Comparing modal noise and FRD of circular and non-circular cross- section fibres

TL;DR

This study compares modal noise and focal-ratio degradation in circular and non-circular optical fibres, demonstrating that mechanical scrambling can significantly reduce modal noise without affecting FRD, thereby improving spectroscopic data quality.

Contribution

It provides a comparative analysis of modal noise and FRD in various fibre geometries and shows how mechanical scrambling enhances signal-to-noise ratio in spectroscopic applications.

Findings

Octagonal fibres reduce modal noise more effectively than circular fibres.

Mechanical scrambling with an excenter reduces modal noise by up to 180%.

Replacing circular fibre with octagonal in a spectrograph increased SNR by 1.6 times.

Abstract

Modal noise is a common source of noise introduced to the measurements by optical fibres and is particularly important for fibre-fed spectroscopic instruments, especially for high-resolution measurements. This noise source can limit the signal-to-noise ratio and jeopardize photon-noise limited data. The subject of the present work is to compare measurements of modal noise and focal-ratio degradation (FRD) for several commonly-used fibres. We study the influence of a simple mechanical scrambling method (excenter) on both FRD and modal noise. Measurements are performed with circular and octagonal fibres from Polymicro Technology (FBP-Series) with diameters of 100, 200 and 300 {\mu}m and for square and rectangular fibres from CeramOptec, among others. FRD measurements for the same sample of fibres are performed as a function of wavelength. Furthermore, we replaced the circular fibre of the STELLA-echelle-spectrograph (SES) in Tenerife with an octagonal and found a SNR increase by a factor of 1.6 at 678 nm. It is shown in the laboratory that an excenter with a large amplitude and low frequency will not influence the FRD but will reduce modal noise rather effectively by up to 180%.