Optical scanners

Abstract: An optical scanning system (46) reduces wobble at a ends of a scan line (62) by twice reflecting a laser beam (48) off a facet (50) of a rotating polygon mirror (52), then passing the beam through a cylindrical scan lens system (58) which will focus the beam in the scan plane to the scan line, and then reflecting the beam off a cylindrical mirror (60) to focus the beam in the cross-scan plane to the scan line.

Description: This invention relates to an optical scanner utilizing a rotary polygon mirror to cause a beam of radiation to trace out a straight line on a scan plane. The present invention aims at reducing the tendency of the light spot to 'wobble', or depart from true linearity at the ends of the scan line. By 'polygon' in this specification is meant a body of polygonal cross-section having polished or mirrored planar facets. Many conventional raster optical scanners utilize a multi-faceted rotary polygon mirror as the scanning element. A collimated beam of light, as for example from a laser, strikes the facets of the rotating polygon, which causes the reflected light to revolve about an axis near the center of rotation of the polygon, and scan a straight line. This reflected light can be utilized to scan a document at the input end of an imaging system, or can be used to impinge upon a photosensitive medium, such as a xerographic drum, in the output mode. Many systems to overcome various optical and other distortions caused by rotating polygon mirror optical scanners are known. Bow is defined as an error in the optical scanning system caused by the beam not being exactly horizontal prior to striking a facet. The scan line deviates from a straight line and is bowed in the middle of the scan line. Wobble is caused by the facet not being exactly parallel to the vertical axis, thereby angling the beam reflected from the facet up or down a small amount. One known rotating polygon mirror optical scanner 10, is shown in Figure 1 of the accompanying drawings. A laser beam 12 strikes a facet 14 of the rotating polygon mirror 16 The laser beam is in the same horizontal plane with the facet and tangentially offset in the scan plane. By known lens elements (not shown), the laser beam 12 is collimated in the scan or horizontal plane, and converges in the cross-scan or vertical plane. The beam is focused in the cross-scan plane to form a horizontal line at the facet. The laser beam 12 reflected from the facet is still collimated in the scan plane and is now diverging in the cross-scan plane. The beam then passes through a scan lens 18. The scan lens may be cylindrical in the scan plane and plano in the cross-scan plane Thus, the scan lens converges the scan portion of the beam, focusing it on the scan line but allows the diverging cross-scan portion of the beam to pass through unaffected. The beam 12 then strikes the cylindrical wobble mirror 20 which reflects the beam to the scan line 22. The wobble mirror is positive and cylindrical in the cross-scan plane and flat in the scan plane. Thus, the wobble mirror converges the previously diverging cross-scan portion of the beam, focusing it on the scan line but allows the converging cross-scan portion of the beam focused by the scan lens on the scan line to pass through unaffected. The collimation, convergence, divergence and focus of the laser beam 12 unfolded in both scan and cross-scan plane is shown in Figure 2. In the scan plane, the laser beam 12 is collimated and is reflected off the rotating mirror facet 14 unaffected. The collimated beam is converged by the cylindrical portion of the scan lens 18, reflected off the wobble mirror 20 unaffected to focus along the scan line 22. In the cross-scan plane, the laser beam 12 is converging and focused along a line at the rotating mirror facet 14. The reflected diverging beam passes through the plano portion of the scan lens 18 unaffected but is converged by reflecting from the positive cylindrical wobble mirror 20 to focus along the scan line 22. Bow is caused by the beam not being exactly horizontal prior to striking the facet. In this known scanner, the beam is defined as horizontal, so there is no bow. However, there is wobble in this optical scanner, caused by the facet not being exactly parallel to the vertical axis. Therefore, the beam reflected off the facet is angled up or down a small amount. The first-order effect of this wobble can be removed by using the wobble mirror. Since, in the cross-scan plane or vertical direction, the beam is focused at the facet, and since the mirror will again focus the beam at the scan line, the angle, either up or down, will be removed. That is, if the beam is going upward a small amount, the mirror, by refocusing the beam, will bend it down an equal amount to strike the scan line at the same spot. What slight wobble remains in the optical scanner 10 is a second-order effect caused by focusing