Speaker : Swagato Mukherjee (EE12S040) M.S Scholar,
Abstract :Silicon electro-optic modulators exploit the plasma dispersion effect in silicon to change the refractive index with an applied voltage. They are realized in Mach-Zender interferometers, where two phase-modulated beams interfere constructively or destructively to give amplitude modulation. Two important design requirement of such modulators is to come up with the length of the modulator required to achieve the pi phase change and to determine the frequency response of the modulator which determines the operational bandwidth of the device.
For determining the length of the modulator it is important to find the effective index of the optical waveguide having a refractive index profile defined by the carrier concentration in the waveguide. We have developed a Beam Propagation Method (BPM) based solver to obtain the effective index of the waveguide. Although a mode-solver could have been used for this as well, a BPM based solver is more versatile as it gives both the eigenmodes as well as, the propagating field solutions. Moreover, BPM can also be used to simulate non-uniform waveguides like dielectric bends, Y-junctions etc., which invariably come up in the design of an optical modulator.
Conventional BPM uses transparent boundary condition (TBC) to suppress reflection from the edges of the computational domain. But TBC fails to handle cases where the beam is diverging at the boundary, as in dielectric bends. So we have explored another BPM scheme named the Boundaryless BPM, provided a new analysis of the cause of reflection in this scheme and introduced an absorbing boundary condition with it to make it suitable for cases where the conventional BPM with TBC fails. With this new scheme, we have simulated dielectric bends and shown the bend loss obtained by this algorithm matching closely with previously published experimental and simulated data.
We have also developed a quasi-static model to simulate the waveguide characteristic of the travelling wave electrode. From this model, we can extract equivalent circuit parameters which help us to determine the frequency response of the device.