How laser output intensity is controlled electrically?

A dither pattern is just a pattern with some pixels filled and some empty to create from a distance the impression of a half tone.

Variable lasers tend to be difficult, when cascade is met the intensity ramps up due to a feedback loop to its maximum. That feedback loop makes them very intense. So they are on or off type devices.

Getting the toner to hold prior to fixing requires a good strong force, if you start varying it you will probably have toner slipping or not being help prior to fixing, so I suspect, but do not know, that using this technique to vary intensity would be quite difficult.

Most lasers used in printers and such are laser diodes, in which the energy is pumped into the semiconductor by passing a current through the depletion region.

More current = more light output. It is very simple to modulate current at high frequencies, although you have to be a bit careful about the inductance of the circuit (and the nonlinear characteristics of the diode) to do this properly.

This is why dithering with a simple "on-off" mechanisms is a nice way to achieve well controlled gray scales without the need to understand and characterize the nonlinearity.

UPDATE

You are asking clarifying questions about the operation of the laser printer which are a bit outside the usual scope of this physics site, but let me try:

First, the printer's "brain" converts the requested image (which may include characters) into a finely rastered "bit image" - in the same way that your video card does, before displaying images on your screen, but at very high resolution. Now you need to know "where is the laser beam pointing" and "what gray scale should I display there".

The first question is answered with a position encoder on the mirror - this produces a precise location of the mirror (as a digital signal), and from the position of the mirror you can compute where the beam is pointing. This takes some calibration that is normally done at the time of assembly.

Once you know where the laser is pointing, you then look up the corresponding location in memory, and convert that value into a current sent into the laser diode. This is usually done with a DAC (digital to analog converter) with a current output.

All this happens very, very quickly. And it results in little bits of charge being removed from some of the plate, while remaining on other parts. The plate then picks up toner where it is charged, the toner is transferred to the paper, and there it is melted into place.

There is a nice description with some diagrams at [this link}(http://www.explainthatstuff.com/laserprinters.html).