Inhomogeneous quarter-wave transformers for a waveguide electro-optic modulator

ABSTRACT Electro-optic frequency shifters, or modulators, have been built with almost 100% efficiency. Conversions over 50% require the interaction of a circularly polarized high power microwave beam traveling synchronously with a circularly polarized laser beam in an electro-optic crystal. We describe the inhomogeneous quarter-wave transfonner structures made for a C02 laser beam modulator with a conversion efficiency in excess of 80%. Conventionalimpedance matching synthesis was adapted to include the requirements for maintaining circular polarization and handling high power. Measured results are presented. 1. INTRODUCTION Electro-optic frequency shifters, or modulators, have been built with almost 100% efficiency.1 Very high conversions, over 50%, require the interaction of a circularly polarized microwave beam traveling synchronously with acircularly polarized laser beam in an electro-optic crystal, such as Cadmium Telluride (CdTe). This paper describes theinhomogeneous quarter-wave transformer structures made for a C02 laser beam modulator with a 3 dB bandwidthgreater than 1 GHz at a center frequency of 16 GHz and a maximum conversion efficiency in excess of 80%.