The impact of surface-polish on the angular and wavelength dependence of fiber focal ratio degradation
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.
- Publication:
-
Ground-based and Airborne Instrumentation for Astronomy IV
- Pub Date:
- September 2012
- DOI:
- 10.1117/12.926568
- arXiv:
- arXiv:1208.0829
- Bibcode:
- 2012SPIE.8446E..5WE
- Keywords:
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- Astrophysics - Instrumentation and Methods for Astrophysics
- E-Print:
- 10 pages, 7 figures, presented at SPIE Astronomical Telescopes and Instrumentation, July 2012