An underwater optical communication method and system based on vortex beam multiplexing

Abstract

The present invention belongs to the technical field of single-photon detection and generation of vortex light beams, and discloses an underwater optical communication method and system based on vortex light beam multiplexing. The method utilizes the orthogonality of vortex light modes to conduct underwater photon counting communication experiments based on multi-channel vortex light beam multiplexing, respectively using multiple spatial light modulators and vortex wave plates to modulate and demodulate vortex light beams, using photon counting detection, using FPGA programming for signal processing, and applying the modulation method of vortex light beams to complete multi-channel parallel information transmission in a simulated seawater channel. The present invention effectively improves the detection accuracy of optical signals to the single-photon level. While maintaining the intensity of the emitted light pulse unchanged, the effective distance of underwater optical communication is successfully extended.

Description

Technical Field

[0001] The present invention belongs to the technical field of single-photon detection and vortex beam generation, and in particular relates to an underwater optical communication method and system based on vortex beam multiplexing. Background Art

[0002] Traditional underwater wireless communication methods, such as hydroacoustic communication and electromagnetic communication, usually have slow transmission rates and low security. However, by utilizing blue-green light in the visible light band, a higher-speed underwater communication system can be achieved due to its relatively small attenuation coefficient in water. Therefore, the use of blue-green laser technology has become an effective means to improve underwater communication performance.

[0003] With the development of quantum information, single-photon detection technology has been promoted. Due to its excellent detection sensitivity, it can reach the level of single photons, which makes it possible to further increase the transmission distance of optical communication while maintaining the constant power of the light source.

[0004] Underwater optical communication has problems such as light pulses are easily absorbed or scattered by water channels, and the distribution of pulse photons is affected by random noise. Using conventional intensity modulation methods, more pulses will be lost at the receiving end, resulting in a large bit error rate during communication, making it difficult to communicate normally. Therefore, it is necessary to take into account high communication rate, low bit error rate, and long communication distance. Photon counting communication is expected to further break through the communication distance and tolerate greater link loss due to its high-sensitivity detection advantage. Vortex beam multiplexing is applied to underwater photon counting communication to achieve high-speed, reliable and stable communication under greater channel loss.

[0005] Through the above analysis, the problems and defects of the prior art are as follows:

[0006] In the existing underwater optical communication technology, the communication rate is low, the bit error rate is high, and the communication distance is short, resulting in low communication efficiency. Summary of the invention

[0007] In order to overcome the problems existing in the related art, the disclosed embodiments of the present invention provide an underwater optical communication method and system based on vortex beam multiplexing. With the rapid increase in marine development activities in recent years, the scope of underwater exploration has expanded rapidly, and there is an urgent need for high-speed, safe and reliable underwater communication technology to meet the huge data transmission tasks in multiple scenarios. As the main underwater communication means at present, underwater acoustic communication has a communication distance of tens or hundreds of kilometers, but the low-frequency characteristics of sound waves limit the bandwidth of underwater acoustic communication, making its communication rate incomparable with underwater optical communication.

[0008] The technical scheme is as follows: an underwater optical communication method based on vortex beam multiplexing, using the orthogonality of vortex light modes, to conduct underwater photon counting communication experiments based on multi-channel vortex beam multiplexing, using multiple spatial light modulators and vortex wave plates to modulate and demodulate vortex beams respectively, using photon counting technology, using FPGA to count photon count values ​​in real time and store them in a fifo cache module, a serial port sending module to upload the photon count data in the fifo to a PC, and the PC completes demodulation and decoding; using the modulation method of the vortex beam to complete multi-channel parallel information transmission in a simulated seawater channel, specifically including the following steps:

[0009] S1, using the transmitting end optical system to transmit the time reference, cyclically encode the periodically emitted binary laser pulse raw data, and multiple lasers of different bands transmit pulse signals according to OOK modulation to generate multiple vortex beams;

[0010] S2, sending the generated multi-path vortex light beams to the receiving end optical system through the water channel;

[0011] S3, using the receiving end optical system to receive the time reference, demodulate the multi-channel vortex light beams, the multi-channel single-photon detectors respectively receive photons, count the photon counts of multiple channels with different bits, restore the laser pulse data, and complete the data demodulation and decoding of the entire communication link, as well as parallel transmission.

[0012] In step S1, multiple lasers of different wavelength bands transmit pulse signals according to OOK modulation, including:

[0013] The FPGA of the control circuit generates two 10MHz electrical signals, which are converted by a phase-locked loop to output pulse signals of set frequency and duty cycle. After waveform shaping and voltage conversion by the voltage comparator, they are output to four lasers respectively, and pulse signals are sent cyclically in a binary data sequence. The data is sent frame by frame. One frame of data contains a frame header sequence, a frame estimation sequence and a data sequence. In the frame header sequence, the first 20 bits "10101010101010101010" are used for clock synchronization, and the last 8 bits are used for frame synchronization. "10101010" is used for frame synchronization. The frame estimation sequence is used to estimate the average photon counts of bit 1 and bit 0. When OOK modulation is used, the data sequence sends 32256 bits.

[0014] The cyclically transmitted pulse signal is converted into an electrical pulse signal by using OOK modulation technology.

[0015] In step S1, the OOK modulation adopts the on-off keying modulation technology in optical communication to transmit information by the presence or absence of light. Light emission indicates the transmission of bit 1, and no light emission indicates the transmission of bit 0. One bit corresponds to one time slot, and the time slot width is the pulse period.

[0016] In step S1, a multi-path vortex beam is generated, including:

[0017] For the light beam 1 and light beam 2 formed by converting the binary data sequence into an electrical pulse signal, a spatial light modulator is used to modulate the phase according to an externally loaded phase map, and the corresponding diffraction phase map is calculated using Matlab programming;

[0018] The generated phase image is loaded into the SLM to realize light field wavefront modulation, generating vortex beams with topological charges of +3 and -3 respectively.

[0019] Furthermore, the control circuit synchronizes the switching of the laser and adjusts the parameters of the vortex beam by changing the parameters of the phase diagram loaded on the spatial light modulator.

