Image forming apparatus

Abstract: A laser beam scanning unit (53) adapted for scanning laser beams on a surface of a charged image bearing member (5) is provided. The laser beam scanning apparatus (53) comprises a semiconductor laser array (21) having a plurality of light emitting portions (21a) and a rotating polygon mirror (3) adapted for deflecting laser beams emitted from the light emitting portions to the image bearing member (5). The light emitting portions (21a) are disposed two-dimensionally on a surface of the semiconductor laser array. The lighting and the amount of light of the light emitting portions (21a) are discretely controlled by a control unit (60).

Description: Technical Field The present invention relates to an image forming apparatus for scanning laser beams and thereby forming a latent image on a image bearing member. Related Art Heretofore, a large number of image forming apparatuses for forming a static latent image on a image bearing member with a laser beam and for printing the image on a paper at a high speed by a electrophotographic process have been used as output units of computers, facsimile machines, multi-functional copy machines, and so forth. In recent years, there has been an urgent need for improving the output speeds of these apparatuses. Accordingly, the apparatuses have been actively improved. For example, an image forming apparatus using a rotating polygon mirror type deflecting unit deflects one laser beam with each facet and draws one scanning line. Thus, to increase the number of scanning lines in a particular time period, provided that the number of small mirror surfaces of the rotating polygon mirror is constant, the number of rotations should be increased. On the other hand, provided that the number of rotations is constant, the number of mirror surfaces of the rotating polygon mirror should be increased. To increase the number of rotations of the rotating polygon mirror, a dynamic or static bearing using pneumatic or hydraulic power is required. These bearings are expensive and difficult to handle. Thus, they are difficult to use in conventional laser printers. In contrast, provided that the number of mirror surfaces of the polygon mirror is increased, since the deflection angle becomes small, the length of the optical path following the deflecting unit becomes large. In addition, the diameter of collimated laser beams entering into an image forming optical system becomes large in proportion to the length of the optical path. Thus, the sizes of the lens and the rotating polygon mirror become large. In particular, when a high resolution is required, since the number of scanning lines are increased, the number of rotations of the polygon mirror and the length of optical path should be further increased. This situation also applies to the case where the deflecting unit is not a rotating polygon mirror. In this case, the scanning frequency and the length of the optical path following the deflecting unit increase. To solve these problems, an exposing technique for writing a plurality of scanning lines with a plurality of laser beams in one scanning sequence has been developed. This technique is referred to as a multi-beam exposing technique. To obtain a plurality of laser beams, a plurality of gas laser (for example, He-Ne) oscillators are used as a light source. In addition, a technique wherein a laser beam generated by one oscillator is time-divided into a plurality of portions by an acousto-optical modulator (AOM) or the like has been developed. As a technique for simplifying the construction of the unit and decreasing the size thereof, for example, as disclosed in Japanese Patent Laid-Open Publication Serial No. SHO 54-7328, a semiconductor laser array where a plurality of light emitting portions for radiating laser beams are integrally disposed on one device is used as a light source. Next, an image forming apparatus using a semiconductor laser array will be described. An image forming apparatus uses a laser array integrally disposed on one substrate as a light source. A beam radiating point of each light emitting portion is disposed at an edge of a semiconductor device substrate. A plurality of laser beams are collimated by a common collimator lens so that they have a particular diameter. Thereafter, the collimated laser beams are introduced into one facet of the rotating polygon mirror (deflecting unit). As the facet rotates, the laser beams are deflected. Next, the laser beams are spotted as an image through an image forming lens. Then, the image spot exposes the image bearing member and thereby a static latent image is formed. Next, in accordance with the electrophotographic process, the latent image is developed, transferred onto a paper, and then fixed. In addition, as disclosed in Japanese Patent Laid-Open Publication Serial No. SHO 54-158251, to decrease the distance of each adjacent scanning line scanned on the image bearing member at a time, the light emitting portions of the laser array are disposed at particular angles to the scanning plane. On the other hand, there is still another need of an image forming apparatus using such a semiconductor laser array which can scan laser beams at a high speed and with a high resolution. However, the conventional image forming apparatus has not satisfactorily accomplished a high-speed and high-resolution laser beam scanning technique. Disclosure of the Invention The present invention has been made with consideration of the above described points. An object of the present invention is to provide a compact image forming apparatus for scanning laser beams at a high speed and with a high resolution. A first feature of the present invention is an image forming apparatus, according to an image forming apparatus, comprising: an image bearing member for forming a static latent image thereon; a changing unit for charging the surface of said image bearing member; a laser beam scanning unit for scanning, with a plurality of laser beams, a surface of said image bearing member which is charged; and a developing unit for causing a developing