Underwater wireless laser communication system and method compatible with multiple modulation formats

Abstract

The invention discloses an underwater wireless laser communication system and method compatible with multiple modulation formats, and belongs to the technical field of underwater wireless laser communication. The system comprises a transmitting end and a receiving end, wherein the transmitting end comprises a modulation module, an optical frequency doubling module and a beam shaping module; the receiving end comprises an optical signal receiving module and a coherent demodulation module; the system can be compatible with various modulation formats, improves the flexibility and adaptability of underwater wireless laser communication, effectively solves the problems of multi-format compatibility and efficient transmission of laser communication in an underwater complex environment, and has the advantages of high data transmission rate, long communication distance, strong anti-interference capability and the like.

Description

Technical Field

[0001] This invention relates to the field of underwater wireless laser communication technology, and in particular to an underwater wireless laser communication system and method compatible with multiple modulation formats. Background Technology

[0002] Since the beginning of the 21st century, humanity has intensified its efforts in ocean research and development, leading to the rapid advancement of marine exploration systems and underwater communication technologies. Underwater communication is primarily categorized into wired and wireless communication. Wired communication requires cables for connection, and its flexibility is limited by the complex seabed environment. Wireless communication, on the other hand, transmits signals via sound waves, electromagnetic waves, or light waves. Among various underwater communication technologies, underwater wireless optical communication has become a highly anticipated technological direction due to its significant advantages, including high transmission rates, strong anti-interference capabilities, good security, and high flexibility.

[0003] In existing technologies, underwater laser communication signals are typically modulated using traditional intensity modulation. The core advantage of this modulation method lies in its simplicity and ease of implementation, meeting the needs of basic underwater communication scenarios. However, it suffers from significant technical drawbacks: firstly, the transmission rate has an inherent upper limit, making it unsuitable for high-speed data transmission applications; secondly, in seawater environments, the signal is easily affected by environmental factors such as water turbulence, light signal scattering, and absorption, leading to decreased communication stability.

[0004] To address the aforementioned issues, coherent modulation underwater wireless optical communication systems have been gradually developed in existing technologies. These systems maintain high receiving sensitivity while achieving high-speed information transmission, thus mitigating the speed limitations of intensity modulation to some extent. Furthermore, domestic and international research has focused on various higher-order modulation formats. Related technical reports indicate that higher-order modulation formats can effectively improve bandwidth utilization and occupy less bandwidth, but their technical implementation complexity is significantly increased, specifically manifested in greater hardware design difficulty and more complex control logic.

[0005] In practical applications, the selection of modulation format for underwater laser communication systems depends on multiple practical conditions: if system complexity and cost are prioritized, lower-order modulation formats are more advantageous; if data transmission rate is the core requirement, higher-order modulation formats must be used. However, existing underwater laser communication systems generally only support a single modulation format and cannot flexibly switch according to actual scenarios (such as different transmission distances, environmental interference intensity, and rate requirements), resulting in poor system adaptability. A single system cannot cover multiple application scenarios, and deploying separate systems with corresponding modulation formats for different scenarios would significantly increase equipment costs and maintenance complexity.

[0006] Therefore, the current field of underwater laser communication urgently needs a technical solution that is compatible with multiple modulation formats and supports flexible switching in order to improve system flexibility and reduce the overall cost in multiple application scenarios. This is also the core technical problem that this invention aims to solve. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides an underwater wireless laser communication system and method compatible with multiple modulation formats, thus solving the problems mentioned in the background section.

[0008] To achieve the above objectives, the present invention specifically adopts the following technical solution: An underwater wireless laser communication system compatible with multiple modulation formats includes a transmitter and a receiver. The transmitter includes a modulation module, an optical frequency doubling module, and a beam shaping module connected in sequence. The modulation module is used to modulate the digital signal to be transmitted into an infrared optical carrier by using a Mach-Zehnder modulator to modulate it into infrared signal light. The optical frequency doubling module is used to amplify the power and convert the wavelength of the modulated infrared signal light to make it a high-power green band signal suitable for underwater transmission. The beam shaping module shapes the wavelength-converted optical signal and sends it into the underwater channel for transmission. The receiving end includes an optical signal receiving module and a coherent demodulation module connected in sequence. The optical signal receiving module is used to receive green band optical signals transmitted underwater; the coherent demodulation module is used to convert the received optical signals into electrical signals and perform coherent demodulation on the electrical signals to obtain the required information.