[0020] Furthermore, in the generation of multi-path vortex beams, the lasers emitted by multiple lasers of different wavelength bands are respectively shaped by single-mode optical fibers, and the laser beams output by the optical fiber collimator are TEM 00 In this model, a polarizing prism is used as a polarizer to adjust the polarization direction of the light beam to horizontal, which meets the requirement of the spatial light modulator to modulate the horizontally polarized light beam. Subsequently, a dichroic mirror and a reflector are used to multiplex the two vortex light beams into one coaxially transmitted vortex light beam, and an attenuator is used to adjust the transmitted light power to simulate channel loss.

[0021] In step S3, demodulating the multi-path vortex light beams includes:

[0022] The received light beams are converged using a beam expander, and the multiplexed vortex beams are converted from linear polarization to circular polarization using a quarter-wave plate. The beams are split into two beams at a 1:1 ratio by a beam splitter and then demultiplexed using a dichroic mirror.

[0023] In step S3, the receiving end optical system receives and demodulates the photon counting signal, and after passing through the optical fiber collimator of the receiving end optical system, it is detected by the single photon detector, and the single photon detector outputs an electrical signal, which is sent to the receiving end FPGA of the computer 2. The receiving end FPGA acts as a main controller to process the single photon pulse received by the single photon detector. First, clock synchronization is performed to establish a common time frame for both ends of the transmitting end optical system and the receiving end optical system. Next, frame synchronization operation is performed to determine the starting position of each frame, and then the photon counting value corresponding to the statistical bit is counted. The photon counting value is counted in real time and stored in the fifo, and the photon counting data in the fifo is uploaded to the computer 2 through the serial port, and the experimental data is divided into groups of data, each group of data corresponds to a frame of bit sequence, and then the bit 1 and 0 are distinguished by the set decision threshold to complete the communication experiment;

[0024] The decision threshold is the decision on the symbols 0 and 1. The process of photon accumulation contains signals and noise. The characteristics of noise photons appear randomly in the time domain. The optimal decision threshold is calculated based on the photon counting statistics of bits 1 and 0. The 01 decision threshold is given by the formula:

[0025] ;

[0026] In the formula, is the average photon count for bit 1, is the average photon count for bit 0.

[0027] Another object of the present invention is to provide an underwater optical communication system based on vortex beam multiplexing, which is implemented by the underwater optical communication method based on vortex beam multiplexing, and the system comprises:

[0028] The transmitting end optical system is used to cyclically encode the original data of the periodically emitted binary laser pulses, and multiple lasers of different wavelengths emit pulse signals according to OOK modulation to generate multiple vortex beams;

[0029] A water channel, used for sending the generated multi-path vortex light beam to a receiving end optical system;

[0030] The receiving end optical system is used to demodulate the multi-channel vortex light beams, respectively receive photons from multi-channel single-photon detectors, count the photon counts of multiple channels with different bits, recover the laser pulse data, and complete the data demodulation and decoding of the entire communication link, as well as parallel transmission.

[0031] Further, the optical system at the transmitting end controls the first laser and the second laser to periodically emit laser pulses through the control circuit carried by the computer 1 through the FPGA contained therein, the light emitted by the first laser is shaped by the single-mode optical fiber and then output by the first optical fiber collimator, the first polarizer as a polarizer adjusts the polarization direction of the emitted light beam to be horizontal, the light emitted by the second laser is shaped by the single-mode optical fiber, only the basic mode of the optical fiber is retained, and then output by the second optical fiber collimator, the second polarizer as a polarizer adjusts the polarization direction of the emitted light beam to be horizontal, the horizontally polarized light beam passes through the first spatial filtering system and the second spatial filtering system composed of two lenses and an aperture, the light beam is expanded to the light waist diameter and the noise is filtered out at the same time, and the light beam is combined by the first dichroic mirror and the first reflector, the light beam continues to be transmitted from the first beam splitter, and a fork grating is loaded on the first spatial light modulator, and after the light beam is modulated, a vortex light beam with a topological charge of +3 is generated;

[0032] The control circuit controls the third laser and the fourth laser to periodically emit laser pulses through the FPGA contained inside. The light emitted by the third laser is output by the third fiber collimator after being shaped by the single-mode fiber. The third polarizer as a polarizer adjusts the polarization direction of the emitted light beam to be horizontal. The light emitted by the fourth laser is output by the fourth fiber collimator after being shaped by the single-mode fiber. The fourth polarizer as a polarizer adjusts the polarization direction of the emitted light beam to be horizontal. The horizontally polarized light beam passes through the third spatial filtering system and the fourth spatial filtering system composed of two lenses and an aperture, and the light beam is expanded to the light waist diameter while filtering out noise. The light beam is combined by the second dichroic mirror and the second reflector. After the light beam continues to be transmitted from the third beam splitter, a fork grating is loaded on the second spatial light modulator. After the light beam is modulated, a vortex light beam with a topological charge of -3 is generated.

[0033] The vortex beam with a topological charge of +3 is output as beam 1 through the first beam splitter and the second beam splitter, and the vortex beam with a topological charge of -3 is output as beam 2 through the third beam splitter and the third reflector. Beam 1 and beam 2 are combined through the second beam splitter, and after the combination, they are emitted into the water channel through the attenuation plate;

[0034] The beam expander composed of lenses in the optical system of the receiving end collects the optical signal. The optical pulse signal passes through the quarter wave plate and reaches the beam splitter. The beam splitter divides the light beam into two beams. One beam passes through the third dichroic mirror to divide the light beam into two beams, and the other beam passes through the fourth reflector and the fourth dichroic mirror to divide the light beam into two beams, a total of four beams, and passes through the first vortex wave plate, the third vortex wave plate, the second vortex wave plate, and the center of the fourth vortex wave plate with topological charges of +3 and -3 respectively, and passes through the first aperture, the third aperture, the second aperture, and the fourth aperture respectively. After that, it is transmitted for a distance in the free space, and the vortex light beam is transformed into a form with a bright spot at the center and concentric light rings around it. The fifth optical fiber collimator, the sixth optical fiber collimator, the seventh optical fiber collimator, and the eighth optical fiber collimator are used to couple the center of the bright spot, and the first single photon detector, the second single photon detector, the third single photon detector, and the fourth single photon detector are used for detection, and the detection data is sent to the receiving end FPGA of computer two.

