Try using a regular "light" entity with the same color of the laser.

Also brush entites dont light like world brushes, and more importantly the dont block light like world brushes. When a world brush covers another surface, the surface behind the world brush is completely removed. If a surface is covered by a brush entity it is not removed, but considered as being in shadow. The face lights using the average accross its surface, so if a large part of the surface is covered its looks very dark. You can crop or carve the behind brushes to form a kind of picture frame, with only the edges visible. This doesnt explain the first brush, but the previous post should fix that.