Abstract:
Method and apparatus for optical scanning includes a sliding bed and a center adjusting unit. The sliding bed predetermines a position of an optical element in a light axis direction. The optical element has at least one lens surface to be have been subjected to a precision-figure transferring and optically scanning at least one light beam generated by a plurality of light sources and deflected by a rotary mirror. The center adjusting unit moves the optical element to make a lateral center of a curvature radius of the optical element exactly on a light axis center. This paper also describes an optical element itself, a method and apparatus for positioning and fixing the optical element. This paper further describes a molding tool for making the optical element.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   The present divisional application claims the benefit of priority under 35 U.S.C. §120 to application Ser. No. 10/965,773, filed Oct. 18, 2004, and under 35 U.S.C. §119 from Japanese application No. 2003-358380, filed on Oct. 17, 2003, the entire contents of both are hereby incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical scanning apparatus, and more particularly to an optical scanning apparatus which uses a multi-layered optical element system. The present invention also relates to an optical element itself, a method and apparatus for positioning and fixing the optical element. The present invention further relates to a molding tool for making the optical element. 
   2. Discussion of the Background 
   Generally, an optical scanning system using a lens system for forming an image of multiple laser beams uses known techniques including a plastic-made image forming lens capable of being instantly multi-layered corresponding to the multiple laser beams and a method of making a multi-layer lens system. Also, an optical scanning apparatus using such method of making a multi-layer lens system is known. 
   In particular, there is an optical scanning apparatus employed in an multi-color image forming apparatus. In such multi-color image forming apparatus, a plurality of light sources generate laser beams which are in turn directed to respective photosensitive members via a deflection mechanism and an image focusing mechanism and form separate color images according to colors on the respective photosensitive members. 
   Further, there is an image forming apparatus such as a digital copier and a laser printer using four photosensitive members in response to increasing demands for a high speed processing and a high image quality. This image forming apparatus forms a color image as follows. The four photosensitive members are previously aligned in parallel to each other along in a direction of sheet transferring. The photosensitive members are exposed simultaneously to light beams generated in accordance with these four photosensitive members to form latent images. The latent images are developed with different color toners: yellow, magenta, cyan, and black toners. The developed toner image in different colors are in turn overlaid in synchronism with each other into a single full-color image. 
   The above-described various image forming apparatuses adopt a plurality of optical scanning mechanisms; however, the adaptation results in upsizing of the apparatus. To attempt to solve this issue, an optical scanning method and apparatus has been introduced. Such optical scanning method and apparatus uses a lens system including a plurality of image focusing lenses made in a multi-layered unified form; however, in this method and apparatus, positioning of such a lens system is not appropriately established. 
   SUMMARY OF THE INVENTION 
   This patent specification describes a novel optical scanning apparatus. In one example, a novel optical scanning apparatus includes a sliding bed and a center adjusting unit. The sliding bed is configured to predetermine a position of an optical element in a light axis direction. The optical element has at least one lens surface to be have been subjected to a precision-figure transferring and optically scanning at least one light beam generated by a plurality of light sources and deflected by a rotary mirror. The center adjusting unit is configured to move the optical element to make a lateral center of a curvature radius of the optical element exactly on a light axis center. 
   The center adjusting unit may include two arms symmetrically arranged relative to the light axis center and configured to engage with respective side ends of the optical element. 
   The center adjusting unit may further include a rotating member configured to move the two arms in synchronism with each other simultaneously at a common speed in opposite directions. 
   The rotating member may include an elastic layer wrapping around a circumferential surface of the rotating member. 
   The above-mentioned optical element positioning apparatus may further include stopper pins configured to restrict moving ranges of the arms. 
   The arms may include stopper ribs configured to engage with the stopper pins. 
   Each of the two arms may include a pressing portion using a shrank spring to predetermine a position of the optical element in a light axis direction. 
