Matlab Gaussian Laser Beam

How do I define a laser beam(gaussian) in Matlab with given beam waist and centroid. The task is to then add noise to this laser beam and calculate the change in the centroid using a centroid detecting algorithm.

[dpb]

Guess first you define "beam waist" and "centroid" for those who know something of Matlab but little (if any) of lasers.

Then, what you mean, specifically, by "define in Matlab" -- a function that returns something like a 2D PDF of intensities or the like, maybe?

The crystal orb is dark on this one, sorry...