problems. First, the beam, vertically, cannot be focused at the facet along the entire scan line, since the field is curved by the difference in glass thickness as the beam scans through thinner and thicker portions of the scan lens. To the extent that the beam is out of focus as it hits the scan line, it will have a small wobble at the ends of the scan line. Another problem with this known scanner 10 is that some of the lens elements used to produce laser beam 12 are very expensive. Another factor in the high cost of optical scanner 10 is the high fabrication and assembly tolerances required for the optical scanning system to work efficiently and properly. It has been proposed that wobble be cancelled by a double reflection from the facets of the rotating polygon, such facets being parallel with the axis of rotation of the rotating polygon, see US-A-3,897,132. After a light beam normally incident on a facet is reflected by the facet, the light beam is returned to the same facet, by an arrangement of even-numbered mirror reflections, for a second reflection. Wobble introduced at the first facet reflection is cancelled by the second facet reflection. However, in such an optical scanning system, with prism facets parallel with the axis of rotation, that is having a zero draft angle, bow effects are excessive. US-A-4,898,437 discloses a rotating polygon mirror optical scanner 24 in Figure 3 for corrected wobble by a double reflection from the facet with the wobble correction being achieved without bow effects. A laser beam 26 strikes a facet 28 of the rotating polygon mirror 30. The beam is in the same vertical plane as the polygon axis, and only sagittally offset. The laser beam 26 is collimated in both the scan and cross-scan planes by lenses (not shown). Rather than having the plane of each facet parallel to the axis of rotation of the mirror, the facets of the mirror are tilted at an angle, 'draft angle', to the axis of rotation. The collimated beam strikes the facet 28 at a first angle of incidence and is reflected to first mirror 32. The beam is then reflected to second mirror 34 where it is reflected back toward facet 28 at a second angle of incidence. The beam is reflected from the facet to the scan lens 36. The first and second mirrors, as well as the facet, are all flat surfaces with no optical power. Thus the beam remains collimated. The beam 26 passes through the scan lens 36. The scan lens is spherical, focusing the beam in both the scan and cross-scan planes. Thus, the scan lens converges both the scan and cross-scan portion of the plane, focusing it along the scan line 38. The collimation, convergence and focus of the laser beam 26 unfolded in both scan and cross-scan plane is shown in Figure 4. In the scan plane, the laser beam 26 is collimated and is reflected off the rotating mirror facet 28, the first mirror 32, the second mirror 34 and the facet 28 again unaffected. The collimated beam is converged by the spherical scan lens 36 to focus along the scan line 38. In the cross-scan plane, the laser beam 26 is collimated and is reflected off the rotating mirror facet 28, the first mirror 32, the second mirror 34 and the facet 28 again unaffected. The collimated beam is converged by the spherical scan lens 36 to focus along the scan line 38. By two further reflections from flat mirrors, the light beam is made to reimpinge upon the same facet, correcting the first-order wobble but now with no effect on bow. Three design variables are used to reduce the effects of bow in the optical scanner 24; namely, the angle of incidence at the first facet reflection, the angle of incidence at the second facet reflection, and the draft angle of the facets of the rotating polygon mirror. The optical scanner of US-A-4,898,437 was developed to perform at resolution standards of scan lines of 12 spots per mm. The features of concern were linearity, no bow over the scan line, diffraction-limited optics, and wobble less than 10 percent of the spot size everywhere along the scan line. The current resolution standard is 16 spots per mm, but the design must be capable of being extended to 24 spots per mm, the next resolution standard. The features of concern are the same, but performance requirements have doubled. The optical scanner of US-A-4,898,437 will perform at 24 spots per mm in all aspects except wobble. As shown in Figure 5 (not drawn to scale), the scan line 40 deviates at the ends 42 from a straight line 44. At the ends of the scan lines, wobble from the higher orders is two to three times larger than with the known scanner having a cylindrical scan lens and a cylindrical wobble mirror. The wobble exceeds 10 percent of 0.15 mm. The end-of-scan-line wobble of the scanner of US-A-4,898,437 is a hard characteristic, which cannot be corrected by adjusting the elements of the optical scanning system. In WO - 90 / 03589 there is described an optical scanner including a rotating