Received: 23 August 2017 DOI: 10.1002/mop.31061 Low-frequency detection of acoustic signals using fiber as an ultrasonic guide with a distant in-fiber Bragg grating Wescley A. Leal | Mateus B. R. Carneiro | Taiane A. M. G. Freitas | Cl audia B. Marcondes | Ricardo M. Ribeiro Laborat orio de Comunicaç~ oes  Opticas (LaCOp) Departamento de Engenharia de Telecomunicaç~ oes, Universidade Federal Fluminense 24.210-240, Niter oi, RJ, Brasil Correspondence Ricardo M. Ribeiro, Laborat orio de Comunicaç~ oes  Opticas (LaCOp) Departamento de Engenharia de Telecomunicaç~ oes, Universidade Federal Fluminense 24.210-240, Niter oi, RJ, Brasil. Email: rmr@pq.cnpq.br Abstract 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 band- width 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 ultra- sonic 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. KEYWORDS acoustic communications, acoustic guide, acoustic monitoring, fiber Bragg grating, optical fiber, ultrasound 1 | INTRODUCTION Some physical media impede electromagnetic propagation: petroleum, 1 metallic layers, 2 energy cables, 3 and so forth. Piezoelectric transducers (PZTs) have been used to generate and detect acoustic signals. PZTs are practical in many appli- cations because they are compact, sensitive and widely avail- able in the market. 4 Acoustic sensors based on optical fibers are immune to electromagnetic interference (EMI), broad- band in their operation and electrically isolated, among other aspects. Optical sensors based on fiber-Bragg gratings (FBG) and interferometers intended for acoustic detection can fea- ture comparable or even higher sensitivity than the traditional PZTs. 5–9 It may often be appropriate—or even necessary—to install an FBG relatively far from the acoustic signals’ point of incidence. In so doing, an FBG automatically becomes protected against strain and permanent damage that may occur at the point of detection. However, the distance implies a reduction in the sensitivity caused by acoustic attenuation along the optical fiber conveying the signals. 6,10–12 The optical response of an FBG is not only sensitive to the acoustic signals of interest but also to temperature, strain and environmental thermo-acoustic fluctuations. Many papers have reported a variety of techniques to simultaneously mea- sure two or more of physical variables that affect an FBG. Most of these papers describe techniques aimed at sensing strain and temperature, though not in real time. Most papers describing the detection of acoustic signals using FBGs are focused on sensing rather than communications. In one exception, the authors reported acoustic phase-shift keying (PSK) modulated communication through a metallic layer by using a PZT as a transmitter and an FBG as a receiver. 9 The present article reports the development of a low- frequency opto-acoustic detector based on a distantly placed FBG—FBG-detector—tested for sensitivity in the time- domain for 43 kHz ultrasonic carriers. The acoustic signals were conveyed by means of 70 cm of optical fiber serving as an acoustic guide. This length, 70 cm, is the largest value ever reported, thus allowing the FBG to be properly pro- tected and passively resistant to damage and strain. More- over, in this article, an original and passive technique FIGURE 1 Experimental set-up of the low-frequency opto-acoustic FBG-based detector in which the grating is distant to the point of incidence of acoustic signals LEAL ET AL. | 813