Validation of Packaged Clad-Etched Fiber Bragg Grating as Underwater Acoustic Sensor

The low intrinsic pressure sensitivity of fiber Bragg grating (FBG) sensors impede their potential applications in the field of hydro-acoustics. This article demonstrates a simple, yet effective way of improving the aggregate hydro-acoustic sensitivity of a bare FBG sensor through clad-etching of the fiber sensor followed by a side-hole polymer packaging. The proposed underwater acoustic sensor system is characterized and validated with a standard lead zirconate titanate (PZT)-based hydrophone under various experimental conditions. This is the first time, to the best of our knowledge, such a high acoustic pressure sensitivity enhancement of
$\sim 50$
dB, in comparison with a bare FBG sensor, is reported using a conventional and highly operationally stable wavelength interrogation technique. The minimum detectable acoustic pressure is calculated to be
$\sim 1$
Pa/
$(\text {Hz})^{1/2}$
with a maximum responsivity of
$\sim 100$
nm/MPa in the investigated frequency range, with excellent performances in terms of linear response and repeatability. The dynamic range, in the present scenario, is only limited by the range of the interrogation system. The proposed underwater acoustic sensor along with the interrogation technique is a highly compact and cost-effective method for underwater acoustic sensing. Thus, the obtained results can be used for developing and producing long, narrow hydro-acoustic quasi-distributed sensor arrays for marine explorations and other applications.