We propose and demonstrate experimentally a direct-detection single-end Brillouin temperature sensing system based on single-photon detector to evaluate the ambient temperature along the sensing link, which is an excellent candidate for the demodulation of frequency shift from Brillouin gain spectrum in traditional Brillouin configurations. In the proposed method, the ratio of the Rayleigh backscattering component and the Brillouin anti-Stokes component is used to demodulate the temperature information along the measured optical fiber link. Proof-of-concept experiment proves that a spatial resolution of 1.2m over 4.2km sensing range with a 1.24°C temperature error can be obtained. Additionally, the proposed direct-detection Brillouin sensor maintains large dynamic range, which boosts its practicability.
We developed a distributed refractive index (RI) sensor based on high performance optical frequency-domain reflectometry (OFDR) by simply bending a piece of standard single mode fiber (SMF) in a U shape. In the U-bent region, cladding modes are excited, which can reach to the boundary of the SMF to sense external RI variation. The cladding modes are then coupled back to the core mode and interfere with the fundamental mode. Thus, the fundamental mode can carry the varied RI information, and distributed index sensing is achieved by measuring the wavelength shifts of the local Rayleigh backscattered spectra. Thanks to the high signal SNR of OFDR, that compensating the bending induced loss, the proposed sensor can be bent in a small bending radius so that a high sensitivity of RI could be achieved. In the experiment, index sensitivity of 39.08 nm/RIU is achieved by imposing a bending radius of 4 mm, when the RI ranges from 1.3330 to 1.3773. Additionally, the proposed sensor maintains buffer coating intact, which boosts its practicability and application flexibility.
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