platforms such as manned submersibles, AUVs (autonomous underwater vehicles), or ROVs (remotely operated vehicles). These mobile platforms move randomly, and relying on acoustic communication for positioning is not very accurate and easily exposes the positioning information, making it difficult for the underwater communication target to obtain its initial orientation.

[0005] Therefore, underwater laser communication equipment needs to be able to scan and capture over a large spatial range in order to establish optical communication links. Summary of the Invention

[0006] The purpose of this invention is to solve the technical problem of difficulty in quickly orienting and establishing optical communication links between underwater randomly moving platforms, and to provide an underwater large-area search and tracking duplex laser communication device and method.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] An underwater wide-range search and tracking duplex laser communication device includes two identical optical communication units, which are respectively installed on the local end equipment and the remote end equipment. The special feature is that the optical communication unit includes a sealed cabin composed of a fixed base and a transparent optical window, a two-dimensional turntable and a main control unit set in the sealed cabin, and a transceiver off-axis optical system set on the two-dimensional turntable.

[0009] The off-axis optical system is positioned close to the transparent optical window and includes a transmitting branch and a receiving branch that are parallel to each other.

[0010] The transmitting branch includes a laser for emitting beacon light and / or communication light, and a laser collimator and a first zoom lens arranged sequentially along the laser transmission direction;

[0011] The receiving branch includes a second zoom lens and a beam splitter arranged sequentially along the beam propagation direction. The beam splitter has a first-level large-range scanning and communication unit and a second-level fine tracking scanning unit arranged sequentially along the transmission and reflection paths of the beam. The first-level large-range scanning and communication unit includes a collimating and focusing unit and a photodetector arranged sequentially along the beam propagation direction. The second-level fine tracking scanning unit includes a reflector, a focusing unit and an image detector arranged sequentially along the beam propagation direction.

[0012] The main control unit is electrically connected to the two-dimensional turntable, the laser, the photodetector, and the image detector, respectively, and is used to control the two-dimensional turntable, modulate the laser output from the laser, and filter and demodulate the laser signals received by the photodetector and the image detector.

[0013] Furthermore, the laser includes a beacon laser and a communication laser, wherein the beacon laser and the communication laser emit beams with the same wavelength. The communication laser is used to emit communication light, and the beacon laser is used to emit beacon light and enhance the power of the communication light.

[0014] Both the beacon laser and the communication laser have a laser collimator and a first zoom lens sequentially arranged at their rear ends along the laser transmission direction.

[0015] Furthermore, the detection field of view of both the photodetector and the image detector is ≥10°;

[0016] The photodetector is a PMT detector, and the image detector is a CCD camera;

[0017] The transparent optical window is a large-diameter transparent optical window with a cylindrical section and a hemisphere, and its inner surface is provided with an anti-reflective coating.

[0018] The two-dimensional turntable adopts an L-shaped layout. The azimuth and pitch rotation are driven by servo motors, and the angle measuring element is a small time grid encoder.

[0019] The beam splitter has a beam splitting ratio of 1:4.

[0020] Furthermore, the collimating and focusing unit includes a first focusing lens, a variable aperture, a shaping lens, a narrowband filter, and a second focusing lens arranged sequentially along the beam propagation direction, and the variable aperture is electrically connected to the main control unit;

[0021] The focusing unit includes a third focusing lens.

[0022] Furthermore, it also includes a tracking processing board, which is electrically connected to the laser, photodetector, and image detector respectively, and wirelessly connected to the main control unit. It is used to receive the modulated electrical signal sent by the main control unit and send it to the laser, as well as amplify the output signal of the photodetector, analyze the output signal of the image detector, and send it to the main control unit.

[0023] Alternatively, it may also include an external synchronous rotating frame mounted on the two-dimensional turntable and the off-axis optical system at both ends, with the electrical wiring between the main control unit and the two-dimensional turntable, laser, photodetector, and image detector fixed on the external synchronous rotating frame.

[0024] This invention also provides an underwater large-area search and tracking full-duplex laser communication method, which employs the aforementioned underwater large-area search and tracking full-duplex laser communication device, and is characterized by including the following steps:

[0025] Step 1: The local main control unit controls the laser to emit beacon light, and the remote main control unit controls the two-dimensional turntable to drive the photodetector to perform a first-level large-area scan and the image detector to perform a second-level fine tracking scan, so that the beacon light spot emitted by the local laser is at the center of the target surface of the remote image detector.

