Underwater wireless optical communication device and method based on single photon detector

Abstract

The application discloses an underwater wireless optical communication device based on a single photon detector, which comprises a first single photon communication device and a second single photon communication device for bidirectional communication, and each of the first single photon communication device and the second single photon communication device comprises a laser emission unit, a signal receiving unit, a signal comprehensive processing unit and a power module; the laser emission unit loads a received modulated electric signal into an optical signal and outputs the optical signal, and simultaneously emits auxiliary alignment laser for communication device alignment; the signal receiving unit converts the received optical signal into an electric signal through a single photon detector and transmits the electric signal to the outside, and simultaneously, the signal receiving unit receives auxiliary alignment laser to complete communication device alignment; the signal comprehensive processing unit converts communication information of a host computer into a modulated electric signal after receiving the communication information, transmits the modulated electric signal to the laser emission unit, demodulates and restores the electric signal transmitted by the signal receiving unit into communication information, and then transmits the communication information to the host computer. The application can improve underwater communication distance and meet the requirements of long-distance wireless optical communication in different water quality environments.

Description

Technical Field

[0001] This invention relates to the field of underwater wireless optical communication technology, and in particular to an underwater wireless optical communication device and method based on a single-photon detector. Background Technology

[0002] With the deepening understanding and development of ocean characteristics, the application of unmanned submersibles, underwater robots, and pre-positioned seabed systems is becoming increasingly widespread, and the number of nodes is gradually increasing, making them an important part of the marine information network. At the same time, the demand for bandwidth for information transmission between underwater platforms has increased significantly. How to connect dynamic and static nodes scattered in various waters, quickly offload data from each network node, improve information transmission capabilities, and meet the communication requirements of high bandwidth, real-time performance, and stealth has become an urgent problem to be solved in the field of underwater communication.

[0003] In recent years, low-loss underwater optical wireless communication (UOWC) technology has attracted widespread attention due to its higher transmission rates and lower transmission link latency, which translates to lower power consumption in the transceiver system. Typical UOWC systems can achieve transmission rates and communication distances on the order of several Gbps / tens of meters and several Mbps / hundred meters, making it an important technology for future deep-sea underwater information transmission. However, in long-distance underwater optical wireless communication, the underwater channel is complex. Water attenuation, scattering caused by large molecular particles, and flat fading caused by underwater turbulence all affect the transmission of the optical link. The energy of the optical signal reaching the receiver after long-distance transmission is very weak, making traditional underwater optical wireless communication technologies insufficient for long-distance communication requirements. In recent years, the development of single-photon detection devices has improved the detection sensitivity of receivers at the device level, which is key to breaking through the effective range of underwater optical wireless communication in the future. However, while the introduction of single-photon detectors helps improve the operating range of underwater wireless optical communication systems in terms of sensitivity, the optical field of view of the system is very small due to the limited target surface size of some single-photon detectors (such as the mature Si-GMAPD product of about 200μm). This poses a significant challenge to link alignment during underwater communication. Simultaneously, the high sensitivity of signal detection also brings problems such as sensitivity to background noise. Under different water quality conditions and varying background noise interference, it is difficult to achieve robust demodulation of information through a single parameter setting, placing higher demands on the environmental adaptability and communication error rate of underwater single-photon wireless optical communication. Therefore, to address the above problems, this invention proposes an underwater wireless optical communication device and method based on single-photon detection, to solve or partially solve the current technical challenges of high-sensitivity underwater wireless optical communication. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by designing an underwater bidirectional wireless optical communication device and method based on a single-photon detector, thereby increasing the underwater communication distance and meeting the requirements for long-distance wireless optical communication in different water quality environments.

[0005] The technical solution adopted in this invention is:

[0006] An underwater wireless optical communication device based on a single-photon detector is provided, characterized in that...

[0007] The first single-photon communication device and the second single-photon communication device include bidirectional communication, each of which includes a laser emitting unit, a signal receiving unit, a signal processing unit, and a power supply module;

[0008] The laser emitting unit loads the received modulated electrical signal into the optical signal and outputs it, while simultaneously emitting an auxiliary alignment laser to align the communication device.

[0009] The signal receiving unit converts the received optical signal into an electrical signal through a single-photon detector and transmits it externally. At the same time, it receives an auxiliary alignment laser to complete the alignment of the communication device.

