Optical humidity sensors have evolved through decades of research and development, constantly adapting to new demands and challenges. The continuous growth is supported by the emergence of a variety of optical fibers and functional materials, in addition to the adaptation of different sensing mechanisms and optical techniques. This review attempts to cover the majority of optical humidity sensors reported to date, highlight trends in design and performance, and discuss the challenges of different applications.

Long-range vibration sensing is an important tool for real-time structural health monitoring. A new, to the best of our knowledge, design of a distributed fiber-optic vibration sensor is introduced and experimentally demonstrated in this study. The proposed system utilizes the transmission of light in the forward direction for sensing, and a self-interference method for laser source simplification. To extract vibration information from phase modulation of light, two Mach–Zehnder interferometers (MZIs) are employed with a 3 × 3 coupler-based differential cross multiplication algorithm for phase calculation. A folded double-ended detection configuration allows the time-of-flight difference via cross correlation (CC) to provide vibration positioning. Experimental results demonstrate a sensing range of up to ∼80 km without optical amplification, accompanied by a position accuracy of 336 m.

Undersea earthquakes can have a major impact on coastal cities, submarine infrastructure and environment. To provide advance warning and condition assessment, remote monitoring is critical. However, the current methods of undersea earthquake monitoring have some limitations, such as limited number of sensors, high deployment cost, severe signal attenuation, etc. In this study, we propose a new approach for super-long range vibration monitoring, by detecting vibration-induced phase change along a single sensing fiber using a double-ended forward-transmission distributed fiber-optic vibration sensing system with a compensated self-interference sensing mechanism. The experiment results show a single-span sensing range of 151.5 km without optical amplification, a spatial resolution of 10 m, and the position error can be as small as 37.6 m. The average sensitivity is ∼12.2 mrad/μϵ, and the average limit of detection is 186.7 pϵ/Hz1/2. The vibration frequency range tested is between 100-500 Hz. The sensing system has several advantages, such as super-long sensing distance, relatively low-coherence laser, anti-electromagnetic interference, etc. This study provides a new technical solution for undersea earthquake monitoring, which can improve the early warning and assessment capabilities of undersea earthquakes.

For distributed fiber-optic sensors, slowly varying vibration signals down to 5 mHz are difficult to measure due to low signal-to-noise ratios. We propose and demonstrate a forward transmission-based distributed sensing system, combined with a polarization-generated carrier for detection bandwidth reduction, and cross-correlation for vibration positioning. By applying a higher-frequency carrier signal using a fast polarization controller, the initial phase of the known carrier frequency is monitored and analyzed to demodulate the vibration signal. Only the polarization carrier needs to be analyzed, not the arbitrary-frequency signal, which can lead to hardware issues (reduced detection bandwidth and less noise). The difference in arrival time between the two detection ends obtained through cross-correlation can determine the vibration position. Our experimental results demonstrate a sensitivity of 0.63 mrad/με and a limit of detection (LoD) of 355.6 pε/Hz^1/2 at 60 Hz. A lock-in amplifier can be used on the fixed carrier to achieve a minimal LoD. The sensing distance can reach 131.5 km and the positioning accuracy is 725 m (root-mean-square error) while the spatial resolution is 105 m. The tested vibration frequency range is between 0.005 Hz and 160 Hz. A low frequency of 5 mHz for forward transmission-based distributed sensing is highly attractive for seismic monitoring applications.

Distributed fiber-optic sensors are widely used for mining safety monitoring, civil engineering structural health monitoring and environmental monitoring. The position accuracy of measurements is of critical concern, as errors can lead to expensive decisions at the wrong location. Accurate fiber length measurement is key to calibrate distributed fiber-optic sensors. Existing methods suffer from time resolution or component delay errors. Optical time-domain reflectometry (OTDR) and its variants are widely used but needs calibration for practical applications. We propose two new methods that can self-calibrate, and can be used to calibrate other methods such as those based on OTDR. One based on a differential pulse loop using dual pulse statistical signal processing, and another is based on a differential pulse loop with multi-pulse sequence analysis, to enhance the timing accuracy. In addition, it is possible to determine the effective index of a single-mode fiber with known fiber length, which is important prerequisite information for practical applications in the field with uncertain laser wavelength and ambient temperature. When using a low-end oscilloscope with a nominal 26 MS/s sampling rate, compared with the measurement results of optical frequency domain reflectometer (OFDR) (LunaOBR4600), the double pulse method has an error of ±6 cm. For the multi pulse method, the measurement error can be as low as 1.75 mm for a fiber length of 2.049 km (relative error of 8.54 × 10 ⁻⁷ ).