[0026] Step 2: Following the method in Step 1, position the beacon beam emitted by the laser at the other end at the center of the target surface of the image detector at this end, thereby establishing a communication link;

[0027] Step 3: Adjust the positions of the second zoom lenses at the local and remote ends respectively to reduce the receiving field of view. Then, the main control units at the local and remote ends control the corresponding lasers to emit communication light.

[0028] Step 4: The communication light from this end and the other end passes through the second zoom lens and beam splitter of the other end in sequence and is then split into a tracking beam and a communication beam. The tracking beam is focused by the focusing unit and then received by the image detector and transmitted to the main control unit. The communication beam is shaped and focused by the collimating and focusing unit and then received by the photodetector and transmitted to the main control unit.

[0029] Step 5: The main control units at both ends process the output signals of the corresponding image detectors in real time, calculate the centroid position of the communication light spot, and adjust the two-dimensional turntable so that the communication light spot is always at the center of the target surface of the image detector. The main control units at both ends process the output signals of the corresponding photoelectric detectors in real time to obtain communication information and complete the large-scale underwater search and tracking duplex laser communication.

[0030] Further, step 1 specifically includes:

[0031] Step 1.1: Set the adjustment time interval and position of the first zoom lens on this end. The main control unit controls the laser to emit beacon light, and then uses a polling method to adjust the position of the first zoom lens according to the set adjustment interval and position, thereby adjusting the beam divergence angle of the beacon light on this end.

[0032] Step 1.2: Power on the other end of the 2D turntable and return to zero to enter the waiting state. Then, adjust the second zoom lens to maximize the receiving field of view of the other end. Set the predetermined scanning trajectory and step distance of the 2D turntable in the first-level large-range scanning and second-level fine tracking scanning states, as well as the output threshold of the photodetector, through the main control unit.

[0033] Step 1.3: Turn on the peer photodetector and image detector. The main control unit determines whether the image detector has received a light spot based on its output signal. If a light spot is received, proceed to step 1.8; otherwise, proceed to step 1.4.

[0034] Step 1.4: The remote main control unit controls the two-dimensional turntable to perform a first-level large-area scan according to the predetermined scanning trajectory and step distance, and records the output signal of the photodetector until the scan of the entire spatial range is completed.

[0035] Step 1.5: The remote main control unit analyzes the output signal of the photodetector. If the output signal of the photodetector is greater than the output threshold, the corresponding spatial region of the maximum light intensity peak range of the photodetector output signal is extracted and identified as the suspected target region. Then, step 1.6 is executed.

[0036] Otherwise, reduce the predetermined step size of the large-scale scan at the other end, reduce the beam divergence angle of the beacon light at this end, and then return to step 1.4;

[0037] Step 1.6: The remote main control unit controls the two-dimensional turntable to perform a secondary fine tracking scan on the suspected target area according to the predetermined scanning trajectory and step distance, and records the output signal of the image detector;

[0038] Step 1.7: The remote main control unit analyzes the output signal of the image detector in real time. If the remote image detector receives a light spot, then proceed to step 1.8; otherwise, reduce the predetermined step distance of the secondary fine tracking scan and return to step 1.6.

[0039] Step 1.8: The remote main control unit calculates the centroid position of the light spot output signal of the image detector, and then adjusts the two-dimensional turntable according to the centroid position of the light spot so that the beacon light spot emitted by the local laser is located at the center of the target surface of the remote image detector 24.

[0040] Further, in step 1.2, the output threshold of the photodetector is obtained by the following method:

[0041] Background noise is recorded using a photodetector, the average output voltage of the background noise is calculated, and then the average output voltage plus 3 times the standard deviation is taken as the output threshold of the photodetector.

[0042] Alternatively, a photodetector can be used to record background noise, calculate the average rate of change of the background noise, and add three times the standard deviation to the average rate of change of the background noise as the output threshold of the photodetector.

[0043] Furthermore, in step 1.1, the beam divergence angle of the local beacon light satisfies the condition that the diameter of the beam emitted through the transparent optical window at least covers the optical communication device at the other end.

[0044] Further, in step 1.2, the predetermined scanning trajectory and step distance of the two-dimensional turntable in the first-level large-range scanning and second-level fine-tracking scanning states are specifically as follows:

[0045] The predetermined scanning trajectory for the first-level large-area scan is a serpentine scan or a rectangular scan, with a step size of 80% to 85% of the field of view of the photodetector;

[0046] The predetermined scanning trajectory for the second-stage precision tracking scan is a spiral scan, with a step size of 80% to 85% of the laser beam divergence angle.

