Diffraction-limited ring beam generated by radial grating

Abstract

Laser beams with ring-shaped intensity distributions have attracted much attention for a wide variety of applications in science and technology. In these applications, commonly used techniques for generating ring-shaped beams involve use of optical elements such as spiral phase plates or axicons. However, the ring-shaped beams formed by these methods have non-diffraction-limited resolution because the practical numerical aperture of a ring-shaped beam is smaller than that of a Gaussian beam. This is due to a reduction of the effective numerical aperture of the propagating beam after passing through the element. To overcome the issue, we propose an optical element composed of a radial grating to obtain a diffraction-limited ring-shaped beam. We found that a key point for optimizing the diffraction-limited ring-shaped beam is fine-tuning of the phase distribution in the ring-shaped beam. In an experiment for femtosecond laser processing with a ring-shaped beam, we demonstrated that the debris on a morphological structure fabricated by single-shot irradiation was considerably reduced because the subsequent pulse did not destroy the melt-solidification structure. Furthermore, we demonstrated the fabrication of an annular structure by single-shot ring-shaped beam irradiation in water without any inhibitory influence due to laser-induced bubble generation.