Research On Underwater Frequency Transmission Technology Based On Blue-green Laser

In the past years,transfer of optical signal in water environment has been widely used in the field of underwater communication.This underwater optical transmission is more flexible and efficient than the traditional underwater communication methods,for example,submarine optical fiber communication,sonar underwater communication,and microwave method.Although the submarine optical fiber network can provide a high-bandwidth communication,it can only be used in the case of static objects in the water.Current available acoustic scheme which launches the acoustic waves in water can reach a long distances(ranging larger than few hundred meters).However,it is still limited by the high attenuation,low bandwidth,time varying multipath propagation,and high latency.The transmission of microwave have a high-bandwidth and high propagation speed.However,the microwave signal is highly attenuated by the water,which significantly limits the distance of the underwater communication(ranging only few meters).With the development of the underwater optical communication,dissemination of time and frequency between the sites in the water environment is becoming more and more indispensable,which can be widely used in the underwater metrology such as submersible synchronization,underwater navigation,and underwater sensing.A prior work shows that a modulated green laser is used to transfer a 100 MHz radio-frequency(RF)signal over a underwater transmission link,where the timing fluctuation introduced by the water turbulence is been suppressed by an electronic phase compensation technique.However,in that technique,the electronic noise on phase shifter and asymmetric round-trip optical path limit there solution of the phase compensation,which constrains the RMS timing fluctuation to picosecond-level.In this article,a method for frequency transmission of optically carried microwave signals over underwater links based on optical phase compensation technology is proposed.The main research contents and innovations are as follows:Firstly,this article has deeply studied the attenuation mechanism of the green laser in the underwater link,simulated the attenuation model of the laser power underwater link,and studied the relationship between the underwater laser power attenuation and the link length through experiments;Independence of underwater laser power attenuation and link temperature;relationship curve of underwater laser power attenuation and salinity.Secondly,this paper studies the influence of underwater turbulence on the refractive index of the underwater link,which in turn causes the mechanism of phase fluctuations The Kolmogorov spectral function and the von Karman spectral function of the influence of atmospheric turbulence on phase noise are introduced into the study of underwater turbulence,and the two models are simulated and compared accordingly,and it is determined that the von Karman spectral function is more suitable for describing the phase of underwater turbulence.noise.Thirdly,an underwater frequency transmission scheme based on optical phase compensation is proposed.The principle and hardware construction of optical phase compensation are introduced.Finally,underwater frequency transmission experiments are carried out,and the results were analyzed and compared.With this transfer technique,a highly-stable 500 MHz radio-frequency(RF)signal was disseminated over a 5-m underwater link for 5000 s,and the characteristic of the timing fluctuation and instability for the transfer was analyzed and measured.The experimental results show the total root-mean square(RMS)timing fluctuation of the transferred RF signal with compensation is about 162 fs with a fractional frequency instability on the order of 2.8×10^-13 at 1 s and 2.7×10^-16 at 1000 s.