Underwater wireless optical communication channel simulation method based on ocean phase screen

Abstract

The invention relates to the field of underwater optical communication channel simulation, and particularly discloses an underwater wireless optical communication channel simulation method based on an ocean phase screen. The method comprises the following steps: S1, performing simulation modeling on an underwater optical communication channel based on an ocean phase screen, a dynamic turbulence simulation theory and an attenuation model; s2, establishing a semi-physical simulation experiment verification platform according to the model; s3, simulating the underwater optical communication system by adopting a Monte Carlo method, and estimating theoretical received signal bit error rates of the underwater optical communication system at different communication rates; s4, carrying out the performance verification of the underwater optical communication system based on an in-air semi-physical simulation experiment platform, and obtaining the error rate data of an experiment receiving signal; and S5, comparing a theoretical result obtained by simulation estimation with an actual result obtained by an experiment, and verifying the feasibility of the channel simulation model and the simulation experiment platform. According to the technical scheme, the limitation of a test site can be broken through, and further-distance research on communication of the underwater wireless optical communication system can be carried out.

Description

Technical Field

[0001] The invention relates to the technical field of underwater optical communication channel simulation, and in particular to an underwater wireless optical communication channel simulation method based on an ocean phase screen. Background Art

[0002] The application prospects of underwater wireless optical communication are very broad. It can be applied to many fields such as ocean exploration, marine resource development, and underwater robot control. It can improve the efficiency and accuracy of ocean development and exploration, and can also provide a more efficient control method for underwater robots and other equipment, and realize the rapid transmission of data when collecting information from underwater preset fixed devices. Therefore, underwater wireless optical communication is gradually developing into an emerging underwater communication method, which is helpful for the rapid acquisition of marine information in the future and is a new direction for people to study and explore. Due to its high carrier bandwidth and communication rate, underwater wireless optical communication has excellent application prospects in the field of underwater information transmission in the future. However, in actual research, due to the complex and changeable seawater channel environment and the difficulty of sea trial experiments, there are few studies on underwater communication systems with a channel distance of more than 50 meters. In order to solve the problem that underwater verification experiments are difficult to implement in the design process of underwater wireless optical communication systems, the study of a simulation experiment verification platform for performance analysis of underwater optical communication systems is helpful to reduce research costs, accelerate the research progress of underwater wireless optical communication systems, and solve practical application problems. Based on this, a method for simulating underwater wireless optical communication channels based on the ocean phase screen was proposed, and a semi-physical simulation experimental platform based on underwater optical channel modeling was built in the air. Summary of the invention

[0003] In view of the above-mentioned problems in the prior art, the present invention provides an underwater wireless optical communication channel simulation method based on an ocean phase screen, which can break the restrictions of the test site on underwater tests, reduce research costs, and help carry out research on longer-distance communication of underwater wireless optical communication systems.

[0004] The technical solution adopted by the present invention is as follows:

[0005] A method for simulating an underwater wireless optical communication channel based on an ocean phase screen, the steps are as follows:

[0006] S1. Based on the ocean phase screen, dynamic turbulence simulation theory and attenuation model, the underwater optical communication channel is simulated and modeled to obtain a simulation model for simulating the underwater optical communication channel in the air for experiments;

[0007] S2. Establish a semi-physical simulation experiment verification platform in the air for underwater optical communication system based on the model obtained in S1;

[0008] S3. Using the Monte Carlo method to simulate the underwater optical communication system, simulate the movement of photons in water, and estimate the theoretical received signal bit error rate of the underwater optical communication system at different communication rates;

[0009] S4. Based on the semi-physical simulation experimental platform in the air, the performance of the underwater optical communication system is verified to obtain the experimental received signal bit error rate data;

[0010] S5. Compare the theoretical results obtained by simulation estimation with the actual results obtained by experiment to verify the feasibility of the channel simulation model and the simulation experiment platform.

[0011] Preferably, in S1, the attenuation model is designed as a seawater attenuation model for light, and the simulation type is clean seawater.

[0012] Preferably, the design process of the attenuation model is to determine the attenuation device that simulates the effect of seawater on light intensity in a semi-physical manner based on the simulated communication distance and the attenuation coefficient and the Beer-Lambert law, wherein the simulated communication distance is 100 meters.

