Building a driver that gives constant current regardless of input/battery voltage is relatively simple to do, altough many manufacturers manage to fail miserably at this.
Maintaining the peak power of a dpss (green) laser is another matter though - the cause of the power drop is mostly thermal in nature, and can only be remedied by good heatsinking of both pump diode and crystal set. To make things worse, some lasers work better when slightly warm, so overkill-heatsinking doesnt give optimum results.