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A single-wavelength passive phase-shifted demodulation technique for the interrogation of a diaphragm-based fiber sensor is proposed. It is based on the configuration of the external Fabry–Perot interferometer (EFPI) with dual-cavity (DC) and the ellipse-fitting algorithm (EFA). The technique can address two main concerns: the orthogonality of the DC-EFPI must be matched, the direct current values and fringe visibility of the DC–EFPI must be excluded or normalized. Experiment results show that the proposed technique demonstrates high accuracy amplitude measurement of reflective surface vibration with a high dynamic range over 135 dB and good effectiveness with various initial phase differences apart from two areas around 0 and π rad. It is a successful solution for interrogating of diaphragm-based fiber sensors. [ABSTRACT FROM AUTHOR]

An ameliorated phase generated carrier demodulation algorithm based on a reference interferometer is proposed. It is theoretically analyzed, numerically simulated, and experimentally verified in this paper. The method of matching the amplitude of the quadrature signal in the reference interferometer algorithm eliminates the harmonic distortion caused by modulation depth fluctuations, suppresses system noise, and improves system stability. In experiments, where the sensed signal amplitude is 0.68 rad, the frequency is 1 kHz, and the modulation depth ranges from 1.5 to 3.5 rad, the ameliorated algorithm has SINAD that reaches 24.5 dB, which is an average improvement of 7.9 dB over the PGC-DCM algorithm based on a reference interferometer and an average improvement of 3 dB over the PGC-Arctan algorithm based on a reference interferometer. The ameliorated algorithm achieves a THD of 0.451%, which consistently maintains the best level of the traditional PGC algorithm based on a reference interferometer. The method provides an effective reference for signal processing in practical applications of fiber-optic sensors. [ABSTRACT FROM AUTHOR]

A compact mode converter (MC) has been demonstrated based on tapered hole-assisted dual-core fiber (HADCF). The MC is fabricated by splicing single mode fiber (SMF) and a piece of HADCF that is composed of a central single mode core (SMC), an eccentric few mode core (FMC), and a large eccentric air hole. The HADCF is tapered in a controllable manner and the fundamental mode LP01 of the central core is coupled to the high-order LP11 mode of the eccentric core as two modes satisfy the phase matching condition. The high-order LP11 mode can be converted effectively across the range from O-band to C-band by only changing the tapered length. With the decreasing taper ratio, the mode conversion wavelength has a blue shift. The measured purity of the LP11 modes in the range from 1310 to 1550 nm are always higher than 95%. In addition, the tapered HADCF-MC is insensitive to the change of the temperature and external refractive index. [ABSTRACT FROM AUTHOR]

A fiber Bragg grating (FBG) has been fabricated in a hollow suspended-core fiber (HSCF). The HSCF is suitable for refractive index sensor because its core is exposed in the large air hole, which can be used as a microfluidic channel. The refractive index sensitivity of the grating is 8.9 nm/RIU in the range from 1.41 to 1.44. The strain and temperature sensitivities of the HSCF-FBG are 0.8648 pm/ $\mu \varepsilon $ and 9.089 pm/°C, respectively, which are similar to the traditional single-mode fiber FBGs. Remarkably, the bending characteristic of HSCF-FBG has a great dependence on the bending direction and the maximum bending sensitivity is 84.11 pm/m−1. Therefore, the proposed HSCF-FBG can be also utilized as a bending sensor with direction recognition. [ABSTRACT FROM AUTHOR]

A novel heterogeneous double period array multicore fiber (DPAMCF) is proposed, in which the spacing between two adjacent cores is approximately equal and the core sizes change periodically. The modes in the DPAMCF form two supermode groups, which can be attributed to double period distribution of heterogeneous cores. Two supermode groups are phase mismatched and the coupling crosstalk between them is suppressed. Fiber Bragg gratings (FBGs) are successfully inscribed in the DPAMCF and the responses of each grating to the axial strain and temperature are measured. The proposed DPAMCF offers a platform to integrate a series of FBG units with two sets of different Bragg wavelengths into a plane at the same longitudinal position. [ABSTRACT FROM AUTHOR]

A compact temperature sensor based on a Mach–Zehnder interferometer (MZI) is demonstrated in a suspended core hollow fiber (SCHF) covered with reduced graphene oxide (rGO). The MZI is constructed by connecting two single-mode fibers with an intermediate section SCHF in offset splicing, which introduces the interference between the fundamental and high-order modes of the suspended core. The SCHF-based MZI without rGO is insensitive to the temperature; however, after employing rGO into the SCHF, the temperature sensitivity of ~179.4 pm/°C in the range of 30 °C–80 °C is obtained experimentally. Compared to microfiber-based sensors, the annular cladding can prevent the sensor from external environmental pollution. The proposed rGO-based all-fiber temperature sensor has good mechanical strength and is well compatible with fiber-optic systems, which makes it a good candidate for temperature monitoring. Meanwhile, the air hole of the SCHF is convenient for forming microfluidic channel to realize gas concentration or humidity sensing. [ABSTRACT FROM AUTHOR]

A novel Mach-Zehnder interferometer (MZI) in an embedded-core hollow optical fiber (ECOF) was demonstrated, which was composed of a pair of long period gratings (LPGs). In order to form an in-fiber MZI in an ECOF, a high-frequency CO2 laser was used to fabricate two similar LPGs with their peak attenuation of approximate 3 dB. The responses of the MZI to the temperature, the axial strain, and the refractive index were measured and discussed. The temperature sensitivity of the MZI is about 48.8 pm/°C, and the sensitivity of the axial strain is 0.5 pm/ $\mu \varepsilon $ . The ECOF has a big air hole, which is easy to form a microfluidic channel. The sensitivity of the internal refractive index can be as high as 1437.4 nm/RIU in the refractive index range of 1.4248~1.4569. The proposed MZI has a high resolution and can be a potential candidate for sensing, especially for on-line microfluidics sensors. [ABSTRACT FROM PUBLISHER]