[0035] Combining all the above technical solutions, the beneficial effects of the present invention are as follows: the present invention adopts a highly sensitive single-photon detector, which has a shorter dead time, reduces the risk of effective information loss, and effectively improves the detection accuracy of optical signals to the single-photon level. While maintaining the intensity of the emitted light pulse unchanged, the effective distance of underwater optical communication is successfully extended. The traditional vortex beam modulation method and the corresponding error correction scheme are used to ensure that information can be effectively transmitted.

[0036] In recent years, with the rapid increase in marine development activities, the scope of underwater exploration has expanded rapidly, and there is an urgent need for high-speed, safe, and reliable underwater communication technology to meet the huge data transmission tasks in multiple scenarios. As the main underwater communication method, underwater acoustic communication can reach a communication distance of tens or hundreds of kilometers, but the low-frequency characteristics of sound waves limit the bandwidth of underwater acoustic communication, making its communication rate incomparable to underwater optical communication. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The accompanying drawings herein are incorporated in and constitute a part of the specification, illustrate embodiments consistent with the present disclosure, and together with the description, serve to explain the principles of the present disclosure;

[0038] Figure 1 is a schematic diagram of an underwater optical communication system based on vortex beam multiplexing provided by an embodiment of the present invention;

[0039] Figure 2 is a flow chart of an underwater optical communication method based on vortex beam multiplexing provided by an embodiment of the present invention;

[0040] Figure 3 is a schematic diagram of an underwater optical communication method based on vortex beam multiplexing provided by an embodiment of the present invention;

[0041] In the figure: 0, control circuit; 1, receiving end FPGA; 2, first laser; 3, second laser; 4, third laser; 5, fourth laser; 6, first fiber collimator; 7, second fiber collimator; 8, third fiber collimator; 9, fourth fiber collimator; 10, first polarizer; 11, second polarizer; 12, third polarizer; 13, fourth polarizer; 14, first spatial filtering system; 15, second spatial filtering system; 16, third spatial filtering system; 17, fourth spatial filtering system; 18, first dichroic mirror; 19, first reflector; 20, second reflector; 21, second dichroic mirror; 22, third reflector; 23, third beam splitter; 24, first beam splitter; 25, second beam splitter; 26 , the second spatial light modulator; 27, the first spatial light modulator; 28, the attenuation plate; 29, the water channel; 30, the beam expander; 31, the quarter wave plate; 32, the beam splitter; 33, the fourth reflector; 34, the third dichroic mirror; 35, the fourth dichroic mirror; 36, the first vortex wave plate; 37, the second vortex wave plate; 38, the third vortex wave plate; 39, the fourth vortex wave plate; 40, the first aperture; 41, the second aperture; 42, the third aperture; 43, the fourth aperture; 44, the fifth fiber collimator; 45, the sixth fiber collimator; 46, the seventh fiber collimator; 47, the eighth fiber collimator; 48, the first single-photon detector; 49, the second single-photon detector; 50, the third single-photon detector; 51, the fourth single-photon detector. DETAILED DESCRIPTION

[0042] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention, so the present invention is not limited by the specific implementation disclosed below.

[0043] The innovation of the present invention is that the present invention utilizes the orthogonality of vortex light modes to design an underwater optical communication method and system using vortex light beam multiplexing modulation, wherein different vortex light beams and different wavelengths are used to improve the communication bandwidth, and single-photon detection technology is used to complete multi-channel parallel information transmission in a simulated seawater channel. The present invention is expected to effectively improve the detection accuracy of optical signals to the single-photon level. Moreover, while maintaining the intensity of the emitted light pulse unchanged, the effective distance of underwater optical communication is successfully extended as much as possible.

[0044] Embodiment 1, as Figure 1 As shown, an underwater optical communication method based on vortex beam multiplexing utilizes the orthogonality of vortex light modes to conduct underwater photon counting communication experiments based on multi-channel vortex beam multiplexing, respectively using multiple spatial light modulators and vortex wave plates to modulate and demodulate vortex beams, using photon counting detection, using FPGA programming for signal processing, and applying the modulation method of vortex beams to complete multi-channel parallel information transmission in a simulated seawater channel, specifically including the following steps:

[0045] S1, using the transmitting end optical system to transmit the time reference, cyclically encode the periodically emitted binary laser pulse raw data, and multiple lasers of different bands transmit pulse signals according to OOK modulation to generate multiple vortex beams;

[0046] S2, sending the generated multi-path vortex light beams to the receiving end optical system through the water channel 29;

[0047] S3, using the receiving end optical system to receive the time reference, demodulate the multi-channel vortex light beams, the multi-channel single-photon detectors respectively receive photons, count the photon counts of multiple channels with different bits, restore the laser pulse data, and complete the data demodulation and decoding of the entire communication link, as well as parallel transmission.

[0048] Embodiment 2, as Figure 2 As shown, an underwater optical communication system based on vortex beam multiplexing provided by an embodiment of the present invention includes a transceiver circuit, a transmitting end optical system (modulation optical path), and a receiving end optical system (demodulation optical path).

[0049] The transmitting end optical system comprises a control circuit 0 (including FPGA), a first laser 2, a second laser 3, a third laser 4, a fourth laser 5, a first fiber collimator 6, a second fiber collimator 7, a third fiber collimator 8, a fourth fiber collimator 9, a first polarizer 10, a second polarizer 11, a third polarizer 12, a fourth polarizer 13, a first spatial filter system 14, a second spatial filter system 15, a third spatial filter system 16, a fourth spatial filter system 17, a first dichroic mirror 18, a first reflector 19, a second reflector 20, a second dichroic mirror 21, a third reflector 22, a third beam splitter 23, a first beam splitter 24, a second beam splitter 25, a second spatial light modulator (SLM) 26, a first spatial light modulator (SLM) 27, and an attenuation plate 28; the control circuit 0 is mounted on a computer 1;

[0050] The receiving end optical system includes a beam expander 30, a quarter wave plate 31 (QWP), a beam splitter 32, a fourth reflector 33, a third dichroic mirror 34, a fourth dichroic mirror 35, a first vortex wave plate 36, a second vortex wave plate 37, a third vortex wave plate 38, a fourth vortex wave plate 39, a first aperture 40, a second aperture 41, a third aperture 42, a fourth aperture 43, a fifth fiber collimator 44, a sixth fiber collimator 45, a seventh fiber collimator 46, an eighth fiber collimator 47, a first single photon detector 48, a second single photon detector 49, a third single photon detector 50, a fourth single photon detector 51, and a receiving end FPGA 1; the receiving end FPGA 1 is mounted on computer 2.

