Abstract

A number of single-mode electrooptical LiNbO₃ channel waveguide modulators and switches are all operating on the principle of mode interference. These devices include the Mach-Zehnder, the crossing channel and the widened X (or zero gap directional coupler) devices. For operation at high microwave frequencies, they are traveling wave devices ^[1,2]. The electrodes of these devices are in the form of coplanar waveguide (CPW), symmetric coplanar strip (SCPS) and asymmetric coplanar strip (ACPS) microwave transmission lines. Although the basic principles of operation of these devices are well known, their design optimization is complex because many parameters are involved ^[3]. We shall describe here a numerical modeling procedure that could be used systematically to optimize the drive power requirement for a given bandwidth. Specifically, we shall show: (1) The bandwidth of interferometric traveling wave devices depends only on the electrode length L, the microwave velocity (represented by the microwave effective index ${\text{n}}_{\text{eff}}^{\text{m}})$ and the frequency dependence of microwave attenuation. (2) Numerical calculation of the microwave attenuation and ${\text{n}}_{\text{eff}}^{\text{m}}$ as a function of the electrode width W, gap of separation G and thickness T has been obtained. Thus the bandwidth requirement can be satisfied by appropriate choice of L, G, W and T. (3) In the first order, the optical waveguide that can be fabricated with the tightest optical mode confinement has the lowest drive power requirement. (4) The microwave drive power (not including the effect of impedance mismatch to the generator) can then be minimized by using the waveguide parameters for tight mode confinement and by varying W, T and G within the allowed range of values that satisfy the bandwidth requirement.

© 1989 Optical Society of America