Wavelength multiplexing of MEMS pressure and temperature sensors using fiber Bragg gratings and arrayed waveguide gratings.

摘要

This thesis presents the design, fabrication and testing of wavelength multiplexing of optically interrogated MEMS pressure and temperature sensors using both fiber Bragg gratings (FBGs) and arrayed waveguide gratings (AWGs). The pressure sensors and temperature sensors have been designed and fabricated using MEMS techniques. For the pressure sensor, fabrication is initiated with a standard fused-silica wafer (Pyrex 7740) polished on both sides that is patterned to form a series of cavities for the Fabry-Perot interferometer. Positive photoresist is patterned and serves as a mask for etching the cavities in the glass wafer with buffered HF. A silicon wafer polished on both sides is then electrostatically bonded to the patterned glass wafer. Bulk etching techniques are used to thin the silicon wafer down to the desired diaphragm thickness, while the other side of the Si/glass assembly is protected. The configuration of a Fabry-Perot temperature sensor involves a thin layer of silicon bonded to a glass wafer. The fabrication process is similar to that of a pressure sensor, but with no air cavity.; Pressure measurements were made over the 0 to 30 psi range while temperature measurements were made over the 24 to 100°C range. Pressure sensor sensitivities of 0.022mv/psi for the multiplexing system using AWGs, and of 0.0072mv/psi for the multiplexing system using FBGs were obtained. The pressure sensor were designed with cavity diameter R0 = 300 mum, cavity depth d0 = 0.64 mum for the sensor operating at 850 nm, and d 0 = 1.1 mum for the sensor operating at 1550 nm. Diaphragm thickness for the two sensors were 14 mum and 15.5 mum. The temperature sensor was fabricated by bonding a silicon wafer with different thickness on the glass wafer. An anti-reflective coating of SiO was deposited on the top surface of the pressure sensor by evaporation. The purpose of this anti-reflective coating was to reduce sensor's temperature sensitivity. An oxidant-resistant encapsulation scheme for the temperature sensor was proposed, fabricated and tested, namely aluminum coated silicon nitride (Al/Si3N4).; The multiplexed sensor system using FBGs has the potential of multiplexing about seventy sensors depending on FBGs bandwidth and the one using AWGs has the potential of multiplexing about eight sensors depending on the number of AWGs channels and the bandwidth of each channel. A dual-wavelength method incorporating a tunable laser was used to interrogate either the applied pressure or temperature experienced by the sensor, while a three-wavelength method was used to simultaneously interrogate pressure and temperature. Experimental results, including response as a function of pressure or temperature, were characterized by good agreement between experimental and theoretical results. There is no observable cross-talk between the multiplexed sensors.