We had to overcome various issues to realize high image quality comparable to that of offset printing by using xerography. The greatest challenge was to integrate multiple beams into a laser for optical scanning at high pixel density.
FUJIFILM Business Innovation resolved this problem with our proprietary technology-a surface-emitting type of semiconductor laser named "VCSEL." By adopting VCSEL as a light source of the Raster Output Scanner (ROS, which is a scanning-type light exposure system), we succeeded in controlling 32 beams in one scan. Thus, we achieved 2,400 dpi resolution in color laser printing with xerography for the first time in the world.
By projecting four beams into a space that occupies one dot in a conventional 600 dpi image, we could attain four times higher resolution (= 2,400 dpi). Moreover, eight sets of beams can be used for simultaneous scanning, thereby enabling a printing speed of more than 80 pages per minute. The latest high-performance image processing Application Specific Integrated Circuit (ASIC) is also implemented, to achieve high-image-quality printing similar to that of production printing by designing the arrangement of radiation points, equalizing the light intensity of each beam, and controlling the write timing.
VCSEL stands for Vertical-Cavity Surface-Emitting Laser diode. For conventional semiconductor lasers (edge-emitting type lasers), there is a set of facet mirrors on the cleaved surface of a semiconductor substrate and radiation occurs in parallel with it. In contrast, VCSEL has an optical cavity perpendicular to the substrate and emits light in the vertical direction.
The edge-emitting lasers used for laser printers and optical communication components have several issues, such as high power consumption, catastrophic optical damage on cleaved facets, as well as the high cost associated with controlling multiple beams.
VCSEL has the following advantages compared with conventional edge-emitting lasers:
- Low cost
Leads to a significant reduction in manufacturing cost because the "cleavage" process (to split materials along crystallographic structural planes) required in the fabrication process to create reflecting mirrors that work as a Fabry-Perot resonator is no longer necessary, and inspection at the wafer level can be conducted. - Lower power consumption
VCSEL operates at one-tenth the current needed by conventional edge-emitting lasers, and also offers higher quantum efficiency. - Easy to fabricate two-dimensional arrays
Multiple-beam lasers can be easily realized by increasing array elements two -dimensionally as permitted by the substrate area, something which is impossible for edge-emitting lasers. - Higher modulation bandwidth
VCSEL can be modulated at higher speeds over 10 GHz, because its active region volume is less than 1% compared with an edge-emitting laser.