GB2094497A

GB2094497A - Optical mechanical semiconductor laser scanning system

Info

Publication number: GB2094497A
Application number: GB8205501A
Authority: GB
Original assignee: Benson Inc
Current assignee: Benson Inc
Priority date: 1981-02-25
Filing date: 1982-02-24
Publication date: 1982-09-15
Also published as: FR2500701B1; JPS57158827A; DE3206819A1; FR2500701A1; CA1178463A; NL8200557A; GB2094497B

Abstract

A system for reproducing a raster scanned line of an image on a photoconductive surface of a cylindrical drum (14), comprises a semiconductor laser monochromatic light source (11), preferably a GaAlAs diode, a means 16 for causing the intensity of the light beam to vary with time in accordance with the spatial distribution of the image along the raster scanned line, and means for reflecting said monochromatic light beam so as to direct the monochromatic light beam to and along a selected straight line on the photoconductive surface of the drum (14) e.g., a rotating holographic disk (12) containing a plurality of diffraction gratings therein (121-1, 121-2, 121-3, 121-4). The means for reflecting the monochromatic light beam may comprise an elliptical reflecting surface (13), a first mirror (15a) for reflecting the monochromatic light beam passed through the scanning disk to said elliptic reflecting surface, and a second mirror (15b) for receiving the monochromatic light beam reflected from the elliptical reflecting surface and for directing the reflected light beam onto a selected straight line on the photoconductive surface of the drum (14). The elliptic reflecting surface (13) can, with little loss in accuracy, be replaced by a circular cylindrical surface. <IMAGE>

Description

SPECIFICATION Improvements in or relating to a laser scanning system This invention relates to laser scanning systems, in particular those used for 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 wellknown manner.

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, United States Patent Specifications Nos. 4,230,902 and 3,951,509.

One problem in these prior 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 to 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 back 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 relatively inexpensive 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.

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 and 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 United States Patent Specification 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, the axis of the cone being perpendicular to the plane. (See, for example, United States Patent Specification 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 August 1 977 issue of "Applied Optics" (Vol.

16, Neo.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", October 15, 1978, Vol. 17, No. 20. See also United States Patent Specification No. 3,951,509 for a description of another technique.

The user 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 in, for example, U.S. Patent Specification 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 a need for a structure which will allow the use of a holographic scanning disk and a monochromatic light source for the purpose or reproducing a straight line image on a photoconductive cylindrical surface which is both inexpensive and accurate.

One aspect of the invention provides a system for reproducing a raster scanned line of an image on a photoconductive surface of a cylindrical drum, the system comprising: a semiconductor laser light source for emitting a monochromatic light beam; a means of causing the intensity of the said light beam to vary with time in accordance with the spatical distribution of the image along said raster scanned line; and a means for reflecting the light beam to and along a selected straight line on the said photoconductive surface of the drum.

Preferably, the system further comprises a means for modulating the intensity of the monochromatic light beam in response to stored information.

Suitably the said means for reflecting comprises a holographic disk having a plurality of diffraction gratings, and a means for rotating the said disk, the disk being so oriented as to allow the monochromatic light beam to pass through a selected diffraction grating in the disk during the formation of a raster scan line on the said photoconductive surface when the disk is being rotated.

Advantageously, the system further comprises a means for synchronizing the rotation of the drum with the rotation of the holographic disk so that the photoconductive surface is properly positioned for the light beam passed through the holographic disk at the start of a scan cycle to be incident on one edge of the said drum and to sweep gradually in a straight line across the surface of the drum parallel to the axis of rotation of the said drum as a selected diffraction grating on the holographic disk is rotated through the said light beam.

Conveniently, the said means for reflecting the monochromatic light beam comprises a reflecting surface.

Preferably, the said means for reflecting also comprises a first mirror for reflecting the light beam to the said reflecting surface; and a second mirror for reflecting the light beam reflected from the said reflecting surface onto a selected straight line on the said photoconductive surface.

Suitably, the said selected diffraction grating in the rotating holographic disk is located at a first focal axis of the said reflecting surface and the said selected straight line on the said photoconductive surface is located at a second focal axis of the said reflecting surface.

Conveniently, the said reflecting surface is elliptical.

