We have presented and demonstrated a fiber optic gas pressure sensor with ultra-high sensitivity based on Vernier effect. The sensor is composed of two integrated parallel Mach-Zehnder interferometers (MZIs) which are fabricated by fusion splicing a short section of dual side-hole fiber (DSHF) in between two short pieces of multimode fibers (MMFs). Femtosecond laser is applied for cutting off part of the MMF and drilling openings on one air hole of the DSHF to achieve magnified gas pressure measurement by Vernier effect. Experimental results show that the gas pressure sensitivity can be enhanced to about −60 nm/MPa in the range of 0-0.8 MPa. In addition, the structure possesses a low temperature cross-sensitivity of about 0.55 KPa/°C. This presented sensor has practically value in gas pressure detection, environmental monitoring and other industrial applications.

We present a relative sensitivity of in-fiber inter-cross demodulation of a Mach- Zehnder interferometer (MZI) based on an etched Multi-core fiber (eMCF). The sensor can measure the external refractive index (RI) and temperature with a large fringe visibility of 15dB. It is tuned using a simple technique of slow chemical etching. When the outer cores of MCF will be exposed to the surrounding, a large difference of relative effective RI is observed, which enhances the sensitivity of the sensor. The sensor's wavelength and intensity responses have displayed that it can function with three different inter-cross-demodulation phenomena. A superior RI sensitivity of 178.20 dB/RIU in the range of 1.334 to 1.370, and temperature sensitivity of 66.73 pm/°C in the range of 30 to 80 °C are obtained, with an adequate linear response. Besides, it can readily resolve the issues of cross-sensitivity. Moreover, it has many advantages including easy fabrication, compact size, multiplex, repeatable, stable and can easily differentiate RI and temperature, which lack others.

A novel magnetic field sensor based on Terfenol-D coated fiber Bragg grating with spiral microstructure was proposed and demonstrated. Through a specially-designed holder, the spiral microstructure was ablated into the fiber Bragg grating (FBG) cladding by femtosecond laser. Due to the spiral microstructure, the sensitivity of FBG coated with magnetostrictive film was enhanced greatly. When the spiral pitch is 50 μm and microgroove depth is 13.5μm, the sensitivity of the magnetic field sensor is roughly 5 times higher than that of non-microstructured standard FBG. The response to magnetic field is reversible, and could be applicable for magnetic field detection.

A Mach-Zehnder interferometer based on double off-axis twisted deformation on single mode fiber (SMF) is proposed and experimentally demonstrated for gas pressure measurements. The off-axis twisted deformation is achieved by twisting a SMF under continuous arc discharging. An open air-gap was created on the cladding of the SMF between the two off-axis deformation points by using a femtosecond laser. The cladding modes transmit through the air-gap whose refractive index is directly determined by the air pressure. The interferometer is applied for high-sensitive pressure sensing with a sensitivity of -5.183 nm/MPa and temperature cross-sensitivity of 13.3 kPa/°C. Consequently, the open air-gap added MZI structure can therefore be considered an excellent candidate as a gas pressure sensor in harsh environments due to its low cost, compact size, ease of fabrication, and all-fiber configuration.

A simple structured ultra-highly sensitive fiber optic gas pressure sensor with low temperature cross sensitivity is proposed and demonstrated. The sensor is composed of two cascaded Mach-Zehnder interferometers (MZIs) which consist of single mode-multimode-single mode fiber structure (SMS) with a micro-machined air cavity in each multimode fiber. The micro air cavities are drilled by a femtosecond laser and act as a gas pressure sensing element. Interference happens between the beams transmitting along the air cavity and the remaining silica core of the multimode fiber. The difference between the fringe intervals of the two MZIs are small enough for generating Vernier effect in gas pressure measurement. The same configuration employed in the two MZIs is to obtain the same temperature response, so that preventing the Vernier effect from appearing in temperature changes. Therefore, the proposed sensor can achieve a high gas pressure sensitivity but is insensitive to temperature variation. Experimental results show that the sensitivity for gas pressure measurement is -82.131 nm/MPa.

This work reports a simple optical fiber Surface Plasmon Resonance (SPR) sensor with ultra-high sensitivity for simultaneous measurement of the dual-parameter. The sensor is based on a D-shaped fiber with a nanolayer coating of Silver (Ag) and Hematite (α−Fe ₂ O ₃ ). The Ag/α− Fe ₂ O ₃ layer is deposited on the longitudinal surface of residual cladding, and a Fiber Bragg Grating (FBG) is inscribed on the single-mode fiber (SMF) for temperature compensation. The α− Fe ₂ O ₃ layer protects the Ag layer from oxidation and effectively enhances the surface plasmon wave while interacting with free electrons. Finite Element Method (FEM) modeling is employed to investigate the refractive index (RI) and temperature response of the SPR sensor. The SPR sensor exhibits an ultra-high RI sensitivity of 8,518 nm/RIU (refractive index unit) in the RI range of 1.33 to 1.40, and a temperature sensitivity of -52 pm/°C in the temperature range of 20°C to 60°C. The SPR performance suggests that it is a potential candidate for various applications of dual-parameter sensing.