Development of optimum structure of a lithium niobate optical modulator considering dielectric loss

Abstract

This work presents the optimization of some design parameters of a LiNbO3 modulator in MATLAB environment using PDE Toolbox. It is seen that asymmetry of coplanar waveguide in terms of the gap-width makes the potential distribution and hence the electric field distribution asymmetric and thus affects the microwave properties of the modulator significantly. In most cases, the symmetric coplanar waveguide electrode are used, however, the asymmetric coplanar waveguide electrode can play a significant role in the design. By introducing deliberate asymmetry, the microwave effective index, characteristic impedance and conductor loss can be controlled to achieve velocity and impedance matching, which in turn enhance optical response of the modulator. In this work, the analysis of coplanar waveguide LiNbO3 optical modulators with asymmetric electrodes is carried out and the microwave propagation characteristics such as the characteristic impedance, Zc, the microwave effective index, Nm, the microwave propagation losses and the frequency response of the modulator are calculated for various structural parameters. It is found that the dielectric loss in the silica buffer layer is larger than that in the LiNbO3 substrate and when these dielectric losses are included, and the resulting bandwidth is reduced significantly in a z-cut substrate. However, it is relatively easier to match both Nm and Zc simultaneously and the resulting optical bandwidth is also greater. For a structure with x-cut substrate, it can also be seen that the bandwidth is reduced significantly when the dielectric loss is considered.