Highly Sensitive Fiber Michelson Interferometer Based on a Liquid-Filled Photonic Crystal Fiber with Dispersion Engineering

Photonic crystal fiber interferometers (PCFIs) with superiority of the unique fiber structure have been extensively developed as the all-fiber based strain () [1], external refractive index (RI) [2], chemical [3] and bending sensors [4]. The majority of the valuable applications on the fiber-optic sensing are all accredited to the air voids structured periodically in the photonic crystal fiber (PCF) which attracts researchers' much attention. Since only one material used in the commercial PCF is the silica. Therefore, filling other material with the optical property that differs to those of the silica is a good way to vary the characteristics of optical modes in the conventional PCF to achieve a more flexible design. Many studies about the filling liquids in the PCFIs for ultrasensitive sensing applications have been proposed [5-8], like fiber Mach-Zehnder interferometers [4-5], fiber Sagnac interferometers [6-7] and fiber Fabry-Perot interferometer [8]. In this study,we demonstrate an ultrasensitive, ultracompact, tip typed, in-line liquid-filled photonic crystal fiber Michelson interferometer (LF-PCFMI) based on material dispersion engineering. The sensor tip consists of a single mode fiber (SMF) splicing a section of photonic crystal fiber (PCF), as shown in Fig.1. In the splice the voids of the PCF are fully collapsed, a collapsed region with length about 300m is formed between SMF and PCF. Afterward we can fabricate arbitrary length of the LF-PCFMI sensor tip by cleaving the side of the PCF. Since the air holes are full broken in the collapsed section, there is noguiding core in the section of PCF.The fundamental mode that inputs from the SMF and propagates into the collapsed region can excite cladding modes into PCF. The core mode and cladding mode are reflected by the PCF endface and interfered. The light propagating in the cladding can be engineered by filling material into the air holes for enhancing sensitivity. Since as we know that the silica-based sensors exhibit low temperature tunability due to a very low thermal expansion coefficient: TEC (~5.5×10-7 C-1) and low thermo-optic coefficient: TOC (dn/dT~510-6 C-1). Here, n denotes the RI of material. Therefore, when the air holes in the cladding of PCF endface was full filled with Cargille ® optical liquid of refractive index (RI) n D =1.452 by duration of the capillary action. The TOC of the filled liquid with the dn/dT of 3.7410-4 C-1 is much larger than that of the silica. The material and waveguide dispersions of the PCF cladding are …