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

Type Conference Paper
Names Arthur D. Eigenbrot, Matthew A. Bershady, Corey M. Wood
Proceedings Title Ground-based and Airborne Instrumentation for Astronomy IV.
Conference Name Proceedings of the SPIE
Volume 8446
Pages 84465W
Date September 1, 2012
DOI 10.1117/12.926568;
ISBN 0277-786X
Library Catalog
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 μm 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 μm (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.
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