Abstract
Generation of a donut-shaped first-order Laguerre–Gaussian (LG01) 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 (HG01) 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:YVO4 laser to generate a (LG01) 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 LG01 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.
Highlights
T HERE has been considerable interest in vortex modes as their unique properties give them wide ranging applications
We describe a method for generating high purity LG01 vortex modes that employs a new design of astigmatic mode converter (AMC) based on spherical mirrors aligned at oblique incidence in combination with a twin-spot end-pumping scheme to generate a high quality H G01 mode
In summary, we have demonstrated an improved method for generating high purity LG01 vortex modes by utilizing a novel twin-spot end pumping scheme to directly generate a high quality H G01 mode from a solid-state laser followed by a novel AMC based on spherical mirrors
Summary
T HERE has been considerable interest in vortex modes as their unique properties give them wide ranging applications. We describe a method for generating high purity LG01 vortex modes that employs a new design of AMC based on spherical mirrors aligned at oblique incidence in combination with a twin-spot end-pumping scheme to generate a high quality H G01 mode. The spherical mirror mode converter comprises two very high reflectivity concave mirrors of equal radius of curvature separated by a distance (2d) and aligned so that the beam strikes both at the same angle of incidence (see Fig. 2). Expressions (21) and (22) yield the required values of the incident beam parameters for the spherical mirror mode converter to yield a pure LG01 mode These can in turn be used to calculate the required beam waist size and its distance from the AMC. It is worth mentioning that higher-order LG0,ł OAM modes could be generated with spherical mirror AMC providing that the appropriate H Gm,n modes can be generated with high purity for the input beam
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