Diamond turned micro machined metal diaphragm based Fabry Perot pressure sensor

Micro-machined Silicon (Si) diaphragm is extensively studied in Fabry Perot Interferometer (FPI) based pressure sensors, but its applications are limited to non-corrosive media due to poor corrosion resistance. Metal alloys such as Copper-Beryllium Alloy (CBA), Stainless Steel (SS) and Ni based alloys have comparable mechanical properties and much better corrosion resistance than Si. But their use as diaphragm for FPI based pressure sensor is much under reported. In this paper, design, analysis and fabrication of an FPI based pressure sensor with CBA diaphragm is discussed. Its sensing probe has two parts, a sensing head and a sleeve, welded together. The sensing head is a three-dimensional monolithic component which consists of pressure sensitive diaphragm and a step for FPI cavity. The sleeve guides the optical fiber to a fixed position such that the cleaved end of the fiber forms an FPI with the diaphragm. Both the sensing head and the sleeve are manufactured by Diamond Turning Machining (DTM) process and have CBA as material of construction. This manufacturing process ensures that flatness on interferometric side of the diaphragm and its parallelism with the fiber end are achieved within 2 mu m; surface finish (R-a) on same side of the diaphragm is less than 10 nm. The sensor showed sensitivity of more than 1 mu m/bar within the measurement range of 0-7 bar. DTM machined sensing probe with all essential FPI features (flatness, surface finish and parallelism) and welded diaphragm in FP pressure sensor are being reported for the first time to the best of our knowledge.