[0009] Furthermore, the modulation module includes a Mach-Zehnder modulator and a laser, a bias controller, and a microwave amplifier, all connected to the Mach-Zehnder modulator. The microwave amplifier receives the digital signal to be transmitted at its input and outputs the amplified digital signal. The bias controller outputs a DC bias voltage to adjust the bias point of the Mach-Zehnder modulator, thereby changing its modulation format. The Mach-Zehnder modulator receives the laser emitted by the laser, the amplified digital signal output by the microwave amplifier, and the DC bias voltage output by the bias controller. The Mach-Zehnder modulator modulates the digital signal to be transmitted onto the carrier laser emitted by the laser, outputting a modulated optical signal.

[0010] Furthermore, the optical frequency doubling module includes a ytterbium-doped fiber amplifier and an optical wavelength conversion unit connected to each other. The ytterbium-doped fiber amplifier is used to amplify the modulated optical signal output by the modulation module and then output it. The optical wavelength conversion unit is used to convert the wavelength of the amplified optical signal to the green band and then output it.

[0011] Furthermore, the beam shaping module shapes the wavelength-converted optical signal and then sends it into the underwater channel for transmission.

[0012] Furthermore, the optical signal receiving module is used to receive green band optical signals transmitted underwater.

[0013] Furthermore, the coherent demodulation module includes a 2×4 optical 90° mixer, two balanced detectors, two analog-to-digital converters, and a digital signal processing unit connected in sequence. One output of the digital signal processing unit is connected to the input of the local oscillator laser. The input of the 2×4 optical 90° mixer is respectively connected to the optical signal received by the optical signal receiving module and the local oscillator light generated by the local oscillator laser. After coherent mixing by the 2×4 optical 90° mixer, four signal beams with relative phase differences of 0°, 180°, 90°, and 270° are output. The 0° and 180° beams and the 90° and 270° beams are received and amplified by the two balanced detectors and converted into two analog electrical signals, which are then output to the two analog-to-digital converters. After receiving the two analog electrical signals, the two analog-to-digital converters convert them into two digital electrical signals, which are then output to the digital signal processing unit for information demodulation. The digital signal processing unit can also output a frequency control signal to control the frequency of the tunable local oscillator laser.

[0014] A method for underwater wireless laser communication compatible with multiple modulation formats, comprising the following steps: At the transmitting end, when information needs to be transmitted, the digital signal to be transmitted is modulated onto the infrared optical carrier according to the actual required modulation format to form infrared signal light. Then, based on the frequency doubling method of generating second harmonics by adding a ytterbium-doped fiber amplifier and nonlinear optical effects, the modulated infrared signal light is amplified in power and its wavelength is converted to the green band. Finally, the converted optical signal is shaped and transmitted. At the receiving end, when a green band optical signal transmitted underwater is received, an intradynastic coherent optical detection method is used to convert the received optical signal into an electrical signal, and then the electrical signal is demodulated to obtain the required information.

[0015] Furthermore, the step of modulating the digital signal to be transmitted onto an infrared optical carrier according to the actual required modulation format to form infrared signal light includes: Connect the input terminal of the microwave amplifier to the digital signal to be transmitted, and output the amplified digital signal at the output terminal. Adjust the output of the bias controller according to the modulation format required for actual use, so that its output is a DC bias voltage used to adjust the bias point of the Mach-Zehnder modulator and thus change its modulation format. The Mach-Zehnder modulator is connected to the carrier laser emitted by the laser, the digital signal to be emitted, and the DC bias voltage used to change the modulation format. After modulation, the signal is modulated onto the carrier laser emitted by the laser, and the modulated optical signal is output.

