Free Space Communication Based On Partially Coherent Light Source

Free space optical(FSO)communication has the advantages of high communication speed,good confidentiality,anti-electromagnetic interference,convenient installation,low power consumption and no electromagnetic spectrum permission.It is widely used in secure communication,temporary communication link construction and other aspects.However,the communication performance can be reduced when transmission links are in the atmosphere due to the intensity scintillation,beam expansion,beam drift and angle-of-arrival fluctuation.Therefore,how to reduce the influence of atmospheric turbulence on free-space optical communication is one of the focuses of current wireless optical communication research.Aiming at the above contents,a wide-spectrum partially coherent optical free space communication system based on supercontinuum is experimentally established in this paper to study its resistance to atmospheric turbulence.The following work mainly completed:1.The coherence of light and the principle of supercontinuum generation are analyzed in detail.Amplified diode pulses after pulse width compression are used as the pump source to generate supercontinuum with partially coherence by pumping highly nonlinear fibers.After the amplified diode pulse passes the Raman gain soliton compression structure,the pulse width is changed from 40 ps to 3.5 ps,and the repetition frequency is 4GHz.Using the ultrafast pulse to pump the high nonlinear fiber,a supercontinuum with a spectral width of more than 200 nm is generated.The coherence of the obtained supercontinuum was measured and analyzed using a Michelson interferometer.Experiments have shown that the obtained supercontinuum spectrum has temporal partial coherence,and the coherence decreases with decreasing pump pulse width.2.The optimal modulation band of the supercontinuum spectrum was selected,and the signal-to-noise ratio of the light source in this band was optimized and improved.The signal-to-noise ratio of three light sources suitable for the modulation at 1540-1560 nm,1560-1580 nm,and 1580-1600 nm of the supercontinuum spectrum was measured.By combining the method of suppressing amplified spontaneous emission noise and optimizing pump power,the signal-to-noise ratio of this band was improved,and the signal-to-noise ratio was increased by 10.02 d B.The Michelson interferometer was used to measure the coherence of the light source after the signal-to-noise ratio was improved,and the coherence was 0.14.3.The experiment introduces the 4Gbit/s atmospheric turbulence simulation channel communication of coherent light source and partially optimized coherent light source before and after optimization.The intensity scintillation index,eye diagram,signal-to-noise ratio and bit error rate of three light sources were measured experimentally.In the atmospheric turbulence channel,the signal-to-noise ratios of the partially coherent light sources before and after optimization are 4.48 d B and 8.15 d B,respectively,and the sensitivity is-38.9 d Bm and-40.4 d Bm,respectively.The scintillation index of the optimized partially coherent light source is 31% lower than that of the coherent light source.The signal-to-noise ratio of the two sources is 8.15 d B and 5.20 d B respectively,and the communication sensitivity is-40.4d Bm and-37.5d Bm respectively.The experimental results show that the communication performance and turbulence resistance of the optimized partially coherent light source are significantly improved.