IMAGE-FORMING DEVICE

Abstract: A laser beam scanner (53) is provided to project a laser beam onto the surface of an electrically charged image carrier (5). The laser scanner (53) has a semiconductor laser array (21) equipped with a plurality of light-emitting portions (21a) and a rotary polygonal mirror (3) that deflects the laser beam from the light-emitting portions (21a) toward the image carrier (5). The light-emitting portions (21a) are two-dimensionally arranged on the surface of the semiconductor laser array. The light-emitting portions (21a) are individually controlled for their turn-on and quantity of light by a controller (60). There is thus provided a high-speed and high-resolution laser beam scanner that has a low scanning frequency and a short optical path.

Description: The present invention relates to an image forming apparatus for scanning laser beams and thereby forming a latent image on an image bearing member. 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. JP-U-61 53 725 discloses a two-dimensional array of light sources, in which each light source is fixed to a support which is then mechanically adjusted to set the distance between the light sources. 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. JP-A-1-44 085 discloses a semiconductor laser array having a plurality of light-emitting portions disposed two-dimensionally on a surface of the array. 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. 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, comprising a image bearing member for forming a static latent image thereon, a charging unit for charging the surface of the image bearing member, a laser beam scanning unit for scanning a plurality of laser beams on the surface of the image bearing member which is charged, and a developing unit for causing a developing agent to adhere on the surface of the image bearing member scanned with the laser beams, wherein the laser beam scanning unit comprises a semiconductor laser array having a plurality of light emitting portions for emitting laser beams, the light emitting portions being formed on a device substrate, and a deflecting unit for deflecting laser beams emitted -from the light emitting portions to the surface of the image bearing member, wherein the light emitting portions are disposed two-dimensionally on a surface of the semiconductor laser array, and wherein the lighting and the amount of light of each of the light emitting portions are discretely controlled. A second feature of the present invention is a laser beam scanning unit, comprising a semiconductor laser array having a plurality of light -emitting portions for emitting laser beams, the light emitting portions being formed on a device substrate, and a deflecting unit for deflecting the laser beams emitted from the light emitting portions, wherein the light emitting portions are disposed two-dimensionally on a surface of the semiconductor laser array, and wherein the lighting and the amount of light of each of the light emitting portions are discretely controlled. Brief Description of Drawings Fig. 1 is a schematic diagram showing a laser scanning optical system of a first embodiment of an 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 array; Fig. 4 is a perspective view showing a light emitting portion of a phase-lock surface-light-emitting type semiconductor laser array; Fig. 5 comprises schematic diagrams showing the relation of scanning lines and spot positions; Fig. 6 comprises schematic diagrams showing an optical resonator of a light emitting portion of the phase-lock surface-light-emitting type semiconductor laser array; Fig. 7 is a schematic diagram showing the conception of a conventional edge emitting type semiconductor laser array; Fig. 8 is an optical side view showing an optical path of a conventional laser scanning optical system; Fig. 9 is a schematic diagram showing the relation of scanning lines and spot positions; Fig. 10 is a graph indicating reflectances of P and S polarization of a conventional metallic mirror; Fig. 11 is a schematic diagram showing a method of adjusting the collimate diameter of a laser beam; Section 1 First Embodiment of Image Forming Apparatus 1-1 Comparison with Related Art Before describing a first embodiment of the present invention, the related art thereof will be first described as conducive to a full understanding of the conception thereof. Fig. 7 shows a conventional semiconductor laser array for use in an image forming apparatus. As shown in the figure, in a semiconductor laser array 1 which emits a laser beam, the spread angle of a laser beam on a plane which includes the optical axis thereof and which is in parallel with the contact surface largely differs from that on a plane which contains the optical axis and which is perpendicular to the contact surface. In the figure, the spread angle p of a conventional laser diode on the plane in parallel with the contact surface is approximately 10 degrees in