Optical MEMS Pressure Sensors Incorporating Dual Waveguide Bragg Gratings on Diaphragms

Abstract

The wavelength shift of single Bragg grating integrated into a waveguide in optical MEMS pressure sensors gives inaccurate result due to the cross sensitivity caused by multiple external factors including temperature. Novel dual waveguide Bragg gratings (WBGs)-based microelectromechanical system pressure sensors with silicon micromachined diaphragm are presented here. The sensor consists of a curved waveguide with two identical WBGs located on the diaphragm. When pressure is applied on the diaphragm, the pitch of the gratings changes, and hence, there is a shift in the corresponding Bragg wavelength. The effect of temperature is assumed to be the same for both the gratings. As temperature shifts the Bragg wavelength of the two gratings equally, the error due to the temperature change on pressure sensitivity is eliminated. This method of pressure measurement effectively cancels out the temperature sensitivity. Three different configurations of the sensor with circular, square, and rectangular diaphragms are designed and simulated using COMSOL Multiphysics. Pressure sensitivity for the circular diaphragm configuration is found to be 2.1 and 0.744 pm/Pa for the two gratings, which are higher than the square and rectangular diaphragm configuration. Hence, the circular diaphragm configuration is better suited for multiplexing. The temperature independent properties of the sensors as well as the adjustability of sensitivity and measurement range by means of altering the dimensions of the sensors are studied. The temperature sensitivity is the same for both the gratings and is found to be 11.6 ppm/°C. Using modulation techniques and suitable designs, these sensors find applications in remote distributed sensing.