1.  Field of the Invention
This invention relates to the production of permanent copies of electronic images and more particularly to the formation of a plurality of lines of information on a recording medium, such as a drum possessing a photoconductive surface, thereby to allow a hard copy of the electronic image to be produced in a well-known manner.  This invention also relates to a laser scanning system and in particular to a low cost laser scanning system using a solid state injection laser. 
2.  Description of the Prior Art
The use of recording media containing photoconductive material on their surfaces for the intermediate storage of images to be reproduced on a material such as paper is well-known in the art.  See, for example, U.S. Pat.  Nos.  4,230,902 and 3,951,509.  One problem in these systems is that the optics required to scan each line of an image across a particular recording medium (such as a rotating drum with a photoconductive material on its surface) in a straight line are quite expensive.  Moreover these optics must be accurately aligned to ensure proper operation of the system.  Holographic (or holofacet) scanning across the drum offers a cost effective method for repetitive mechanical scanning of a monochromatic light beam which is modulated as a function of time in order to replicate the line of information being stored on the photoconductive drum.  This type of raster scanning technique of an image is particularly adapted for use with systems wherein the image to be reproduced is stored in a computer memory in the form of bits of information (representing either a black or white dot) derived by raster scanning each line of the image to be reproduced.  The monochromatic light beam is then modulated as a function of the status of each bit in each raster scanned line and is deflected along a line on the drum parallel to the rotation axis of the drum.  The drum is rotated at a slow rate relative to the formation of each line of raster-scanned information on the surface of the drum.  A holofacet scanning system offers a way of deflecting the monochromatic light beam along a line on the photoconductive drum surface parallel to the axis of rotation of the drum surface which is relatively inexpensive. 
A holographic scanning disk suitable for this purpose contains a plurality of diffraction gratings mounted around the outer area of the disk.  The disk is preferably arranged to be nonperpendicular to the light beam from the monochromatic light source such that the light beam intersects the diffraction gratings as the disk rotates.  By arranging the disk to be nonperpendicular to the light beam, the adverse effects of disk wobble are minimized.  As disclosed in U.S. Pat.  No.  3,721,486, the rotation of the disk results in the diffracted light beam producing a focused spot which sweeps over a plane surface in an arc.  The arc comprises the intersection of a cone with a plane wherein the axis of the cone is perpendicular to the plane. ( See, for example, U.S. Pat.  No.  4,094,576.)  One technique for straightening this line is disclosed by Ih in a paper entitled "Holographic Laser Beam Scanners Utilizing an Auxiliary Reflector" published on page 2137 of the Aug. 1977 issue of Applied Optics (Vol.  16, No.  8).  Another technique for doing this is disclosed in a paper by Pole et al.  published in an article entitled "Holographic Light Deflection" on page 3294 of Applied Optics, Oct. 15, 1978, Vol.  17, No.  20.  See also U.S. Pat.  No.  3,951,509 for a description of another technique. 
The use of laser scanning systems for the purpose of producing a line of information on a photoconductive surface in a copier system is well known.  A typical system of this type is disclosed for example in U.S. Pat.  No.  3,750,189.  One problem in such a system is to provide a source of collimated, monochromatic light.  Typically, this is provided by use of a modulated gas laser.  Gas lasers and their modulators, as well as their associated scanning systems, unfortunately are quite expensive.  Accordingly, there is a need for a low cost scanning system with the advantages of a gas laser scanning system. 
There is also need for a structure which will allow the use of a holographic scanning disk and a monochromatic light source for the purpose of reproducing a straight line image on a photoconductive cylindrical surface which is both inexpensive and accurate.