## 5 Results and Discussion
phase difference between the two arms of the interferometer, is analyzed. As the external condition (e.g., gas concentration) changes, it affects the refractive index of the PHMB polymer, resulting in the shift in the effective refractive index in the sensing arm of the MZI, leading to a shift in the interference pattern. This shift causes a change in the intensity of light at specific wavelengths. The transmission spectrum of the device is plotted over a wavelength range of 1450 nm to 1580 nm, with varying gas concentrations, as shown in Figure 5b. The geometric parameters of the device are fixed, with w = 250 nm, g = 100 nm, L\({}_{\text{DC}}\) = 6 \(\upmu\)m, and L = 5 \(\upmu\)m. At the operational wavelength of 1535.5 nm, destructive interference occurs, resulting in a dip in the transmission spectrum that becomes more pronounced as the gas concentration increases. Figure 5c illustrates the power measured at Output_1 at the wavelength of 1535.5 nm under different gas concentrations. It is observed that the power decreases from \(-\)7.15 dB to \(-\)15.38 dB as the gas concentration rises from 0 ppm to 366 ppm. However, beyond 366 ppm, a slight increase in power is noted, which may be attributed to weak destructive interference. Additionally, the H-field distribution within the device is presented for gas concentrations of 0 ppm and 366 ppm in Figure 5d,e, respectively. These figures provide a visual representation of the field changes due to varying gas concentrations. The transmission spectrum of the LT-MZI structure under varying CO\({}_{2}\) gas concentrations is methodically depicted for two distinct arm lengths, L = 5 \(\upmu\)m and L = 10 \(\upmu\)m, illustrated in Figure 6a,b, respectively. Analyzing these figures reveals a noticeable trend: as the CO\({}_{2}\) gas concentration escalates from 0 to 524 ppm, an observable red shift in the  
Figure 5: (a) Schematic representation of MZI structure without end loop (red arrows indicate the light propagation direction), (b) transmission spectrum versus CO\({}_{2}\) gas concentration, (c) power decay at Output_1 for the operational wavelength of 1535.5 nm versus CO\({}_{2}\) gas concentration. Normalized H-field distribution in the device at the wavelength of 1535.5 nm for the gas concentrations of (d) 0 ppm and (e) 366 ppm. interference dip occurs. However, the magnitude of this wavelength shift is intrinsically linked to the length of the MZI arms. The sensitivity of an MZI structure increases with longer arm lengths (L = 15 \(\upmu\)m) compared to shorter ones (L = 5 \(\upmu\)m) as shown in Figure 6c. Firstly, longer arms provide a greater interaction length for the propagating optical signal within the device, allowing for more opportunities for the signal to experience phase changes induced by external stimuli, such as refractive index variations induced by the change in the properties of PHMB deposited on the sensing arm. This increased interaction length enhances the sensitivity of the MZI to these external perturbations. Additionally, longer arm lengths typically result in a larger optical path length imbalance between the two arms of the interferometer. This imbalance amplifies the phase difference between the two arms, leading to more significant interference pattern changes in response to minute changes in the external conditions. Furthermore, longer arms inherently possess a higher spatial resolution, enabling the detection of smaller changes in the environment or sample properties. The sensitivity of the proposed device was estimated at 7.63 pm/ppm, 34.46 pm/ppm, and 74.78 pm/ppm for the arm lengths (L) of 5 \(\upmu\)m, 10 \(\upmu\)m, and 15 \(\upmu\)m, respectively. Furthermore, Table 3 showcases not only the footprint of the device but also the sensitivity it offers. \begin{table}
\begin{tabular}{c c c} \hline \hline
**L (\(\upmu\)m)** & **Device Footprint (\(\upmu\times\upmu\)m)** & **Sensitivity (pm/ppm)** \\ \hline
5 & \(45\times 16\) & 7.63 \\
10 & \(55\times 16\) & 34.46 \\
15 & \(65\times 16\) & 74.78 \\ \hline \hline \end{tabular}
\end{table}