# Switchable Fiber Ring Laser Sensor for Air Pressure Based on Mach-Zehnder Interferometer
## 3 Results and Discussion  
Based on the conventional ring fiber laser cavity, as shown in Figure 6, the elements described in Figure 3 were closed to form a ring cavity by a 90/10 optical fiber coupler, allowing us to return 90% of the signal to the system as feedback. This cavity length increment provided a flat spectrum of 1060 to 1120 nm [27]. The response of the laser is demonstrated in Figure 7. A single-line emission was generated at 1063.04 nm for the initial conditions with a side mode suppression ratio (SMSR) of 34.17 dB. These results of emissions, as well as all the results shown in the manuscript, were recorded using an OSA resolution of 0.2 nm. Figure 8 shows that when the pressure inside the gas cell increases, the laser emission shifts to the right of the reference until it reaches 25 PSI, and then, switches to the other side at 30 and 40 PSI. Figure 6: Experimental setup of the fiber laser. Figure 7: Reference laser emission. The result shows that by increasing the air pressure inside the cell, the TCFMI undergoes stress. The modes inside of the TCFMI are affected by changes in the refractive index due to the photoelastic effect. It is essential to note that the fiber laser is switchable. Table 2 shows the wavelength of the laser emission peaks as the air cell pressure increases from 0 to 40 PSI. As shown in Figure 9, a switchable fiber laser response is observed within a specific range. In the area delimited in blue, as the pressure increases from 0 to 5 PSI, it shifts 0.68 nm upon reaching 10 PSI; in the region of red color, represented in Figure 10, the relationship between the pressure and the laser wavelength presents a certain linearity. As the pressure increases, the wavelength of the laser emission shifts toward wavelengths in the red region. A sensitivity of 0.0744 nm/PSI in the range of 10-25 PSI with a step of 5 PSI is achieved for the laser. The switching occurs at a pressure of 30 PSI, as seen in the green region. Maintaining a constant pressure of 30 PSI inside the cell, the output spectrum was scanned for 30 min and recorded after 5 min intervals (see Figure 11a) with a resolution of 0.2 nm. This experimentation was conducted at ambient room temperature to confirm the laser's operational stability. Furthermore, it allowed for the examination of alterations in laser emissions when transitioning pressure between 25 and 30 PSI, and vice versa, over 30 min. Notably, it was observed that the laser emission reverted to 1075.68 nm (as shown in Figure 11b). \begin{table}
\begin{tabular}{c l l} \hline \hline  & REF & \(\lambda\)1 = 1063.04 nm \\ \hline  & 5 PSI & \(\lambda\)2 = 1063.72 nm \\ \hline  & 10 PSI & \(\lambda\)3 = 1074.48 nm \\ \hline  & 15 PSI & \(\lambda\)4 = 1075.28 nm \\ \hline  & 20 PSI & \(\lambda\)5 = 1075.40 nm \\ \hline  & 25 PSI & \(\lambda\)6 = 1075.68 nm \\ \hline  & 30 PSI & \(\lambda\)7 = 1059.20 nm \\ \hline  & 40 PSI & \(\lambda\)8 = 1060.04 nm \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Wavelength of laser emissions. Figure 8: Laser emission spectra for different pressure values. The sensitivities achieved were compared with prior MZI optical fiber structures in Table 3. The sensitivity of the proposed sensor is satisfactory, like those reported in other works of air-channel-based structures. However, the proposed sensor has a simple fabrication process and high resolution. Figure 10: Zoomed-in view of the red area of Figure 9 in response of the shift in the wavelength. \begin{table}
\begin{tabular}{c c} \hline \hline
**Pressure Sensor** & **Sensitivity** \\ \hline This work—TCFMI & 0.0925 nm/PSI \(\approx\) 13.23 nm/MPa \\ \hline Twin-core fiber with a micro-channel in one core [29] & \(-\)9.6 nm/MPa \\ \hline An MZI based on core-cladding interference with a pair of off-axis twisted distortion points [35] & \(-\)5.183 nm/MPa \\ \hline Cascaded fiber MZIs for simultaneous measurement of pressure [40] & 78.553 nm/MPa \\ \hline MZI with a micro-channel across the fiber core [49] & \(-\)8.239 nm/MPa \\ \hline \hline \end{tabular}
\end{table}