Rotating vs. Sliding Beam Expander Focusing Mechanisms

Laser beam expanders increase the diameter of a collimated input beam to a larger collimated output beam and are critical components of a wide range of laser systems. Beam expanders typically have a mechanical focusing mechanism in order to adjust for different input divergences or wavelengths. Variable beam expanders allow for enough mechanical adjustment to change their magnification, or expansion power. There are two main types of beam expander focusing mechanisms: rotating and sliding.

As their name implies, rotating focusing mechanisms, such as threaded focusing tubes, rotate the optical elements of the beam expander during translation. While they are typically less expensive than sliding focusing mechanisms due to their simplified mechanics, they can suffer from beam wander, also known as pointing error (Figure 1).

Figure 1: An exaggerated depiction of beam wander that may occur due to a rotating focusing mechanism

Sliding focusing mechanisms, such as helicoid barrels, translate the internal optical elements without rotation, which minimizes beam wander. The mechanics required for sliding focusing mechanisms, however, are more complex than those in rotating focusing mechanisms, which generally increases system cost. For this reason, sliding focusing mechanisms typically outperform rotating focusing mechanisms for a cost premium.

Poorly designed sliding beam expander focusing mechanisms can potentially have too much freedom of movement, introducing a pointing error that does not rotate during adjustment. This pointing error may even be greater than that of rotating focusing mechanisms. Laser beam expanders with sliding focusing mechanisms also tend to be heavier than rotating focusing mechanisms and have a larger outer diameter due to their more complex mechanics, making rotating focusing mechanisms beneficial for size and weight sensitive systems.