Study On Focal Ratio Degradation Characteristics Of Astronomical Optical Cable For LAMOST

It has become an important research direction in the field of astronomical observation to analyze the physical properties and evolution trend of stars by starlight spectrum.In the 1970 s,optical fiber was used in the astronomical observation system,which opened a new era of astronomical spectral observation and analysis.Many countries,including China,are actively constructing large aperture telescopes and deploying wide field multi-object spectrometer systems.Optical fiber is used to transmit focal starlight to spectrometers with low loss,and the scale varies from hundreds to tens of thousands.It makes the decoupling of collection and processing come true,and provides conditions for the development of large-scale telescope systems.Worldwide,sky survey telescopes represented by SDSS have made major discoveries in the field of astronomy since the 21 st century.In China,LAMOST has published more than20 million spectra of stars,becoming the largest single spectral database in the world.It has been more than a decade since LAMOST began operating,it shows performance attenuation because of a large number of aging optical fibers,the renewal and maintenance of optical fibers are increasingly prominent,optical cable solutions emerge at the historic moment.In China,due to the special requirements on transmission performance such as focal ratio degradation,the manufacturing of astronomical optical fiber and optical cable starts later and the technology is not mature.In order to promote the development of domestic astronomical optical fiber and optical cable,this project cooperates with YOFC(Yangtze Optical Fibre and Cable Joint Stock Limited Company)to test focal ratio degradation and other transmission performance of optical cable designed for LAMOST.The reason analysis and suggestions for making cable will be given later.Firstly,the transmission process of light in optical fiber is analyzed in field of ray optics,and the transmission condition of guide mode is explained.Then the focal ratio degradation test system based on CCD imaging method is designed and built according to the definition of focal ratio,which consists of incident coupling and outgoing focal ratio measurement.The incidence system can obtain the incident light cone with controllable focal ratio by controlling the range of parallel light and converging it.The exit system can obtain the focal ratio by processing data which is shot with CCD and stepper motor in multiple distances.In order to ensure the accuracy and stability of the measurement,the plane reflection method is used to ensure the vertical incidence at the coupling end.The optical fiber end face observation system is set up to monitor the incidence coupling and the optical power meter ensures the highest incidence coupling efficiency.In data processing,reasonable ambient light deduction and spot range selection are used to improve the goodness of fit.The transmission characteristics of the LAMOST fiber cable sample and its microbeamcorresponding sample fiber are measured by using the system and method which are showed above.The energy transmission efficiency of the LAMOST Polymicro fiber is 90%.The energy transmission efficiency of the fiber cable samples is 91±4%,which is basically consistent with Polymicro fiber,and meets the usage standards.In the spectral efficiency experiment,the performance of the fiber made with the same prefabricated rod is similar.,the fiber made by self-made prefabricated rod has good performance in visible band,and its efficiency is superior to Polymicro fiber after 427 nm.The fiber made by imported prefabricated rod attenuates seriously after 700 nm,and its performance is relatively better in the ultraviolet band.In the experiment of focal ratio degradation characteristics,in order to clarify the influence of cable making technology on focal ratio degradation,the focal ratio degradation test is carried out when optical fiber is in different states: cable,microbeam and bare fiber.The experimental results show that the fiber of cable is better than Polymicro fiber in focal ratio degradation,but the focal ratio degradation is aggravated in the manufacturing process of microbeam,and is affected by the length of microbeam tube.Combined with the cable structure design and experimental measurement,the results show that the cable sample still cannot meet the use standards of LAMOST.It is suggested to improve the toughness and size of the microbeam to reduce the stress on the fiber,and to use a single type of fiber to make the cable to average the internal force.