polygon, in which the post-polygon optics includes a lens with two spherical surfaces, a lens with orthogonal cylindrical surfaces, and a cylindrical mirror having power in the cross-scan direction. It is an object of this invention to reduce end-of-scan-lines wobble in a rotating polygon mirror optical scanner, while providing no wobble over the rest of the scan line and no bow over the scan line. In accordance with the present invention, there is provided an optical scanning system for reducing wobble at the ends of a scan line in a scan plane, comprising: a rotary polygon having a plurality of planar facets each inclined at an angle to the axis of rotation; means for directing a beam of collimated radiation at the polygon from a location offset to one side of the scan plane, so as to fall on the facets in turn; and, as consecutive items in the beam path: a mirror system for causing an even number of reflections of the beam after its first reflection from a facet before it falls on the same facet a second time; a cylindrical scan lens for focussing the beam in the scan plane after its second reflection from the same facet, and a cylindrical mirror for focussing the beam in a cross-scan plane on to the scan line after it has emerged from the cylindrical scan lens. In the system of the invention, a collimated laser beam is initially twice reflected off a draft-angled facet of a rotating polygon mirror, to eliminate bow and wobble. The still-collimated beam is first passed through a cylindrical scan lens system, which will focus the beam in the scan plane to the scan line, and then reflected off a cylindrical mirror to focus the beam in the cross-scan plane. The present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of a perspective elevation of a known rotating mirror scanner; Figure 2 is a schematic illustration of the unfolded beam in the scan and cross-scan planes travelling through the scanner of Figure 1; Figure 3 is a schematic illustration of a perspective elevation of another known rotating mirror scanner; Figure 4 is a schematic illustration of the unfolded beam in the scan and cross-scan planes travelling through the scanner of Figure 3; Figure 5 is a schematic illustration (not drawn to scale) of the scan line of the scanner of Figure 3 in comparison with a straight line; Figure 6 is a schematic illustration of a perspective elevation of the rotating mirror scanner of the present invention, and Figure 7 is a schematic illustration of the unfolded beam in the scan and cross-scan planes travelling through the scanner of Figure 6. Reference is now made to Figure 6, wherein there is disclosed an optical scanner 46 for reducing wobble at the ends of the scan lines. A laser beam 48 strikes a facet 50 of the rotating polygon mirror 52. The beam is in the same vertical plane as the polygon axis, and only sagittally offset. The laser beam 48 is preferably elliptical in cross-section with the major axis or wide portion of the beam in the scan plane, and the minor axis or narrow portion of the beam in the cross-scan plane, and collimated in both planes by lens elements (not shown) or as a natural result of the use of laser diodes to produce the beam. If not elliptical, the beam can be circular in section. The facets of the rotating mirror are tilted at an angle, the draft angle, to its axis of rotation. The collimated beam strikes the facet 50 at a first angle of incidence and is reflected to first mirror 54. The beam is then reflected to second mirror 56 where it is reflected back toward facet 50 at a second angle of incidence. The beam is reflected from the facet to the scan lens 58. Two flat mirrors are used in this embodiment of the invention, although the only requirement is that there be an arrangement of even-numbered mirror reflections between the beam striking the facet the first and the second time. The first and second mirrors, as well as the facet, are all flat surfaces with no optical power. Thus the beam remains collimated. The beam 48 passes through a cylindrical scan lens system 58 which consists of at least one cylindrical scan lens. The scan lenses are cylindrical in the scan plane and plano in the cross-scan plane. Thus, the scan lens converges the scan portion of the beam, focusing it on a scan line 62, but allows the collimated cross-scan portion of the beam to pass through unaffected. For the purposes of this embodiment of the invention, a single cylindrical scan lens is used. The beam 48 then strikes a cylindrical mirror 60 which reflects the beam to the scan line 62. The mirror is positive and cylindrical in the cross-scan plane, and flat in the scan plane. Thus, the cylindrical mirror converges the collimated cross-scan portion of the beam, focusing it on the scan line 62 but allowing the converging cross-scan portion of the beam focused by the scan lens on the scan line to pass through unaffected. Since the beam is focused to a