RMSTCAF: The rapid measurement system for transmission characteristics of astronomical fibers

Abstract

The fiber link between a telescope and a spectrograph is an attractive solution to improve the performance of astronomical instruments. The throughput of the observing system is mainly current concerned by the astronomer. The throughput of fiber link system is limited by the transmission efficiency (TE) and focal ratio degradation (FRD) of the fibers.The rapid measurement system for transmission characteristics of astronomical fibers (RMSTCAF) is an instrument to measure the output focal ratio (OFR), TE, the throughput of the fiber in a certain focal ratio and fiber endface quality. The number of optical fibers used in multi-object spectroscopy (MOS) and integral field spectroscopy (IFS) are increasing. It will be a time-consuming and huge burden to measure the FRD and TE of these fibers, especially the dynamic characteristics of high density optical cables. We designed this instrument to solve this problem.The system uses the encircled energy (EE) of the output energy of the fiber to determine the focal ratio of the optical fiber. With two measurement arms, the RMSTCAF provides feedback compensation to reducing the influence of incident light source fluctuation and realizes the closed-looped real-time measurement of the optical fiber transmission characteristics. The powermeters with double probes was used to suppress the environmental stray light to improve the measurement precision of the system with different wavelength. Used this system, we measured the critical focal ratio of the optical fiber with Numerical Aperture (NA) 0.128, the OFR average value within EE95 is F/3.83, compared with theoretical Fcritical/3.87, the error is 1.1%. We repeatedly connected the fiber with NA0.124 to the system and measured TE and OFR within EE90 and EE95. The standard deviation of them is 0.002, 0.03 and 0.02.As a universal measuring platform, RMSTCAF can meet the measurement requirements of minimum core diameter of 30 μm, minimum incident focal ratio F/2.6 and wavelength range of 400–850 nm. It has good repeatability and environmental adaptability. It would provide effective data for the design and evaluation of astronomical instruments with fiber link.