Multiple beam optical modulation system
This invention is a system for producing two scanning beams from one laser beam in a raster output scanner. The invention discloses the separation of one laser beam into two which are separately modulated and polarized. These two beams are then brought back together and are used to expose a light sensitive surface such as a xerographic plate. If the beams are brought back together to a distance which equals one or more spaces between scan lines, then this invention can be used to generate two scan lines from one beam, and therefore, two scans from one facet of a rotating polygon. 
     BACKGROUND OF THE INVENTION 
     It is common for an electronic printer to use a scanning laser light source and a rotating polygon to generate a beam to impinge on the photoconductor. The laser beam strikes the photoconductor and, in the well known manner, discharges the drum, leaving a charge pattern on the surface of the photoconductor in the shape of the information to be printed. Then the drum is developed with toner which is transferred to an output document. 
     A problem to be considered is scanning speed. Rotating the polygon faster is an obvious solution, but bearing life imposes an upper limit on polygon speed of rotation. One improvement used in the past is to scan with two beams, which allows the generation of two scans per polygon facet. An example is commonly assigned U.S. Pat. No. 4,686,542, High Speed, High Resolution Raster Output Scanner. The single original beam is separated into two beams which are individually modulated and used to generate two raster output scans per polygon facet. The problem with this arrangement is that a certain minimum distance must be maintained between scan lines at the photoreceptor plane or the beams will interfere with each other to produce artifacts, and this limits the number of lines per inch at the output if the apparatus is being used to generate two consecutive scan lines. In the alternative, a higher density of scan lines per inch can be produced by using two beams to produce non-consecutive scan lines. That is, each two scan lines generated are separated by one, two or more scan lines. Of course, in this case, the display information must be supplied in the proper sequence also. The present invention described below provides a method of generating two non-interacting but closely spaced beams per facet. 
     SUMMARY OF THE INVENTION 
     This invention solves the problem by separating the original beam into two beams which are polarized at right angles to each other. Then, after modulation, when the beams are brought together at the photoreceptor plane, there will be no optical interference. The beams can be brought very close to each other to generate consecutive scan lines of high density. 
     The most basic form of this invention is the combination of a beam splitter to break the single input beam into two beams which are separated by a predetermined distance and by a polarization of ninety degrees, an acousto-optic (A/O) modulator which modulates the two beams without a significant amount of crosstalk, and a beam recombiner which brings the beams back together to a separation of one scan line. Facet tracking is also accomplished in the modulator, i.e., the beam tracks the current facet as it swings through its arc. Finally, there is no interference between the two beams when they are ultimately brought together to a one scan line separation since they are polarized ninety degrees apart at that point. 
     One problem that remains is that in this configuration one beam travels a longer distance and therefore both beams will not focus at the same point. In this situation a birefringent waist location compensator can be used to bring the waists back together again. An alternative is to use a half wave plate between the modulator and the beam recombiner. In this case one beam is diverted in the beam splitter and the other is diverted in the recombiner. Therefore both beams will focus at the same plane. 
     All of the above embodiments require a polarization insensitive modulator. If a polarization sensitive modulator is used, the system will require a half wave plate for one beam between the splitter and the modulator, and another for the other beam between the modulator and the recombiner. In this configuration, the beams will have the same polarization at the modulator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the invention, reference may be had to the following detailed description of the invention in conjunction with the drawings wherein: 
     FIG. 1 shows an overall schematic diagram of one embodiment of the modulator. 
     FIG. 2 shows an embodiment having a waist compensator. 
     FIG. 3 shows an embodiment with equal beam path lengths, 
     FIG. 4 shows an embodiment which uses two half wave plates. 
     FIG. 5 is a diagram of an acousto-optic cell. 
     FIG. 6 is a schematic diagram of the A/o cell. 
     FIG. 7 is a schematic diagram of a beam splitter. 
     FIG. 8 is a schematic diagram of a beam recombiner. 
     FIG. 9 is a side view of the modulator. 