   This patent specification further describes a novel optical element positioning method. In one example, a novel optical element positioning method includes the steps of predetermining and moving. The predetermining step predetermines a position of an optical element in -a light axis direction, in which the optical element has at least one lens surface to be have been subjected to a precision figure transferring and optically scans at least one light beam generated by a plurality of light sources and deflected by a rotary mirror. The moving step moves the optical element to make a lateral center of a curvature radius of the optical element exactly on a light axis center. 
   This patent specification further describes a novel optical scanning device for optically scanning a plurality of light beams generated by at least one light source and deflected by a rotary mirror. In one example, a novel optical scanning device include a first lens surface and a rib. The first lens surface includes a surface having been subjected to a precision-figure transferring. The rib is formed at a circumferential surface edge of the first lens surface and including a rib portion at each lateral side of the circumferential surface edge configured to be used to determine a lateral position of the optical scanning device. 
   The rib portion may be projected from other portions of the rib. 
   The above-mentioned optical scanning device may further include first and second side surfaces and a second lens surface. The first side surface is immediately adjacent to the first lens surface and includes a plurality of first reference surfaces. The second side surface is immediately adjacent to the first lens surface and includes a plurality of second reference surfaces. The second lens surface is immediately adjacent to the first and second side surfaces and opposite to the first lens surface. 
   The first side surface may include a depression. 
   The depression may be formed by a block-separation figure forming process. 
   The plurality of first reference surfaces may be formed in a cylindrical projection shape. One of the plurality of first reference surfaces may be formed at an approximately center of the first side surface in a lateral direction close to the first lens surface, and at least two of the plurality of first reference surfaces may be formed at respective ends of the first side surface in the lateral direction. 
   The optical scanning device may be made of plastic material. 
   The optical scanning device may include an f-theta lens. 
   This patent specification further describes a novel optical scanning device for optically scanning a plurality of light beams generated by at least one light source and deflected by a rotary mirror. In one example, a novel optical scanning device includes a plurality of optical scanning elements. Each one of the plurality of optical scanning elements includes a first lens surface and a rib. The first lens surface includes a surface having been subjected to a precision-figure transferring. The rib is formed at a circumferential surface edge of the first lens surface and includes a rib portion at each lateral side of the circumferential surface edge configured to be used to determine a lateral position of the optical scanning device. 
   The rib portion may be projected from other portions of the rib. 
   Each of the plurality of optical scanning elements may further include first and second side surfaces and a second lens surface. The first side surface is immediately adjacent to the first lens surface and includes a plurality of first reference surfaces. The second side surface is immediately adjacent to the first lens surface and includes a plurality of second reference surfaces. The second lens surface is immediately adjacent to the first and second side surfaces and opposite to the first lens surface. 
   The first side surface may include a depression. 
   The depression may be formed by a block-separation figure forming process. 
   The plurality of first reference surfaces may be formed in a cylindrical projection shape. One of the plurality of first reference surfaces may be formed at an approximately center of the first side surface in a lateral direction close to the first lens surface, and at least two of the plurality of first reference surfaces may be formed at respective ends of the first side surface in the lateral direction. 
   The plurality of optical scanning elements may be formed in shapes equivalent to each other. 
   The plurality of optical scanning elements may be made of plastic material. 
   Each of the plurality of optical scanning elements may include an f-theta lens. 
   Each of the plurality of optical scanning elements may be fixed with an adhesive agent coated around a center area of the first side surface in the lateral direction except for an area of the depression. 
   The adhesive agent may include a ultra violet radiation curing adhesive agent. 
   The plurality of optical scanning elements may be fixed stepwise one to another into a multi-layered form. 
   The plurality of optical scanning elements may be fixed at one time into a multi-layered form. This patent specification further describes a novel optical scanning apparatus. In one example, a novel optical scanning apparatus includes a plurality of light sources, a rotary mirror, and an optical scanning device. The plurality of light sources generate a plurality of light beams. The rotary mirror is configured to deflect the plurality of light beams. The optical scanning device is configured to optically scan the plurality of light beams generated by the plurality of light sources and deflected by the rotary mirror. The optical scanning device includes a plurality of optical scanning elements. Each one of the plurality of optical scanning elements includes a first lens surface and a rib. The first lens surface includes a surface having been subjected to a precision-figure transferring. The rib is formed at a circumferential surface edge of the first lens surface and includes a rib portion at each lateral side of the circumferential surface edge configured to be used to determine a lateral position of the optical scanning device. 
