Polarization independent optical switches using an off diagonal electrooptic coefficient in lithium tantalate

Abstract

Polarization independent optical coupled waveguide switches have been produced in lithium tantalate using the recently developed zinc vapor diffusion technique. Diffusion and fabrication process are described in detail. Optical planar waveguides fabricated by this method in X, Y, and Z-cut LiTaO3 are characterized and compared in terms of their respective effective guide depth, effective refractive index change, and surface index change. Directional couplers of various dimensions produced in X-cut LiTaO3 are described and, their coupling coefficient and transfer length values are presented for both TE and TM input polarizations. The coupling coefficients are found to decrease exponentially as the guide separation is increased. It is shown that in spite of polarization dependent differences in the effective guide refractive indexes, directional couplers for which l[TM] = l[TE] are possible to fabricate. The optical characteristics of polarization independent coupled switches made in X-LiTaO3 are discussed. A switch having 0.39 cm electrooptic interaction length showed V[π] = 70.5 volts with switching efficiencies of 91% and 87% for the TM and TE polarizations respectively. Values for the optical/electrical fields overlap factor ranging between 0.04-0.06 were estimated in the tested devices.