[0047] Compared with the prior art, the present invention has the following beneficial technical effects:

[0048] 1. The present invention provides an underwater large-area search and tracking duplex laser communication device, which uses a photodetector in conjunction with a two-dimensional turntable for primary large-area scanning and an image detector in conjunction with a two-dimensional turntable for secondary fine tracking scanning. This ensures a large scanning range and features high speed and high precision. For underwater non-cooperative target scenarios, it can quickly narrow down the uncertain area and complete the initial pointing. Then, by obtaining the centroid position of the laser spot, it can locate and track the target, realize the discovery and capture of underwater random moving platforms, and quickly establish an optical communication link. At the same time, it effectively improves the communication distance and communication rate. The overall layout of the device is compact and miniaturized, making it easy to apply in engineering.

[0049] 2. In the underwater wide-range search and tracking duplex laser communication device provided by the present invention, the transmitting branch and the receiving branch are parallel, which can ensure that there is no interference between the transmitting and receiving signals during the communication process;

[0050] 3. In the underwater large-area search and tracking duplex laser communication device provided by the present invention, a reflector is set in the reflected light path of the beam splitter prism so that the image detector branch and the photoelectric detector branch are set in parallel, which can reduce the system volume and meet the system size constraints.

[0051] 4. In the underwater wide-range search and tracking duplex laser communication device provided by the present invention, two laser emitters are set up, and the two lasers emit beams with the same wavelength. One of them is used to emit beacon light, which is used for the tracking and scanning of the device on the one hand, and to enhance the communication optical power when the communication optical power is insufficient on the other hand.

[0052] 5. The underwater wide-range search and tracking duplex laser communication device provided by the present invention uses a PMT detector, which has a large field of view, high sensitivity, and fast data processing speed. For underwater non-cooperative target scenarios, it can quickly narrow down the uncertain area.

[0053] 6. In the underwater large-range search and tracking duplex laser communication device provided by the present invention, the two-dimensional turntable adopts a servo motor and a small time encoder, which avoids the large friction torque of the dynamic seal and has fast response and high precision performance.

[0054] 7. In the underwater wide-range search and tracking duplex laser communication device provided by the present invention, the sealed cabin adopts a transparent optical window and a fixed base, which can protect the equipment from the underwater environment without interfering with the light transmission. The anti-reflective coating on the inner surface of the transparent optical window can maintain the excellent optical transmittance of the transparent optical window.

[0055] 8. In the underwater large-scale search and tracking duplex laser communication device provided by the present invention, the electrical wiring is fixed by an external synchronous rotating frame to keep the connecting line relatively stationary, avoiding the wiring along the axis of the two-dimensional turntable, or the data is preliminarily processed by the tracking processing board before being wirelessly transmitted to the main control unit, which solves the problem of the connecting line winding and twisting during rotation under the constraints of a compact layout.

[0056] 9. The underwater wide-range search and tracking duplex laser communication device provided by the present invention includes a variable aperture. The main control unit can adjust the size of the variable aperture in real time according to the noise count in the bit time slot to reduce the interference of the background light signal.

[0057] 10. The present invention provides an underwater large-area search and tracking duplex laser communication method, which first uses a photodetector to scan the entire space range, establishes a suspected target area based on the peak change of its output signal during the search period, and then uses an image detector to perform a fine tracking scan of the suspected target area. The two-dimensional turntable is adjusted according to the position of the center of mass of the light spot calculated based on the output signal of the image detector, so that the position of the center of mass of the light spot is at the center of the target surface of the image detector, thereby realizing the detection and capture of underwater randomly moving targets.

[0058] 11. In the underwater wide-range search and tracking duplex laser communication method provided by the present invention, when the device is in the staring scanning state, the receiving branch is in the wide field of view state, and captures the receiving beam of the other end over a wide range; when the alignment, capture and tracking state is completed and the communication link is established, the receiving branch is in the small field of view state to avoid the background light degrading the communication quality. Attached Figure Description

[0059] Figure 1 This is a schematic diagram of the structure of the optical communication device in an embodiment of the present invention;

[0060] Figure 2 This is a schematic diagram of the off-axis optical system for transmitting and receiving in an embodiment of the present invention;

[0061] Figure 3 This is a schematic diagram of the optical communication device in an embodiment of the present invention;

[0062] Figure 4 This is a flowchart of the underwater large-scale search and tracking duplex laser communication method in an embodiment of the present invention;

[0063] The annotations in the attached figures are explained as follows:

[0064] 1-Sealed chamber, 2-Two-dimensional turntable, 3-Off-axis optical system for transmitting and receiving, 4-Main control unit, 5-Transparent optical window, 6-Fixed base, 7-Sealing gasket, 8-Sealing ring, 9-Transmitting branch, 10-Receiving branch;

