A high-temperature sensor based on a Mach-Zehnder interferometer (MZI) in a conventional single-mode optical fiber is proposed and fabricated by concatenating two microcavities separated by a middle section. A femtosecond laser is used to fabricate a microhole on the center of a fiber end. Then a micro-air-cavity is formed by splicing the microholed fiber end with a normal fiber end. The interferometer is applied for high-temperature sensing, in the range of 500-1200 °C, with a sensitivity of 109 pm/°C that, to the best of our knowledge, is highest in silica fiber temperature sensors. Also, the interferometer is insensitive to external refractive index (RI), which is desirable for temperature sensors.

A taper-based Mach-Zehnder interferometer (MZI) embedded in a thinned optical fiber is demonstrated as a highly sensitive refractive index (RI) sensor. A RI sensitivity of 2210.84 nm/RIU (refractive index unit) is obtained at the external RI of 1.40, which is ten times higher than that of normal taper- and long-period fiber grating (LPFG)-based sensors. The sensitivity can be further improved by decreasing the diameter of the thinned fiber and increasing the interferometer length of the MZI. The proposed MZIs have lower temperature sensitivities compared with normal fiber sensors, which is a desirable merit for RI sensors to reduce the cross sensitivity caused by thermal drift.

A diaphragm-free fiber gas pressure sensor based on the extrinsic Fabry-Perot interferometer (EFPI) is proposed and experimentally demonstrated. The sensor is fabricated by fusion splicing a sandwich structure of a single mode fiber, a hollow core fiber (HCF), and a coreless fiber. A femtosecond (fs) laser is employed to drill a micro-channel on the side wall of the HCF to allow gas entering/exiting the HCF. The refractive index of the gas has a linear relationship with the gas pressure. Thus, the gas pressure can be detected by investigating the optical path difference of the EFPI. The experimental results show that the proposed sensor exhibits a high gas pressure sensitivity of ∼1.80 µm/MPa from vacuum to 10 MPa at room temperature. Pressure responses under high temperatures are also investigated. The proposed sensor has properties of robust structure, miniature size, large measuring range, and high sensitivity.