[0016] Furthermore, the steps of amplifying the modulated infrared signal light and converting its wavelength to the green band using a frequency doubling method based on a ytterbium-doped fiber amplifier and nonlinear optical effects to generate a second harmonic include: The optical signal modulated by the Mach-Zehnder modulator is amplified by a ytterbium-doped fiber amplifier. The amplified optical signal is first expanded and collimated by a collimating lens. Then, the polarization direction of the fundamental frequency light is adjusted by a half-wave plate to make it parallel to the LBO frequency doubling crystal to meet the polarization requirements of temperature phase matching. Finally, it is focused into the LBO frequency doubling crystal by a focusing lens. The temperature of the LBO frequency doubling crystal is controlled by a temperature controller circuit. The waveguide direction is on the same optical axis as the incident light direction. The LBO frequency doubling crystal converts the incident optical signal into an output optical signal with a wavelength in the green band.

[0017] Furthermore, the steps of converting the received optical signal into an electrical signal using an intrinsic coherent optical detection method, and then demodulating the electrical signal to obtain the desired information, include: The system receives the green band optical signal after underwater transmission and simultaneously and perpendicularly incidents it on a 2×4 optical 90° mixer for coherent mixing with the local oscillator light, outputting four optical signals with relative phase differences of 0°, 180°, 90° and 270°. The 0° and 180° light beams and the 90° and 270° light beams are detected and received by two balanced detectors, and then amplified and converted into two analog electrical signals. The two analog electrical signals are transmitted to two analog-to-digital converters to convert them into two digital electrical signals. The two digital electrical signals are output to the digital signal processing unit for information demodulation to obtain the required information; The digital signal processing unit can also output frequency control signals to control the frequency of the tunable local oscillator laser.

[0018] Compared with the prior art, the present invention provides an underwater wireless laser communication system and method compatible with multiple modulation formats, which has the following advantages: 1. This invention achieves dynamic switching between multiple modulation formats such as OOK, BPSK, and DPSK by combining a Mach-Zehnder modulator in the modulation module with a bias controller. Users can flexibly select the modulation method according to the actual scenario (such as transmission distance, rate requirements, or environmental interference intensity), avoiding the limitation of existing systems that only support a single format. This significantly improves the system's versatility and flexibility, and reduces equipment costs and maintenance complexity in multi-scenario applications.

[0019] 2. This invention converts 1064nm infrared carrier light into a 532nm green light signal using an optical frequency doubling module (including a ytterbium-doped fiber amplifier and an optical wavelength conversion unit). Green light has a lower attenuation coefficient and scattering loss underwater. Combined with power amplification technology, this effectively extends the communication distance and improves the signal penetration ability in turbulent or turbid water environments. The ytterbium-doped fiber amplifier employs a master oscillation power amplifier structure, boosting the signal power to the watt level through multi-stage amplification. The LBO frequency doubling crystal achieves efficient wavelength conversion through temperature control and polarization matching, thereby ensuring high-speed, high-stability data transmission.

[0020] 3. This invention employs an intrinsic coherent detection scheme at the receiving end, extracting the amplitude and phase information of the signal through a 2x4 optical 90° mixer and a balanced detector. This method boasts extremely high receiving sensitivity, effectively suppressing interference from water disturbances, background light noise, and multipath effects, ensuring reliable communication with a low bit error rate in complex underwater environments. Furthermore, frequency offset compensation and phase recovery are performed through a digital signal processing unit, simultaneously supporting asynchronous demodulation (for OOK) and synchronous demodulation (for BPSK / DPSK), improving the system's versatility and flexibility, and further enhancing its anti-interference capability and security. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the transmitter structure of an underwater wireless laser communication system compatible with multiple modulation formats according to the present invention; Figure 2 This is a schematic diagram of the structure of the present invention that achieves compatibility with multiple modulation formats through a Mach-Zehnder modulator; Figure 3 This is a schematic diagram of the ytterbium-doped fiber amplifier based on the master oscillation power amplifier structure of the present invention; Figure 4 This is a schematic diagram of the optical wavelength conversion unit of the present invention; Figure 5 This is a schematic diagram of the receiver structure of an underwater wireless laser communication system compatible with multiple modulation formats according to the present invention. Detailed Implementation

[0022] The present invention will now be further described with reference to the accompanying drawings and specific preferred embodiments.

