Light Intensity Fibre Optic Sensor for MEMS displacement and vibration metrology

Micro- and Electro-Mechanical Systems (MEMS) form a group of transducers used for very precise mass sensing (in the range of femtograms) and force detection (in the range of piconewtons) changes. These can be read by the resonant frequency shift or amplitude of mechanical vibration changes. Monitoring of optical path is one of the ways in which this detection can occur. The use of fibre construction makes the read-out system even more attractive. There are two popular types of Fibre Optics Displacement Sensors (FODS). The first – with amplitude of optical wave modulation – intensity sensors. The second – with phase modulation – fibre optics interferometers. In this paper Intensity Fibre Optic Displacement Sensor (I-FODS) for MEMS displacement metrology has been presented. A system formed by simple optical fibre optics, light source and photodetector had been tested and evaluated in order to obtain the highest possible sensitivity and resolution, using low-cost standard telecommunications equipment. The sensor׳s system is dedicated to MEMS amplitude of vibrations metrology and allows displacement monitoring in the range up to 400 μm with the resolution of 15 nm and the sensitivity of 0.19 V/nm for a mirror reflecting surface. The paper shows Quartz Tuning Fork (QTF) as an example of the MEMS with a very rough surface and its resonance frequency is equal to about 32 kHz. The designed I-FODS allows the QTF amplitude measurements with the resolution of 92 nm and the sensitivity of 0.017 mV/nm.