agent to adhere on the surface of said image bearing member scanned with the laser beams; wherein said laser beam scanning unit comprises: a monolithic semiconductor laser array having a plurality of light emitting portions for emitting laser beams, said light emitting portions being formed on a single device substrate; a collimator lens having a focal length fc, for collimating said laser beams emitted from the plurality of light emitting portions; and a deflecting unit for deflecting laser beams emitted from colimator lens to the surface of said image bearing member; a scanning optical system for two-dimensionally forming spots on a surface of said image bearing member with the laser beams deflected by said deflecting unit, said image bearing member being moved in a direction substantially perpendicular to a scanning direction of said laser beams so that the spots are two-dimensionally formed; wherein the lighting and the amount of light of each of said light emitting portions are discretely controlled; said light emitting portions are so two-dimensionally disposed on said single device substrate that said two-dimensionally formed spots on the surface of said image bearing member form their own separate scanning lines, and when a distance between most spaced two of the plurality of light emitting portions on the semiconductor laser array ie δ max, fc/δ max is 25 or more. A second feature of the present invention is a laser beam scanning unit, according to a laser beam scanning unit, comprising: a monolithic semiconductor laser array having a plurality of light emitting portions for emitting laser beams, said light emitting portions being formed on a single device substrate; a deflecting unit for deflecting the laser beams emitted from said collimator lens; a collimator lens having a focal length fc, for collimating said laser beams emitted from the plurality of light emitting portions; and a scanning optical system for two-dimensionally forming spots on a surface of a member to be scanned with the laser beams deflected by said deflecting unit, said member to be scanned being moved in a direction substantially perpendicular to a scanning direction of said laser beams so that the spots are two-dimensionally formed; wherein the lighting and the amount of light of each of said light emitting portions are discretely controlled; and said light emitting portions are so two-dimensionally disposed on said single device substrate that said two-dimensionally formed spots on the surface of said member to be scanned form their own separate scanning lines, and when a distance between most spaced two of the plurality of light emitting portions on the semiconductor laser array is δ max, fc/δ max is 25 or more. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a schematic diagram showing a laser scanning optical system of an embodiment of the image forming apparatus in accordance with the present invention; Fig. 2 is a side view showing the image forming apparatus; Fig. 3 is a sectional view showing an optical resonator of a surface light emitting type semiconductor laser; Fig. 4 comprises schematic diagrams showing the relations of scanning lines and spot positions; Fig. 5 is an optical side view showing an optical path of a conventional laser beam scanning optical system; Fig. 6 is an optical side view showing an optical path of a conventional multi-beam scanning system; Fig. 7 is a schematic diagram showing a conventional edge emitting type semiconductor laser; Fig. 8 is an optical side of an optical path including a conventional tilt angle compensation lens. Embodiment of Image Forming Apparatus 1-1 Comparison with Related Art Before describing an embodiment of the present invention, so as to easily understand the conception thereof, the related art thereof will be described. Fig. 5 shows a sectional view of an optical path of a conventional image forming apparatus. In the figure, a laser beam which is emitted from a semiconductor laser 301 is radiated with a spread angle . This beam is shaped to a nearly parallel beam by a collimator lens 302 of a focal length fc. Each beam is collected on the reflecting surface 308 of the rotating polygon mirror by a tilt angle compensation lens 307. The beam which is deflected by the rotating polygon mirror 308 becomes a parallel beam after passing through a second tilt angle compensation lens 307'. Thereafter, the beam is imaged as a spot on a image bearing member by a scanning lens 304 of a focal length fi. On a plane in parallel with the scanning plane, since the tilt angle compensation lenses 307 and 307' do not have optical powers, the beam is kept parallel. In other words, the beam is imaged as a line image on the reflecting surface 308 of the rotating polygon mirror. Next, the operations of the tilt angle compensation lenses 307 and 307' will be described. The inclination of each reflecting surface 308 of the rotating polygon mirror has an error of the order of several ten seconds in angles to the rotating axis. Thus, the image position of the beam reflected to this surface has a deviation in the sub scanning direction on the surface of the image bearing member due to the effect of "optical lever". This deviation is too large to ignore in comparison with pitches of scanning lines. To prevent this deviation, as disclosed in Japanese Patent Laid-Open Publication Serial No. SHO 48-49315, a tilt angle compensation lens 307' which allows each reflecting surface of the polygon mirror and the surface of the image bearing member (the surface on which an image is formed) to be placed in optical conjugate positions. This tilt angle compensation lens 207' is normally a cylindrical lens or a toric lens which has an optical power only on a sub scanning plane. Even if the reflecting surface is inclined as shown with 308' of Fig. 33, the beam is always imaged in the same position on the image forming surface. However, in recent years, as the skills for using computers are improved, the needs of high output speeds of image forming apparatuses are becoming strong. Accordingly, the modification of the apparatuses