[0013] Preferably, in S1, the generation process of the ocean phase screen is to derive the ocean turbulence refractive index power spectrum by the Hill first model, and use the ocean turbulence refractive index power spectrum to generate the ocean phase screen by using the power spectrum inversion method.

[0014] Preferably, the ocean phase screen needs to add subharmonics for compensation when simulating and modeling the underwater optical communication channel.

[0015] Preferably, the subharmonic compensation adopts a method of resampling near the zero point, sampling the missing low-frequency components multiple times to make up for them, and superimposing them on the original phase screen to achieve the effect of compensating its low-frequency components.

[0016] Preferably, in S1, the Greenwood frequency of the switching frequency after the ocean phase screen is introduced into the spatial light modulator is determined according to Taylor turbulence freezing theory to realize dynamic simulation of ocean turbulence, and the calculation process is:

[0017] Step 1: Arrange the expression of Greenwood frequency, which is:

[0018]

[0019] Where, the optical path is from s1 to s2, f c is the Greenwood frequency, k = 2π / λ represents the wave number, V is the seawater velocity, C n 2 is the equivalent structure coefficient of the corresponding ocean Kolmogorov refractive index power spectrum;

[0020] Step 2: Get the WSF of a plane wave in some asymptotic state. The process is:

[0021] Under the Rytov approximation, the wave structure function (WSF) of a plane wave propagating through isotropic and uniform turbulence is defined as:

[0022]

[0023] Where D pl (ρ,L) is the wave structure function between two points with a propagation distance of L and a spacing of ρ on the beam propagation cross section, Φ n (κ) is the uniform isotropic seawater refractive index power spectrum, J0(·) is the zero-order Bessel function;

[0024] Substituting the uniform isotropic seawater refractive index power spectrum into the above formula, the WSF can be written as follows by expanding the zero-order Bessel function in the power series:

[0025]

[0026] Where a = 8.284A T η 4 / 3 ,b=12.978A T η 2 ,c=8.284A S η 4 / 3 ,d=12.978A S η 2 ,e=8.284A IS η 4 / 3 ,f=12.978A IS η 2 ,g=2.35η 2 / 3 ,A=8π 2 k 2 (0.388×10 -8 )ε -1 / 3 χ T ω -2 ;

[0027] After calculation, we can get the WSF of a plane wave in certain asymptotic states:

[0028]

[0029] Step 3: Compare the WSF of a plane wave in some asymptotic states with the original WSF to obtain the equivalent structural coefficient of the Nikishov ocean turbulence refractive index power spectrum based on the Hill first model. The original WSF is:

[0030]

[0031] The equivalent structure coefficient of the Nikishov ocean turbulence refractive index power spectrum based on the Hill first model is:

[0032]

[0033] Step 4: Substitute the equivalent structural coefficient of the Nikishov ocean turbulence refractive index power spectrum based on the Hill first model into the simulated seawater flow rate to obtain the dynamic simulation Greenwood frequency of the corresponding ocean turbulence.

[0034] Preferably, in S2, the semi-physical simulation experiment verification platform of the underwater optical communication system in air is generally composed of an optical signal transmitting end, a channel simulation part, a signal receiving end and a data processing part, and the channel simulation part and the signal receiving end are carried out in an optical darkroom.

[0035] Preferably, in S3, a Monte Carlo method is designed to simulate the movement of photons in water.

[0036] Preferably, the optical signal transmitting end is composed of a signal transmitter and a pulse laser, the channel simulation part includes an aperture, a beam expander, an attenuation plate, a spatial light modulator, a beam splitter, a reflector, and a concave lens, the signal receiving end is composed of a single photon detector, and the data processing part is composed of a filter and a filter.

[0037] Preferably, a method for simulating an underwater wireless optical communication channel based on an ocean phase screen, wherein the specific steps of simulating an underwater wireless optical communication channel on a semi-physical simulation experimental platform are as follows:

[0038] Step 1: A host computer writes a random binary sequence signal as an analog communication signal and transmits it to the signal transmitter to drive the pulse laser to emit laser;

[0039] Step 2: The laser passes through an aperture, a beam expander, and an attenuator to simulate the effect of seawater on light intensity, and then passes through a spatial light modulator to simulate the effect of seawater on light phase. The two beams of light are combined by a beam splitter.

