A fiber-optic vector magnetic field sensor based on a large-core-offset Mach–Zehnder interferometer (MZI) infiltrated by magnetic fluid (MF) is proposed and demonstrated in this paper. By large-core-offset fusion splicing of a short single-mode fiber (SMF) between a lead-in SMF and a coupling multi-mode fiber, the MZI with a sub-millimeter length is formed, which is then sealed in an MF-infiltrated glass capillary. Through the MF’s refractive index modulation by external magnetic field, the phase of the light passing through the MZI is altered. As a result, the transmission spectrum can be monitored for the magnetic field measurement. Furthermore, from the axial-asymmetry of the large-core-offset MZI structure, the proposed sensor possesses vectorial magnetic-field-sensing ability. Experiments show that the MF-infiltrated large-core-offset MZI vector magnetic-field sensor can achieve a high wavelength sensitivity of 96.68 pm/Oe in a magnetic field range of 50–130 Oe.

A white light non-scanning correlation interrogation system was proposed and built to interrogate absolute length of the air cavity of fiber-optic compound Fabry–Perot pressure sensors for the extraction of pressure value. By carefully choosing thickness range and tilt angle of the optical wedge used for cavity length matching, correlation interferometric signal of the basal cavity can be naturally filtered out. Based on peak positioning by Fourier transform, bandpass filtering in frequency domain, inverse Fourier transform back to time domain, envelope fitting and zero fringe finding through a gravity center method, cavity length can be determined with an accuracy of 0.04%. The system was used for the interrogation of a fiber-optic compound Fabry–Perot pressure sensor under different pressures. For a pressure range of 0.1~2.9 Mpa, the linear relationship between the air cavity length and the gas pressure imposed was successfully extracted.

This study presents an extrinsic Fabry–Perot interferometric (EFPI) fiber-optic strain sensor with a very short cavity. The sensor consists of two vertically cut standard single-mode fibers (SMFs) and a glass capillary with a length of several centimeters. The two SMFs penetrate into the glass capillary and are fixed at its two ends with the use of ultraviolet (UV) curable adhesives. Based on the use of the lengthy glass capillary sensitive element, the strain sensitivity can be greatly enhanced. Experiments showed that the microcavity EPFI strain sensor with initial cavity lengths of 20 μm, 30 μm, and 40 μm, and a capillary length of 40 mm, can yield respective cavity length–strain sensitivities of 15.928 nm/με, 25.281 nm/με, and 40.178 nm/με, while its linearity was very close to unity for strain measurements spanning a range in excess of 3500 με. Furthermore, the strain–temperature cross-sensitivity was extremely low.

A fiber-optic, extrinsic Fabry–Perot interferometric (EFPI), dual-cavity sensor made of sapphire was fabricated and interrogated by a dual-segment, low-coherence Fizeau interferometer to achieve simultaneous pressure and temperature measurements. The fiber-optic EFPI, dual-cavity sensor had an initial basal cavity length of 680 µm and an vacuum cavity length of 80 µm and was experimentally tested based on temperature and pressure measurements. It was demonstrated that simultaneous pressure and temperature measurement could be achieved in the respective pressure and temperature ranges of 0.1–3 MPa and 20–350 °C.

A simple in-fiber breathing monitoring sensor based on a micro-tapered long period fiber grating (MT-LPFG) structure was proposed and demonstrated. The MT-LPFG structure was manufactured through the periodical tapering of an optical fiber utilizing a CO ₂ laser fusion splicer. Without the using of a humidity sensitive film, the MT-LPFG structure can directly and dynamically monitoring relative humidity with a fast response, based on which, the breathing process can be monitored in real time through the monitoring of its transmission affected by the humidity. Experiments show that breathing rate and breathing status during respiration were successfully monitored by placing the sensor structure nearby the nose in a breathing mask. A rise time of 0.237 s and a fall time of 0.299 s were realized in the breathing process monitoring. For its advantages of simple fabrication, miniature size, and fast response, the proposed sensor may found applications in visual observation of the patient’s breathing condition.