title: Measuring humidity with planar polyimide light guides.
abstract: Planar polyimide light guides can be used for optical detection of humidity. In a phase-matched guide a TE mode and a TM mode of different orders are influenced differently by a humid atmosphere. With crossed polarizers a phase shift induced by the moisture is detectable in the form of an oscillating output signal. For fully cured and thus well-defined and stable fluorinated polyimide light guides a sensitivity curve is presented for the humidity range of 10-75% relative humidity. A special sensitivity is obtained by using a four-layer waveguide arrangement with two layers of polyimide. The shift of the effective mode indices is also measured for each polarization by using a half-integrated interferometer. The results are interpreted in terms of a refractive-index increase that is due to water indiffusion and a swelling process for higher humidities. The measurement in the waveguide interferometer required a moisture-insensitive coating of the reference channel. This was achieved by the use of fluorinated polymers.
authors.forename: H
authors.initials: H
authors.lastname: Franke
authors.forename: D
authors.initials: D
authors.lastname: Wagner
authors.forename: T
authors.initials: T
authors.lastname: Kleckers
authors.forename: R
authors.initials: R
authors.lastname: Reuter
authors.forename: H V
authors.initials: HV
authors.lastname: Rohitkumar
authors.forename: B A
authors.initials: BA
authors.lastname: Blech