[0010] After receiving the communication information from the host computer, the signal processing unit converts it into a modulated electrical signal and transmits it to the laser emitting unit. It also demodulates the electrical signal transmitted by the signal receiving unit and restores it to the communication information before transmitting it to the host computer.

[0011] According to the above technical solution, the laser emitting unit includes two optical paths. One optical path includes a beacon laser and a first collimator; the other optical path includes multiple source lasers, a beam combiner, a second collimator, a dichroic mirror, and a beam expanding optical system. The beam combiner is used to combine the lasers output from the multiple source lasers into one beam before outputting it. The first collimator is used to collimate the beacon laser before outputting it. The second collimator is used to collimate the source lasers before outputting them. The dichroic mirror is connected to the first collimator and the second collimator to combine the collimated source lasers with the beacon lasers and output them to the beam expanding optical system.

[0012] Following the above technical solution, the source laser of one laser emitting unit has the same wavelength as the beacon laser of another laser emitting unit.

[0013] According to the above technical solution, the signal receiving unit includes two optical paths, one of which specifically includes a receiving optical system, a dichroic mirror, a filter assembly, a first converging optical system, and a single-photon detector connected in sequence. The receiving optical system receives the laser emitted by the laser emitting unit. The dichroic mirror is used to separate the source laser from the beacon laser. The source laser is filtered by the filter assembly and then output to the first converging optical system to converge the source laser energy to the target surface of the single-photon detector. The single-photon detector is used to receive the source laser energy and convert it into a digital pulse signal, which is then output to the signal processing unit.

[0014] The other path includes a second converging optical system and a camera. The second converging optical system is connected to a dichroic mirror, and the camera is connected to an external computer. The second converging optical system is used to converge the beacon laser energy to the target surface of the camera. The camera receives the beacon laser spot and completes the alignment of the emission and reception optical axes according to the position of the spot.

[0015] According to the above technical solution, the bandpass center wavelength range of the filter assembly of the signal receiving unit of the first single-photon communication device covers the wavelengths of multiple source lasers of the laser emitting unit of the second single-photon communication device, and the cutoff wavelength range covers the wavelengths of multiple source lasers of the first laser emitting unit.

[0016] Following the above technical solution, the bandpass center wavelength range of the filter assembly of the receiving unit of the second single-photon communication device covers the wavelengths of multiple source lasers of the laser emitting unit of the first single-photon communication device, and the cutoff wavelength range covers the wavelengths of multiple source lasers of the laser emitting unit of the second photon communication device.

[0017] Following the above technical solution, the single-photon detector is a Geiger-mode APD single-photon detector, a photon-counting PMT single-photon detector, or a superconducting nanowire single-photon detector.

[0018] Following the above technical solution, the camera is a CCD camera or a CMOS camera.

[0019] According to the above technical solution, the signal processing unit includes an information transmission module, a level conversion module, a modulation function module, a demodulation function module, and a system initialization module.

[0020] This invention also provides an underwater wireless optical communication method based on a single-photon detector. This communication method, based on the underwater wireless optical communication device based on a single-photon detector described above, includes the following steps:

[0021] The first step is to align the transmitting and receiving optical axes of the first and second single-photon communication devices.

[0022] The second step involves emitting low-energy, medium-energy, and high-energy lasers through a laser emitting unit, and then using the scattered echo counts collected by a signal receiving unit to assess the absorption and scattering coefficients of the water quality environment. The average value of the three measurements is taken as the current absorption and scattering coefficients of the water quality environment.

[0023] The third step is to determine the current water quality conditions based on the sum of the absorption coefficient and the scattering coefficient. When the water quality is lower than the attenuation coefficient of clean seawater, low-energy, high-repetition-rate modulation is set to achieve the output of the source laser; when the water quality is higher than the attenuation coefficient of clean seawater, high-energy, low-repetition-rate modulation is set to achieve the output of the source laser.

[0024] The fourth step involves the signal receiving unit using a single-photon detector to statistically analyze the background noise photon count rate and the maximum information photon count rate for a single symbol period. It then sets the minimum pulse interval threshold and signal recovery decision threshold for online signal demodulation to adapt to communication under different background environments and transmission distances. Steps three and four are repeated to achieve continuous communication transmission. The first step includes using two single-photon communication devices—beacon lasers and a camera—to achieve assisted alignment, ensuring that the center of the converged laser spot at both ends falls on the camera's alignment center.

