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This paper reports a novel optical fiber-based micro-shear stress sensor utilizing a flexible membrane and double SU-8 resist structures as a moving micro-mirror, together with an optical fiber as a micro-Fabry-Perot interferometer. This sensor can be employed in air or liquid environments with high sensitivity because of its waterproof design. Through UV lithography processes on thick SU-8 resist, the roughness of the reflection surface has approached 7 nm (Ra value), suitable for optical applications. A single-mode optical fiber is employed for detecting the displacement of the floating element induced by shear stress. Tests have been carried out successfully and a detection possibility of 10 nm-displacement of the floating element and a displacement sensitivity of 0.128 nm/nm (spectrum shift/floating element displacement) has been demonstrated. The temperature coefficient of this fiber-optic sensor has been characterized to be 3.4 nm/K linearly from 25 to 48°C. Fluid tests have also been performed by placing the sensor inside the inner wall of a precisely machined rectangular channel and the result shows a sensitivity of 0.4 nm/ml/min (spectrum shift/flow rate), corresponding to a shear stress resolution of 0.65 Pa/nm (shear stress/spectrum shift). The minimum detectable shear stress is thereby estimated as 0.065 Pa from the reading resolution of the spectrometer of 0.1 nm, comparable to its counterparts with resolutions from 0.1-1 Pa.
Date of Publication: Dec. 2003