Slightly Tapered Optical Fiber with Inner Air-cavity for Simultaneous Refractive Index and Temperature Sensing

A fiber in-line Mach-Zehnder interferometer based on inner air-cavities for simultaneous refractive index and temperature sensing is presented. The inner air-cavities are fabricated by use of femtosecond laser micromachining, fusion splicing and slightly tapering techniques. The transmission spectrum of the interferometer exhibits a number of resonance wavelength dips corresponding to different orders of cladding modes. By tracking the shift of two dip wavelengths, the changes of refractive index and temperature can be obtained by use of a matrix method. The refractive index and temperature sensitivity achieved are 103.00 nm/RIU (refractive index unit) and 73.07 pm/degrees C respectively.