Focal ratio degradation for fiber positioner operation in astronomical spectrographs

TL;DR

This study investigates focal ratio degradation (FRD) in optical fibers used in astronomical spectrographs, emphasizing the impact of angular misalignments over stress, with implications for fiber positioner design.

Contribution

Developed a compact experimental setup and model to separate FRD effects from angular misalignment, applied to fibers in the Subaru PFS, revealing angular misalignment as the dominant factor.

Findings

Angular misalignment effects dominate FRD measurements.

No significant FRD increase due to stress was observed.

The results inform fiber positioner design for astronomical spectrographs.

Abstract

Focal ratio degradation (FRD), the increase of light's focal ratio between the input into an optical fiber and the output, is important to characterize for astronomical spectrographs due to its effects on throughput and the point spread function. However, while FRD is a function of many fiber properties such as stresses, microbending, and surface imperfections, angular misalignments between the incoming light and the face of the fiber also affect the light profile and complicate this measurement. A compact experimental setup and a model separating FRD from angular misalignment was applied to a fiber subjected to varying stresses or angular misalignments to determine the magnitude of these effects. The FRD was then determined for a fiber in a fiber positioner that will be used in the Subaru Prime Focus Spectrograph (PFS). The analysis we carried out for the PFS positioner suggests that effects of angular misalignment dominate and no significant FRD increase due to stress should occur.