*1.1.3 Classification of fiber optic gas sensing (FOS) system*

The fiber optic sensing region is primarily divided into four kinds of approaches namely the Fabry-Perot Interferometer (FPI), Michelson Interferometer (MI), Mach-Zehnder Interferometer (MZI), Sagnac Interferometer (SI), based on interferometric techniques [47, 48] as shown in **Figure 5**. These approaches are known for

**Figure 5.** *Fiber optic sensors based on interferometric techniques.*

*Fiber Optic Sensors for Gas Detection: An Overview on Spin Frustrated Multiferroics DOI: http://dx.doi.org/10.5772/intechopen.106863*

their diverse geometries, operating principles, and their sensitivity. The Michelson, Mach-Zehnder, and Sagnac interferometers use a two-beam interference principle, whereas the Fabry-Perot interferometer uses a multi-beam interference principle. An interferometer's basic operation is to split an incident light beam into two portions, the reference beam and the sensing beam, the latter of which is changed by a variable of interest that we want to measure. The two beams are then recombined to form an interference pattern that allows information about the desired variable to be recovered. Other fiber-optic geometries based on the SPR principle, gratings, and the evanescent wave absorption (EWA) principle exist. An overview of various kinds of Fiber optic gas sensors is displayed in **Figure 6**.

#### *1.1.3.1 Fabry-Perot interferometer (FPI)*

Because of its simplicity, sensitivity, and cost-effectiveness, this is the most commonly used interferometric geometry for chemical sensing. The FPI is made up of two parallel reflecting mirrors that are spaced apart. It's known as an etalon. A single light beam enters the FP cavity and is multiplicatively internally reflected. The superposition of both reflected and transmitted beams at the two parallel surfaces causes interference. The FP cavity can be created inside or at the end of an optical fiber in the case of FOS. Extrinsic FPI and intrinsic FPI are the two types of FPI that exist. An extrinsic FPI uses the reflections from the cavity surfaces to create a cavity at the end of the optical cable. As a result, an extrinsic FPI is defined as a cavity material that is not made up entirely of fiber. The cavity and reflections arise within the fiber itself in the case of an intrinsic FPI [49]. The FPI is easy to build and responds well to external inputs.

**Figure 6.** *An overview of various kinds of fiber optic gas sensors.*
