Controlling current in a PWM circuit
There is quite a lot of information available on the web, however knowing good search terms is an obstacle. 

You need a constant current source to reliably drive a LASER.

I recommend looking at Sam's LASER FAQ and specifically Communications Systems Using Diode Lasers which, I think may provide solutions to your problem.

Current control is very important to ensure a LASER works properly. 

Comment:
I don't think current modulation is easy. If it is to ensure the LASER operates correctly then it is addressed at that web site.

I'll quote a useful part:


  Modulating a laser to complete cutoff is a very, very bad game for all
  sorts of reasons:
  
  
  The light versus current behavior is hideously non-linear below about    10% of full output, so you really need dynamic feedback
  control - but    the photodiodes tend to be slow, so that's not on.
  Spectral quality, and beam shape, go to blazes at low power levels
  lasers don't turn on from fully-off anywhere near as fast as they can    vary intensity around the 50% level.
  It's pig-difficult to design the modulation circuit so it is guaranteed never to overshoot the current that gives 100% full light output (which is essential, because even very brief over-power transients dramatically shorten the laser's life).
  


That link seems like a good place to start. At best it solves the problem, at worst it gives you enough understanding to be able to write good search queries.