Dynamic Strain Measurement with Fiber Bragg Grating Sensor System for Smart Structure

Fiber Bragg grating (FBG) sensor systems are being widely used as temperature measurement and strain measurement systems for aerospace structures, civil structures, and high rise buildings. However, the systems are rather complicated because the wavelength change must be measured using several kinds of optical filters such as Fabry-Perot filters, edge filters, and bandwidth filters. In this paper, FBG sensor system for dynamic strain measurement is proposed. The system consists of SLD (Super Luminescence Diode), diode driver, FBG sensors and the photo diode as a detector. Neither Fabry-Perot filter nor edge filter is applied. SLD has its own intensity slope according to various wavelengths. The slope is very linear at certain wave length range. In the concrete beam experiments, the 1525 nm center wavelength FBG is employed as a sensor and the grating shows good linear responses to the dynamic loads. The data compare to those of electric strain gauges. The system also has a potential to be multiplexed by the pulse modulations in the time domain. If the FBGs have different wavelengths, they can be placed in the same fiber and if the FBGs have the same wavelength, they must be in the separate optical fiber and connected with couplers in order to be multiplexed. Introduction Fiber optic sensors are primary candidates for structural monitoring, which is progressively being utilized for advanced structures. Several of previous studies performed to the various fiber optic sensors [1-5]. Recently, Fiber Bragg Grating (FBG) sensors became dominant and they are very useful for sensor application and filter application [6-7]. The performance of the ordinary FBG sensor strain measurement system, which uses Fabry-Perot filter for scanning wavelength [8], has limitation for application because of hysteresis characteristics of PZT element in the filter, slow scan rate of the filter and the high cost of system. Band path filter or edge filter can be used for signal processing system for FBG. For the band path filter, the important things are flatness of the plate of the filter and the suppression of the side modes. The band width should be big enough to cover the 5~6 nano meter shift which occurs under the strain of 5,000~6,000 micro strain. For the edge filter, the smoothness and linearity of edge are important and the slope and the band width must be controlled. The temperature dependence must be eliminated, but it is very difficult. Therefore, most probable sensor system is no filter system. In this paper, FBG sensor system with SLD(Super Luminescent Diode) source and without any filter is proposed. It emits the light in the wide range wavelength. Especially it shows good linearity in 1510~1540nm wavelength. SLD light source emits different intensity according to the wave length. Therefore the variation of the wavelength in FBG due to strain varies the reflected intensity. Key Engineering Materials Vols. 270-273 (2004) pp 2114-2119 online at http://www.scientific.net © (2004) Trans Tech Publications, Switzerland Online available since 2004/Aug/15 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 130.203.133.33-14/04/08,14:54:13) Title of Publication (to be inserted by the publisher) Principles of Fiber Bragg Gratings Among typical fiber optic sensors, fiber Bragg grating sensors (FBGs) and fiber optic Fabry-Perot sensors are widely used. Fiber optic Fabry-Perot sensor is very sensitive but it is very difficult to produce by the automated mass production process and its signal treatment is very complicated. On the other hand, fiber Bragg gratings can be produced by mass production equipments and they have very good reproducibility. Their signal processing is rather simple. Many of previous investigators chose fiber gratings as sensors and we also choose FBGs for structural monitoring sensors.