UBC Theses and Dissertations
Linbo₃ Y-branch optical modul Lai, Winnie Chelsea
Y-branch optical modulators are potentially very useful in optical communications because of their non-interferometric nature, making them easier to fabricate and to control than other types of electro-optic switches. They can be used as digital optical switches, time division multiplexers, or in conjunction with a resonator as its electrode to form a high speed optical commutator switch. The main problem with Y-branch modulators to date is that they all have very small branch angles, e.g., less than 0.2°, and hence long electrodes, thereby increasing the device capacitance and reducing the switching speed. By studying a Y-branch optical modulator numerically and experimentally, our objective is to design a Y-branch modulator which has a short electrode and still offers high on/off ratios and high percentage guided power. By using the effective index method along with the 2-D split-step finite difference beam propagation method, a z-cut titanium indiffused lithium niobate Y-branch modulator is simulated for a free space wavelength of 632.8 nm. The parameters varied in the simulations are: maximum refractive index change at the surface of the waveguide, electrode length, branch angle, and applied modulating voltage. Based on the simulation results, a maximum refractive index change of 0.0042 and an electrode length of two-horn-length are used to provide good on/off ratios and percentage guided power while keeping the electrode short. Since the on/off ratios increase with branch angle while the percentage guided power decreases with branch angle, a range of angles between 1.0° and 1.5° are found to provide the preferred operating characteristics. Y-branch modulators with branch angles ranging from 0.5° to 3.0°, and with electrodes of two-horn-length as well as three-horn-length, are fabricated. The fabrication parameters are as specified in the simulations, e.g. waveguides are formed by diffusing 4 µm wide titanium strips at 1050 °C for 6 hours. Previous fabrication problems such as dust accumulation and surface guiding are alleviated. The devices are tested by launching polarized light from a helium neon laser into a polarization-maintaining fibre and then endfire coupling the light into the fundamental TM-like mode of the waveguides. The on/off ratios and percentage guided powers are measured for both devices with two-horn-length and three-horn-length electrodes. These measured results generally compare well with the theoretical values and the behaviours of all the Y-branch are as predicted. Using the 1.5° Y-branch with the two-horn-length electrode (300 µm) as an example, the experimental on/off ratio is 40:1 with a 66% guided power at 75 V while the theoretical values are 44:1 with 62% guided power. We have demonstrated that a Y-branch electro-optic modulator with high on/off ratios and percentage guided power can be realized with short electrodes.
Item Citations and Data