[0040] Step 3: After aggregation, focus and filter out other wavelength light sources, and use a single photon detector to detect the number of photons and output a photon counting pulse signal.

[0041] The present invention has the following characteristics and advantages:

[0042] (1) Simulate the real environment of ocean optical communication channels under laboratory conditions, and the platform is simple and easy to operate;

[0043] (2) The platform can simulate various marine environments, break the restrictions of the test site on underwater experiments, reduce research costs, and help carry out research on underwater wireless optical communication systems over longer distances, simulating underwater wireless optical communications more efficiently, faster, and more accurately.

[0044] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments, but the embodiments should not be construed as limiting the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 It is an overall framework diagram of the basic components of a semi-physical simulation experiment platform of an underwater wireless optical communication channel simulation method based on an ocean phase screen of the present invention. DETAILED DESCRIPTION

[0046] Unless otherwise defined, technical or scientific terms used in the present invention shall have the common meanings understood by one having ordinary skills in the field to which the present invention belongs.

[0047] In order to make the technical solutions, advantages and purposes of the present invention clearer, the technical solutions of the embodiments of the present invention are clearly and completely described below. The described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the described embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work belong to the protection scope of this application.

[0048] The present invention proposes a method for simulating an underwater wireless optical communication channel based on an ocean phase screen, the steps of which are as follows:

[0049] S1. Based on the ocean phase screen, dynamic turbulence simulation theory and attenuation model, the underwater optical communication channel is simulated and modeled to obtain a simulation model that can simulate the underwater optical communication channel in the air for experiments;

[0050] The attenuation model is designed as a seawater attenuation model for light. The simulation type is clean seawater. Based on a simulated communication distance of 100 meters, the attenuation device that simulates the effect of seawater on light intensity in the semi-physical environment is determined according to the attenuation coefficient and the Beer-Lambert law.

[0051] The generation process of the ocean phase screen is to derive the ocean turbulence refractive index power spectrum from the Hill first model and use the power spectrum inversion method to generate the ocean phase screen.

[0052] Furthermore, since the construction of the phase screen using the power spectrum inversion method needs to be completed based on the periodic Fourier transform, the preset phase screen size is limited. When there is a long-period component larger than the spatial size of the phase screen in the numerical simulation, the sampling effect of this method on this component is greatly reduced, and the long sampling period corresponds to the low-frequency component, that is, the low-frequency sampling of this model is insufficient. At the same time, when the phase screen size is L, the spectrum sampling interval is Δk=2π / L. Since the power spectrum of the ocean turbulence refractive index is mostly high-frequency components, and the low-frequency components mainly exist near the zero point, that is, in the spectrum region (-Δk / 2, Δk / 2) where the sampling samples are missing, the power spectrum less than Δk / 2 will cause low-frequency errors, which will lead to the loss of low-frequency components when the large-scale fluctuations of the phase screen cannot be simulated.

[0053] When simulating and modeling underwater optical communication channels based on the ocean phase screen, subharmonic compensation needs to be added; subharmonic compensation adopts the method of resampling near the zero point, sampling the missing low-frequency components multiple times to make up for them, and superimposing them on the original phase screen to achieve the effect of compensating its low-frequency components.

[0054] Taylor's turbulence freezing hypothesis theory describes the temporal characteristics of turbulent phase disturbances. According to this theory, the random phase screen representing the entire frozen turbulence and the Greenwood frequency representing the switching frequency can be determined to achieve dynamic simulation of ocean turbulence.

[0055] To find the equivalent Greenwood frequency of dynamic turbulence in seawater environment, it is necessary to find the equivalent structure coefficient of the corresponding ocean Kolmogorov refractive index power spectrum.