is actively performed. However, a deflecting unit using a rotating polygon mirror deflects one laser beam per facet and draws one scanning line. Thus, when the number of scanning lines in a predetermined time period is increased, provided that the number of surfaces of the rotating polygon mirror is not changed, the number of rotations thereof increases. On the other hand, provided that the number of rotations is not changed, the number of surfaces of the rotating polygon mirror increases. When the number of rotations of the rotating polygon mirror is increased, one of bearings using dynamic or static pressure of gas or liquid should be used. However, these bearings are expensive in cost and difficult to handle. Thus, so far, these bearings could not be used for laser beam printers. On the other hand, when the number of surfaces of the rotating polygon mirror is increased, the deflection angle becomes small. Thus, the length of the optical path following the deflecting unit becomes long and the collimate diameter of the laser beam entered into the image forming optical system proportionally becomes large. Thus, the sizes of the lens and the rotating polygon mirror become large. In particular, when a high resolution of the image forming apparatus is also required along with a high output speed thereof, since the number of scanning lines also increases, a larger number of rotations and a longer optical path are further required. This situation also applies to an deflecting unit which is other than the rotating polygon mirror. In this situation, the scanning frequency becomes high and the length of the optical path following the deflecting unit becomes long. To solve this problem, a new exposing technique where a plurality of scanning lines are written with a plurality of laser beams in one scanning sequence was developed. This exposing technique is referred to as a multi-beam exposing technique. To obtain a plurality of laser beams, a plurality of gas laser (for example, He-Ne) oscillators are used as light sources. Alternatively, a technique where a laser beam generated by one oscillator is time-divided into a plurality of portions by an acousto-optical modulator (AOM) or the like was developed. However, as a technique for simplifying the apparatus and for reducing the size thereof, as disclosed in Japanese Patent Laid-Open Publication Serial No. SHO 54-7328, a technique where a semiconductor laser array integrally having a plurality of light emitting portions on one device is used as a light source has been proposed. This technique is referred to as a multi-beam laser scanning technique. However, when a plurality of laser beams with parallel optical axes are entered into a collimator lens, the optical axes tend to spread out with large angles. Thus, the size of the reflecting surface of the deflecting unit and the size of the lens constructing the optical system become very large in comparison with the technique where only one laser beam is used. Fig. 6 is a sectional view showing an optical path from a semiconductor laser array to a image bearing member according to the multi-beam laser scanning technique. Now, for simplicity, consider a multi-beam laser scanning optical system using two laser beams, a convex collimator lens, and a convex image forming lens. The two laser beams spaced apart by δ and emitted from a semiconductor laser array 321 are collimated by a collimator lens 302 of a focal length fc. Since the semiconductor laser array 321 is disposed at an object side focal point of the collimator lens 302, the two laser beams are intersected at an image side focal point F thereof. When a spot 306 of d 0 = 100 µ m is formed on an image plane 311, if fi is 200 mm, the diameter Wc of the beam entered into the scanning lens, that is, the collimate diameter, is given by the formula (1). The spot diameter or the beam diameter is a diameter where the intensity of the cross section of a beam is the power of the peak intensity times (1/e 2 ). The distribution of this intensity accords with the Gaussian distribution. WC = d 0 1+ ( 4λfi πd 0 2 ) 2 However, λ is the wavelength of the laser, which is 780 nm. Thus, in this example, the collimate diameter Wc is approximately 2 mm. In the so-called edge emitting type laser diode which has been conventionally used, as shown in a conceptual schematic diagram of Fig. 7, the beam spread angle of a plane which includes the optical axis and which is in parallel with the contact surface largely differs from that of a plane which includes the optical axis and which is perpendicular to the contact surface. In the case of the conventional semiconductor laser, the spread angle p on a plane in parallel with the contact surface is approximately 10 degrees in full width at half maximum. However, on a plane perpendicular to the contact surface, due to an influence of diffraction, the spread angle t becomes approximately 30 degrees in full width at half maximum. In addition, it is difficult to freely set the spread angles t and p and the ratio thereof (namely, the ratio of longer diameter and shorter diameter of the ellipse of the beam). To effectively use most of the radiated beam, it is necessary to raise the coupling efficiency of the semiconductor laser array and the collimator lens. To obtain the collimate diameter of 2 mm, the focal length fc of the collimator lens 2 should be approximately 3 mm. On the other hand, in the conventional semiconductor laser array, the distance δ of each adjacent light emitting portion cannot be set to 100 µ m or less due to the mutual interference thereof. The distance from the image side focal point F of the collimator lens 302 to the reflecting surface 308 of the deflecting unit should be spaced apart by h due to the presence of each element of the scanning unit. On the other hand, when the distance between most spaced two of a plurality of light emitting portions on the semiconductor laser array 321 is δmax, the distance q of these two beams on the reflecting surface 308 of the deflecting unit can be expressed by the following formula. q = δmax · h fc For