Measurement of differential pressure by optical interferometry with multimode tunable lasers

Abstract

Determining the deformation of a capacitive membrane is a well-known pressure measurement technique. A novel technique is presented for pressure measurement based on low coherence speckle interferometry (LCSI) to optically evaluate the deformation of a membrane submitted to a pressure differential. By processing the low spatial frequency speckle fringe pattern through four-stepping and phase unwrapping methods the membrane contour was determined. A tunable multimode red diode laser is the light source. A circular aluminum membrane was submitted to pressure values up to 90 kPa resulting in a curve of the central membrane deformation as a function of the pressure. An External Cavity Diode Laser (ECDL) was used for laser tuning thus changing the interferogram spatial frequency and the sensitivity of the measurement process. The interferometry results were compared with those obtained experimentally by a feeler clock and by an analytical expression. The possibility of applying this technique in quasi real-time testing is explored.