Study and design on a facilitated system of fiber grating sensor

Abstract

A facilitated fiber Bragg grating detection system for detecting temperature is presented. The scheme uses a processing fiber Bragg grating that is stuck on a cantilever beam and is identical to one used as a sensor to demodulate the temperature signal. A step motor that is controlled by PLC (Programmable Logic Controller) stretches the cantilever beam periodically, and the system of two gratings produces a carrier at this frequency with its phase modulated by the temperature signal applied to the sensing grating. The temperature signal, which is demodulated by this system, is transmitted to PC then processed and displayed in PC. This system's configuration is simple, sensitivity is high, and cost is low.The fiber Bragg grating ( FBG) has gained a considerable research effort since 1989, when the FBG was discovered by Meltz, particularly on systems using these devices for sensing applications because of its ease in handling, low cost ,small size, and inherent advantages incorporated with fiber communication system[1][2] [3]. The basic principle of operation commonly used in a sensor system basing on FBG is to monitor the shift in wavelength of the returned "Bragg" signal with the changes in the measurand (e.g., strain, temperature). The dependence between the temperature drift and the Bragg wavelength shift caused by the thermal expansion of the FBG can be theoretically predicted and has been experimentally verified. To detect such a wavelength shift of a FBG, many FBG sensor configurations have been proposed, such as spectrum analyzer, acousto-optic tunable filter, Fabry-Perot filter, Mach-Zehnder interferometer, matched fiber grating filter, laser-sensor combinations, and so on. In order to reduce cost, complexity and improve detection resolution of the system, we analyze the feasibility of using a FBG filter based on tunable cantilever beam to demodulate of the signal from the sensor FBG and verify the result in theory. Then, one scheme that consists in the filter based technique which uses a second FBG for the interrogation of the signal of temperature from the sensor FBG has been pointed out in this paper, in our scheme, the second FBG is stuck on cantilever beam that is connected to a step motor, which can tune the second FBG.In this paper, we illustrate important points in regard to the proposed FBG temperature detecting system. First, the configuration of demodulation part of the system that uses a matched FBG stuck on cantilever beam. Experimental results show that a fairly high precision of wavelength can indeed be obtained by using the proposed method. Also, we describe the FBG sensor's configuration and parameter of temperature characteristic. The rest of this paper is organized as follows. In Section II, basic theory of temperature sensitivity enhancement of FBG, displacement of the free-end of the cantilever beam, and reflective spectrum correlation of two gratings are given. Experimental results of the system are shown in Section III, the results of both temperature sensitivity enhancement of FBG a...