An alpha channel isnt just black/white, infact the brightness determines how opaque or translucent it is displayed/rendered.
Or just use the alternative that i mentioned (and tbh would recommend for this more) is to just use additive rendering.
In blender just select your faces and set them to be additive (add) in the alpha blending section of the texture face panel.
(Since you just want the laser to be affected obviously just select those faces and set them to additive.)