Alternatively, the said reflecting surface comprises a circular cylindrical reflective surface closely approximating to the elliptical reflective surface required to produce a straight raster scan on the photoconductive surface of the drum.

Advantageously, the said light source is a GaAIAs laser diode.

Suitably, the means for reflecting also comprises a means for collimating the said light beam, a cylindrical lens, and a spherical lens, the cylindrical lens and the spherical lens being adapted to focus the light beam after collimation onto the surface of the holographic disk.

Conveniently, the cylindrical lens has its axis of rotation perpendicular to the junctions of the said GaAIAs diode.

Preferably, the said means for collimating comprises a lens having a numerical aperture structure of substantially 0.65 and a focal length of substantially 9 mm; the said cylindrical lens has a focal length of substantially 300 mm; and the said spherical lens has a focal length of substantially 1000 mm.

A second aspect of the invention provides a system for reproducing a raster scanned line of an image on a photoconductive surface of a cylindrical drum, the system comprising: a semiconductor laser light source for emitting a monochromatic light beam; a means for causing the intensity of the said light beam to vary with time in accordance with the spatial distribution of the image along said raster scanned line; and a means for reflecting the light beam to and along a selected straight line on the said photoconductive surface of the drum, the said means for reflecting comprising a first mirror for reflecting the said light beam to a reflecting surface for reflection therefrom as a secondary beam, and a second mirror for reflecting the said secondary light beam onto the said straight line.

A third aspect of the invention provides a system for reproducing a raster scanned line of an image on a photoconductive surface of a cylindrical drum, the system comprising: a semiconductor laser light source for emitting a monochromatic light beam; a means for causing the intensity of the said light beam to vary with time in accordance with the spatial distribution of the image along said raster scanned line; and a means for reflecting the light beam to and along a selected straight line on the said photoconductive surface of the drum, wherein the said means for reflecting comprises a hoiographic disk having a plurality of diffraction gratings, and a means for rotating the said disk, the disk being so oriented as to allow the monochromatic light beam to pass through a selected diffraction grating in the disk during the formation of a raster scan line on the said photoconductive surface when the disk is being rotated, and the means for reflecting also comprises a means for collimating the said light beam, a cylindrical lens, and a spherical lens, the cylindrical lens and the spherical lens being adapted to focus the light beam after collimation onto the surface of the holographic disk.

Thus it has been found possible to provide a structure which alters from a curved to a straight line a repetitively scanned raster line from a monochromatic light source which is passed through a holographic scanning disk, by employing a simple optical system for correcting the curvature of the monochromatic light beam passed through the holographic scanning disk to yield a straight line scan.

In a preferred embodiment of this invention, a monochromatic light beam passed through a holographic scanning disk containing a diffraction grating is reflected off the interior surface of an elliptical cylinder, one focal axis of which is the desired raster scan line on the cylindrical photoconductive surface of the drum, and the other focal axis of which passes through the diffraction zone on the holographic scanning disk.

It has been found that the monochromatic beam passed through the holographic scanning disk is thus correctly focused as a straight line on the cylindrical drum provided with the photoconductive material. This straight line is parallel to the rotational axis of the cylindrical drum.

In one embodiment, the surface of the elliptical cylinder can be replaced by a circular cylindrical surface, particularly if the optical system is so designed that the two focal points of the elliptical mirror are required to lie very close together. The resulting structure has been found to be easily manufactured and approximates to an elliptical cylinder of very slight eccentricity. The replacement of the elliptical mirror by a circular cylindrical mirror has been found to result in a structure which is relatively inexpensive to manufacture and which yields satisfactory accuracy for most applications.

In accordance with this invention it has been found possible to provide a system which uses low cost components, including a solid state injection laser and a holographic scanner, but which provides the resolution, accuracy and speed of a higher cost system using a standard collimated, monochromatic light beam from a gas laser whose power output is modulated by an external modulator of well-known design.

In one such embodiment, a laser system employs a solid state laser diode producing a divergent beam of non-circular cross section, together with an anamorphic optical system to produce a monochromatic light beam which is collimated at one point and which has a circular cross section at the image plane. The system further employs a holographic disk for directing a light beam in a circular arc across a plane and an elliptical mirror located at an appropriate point to correct the curved arc output from the holographic laser scanner to a straight line on the surface of a photoconductive drum.

