Natural Science Forum / Physics / Optics / June 2005

I agree about the dubious quality of direct beam profile measurements at
 NA > 0.05 or so.  It's much better to use a microscope objective to
collect the beam and measure the image.  The only serious difficulty is
if you care about the focal position, which is not easy to measure
accurately that way.

If you have the time and interest, a simple interferometer e.g. a Smartt
point-source device, can do an excellent job on small foci, for the same
reasons as the microscope objective--the measurements are done where the
beam is much larger, and so the measurements are better defined.
Maxwell's equations will allow you to transform the measurement plane to
the focal region.  You'll need to place the Smartt plate a little out of
focus, so that the beam doesn't all go through the pinhole.

You can also make a diameter measurement by blowing small aerosol
particles (e.g. polystyrene latex spheres) across the beam and measuring
the scattered light with a PMT.  Because Gaussians are separable (a
circular Gaussian is exp(-(x**2+y**2)) = exp(-x**2) exp(-y**2), the
pulse width you measure will be independent of lateral offset, and just
sorting the shortest pulses will give you the minimum diameter.  If the
beam isn't at least approximately Gaussian, this doesn't work as well,
but you can think of it as sort of a tomographic approach.

Cheers,

Phil Hobbs