WO2005076053A1

WO2005076053A1 - Scanner systems

Info

Publication number: WO2005076053A1
Application number: PCT/EP2005/050514
Authority: WO
Inventors: David K. Towner; Michael Plotkin; Haim Livne
Original assignee: Hewlett-Packard Indigo B.V.
Priority date: 2004-02-06
Filing date: 2005-02-07
Publication date: 2005-08-18
Also published as: WO2005076053A9

Abstract

A scanner system including a photoreceptor, a plurality of laser beams, a scanning reflecting surface that receives the plurality of beams and directs them as scanning beams toward the photoreceptor and an aperture adjacent to the reflecting surface. As a single beam scans along the photoreceptor surface, a focal surface of the beam has a given deviation as a function of the scan position. The positions of the various beams at the reflecting surface are different due to a placement of the aperture and additional deviation of the plurality of beams in the focal surface caused by said placement is less than the given deviation.

Description

SCANNER SYSTEMS FIELD OF THE INVENTION The present invention is in the field of imaging and in particular deals with methods and apparatus for scanning. BACKGROUND OF THE INVENTION In laser printers, a modulated light beam (or a plurality of beams) scans the surface of a moving photosensitive element such as a rotating cylindrical photoreceptor to produce an image. The light beam scans the photosensitive surface in a scan direction that is generally perpendicular to the direction of surface motion to produce each scan line of the image. For a scanning system comprising a rotating cylindrical photoreceptor, the cylinder rotates so that each scan line falls on a different azimuthal location on the surface. In some scanning systems, multiple scan lines are scanned simultaneously by a plurality of beams. One very common scanning system utilizes scanning reflecting surface such as a rotating reflecting polygon to cause the beam to scan across the moving photoreceptor surface. Before reaching the rotating polygon and between the polygon and the photoreceplor surface the beam passes through an optical system which, inter alia, focuses the beam onto the photoreceptor surface and converts the beam motion from a constant angular velocity scan at the polygon to a constant linear velocity scan across the photoreceptor. Mirrors or prisms are often used to "fold" the optical system so that it is more compact and or to control the placement of the scanned beam on the photoreceptor. The structure of such a system is relatively straightforward when a single beam is scanned across the photoreceptor. It may not be possible with current technology, however, to simultaneously meet the system requirements for laser power, scan velocity, modulation rate and polygon speed in a high speed printer using a ^"single scanning beam, especially if the print resolution is also high. Thus, high speed, high resolution, printers generally utilize multiple scanning beams. Figs. 1A and IB show a prior art multi-beam scanning system 10. The view is perpendicular to the scan plane. The described system is used in the H-P Indigo 3000 Press. In this scanning system, a multi-beam laser source 12 emits a plurality of beams of laser light which are collimated by a collimator 13 and size limited by an aperture stop 20. In the described system multi-beam laser source 12 comprises a linear array of twelve laser sources 23 as shown in Fig. 1C and thus produces twelve beams which propagate through the optical system and produce twelve focused spots on the surface of photoreceptor 50 in Figs. 1A and IB. Since the focused spots on the photoreceptor surface are further apart than the distance between scan lines, the multi-beam laser source is oriented at an angle to the scan direction "SCAN". For ease of visualization, in Fig. 1A, only the central rays 14, 16 and 18, often referred to as "chief rays", from three of the beams are shown. In Fig. IB, three rays 15, 16 and 17 from the same beam are shown. Ray 16 is the same central ray indicated by reference label 16 in Fig. 1A, while rays 15 and 17 are "marginal rays" which define the edges of the beam. Aperture stop 20 limits the beams so that all of the beams have the same cross-sectional size and shape at the aperture. Aperture stop 20 also defines the chief ray and the marginal rays for each laser source, the chief ray being the ray passing through the center of the aperture stop and the marginal rays being the rays which just touch the aperture stop. Although all twelve beams are spatially superimposed at the aperture stop, each beam passes through the aperture at a slightly different angle, because the respective sources are at different positions in the object field of the collimator. After passing through aperture stop 20, the beams are optionally shaped by an optional anamorphic element, such as an anamorphic prism assembly 22. The beams then pass through an optional polarizing beam splitter (PBS) 24. The PBS assures that each beam is linearly polarized with a constant polarization direction in order to achieve accurate and stable beam power measurement. En practice, the individual beams emitted by multi-beam laser source 12 may have slightly different polarizations which may also vary slightly with time and operating conditions. To reduce the overall size of the scanning system 10, a 180° turning prism 26 and one or more mirrors (28 and 30 are shown) are optionally provided. In an embodiment of the invention, mirror 30 is a beam splitter. A small amount of the beam power passes through beam splitter 30 to a power sensor 32, which is used to control the beam power in a closed loop system (not shown). Other layouts, utilizing a different number of reflectors and different reflector arrangements can be used, to suit the available space. Two relay lenses 34 and 36 are situated between prism 26 and mirror 30 to form an afocal pupil relay. Because the pupil relay of scanning system 10 is afocal, the relay preserves the collimation of the input beams, Le., the beams leaving lens 36 are collimated, as are the beams entering lens 34.