So that this invention may be more readily understood and so that further features may be appreciated, apparatus in accordance with the invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 illustrates diagrammatically a system in accordance with this invention; Figure 2 shows schematically in plan view a typical holographic scanning disk forming part of the system of Figure 1; Figure 3 is a diagram showing the relationship between the light beam incident on the disk of Figure 2 and the light beam diffracted by the disk; Figure 4 is a schematic drawing of a low cost system for producing the laser scanning beam; Figure 5a illustrates a solid state laser diode used in the system of Figure 1;; Figures Sb and Sc illustrate the divergent angles of the light beam emitted by the diode of Figure 5a, parallel and perpendicular, respectively to the junction of the diode; and Figures 6a and 6b illustrate an optical system forming part of an embodiment of this invention.

As shown in Figure 1, a monochromatic light source 11 produces a monochromatic light beam 1 a which is directed toward the holographic scanning disk 12. The disk 12 is mounted to rotate about an axis 1 2a, and is driven by an electric motor of a well-known design in such a manner that the rotation of the disk 12 is synchronized with the readout of information used to modulate the monochromatic light beam 1 The information used to modulate light beam 11 a is derived from an information source 1 6 which is shown schematically only. The source 1 6 is of well-known construction and might, for example, comprise a computer memory and appropriate accessing and logic circuitry. The structure for modulating monochromatic light beam 11 a is also well-known in the art and thus will not be described.

The modulated light beam 11 a is passed through a diffraction grating (of well-known design) such as any one of the gratings 121-1 to 121 -M on the scanning disk 12 shown in Figure 2 (where "M" is selected integer equal to the number of diffraction gratings on the disk 12). In Figure 2, M is shown as four (4) but a typical disk will have many more gratings; for example, twelve are used in one embodiment. The rotational velocity of the disk 1 2 is so synchronized that any one of the diffraction gratings 1 21 -m (where "m" is an integer which can vary from 1 to M) first intercepts the monochromatic light beam 1 1a at the time this light beam is modulated with the first bit of information to be stored in a straight line on the photoconductive surface 1 4b of a photoconductive drum 14.The diffraction grating 1 21 -m then sweeps across the modulated light beam in synchrony with the moduiation of this beam (i.e. during the period in which the beam is modulated) so that the information in one raster line stored in the information source 1 6 is stored on a corresponding line on the photo-conductive surface 1 4b of the drum 14. The light beam 11b emerging from the grating would sweep in an arc across the surface 1 4b unless compensated.

Thus, to compensate, an elliptical mirror 13 is provided and so located that a first planar mirror 1 spa, located near one focal axis of the elliptical mirror 13, reflects the light beam 11 b from the grating 121 -m (which is at one reflected focal axis of the mirror 13) into path 11 c at the start of the scan. The light from the said path 11 C is reflected by the elliptical mirror 1 3 onto the second focal axis of the elliptical mirror 13 (located on a line on the photoconductive surface 1 4b of the drum 14) via a second planar mirror 1 sub. The light is thus reflected onto a particular straight line on the photoconductive surface 1 4b of the drum 14.As the disk 12 rotates, the light beam 1 1b emerging from the diffraction grating 1 21 -m continues to be reflected from the first planar mirror 1 5a to the surface of the elliptical mirror 13. Thus, at the middle of the scan, this light follows a line 11 d which reaches the top of the arc being formed by this light beam and is then reflected back along another line 11 fto the second planar mirror 1 5b and thence to the photoconductive surface 1 4b of the drum 14.

Thus the sweep of the beam 11 b emerging from the grating as reflected by the first planar mirror 1 5a across the surface of the elliptical mirror 13 in an arc has been converted to a substantially straight line formed along the photoconductive surface 1 4b of the drum 14. This line is parallel to the axis 1 4a of rotation of the drum 14, which axis 1 4a is in turn perpendicular to the plane of the drawing of the drum shown in Figure 1.

In one preferred embodiment the diffraction disk 12 is positioned non-perpendicularly to the incident light beam 11 a (see Figure 3). In this manner, the effect of disk wobble (a change in angle fl) on the beam 1 b diffracted from the disk 12 (a change in angle p) may be minimized.

