Optical unit for use in laser beam printer apparatus

Abstract: An optical unit (2) according to the invention includes a primary lens (22) for converging a laser beam L emitted from a semiconductor laser element (10), a cylindrical lens (26) for further converging the converged laser beam (L), and first and second plastic lenses (62, 64) having optical characteristics offsetting each other in at least one of first and second directions perpendicular to each other. By virtue of the primary lens (22) and cylindrical lens (26), devices included in the optical deflector on the front side thereof can be made small, and also the arrangement of the lenses can be simplified. Devices of the same on the rear side thereof incorporate lenses including a planar surface, which reduces the required manufacturing cost.

Description: This invention relates to an optical unit according to the pre-characterizing part of claim 1, e.g. for use in a laser beam printer apparatus. An optical unit of this type is known from EP-A-0 431 603. An optical unit for use in a laser beam printer has in general a semiconductor laser for generating light beams, a plurality of mirrors formed rotatable, an optical deflector for deflecting a light beam generated from the laser, and lenses for transforming into light beams having a desired beam spot size the light beam directly supplied from the laser and the light beam deflected by the deflector. The lenses are sorted into a first lens group, i.e., a pre-deflection-stage optical system and a second lens group, i.e., a post-deflection-stage system. The first lens group is interposed between the optical deflector and laser, and includes a glass or a plastic lens for transforming the light beam supplied from the laser, into a parallel beam. The optical deflector rotates the mirrors to thereby deflect the light beam toward the object or photoconductor. The second lens group includes a focusing lens for transforming the deflected light beam into a beam having a spot size most suitable for a laser beam printer apparatus, and an f lens for causing an angle , at which the beam is continuously reflected by the mirrors of the deflector, to be proportional to the position of the beam converged on the object in the main scanning direction. The light beam generated from the laser is transformed into a parallel beam via the first lens group serving as a collimating lens, and then is deflected by the optical deflector to a desired direction, i.e., the light beam is reflected in accordance with rotation of the mirrors. The deflected light beam is transformed by the f lens and focusing lens arranged in the second lens group into a beam of a desired shape, which converges onto a desired surface portion of the photoconductor. In the above-described optical unit, the laser is movable so that the center line of the light beam generated from the laser could be aligned with an optical axis defined between the laser and photoconductor. Further, at least one of the first and second lens groups incorporates a source-object adjusting mechanism for adjusting the length of an optical path between the laser and photoconductor. The source-object adjusting mechanism is provided for causing the light beam from the laser to converge onto the photoconductor even when the beam spot size of the light beam is varied due to a change in the central portion of the laser, in the focal length of each lens, or in temperature or humidity. In general, a mechanism of this type includes a lens for first transforming a laser beam, which should be finally converged, into a parallel beam, and a lens for transforming the parallel beam into a converged beam. That is, two or more lenses, which complimentarily act upon one another, for example, they have the same power but opposite polarities, are provided between the laser and photoconductor, thereby accurately converging the laser beam onto the photoconductor. However, if the lenses acting complementarily upon one another are interposed between the laser and optical deflector, i.e., the first lens group, adjustment is necessary so as to accurately converge the laser beam onto the photoconductor, which makes it complex to assemble the optical unit. Moreover, provision of complementary lenses inevitably makes the optical unit large. An optical unit incorporating such complementary lenses, in which a laser beam is guided to a deflector after it is transformed into a parallel beam, is disclosed, for example, in USP Nos. 4,643,516 and 4,715,699. It is the object of the invention to provide an optical unit which is easy to adjust a focal point at the time of assembly thereof, wherein the optical unit is of a compact size having a stable optical characteristics. To attain the objects, an optical unit comprises the features of claim 1. The laser beam generated from the laser is transformed into a beam converged in the sub and main scanning directions before it is deflected by the deflector. The converged laser beam is guided to the photoconductor through lenses having a power in only one of the main or sub scanning direction. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a schematic plan view, showing the optical path of an optical unit of the invention for use in a laser beam printer apparatus; Fig. 2A is a sectional view, showing the optical path of a laser beam taken along an optical axis extending from a semiconductor laser employed in the unit of Fig. 1 to an optical deflector employed in the same, and also taken in a sub-scanning plane perpendicular to a main-scanning plane; Fig. 2B is a sectional view, showing the optical path of the laser beam taken along an optical axis extending from the optical deflector to a photocell, and also taken in the sub-scanning plane perpendicular to the main-scanning plane; Fig. 3 is a perspective view, showing a structure fixing the semiconductor laser, a glass lens, and an aperture stop; Fig. 4 is a schematic view, showing the relationship in the position of the glass lens and that of the aperture stop shown in Fig. 3; Fig. 5 is a schematic plan view, useful in explaining how the optical path extends between the deflector and photoconductor in a case where only a first plastic lens is used in the optical unit; Figs. 6A and 6B show a change in the position of an image obtained by the structure shown in Fig. 5 -- Fig. 6A being a graph, indicating a change in the distance between the optical deflector and the photoconductor; Fig. 6B being a graph, indicating a change in the f characteristic on the photoconductor; and Figs. 7A and 7B show a change in the position of an image obtained by the structure shown in Fig. 1 -- Fig. 6A being a graph, indicating a change in the distance between the optical deflector and the photoconductor; Fig. 6B being a graph, indicating a change in the f characteristic on the photoconductor. The invention will be explained with reference to the accompanying drawings showing an embodiment thereof. As is shown in Figs. 1 - 2B, the optical unit 2 includes a semiconductor laser element (i.e., light source) 10 for generating a light beam L to an object 70. The optical unit 2 contains a first lens group (i.e., first optical means) 20 being arranged after the laser element 10, having a plurality of lenses, for transforming the beam spot size of the laser beam L into a desired size. The optical unit 2 has an optical deflector 40, arranged between the first lens group 20 and the object 70, for deflecting the transformed beam L to a desired direction. The optical unit 2 further includes a second lens group (i.e., second optical means) 60 arranged between the deflector 40 and the object 70, having a plurality of lenses, for aligning the deflected beam L on a photoconductor (i.e., object) 70, and also for adjusting the beam spot size to a predetermined value. The optical unit 2 further contains a monitor optical unit (not shown) for monitoring horizontal synchronization of the laser beam L occurring in a main scanning direction in which the laser beam L is deflected toward the photoconductor 70 through the first and second lens groups 20 and 60. The first lens group 20 has a primary lens 22 made from glass, e.g., BK 7, and coupled thereto at least one aspherical plastic lens serving as light-entrance and/or light-outgoing surfaces, an aperture stop 24, and a secondary lens 26 arranged in the order mentioned. The primary lens 22 converges the laser beam L in the main or sub scanning direction. The lens 22 is supported by a cylindrical lens barrel 30 secured to a lens housing 8, for example, as shown in Fig. 3. The aperture stop 24 is provided for keeping at a constant value the amount of the laser beam L supplied from the laser 10 to the photoconductor 70, and is secured at a location as shown in Fig. 4. The secondary lens 26 is a cylindrical lens having a power in the sub scanning direction only, and is secured at a desired portion of the housing 8, as shown in Fig. 3. The optical deflector 40 includes a polygonal mirror 42 having reflecting surfaces 42a - 42f and a motor 48. The deflector 40 deflects toward the photoconductor 70 the beam L whose beam spot size is transformed into a desired one by the first lens group 20. Specifically, the beam L is continuously reflected at different angles in accordance with rotation of the polygonal mirror 42. The reflecting surfaces 42a - 42f consist of planar mirrors having a high smoothness. The mirror 42 is secured to a shaft 48a of the motor 48 with a stop ring 44 and a leaf spring 46 interposed therebetween. The second lens group 60 includes a first plastic lens (tertiary lens) 62, a second plastic lens (fourth lens) 64, and a dust-proof cover glass 66 enclosing the above-described deflector, lenses, mirrors, etc. The first and second plastic lenses 62 and 64 each have at least one planar surface, and form an achromatic structure there between. These lenses 62 and 64 in combination have a power for converging the beam L, which is converged by the second lens 26 in the sub scanning direction only, also in the main scanning direction, and have a power for aligning on the photoconductor 70 the beam L which moves in accordance with the rotation of the deflector 42, and adjusting the beam spot size to a predetermined size at all locations corresponding to every rotational angle  of the reflecting surfaces 42a - 42f of the deflector 42. In other words, curvatures of the lenses 62 and 64 are determined so as to satisfy the equation h = f, where