The generation of a stable collimated ring-shaped beam is compulsory for the following application. The principle setup involves monolithic beam expansion to optimize the illumination of the beam shaper and reduction to reduce the in-coming beam for the axicon. The width of the generated ring beam equals 50% of the incoming beam diameter. In order to study the focusing quality a plano-convex spherical lens is placed as the last optical element of the setup.
By reducing the incoming beam the focusing of a spheric lens can be improved since the effect of spherical aberration will be reduced. The resulting spot diagram for this spherical lens and an incoming ray bundle of 22 mm, without the axicon in the setup, is shown first in Fig. 3 (a), the spot diameter equals 400 µm. The three following figures show spot diagrams of the same lens with different ring-shaped incoming beams with diameter 4/2/1 mm, resulting in a ring width of 2/1/0,5 mm. The last spot diagram Fig. 3 (d) shows a spot size of 40 µm and meets the criteria of diffraction limited focusing. This phenomenon can be explained by zonal decomposition of the spherical surface. Each zone has its own focal plane which causes spherical aberration.
By choosing just one of these zones with the ring-shaped incoming beam the spherical aberration can be avoided.
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