Fully and Wideband Distributed Acoustic Sensing With High Sensitivity by an Air-Ring Microstructure Optical Fiber

Fiber-based distributed acoustic sensing (DAS) technique has recently captured great attention of academia and industry. The fiber under test (FUT) gains extreme importance for DAS system. However, it is difficult for the commercially available telecom fibers to equip DAS system with long distance, high sensitivity and full distribution. In this work, we have proposed and demonstrated an air-ring microstructure optical fiber (AR-MOF) for fully distributed wideband acoustic sensing with high sensitivity for the first time, to our best knowledge. The cladding of the AR-MOF is constructed by an air-ring for enhancing acousto-optic coupling, and a large mode-area silica core is explored for enlarging the backscattering capture factor. By integrating a 2.2-km AR-MOF into a phase-sensitive optical time-domain reflectometry (Φ-OTDR) system, we have achieved high-sensitivity acoustic detection along the entire FUT within the frequency range from 50 Hz to 5000 Hz. The DAS system shows a maximum acoustic sensitivity of −92.5 dB re rad/μPa at 50 Hz, and an average acoustic sensitivity of −129.4 dB re rad/μPa at the flat area from 500 Hz to 5000 Hz. Compared with previous reports, our AR-MOF based DAS system shows the best sound pressure level sensitivity in such a wide bandwidth. Benefiting from the technical maturity of stack-drawing method and avoidance of nanoparticle doping, the AR-MOF allows for highly sensitive and long-distance DAS system with low attenuation, showing great potential in smart grid, protecting gas pipelines, monitoring of a railway tunnel, illuminating earth phenomenon, real-time intrusion threat detection, marine geophysics, and etc.

I. Introduction

Fiber-based distributed acoustic sensing (DAS) technique has recently captured great attention of academia and industry, which has been explored in a wide range of application fields, such as ocean monitoring [1], assessing volcanic eruptions potential [2], earthquake imaging [3], marine geophysics [4], and etc. Because the magnitude of phase variation of Rayleigh backscattered signal (RBS) along optical fibers is linearly related to the magnitude of external acoustic vibrations applied, the phase-sensitive optical time-domain reflectometry (Φ-OTDR) is a promising technique for DAS system, in which the location, amplitude and frequency of external acoustic signals can be demodulated simultaneously [5]. The fiber under test (FUT) gains extreme importance for Φ-OTDR system, which is used as a transducer to convert the acoustic signal into an optical signal that can be measured at the detector [6], [7], [8], [9], [10]. Importantly, the fiber attenuation limits the sensing length of DAS system [11]; the strain-optics and elastic-optics properties of the FUT determine the acoustic sensitivity [12]; the acoustic detection bandwidth is limited by the flat response region of FUT [13]. In order to improve the DAS performance, a number of efforts have been made in terms of sensing fiber, which can be typically categorized as standard single-mode fiber (SMF) or specialty optical fiber.