   This patent specification further describes a novel plastic lens molding tool. In one example, a novel plastic lens molding tool includes at least two mirror surface blocks and at least two nested blocks. The at least two nested blocks are respectively arranged in contact with the at least two mirror surface blocks on a one-to-one basis to respectively form a reference rib groove on a top of a border surface between one of the at least two mirror surface blocks and one of the at least two nested blocks in contact to each other. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
       FIG. 1  is a schematic diagram for explaining an optical scanning apparatus according to an exemplary embodiment of the present invention; 
       FIG. 2  is a schematic diagram for explaining an optical scanning apparatus according to another exemplary embodiment of the present invention; 
       FIGS. 3 and 4  are schematic diagrams for explaining an optical scanning apparatus according to another exemplary embodiment of the present invention; 
       FIG. 5  is a schematic diagram for explaining a positioning tool engaged with an exemplary optical scanning device; 
       FIGS. 6 and 7  are schematic diagrams for explaining a centering mechanism included in the positioning tool of  FIG. 5 ; 
       FIGS. 8 and 9  are schematic diagrams for explaining the optical scanning device of  FIG. 5 ; 
       FIGS. 10 and 11  are schematic diagrams for explaining a plastic lens molding tool; 
       FIGS. 12 and 13  are schematic diagrams for explaining an exemplary procedure of determining the position of the optical scanning device with the positioning tool of  FIG. 5 ; and 
       FIGS. 14 and 15  are schematic diagrams for explaining a method of fixing the optical scanning device in a single-layer form and a multi-layered form using an adhesive agent. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, particularly to  FIG. 1 , an optical scanning apparatus according to an exemplary embodiment of the present invention is explained.  FIG. 1  illustrates the optical scanning apparatus  100  which is typically used in an image forming apparatus such as a digital copying machine, a laser printer, and so forth. 
   The optical scanning apparatus  100  of  FIG. 1  includes a case  1 , a laser diode  2 , a cylindrical lens  3 , a polygon scanner unit  4 , an optical element  5 , and a mirror  6 .  FIG. 1  also illustrates a photosensitive member  7  which is arranged outside the optical scanning apparatus  100  but within the image forming apparatus. This photosensitive member  7  is used as an image carrying member for carrying an image written by the optical scanning apparatus  100 . The optical element  5  is an f-theta lens, for example. 
   As illustrated in  FIG. 2 , it is possible to employ an optical system using two photosensitive members  7  in the image forming apparatus based on the structure of the optical scanning apparatus  100  of  FIG. 1 . Furthermore, another optical system employing more than two photosensitive members can be achievable. 
   The case  1  of the optical scanning apparatus  100  is made of resin material, for example, and accommodates various optical components therein including the laser diode  2 , the cylindrical lens  3 , the polygon scanner unit  4 , and the optical element  5 . The laser diode  2  emits laser light. The cylindrical lens  3  condenses the laser light emitted by the laser diode  2  and directs the laser light to the polygon scanner unit  4 . The polygon scanner unit  4  includes a rotary polygon mirror  4   a  which has a plurality of mirrors and is driven to rotate at a constant rotation speed so that laser light impinges on one of the plurality of mirrors is deflected at a constant angular velocity to form a scanning laser beam. The scanning laser beam generated by the deflection of the rotary polygon mirror  4   a  passes through the optical element  5  and is reflected by the mirrors  6  towards the photosensitive members  7 . 
   In the case of optical system illustrated in  FIG. 2 , a dual laser beam system is employed in which two scanning laser beams are generated. Each of the two laser beams is caused to irradiate a corresponding one of the photosensitive members  7  via the optical element  5  and the mirrors  6 . 
   Each one of the scanning laser beams impinging on the corresponding one of the photosensitive members  7  scans the surface of the photosensitive member  7  in a main scanning direction at a constant velocity by an action of the optical element  5 . 