[0025] The second step includes:

[0026] 1) The beacon laser is set to a fixed frequency output, initially emitting N pulses with low single-pulse energy. L A pulse triggers the detector and data acquisition device to acquire T. L The water absorption coefficient A at low pulse energy was obtained by exponentially fitting the backscattered signal based on the scattering waveform. L Scattering coefficient B L

[0027] 2) Send medium-energy N using the same method in sequence. M A pulse and high-energy N H The pulse was used to obtain the corresponding medium-energy water absorption coefficient A. M Scattering coefficient B M and the absorption coefficient A of high-energy water bodies H Scattering coefficient B H The average of the three measurements is the current absorption coefficient A and scattering coefficient B of the water quality environment.

[0028]

[0029]

[0030] The third step includes:

[0031] When the sum of the absorption coefficient A and the scattering coefficient B is less than 0.151m -1(Attenuation coefficient of clean seawater) indicates that the current water quality is good, and low-energy, high-repetition-rate modulation is set to achieve the output of the source laser; when the sum of the absorption coefficient A and the scattering coefficient B is higher than 0.151m -1 This indicates that the current water quality is poor, so high-energy, low-repetition-rate modulation is set to achieve the output of the source laser.

[0032] The energy output of a source laser is set by the duration of a single laser pulse. At low energy output, the pulse duration T... s Short; during high energy output, the light emission duration T s long.

[0033] The modulation frequency of the source laser and the communication frequency R b Directly related, due to the discrete pulse output characteristics and dead-time constraints of single-photon detection, within a single symbol period of communication (1 / R). b The source laser does not maintain a high level continuously; instead, it employs a spread-spectrum output method, meaning that the output is maintained at 1 / R within a single symbol period. b N pulses are emitted, where N is the number of spread spectrum pulses, to avoid the loss of capability caused by the dead time of the single-photon detector.

[0034] The fourth step includes:

[0035] a) With the source laser and beacon laser off, turning on the single-photon detector will cause it to output discrete pulses due to environmental noise, dark counting, and other factors. The period 1 / R of a single symbol is then calculated using the signal processing unit. b Maximum count rate N of internal noise DCR .

[0036] b) With the laser source working normally, the single-photon detector outputs a pulse after receiving the laser signal. The maximum count rate N of the single-photon detector is counted within a single symbol period (1 / R) of the communication. SIG ;

[0037] c) To ensure a low bit error rate in communication, it is necessary to calculate the noise counting rate N. DCR and maximum signal count rate N SIG Comprehensive setting of minimum pulse interval threshold T inter Signal recovery decision threshold N RECOVER The value can be:

[0038]

[0039]

[0040] That is, during signal demodulation, when the time interval between the first pulse signal and the second pulse signal of the single-photon detector is greater than T... interIf the current pulse is considered a noise photon, no further counting or statistics will be performed, and the system will wait for a signal photon again; if the interval between the first and second pulse signals of the single-photon detector is less than or equal to T... inter When the pulse is considered to be a signal photon, counting and statistics for subsequent single symbol periods begin. When the count value is greater than or equal to N... RECOVER When the count value is less than N, the symbol is "1". RECOVER When the symbol is zero, it is considered to be "0".

[0041] d) Repeat steps b) to c) to complete the demodulation of one frame of communication data.

[0042] The beneficial effects of this invention are: by using a single-photon detector as a receiver, this invention can achieve more sensitive signal detection and improve the system's operating range.

[0043] Secondly, taking advantage of the high sensitivity of single-photon detectors, optical isolation is used to suppress the backscattering effect of laser light from the same-side source, thus ensuring bidirectional full-duplex communication capability.

[0044] Third, the water quality is assessed based on the backscattered echo count values ​​at different low, medium, and high energies of the beacon laser. The output energy and modulation frequency of the source laser are adaptively modulated to meet the communication requirements of different water quality environments, thereby improving the robustness of the system.