[0056] According to Taylor's turbulence freezing theory, the Greenwood frequency of the switching frequency after the ocean phase screen is introduced into the spatial light modulator is determined, and the calculation process is:

[0057] Step 1: Arrange the expression of Greenwood frequency, which is:

[0058]

[0059] Where, the optical path is from s1 to s2, f c is the Greenwood frequency, k = 2π / λ represents the wave number, V is the seawater velocity, C n 2 is the equivalent structure coefficient of the corresponding ocean Kolmogorov refractive index power spectrum;

[0060] Step 2: Get the WSF of a plane wave in some asymptotic state. The process is:

[0061] Under the Rytov approximation, the wave structure function (WSF) of a plane wave propagating through isotropic and uniform turbulence is defined as:

[0062]

[0063] Where D pl (ρ,L) is the wave structure function between two points with a propagation distance of L and a spacing of ρ on the beam propagation cross section, Φ m (κ) is the uniform isotropic seawater refractive index power spectrum, J0(·) is the zero-order Bessel function;

[0064] Substituting the uniform isotropic seawater refractive index power spectrum into the above formula, the WSF can be written as follows by expanding the zero-order Bessel function in the power series:

[0065]

[0066] Where a = 8.284A T η 4 / 3 ,b=12.978A T η 2 ,c=8.284A S η 4 / 3 ,d=12.978A S η 2 ,e=8.284A IS η 4 / 3 ,f=12.978A IS η 2 ,g=2.35η 2 / 3 ,A=8π 2 k 2 (0.388×10 -8 )ε -1 / 3 χ T ω -2 ;

[0067] After calculation, we can get the WSF of a plane wave in certain asymptotic states:

[0068]

[0069] Step 3: Compare the WSF of a plane wave in some asymptotic states with the original WSF to obtain the equivalent structural coefficient of the Nikishov ocean turbulence refractive index power spectrum based on the Hill first model. The original WSF is:

[0070]

[0071] The equivalent structure coefficient of the Nikishov ocean turbulence refractive index power spectrum based on the Hill first model is:

[0072]

[0073] Step 4: Substitute the equivalent structural coefficient of the Nikishov ocean turbulence refractive index power spectrum based on the Hill first model into the simulated seawater flow rate to obtain the dynamic simulation Greenwood frequency of the corresponding ocean turbulence.

[0074] S2. Establish a semi-physical simulation experiment verification platform in the air for underwater optical communication system based on the model obtained in S1;

[0075] The semi-physical simulation experiment verification platform of underwater optical communication system in air is generally composed of optical signal transmitting end, channel simulation part, signal receiving end and data processing part, and the channel simulation part and signal receiving end are carried out in an optical darkroom;

[0076] The optical signal transmitting end is composed of a signal transmitter and a pulse laser. The channel simulation part includes an aperture, a beam expander, an attenuator, a spatial light modulator, a beam splitter, a reflector, and a concave lens. The signal receiving end is composed of a single photon detector. The data processing part is composed of filters and filters.

[0077] S3. Using the Monte Carlo method to simulate the underwater optical communication system, simulate the movement of photons in water, and estimate the theoretical received signal bit error rate of the underwater optical communication system at different communication rates;

[0078] S4. Based on the semi-physical simulation experimental platform in the air, the performance of the underwater optical communication system is verified to obtain the experimental received signal bit error rate data;

[0079] S5. Compare the theoretical results obtained by simulation estimation with the actual results obtained by experiment to verify the feasibility of the channel simulation model and the simulation experiment platform.

[0080] like Figure 1 As shown, the present invention is built on a platform and includes: 1 signal transmitter, 1 pulse laser, 1 aperture, 1 beam expander, 2 attenuation plates, 1 beam splitter prism, 1 reflector, 1 spatial light modulator, 1 concave lens, 1 filter, 1 single photon detector, and 2 computers.

[0081] Signal transmitter: A computer is connected to the signal transmitter, and the designed random binary sequence signal is loaded into the signal transmitter, and the signal transmitter is connected to the pulse laser;

[0082] Aperture: controls the size of the aperture;

[0083] Pulse laser: A green laser with a wavelength of 520nm and a power of 50mW is used as the emission light source, and the laser is triggered by a TTL signal;

[0084] Beam expander: diverges the laser;

[0085] 2 attenuators: neutral density filters with 0.1% and 32% transmittance to simulate the effect of clean seawater on light intensity;

[0086] Beam splitter: It can split a beam of light into two beams, one of which keeps the same direction, and the other is changed to 90° from the original direction.

