Via precise selleck chemicals refractive index measurement, other properties such as liquid concentration, the gas density and ambient temperature can be determined. Therefore, the precise measurement of refractive index is very important. Fiber-optic Fabry-Perot sensors have been developed as extremely high-precision sensing devices for precise measurement of RI [1,2], temperature [3�C6], strain [6,7], pressure [8], and displacement [9], etc. Fiber-optic sensors have the advantages of high sensitivity, small size, resistance to electromagnetic interference, fast response, potential for multiplexing and high tolerance to harsh environments, etc. Various RI sensors have been developed based on different fiber-optic schemes, such as photonic crystal fiber (PCF) [10], fiber tapers [11�C13], multimode interferometric sensors [13,14], fiber Bragg grating (FBG) sensors [15�C18], long period fiber gratings (LPFGs) [19,20], and fiber-optic Fabry-Perot sensors [1,2,21].
Among them, fiber-optic interferometric RI sensors have been widely used owing to their properties of high precision and ease of fabrication.Multiplexing can achieve multi-point simultaneous detection, large area detection, and it can further reduce system costs. The extrinsic fiber Fabry-Perot interferometric sensors (EFPIs) have many advantages, but it is difficult to multiplex multiple sensors and this leads to a high average cost of single sensors [22]. The spatial frequency division multiplexing method [23] can increase the number of sensors used in one fiber-optic sensing system, thus reducing GSK-3 the cost.
Lee and co-workers have reported a temperature and refractive index Fabry-Perot interferometric sensor [1], and used it for hydrogen gas measurement CHIR99021 GSK-3 inhibitor [24]. In this paper, the multiplexing capability of fiber-optic interferometric sensors based on the graded-index multimode fibers (GI-MMFs) is estimated. The refractive index and temperature responses of such sensors are tested under multiplexing condition. In addition, the principle of such sensors is further investigated by analyzing the spot distribution on the fiber end via a near-field optical profiler.2.?Principle of Fiber-Optic Interferometric Sensor Based on GI-MMFThe sensors investigated by this paper are based on the periodic focusing effect of GI-MMFs and the three-beam interferometer [25,26]. The GI-MMF has a core diameter of 62.5 ��m with high Ge doping concentration, and the numerical aperture was 0.275. Firstly, the GI-MMF was chemically etched by 40% hydrofluoric acid solution. This process is quite simple and cost-effective. The etching time was about one and a half minutes to obtain a microscale hole on the fiber tip. The etched fiber tip was fusion spliced to a single-mode fiber to form an air cavity.