Development and characterisation of phosphate glasses for athermalisation

Abstract

Lightwave devices such as gratings have a drawback where the central wavelength of the devices is sensitive to changes in temperature, which compromises their performance. This problem is particularly acute in systems that employ Wavelength Division Multiplexing (WDM), where the stability of wavelength with regards to temperature is essential. An athermal waveguide, which is inherently insensitive to temperature, is an ideal choice to solve this problem instead of using a passive temperature control unit, which could limit the optimum performance of a WDM system.

Generally, athermal glasses must exhibit negative refractive index change with temperature (dn/dT) to counter the effect of thermal expansion. Phosphate glasses are ideal candidates for athermalisation as they exhibit negative dn/dT. An interferometer set-up has been constructed to specifically measure dn/dT of the glasses.

Three different phosphate glass families were prepared and characterised in terms of their optical and thermal properties. The properties of the potassium aluminophosphate glasses were strongly influenced by the strengthening effect of Al³⁺, the trend observed for the binary phosphate glasses depended on the availability of terminal oxygens to coordinate the cations while the change in the properties of borophosphate glasses depended on the effect and coordination number of B³⁺. Glasses with negative dn/dT have been achieved from the various compositions of glasses studied. The results showed that there was a minimum thermal expansion coefficient (α) for each glass system, beyond which α would overcome the effect of polarisability change with temperature so that the glasses exhibited negative dn/dT. Semi-empirical equations linking dn/dT and α have been defined, enabling the estimation of dn/dT of any glass within the same system. This research work represents the first comprehensive measurement of dn/dT of phosphate glasses.

Both fibre and planar waveguides were fabricated. Extrusion was used to prepare the preform for fibre drawing. The extrusion process used a two-die assembly to extrude dissimilar glasses, and is the first demonstration of its kind. The spin-coating process was used to deposit layers of glass on glass substrates to produce planar structures. The success of the extrusion and spin-coating work showed that these processes could be used for combinations of glasses with dissimilar thermal properties. Femtosecond writing was investigated on the phosphate glasses and channel waveguides have been successfully defined.

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