[0051] The control circuit 0 carried by the computer 1 controls the first laser 2 and the second laser 3 to periodically emit laser pulses through the FPGA contained therein. The light emitted by the first laser 2 is shaped by the single-mode optical fiber and then output by the first optical fiber collimator 6. The first polarizer 10, which is a polarizer, adjusts the polarization direction of the emitted light beam to be horizontal. The light emitted by the second laser 3 is shaped by the single-mode optical fiber, that is, other modes are filtered out, and only the basic mode of the optical fiber is retained. Then, it is output by the second optical fiber collimator 7. The second polarizer 11, which is a polarizer, adjusts the polarization direction of the emitted light beam to be horizontal. The horizontally polarized light beam passes through the first spatial filtering system 14 and the second spatial filtering system 15 composed of two lenses and an aperture, and the light beam is expanded to the light waist diameter and the noise is filtered out at the same time. The light beam is combined by the first dichroic mirror 18 and the first reflector 19. After the light beam continues to pass through the first beam splitter 24, a fork grating is loaded on the first spatial light modulator (SLM) 27. After the light beam is modulated by the SLM, a vortex light beam with a topological charge of +3 is generated.

[0052] The control circuit 0 carried by the computer 1 controls the third laser 4 and the fourth laser 5 to periodically emit laser pulses through the FPGA contained therein. The light emitted by the third laser 4 is output by the third fiber collimator 8 after being shaped by the single-mode fiber. The third polarizer 12, which serves as a polarizer, adjusts the polarization direction of the emitted light beam to be horizontal. The light emitted by the fourth laser 5 is output by the fourth fiber collimator 9 after being shaped by the single-mode fiber. The fourth polarizer 13, which serves as a polarizer, adjusts the polarization direction of the emitted light beam to be horizontal. The horizontally polarized light beam passes through the third spatial filter system 16 and the fourth spatial filter system 17 composed of two lenses and an aperture, expands the light beam to the light waist diameter and filters out noise at the same time, and is combined by the second dichroic mirror 21 and the second reflector 20. After the light beam continues to be transmitted through the third beam splitter 23, a fork grating is loaded on the second spatial light modulator (SLM) 26. After the light beam is modulated by the SLM, a vortex light beam with a topological charge of -3 is generated.

[0053] The vortex beam with a topological charge of +3 is output as beam 1 through the first beam splitter 24 and the second beam splitter 25, and the vortex beam with a topological charge of -3 is output as beam 2 through the third beam splitter 23 and the third reflector 22. The beam 1 and the beam 2 are combined through the second beam splitter 25, and after the combination, they are emitted into the water channel 29 through the attenuation plate 28;

[0054] The beam expander 30 composed of lenses in the optical system of the receiving end collects the optical signal. The optical pulse signal passes through the quarter wave plate 31 and reaches the beam splitter 32. The beam splitter 32 splits the light beam into two beams. One beam passes through the third dichroic mirror 34 to split the light beam into two beams. The other beam passes through the fourth reflector 33 and the fourth dichroic mirror 35 to split the light beam into two beams, a total of four beams, and passes through the first vortex wave plate 36, the third vortex wave plate 38, the second vortex wave plate 37, and the fourth vortex wave plate 39 with topological charges of +3 and -3 respectively. And respectively pass through the first aperture 40, the third aperture 42, the second aperture 41, and the fourth aperture 43, and then transmit a certain distance in the free space. It is observed that the vortex light beam is transformed into a shape with a bright spot at the center and concentric light rings around it. The center of the bright spot is coupled with the fifth fiber optic collimator 44, the sixth fiber optic collimator 45, the seventh fiber optic collimator 46, and the eighth fiber optic collimator 47, and is detected with the first single-photon detector 48, the second single-photon detector 49, the third single-photon detector 50, and the fourth single-photon detector 51.

[0055] Embodiment 3, the underwater optical communication method based on vortex beam multiplexing provided by the embodiment of the present invention includes photon counting signal processing and time synchronization between the transmitting end and the receiving end; specifically includes:

[0056] Step 1: The optical system at the transmitting end first cyclically transmits the binary data sequence to be transmitted, and converts the binary data sequence into an electrical pulse signal by adopting OOK modulation technology; the control circuit 0 (including FPGA) is responsible for controlling the laser to cyclically transmit the pulse signal at a specific frequency;

[0057] Step 2: In the underwater optical communication system, the optical system at the receiving end uses a single-photon detector to receive the received pulse signal. In order to correctly restore the information, it is stored through the receiving end FPGA 1 and uploaded to the computer 2. The received signal is precisely processed to determine the starting position of each frame, that is, to achieve time synchronization of the communication experiment between the transmitting end optical system and the receiving end optical system.

[0058] Step 3: After time synchronization is completed, the communication experiment officially begins, accurately converting the optical signal processed in the above steps into original binary data, completing the data transmission and interpretation of the entire communication link. Due to the high-quality vortex beam generation, precise optical path alignment and clearer water quality, the vortex beam still maintains a good morphological structure after reaching the receiving end. Photon counting communication further breaks through the communication distance due to its high-sensitivity detection advantage, tolerates greater link loss, and applies vortex beam multiplexing to underwater photon counting communication, achieving high-speed, reliable and stable communication under greater channel loss.

[0059] Exemplarily, the control circuit 0 performs brightness control on the first laser 2 and the second laser 3 in the optical system at the transmitting end.

[0060] The brightness control uses the on-off keying modulation technology in optical communication to transmit information through the presence or absence of light. Lighting indicates the transmission of a bit "1", and no light indicates the transmission of a bit "0". One bit corresponds to one time slot, and the time slot width, that is, the pulse period, is determined by the modulation bandwidth.

