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A temperature sensor based on composite multicavity Fabry-Perot interferometers (FPIs) and a sensitivity-enhanced differential phase demodulation method is proposed and demonstrated. The structure was fabricated by partially filled polydimethylsiloxane (PDMS) into a segment of hollow-core fiber (HCF) which is spliced with a single-mode fiber (SMF). In this structure, composite multicavity FPIs are formed, including an air-cavity FPI, a PDMS-cavity FPI and an air-PDMS cavity FPI. The contribution of the different FPIs to the superimposed spectrum is analyzed from the simulated and experimental perspectives. The results demonstrate that the superimposed output spectrum is mainly produced by the air-cavity FPI and the air-PDMS cavity FPI. When the temperature increases, the lengths of the PDMS-cavity and the air-PDMS cavity are enhanced due to the volume expansion of PDMS, and the length of the air-cavity decreases because the air-cavity is compressed by the expanded PDMS. The cavity length variation causes the phase variation in the air-cavity FPI to be the opposite to that in the PDMS-cavity FPI or air-PDMS cavity FPI. Therefore, the phase demodulation sensitivity is enhanced by calculating the differential phase between the air-cavity FPI and the air-PDMS cavity FPI. The experimental results show that the sensitivity amplification factor of differential phase to the single phase of the air-cavity FPI or the air-PDMS cavity FPI is from approximately 1.44–3.25. This phase demodulation method is simpler, faster, more direct, and more convenient than wavelength demodulation from the perspective of demodulating composite interferometers. [ABSTRACT FROM AUTHOR]

An optical fiber sensing and signal demodulation technology based on Mach-Zehnder interferometer (MZI) and orbital angular momentum (OAM) beam was proposed in this work. One path of the MZI containing only single mode fiber (SMF) emits Gaussian beam, and the other path which contains a segment of two-mode fiber (TMF) generates OAM beam. The OAM beam was generated by offset splicing a segment of TMF with SMF, and then rotating the TMF by using a customized fiber rotator. The interference between the Gaussian beam and the OAM beam generates spiral interference pattern, which can rotate with the change of phase difference between these two beams. A segment of SMF in the Gaussian beam path was used as a sensing fiber, which was placed on a thermostat. When the temperature changes, the phase difference between the Gaussian beam and the OAM beam was changed owing to the thermo-optic effect and thermal expansion effect of the sensing fiber, resulting in the rotation of spiral interference pattern. An optical phase difference demodulation method based on spiral interference pattern feature extraction technology was proposed to demodulate optical phase difference between these two beams. Experimental result shows that the sensing system has a linear temperature measurement sensitivity of 12.67 rad/°C. The temperature measurement resolution is 0.0000122 °C in theory and about 0.005 °C achieved in experiment. [ABSTRACT FROM AUTHOR]

A novel and in-line fiber low-frequency vibration sensor based on Butterfly-Shape Mach-Zehnder Interferometer (BSMZI) is proposed and demonstrated experimentally. The sensing element is a tapered hollow-core fiber (HCF) sandwiched between two single-mode fibers (SMFs). The frequency measured by the Butterfly-Shape vibration sensor can well match with the applied frequency, and the presence of the higher harmonics can verify that the sensor does not distort the measured vibration signal. The maximum relative error of vibration frequency is 0.27%. The intensity of vibration signal (of the fiber sensor) is linearly proportional to the driving power of vibration platform. The vibration measurement sensitivity is 13.575 dB/w in the range of 0.1 w to 0.5 w under a constant vibration frequency of 50 Hz, and the linear fitting coefficient is 0.9556. The proposed sensor has a negligible temperature sensitivity in the range of 30.3 °C–69.5 °C. Because of its compact structure, easy preparation, low production cost, high frequency measurement accuracy of low-frequency vibration, and good linearity of vibration sensing, the proposed BSMZI sensor has potential application prospect in low-frequency vibration sensing field. [ABSTRACT FROM AUTHOR]

