Analysis of velocity-matched Ti:LiNbO3 optical intensity modulators with an extended point-matching method

The point-matching method has been extended to analyse a travelling-wave electrode structure for shielded velocity-matched Ti:LiNbO₃ Mach-Zehnder optical modulators. This method provides a simple and fast approach to analysing the coplanar waveguide with buffer layer, cover shield, and electrodes of finite thickness. The calculated characteristic impedances and microwave effective indices agree well with those obtained using the spectral domain technique and the finite-element method. It is found that in the coplanar waveguide without cover shield it is difficult to achieve velocity matching. The coplanar waveguide with cover shield can be used to achieve velocity matching, but the corresponding characteristic impedance is lower than 50. Both velocity matching and impedance matching can be obtained using a narrow centre electrode. The calculated electric field has been used to estimate the modulation efficiency of the device. Velocity matching can be achieved by adjusting the height of the cover shield, but a compromise must be made between high interaction efficiency and low microwave reflection when choosing the buffer layer thickness and the electrode thickness.