Single-frequency Nd:YAG laser with LG01 donut mode output

Laguerre-Gaussian (LG0n) laser beams with a donut-shaped profile are attracting growing interest due to a wealth of applications in areas such as optical manipulation of particles, trapping and guiding of atoms, laser drilling and writing and super-resolution microscopy. Here we report a diode-end-pumped single-frequency Nd:YAG laser using a novel fiber-based pump beam conditioning element to achieve lasing on the lowest order donut (LG01) mode. The pump conditioning element was fabricated from a capillary (hollow-core) silica fiber with a 200 µm outer diameter and a 105 µm diameter inner air hole. The fiber was tapered and spliced to a fiber-coupled laser diode with a maximum output power of 4 W at 808 nm. The capillary fiber yielded ~3.5 W of output in a beam with a ring-shaped near-field profile. The latter was imaged into a Nd:YAG rod in a two-mirror plano-concave resonator to selectively excite the LG01 mode. The laser produced up to 1.71 W of multi-axial-mode output at 1064 nm for 3.5 W of incident pump power. The corresponding slope efficiency was 57%. Single-axial-mode operation was achieved by inserting quarter-wave plates at both ends of the Nd:YAG rod to suppress spatial hole- burning together with an intracavity etalon. Using this set-up, we obtained 810 mW of single-frequency LG01 mode output with a well-defined sense of spiral phase for ~3.1 W of incident pump power. The beam propagation factor (M2) was measured to be ~2.09 and hence in close agreement with the theory. The prospects for further improvement in performance will be discussed