<title>Digital demodulation of a twin-grating fiber optic sensor</title>

Abstract

New digital demodulation algorithm for recently announced twin-grating fiber-optic sensor is presented. The sensor is built with two short, identical, low reflectance, closely spaced Bragg gratings imprinted in a telecommunications fiber. Because of such a design the reflection spectrum of this sensor is a sinusoidal wave within an envelope of the reflection spectrum of a single Bragg grating. Similar to traditional sensor based on Bragg gratings, the change of the temperature or strain leads to a shift of the reflection spectrum along the wavelength axis. However the reflection spectrum of the twin-grating sensor has more complicated structure and instead of measuring the position of gravity center of the spectrum we use more sophisticated algorithm to evaluate the measurands. Two computational methods are used to estimate the shift of the spectrum: evaluation of the phase of the sinusoidal wave and calculation of the position of the envelope. This combination of two different methods provides high accuracy of temperature or strain measurements. The ability of this sensor to provide simultaneous measurement of both temperature and axial strain is also demonstrated using computer simulation of twin-grating sensor in a Hi-Bi fiber.