Abstract

We discuss the use of the resonant FTR (frustrated total reflection) concept to enhance the sensitivity of internal reflection optical sensors. In a resonant FTR cavity, radiation of a given wavelength is trapped and bounced back and forth until, if even a minute amount of absorption is present, the radiation is eventually totally absorbed.¹ Such a cavity, tuned on the absorption line of the substance to be detected in a gas or in a solution, can be straightforwardly incorporated in a conventional internal reflection optical sensor, leading to a sensitivity enhancement of the order of 2 or 3 orders of magnitude. We discuss in particular the detection of CH₄ in air and show that the resonant FTR approach allows the detection of concentrations of 0.01%, which represents a 2 order of magnitude improvement over the sensitivity threshold recently achieved with fiber optic sensors.² We also discuss the resonant FTR approach in comparison with the plasmon resonance approach, which has been recently proposed by several groups, and show that, although they have comparable sensitivity, the FTR approach has important practical advantages.

© 1989 Optical Society of America