Aprocess that modifies light waves so they vibrate in a single plane is referred to as

The process you're fishing for is "polarization", but that's a
misleading description.

Polarization doesn't do anything to change the light waves. 
It simply filters out (absorbs, as with a polarizing filter) the
light waves that aren't vibrating in the desired plane, and
allows only those that are to pass.

The intensity of a light beam is always reduced after
polarizing it, because much (most) of the original light
has been removed.

A laser light source may be thought of as an exception,
since everything coming out of the laser is polarized.