[0065] 11-Laser, 12-Laser collimator, 13-First zoom lens; 14-Second zoom lens, 15-Beam splitter prism, 16-First focusing lens, 17-Variable aperture, 18-Shaping lens, 19-Narrowband filter, 20-Second focusing lens, 21-Photodetector; 22-Mirror, 23-Third focusing lens, 24-Image detector. Detailed Implementation

[0066] The underwater large-area search and tracking duplex laser communication device and method proposed in this invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. Those skilled in the art should understand that these embodiments are merely used to explain the technical principles of this invention and are not intended to limit the scope of protection of this invention.

[0067] An underwater wide-range search and tracking duplex laser communication device includes two identical optical communication units, which are respectively installed on the local end device and the remote end device, such as... Figure 1 As shown, the optical communication device includes a sealed chamber 1, a two-dimensional turntable 2 and a main control unit 4 disposed in the sealed chamber 1, and a transceiver off-axis optical system 3 disposed on the two-dimensional turntable 2.

[0068] The sealed chamber 1 consists of a fixed base 6 and a transparent optical window 5. The fixed base 6 and the transparent optical window 5 are sealed together by a sealing gasket 7 and a sealing ring 8. The transparent optical window 5 is a large-diameter transparent optical window with a cylindrical section and a hemisphere, and its inner surface is coated with an anti-reflective coating. The sealed chamber 1 has a large range of light transmission windows and provides a large range of rotation space for the two-dimensional turntable 2. It can protect the equipment from the underwater environment without interfering with light transmission. On the one hand, it has good high pressure resistance and sealing performance, which can ensure that the inside of the optical communication device is completely isolated from the external environment, preventing water, dust, gas and other substances from entering. On the other hand, the combination of the anti-reflective coating and the transparent optical window 5 can maintain excellent optical transmittance, especially in a specific spectral range.

[0069] The two-dimensional turntable 2 adopts an L-shaped layout. The azimuth and pitch rotation are driven by servo motors, and the angle measuring element adopts a small time grid encoder. The response time of the control command is less than 20ms.

[0070] like Figure 2 As shown, the off-axis optical system 3 is positioned near the transparent optical window 5 and includes a transmitting branch 9 and a receiving branch 10 that are parallel to each other. The transmitting and receiving branches are parallel, and the overall optical system weighs ≤200g, which can reduce the driving torque of the servo motor and achieve a compact layout.

[0071] like Figure 3As shown, the transmitting branch 9 includes a laser 11 for emitting beacon light and / or communication light, and a laser collimator 12 and a first zoom lens 13 arranged sequentially along the laser transmission direction. In other embodiments, there can be two lasers 11, with a laser collimator 12 and a first zoom lens 13 arranged sequentially at the rear end of each laser along the laser transmission direction. The two lasers emit beams of the same wavelength, one of which is used to emit beacon light, which is used for tracking and scanning of the device, and also to enhance the transmitted light power when the communication light power is insufficient. The transmitting branch can emit continuous laser with a variable beam divergence angle for scanning the peer communication terminal, a process assisted by the first zoom lens 13; and after the two optical communication devices complete the alignment, capture, and tracking of the beams, they modulate the signal onto an optical carrier to transmit communication light.

[0072] The receiving branch 10 includes a second zoom lens 14 and a beam splitter 15 arranged sequentially along the beam propagation direction. Along the beam propagation direction, the transmission path of the beam splitter 15 includes a first focusing lens 16, a variable aperture 17, a shaping lens 18, a narrowband filter 19, a second focusing lens 20, and a photodetector 21. Along the beam propagation direction, the reflection path includes a reflecting mirror 22, a third focusing lens 23, and an image detector 24. The narrowband filter 19 enhances the quality of the received signal by filtering out non-target light wavelengths. The variable aperture 17 is electrically connected to the main control unit 4. The beam splitter 5 is used to split the received beam into a tracking beam and a communication beam, with a splitting ratio of 1:4. In other embodiments, this splitting ratio can be adjusted. The tracking beam enters the image detector 24 for fine tracking and alignment, while the communication beam enters the photodetector 21 for coarse tracking and communication. The photodetector 21 and the image detector 24 can be interchanged according to structural requirements.

