Patent Publication Number: US-6343092-B1

Title: Light source device including semiconductor lasers

Description:
This application is a Continuation of application Ser. No. 09/055,902 Filed on Apr. 7, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a light source device for use in an image forming apparatus and including a semiconductor laser, particularly a plurality of semiconductor lasers. 
     A light source device including a semiconductor laser and a collimator lens is extensively used in a digital copier, laser printer, facsimile apparatus or similar image forming apparatus. As for optical characteristics, the light source device is required to have the directionality (optical axis characteristic) and parallelism (collimation characteristic) of a laser beam to issue from the device. To satisfy such characteristics, it is a common practice to adjust the emission point of the laser and the collimator lens relative to each other in the directions of three axes x, y and z. The required positional accuracy is less than the order of micron. Therefore, the light source device of the type described should be adjustable in position in the three directions x, y and z and should be fixed in place at its adjusted position. 
     Adhesive used to affix the collimator lens contracts during curing. It is therefore necessary to reduce the adverse influence of the contraction on the optical characteristics. Particuarly, the accuracy of the light source device is severely restricted in the direction z (optical axis direction), so that the device must be so constructed as to obviate the contraction in the direction z. It is therefore preferable that the adhesive layer be substantially parallel to the optical axis or z axis, and that the contraction be limited to one of the x axis direction and y axis direction in order to facilitate adjustment. 
     Light source devices using a semiconductor laser and a collimator lens are taught in, e.g., Japanese Patent Laid-Open Publication Nos. 5-88061, 5-136952, and 5-273483. The conventional light source devices, however, have some problems left unsolved, as follows. 
     (1) The devices are expensive because they need a number of parts. 
     (2) Displacements occur in the three directions x, y and z during assembly, lowering the accuracy of directionality of the laser. 
     (3) To affix the collimator lens, use cannot be made of photo-curable adhesive capable of curing rapidly in a desired configuration and having high reliability. 
     On the other hand, a multibeam scanning device capable of scanning a photoconductive element with a plurality of laser beams is available with a digital copier, laser printer or similar image forming apparatus. This type of scanning device includes a plurality of semiconductor lasers arranged in the subscanning direction. Laser beams issuing from the lasers are so combined as to lie on optical axes adjoining each other, and then output in one direction. A light source device for use in such a scanning device is, of course, required to have an accurate beam pitch, i.e., distance between the laser beams in the direction y. 
     Light source devices for emitting a plurality of laser beams are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 7-181410 and 7-181412. However, the devices taught in these documents have the following problem (4) in addition to the problems (1)-(3). 
     (4) The beam pitch of the semiconductor lasers cannot be accurately maintained due to the variation of temperature around the light source device. 
     The conventional light source devices have problems to be discussed later in addition to the problems (1)-(4). 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a cost-effective and accurate light source device needing a minimum number of parts, obviating displacements during assembly, and allowing collimator lenses to be affixed by photo-curable adhesive. 
     It is another object of the present invention to provide a light source device allowing a minimum of variation to occur in the distance between semiconductor lasers in the beam pitch direction even when temperature around the device varies. 
     In accordance with the present invention, a light source device includes a base formed with a plurality of through bores. A plurality of semiconductor lasers are positioned at the rear side of the base, and each is received in the respective through bore of the base. A plurality of collimator lenses are respectively adhered to a plurality of lens support portions formed on the front side of the base. The collimator lenses each is positioned coaxially with the optical axis of the respective semiconductor laser. A plurality of apertures each shapes a laser beam to issue from the respective collimator lens. A beam combining optical element combines laser beams to respectively issue from the semiconductor lasers to thereby output laser beams lying substantially on a single optical axis. The lens support portions each has a center line extending substantially perpendicularly to a beam pitch direction of the laser beams output from the collimator lenses. 
     Also, in accordance with the present invention, a light source device is made up of a plurality of semiconductor lasers, a base formed with through bores for respectively press-fitting the semiconductor lasers, a plurality of lens support portions formed on the base, a plurality of collimator lenses respectively adhered to the lens support portions via adhesive layers, an optical element for combining laser beams output from the collimator lenses to thereby produce beams adjoining each other, and a case mounted to the base for covering the collimator lenses and optical element. The adhesive layers each has a center thereof shifted outward of the respective bore in a beam pitch direction, so that the adhesive layers each thermally expands inward. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become apparent from the following detailed description taken with the accompanying drawings in which: 
     FIG. 1 is a section showing a conventional light source device; 
     FIG. 2 is an exploded perspective view showing another conventional light source device; 
     FIG. 3 is an exploded perspective view showing a first embodiment of the light source device in accordance with the present invention; 
     FIG. 4 is a front view showing lens support portions included in the first embodiment for supporting collimator lenses; 
     FIG. 5 is an exploded perspective view showing a second embodiment of the present invention; 
     FIG. 6 is a front view showing lens support portions included in the second embodiment for supporting collimator lenses; 
     FIG. 7 is an exploded perspective view showing a third embodiment of the present invention; 
     FIG. 8 is a front view showing a method of affixing collimator lenses to a base included in the third embodiment; 
     FIG. 9A is a perspective view showing a base representative of a fourth embodiment of the present invention; 
     FIG. 9B is a front view of the base of FIG. 9A; 
     FIG. 10 is a front view showing a modification of the fourth embodiment; 
     FIG. 11 is an exploded perspective view showing a fifth embodiment of the present invention; 
     FIG. 12 is an exploded perspective view showing a sixth embodiment of the present invention; 
     FIG. 13 is a perspective view of the sixth embodiment; 
     FIG. 14 is a front view of the sixth embodiment; 
     FIG. 15 is an exploded perspective view showing a seventh embodiment of the present invention; 
     FIG. 16 is a perspective view showing a base included in the seventh embodiment; 
     FIG. 17 is a front view of the base of FIG. 16; 
     FIG. 18 is an exploded perspective view showing an eighth embodiment of the present invention; 
     FIG. 19 is a perspective view showing a base included in the eighth embodiment; 
     FIG. 20 is a front view of base shown in FIG. 19; 
     FIG. 21 is a vertical section showing a ninth embodiment of the present invention; 
     FIG. 22 is an exploded perspective view showing a tenth embodiment of the present invention; 
     FIG. 23 is a perspective view of a base included in the tenth embodiment; 
     FIG. 24 is a front view of the base of the tenth embodiment; 
     FIG. 25 is a partly taken away bottom view of the base included in the tenth embodiment; 
     FIG. 26 is an exploded perspective view showing in eleventh embodiment of the present invention; 
     FIG. 27 is a perspective view showing a twelfth embodiment of the present invention; 
     FIGS. 28A and 28B each shows a specific arrangement of notches formed in a base included in the twelfth embodiment; 
     FIG. 29 is a front view showing a thirteenth embodiment of the present invention; 
     FIG. 30 is a section along line A—A of FIG. 29; 
     FIG. 31A is an enlarged front view showing an adhesive layer included in the thirteenth embodiment; 
     FIG. 31B shows directions in which the adhesive layer of FIG. 31A expands; 
     FIG. 31C demonstrates a condition wherein the expansion of the base and that of the adhesive layer cancel each other and appear in the distance between collimator lenses; 
     FIG. 32 is a perspective view of a case representative of a fourteenth embodiment of the present invention; 
     FIG. 33 is a horizontal section showing a base also included in the fourteenth embodiment; 
     FIG. 34A is a section showing a fifteenth embodiment of the present invention; 
     FIG. 34B is a front view of a base included in the fifteenth embodiment; and 
     FIGS. 35A and 35B are fragmentary views showing a sixteenth embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     To better understand the present invention, reference will be made to a conventional light source device including a semiconductor laser and a collimator lens, shown in FIG.  1 . The light source device to be described is taught in Japanese Patent Laid-Open Publication No. 5-88061 mentioned earlier and emits a single laser beam. As shown, the device includes a base  101  formed with a stepped hole  102 . A semiconductor laser  103  is press-fitted in the stepped hole  102 . A flange  105  is fastened to the base  101  by two screws  104  and formed with a through bore  106  aligning with the stepped hole  102 . The left end of the bore  106 , as viewed in FIG. 1, is implemented as an inlet portion  106   a  greater in diameter than the other portion by about 0.1 mm. A hollow cylindrical lens holder  107  is received in the bore  106 , but spaced from the wall of the bore  106  by about 0.01 mm to 0.03 mm. A collimator lens  108  is held in the lens holder  107  for converting a laser beam to a parallel beam. 
