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For low fineness extrinsic Fabry-Perot inter-ferometer (EFPI) sensors, the discrete gap transform (DGT) algorithm can be usually adopted to acquire the cavity length, and the magnitude of its demodulation error is often larger than several hundred picometer. In this paper, three-parameter cosine fitting (TPCF) method is proposed to compensate for the EFPI cavity length error caused by traditional spectrum truncation, which can improve the demodulation accuracy by 3800 times of that of DGT. This method has a large cavity length demodulation range of 20 μm ∼30000 μm with a maximum error of 65 pm, which was proved by numerical simulation. The experiment shows that when the cavity length is 31 μm and 282.3 μm, the demodulation resolutions with a confidence interval of 68.27% are 15.142 pm and 27.582 pm, respectively, and its actual demodulation time is no more than 0.021 s. Compared with the traditional cavity length demodulation algorithms, the proposed algorithm has broader demodulation range, higher demodulation accuracy and resolution. Moreover, it will be unrestricted by wavelength shift for more than one period and can realize real-time fast demodulation. [ABSTRACT FROM AUTHOR]

In this paper, a novel platform for simultaneous measurement of relative humidity (RH) and temperature using dual Fabry–Perot interferometers (FPIs) based on C-shaped fiber was demonstrated. The sensor was composed by splicing two sections of C-shaped fiber between single mode fiber (SMF). Polydimethylsiloxane (PDMS) and polyvinyl alcohol (PVA) are filled in the two sections of C-shaped fibers to increase sensitivity to temperature and RH. This is the first demonstration that solid polymer materials can be added to the C-shaped fiber interferometers for sensing. In our experiment, RH sensitivities of -0.128 nm/%RH and 0.038 nm/%RH in the range of 20%RH to 45%RH, and temperature sensitivities of 0.022 nm/°C and -0.722 nm/°C in the temperature range of 15 °C to 45°C, were acquired for dual FPIs, respectively. We verified that it is possible to use the sensitivity matrix method to measure two parameters simultaneously. In addition, it has the benefits of simple structure, excellent stability and high sensitivity, and has a broad application prospect in agriculture, food processing and environmental measurement. [ABSTRACT FROM AUTHOR]

A ring-shaped structure based on polymer coated no-core fiber (NCF) is presented for simultaneous temperature and pressure measurements. The sensing structure is fabricated by splicing the partially coated NCF between single-mode fibers and bending it into a loop. The anti-resonance effect is formed in the NCF and the resonant dips have wavelength shift responses to the temperature and displacement variations. It should be noted that the increase of temperature leads to a decrease in the polymer coating refractive index and changes the displacement measurement sensitivity, thus, the compensation algorithm is required to resolve this cross-sensitivity problem. Combining the ring-shaped structure with the elastic diaphragm encapsulation can convert seawater pressure into structural deformation displacement to achieve pressure measurement. The experimental and simulation results show that this sensor offers a high temperature sensitivity of −5.098 nm/°C with the resolution of $2\times 10^{-4}$ °C and a pressure sensitivity of −2.368 nm/MPa with the resolution of $0.4\times 10^{-3}$ MPa. In addition, the sensing performances could be changed by adjusting the NCF size and the ring-shaped structure diameter, which enables this sensing structure to have a broader application potential for seawater parameter measurement. [ABSTRACT FROM AUTHOR]

A novel, highly sensitive dual-parameter sensor based on tandem-interferometric arrangement of Fabry–Pérot interferometer and Mach–Zehnder interferometer (MZI) has been designed and implemented in this paper. The sensor is composed of a single-mode fiber (SMF), hollow-core fiber, no-core fiber (NCF), and photonic crystal fiber (PCF). Those can simultaneously form a Fabry–Pérot interferometer and a MZI. The both interferometers are able to measure strain. Meanwhile, the PCF was filled with liquid crystal to measure electric field intensity. Hence, simultaneous measurement of electric field and strain can be achieved. The sensitivities of electric field and strain can reach up to 10.5 nm/(kV/mm) and 1.25 pm/ $\mu \varepsilon $ , respectively. [ABSTRACT FROM PUBLISHER]

A novel, highly sensitive dual-parameter sensor based on a composite interference structure has been designed and implemented in this paper. The sensor was composed of a Fabry–Pérot interferometer, where the microcavity was filled by magnetic fluid (MF) and a Mach–Zehnder interferometer, where the photonic crystal fiber was filled with ethanol. The refractive index of the MF and ethanol would be changed with the increase of magnetic field and temperature. Hence, simultaneous measurement of temperature and refractive index can be achieved by monitoring the reflected spectrum and transmitted spectrum. The sensitivities of MF and ethanol can reach up to 0.033 nm/Gs and 0.236 nm/°C, respectively. [ABSTRACT FROM PUBLISHER]

