The waveguide design for wide band Ti : LiNbO3 Mach-Zehnder intensity modulators /

In this thesis, the waveguide design characterization for wide-band Ti : LiNbO3 Mach-Zehnder electro-optical interferometric intensity modulators (Z-cut substrate) has been performed using a combination of the effective index method and the 2D finite-difference vectorial beam propation method (FD-VBPM). For the passive aspect of the device, to minimize the propagation loss, the coherent coupling effect of radiation modes has been studied as well as the effectiveness of a cosine-generated Y-branch and a notched Y-branch in comparison with the conventional sharp-bend Y-branch. By making use of the coherent coupling effect on a notched Y-branch structure, a propagation loss figure of 0.145 dB/cm has been achieved. For the active aspect including the electro-optical effect, the minimum seperation of the two waveguide arms in order to achieve an extinction ratio above 20 dB has been decided and the corresponding modulation curve obtained. The best achieved extinction ratio is 21.8 dB with a 15 mum seperation between the inner edges of two branching arms for a 80 GHz electrode design. Two different electrode structures have been design-tested and compared. Tolerance of the alignment between the optical waveguides and the electrodes has also been determined