Focused, multiple-pass cell for Raman scattering

A simple optical system is described that makes use of a unique property of ellipsoidal mirrors, viz., light brought to one focus will be reflected alternately through the two foci and collapse to the major axis. This system consists of an on-axis ellipsoidal mirror facing a coaxial flat-spherical mirror assembly that is positioned at the minor axis. Calculations indicate that gains of the order of 500 in the light flux at the point of observation should be attainable with low-eccentricity ellipsoids. Raman-scattered light from atmospheric N(2) was obtained with a system employing a 0.2 eccentricity ellipsoid. An experimental gain of 93 was determined by the ratio of the scattering with the system to the scattering obtained with one beam. This result is in good agreement with the theory.