I would like to use POP to measure the size and profile of a beam as it approaches its focal position, however, I am struggling to configure this correctly. The model is a recreation of an experiment I have setup in the lab as I would like to compare the results. The input beam has a very low divergence value (~0.04 mrads, NA 0.000035), and a single achromat lens is used to focus the beam. The position of the lens relative to the source has been set to give a diameter of  ~ 2 mm at this surface. Using the tutorial https://support.zemax.com/hc/en-us/articles/1500005488601-Using-Physical-Optics-Propagation-POP-Part-1-Inspecting-the-beams I cofnigured the aperture type and POP seetings to create the approprite beam definiton and launch conditions.  I then placed various dummy surfaces throughout the system to assess the beam waist.

The problem arises when the beam reaches the lens surface (surface 3). No matter how much I change the size of the sampling grids of the previous surfaces (1 and 2), the model returns a non spherical profile, and all subsequent surfaces (4 to have a very small non-gaussian profile.

But then at the focal plane, I get something that looks like this.

I have a feeling I am not configuring the model correctly; I really need some help! I have attached the Zemax files for clarity. Any assistance with this would be most appreciated :)

Chris