full width at half maximum. On the other hand, the spread angle t on the plane perpendicular to the contact surface is as large as 30 degrees in full width at half maximum due to the influence of diffraction. In addition, it is difficult to freely set the values of the spread angles t and p and the ratio thereof (in other words, the ratio of the longer diameter and the shorter diameter of the ellipse). As a result, the position of the beam waist on the parallel plane differs from that of the vertical plane by δ. The value of δ is generally referred to as an astigmatic difference. Due to this astigmatic difference, the beam which leaves a collimator lens is not in parallel with either the scanning plane or the direction perpendicular thereto or both of them. Thus, the beam cannot be precisely spotted on an image bearing member, but it has an aberration. Since the focal length of an image forming lens for use in a conventional laser printer is long and the spot diameter thereof is large, this aberration does not lead to a considerable problem. However, as the need for high resolution printers arises in recent years, aberration is becoming a critical problem to solve. As one of the techniques for solving this problem, a beam shaping optical system which comprises a so-called anamorphic lens set where the power on the vertical plane differs from that on the horizontal plane is used to compensate for the astigmatic difference. However, such a beam shaping optical system is liable to raise the cost of the final product and to increase the size thereof. In addition, this system cannot be easily applied to an application which scans a plurality of laser beams. Moreover, since a laser beam is emitted from an end face of the semiconductor laser array 1, the light emitting portion of the laser beam should be necessarily disposed one-dimensionally in line. To obtain a large number of laser beams, since the laser beams are arranged in line, the effective diameter of the optical system becomes large. Further, because the size of the spread angle is large, the focal length of the collimator lens which collimates the beams becomes as small as several millimeters. Even if the distance between the semiconductor laser array and the collimator lens slightly varies for example by the order of several 10 µm, the resultant rays (parallel beams) are not collimated rays. Thus, the diameter of the beam introduced into an image forming optical system deviates and the size of the image spot on the image bearing member varies. Therefore, the allowable ranges of the semiconductor laser and the collimator lens become very small. As a result, the producibility is low. In addition, the position of the collimator lens which has been precisely adjusted is moved due to temperature change of the adjacent portions of the optical system and aging deformation due to aging of the constructional parts. Thus, the diameter of the image spot varies and thereby the image quality deteriorates. Furthermore, when a plurality of laser beams having parallel optical axes enter the collimator lens, the optical axes are spread with large angles. Now, for simplicity, a laser scanning optical system using two laser beams, a convex collimator lens, and a convex image forming lens will be considered. Fig. 8 is an optical side view showing the optical path of this optical system. The two laser beams spaced apart by d and emitted from the semiconductor laser array 1 are collimated by the collimator lens 2 of a focal length fc. Since the semiconductor laser array 1 is disposed at an object side focal point of the collimator lens 2, the two laser beams are intersected at an image side focal point F. To form images of the two laser beams which are nearly parallel on an image plane 11, the image forming lens 4 of the focal length fi is disposed in such a way that the object side focal point thereof matches the image side focal point F of the collimator lens 2. Since the mirror surface of the deflecting unit does not have an optical power, it is omitted in the figure. When a spot 6 of 100 µm is imaged on the image plane 11, if fi is 200 mm, the diameter Wc of the beam introduced into the image forming lens, that is, the collimate diameter, is approximately 2 mm. 