[0045] Finally, based on the background noise photon count and maximum information photon count of a single symbol period obtained by the single-photon detector, the online signal demodulation threshold setting is completed. This can adapt to different background environments and transmission distance requirements, reduce the communication bit error rate, and improve the environmental adaptability of single-photon communication. Attached Figure Description

[0046] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0047] Figure 1 This is a schematic diagram of bidirectional communication of a single-photon communication device according to an embodiment of the present invention;

[0048] Figure 2 This is a schematic diagram of the laser emitting unit of the single-photon communication device according to an embodiment of the present invention;

[0049] Figure 3 This is a schematic diagram of the signal receiving unit of the single-photon communication device according to an embodiment of the present invention;

[0050] Figure 4 This is a schematic diagram of the signal processing unit of the single-photon communication device according to an embodiment of the present invention. Detailed Implementation

[0051] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0052] like Figure 1 As shown, the underwater bidirectional wireless optical communication device based on a single-photon detector in this embodiment of the invention specifically includes a single-photon communication device 1 and a single-photon communication device 2. The two devices are independent communication modules and can serve as transmitters and receivers for each other to achieve bidirectional communication.

[0053] The single-photon communication device 1 includes a laser emitting unit 11, a signal receiving unit 12, a signal processing unit 13, and a power supply module 14. The single-photon communication device 2 includes a laser emitting unit 21, a signal receiving unit 22, a signal processing unit 23, and a power supply module 24.

[0054] The laser emitting units 11 and 22 function the same as the optical path arrangement. The laser emitting units can load the received modulated electrical signal into the optical signal to achieve the transmission of communication information, and can also emit auxiliary alignment lasers to complete the alignment of the communication device. For example... Figure 2 As shown, the laser emitting unit specifically includes two optical paths. One path includes a beacon laser and collimator b1, while the other path includes source lasers a1-aN, a beam combiner, collimator b2, a dichroic mirror c, and a beam expander optical system. The beacon laser is a high-power, continuous laser and serves as an auxiliary alignment light source. The source lasers a1-aN are high-modulation-frequency, high-extinction-ratio lasers used for optical transmission of communication information, with external modulation pulses used to control light output and shutdown. The source lasers 1a-1N of laser emitting unit 11 have the same wavelength as the beacon laser of laser emitting unit 22, and the source lasers 1a-1N of laser emitting unit 22 have the same wavelength as the beacon laser of laser emitting unit 11. The beam combiner combines the laser outputs from multiple source lasers into a single output; a fiber optic beam combiner can be used. Collimator b1 collimates the beacon laser before output; collimator b2 collimates the source lasers before output. The dichroic mirror c is used to combine the collimated source laser and the beacon laser into a single beam before outputting it to the beam expander optical system. The beam expander optical system expands the combined laser beam before outputting it, reducing the exit divergence angle and increasing the communication distance.

[0055] The signal receiving units 12 and 21 have the same function and optical path arrangement. The signal receiving unit can convert the received optical signal into an electrical signal and transmit it externally. It can also receive auxiliary alignment lasers to complete the alignment of the communication device. For example... Figure 3 As shown, the signal receiving unit includes two optical paths. One path specifically includes a receiving optical system, a dichroic mirror, a filter assembly, a converging optical system d1, and a single-photon detector connected in sequence. The receiving optical system receives the laser emitted by the laser emitting unit and outputs it to the signal processing unit through the single-photon detector. The other path includes a connected converging optical system d2 and a camera. The converging optical system d2 is connected to the dichroic mirror, and the camera is connected to an external computer (used to transmit user data to the signal processing unit). The receiving optical system receives laser energy and transmits it to the dichroic mirror. The dichroic mirror separates the source laser from the beacon laser, with one path used for communication and the other for alignment. The filter assembly is a bandpass filter group. The bandpass center wavelength range of the filter assembly of the signal receiving unit 12 covers multiple source lasers of the laser emitting unit 22, and the cutoff wavelength range covers multiple source lasers of the laser emitting unit 11. The passband center wavelength range of the filter assembly in signal receiving unit 21 covers multiple source lasers of laser emitting unit 11, and the cutoff wavelength range covers multiple source lasers of laser emitting unit 22. A converging optical system d1 is used to converge the source laser energy to the target surface of a single-photon detector. The single-photon detector receives the source laser energy and converts it into a digital pulse signal; examples include Geiger-mode APD single-photon detectors, photon-counting PMT single-photon detectors, or superconducting nanowire single-photon detectors. A converging optical system d2 is used to converge beacon laser energy to the camera target surface. The camera receives the beacon laser spot and aligns the emission and reception optical axes according to the spot position; examples include CCD cameras and CMOS cameras.