[0073] The photodetector 21 employs a PMT detector for primary large-area scanning, featuring high sensitivity, fast response, and a wide field of view. This allows the two-dimensional turntable 2 to perform a wide-area search. Based on the peak abrupt changes in the PMT detector's response signal within the search range, a suspected target area is established, achieving a rough initial pointing. The image detector 24 uses a CCD camera for secondary fine-tracking scanning. It begins scanning the suspected target area along spiral, rectangular, or other trajectories to capture the target light spot, thus entering tracking mode and aligning the beams at both ends of the communication. The detection fields of photodetector 21 and image detector 24 are similar, both ≥10°. The target surfaces of the PMT detector and the CCD camera are comparable, differing in size by ±2mm.

[0074] The main control unit 4 uses an FPGA and is electrically connected to the two-dimensional turntable 2, laser 11, photodetector 21, and image detector 24 respectively. It is used to control the two-dimensional turntable 2, modulate the laser output from the laser 11, and filter and demodulate the laser signals received by the photodetector 21 and the image detector 22.

[0075] There are fiber optic and electrical connections between the off-axis transceiver optical system 3, the two-dimensional turntable 2, and the main control unit 4. Under the constraint of a compact layout, two methods can be used to solve the problem of entanglement and twisting of the connecting lines during rotation. One method is to use external cabling. An external synchronous rotation frame is installed on the two-dimensional turntable 2 and the off-axis transceiver optical system 3 to fix the fiber optic and electrical connections of the off-axis transceiver optical system 3, keeping the connecting lines relatively stationary. The second method is to use a tracking processing board. The tracking processing board is electrically connected to the laser 11, photodetector 21, and optical detector 24, and wirelessly connected to the main control unit 4. The tracking processing board amplifies the output signal of the photodetector 21 and analyzes the output signal of the image detector 24. Then, high-speed Wi-Fi is used for wireless data exchange with the main control unit 4, with a data delay of no more than 10ms.

[0076] When the device starts working, the system is in staring scanning mode, and the zoom optical system is in a large field-of-view state, capturing the receiving beam from the other end over a wide area. After alignment, capture, and tracking are completed and the communication link is established, the zoom optical system is in a small field-of-view state to avoid background light degrading communication quality. A 1:4 beam splitter prism 5 is placed parallel to the zoom optical system to split the incident light, with 20% of the light energy being split and sent to the image detector 24 for beam fine tracking and real-time follow-up. 80% of the light energy is used for communication. The first focusing lens 16, the variable aperture 17, the shaping lens 18, and the second focusing lens 20 collimate and focus the beam, the narrowband filter 19 filters the signal light, and the photodetector 21 is placed at the focal plane of the focusing lens to convert the received light signal into an electrical signal. The main control unit 4 synchronizes, filters, and demodulates the light signal output by the photodetector 21, and simultaneously adjusts the size of the variable aperture 17 in real time according to the noise count within the bit time slot to reduce interference from the background light signal.

[0077] This embodiment also provides an underwater large-area search and tracking full-duplex laser communication method, employing the aforementioned underwater large-area search and tracking full-duplex laser communication device, such as... Figure 4 As shown, it includes the following steps:

[0078] Step 1: The local main control unit 4 controls the laser 11 to emit beacon light, and the remote main control unit 4 controls the two-dimensional turntable 2 to drive the photodetector 21 to perform a first-level large-area scan and the image detector 24 to perform a second-level fine-tracking scan, so that the beacon light spot emitted by the local laser 11 is at the center of the target surface of the remote image detector 24. Specifically:

[0079] Step 1.1: Set the adjustment time interval and position of the local first zoom lens 13. The main control unit 4 controls the laser 11 to emit beacon light. The local main control unit 4 modulates and generates a modulation electrical signal. After the electrical signal is amplified, it drives the laser 11 to emit beacon light. After transmission via fiber optic pigtail, the light is emitted using the laser collimator 12. Then, the position of the first zoom lens 13 is adjusted according to the set adjustment interval and position using a polling method, thereby adjusting the beam divergence angle of the local beacon light. The beam divergence angle of the beacon light should meet the following requirement: the diameter of the beam emitted through the transparent optical window 5 should at least cover the optical communication device at the other end.

[0080] Step 1.2: The two-dimensional turntable 2 at the other end is powered on and returns to zero to enter the waiting state. The photodetector 21 is in a staring state. Then, the second zoom lens 14 is adjusted to maximize the receiving field of view at the other end. The main control unit 4 sets the predetermined scanning trajectory and step size of the two-dimensional turntable 2 in the first-level large-range scanning and second-level fine-tracking scanning states, as well as the output threshold of the photodetector 21. The predetermined scanning trajectory for the first-level large-range scanning is a serpentine or rectangular scan, and the step size is determined according to the field of view of the photodetector 21; in this embodiment, it is 80% of the field of view of the photodetector 21. The predetermined scanning trajectory for the second-level fine-tracking scanning is a spiral scan, and the step size is determined according to the beam divergence angle of the laser 11; in this embodiment, it is 80% of the beam divergence angle of the laser 11. The output threshold of the photodetector 21 is obtained through the following method:

[0081] The background noise is recorded using photodetector 21, the average output voltage of the background noise is calculated, and then the average output voltage plus 3 times the standard deviation is taken as the output threshold of photodetector 21.

