| Femtosecond pulse has a large variety application in physics, biology, chemistry, optical communication and has unique advantages on the fields such as fast chemical dynamics, in-vivo investigation and supermicromachinery. Recent years, the femtosecond pulse delivery schemes usages of large mode area fiber, photonics crystal fiber have a lot of progress. In this paper, two kinds of femtosecond pulse delivery schemes usage of long period fiber grating (LPFG) including LPFG based on three-layered few-mode fiber and period-chirped LPFG based on multi-mode fiber (MMF-CLPFG) as mode convertor are proposed.The proposed three-layered few-mode fiber is based on a special multi-layer structure, the dispersion of LP02 in 800nm band is -499.06ps/km?nm and mode area is 124μm2. With the guidance of design technology of using turn around point parameter in phase matching curve as grating period, new-type broad LPFG (the conversion efficiency of LP01-LP02 is -42.56dB, the band of -10dB is 25nm) based on three-layered few-mode fiber is obtained. The numeric simulation of 100fs, 100kW peak power pulse propagation with LP02 in three-layered few-mode fiber is carried out. The result shows the nonlinear effects are decreased. SPM-induced broadening and Self-Steepening induced asymmetric spectrum distortion is restrained.We also theoretically investigate the mode characteristics of MMF and a scheme to control the transmission spectrum by optimizing the parameters of MMF-CLPFG, besides the dispersion property of MMF. After optimization chirp parameter, we design MMF-CLPFG with a broad transmission spectrum (60nm breadth of coupling efficiency>90%). Usage this kind of MMF-CLPFG, we convert and prechirp the 75fs, 18kW peak power pulse into LP02 and numerically simulate its propagation in MMF. The result shows the SPM-induced distortion of the pulse spectrum is little and the prechirps introduced by MMF-CLPFG make pulse compression and offset the nonlinear frequency chirp caused by nonlinearities. |
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