Study On The Performance Of FRA-based Fiber Optic Hydrophone Remote Relay Transmission System

The long-distance non-relay optical fiber hydrophone system is an important part of underwater early warning detection,and it is an important way to extend the early warning detection range and expand the early warning detection range.In order to obtain higher signal-to-noise ratio and longer transmission distance,the on-line fiber Raman Amplifier(FRA)is usually used to amplify the analog optical signal in the long-distance relay-free optical fiber hydrophone system.However,the non-linear effect of fiber Raman amplification will lead to the deterioration of the performance of the long-distance relayfree transmission system of fiber-optic hydrophone and the decrease of the noise level of the fiber-optic hydrophone system.Based on the basic principle of fiber Raman,the Raman amplification gain and noise characteristics of the long-distance non-relay transmission system of fiber-optic hydrophone are studied theoretically and experimentally in order to optimize the Raman amplification parameters and improve the performance of the long-distance non-relay fiber-optic hydrophone system.Specific research contents and innovations are as follows:1.Distributed FRA theory model is used to analyze the performance of long-distance transmission fiber optic hydrophone system.The Raman equation is numerically calculated by shooting method,and the power distribution of signal light and pump light in FRA amplified transmission fiber is simulated.The modulation instability,stimulated Brillouin scattering and intensity noise transfer in FRA amplified transmission process are analyzed.Secondly,the long-distance relay-less amplification transmission system of optical fiber hydrophone is built,and the experiments of gain characteristics,line width broadening and noise characteristics of co-and reverse Raman amplification are carried out.Compared with reverse FRA amplification,co-directional FRA amplification can obtain higher optical signal-to-noise ratio(OSNR)output light in long-distance non-relay fiber optic hydrophone system.However,co-directional FRA amplification is more likely to produce Modulation Instability(MI)effect,and the phase noise level of output light is about 2 dB higher than that of reverse FRA amplification.When the power is too high,stimulated Brillouin Scattering(SBS)and MI effect will also lead to the broadening of the line width of signal light.SBS effect broadens the line width by 13.9%,MI effect broadens the line width by 89%,and MI effect has greater impact than SBS effect.The higher the relative intensity noise(RIN)of the pump source,the higher the RIN corresponding to the signal light.Co-directional FRA amplification is more sensitive to RIN of pump source than reverse FRA amplification.With the increase of RIN in signal light,the fluctuation of phase noise will be more and more serious.RIN transfer in FRA amplification process will increase the phase noise index of signal light.The noise transfer caused by the deterioration of RIN noise will nearly double the effect of phase noise,that is,the phase noise will increase about 2 dB correspondingly with the increase of 1 dB of RIN noise.However,RIN transfer during FRA amplification does not affect the signal-to-noise ratio of signal light.Thirdly,the amplification performance optimization of the second-order FRA fiber optic hydrophone long-distance transmission system is studied,and the transmission upper limit distance farther than the first-order FRA amplification is obtained.On the premise of the same noise level,the relay-free transmission distance of the fiber-optic hydrophone with the same direction second-order FRA is 15 km longer than that of the first-order FRA,and the reverse second-order FRA can be extended by 10 km.