Spherical mirrors for collimation

Collimator Mirror

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.

Zemax trace of tilted spherical mirrors

Spherical mirrors used in laser beam expander for collimation

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.

3 thoughts on “Spherical mirrors for collimation

  1. i have a 150mm beam expander i would like to expand its output as much as affordably possible by a factor of 2 to 5x more using mirrors for hobby use, any ideas of how to do so?

    • In our blog example above

      1) The magnification is set by the ratio of the focal length of the concave mirror to the focal length of the convex mirror. Since these are spherical mirrors we prefer to say focal length = 0.5 X Radius Of Curvature
      2) any focus (or defocus) of the beam is set by the distance between the mirrors.

      These two factors are simultaneous, so your options are

      A) Replace both mirrors with new ones. The focal lengths being designed for the new magnification you want, AND for the current spacing.

      B) Replace one mirror, sensibly the output mirror for the new magnification you want AND be prepared to change the distance between the mirrors

      As the magnification is increased be prepared for some drop, potentially considerable drop in optical quality. Using steeply curved spherical mirrors ( or “fast” mirrors) off axis can give a rapid increase in optical aberrations

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