Travelling wave electrode optimisation for high speed electro-optic modulators using the Fourier series method

The performance of Z-cut Mach-Zehnder high speed LiNbO₃ amplitude modulators with a CPW travelling wave electrode, has been analysed and optimally designed in detail by the Fourier series method. The effect of electrode and buffer layer thickness on the electric field distribution in the substrate, effective microwave refractive index and characteristic impedance of CPW electrode, are discussed. Calculation shows that increasing the thickness of the electrode and SiO₂ buffer layer can improve the phase velocity mismatch between electric and optical waves, and the drive power required by the modulator with thin buffer layer and thick electrode is lower than that required by the modulator with thick buffer layer and thin electrode. In the optimisation procedure the graphic method without iteration is used with the help of a software called “GRAPH”. The optimum rate of electrode width with gap is near by 0.75 and is independent of the rate of electrode thickness with gap. The on/off voltage lower than 4.1 V is achieved by a modulator with electrode length 59 mm for 18 GHz bandwidth