Electrooptic interaction in oversized waveguide modulators

The electrooptic characteristics of oversized waveguide modulators are examined theoretically in terms of the bandwidth and spatial-coherence-conservation properties required for space laser-communication techniques. The differential equations describing the conversion from TE to TM mode, based on the mode-coupling theory, are presented and solved for the general case and for oversized modulators, with allowance for the orientation of the electrooptic tensor relative to the coordinates of the waveguide and for the differences and inhomogeneities of the optical and electrical field distributions. The numerical calculation of equivalent phase retardation is demonstrated using the parameters of a lightpipe CeTe modulator suitable for use with a CO2 laser at 10.6 microns; good agreement with experimental data is found.