[0082] Alternatively, the background noise can be recorded using photodetector 21, the average rate of change of the background noise can be calculated, and the average rate of change of the background noise plus three times the standard deviation can be used as the output threshold of photodetector 21.

[0083] Step 1.3: Turn on the peer photodetector 21 and image detector 24. The main control unit 4 determines whether it has received a light spot based on the output signal of the image detector 24. If a light spot is received, proceed to step 1.8; otherwise, proceed to step 1.4.

[0084] Step 1.4: The remote main control unit 4 controls the two-dimensional turntable 2 to perform a first-stage large-area scan according to the predetermined scanning trajectory and step distance, and records the output signal of the photodetector 21 until the entire spatial range (azimuth ±180°, elevation ±90°) is scanned. The first-stage large-area scan sampling grid search method divides the entire field of view of the optical window (azimuth ±180°, elevation ±90°) into 16 rectangular grids, each grid being the same size as the detection field of view of the photodetector 21, and adopts a serpentine traversal search order.

[0085] Step 1.5: The main control unit 4 at the other end analyzes the output signal of the photodetector 21. If the output signal of the photodetector 21 is greater than the output threshold, the spatial region corresponding to the maximum light intensity peak range of the output signal of the photodetector 21 is extracted and identified as the suspected target region. Then, step 1.6 is executed.

[0086] Otherwise, reduce the predetermined step size of the large-scale scan at the other end, reduce the beam divergence angle of the beacon light at this end, and then return to step 1.4.

[0087] Step 1.6: The remote main control unit 4 controls the two-dimensional turntable 2 to perform a secondary fine-tracking scan on the suspected target area according to the predetermined scanning trajectory and step distance, and records the output signal of the image detector 24. The specific method of the secondary fine-tracking scan is to perform a spiral scan starting from the center of the suspected target area.

[0088] Step 1.7: The peer control unit 4 performs real-time analysis on the output signal of the image detector 24. If the peer image detector 24 receives a light spot, then proceed to step 1.8; otherwise, reduce the predetermined step distance of the secondary fine tracking scan and return to step 1.6.

[0089] Step 1.8: The remote main control unit 4 calculates the position of the centroid of the light spot of the output signal of the image detector 24, and then adjusts the two-dimensional turntable 2 according to the position of the centroid of the light spot so that the beacon light spot emitted by the local laser 11 is at the center of the target surface of the remote image detector 24.

[0090] Step 2: Following the method in Step 1, position the beacon light spot emitted by the laser at the other end 11 at the center of the target surface of the image detector 24 at the local end, thereby establishing a communication link.

[0091] Step 3: Adjust the positions of the second zoom lens 14 at the local end and the remote end respectively to reduce the receiving field of view. Then, the main control unit 4 at the local end and the remote end respectively control the corresponding laser 11 to emit communication light.

[0092] Step 4: The communication light from the local end and the remote end passes through the second zoom lens 14 and the beam splitter 15 of the remote end in sequence and is divided into a tracking beam and a communication beam. The tracking beam is focused by the focusing unit and then received by the image detector 24 and transmitted to the main control unit 4. The communication beam is shaped and focused by the collimating focusing unit and then received by the photodetector 21 and transmitted to the main control unit 4.

[0093] Step 5: The main control units 4 at the local and remote ends process the output signals of the corresponding image detectors 24 in real time, calculate the centroid position of the communication light spot, and adjust the two-dimensional turntable 2 so that the communication light spot is always at the center of the target surface of the image detector 24; the main control units 4 at the local and remote ends process the output signals of the corresponding photodetectors 21 in real time to obtain communication information and complete the underwater large-scale search and tracking duplex laser communication.

[0094] Throughout the entire acquisition and tracking link establishment process, the communication sensitivity of the optical communication link should not exceed the saturation optical power of photodetector 21 and image detector 24.

