2-20 Phaser 6121MFP Service Manual
Theory of Operation
Laser Unit
The Laser Unit in the Phaser 6121 printer has two laser diodes. The laser diode 
control circuitry adjusts the light intensity for each beam automatically. Image 
data is transmitted to the laser diodes in the Laser Assembly as digital signals. The 
laser diodes convert the image data from digital signals to optical signals. The 
Laser Assembly monitors and adjusts the intensity of the laser beams to attain a 
stable electrostatic image. The Laser Assembly is replaced as a complete assembly. 
Functional components within the Laser appear in the following.
Imaging Unit Exposure
The surface of the Imaging Unit is irradiated with laser light to form an 
electrostatic latent image. The polygon mirror has four faces. The two-beam laser 
array consists of two laser diodes arranged vertically. Two lines are scanned 
through a single face of the polygon mirror.
s6121mfp-117
Semiconductor Laser
SOS Sensor
Polygon Mirror
G1 Lens
G2 Lens
SOS Mirror
Return Mirror
s6121mfp-118
Photo
Conductor
Polygon Mirror
Laser Diode
Beam A
Photo Conductor Surface
Beam B
Return Mirror
G2 Lens
G1 Lens