[0061] The control circuit 0 includes an FPGA (field programmable gate array) development board and a voltage comparator;

[0062] Computer 1 and Computer 2 perform overall control and coordinate the operation of the transmitting end optical system and the receiving end optical system. Control circuit 0 is responsible for synchronizing the switch of the laser and adjusting the parameters of the vortex beam, that is, adjusting the parameters of the vortex beam by changing the parameters of the phase map loaded on the SLM;

[0063] The vortex beam is adjusted by a spatial light modulator (SLM), which can achieve pure phase modulation. The spatial light modulator SLM needs to modulate the phase according to the externally loaded phase diagram, and the corresponding diffraction phase diagram is calculated using Matlab programming;

[0064] The first laser 2 and the third laser 4 use a wavelength of 450nm, and the second laser 3 and the fourth laser 5 use a wavelength of 520nm, and are all controlled by the FPGA of the control circuit 0 to periodically emit laser pulses;

[0065] For example, in the entire communication link, the clock synchronization of the transmitting optical system and the receiving optical system cooperate with each other to ensure the accurate transmission and analysis of information. At present, the laboratory has realized the clock synchronization technology matching FPGA. The innovation of the present invention focuses on the optical path. The transmitting optical system is increased to four paths, and lasers of different bands are used.

[0066] Among them, single-mode fiber is used to shape the beam emitted by the laser;

[0067] The FPGA of the control circuit 0 generates two 10 MHz electrical signals, which are respectively output to four lasers for direct modulation after waveform shaping and voltage conversion by the voltage comparator.

[0068] The spatial filtering system includes two lenses and an aperture, which performs spatial filtering and beam expansion. The aperture blocks most of the high-frequency noise and optimizes the light spot quality.

[0069] The spatial light modulator (SLM) is used to adjust the light beam 1 and the light beam 2. A fork-shaped grating is loaded on the spatial light modulator (SLM) to generate vortex light beams with topological charges of +3 and -3 respectively.

[0070] The lasers emitted by the first laser 2 and the second laser 23, the lasers emitted by the third laser 4 and the fourth laser 5 are respectively subjected to mode shaping by the single-mode optical fiber, and the laser beams outputted by the optical fiber collimator are TEM 00 The polarizing prism is used as a polarizer to adjust the polarization direction of the light beam to horizontal, so as to meet the requirement of the spatial light modulator (SLM) to modulate the horizontal polarization direction light beam. Then, the two vortex beams are multiplexed into one coaxially transmitted vortex beam (OAM beam) using a dichroic mirror and a reflector. The attenuator is used to adjust the transmitted light power and simulate the channel loss.

[0071] In the receiving end optical system, the beam expander 30 includes two lenses for improving the light receiving efficiency;

[0072] The orders of the first vortex wave plate 36 and the second vortex wave plate 37 are q=3 / 2 and q=-3 / 2 respectively, and are used to devortex the two beams after vortex multiplexing;

[0073] Single photon detector (PMT), output pulse width is 10ns, pulse pair resolution is 20ns. Used to collect optical signals;

[0074] The receiving end optical system first uses a beam expander 30 to converge the received light beam, and converts the multiplexed vortex light beam (OAM) light beam from linear polarization to circular polarization through a quarter wave plate (QWP) 31. The light beam is split into two beams at a ratio of 1:1 by a beam splitter 32, and then becomes four beams through a dichroic mirror (including a third dichroic mirror 34 and a fourth dichroic mirror 35). The first vortex wave plate 36, the third vortex wave plate 38, the second vortex wave plate 37, and the fourth vortex wave plate 39 are used to realize demultiplexing respectively;

[0075] After passing through the optical fiber collimator of the receiving optical system, the single photon detector detects it, and the single photon detector outputs an electrical signal and sends it to the receiving FPGA 1 (chip) of Computer 2. FPGA 1 at the receiving end acts as the main controller to process the single photon pulse received by the single photon detector (SPD). First, clock synchronization is performed to establish a common time frame for both the transmitting optical system and the receiving optical system. Next, frame synchronization is performed to determine the starting position of each frame, and then the photon count value corresponding to the statistical bit is counted. The receiving optical system can only count correctly if the starting position of each pulse cycle is accurately known. FPGA 1 at the receiving end counts the photon count value in real time and stores it in the fifo. The photon count data in the fifo is uploaded to Computer 2 through the serial port, and Computer 2 completes the demodulation and decoding.

[0076] The transmitting end optical system and the receiving end optical system of the present invention are applied to the communication between underwater devices;

[0077] The communication between the underwater devices is the communication between the underwater sensors.

[0078] For example, in communication, in order to convert the pulse signal output by the single-photon detector into correct information, the signal needs to be stored through the programming of FPGA 1. After uploading to computer 2, a large number of experimental data are divided into groups of data, each group of data corresponds to a frame of bit sequence, and then the bit 1 and 0 are distinguished by the set decision threshold to complete the communication experiment;

[0079] The decision threshold is the decision on the symbols 0 and 1. The process of photon accumulation contains signals and noise. The characteristic of noise photons is that they appear randomly in the time domain, and the performance of signal light is related in the time domain. The optimal decision threshold is calculated based on the photon counting statistics of bits 1 and 0;

[0080] The bits 1 and 0 are information to be sent in the optical communication experiment;

[0081] The photon counting is that the single photon detector transmits the received pulse photon signal to FPGA 1, and the fifo in FPGA 1 stores the accumulated photon signal;

[0082] The original information of the optical system at the transmitting end is binary data. The data sequence is circulated and converted into an electrical pulse signal through OOK modulation. The FPGA of the control circuit 0 outputs a pulse signal with a certain frequency and duty cycle after frequency conversion through a phase-locked loop. After passing through a comparator, it is connected to the laser (the first laser 2, the second laser 3, the third laser 4, and the fourth laser 5), thereby controlling the laser to circulate and send pulse signals at a specific frequency.

[0083] After receiving the photon counting signal output by the single photon detector SPD, the receiving end FPGA 1 performs clock synchronization to establish a common time frame for both ends of the transmitting end optical system and the receiving end optical system;

[0084] The OOK modulation is a modulation method in optical communication. OOK modulation, namely on-off keying modulation, transmits information by the presence or absence of light. Light emission indicates the transmission of bit 1, and no light emission indicates the transmission of bit 0. One bit corresponds to one time slot, and the time slot width is also the pulse period.

