Using a laser
A laser module contains a solid state laser diode and some control electronics. Most of these modules expect to be provided 3 volts, and they draw about 40mA.
If you want to run a laser module from a 6V supply or battery, you can put a resistor in series with it, of about 100 ohms. The "extra" three volts, across the 100 ohm resistor, corresponds to only 33mA, which is less than the current laser module normally handles, so the laser module is safe. Higher values than 100 ohms will make the laser dimmer. Lower vales may burn it out. For a 12V battery, use 300 ohms, etc.
You can also operate a laser diode directly from a digital output line of a PIC or digital logic chip. Even though the Logic High voltage for digital logic is nominally 5V, most chips limit their maximum current severely, typically to 25mA. So, connected directly to a laser module, the logic level will "sag" to about 3V and the laser module will illuminate nicely.