In photonics since 1999. Benelux technical support, answer within one working day.
Beam expanders: why the collimation matters more than the magnification
An afocal beam expander enlarges a collimated beam and, in doing so, reduces its divergence by the same factor. That reduced divergence is usually the real reason to fit one: a larger beam stays collimated further and focuses to a smaller spot.
THE SHORT ANSWER
An afocal beam expander enlarges a collimated beam and, in doing so, reduces its divergence by the same factor. That reduced divergence is usually the real reason to fit one: a larger beam stays collimated further and focuses to a smaller spot.
| Type | Afocal, no focal plane |
|---|---|
| Typical magnification | 2x to 7x, variable |
| Divergence change | Reduced by the magnification |
| Coating | Specified per wavelength |
| Mount thread | W 0,8 inch typical |
| Input requirement | Collimated beam |
The name describes the visible effect and hides the useful one. Yes, the beam comes out larger. The reason that matters is what happens to divergence.
The reciprocal relationship
In an afocal system the product of beam diameter and divergence angle is conserved. Expand the diameter by five and the far-field divergence drops by five. A beam that would have doubled in width over ten metres now barely changes.
The same relationship governs focusing. Spot size at the focus of a lens is proportional to wavelength and focal length and inversely proportional to the input beam diameter. If you need a smaller spot, filling the aperture of the focusing optic is the lever you have.
Filling an aperture properly
Scan lenses and microscope objectives quote spot sizes that assume the entrance pupil is filled. Underfill it and you get a larger spot from an expensive lens, then blame the lens. An expander sized to the pupil recovers the performance you paid for.
What it will not fix
Beam quality is conserved too. An expander scales the beam, it does not clean it. If M squared is 3 going in it is 3 coming out, and the focused spot will be three times the diffraction limit at whatever diameter you chose. Beam quality problems are solved at the source or with a spatial filter, not with magnification.
When to use it
- You need a smaller focused spot: expand before the focusing lens
- The beam must travel a long distance without growing
- A scan lens or objective needs its aperture filled to reach its specified spot size
- Power density at the optics is too high and a larger beam spreads the load
What to watch out for
- Expanders are afocal and assume a collimated input; feed one a diverging beam and the output is not collimated
- Magnification multiplies pointing error as well as beam size
- A variable expander trades a little collimation accuracy for adjustability
- Expanding the beam does not improve M squared: a poor beam stays poor, just larger
Still deciding?
Tell us the wavelength, the aperture and what you are measuring. We answer with a part number, a price and a lead time, usually the same working day.