*1.1.1.2 Multi-mode fiber (based on number of modes)*

The light ray flowing through multimode fiber can take on a variety of modes. The cladding has a diameter of (40 μm) and the core has a diameter of (70 μm). The difference in relative refractive index is also bigger than in single mode fiber. Due to

**Figure 2.**

*(a) Single mode optical fiber; and (b) multi mode optical fiber.*

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

multimode dispersion, signal quality suffers. Due to signal dispersion and attenuation, it is not ideal for long-distance transmission [42] (shown in **Figure 2b**).

#### *1.1.1.3 Step index fiber (based on refractive index)*

The refractive index of step-index fibers is discontinuous at the interface between the cladding and the core, and the core is constant. Light rays pass through it as meridional rays that cross the fiber axis at the core-cladding interface on every reflection as shown in **Figure 3a**.

#### *1.1.1.4 Graded index fiber (based on refractive index)*

The core of this fiber has a non-uniform refractive index that declines gradually from the centre to the core-cladding interface. The refractive index of the cladding is uniform. Light rays in the form of skew rays or helical rays propagate through it as shown in **Figure 3b**. At no point does it cross the fiber axis. The refractive index of the core in a gradient index fiber is higher in the centre and gradually drops as the interface approaches [43, 44].

#### *1.1.2 Design of a basic fiber optic gas sensing system*

A gas sensor is a device that measures target gas molecules present in a given environment. When gas molecules encounter the sensor's solid receptors, a potential difference occurs, that is converted into an electrical signal. The sensor's gas sensitivity and the selectivity are dependent upon the reaction of the sensing materials with the gases [45]. The sensing materials basically used for all types of gas sensors include polymers, organic monolayers, ceramics, semiconductors, porous nanomaterial and nanostructured materials (nanorods, nanotubes, nanodots). Optical fiber sensors have been projected as a good choice, to considering their benefits compared to other available sensing technologies, as they are immune to electromagnetic interferences [46], do not require any electric power to work, possibility of multiplexation, ability to perform in remote areas and harsh environmental conditions [27]. A block diagram of a common fiber optic gas sensor is shown in **Figure 4**. A light source, a signal input optical fiber, a signal output optical fiber, and a detector make up a fiber-optic gas sensing system (optionally the system may include other components such as an optical modulator and a demodulator). The essential idea is that the light from the source is transferred to the sensor element via the incident optical fiber.

**Figure 3.** *(a) Step index optical fiber; and (b) graded index optical fiber.*

**Figure 4.**

*Block diagram of extrinsic and intrinsic fiber optic gas sensor.*

The measured parameters are obtained by modulating the optical properties of light, such as intensity, wavelength, frequency, phase, and polarisation state, in the sensor element and sending them to the optical detector through the outgoing fiber. Intrinsic and extrinsic optical fiber sensors are two types of sensors. The intrinsic optical fiber sensor relies on the optical fiber's own sensitivity to environmental variables. In an intrinsic sensor, specific regions in the fiber cable perform sensing function, thereby detector is not exposed to the light source. However, in extrinsic sensors, the fiber optic cable is used as a data transmission line, wherein the optical cable carries light from an optical source externally to the sensing region. Most of the fiber optic sensors used in VOC gas detection are intrinsic sensors.
