Picosecond pulsed flat-top beam in a mode-locking all-fiber laser

Ow7ing to the flat center and steep edge, the flat-top beam is widely used in the fields of micromachining and optical image processing. Here, we propose an efficient scheme to generate a picosecond pulsed flat-top beam in a mode-locking all-fiber laser. After utilizing an orthogonal polarization method for complete incoherence and a high-precision all-fiber optical delay line (ODL) for rigorous time synchronization, the pulsed fundamental mode (LP₀₁) and the pulsed vortex beam (VB) are superimposed to generate a pulsed flat-top beam. The pulsed flat-top beam has a duration of 6.10 ps, with a normalized root mean square (NRMS) variation of 0.049 and a steep degree value of 0.876, indicating an excellent beam quality. In addition, the effect of coherence between the two superimposed beams on the quality of the combined beam has also been investigated. This is the first, to the best of our knowledge, demonstration of a picosecond scale pulsed flat-top beam in the mode-locking all-fiber laser, which may greatly promote its application in laser processing and biomedicine.