Discussion

Astigmatism in the Reflected Beam and Dichroic Thickness

The laser beam becomes astigmatic after it reflects off of the dichroic. This means that the beam's focal plane in the sample space for one transverse direction is different from its perpendicular direction.

This is a common problem for applicatons requiring laser beam shaping. The solution is to buy thicker dichroics, such as 2 mm or even 3 mm dichroics.

Of course, this comes with tradeoffs. For one, thicker dichroics are more expensive. Additionally, many dichroic mounts are not designed to hold dichroics thicker than 1 mm.

The Thorlabs FFM1 filter holder, mounted inside a cage cube, can hold dichroics up to 3 mm thick and is compatible with kinematic base plates which facilitate alignment.