   With reference to  FIGS. 3 and 4 , another optical scanning apparatus  200  is explained, which is used in a color image forming apparatus such as a digital color copying machine, a color laser printer, and so forth. 
   The optical scanning apparatus  200  of  FIG. 3  employs four laser beam scanning system for writing an image in color. As illustrated in  FIG. 3 , the optical scanning apparatus  200  includes a case  1   a , four combinations of the laser diode  2  and the cylindrical lens  3 , the polygon scanner unit  4 , two sets of the optical element  5  and the mirror  6 .  FIG. 3  also illustrates two sets of the photosensitive member  7  which is arranged outside the optical scanning apparatus  200  but within the color image forming apparatus. 
     FIG. 5  illustrates a positioning tool  11  for determining appropriate positions of the optical components such as the optical element  5  in the optical scanning apparatuses  100  and  200 . As illustrated in  FIG. 5 , the positioning tool  11  includes a centering mechanism  12  and arms  14  and  15 . The centering mechanism  12  is arranged between the arms  14  and  15  to form, as a whole, a U-like shape. Each of the arms  14  and  15  includes a projection portion  13  and a spring holder  16 . The projection portion  13  is formed on an end of each of the arms  14  and  15 . The spring holder  16  is formed at a side of the projection portion  13  and is configured to hold a spring  16   a  in a shrank state. Reference numeral  23  denotes a light axis center. 
   Further details of the centering mechanism  12  are explained with reference to  FIGS. 6 and 7 . As illustrated in  FIG. 6 , the centering mechanism  12  is formed approximately in a rectangular shape, and includes a slide rail  19 , a slide base, and a reference base  21 . The reference base  21  is used as a centering reference, and the slide rail  19  is arranged on and formed in one piece with the reference base  21 . As illustrated in  FIG. 7 , the slide rail  19  has a cross sectional profile of a reversed trapezoid with an upper base greater than a lower base. 
   The slide base  20  has a slide hollow  20   a  at an approximate center of the bottom and is movably mounted on the reference base  21  such that the slide hollow  20   a  is engaged with the slide rail  19 , and is therefore movable along the slide rail  19  in directions orthogonal to the surface of  FIG. 7 , that is, in directions parallel to the light axis center  23 . 
   As also illustrated in  FIG. 7 , the centering mechanism  12  further includes three reference pins  22  which are arranged underneath the reference base  21 . The three reference pins  22  have a cylindrical shape and are engaged with holes formed in the surface of the case  1  so that the centering mechanism  12  is appropriately positioned relative to the case  1 . One of the three reference pins  22  is arranged at a position close to the optical element  5  and at which it axis intersects the light axis center  23 . The rest of the three reference pins  22  are arranged at positions away from the optical element  5  and away from each other relative to the light axis center  23  and at which their axes are in parallel to the light axis center  23 . 
   The slide base  20  is provided with a cylindrically-shaped rotating member  18  to an inner wall thereof at a position corresponding to a center area of the slide rail  19 , and such rotating member  18  includes an outer elastic member  18   a  arranged around a perimeter of the rotating member  18 . Furthermore, the rotating member  18  and the outer elastic member  18   a  have a common axis arranged on the light axis center  23 . 
   As illustrated in  FIG. 6 , the rotating member  18  is sandwiched between the arms  14  and  15  via the outer elastic member  18   a . The arms  14  and  15  are provided with stopper ribs  14   a  and  15   a , respectively, to their ends. The slide base  20  is provided with a pair of stopper pins  17  to sandwich each of the stopper ribs  14   a  and  15   a , as illustrated in  FIG. 6 . 
   There is a mechanical relationship between the arms  14  and  15  and the elastic member  18   a  such that a driving force is transmitted by a friction force. Namely, when the rotating member  18  is rotated together with the elastic member  18   a , the arms  14  and  15  are caused to slide in lateral directions opposite to each other. In this process, the lateral movements of the arms  14  and  15  are restricted by the stopper ribs  17 . 