Claims

1. An underwater wide-range search and tracking full-duplex laser communication method, wherein the underwater wide-range search and tracking full-duplex laser communication device comprises two identical optical communication units, which are respectively installed on the local end device and the remote end device, characterized in that: The optical communication device includes a sealed cabin (1) consisting of a fixed base (6) and a transparent optical window (5), a two-dimensional turntable (2) and a main control unit (4) set inside the sealed cabin (1), and a transceiver off-axis optical system (3) set on the two-dimensional turntable (2); The off-axis optical system (3) is positioned close to the transparent optical window (5) and includes a transmitting branch (9) and a receiving branch (10) that are parallel to each other. The transmitting branch (9) includes a laser (11) for emitting beacon light and / or communication light, and a laser collimator (12) and a first zoom lens (13) arranged sequentially along the laser transmission direction; The receiving branch (10) includes a second zoom lens (14) and a beam splitter (15) arranged sequentially along the beam transmission direction. The beam splitter (15) is provided with a first-level large-range scanning and communication unit and a second-level fine tracking scanning unit on the transmission and reflection paths, respectively. The first-level large-range scanning and communication unit includes a collimating and focusing unit and a photodetector (21) arranged sequentially along the beam propagation direction. The second-level fine tracking scanning unit includes a reflector (22), a focusing unit and an image detector (24) arranged sequentially along the beam propagation direction. The main control unit (4) is electrically connected to the two-dimensional turntable (2), laser (11), photodetector (21), and image detector (24) respectively, and is used to control the two-dimensional turntable (2), modulate the laser output by the laser (11), and filter and demodulate the laser signals received by the photodetector (21) and the image detector (22). Its characteristic is that it includes the following steps: Step 1: The local main control unit (4) controls the laser (11) to emit beacon light, and the remote main control unit (4) controls the two-dimensional turntable (2) to drive the photodetector (21) to perform a first-level large-area scan and the image detector (24) to perform a second-level fine tracking scan, so that the beacon light spot emitted by the local laser (11) is located at the center of the target surface of the remote image detector (24); specifically: Step 1.1: Set the adjustment time interval and position of the local first zoom lens (13), the main control unit (4) controls the laser (11) to emit beacon light, and then uses a polling method to adjust the position of the first zoom lens (13) according to the set adjustment interval and position, thereby adjusting the beam divergence angle of the local beacon light; Step 1.2: Power on the two-dimensional turntable (2) at the other end and return to zero to enter the waiting state. Then adjust the second zoom lens (14) to maximize the receiving field of view at the other end. Set the predetermined scanning trajectory and step distance of the two-dimensional turntable (2) in the first-level large-range scanning and second-level fine tracking scanning states, as well as the output threshold of the photodetector (21) through the main control unit (4). Step 1.3: Turn on the peer photodetector (21) and image detector (24). The main control unit (4) determines whether the image detector (24) has received a light spot based on the output signal of the image detector (24). If a light spot is received, proceed to step 1.8; otherwise, proceed to step 1.

4. Step 1.4: The main control unit (4) controls the two-dimensional turntable (2) to perform a first-level large-range scan according to the predetermined scanning trajectory and step distance, and records the output signal of the photodetector (21) until the scan of the entire spatial range is completed. Step 1.5: The main control unit (4) at the other end analyzes the output signal of the photodetector (21). If the output signal of the photodetector (21) is greater than the output threshold, the corresponding spatial region of the maximum light intensity peak interval of the output signal of the photodetector (21) is extracted and identified as the suspected target region. Then, step 1.6 is executed. Otherwise, reduce the predetermined step size of the large-scale scan at the other end, reduce the beam divergence angle of the beacon light at this end, and then return to step 1.4; Step 1.6: The remote main control unit (4) controls the two-dimensional turntable (2) to perform secondary fine tracking scanning on the suspected target area according to the predetermined scanning trajectory and step distance, and records the output signal of the image detector (24); Step 1.7: The peer control unit (4) performs real-time analysis on the output signal of the image detector (24). If the peer image detector (24) receives a light spot, then step 1.8 is executed; otherwise, the predetermined step distance of the secondary fine tracking scan is reduced, and the process returns to step 1.

