**1. Introduction**

The detection of gases is webbed across a plethora of application fields. Primary application extended over sector of petrochemical, where sensors were incorporated to safeguard (namely by detecting the leakage of highly toxic or flammable gases). The monitoring of feed stocks, as well as the key species parameters utilized in products as well as processes, keeps on revising depending upon the requirements with passage of time and day-to-day advancement in the field of science and technology [1, 2]. Several fields of science, namely atmospheric science, encourage the application of extremely sensitive gas detectors in order to detect, measure and develop the understanding of properties and trajectories of numerous gases, namely the greenhouse gases [3]. In the field of medical, investigation is made over several gases (namely nitric oxide (NO), ethane, ammonia (NH3), etc.) having potential candidature for



#### **Table 1.**

*Summary of the advantages, disadvantages and the myriad of applications for the listed methods of gas sensing.*

biomarker gases application in fields of different for diagnostics, namely respiratory, etc. Chromatographs, analytical laboratory equipment has traditionally being utilized on larger scale for quantitative gas detection, but the equipment lacks the real-time data sampling [4]. Other than this sensor, semiconductor gas sensors or electrochemical devices were utilized. Other devices, namely Pellistorbeing, a potential sensor device (for the detection of the flammable gases closer to the lower range of the explosive limit but zero parts per million) for combustion on catalyst beads [5], semiconductor gas sensors can be very sensitive down to ppm [6], but they lack and suffer from the drift and erratic response to other gases as well as changing humidity levels. Other than this, electrochemical gas sensor being highly sensitive (ppm or ppb levels) and relatively specific to an individual gases suffers limited lifetimes and cross-response issues [7]. In comparative to this (**Table 1**), carefully designed optical absorption-based gas sensors are enriched with highly sensitivity, responsivity (approximately, 1 s), negligible drift, high-level gas specificity and seize to any cross response to other gases . They are blessed with highly controlled processes since the measurement corresponding to the detection can be made in real time and in situ devoid of any disturbance to sample under investigation [8].

Optical gas sensors act as a bridge among the sensors available at cheaper rates (with substandard performance) and costlier laboratory equipments. Optical gas sensors involve the transduction method which through direct measurement of molecules physical properties (e.g. the absorption of molecule at a particular wavelength) causes reduction in the drift. Since the intensity of the incident beam can be determined, the measurements are self-referenced and are compelled to be inherently reliable. Gas detection applications are webbed over the broader range of gas concentrations (proportion present in the air (or the other matrix) by the total volume). At standard temperature and pressure, gases mostly behave as an ideal gas and are approximately equal to molar concentration present in the given matrix. The study of widely used gas sensing methods based on the optical absorption measurements at particular wavelengths is the main focus of this article. Being non-dispersive gas sensing methods utilizing spectro-photometry, non-dispersive infrared, tunable diode laser spectroscopy and photo-acoustic spectroscopy [9–11].

Optical gas sensing proved to be the uncomplicated method and accomplish superior sensitivity, stability and selectivity over all other non-optical methods with much longer lifetime. Optical-based gas sensing has relatively shorter response time, enabling the detection on-line in real time. The performance of these sensors is immune to the any environment changes or catalyst poisoning triggered by the presence of specific gasses, etc. Optical gas sensing is spectroscopic investigation, but their applications on gas sensors are hampered because of miniaturization as well as comparatively high cost. That is the reason why only limited commercially available gas sensors based on the optical principle are available. Present review focusses over the general overview of optical-based gas sensors.
