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We have proposed and experimentally demonstrated an ultrasensitive fiber-optic temperature sensor based on two cascaded Fabry–Perot interferometers (FPIs). Vernier effect that significantly improves the sensitivity is generated due to the slight cavity length difference of the sensing and reference FPI. The sensing FPI is composed of a cleaved fiber end-face and UV-cured adhesive while the reference FPI is fabricated by splicing SMF with hollow core fiber. Temperature sensitivity of the sensing FPI is much higher than the reference FPI, which means that the reference FPI need not to be thermally isolated. By curve fitting method, three different temperature sensitivities of 33.07, −58.60, and 67.35 nm/°C have been experimentally demonstrated with different cavity lengths ratio of the sensing and reference FPI, which can be flexibly adjusted to meet different application demands. The proposed probe-type ultrahigh sensitivity temperature sensor is compact and cost effective, which can be applied to special fields, such as biochemical engineering, medical treatment, and nuclear test. [ABSTRACT FROM AUTHOR]

In this paper, we proposed a phase interrogation method for diaphragm-based interferometric optical fiber acoustic sensor based on the spectral coding process. The information of the sound wave is coded on the optical spectrum of the sensor head by wavelength scanning, which builds a map between time and wavelength. The acoustic signal can be reconstructed from the sound-encoded spectrum. The sound-induced phase variation of the sensor can be interrogated by comparing the sound-encoded spectrum with the initial spectrum curve, which can be acquired from the sinusoidal fitting method. In order to eliminate the phase ambiguity, a diaphragm-based Michelson interferometer is set up using a commercial 3 × 3 fiber coupler and two synchronized channels with 120° phase difference are employed for the interrogation. Experimental results indicate that the proposed method can provide stable demodulation results under random environmental fluctuations, while the commonly used edge filtering technique shows fluctuations of more than 10 dB under the same condition. [ABSTRACT FROM AUTHOR]

A novel fiber sensor for curvature and acoustic wave measurement, which is based on a thin core ultralong period fiber grating (TC-ULPFG), has been proposed and experimentally demonstrated. By tracking the power variation of different resonant wavelength caused by TC-ULPFG, high curvature sensitivity of 97.77 dB/m–1 is achieved, to best of our knowledge, which is the highest than other structures at the same measurement range. Thus, the desired curvature property of the TC-ULPFG is used for acoustic measurement. The polyethylene terephthalate film is selected as a transducer, on which TC-ULPFG is tightly pasted. The acoustic pressure sensitivity of 1.89 V/Pa is two orders higher than other structures based on the diaphragm transducer, and the noise-limited minimum detectable pressure is 1.94 mPa/Hz1/2 at 200 Hz. In addition, the frequency fluctuations are nearly ±0.4 dB from 70 to 200 Hz and ±0.2 dB from 1 to 3 kHz. Therefore, the proposed optical fiber acoustic sensor has a flat frequency response at a relatively lower frequency. The TC-ULPFG shows many advantages, including high sensitivities of curvature, high acoustic pressure sensitivity, easy fabrication, simple structure, and low cost. [ABSTRACT FROM PUBLISHER]

An asymmetrical thin-core long-period fiber grating (ATC-LPFG) cascaded with an inline Mach–Zehnder interferometer (MZI) for dual-parameter sensing has been demonstrated. In addition, bending direction is also determined at the same time. A section of thin-core fiber (TCF) is fusion spliced between two single-mode fibers (SMFs). ATC-LPFG is 4 cm away from the first splicing point to form an inline MZI. The transmission spectrum consists of four dominant resonant wavelengths generated by multimode interference and loss peaks of ATC-LPFG. Two resonant wavelengths are the main loss peaks of ATC-LPFG, and additional two resonant wavelengths are caused by multimode interference. Bending direction is determined by former two resonant wavelengths; curvature and temperature sensitivities are measured by the resonant wavelength of multimode interference and ATC-LPFG. Cross sensitivity can be overcome because the resonant wavelength is generated by different mechanisms. The experimental results indicate that the sensitivities of curvature and temperature are 28.82 dB/m−1 and 67.3 pm/°C, respectively. [ABSTRACT FROM PUBLISHER]

An infrasound sensor based on an extrinsic fiber-optic Fabry–Pérot interferometer structure is reported and demonstrated. The transducer part of this sensor is a composite film which is composed of a 3- \mu \textm -thick small round aluminum foil and a $\sim 50$ - \mu \textm -thick polymer polyethylene terephthalate film. The infrasound interference will introduce the vibration of the film and result in the changes of the FP cavity length synchronously, as well as the interference spectral shift. As a consequence, we will obtain the synchronous changes of reflected power by inputting a single wavelength laser. Theoretical simulation and experimental validation are both carried out to test the performance of our fiber infrasound sensor. Results show that low-frequency infrasound signal from 1 to 20 Hz can be detected with low distortion. The proposed sensor has its superiorities and can be useful for low-frequency infrasound sensing. [ABSTRACT FROM PUBLISHER]

A low-cost way of achieving a high sensitivity optical fiber strain sensor by introducing higher-order interference modes using a torsional multimode fiber (MMF) instead of normal MMF based on single-mode–multimode–single-mode (SMS) structure is proposed and the coupling mechanism of twist fiber is investigated theoretically. The sensor is fabricated by simple process of heating and twisting a small region of MMF. According to this method, the shift of multimode interference spectrum caused by an axial strain will be greatly magnified. Different strain sensitivities can be easily realized by controlling the torsional number of circles. The experimental results indicated a high strain sensitivity of 42.5 pm/ μ ε at most. [ABSTRACT FROM AUTHOR]