A cylindrical lens 42 shapes the beam so that it is focused in the cross-scan direction on the surface 38 of a polygon 40 to desensitize scan line placement in printed output to polygon wobble or facet-to-facet tilt error, as is well known in the ait. The pupil relay images aperture stop 20 onto the reflecting surface 38 of rotating polygon 40. Only a single position of polygon 40, corresponding to a beam position at one end of the scan, is shown in Figs. 1A and IB. As the polygon rotates in the direction shown by arrow 48, the beams scan across the surface of a photoreceptor 50 from position "A" to position "B". The laser beams, after reflection from polygon surface 38 pass through f-? lens system 44 and are optionally reflected from an optional long fold mirror (shown as a line at reference 46), such as for example shown in US patent 5,268,687, to Peled et al, or as described in a PCT patent application titled "High Performance Dynamic Mirror" filed on February 5, 2004, in the Israel Receiving office and designating the United States, the disclosures of both of which are incorporated herein by reference. The purposes of optional mirror 46 are to control the position of the scan lines on a photoreceptor surface and to fold the optical system to reduce the size of Ihe scanning system 10. However, since this mirror does not affect Ihe performance of the optics which are the subject of the present discussion, the presence of the mirror is ignored in further discussion. As indicated in Fig. 1A, at reference A, the spots formed by beams containing central rays 14, 16 and 18 are slightly offset in the scan direction (corresponding to die image of the offset in the "scan" direction on the source (see Fig. 1 C). As the polygon rotates, each of the beams scans the length of the photoreceptor with a slight delay between the times they reach the same position in the scan direction. Each of the laser sources emitting the beams is modulated by data that takes account of this delay. SUMMARY OF THE INVENTION In an aspect of the invention an aperture stop is placed, in the pre-polygon beam path, near the polygon surface. This aperture stop replaces the pupil located at the polygon surface in the above described prior art system. The focus of a scanning beam moves along a focal surface as the beam is scanned across the photoreceptor (e.g., a photosensitive drum). In general, the focal surface is curved rather than straight and does not coincide with the straight surface of the photoreceptor. Instead, the distance between the focal surface and the photoreceptor surface (referred to herein as focal plane deviation) varies along the scan due to residual field curvature in the optical design as well as manufacturing tolerances of the optics and especially of the f-? lens. This focal plane deviation reduces the allowable focus error, also called "depth of focus", of the system by an amount equal to the deviation. For any beam traversing the same optical path, the focal plane deviation is substantially the same. A primary purpose of the pupil relay in the prior art system of Fig. 1, is to image aperture stop 20 onto the reflecting surface 38 of rotating polygon 40. Such an image of an aperture stop is customarily referred to as a "pupil". Because a pupil is image conjugate to the aperture stop in an optical system, rays pass through a pupil in very nearly the same way they pass through the aperture stop itself. Consequently, a pupil can serve many of the optical functions of an aperture stop, with the advantage of not requiring a physical element. In the prior art system of Fig. 1, the position and size of the beams are the same for all the beams at the polygon face because the polygon face is at a pupil location. If the position of the beams at the polygon were different, then the paths of the beams through the f-? lens would also be different and the focal surface curvature would be different for the different beams. ^"When the beams do not have the same position at Ihe polygon, focal surface offsets and tilts generally result among the focal surfaces scanned by each of the beams. This variation in focal surface curvature from beam to beam would result in a substantial reduction in the depth of focus of the system. Under these circumstances the term "focal plane deviation" includes said offsets and tilts. Although providing a pupil at d e polygon surface miiiirnizes focal plane deviation, it requires the use of a relay lens system and a much longer optical path. Furthermore, the requirement for producing a pupil on the polygon surface makes adjustment of the system more complex. The present inventors have found that when an aperture is spaced from the polygon then there is an increase in focal plane deviation, as expected, and a corresponding reduction in the effective depth of focus for the reasons given above. Although this non-ideal placement of the aperture stop has been regarded as unsuitable for use in a multi-beam scanner, the present inventors have found that if the distance between the aperture stop and the polygon surface is reduced to less than approximately 50 mm (more preferably less than 40 mm and most preferably less than approximately 20 mm), the focal plane deviation between the various beams are less, in fact substantially less, than the focal plane deviation for a particular beam as it scans across the photoreceptor. For example, the increase in focal plane deviation can be as low as 75%, 50%, 25% or even as low as 10% or less of the "normal" focal plane deviation for a single beam. In an exemplary embodiment of the invention, the nominal beam path incident on the polygon is in the nominal scanning plane, Le., the plane perpendicular to the axis of rotation of the polygon, also containing the normals to the polygon faces and the reflected scanning beam. However, this orientation limits how near the aperture stop can be to the polygon, since the aperture stop cannot be permitted to block the reflected beams (the scanning beams directed towards the photoreceptor). In an alternative embodiment of the invention, the beam path is at an angle to the plane perpendicular to the axis of rotation of the polygon. This allows the aperture stop to be closer to the polygon surface without interfering with the reflected beams. There is thus provided, in accordance with an embodiment of the invention, a scanner system comprising: a photoreceptor; a plurality of laser beams; a scanning reflecting surface that receives the plurality of beams and directs them as scanning beams toward the photoreceptor; and an aperture adjacent to the reflecting surface, wherein, as a single beam scans along the photoreceptor surface, a focal surface of the beam has a given deviation as a function of the scan position; and wherein, die positions of the various beams at die reflecting surface are different due to a placement of the aperture; and wherein additional deviation of the plurality of beams in the focal surface caused by said placement is less than the given deviation. Optionally, the additional deviation is less than or equal to 75%, 50%, 25% or 10% of the given deviation. Optionally, the aperture is less than or equal to 50, 40, 20 or 10 mm from the reflecting surface. In an embodiment of the invention, the aperture is other than round. Optionally, the scanner includes a collimator that collimates the beams after they leave a source of the beams.