Referring to Figure 3, it is well-known that: d(sin a+sin ss)=A (1) where d=the pitch of the diffraction gratings; =angle of incidence of light beam 1 1a; angle of diffraction of light beam 11 b; A=wavelength of the emitted light; and 0=a+ss, by definition.

Thus, sin +sin (0a)=A (2) d Utilizing a monochromatic light source and fixed diffraction gratings, d is constant.

By differentiating equation (2) with respect to a, and setting equal to zero, an angle a may be found where the effect of disk wobble (åS) is minimized: dO O=cos a+cos (S -cos (6-a) (3) da dO cos a-cos (0-a) ---- =0 -------------- =o (4) da -cos (B--a) cos a=cos (8--a) (5) 0=2 (6) In order to minize the effect of disk wobble (##), the system must be designed such that 0=2 or, in other words, a=ss.

The elliptical mirror 13 can, for simplicity, be replaced by a circular cylindrical mirror. Since a circle is merely an ellipse with equal major and minor axes, the use of a circular cylinder can, in some circumstances, sufficiently approximate an ellipse to allow satisfactory raster scanning of the image line on the photoconductive surface 1 4b. In this embodiment the accuracy with which the approximation is achieved depends upon the separation of the focal axes of the "ellipse'! and the desired linearity of each raster scan line formed on the photoconductive surface 1 4b of the cylinder 14.

The equation of an ellipse is given as:

The eccentricity of an ellipse is defined as:

is equal to the distance from the centre of the ellipse to either focus. The distance from one focus to a point on the ellipse to the other focus is equal to 2a. Thus, in a typical system where the distance from the centre of the ellipse to a focus is approximately 25 millimetres and the distance between foci through a point on the ellipse is approximately 1000 millimetres, the eccentricity is 0.05, or five percent. Thus, in this system, utilizing a circular cylindrical mirror (whose eccentricity is zero, by definition) as an approximation of an elliptical mirror will introduce an inaccuracy of at most only five percent at the extremities of the scan. This is sufficiently accurate for most uses of this apparatus.

In a preferred embodiment, a solid state laser diode 21, such as is shown schematically in Figures 5a, 5b and 5c, is used as the source of the monochromatic light beam 11 a, rather than a gas laser with external modulator. In this preferred embodiment the solid state injection laser diode comprises a GaAIAs laser diode 21 of well known construction containing a P-type region 22a, an N-type region 22b and a PN junction region 23 between the two. This diode produces a divergent beam of non-circular cross-section.The angle ss of divergence of the beam in a direction perpendicular to the plane of the junction (as shown in Figure 5c) is typically much greater than the angle a of divergence of the beam in a direction parallel to the plane of the junction (as shown in Figure 5b). For example, the divergence of the beam of the GaAIAs solid state diode 21 perpendicular to its junction is typically about 40 degrees while the divergence of the beam in the direction parallel to its junction is typically about 10 degrees. The angle of divergence of the beam is a function of the size of the laser diode junction 23 (as junction size increases, divergence decreases). The solid state laser diode typically has not been used as an optical source for a laser scanning system because it produces a noncircular, diverging beam.On the contrary, a gas laser produces a collimated beam of monochromatic light with a circular cross section and thus has been used for this purpose. The spot size of a beam produced from a laser diode is given by the following equation: 4# do= --- F # where do is the spot size, A is the wavelength of the light beam produced by the diode, and F is a measure of the divergence of the light beam equal to the ratio of the focal length to the diameter of the aperture through which the beam is passed.

With a solid state laser diode, the beam divergence of the system in the direction perpendicular to the junction differs from the divergence in the direction parallel to the junction.

In the apparatus being described, a solid state laser diode is rendered usable in a laser scanning system such as is used in a laser scanning printer by using a specially designed anamorphic lens system. In particular, the system employs two cylindrical lenses with different radii of curvature in mutually perpendicular planes. These cylindrical lenses are located in different positions in the optical path of the output beam.

