using just 2 batteries won't be enough overhead voltage to use a regulator. We switch laser diodes really fast with no problems on the products we build at my job. but the laser diodes also have current and voltage regulation built into them (its in the housing of the laser diode). I've used the laserdiodes from laser pointers for home projects and had to limit them to under 5volts, but those I couldnt pwm them after a certain fregeuncy, I just ran them from 3 volts (2AA batt). The data sheet should be able to tell you the max. requirements.