[0085] The clock synchronization is to keep the arrival time of the rising edge of the crystal oscillator at both the transmitting end and the receiving end consistent;

[0086] The present invention utilizes the orthogonality of vortex light modes, analyzes underwater photon counting communication experiments based on dual-path vortex beam multiplexing, respectively uses spatial light modulator (SLM) and vortex wave plate to modulate and demodulate vortex beams, uses photon counting detection, uses FPGA programming for signal processing, and uses the modulation method of vortex beams to realize four-channel parallel information transmission in a simulated seawater channel. Figure 3 shown.

[0087] In the above embodiments, the description of each embodiment has its own emphasis. For parts that are not described or recorded in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0088] Since the information interaction, execution process, etc. between the above-mentioned devices / units are based on the same concept as the method embodiment of the present invention, their specific functions and technical effects can be found in the method embodiment part and will not be repeated here.

[0089] Those skilled in the art can clearly understand that for the convenience and simplicity of description, only the division of the above-mentioned functional units and modules is used as an example for illustration. In practical applications, the above-mentioned function allocation can be completed by different functional units and modules as needed, that is, the internal structure of the device can be divided into different functional units or modules to complete all or part of the functions described above. The functional units and modules in the embodiments can be integrated in one processing unit, or each unit can exist physically separately, or two or more units can be integrated in one unit. The above-mentioned integrated unit can be implemented in the form of hardware or in the form of software functional units. In addition, the specific names of the functional units and modules are only for the convenience of distinguishing each other, and are not used to limit the scope of protection of the present invention. The specific working process of the units and modules in the above-mentioned system can refer to the corresponding process in the aforementioned method embodiment.

[0090] An embodiment of the present invention further provides a computer device, which includes: at least one processor, a memory, and a computer program stored in the memory and executable on the at least one processor, wherein the processor implements the steps in any of the above-mentioned method embodiments when executing the computer program.

[0091] An embodiment of the present invention further provides a computer-readable storage medium, wherein the computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the steps in the above-mentioned method embodiments can be implemented.

[0092] An embodiment of the present invention further provides an information data processing terminal, which, when executed on an electronic device, provides a user input interface to implement the steps in the above method embodiments. The information data processing terminal is not limited to mobile phones, computers, and switches.

[0093] An embodiment of the present invention further provides a server, which is used to provide a user input interface to implement the steps in the above method embodiments when executed on an electronic device.

[0094] If the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the present application implements all or part of the processes in the above-mentioned embodiment method, which can be completed by instructing the relevant hardware through a computer program. The computer program can be stored in a computer-readable storage medium, and the computer program can implement the steps of the above-mentioned method embodiments when executed by the processor. Among them, the computer program includes computer program code, and the computer program code can be in source code form, object code form, executable file or some intermediate form. The computer-readable medium may at least include: any entity or device that can carry the computer program code to the camera device / terminal device, recording medium, computer memory, read-only memory (ROM), random access memory (RAM), electrical carrier signal, telecommunication signal and software distribution medium. For example, a USB flash drive, a mobile hard disk, a disk or an optical disk.

[0095] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with the technical field within the technical scope disclosed by the present invention and within the spirit and principles of the present invention should be covered within the protection scope of the present invention.

Claims

1. An underwater optical communication method based on vortex beam multiplexing, characterized in that: The method uses the orthogonality of vortex light modes to conduct underwater photon counting communication experiments based on multi-channel vortex beam multiplexing. Multiple spatial light modulators and vortex wave plates are used to modulate and demodulate vortex beams respectively. Photon counting technology is used to use FPGA to count photon count values ​​in real time and store them in a fifo cache module. The serial port sending module uploads the photon counting data in the fifo to a PC, which completes demodulation and decoding. The modulation method of the vortex beam is used to complete multi-channel parallel information transmission in a simulated seawater channel. The method specifically includes the following steps: S1, using the transmitting end optical system to transmit the time reference, cyclically encode the periodically emitted binary laser pulse raw data, and multiple lasers of different bands transmit pulse signals according to OOK modulation to generate multiple vortex beams; S2, sending the generated multi-path vortex light beam to the receiving end optical system through the water channel (29); S3, using the receiving end optical system to receive the time reference, demodulate the multi-channel vortex light beams, the multi-channel single-photon detectors respectively receive photons, count the photon counts of different bits of multiple channels, and restore the laser pulse data to complete the data demodulation and decoding of the entire communication link, as well as parallel transmission; In step S1, the optical system at the transmitting end controls the first laser (2) and the second laser (3) to periodically emit laser pulses through the FPGA contained in the control circuit (0) carried by the computer 1. The light emitted by the first laser (2) is shaped by the single-mode optical fiber and then output by the first optical fiber collimator (6). The first polarizer (10) as a polarizer adjusts the polarization direction of the emitted light beam to be horizontal. The light emitted by the second laser (3) is shaped by the single-mode optical fiber, and only the basic mode of the optical fiber is retained. Then, the light is output by the second optical fiber collimator (7) as the starting point. The second polarizer (11) of the polarizer adjusts the polarization direction of the emitted light beam to be horizontal, and the horizontally polarized light beam passes through a first spatial filter system (14) and a second spatial filter system (15) composed of two lenses and an aperture, respectively, to expand the light beam to the light waist diameter and filter out noise, and then is combined by a first dichroic mirror (18) and a first reflector (19). After the light beam continues to be transmitted through the first beam splitter (24), a fork grating is loaded on the first spatial light modulator (27), and after the light beam is modulated, a vortex light beam with a topological charge of +3 is generated; The control circuit (0) controls the third laser (4) and the fourth laser (5) to periodically emit laser pulses through the FPGA contained therein. The light emitted by the third laser (4) is shaped by the single-mode optical fiber and then output by the third optical fiber collimator (8). The third polarizer (12) as a polarizer adjusts the polarization direction of the emitted light beam to be horizontal. The light emitted by the fourth laser (5) is shaped by the single-mode optical fiber and then output by the fourth optical fiber collimator (9). The fourth polarizer (13) as a polarizer adjusts the polarization direction of the emitted light beam to be horizontal. The horizontally polarized light beam passes through a third spatial filter system (16) and a fourth spatial filter system (17) composed of two lenses and an aperture, respectively, to expand the light beam to a light waist diameter and filter out noise. The light beam is combined by a second dichroic mirror (21) and a second reflector (20). After the light beam continues to pass through the third beam splitter (23), a fork grating is loaded on the second spatial light modulator (26). After the light beam is modulated, a vortex light beam with a topological charge of -3 is generated. A vortex light beam with a topological charge of +3 is output as a light beam 1 through a first beam splitter (24) and a second beam splitter (25), and a vortex light beam with a topological charge of -3 is output as a light beam 2 through a third beam splitter (23) and a third reflector (22). The light beam 1 and the light beam 2 are combined through the second beam splitter (25), and after the combined light beams are emitted into a water channel (29) through an attenuation plate (28); In step S3, a beam expander (30) composed of lenses in the optical system of the receiving end collects the optical signal, and the optical pulse signal passes through a quarter wave plate (31) to reach a beam splitter (32), and the beam splitter (32) splits the light beam into two beams, one beam passes through a third dichroic mirror (34) to split the light beam into two beams, and the other beam passes through a fourth reflector (33) and a fourth dichroic mirror (35) to split the light beam into two beams, a total of four beams, and respectively pass through the center of a first vortex wave plate (36), a third vortex wave plate (38), a second vortex wave plate (37), and a fourth vortex wave plate (39) with topological charges of +3 and -3; and respectively pass through a first aperture (40), a third aperture (42), a second aperture (41), and a fourth aperture (43), and then transmit a certain distance in free space, and the vortex light beams are respectively converted into a form with a bright spot at the center and concentric light rings around it; The vortex light beam is converted into a light beam having a bright spot at the center and concentric light rings around it by passing through the first aperture (40), and the center of the bright spot is coupled with a fifth optical fiber collimator (44), and the light beam is detected by a first single-photon detector (48), and the detection data is sent to a receiving end FPGA (1) of a second computer; The vortex light beam is converted into a light beam having a bright spot at the center and concentric light rings around it by passing through the second aperture (41), and the center of the bright spot is coupled with a sixth optical fiber collimator (45), and a second single-photon detector (49) is used for detection, and the detection data is sent to the receiving end FPGA (1) of the second computer; The vortex light beam is transformed into a light beam having a bright spot at the center and concentric light rings around it by passing through the third aperture (42), and the bright spot center is coupled by a seventh optical fiber collimator (46), and the light beam is detected by a third single-photon detector (50), and the detection data is sent to a receiving end FPGA (1) of the second computer; The vortex light beam is converted into a light beam with a bright spot at the center and concentric light rings around it by passing through the fourth aperture (43). The center of the bright spot is coupled with an eighth optical fiber collimator (47), and detected by a fourth single-photon detector (51), and the detection data is sent to the receiving end FPGA (1) of the second computer.

