E-O Polymer Based Optical Phase Modulator and Its Performance Enhancement using Slow-Wave Structures
Authors:   , Receveur Kevin , Daryoush Afshin S. 
Published: July 2021

Electro-optic (EO) polymers have proved to be promising for use in optical modulators due to their broad bandwidth and higher electro-optic coefficient compared to conventional inorganic ceramics, while maintaining compatibility with Si-Photonics hence lowering the manufacturing cost. While modulator designs using EO polymers have been explored previously, these designs require a long length (>1cm) to obtain a phase shift of 180° at reasonable voltage levels. In order to produce a modulator with a smaller footprint, the device must have a low figure of merit of V[pi]xL. Conventional EO polymer-based modulators offer decent bandwidth characteristics (>50GHz), however, the length - half wave voltage tradeoff limits the size and the practicality. In recent years, the use of slow-wave photonic crystal (PhC) structures has been shown to enhance the half wave voltage without increasing the modulator length. The goal of this work is to enhance phase sensitivity performance of an electro-optic (EO) polymer based phase modulator (PM) design by utilizing PhC structures. The addition of a PhC structure produces a slow-wave effect, which allows for control of the group velocity of the optical wave in the core of the integrated optic channel. PhC structures consist of a base substrate with a second material added along periodic lattice, in close proximity to the core of the optical waveguide. Analysis and numerical calculation group velocity in 1D and 2D PhC structures showed an increase in the effective group index of refraction, indicating a slowed group velocity of the lightwave. These structures were modeled using MATLAB by considering periodic structures to obtain the sensitivity of the group index of refraction in relation to cell size and periodicity. The PhC structure was analytically modeled and then verified using periodic boundaries utilizing finite element method (FEM). The FEM based HFSS simulation has included 1D and 2D PhC topologies using material combinations of PMMA/Air or PMMA/Si₃N₄ structures. The primary goal of this thesis is then achieved by adding the modeled slow wave structure to a traveling-wave EO polymer-based phase modulator as either a substrate or superstrate to the optical core. To quantify the phase modulation sensitivity improvement, the traveling wave phase modulator was modeled first using FEM and beam propagation methods (BPM) to model RF and optical characteristics. Then, both substrate and superstrate topologies are introduced to the PM design. Utilizing both techniques, the EO polymer based PM design with an added PhC structure reduces the length voltage product required to achieve optical phase of 180°. Phase improvement due to the added structure was achieved through modulator simulation, yielding an improved effective EO coefficient of 162.2 pm/V compared to 67.9pm/V from the original modulator. The optimal topology a high phase improvement was chosen to be the superstrate design using a 2D PMMA/Air topology, whereas the most practical manufacturing challenge was chosen to be the 2D PMMA/Si₃N₄ topology with a superstrate design.