The lens system used in this apparatus is shown in Fig. 6a looking along the plane of the PN junction 21 a of the laser diode 21 and in Figure 6b looking perpendicular to the plane of the PN junction 21 a of the laser diode 21. Thus, in Figure 6a, the solid state laser diode 21 produces a diverging output beam 38a shown to diverge with the angle P in a plane perpendicular to the plane of the PN junction 21 a and shown in Figure 6b to diverge with the angle a in the plane parallel to the PN junction 21 a.The beam 38a is then passed through a first lens 31 (which may be either spherical or anamorphic) providing a collimated beam 38b having a first diameter of 12 mm perpendicular to the PN junction of diode 21 and a second diameter of 4 mm in the direction parallel to the plane of the PN junction of diode 21. The beam 38b is then passed through a second, cylindrical, lens 32 and a third lens 33 (which may be either spherical or anamorphic) which are so oriented as to focus the beam 38d in a plane parallel to the PN junction of diode 21 onto and through a holographic scanning disk 34 of well known construction. The scanning disk 34 has formed in a well known manner on one surface a plurality of diffraction gratings.In one preferred embodiment, the scanning disk 34 contains a plurality of sets of diffraction gratings, each set containing a plurality of parallel diffraction gratings. The passage of the beam 38d through one set of diffraction gratings of the holographic disk 34 results in a diffracted beam of the first order being produced on the other side of the holographic disk 34. This beam is then rotated across a reflecting surface 35a on a cylindrical mirror 35 as the holographic disk 34 rotates about its X-axis. The resulting beam 38e, sweeps a point of circular cross-section in an arc over the reflecting surface 35a of the mirror 35. The reflecting surface 35a is preferably elliptical, but in many instances by proper selection of the focal lengths, can be made circular without significant loss of accuracy.

The beam passed through the holographic disk 34 is swept across the reflecting surface 35a from point 35b to point 35c. In sweeping this arc in a direction perpendicular to the junction 21a of diode 21, the beam moves vertically from points 35dto 35e, as shown in Figure 6b. Point 35e is the lowest point reached by the beam 38e, at the beginning and end of the scan, whereas point 35d is the highest point reached by this beam during the mid-point of the scan.

The beam 38f reflected from the reflecting surface 35a of the mirror 35 is then transmitted to a drum 36 upon a line parallel to the rotational axis Y of the drum 36 upon the surface of which a photoconductive coating is formed. The drum 36 is thus the recipient of a raster of lines of information represented by the intensity of the light beam 38f derived from the laser diode 21.

By modulating the current through the PN junction 21 a of this diode, the intensity of the light beam emitted by the laser diode 21 is varied in a well-known manner, thereby to vary the intensity of the beam 38fin a line on the drum 36.

The result is to store different levels of charge on the line on the drum 36. The drum 36 is then used to make a copy of the information used to modulate the output signal from the diode 21.

Figure 4 illustrates schematically the structure of an embodiment of this invention. As shown in Figure 4, a laser diode 111 produces an output beam which is passed through a collecting lens 112 which ccllects and collimates the output light from the diode 111. The output signal from the collecting lens 112 has an elliptical cross section because of the difference in the divergence angles of the light beam perpendicular and parallel to the junction of the laser. Taking advantage of this difference, the cylindrical lens 11 3, with its axis of rotation perpendicular to the junction, focuses the minor axis of the light onto a holographic scanner disk 11 5 while an anamorphic or spherical lens 114 focuses the major axis through the scanner onto the image plane 119.This allows the focused beam of the hologram to be located at one of the axes of an elliptical cylindrical mirror 11 6. The mirror 11 6 re-images the beam and corrects the arcuate scan generated by the parallel grating holographic scanner by imaging this beam to the other focal axis of the cylindrical mirror 116, thus reproducing a straight line raster scan which is sharply focused at the image plane.

