Characterization of mid-infrared single mode fibers as modal filters

Details Characterization of mid-infrared single mode fibers as modal filters Characterization of mid-infrared single mode fibers as modal filters

Nov 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007apopt..46.7957k&link_type=abstract

Applied Optics IP, vol. 46, Issue 32, pp.7957-7962

Physics

Optics

11

Fiber Characterization, Fiber Optics, Infrared, Fibers, Single-Mode

Scientific paper

We present a technique for measuring the modal filtering ability of single mode fibers. The ideal modal filter rejects all input field components that have no overlap with the fundamental mode of the filter and does not attenuate the fundamental mode. We define the quality of a nonideal modal filter Qf as the ratio of transmittance for the fundamental mode to the transmittance for an input field that has no overlap with the fundamental mode. We demonstrate the technique on a 20 cm long mid-infrared fiber that was produced by the U.S. Naval Research Laboratory. The filter quality Qf for this fiber at 10.5 μm wavelength is 1000±300. The absorption and scattering losses in the fundamental mode are approximately 8 dB/m. The total transmittance for the fundamental mode, including Fresnel reflections, is 0.428±0.002. The application of interest is the search for extrasolar Earthlike planets using nulling interferometry. It requires high rejection ratios to suppress the light of a bright star, so that the faint planet becomes visible. The use of modal filters increases the rejection ratio (or, equivalently, relaxes requirements on the wavefront quality) by reducing the sensitivity to small wavefront errors. We show theoretically that, exclusive of coupling losses, the use of a modal filter leads to the improvement of the rejection ratio in a two-beam interferometer by a factor of Qf.

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