Impact of surface-polish on the angular and wavelength dependence of fiber focal ratio degradation

TL;DR

This study investigates how the surface polish level of multimode fibers affects focal-ratio degradation and throughput across various wavelengths, revealing that finer polishing reduces FRD and increases throughput, especially at redder wavelengths.

Contribution

It provides detailed measurements of FRD and throughput dependence on surface polish levels and challenges existing micro-bending models for FRD.

Findings

FRD decreases with finer surface polish, especially above 5 microns grit.

Throughput improves with finer polish, reaching 90% at 790 nm.

Surface scattering is the main cause of throughput loss at coarser polish levels.

Abstract

We present measurements of how multimode fiber focal-ratio degradation (FRD) and throughput vary with levels of fiber surface polish from 60 to 0.5 micron grit. Measurements used full-beam and laser injection methods at wavelengths between 0.4 and 0.8 microns on 17 meter lengths of Polymicro FBP 300 and 400 micron core fiber. Full-beam injection probed input focal-ratios between f/3 and f/13.5, while laser injection allowed us to isolate FRD at discrete injection angles up to 17 degrees (f/1.6 marginal ray). We find (1) FRD effects decrease as grit size decreases, with the largest gains in beam quality occurring at grit sizes above 5 microns; (2) total throughput increases as grit size decreases, reaching 90% at 790 nm with the finest polishing levels; (3) total throughput is higher at redder wavelengths for coarser polishing grit, indicating surface-scattering as the primary source of loss. We also quantify the angular dependence of FRD as a function of polishing level. Our results indicate that a commonly adopted micro-bending model for FRD is a poor descriptor of the observed phenomenon.