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 x (l/e 2 ). The distribution of this intensity accords with the Gaussian distribution. To obtain the above-mentioned beam diameter of 2 mm, the focal length fc of the collimator lens 2 should be approximately 3 mm. As shown in Fig. 8, the distance d' between these spots is obtained by multiplying the ratio of fc and fi by d. In the conventional semiconductor laser arrays, it is difficult to set the distance between each light emitting portion to 100 µm or below due to mutual interference thereof. Thus, in this example, the spot distance d' on the image plane can be expressed as follows. d' = fi fc × d = 200 3 × 0.1 = 6.6 (mm) In addition, when a so-called tilt angle compensation optical system which compensates for the difference of the tilt angle of each facet of a rotating polygon mirror is used, the angle made of the optical axis of each laser beam sometimes becomes large depending on the relative distance between each lens and the collimator lens. To overcome this problem, various countermeasures such as adding another lens are considered. For example, as disclosed in Japanese Patent Laid-Open Publication Serial No. SHO 58-211735, a construction for compensating for the mutual angle of the optical axis of each laser beam by using a prism has been proposed. However, this construction leads to a complicating the optical system. Thus, the cost of the final product rises and the adjustment of the optical system becomes difficult. In Fig. 8, for simplicity, the tilt angle compensating lens is omitted. Next, referring to Fig. 9, the relation of spot positions and scanning lines on a conventional image bearing member is shown. In this example, there are four spots. In other words, images are formed with four laser beams. As described above, since the laser scanning optical system is an enlarging optical system, the distance of each adjacent spot on the semiconductor laser array is enlarged on the image bearing member to d' as shown in the figure. Normally, the distance of each adjacent spot on the image bearing member is much larger than the distance P of each adjacent scanning line 9. For example, when the resolution is 300 dpi (that is, the number of dots per inch (= 25.4 mm)), although P = 25.4 / 300 = 84.7 µm, the distance of each adjacent spot becomes as large as 6.7 mm. Thus, the angle α made of a line 12 connecting the center of the spot 6 and each scanning line 9 becomes very small as follows. α = sin -1 P d' = 0.72 (deg) In addition, the line which connects the light emitting portions on the semiconductor laser array 1 (that is, the edge of the contact surface) should have an angle of a to the scanning plane. As the value of α becomes small, very fine adjustment is required. Generally, a laser beam emitted from a semiconductor laser is linearly polarized. The direction of the plane of polarization of the laser beam is determined by the inclination of the contact surface of the semiconductor laser array. However, the reflectance on a reflecting surface depends on the incidence angle to the mirror surface. In addition, the reflectance of P polarized light differs from that of S polarized light. Fig. 10 shows respective reflectances Rp and Rs of P polarized light and S polarized light on a metallic mirror. As the polygon mirror rotates, the incidence angle of the beam relative to the mirror surface varies. Thus, as shown in the figure, the amount of light of the laser beam represented as a composition of P polarized light and S polarized light also varies. In particular, when the deflection angle of the polygon mirror is large, the amount of light of the laser beam remarkably varies. To overcome this problem, as disclosed in Japanese Patent Laid-Open Publication Serial No. SHO 58-42025, a technique for inclining the plane of polarization about the rotating axis of the rotating polygon mirror by 45 degrees has been proposed. However, as described above, in the edge emitting type semiconductor laser array 1, due to the restriction of the distance of each adjacent scanning line, the inclined angle is also restricted. Thus, this technique cannot be used. In this case, the plane of polarization should be rotated with a 1/4λ plate or the like. In addition, each laser beam emitted from the semiconductor laser array 1 enters the same collimator lens 2. At this point, as shown in Fig. 8, the collimate diameter Wc of each laser beam is determined by the spread angle  thereof and the distance fc between the semiconductor laser array 1 and the collimator lens 2. However, since this distance fc of each adjacent laser beam is the same, the collimate diameter Wc is determined by only the spread angle  of the laser beam. Nevertheless, in the conventional edge emitting type semiconductor laser array 1, the spread angle varies depending on each light emitting portion, so does the collimate diameter Wc of each laser beam. As a result, the size of each spot where each collimated laser beam is imaged also varies. In a conventional laser scanning optical system which uses only one (single) laser beam, as shown in Fig. 11, with aperture stop 13 disposed either in the front or behind the collimator lens 2, the beam is shaped so as to adjust the collimator diameter Wc. However, as shown in Fig. 8, when a plurality of laser beams in a bundle are used, the aperture stop can be disposed at only the a focal point of the collimator lens. Generally, in a conventional semiconductor laser array, unless the current which flows in an optical resonator exceeds a predetermined value, laser oscillation does not take place. This current value is referred to as the threshold current value. In the conventional semiconductor laser array, the threshold current value is as the high as several 10 mA. The heat generated by the current adversely affects the characteristics of the laser beams, such as shifting the oscillation wavelength. Thus, the heat generated by the semiconductor laser array is a problem to be solved. In particular, a semiconductor laser array which emits a plurality of laser beams has a number of heat sources equal to the number of light emitting parts. This heat has been an obstacle in integrally constructing a large number of light emitting parts. 