A novel refractive index (RI) sensor based on photonic crystal fiber Mach–Zehnder interferometer (PCF-MZI) was proposed. It was realized by cascading a section of PCF with half-taper collapse regions (HTCRs) between two single mode fibers (SMFs). The relationship between RI sensitivity and interference length of the PCF-MZI was firstly investigated. Both simulation and experimental results showed that RI sensitivity increased with the increase of interference length. Afterwards, influence of HTCR parameters on RI sensitivity was experimentally investigated to further improve the sensitivity. With intensification of arc discharge intensity in HTCR fabrication process, HTCR with larger maximum taper diameter and longer collapsed region length was obtained, which enhanced evanescent field of the PCF-MZI and then generated higher RI sensitivity. Consequently, a high RI sensitivity of 181.96 nm/refractive index unit (RIU) was achieved in the RI range of 1.3333–1.3574. Increasing arc discharge intensity in HTCR fabrication process has the capacity to improve RI sensitivity of PCF-MZI and meanwhile provides higher mechanical strength and longer sensor life compared to the traditional method of tapering the fiber, which improves the RI sensitivity at the cost of reducing mechanical strength of the sensor. This PCF-MZI was characterized by high RI sensitivity, ease of fabrication, high mechanical strength, and robustness. [ABSTRACT FROM AUTHOR]

Mach–Zehnder interferometers have attracted attention due to their flexible structures and extensive applications. In this study, a simple and highly sensitive curvature sensor based on an asymmetrical Mach–Zehnder interferometer comprising an up-taper and a peanut-like section was theoretically and experimentally characterized. A simulation was conducted to investigate the influence of structural parameters on the interference spectrum. Experimental measurements show that a high curvature sensitivity of −35.3 nm/m−1from 1.35 to 2.20 m−1was obtained and high mechanical strength was demonstrated. The temperature–curvature cross-sensitivity of 4.44 × 10−4 m−1/°C was lower than the curvature resolution. The high curvature sensitivity and low temperature cross-sensitivity make this sensor a candidate for practical applications. [ABSTRACT FROM PUBLISHER]

A novel Mach–Zehnder Interferometer (MZI) formed by cascading two vase-shaped tapers in a single mode fiber (SMF) is proposed in this work. The vase-shaped taper is fabricated by fusion splicing a fiber sphere with a fiber tip by applying strong arc discharge. The curvature sensing characteristic of the proposed vase-shaped taper-based MZI is experimentally investigated, displaying that the interference spectrum has a blue shift with the increase of applied curvature. The sensor realizes a large range measurement of curvature from 0.9284 m −1 to 4.0470 m −1 with sensitivities of −8.41 nm/m −1 and −16.72 nm/ m −1 in the curvature range of 0.9284 m −1 to 2.4564 m −1 and 2.6025 m −1 to 4.0470 m −1 , respectively. Its temperature-curvature cross-sensitivities are −0.0021 m −1 / °C and −0.0011 m −1 / °C in the two measurement ranges, which are lower than the corresponding curvature resolutions which are 0.0024 m −1 and 0.0012 m −1 . The proposed sensor exhibits the advantages of low cost, negligible temperature cross-sensitivity, fine curvature sensitivity and large measurement scale. [ABSTRACT FROM AUTHOR]

All-fiber Mach-Zehnder interferometers (MZIs) working as curvature sensors have been attracting increasing attention. However, previously reported MZI-based curvature sensors with offset or abrupt-taper structures suffer from weak mechanical strength and low curvature sensitivity. In this letter, a robust and highly-sensitive MZI-based curvature sensor formed by cascading two up-tapers in a single mode fiber (SMF) is proposed and experimentally demonstrated. By optimizing structural parameters of the MZI both in simulation and in experiment, a high quality interference spectrum with a maximum extinction ratio (ER) of around 20 dB is obtained, which is crucial in curvature sensing with spectrum intensity variation and interference dip identifiability being considered. Experimental results show that a high curvature sensitivity of −35.41 nm/m −1 is obtained in the range of 0.6846 m −1 – 1.0606 m −1 , which is 2.4 times higher than that of the interferometer with single-mode-multimode-single-mode (SMS) structure. The two up-tapers, featured by great mechanical strength, are fabricated by only fusion pushing together a whole SMF without cleaving procedure, contributing to excellent repeatability and consistency in the sensor fabrication process. Such an all-fiber sensor exhibits advantages of simple configuration, ease of fabrication, excellent mechanical strength, low cost, and high curvature sensitivity. [ABSTRACT FROM AUTHOR]

