## 5 Results and Discussion
### Strain Response of the Intensity-Modulated DWDM PM-PCF Sagnac Loop Sensor  
The proposed sensor setup as depicted in Figure 2 was investigated by monitoring the output light after passing the Sagnac loop and the DWDM due to elongation of PM-PCF. The fiber was stretched over a range of 0-2000 \(\mu\varepsilon\) in steps of 250 \(\mu\varepsilon\). The output transmission spectra are shown in Figure 5.  
It could be observed from Figure 5 that the intensity of the output light is different due to the applied strain, i.e., the elongation of PM-PCF influences the spectrum of PM-PCF, which shifts towards longer wavelengths and thus the light coupling out from the Sagnac loop is accordingly modulated by the DWDM. The integral over the output spectrum corresponds to measured optical power, which determines the elongation of the fiber. An increase in strain causes an increase in output light. The intensity levels of the output spectrum are different due to the influence of edge slope spectrum of the interferometer. Thus, an investigation into the optical power change was conducted using the  
Figure 4: (**a**) The interferometer spectrum shift due to strain, (**b**) the strain response of a selected wavelength dip from the interferometer. reference (\(P_{\textit{rpf}}\)) and output power (\(P_{\textit{out}}\)), which eliminates the fluctuation of incident light. Multiple measurements were performed in order to examine the proposed experimental setup. The relationship between the change of optical power and the axial strain is presented in Figure 6.  
The analysis of measurement data shows that the change in optical power exceeds 11 dB (\(\sim\)11.2 dB) within 2000 \(\upmu\)e with a negligible deviation between the performed measurements, maximally \(\pm\) 0.03 dB. A nonlinear response to strain is observed, which could be a result of the spectral correlation between the interferometer and DWDM. The quadratic function was selected as a fitting function (\(R^{2}\) = 0.999) determined by the following equation:  
\[\Delta P=-1.925\cdot 10^{-6}\cdot S^{2}+0.009454\cdot S+0.03112 \tag{7}\]  
In the equation above \(S\) refers to the strain value applied on PM-PCF (\(\upmu\varepsilon\)) and \(\Delta P\) corresponds to the change in output power (dB). To determine sensitivity at given strain values, the first order derivative of the fitting function was calculated as follows:  
\[\frac{\partial(\Delta P)}{\partial S}=-3.85\cdot 10^{-6}\cdot S+0.009454 \tag{8}\]  
Thus, the sensitivity is approximately 0.01 dB/\(\upmu\)e at the initial value (0 \(\upmu\varepsilon\)) and 0.002 dB/\(\upmu\varepsilon\) at 2 m\(\upmu\), respectively. Assuming the resolution of the standard optical power meter used in the experiment  
Figure 5: The output light spectra for different stretched PM-PCF. Figure 6: The optical power change as a function of axial strain. (0.01 dB), the resolution of the proposed setup changes within the range of 1 \(\mu\epsilon\) to 5 \(\mu\epsilon\) was the result of the measurement. Due to the use of an optical spectrum analyzer with a resolution of 0.01 nm and assuming the use of the experimental PM-PCF Sagnac interferometer (sensitivity of 0.98 pm/\(\mu\epsilon\) according to the Figure 4), the possible resolution could be \(\sim\)10 \(\mu\epsilon\), which is lower than expected from the proposed intensity-modulated sensor setup. The setup presented by [PERSON] et al. had a sensitivity of 0.23 pm/\(\mu\epsilon\) and a resolution of \(\sim\)43 \(\mu\epsilon\)[16]. A similar setup demonstrated by [PERSON] achieved a sensitivity of \(\sim\)1.3 pm/\(\mu\epsilon\)[17]. Another comparable sensitivity was achieved by [PERSON] et al. [22], i.e., 1.21 pm/\(\mu\epsilon\) or 1.11 pm/\(\mu\epsilon\) depending on whether PM-PCF is uncoated or coated (acrylate). Thus, compared to wavelength-based sensor setups employing this type of PCF [16; 17], the presented sensor exhibits a higher resolution (\(\sim\)1 to 5 \(\mu\epsilon\)) and reduces system cost through the replacement of OSA (Optical Spectrum Analyzer) by optical power meters.