Gallium lanthanum sulphide based glass fibres for passive MIR delivery applications

Brady, D.J., Schweizer, T., Wang, J. and Hewak, D.W. (1997) Gallium lanthanum sulphide based glass fibres for passive MIR delivery applications. Quantum Electronics Conference (QE13), , Cardiff, United Kingdom. 08 - 11 Sep 1997.

Abstract

Applications such as remote sensing, imaging and laser power delivery (for medical or industrial applications) using mid infra-red wavelengths require stable and non toxic low loss MIR transmitting glass fibres. The silica transmission window extends to only 2µm making silica fibres unsuitable for such applications. Mid infra-red transmitting fibres are currently either fluorozirconate based, with the disadvantages of being hygroscopic, and a relatively small transmission window (~1-3µm), or As₂X₃ (X = S, Se) based with the disadvantage of toxicity should the fibre burn or degrade. Gallium lanthanum sulphide based glasses are proposed as high quality alternatives as they have a high glass transition temperature which results in increased stability, are non toxic, and have a wide transmission window of 1-8µm [1]. One limiting factors in the performance of a fibre for the passive applications listed above is the optical loss of the fibre. This has three main physical causes: The electronic edge at the UV end of the spectrum is from electrons excited from the valence band across the bandgap of the material or to defect states in the bandgap, the fundamental glass absorption in the IR is from the multiphonon edge by excitation of optic phonon modes in the glass with phonon generation and subsequent photon absorption, and Rayleigh scattering caused by fluctuations in the refractive index of the glass caused by density and compositional fluctuations on a scale much smaller than the wavelength of light.
In this paper we present predictions for the minimum loss in glasses of composition; 70Ga₂S₃:30La₂S₃, 72.5Ga₂S₃:27.5La₂O₃ and 65Ga₂S₃:25CsCl:10La₂S₃. Measurement of loss from the fundamental glass absorptions (The electronic and multiphonon edges) was achieved using a spectrophotometer and Fourier transform infra-red spectroscopy respectively with thin slices of the glasses. Rayleigh scattering properties were calculated according to the method of Lines [2] from material parameters. The minimum predicted loss is given by the addition of the loss due to Rayleigh scattering and fitted data of the fundamental glass absorptions. Fiberization of gallium lanthanum sulphide glasses is routinely possible and fibre loss measurements in the NIR and MIR are also presented.

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Contributors

Author: D.W. Hewak

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