Fig. 1. (a) Geometrical evolution from a Gaussian beam to a donut-shaped beam. (b),(d) The top view and side view of a beam focused by a cylindrical lens. (c),(e) The top view and side view of a donut-shaped beam formed with an axicon.

Fig. 2. CLSM images of fs laser fabrication by (a) an axicon-generated donut-shaped beam and (b) a cylindrical directly focused beam. (c),(d) The corresponding cross-sectional profiles.

Fig. 3. Donut-shaped beam polished surfaces with different scanning speeds. (a) 0.1  mm/s, (b) 0.2  mm/s, (c) 0.4  mm/s, (d) 0.6  mm/s, (e) 0.8  mm/s, (f) 1.0  mm/s, (g) 1.2  mm/s, and (h) 1.4  mm/s.

Fig. 4. Influence of (a) the scanning speed and the laser pulse energy, and (b) the scanning times on the roughness of processed surfaces by the donut-shaped beam.

Fig. 5. Relationship between the scanning width and the relative distance from the lens to the objective.