Materials for infrared fibre optics

This review discusses the development of materials for fibres from early work in the 1960s on oxide glass, chalcogenide glass and crystalline halides which was way ahead of its time; through the huge development on silicate fibres which has led to our present-day new IR optical communications capability; to the frontiers of current research on mid IR fluoride glass fibres and far IR glass, hollow core and crystalline fibres. Fibre fabrication technology was rapidly developed during the 1970s allowing losses of a fraction of a dB/km to be achieved at near infrared wavelengths. In the 1980s optical communications have become possible using multimode silicate glass fibres in the near IR operating at 0.8-0.9 μm, at 1.3 μm and at 1.55 μm and using monomode fibres operating at 1.3 μm and 1.55 μm. The loss in these fibres at 1.55 μm is about 0.2 dB/km which is very nearly the intrinsic loss limit of GeO₂-SiO₂ glass. To achieve repeaterless transmittance over longer distances than possible with silicate fibres it is necessary to fabricate fibre from a lower loss medium. In the infrared spectral region attenuation in a material is dominated by Rayleigh scattering and multiphonon absorption. Thus if materials transmitting farther into the infrared can be utilised then potentially much lower losses can be achieved since Rayleigh scatter has a λ⁻⁴ dependence. It may be possible to achieve of the order of 10⁻² dB/km loss in a fibre at 2.55 μm and perhaps lower losses at longer wavelengths of 3 to 4.5 μm. For these reasons researchers are now addressing the problems of making mid IR fibres from amongst the fluorozirconate and fluorohafnate glasses. There are very different applications for fibres in the far infrared spectral region mainly requiring path lengths of a few centimeters to a few meters for sensor and power delivery devices. In spite of the fact that only short lengths of fibre are required, there are problems in finding sufficiently low loss materials which also demonstrate mechanical and environmental integrity. Research is being carried out amongst the chalcogenide glasses, hollow core oxide glasses, monocrystalline halides and polycrystalline halides since there is no obvious front running material which is able to satisfy all requirements.