First, OP, don't stress too much-- this is something that happens from time to time, and your dog was predisposed to being obsessive. We treated plenty of these when I was on rotation in the behavior service.
Second, the best way to solve any serious behavior problem is to see a BOARD-CERTIFIED BEHAVIOR THERAPIST. This is as opposed to a "trainer." There are good trainers, and there are bad trainers, and the only way to know which is which is to have a lot of knowledge of animal behavior-- in which case, you wouldn't need a trainer. Remember, if you wanted, you could call yourself a trainer tomorrow, start a web page, and start charging people for lessons; no education required.
http://www.dacvb.org/, click on "find a behaviorist."
If your pet is not a danger to himself or others, then you may wish to try some things at home first. The general principles for most behavior problems are as follows:
0) Safety! This doesn't really apply to your dog, but in general, the most important thing is to avoid putting the pet, other animals, or humans in harms way.
1) Avoidance! When a dog engages in a behavior, it is because he finds that behavior innately rewarding. Something about it is positive or beneficial. Therefore, any time the behavior is practiced, the neural circuit involved with the behavior is being strengthened.
After this, there is more variation. In your case, the steps would be as follows:
2) Improve your pet's drive to follow your commands. You should do this while AVOIDING any kind of dominance-based training.* Some tips include working on "focus" commands like "look" and "touch" (YouTube is a great resource for learning how to teach these tricks.) You might also work on recall commands.
This is a really important step, and it will take you several weeks to a few months to really do a good job. Be patient, work in small, short sessions, and always use plenty of positive reinforcement.
A more general therapy package to teach discipline can be found [here[(https://humanebroward.com/downloads/behavior%20handout%20-%20Nothing%20in%20life%20is%20Free.pdf).
3) Generally, the final step is "desensitization and counter-conditioning." This is the step that has been suggested below, regarding feeding and moving the light around. To be honest, I am struggling with whether this is appropriate or not, and if you were a client of mine, I would probably call up a boarded behaviorist and ask about it. In this case, the level of stimulation may be so high that you simply cannot use DS/CC. What you may be able to do is desensitize and counter-condition to things like shadows and reflections. I think, however, the laser should be left in a drawer from here on out. For more on DS/CC, see here.
For more reading on compulsive behaviors in general, check out this excellent article.
*If you don't know, briefly, DBT is based on the errant belief that dogs are "pack animals" when in fact they are not (they are highly social/gregarious animals with loose and flexible organization, hunting, and feeding arrangements); it is the belief that most problems are caused by a desire to be dominant over other things (eg, there are videos of The Dog Whisperer arguing that a dog in your situation is trying to "dominate" the light source). This is unscientific. The biggest problem with broad use of DBT is that even when it provides results, it does so at the cost of increasing anxiety. Anxiety is often a contributor to compulsive behaviors, and may also lead to aggressive behaviors.