Rotating vs. Sliding Beam Expander Divergence Adjustment

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 divergence adjustment 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 divergence adjustment: rotating and sliding.

As their name implies, rotating divergence adjustment, such as threaded focusing tubes, rotate the optical elements of the beam expander during translation. While they are typically less expensive than sliding divergence adjustment 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 divergence adjustment

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

Poorly designed sliding beam expander divergence adjustment 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 divergence adjustment. Laser beam expanders with sliding divergence adjustment also tend to be heavier than rotating divergence adjustment and have a larger outer diameter due to their more complex mechanics, making rotating divergence adjustment beneficial for size and weight sensitive systems.