Optimization of the Optical Measuring Converter’s Geometric Properties for the Microelectromechanical Pressure Sensor

Abstract

Nowadays development and production of microelectromechanical systems is one of the most promising directions in the world's economy. One of the innovative fields in the development of microsystems is integration of optical devices as measuring transducers. Goal of the paper is optimization of the optical measuring transducer geometry for microelectromechanical pressure sensor which provides a required value and linearity of the optical transmission coefficient. Microelectromechanical pressure sensor comprises the optical measuring transducer represented by a pair of waveguides that form an optical directional coupler. Linearity of the optical transmission coefficient of the transducer is provided by the selection of the initial gap between the waveguides at the linear section of the curve representing dependence of the optical transmission coefficient on the gap. Calculation of the required characteristics of such transducer requires a combination of the Finite-difference time-domain method and the mode overlapping method. This allows calculating the magnitude of the optical transmission coefficient for different geometric parameters of the transducer. Two models of the directional coupler with silicon and silicon nitride waveguides were used to determine dependencies of the transducer's optical transmission coefficient on the optical coupling length and the waveguide bending radius. The data obtained were used to plot the dependencies of the transmission coefficient on the gap between the waveguide for both models. The plots show the optimal initial gaps and the length of the linear section. The results allow designing a device with predetermined working section at which the optical measuring transducer can measure membrane displacements that are linearly proportional to an acting pressure. Such working section is characterized by the initial gap in the middle of the linear section that amounts to 500 nm for silicon waveguides and 600 nm for silicon nitride ones. The linear section of the transducer's transmission characteristic for the waveguides of both types is estimated as ±80 nm in relation to the initial gap. In this section, the optical transmission coefficient of the transducer with silicon waveguides alters from 0 to 0.86 which corresponds to a value of 5.375‧106 m-1. For the waveguide with silicon nitride waveguides the coefficient varies from 0.09 to 0.53 which corresponds to a value of 2.75‧106 m-1. The computer aided analysis methods allow for determining the optimal geometry of the evanescent coupling-based measuring transducer that is used in the microelectromechanical pressure sensor. The presented models of two transducers with two waveguides made of different materials demonstrate different characteristics. The optimal parameters for each of the models are achieved at different waveguide bending radii and optical coupling lengths.