     A printed circuit board  109  is formed with a positioning hole  110 . A guide pin  111  protrudes from the end face of the base  101  and is received in the hole  110 . The tip of the guide pin  111  is melted by heat and crushed thereby, as indicated by a phantom line  11 ′, thereby affixing the base  101  and printed circuit board  109  to each other. Leads  112  extending out from the laser  103  are passed through holes formed in the circuit board  109  and soldered to a wiring pattern provided on the rear of the circuit board  109 . 
     The flange  105  is positioned in the directions x and y such that the emission point of the laser  103  aligns with the optical axis of the collimator lens  108 . Thereafter, the flange  105  is affixed to the base  101  by the screws  104 . The flange  105  is formed with a notch  113  communicated to the inlet portion  106   a . After the lens holder  107  has been adjusted in the direction x so as to bring the light source position of the laser  103  into alignment with the focus of the collimator lens  108 , adhesive is filled in the bore  106  via the notch  113  in order to affix the lens holder  107  to the flange  105 . 
     An aperture forming member  114  is a shield cap for separating and shaping only the center portion of the beam output from the collimator lens  108 . The member  114  is formed with an aperture  114   a  for selecting the above portion of the beam and a lug  114   b  mating with the flange  113 . In this condition, the member  114  is affixed to the flange  105 . 
     When the above light source device is mounted to, e.g., the body of a digital copier or that of a laser printer, a flat surface  105   a  included in the flange  105  and perpendicular to the optical axis is used as a reference surface. In addition, the surface  105  serves as a reference surface for the adjustment of optical characteristics. 
     The laser beam issuing from the laser  103  is collimated by the collimator lens  108 . The center portion of the resulting parallel beam is passed through the aperture  114   a  of the aperture forming member  114 . The beam output via the aperture  114   a  is incident to a photoconductive element via a polygonal mirror or similar deflector and an f-theta lens or similar optical element so as to form an image on the photoconductive element, although not shown specifically. 
     The light source device having the above configuration has the following problems (1)-(6). 
     (1) The adjusting portion assigned to the directions x and y (optical axis characteristic) and the adjusting portion assigned to the direction z (collimation characteristic or focal direction) each has an independent structure. This increases the number of parts and the cost of the device. 
     (2) The laser beam issuing from the laser  103  has a certain spread and is not always entirely incident to the collimator lens  108 . Semiconductor lasers are standardized by laws from the safety standpoint. A beam issuing from a semiconductor laser should preferably be prevented form leaking in directions other than the direction of the optical axis, not only during actual operation but also during adjustment. That is, the flange  105  and base  101  must be formed of materials capable of shielding the laser beam. 
     (3) The adhesive used to affix the lens holder  107  should advantageously be implemented by ultraviolet (UV)-curable adhesive. UV-curable adhesive cures rapidly and reduces the production tact and is reliable. However, because the base  101  and flange  105  are opaque for UV rays, UV rays radiated via the clearance filled with the UV-curable adhesive cannot be evenly incident to the entire adhesive, resulting in irregular curing or local curing. Consequently, a strain ascribable to contraction acts on the assembly unevenly and displaces the lens holder  107  and causes the structural elements to crack. A material opaque for infrared rays, red light or similar laser beam is also opaque for UV rays shorter in wavelength than the laser beam. To allow such a material to transmit only UV rays, it is necessary to add a special filter to the assembly or provide the flange  105  itself with a special coating. This noticeably increases the cost of the assembly. It follows that the adhesive for affixing the collimator lens  108  cannot be implemented by UV-curable adhesive. 
     (4) The adhesive exists on the entire circumference of the lens holder  107 , i.e., in the directions x, y and z. Therefore, the contraction of the adhesive during curing is not definite in the directions x or y, resulting in a scatter in the positional accuracy in the directions x and y. To guarantee positional accuracy after adhesion, it is necessary to provide the initial position with some offset taking account of the contraction. However, if the directions of contraction are not constant, it is difficult to provide the initial position with an offset and prevent accuracy in the directionality of the laser (optical axis characteristic) from falling. 
     (5) After the adjustment in the directions x and y, the flange  105  is affixed to the base  101  by the screws  104 . This is undesirable because screw seats formed on the end face of the base  101  and the flange  105  bite each other when the screws  104  are driven. As a result, displacements in the directions x and y are apt to occur and lower the accuracy of the directionality of the laser. 
     (6) Because the adhesive is introduced into the clearance via the notch  23 , a strain and therefore a scatter in the positional accuracy occurs due to the partial contraction of the adhesive during the inflow of the adhesive or the irregular flow of the adhesive. 
     FIG. 2 shows a light source device of the type emitting a plurality of (two) beams and taught in each of Japanese Patent Laid-Open Publication Nos. 7-181410 and 7-181412 also mentioned earlier. As shown, the light source device includes two bases  201  each having a stepped hole like the base  101  of FIG.  1 . Two semiconductor lasers  203  are respectively press-fitted in the two stepped holes. The bases  201  are fastened to a flange  205  by four screws  204 . Through bores  205   a  are formed in the flange  205  in alignment with the semiconductor lasers  203 . Hollow cylindrical lens holders  207  each is received in the respective bore  205   a , but spaced from the wall of the bore  205   a  by about 0.01 mm to 0.03 mm. Each lens holder  207  has a collimator lens  208  thereinside for converting a laser beam to a parallel beam. The bases  201  each is adjusted in the directions x and y such that the emission point of the respective laser  203  aligns with the optical axis of the associated collimator lens  208 . Then, the bases  201  are affixed to the flange  205  by the screws  204 . 
     The bores  205   a  of the flange  205  each is formed with notches  206   a . After the lens holders  207  have been so adjusted in the direction z as to bring the emission points of the lasers  203  into alignment with the focuses of the associated collimator lenses  208 , adhesive is fed via the notches  206   a  in order to affix the lens holders  207  to the flange  205 . 
     Aperture forming members  209  each separates and shapes the center part of the beam output from the associated collimator lens  208 . Each aperture forming member  209  is formed with an aperture  209   a  aligning with the optical axis of the parallel beam output from the associated collimator lens  208 . The parallel beams output from the apertures  209   a  are combined by a beam combining prism  210  to turn out beams  211  existing on substantially the same optical axis. The angles of the output optical axes are finely adjusted such that the two beams  211  have a pitch implementing a desired pitch in the subscanning direction on an image forming plane (i.e., pitch in the direction of lines in the case of simultaneous two-line writing). This method corresponds to the adjustment of the bases  201  in the direction y. 
     As shown in FIG. 2, the aperture forming members  209  and prism  210  are received in a case  212 . The case  212  is positioned by the bores  205   a  of the flange  205  and positioning portions, not shown, included in the case  212 , and then affixed to the flange  205  via screw holes formed in its four corners. The flange  205  is formed of metal (particularly aluminum) in order to prevent the heat radiation of the lasers  203  and the adjusted beam pitch from being effected as far as possible. The case  212  is implemented as a resin molding from the cost standpoint. 
     The light source device shown in FIG. 2 has the following problems (1)-(3). 
     (1) The characteristic of the adhesive after curing is equivalent to the characteristic of resins in general. Usually, the coefficient of linear expansion of resin becomes greater than that of metal, causing resin to contract noticeably in the event of temperature variation. Even when the bases  201  and flange  205  are formed of metal having a small coefficient of linear expansion, the adhesive layers intervening between the collimator lenses  208  cause the pitch or distance between the lenses  208  to vary noticeably when temperature varies. This varies the angles of the beams  211  output from the prism  210  and the distance between them and therefore the pitch on the image writing plane in the subscanning direction, thereby deteriorating image quality. 
