| Because of its low loss and long transmission distance,optical fibers are widely used in data transmission between telescopes and spectrometers.The application of optical fiber can not only reduce the loss of optical energy,but also increase the flexibility of the connection between the spectrometer and the telescope.Installing multiple optical fibers on the focal plane of the telescope can achieve simultaneous observation of multiple targets,which greatly improves the efficiency of astronomical spectrum acquisition.As the astronomical telescope with the highest astronomical spectrum acquisition rate in my country,LAMOST has 4,000 optical fibers installed on the focal plane.The use of LAMOST is of great significance to the development of astronomy in my country.The transmission characteristics of the optical fiber directly affect the observation efficiency and quality of the telescope.With the continuous development of astronomy,astronomical observations put forward higher requirements on the transmission characteristics of optical fibers.Transmission efficiency and focal ratio degradation characteristics are two important factors for evaluating the quality of astronomical optical fibers.This paper designs and produces a real-time online measurement system for astronomical fiber transmission characteristics.The system can simultaneously measure the fiber transmission efficiency and focal ratio degradation.The reference optical path is used to realize the feedback compensation for the incident light source fluctuations,avoiding the measurement errors caused by the incident light source fluctuations,and thus realizing the closed-loop real-time measurement of the fiber transmission characteristics.The measurement system is divided into two independent parts: the control device of the fiber entrance end and the measurement device of the fiber exit end.The data transmission is performed by Lo Ra spread spectrum communication between the front and back parts.The system can realize the non-destructive measurement of the optical fiber on the telescope at the telescope site,and can guide the repair and replacement of the optical fiber on the telescope.This paper analyzes the transmission of light in large core diameter step fibers,and explains the mechanism of fiber focal ratio degradation from the perspective of fiber mode.The mode coupling phenomenon that occurs during the transmission process will change the power distribution in the fiber,and then the fiber focal ratio degradation will occur.In the process of using the telescope,the optical fiber on it will inevitably be affected by external forces to cause focal ratio degradation.Accurate measurement of the focal ratio degradation of astronomical optical fibers is of great significance for improving the telescope spectral efficiency.In order to verify the effectiveness of the system,two types of optical fibers of 4 micrometers and 35 micrometers were used to test the transmission efficiency and focus of the large-core fiber ordered by YOFC for astronomy and the fiber made by Polymicro currently used by LAMOST.Measurements and comparisons with respect to degradation were made.The measurement system adopts the design of the space optical path.During the debugging and use of the optical path,there will be various errors.In this paper,the influence of stray light in the system is analyzed,and the software simulation method is used to analyze the influence of many factors that have an effect on the measurement of fiber focal ratio degradation.The improvement of the measurement method and the optimization of the system are proposed. |
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