Mode-locked fiber lasers with high-purity high-order mode generation using a grating combination

We propose an all-fiber passively mode-locked laser for high-purity high-order mode generation by combining a two-mode long period fiber grating (TM-LPFG) and a two-mode fiber Bragg grating (TM-FBG). The grating combination fully plays respective advantages: highly efficient mode conversion of the LPFG, mode selection and spectrum filtering of the TM-FBG. A semiconductor saturable absorber (SESAM) is used to achieve passively mode-locking. As a result, the mode-locked cylindrical vector beam (CVB) pulse with a 3-dB spectral width of 0.08 nm at a repetition rate of 12.88 MHz has been obtained, and the signal to noise ratio (SNR) is as high as 68 dB. Moreover, the azimuthally and radially polarized CVBs obtained after degeneracy elimination have high purity of 95.6% and 98.3%, respectively. This laser may have the potential for generation of high-purity CVB pulse.