Microwave characterization of lithium niobate electrooptic modulators with traveling wave electrodes

The analysis of traveling-wave electrooptic modulator on z-cut and x-cut lithium-niobate substrates is carried out using the finite element method based on a quasi-TEM approximation. The microwave effective index, characteristic impedance, and frequency dependent attenuations are calculated. Optical frequency response is also calculated and hence the bandwidth is estimated for velocity matching and impedance matching conditions. Bandwidth increases significantly for simultaneous velocity and impedance matching but is limited by the microwave losses in the dielectric material. Optical 3-dB bandwidth of more than 140 GHz can be achieved with x-cut LiNbO₃ substrate, when two slots are incorporated in the structure.