DEEM, a versatile platform of FRD measurement for highly multiplexed fibre systems in astronomy

Abstract

We present a new method of DEEM, the direct energy encircling method, for characterising the performance of fibres in most astronomical spectroscopic applications. It's a versatile platform to measure focal ratio degradation (FRD), throughput, and point spread function (PSF). The principle of DEEM and the relation between the encircled energy (EE) and the spot size were derived and simulated based on the power distribution model (PDM). We analysed the errors of DEEM and pointed out the major error source for better understanding and optimisation. The validation of DEEM has been confirmed by comparing the results with conventional method which shows that DEEM has good robustness with high accuracy in both stable and complex experiment environments. Applications on the integral field unit (IFU) show that the FRD of 50µm core fibre is substandard for the requirement which requires the output focal ratio to be slower than 4.5. The homogeneity of throughput is acceptable and higher than 85 per cent. The prototype IFU of the first generation helps to find out the imperfections to optimise the new design of the next generation based on the staggered structure with 35µm core fibres of N.A.=0.12, which can improve the FRD performance. The FRD dependence on wavelength and core size is revealed that higher output focal ratio occurs at shorter wavelengths for large core fibres, which is in agreement with the prediction of PDM. But the dependence of the observed data is weaker than the prediction.

Focal Ratio Degradation for Fiber Positioner Operation in Astronomical Spectrographs

“…There are two primary methods to measure FRD: a cone test,also known as a solid angle test, and a ring test, also known as a collimated beam test (Haynes et al, 2011;Yan et al, 2018). The cone test involves a uniform beam of known solid angle that fills a fiber.…”

A fiber bundle structure with uniform transmission characteristics for high-density astronomical optical cables

“…A schematic diagram of the platform is shown in Figure5. Details about the measurement methods and theories were described byYan et al (2018). As illustrated in Figure6, EE characterizes the percentage of energy in the measured focal ratio to total energy.…”