[0023] like Figures 1-5 As shown, the present invention provides an underwater wireless laser communication system compatible with multiple modulation formats, including a transmitter and a receiver.

[0024] The structure of the transmitter is as follows: Figure 1 As shown, it includes a modulation module, an optical frequency doubling module, and a beam shaping module connected in sequence.

[0025] The modulation module includes a laser, a Mach-Zehnder modulator, a bias controller, and a microwave amplifier. The laser is a 1064nm distributed Bragg reflector semiconductor laser, which can emit infrared light signals with a wavelength of 1064nm. The input of the microwave amplifier is connected to the digital signal to be emitted, and the output is the amplified digital signal. The output of the bias controller is a DC bias voltage used to adjust the bias point of the Mach-Zehnder modulator, thereby changing its modulation format. Then, the Mach-Zehnder modulator is connected to the infrared signal light, the amplified digital signal, and the DC bias voltage respectively. The Mach-Zehnder modulator can then modulate the digital signal to be emitted onto the carrier laser emitted by the laser according to the actual required modulation format and output the modulated light signal.

[0026] like Figure 2 The diagram illustrates the structure of a Mach-Zehnder modulator that achieves compatibility with multiple modulation formats. When the bias controller is adjusted to output a bias voltage that allows the Mach-Zehnder modulator to operate at the quadrature propagation point, the modulator operates at the quadrature propagation point in PUSH-PULL mode, and the modulation format of the output modulated signal light is OOK (intensity modulation). When the bias controller is adjusted to output a bias voltage that allows the Mach-Zehnder modulator to operate at the minimum propagation point, the modulator operates at the minimum propagation point in PUSH-PUSH mode, and the modulation format of the output modulated signal light is BPSK (coherent modulation). When the Mach-Zehnder modulator operates at the minimum propagation point, differential precoding is performed on the digital signal to be transmitted before it is input to the modulator, thus enabling the output modulated signal light to have a DPSK modulation format. Therefore, this invention can achieve compatibility with multiple modulation formats.

[0027] The optical frequency doubling module includes a ytterbium-doped fiber amplifier and an optical wavelength conversion unit connected to each other. The ytterbium-doped fiber amplifier is used to amplify the modulated optical signal output by the modulation module and then output it. The optical wavelength conversion unit is used to convert the wavelength of the amplified optical signal to the green band and then output it.

[0028] like Figure 3The diagram shows the structure of a ytterbium-doped fiber amplifier based on a master oscillator power amplifier structure. It includes a pre-amplification stage and a main amplification stage, employing a fully polarization-maintaining fiber architecture to maintain a single polarization state of the signal light, improving polarization stability and reducing nonlinear effects within the light beam. The pre-amplification stage uses a forward-pumped structure. The infrared signal light, modulated by the modulation module with a wavelength of 1064nm and a power of 35μW, first passes through a first fiber isolator and a wavelength division multiplexer, entering the first-stage polarization-maintaining ytterbium-doped single-mode fiber. After the fiber filter removes the amplifier's spontaneous emission noise, the first-stage pre-amplification is completed. It then passes through a second fiber isolator and a wavelength division multiplexer before entering the second-stage polarization-maintaining ytterbium-doped single-mode fiber for the second-stage pre-amplification. A low-noise wavelength-locked pump source with a maximum output power of 440mW and a center wavelength of 976nm is split by a 50 / 50 beam splitter and coupled into the two stages of polarization-maintaining ytterbium-doped single-mode fibers through two wavelength division multiplexers, providing pump energy for the signal light amplification. After two stages of pre-amplification, the signal light power can be increased to 37mW. The main amplification stage still adopts a forward pumping structure. The infrared signal light after two stages of pre-amplification first passes through an optical fiber isolator, and then is coupled to a pump source with a maximum power of 9W and a center wavelength of 976nm through a (1+1)×1 combiner into the first-stage polarization-maintaining ytterbium-doped double-clad fiber for the first main amplification stage. The output signal light passes through an optical fiber isolator to protect the previous stage optical path, and then passes through a 99 / 1 beam splitter. The 1% port is connected to a photodetector. The photodetector senses changes in light intensity and feeds back to the laser driver module for automatic correction. When the output power shows a nonlinear change, it indicates that a nonlinear effect has occurred in the optical path, and the power output needs to be reduced. The remaining 99% of the signal light after the first-stage main amplification is coupled to a pump source with a maximum power of 27W and a center wavelength of 976nm through a (1+1)×1 combiner into a second-stage polarization-maintaining ytterbium-doped double-clad fiber for a second main amplification. Afterward, the residual pump light is filtered out by a cladding light filter, effectively improving the output beam quality. After two stages of main amplification, the signal light power can be increased to 15W.

