Fabrication Of Micro Cavity In Single-multimode Fiber Cascaded Structure By Femtosecond Laser And Its Sensing Properties

The fiber micro-cavity with double openings have great application in refractive index and temperature sensing owing to their excellent properties such as simple fabrication and easy to fill. The Mach Zehnder interferometers(MZI)based on the micro cavity in a single-multi mode fiber cascaded structure can show high quality interference spectrum. It is with the advantages of high sensitivity,good linearity, and compact structure. In addition, when the length of multi mode optical fiber reaches a certain value, the mode interference can be formed in the single-multi mode fiber cascaded structure. Its refractive index sensitivity can be improved effectively by fabricating micro cavity in the structure through using femtosecond laser-induced water breakdown.we have proposed and fabricated an MZI sensing structure based on single-multimode fiber cascaded structure, which could be used for measurement of refractive index(RI) and temperature. Firstly, we calculated and simulated the transmission spectrum of the micro-cavity in the single-multi mode cascaded structure based on the theory of Mach Zehnder interferometer. The influence of the micro cavity size and relative position on the transmission spectrum is studied, and the optimal parameters of the micro cavity with high quality interference spectrum are determined. Besides, by using the coupled wave theory, we studied the light transmission characteristics of the single-multimode optical fiber cascaded structure. The influence of the length of the cascaded fiber on the transmission spectrum was analyzed by using Beam PROP. Secondly, based on the optimal parameters, the micro cavity in single-multimode optical fiber cascaded structure was fabricated by using the femtosecond laser-induced water breakdown, which formed a high quality MZ interferometer. The spectral transmission loss is 10 d B,and the contrast is 35 d B.The refractive index and temperature sensing experiment is carried out. The results show that the refractive index sensitivity is–10053.45nm/RIU in the range of 1.333-1.3367 and the temperature sensitivity is43.9pm/oC in the air. We sealed the polymer material into the micro-cavity, and obtained an ultrasensitive temperature sensor with the maximum sensitivity of 5.6nm/?C.we have proposed and fabricated an single-multi mode fiber cascaded structure which forms mode interference, and improved its refractive index sensitivity by fabricated single micro cavity and double micro cavities in the structure. Firstly, the transmission spectra of single-multimode optical fiber cascaded structures with single micro cavity and double micro cavities in themwere simulated, and their refractive index sensing properties were investigated theoretically.Secondly, the single-multi mode fiber cascaded structure which forms mode interference was prepared by using fiber cutting and fusion method, and the refractive index sensing experiment is carried out. The refractive index sensitivity is 51.88nm/RIU. After that we fabricated the single micro cavity, symmetrical double micro cavities in the structure respectively by using the femtosecond laser-induced water breakdown. The spectra do not show obviously changes, and only the center wavelength shifts to short wavelength direction with contrast increasing. Their refractive index sensitivities are increased to 685.68nm/RIU and2727.09nm/RIU respectively.