BARC NEWSLETTER RESEARCH ARTICLE BARC NEWSLETTER 30 I ISSUE NO. 327 I JULY - AUGUST 2012 30 I ISSUE NO. 330 I JAN. - FEB. 2013 TECHNOLOGY DEVELOPMENT ARTICLE Development of DSP Based Signal Acquisition and Processing System for Extrinsic Fabry-Perot Interferometric (EFPI) Fiber Optic Sensors Ashwin Rathod, Shivam Mishra and Shrinkhla Ghildiyal Precision Engineering Division and Sushil Bahuguna, Sangeeta Dhage and S. Mukhopadhyay Control Instrumentation Division Introduction The advent of fiber optic sensors has brought numerous potential advantages over conventional electrical signal based sensors, such as small size and weight, immunity to electromagnetic interference, lack of sparking hazard, larger bandwidth, easy interface with data communication system, higher accuracy and resolution, and high multiplexing potentials. Along with this, the low optical loss of the fibers makes it possible to locate sensors far from the signal processing electronics. Their ability to withstand high level of radiation, temperature and pressure makes them ideal for applications in harsh environments. The optical fiber sensing proves to be a remarkably versatile approach in the field of measurement. Indigenous development of Fiber Optic Sensors based on EFPI principle has been carried out for Pressure (Gauge), Temperature and Low Pressure (Absolute) transducers. The article presents the development of a DSP based signal acquisition and processing system for Extrinsic Fabry-Perot Interferometric (EFPI) sensors, jointly by Precision Engineering Division and Control Instrumentation Division. It describes the basic concepts of EFPI sensors and the algorithm used to estimate its cavity length. The hardware configuration and the implementation issues are covered in detail. The experimental results of an EFPI temperature sensor interfaced with this hardware have also been presented. Fiber Optic Sensing In an EFPI sensor, two mirrors (either or both mirrors could be ends of fibers, separated by an air cavity or any dielectric material other than fiber) form the interferometer. The distance between the two mirrors is called the cavity length or gap length. Fiber optic white-light interferometry (WLI) has been widely used to measure the cavity length of an EFPI. This technique allows absolute and unambiguous measurements over a wide range and is insensitive to the light source instability. WLI uses a broad band light source to illuminate the sensor and a spectrometer senses the optical signal. A change in the physical parameter, to be measured, causes a calibrated change in the cavity length of the EFPI. Hence the primary objective is to get an accurate and reliable measure (calibration) of cavity length. By using different transduction principles, different type of EFPI sensors can be developed. Fig. 1: EFPI configuration formed using a fiber end and a diaphragm.