Design of new multipass gas cells with circular and elliptical cross-section

Two new types of multipass gas cells with circular and elliptical cross-sections were developed. As for a cylindrical cell, the reflected rays in the circle would be an irregular trace without superposition and termination, if its central angle is a bit more than 2 pi/3 for an example. Each two adjacent reflected points have the same optical path difference and optical light passes through different optical distances before different reflection points. As for a cylindroidal cell, if the optical ray passes through one focus point of the ellipse, it must also pass through the other. The light will exit finally from the termination point of an axis. The optical distance depends on the eccentricity of the ellipse and the size of the emergent window. In this article, the detailed mathematical derivations of the optical multireflection traces for the two structures are presented. The unified formula is developed. It is found that these designs have small volumes and adjustable long optical paths. For example, the optical distances in the two structures are about 625 mm for a circle with 50 mm radius. The performance of the system can be enhanced by coating high total reflection film, increasing the processing and positioning precisions, and using highly collimating laser beam. (C) 2014 Wiley Periodicals, Inc.