Focus a laser with a spherical mirror ?

We often get asked this question, the attraction of using a spherical mirror to focus a beam is obvious. Spherical mirrors are low cost, readily available, easy to test, easy to mount and align. Some laser users understand that the scheme on the left, of using a mirror at 45 degree incidence to turn and focus a beam, is probably not quite as easy as it seems.  And so it turns out to be. A spherical mirror surface is a perfect surface for imaging a point located exactly at it’s radius of curvature, exactly back to it’s radius of curvature ! .  This is why, even for the simple imaging of a far away object, rather than one located at the mirrors radius of curvature such as a star, a parabolic surface is required. Isaac Newton who studied a few miles from our factory in Cambridge knew this, and made his own parabolic mirror for his telescope.  A customer asked us if a CO2 laser was focused using a spherical mirror at 45 degrees incidence what would the focal spot look like.

This can be modelled with a lens design programme like Zemax, which we did. To even be in with a chance of having something useful we looked at a long focal length, 300mm. The size and shape of the spot above and below focus can be seen on our web site, a rough guess of 5mm would be about right, and nastily elliptical. A better spot diagram at 254mm away from the mirror is on our website too. Obviously astigmatic spot would be hopeless for laser cutting, but for crude surface heat treatment, or this case firing the beam into an instrument entrance it was good enough.

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