Sensors and Actuators A 138 (2007) 76–80 Research of a novel fiber Bragg grating underwater acoustic sensor Xingjie Ni, Yong Zhao ∗ , Jian Yang Department of Automation, Tsinghua University, Beijing 100084, PR China Received 9 November 2006; received in revised form 20 April 2007; accepted 20 April 2007 Available online 27 April 2007 Abstract A new type of optical fiber underwater acoustic sensor (hydrophone) constructed with two fiber Bragg gratings (FBG) and a self-demodulation method is presented in this paper. Unlike other FBG sensors which have separate sensing and demodulating elements, this FBG hydrophone has only a single structure for both sensing and demodulating using a pair of matched FBGs. The cylindrical structure of the hydrophone solves the problems of low sensitivity and cross-sensitivity of the bare fiber Bragg grating acoustic sensors. Theoretical analysis of measurement sensitivity is described. Simulation and preliminary experimental results indicate that the measurement sensitivity can reach 0.78 nm/MPa for underwater acoustic pressure in range of 100–200 dB re 1 Pa. © 2007 Elsevier B.V. All rights reserved. Keywords: Fiber Bragg grating; Acoustic sensor; Optical hydrophone; Self-demodulation 1. Introduction Fiber optic hydrophone is a new type of hydrophone came with the development of the optic fiber optoelectronics. J.A. Bucaro (1997) [1] first reported their research on applying optic fiber technology to the underwater acoustic sensor sys- tem. Fiber Bragg grating sensors not only have the advantages of conventional fiber optic sensors, but also have some other unique features: FBG sensors are immune to the disturbance to front-end optical fibers; An FBG sensor uses reflection of narrow wavelength bandwidth and therefore can be read- ily wavelength/time/spatial division multiplexed for multipoint sensing or simultaneous measurement of various quantities [2]. FBG was first applied to measure the underwater acoustic by Nobuaki Takahashi (2000) et al. [3]. A FBG reflects selectively incident light when its wavelength is equal to the Bragg reflec- tion wavelength. When there is a pressure applied to an FBG, the Bragg reflection wavelength shifts because of not only the physical lengthening/shortening effect due to the elasticity of the fiber but also the change in the index of refraction of the fiber core due to the photoelasticity. They made use of this char- acteristic to detect the signal of underwater sound field and got wonderful results including high linearity, wide dynamic range, ∗ Corresponding author. Tel.: +86 10 62795961; fax: +86 10 62786911. E-mail address: zhaoy18@mail.tsinghua.edu.cn (Y. Zhao). small probe size and so on. However, the Bragg wavelength shift induced by acoustic pressure is very small, which is about 6 pm/MPa [4] and this caused a lot difficulty in signal detection and demodulation. In this paper, we developed a novel underwater acoustic sen- sor based on fiber Bragg gratings driven by elastic materials, which significantly improves the underwater acoustic sensitiv- ity of the FBG sensor. Moreover, the sensor equips two FBGs for measuring and has differential output, which leads to fur- ther improvement of sensor’s sensitivity and immunity to the environment temperature change. Besides, a self-demodulation method is used in the sensor unit, which makes the structure more simplified. 2. Principles and sensor construction Figs. 1 and 2 show the structure of the fiber Bragg grating underwater acoustic sensor probe. The rigid structure which has an ‘I’ shape cross section in Fig. 1 is made of inelastic material. There are two elastic elements attached to the inner and outer side of the rigid structure. FBG1 is attached on the surface of the outer elastic elements and FBG2 is attached on the surface of the inner one. Fig. 3 shows the system configuration of the fiber Bragg grating underwater acoustic sensor. The light from an optical broad-band source (BBS) transmits selectively through FBG1. 0924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.04.055