Abstract: A constant power laser driver for a laser diode in a laser printer reduces laser shadow by maintaining the junction temperature of the laser diode at a constant level. The constant temperature is maintained by a reverse or zener current through the laser diode when the laser is in an off state. The zener current is determined so that the product of the reverse laser current and reverse breakdown voltage is equal to the product of the forward bias voltage drop and forward current when the laser is on. Since these products are substantially equal to the current dissipated in the diode, equal amounts of power are dissipated as heat in the laser diode junction both when the laser is in a writing state and in a non-writing state. As a result, the junction temperature remains relatively constant and the light output remains constant when the laser is turned on, regardless of the length of time it was previously in the opposite state.

Abstract: A high resolution laser printer utilizes a first cylindrical lens to focus a laser beam in the sagittal direction prior to the facets of a rotating polygon. The light reflected from the polygon is focused in the horizontal direction at the photoconductor drum of an electrophotographic process by a scan lens assembly. Following the scan lens assembly, there is a second cylindrical lens which also focuses the laser beam in the horizontal direction at the photoconductor. The first and second cylindrical lenses are positioned to eliminate errors in the facets of the polygon. Scan velocity deviations resulting from the optical design are corrected by storing correction information in a memory and using this information to vary the rate at which the laser is modulated with data.

Abstract: A dual mode laser printer utilizes a program-controlled microprocessor to switch between a high resolution, slow speed mode and a low resolution, high speed mode. In the printer a laser beam is modulated at a rate which is a first division of a system clock rate. This beam is caused to focus on a recording medium and, due to a rotating multi-faceted polygon, the beam sweeps across the medium. The medium is moved according to a second division of the system clock. By means of the microprocessor, the printer can be in either: (1) the high resolution mode in which the laser beam is modulated at a high rate due to the first divided signal and the medium moves at a slow rate due to the second divided signal, or (2) the low resolution mode in which the light beam is modulated at a lower rate due to the first divided signal and the medium moves at a faster rate due to the second divided signal.