Heterodyne detection for Fiber Bragg Grating sensors

In this paper, we present a Fiber-Bragg-Grating-based temperature sensor. The technique employs heterodyne detection using two Fiber Bragg Gratings. One of the gratings is used as a reference (local oscillator) and the second as a sensing arm. This sensor uses a Folded Mach-Zehnder interferometer. As the temperature changes, the Bragg wavelength of the FBG shifts. The heterodyne detection is used to detect the frequency di erence between the reference and sensing signals that is caused by the temperature change. The dynamic range and sensitivity of the sensor were analyzed and presented. c © 2000 Published by Elsevier Science Ltd. All rights reserved. Optical ber sensors became devices of choice for many applications over the years [1]. A wide range of ber sensors have been introduced based on Fiber Bragg Gratings (FBG). Fiber sensors use a variety of techniques to detect the measurands. Some of these based on intensity [2], phase [3], polarization [4], or wavelength changes [5]. Phase sensors are usually based on Mach-Zehnder (MZ) interferometer. The MZ interferometer is an optical instrument capable of high-resolution phase analysis [6]. Recently, FBGs were developed, and enjoyed wide spread applications in telecommunications and sensing. The Bragg wavelength of FBG is very sensitive to the environment. This property is exploited in the sensing applications. It has been demonstrated that the use of interferometric detection of the wavelength shift of a Bragg Grating Sensors can yield very high sensitivity of grating temperature and strain [7,8]. FBG has become the most attractive intrinsic ber sensor in recent years for various reasons [9]. One of the major advantages of this type of sensor is attributed to wavelength-encoded information provided by Bragg grating when a ected by the measurands. Since the wavelength is an absolute parameter, signals from a FBG may be processed such that its information remains immune from power uctuations along the optical path. Other advantages are the small size, rugged and intrinsic ∗ Corresponding author. Tel.: +1-256-890-6316; fax: +1-256-8906803. 1 Hedi Bellil is also with Institut Superieur des Etudes Technologiques de Tunis, Tunisia. nature of these structures, as well as their multiplexing capabilities. FBG temperature sensors have been developed with the characteristic advantages of wavelength encoding distributed sensing, and low cost. In this kind of sensors, as the sensed information is encoded directly into wavelength, the output does not depend on the total light levels, losses in the couplers, or light source power uctuations. In this paper, we present a new technique for measuring the Bragg wavelength shift using heterodyne detection. In such sensors, we detect the wavelength shifts of the re ected light, induced by the temperature changes in the sensing FBG element. The system used in this temperature sensor is shown in Fig. 1. It is in architecture we name Folded Mach-Zehnder (FMZ) interferometer. The FMZ is made of single-mode bers with two inscribed identical Bragg gratings. One of these gratings is along the reference arm while the other is along the sensing arm. The light from the broadband source is launched into the single-mode ber and split by a coupler into the two arms of the FMZ interferometer. The light waves will be re ected by the FBGs and are then combined by couplers C1 and C2 at the output. The phase di erence between the two output waves is measured using heterodyne detection. This phase shift is caused by the change in the Bragg wavelength of the sensing arm caused by the temperature change. The Bragg wavelength of the re ected beam from the FBG is given by