[0056] Signal processing units 13 and 23 are identical. After receiving communication information from the host computer, the signal processing unit converts it into a modulated electrical signal and transmits it to the laser emitting unit. It also demodulates the electrical signal transmitted by the signal receiving unit, recovers it as communication information, and transmits it back to the host computer. Simultaneously, it can control the on / off state of the beacon laser and the camera as needed. For example... Figure 4 As shown, the signal processing unit includes an information transmission module, a level conversion module, a modulation module, a demodulation module, and a system initialization module.

[0057] The system comprises several modules: an information transmission module receives information from the host computer and converts it into data to be transmitted to the modulation module; a modulation module encodes the data and transmits it to the level conversion module in high (represented by the symbol "1") and low (represented by the symbol "0") voltage levels; the level conversion module applies high and low voltage levels to the laser based on the laser driver power supply, with high voltage levels resulting in light output and low voltage levels in the absence of light. Simultaneously, the discrete pulses output by the single-photon detector are converted into voltage levels detectable by the demodulation module; the demodulation module demodulates the discrete pulses output by the single-photon detector and then transmits the demodulated information to the computer via the information transmission module; and a system initialization module initializes the other modules.

[0058] In the two single-photon communication devices, the laser transmitting unit and the signal receiving unit are arranged symmetrically to ensure that the transmitting and receiving ends are aligned at close range.

[0059] The underwater wireless optical communication method based on a single-photon detector includes the following steps:

[0060] The first step is to align the transmitting and receiving optical axes of single-photon communication device 1 and single-photon communication device 2 using a beacon laser and a camera.

[0061] The second step involves evaluating the absorption and scattering coefficients of the water environment by counting the scattered echoes collected by a single-photon detector based on the low-energy, medium-energy, and high-energy lasers emitted by the beacon laser.

[0062] The third step involves adaptively adjusting the single-pulse energy and modulation frequency of the source laser based on the evaluated absorption and scattering coefficients, and then sending a frame of communication information.

[0063] In the fourth step, the single-photon detector in the signal receiving unit converts the received optical signal into an output discrete pulse digital signal. The signal processing unit demodulates the discrete pulse digital signal to recover a frame of communication information. Repeating steps three and four achieves continuous communication transmission. In bidirectional communication, single-photon communication device 1 and communication device 2 act as information transmitters and receivers for each other.

[0064] The first step includes:

[0065] (1) The beacon laser in the single-photon communication device 1 outputs continuously, and the camera 2 in the single-photon communication device 2 adjusts the angle and height of the device 2 according to the position of the converging spot, so that the center of the laser converging spot falls on the center position of the camera.

[0066] (2) Turn off the beacon laser in single-photon communication device 1 and continuously output the beacon laser in single-photon communication device 2. Observe whether the position of the converging spot is at the camera center of single-photon communication device 1. If so, the receiving and transmitting light axes of single-photon communication devices 1 and 2 are aligned. When the converging spot deviates significantly from the camera center of single-photon communication device 1, the internal optical axes of single-photon communication devices 1 and 2 are offset, and bidirectional communication is impossible.

[0067] The second step includes:

[0068] 1) The beacon laser is set to a fixed frequency output, initially emitting N pulses with low single-pulse energy. L A pulse triggers the detector and data acquisition device to acquire T. L The water absorption coefficient A at low pulse energy was obtained by exponentially fitting the backscattered signal based on the scattering waveform. L Scattering coefficient B L ;

[0069] 2) Send medium-energy N using the same method in sequence. M A pulse and high-energy N H The pulse was used to obtain the corresponding medium-energy water absorption coefficient A. M Scattering coefficient B M and the absorption coefficient A of high-energy water bodies H Scattering coefficient B H The average of the three measurements is the current absorption coefficient A and scattering coefficient B of the water quality environment.

[0070]

[0071]

[0072] The third step includes:

[0073] When the sum of the absorption coefficient A and the scattering coefficient B is less than 0.151m -1 (Attenuation coefficient of clean seawater) indicates that the current water quality is good, and low-energy, high-repetition-rate modulation is set to achieve the output of the source laser; when the sum of the absorption coefficient A and the scattering coefficient B is higher than 0.151m -1 This indicates that the current water quality is poor, so high-energy, low-repetition-rate modulation is set to achieve the output of the source laser.

[0074] The energy output of a source laser is set by the duration of a single laser pulse. At low energy output, the pulse duration T... s Short; during high energy output, the light emission duration T s long.