[0029] like Figure 4The diagram shows the structure of the optical wavelength conversion unit. The 1064nm optical signal, after power amplification, is first expanded and collimated by a collimating lens. Then, a half-wave plate adjusts the polarization direction of the fundamental frequency light to make it parallel to the LBO frequency doubling crystal, satisfying the polarization requirement for temperature phase matching. Finally, the light is focused into the LBO frequency doubling crystal by a focusing lens. A temperature controller controls the temperature of the LBO frequency doubling crystal to ensure it operates at its optimal temperature, with the waveguide direction aligned with the incident light direction on the same optical axis. The LBO frequency doubling crystal converts the incident light signal into a 532nm green band optical signal through frequency doubling. The output 532nm light signal is then collimated by a second collimating lens and output to a beam splitter. The beam splitter separates the residual 1064nm fundamental frequency light, resulting in a high-power 532nm green carrier signal more suitable for transmission in seawater channels.

[0030] The beam shaping module then reshapes the green light signal, which has undergone power amplification and wavelength conversion, before sending it into the underwater channel for transmission.

[0031] The structure of the receiving end is as follows Figure 5 As shown, it includes an optical signal receiving module and a coherent demodulation module connected in sequence.

[0032] The optical signal receiving module is used to receive the green band optical signal transmitted underwater to provide sufficient signal input for subsequent modules. Specifically, it is an optical receiving antenna. The receiving end uses the optical receiving antenna to collect the divergent and weak signal light, and the local oscillator laser generates the local oscillator light. The signal light collected by the optical receiving antenna and the local oscillator light generated by the local oscillator laser are input to the coherent demodulation module for coherent demodulation.

[0033] The coherent demodulation module includes a 2×4 optical 90° mixer, two balanced detectors, two analog-to-digital converters, and a digital signal processing unit connected in sequence. The inputs of the 2×4 optical 90° mixer are the optical signal received by the optical signal receiving module and the local oscillator light generated by the local oscillator laser. The frequency of the local oscillator light is then adjusted so that the frequency difference between the local oscillator light and the signal light is much smaller than the noise bandwidth of the communication signal; that is, the receiver operates in an intrinsic state. After coherent mixing by the 2×4 optical 90° mixer, the outputs have relative phase differences of 0°, 180°, 90°, and 2... Four beams of signal light at 70°, two beams at 0° and 180°, and two beams at 90° and 270° are received and amplified by two balanced detectors, then converted into two analog electrical signals and output to two analog-to-digital converters (ADCs). The ADCs convert these analog signals into two digital electrical signals and output them to a digital signal processing unit (DSP). The DSP performs preprocessing on the signals, including frequency offset compensation, clock recovery, and phase recovery, while simultaneously extracting the amplitude and phase information. Subsequently, the channel is estimated, denoised, and compensated to ultimately recover the transmitted information. During multi-mode compatible coherent demodulation, if the signal light is an OOK modulated signal, detection is performed using the carrier envelope; this method is called asynchronous demodulation. If the signal light is a BPSK / DPSK modulated signal, detection is performed using the carrier frequency and phase; this method is called synchronous demodulation. The DSP can also output a frequency control signal to control the frequency of the tunable local oscillator laser. The balanced detectors can be avalanche diodes.

