Recently, printing systems utilizing lasers to reproduce information or laser sensitive mediums have been announced by various manufacturers. Typically, the system includes a laser for generating a laser beam, a modulator, such as an acousto-optic modulator, for modulating the laser beam in accordance with input information to be reproduced, a multi-faceted scanner for scanning the modulated laser beam across a medium on a line to line basis, various optical components to focus and align the laser beam onto the medium, the laser sensitive recording medium itself and, if necessary, a means for developing the information in human readable form. A typical representation of printing systems utilizing lasers is shown, for example, in U.S. Pat. No. 3,922,485, the patent disclosing the use of a xerographic drum as the recording medium and means for developing the latent electrostatic image formed on the drum.
In some printing systems it is required that a cylindrical lens be interposed between the modulated laser beam and the scanner to provide a fan fold light output beam which fully illuminates at least one facet of the scanner. The cylindrical lens is preferably aligned with the laser beam to redistribute the energy of the modulated laser beam in a manner whereby the energy distribution of the laser beam incident on the recording medium is substantially symmetrical about the start of scan and end of scan positions of the medium, maximum energy ideally occuring at the center of the scanline. This is particularly important in those systems which utilize a xerographic recording medium.
In general, the laser portion of the overall printing system is regarded as the "weak link" in that the laser would be the first component to fail or in some way become inoperative. The accepted approach, from a field service standpoint, to make the printing system operative, would be to remove the defective laser and replace it with another laser, either a new laser or a repaired laser. In regards to the configuration discussed above, it has been determined that the relative positioning of the laser beam with respect to the cylindrical lens is extremely critical, this critically affecting the energy distribution of the laser beam which scans the medium. If the laser beam is moved with respect to the cylindrical lens a small distance, say for example 0.001 of an inch in the scanning direction, the beam energy incident on the polygon facet will shift a substantial fraction of the facet aperture in the scanning or tangential direction, the displacement being determined by the magnification factor of the cylindrical lens which in turn will effect the beam energy distribution at the surface of the recording medium. The relatively tight tolerance required between the laser beam and the cylindrical lens has made it extremely time consuming and costly to replace the laser independently of the cylindrical lens and maintain the necessary tight tolerances therebetween.
Therefore, what is desired in a laser printing system is an arrangement wherein tolerances between the laser beam and certain of the down stream optical elements can be maintained within acceptable limits without the attendant increased costs and time delays which would otherwise occur.