2. The underwater optical communication method based on vortex beam multiplexing according to claim 1, characterized in that: In step S1, multiple lasers of different wavelength bands transmit pulse signals according to OOK modulation, including: The FPGA of the control circuit (0) generates two 10MHz electrical signals, and outputs pulse signals of set frequency and duty cycle after frequency conversion by a phase-locked loop. After waveform shaping and voltage conversion by a voltage comparator, the signals are output to four lasers respectively, and the pulse signals are cyclically sent in a binary data sequence. The data is sent frame by frame. One frame of data includes a frame header sequence, a frame estimation sequence and a data sequence. In the frame header sequence, the first 20 bits "10101010101010101010" are used for clock synchronization, and the last 8 bits "10101010" are used for frame synchronization. The frame estimation sequence is used to estimate the average photon count of bit 1 and bit 0. When OOK modulation is used, the data sequence sends 32256 bits. The cyclically transmitted pulse signal is converted into an electrical pulse signal by using OOK modulation technology.

3. The underwater optical communication method based on vortex beam multiplexing according to claim 1, characterized in that: In step S1, the OOK modulation adopts the on-off keying modulation technology in optical communication to transmit information by the presence or absence of light. Light emission indicates the transmission of bit 1, and no light emission indicates the transmission of bit 0. One bit corresponds to one time slot, and the time slot width is the pulse period.

4. The underwater optical communication method based on vortex beam multiplexing according to claim 1, characterized in that: In step S1, a multi-path vortex beam is generated, including: For the light beam 1 and light beam 2 formed by converting the binary data sequence into an electrical pulse signal, a spatial light modulator is used to modulate the phase according to an externally loaded phase map, and the corresponding diffraction phase map is calculated using Matlab programming; The generated phase map of the modulation phase is loaded into the spatial light modulator SLM to realize light field wavefront modulation, generating vortex beams with topological charges of +3 and -3 respectively.

5. The underwater optical communication method based on vortex beam multiplexing according to claim 4 is characterized in that: The control circuit (0) synchronizes the switching of the laser and adjusts the parameters of the vortex beam by changing the parameters of the phase diagram loaded on the spatial light modulator.

6. The underwater optical communication method based on vortex beam multiplexing according to claim 4, characterized in that: In the generation of multi-path vortex beams, the lasers emitted by multiple lasers of different wavelength bands are respectively shaped by single-mode optical fibers, and the laser beams output by the optical fiber collimator are TEM 00 The polarization wave plate is used as a polarizer to adjust the polarization direction of the light beam to horizontal, which meets the requirement of the spatial light modulator to modulate the horizontal polarization direction light beam. Then, the two vortex light beams are multiplexed into one coaxially transmitted vortex light beam using a dichroic mirror and a reflector, and the transmitted light power is adjusted using an attenuator to simulate channel loss.

7. The underwater optical communication method based on vortex beam multiplexing according to claim 1, characterized in that: In step S3, demodulating the multi-path vortex light beams includes: The received light beam is converged by using a beam expander (30), and the multiplexed vortex light beam is converted from linear polarization to circular polarization by using a quarter wave plate (31). The light beam is split into two beams at a ratio of 1:1 by a beam splitter (32), and then demultiplexed by using a dichroic mirror.