example, when the number of beams is four and the distance of each adjacent light emitting portion disposed in line on the semiconductor laser array is 0.1 mm, δmax becomes 3 x δ = 0.3 mm. When the distance h from the image side focal point F of the collimator lens 302 to the reflecting surface 308 is 50 mm, q becomes 5 mm. Thus, the size of the reflecting surface should be at least the value where the distance q and the laser beam collimator diameter are added. In other words, the size of the rotating portion of the deflecting unit becomes large and thereby the bearing is exposed to a large load. In addition, the deflecting unit is unbalancedly rotated. According to the formula (2), as the value of fc/δmax decreases, the value of q increases. Next, the case where the above-mentioned tilt angle compensation lenses 307 and 307' are added in the scanning optical system will be considered. Since the tilt angle compensation lenses are anamorphic optical elements, the optical characteristics on the scanning plane differ from those on the sub scanning plane. As described above, since the tilt angle compensation lenses do not have optical powers on the scanning plane, by applying the above-mentioned formula (2), the distance q of most spaced two beams on the reflecting surface can be obtained on the scanning plane. Thus, it is enough to consider only the sub scanning plane. Fig. 8 is a sectional view showing an optical path on the sub scanning plane including the tilt angle compensation lens 307. The figure shows elements disposed from the semiconductor laser array 321 to the reflecting surface 308 of the deflecting unit. As described above, on the sub scanning plane, each beam is imaged as a line image on the reflecting surface 308 of the deflecting unit. Each line image is formed with a distance q on the right of the reflecting surface in the sub scanning direction. When the focal length of the tilt angle compensation lens 307 on the left of the beam deflecting unit is ft, the distance from the image side focal point F of the collimator lens 302 to the tilt angle compensation lens 307 is t1, and the distance from the tilt angle compensation lens 307 to the reflecting surface 308 of the deflecting unit is t2, provided that symbols in Fig. 6 are used, the mutual distance q' of most spaced two of laser beams entered into the tilt angle compensation lens 307 and the mutual distance q of these beams which are entered into the reflecting surface 308 are given by the following formulas (3) and (4), respectively. q' = δmax · t1 fc q = -q' · -ft·t1+t2·t1-fc·t2 ft · t2 Generally, to allow a collimated laser beam to have a beam waist on the scanning plane, the relation of ft = t2 should be satisfied. When t1 < ft, two beams are not intersected on the imageside. Thus, the relation of q > q' takes place. As the value of t1 decreases, the value of q increases. For example, when t1 is 20 mm, ft is 30 mm, and t2 is 30 mm, then q' becomes 2 mm and q becomes 3 mm. The formula (4) contains q'. By applying the formula (3), as the value of fc/δmax becomes large, the value of q' becomes small. At this point, since the relation of t1 + t2 = h is satisfied, the above-mentioned calculations can be applied on the scanning plane. In other words, in this example, the distance of most spaced two of beams in the sub scanning direction is small. This situation can also apply to the mutual distance of each of plurality of laser beams which are entered into the scanning lens 4. In other words, when the incident position of the above-mentioned tilt angle compensation lens is the incident position of the scanning lens, the formula (3) can be applied. The distance from the collimator lens to the scanning lens is larger than that of the above-mentioned example. Thus, the size of the scanning lens should be further increased. Generally, the collimator lens has the highest optical power in the optical elements which constructs the laser beam scanning optical system. In other words, the focal length of the collimator lens is the shortest in the other constructional elements of the laser beam scanning optical system. Thus, on the path where a plurality of laser beams radiated from a semiconductor laser array reach a image bearing member through an optical system, when the laser beams pass through the collimator lens, the mutual angle of each laser beam is most largely changed. To prevent this problem, a technique for adding various optical elements which cause the positions of a plurality of laser beams reflected to be narrowed has been proposed. For example, in Japanese Patent Laid-Open Publication Serial No. SHO 56-69611, an afocal optical system is disposed behind a collimator lens so as to collect each beam on a reflecting surface. However, the addition of such an optical system results in complicating the construction of the scanning optical system. Thus, it is not suitable from standpoints of cost, adjustment, and reliability. In addition, as described above, when a plurality of laser beams travel on different optical paths, the optical system should be designed so that the aberration and size of each image spot meet predetermined values for each laser beam. Thus, the number of design steps increases and the development period of the image forming apparatus becomes long. In addition, it is difficult to obtain designed solutions where all the laser beams in any positions of the scanning region satisfy the designed specifications. In other words, such requirements lead to a critical problem of an image forming apparatus with a high resolution or small diameter of image spots which requires advanced designing techniques. Moreover, the laser scanning optical system which satisfies such difficult requirements should have larger reflecting surfaces of the scanning unit and larger effective diameters of the constructional lenses than those of the conventional laser beam scanning optical system using one laser beam. In addition, the construction of the former is more complicated than that of the latter. In other words, the former apparatus requires a large number of lenses and