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Optical microfiber-based refractometer, which relies on modal interference operating near the dispersion turning point (DTP), has attracted a high level of research interest. The ultrahigh sensitivity can be achieved by mitigating the group effective refractive index difference between the modes in the coupling. Through modifying the microfiber diameter, the group effective refractive index difference can be modified to zero which is defined as DTP. However, all the DTP-based sensors report low fabrication tolerance and narrow operation bandwidth of probing wavelength for the high-sensitivity region where the refractive index (RI) sensitivities higher than 104 nm/RIU can be achieved, which extremely limits the practical applications. To address the challenge, we manipulate and optimize the mode dispersion by using a tapered specially-designed single stress-applying fiber (SSAF). By introducing the stress-applying part (SAP) into the cladding, the wavelength sensitivity of the group effective RI difference can be significantly reduced through the flexibly engineerable control over the material index. In this way, the operation bandwidth is dramatically broadened for 500 nm in comparison with that of conventional microfiber with the same diameter based on DTP, indicating a broad range of optional probing wavelength and high fabrication tolerance. We experimentally verify the ultrahigh sensitivity and broadband operation around the DTP using an SSAF-based microfiber with a waist diameter of about 2.3 μm. The maximum RI sensitivity of 30563 nm/RIU is obtained. [ABSTRACT FROM AUTHOR]

Acoustic measurement is the most widely used non-electric method to detect partial discharge (PD) signals by detect the ultrasonic waves arisen from partial discharge. To solve the shortages of low sensitivity and susceptibility to interferences, a gold diaphragm based Fabry-Perot interferometer ultrasonic sensor is proposed and experimentally demonstrated. Its high acoustic sensitivity and directional insensitivity shows great capability for PD detection and localization. Experimental results illustrate a well acoustic response from 20 kHz to 150 kHz when standard ultrasonic signals applied. The frequency range covers the acoustic frequencies derived from PD process. The ultrasonic signals from a discharge source is measured by the proposed fiber optic sensor with difference distances and incident angles. The output shows an attenuation of 1.32 dB/m and the sensor can detect weak signals as far as 3 m. The directional deviation coefficients are less than 10% for each measuring distance which exhibits directional independence. Two localization techniques with a sensor array (four sensors) are proposed and simulations are carried out to verify them feasible. The fiber-optic ultrasonic sensor may have a great potential in stability maintenance of electrical power systems due to its low cost, compact structure, environmentally robustness and simple manufacturing. [ABSTRACT FROM AUTHOR]

A new optical fiber twist sensor with temperature compensation mechanism is designed and experimentally demonstrated. The main structure of this twist sensor is a torsion perturbation point cascaded with single mode-multimode-single mode (SMS) fiber structure. The experimental results show that the twist sensor has ability to distinguish the twist direction with sensitivities of 9.5 pm/° and 34.7 pm/° at the chosen dips in the range from −100° (CCW) to 140° (CW). Furthermore, the temperature response characteristic of the sensor is also researched through experiment. Temperature sensitivities of 71.8 pm/°C and 65.2 pm/°C can be obtained at the same dips in the range from 20 °C to 75 °C. In addition, the temperature's influence can be compensated with post-measurement calculation through the cross-matrix. The theoretical resolutions of twist and temperature simultaneous measurement are ±0.084° and ±0.32 °C, respectively. [ABSTRACT FROM AUTHOR]

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 phase interrogation method for a fiber-optic acoustic sensor based on a short-cavity extrinsic Fabry–Perot interferometer (EFPI) is proposed with two wavelengths. A multichannel tunable optical filter is employed, which selects out two monochromatic beams from the broadband interference spectrum of EFPI with fixed wavelength interval. The influence of the wavelength interval on the recovery of the phase signal has been theoretically and experimentally analyzed. To get the best sensitivity, the wavelength interval is fixed at one quarter of the period of sensor interference fringes. An optimized differential cross multiplication algorithm is utilized to demodulate the acoustic signal, which can eliminate the impact of optical power imbalance between the two light paths. This system may be a universal phase demodulation unit for short-cavity EFPI acoustic sensors. The EFPI acoustic sensing head is formed by an aluminum-attached polyethylene terephthalate membrane and a cleaved fiber tip. The sensor interrogated by the proposed demodulation system demonstrated large dynamic range in low-frequency domain under high sound pressure. A signal-to-noise ratio (SNR) of ∼53 dB is obtained at 80 Hz. These features make the sensor especially suitable for noise detection. [ABSTRACT FROM PUBLISHER]

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]