     FIG. 10 is a top view of the modulator. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is the most basic embodiment of the invention. A single beam 20 enters the polarizing beam splitter 21 which is made from a material with large birefringence such as calcite or rutile. This beam splitter 21 is used to split the one input beam from a polarized laser 20, polarized at 45 degrees with respect to the plane formed by the incident beam and the crystal axis, into two orthogonally polarized beams with a final exit separation distance equal to the distance between the two electrodes of the A/O or Bragg cell modulator 22. 
     The A/O modulator 22 has two functions, to provide equal facet tracking for both beams and to modulate the beams independently. The beam is turned on and off according to the video data stream input to its driver and will reach the printer photoconductive surface while tracking the current polygon facet. Finally the beams are brought together by a beam recombiner 23 to form two scan lines. 
     One problem with this configuration is that the upper beam 24 travels a longer (or shorter, depending on the crystal and its orientation) path than the lower beam 25. Therefore, the waist of the upper beam will occur to the left (right) of that of the lower beam 25. The waist is the narrowest portion of the beam. This waist shift can be corrected by using a waist compensator 26, as shown in FIG. 2. The waist compensator 26 has different light speed transmission as a function of polarization, and therefore will shift the waist of one beam with respect to the other. Therefore, both beams will focus to the narrowest spot possible at the same plane. 
     An embodiment that will not have to compensate for the waist shift is shown in FIG. 3. Here a half wave plate 27 is used between the A/O modulator 22 and the beam recombiner 23. The result is that both beam paths are of equal length. 
     A problem with all of the above embodiments is that a polarization insensitive A/O cell is preferred, thereby allowing the same amount of power to be used to modulate beams of any polarization, and such a cell material is not always available for the optical wavelengths and/or carrier frequencies required by the system. The embodiment shown in FIG. 4 equalizes the polarization of both beams prior to entering the A/O cell 22. Therefore, a polarization sensitive medium can be used. The polarization can be equalized by inserting a half wave plate 28 in the path of the lower beam 25 prior to the A/O cell 22 and a second half wave plate 29 in the path of the upper beam 24 after the A/O cell 22. Thus, the beams have the same polarization while passing through the A/O cell. The half wave plate 28,29 material can be, for example, crystal quartz designed for light of 6328 Angstroms. 
     As shown in FIG. 9, as the beam 40 exits the recombiner 23, it is parallel to, but displaced above, the system center line 41, but it is convenient in an optical system that the output beam is on line with the input. Therefore a sagital plate 42 is provided. This element is a rectangular piece of optical glass, fine annealed, grade A, and is adjusted to the angle required to bring the beam 40 down to the centerline 41. 
     In this system the beam is OFF when the modulator does not defract the beam, and the beam is ON when the defracted first order beam is directed exactly along the centerline 46. These relationships are shown in FIG. 10 which is a top view of the elements of FIG. 9 and which shows the collinearity wedge 45. This is an almost-rectangular piece of fine annealed, grade A, optical glass, in that one surface deviates from its rectangular form by one or several degrees, and is adjusted so that when the beam 43 is not deflected by the modulator it will be angled downward as shown. When properly adjusted, the first order beam 44 from the modulator will be aligned with the center line 46. 
     The A/O cell 22 in FIGS. 1-4 is shown in more detail in FIGS. 5 and 6. The bulk of the unit is an acousto-optic medium 36, attached to which is a piezoelectric acoustic transducer 34. The cell has two ground 37 and two rf 35 vacuum metalised gold electrodes. Two rf signals are applied by wires 33 between electrodes and ground, to form two acoustic fields 38 which modulate the beams and also provide facet tracking. There is a lens, not shown, ahead of the cell to focus the beams into the shape of the Gausian spots shown in FIG. 6 so that the entire beam is within the acoustic field. The wires are ultrasonically welded to the gold electrodes and are reinforced with beads of epoxy. The distance between electrodes must be at least several times the electrode width to provide non-interfering acoustic fields for each of the optical beams for independent modulation. The acousto-optic medium can be one of the varieties of optical grade glass or single crystals of TeO 2 , LiNbO 3 , PbMoO 4 , GaP, etc. The first order beam is the one used as the output to the remainder of the output scanner, so that if there is a field applied to the modulator, the beam is ON. When there is no acoustic field, the beam is OFF. In addition, the frequency of the applied rf is changed to change the deflection angle. This accomplishes facet tracking. 
