Littrow Prism Beam Expansion for Laser Cooling

Melissa E. Friedman, Oleg Kritsun, Harold J. Metcalf
Department of Physics and Astronomy, Stony Brook University

A collimated atomic beam improves the focusing of neutral Rydberg helium atoms by an inhomogeneous magnetic field. The atoms are emitted from a diffuse source one meter away and have a range of transverse velocities. Transverse laser cooling decreases these velocities, thereby collimating the beam. Expansion of the light beam along the atomic beam axis increases the interaction time with the atoms, which is otherwise limited by their high longitudinal velocities. Laser cooling experiments often use telescopes with cylindrical lenses for beam expansion. We can replace these telescopes with Littrow prisms (30-60-90 right triangles). A prism beam expander preserves optical table space, provides flexibility in magnification, and requires less alignment precision than one made from a telescope. Prisms bend light based on their index of refraction and the light wavelength, causing beam expansion at the correct angles of incidence. Monochromatic, linearly polarized coherent light strikes our prism beam expander surfaces close to Brewster's angle, minimizing power loss from reflection. Our experiment includes a 389 nm laser beam and four prisms made from BK-7 glass. The total magnification, overall light transmission, and displacement of the beam depend on the prisms' relative angles. This project was funded by ONR grant 23792 and NSF grant 22179.

Melissa Friedman
March 2004
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