Distributed optical fiber sensors (DOFS) offer unprecedented features, the most unique one of which is the ability of monitoring variations of the physical and chemical parameters with spatial continuity along the fiber. Among all these distributed sensing techniques, optical frequency domain reflectometry (OFDR) has been given tremendous attention because of its high spatial resolution and large dynamic range. In addition, DOFS based on OFDR have been used to sense many parameters. In this review, we will survey the key technologies for improving sensing range, spatial resolution and sensing performance in DOFS based on OFDR. We also introduce the sensing mechanisms and the applications of DOFS based on OFDR including strain, stress, vibration, temperature, 3D shape, flow, refractive index, magnetic field, radiation, gas and so on.

Optical fiber surface plasmon resonance (SPR) sensors point toward promising application potential in the fields of biomarker detection, food allergen screening, and environmental monitoring due to their unique advantages. This review outlines approaches in improving the fiber SPR sensing performance, e.g., sensitivity, detection accuracy, reliability, cross-sensitivity, selectivity, convenience and efficiency, and corresponding sensing applications. The sensing principles of SPR sensors, especially the performance indicators and their influencing factors, have been introduced. Current technologies for improving the fiber SPR performance and their application scenarios are then reviewed from the aspects of fiber substrate, intrinsic layer (metal layer), and surface nanomaterial modification. Reasonable design of the substrate can strengthen the evanescent electromagnetic field and realize the multi-parameter sensing, and can introduce the in situ sensing self-compensation, which allows corrections for errors induced by temperature fluctuation, non-specific binding, and external disturbances. The change of the intrinsic layer can adjust the column number, the penetration depth, and the propagation distance of surface plasmon polaritons. This can thereby promote the capability of sensors to detect the large-size analytes and can reduce the full width at half-maximum of SPR curves. The modification of various-dimensionality nanomaterials on the sensor surfaces can heighten the overlap integral of the electromagnetic field intensity in the analyte region and can strengthen interactions between plasmons and excitons as well as interactions between analyte molecules and metal surfaces. Moreover, future directions of fiber SPR sensors are prospected based on the important and challenging problems in the development of fiber SPR sensors.

Protein assays show great importance in medical research and disease diagnoses. Liquid crystals (LCs), as a branch of sensitive materials, offer promising applicability in the field of biosensing. Herein, we developed an ultrasensitive biosensor for the detection of low-concentration protein molecules, employing LC-amplified optofluidic resonators. In this design, the orientation of LCs was disturbed by immobilized protein molecules through the reduction of the vertical anchoring force from the alignment layer. A biosensing platform based on the whispering-gallery mode (WGM) from the LC-amplified optofluidic resonator was developed and explored, in which the spectral wavelength shift was monitored as the sensing parameter. The microbubble structure provided a stable and reliable WGM resonator with a high Q factor for LCs. It is demonstrated that the wall thickness of the microbubble played a key role in enhancing the sensitivity of the LC-amplified WGM microcavity. It is also found that protein molecules coated on the internal surface of microbubble led to their interactions with laser beams and the orientation transition of LCs. Both effects amplified the target information and triggered a sensitive wavelength shift in WGM spectra. A detection limit of 1 fM for bovine serum albumin (BSA) was achieved to demonstrate the high-sensitivity of our sensing platform in protein assays. Compared to the detection using a conventional polarized optical microscope (POM), the sensitivity was improved by seven orders of magnitude. Furthermore, multiple types of proteins and specific biosensing were also investigated to verify the potential of LC-amplified optofluidic resonators in the biomolecular detection. Our studies indicate that LC-amplified optofluidic resonators offer a new solution for the ultrasensitive real-time biosensing and the characterization of biomolecular interactions.

Pressure-temperature cross-sensitivity and its accompanying temperature-related stability is a nerve-wracking obstruction for pressure sensor performance in a wide temperature range. To solve this problem, we propose a novel (to the best of our knowledge) all-silicon dual-cavity optical Fabry–Perot interferometer (FPI) pressure sensor. The all-silicon structure has high intrinsic reflectivity and is able to eliminate the influence of thermal-expansion-mismatch-induced stress and chemical-reaction-induced gas generation, and therefore, in essence, enhances measurement accuracy. From the experiment results, the pressure-temperature cross-sensitivity is reduced to be ∼ 5.96 Pa / ° C , which presents the lowest pressure-temperature cross-sensitivity among the FPI pressure sensors with the capability of surviving high temperatures up to 700°C thereby opening the way for high-precision pressure monitoring in various harsh and remote environments.

In unmanned monitoring such as perimeter security, pipeline detection, railway security, and so on, it is still a challenging task to accurately locate the irregular vibration events. Compared with traditional vibration detection technologies, distributed optical fiber vibration sensors (DOFVSs) have superior advantages such as high spatial resolution, large monitoring range, good concealment, excellent flexibility, and simple sensing structure. The DOFVS can be mainly classified into two types, i.e., the interferometer-based DOFVS (IDOFVS) and the backscattering-based DOFVS (BDOFVS). In particular, IDOFVS has unique sensing advantages, such as high stability, simple structure, high sensitivity, and wide frequency response. In this article, the development of long-distance IDOFVS in recent years has been summarized. The sensing principle of IDOFVS has been theoretically explained. The specific classification of the IDOFVS and the corresponding sensing structures are described in detail. In addition, the long-distance distributed vibration sensing structure and processing algorithms, such as denoising and locating algorithms, are analyzed. Finally, the performance of recent long-distance IDOFVS has been compared and the development direction has been discussed.

Liquid crystals (LCs) have been widely used as sensitive elements to construct LC biosensors based on the principle that specific bonding events between biomolecules can affect the orientation of LC molecules. On the basis of the sensing interface of LC molecules, LC biosensors can be classified into three types: LC–solid interface sensing platforms, LC–aqueous interface sensing platforms, and LC–droplet interface sensing platforms. In addition, as a signal amplification method, the combination of LCs and whispering gallery mode (WGM) optical microcavities can provide higher detection sensitivity due to the extremely high quality factor and the small mode volume of the WGM optical microcavity, which enhances the interaction between the light field and biotargets. In this review, we present an overview of the basic principles, the structure, and the applications of LC biosensors. We discuss the important properties of LC and the principle of LC biosensors. The different geometries of LCs in the biosensing systems as well as their applications in the biological detection are then described. The fabrication and the application of the LC-based WGM microcavity optofluidic sensor in the biological detection are also introduced. Finally, challenges and potential research opportunities in the development of LC-based biosensors are discussed.