Light scrambling and focal ratio degradation of thin multimode fibers with different core geometries

TL;DR

This study investigates how different core geometries in multimode fibers affect their scrambling properties and focal ratio degradation, revealing that octagonal cores improve far-field uniformity and reduce FRD compared to circular cores.

Contribution

It provides new insights into the impact of core shape on scrambling and FRD, especially highlighting the benefits of octagonal-core fibers for astronomical spectrographs.

Findings

Octagonal-core fibers offer more uniform far-field output.

Octagonal cores exhibit less FRD with f/3 beam.

Near-field scrambling is excellent across tested fibers.

Abstract

The performance of fiber-fed astronomical spectrographs is highly influenced by the properties of fibers. The near-field and far-field scrambling characteristics have a profound impact on the line spread function (LSF) of the spectra. Focal ratio degradation (FRD) influences the output beam size, thereby affecting the throughput, as well as the size of the collimator and dispersion elements. While previous research has indicated that these properties depend on the shape of the fiber core and showed that non-circular core fibers can yield uniform near-field scrambling, the result remains inconclusive for far-field. In this study, we investigate the near-field and far-field scrambling properties, along with the FRD, of 50-micron core fibers with different core geometries. We find that in addition to excellent near-field scrambling, octagonal-core fibers can also produce more uniform far-field output when compared to circular-core fibers. They also have less FRD effect when being fed with a f/3 beam.