Displacement measurements are of significant importance in a variety of critical scientific and engineering fields such as gravitational wave detection, geophysical research, and manufacturing industries. Due to the inherent advantages such as compactness, high sensitivity, and immunity to electromagnetic interference, in recent years, fiber optic sensors have been widely used in an expansive range of sensing applications, ranging from infra-structural health monitoring to chemical and biological sensing. Of particular interest here, fiber optic displacement sensors have gained wide interest and have evolved from basic intensity modulation-based configurations to more advanced structures, such as fiber Bragg grating-based, interferometric configurations, etc. This paper reviews specifically the advanced fiber optic displacement sensing techniques that have been developed in the past two decades. Details regarding the working principle, sensor design, and performance measures of fiber Bragg gratings-based, interferometers-based (including the Fabry-Perot interferometer, the Michelson interferometer, and the multimode interferometer), microwave photonics-based, and surface plasmon resonance-based fiber optic displacement sensors are given. Challenges and perspectives on future research in the development of practical and high-temperature tolerant displacement sensors are also discussed.

Fabry-Perot interferometers have found a multitude of scientific and industrial applications ranging from gravitational wave detection, high-resolution spectroscopy, and optical filters to quantum optomechanics. Integrated with optical fiber waveguide technology, the fiber-optic Fabry-Perot interferometers have emerged as a unique candidate for high-sensitivity sensing and have undergone tremendous growth and advancement in the past two decades with their successful applications in an expansive range of fields. The extrinsic cavity-based devices, i.e., the fiber-optic extrinsic Fabry-Perot interferometers (EFPIs), enable great flexibility in the design of the sensitive Fabry-Perot cavity combined with state-of-the-art micromachining and conventional mechanical fabrication, leading to the development of a diverse array of EFPI sensors targeting at different physical quantities. Here, we summarize the recent progress of fiber-optic EFPI sensors, providing an overview of different physical and mechanical sensors based on the Fabry-Perot interferometer principle, with a special focus on displacement-related quantities, such as strain, force, tilt, vibration and acceleration, pressure, and acoustic. The working principle and signal demodulation methods are shown in brief. Perspectives on further advancement of EFPI sensing technologies are also discussed.

High-temperature measurements are of significant importance in various harsh-environment engineering fields such as fossil fuel production, and the metallurgical and aviation industries. In recent years, there is a steady trend to shift from conventional electronic sensors to optical fiber sensors for high-temperature applications. In particular, optical fiber sensors are small in size, immune to electromagnetic interference, readily applicable for remote sensing, have high elasticity, and incorporate capabilities for multiplexing and distributed sensing. However, commonly used fused silica optical fiber sensors exhibit severe limitations at ultra-high temperatures due to significantly degraded optical and mechanical properties at temperatures >1000°C. The excellent optical transparency, thermal and chemical stability, mechanical robustness, and high melting temperature (~2040°C) of single-crystal sapphire fibers make them a strong candidate for sensing applications in high-temperature environments. Translation of the sensing schemes from mature silica fiber sensors to sapphire fiber sensors has undergone tremendous growth and advancements in the past two decades. However, hurdles to the development of a near-term deployable sapphire fiber sensing system have proven persistent due to the highly multimodal nature of sapphire fibers. This paper reviews sensing techniques that have been implemented with sapphire fibers recently. The aim is to provide a comprehensive summary of past research on sapphire fiber sensing systems. Perspectives on further research into the challenging yet promising arena are also discussed.

Gas sensor technology is widely utilized in various areas ranging from home security, environment and air pollution, to industrial production. It also hold great promise in non-invasive exhaled breath detection and an essential device in future internet of things. The past decade has witnessed giant advance in both fundamental research and industrial development of gas sensors, yet current efforts are being explored to achieve better selectivity, higher sensitivity and lower power consumption. The sensing layer in gas sensors have attracted dominant attention in the past research. In addition to the conventional metal oxide semiconductors, emerging nanocomposites and graphene-like two-dimensional materials also have drawn considerable research interest. This inspires us to organize this comprehensive 2020 gas sensing materials roadmap to discuss the current status, state-of-the-art progress, and present and future challenges in various materials that is potentially useful for gas sensors.