Method for Generating High Purity Laguerre–Gaussian Vortex Modes

Generation of a donut-shaped first-order Laguerre-Gaussian (LG
₀₁
) vortex mode via a method designed to yield high mode purity is reported. Our approach utilizes a novel twin-spot end-pumping scheme to directly excite the first order Hermite-Gaussian (HG
₀₁
) mode in a solid-state laser, followed by a novel astigmatic mode converter based on spherical (concave) mirrors aligned at oblique incidence. A simple theoretical model for the mode converter is derived and from this the design approach is explained along with the potential benefits compared with conventional schemes-based cylindrical-lens astigmatic mode converters, particularly for power scaling. As a proof-of-principle and to confirm the benefits of this scheme in terms of high mode purity we have applied it to an end-pumped Nd:YVO
₄
laser to generate a (LG
₀₁
) beam with a controllable sense of azimuthal phase and hence orbital angular momentum. A method for characterizing the resulting beam-based on analysis of the spiral interference pattern derived with the aid of a Mach-Zehnder interferometer is described and yields a value for the LG
₀₁
mode purity of 94%. Common sources of mode impurity are identified and the prospects for scaling to higher power whilst maintaining high mode purity are considered.