h represents a point of the light beam L which arrived at the photoconductor 70 (i.e., a distance between the optical axis 0 and the beam L on the photoconductor 70), and f the focal length of the combined structure of the lenses. In addition to the optical characteristic given by the equation h = f, the lenses 62 and 64 have the following optical characteristics: the lens 62 has a curvature determined by a part of a circumference of a circle having a predetermined radius, and the lens 64 has such an optical characteristic as can offset the optical characteristic given to the beam L by the lens 62 (that is, the curvature of the lens 64 increases as the distance from the optical axis 0 increases). This means that the above-described powers are imparted to the lenses 62 and 64, respectively, and hence the lenses can be easily formed by molding. Further, in general, a mirror (not shown) is interposed between the second plastic lens 64 of the second lens group 60 and the photoconductor 70, for bending the laser beam L to thereby reduce the size of the image of the optical unit 2. As is shown in Fig. 3, the first lens group 20 is constructed on the lens housing 8 as one piece. The primary lens 22 is received in the lens barrel 30. The barrel 30 is secured to a desired portion of the housing 8 by being urged against one end of the housing with a pressing element 32 formed of an elastic member such as a copper plate, a phosphor bronze spring, or a stainless spring, etc. The lens barrel 30 can be slightly moved by means of a dedicated tool, not shown (such as a micro head apparatus). A clearance between the barrel 30 and the housing 8 is filled with an adhesive F. The secondary lens 26 is fitted in a depression formed in that predetermined portion of the housing 8 through which the optical axis of the primary lens 22 extends, and the laser beam L will pass, and is attached to the depression by means of the adhesive F. The laser 10 is fixed to the laser holder 36 by means of the adhesive F. The holder 36 is movable in the direction perpendicular to the optical axis 0, and is secured to the housing 8 after the laser 10 is aligned with the optical axis 0. Fig. 4 shows the relationship between the primary lens 22 and aperture stop 24. In the figure, reference numeral 11 denotes the actual light-emission point of the laser 10, reference numeral 111 an imaginary light-emission point, and reference numeral 124 an imaginary position of the stop 24. It is known that the amount of the beam L passing through the stop 24 will be greatly changed if the light-emission point is moved to the imaginary point 111. If the stop 24 is located in the imaginary point 124 (which differs from the rear focal point of the primary lens 22), the amount of the beam L will be approx. half of that obtained in the actual case. If, on the other hand, the stop 24 is located in the rear focal point of the primary lens 22, the amount of the beam L will be kept at a constant value even if the light-emission point is moved. Then, the movement of the laser beam L in the optical unit shown in Figs. 1 - 4 will be explained. The laser beam L output from the laser 10 is transformed into a converged beam by the primary lens 22, and is guided to the secondary lens 26 after it is restricted to have a predetermined amount by the aperture stop 24. The secondary lens 26 is cylindrical, and has a power for acting in the sub scanning direction. The power is determined so that the beam L could have a minimum beam spot size on the mirror surfaces of the deflector 40 in the sub scanning direction. The laser beam L converged in the main scanning direction by the primary lens 22 is transformed by the secondary (cylindrical) lens 26 to have convergence also in the sub scanning direction, so as to have a minimum spot size on each mirror surface of the deflector 40. The laser beam L guided to the optical deflector 40 is reflected toward the first plastic lens 62 by the reflecting surfaces 42a - 42f at a constant angular speed. If a single lens 162 as shown in Fig. 5 is used to satisfy the equation h = f, (Note that "h" and "", which are shown in Fig. 1, are respectively indicated by "H" and "" in Fig. 5), that undesired characteristic known as distortion or field curvature which makes it difficult to obtain a minimum beam spot size may increase for the following reasons, as the distance h from the optical axis 0 increases, or as the reflection angle of the laser beam L formed on the reflecting surfaces 42a - 42f increases. In a case where a beam L deflected by a planar mirror surface 142 of an optical deflector 140 is converged by means of the single lens 162, the deviation d between the actual destination position (a set of actual destination points H) 172 of the converged beam L (i.e., the actual destination points H in which the beam L has a predetermined spot size) and the position of the photoconductor 170 increases, as the angle  formed between the optical axis 0, extending from the mirror surface 142 to the photoconductor 170, and the beam L deflected by the deflector increases. Accordingly, the above-described distortion and the degree of the field curvature increase (the actual destination position (the set of points H) 172 