A lasing wavelength stabilized simultaneous multipoint acoustic sensing system using pressure-coupled fiber Bragg gratings Jung-Ryul Lee a,n , See Yenn Chong a , Chang-Yong Yun a , Dong-Jin Yoon b a Department of Aerospace Engineering, Chonbuk National University, 664-14 Duckjin-dong, Duckjin-gu, Jeonju, Chonbuk 561-756, Republic of Korea b Safety Measurement Center, Korea Research Institute of Standards and Science, 209 Gajeong-ro Yuseong-gu, Daejeon, Republic of Korea article info Article history: Received 5 June 2010 Received in revised form 17 July 2010 Accepted 15 August 2010 Keywords: Fiber Bragg grating Tunable laser Lasing wavelength stabilization Integrated structural health management Acoustic emission abstract A fiber Bragg grating (FBG) sensor head, using a pressure coupling mechanism, was designed for broadband frequency response and structural strain-free characteristic. The pressure-coupled sensor heads were connected to a simultaneous multipoint acoustic sensing system based on a tunable laser. An intelligent lasing wavelength stabilization algorithm capable of identifying the direction of spectrum movement, the wavelength shifting speed, and a fiber bending event was developed so that the simultaneous multipoint acoustic sensing system could be used in environments with rapid temperature variations. The lasing wavelength feedback control algorithm updated the lasing wavelength into the steep slope of the FBG spectrum even under conditions of rapid temperature change. The averaging lasing wavelength updating time was only 21 s because the system can decide a minimal size in scan window by finding the FBG spectrum shifting speed and direction in real time. The system was able to update the lasing wavelength which missed the steep slope of the FBG spectrum under maximum temperature variation rates 0.3014 and 0.3246 1C/s. The proposed system detected simultaneous impact waves at multiple points under conditions of rapid temperature change and change in dynamic strain. & 2010 Elsevier Ltd. All rights reserved. 1. Introduction Acoustic emission (AE) is defined as transient elastic waves in the audible and ultrasonic regions. These waves are generated by the rapid release of energy within a material undergoing fracture or deformation due to material degradation, reversible processes, fabrication processes, or leak and flow [1]. AE detection and ultrasonic transmission and reception technologies are widely used for in-field nondestructive testing (NDT) and integrated structural health management (ISHM). Acoustic waves selected in the frequency range of up to 100 MHz [2] are generated and detected by piezoelectric transducers in NDT and ISHM. However, from the viewpoint of ISHM, conventional piezoelectric transdu- cers are susceptible to electromagnetic interference (EMI) and temperature change. In addition, piezoelectric sensor networks are usually too large and heavy, especially for aerospace applications, and are often inappropriate for long-distance, long- time, and large structure monitoring. In the last decade, fiber optic acoustic sensor networks have been proposed to overcome the limitations of conventional piezoelectric sensor networks. Fiber optic acoustic sensor networks have potential advantages for ISHM due to their multi-functional sensing ability, small size, flexibility, durability, EMI immunity, corrosion resistance, and exceptional embedding and communication capabilities. Prior studies of fiber optic acoustic sensing were categorized into three categories: single fiber intensiometric, fiber optic interferometric, and fiber Bragg grating (FBG) techniques [1]. In the category of single fiber intensiometric techniques, Chen et al. [3] developed an ultrasonic fiber optic sensor based on a 2  2 fused-tapered optical fiber coupler that incorporated mechanical strain amplification and increased the frequency response range from several tens of kHz to several hundred kHz. Later work [4] incorporated in-line multiplexing of two sensors and demonstrated linear localization of a pencil lead break AE source on an aluminum plate. However, AE sensing capability in a state accompanying structural strain and environ- mental temperature change has not been reported, which is very important so that the sensor can be used as a built-in AE sensor for ISHM. Kim et al. [5] developed a filtering wavelength stabilized gold-deposited extrinsic Fabry–Perot interferometric (EFPI) sensor system composed of a broadband light source, the fiber Fabry–Perot tunable filter, and a control-circuit board. They demonstrated the detection of fracture-induced AE by compen- sating for the low frequency phase drift resulted from quasi-static strains and temperature variations. However, the filtering wave- length stabilization algorithm was unable to operate under Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/optlaseng Optics and Lasers in Engineering 0143-8166/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2010.08.009 n Corresponding author. Tel.: + 82 63 270 4637; fax: + 82 63 270 2472. E-mail address: leejrr@jbnu.ac.kr (J.-R. Lee). Optics and Lasers in Engineering 49 (2011) 110–120