[0034] Meanwhile, this invention also provides an underwater wireless laser communication method compatible with multiple modulation formats, adopting... The above-mentioned underwater wireless laser communication system, which is compatible with multiple modulation formats, includes the following steps: 1. At the transmitting end, when information needs to be transmitted, the digital signal to be transmitted is modulated onto the infrared optical carrier according to the actual required modulation format to form infrared signal light; 2. Then, based on the frequency doubling method of generating second harmonics using a ytterbium-doped fiber amplifier and nonlinear optical effects, the modulated infrared signal light is amplified in power and its wavelength is converted to the green band. 3. The converted optical signal is then shaped and emitted as an output; 4. At the receiving end, when the green band optical signal transmitted underwater is received, the received optical signal is converted into an electrical signal using an intradynastic coherent optical detection method, and then the electrical signal is demodulated to obtain the required information.

[0035] Step 1, which modulates the digital signal to be transmitted onto an infrared optical carrier according to the required modulation format to form infrared signal light, includes: 1.1 Start the infrared laser, microwave amplifier, bias controller, and Mach-Zehnder modulator; 1.2 Connect the input terminal of the microwave amplifier to the digital signal to be transmitted, and output the amplified digital signal at the output terminal; 1.3 According to the modulation format required by the actual use, adjust the output of the bias controller so that its output is used to adjust the bias point of the Mach-Zehnder modulator and thus change its modulation format. When the required modulation format is OOK, set the bias point to the quadrature bias point. When the required modulation format is BPSK, set the bias point to the minimum transmission point. When the required modulation format is DBPSK, perform differential precoding on the digital signal to be modulated based on the BPSK setting. 1.4 The carrier laser with a wavelength of 1064nm emitted by the laser, the digital signal to be emitted, and the DC bias voltage used to change the modulation format are respectively connected to the Mach-Zehnder modulator, and after modulation, they are modulated onto the carrier laser emitted by the laser to output the modulated optical signal.

[0036] Step 2, the steps of amplifying the modulated infrared signal light and converting its wavelength to the green band using a frequency doubling method based on a ytterbium-doped fiber amplifier and nonlinear optical effects to generate a second harmonic, include: 2.1 The optical signal modulated by the Mach-Zehnder modulator is passed through a pre-amplification stage based on a ytterbium-doped fiber amplifier with a master oscillation power amplifier to perform two-stage pre-amplification of the signal optical power; 2.2 The power of the signal light after two stages of pre-amplification is then further amplified by two stages of main amplification, ultimately increasing the power of the infrared signal light from the milliwatt level to the watt level. 2.3 The amplified optical signal is first expanded and collimated by a collimating lens. Then, the polarization direction of the fundamental frequency light is adjusted by a half-wave plate to make it parallel to the LBO frequency doubling crystal to meet the polarization requirements of temperature phase matching. Finally, it is focused into the LBO frequency doubling crystal by a focusing lens. The temperature of the LBO frequency doubling crystal is controlled by a temperature controller circuit to keep it working at the optimal temperature. The waveguide direction is on the same optical axis as the incident light direction. The LBO frequency doubling crystal converts the incident optical signal into a green band optical signal with a wavelength of 532nm through frequency doubling. 2.4 The output 532nm optical signal is then collimated by the second collimating lens and output to the beam splitter. The beam splitter separates the residual 1064nm fundamental frequency light, thereby obtaining a high-power green light carrier signal with a wavelength of 532nm that is more suitable for transmission in the seawater channel.

[0037] In step 3, the main operation is to reshape the 532nm wavelength optical signal after it has been processed by the optical frequency doubling module by the beam shaping module in order to improve the stability of underwater transmission.

[0038] Step 4, which involves receiving a green band optical signal transmitted underwater, converting the received optical signal into an electrical signal using an intrapolated coherent optical detection method, and then demodulating the electrical signal to obtain the desired information, includes the following steps: 4.1 First, the optical signal receiving module is used to receive the green band optical signal transmitted underwater. Specifically, the optical receiving antenna is used to collect the divergent and weak signal light. 4.2 The local oscillator laser then generates local oscillator light, and the signal light collected by the optical receiving antenna and the local oscillator light generated by the local oscillator laser are input into the coherent demodulation module for coherent demodulation.

