Reflective Beam Expanders for collimating infrared lasers (like CO₂ flying optics systems) are robust devices. They can be simple to align and adjust, and with careful optical design they can use inexpensive spherical mirrors and still give near diffraction limited results. There is often no need to use aspheric mirrors such as Off Axis Parabaloids for shallow tilt angles, large F numbers, and infra red wavelengths. Mirror collimator design needs to consider two factors
Firstly – Gaussian beam propagation parameters such as waist location, 1/e2 diameters, M2, Rayleigh Range. If you don’t understand these terms you should talk to us urgently!. I’ll confess we use a very old spreadsheet based Gaussian beam propagation analysis software that runs under MS DOS, but it works fine.
Secondly Geometrical or “traditional” optics parameters such Seidel aberrations. As the mirrors are used at a slight tilt, astigmatism is an issue, although of course chromatic problems aren’t an issue with mirror systems. We use Zemax software for this sort of modelling.
Our Gold coated mirrors have high reflectivity in the visible and infra red so a visible laser can be used to safely align the beam expander. We have designed and assembled beam expanders with magnifications from 1.2x to 10x, and apertures as large as 125mm. For customers who have their own mechanical capability, we can simply supply the optical design and optical components to you. Spherical concave and convex mirrors are also used as rear mirrors in resonators, and in adjustable beam path devices. We have a wide range of tooling for curved mirrors, from 10mm Radius Of Curvature (ROC) out to 100 metres.