Thermal lens effects on the intracavity generation and frequency conversion of Bessel–Gauss beams using a diffractive axicon in a diode-side-pumped continuous-wave Nd:YAG laser

This study presents a novel approach, to our knowledge, for the intracavity generation of quasi-non-diffracting Bessel-Gauss (BG) beams using a diffractive axicon in a diode-side-pumped Nd:YAG laser. The beam's intensity profile was characterized under various conditions using 2D and 3D profilers, revealing a stable central lobe in the near field (40 mm) that transitions to multiple lobes in the far field (250 mm). The thermal lensing effect in the Nd:YAG crystal was investigated, demonstrating that increased pump current (11-14 A) enhances thermal lensing, reducing the non-diffracting range of the BG beam, which we call the BG beam length, from 12.5 to 9.5 cm. Second-harmonic generation at 532 nm was achieved using a KTP crystal, despite far-field distortions due to the green problem and crystal size limitations. Cavity stability analysis for plano-plano and plano-concave configurations showed stable operation across thermal lens focal lengths (490-1130 mm). Comparison of intracavity and extracavity beam generation revealed higher output power for the extracavity method at low currents, with the intracavity method outperforming at higher currents. The plano-concave cavity produced multiple concentric rings, unlike the single-ring profile of the plano-plano cavity, due to a larger beam diameter at the axicon, enabling precise beam shaping.