[0075] The modulation frequency of the source laser and the communication frequency R bDirectly related, due to the discrete pulse output characteristics and dead-time constraints of single-photon detection, within a single symbol period of communication (1 / R). b The source laser does not maintain a high level continuously; instead, it employs a spread-spectrum output method, meaning that the output is maintained at 1 / R within a single symbol period. b N pulses are emitted, where N is the number of spread spectrum pulses, to avoid the loss of capability caused by the dead time of the single-photon detector.

[0076] The fourth step includes:

[0077] a) With the source laser and beacon laser off, turning on the single-photon detector will cause it to output discrete pulses due to environmental noise, dark counting, and other factors. The period 1 / R of a single symbol is then calculated using the signal processing unit. b Maximum count rate N of internal noise DCR .

[0078] b) With the laser source working normally, the single-photon detector outputs a pulse after receiving the laser signal. The maximum count rate N of the single-photon detector is counted within a single symbol period (1 / R) of the communication. SIG ;

[0079] c) To ensure a low bit error rate in communication, it is necessary to calculate the noise counting rate N. DCR and maximum signal count rate N SIG Comprehensive setting of minimum pulse interval threshold T inter Signal recovery decision threshold N RECOVER The value can be:

[0080]

[0081]

[0082] That is, during signal demodulation, when the time interval between the first pulse signal and the second pulse signal of the single-photon detector is greater than T... inter If the current pulse is considered a noise photon, no further counting or statistics will be performed, and the system will wait for a signal photon again; if the interval between the first and second pulse signals of the single-photon detector is less than or equal to T... inter When the pulse is considered to be a signal photon, counting and statistics for subsequent single symbol periods begin. When the count value is greater than or equal to N... RECOVER When the count value is less than N, the symbol is "1". RECOVER When the symbol is zero, it is considered to be "0".

[0083] d) Repeat steps b) to c) to complete the demodulation of one frame of communication data.

[0084] In summary, this invention utilizes a single-photon detector as the receiver to achieve more sensitive signal detection and improve the system's operating range. Secondly, leveraging the high sensitivity of the single-photon detector, optical isolation is employed to suppress backscattering effects from the same-side source laser, ensuring bidirectional full-duplex communication capability. Thirdly, water quality is assessed based on backscattered echo counts at different low, medium, and high energies of the beacon laser, and the source laser's output energy and modulation frequency are adaptively modulated to meet communication requirements in varying water quality environments, thus improving system robustness. Finally, based on the background noise photon count and maximum information photon count obtained from the single-photon detector within a single symbol period, an online signal demodulation threshold is set, adapting to different background environments and transmission distances, reducing the communication error rate, and improving the environmental adaptability of single-photon communication.

[0085] It should be understood that those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims.

Claims

1. An underwater wireless optical communication device based on a single-photon detector, characterized in that, The first single-photon communication device and the second single-photon communication device include bidirectional communication, each of which includes a laser emitting unit, a signal receiving unit, a signal processing unit, and a power supply module; The laser emitting unit loads the received modulated electrical signal into the optical signal and outputs it, while simultaneously emitting an auxiliary alignment laser to align the communication device. The signal receiving unit converts the received optical signal into an electrical signal through a single-photon detector and transmits it externally. At the same time, the signal receiving unit receives an auxiliary alignment laser to complete the alignment of the communication device. After receiving the communication information from the host computer, the signal processing unit converts it into a modulated electrical signal and transmits it to the laser emitting unit. It also demodulates the electrical signal transmitted by the signal receiving unit and restores it to the communication information before transmitting it to the host computer. The laser emitting unit includes two optical paths. One optical path includes a beacon laser and a first collimator. The other optical path includes multiple source lasers, a beam combiner, a second collimator, a dichroic mirror, and a beam expanding optical system. The beam combiner is used to combine the laser outputs from the multiple source lasers into one output. The first collimator is used to collimate the beacon laser before outputting it; the second collimator is used to collimate the source laser before outputting it. The dichroic mirror is connected to the first collimator and the second collimator to combine the collimated source laser and the beacon laser and output them to the beam expander optical system. The signal receiving unit includes two optical paths. One path specifically includes a receiving optical system, a dichroic mirror, a filter assembly, a first converging optical system, and a single-photon detector, connected in sequence. The receiving optical system receives the laser emitted by the laser emitting unit. The dichroic mirror is used to separate the source laser from the beacon laser. The source laser is filtered by the filter assembly and then output to the first converging optical system to converge the source laser energy onto the target surface of the single-photon detector. The single-photon detector receives the source laser energy and converts it into a digital pulse signal, which is then output to the signal processing unit. The other path includes a second converging optical system and a camera. The second converging optical system is connected to a dichroic mirror, and the camera is connected to an external computer. The second converging optical system is used to converge the beacon laser energy to the target surface of the camera. The camera receives the beacon laser spot and completes the alignment of the emission and reception optical axes according to the position of the spot. The process of aligning the communication devices specifically involves: In the first single-photon communication device, the beacon laser outputs continuously. In the second single-photon communication device, the camera adjusts the angle and height of the second single-photon communication device according to the position of the converging laser spot, so that the center of the laser converging spot falls on the center position of the camera. Turn off the beacon laser in the first single-photon communication device, and continuously output the beacon laser in the second single-photon communication device. Observe whether the position of the converging spot is at the center of the camera of the first single-photon communication device. If so, the receiving and transmitting axes of the two single-photon communication devices are aligned.