   As described above, this positioning tool  11  has an exemplary structure in which the arms  14  and  15  are configured to engage with lateral edges of an optical element (e.g., the optical element  5 ) and to laterally slide in the opposite directions in synchronism with each other through the rotating member  18 . 
   With reference to  FIGS. 8 and 9 , the optical element  5  according to an exemplary embodiment of the present invention, which is the plastic lens, is explained more in detail. As illustrated in  FIGS. 8 and 9 , the optical element  5  includes lens surfaces  25   a  and  25   b  and side surfaces  26   a  and  26   b . The lens surfaces  25   a  and  25   b  and the side surfaces  26   a  and  26   b  are those subjected to a highly-precision figure transferring in a plastic molding process and those which light (e.g., the laser beam) enters and exits. The lens surface  25   b , from which the laser beam exits, is curbed so that the center portion of the lens has the greatest wall thickness. 
   The side surfaces  26   a  and  26   b  are arranged in parallel to a direction that the laser beams pass through. The side surface  26   a  is provided with first reference surfaces  27 , as illustrated in  FIG. 8 , formed in a cylindrical shape slightly projecting from the side surface  26   a  at an approximately center in the lateral direction close to the curbed lens surface  25   b  and at left and right ends in the lateral direction. The side surface  26   b  includes a second reference surface  30   a  and a depression  30   b . The second reference surface  30   a  is made in a highly precision manner; however, the depression  30   b  is referred to as an imperfect figure-transferred surface and is not made in a highly precision manner. 
   The optical element  5  is provided with an extended portion  28  at each one of the left and right ends in the lateral direction. The optical element  5  is further provided with a third reference surface  24  formed underneath each of the extended portions  28 . The third reference surfaces  24  are used to determine the fixing positions of the optical element  5 , together with the first reference surfaces  27 . 
   The curbed lens surface  25   b  is provided with long ribs  29  and short ribs  29   a  along its circumferential edges. These ribs  29  and  29   a  are used to determine the fixing position of the optical element  5  in a highly accurate manner. 
   To form an optical element having an imperfect-figure-transfer surface with a depression, a plastic molding process generally uses a slidable cavity block. In such process, a pair of molding tools are prepared, in which at least one cavity is formed using figure-transfer surfaces and several cavity blocks. 
   This process is conducted as follows. The molding tools are maintained, by an application of heat, at a predetermined temperature lower than a temperature at which plastics are softened. Then, a plastic pressure is generated at the transfer surfaces so as to make the plastic closely contacted the transfer surfaces. After that, the plastic is cooled down to a temperature lower than the softening temperature. At this time, the slidable cavity block is slid away from the plastic filled inside the cavity, so that a space is forcibly formed between the slidable cavity block and the plastic. This step is referred to as a block-separation figure forming. As a result, the surface of the plastic exposed to the space is provided with a depression according to the cooling process. 
   With reference to  FIGS. 10 and 11 , a mirror surface block and a nested block of a plastic molding tool used in the plastic molding process according to an exemplary embodiment of the present invention are explained. In  FIGS. 10 and 11 , reference numeral  31  denotes a nested block, reference numeral  32  denotes a nested line, reference numeral  33  denotes a mirror surface block, and reference numeral  34  denotes a reference rib groove. The nested line  32  is a contact surface (i.e., a border surface) of the mirror surface block  33  and the nested block  31 . As illustrated in an enlarged view of  FIG. 11 , the mirror surface block  33  and the nested block  31  are made to form the reference rib groove  34  at an end of the nested line  32  so as to produce the optical element  5  that has transfer surfaces to which a curvature-radius center position of the mirror surface block  33  is transferred in a high precision manner. 
   Referring to  FIGS. 12 and 13 , an exemplary procedure of the positioning operation with respect to the optical element  5  is explained.  FIG. 12  illustrates an enlarged view of the engagement between the arm  14  and the optical element  5 .  FIG. 13  illustrates a movement of the arms  14  and  15 , engaged with the optical element  5 , from the side edges towards the light axis center  23  of the optical element  5 . As a preparatory step, the optical element  5  is placed on the case  1  and the positioning tool  11  is also placed on the case  1  such that the reference pins  22  provided underneath the reference base  21  are engaged with reference holes (not shown) of the case  1 . 