accurate adjustments of lens positions. Thus, the conventional production facilities cannot be commonly used. 1-2 Construction of Present Invention Fig. 2 is a schematic diagram showing the overall construction of an image forming apparatus in accordance with the present invention. The process for obtaining a print result on an image transfer material 351 accords with a so-called electrophotographic process. As a image bearing member 305 of an electrophotographic printer using a semiconductor laser as a light source, an organic photoconductor (OPC) with an increased sensitivity in a longer wavelength region has been widely used. This image bearing member 305 is charged to a predetermined surface potential by a charger 352. Thereafter, a laser beam scanning unit 353 performs a light writing process, that is, a light exposing process. In accordance with image information from the laser beam scanning unit 353, a plurality of laser beams 354 whose light intensities are individually modulated are scanned on image bearing member 305 in the axial direction thereof, thereby generating electric charges which neutralize the surface potential only for the exposed portion. Thus, the absolute value of the surface potential of this portion becomes low. As a result, on the image bearing member 305, a distribution of surface potentials in accordance with the image, that is, a static latent image is formed. A developing unit 355 selectively adheres a developing agent in accordance with the surface potentials to the image bearing member 305. Thus, the static latent image is developed. This developing agent is transferred to a transfer material 351 (normally, a paper) by a transferring unit 356. The developing agent on the transfer material 351 is fixed with a thermal pressure by a fixing unit 357. Thereafter, the transfer material 351 is unloaded from the apparatus. Fig. 1 is a schematic diagram showing the construction of a laser beam scanning optical system in accordance with the present invention. In the laser beam scanning unit 353 shown in Fig. 2, the laser beams 354 are folded back and downwardly emitted. However, in this example, the illustration of the laser beams is simplified. In the figure, laser beams emitted from a plurality of light emitting portions 341a of a monolithic semiconductor laser array 341 are collimated to laser beams with a predetermined beam diameter by a collimator lens 302. The laser beams are entered into a rotating polygon mirror 303. As the polygon mirror 303 rotates, these laser beams are deflected. The laser beams which pass through an image forming lens 304 are imaged at spots 306 on the image bearing member 305. The lighting and the amount of light of each of the light emitting portions 341a is discretely controlled by a control unit 360. As a semiconductor laser array having such characteristics, a so-called surface light emitting type semiconductor laser array is used. It is more preferable to use a surface light emitting type semiconductor laser array with light emitting portions in which a group II-VI compound semiconductor is embedded. Fig. 3 is a sectional view showing one of the light emitting portions 341a two-dimensionally disposed on a device substrate 341 of the surface light emitting type semiconductor laser array. As shown in the figure, on the Ga-As device substrate 322, a semiconductor laminate reflecting layer 322 is formed. The reflecting layer 322 is composed of several ten layers of two types of Al-Ga-As compounds. On the reflecting layer 322, a clad layer 324, an active layer 325, a clad layer 326, and a contact layer 327 are disposed, each of which is composed of Al-Ga-As compounds. On the contact layer 327, a SiO2 dielectric laminate reflecting layer 328 is formed. On the entire rear surface of the Ga-As substrate, a window-shaped electrode 329 is formed. In addition, in the periphery of the dielectric laminate reflecting layer 328, a window-shaped electrode 330 is formed. Thus, all the parts formed on the Ga-As substrate compose an optical resonator. A light beam which is generated on the active layer 325 reciprocatively travels between the upper reflecting layer 327 and the lower reflecting layer 323 in the direction perpendicular to the surface of the device substrate. Thus, the light beam is oscillated. As a result, the optical axis of the laser beam 331 is nearly perpendicular to the surface of the device substrate. In the periphery of the optical resonator, a group II-VI compound semiconductor is embedded as an embedded layer 332. As a group II-VI compound semiconductor, it is preferable to use a group II-VI compound which contains two, three, or four elements selected from both group II elements Zn, Cd, and Hg and group VI elements O, S, Se, and Te. In addition, it is preferable to match the lattice constant of the compound with that of the semiconductor layers composed of the clad layer 324, the active layer 325, and the clad layer 326. Since the electric resistance of the group II-VI compound semiconductor is very large, the current is effectively closed in the optical resonator. In addition, since the refractive index of the embedded layer 332 differs from that of the Al-Ga-As semiconductor layer, the beam which travels in the optical resonator in the direction just or nearly perpendicular to the surface of the device substrate is totally reflected at the interface with the embedded layer 332. Thus, the beam is effectively closed in the optical resonator. Therefore, when such a semiconductor laser array is used, the laser oscillation starts with a very small amount of current in comparison with the conventional laser. In other words, the threshold value of the semiconductor laser array is lower than that of the conventional semiconductor laser. Thus, the amount of lost heat is small. In Fig. 3, a diode is formed on the Ga-As device substrate 322. The light generated in the active layer 325 reciprocally travels between the reflection layers 323 and 328. Thus, a laser oscillation takes place. A laser beam 331 is emitted from the reflection layer 328 whose reflectance is smaller than