A new optical fiber curvature sensor dominated by three-core single mode fiber (TCSMF) is proposed and demonstrated. The sensing structure is formed by cascading multimode fiber (MMF), TCSMF and MMF together, namely, the SMF-MMF-TCSMF-MMF-SMF structure. Three core modes in the TCSMF interfere with each other and then form the interference spectrum. The interference phase difference is changed due to the elastic-optic effect as bending deformation inflicted on the fiber. Thus the spectrum can be shifted accordingly. The two short segments of MMF can significantly improve the coupling efficiency between the fundamental mode and three core modes, and get high fringe visibility. Experiments show that the highest curvature sensitivity of −28.29 nm/ \textm^-1 is achieved in the range of 2.79–3.24 \textm^-1 . Moreover, this sensor possesses the temperature-insensitive characteristic on account of the similar thermo-optic coefficient among three cores. [ABSTRACT FROM PUBLISHER]

A novel, highly sensitive double-parameter sensor based on the composite interference structure has been designed and implemented in this paper. It can be regarded as a cascade of the traditional single-mode fiber (SMF)–hollow-core fiber (HCF)–photonic crystal fiber (PCF)-SMF structure, which forms a reflected Fabry–Perot interferometer and a transmitted Mach–Zehnder interferometer at the same time. The HCF is filled with ethanol, which is sensitive to temperature. Then the simultaneous measurement of the temperature and the refractive index (RI) can be achieved by monitoring the reflected spectrum and transmitted spectrum. The fact that these two spectrums do not interfere with each other makes it easier to obtain a demodulation method for simultaneous temperature and RI measurement that does not involve complex fast Fourier transform and matrix computation. The sensitivities of the temperature and the RI can reach up to 0.454 nm/°C and 178.7 nm/RIU, respectively. [ABSTRACT FROM AUTHOR]

A novel double-parameter sensor based on a composite interference structure has been designed and implemented in this letter. It can be regarded as a cascade of the conventional single-mode fiber (SMF)–multimode fiber–photonic crystal fiber–SMF structure, which forms a reflected Fabry–Perot interferometer and a transmitted Mach–Zehnder interferometer at the same time. In addition, a simultaneous measurement of temperature and refractive index has been achieved by monitoring the correlative moves of reflected spectrum and transmitted spectrum. The fact that these two spectra do not interfere with each other makes it easier to obtain a demodulation method that does not involve complex fast Fourier transform and matrix computation. The sensitivities of temperature and refractive index can reach up to 27.5 pm/°C and 108 nm/RIU, respectively. [ABSTRACT FROM PUBLISHER]

A robust optical fiber modal Mach–Zehnder (MZ) interferometer is proposed, which is embedded in a fiber ring laser system as a tunable filter to improve the detection limit (DL) due to its narrow full width half maximum (FWHM) and high signal-to-noise radio (SNR). The sensing element is composed of two cascaded up-taper joints fabricated by shoving fusion. After filtering, the laser spectrum possesses the sensing characteristics that the interferometer possessed. Temperature and refractive index (RI) are investigated as measurands in experiments and the response sensitivities are 92.1 pm/°C and 44.8 nm/RIU, respectively. Accordingly, the DLs for these two measurands are calculated as 0.19 °C and 3.86 \times 10^ - 4 (RIU). Moreover, the axis strain response is demonstrated to be much lower than many modal interferometers even under the help of fiber ring laser. Thus, the property contributes to the resistance of the outer force and disturbance. Compared to the simple sensing device, the proposed fiber ring laser sensor significantly improves the DL, which is one of the most crucial parameters. It is the first time to combine the fiber ring laser device with the up-taper MZ interferometer as far as we known. It is a feasible balanced way to improve the sensing capability without the expense of mechanical strength of sensing element. [ABSTRACT FROM PUBLISHER]

A novel sensitivity-enhanced optical fiber sensor has been presented and demonstrated to measure liquid temperature and refractive index (RI) based on the reflective single-mode–multimode–single-mode fiber structure. The sensing part is packaged in a segment of capillary tube by hot glue. Due to the high coefficient of thermal expansion of hot glue, the increasing temperature bends the fiber in the tube. The measurement is achieved via monitoring the spectral shift of the modal interference between the core and the cladding modes in the ended single-mode fiber, which is well carved to provide a mirror interface. The Fresnel reflection occurred in the interface is utilized to measure liquid RI and temperature. The highest sensitivity of −92.6 pm/°C and −67.9 dB/RIU are achieved, respectively. This structure has many advantages, such as high sensitivity, expectation for simultaneous measurement of liquid temperature and RI, convenient operation, and low cost of fabrication. [ABSTRACT FROM PUBLISHER]