     (2) The adhesive layers exist on the entire circumferences of the collimator lenses  208 . Therefore, when each collimator lens  208  adjusted in the direction z (collimation characteristic) is affixed by the adhesive, it is displaced due to contraction in both of she directions x and y. As a result, the positional accuracy (optical axis accuracy) of each lens  208  varies in the directions x and y after the adhesive has cured. To adjust the distance between the two beams  211  with higher accuracy, it is necessary to loosen the screws of either one of the bases  201  after the fixation of the lenses  208 , again adjust the position in the directions x and y (particularly y), and then drive the screws in order to affix the base  201  to the flange  205 . However, while the required accuracy in the direction z (collimation characteristic or focal direction) is less than the order of micron, a stress occurs in the direction z in the event of fastening and brings about a displacement of the order of micron in the direction z. Consequently, this method degrades accuracy in the direction z although implementing accurate adjustment of the distance between the beams. That is, when the distance between the beams is varied on an image plane due to, e.g., the scatters of optical parts other than the light source device, and if it is adjusted via the light source device, accuracy in the direction z is deteriorated. 
     (3) The lasers  203  each is affixed to the respective base  201  which is, in turn, fastened to the flange  205  by the respective two screws  204 . Then, the flange  205  is fastened to the case  212  by screws. This is undesirable because each part is apt to deform or move due to the fastening force, causing the beam pitch to vary. 
     Preferred embodiments of the light source device in accordance with the present invention will be described hereinafter. 
     1st Embodiment 
     Referring to FIGS. 3 and 4, a light source device embodying the present invention is shown and includes a base  41  formed with two through bores  41   a . Two semiconductor lasers  42  are respectively press-fitted in the rear parts of the bores  41   a . In the illustrative embodiment, two collimator lenses  43  are directly adhered to the base  41 . Specifically, lens support portions  41   b  are molded integrally with the front of the base  41 , and each has an arcuate section slightly greater in diameter than the collimator lenses  43 . The diameter of each lens support portion  41   b  may be about 0.2 mm. The lens support portions  41   b  each has an axis aligning with the optical axis of the associated laser  42 . If desired, each lens support portion  41  may be provided with an arcuate section slightly greater in the radius of curvature than the collimator lens  43  by the thickness of an adhesive layer. 
     The lens support portions  41   b  each has a length, as measured in the direction of the optical axis (direction z), greater than the thickness of the collimator lens  43  (direction z), so that excess adhesive is prevented from deposing on unexpected portions. The portions  41   b  each has an arcuate section smaller than a semicircle, as seen in a front view. As shown in FIG. 4, the arcuate section should preferably extend over about 60 degrees and be symmetrical in the right-and-left direction with respect to a center line C in order to facilitate position adjustment and adhesion. The center lines C of the lens support portions  41   b  are substantially perpendicular to the subscanning pitch direction (direction y) of the two beams to issue from the collimator lenses  43 . 
     The collimator lenses  43  are transparent for UV rays. While plastics and glass, for example, are transparent for UV rays, glass is desirable from the optical characteristic standpoint. As shown in FIG. 4, during assembly, each collimator lens  43  is held by a chuck  47  movable in the three directions x, y and z and then positioned on the respective lens support portion  41   b  coaxially with the associated laser  42 . 
     UV-curable adhesive  46  is filled in a gap formed between the adhesion surface  41 c of each support portion  41   b  and the circumference of the associated collimator lens  43 . Subsequently, the collimator lens  43  is finely adjusted to its position implementing an expected optical characteristic, and then the chuck  47  is fixed in place. Thereafter, as shown in FIG. 4, UV rays L are radiated from a UV radiator  48  toward the adhesive  46  from above the collimator lens  43 . The UV rays L are incident to the adhesive  46  via the collimator lens  43  and causes it to cure evenly. 
     The curing of the adhesive is effected with each of the two collimator lenses  43 . As a result, an adhesive layer  46  is formed between the adhesion surface  41   c  of each lens support portion  41   b  and the associated collimator lens  43 . The adhesive layer  46  is therefore about 0.2 mm thick and symmetrical in the right-and-left direction and has a thickness in the direction substantially perpendicular to the subscanning pitch direction (direction y). Each collimator lens  43  is affixed to the respective lens support portion  41   b  by the adhesive layer  46  while Preserving its expected optical characteristic. That is, the lens support portions  41   b  are symmetrical with respect to a line perpendicular to the beam pitch direction. 
     Particularly, each lens support portion  41   b  has a symmetrical arcuate section extending over about 60 degrees, as shown in FIG.  4 . Such a configuration allows the chuck  47  to chuck the collimator lens  43  surely and easily. In addition, the UV rays L issuing from the UV radiator  48  can illuminate the entire adhesion surface  41   c  via the collimator lens  43 , insuring the even and complete curing of the adhesive. The fully set uniform adhesive layer obviates the displacement of the collimator lens  43  ascribable to irregular curing or local curing. 
     Because strains ascribable to the contraction of the adhesive occur symmetrically in the right-and-left direction and cancel each other, a strain occurs only in the direction x. The strain in the direction x can be provided with fine offset taking account of the contraction of the adhesive. It follows that the beams output from the two collimator lenses  43  have desirable directionality (optical axis characteristic) in the direction y, and of course have an accurate beam pitch (accuracy in the distance in the direction y or subscanning direction). 
     In addition, because each adhesive layer is symmetrical in the right-and-left direction or direction y, the expansion and contraction of the adhesive layer ascribable to the ambient temperature is cancelled in the direction y, i.e., limited to the direction x. This further enhances the accuracy of the distance between the two beams. 
     As shown in FIG. 3, an aperture forming member  44  is used to select the center portion of the parallel beam output of each collimator lens  43 . For this purpose, the aperture forming member  44  is formed with two apertures  44   a  respectively aligning with the optical axes of the collimator lenses  43 . The parallel beams output from the two apertures  44   a  are combined by a beam combining prism, or beam combining means,  45  to turn out beams existing on substantially the same axis. The combined beams are incident to scanning optics, not shown, for writing an image. The angles of the output optical axes are finely adjusted such that the two beams have a pitch implementing a desired pitch in the subscanning direction on an image forming plane (i.e., pitch in the direction of lines in the case of simultaneous two-line writing). This method corresponds to the adjustment of the collimator lens  43  in the direction y. If desired, the beam combining means may be implemented by a combination of a mirror and a half-mirror. 
     The aperture forming member  44  and prism  45  are mounted to the base  41  by a mounting member, not shown. At this instant, two contiguous circular stepped portions  41   d  are used for positioning while four holes  41   e  are used for mounting. 
     As stated above, the illustrative embodiment achieves various unprecedented advantages, as enumerated below. 
     (1) The center line C of each lens support portion  41   b  is substantially perpendicular to the direction of the pitch of two beams output from the collimator lenses  43  (direction y). Therefore, the influence of contraction of the adhesive  46  due to curing does not act in the above beam pitch direction. This, coupled with the fact that the influence of expansion and contraction of the adhesive  46  ascribable to the varying temperature does not act in the beam pitch direction, provides the light source device with an accurate beam pitch and allows it to preserve its accuracy stably. 
     (2) The collimator lenses  43  each is directly affixed to the respective lens support portion  41   b  molded integrally with the base  41 . Therefore, the number of parts and therefore the cost of the light source device is reduced. Further, because holders or similar members do not intervene between the collimator lenses  43  and the lens support portions  41   b , the device is free from the influence of the errors of such intermediary members. In addition, directly affixing the lenses  43  to the lens support members  41   b  obviates the need for screws or similar fastening means. The device is therefore free from displacements particular to fastening. 
     (3) The lens support portions  41   b  are symmetrical with respect to a line perpendicular to the beam pitch direction, so that strains ascribable to the contraction in the beam pitch direction cancel each other. This limits the contraction to the direction perpendicular to the beam pitch direction and thereby further enhances the directionality of contraction and therefore accurate position adjustment. 
     (4) The lens support portions  41   b  each has an arcuate section whose diameter is slightly greater than the diameter of the collimator lens  43 . Therefore, when each lens support portion  41   b  and associated collimator lens  43  are positioned coaxially with each other, the adhesive layer between them has a uniform thickness and can therefore cure evenly. This protects the collimator lenses  43  from displacement. The radius of curvature of each lens support portion  41   b  greater than that of the collimator lens  43  by the thickness of the adhesive layer further enhances the above effect. 
