The present invention relates to a laser printing system utilizing a multiple wavelength, multiple diode laser and, more particularly, to an improved system which further includes a dispersive element positioned in the optical scanning path to cause the diode outputs to be scanned as adjacent lines at a photosensitive image surface.
It is known in the scanning art to utilize a multiple laser beam source for scanning a plurality of lines on a recording medium surface simultaneously with a plurality of laser beam outputs. This technique is finding increasing usage because it substantially increases the effective printing speed of a system compared to the conventional laser ROS printer using a single output laser source. U.S. Pat. No. 4,637,679 discloses a multi-beam scanning system utilizing two semi-conductor laser sources LD1, LD2, which are arranged as shown in FIG. 2 of the patent. Their output beams are formed into parallel light flux which are polarized perpendicularly to each other. Each beam is independently modulated by signals of the scanning line of an odd number and an even number, respectively, and it then becomes possible to write information as multiple scanning lines. Further advances in the art have led to the laser emitter sources being formed within the same semi-conductor chip. An example of this type of scan system is found in U.S. Pat. No. 4,445,125 assigned to the same assignee as the present invention.
One of the problems encountered with the prior art multiple beam laser scanning systems is that because the distance (pitch) between the multiple diodes is large compared with the emitting area of the diode (typically a minimum of five times larger), diode outputs are imaged on the photoreceptor several scan lines apart. In the prior art, the problem has been addressed by using an electronic buffer to first store the scan lines of information and print them in interlaced fashion. Such buffers, however, add to the cost and complexity of the system.
The present invention addresses the above problem by using a multiple diode laser with each diode having differing output wavelengths. A dispersive element, such as a prism or diffraction grating, is then positioned along the optical path for bringing focused beams closer together when they are imaged at the photoreceptor.
The use of dispersive elements in scanning systems to accomplish various functions is known in the art. A Toshiba publication 63-155018 discloses placing prisms in the object path of a polygon scanner to generate parallel beams, but the beams, not being of different wavelength, are not brought together as adjacent lines as is disclosed in the present invention. Other relevant prior art references are: U.S. Pat. No. 4,268,110 to Ford discloses a facet angle corrector for a multi-faceted optical scanner which reduces banding in the output image. The corrector comprises a corrector plate supporting a plurality of transparent discs having a slight wedge angle to precisely control the angle at which the beam is to be bent. See FIG. 1 of the patent which shows laser source 10, light beam 11 expanded light beam 12, beam expanding optics 13, pyramidal scanner 14 corrector means 31, plate 32, and discs 36 having a light wedge. U.S. Pat. No. 4,770,507 to Arimoto et al. discloses an optical system for converting the shape of a light beam using the refraction of prisms by using at least two types of prisms having different values of refractivity. Geometrical properties of the optics are stable regardless of changes in light beam wavelengths. U.S. Pat. No. 4,828,371 to McCaslin, assigned to Xerox Corporation, discloses diode laser beam correction optics which use an expansion prism pair separated by a rotationally adjustable mirror. Beam expansion can be controlled by varying the rotation and translation of the prisms. See FIG. 3 of the patent which shows prisms 42 and 44, mirror 46, and diode laser 13. U.S. Pat. No. 3,871,750 to Mecklenborg discloses a laser beam expansion apparatus for an optical disk drive. The apparatus has prisms 22 and 24, mirror 14, beam splitter 15, optical disc 16, and laser 13. U.S. Pat. No. 4,796,964 to Connell et al., assigned to Xerox Corporation, discloses a method of utilizing a multiple emitter solid state laser in a raster output scanner. Plural overlapped laser beams are focused onto a recording medium by selectively sequencing the lasers to that there will not be any nonuniformity due to optical interference of the laser beams. See scanning imaging lens 48 in FIG. 1 of the patent which produces overlapping laser spots on the recording surface.
More particularly, the invention relates to a multi-beam scanning system comprising: a laser light source for generating a plurality of light beam outputs, each output having a different wavelength from the other, optical scanning means for receiving said output beams and directing the beams along an optical path as a plurality of scanning beams to simultaneously form a plurality of focused adjacent lines on the surface of a photosensitive plane, the scanning means further including a dispersive element positioned along the optical path so as to bring the focused scanning beams into adjacent lines, respectively, at the photoreceptor.
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