Rays and beams
Beam expander
Two lenses spaced by the sum of their focal lengths widen a beam and keep it parallel. A wider beam spreads less and can be focused tighter, so expanders sit in front of most focusing optics.
What you are looking at
The beam focuses after the first lens, spreads, and is caught by the second, which sends it on four times wider.
How the bench is set up
Laser (0.5 mm waist), lenses f₁ = 50 mm and f₂ = 200 mm 250 mm apart, and a beam profiler.
The theory
A Keplerian telescope magnifies the beam width by
What to measure
Width ratio f₂/f₁
- Read the profiler: about 2 mm, four times the input.
- Move the second lens by a few millimetres and watch the beam stop being collimated.
What the model leaves out
- The lenses are thin and ideal.
Things to try
- Swap the first lens for f = −50 mm and move the second to 150 mm: a Galilean expander with no focus inside.
References
- A. E. Siegman (1986), Lasers, University Science Books.
More experiments, and how the lab works, are in the laser lab’s overview.