that of the reflection layer 328 in the direction perpendicular to the surface of the device substrate. In addition, since the sectional area (near field pattern) of the laser beam emitting portions of the surface light emitting type semiconductor laser array is larger than that of the conventional edge emitting type semiconductor laser, the spread angles of the laser beams are small. Although the value of the spread angle just depends on the area of the light emitting window, the area can be precisely controlled by an etching process or the like. Thus, the spread angle can be kept constant. For example, a laser beam with a spread angle of approximately 8 degrees in full width at half maximum can be satisfactorily obtained. Moreover, in the surface light emitting type semiconductor laser, since current and light can be effectively closed in an optical resonator of the laser, the amount of heat generated by each light emitting portion can be decreased. Moreover, when a plurality of light emitting portions are adjacently disposed, mutual optical, electrical, and thermal interferences can be remarkably decreased in comparison with the conventional edge emitting type semiconductor laser array. Thus, the distance of each adjacent light emitting portion can be decreased in comparison with the conventional semiconductor laser. In other words, the order of 50 µ m can be accomplished. As described in the paragraph of the related art, to obtain collimated beams with a diameter of 2 mm by using the above-mentioned surface light emitting type semiconductor laser, the focal length fc of the collimator lens should be approximately 8 mm. In addition, since the distance δ of each adjacent light emitting portion on the semiconductor laser array 341 can be set to as narrow as 50 µ m, in the case where four beams are disposed in line, δmax becomes 150 µ m. When the reflecting surface of the deflecting unit is disposed in the same position (against the collimator lens) as the embodiment of the related art, the distance q of each adjacent beam reflection position on the reflecting surface becomes 1/5 times the value of the embodiment of the related art. The value of fc/δmax of the embodiment of the related art is approximately 10. On the other hand, according to the present invention, the value of fc/δmax becomes approximately 53. Like the embodiment of the related art, when the distance h from the collimator lens to the reflecting surface is 50 mm, the distance q of each adjacent beam reflection position becomes approximately 0.94 mm. This value is not large in comparison with the collimate diameter Wc of the beams. In particular, consider the case where the spot diameter on the image bearing member is set to 50 µ m so as to form an image with a much higher resolution. By applying the above-mentioned formula, the collimate diameter Wc is doubled (namely, approximately 4 mm). Thus, the focal length fc of the collimator lens is also doubled. The distance q of each adjacent beam reflection position on the reflecting surface is halved. When each beam is tracked, the distance of each adjacent beam in any position on the optical axis is much smaller than the collimate diameter. Thus, even if an optical system deals with a plurality of laser beams, it is possible to design the optical system by considering only one typical beam. Thus, the design work of the laser scanning optical system is very simplified. When the accuracy of the image spots is not important, the conventional laser beam scanning optical system with one laser beam can be used as it is. In addition, in the surface light emitting type semiconductor laser, since the light emitting portions can be disposed at any positions where they do not interfere each other, they can be two-dimensionally disposed on the device. The laser beams radiated from the light emitting portions disposed on the device substrate are magnified by the optical magnification M of the scanning optical system, the distance of each adjacent light emitting portion being δ. Thereafter, the resultant beams are imaged at spots on the image bearing member, the distance of each adjacent spot being δ'. The value of M is nearly equal to the ratio of the focal length of the collimator lens and that of the scanning lens. Now, in an exposing system which scans the image bearing member with four laser beams, consider the relation between scanning lines and image spots. In addition, assume the case where four adjacent scanning lines are drawn with one scanning sequence. Now, the distance of most spaced two of image spots is represented with δ'max. When the image spots are disposed as shown in Fig. 4 (a), the value of δ'max can be decreased in comparison with the case where they are disposed in line as shown in Fig. 4 (b). The positions of the image spots on the image bearing member are similar to those of the light emitting portions on the semiconductor array. Alternatively, when the tilt angle compensation optical system is used, a mapping relation where the sub scanning direction is multiplied by a particular magnification takes place. Thus, in the same optical system, when the value of δ'max is small, the value of δmax is also small. Thus, in the arrangement shown in Fig. 4 (b), the value of q in the formula (2), (3), or (4) becomes small. Thus, the size of the reflecting surface of the deflecting unit can be accordingly decreased. As a result, the effect of the present invention can be further improved. In the above example, the case where four laser beams are used was considered. When the number of laser beams is further increased, the light emitting portions can be freely disposed on the semiconductor laser array so that the spots are disposed at the closest positions on the image bearing member. As a result, more significant effects can be obtained than the above-mentioned construction. In Fig. 4 (c), an example of the arrangement of image spots according to scanning line in the case where eight laser beams are used is shown. In other words, by applying the formula (2), when the light emitting portions are disposed in line, δmax = 7 x δ. However, in the arrangement shown in Fig. 4 (c), the value of q can be calculated substantially as δmax = 3 x δ in designing the optical system. Thus, the effects of the present invention can be further improved. In addition, since the scanning direction of the image spot 306a is the same as that of the image spot 306e, the corresponding light emitting portions can be driven in the same timing. The focal length of a collimator lens in the case where a surface light emitting type semiconductor laser array is used is larger than that in the case where the conventional edge emitting type semiconductor laser is used. Thus, the allowance of the distance between the semiconductor laser array and the collimator lens in the direction of the optical axis can becomes large. As a result, the adjustment work in the production stage can be simplified. In addition, the collimator lens can have a resistance to the deviation of its position due to temperature variation and aged tolerance. As described above, according to the image forming apparatus of the present invention, a plurality of laser beams radiated from a semiconductor laser array are collimated by a collimator lens. The resultant laser beams are deflected by a beam deflecting unit. The resultant laser beams are imaged as spots on a image bearing member through a scanning lens. Thus, an optical writing operation is performed. The focal length fc of the collimator lens of the present invention is larger than that of the embodiment of the related art. In addition, the distance δ of each adjacent light emitting portion on the semiconductor laser array is small. In particular, when the surface light emitting type semiconductor laser array is used, since the spread angles of radiated laser beams are small, the focal length fc of the collimator lens becomes long. Moreover, since the amount of heat generated in each light emitting portion is small and the electric and optical interferences of thereof are small, the distance thereof can be further decreased. In the case where a tilt angle compensating lens is not used, the distance q of most spaced laser beams disposed in line on the reflecting surface of the deflecting unit is given by the above-mentioned formula (2). In the case where the tilt angle compensating lens is used, the distance q' of most spaced beams on the tilt angle compensating lens and the distance q of that on the reflecting surface of the deflecting unit are given by the formulas (3) and (4), respectively. By applying the formulas (2) and (3), it is found that q and q' are reversely proportional to fc/δmax. Moreover, by applying the formula (4), since q is proportional to q', they are also reversely proportional to fc/δmax. In this case, on the scanning plane, the formula (2) can be applied. In other words, when the inverse number of fc/δmax is multiplied by the value equivalent to the size in the direction of the optical axis, the distance of each beam in the direction perpendicular to the optical axis can be obtained. Generally, in a small image forming apparatus which can print data on papers in A4 or similar size, the distance of each adjacent optical element in the direction of the optical axis and the focal length thereof are approximately 50 mm. This value is represented with Z. On the other hand, when converted from the resolution, the collimate diameter of the laser beams is approximately 2 mm. When the maximum distance of most spaced two of beams on each lens surface and the reflecting surface is limited to the collimate diameter, the value of δmax/fc x Z is preferably 2 mm or less. Thus, the value of fc/δmax is preferably 25 or more. In addition, when a plurality of laser beams are entered into the scanning lens and the distance of most spaced two of beams is 2 mm, the value of Z should be approximately 100 mm. Thus, according to the above-mentioned calculation, the value of fc/δmax is preferably 50 or more. As described above, when the maximum value of the distance of each adjacent beam on each lens and the reflecting surface is nearly the same as the value of the collimate diameter of the laser beams, the sizes of each lens and the reflecting surface are not remarkably larger than the size of an optical system which scans one laser beam. In addition, when the distance of each adjacent laser beam is smaller than the collimate diameter, a plurality of laser beams can be substantially treated as one laser beam in designing the optical system. The above-mentioned embodiment is only one embodiment of the present invention. When the spread angle of each beam is small and the distance of each adjacent light emitting portion is small, the same effects can be obtained. In addition, it should be noted that even if the rotating mirror is constructed and/or produced by using a different method such as a plastic injection molding method, the similar effects of the present invention can be attained. Moreover, besides the rotating polygon mirror as the deflecting unit, with a galvano mirror or the like, the same effects can be similarly acquired. Further, the construction of the surface light emitting type laser described in the above-mentioned embodiment is only an example which can be accomplished. When the above-mentioned relation of the focal distance of the collimate lens and the distance of each adjacent light emitting portion is satisfied, the same effects of the present invention can be obtained regardless of the construction thereof. It is obvious that the scope of the applications of the image forming apparatus of the present invention includes facsimile machines and display units as well as printing apparatus such as the printers and copy machines. In these apparatuses and units, the same effects of the present invention can be attained. 1-3 Effects As described above, according to the image forming apparatus of the present invention, in an exposing technique using a plurality of laser beams, with a semiconductor laser array which satisfies predetermined conditions with respect to the focal length of a collimator lens and the distance of each adjacent light emitting portion, the size of the reflecting surface of a scanning unit and the effective diameter of each lens can be decreased without necessity of adding auxiliary optical elements. Thus, the size of the scanning optical system or the image forming apparatus can be reduced and thereby the cost thereof can be decreased. In addition, since a plurality of laser beams travel on almost the same optical path, a scanning optical system can be designed in the same manner as that with one laser beam. Thus, the number of designing steps of the system can be remarkably reduced and the developing period thereof can be shortened. Moreover, the scanning optical system with one laser beam can be used. Thus, the producibility is remarkably improved. Further, when a plurality of laser beams enter each optical system which constructs the scanning optical system, provided that the distance of most spaced two of beams is shorter than the collimate diameter of the laser beams, the conventional scanning optical system with one laser beam can be used as it is. In other words, without any modification of the scanning optical system of the image forming apparatus using one laser beam, by increasing the number of laser beams, an image forming apparatus with a high speed can be produced. Thus, unexpected benefits can be obtained in producing the products. Moreover, since the spread angles of laser beams are small, the distance between the collimator lens and the semiconductor laser array can be increased. The adjustment allowance in the direction of the optical axis of the collimator lens is increased. Thus, as well as the increase of producibility, without influences of aged deterioration and temperature change, images with a predetermined spot diameter can be exposed. As a result, the image quality is improved. Industrial Utilization The image forming apparatus according to the present invention can print data on papers at a high speed by using an electrophotographic process. The image forming apparatus can be used as an output unit of a computer, a facsimile machine, a multi-functional copy machine, and so forth.

Claims: An image forming apparatus, comprising : an image bearing member (311) for forming a static latent image thereon; a changing unit for charging the surface of said image bearing member (311) ; a laser beam scanning unit (304) for scanning, with a plurality of laser beams, a surface of said image bearing member (311) which is charged; and a developing unit for causing a developing agent to adhere on the surface of said image bearing member (311) scanned with the laser. beams; wherein said laser beam scanning unit (304) comprises: a monolithic semiconductor laser array (321) having a plurality of light emitting portions (341a) for emitting laser beams, said light emitting portions (341a) being formed on a single device substrate (341); a collimator lens (302) having a focal length fc, for collimating said laser beams emitted from the plurality of light emitting portions; and a deflecting unit (308) for deflecting laser beams emitted from collimator lens (302) to the surface of said image bearing member (311); a scanning optical system for two-dimensionally forming spots on a surface of said image bearing member with the laser beams deflected by said deflecting unit (308), said image bearing member being moved in a direction substantially perpendicular to a scanning direction of said laser beams so that the spots are two-dimensionally formed; wherein the lighting and the amount of light of each of said light emitting portions (341a) are discretely controlled; said light emitting portions (341a) are so two-dimensionally disposed on said single device substrate that said two-dimensionally formed spots on the surface of said image bearing member form their own separate scanning lines, and when a distance between most spaced two of the plurality of light emitting portions on the semiconductor laser array (321) is δ max, fc/ δ max is 25 or more. The image forming apparatus according to claim 1, wherein each of the laser beams emitted from the light emitting portions has a spread angle of 20 degrees or less in full width half maximum. A laser beam scanning unit, comprising: a monolithic semiconductor laser array (321) having a plurality of light emitting portions (341a) for emitting laser beams, said light emitting portions (341a) being formed on a single device substrate; a deflecting unit (308) for deflecting the laser beams emitted from said collimator lens (302); a collimator lens (302) having a focal length fc, for collimating said laser beams emitted from the plurality of light emitting portions; and a scanning optical system for two-dimensionally forming spots on a surface of a member to be scanned with the laser beams deflected by said deflecting unit (308), said member to be scanned being moved in a direction substantially perpendicular to a scanning direction of said laser beams so that the spots are two-dimensionally formed; wherein the lighting and the amount of light of each of said light emitting portions (341a) are discretely controlled; and said light emitting portions (341a) are so two-dimensionally disposed on said single device substrate that said two-dimensionally formed spots on the surface of said member to be scanned form their own separate scanning lines, and when a distance between most spaced two of the plurality of light emitting portions on the semiconductor laser array (321) is δ max, fc/ δ max is 25 or more. The laser beam scanning unit according to claim 3, wherein each of the laser beams emitted from the light emitting portions has a spread angle of 20 degrees or less in full width half maximum.

Inventors: INOUE NOZOMU, KOGA YOSHIRO, KURIHARA HAJIME, TAKADA KYU, INOUE, NOZOMU, KOGA, YOSHIRO, KURIHARA, HAJIME, TAKADA, KYU, Inoue, Nozomu, Seiko Epson Corporation, Koga, Yoshiro, Seiko Epson Corporation, Kurihara, Hajime, Seiko Epson Corporation, Takada, Kyu, Seiko Epson Corporation
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1992
Main classification: H04N1
IPC codes: H04N1, B41J2, G06K15, G02B26