6. Step 1.8: The remote main control unit (4) calculates the position of the centroid of the output signal of the image detector (24), and then adjusts the two-dimensional turntable (2) according to the position of the centroid of the spot so that the beacon light spot emitted by the local laser (11) is at the center of the target surface of the remote image detector (24). Step 2: Following the method in Step 1, position the beacon light spot emitted by the laser at the other end (11) at the center of the target surface of the image detector (24) at the local end, thereby establishing a communication link; Step 3: Adjust the positions of the second zoom lens (14) at the local end and the remote end respectively to reduce the receiving field of view. Then, the main control unit (4) at the local end and the remote end respectively control the corresponding laser (11) to emit communication light. Step 4: The communication light from this end and the other end passes through the second zoom lens (14) and beam splitter (15) of the other end in sequence and is divided into a tracking beam and a communication beam. The tracking beam is focused by the focusing unit and then received by the image detector (24) and transmitted to the main control unit (4). The communication beam is shaped and focused by the collimating focusing unit and then received by the photodetector (21) and transmitted to the main control unit (4). Step 5: The main control units (4) of the local end and the remote end process the output signals of the corresponding image detectors (24) in real time, calculate the centroid position of the communication light spot, and adjust the two-dimensional turntable (2) so that the communication light spot is always at the center of the target surface of the image detector (24); the main control units (4) of the local end and the remote end process the output signals of the corresponding photoelectric detectors (21) in real time, obtain communication information, and complete the underwater large-scale search and tracking duplex laser communication.

2. The underwater large-area search and tracking duplex laser communication method according to claim 1, characterized in that: The laser (11) includes a beacon laser and a communication laser. The beacon laser and the communication laser emit beams with the same wavelength. The communication laser is used to emit communication light, and the beacon laser is used to emit beacon light and enhance the power of the communication light. The beacon laser and the communication laser are each provided with a laser collimator (12) and a first zoom lens (13) in sequence along the laser transmission direction at their rear ends.

3. The underwater large-area search and tracking duplex laser communication method according to claim 1 or 2, characterized in that: The detection field of view of both the photodetector (21) and the image detector (24) is ≥10°; The photodetector (21) is a PMT detector, and the image detector (24) is a CCD camera; The transparent optical window (5) is a large-diameter transparent optical window with a cylindrical section and a hemisphere, and its inner surface is provided with an anti-reflective coating; The two-dimensional turntable (2) adopts an L-shaped layout. The azimuth rotation and pitch rotation are driven by servo motors, and the angle measuring element adopts a small time grid encoder. The beam splitter (15) has a beam splitting ratio of 1:

4.

4. The underwater large-area search and tracking duplex laser communication method according to claim 3, characterized in that: The collimation and focusing unit includes a first focusing lens (16), a variable aperture (17), a shaping lens (18), a narrowband filter (19), and a second focusing lens (20) arranged sequentially along the beam propagation direction. The variable aperture (17) is electrically connected to the main control unit (4). The focusing unit includes a third focusing lens (23).

5. The underwater large-area search and tracking duplex laser communication method according to claim 4, characterized in that: It also includes a tracking processing board, which is electrically connected to the laser (11), photodetector (21), and image detector (24) respectively, and wirelessly connected to the main control unit (4). It is used to receive the modulated electrical signal sent by the main control unit (4) and send it to the laser (11), as well as amplify the output signal of the photodetector (21), analyze the output signal of the image detector (24), and send it to the main control unit (4). Alternatively, it may also include an external synchronous rotating frame mounted on the two-dimensional turntable (2) and the off-axis optical system (3) at both ends, with the electrical wiring between the main control unit (4) and the two-dimensional turntable (2), the laser (11), the photodetector (21), and the image detector (24) fixed on the external synchronous rotating frame.

6. The underwater large-area search and tracking duplex laser communication method according to claim 5, characterized in that, In step 1.2, the output threshold of the photodetector (21) is obtained by the following method: The background noise is recorded using a photodetector (21), the average output voltage of the background noise is calculated, and then the average output voltage plus three times the standard deviation is taken as the output threshold of the photodetector (21). Alternatively, the background noise can be recorded using a photodetector (21), the average rate of change of the background noise can be calculated, and the average rate of change of the background noise plus three times the standard deviation can be used as the output threshold of the photodetector (21).

7. The underwater large-area search and tracking duplex laser communication method according to claim 6, characterized in that: In step 1.1, the beam divergence angle of the local beacon light satisfies the condition that the diameter of the beam emitted through the transparent optical window (5) at least covers the optical communication device at the other end.

8. The underwater large-area search and tracking duplex laser communication method according to claim 7, characterized in that: In step 1.2, the predetermined scanning trajectory and step distance of the two-dimensional turntable (2) in the first-level large-range scanning and second-level fine-tracking scanning states are specifically as follows: The predetermined scanning trajectory for the first-level large-area scan is a snake scan or a rectangular scan, with a step size of 80% to 85% of the field of view of the photodetector (21); The predetermined scanning trajectory for the secondary precision tracking scan is a spiral scan, with a step size of 80% to 85% of the divergence angle of the laser beam (11).

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