Analysis of the phase modulation characteristics of electro-optic or acousto-optic waveguide modulators reveals that a significant increase of modulation sensitivity, per unit of applied field, above that of conventional, bulk devices can occur in multimode waveguides with tight confinement. In the limit, an enhancement of as much as a factor of 3 is theoretically attainable. Explicit formulas for relative modulation sensitivity are given for both TE- and TM-mode polarizations, and illustrative curves are presented for a wide variety of structural parameters. Semiquantitative experimental verification of the effect is presented, using results of a GaAs electro-optic thin-film modulator at 10.6 µm.
James F. Lotspeich, "Modulation-sensitivity enhancement in electro-optic slab waveguides," J. Opt. Soc. Am. 65, 797-803 (1975)