[0087] Reflector: reflects the light beam changed by the beam splitter;

[0088] Spatial light modulator: A computer is connected to the spatial light modulator, and the designed phase screen is loaded in the software of the spatial light modulator and switched at a frequency of 10 Hz;

[0089] Concave lens: converges the received light beam, which helps the detector detect more photons;

[0090] Filter: Filter out light sources with other wavelengths, leaving only the light source with a wavelength of 520nm;

[0091] Single photon detector: As the photon receiving end of the experimental platform, it can detect and count photons (the detection photon number range is 10-10 7 ), output photon counting pulse signal to detect extremely weak target signals. When using this device, it is necessary to strictly avoid light. Therefore, the light path detection part of this experiment needs to be carried out in an optical darkroom.

[0092] The specific steps for simulating underwater wireless optical communication channels in the semi-physical simulation experimental platform are as follows:

[0093] Step 1: A host computer writes a random binary sequence signal as an analog communication signal and transmits it to the signal transmitter to drive the pulse laser to emit laser;

[0094] Step 2: The laser passes through an aperture, a beam expander, and an attenuator to simulate the effect of seawater on light intensity, and then passes through a spatial light modulator to simulate the effect of seawater on light phase. The two beams of light are combined by a beam splitter.

[0095] Step 3: After aggregation, focus and filter out other wavelength light sources, and use a single photon detector to detect the number of photons and output a photon counting pulse signal.

[0096] The present invention simulates the influence of seawater on the light phase, designs an ocean phase screen and loads it into a spatial light modulator; simulates the influence of seawater on light intensity, designs an attenuation model and calculates the attenuation coefficients of two attenuation plates of 0.1% and 32%; loads a random phase screen generated when the laser is transmitted for 100 meters and a neutral density attenuation plate with a transmittance of 0.1% and 32% into a platform channel simulation module, and simulates the influence of the seawater channel on the laser when it propagates for 100 meters in clean seawater.

[0097] Therefore, the present invention proposes a method for simulating underwater wireless optical communication channels based on an ocean phase screen. The method applies the designed ocean phase screen to simulate the phase change of the underwater wireless optical communication channel, loads the designed phase screen into the spatial light modulator, and uses an attenuation plate to simulate the light intensity change to form an overall underwater optical communication channel. The entire platform is compared with lake trials, sea trials and other tests, which greatly reduces the test cost, while simulating the real channel environment of underwater optical communication, and can be applied to wireless optical communication channel simulation in various underwater environments.

[0098] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for simulating underwater wireless optical communication channels based on an ocean phase screen, characterized in that: Here are the steps: S1. Based on the ocean phase screen, dynamic turbulence simulation theory and attenuation model, the underwater optical communication channel is simulated and modeled to obtain a simulation model for simulating the underwater optical communication channel in the air for experiments; S2. Establish a semi-physical simulation experiment verification platform in the air for underwater optical communication system based on the model obtained in S1; S3. Using the Monte Carlo method to simulate the underwater optical communication system, simulate the movement of photons in water, and estimate the theoretical received signal bit error rate of the underwater optical communication system at different communication rates; S4. Based on the semi-physical simulation experimental platform in the air, the performance of the underwater optical communication system is verified to obtain the experimental received signal bit error rate data; S5. Compare the theoretical results obtained by simulation estimation with the actual results obtained by experiment to verify the feasibility of the channel simulation model and the simulation experiment platform.

2. According to claim 1, a method for simulating underwater wireless optical communication channels based on ocean phase screen is characterized in that: In S1, the attenuation model is designed as the attenuation model of seawater to light, and the simulation type is clean seawater.

3. The underwater wireless optical communication channel simulation method based on the ocean phase screen according to claim 2 is characterized in that: The design process of the attenuation model is to determine the attenuation device that simulates the effect of seawater on light intensity in the semi-physical environment based on the simulated communication distance and the attenuation coefficient and the Beer-Lambert law.

4. The underwater wireless optical communication channel simulation method based on the ocean phase screen according to claim 1 is characterized in that: In S1, the generation process of the ocean phase screen is derived from the power spectrum of the ocean turbulence refractive index using the Hill first model, and then generated using the power spectrum inversion method using the power spectrum of the ocean turbulence refractive index.

