| The abundant resources in the ocean would produce huge economic benefits.It has become a major development strategy for most countries to study marine technologies and exploit resources.With the exploitation of the ocean,the demand for underwater communication is radically increasing.Compared with radio frequency communication and acoustic communication,underwater wireless optical communication has lots of advantages such as high bandwidth,low delay,less energy consumption,small volume and so on.It plays a vital role in the fields of ocean exploration,environmental monitoring and resource development.In this paper,the inherent optical properties of seawater are introduced firstly and analyze the absorption and scattering characteristics of the main components of seawater.The common scattering phase functions are compared;the main research methods of underwater channel are described.The Monte Carlo method and tank experiment method are selected to study the transmission characteristics of seawater channel.According to Monte Carlo method,it is used to simulate the underwater transmission process of photons,and the photon transmission model is constructed to calculate the channel transmission characteristics,including power,impulse response and power distribution,and the influence of parameters such as sea water type,transmission distance and divergence angle on the channel is emphatically analyzed.The simulation results show that the time domain width changes from 0.32 ns to 0.8 ns with the increase of divergence angle,even if the transmission distance is doubled,the power similarity between pure seawater,clear seawater and coastal seawater can still reach 98%.Based on the difference of attenuation coefficient of seawater,when the absorption of seawater occupies the main attenuation,the transmission characteristics are analyzed by replacing the long distance with the short distance according to the power similarity.And when the scattering effect is the main body,the area ratio is used to analyzed the long distance underwater transmission characteristic.In addition,the underwater wireless optical transmission system is designed,in which a 520 nm laser is selected as the light source at the transmitter,and an avalanche photo diode is used as the photon detector at the receiver.According to On Off Keying(OOK)modulation technology,the CH340G circuit and multiplier circuit at the transmitter are designed to realize the modulation function and the active filter circuit and voltage comparison circuit at the receiver are designed to obtain the demodulation output.At the same time,the performance test of the module shows that the circuit modules of the transmitter and receiver are in normal working condition,and there is no error code in the process of information transmission.Finally,the underwater wireless optical transmission characteristics are obtained by the laboratory experiment of water tank.The power measurement and attenuation coefficient of the laser passing through the water quality of the tap water,the sea crystal,chlorophyll and Maalox of different concentration were carried out.The absorption and scattering effect of various water qualities were analyzed according to the light distribution and beam quality value.Maalox had the strongest scattering effect,and the M~2 value increased by 7.5%.The modulation/demodulation module is used to successfully complete the Chinese and English text transmission in the 21 m tap water channel,6 m sea crystal water channel,12 m chlorophyll water channel and 6 m Maalox water channel,and no error coded,no chaotic code and lag occurs.The video transmission was completed in the 9 m water quality channel,and the images were highly identifiable. |
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