Mode-Locked Tm³⁺-Doped Fiber Laser With CV Mode Output Using a Cascading Chirped LPFG

Abstract

A passively mode-locked Tm3+-doped fiber laser based on the mechanism of nonlinear polarization rotation and cascading chirped long period fiber grating (LPFG) to generate ultrafast cylindrical vector (CV) mode output is proposed and demonstrated. The fiber laser output traditional soliton pulses operating at 1878.8 nm with a 3 dB bandwidth of 3.94 nm. The pulse duration of the laser is ~1.05 ps with a repetition rate of 18.766 MHz. A cascading chirped LPFG, fabricated in a few-mode fiber by a CO2 laser, serves as a broadband transverse mode converter to achieve ultrafast CV mode generation. The LPFG has a 10 dB bandwidth (corresponding to conversion efficiency of 90%) of 20 nm from 1877 nm to 1897 nm, which perfectly covers the wide mode-locked spectrum. High purity radially and azimuthally polarized modes are obtained through adjusting the polarization controllers, and the mode purity is calculated to be about 95.1% and 94.9% for TE01 and TM01. To our best knowledge, this is the first mode-locked fiber laser emitting CV modes in $2~\mu \text{m}$ waveband using a broadband LPFG as a mode converter.