is shifted to the "- (minus)" side) increases as the angle  increases. Fig. 5 shows a case where the actual destination position 172 is closer to the deflector than the position of the photoconductor 170. In the invention, the distortion or field curve is reduced by using the first and second plastic lenses 62 and 64 in combination (as is aforementioned, at least one of the light-entrance surface and light-outgoing surface of each of the lenses 62 and 64 comprises a flat surface, and the lenses in combination have an achromatic function). Figs. 6A, 6B, 7A, and 7B show optical characteristics of the lenses 62 and 64. As is shown in Fig. 6A, where only the plastic lens 62 is used for passing the laser beam L deflected by the rotating polygonal mirror 42, the deviation d of the actual destination position (i.e. the distance from the position of the photoconductor 70) increases as the angle  increases. Further, as is shown in Fig. 6B, the above-described f characteristic is increased as the angle  increases. This means that the deviation of the actual destination position of the laser beam (i.e., one of the actual destination point H corresponding to the actual destination position 172 in Fig. 5) from the theoretical image-forming position determined by the equation h = f increases, and that the actual destination point H becomes larger than the theoretical destination point h (Fig. 1). As can be understood from Fig. 7A, the combination of the first and second plastic lenses 62 and 64 improves the optical characteristics shown in Figs. 6A and 6B, that is, it enables the deviation d to be substantially 0. Further, as can be understood from Fig. 7B, the combination provides the f characteristic which enables the actual destination point H to be equal to the theoretical destination point h. More specifically, the laser beam L reflected on the reflecting surfaces 42a - 42f is adjusted by the first plastic lens 62 such that a) the angle  at which the beam passes is proportional to a revolution of each surface 42a - 42f in the vicinity of the optical axis 0 in the main scanning direction, so as to have a predetermined spot size on the photoconductor 70 irrespective of the revolution of one of the surface; and that b ) the beam L is adjusted by the lens 62 at horizontal opposite end portions thereof to have an angle larger than the angle  in the case of a ) and to have a spot size larger than the predetermined size on the photoconductor 70 (i.e., the beam is transformed to converge into a point remoter from the lens 62 than the photoconductor 70, in other words, the laser beam L is supplied by the lens 62 with a relatively large field curvature and a relatively undesirable f characteristic. The laser beam L is then guided to the second plastic lens 64, where the optical characteristics of the beam are kept as they are in the vicinity of the optical axis 0, and the beam passing the horizontal opposite ends of the lens 64 is bent in the main scanning direction by a sufficient power at an angle smaller than the case in the vicinity of the optical axis 0. As a result, the beam L passes substantially linearly between the lens 64 and photoconductor 70, and is converged onto a desired portion of the photoconductor 70 with the predetermined spot size. Each time one scanning is performed in the main scanning direction, part of the beam L having passed the lens 64 is reflected toward a horizontal synchronization-detecting mirror (not shown), and is guided to a synchronization signal detector for detecting the horizontal synchronization of the beam L. The photoconductor 70 is rotated by a driving apparatus (not shown) in a predetermined direction, and the beam L for forming an image to be output from the laser beam printer apparatus is continuously radiated onto the outer peripheral surface of the photoconductor 70. An exposed image obtained by radiating the beam L onto the photoconductor 70 is developed by visible image-forming means (not shown), and then copied on a copying medium. Then, that spot size of the laser beam L which is obtained on the photoconductor 70 in a case where the second plastic lens 64 is arranged close to the photoconductor 70 will be explained. Since the optical unit 2 shown in Figs. 1 - 4 is used in a laser beam printer apparatus, etc., a fine powder such as a toner serving as the visible image-forming means for visualizing the laser beam L (exposed image) exists around the photoconductor 70. Accordingly, in the above case, the distance between the photoconductor 70 and second plastic lens 64 must be set to such a value so that it enables the beam L to be prevented from being diffused or diffracted by the toner even if the beam L has a sufficiently fine spot size, or enables the beam L to have a sufficient spot size even if the beam is interrupted by the toner. The diameter of the toner is, in general, approx. 10 - 20 µm, so that it is preferable to restrict variations of energy within a range of approx. 