point in both the scan and cross-scan planes at the scan line, the beam in the scan plane which is focused by the scan lens, and the beam in the cross-scan plane which is focused by the cylindrical mirror, have the same f/number, if the collimated beam at the polygon has an elliptical section and the distance between the scan lens and the cylindrical lens is chosen appropriately. The collimation, convergence and focus of the laser beam 48 unfolded in both scan and cross-scan plane is shown in Figure 7. In the scan plane, the laser beam 48 is collimated and is reflected off the rotating mirror facet 50, the first mirror 54, the second mirror 56 and the facet 50 again unaffected. The collimated beam is converged by the cylindrical portion of the scan lens 58, reflected off the cylindrical mirror 60 unaffected to focus along the scan line 62. In the cross-scan plane, the laser beam 48 is collimated and is reflected off the rotating mirror facet 50, the first mirror 54, the second mirror 56 and the facet 50, again unaffected. The collimated beam passes through the plano portion of the scan lens 58 unaffected but is converged by reflecting from the positive cylindrical mirror 60 to focus along the scan line 62. The cylindrical mirror 60 does not form an image of the facet at the scan line, as does the known cylindrical wobble mirror. The double facet reflection of the present invention corrects the wobble at the center of the scan line. The cylindrical mirror 60 reduces the wobble at the ends of the scan line. The known cylindrical wobble mirror has zero field curvature, no matter what its optical power, while the corresponding known cylinder lens has strong field curvature as a necessary result of the strong optical power needed to image the facet at the scan line. Field curvature becomes an important consideration when the spots per mm exceed 12. The lens components in US-A-4,898,437 were spherical, to focus both the scan and cross-scan planes at the scan line. The wobble at the ends of the scan line is because of the cross-scan field curvature of the spherical scan lens. To eliminate, or greatly reduce, the wobble at the ends of the scan line, the cylindrical scan lens S8 of the present invention is plano in the cross-scan plane and has no power. Even with no power in the cross-scan plane, the cylindrical scan lens can still create field curvature if the beam is diverging or converging in the cross-scan plane. Thus, the beam is collimated in the cross-scan plane when passing through the cylindrical scan lens and is unaffected by the lens. The beam is focused on the scan line by the cylindrical mirror. There are no other components with power in the cross-scan plane. The present invention uses a draft angle on the facets to take out bow, and uses reflections from two mirrors between reflections off the same facet to take out the first-order effect of wobble. The second-order effect of wobble is removed by not focusing the light beam at the facet. Since there is no focus at the facet, there is no second-order wobble caused by the focus. Wobble is reduced at the ends of the scan line by a five-fold factor.

Claims: An optical scanning system (46) for reducing wobble at the ends of a scan line (62) in a scan plane, comprising: a rotary polygon (52) having a plurality of planar facets (50) each inclined at an angle to the axis of rotation; means for directing a beam (48) of collimated radiation at the polygon from a location offset to one side of the scan plane, so as to fall on the facets in turn; and, as consecutive items in the beam path: a mirror system (54, 56) for causing an even number of reflections of the beam after its first reflection from a facet before it falls on the same facet a second time; a cylindrical scan lens (58) for focussing the beam in the scan plane after its second reflection from the same facet, and a cylindrical mirror (60) for focussing the beam in a cross-scan plane on to the scan line (62) after it has emerged from the cylindrical scan lens. The system as claimed in claim 1, in which the beam is elliptical in transverse cross-section. The system as claimed in claim 2, in which the elliptical beam has its major axis in the scan plane, and its minor axis in the cross-scan plane. The system as claimed in any preceding claim, in which the source of the beam is a laser diode. The system as claimed in any preceding claim, in which the cylindrical scan lens is cylindrical in the scan plane, and plano in the cross-scan plane. The system as claimed in any preceding claim, in which the cylindrical mirror is cylindrical in the cross-scan plane, and flat in the scan plane. The system as claimed in any preceding claim, in which the mirror system comprises two flat mirrors.

Inventors: BRUEGGEMANN HARRY P, BRUEGGEMANN, HARRY P.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1991
Main classification: G02B26
IPC codes: H04N1, G02B27, G02B26, G02B5