The production of thin films whose refractive index is measurand specific, for use in an
interferometric fiber optic chemical sensor, is discussed. The problem of making such coatings has
been tackled by a system we have termed the "guest-host" approach, in which an active liquid
whose index varies with measurand, is contained within a porous glass host of fixed index.
Suitable porous silica-titania glass films have been produced via the sol-gel process. The use of this
system enables the index of the glass to be varied, so that the composite index of the liquid filled
film can be tailored to that required by the optical system. The sol-gel method developed is based
upon the hydrolysis and polymerisation of metal alkoxides, in an acidic aqueous/alcoholic solution.
Thin film slab waveguides were deposited in order to measure the light scattering losses, which
were found to be typically ''1dB/cm. The porosity of films was studied using a new technique
developed in which water adsorption isotherms are plotted using ellipsometry. The pore size was
found to be very small of pore diameter in the nanometer range, and the total porosity -1O%. Both
of these factors were increased by the removal of residual organic material, using hydrogen
peroxide. Finally the use of pH indicator dyes as a liquid fill is discussed, to produce a pH sensor.