8. The underwater optical communication method based on vortex beam multiplexing according to claim 1, characterized in that: In step S3, the receiving end optical system receives and demodulates the photon counting signal, and after passing through the optical fiber collimator of the receiving end optical system, the photon counting signal is detected by the single photon detector, and the single photon detector outputs an electrical signal, which is sent to the receiving end FPGA (1) of the computer 2. The receiving end FPGA (1) serves as a main controller to process the single photon pulse received by the single photon detector. First, clock synchronization is performed to establish a common time frame for both ends of the transmitting end optical system and the receiving end optical system. Next, frame synchronization operation is performed to determine the starting position of each frame. After that, the photon counting value corresponding to the statistical bit is counted. The photon counting value is counted in real time and stored in a fifo. The photon counting data in the fifo is uploaded to the computer 2 through the serial port, and the experimental data is divided into groups of data, each group of data corresponds to a frame of bit sequence, and then the bit 1 and 0 are distinguished by a set decision threshold, and the communication experiment is completed. The decision threshold is the decision of the 0 and 1 symbols. The process of photon accumulation contains signals and noise. The characteristics of noise photons appear randomly in the time domain. The optimal decision threshold is calculated based on the photon count statistics of bits 1 and 0. The calculation formula for the optimal decision threshold of the photon count of bits 1 and 0 is: Where n1 is the average photon count of bit 1, and n0 is the average photon count of bit 0.

9. An underwater optical communication system based on vortex beam multiplexing, characterized in that: The system runs the underwater optical communication method based on vortex beam multiplexing according to any one of claims 1 to 8, specifically comprising: The transmitting end optical system is used to cyclically encode the original data of the periodically emitted binary laser pulses, and multiple lasers of different wavelengths emit pulse signals according to OOK modulation to generate multiple vortex beams; A water channel (29) is used to send the generated multi-path vortex light beam to a receiving end optical system; The receiving end optical system is used to demodulate the multi-channel vortex beams, receive photons respectively by multi-channel single-photon detectors, count the photon counts of different bits of multiple channels, recover the laser pulse data, and complete the data demodulation and decoding of the entire communication link, as well as parallel transmission; The optical system at the transmitting end controls the first laser (2) and the second laser (3) to periodically emit laser pulses through an FPGA contained in a control circuit (0) carried by a computer. The light emitted by the first laser (2) is shaped by a single-mode optical fiber and then output by a first optical fiber collimator (6). The first polarizer (10) as a polarizer adjusts the polarization direction of the emitted light beam to be horizontal. The light emitted by the second laser (3) is shaped by a single-mode optical fiber, and only the basic mode of the optical fiber is retained. The light is then output by a second optical fiber collimator (7) as a polarizer. The second polarizer (11) adjusts the polarization direction of the emitted light beam to be horizontal. The horizontally polarized light beam passes through a first spatial filter system (14) and a second spatial filter system (15) composed of two lenses and an aperture, respectively, to expand the light beam to a light waist diameter and filter out noise. The light beam is combined by a first dichroic mirror (18) and a first reflector (19). After the light beam continues to be transmitted through a first beam splitter (24), a fork grating is loaded on a first spatial light modulator (27). After the light beam is modulated, a vortex light beam with a topological charge of +3 is generated. The control circuit (0) controls the third laser (4) and the fourth laser (5) to periodically emit laser pulses through the FPGA contained therein. The light emitted by the third laser (4) is shaped by the single-mode optical fiber and then output by the third optical fiber collimator (8). The third polarizer (12) as a polarizer adjusts the polarization direction of the emitted light beam to be horizontal. The light emitted by the fourth laser (5) is shaped by the single-mode optical fiber and then output by the fourth optical fiber collimator (9). The fourth polarizer (13) as a polarizer adjusts the polarization direction of the emitted light beam to be horizontal. The horizontally polarized light beam passes through a third spatial filter system (16) and a fourth spatial filter system (17) composed of two lenses and an aperture, respectively, to expand the light beam to a light waist diameter and filter out noise. The light beam is combined by a second dichroic mirror (21) and a second reflector (20). After the light beam continues to pass through the third beam splitter (23), a fork grating is loaded on the second spatial light modulator (26). After the light beam is modulated, a vortex light beam with a topological charge of -3 is generated. A vortex light beam with a topological charge of +3 is output as a light beam 1 through a first beam splitter (24) and a second beam splitter (25), and a vortex light beam with a topological charge of -3 is output as a light beam 2 through a third beam splitter (23) and a third reflector (22). The light beam 1 and the light beam 2 are combined through the second beam splitter (25), and after the combined light beams are emitted into a water channel (29) through an attenuation plate (28); A beam expander (30) composed of lenses in the optical system at the receiving end collects the optical signal. The optical pulse signal passes through a quarter wave plate (31) and reaches a beam splitter (32). The beam splitter (32) splits the light beam into two beams. One beam passes through a third dichroic mirror (34) to split the light beam into two beams. The other beam passes through a fourth reflector (33) and a fourth dichroic mirror (35) to split the light beam into two beams. There are four beams in total. The beams pass through the centers of a first vortex wave plate (36), a third vortex wave plate (38), a second vortex wave plate (37), and a fourth vortex wave plate (39) with topological charges of +3 and -3 respectively; and pass through a first aperture (40), a third aperture (42), a second aperture (41), and a fourth aperture (43) respectively. After that, the beams are transmitted for a certain distance in free space. The vortex light beams are converted into a form with a bright spot at the center and concentric light rings around them. The vortex light beam is converted into a light beam having a bright spot at the center and concentric light rings around it by passing through the first aperture (40), and the center of the bright spot is coupled with a fifth optical fiber collimator (44), and the light beam is detected by a first single-photon detector (48), and the detection data is sent to a receiving end FPGA (1) of a second computer; The vortex light beam is converted into a light beam having a bright spot at the center and concentric light rings around it by passing through the second aperture (41), and the center of the bright spot is coupled with a sixth optical fiber collimator (45), and a second single-photon detector (49) is used for detection, and the detection data is sent to the receiving end FPGA (1) of the second computer; The vortex light beam is transformed into a light beam having a bright spot at the center and concentric light rings around it by passing through the third aperture (42), and the bright spot center is coupled by a seventh optical fiber collimator (46), and the light beam is detected by a third single-photon detector (50), and the detection data is sent to a receiving end FPGA (1) of the second computer; The vortex light beam is converted into a light beam with a bright spot at the center and concentric light rings around it by passing through the fourth aperture (43). The center of the bright spot is coupled with an eighth optical fiber collimator (47), and detected by a fourth single-photon detector (51), and the detection data is sent to the receiving end FPGA (1) of the second computer.

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