The use of slow-wave optical propagation to promote highly efficient electrooptic modulation of light is investigated theoretically. The proposed modulators utilize a traveling wave (TW) design in which a grating integrated with a single-mode waveguide induces coupling between forward- and reverse-propagating waves. This contradirectional coupling leads to a reduction in the average optical propagation speed in the forward direction. The "slow" waveguide structures provide two features which facilitate improved modulator performance over conventional "fast" TW designs: 1) optical/microwave velocity matching in substrates with high electrooptic coefficients and dielectric constants and 2) enhancement of electrooptic phase shift due to the "dwell time" of the light in the modulation region. For the ideal case of perfect velocity matching, these two factors lead to a potential improvement of nearly an order of magnitude in electrical power dissipation over velocity-matched designs in the conventional lithium niobate (LN) substrate material. Additional orders-of-magnitude improvement in the required electrical power could result from the use of tungsten bronze substrates such as strontium barium niobate (SBN), which have much higher electrooptic coefficients than LN. The prediction of a large reduction in electrical power dissipation is confirmed by calculations for specific slow-wave designs utilizing multireflector etalons in SBN, although response speed limitations result from the fact that perfect velocity matching is not achieved.
Henry F. Taylor, "Enhanced Electrooptic Modulation Efficiency Utilizing Slow-Wave Optical Propagation," J. Lightwave Technol. 17, 1875- (1999)