5. The underwater wireless optical communication channel simulation method based on the ocean phase screen according to claim 4 is characterized in that: When the marine phase screen is used to simulate and model the underwater optical communication channel, subharmonic compensation needs to be added.

6. The underwater wireless optical communication channel simulation method based on the ocean phase screen according to claim 5 is characterized in that: The subharmonic compensation adopts a method of resampling near the zero point, sampling the missing low-frequency components multiple times to make up for them, and superimposing them on the original phase screen to achieve the effect of compensating its low-frequency components.

7. The underwater wireless optical communication channel simulation method based on the ocean phase screen according to claim 1 is characterized in that: In S1, the Greenwood frequency of the switching frequency after the ocean phase screen is introduced into the spatial light modulator is determined according to Taylor's turbulence freezing theory to realize the dynamic simulation of ocean turbulence. The calculation process is as follows: Step 1: Arrange the expression of Greenwood frequency, which is: Where, the optical path is from s1 to s2, f c is the Greenwood frequency, k = 2π / λ represents the wave number, V is the seawater velocity, C n 2 is the equivalent structure coefficient of the corresponding ocean Kolmogorov refractive index power spectrum; Step 2: Get the WSF of a plane wave in some asymptotic state. The process is: Under the Rytov approximation, the wave structure function (WSF) of a plane wave propagating through isotropic and uniform turbulence is defined as: Where D pl (ρ,L) is the wave structure function between two points with a propagation distance of L and a spacing of ρ on the beam propagation cross section, Φ n (κ) is the uniform isotropic seawater refractive index power spectrum, J0(·) is the zero-order Bessel function; Substituting the uniform isotropic seawater refractive index power spectrum into the above formula, the WSF can be written as follows by expanding the zero-order Bessel function in the power series: where a = 8.284 A T η 4 / 3 , b = 12.978 A T η 2 , c = 8.284 A S η 4 / 3 , d = 12.978 A S η 2 , e = 8.284 A IS η 4 / 3 , f = 12.978 A IS η 2 , g = 2.35η 2 / 3 , A = 8π 2 k 2 (0.388×10 -8 )ε -1 / 3 χ T ω -2 ; After calculation, we can get the WSF of a plane wave in certain asymptotic states: Step 3: Compare the WSF of a plane wave in some asymptotic states with the original WSF to obtain the equivalent structural coefficient of the Nikishov ocean turbulence refractive index power spectrum based on the Hill first model. The original WSF is: The equivalent structure coefficient of the Nikishov ocean turbulence refractive index power spectrum based on the Hill first model is: Step 4: Substitute the equivalent structural coefficient of the Nikishov ocean turbulence refractive index power spectrum based on the Hill first model into the simulated seawater flow rate to obtain the dynamic simulation Greenwood frequency of the corresponding ocean turbulence.

8. The underwater wireless optical communication channel simulation method based on the ocean phase screen according to claim 1 is characterized in that: In S2, the semi-physical simulation experiment verification platform of the underwater optical communication system in the air is generally composed of an optical signal transmitting end, a channel simulation part, a signal receiving end and a data processing part, and the channel simulation part and the signal receiving end are carried out in an optical darkroom.

9. The underwater wireless optical communication channel simulation method based on the ocean phase screen according to claim 8 is characterized in that: The optical signal transmitting end is composed of a signal transmitter and a pulse laser. The channel simulation part includes an aperture, a beam expander, an attenuator, a spatial light modulator, a beam splitter, a reflector, and a concave lens. The signal receiving end is composed of a single photon detector. The data processing part is composed of filters and filters.

10. The underwater wireless optical communication channel simulation method based on the ocean phase screen according to claim 1 is characterized in that: The specific steps for simulating underwater wireless optical communication channels in the semi-physical simulation experimental platform are as follows: Step 1: A host computer writes a random binary sequence signal as an analog communication signal and transmits it to the signal transmitter to drive the pulse laser to emit laser; Step 2: The laser passes through an aperture, a beam expander, and an attenuator to simulate the effect of seawater on light intensity, and then passes through a spatial light modulator to simulate the effect of seawater on light phase. The two beams of light are combined by a beam splitter. Step 3: After aggregation, focus and filter out other wavelength light sources, and use a single photon detector to detect the number of photons and output a photon counting pulse signal.

Images