A novel refractive index (RI) sensor based on an asymmetrical Mach–Zehnder interferometer (MZI) with two different step-like tapers is proposed. The step-like taper is fabricated by fusion splicing two half tapers with an appropriate offset. By further applying offset and discharging to the last fabricated step-like taper of MZI, influence of taper parameters on interference spectrum is investigated using only one device. This simple technique provides an on-line method to sweep parameters of step-like tapers and speeds up the optimization process of interference spectrum, meanwhile. In RI sensing experiment, the sensor has a high sensitivity of −185.79 nm/RIU (refractive index unit) in the RI range of 1.3333–1.3673. [ABSTRACT FROM AUTHOR]

A hybrid fiber-optic sensor consisting of a micro extrinsic Fabry-Perot Interferometer (MEFPI) and an etched fiber Bragg grating (FBG) is proposed, which can measure strain and magnetic field simultaneously. The etched FBG is sealed in a capillary with ferrofluids to detect the surrounding magnetic field. FBG with small diameter will be more sensitive to magnetic field is confirmed by simulation results. The MEFPI sensor that is prepared through welding a short section of hollow-core fiber (HCF) with single-mode fiber (SMF) is effective for strain detection. The experiment shows that strain and magnetic field can be successfully simultaneously detected based on hybrid MEFPI/FBG sensor. The sensitivities of the strain and magnetic field intensity are measured to be up to 1.41 pm/με and 5.11 pm/mT respectively. There is a negligible effect on each other, hence simultaneously measuring strain and magnetic field is feasible. It is anticipated that such easy preparation, compact and low-cost fiber-optic sensors for simultaneous measurement of strain and magnetic field could find important applications in practice. [ABSTRACT FROM AUTHOR]

The paper proposed and studied a Mach-Zehnder mode interferometric refractive index sensor, which is based on splicing points tapered SMF-PCF-SMF (SMF, single-mode fiber; PCF, photonic crystal fiber) structure. For the reason that the effective refractive index of photonic crystal fiber cladding high-order modes near fiber core are more sensitive to surrounding refractive index changes, the refractive index measurement sensitivity of splicing points tapered SMF-PCF-SMF Mach-Zehnder mode interferometer can be enhanced further through tapering the splicing points. Relations between refractive index measurement sensitivity and photonic crystal fiber length and taper waist diameter are studied through numerical simulations and experiments. Simulation and experimental results show that sensitivity will be increased with the increase of photonic crystal fiber length and the decrease of taper waist diameter. In the refractive range of 1.3333–1.3737, splicing points tapered SMF-PCF-SMF Mach-Zehnder mode interferometer with PCF length of 4 cm and taper waist diameter of 60.4 μm has refractive index measurement sensitivity of 260.8 nm/RIU, compared with sensitivity of 224.2 nm/RIU of direct splicing SMF-PCF-SMF Mach-Zehnder mode interferometer with PCF length of 4 cm, the sensitivity increased by 16.3%. The research shows that the sensing structure is with good linearity and repeatability. [ABSTRACT FROM AUTHOR]

• A novel and compact temperature and relative humidity sensor was proposed and implemented. • The sensor was based on compact Mach-Zehnder interferometer and Fabry-Perot interferometer. • Detection of temperature and RH can be achieved by monitoring the spectrums of FPI and MZI. • The sensitivity to RH is −0.132 nm/%RH with resolution of 0.15 %RH. • The sensitivity to temperature is 0.37 nm/°C with the resolution of 0.05 °C. A novel and compact temperature and relative humidity (RH) simultaneously measurement sensor based on compact Mach-Zehnder interferometer (MZI) and Fabry-Perot interferometer (FPI) is proposed and experimental implemented. The sensing structure is made by splicing a 200 μm single mode fiber (SMF) between two 10 cm SMFs with large core-offset. The core-offset distance is 62.8 μm coated with RH sensitive material graphene quantum dots and polyvinyl alcohol (GQDs-PVA). The transmitted structure acts as MZI, and the reflected structure acts as FPI, then the simultaneous detection of temperature and RH can be achieved by monitoring the spectrums of FPI and MZI, respectively. The sensitivity to RH is −0.132 nm/%RH with resolution of 0.15 %RH, and the sensitivity to temperature is 0.37 nm/°C with the resolution of 0.05 °C. The proposed sensor is compact, high sensitivity, simple making and low cost. [ABSTRACT FROM AUTHOR]