Fiber optic gas sensors
A lot of effort has been dedicated during the past decades to the detection of gaseous species. Precise and rapid detection of minute gas concentrations is required for many applications and it is highly relevant in many industrial fields . In particular, gas sensors are widely employed in air quality monitoring, industrial process control, disease diagnosis or prognosis or alerting the population or workers from dangerous and toxic gases in the atmosphere or working environment among others. Recent changes in national and international regulations have lowered the minimum allowable concentrations of different gaseous species, which has increased the demand for better, faster and more precise gas sensors that permit to control the air pollution in our cities or working centers  (see Figure 1).
Gas sensing detection techniques are based on the detection of physicochemical modifications of the environment or sensing materials associated to their interactions with gaseous species. Typical materials used for this purpose comprise metal oxide nanoparticles (tin oxide, zinc oxide, tungsten oxide, etc.), polymers (polyaniline, polypyrrole, …), decorated carbon nanotubes or graphene oxide thin-films among others. Concerning the materials and their combinations, many of them have been explored together with their sensitivity with temperature in order to maximize the selectivity to particular gaseous species .
As regard as the gas detection method used we can distinguish among a wide variety of techniques, such as mass spectrometry, calorimetry, gas chromatography, acoustic, optical  as well as other methods based on the measurement of the electrical properties of the materials . A particular case consists of the utilization of optical fibers for the fabrication of gas sensors. Here, gas sensing devices take advantage of the inherent properties of optical fiber, such as small size, flexibility, appropriate for confined spaces and immunity to electromagnetic interferences. Optical fibers are also resistant to high temperatures, corrosive environments, high humidity and do not transport electricity making them highly suitable candidates for applications in explosive atmospheres. Previous advantages enable the utilization of optical fibers in niche applications where no other technology can be used [3,4]. In addition, the use of optical fiber allows not only the transmission of light to remote zones, but also the use of optical fiber itself as the sensing part with easy multiplexing capabilities or even the possibility to perform distributed measurements.
A main classification of optical fiber sensors, which can be also extrapolated to optical fiber gas sensors is associated to the place where the transduction takes place. Here, we can differentiate among intrinsic optical fiber gas sensors where sensing takes place within the fiber itself and extrinsic optical fiber gas sensors where sensing takes place outside the fiber as it is schematically represented in Figure 2.
Particularly, optical fiber gas sensors have been obtained using different optical fiber configurations, such as single mode fiber (SMF), multimode fiber (MMF), D-shape fiber, or photonic crystal fibers as well as diverse interrogation techniques already described in some of our previous blog posts, such as interferometry, long periods fiber gratings or resonances , among others.
The application of optical fiber gas sensors is wide and heterogeneous. In the case of industrial processes they have been exploited for the detection of NOx in coal power plants, H2S in sewage plants or volatile compounds (ethanol) in wine industry. Fiber optic gas sensors are also relevant in human wellbeing for the monitorization of dangerous gases including volatile compounds in working environments as well as indoor and outdoor air pollutants, such as NOx that can cause respiratory and neurological disorders in elevated concentrations. Moreover, fiber optic gas sensors associated to their small size and flexibility, can also be employed in the healthcare system for real time and in vivo monitoring to monitor of several gaseous species associated to different diseases like ammonia, which is used for detecting liver dysfunction, or acetone, which is used for detecting diabetes.
Finally, it is important to remark that optical fiber gas sensors are still at a very early stage but they have already demonstrated an outstanding performance with ample possibilities of improvement thanks to the continuous advancement and recent progresses in the fields of photonics and nanotechnology. Nowadays, the development of some of these sensors is still in a research phase but every new research work pushes these devices closer to their final applications. The rapid development and outstanding advantages of optical fiber technology for gas sensing applications mentioned above reveals a very interesting opportunity and the tremendous potential of these devices in order to fulfil the increasing and highly demanding needs for gas sensors.
2. Ahmed Abdul-Wahab, S.A.; En, S.C.F.; Elkamel, A.; Ahmadi, L.; Yetilmezsoy, K. A review of standards and guidelines set by international bodies for the parameters of indoor air quality. Atmos. Pollut. Res. 2015, 6, 751–767.
5. Vitoria, I., Zamarreño, C.R., Ozcariz, A., Matias, I.R. Fiber optic gas sensors based on lossy mode resonances and sensing materials used therefor: A comprehensive review. Sensors 2021, 21(3), pp. 1–26, 731