The power would be very low, probably no more than one or two millwatts. In the printer all it has to do is project the image on to the toner drum and given that it's sharply focussed the power per area only has to be very small.
In general you will find they have 3 pins, one is usually bonded to the base and will be the common negative (ground) side, one will be the laser diode anode (+) side and the other will be the photo-sensor anode side. The 'lasing' will only happen over a very limited current range and exceeding the current will immediately fry the diode. The control IC will use the sensor output to regulate the laser diode current to keep it within range, regulate the light output by adjusting the current and of course provides the on/off control for actually projecting the raster on the drum. As far as I know, the laser power is not adjusted to produce gray levels, that is achieved by dithering the pixels.
Brian.