laser beam is a Gaussian beam. At its waist postion (inside the laser cavity), the optical wave of this beam is close to a plane wave. After a gaussian beam passes through a lens, it's changed into another Gaussian beam. under certain conditions(one condition is a lens with short focal length...
put a pinhole at the first lens back focal length position and use another lens with the pinhole at its front focal length point, I think it will work if i got your question.
