ABCD matrix formalism for real laser

[kelly0303]

Hello! I have an optical setup and a laser producing a gaussian beam and I would like to get the waist of the beam at the end of the setup (i actually want to know what focal length to use for some lenses, such that I get a certain beam waist in the end). Getting the ABCD matrix for my setup is straightforward (I don't need to be super accurate, e.g. I don't account for the lenses thickness), but I am not sure about the initial properties of the beams. For a gaussian beam (which is my case), the 2 parameters that get propagated using ABCD formalism are the waist and the radius of the wavefront of the beam. I have the waist at the beginning of my setup from the laser specifications (they provide waist vs distance from the laser head), but I can't seem to find any information about the wavefront and I am a bit confused about how to obtain that. In their waist vs length plot there is a minimum, can I assume that the wavefront is zero there, or is that not generally the case? Thank you!