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.
|