Low‐frequency detection of acoustic signals using fiber as an ultrasonic guide with a distant in‐fiber Bragg grating

This article shows the development of a photonic detector of low‐frequency ultrasonic signals. These signals latter reach a sensitive single‐mode optical fiber as an acoustic guide conveying the signals (around 43 kHz) over 70 cm to a distant and protected in‐fiber Bragg grating. The grating reflects a spectrum centered around 1550 and 2.6 nm bandwidth and is interrogated by a tunable laser in their linear and most sensitive region. As a result, real‐time passive demodulation of communication or monitoring of ultrasonic signals propagating through solid materials is possible. The detector may also be applied to other physical media. In this study, a new passive technique to thermally desensitize the fiber‐Bragg gratings (FBG) is also proposed, though the proposed technique is not experimentally tested.