Characterization of mid-infrared single mode fibers as modal filters
2007
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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