Dense pattern optical multipass cell,
U. S. Patent 7,477,377 issued January 13, 2009
Spectroscopic measurements of gas concentration are more senstitive when the light makes a longer path through the gas. Multiple pass optics have been developed to allow a long path to be folded into a small volume. This invention is a multiple pass optical cell comprising a pair of opposed cylindrical mirrors. A hole in one mirror lets light into and out of the cell. By adjusting both the twist angle and the mirror spacing, various pathlengths can be produced. At a twist angle of 90 degrees (far right column) the system acts somewhat like a Herriott cell. The picture at right summarizes the many possible solutions. Larger, bluer spots correspond to solutions with a small number of passes, while small, red spots correspond to large number of passes. The ideal path length depends on a number of factors, including mirror reflectivity and desired measurement range.
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