Optical Properties Of BeF2 CVD And Melted Glasses

Abstract

This paper describes a preliminary study of the refractive index dispersion in the beryllium fluoride (BeF2) melted glasses doped with aluminum fluoride (A1F3). It is shown that these glasses have very low dispersion, an order of magnitude lower than silica glass. It is further shown that doping BeF2 with A1F3 increases the refractive index of the base glass. Thus, an optical waveguide fiber design with BeF2 as the cladding and A1F3 doped BeF2 as the core is feasible. Also, the index dispersion of a melted BeF2 glass is compared with that of a CVD (chemical vapor deposition) glass. In the past it has been shown that fluoride glasses possess excellent transparency from the near UV to mid IR. The most interesting properties of fluoride glasses are that they possess low refractive index, low dispersion and low Rayleigh scattering. Hence, the potential for making an ultra low loss, ultra long optical fiber exists. The major intrinsic losses in a pure fluoride glass fiber would be the Rayleigh scattering which is proportional to the inverse fourth power of wavelength. Intrinsic attenuation losses of 0.001 dB/km are possible in high-purity fibers in the 2 to 4 gm wavelength range. Major efforts in fluoride glass technology have focused on fiber fabrication with zirconium tetrafluoride-based glasses, melted conventionally. Such systems can contain from two to five components. An alternate approach is the CVD of fluoride glass systems being developed at Corning. An organo-metallic compound has been processed to fabricate BeF2 glass. The ZrF4 -based glasses do not lend themselves to CVD processes because of the problems in finding suitable precursors with the required vapor pressures.