Femtosecond pulse delivery using a chirped long-period grating of multi-mode fiber for mode conversion

A period-chirped long-period fiber grating (CLPFG), using ordinary multi-mode fiber (MMF), for broadband mode conversion (60 nm bandwidth of coupling efficiency >90%) has been proposed. Through using such a MMF-CLPFG, a broad-spectrum pulse can be converted into LP ₀₂ for femtosecond pulse distortion-free delivery in MMF. The simulation shows that the pulse suffers less nonlinear distortion than that in a single-mode fiber. In addition, the negative pre-chirp introduced by MMF-CLPFG can offset the linear and positive SPM-induced frequency chirp and thus compress the pulse. Using this scheme, an initial 18 kW peak power 75 fs pulse distortion-free delivery in MMF has been numerically demonstrated.