Fabry-Perot Diaphragm Fiber Optic Sensor (DFOS) for Acoustic Detection Y. Sun * , G. Feng * , G. Georgiou*, I. Padron*, N. Edip * , K. Noe ** and K. Chin * * New Jersey Institute of Technology, Materials Science Program, 07031 USA, ys2@njit.edu ** PSE&G Corporation, Newark, NJ, USA, Karen.Noe@pseg.com ABSTRACT The diaphragm fiber optic sensor (DFOS) solely based on Fabry-Perot multiple beam interference has been designed and fabricated with Micro-electric mechanical system (MEMS) technology. The silicon diaphragm with an embossed center was designed with an interference gap width kept accurately. The DFOS is verified to be truly and purely of Fabry-Perot type via a critical test. Parallel testing of DFOS in comparison with a Piezoelectric (PZT) sensor shows that DFOS has high sensitivity. Fabry-Perot DFOS has demonstrated excellent performances in on-line monitoring of Partial Discharge (PD) in power transformers. Keywords: MEMS, Fabry-Perot, fiber optic, acoustic, diaphragm with embossed center 1 INTRODUCTION A diaphragm fiber optic sensor (DFOS) utilizes a diaphragm as the sensing element to detect static and dynamic pressure (acoustic wave). The fiber is used to deliver a steady probing light and to receive the reflected light modulated by the signals under detection. In recent years diaphragm-based fiber optic sensors have been developed for static pressure and acoustic signal detection due to its high sensitivity, flexibility, and versatility of its diaphragm-fiber structure, as well as resistance to electromagnetic interference (EMI) caused noise, and easiness for multiplexing and integration [1-4]. In this research, the silicon diaphragm was designed to have an embossed center and micro-channels in order to improve efficiency, alignment, linearity, and Q-point stability which are still critical issues in diaphragm-based fiber optic sensor design. Currently an imminent application of DFOS is detecting partial discharge (PD) in high voltage transformers in the power industry. PD is an electrical discharge that occurs in an insulation system where the charge does not completely bridge the electrodes. It is a well-known phenomenon and a precursor to complete insulation failure. Partial discharge within the power transformer could lead to degradation of the insulation system and may therefore result in catastrophic failures [5-9]. It is a big concern for the power industry. As a consequence, there is a strong need for a capable sensor to detect and study PD. 2 SENSOR SYSTEM DESIGN 2.1 Sensor System Configuration Figure 1: Schematic of DFOS system. The acoustic detection system is illustrated schematically in Figure 1. Acoustic signal is detected by using the developed DFOS system, which consists of a DFOS, a 1527 nm DFB laser, a 3-port fiber circulator, a photodiode and a filter and amplification circuit. The light from DFB laser propagates along the single mode fiber to the DFOS through the circulator and interferes inside the DFOS. The modulated light propagates back in the third leg of the circulator and is detected by the photodiode. The optical signal is turned into electric signal. Its output voltage is filtered and amplified and finally processed and collected by an oscilloscope. 2.2 Sensor Operation Principle Fabry-Perot interferometric device is based on the interference of multiply reflected beams. The interference gap L corresponding to applied pressure is determined by measuring the inference spectrum. The spectrum of the ratio of output and input optical power is defined as. I (0) I ( i ) = 2 R a ! 2 R g cos " 1 + R g 2 ! 2 R g cos " (1) where R a is the arithmetic mean reflectance of the interfaces, R g is the geometric mean reflectance of the interfaces, and φ is the phase shift of the light propagating across the interference gap L. The mechanism of DFOS is usually described by the Fabry-Perot interference of multiply reflected beams between the diaphragm surface and the fiber end surface. The structure of the Diaphragm Fiber Optic Sensor (DFOS) is shown in Figure 2. The diaphragm, as the sensing NSTI-Nanotech 2007, www.nsti.org, ISBN 1420061844 Vol. 3, 2007 229 229