The diode laser systems emit light from their emitters which is not collimated in their vertical and horizontal axis. In com- parison, the divergence in the y-axis is significantly higher than the divergence in the x-axis.

Due to this observation, the y-axis is called fast-axis while the x-axis is called slow- axis. High power diode manufacturer provide specifications for the divergence distribution of the lasers.

In common laser system, only a convex lens is used to zoom the laser beam, resulting in the rectangular laser spot, like Atomstack A5 and many other laser engravers from other brands, however in high-end laser engravers, such as Atomstack A5 pro, A5 M30 and A5 M40, besides convex lens, an additional lens called Fast-Axis-Collimating Lens is used.

FAC lens is an optical lens that collimates light spreading from a semiconductor laser (LD) in the fast-axis direction. LD have a large divergence angle in the fast-axis direction, so the output light cannot be efficiently used unless collimated. The FAC lens collimates light spreading from a LD into a narrow beam with a radiation angle of several milliradians (mrad) or less so that the diverging light can be efficiently utilized.

In this case, instead of a rectangular spot, laser beam pass through FAC lens and Convex Lens is zoomed in a more squared spot. We call it compressed-spot laser. Adding a FAC lens to our laser system won’t noticeably reduce the laser power, and since laser beam is more concentrated, the power density is much higher than those laser engravers don’t have FAC lens.