Optical detection of water/alcohol vapours by polymide lightguides

Abstract Planar polyimide lightguides are used as optical sensors for the detection of H 2 O/alcohol vapours. The waveguide birefringence is monitored while it is exposed to a defined atmosphere. The diffusion of the vapour molecules into the polyimide film causes a detectable phase shift between two index-matched waveguide modes of different polarization (TE- and TM-modes). The phase shift, i.e., the birefringence ΔN of the lightguide, is a direct measure of the partial vapour pressure. The value of the birefringence and the time constants for the dynamic behaviour strongly depend on the type of vapour molecules. In the case of differing time constants, a selectivity in mixed atmospheres can be achieved by evaluating the measured data.