Instead of heating photons from INSIDE of laser try from outside of void, they will head up, photons and you beam will not split up, you can also use plasma or uran to heat even more, you can also extend barrel so they heat up faster and more.
marek11 is right, I used plasma to heat up my laser and my barrel was almost twice as large as this. put a few clones that produce plasma around the barrel. this will heat the void which in turn heats the photons.