     (5) Because each lens support portion  41   b  has an arcuate section smaller than a semicircle, the adhesive layer covers only less than one half of the circumference of the collimator lens  43  and gives the contraction of the adhesive  46  directionality. The initial position of the collimator lens  43  can therefore be provided with an offset taking account of the contraction of the adhesive  46  to occur, enhancing positional accuracy after the curing of the adhesive  46 . In addition, light for curing the adhesive  46  can be directed toward the side ends of the collimator lenses  43  via the open sides of the lens support portions  41   b , obviating irregular curing more positively. 
     (6) Gaps, grooves or similar non-adhesion portions prevent the adhesive from bridging the collimator lenses  43  and base  41  in the optical axis direction. Therefore, even when the adhesive  46  is introduced in an excessive amount, it is prevented from directly depositing on the walls of the base  41  and exerting its contracting force on the collimator lenses  43  in the direction z. This additionally increases the positional accuracy in the optical axis direction or direction z. 
     The UV-curable adhesive used in the above embodiment may be replaced with any other photo-curable adhesive. Also, the light source device may be so constructed as to emit three or more beams, as needed. 
     2nd Embodiment 
     Referring to FIGS. 5 and 6, a second embodiment of the light source device in accordance with the present invention is shown and includes a first base  51  and a second base  54 . The first base  51  is formed with through bores  51   a  and  51   b . A semiconductor laser  52   a  is press-fitted in the rear part of the bore  51   a . A collimator lens  53   a  is directly adhered to the first base  51 . Specifically, a lens support portion  51   c  is molded integrally with the front of the first base  51  and has an arcuate section slightly greater in diameter than the collimator lens  53   a . The diameter of the lens support portion  51   c  may be about 0.2 mm. The lens support portion  51   c  has an axis aligning with the optical axis of the laser  52   a . The lens support portion  51   c  has a length, as measured in the direction of the optical axis (direction z), greater than the thickness of the collimator lens  53   a  (direction z), so that excess adhesive is prevented from deposing on unexpected portions. 
     The portion  51   c  has an arcuate section smaller than a semicircle, as seen in a front view. The second base  54  is formed with a through bore  54   a  and a circular stepped portion  54   b . A semiconductor laser  52   b  is press-fitted in the rear part of the bore  54   a . A collimator lense  53   b  is directly adhered to the second base  54 . Specifically, a lens support portion  54   c  is molded integrally with the front of the base  54  and has an arcuate section slightly greater in diameter than the collimator lens  53   b . The diameter of the lens support portion  54   c  may be about 0.2 mm. The lens support portion  54   c  has an axis aligning with the optical axis of the laser  52   b . The lens support portion  54   c  has a length, as measured in the direction of the optical axis (direction z), greater than the thickness of the collimator lens  53   b  (direction z), so that excess adhesive is prevented from deposing on unexpected portions. The base  54  is temporarily positioned by the bore  51   b  of the base  51  and the stepped portion  54   b  of the base  54 , and then fastened to the base  51  by two screws  55 , holes  54   d , and threaded holes  51   d.    
     As shown in FIG. 6, the arcuate section of each of the lens support portions  51   c  and  54   c  should preferably extend over about 60 degrees and be symmetrical in the right-and-left direction in order to facilitate position adjustment and adhesion. The center lines C 1  and C 2  of the arcuate lens support portions  51   c  and  54   c , respectively, are substantially perpendicular to the subscanning pitch direction (direction y) of two beams to issue from the collimator lenses  53   a  and  53   b.    
     The collimator lenses  53   a  and  53   b  are transparent for UV rays. While plastics and glass, for example, are transparent for UV rays, glass is desirable from the optical characteristic standpoint. As shown in FIG. 6, during assembly, each collimator lens  53   a  or  53   b  is held by a chuck  57  movable in the three directions x, y and z and then positioned on the respective lens support portion  51   c  or  54   c  coaxially with the associated laser  52   a  or  52   b.    
     UV-curable adhesive  56   a  is filled in a gap formed between the adhesion surface  51   e  of the support portion  51   c  and the circumference of the collimator lens  53   a . Subsequently, the collimator lens  53   a  is finely adjusted to its position implementing expected optical characteristics, and then the chuck  57  is fixed in place. Thereafter, as shown in FIG. 6, UV rays L 1  are radiated from a UV radiator  58   a  toward the adhesive  56   a  from above the collimator lens  53   a . The UV rays L 1  are incident to the adhesive  56   a  via the collimator lens  53   a  and causes it to cure evenly. 
     Adhesive  56   b  intervening between the other adhesion surface  54   e  and the collimator lens  53   b  is also caused to cure by UV rays issuing from a UV radiator  58   b.    
     As a result, an adhesive layer  56   a  is formed between the adhesion surface  51   c  of the lens support portion  51   c  and the collimator lens  53   a . The adhesive layer  56   a  is therefore about 0.2 mm thick and symmetrical in the right-and-left direction and has a thickness in the direction substantially perpendicular to the subscanning pitch direction (direction y). Likewise, an adhesive layer  56   b  is formed between the adhesion surface  54   e  of the lens support portion  54   c  and the collimator lens  53   b . The adhesive layer  56   b  is identical with configuration with the adhesive layer  56   a . The collimator lens  53   a  and  53   b  each is affixed to the respective lens support portion  51   c  or  54   c  by the adhesive layer while preserving its expected optical characteristic. That is, the lens support portions  51   c  and  54   c  are symmetrical with respect to a line perpendicular to the beam pitch direction. 
     Particularly, each lens support portion  51   c  or  54   c  has a symmetrical arcuate section extending over about 60 degrees, as shown in FIG.  6 . Such a configuration allows the chuck  57  to chuck each collimator lens  53   a  or  53   b  surely and easily. In addition, the UV rays L 1  and L 2  issuing from the UV radiators  58   a  and  58   b , respectively, each can illuminate the entire adhesion surface  51   e  or  54   e  via the collimator lens  53   a  or  53   b , insuring the even and complete curing of the adhesive. The fully set even adhesive layer obviates the displacement of the collimator lens ascribable to irregular curing or local curing. 
     Because strains ascribable to the contraction of the adhesive occur symmetrically in the right-and-left direction and cancel each other, a strain occurs only in the direction x. The stain in the direction x can be provided with fine offset taking account of the contraction of the adhesive. It follows that the beams output from the two collimator lenses have desirable directionality (optical axis characteristic) in the direction y, and of course have an accurate beam pitch (accuracy in the distance in the direction y or subscanning direction). In addition, because each adhesive layer is symmetrical in the right-and-left direction or direction y, the expansion and contraction of the adhesive layer ascribable to the ambient temperature is cancelled in the direction y, i.e., limited to the direction x. This further enhances the accuracy of the distance between the two beams. 
     After the collimator lens  53   b  has been adhered to the second base  54 , the base  54  can be adjusted in position in the directions x and y (particularly y) by loosening the screws  55 . Therefore, when the beam pitch or distance on an image surface differs from expected one due to an extrinsic scatter, it is possible to readjust the base  54 , i.e., the beam distance in any one of the directions x and y. 
     In the above embodiment, the second base  54  is fastened to the first base  51  first. Alternatively, after the collimator lens  53   b  has been adhered to the second base  54 , the second base  54  may be affixed to the first base  51  carrying the collimator lens  53   a  therewith. This will be followed by the adjustment of the distance between the two beams. 
     As shown in FIG. 5, an aperture forming member  59  is used to select the center portion of the parallel beam output of each collimator lens  53   a  or  53   b . For this purpose, the aperture forming member  59  is formed with two apertures  59   a  and  59   b  respectively aligning with the optical axes of the collimator lenses  53   a  and  53   b . The parallel beams output from the two apertures  59   a  and  59   b  are combined by a beam combining prism, or beam combining means,  60  to turn out beams existing on substantially the same axis. The combined beams are incident to scanning optics, not shown, for writing an image. The angles of the output optical axes are finely adjusted such that the two beams have a pitch implementing a desired pitch in the subscanning direction on an image forming surface (i.e., pitch in the direction of lines in the case of simultaneous two-line writing). This method corresponds to the adjustment of the collimator lens in the direction y. 
     The aperture forming member  59  and prism  60  are mounted to the first base  51  by a mounting member, not shown. At this instant, the circular stepped portion  51   f  of the first base  51  is used for positioning while four holes  51   g  are used for mounting. 
     As stated above, the illustrative embodiment achieves the following various advantages. 
