Initial measurements of focal ratio degradation in ZBLAN fluoride fibers for K band spectroscopy

The applicability of optical fibers to astronomical spectroscopy in the infrared depends heavily on the fiber's wavelength-dependent transmission properties. Standard silica fibers typically allow transmission at wavelengths as red as 1550nm but absorb strongly at redder wavelengths, limiting their use for the infrared instruments. Fibers using fluoride glasses transmit redward of this range, opening up the infrared K band, corresponding to a window of transmission in the Earth's atmosphere. In order to further assess their feasibility for astronomical instrumentation, the characteristics of these fibers must be well-known. Here initial measurements and analysis of the focal ratio degradation (FRD) of two types of fluoride fibers are reported.