Vapour sorption, wavelength tracking and thermo-optic properties of dual slab waveguide interferometers

Abstract

The dual slab waveguide interferometer is introduced as a device which has many applications in various research areas. Reported is its ability to provide details on the mechanism for the vapour sorption of thin polymer films, the development of the interferometer as a wavelength tracking device for the telecommunications industry and a method to characterise the thermo-optic properties of III-V semiconductor alloys. The vapour sorption mechanisms of thin films of polymers Polyisobutylene (PIB) and Polyvinylpyrrolidone (PVP) on exposure to several solvents are investigated. Coating interferometer chips with a thin layer of polymer and monitoring the interference fringe pattern for changes due to the exposure to a solvent vapour provides information on the mechanism for vapour sorption as one of swelling rather than void-filling. Interferometer sensitivities to vapour concentrations are linear and depend on refractive index differences between polymer and condensed vapour. An interferometer manufactured from III-V semiconductor compounds is developed to produce a device which can operate as a wavelength tracker. Sensitivities of over 6 mrad / pm and 7 mrad / pm for single and dual quaternary systems respectively indicate that a device of length around 5 mm would be capable of detecting picometer input wavelength changes including thermal background noise. The sensitivity to thermal changes provides a simple method for determining the thermo-optic coefficient of two Indium Gallium Arsenide Phosphide (InGaAsP) alloy compositions as (3.15±0.08)x 10(^-4)K(^-1) and (2.60±0.017)x10(^-4)k(6-1) forInGaAsP compounds with bandgap wavelengths around 1.3 μm and 1.15 μm respectively.