Optionally, the locus of marginal rays at the collimator has substantially the same shape as a clear aperture of the collimator. Optionally, the locus of marginal rays at the collimator is incrementally smaller than the size of the collimator clear aperture. Optionally, the scanner

includes an anamorphic element between the collimator and the aperture. Optionally, the aperture has an elliptic shape. In an embodiment of the invention, the scanning reflecting surface is the surface of a rotating polygon. BRIEF DESCRIPTION OF THE DRAWINGS The following non-limiting description of exemplary embodiments of the invention should be read in conjunction with the attached figures. In the drawings similar elements in the various embodiments are designated by the same reference numbers. Fig. 1A is a planarized optical layout of a multi-beam scanner, showing the central rays of three of the separate beams, in accordance with the prior art. Fig. IB is a planarized optical layout identical to Fig. 1A showing the central ray and two marginal rays of a single beam; Fig. 1C is a schematic showing a layout of laser source positions for a multi-element laser source; and Fig. 2 is a planarized optical layout of a multi-beam scanner, in accordance with an embodiment of the invention, showing the central rays of three of the beams. DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS Fig. 2 shows a planarized optical layout in the scan plane of a scanner 100 in accordance with an embodiment of d e invention. In Fig. 2, only die central rays of the three beams are shown, for clarity of presentation. As in the embodiment of Fig. 1, a multi-beam laser source 12 emits a plurality of beams of laser light which are collimated by a collimator 20. The beams are optionally shaped by an optional anamorphic element, such as an anamorphic prism 22. The beams are then reflected by mirror 30 and pas^'s through an optional polarizing beam splitter (PBS) 24. Cylindrical lens 42 has the same function as the like element in Fig. 1. Note that since the path is much shorter, only a single folding mirror 30 is used. Aperture stop 102 is placed as close to polygon 40 as practical. The proximity of the aperture stop to the polygon is limited primarily by the need to maintain an unobstructed path for the scanning beams after reflection from the polygon. Generally, the aperture stop can be placed as close to polygon surface 38 as 10 to 20 mm. However, in many situations it can be as far as 40 or 50 mm or more from the polygon surface. While these values were determined for a scan width of 320 mm on the photoreceptor and for the scanner geometry shown in Fig. 2, they are believed to be generally correct for other scanners. These allowable distances may differ for different designs and for a particular design allowable distances can be determined using optics design programs or experimentally. Generally, for a given design, the allowable distances scale approximately linearly with size. If a closer placement is necessary to meet the conditions described below, or if physical constraints require it, the beams can be incident on surface 38 from a direction outside the plane of rotation of the polygon. Thus, the beams reflected from the polygon will pass above or below the aperture stop. In general, especially when an anamorphic or astigmatic element is present in the beam path, the aperture stop is optionally not round. Rather is may have a substantially elliptical shape to maximize utilization of the collimator optics. In order to describe the conditions under which the aperture stop and the collimator are said to be "matched", the term "marginal ray" is used to describe any ray that grazes or just touches the aperture stop. At the aperture stop, the marginal rays simply delineate the perimeter of the aperture opening. When these rays are traced backward through the optical system to the collimator, they establish the clear aperture requirements for the collimator optics. Optionally, the locus of marginal rays for all beams at the output side of the collimator has substantially the same shape as the clear aperture of the collimator, where "clear aperture" denotes