Mode expansion theory and application in step-index multimode fibres for astronomical spectroscopy

TL;DR

This paper introduces a new wave optics-based numerical model for simulating mode expansion in step-index multimode fibers, improving understanding of modal effects in astronomical spectroscopy instruments.

Contribution

It presents a novel numerical field propagation model that accurately simulates modal noise, scrambling, and focal ratio degradation in multimode fibers for astronomical applications.

Findings

Effective simulation of near- and far-field fiber outputs

Assessment of fiber performance at the detector level

Enhanced understanding of modal effects in spectroscopy fibers

Abstract

In astronomical spectroscopy, optical fibres are abundantly used for multiplexing and decoupling the spectrograph from the telescope to provide stability in a controlled environment. However, fibres are less than perfect optical components and introduce complex effects that diminish the overall throughput, efficiency, and stability of the instrument. We present a novel numerical field propagation model that emulates the effects of modal noise, scrambling, and focal ratio degradation with a rigorous treatment of wave optics. We demonstrate that the simulation of the near- and far-field output of a fiber, injected into a ray-tracing model of the spectrograph, allows to assess performance at the detector level.