Generation Of Supercontinuum Source And Its Atmospheric Transmission Characteristics

Free space optical(FSO)communication is a kind of communication mode without optical fiber transmission,while its transmission medium is the space atmosphere.Due to its high speed,good confidentiality,easy installation and no spectrum license,free space optical communication has great potential in satellite-ground,inter-satellite communication network and "last kilometer" access,and it can also be used as an auxiliary means of microwave communication and optical fiber communication.However,the atmospheric turbulence will lead to beam drift,beam expansion and light intensity scintillation of laser carriers,which will affect the communication quality.In order to suppress the influence of atmospheric turbulence in free space optical communication,some methods such as adaptive optics,spatial diversity,aperture averaging and partially coherent beams are proposed.The partially coherent light source is one of the important means to suppress atmospheric turbulence in free space optical communication system for its mature technology and good compatibility.However,previous methods of generating partially coherent light,such as rotating phase screen and using spatial light modulator,have the problem of low modulation rate,hence not suitable for high-speed free space optical communication systems.Various theories and experiments have verified that the supercontinuum spectrum is a partially coherent light source,and its repetition rate is determined by the pump pulse.Therefore,it is feasible to get high-speed partially coherent optical carriers suitable for free space optical communication by using high repetition rate pulse sources.However,the signal quality of partially coherent optical carriers is greatly deteriorated by its high pulse jitter(i.e.,low signal-to-noise ratio(SNR))due to the influence of non-linear effects in supercontinuum generation process.Toward to the demand of space optical communication,the generation and optimization of high repetition rate partially coherent optical carriers are achieved,and a free space optical communication system is built based on the optimized optical carriers.i.In this paper,the principle of supercontinuum generation in optical fibers is analyzed in detail.Various non-linear effects affecting supercontinuum generation are explained elaborately.The mechanism of intense pulse jitter caused by modulation instability and high-order soliton splitting is introduced.After using amplified diode pulse as pump source and introducing external trigger into the supercontinuum pump structure,the experimental results show that the pulse jitter factor reduces from 0.1346 to 0.066,and the corresponding optical carrier SNR increases from 14.93 d B to 20.7 d B.ii.In this paper,Kolmogorov turbulence theory is described in detail,and the principle of using partially coherent optical carrier to restrain the effect of atmospheric turbulence is verified by formula derivation.A simulation pool of atmospheric turbulence based on the principle of hot air convection is experimentallly built,and the simulated atmospheric turbulence device is calibrated.iii.We experimentally demonstrated a free space optical communication system based on the optimized partially coherent optical carrier,and its transmission performance such as intensity jitter,bit error rate(BER),and received SNR are analyzed.Comparing with the FSO system using coherent source and unoptimized partially coherent source,the results show that the use of partially coherent optical carriers does reduce the impact of turbulence on the communication system,and the optimization method we adopted further improves the communication quality of the system.This communication system based on low jitter supercontinuum light source is well compatible with the current free space optical communication system.Undoubtably,it is an effective method to suppress turbulence and improve the quality of space optical communication.