Low-OH-absorption fluoride glass infra-red optical fibres

Ultralow losses of 10¿2 to 10¿3 dB/km have been predicted in fluoride glass optical fibres between 2 and 4.5 ¿m. At 2.9 ¿m, in the central part of this spectral region, the OH¿ ion presents its fundamental stretching vibration which gives rise to a strong and broad absorption. The attenuation at 2.9 ¿m in a step-index fluorozirconate glass fibre has been brought down to 58 dB/km, by reducing the OH¿ concentration in the glass. It is the lowest attenuation yet reported at this wavelength in any optical fibre.