The rapid growth of industry and the global drive for modernization have led to an increase in gas emissions, which present significant environmental and health risks. As a result, there is a growing need for precise and sensitive gas-monitoring technologies. This review delves into the progress made regarding photoacoustic gas sensors, with a specific focus on the vital components of acoustic cells and acoustic detectors. This review highlights photoacoustic spectroscopy (PAS) as an optical detection technique, lauding its high sensitivity, selectivity, and capability to detect a wide range of gaseous species. The principles of photoacoustic gas sensors are outlined, emphasizing the use of modulated light absorption to generate heat and subsequently detect gas pressure as acoustic pressure. Additionally, this review provides an overview of recent advancements in photoacoustic gas sensor components while also discussing the applications, challenges, and limitations of these sensors. It also includes a comparative analysis of photoacoustic gas sensors and other types of gas sensors, along with potential future research directions and opportunities. The main aim of this review is to advance the understanding and development of photoacoustic gas detection technology.

We report a comprehensive model that quantifies analytically and numerically the heat that can be extracted by anti-Stokes fluorescence (ASF) from a fiber doped with a quasi-two-level laser ion. This model is used to investigate the effects on cooling of all relevant fiber and pump parameters, as well as amplified spontaneous emission. Simulations of a typical Yb-doped ZBLANP single-mode fiber show that for short enough fibers the heat extraction is relatively uniform along the fiber length. There is an optimum pump wavelength and power that maximizes the heat extracted per unit length. At this power, the coolest point is at the fiber input end. At higher powers, the coolest spot moves further down the fiber. The total heat extracted from a fiber, important for payload cooling, depends on the fiber absorptive loss, the pump wavelength, and the pump power. Simple expressions are derived to predict the optimum dopant concentration that maximizes heat extraction and the maximum tolerable absorptive fiber loss above which cooling is unobtainable. In a fiber with negligible residual absorption, the cooling efficiency is predicted to be 3.7%. In the modeled fiber, it is reduced to 1.7% in part by concentration quenching, but mainly due to the fiber absorptive loss (~15 dB/km). Since the total extracted heat increases linearly with core radius and dopant concentration (up to a limit determined by concentration quenching), highly doped multimode fibers are strong candidates for payload cooling. This model can be straightforwardly expanded to design and optimize fiber lasers and amplifiers that utilize ASF for cooling.

We demonstrate the full bandwidth measurement of the intermodal four-wave mixing efficiency for a 4.7-km 3- spatial-mode few-mode fiber. Large fluctuations of inter-modal four-wave mixing between the LP01 and LP11 modes are experimentally observed. These fluctuations exhibit a strong wavelength dependence which can be persistent in time. The fluctuations are also shown to display some polarization dependence.

Brillouin scattering-based distributed fiber optic sensing (DFOS) technologies such as Brillouin optical time domain reflectometry (BOTDR) and Brillouin optical time domain analysis (BOTDA) have broad applicability for the long term and real-time monitoring of large concrete structures, underground mine excavations, pit slopes, and deep subsurface wellbores. When installed in brittle media, however, the meter scale spatial resolution of the BOTDR/A technology prohibits the detection or measurement of highly localized deformations, such as those which form at or along cracks, faults, and other discontinuities. This work presents a novel hybrid fiber optic cable with the ability to self-anchor to any brittle installation media without the need for manual installation along fixed interval points. Laboratory scale testing demonstrates the ability of the hybrid fiber optic cable to measure strains across highly localized deformation zones in both tension and shear. In addition, results show the applicability of the developed technology for strain monitoring in high displacement environments. Linear relationships are proposed for use in estimating the displacement magnitude along discontinuities in brittle media from strain signals collected from the hybrid fiber optic cable. The hybrid fiber optic cable has broad potential applications, such as geomechanical monitoring in underground mines, surface pits, large civil infrastructure projects, and deep subsurface wellbores. The benefits of fiber optic sensing, such as the intrinsic safety of the sensors, the long sensing range, and real time capabilities make this a compelling technique for long term structural health monitoring (SHM) in a wide range of industrial and civil applications.

This review explores various advanced photonic-assisted techniques for microwave frequency measurement, highlighting their distinct advantages and challenges in detecting multi-tone and broadband microwave frequency signals. Different optical processing techniques for instantaneous frequency measurement, including frequency-to-time mapping techniques, are discussed in detail. The application of multicore and few-mode fibers, artificial intelligence-enhanced, and complex modulation techniques are also discussed. These recent advances collectively push the boundaries of microwave frequency measurement, offering robust and scalable solutions for various applications.

Large fluctuations of inter-modal four-wave mixing between the LP01 and LP11 modes of a few-mode fiber are experimentally observed. These fluctuations exhibit strong frequency dependence and can be persistent in time.