Abstract

We demonstrate electro-optic sensing of a microwave signal by total internal reflection of light at the critical angle. A mode-locked laser beam and a microwave frequency electro-optically mix in a nonlinear crystal near the interface. This process, described by using generalized nonlinear optical formalism at the boundary, provides a sensitive mechanism for studying the electro-optic effect in reflection. To experimentally verify this phenomenon, we use a KDP crystal with a coplanar waveguide on its surface that carries a 109 MHz microwave signal. A100 MHz mode-locked laser is focused through a prism onto the opening in the coplanar waveguide and is internally reflected off the KDP surface. In the region of overlap inside the KDP crystal, the laser beam's evanescent electric field and the microwave signal's electric field interact electro-optically to produce a difference frequency sideband (9 MHz) on the reflected light beam. The strength of the difference frequency signal peaks at the critical angle; when we change the incoming beam's angle by one degree, the signal level drops 25 dB. We have experimentally investigated the signal dependence on polarization, light intensity, and microwave signal power, agree qualitatively with theory.

© 1990 Optical Society of America