the largest opening through which the collimator is capable of transmitting light from the multi-element laser source. Optionally, alternatively or additionally the locus of marginal rays for all beams at the collimator is incrementally smaller than the clear aperture of the collimator. The first of these conditions allows for optimal utilization of the clear aperture of the collimator optics. The second of these conditions avoids vignetting by the collimator optics. When both conditions are met, the collimator and the aperture stop are matched. Optimally, the position of aperture stop 102 should be close enough to surface 38 so that the overall focal plane deviations are not increased by more than 10%. However, the inventors have found that if the increase is less than 25% or even less than 50%, in some cases, the increased focal plane deviations still allow for enough margin to achieve focal spot size and spot size uniformity requirements. The invention has been described in the context of the best mode for carrying it out. It should be understood that not all features shown in the drawing or described in the associated text may be present in an actual device, in accordance with some embodiments of the invention. Furthermore, variations on the method and apparatus shown are included within Ihe scope of the invention, which is limited only by the claims. Also, features of one embodiment may be provided in conjunction with features of a different embodiment of the invention. As used herein, the terms "have", "include" and "comprise" or their conjugates mean "including but not limited to."

Claims

1. A scanner system comprising: a photoreceptor; a plurality of laser beams; a scanning reflecting surface that receives the plurality of beams and directs them as scanning beams toward the photoreceptor; and an aperture adjacent to the reflecting surface, wherein, as a single beam scans along the photoreceptor surface, a focal surface of the beam has a given deviation as a function of the scan position; and wherein, the positions of the various beams at the reflecting surface are different due to a placement of the aperture; and wherein additional deviation of the plurality of beams in the focal surface caused by said placement is less than the given deviation.

2. A scanner system according to claim 1 wherein the additional deviation is less than 75% of the given deviation.

3. A scanner system according to claim 1 wherein the additional deviation is less than 50% of the given deviation.

4. A scanner system according to claim 1 wherein the additional deviation is less than 25% of the given deviation.

5. A scanner system according to claim 1 wherein the additional deviation is less than 10% of the given deviation.

6. A scanner according to any of the preceding claims wherein the aperture is less than or equal to 50 mm from the reflecting surface.

7. A scanner according to claim 6 wherein the aperture is less than or equal to 40 mm from die reflecting surface.

8. A scanner according to claim 6 wherein the aperture is less that or equal to 20 mm from the reflecting surface.

9. A scanner according to claim 6 wherein the aperture is less that or equal to 10 mm from the reflecting surface.

10. A scanner according to any of the preceding claims wherein the aperture is other than round.

11. A scanner according to claim 10 wherein the scanner includes a collimator that collimates the beams after they leave a source of the beams.

12. A scanner according to claim 10 or claim 11 wherein the locus of marginal rays at the collimator has substantially the same shape as a clear aperture of the collimator.

13. A scanner according to claim 12 wherein the locus of marginal rays at the collimator is incrementally smaller than the size of the collimator clear aperture.

14. A scanner according to claim 12 or claim 13 wherein die scanner includes an anamorphic element between the collimator and the aperture.

15. A scanner according to claim 10 wherein the aperture has an elliptic shape.

16. A scanner according' 'to any of the preceding claims wherein the scanning reflecting surface is the surface of a rotating polygon.