   As illustrated in  FIG. 12 , the centering mechanism  12  is moved towards the optical element  5  by using the slide rail  19  until the spring holders  16  of the arms  14  and  15  push the extended portions  28  of optical element  5  so that the third reference surface  24  of each of the extended portions  28  contacts a positioning rib  37  provided to the case  1 . Thereby, a direction of beam pass is determined. 
   Then, as illustrated in  FIG. 12 , the projection portions  13  of the arms  14  and  15  are caused to contact the short ribs  29   a . When one of the arms  14  and  15  is moved towards the light axis center  23 , the arm&#39;s sliding movement is transmitted to the rotating member  18  and is converted into a rotating movement. This rotating movement causes the other arm to slide in an opposite direction, as indicated by arrows in  FIG. 13 . Thus, via the rotating member  18 , the arms  14  and  15  can be slid simultaneously at the same speed. As a result, the optical element  5  can be positioned with the center of the curvature-radius in the lateral direction exactly on the light axis center  23 . In this way, the positioning with respect to the scanning direction can be achieved in a highly accurate manner. 
   The outer elastic member  18   a  provided to the rotating member  18  facilitates the manipulation of the arms  14  and  15  at the simultaneous and common speed. This leads to a reduction of time to adjust the position of the optical element  5 . 
   In a case a plurality of optical elements (e.g., the optical element  5 ) are arranged in a multi-layered shape, the molding tool is arranged to have the reference rib groove  34  formed between the mirror surface block  33  and the nested block  31 , as illustrated in  FIG. 11 , so as to make the optical element  5  in which the center position of the curvature radius is transferred in a high precision manner. With this structure, the center position of the curvature radius of the optical element  5  can be determined in a highly precision manner by using the same projection portion  13 , regardless of an element layer number. In addition, since the optical elements  5  used in the multi-layer element have the same shape, the positioning can easily be conducted, regardless of an element layer number. 
   Referring to  FIGS. 14 and 15 , an exemplary way of coating an adhesive agent to the side surface  26  of the optical element  5  is explained. As illustrated in  FIG. 14 , the optical element  5  is coated with an adhesive agent  38  at a center area of the side surface  26   b  in the lateral direction. The adhesive agent  38  is a UV (ultra violet) radiation curing adhesive. The optical element  5  may thermally be expanded due to an environmental temperature, which expansion will change an optical characteristic such as an image forming characteristic of the optical element  5 . Therefore, the adhesive agent  38  is coated on a limited area, namely the center area of the side surface  26  in the lateral direction so as not to suffer from the thermal expansion. 
   After having coated with the adhesive agent  38 , a light source  39  is set, as illustrated in  FIG. 15 , and the adhesive agent  38  is exposed to a UV radiation from the light source  39 . As a result, the adhesive agent  38  is cured and the optical element  5  is securely adhered to the case  1  of the optical scanning apparatus  100 , for example. This adhering process can be used either the single layer type or the multi-layer type of the optical element  5 . 
   Since the above adhering fixes the optical element  5  with the center area of the side surface  26   b  in the lateral direction, deviations of the positioning of the optical element  5  with respect to the lateral direction and the light axis direction can be suppressed. 
   In the case of multi-layered optical element fixed with the adhesive agent  38 , since every optical element has the same thermal expansion coefficient, deviations of the positioning of the optical elements with respect to the lateral direction and the light axis direction can be suppressed. 
   As described above, the center position of the curvature radius of the mirror surface block  33  can be accurately transferred due to the reference rib groove  34  formed between the mirror surface block  33  and the nested block  31 . At a time of adhering, the adhesive agent  38  is previously coated to the adhering surfaces of the bottom optical element  5  and the case  1  and also the adhering surfaces of the upper optical elements  5 . Then, the positioning is made with respect to the center position of the curvature radius in the lateral direction of the lens surface  25   b  and at the same time the position of the multi-layered optical element. Thereby, the multi-layered optical element can be fixed in a highly precision manner. 
   Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein.