Abstract:
A six sided refractive / reflective optical element controls the separation between four parallel laser beams. Two beams will be reflected off the optical element and two beams will be refracted within the optical element with each reflection and refraction being off a different side of the optical element to form four closely spaced parallel light beams.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application contains subject matter that is related to subject matter of patent application Ser. No. 09/428,390, filed on Oct. 28, 1999, now U.S. Pat. No. 6,301,054 entitled “Optical Element for Multiple Beam Separation Control” by Chuong Van Tran, commonly assigned to the same assignee herein and herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a multiple beam spacer and, more particularly, to a refractive / reflective optical element which controls the separation between four parallel laser beams. 
     Printing systems will use a raster output scanning (ROS) system to have a modulated light beam strike the facets of a rotating polygon mirror and scan in a raster pattern across a photoreceptor. High speed or color printing requires a multiple beam light source. 
     One possible multiple beam light source is a laser diode array. However, placing two or more laser diodes in a single array creates practical difficulties including phase locking of the emitted laser beams and electrical and thermal interference between the adjacent laser beams. 
     Another approach to providing a multiple beam light source is to integrate individual laser diodes to form the multiple light beam source. In a ROS system, it is beneficial to have the rotating polygon as thin as possible. Thicker polygons cost more and require larger, higher power and more expensive motors and drivers. A four parallel beam ROS therefore requires that the four beams be closely spaced in order to enable a thin polygon. 
     The major problem with integrating individual laser diodes into a multiple beam light source is the large spacing between the individual laser diodes caused by the physical size of the laser diodes themselves. The spacing or pitch between two adjacent individual laser diodes can be 100 microns or larger while the required spacing of the two adjacent light beams for printing uses is 25 microns or less, a difference of a factor of four or greater. Also multiple laser beam systems are often required to have beam to beam spacing that is considerably different in different parts of the system. 
     A beam spacer uses optical elements to expand or contract the pitch or spacing between light beams. 
     Current technology can use mirrors and lenses as beam spacers. However, manufacturing these optical elements on such a small micron scale requires expensive, extensive fabrication and aligning the various optical elements on an even smaller scale mandates a precision assembly. 
     Beam combiners, as their name indicates, are optical elements that combine two or more light beams into a single overlapping composite beam. These are distinctly different optical elements from beam spacers which move light beams closer without combining the beams. 
     Beam splitter prisms can be used as beam spacer elements but this approach reduces the intensity of the output beam by half due to light loss caused by splitting the beam. A tilt plate and a pair of beam steering prisms (or a second tilt plate) are used to split a wide horizontal beam into two smaller vertically aligned beams in U.S. Pat. No. 5,557,475 to improve the brightness symmetry of the beam. 
     One possible beam spacer is found in U.S. Pat. No. 5,566,024, commonly assigned as the present application and herein incorporated by reference. Two sets of two single blazed binary diffractive optical elements form a beam spacer for contracting two wider spaced parallel beams into two closely spaced parallel beams. 
     It is an object of the present invention to provide a multiple beam separation spacer of a refractive / reflective optical element which controls the separation between four parallel laser beams. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a six sided refractive / reflective optical element will control the separation between four parallel laser beams. A first laser beam will be reflected off a first side of the optical element. A second laser beam will be refracted from a second side to a third side within the optical element. A third laser beams will be refracted from a fourth side to a fifth side within the optical element. A fourth laser beam will be reflected off a sixth side of the optical element. These reflections and refractions by the optical element beam spacer will form four closely spaced parallel light beams. 
     Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a schematic view of the refractive / reflective optical element beam spacer of the present invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     Reference is now made to FIG. 1 illustrating the single refractive / reflective optical element beam spacer  100  for spacing four laser beams of the present invention. 
     The refractive / reflective optical element  100  is a single solid element formed of a material that transmits light of the wavelength of the incident light beams and has a uniform index of refraction throughout the material. Examples would include plastics and BK7 glass, as are known in the art. 
     The optical element  100  has six flat surfaces and has a diamond pattern with a right triangle concave insert on the back corner. 
     The optical element has a first incident surface  102  adjacent to a second incident surface  104 . The optical element has a first output surface  106  adjacent to a second output surface  108 . The optical element has a fifth reflective surface  110  which is adjacent to the first incident surface  102  and the first output surface  106 . The optical element has a sixth reflective surface  112  which is adjacent to the second incident surface  104  and the second output surface  108 . 
     The first incident surface  102  is at a specified angle  114  relative to the optical axis  101 . The first output surface  106  is at the same specified angle  114  relative to the optical axis  101 . The first incident surface  102  is parallel to the first output surface  106 . 
     The second incident surface  104  is at the same specified angle  114  relative to the optical axis  101 . The second output surface  108  is at the same specified angle  114  relative to the optical axis  101 . The second incident surface  104  is parallel to the second output surface  108 . 
     A first laser diode  116  will emit a first laser beam  118 . A second laser diode  120  will emit a second laser beam  122 . A third laser diode  124  will emit a third laser beam  126 . A fourth laser diode  128  will emit a fourth laser beam  130 . The laser diodes  116 ,  120 ,  124  and  128  may be in array or be individual diodes separate from the other diodes. 
     The wavelengths of the four laser beams  118 ,  122 ,  126  and  130  are the same. The four laser beams  118 ,  122 ,  126  and  130  are parallel and adjacent beams are separated by a first spacing distance  132 . 
     The second and third light beams  122  and  126  are symmetric around the optical axis  101  and equally spaced from the optical axis  101  in the direction of light propagation. The first and fourth light beams  118  and  130  are symmetric around the optical axis  101  and equally spaced from the optical axis  101  in the direction of light propagation. The refractive / reflective optical element  100  is symmetric around the optical axis  101  in the direction of light propagation. 
     The first laser beam  118 , after emission by the first laser diode  116 , win be reflected by the first turn mirror  134 . The reflected first laser beam  118  will be incident on the fifth reflective surface  110  of the beam spacer  100  and reflected parallel to the optical axis  101  to the scan line  136 . The scan line  136  is perpendicular to the optical axis  101 . 
     The second light beam  122 , after emission by the second laser diode  120 , will be incident upon the first input surface  102  of the beam spacer  100  at an angle θ. The second laser beam  122  will refract at the input surface  102  and travel through the length  138  of the beam spacer  100 . 
     The second laser beam  122  will then refract at the first output surface  106  of the beam spacer  100  at an angle θ. The angle of incidence θ and the exit angle θ are the same. The first input surface  102  of the beam spacer  100  is parallel to the first output surface  106 . 
     The second laser beam  122  will be refracted parallel to the optical axis  101  to the scan line  136 . 
     The second laser beam  122  and the adjacent first laser beam  118  are separated by a second spacing distance  140  at the scan line  136 . The second spacing distance  140  after refraction and reflection of the beams by the beam spacer  100  is less than the first spacing distance  132  before the beam spacer. As seen in this Figure the second laser beam  122  has been displaced laterally by the beam spacer  100  to be closer in spacing or pitch to the adjacent beam  118 . The amount of lateral displacement is proportional to the length  138  of the beam spacer. 
     The third laser beam  126 , after emission by the third laser diode  124 , will be incident upon the second input surface  104  of the beam spacer  100  at an angle θ, the same angle of incidence as the first laser beam  118 . The third laser beam  126  will refract at the second input surface  104  and travel through the length  142  of the beam spacer  120 . The length  142  of the third laser beam  126  through the beam spacer  100  is the same as the length  138  of the first laser beam  118  through the beam spacer  100 . 
     The third laser beam  126  will then refract at the second output surface  108  of the beam spacer  100  at an angle θ, the same exit angle as the first beam. The angle of incidence θ and the exit angle θ for the third laser beam are the same. The second input surface  108  of the beam spacer is parallel to the second output surface  104 . 
     The third laser beam  128  will be refracted parallel to the optical axis  101  to the scan line  136 . 
     The third laser beam  126  and the adjacent second laser beam  122  are separated by the second spacing distance  140  at the scan line  136 . This second spacing distance  140  is the same spacing distance between the second laser beam  122  and the first laser beam  118 . 
     The second spacing distance  140  after refraction by the beam spacer  100  is less than the first spacing distance  132  before the beam spacer. As seen in this Figure the third laser beam  126  has been displaced laterally by the beam spacer  100  to be closer in spacing or pitch to the adjacent beam  122 . The amount of lateral displacement is proportional to the length  142  of the beam spacer. 
     The fourth laser beam  130 , after emission by the fourth laser diode  128 , will be reflected by the second turn mirror  144 . The reflected fourth laser beam  130  will be incident on the sixth reflective surface  112  of the beam spacer  100  and reflected parallel to the optical axis  101  to the scan line  136 . 
     The fourth laser beam  130  and the adjacent third laser beam  126  are separated by the second spacing distance  140  at the scan line  136 . This second spacing distance  140  is the same spacing distance between the third laser beam  126  and the second laser beam  122  and between the second laser beam  122  and the first laser beam  118 . 
     The second spacing distance  140  after refraction by the beam spacer  100  is less than the first spacing distance  132  before the beam spacer. As seen in this Figure, the fourth laser beam  130  has been displaced laterally by the beam spacer  100  to be closer in spacing or pitch to the adjacent beam  126 . 
     The optical element  100  of FIG. 1 has six flat surfaces and has a diamond pattern with a right triangle concave insert on the back corner. The optical element could alternately have a flat front surface between the first and second incident surfaces or a flat back surface between the first and second output surfaces. 
     The input and output surfaces of the beam spacer typically have an antireflection coating to increase refraction of the laser beam. 
     The beam spacer of the present invention will also closely space three or two beams with the present design. 
     While the invention has been described in conjunction with specific embodiments, it is evident to those skilled in the art that many alternatives, modifications and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all such alternatives, modifications and variations as fall within the spirit and scope of the appended claims.