Low-loss fluoride optical fibers for midinfrared optical communication

Attempts to improve transmission loss characteristics in fluoride fibers are described. Optical loss in current fibers is dominated by two major extrinsic loss factors, defect scatterers and impurities. Scatterer analysis using a Raman microprobe has revealed that the majority of them are ZrO
2
crystallites. These crystallites dominate the fiber scattering characteristics, having both wavelength independent and Rayleigh wavelength
^-4
dependencies according to their size. Excess loss due to OH groups which causes absorption at around 2.9 μm is quantified as 2000-5000 dB/km/ppm, depending on glass composition. These results suggest that further efforts in glass synthesis should concentrate on eliminating oxide and hydroxide impurities, and on the further purification of the raw materials. The key for realizing high-quality, low-loss, and long-length fluoride fibers is currently related to whether or not oxide scatterers can be completely eliminated.