Underwater Acoustic Sensors Based on Fiber Bragg Gratings

Abstract

We report on recent results obtained with a fiber optic hydrophone based on the intensity modulation of the laser light in a FBG (Fiber Bragg Grating) under the influence of the sound pressure. In order to control the behavior of the hydrophone in terms of sensitivity and bandwidth, FBGs have been coated with proper materials, characterized by different elastic modulus and shapes. In particular, new experiments have been carried out using a cylindrical geometry with two different coating, showing that the sensitivity is not influenced by the shape but by the transversal dimension and the material characteristics of the coating.

Resonant hydrophones based on coated fiber Bragg gratings. Part II: experimental analysis

“…To this aim, a tunable laser was used to lock the operating wavelength on the linear edge of the grating response. Since spectrum of FBG shifts without changing its shape [3,[6][7], the change in reflection at the laser wavelength is also expected to be proportional to pressure. The optoelectronic setup thus consists in a stable tuneable laser (Yokogawa AQ2200-136), a 2x1 coupler capable to split transmitted and reflected signals whereas a photodiode provides an electrical output proportional to the optical power of the laser reflected from the grating edge.…”

“…Finally, when the coating size approaches the acoustic wavelength, a decrease in the sensitivity is observed that can be attributed to acoustic diffraction. In order to validate the numerical results, here, we report the experimental analysis of the acoustic performance of cylindrically coated FBG hydrophones with different diameters in an extended frequency range (4Khz-35Khz) compared with our previous work [6][7]. In particular, here we focused the attention on two sensors with height fixed to 30mm and coating diameter of 5 mm and 10 mm, respectively (here named 5-30 and 10-30 respectively).…”

“…In particular, underwater acoustic measurement is an important area of interest and numerous sensors based on FBG [1][2][3] or fiber grating laser [4][5] have been demonstrated: the former configurations (based on FBG) are very easy to use whereas the one based on active FBGs are more sensitive. While the most of works involve bare FBG demonstrating low sensitivity due to the high Young module of silica fiber, the authors efficiently demonstrated the possibility to enhance the FBG sensitivity to sound pressure by coating the fiber (hosting grating) with appropriate polymeric coatings in terms of shape, size and materials properties [6][7]. In the last papers a theoretical/numerical analysis was provided only for static pressure measurements based on Hocker analysis [8].…”

Highly sensitive fiber Bragg grating-based pressure sensor using side-hole packaging

“…Previous studies measure 3 pm of shifts in the Bragg wavelength when a pressure of 1 MPa is applied on a bare FBG sensor [5]. This prevents the FBG sensors from large deformation under acoustic pressure [6]. This eventually results in the low sensitivity of the FBG-based sensors in their use in underwater applications.…”

Analysis of ultrasonic frequency response of surface attached fiber Bragg grating

“…The most significant performance is their immunity to electromagnetic interference, which enables applications under harsh environment [4]. In recent years, FBG has become an attractive candidate for ultrasonic wave detection, because it can be easily embedded in the structure of the body being monitored and is ideal for multiplexing [5,6].…”