15% or less. If the energy distribution of the beam spot is in accordance with Gaussian distribution, then the energy variations can be restricted to 15% or less. Thus, in the embodiment, the spot size of the beam L to be radiated onto the photoconductor 70 is set to 100 - 200 pm. As is described above, in the invention, the laser beam generated from the semiconductor laser element is transformed into a beam having convergence in the main and sub scanning directions, before it is deflected by the deflector toward a desired direction. The converged laser beam is guided to the photoconductor via a pair of lenses each having a power in only one of the main and sub scanning directions. By virtue of this structure, it is easy to arrange the lenses so that they could transform the laser beam to have a desired spot size. The laser beam deflected by the optical deflector is further deflected by one of the lenses which is located upstream of the other, to advance at an angle  proportional to the rotational angle of the optical deflector in the vicinity of the optical axis, and is then guided toward the photoconductor to have a desired spot size on the photoconductor. On the other hand, the laser beam having passed the horizontal opposite ends of the first lens is deflected to advance at an angle larger than the rotational angle of the optical deflector, and is guided toward the photoconductor to have a spot size larger than the desired size on the photoconductor. Subsequently, the optical characteristics of the laser beam passing in the vicinity of the optical axis are maintained as they are by the other of the lenses, while the laser beam passing the horizontal opposite ends of the other lens is deflected with a sufficient power thereof at an angle smaller than the rotational angle of the optical deflector. Accordingly, the laser beam, which has a predetermined spot size irrespective of the rotational angle of the polygonal mirror, is converged onto a desired portion of the photoconductor, and forms aligned images with the lapse of time. The above-described structure utilizing planar surfaces as mirror surfaces facilitates the arrangement of the lenses.

Claims: An optical scanner unit (2) comprising: light beam generating means (10) for generating a light beam (L) to illuminate a recording medium (70); first optical means (20) for converging the light beam at first and second convergence ratios in a first direction and a second direction perpendicular to the first direction, said first optical means having a first lens (22) for converging the light beam and a second lens (26) for further converging the light beam; means (40) for linearly deflecting the light beam having passed the first optical means, said deflecting means having a plurality of rotating deflecting surfaces (42a-42f); and second optical means (60) for further converging the deflected light beam towards the recording medium (70) at convergence ratios in the first and second directions different from the convergence ratios provided by the first optical means; characterized in that said first lens (22) comprises a spherical glass lens and an aspherical plastic lens coupled thereto; said second lens (26) further converges the light beam converged by the first lens (22) in only one of the first and second directions; said second optical means (60) comprises a third lens (62) and a fourth lens (64) which are positioned so that the light beam deflected by the deflecting means (40) is further deflected by an angle (THETA) which is proportional to the angle by which the deflecting means deflects the light beam, wherein said third lens (62) is configured so that the deflected light beam from the deflecting means, after having passed the peripheral regions of said third lens, is deflected by an angle which is greater than said deflection angle of the deflecting means in order to provide for a predetermined spot size of the light beam on the recording medium, irrespective of the deflection angle of the deflecting means. The optical scanner unit (2) according to claim 1, characterized in that said first lens (22) is a converging lens, interposed between the light beam generating means (10) and the deflecting means (40), for converging the light beam in the first and second directions. The optical scanner unit (2) according to claim 1 or 2, characterized in that said second lens (26) comprises a cylindrical lens, interposed between the first lens (22) and the deflecting means (40). The optical scanner unit (2) according to claim 1, 2 or 3, characterized in that said third lens (62) is provided downstream of the deflecting means (40); and said fourth lens (64) is interposed between the third lens (62) and the recording medium (70), serves as substantially one lens together with the third lens (62), and has regions with refraction factors which increase in accordance with the distance from an optical axis. The optical scanner unit according to anyone of claims 1 to 4, characterized in that the first optical means (20) converges the light beam to a desired spot size on the surface of the deflecting means (40) and that the second optical means (60) focusses on the recording medium the light beam reflected from said surface of the deflecting means (40).

Inventors: OMURA KEN, YAMAGUCHI MASAO, OMURA, KEN, YAMAGUCHI, MASAO, OMURA, KEN, C/O INTELLECTUAL PROPERTY DIV., YAMAGUCHI, MASAO, C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1992
Main classification: G02B26
IPC codes: G02B26, G02B27