     (1) The first base  51  is provided with the bore  51   a  defining the optical axis of at least one beam, the semiconductor laser  52   a  received in the bore  51   a , the collimator lens  53   a  coaxial with the laser  52   a , and the lens support portion  51   c  also coaxial with the laser  52   a . The second base  54  is provided with the through bore  54   a  defining the optical axis of another beam, the semiconductor laser  52   b  received in the bore  54   a , the collimator lens  53   b  coaxial with the laser  52   b , and the lens support portion  54   c  also coaxial with the laser  52   b . Therefore, when a desired distance between the beams is not set up on an image writing surface due to, e.g., the scatter of any optical part other than the light source device, the distance can be adjusted without varying the characteristic of the device in the direction z (collimation characteristic), thereby insuring desirable image quality. 
     (2) The collimator lenses  53   a  and  53   b  are respectively directly affixed to the lens support portions  51   c  and  54   c  molded integrally with the bases  51  and  54 . Therefore, the number of parts and cost of the light source device are reduced. Further, because holders or similar members do not intervene between the collimator lenses  53   a  and  53   b  and the lens support portions  51   c  and  54   c , the device is free from the influence of the errors of such intermediary members. In addition, directly affixing the lenses  53   a  and  53   b  to the lens support members  51   c  and  54   c  obviates the need for screws or similar fastening means. The device is therefore free from displacements ascribable to fastening. 
     (3) The lens support portions  51   c  and  54   c  are symmetrical with respect to a line perpendicular to the beam pitch direction, so that strains ascribable to the contraction in the beam pitch direction cancel each other. This limits the contraction to the direction perpendicular to the beam pitch direction and thereby further enhances the directionality of contraction and therefore accurate position adjustment. 
     (4) The lens support portions  51   c  and  54   c  each has an arcuate section whose diameter is slightly greater than the diameter of the collimator lens  53   a  or  53   b . Therefore, when each lens support portion  51   c  or  54   c  and associated collimator lens  53   a  or  53   b  are positioned coaxially with each other, the adhesive layer between them has a uniform thickness and can therefore cure evenly. This protects the collimator lenses  53   a  and  53   b  from displacement. The radius of curvature of each lens support portion  51   c  or  54   c  greater than that of the collimator lens  43  by the thickness of the adhesive layer further enhances the above effect. 
     (5) Because each lens support portion  51   c  or  54   c  has an arcuate section smaller than a semicircle, the adhesive layer covers only less than one half of the circumference of the collimator lens  53   a  or  53   b  and gives the contraction of the adhesive directionality. The initial position of each collimator lens  53   a  or  53   b  can therefore be provided with an offset taking account of the contraction of the adhesive to occur, enhancing positional accuracy after the curing of the adhesive. In addition, light for curing the adhesive can be directed toward the side ends of the collimator lenses  53   a  and  53   b  via the open sides of the lens support portions  51   c  and  54   c , obviating irregular curing more positively. 
     (6) Gaps, grooves or similar non-adhesion portions prevent the adhesive from bridging the collimator lenses  53   a  and  53   b  and bases  51  and  54  in the optical axis direction. Therefore, even when the adhesive is introduced in an excessive amount, it is prevented from directly depositing on the walls of the bases  51  and  64  and exerting its contracting force on the collimator lenses  53   a  and  53   b  in the direction z. This additionally increases the positional accuracy in the optical axis direction or direction z. 
     The UV-curable adhesive used in the above embodiment may be replaced with any other photo-curable adhesive. Also, the light source device may be so constructed as to emit three or more beams. 
     3rd Embodiment 
     Referring to FIGS. 7 and 8, a third embodiment of the light source device in accordance with the present invention is shown and includes a substantially rectangular flat base  1 . Two through bores  1   a  are formed substantially at the center of the base  11  and positioned side by side in the direction y. Two semiconductor lasers  2  are respectively press-fitted in the rear parts of the bores  1   a . Two collimator lenses  3  are directly adhered to the base  1 . Specifically, lens support portions  1   b  are molded integrally with the front of the base  1 , and each has an arcuate section slightly greater in diameter than the collimator lenses  3 . The diameter of each lens support portion  1   b  may be about 0.2 mm. The lens support portions  1   b  each has an axis aligning with the optical axis of the associated laser  2 . The lens support portions  1   b  each has a length, as measured in the direction of the optical axis (direction z), greater than the thickness of the collimator lens  3  (direction z), so that excess adhesive is prevented from deposing on unexpected portions. The portions  1   b  each has an arcuate section smaller than a semicircle, as seen in a front view. 
     As shown in FIG. 8, the arcuate section should preferably extend over about 60 degrees and be symmetrical in the right-and-left direction in order to facilitate position adjustment and adhesion. The center lines C of the lens support portions  1   b  are substantially perpendicular to the subscanning pitch direction (direction y) of the two beams to be output from the collimator lenses  3 . 
     An aperture forming member  4  is formed with apertures  4   a  each for selecting a light beam and is received in a case  9 . A beam combining optical member  4  is implemented as a prism capable of combining two beams  10  output via the apertures  4   a  into beams existing substantially on the same axis. The optical member  4  is also received in the case  9 . If desired, the optical member  4  may be implemented as a combination of a mirror and a half-mirror. 
     As shown in FIG. 8, in the event of assembly, each collimator lens  3  is held by a chuck  7  movable in the three directions x, y and z. The chuck  7  positions the lens  7  on the associated lens support portion  1   b  coaxially with the optical axis of the associated laser  2 . Subsequently, UV-curable adhesive is filled in a gap formed between the adhesion surface the lens support portion  1   b  and the circumference of the collimator lens  3 , forming an adhesive layer  6 . Then, the collimator lens  3  is finely adjusted to its position implementing an expected optical characteristic, and then the chuck  7  is fixed in place. Thereafter, as shown in FIG. 8, UV rays are radiated from a UV radiator  8  toward the adhesive layer  6  from above the collimator lens  3 . The UV rays are incident to the adhesive layer  6  via the collimator lens  3  and cause it to cure evenly. The curing of the adhesive is effected with each of the two collimator lenses  3 . As a result, the adhesive layer  46  is formed between each lens support portion  1   b  and the associated collimator lens  3 . The adhesive layer  6  is therefore about 0.2 mm thick and symmetrical in the right-and-left direction and has a thickness in the direction substantially perpendicular to the subscanning pitch direction (direction y). Each collimator lens  3  is affixed to the respective lens support portion  1   b  by the adhesive layer  6  while preserving its expected optical characteristic. 
     The adhesive layers  6  each has a length, as measured in the optical axis direction smaller than the length of the associated lens support portion  1   b , so that a gap is formed between the adhesive layer  6  and the base  1 . While the light source device is in operation, the adhesive layers  6  expand due to temperature elevation. At this instant, should the adhesive layers  6  and base  1  be held in close contact with each other, the collimator lenses  3  would move in the optical axis direction due to the expansion of the adhesive layers  6 . The gap between each adhesive layer  6  and the base  1  allows the layer  6  to freely move to both sides of the associated lens  3  and prevents it from moving the lens  3 . 
     The case  9  is positioned by positioning holes  1   c  formed in the base  1  and positioning recesses, not shown, formed in the case  9 . After four threaded holes  9   a  formed in the case  9  and four holes  1   d  formed in the base  1  have been aligned, the case  9  is fastened to the base  1  by four screws  11 . 
     4th Embodiment 
     FIGS. 9A and 9B show a base  21  representative of a fourth embodiment of the present invention and including a single lens support portion  21   b . As for the rest of the construction, this embodiment is identical with the third embodiment shown in FIG.  7 . 
     Assume that the conventional light source device shown in FIG. 2 is mounted to a digital copier or a laser printer. Then, even if the device initially has expected optical characteristics, the flange  205 , for example, deforms due to a stress ascribable to assembly or expansion and contraction ascribable to temperature variation in the machine. This embodiment pertains to a light source device generally implemented as enlargement type optics. Therefore, even the slightest displacement of any part ascribable to deformation would be enlarged on reaching a writing position assigned to a photoconductive element and would turn out a noticeable displacement, critically effecting the optical characteristics. Particularly, deformation in the direction y deteriorates the parallelism of the individual laser beam. 
