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