All-fiber mode-locked cylindrical vector beam laser using broadband long period grating

Abstract

We propose and demonstrate an all-fiber passively mode-locked laser based on the mechanism of nonlinear polarization rotation and two-mode fiber long period grating (TMF-LPG) to generate ultrafast cylindrical vector beam (CVB) output. The central wavelength of the laser is 1567.2 nm, and 3 dB bandwidth is measured to be 5.64 nm. The pulse duration of the mode-locked laser is 970 fs and the repetition rate is 20.47 MHz. An eight-period TMF-LPG written in a two-mode fiber by a CO2 laser serves as a broadband transverse mode converter to achieve ultrafast CVBs generation with broader spectrum. The 15.0 dB bandwidth (corresponding to conversion efficiency greater than 96%) of the LPG is 75 nm, which perfectly covers the wide mode-locked spectrum. High purity radially and azimuthally polarized beams are obtained through adjusting the polarization controllers, and the mode purity is calculated to be 94.35%. The slope efficiency of the CVBs’ output is about 5.84%.