     The fourth embodiment solves the above problem by allowing a minimum of deformation to occur in the light source device. Should the machine body for accommodating the light source device be rearranged, other various portions would be effected and would bring about extra costs. In this sense, the scarcely deformable structure is desirable. 
     As shown in FIGS. 9A and 9B, the base  21 , like the base  1  of FIG. 7, has through bores  21   a  for receiving the semiconductor lasers  2 . The base  21  is characterized in that a single continuous lens support portion  21   b  is substituted for the two lens support portions  1   b . Specifically, the lens support portion  21   b  has two lens support portions  21   b   1  connected to each other by a straight tie portion  21   b   2 . The tie portion  21   b   2  is substantially parallel to the beam pitch direction or direction y and uniform in thickness in the same direction. Notches  21   b   3  are respectively formed between the lens support portions  21   b   1  and the tie portion  21   b   2 , and each extends in the optical axis direction. The notches  21   b   3  prevent adhesive from reaching the tie portion  21   b   2  when it is filled for affixing the collimator lenses  3 . The lens support portions  21   b   1  are symmetrical with each other with respect to the tie portion  21   b   2 . 
     Because the tie portion  21   b   2  extends in the direction y, it prevents the base  21  from deforming in the direction y and thereby effectively obviates errors in the distance between the two beams  10  and the angle of the individual beam. 
     FIG. 10 shows a base  22  representative of a modification of the fourth embodiment. As shown, a line connecting the optical axes of the two collimator lenses  3  is inclined with respect to the y axis. The difference between the base  21 , FIGS. 9A and 9B, having the lenses  3  arranged in parallel to the axis y, and the base  22  stems from a difference in the structure of writing optics, although not described specifically. The base  22  includes a lens support portion  22   b  which is also inclined. The lens support portion  22   b  is made up of two lens support portions  22   b   1  and a straight tie portion  22   b   2  connecting them together. The tie portion  22   b   2  is slightly thinner than the tie portion  21   b   2 , FIGS. 9A and 9B, and stepped, as illustrated. The tie portion  22   b   2  may be slightly inclined, a shown and described, although it will be most effective when extending in parallel to the y axis. 
     5th Embodiment 
     FIG. 11 shows a fifth embodiment of the present invention. This embodiment is essentially similar to the embodiment of FIG. 7; identical structural elements are designated by identical reference numerals and will not be described in order to avoid redundancy. In the conventional structure shown in FIG. 2, the flange  205  is fastened to the case  212  by the screws inserted into the four holes  205   b  formed in the corners of the flange  205 . This gives rise to a problem that if the flange  205  and case  212  are different in the coefficient of linear expansion, then the temperature elevation of the light source device distorts the flange  205  and thereby disturbs the distance between the two beams  211  and the parallelism of the same (beam pitch accuracy). 
     In the illustrative embodiment, a single hole  1   d  is formed in the base  1  in the vicinity of the center of the base  1 . A female threaded hole  1   d  is formed in the case  9 . A screw  11  is driven into the threaded hole of the case  9  via the hole  1   d  of the base  1 , so that the base  1  is fastened to the case  9  at a single point. 
     In the above configuration, even when the base  1  and case  9  are different in the coefficient of linear expansion, each of them can expand and contrast independently of the other. This frees the base  1  from distortion and insures the accurate beam pitch between the two beams  10 . The base  1  and case  9  each can be formed of any desired material and therefore implemented even as an inexpensive plastic molding. Moreover, if the hole  1   d  for the screw  11  is positioned on a center line C′ intermediate between the center lines C of the lasers  2  extending in the direction y, it is possible to protect the base  1  from deformation and to enhance the stability of the relative position of the base  1  and case  9  during assembly, i.e., accurate beam pitch. 
     6th Embodiment 
     FIGS. 12-14 show a sixth embodiment of the present invention also constituting an improvement over the conventional light source device of FIG.  2 . In the device shown in FIG. 2, the case  212  is mounted to the flange  205  after the optical element  210  has been mounted to the case  212 . This is undesirable because any positional error of the optical element  210  effects the pitch accuracy of the beams  211 . Moreover, while the case  212  should preferably be implemented as an inexpensive plastic molding, a plastic molding is noticeably susceptible to temperature. Particularly, the displacement of a plastic molding (especially in the beam pitch direction) ascribable to expansion or contraction has critical influence on accuracy. The sixth embodiment is a solution to such problems. 
     As shown in FIG. 12, the base  1  additionally includes an optical element support portion  23  extending out from the opposite lens support portions  1   b . An aperture forming member  14  is positioned downstream of the optical element  5  in the direction of beam propagation and therefore formed with a single aperture  14   a.    
     The optical element support portion  23  includes a reference surface  23   a  and a surface  23   b , as illustrated. The optical element  5  is positioned by the reference surface  23   a  and affixed to the surface  23   b  by an adhesive layer  24  (see FIG.  14 ). In this configuration, even when the adhesive layer  24  expands or contracts due to temperature variation, no changes occur in the direction y (beam pitch direction). The adhesive should preferably be reliable photo-curable adhesive and should preferably be identical with the adhesive used to affix the collimator lenses  3  from the easy production standpoint. Subsequently, the collimator lenses  3  are positioned and then adhered, as described with reference to FIG.  8 . 
     All of the semiconductor lasers  2 , collimator lenses  3  and optical element  5  needing high positional accuracy are supported by the base  1 , as stated above. With this configuration, it is easy to implement required assembly accuracy and maintain it. In addition, the optical element support portion  23  extends in the direction y and effectively obviates distortion in the direction y. 
     While the optical element support portion  23  has been shown and described as being molded integrally with the lens support portions  1   b , it may be implemented as an independent member and mounted to the base  1 . 
     7th Embodiment 
     Referring to FIGS. 15-17, a seventh embodiment of the present invention will be described. In the conventional light source device shown in FIG. 2, the aperture forming member  209  is mounted to the case  212  and then mounted to the flange  205  together with the case  212 . This brings about a problem that any error in the position of the aperture  209   a  translates into an error in the posit on for emission. Further, the semiconductor lasers  203  are mounted to the base  201  while the lens holders  207  and collimator lenses  3  are affixed to the flange  205 . After the adjustment of the optical characteristics, the aperture forming member  209  is mounted. This brings about another problem that a scatter in the pitch of the two apertures  209   a  results in an error in optical characteristic after the mounting of the aperture forming member  209  and thereby lowers the beam pitch accuracy. 
     In light of the above, the seventh embodiment includes an elastic aperture forming member  25  implemented as, e.g., a plastic molding having a generally U-shaped cross-section. The aperture forming member  25  is mounted to the base  1  in such a manner as to embrace the optical element  5  positioned on the optical element support portion  23 . A single aperture  25   a  is formed in the aperture forming member  25  because the member  25  is located downstream of the optical element  5  in the direction of beam propagation. The aperture forming forming member  25  includes opposite walls for retaining the optical element  5 , as illustrated. The walls are formed with rib-like projections  25   b  convex toward each other. The optical element support portion  23  is formed with a groove  23   c  capable of mating with one of the projections  25   b . This configuration prevents the member  25  from easily slipping out of the base  1  and increases the retaining force of the member  25 . 
     In the illustrative embodiment, the aperture forming member  25  is mounted to the base  1  after the adjustment of the optical characteristics of the base  1 . This successfully maintains the initially set emission points with accuracy and prevents the member  25  from effecting the beam pitch. Moreover, the member  25  can be easily positioned without resorting to any extra part. In addition, the pitch of the aperture and therefore the beam pitch accuracy is free from the influence of deformation of the case  9 , so that the case  9  can be implemented by an inexpensive plastic molding in order to reduce the cost of the light source device. 
     8th Embodiment 
     FIGS. 18-20 show an eighth embodiment of the present invention. As shown, an aperture forming member  26  is molded integrally with the optical member support portion  23  of the base  1 . Two apertures  26   a  are formed in the aperture forming member  26  such that they respectively align with the centers of the collimator lenses  3 . If desired, the aperture forming member  26  may be implemented as an independent member and affixed to the optical support portion  23  by, e.g., adhesive. 