[0039] 4.3 The coherent demodulation module includes a 2×4 optical 90° mixer, two balanced detectors, two analog-to-digital converters and a digital signal processing unit connected in sequence. The input terminals of the 2×4 optical 90° mixer are respectively connected to the optical signal received by the optical signal receiving module and the local oscillator light generated by the local oscillator laser. 4.4 The frequency of the local oscillator light is then adjusted so that the frequency difference between the local oscillator light and the signal light is much smaller than the noise bandwidth of the communication signal. That is, the receiver operates in an internal difference state. After 2×4 optical 90° coherent mixing, four signal lights with relative phase differences of 0°, 180°, 90° and 270° are output. 4.5 The 0° and 180° light paths and the 90° and 270° light paths are each received by two balanced detectors, and can be amplified by avalanche diodes and converted into two analog electrical signals. 4.6 The two analog electrical signals are output to two analog-to-digital converters, which convert them into two digital electrical signals; 4.7 The two digital electrical signals are then output to the digital signal processing unit to perform frequency offset compensation, clock recovery and phase recovery to complete signal preprocessing. At the same time, the amplitude and phase information contained in the signal are extracted. Subsequently, the channel is estimated, denoised and compensated to finally recover the information data it carries.

[0040] 4.8 In multi-mode compatible coherent demodulation, if the signal light is an OOK modulated signal, detection is performed using the carrier envelope; this detection method is called asynchronous demodulation. If the signal light is a BPSK / DPSK modulated signal, detection is performed using the carrier frequency and phase; this detection method is called synchronous demodulation. The digital signal processing unit can also output a frequency control signal to control the frequency of the tunable local oscillator laser.

[0041] Finally, it should be noted that the above description is merely for specific embodiments of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An underwater wireless laser communication system compatible with multiple modulation formats, comprising a transmitter and a receiver, characterized in that: The transmitting end includes a modulation module, an optical frequency doubling module, and a beam shaping module connected to each other. The modulation module is used to modulate the digital signal to be transmitted into infrared light by using a Mach-Zehnder modulator in various formats, thereby modulating it onto an infrared light carrier to form infrared signal light. The optical frequency doubling module is used to amplify the power and convert the wavelength of the modulated infrared signal light to make it a high-power green band signal suitable for underwater transmission. The beam shaping module shapes the wavelength-converted light signal and sends it into the underwater channel for transmission. The receiving end includes an optical signal receiving module and a coherent demodulation module connected in sequence. The optical signal receiving module is used to receive green band optical signals transmitted underwater; the coherent demodulation module is used to convert the received optical signals into electrical signals and perform coherent demodulation on the electrical signals to obtain the required information.

2. The underwater wireless laser communication system compatible with multiple modulation formats according to claim 1, characterized in that: The modulation module includes a Mach-Zehnder modulator and a laser, a bias controller, and a microwave amplifier, all connected to the Mach-Zehnder modulator. The microwave amplifier receives the digital signal to be transmitted at its input and outputs the amplified digital signal. The bias controller outputs a DC bias voltage to adjust the bias point of the Mach-Zehnder modulator, thereby changing its modulation format. The Mach-Zehnder modulator receives the laser emitted by the laser, the amplified digital signal output by the microwave amplifier, and the DC bias voltage output by the bias controller. The Mach-Zehnder modulator modulates the digital signal to be transmitted onto the carrier laser emitted by the laser, outputting a modulated optical signal.

3. The underwater wireless laser communication system compatible with multiple modulation formats according to claim 1, characterized in that: The optical frequency doubling module includes a ytterbium-doped fiber amplifier and an optical wavelength conversion unit connected to each other. The ytterbium-doped fiber amplifier is used to amplify the modulated optical signal output by the modulation module and then output it. The optical wavelength conversion unit is used to convert the wavelength of the amplified optical signal to the green band and then output it.

