Approximate analytic astigmatism of unit-magnification multipass system

Biconic White multipass cell design based on a skew ray-tracing model

A biconic White multipass cell (bi-WMPC), designed as a compact, zero-geometrical-loss, anastigmatic optical system, capably used with an extended divergent source, is proposed. First, a skew ray-tracing model is developed for a conventional White-type multipass cell (WMPC), based on which the astigmatism is calculated, together with its sensitivity coefficients in relation to all-important optical structural parameters in various configurations. Next, a generalized bi-WMPC is designed to suppress the astigmatism, which leads to much smaller reflection spots on the field mirror compared to the conventional design. The demonstration of an optimized bi-WMPC initialized from a commercial WMPC (52 pass with a 0.8 m base length) suggests a 53 fold reduction of the wavefront error value from 79.187λ to 1.493λ, as well as the path-to-volume ratio (PVR) increase from 20.8 to 35.2 m/L. These improvements are critical for the design of a compact WMPC with a path length of tens to hundreds of meters. Presently developed skew ray-tracing models for a WMPC can also be applied to other freeform surfaces to further compensate the inherent aberrations induced by multiple off-axis reflections.