2. The underwater wireless optical communication device based on a single-photon detector according to claim 1, characterized in that, The source laser of one laser emitting unit has the same wavelength as the beacon laser of another laser emitting unit.

3. The underwater wireless optical communication device based on a single-photon detector according to claim 1, characterized in that, The bandpass center wavelength range of the filter assembly of the signal receiving unit of the first single-photon communication device covers the wavelengths of multiple source lasers of the laser emitting unit of the second single-photon communication device, and the cutoff wavelength range covers the wavelengths of multiple source lasers of the first laser emitting unit.

4. The underwater wireless optical communication device based on a single-photon detector according to claim 1, characterized in that, The passband center wavelength range of the filter assembly of the receiving unit of the second single-photon communication device covers the wavelengths of multiple source lasers of the laser emitting unit of the first single-photon communication device, and the cutoff wavelength range covers the wavelengths of multiple source lasers of the laser emitting unit of the second single-photon communication device.

5. The underwater wireless optical communication device based on a single-photon detector according to claim 1, characterized in that, The single-photon detector can be a Geiger-mode APD single-photon detector, a photon-counting PMT single-photon detector, or a superconducting nanowire single-photon detector.

6. The underwater wireless optical communication device based on a single-photon detector according to claim 1, characterized in that, The camera is either a CCD camera or a CMOS camera.

7. The underwater wireless optical communication device based on a single-photon detector according to claim 1, characterized in that, The signal processing unit includes an information transmission module, a level conversion module, a modulation function module, a demodulation function module, and a system initialization module. The information transmission module is used to receive information transmitted from the host computer and convert it into data to be transmitted to the modulation function module. The modulation module encodes the data to be transmitted and transmits it to the level conversion module in high and low voltage levels. The level conversion module applies high and low voltage levels to the laser based on the laser driver power supply; high voltage outputs light, and low voltage does not output light. At the same time, it converts the discrete pulses output by the single-photon detector into a level signal that can be detected by the demodulation module. The demodulation module demodulates the discrete pulses output by the single-photon detector and then transmits the demodulated information to the computer through the information transmission module. The system initialization module is used to complete the initialization settings of other modules.

8. An underwater wireless optical communication method based on a single-photon detector, characterized in that, This communication method, based on the underwater wireless optical communication device using a single-photon detector as described in claim 1, includes the following steps: The first step is to align the transmitting and receiving optical axes of the first and second single-photon communication devices. The second step involves emitting low-energy, medium-energy, and high-energy lasers through a laser emitting unit, and then using the scattered echo counts collected by a signal receiving unit to assess the absorption and scattering coefficients of the water quality environment. The average value of the three measurements is taken as the current absorption and scattering coefficients of the water quality environment. The third step is to determine the current water quality conditions based on the sum of the absorption coefficient and the scattering coefficient. When the water quality is lower than the attenuation coefficient of clean seawater, low-energy, high-repetition-rate modulation is set to achieve the output of the source laser; when the water quality is higher than the attenuation coefficient of clean seawater, high-energy, low-repetition-rate modulation is set to achieve the output of the source laser. In the fourth step, the signal receiving unit uses a single-photon detector to count the background noise photon count rate and the maximum information photon count rate of a single symbol period environment, and sets the minimum pulse interval threshold and signal recovery decision threshold for online signal demodulation to adapt to communication under different background environments and transmission distances. The third and fourth steps are repeated to achieve continuous communication transmission.

Images