As shown in Figure 4, the lens system associated with the solid state laser diode comprises several components. The collecting lens 112 has a numerical aperture structure of 0.65 and a focal length of 9 millimeters. From the collecting lens 112 (corresponding to the lens 31 shown schematically in Figure 6a and 6b), the beam is passed through the cylindrical lens 113 which focuses light only in the plane shown in the side view in Figure 6b. This lens has a focal length of 300 millimeters. The light beam from the cylindrical lens 113 (corresponding to the second lens 32 in Figures 6a and 6b) is then transmitted to a spherical lens 114 (corresponding to the third lens 33 in Figures 6a and 6b). The spherical lens 114 has a 1000 millimetres focal length. Thus the light from the spherical lens 114 is passed through the holographic scanner disk 115 which is rotated at a rate synchronized with the rate at which data are being read onto a line on the photoconductive surface 119 (corresponding to the photoconductive surface of the drum 36 in Figures 6a and 6b). This light then creates a first order diffraction beam which is reflected from a first flat mirror 118 (shown in Figure 4, but not shown in Figures 6a and 6b, for simplicity) and is then reflected off the elliptical cylindrical mirror 11 6 back to a second flat mirror 11 7 (shown in Figure 4 but not shown in Figures 6a and 6b, for simplicity) and then onto the photoconductive surface 11 9 (Figure 4) of the drum corresponding to the drum 36 in Figures 6a and 6b.

The scanning system of this apparatus has a large depth of focus thus eliminating the nebd for flat field correction. This large depth of focus is due to the large F of the system. As previously mentioned, F is a measure of the divergence of the light beam and is equal to the ratio of the focal length to the diameter of the aperture through which the beam is passed. Thus, in a first axis, the F of the light beam is equal to 1000 mm 12 mm (The spherical lens 114 has a 1 000 mm focal length and the light beam has a first diameter of 12 mm perpendicuiarto the PN junction of diode 21.) In the axis parallel to this first axis, the F of the light beam is equal to 300 mm ~75.

4 mm (The cylindrical lens 113 has a 300 mm focal length and the light beam has a second diameter of 4 mm in the direction parallel to the plane of the PN junction of diode 21).

To simplify the geometry of the elliptical cylindrical mirror 116, (corresponding to the cylindrical mirror 35 in Figures 6a and 6b), both focal axes are so chosen as to be close to each other so that the slightly eccentric elliptical cylinder is approximated by a circular cylinder.

This has been found to reduce the complexity of manufacture.

This system in accordance with the invention described above has been found to provide several advantages: 1. The scanner has a high duty cycle due to the small ratio of the width of the focussed beam on the hologram to the width of each facet of the hologram.

2. The scanner system is relatively insensitive to wobble or translation of the hologram disk due to the nature and positioning of the hologram and the re-imaging of the elliptical cylindrical mirror 116.

3. The scanner provides curved scan correction to a striaght line raster scan at the image plane.

4. The scanner has a large depth of focus, eliminating the need for flat field correction. (This is particularly important to allow a quality image to be reproduced on the image plane).

5. The scanner provides self-alignment and a fixed focal point of the scanned beam when the wave-length source is changed.

6. The scanner beam shapes for better image resolution.

7. The scanner system has a low cost and is easily fabricated.

Claims

1. A system for reproducing a raster scanned line of an image on a photoconductive surface of a cylindrical drum, the system comprising: a semiconductor laser light source for emitting a monochromatic light beam: a means for causing the intensity of the said light beam to vary with time in accordance with the spatial distribution of the image along said raster scanned line; and a means for reflecting the light beam to and along a selected straight line on the said photoconductive surface of the drum.

2. A structure as in claim 1 further including a means for modulating the intensity of the monochromatic light beam in response to stored information.

3. A system as in claim 1 or 2, wherein the said means for reflecting comprises a holographic disk having a plurality of diffraction gratings and a means for rotating the said disk, the disk being so oriented as to allow the monochromatic light beam to pass through a selected diffraction grating in the disk during the formation of a raster scan line on the said photo-conductive surface when the disk is being rotated.

4. A system as in claim 3 further including means for synchronizing the rotation of the drum 'with the rotation of the holographic disk so that the photoconductive surface is properly positioned for the light beam passed through the holographic disk at the start of a scan cycle to be incident on one edge of the said drum and to sweep gradually in a straight line across the surface of the drum parallel to the axis of rotation of the drum as a selected diffraction grating on the holographic disk is rotated through the said light beam.

5. A system as in any one of the preceding claims wherein the said means for reflecting the monochromatic light beam comprises a reflecting surface.

6. A system as in claim 5 wherein the said means for reflecting also comprises a first mirror for reflecting the light beam to the said reflecting surface, and a second mirror for reflecting the light beam reflected from the said reflecting surface onto a selected straight line on the said photoconductive surface.