     The aperture forming member  26  is constructed integrally with the base  1  and allows the optical characteristics to be adjusted on the basis of the beams passed through and shaped by the apertures  26   a . This guarantees the optical characteristics while absorbing errors in the accuracy and positions of the apertures  26   a , thereby providing the light source device with high accuracy. Furthermore, the aperture forming member  26  formed integrally with the base  1  reduces the number of parts, and in addition the case  9  can be implemented by an inexpensive plastic molding. The resulting light source device is low cost. 
     9th Embodiment 
     FIG. 21 shows a ninth embodiment of the present invention. As for optical characteristics, a light source device is required to have the directionality (optical axis characteristic) and parallelism (collimation characteristic) of a laser beam to issue therefrom, as stated earlier. In practice, however, even if a collimator lens is brought to a desired position, it is displaced due to, e.g., the contraction of adhesive ascribable to curing and deformation ascribable to fastening. As a result, the optical characteristic after adhesion is extremely unstable. This embodiment solves this problem. 
     As shown in FIG. 21, the base  1  is fitted in the right end of the case  9 . The semiconductor lasers  2  and collimator lenses  3  are affixed to the base  1 . An aperture forming member  27  is formed with two apertures  27   a  and implemented as a thin elastic plate. A lug  27   b  is positioned at the center of the aperture forming member  27 . A seat  9   a  is formed integrally with the case  9  and supports the bottom left edge of the optical element or prism  5 , as viewed in FIG.  21 . The lug  27   b  of the aperture forming member  27  elastically supports the intermediate portion of the right end of the element  5 . A screw or adjusting means  28  extends throughout the wall of the case  9  and supports the top left portion of the element  5 . The seat  9   a  is a stationary fulcrum defining the center of rotation. The adjusting means  28  should only be capable of finely adjusting the angular position of the element  5  and is therefore not limited to a screw. 
     Because the seat  9   a  of the case  9  is flat, it is held in line-to-line contact with the optical element  5 . The element is therefore angularly movable about the seat  9   a  in a y-z plane (or about the x axis). 
     When the screw  28  is turned, its length within the case  9  varies and causes the optical element  5  contacting the seat  9   a  to rotate over a small angle in the y-z plane. As a result, the distance between a first reflection surface  5   a  and a second reflection surface  5   b  included in the element  5  in the direction y is varied to, in turn, vary the distance between the two beams  10  (pitch in the subscanning direction) in the direction y. 
     The angular movement of the optical element  5  varies only the distance between the beams  10  in the direction y, i.e., it does not effect the accuracy in the direction x or the parallelism of each beam  10 . That is, the rotation mechanism is capable of adjusting only the beam pitch without effecting the other optical factors. This is also true with three or more beams. 
     The screw constituting the adjusting means  28  may be replaced with a screw extending through the lug  27   b , in which case the aperture forming member  27  will be formed of a material deformable little. To turn the screw positioned at the lug  27   b , the base  1  may be formed with a hole for allowing a driver to be inserted toward the screw therethrough. 
     10th Embodiment 
     FIGS. 22-25 show a tenth embodiment of the present invention. In the conventional light source device discussed with reference to FIG. 2, the flange  205  and case  211  formed of metal and resin, respectively, are fastened together by the four screws  204 . As a result, when the temperature of the device rises, the flange  205  is caused to bend due to a difference in the coefficient of linear expansion between the two materials. Particularly, a bend in the beam pitch direction or direction y disturbs the parallelism of the individual beam; even the slightest error in parallelism is critical because of the enlargement type optics. 
     Further, stresses in the directions x and y which are causative of the above deformation are noticeably different in size and cause the flange  205  to deform in combination. As a result, the flange  205  deforms in a more complicate manner. This, coupled with the fact that the deformation of the flange  205  depends on the rigidity of the same, causes the beam pitch to vary in a complicate manner and results in defective images. This embodiment solves this problem. 
     As shown in FIGS. 22-25, a base  30  is made up of opposite mount portions  30   a , an intermediate light source portion  30 , and narrow bridge portions  30   c  respectively bridging the mount portions  30   a  and light source portion  30   b . The semiconductor lasers  2  and collimator lenses  3  are affixed to the light source portion  30 . The base  30  is therefore identical with the base  1  of FIG. 7 except for the narrow bridge portions  30   c . As for the rest of the construction, too, this embodiment is identical with the embodiment of FIG.  7 . 
     Each bridge portion  30   c  is positioned at the intermediate between an upper and a lower screw hole  30  formed in the adjoining mount portion  30   a , and is coincident with the x axis direction. The bridge portions  30   c  are shown as being as thick as the mount portions  30   a  and light source portion  30   b , but they may be thinner than them, if desired. In the illustrative embodiment, each mount portion  30   a  is symmetrical in the up-and-down direction. Even when the mount portion  30   a  is not symmetrical in the above direction, the bridge portion  30   c  should preferably be positioned at the intermediate between the upper and lower screw holes  30   d.    
     Assume that the case  9  has a greater coefficient of linear expansion than the base  30 . Then, as shown in FIG. 24, an increase in ambient temperature causes forces f 1  and f 2  to act on the front of the base  30  in such a manner as to extend it in the directions x and y, respectively. However, only pulling forces f 3  act on the light source portion  30   b  via the bridge portions  30   c . Therefore, the base  30  is free from the complicate bend stated earlier although its bridge portions  30   c  deform. Consequently, the bend of the light source portion  30   b  does no occur or occurs little in the directions x and y. Particularly, deformation in the direction y having noticeable influence on the beam pitch is substantially obviated, so that the accurate beam pitch is maintained. 
     While the illustrative embodiment includes two bridge portions  30   c , it may include only one of them or three or more bridge portions in consideration of limitations on the device or in matching relation to an application. 
     11th Embodiment 
     An eleventh embodiment of the present invention will be described with reference to FIG.  26 . The base  1  and case  9  formed of aluminum and resin, respectively, are different in the coefficient of linear expansion, as stated previously. This, coupled with the fact that the base  1  and case  9  are fastened by the screws  11  at their four corners, causes the base  1  to bend when the ambient temperature rises. As a result, the beam pitch is disturbed and brings about defective images. 
     In light of the above, this embodiment includes a back plate  31  positioned at the side opposite to the case  9  with respect to the base  1 . The back plate  1  is formed of a material having substantially the same coefficient of linear expansion as the material of the case  9 , e.g, of the same material as the case  9 . The back plate  1  is a rectangular plate having the same size as the base  1 . Holes  31   a  for the screws  11  are formed in the four corners of the back plate  1  while two holes  31   b  for the semiconductor lasers  2  are formed at the center of the back plate  1 . 
     The base  1  is sandwiched between the case  9  and the back plate  31  substantially identical in size in the directions x and y and fastened to them by the screws  11 . When temperature around the light source device rises, the case  9  and back plate  31  having the same coefficient of linear expansion expand in exactly the same manner as each other. The deformations occurring at both sides of the base  1  cancel each other and cancel the deformation of the base  1  and prevent the base  1  from bending. This protects the beam pitch from disturbance. 
     If the case  9  and back plate  31  are noticeably different in bending rigidity, then the base  1  may be caused to bend. In case of this kind of bend, the bending rigidity of the back plate  31  should preferably be substantially the same as the bending rigidity of the case  9 . 
     12th Embodiment 
     Reference will be made to FIGS. 27,  28 A and  28 B for describing a twelfth embodiment of the present invention. FIG. 27 shows the configuration the rear of the base  1 . As shown, the two through bores  1   a  are formed in the center of the base  1 . Annular ribs  1   e  each surrounds the respective bore  1   a  on the rear of the base  1  for receiving the metallic flange of the semiconductor laser  2 . In FIG. 28, two notches If are formed in each rib  1   e  on a line Y extending in the beam pitch direction (direction y) through the centers of the ribs  1   e.    
     The annular ribs  1   e  have an inside diameter slightly smaller than the outside diameter of the lasers  2 . Each laser  2  is press-fitted in the respective bore  1   a  while causing the notches  1   f  to spread. Thereafter, the notches  1   f  tend to restore their original positions and thereby increase the retaining force acting on the laser  2 . This insures the accurate position of the laser  2 . 
