A typical AOM uses a fixed drive frequency to produce effectively a diffraction grating. By amplitude modulating the drive signal the intensity of the light transiting the AOM can be controlled in a pretty linear fashion. In an image setter you want to have multiple frequencies individually selected to essentially print the way old fashioned line printers printed letters - except in an image setter you're printing dots in a grid on a specific angle for each color of ink. (With a bit of dithering to avoid moire patterns within the "screen".) So this AOM probably didn't have an amplitude modulation capability - you'd control the power of the laser instead.
"There are painters who transform the sun into a yellow spot, but there are others who, with the help of their art and their intelligence, transform a yellow spot into the sun." Pablo Picasso