The emitting facet of the single-mode laser diode is typically 1-micron high and 3-5 microns wide. Its output beam is highly divergent in a so-called "fast-axis" direction perpendicular to the emitter and less so along the facet or "slow-axis". The collimation of such an asymmetrically divergent beam with a combination of the fast- and slow-axis collimating lenses, or FAC and SAC lenses, yields a rectangular beam cross-section not convenient for all applications.

Alternatively, the Fast-Axis Reducer (FAR) lens collimates the fast-axis of a laser diode beam with its convex acylindrical surface. Then, its concave cylindrical surface re-introduces the fast axis beam divergence equal to that of the slow axis. The appropriate lens thickness takes care of beam astigmatism. Pairing the Fast Axis Reducer with a rotationally symmetric lens produces a collimated and round beam.