     The beam splitter 21 of FIG. 1 is a polarizing beam splitter with preferably large birefringence such as calcite, rutile, etc. This unit is used to split one incident beam coming in at forty-five degrees with respect to the crystal axis into two orthogonally polarized beams with a final exit separation distance equal to the distance between the two electrodes on the Bragg cell, as illustrated in FIG. 7. 
     Placing the polarization angle of the input beam at 45 degrees with respect to the beam splitter will provide the effect of having equal energy in the horizontally and vertically polarized components of the beam. The energy in the two output beams will thus be equal, which is the desired condition. Circularly polarized light would have the same effect. 
     The same result could be obtained if an unpolarized source were to be used. Unfortunately, so called &#34;unpolarized lasers&#34; are not truly unpolarized, but generate orthogonally polarized beams that have somewhat random or &#34;uncontrolled&#34; polarization properties. The problem of using such a laser is that the intensities of the orthogonally polarized components do not maintain a constant ratio, but vary in what is commonly called &#34;polarization noise&#34;. This type of laser could still be used by alignment of the two polarization components at 45 degrees with respect to the beam splitter. Each of the two output beams will thus contain equal components of the polarization noise, which tend to cancel. However, a potential problem still exists in that without special precautions in the design of the laser, the angle of the two orthogonal components may change with time, and upset the noise cancellation effect. Since polarization noise and polarization angle exhibit good stability in polarized lasers, such lasers are presently preferred. 
     In order to achieve the proper alignment of the laser polarization angle, a means to rotate the laser in its mount can be provided. However, unless the laser rotation axis is concentric with the beam axis, rotating the laser will introduce beam misalignment. A more satisfactory method of aligning the polarization angle is by introducing a half wave or full wave linear retarder 30 into the laser beam preceding the beam splitter 21. Rotation of the half wave retarder will cause rotation of the polarization angle at twice the rate, i.e. a two degree rotation of the retarder will produce a four degree rotation of the polarization angle. A full wave retarder will produce rotation at the same rate. Since the intent is to provide an adjustment to closely balance the intensity of the two beams at the photoreceptor, other combinations of retarders, such as the liquid crystal polarization rotator, can be used to produce rotation of an elliptically polarized beam, thus providing a finer adjustment. 
     Another consideration is that the polygon has reflectance properties that vary in a different manner for the horizontally and vertically polarized light, as the angle of incidence changes with polygon rotation. Thus, the ratio of intensities of the two beams will vary along the scan line. If this variation can not be tolerated, then a quarter wave retarder 31 can be placed into the two beams, following the beam combiner 23, with its axis at a 45 degree angle relative to the axis of the beam recombiner. This serves to convert the two linear but orthogonally polarized beams into clockwise and counter-clockwise circularly polarized beams. Polygon rotation will affect both beams similarly, and the intensity ratio will remain constant throughout the scan. A halfwave retarder could also provide a similar result by aligning its axis at 22.5 degrees relative to the beam combiner axis. This arrangement will introduce a 45 degree rotation of the polarization angle of both beams, and again both beams will be affected in a similar manner by the polygon. 
     The beam recombiner 23 of FIG. 1 is used immediately after the two beams have been modulated-deflected. It has the same construction as the beam splitter 21 but is positioned in reverse, so that the two beams come together and two separate scan lines are formed. The beam recombiner is slightly shorter in length than the beam splitter 21 so that when the two beams exit the unit they are a small distance apart, equivalent to one scan line at the photoreceptor after imaging by the scanning optics. This is illustrated in FIG. 8. The distance between scan lines can be adjusted as shown, by adjusting the angle of the recombiner with respect to the beam. 
     While the invention has been described with reference to a specific embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention. In addition, many modifications may be made without departing from the essential teachings of the invention.