Study Of Focal Ratio Degradation And Mode-Disturbing Of The Fiber In LAMOST

Astronomy is an observational science and the large field and the ability of observing multi-objects in deep depth are needed for the modern astronomical telescope.And introducing optical fiber into telescope is to meet the commands of astronomical observation,promoting the development of astronomy.The LAOMST in China is equipped with 4000 large-core fibers of 320?m to transmit the light from celestial bodies in out space,making it the most efficient spectrum telescope in the world.In this paper,the focal ratio degradation(FRD),laser speckles in fiber and repressing the annular spot was studied theoretically and experimentally using the fibers in LAMOST.The boundary conditions and the classification of transmitting lights in fiber is given according to geometric optics and the reason of forming laser speckle in fiber and generating the annular spot is analyzed by using mode theory,which provide the basement of solution for reference.Considering the practicing situation of LAMOST in sky survey,the results of experiments to investigate the influences caused by the 353ND glue,substrates of silica or metal and the stress bearing on fiber show that the output focal ratio remains steady but non-uniform stress leads worse FRD.A pointolite can be easily obtained by using laser as light source and the incident angle be efficiently controlled.In the meantime,laser speckle occurs for the high coherence of laser,so the mode-disturbing device by vibrating the fiber of different cores,100gm,125?m and 320?m,is introduced to efficiently reduce the laser speckle and decrease the contrast in 70Hz,especially the contrast of the fiber in LAMOST down to 0.277.On this base,repressing the annular spot by fiber-axis rotation method between 540°?720° can obtain center field from the output end with energy efficiency larger than 83%.