Frequency-doubled pico-second vortex fiber laser formed by a stressed Yb-doped fiber MOPA system

Summary form only given. High-power optical vortex lasers, featuring a doughnut-shaped spatial form and an orbital angular momentum (characterized by a topological charge, L) due to a spiral wavefront, have the potential to provide new technologies, such as the formation of chiral nano-structures [1]. To date, several researchers have demonstrated the production of high-power optical vortex outputs by a laser resonator with annular-shaped pumping and a side-pumped laser cavity with a bounce geometry. However, in most of the previous vortex laser work, the helicity (clockwise or counter-clockwise) of the spiral wavefront in the vortex output was randomly chosen by the cavity alignment. Recently, we have successfully demonstrated a high-power (>25W), pico-second vortex fiber laser formed by a stressed large-mode-area fiber amplifier in combination with a mode-locked Nd:YVO₄ master laser [2]. In this system, the wavefront helicity of the optical vortex output was selectively controlled merely by tuning the stress in the fiber amplifier.In the present work, we achieved a frequency-doubling of the pico-second vortex fiber laser. Also, the wavefront helicity of the output was controlled. As discussed in our previous publication [2], walk-off in a nonlinear crystal, such as KTP, frequently induces a spatial separation of vortices in the frequency-doubled vortex output. To avoid this phenomenon (spatial separation of vortices), a nonlinear crystal of LiB3O5 (LBO), permitting non-critical phase-matching, was used.