Investigation Of A Device Based On Energy Method For Measuring Focal Ratio Of Astronomical Fibers

The optical fibers are widely used in astronomy,for its convenience to transmit the spectrum of celestial body to the spectrometer.The optical fibers play crucial roles in telescopes' observations as a kind of important optical transmission medium.In the whole telescope system,the number of optical fibers can be up to hundreds or even thousands,so the transmission properties and various parameters of optical fibers are very significant.There are two most important factors to measure the transmission quality of optical fibers.One is the spectral transmittance of fibers,and the other one is the focal ratio degradation(FRD)of fibers.However,the traditional methods of measuring the FRD have many limitations,and they are inefficient.A method to quickly measure FRD of astronomical fibers based on energy is designed.This method has high flexibility and good stability.And it's easy to operate and calculate,avoiding the additional calculation of the spot size from a CCD image.Our device for measuring FRD based on energy for astronomical fibers is composed of two parts,including the incident system and the measurement system.The incident system can accurately control the input focal ratio coupling into the astronomical optical fibers.In order to compensate the fluctuation of light intensity,we insert a splitter into the output optical path of fibers so that we can get the reference beam and the testing beam.The intensity of the reference beam is used to correct the measuring errors of the total light intensity fluctuation of the testing beam.The Encircled Energy(EE)can be set and controlled by the control panel in LabVIEW.The validity of feasibility and stability has been tested before its applications on other fibers and a Integral Field Unit(IFU).The experiments results show that the system is very stable with good performance of high efficiency.And the device proposed in this paper can be easily implemented to measure each fiber of the IFU.The homogeneity of FRD and transmission of whole IFU can be conducted.