1. Field of the Invention
The present invention relates generally to optical light beam expander apparatus and more particularly to a multiple-prism light beam expander system having particular applicability for use in dye lasers to narrow the bandwidth of the output laser beam.
2. Description of the Prior Art
A simple dye laser includes an optically pumped dye cell with a diffraction grating disposed on one side of the cell and an output mirror disposed on the other side. The diffraction grating is used as the wavelength selection device and permits the laser wavelength to be tuned within the fluorescence band of the particular dye used. While the output power of this basic type of dye laser is excellent, the linewidth is relatively poor because the cross sectional area of the beam cast onto the diffraction grating is small. To improve (i.e., reduce) the linewidth, some type of beam expanding device is usually inserted between the dye cell and the grating to expand the laser beam on the grating.
A telescope expander developed by Hansch was the first beam expander breakthrough and has been used as the classical design since it was disclosed in Applied Optics, Volume 11, pp. 895-898, in April of 1972. Such apparatus is, however, subject to the disadvantages that it is expensive, it has relatively poor beam quality due to imperfect, small radius-of-curvature spherical surfaces, it adds significantly to the cavity length, it does not permit variation of the beam expansion, it expands the beam in two dimensions so that the intracavity etalon must be large and of extremely high quality, and the grating rotation mechanism must be extremely precise so that the grating grooves remain orthogonal to the laser axis as the grating rotates.
Two single-prism designs disclosed by Stokes et al in Optics Communications, Volume 5, No. 4, Page 267, July of 1972, and Hanna et al in Optical and Quantum Electronics 7 (1975), 115-119, have also been used with varying degrees of success to overcome the deficiencies of the telescope expander. However, neither has been satisfactory in that overcoming some of the deficiencies of the telescope has invariably introduced others. For example, these types of expanders are not achromatic, they must sacrifice power or tuning range in order to increase expansion (to reduce laser linewidth), they do not produce an output beam that is completely polarized, and they are not colinear with the optical cavity and thereby make alignment of the system difficult.
Another prior art dye laser structure using a cylindrical lens beam expanding system is disclosed in the U.S. Pat. to Itzkan et al, No. 3,868,590.