7. A system as in claim 6 when dependent on claim 3, wherein the said selected diffracting grating in the rotating holographic disk is located at a first focal axis of the said reflecting surface and the said selected straight line on the said photoconductive surface is located at a second focal axis of the said reflecting surface.

8. A system as in any one of claims 5 to 7, wherein the said reflecting surface is elliptical.

9. A system as in any one of claims 5 to 7, wherein the said reflecting surface comprises a circular cylindrical reflecting surface closely approximating to the elliptic reflective surface required to produce a straight raster scan on the photoconductive surface of the drum.

10. A system as in any one of the preceding claims wherein the said light source is GaAIAs laser diode.

11. A system as in any one of the preceding claims wherein the means for reflecting also comprises a means for collimating the said light beam, a cylindrical lens, and a spherical lens, the cylindrical lens and the spherical lens being adapted to focus the light beam after collimation onto the surface of the holographic disk.

1 2. A system as in claim 11 when dependent on claim 10, wherein the cylindrical lens has its axis of rotation perpendicular to the junctions of the said GaAIAs diode.

13. A system as in claim 11 or 12, wherein the said means for collimating comprises a lens having a numerical aperture structure of substantially 0.65 and a focal length of substantially 9 mm; the said cylindrical lens has a focal length of substantially 300 mm; and the said spherical lens has a focal length of substantially 1000 mm.

1 4. A system for reproducing a raster scanned line of an image on a photoconductive surface of a cylindrical drum, the system comprising: a semiconductor laser light source for emitting a monochromatic light beam; a means for causing the intensity of the said light beam to vary with time in accordance with the spatial distribution of the image along said raster scanned line; and a means for reflecting the light beam to and along a selected straight line on the said photoconductive surface of the drum, the said means for reflecting comprising a first mirror for reflecting the said light beam to a reflecting surface for reflection therefrom as a secondary beam, and a second mirror for reflecting the said secondary light beam onto the said straight line.

1 5. A system as in claim 14 wherein the said means for reflecting comprises a holographic disk having a plurality of diffraction gratings, and a means for rotating the said disk the disk being so oriented as to allow the monochromatic light beam to pass through a selected diffraction grating in the disk during the formation of a raster scan line on the said photoconductive surface when the disk is being rotated.

1 6. A system as in claim 1 5 wherein the said selected diffraction grating in the rotating holographic disk is located at a first focal axis of the said reflecting surface and the said selected straight line on the said photoconductive surface is located at a second focal axis of the said reflecting surface.

1 7. A system as in claim 1 5 or 1 6 further including means for synchronizing the rotation of the drum with the rotation of the holographic disk so that the photoconductive surface is properly positioned for the light beam passed through the holographic disk at the start of a scan cycle to be incident on one edge of the said drum and to sweep gradually in a straight line across the surface of the drum parallel to the axis of rotation of the drum as a selected diffraction grating on the holographic disk is rotated through the said light beam.

1 8. A system as in any one of claims 14 to 17 further including a means for modulating the intensity of the monochromatic light beam in response to stored information.

19. A system as in any one of claims 14 to 18 wherein the said reflecting surface is elliptical.

20. A system as in any one of claims 14 to 18 wherein the said reflecting surface comprises a circular cylindrical reflective surface closely approximating to the elliptical reflective surface required to produce a straight raster scan on the photoconductive surface of the drum.

21. A system as in any one of claims 14 to 20 wherein the said light source is a GaAIAs laser diode.

22. A system as in any one of claims 14 to 21 wherein the means for reflecting also comprises a means for collimating the said light beam; a cylindrical lens, and a spherical lens, the cylindrical lens and the spherical lens being adapted to focus the light beam after the collimation onto the surface of said holographic disk.

23. A system as in claim 22 when dependent on claim 21 wherein the cylindrical lens has its axis of rotation perpendicular to the junctions of the said GaAIAs diode.

24. A system as in claim 22 or 23 wherein the said means for collimating comprises a lens having a numerical aperture structure of substantially 0.65 and a focal length of substantially 9 mm, the said cylindrical lens has a focal length of substantially 300 mm, and the said spherical lens has a focal length of substantially 1000 mm.

