Monitoring humidity by polyimide lightguides

Planar waveguides of the commercially available polyimide ‘‘PI 2566’’ and polyamide-imide ‘‘AI lite’’ are spin coated on glass substrates. TE and TM polarized modes are launched simultaneously into the phase-matched planar lightguide. This polymer guide is sensitive to changes in its birefringence. Polyimide films in the ‘‘as-dried’’ and fast cured state turn out to be very sensitive to moisture. Changes in the vicinity of the waveguide can be monitored instantly.