Design of a vertical cavity optical micro-electromechanical filter

Abstract

Based on Micro-Opto-Electro-Mechanical System (MOEMS) technology, vertical cavity devices with an air-gap enable very efficient ultra-wide continuous tuning with only a single control parameter. The vertical cavity filters and Lasers consist of an air-gap cavity embedded between two highly reflective Bragg mirrors. Vertical Cavity Surface Emitting Lasers (VCSELs) have an active layer additionally integrated in the cavity. The ultra-wide continuous tuning is achieved by micro-electromechanical actuation of the mirror membranes. Our research activities concentrated on Bragg mirror material system with InP/air-gap mirror structures. The resonator quality depends on the refractive index contrast between the Bragg mirror layer materials and on the number of periods. The advantage of the semiconductor/air-gap system is the very high refractive index contrast ensuring a high reflectivity of more than 99.8% with only three layer periods. The semiconductor/air-gap structures forming the membranes of one Bragg mirror are n-doped while the other mirror membranes are p-doped forming a diode structure. Due to device miniaturization very efficient tuning is achieved by electrostatic actuation. The investigations presented focus on an InP/air-gap Fabry-Pérot-filter for air =1.55μm with an air-gap cavity thickness equal to a multiple of 1⁄2 air embedded between two InP/air Bragg mirrors. The InP membranes have a thickness of of 3⁄4 InP , the air-gaps in between are 1⁄4 air thick. The tuning behaviour is determined by the mechanical flexibility of the membranes and by the optical properties of the vertical cavity. The electrostatic deflection due to a reverse voltage is calculated with FEM simulations incorporating the effect of stress in the membranes. The results of the model calculations compared very well with measured values. This study describes the optical micro-electromechanical design of the novel low cost device structure applicable in optical filters, vertical cavity surface emitting lasers (VCSELs) and a host of other optical applications. We also present a solution to one of the most challenging problems in the design of micro-electromechanical devices consisting of thin-walled structures - that is residual stress. Residual stress alters device characteristics if not taken into consideration during design and characterisation.