# A FBG Intensity Modulation System Combined with an Optical Whispering Gallery Mode Edge Filter
## 1 Introduction  
The conventional electrical strain gauge still has some inevitable defects, although it has been developed for decades. The flaws, for instance include the noises induced by the unstable input voltage, electromagnetic interference, impairment by vapor corrosion, and the limited room of the wire distribution, among others. Consequently, we employ optical fiber sensors to realize a strain sensing system to reduce these weaknesses. Optical fibers have some specific properties that include being light in weight, small in size, high in sensitivity, immune to electromagnetic interference, and resistant to corrosion, among others. Therefore, optical fibers are not only used extensively in the field of communications but are also increasingly being used in the fields of mechanics, aviation engineering, and biotechnology. Applications of optical fibers in theoretical research, the production of optical fiber elements and optical fiber sensors, and the use of optical fibers in mechanisms of modulation and demodulation have thus been widely studied [1; 2]. In this study, we utilized the operating principle of the whispering gallery mode (WGM) to fabricate the WGM edge filter and combined that with a fiber Bragg grating (FBG) sensor to build up an intensity modulation system. The whispering gallery phenomenon refers to the observations that sonorous vibrations have a tendency to cling to a concave surface [3]. The micropotent optical fiber-induced WGM is sensitive to the outside changes and can be adopted as the optic fiber sensor [4; 5; 6; 7; 8] and the edge filter [9; 10; 11]. There are a variety of applications for WGM in bent optical fiber. [PERSON] _et al._ first determined that a smaller diameter and a larger bending angle would induce an increasingly apparent interference spectrum [4]. [PERSON] _et al._[5] also showed that bent optical fibers would radiatelight to the cladding and cause internal total reflection at the cladding and air boundary which would result, in turn, in the formation of WGM. In this way, a temperature sensor element was realized, and [PERSON] _et al._ reported that the greater the bending angle was, the greater the amplitude that occurred [5]. [PERSON] _et al._ utilized the WGM to produce a temperature sensor and observed that a wavelength drift of the two main peaks in the optical spectra was caused by increasing the temperatures [6]. Subsequently, [PERSON] _et al._ further demonstrated the relationship between the single-mode fiber bending loss and temperatures. They discovered that if the bending angle was increased, the transmission loss of the optical fiber became more severe [7]. More recently, [PERSON] _et al._ demonstrated that two photodiodes could be placed at both ends of a slightly bent optical fiber to measure the output power and to use the translation platform displacement to adjust the bending losses, which formed a simple power measurement system [8]. A number of important studies on optical fiber edge filters have also been conducted. First, [PERSON] _et al._ used bent optical fiber as an edge filter and constructed a system for the measurement of wavelengths. The experimental results showed that the effective measurement range for the wavelengths was from 1500 to 1550 nm [9]. Then, [PERSON] _et al._ built a proportional wavelength measurement system using the same technique [10]. In 2008, [PERSON] _et al._ reported that they used a 3 dB coupler to build a proportional wavelength measurement system [11]. Although the applications of WGM seem to have already been largely developed, it is still not easy to control the factors that affect the performance of the bent optical fibers. Hence, there has been no research thus far adopting and discussing the effect of using the WGM to model an intensity modulation system. In this paper, we utilized bending single-mode fibers to induce the WGM effect and combined that with a FBG sensor to construct an intensity modulation system.