## 4 Experimental Setup  
The experimental sensor setup is presented in Figure 2 and consists of a broadband light source (superluminescent light-emitting diode, \(\lambda_{\text{peak}}\) = 1544.4 nm, FWHM = 45.5 nm, Thorlabs), two fiber couplers 1 \(\times\) 2 (split ratio--95:5), a 3-port circulator, a fiber coupler 1 \(\times\) 2 (split ratio--50:50), a polarization controller, polarization-maintaining photonic crystal fiber (PM-PCF), a dense wavelength division multiplexer (DWDM, 100 GHz, 8 channels, Fiberon) and two hand-held optical power meters (OPM, Detector: InGaAs, Resolution 0.01 dB, Grandway). The used PM-PCF is commercially available and its properties are herein presented: core diameter 6.3/4.4 \(\pm\) 0.5 \(\upmu\)m, outer cladding 125 \(\pm\) 5 \(\upmu\)m, coating diameter 240 \(\pm\) 10 \(\upmu\)m, length--40 cm, birefringence: \(\geq\)4 \(\times\) 10\({}^{-4}\), attenuation \(\leq\)3 dB/km at 1550 nm. The image of the cross-section of fiber is fully presented as an inset in Figure 2.  
The first fiber coupler 1 \(\times\) 2 (95:5) was used to eliminate fluctuations from the light source. The fluctuation of any light source output power could be observed in a function of time. In order  
Figure 1: The proposed sensor network based on DWDM (Dense Wavelength Division Multiplexer) PM-PCF (Polarization-Maintaining Photonic Crystal Fiber) Sagnac loops. Figure 2: The scheme for the intensity-modulated PM-PCF based Sagnac loop strain sensor. Inset: The image of PM-PCF cross-section. to prevent fluctuations, the fiber coupler is proposed to be implemented in the setup. Otherwise, the sensor output power value could be disturbed by light source power variation, thereby affecting strain determination accuracy. A circulator was placed within the sensor setup so as to provide the light propagation in the proper direction and to prevent any light reflections from affecting the superluminescent diode. In order to prepare the Sagnac interferometer, the splicing process of PM-PCF to SMF (single-mode fiber) was taken into consideration to enhance repeatability and minimize losses. Following the parameters presented by [PERSON] et al. [23], the PM-PCF was spliced to SMF using a commercial fusion splicer (FSU975, Ericsson). Additionally, appropriate adjustment of the polarization state within the Sagnac loop was made to provide adequate interferometer fringe visibility. A DWDM was incorporated to modulate the intensity of light by adjusting its spectrum with the edge slope of the PM-PCF Sagnac interferometer. Both spectrum of the DWDM channel and the Sagnac loop interferometer are shown in Figure 3.  
Spectral analysis from Figure 3 reveals that the interferometer fringe spacing is approximately 6.5 nm (near \(\lambda\) = 1550 nm) with a fringe visibility of \(\sim\)20 dB. The selected DWDM channel spectrum overlaps the slope of the interferometer spectrum, which ensures adequate operation of the proposed sensor. A shift in the interferometer spectrum provides different values of output optical power. The slope of the interferometer spectrum is also crucial for sensor setup capabilities as it is directly related to the fringe spacing and the measurement range of the sensor. Hence, the length of the PM-PCF needs to be controlled in order to meet specific requirements. In summary, Table 1 presents all components used within the experimental sensor setup with specified parameters.