Off-axis cavity absorption cell

An absorption cell forming a coupling aperture and comprising opposed astigmatic mirrors that define an actual closed optical path from the coupling aperture and back to it, consisting of an odd number N of cycles through the absorption cell, in which the locations of the spots at which the actual path hits one of the mirrors, as a function of those spots' order of occurrence, exhibits motion consisting essentially of only two frequency components k.sub.x /N and k.sub.y /N associated with high-frequency and low-frequency axes, respectively, along which the contribution of the other frequency component is essentially zero, where k.sub.x and k.sub.y are different integers and their common factors, other than unity, with N are distinct, each of the mirrors having principal radii r.sub.x ' and r.sub.y ' related to the principal radii r.sub.x and r.sub.y >r.sub.x of a prototype cell by the relationships r.sub.x '<r.sub.x /(1+.DELTA.e) and r.sub.y '>r.sub.y /(1-.DELTA.e), where .DELTA.e=2.times.10.sup.-4, the axes of the prototype cell's mirrors are oriented with their principal axes aligned and define a prototype closed optical path consisting of the same odd number N of cycles through the prototype cell, and the locations at which the prototype path strikes a mirror make k.sub.x cycles in the direction of one principal axis and k.sub.y cycles in the direction of the other principal axis.