Research On Femtosecond Laser Fabrication And Sensing Characteristics Of Long Period Fiber Gratings

In the information field of optical communication and sensing, long period fiber gratings have developed into an irreplaceable new optoelectronic device, for its unique structural features, a variety of functions and excellent performance. Based on the ultra-sophisticated requirements of its structure and performace, the optical analysis and manufacturing technology has always been the research hot point and difficult point. The new femtosecond laser micromachining technology with an ultra-strong power and an ultra-short pulse width can meet the manufacturing requirements of cross-scale, high-precision, high-performance and diversity. In this paper, we use fibers as a vector, use fiber gratings as a research object and combine femtosecond laser processing technique with a large number of experiments to do the research of long period fiber gratings’femtosecond laser processing method and performance characteristics. The main contents of this paper are showed as followed:1、The optical simulation model is established based on the classical coupled-mode theory to analyze the mode coupling process of long period fiber gratings and simulate the transmission spectrum with specific parameters. The change trend of the transmission spectrum along with the modulation depth and the length of gratings is simulated theoretically.2、According to study of the interaction mechanism between femtosecond laser pulse and transparent dielectric material, two types experiments of fiber core-modified gratings processed by objective focusing laser pulses and fiber cladding-removal gratings processed by cylindrical lens focusing laser pulses are carried out. And the differences of the transmission spectrum, the structure and the mechanical property between two types gratings are studied in detail.3、A series of experiments about the grating period, grating length and modulation depth’s impact on the transmission spectrum during the processing period are conducted and the results proved to be identical with the theory simulation. According to the results:The resonant wavelength moves to the longer wavelength while the resonant peak remain unchanged when the period increases. The resonant peak increases with the grating length or the modulation depth while the resonant wavelength almost keeps no drift in the state of under coupling, while the situation is opposite in the state of over coupling.4、The theoretical analysis of the long period fiber gratings’ temperature and refractive index sensing properties are conducted and the experiments to study the properties of two types grating are carried out. The experimental results show that the fiber core modified grating has a low temperature sensitivity and a high refractive index sensitivity while the fiber cladding-removal grating has a very high temperature sensitivity and a very low refractive index sensitivity, pictures41, tables5, reference papers60...