# A FBG Intensity Modulation System Combined with an Optical Whispering Gallery Mode Edge Filter
## 3 Experimental Section  
Experimental Setup for Resonant Wavelength Measurement of the Whispering Gallery Mode (WGM) Edge Filter  
For the optical fiber utilized in this experiment, the coating layer was first removed. In order to moderate the bending-induced internal stress and allow for a smaller bent radius, the single-mode optical fiber was immersed in the buffered oxide etch (BOE) solution to partially etch the cladding layer. After that, the diameter of the etched fiber is approximately 75 \(\upmu\)m, and then the etched one was sandwiched in a plastic plate and fixed on a micro-translation stage in a U shape. The arrangement of the U-shaped bending optical fiber is shown in Figure 2. When the superluminescent diode (SLD) broadband light source transmitted the light into the etched optical fiber, an interference spectrum occurred due to the bending of the fiber. Moreover, we could not only control the distances of the micro-translation stage to adjust the bending curvature but could also tune the wavelength of the attenuation band of the WGM edge filter. Figure 3 illustrates the interference spectra of the WGM edge filter with a different bent radius from 4.52 to 4.66 mm. According to Equations (1)-(5), we can find that when the bent radius of the fiber decreases, the bend loss increases and we can obtain the result from Figure 3. However, while the bent radius of fiber still decreases, the bend loss will become slightly smaller. That is because some light would reenter due to coupling by the leaky core mode and cladding mode to the fundamental mode. Figure 4 shows the relationship of the bending-induced resonant wavelength and the distinct bent radius for WGM edge filter. From the results of the bending fiber experiment, we can obtain a filter in a fast method to replace the expensive electric filter to achieve an all-fiber measurement system. In the experiments of the static and dynamic mode, the peak of the resonant wavelength of the FBG sensor was 1553.30 nm. According to the results of performances of the WGM edge filter, the corresponding resonant wavelengths of the WGM edge filter were 1531.10, 1542.05, 1547.40 and 1551.40 nm when the bending radius of the optical fiber was set at 4.62, 4.60, 4.59 and 4.58 mm, respectively. Figure 5 shows the corresponding positions of the resonant wavelength between the FBG sensor and the WGM edge filter. The purpose of above-mentioned procedure is to ensure the intensity modulation system can acquire better signals for experiments. Figure 3: The interference spectra of the WGM edge filter with different bent radii for etched single-mode fiber. Experimental Setup for Static and Dynamic Mode Measurement of the Fiber Bragg Grating (FBG) Intensity Modulation System Based on the WGM Edge Filter  
The main purpose of the experiments was to test the feasibility of the intensity modulation system. We utilized a FBG sensor combined with a WGM edge filter as the modulation mechanism. The interference spectra of the FBG sensor and WGM edge filter were measured via the interrogation mechanism, and those were sequentially transformed into electric signals to analyze and compare with the signals of the strain gauge under different frequencies generated by the PZT. Before we began utilizing the intensity modulation system to measure the dynamic vibrating signals, we had to execute a static mode experiment to calibrate the voltage and signals of the modulation system to lower noise signals. Moreover, we could investigate the characteristics of the vibrating interrogation system through the results of the static experiment. Figure 6 shows the experimental setup of the FBG intensity modulation system based on the WGM edge filter. First, we applied a power supply to generate the external voltage, with the range of the voltage being from 0 to 5 V in increments of 0.5 V. Additionally, we used a photo detector (PD) to transform the optical signals into electric signals in order to compare them with the signals of the strain gauge.