**Factors affecting gas sensing mechanism:**

Several factors should be taken into account when assessing the effectiveness of gas sensing techniques (**Figure 1** depicts the classification of gas sensor):


The operating stability of gas sensors that are intended for the market must also be guaranteed; or they must display a highly stable as well as repeatable signal over time.

**Figure 1.** *Chart representing the classification of gas sensors.*

#### *Optical Gas Sensors DOI: http://dx.doi.org/10.5772/intechopen.108971*

But, the instability of the gas sensor is caused by a number of reasons such as design mistakes, structural modifications (such as changes in grain size or grain network), phase transitions (which typically refers to separation of the additives doped with sensing materials), poisoning brought on by the chemical processes as well as change in the environment.

The use of optical techniques for gas sensing is typically simple and can produce results with more sensitivity, stability as well as selectivity in respect to other non-optical techniques while having a significantly longer lifetime. Their quick response time makes on-line real-time detection possible. The changes in the atmosphere or poisoning of the catalyst brought on by particular gases, etc. will not affect performance. The majority of optical techniques utilized for sensing gas are based on spectroscopy.

### **2.1 Basic principles**

There has been an extensive discussion about the principles of molecular absorption spectroscopy [12, 13]. Optical gas sensing techniques have been commercialized and while the majority are absorption-based, several other methods also play a significant role. Myriad of chemical substances show high absorption mainly in the region dominated over ultraviolet–visible range, near-infrared (NIR) as well as mid-infrared range of electromagnetic spectrum. Each substance has its unique set of absorption lines or bands that serve as the foundation for its detection and quantification. The parameters of the absorption spectra show variation in the various spectral bands displayed in **Table 2**.

Optical gas sensors work on the principle of the Beer and Lambert law and relation for the same is [14].

$$I(\mathcal{X}) = I\_\circ(\mathcal{X})e^{-ad} \tag{1}$$

were, (λ ) *<sup>o</sup> I and I* are the detected as well as emitted optical intensity at a particular wavelength ( λ) , (respectively)

α λ( )[*L g* / ] = Absorption coefficient of the gas.

*cg L* [ / ] = Concentration of gas.

l[*m*] = beam-gas interaction path length.

**Figure 2** depicts a sensor comprised of different parts, namely


The spectroscopic investigation relied mainly on the spectrometry absorption as well as emission. Absorption analysis is completely based on the Beer and Lambert


#### **Table 2.**

*Origin of absorption spectra in the various regions of the electromagnetic spectrum.*

law and is basically concentration-dependent absorption of the photons at a particular wavelength of the gas. Other than this plethora of improvised techniques of absorption spectrometry, namely Differential Optical Absorption Spectroscopy, Raman Light Detection, Tunable Diode Laser Absorption Spectroscopy, Differential Absorption LIDAR and Intra-Cavity Absorption Spectrometry (ICAS), etc. [15–17]. Spectrometry based on emission is observed when the atoms sitting in the excited levels on their transition to the ground state emits photons. Emission-based techniques are namely Laser-Induced Breakdown Spectroscopy, Fourier Transform Infrared Analysis (FT-IR) can be utilized for both the spectroscopy, namely absorption and emission. Furthermore, the photo-acoustic and correlation spectroscopy techniques also come in the category of spectroscopic analysis. These spectroscopic techniques are generally employed to gas detectors. This can contribute to the designing of the more complicated system at a higher cost level and can incorporate exceptional sensitivity, selectivity and reliability as compared to the other available gas sensors. Extensively utilized optical-based sensors utilize infrared source. The infrared source-based gas sensors work on the principle of absorption spectrometry. That implies that each and every gas is having a specific absorption property to infrared radiation of varying wavelength and results in unique fingerprint of infrared absorption. This implies that each and every gas has some unique property of absorption to infrared radiation range with varying wavelengths and thus shows a rare fingerprint of infrared absorption. Gas sensors based on infrared source consist of mainly three major parts, namely (depicted in **Figure 2**),


#### iii.Infrared Detector

At the primary state, the infrared source emits broad-band of radiation wavelength containing the wavelength range corresponding to the absorption of target gas. After that the gas sample present in the gas cell absorbs the incoming radiation beam of their specific wavelength. Later on the optical filters are utilized to screen out the entire wavelength of the radiation except the radiation wavelength absorbed by the target gas. Thus, the gas of interest can be easily detected as well as measured just by employing infrared detector. The entire setup is named as Non-Dispersive Infrared gas sensor.

Non-dispersive sensing technique is utilized, wherein the beam (unfiltered) is utilized for the interaction with the gas. This sensing technique encourages the selective detection of beam (λ ) , just by filtering the beam detected by characteristic absorption spectra of the molecular species. Non-dispersive infrared (NDIR) sensors conventionally configured with infrared emitters and infrared detectors [18]. Sensors

#### **Figure 2.**

*Beer and Lambert law–based optical gas sensor; (A) off state (no any signal being detected, (B) on state maximum detected signal, (C) decreased detection with gas concentration.*

work on the same principle of Beer–Lambert's Law, if utilized for different spectral regions or configured for acoustic detectors.

A different gas cell consisting of the reference gas other than the single detector mode can be incorporated to uplift the overall accuracy of the infrared sensing method just by truncating the ambient environmental factors without inducing much more complexity. **Figure 2** depicts the layout of two detectors. Just to keep the radiation parameters of infrared source identical, gas cell utilizes the reflected infrared beams from the single source just utilizing the mirror property. The assortment of the range of wavelength of the infrared beam utilized as source impacts a lot on the concluding detection result. Spectral region of the mid-infrared gains more attention as it encourages stronger molecular absorption as compared to spectral region of near-infrared. The conventional mid-infrared laser sources suffer major drawbacks, namely wavelength tunability, lower power of output, complexity, requirement of coolants. Plethora of lasers have been developed and investigated in order to overrule the above mentioned disadvantages, e.g. Quantum-Cascade Lasers blessed with tunability, highly narrow line-width as well as lower average power for the operation at the room temperature.

Myriad of topologies have been employed for the fabrication of optical gas sensors. The commonly utilized sensors are based on gas cells formed between face-to-face configured emitters and optical detectors. Several approaches followed for the miniaturization of the gas cell include utilization of enhancement layers namely, Utilization of photonic crystals [19], optical cavities [20], multi-pass cells [21], gas enrichment layers [22–24], in order to enhance beam-gas interaction, the planar configurations of the emitters as well as detectors, utilization of waveguides for the interaction of evanescent field. Optical detectors, namely photodiode, thermopile or pyroelectric, acoustic detector are mainly implemented in order to detect the beam being absorbed ( *I*(λ) ) [25–28]. The mechanism of lock-in detection is employed to

extract the sensor response signal ( *I*(λ ) ) and further known frequency is utilized to modulate the emitter. The reference detector is frequently utilized in order to compensate for the variation in emitted beam. Further, additional sensors can be utilized in order to compensate for the several parameters of environment, namely temperature, pressure, etc.