4. The underwater wireless laser communication system compatible with multiple modulation formats according to claim 1, characterized in that: The coherent demodulation module includes a 2×4 optical 90° mixer, two balanced detectors, two analog-to-digital converters, and a digital signal processing unit connected in sequence. One output of the digital signal processing unit is connected to the input of the local oscillator laser. The input of the 2×4 optical 90° mixer is respectively connected to the optical signal received by the optical signal receiving module and the local oscillator light generated by the local oscillator laser. After coherent mixing by the 2×4 optical 90° mixer, four signal beams with relative phase differences of 0°, 180°, 90°, and 270° are output. The 0° and 180° beams and the 90° and 270° beams are received and amplified by the two balanced detectors and converted into two analog electrical signals, which are then output to the two analog-to-digital converters. After receiving the two analog electrical signals, the two analog-to-digital converters convert them into two digital electrical signals, which are then output to the digital signal processing unit for information demodulation. The digital signal processing unit can also output a frequency control signal to control the frequency of the tunable local oscillator laser.

5. An underwater wireless laser communication method compatible with multiple modulation formats, characterized in that, The method includes: at the transmitting end, when it is necessary to transmit a digital signal, the digital signal to be transmitted is modulated onto an infrared optical carrier according to the actual required modulation format to form an infrared signal light; then, based on the frequency doubling method of generating second harmonics by adding a ytterbium-doped fiber amplifier and nonlinear optical effects, the modulated infrared signal light is amplified in power and its wavelength is converted to the green band; then, the converted optical signal is shaped and transmitted to the underwater channel for transmission. At the receiving end, when the green band optical signal transmitted in the underwater channel is received, the received optical signal is converted into an electrical signal using an intrapolated coherent optical detection method, and then the electrical signal is demodulated to obtain the required information.

6. The underwater wireless laser communication method compatible with multiple modulation formats according to claim 5, characterized in that: The step of modulating the digital signal to be transmitted onto an infrared optical carrier according to the actual required modulation format to form infrared signal light includes: The infrared carrier light emitted by the laser is sent to the optical signal input terminal of the Mach-Zehnder modulator through an optical fiber. The Mach-Zehnder modulator receives the optical signal sent through the optical fiber, the digital signal to be emitted, and the bias voltage used to switch the modulation format of the Mach-Zehnder modulator. After modulation with the actual required modulation format, the digital signal to be emitted is modulated onto the infrared carrier light emitted by the laser, and the modulated optical signal is output.

7. The underwater wireless laser communication method compatible with multiple modulation formats according to claim 5, characterized in that: The frequency doubling method based on ytterbium-doped fiber amplifiers and nonlinear optical effects to generate second harmonics amplifies the modulated infrared signal light and converts its wavelength to the green band, including: The optical signal modulated by the Mach-Zehnder modulator is amplified by a ytterbium-doped fiber amplifier. The amplified optical signal is first expanded and collimated by a collimating lens. Then, the polarization direction of the fundamental frequency light is adjusted by a half-wave plate to make it parallel to the LBO frequency doubling crystal to meet the polarization requirements of temperature phase matching. Finally, it is focused into the LBO frequency doubling crystal by a focusing lens. The temperature of the LBO frequency doubling crystal is controlled by a temperature controller. The waveguide direction is on the same optical axis as the incident light direction. The LBO frequency doubling crystal converts the incident optical signal into an output optical signal with a wavelength in the green band.

8. The underwater wireless laser communication method compatible with multiple modulation formats according to claim 5, characterized in that: The steps of converting the received optical signal into an electrical signal using an intrinsic coherent optical detection method, and then demodulating the electrical signal to obtain the desired information, include: The system receives the green band optical signal after underwater transmission and simultaneously and perpendicularly incidents it on a 2×4 optical 90° mixer for coherent mixing with the local oscillator light, outputting four optical signals with relative phase differences of 0°, 180°, 90° and 270°. The 0° and 180° light beams and the 90° and 270° light beams are detected and received by two balanced detectors, and then amplified and converted into two analog electrical signals. The two analog electrical signals are transmitted to two analog-to-digital converters to convert them into two digital electrical signals. The two digital electrical signals are output to the digital signal processing unit for information demodulation to obtain the required information; The digital signal processing unit can also output frequency control signals to control the frequency of the tunable local oscillator laser.

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