Elaborated on a docstring comment. Minor.
Wrote a sorta-functioning make_radial_profile_of_diffracted_power.

Seems to work.
Included sigma_over_lambda everywhere. A couple of small style changes too.
Expanded documentation and improved style.
Reformulated the plot to show -z_max to z_max instead of 0 to z_max.
Made z_range an adjustable kwarg in profile_of_diffracted_power.
Now I do a funny ad-hoc "normalization" of the diffracted E field.

It feels a little cheesy but it works and Bryan recommended it.