We propose and demonstrate a high-frequency interferometric optical fiber hydrophone based on acoustic resonance isolation. Novel air chambers are introduced into common type of mandrel sensitization structure to prevent acoustic resonance which occurs inside the inner cylindrical cavity of hydrophone frame at specific high frequency acoustic filed. 1kHz-30kHz frequency response of this new kind of hydrophone is measured in a lake. Experiment shows that, by this acoustic structure optimization, the working frequency bandwidth of optical fiber hydrophone, whose acoustic pressure sensitivity is-147dB, is expanded from 10kHz to above 30kHz.
In a phase extraction based phase-sensitive optical time domain reflectometry (Φ-OTDR), external perturbation induced phase variation of Rayleigh backscattered light-wave (RBL) is obtained from time varying interference signal that is comprised of two RBLs with a spatial shift along the sensing fiber. In this paper, the phase of the interference signal in the phase extraction based Φ-OTDR is studied. Derivation is performed on the interference signal considering the interference of multiple RBL within probe pulse covered fiber section. Theoretical analysis and experimental results reveal that the phase of interference signal are wavelength independent while the intensity of interference signal are wavelength dependent.
In this papers, the influence of the line-width of probe light on the phase noise of phase extracted based phase-sensitive optical time domain reflectometry (Φ-OTDR) is theoretical analyzed and experimentally investigated. Analysis indicates that broad line-width probe light suffers time varying wavelength drift and high level of laser phase noise, and thus guarantees high level phase noise of the Φ-OTDR. In distributed acoustic sensing along 500 m sensing fiber, the phase noise is evaluated for probe lights of different line-widths, and experimental results display that the phase noise increases as the line-width of probe light broadens.
A phase-sensitive optical time-domain reflectometry (∅-OTDR) implements distributed acoustic sensing (DAS) due to its ability for high sensitivity vibration measurement. Phase information of acoustic vibration events can be acquired by interrogation of the vibration-induced phase change between coherent Rayleigh scattering light from two points of the sensing fiber. And DAS can be realized when applying phase generated carrier (PGC) algorithm to the whole sensing fiber while the sensing fiber is transformed into a series of virtual sensing channels. Minimum detectable vibration of a ∅-OTDR is limited by phase noise level. In this paper, nonuniform distribution of phase noise of virtual sensing channels in a ∅-OTDR is investigated theoretically and experimentally. Correspondence between the intensity of Rayleigh scattering light and interference fading as well as polarization fading is analyzed considering inner interference of coherent Rayleigh light scattered from a multitude of scatters within pulse duration, and intensity noise related to the intensity of Rayleigh scattering light can be converted to phase noise while measuring vibration-induced phase change. Experiments are performed and the results confirm the predictions of the theoretical analysis. This study is essential for acquiring insight into nonuniformity of phase noise in DAS based on a ∅-OTDR, and would put forward some feasible methods to eliminate the effect of interference fading and polarization fading and optimize the minimum detectable vibration of a ∅-OTDR.
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