Peter Reilley wrote...
You cannot get better than the 1 to 2 mrad divergence
with a lens or a slit or the expensive lasers would do it.

I understand, and you are correct, of course. I did not mean to suggest
anything else. I only intended to advise the OP that she could get a
finer line with a slit, as she in fact may well be able to do.

To be sure, beam divergence is always increased by any close aperture,
including a slit. I hope I didn't lead anyone to think otherwise.

When a slit does effectively narrow the beam, I suspect it accomplishes
this by blocking part of the diffraction pattern produced by whatever is
already in the optical path. If this is the case, my additional
suggestion that the slit should be very narrow was bad; it should be no
narrower than the "main" beam width at the point of the aperture. This
should minimize the beam divergence caused by the slit.

I should also point out that there is some distance at which a slit would
only worsen the problem, and that the farther the slit is from the laser
source, the better.

Jim