     The above configuration is satisfactory so long as each notch  1   f  spreads evenly in the right-and-left direction due to the insertion of the laser  2 . However, the spread of the notch  1   f  is not always even in the right-and-left direction. If the spread is not even in the above direction, then the center of the notch  1   f  in the widthwise direction shifts and displaces the laser  2 . In addition, the displacement is not constant and cannot be estimated. The displacement in the direction y would result in an error in beam pitch and therefore defective images. On the other hand, the displacement in the direction x does not bring about any defective image only if the read timing is corrected. It is therefore desirable to limit the displacement to the direction x. 
     In the illustrative embodiment, as shown in FIG. 27, the two notches  1   f  are formed in each rib  1   e  face to face on the line Y extending in the beam pitch direction (direction y) through the center of the rib  1   e . This configuration successfully limits the displacement to the direction x and thereby obviates the influence on the beam pitch ascribable to the displacement in the direction y. 
     FIGS. 28A and 28B each shows an alternative case in which a single notch  1   f  is formed in each rib  1   e . In this case, the spread of the notch  1   f  causes the rib  1   e  to deform mainly in the direction opposite to the notch  1   f  and perpendicular to the widthwise direction of the notch  1   f , as indicated by an arrow. In light of this, as shown in FIG. 28A, the notches  1   f  should preferably be positioned only on one side (top) of the ribs  1   e  on the line Y extending through the centers of the ribs  1   e . Of course, the notches  1   f  may be positioned only at the other side (bottom) of the ribs  1   e  on the line Y. In this configuration, shifts in the direction y occur in the same direction in the upper and lower ribs  1   e  and cancel each other, reducing the displacement in the direction y. 
     Alternatively, as shown in FIG. 28B, one notch  1   f  may be positioned at the bottom of the upper rib  1   e  while the other notch  1   f  may be positioned at the top of the lower rib  1   e . This, however, causes displacements in the y direction to occur in opposite directions. Such displacements would be added up and effect the beam pitch. 
     The notches  1   f  of the ribs  1   e  are capable of spreading without regard to the beam pitch direction. Therefore, the accuracy of the lasers  2  in the beam pitch direction can be maintained. 
     13th Embodiment 
     FIGS. 29,  30  and  31 A- 31 C show a thirteenth embodiment of the present invention also including the lens support portions  1   b  each having an arcuate contact surface. The collimator lenses  3  each is affixed to the arcuate adhesion surface of the associated lens support portion  1   b  by the UV-curable adhesive layer  6 , as described with reference to FIG.  8 . However, the problem is that the coefficient of linear expansion of the adhesive layer  6  is far greater than the coefficient of linear expansion of the base  1  after the curing of the layer  6 . 
     On the other hand, as shown in FIGS.  30  and  31 A- 31 C, when the temperature of the base  1  rises, the base  1  expands and increases the distance L between the collimator lenses  3  by ΔL/2 outward of the base  1 . The adhesive layer  6  also expands, as indicated by a phantom line  6 ′ in FIG.  31 A. However, because the outward expansion of the adhesive layer  6  is limited by the lens support portion  1   b , the layer  6  expands toward the center of the collimator lens  3 . Stated another way, the thickness of the adhesive layer  6  increases inward by ΔS. At this instant, if the center of the adhesive layer  6  is deviated from the center of the collimator lens  3  by δ, as shown in FIG. 31A, then the increment ΔS of the thickness of the layer  6  due to expansion includes a component ΔS′ directed toward the center of the base  1 , as shown in FIG.  31 B. That is, the adhesive layer  6  expands inward in the beam pitch direction. 
     In FIG. 31C, the direction in which the distance L between the collimator lenses  3  increases by ΔL (ΔL at each side) and the direction in which each lens  3  is displaced by ΔS′ in the direction y are opposite to each other or cancel each other. Specifically, if the center of each adhesive layer  6  is shifted outward of the base  1  by a suitable amount (e.g. δ), then the distance L between the collimator lenses  3  is equal to a value produced by subtracting the expansion component ΔS′ of the adhesive layer  6  in the direction y from the original expansion (ΔL/2). It is therefore possible to compensate for the expansion ascribable to temperature elevation and thereby confine the beam pitch accuracy in a required range or fully prevent it from varying. 
     14th Embodiment 
     FIGS. 32 and 33 show a fourteenth embodiment of the present invention. Assume that the base  1  and case  9  are respectively formed of aluminum and resin, as stated with reference to FIG.  2 . Then, when the light source is subjected to high temperature, the case  9  expands noticeably due to a difference in the coefficient of linear expansion and causes the base  1  to deform. This disturbs the beam pitch and thereby renders images defective. 
     In the illustrative embodiment, a case  33  is formed with slits  33   a  at both sides thereof. The slits  33   a  reduce the bending rigidity of the case  33 . As a result, as shown in FIG. 33, even when the case  33  deforms due to expansion, the deformation does not cause the base  1  to deform. Particularly, when the slits  33   a  are formed in both sides of the case  33  extending in the beam pitch direction, a bend in the direction y can be effectively obviated. The embodiment therefore protects the beam pitch from disturbance and thereby obviates defective images. 
     15th Embodiment 
     FIGS. 34A and 24B show a fifteenth embodiment of the present invention. The flange  205  and case  212  are fastened to each other by the four screws, as stated with reference to FIG.  2 . This brings about a problem that the thermal stress of the case  212  is transferred to the flange  205  and causes the flange  205  to bend. This embodiment is a solution to this problem. 
     As shown in FIG. 34A, in the illustrative embodiment, three of the four holes  1   d  assigned to the screws  11  are implemented as elongate slots  1   f . An elastic member  32  formed of, e.g., rubber is held between each portion of the base  1  surrounding one slot  1   f  and the screw  11  inserted in the slot  1   f  so as to obviate shaking. 
     In the above configuration, the case  9  and base  1  are affixed to each other at a single point and movable in the x-y plane within the range of the clearances between the other screws  11  and the slots  1   f . It follows that even when the case  9  expands due to temperature elevation, the base  1  is prevented from bending so long as the clearances are available between the screws  11  and the slots  1   f . If desired, the slots  1   f  may be replaced with circular bores greater in diameter than the bore  1   d.    
     The accuracy required of the beam pitch is extremely high in the direction y, but not so high in the direction x. In light of this, as shown in FIG. 34B, only two lower bores  1   d  positioned side by side in the direction x may be implemented as the elongate slots  1   f.    
     16th Embodiment 
     FIGS. 35A and 35B show a sixteenth embodiment of the present invention including an elastic lug  34  formed of a plastic in place of the screw  11 . The lug  34  is molded integrally with the case  9  at the position where the female screw is formed. The lug  34  is made up of a plurality of (four in the embodiment) posts  34   a  arranged in an annular configuration, lock pieces  34   b  formed at the ends of the posts  34   a , and guide slants  34   c  formed on the ends of the lock pieces  34   b . The base  1  is formed with the elongate slot  1   f . While the shank portion of the lug  34  constituted by the posts  34   a  has a diameter smaller than he longer diameter of the slot  1   f , the head portion of the lug  34  constituted by the lock pieces  34   b  has a diameter greater than the shorter diameter of the slot  1   f.    
     The base  1  is mounted to the case  9  by the following procedure. After the base  1  has been aligned with the case  9 , each lug  34  of the case  9  is inserted into the associated slot  1   f  of the base  1 . When the base  1  is pressed against the case  9 , the posts  34   a  of the lug  34  elastically bend inward at the same time due to the guide slants  34   c . As a result, the lug  34  is passed through the slot if until its lock portions  34   b  protrude to the other side of the base  1 . As a result, the posts  34   a  elastically restore their straight positions. In this condition, the base  1  is prevented from separating from the case  9  because the lock portions  34   b  have a greater diameter than the slot  1   f . In addition, the base  1  is biased against the case  9  due to the elasticity of the lock pieces  34   a . The base  1  and case  9  may be locked by the lug  34  at all of the four corners or at two or three of them; in the latter case, the lug  34  will be combined with the screw  11 . 
     As described above, the third to sixteenth embodiments achieve the following advantage. When the distance between the collimator lenses increases in the beam pitch direction due to the thermal expansion of the base, the adhesive layer whose center is shifted outward expands in such a manner as to cancel the expansion of the base. This successfully reduces the displacement of the lenses in the beam pitch direction and thereby reduces the variation of the distance between the semiconductor lasers in the beam pitch direction. 
     Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.