A Comparative Research On The Schemes Of Optical Fiber Coupling For Space Laser Communication

In the space laser communication system,the far-field space laser is usually coupled into the optical fiber by the optical antenna and transmitted to the detector by the optical fiber.In order to obtain higher receiving light energy,the pre-amplification technology is usually used to amplify the received laser,but the mature optical amplification devices are all based on single-mode fiber,and the coupling of space laser to singlemode fiber is difficult.When the laser is transmitted in the atmospheric link,it will be interfered by atmospheric turbulence,as well as the absorption and scattering from suspended particles and gas molecules.These phenomena will affect the transmission of space laser,and eventually lead to the further reduction of the coupling energy of space laser fiber,which will affect the performance of the communication system seriously.In order to realize the optical energy pre-amplification and seek the high efficiency optical fiber coupling scheme,three kinds of spatial laser to fiber coupling schemes are studied: single-mode fiber coupling,few-mode fiber coupling and multi-mode to single-mode fiber coupling(The multi-mode fiber is aligned with the single-mode fiber without eccentricity,and the single-mode fiber transmits the optical energy to the preamplifier).The main research contents include the following parts:(1)Taking single-mode,2 mode and the fundamental mode in multi-mode fiber as the research object,the law of coupling efficiency changing with system parameters under ideal alignment is studied,then the coupling efficiency of the three coupling schemes is analyzed when there have alignment error.(2)Based on the existing single-mode fiber coupling theory,the effects of adding atmospheric turbulence in the coupling research of few-mode fiber and multi-mode to single-mode fiber are analyzed and compared innovatively.The results show that with the increase of alignment error or turbulence intensity,the coupling efficiency of scheme 1 and 2 decreases rapidly,while that of scheme 3 decreases less;In the case of little error or weak turbulence,the coupling efficiency of the first two schemes is higher,while in the case of large error or medium and strong turbulence,the latter two schemes are better.In addition,the results of the study on the coupling scheme of multi-mode to single-mode fiber show that when the normalized turbulence intensity remains unchanged,the total coupling efficiency changes periodically with the length of the multi-mode fiber,so the length of the multi-mode fiber needs to be controlled to maximize the total coupling efficiency.(3)The single-mode fiber coupling experiment with a distance of 6.3km is carried out in the real weather environment,there are few reports on the near ground,medium and long distance fiber coupling experiments at present,analyze the relationship between coupling efficiency and weather and time.Through the measurement of the angle of arrival fluctuation,the normalized turbulence intensity is inversely performed,and the coupling efficiency of single-mode fiber is obtained by substitute into the theoretical model.The coupling efficiency obtained by inversion is compared with that of direct measurement.The results show that the coupling efficiency of inversion measurement are in good agreement with that of direct measurement.