25. A system for reproducing a raster scanned line of an image on a photoconductive surface of a cylindrical drum, the system comprising: a semiconductor laser light source for emitting a monochromatic light beam, a means for causing the intensity of the said light beam to vary with time in accordance with the spatial distribution of the image along said raster scanned line, and a means for reflecting the light beam to and along a selected straight line on the said photoconductive surface of the drum, wherein the said means for reflecting comprises a holographic disk having a plurality of diffraction gratings, and a means for rotating the said disk, the disk being so oriented as to allow the monochromatic light beam to pass through a selected diffraction grating in the disk during the formation of a raster scan line on the said photoconductive surface when the disk is being rotated, and the means for reflecting also comprises a means for collimating the said light beam, a cylindrical lens, and a spherical lens, the cylindrical lens and the spherical lens being adapted to focus the light beam after collimation onto the surface of the holographic disk.

26. A system as in claim 25 wherein the said light source is a GaAIAs laser diode.

27. A system as in claim 26 wherein the cylindrical lens has its axis of rotation perpendicular to the junction of the said GaAIAs diode.

28. A system as in any one of claims 25 to 27 wherein the said means for collimating comprises a lens having a numerical aperture structure of substantially 0.65 and a focal length of substantially 9 mm, the said cylindrical lens has a focal length of substantially 300 mm, and the said spherical lens has a focal length of substantially 1000 mm.

29. A system as in any one of claims 25 to 28 further comprising a means for modulating the intensity of the monochromatic light beam in response to stored information.

30. A system as in any one of claims 25 to 29 further including means for synchronizing the rotation of the drum with the rotation of the holographic disk so that the photoconductive surface is properly positioned for the light beam passed through the holographic disk at the start of a scan cycle to be incident on one edge of the said drum and to sweep gradually in a straight line across the surface of the drum parallel to the axis of rotation of the drum as a selected diffraction grating on the holographic disk is rotated through the said light beam.

31. A system as in any one of claims 25 to 30 wherein the said means for reflecting the monochromatic light beam comprises a reflecting surface.

32. A system as in claim 31 wherein the said means for reflecting also comprises a first mirror for reflecting the light beam to the said reflecting surface, and a second mirror for reflecting the light beam reflected from the said reflecting surface onto a selected straight line on the said photoconductive surface.

33. A system as in claim 31 or 32 wherein the said reflecting surface is elliptical.

34. A system as in claim 31 or 32 wherein the said reflecting surface comprises a circular cylindrical reflective surface closely approximating to the elliptical reflective surface required to produce a straight raster scan on the photoconductive surface of the drum.

35. A system as in any one of claims 25 to 34 wherein the said selected diffraction grating in the rotating holographic disk is located at a first focal axis of the said reflecting surface and the said selected straight line on the said photoconductive surface is located at a second focal axis of the said reflecting surface.

36. A system according to claim 1 substantially as described herein with reference to, and as shown in, Figures 1 to 3 of the accompanying drawings.

37. A system according to claim 1 substantially as described herein with reference to, and as shown in, Figures 1 to 3 as modified by Figures 5a,5b and Sc of the accompanying drawings.

38. A system according to claim 1 substantially as described with reference to Figures 1 to 3 as modified by Figures 6a and 6b of the accompanying drawings.

39. A system according to claim 14 substantially as described herein with reference to, and as shown in, Figures 1 to 3 of the accompanying drawings.

40. A system according to claim 14 substantially as described herein with reference to, and as shown in, Figures 1 to 3 as modified by Figures 5a,5b and Sc of the accompanying drawings.

41. A system according to claim 14 substantially as described herein with reference to, and as shown in, Figures 1 to 3 as modified by Figures 6a and 6b of the accompanying drawings.

42. A system according to claim 25 substantially as described herein with reference to, and as shown in, Figures 1 to 3 of the accompanying drawings.

43. A system according to claim 25 substantially as described herein with reference to, and as illustrated in, Figures 1 to 3 as modified by Figures 5a,5b and Sc of the accompanying drawings.

44. A system according to claim 25 substantially as described herein with reference to, and as shown in, Figures 1 to 3 as modified by Figures 6a and 6b of the accompanying drawings.

45. Any novel feature or combination of features described herein.