**3. Applications**

A side-polished fiber (SPF) coated with cholesteric liquid crystal film (CLCF) has been employed for Volatile Organic compound (VOC) sensing, where a rise in the concentration of VOC on CLCF causes a rise in the pitch of the resulting light. As a result, the resonant dips exhibit a blue shift that can be linked to the exposure to VOCs. For acetone, tetrahydrofuran and methanol gas, the sensitivities of CLCF-SPF have been observed to be 3.46, 7.08 and 0.52 nm-L/mmol, respectively. It can be observed that the overall sensitivity of the CLCF-SPF rises with the VOCs' molar mass.


**Figure 3** depicts photonic crystal gas sensor, as another type of optical gas sensor that is similar to one described above. The photonic crystals are mainly synthetic optical materials having periodic variations in value of the refractive index. According to photonic band gap theory, beams having the wavelength in range of photonic crystal band gap can be bound in and transmitted down the air channel having little energy loss in the case where the cladding of fiber has a higher refractive index compared to the photonic crystal core with air channels. Additionally, air passages act as the cells for the insertion of the molecules of the target gas, which alters core's refractive index of photonic crystal and hence, alters the output illumination.

**Figure 3.** *Schematic illustration of (A) photonic crystal gas sensor and (B) fiber-optic gas sensor.*

Three patterns of varying sizes on a single substrate have been used to identify different VOCs using Si-based photonic crystals. By using electrochemical anodization to create mesopores of various sizes, the designs are etched onto the Si wafer. On a Si substrate, 8 nm of diameter mesopores were created in multilayers with 178, 229 and 300 nm of the vertical spacing, allowing the refection of 430, 580 and 740 nm photons. Introduction of the analytes over this photonic crystal with several layer patterns raises the effective refractive index and changes the permitted wavelength of the reflected beam. When sensing capabilities of the devices were examined using methanol, ethanol, etc. in the nitrogen carrier gas presence, LOD in the ppm range was observed. Using this technique, analytes by calculating the shift in wavelength in time of each VOC.

Researchers have looked into self-assembling silica nanospheres to create photonic crystals for ethanol, water and carbon disulfide (CS2) detection. On silica colloid drying over substrate and then annealing it at the temperature of 600°C for the sintering, the silica photonic crystal can be created. To improve the interaction with analytes, HKUST-1 can be coated on the nano-structured silica. Each silica nano-sphere has a diameter of around 300 nm and is arranged in a face-centred cubic configuration. For a gas-sensing test, when analytes are present, NIR light can be shone in the direction of the silica nanostructure. The predicted detection limit for water, ethanol and CS2 is 2.6, 0.3 ppm and CS2 is 0.5 ppm, according to tests on the sensing ability of these substances.

### **4. Conclusion**

To conclude, optical gas sensing remains an important field that complements other gas detection technologies. The opportunities afforded by new technology, together with the challenges that remain, will make this an exciting and rapidly developing field for many years to come. Commercially available gas sensors are

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

rarely based on the optical principles, present article focuses over the broad overview of techniques involved in gas analysis and illustration of the detailed introduction of the optical-based gas sensors. Optical sensors including fiber-optic and photonic crystal gas sensors operate based on the detection of the light propagation through the device. Fiber-optic sensors detect analytes by measuring optical property changes of light such as wavelength, which is introduced on the polymeric sensing layer through optical fiber. These sensors have many advantages in high sensitivity, stability to environmental factors and long lifetime. However, needs of optical fiber in the structure make it difficult to be miniaturized. Photonic crystal sensors employ periodic arrangements of dielectric materials with various refractive indexes. Mid-infrared PhCs are a common example of these devices to detect common gases such as CO. These sensors can be developed using advanced micro-machining technology and, therefore, the dimension and shape of the device pattern can be precisely controlled, which on the other hand increases the fabrication cost.