1-2 Organization of the Present Invention Next, an embodiment of the present invention will be described. Fig. 2 is a schematic diagram showing the overall structure of an image forming apparatus in accordance with the present invention. The process for obtaining a print result on an image transfer material 51 accords with the so-called electrophotographic process. As a image bearing member 5 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 is widely used. This image bearing member 5 is charged to a predetermined surface potential by a charger 52. Thereafter, a laser beam scanning unit 53 performs a light writing process, that is, a light exposing process. In accordance with image information from the laser beam scanning unit 53, a plurality of laser beams 54 whose light intensities are individually modulated are scanned on image bearing member 5 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 5, a distribution of surface potential in accordance with the image, that is, a static latent image is formed. A developing unit 55 selectively adheres a developing agent in accordance with the surface potentials to the image bearing member 5. Thus, the static latent image is developed. This developing agent is transferred to a transfer material 51 (normally, a paper) by a transferring unit 56. The developing agent on the transfer material 51 is fixed with a thermal pressure by a fixing unit 57. Thereafter, the transfer material 51 is unloaded from the apparatus. Fig. 1 is a schematic diagram showing the organization of a laser beam scanning unit 53 for use in the image forming apparatus of the present invention. In the laser beam scanning unit 53 shown in Fig. 2, the laser beams 54 are folded back and downwardly emitted. However, in the figure, the illustration of the laser beams is simplified. In Fig. 1, a semiconductor laser array 21 comprises a plurality of light emitting portions 21a which are disposed two-dimensionally on a device substrate 22 (see Fig. 3). Laser beams emitted from the light emitting portions 21 are collimated to laser beams with a predetermined beam diameter by a collimator lens 2. The lighting and the amount of light of each of the light emitting portions 21a is discretely controlled by a control unit 60. The laser beams are introduced into one facet of a rotating polygon mirror 3. As the polygon mirror 3 rotates, these laser beams are deflected. The laser beams which pass through an image forming lens 4 are imaged at spots 6 on the image bearing member 5. In Fig. 1, for simplicity, a tilt angle compensation lens is omitted. As the semiconductor laser array 21 having such characteristics, it is preferable to use a so-called surface light emitting type semiconductor laser array. 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 21a of the surface light emitting type semiconductor laser array 21. In the figure, each of the light emitting portions two-dimensionally disposed on the device substrate 22 is provided with one optical resonator. As shown in the figure, on the Ga-As device substrate 22, a semiconductor laminate reflecting layer 23 is formed. The reflecting layer 23 is composed of several tens of layers of two types of Al-Ga-As compounds. On the reflecting layer 23, a clad layer 24, an active layer 25, a clad layer 26, and a contact layer 27 are disposed, each of which is composed of Al-Ga-As compounds. On the contact layer 27, a SiO 2 dielectric laminate reflecting layer 28 is formed. On the entire rear surface of the Ga-As substrate, a window-shaped electrode 29 is formed. In addition, in the periphery of the dielectric laminate reflecting layer 28, a window-shaped electrode 30 is formed. Thus, all the parts formed on the Ga-As substrate constitute an optical resonator. A light beam which is generated on the active layer 25 reciprocatively travels between the upper reflecting layer 28 and the lower reflecting layer 23 in the direction perpendicular to the surface of the device substrate 22. Thus, the light beam is oscillated. As a result, the optical axis of the laser beam 31 is substantially perpendicular to the surface of the device substrate 22. In the periphery of the optical resonator, a group II-VI compound semiconductor is embedded as an embedded layer 32. As the 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 24, the active layer 25, and the clad layer 26. Since the electric resistance of the group II-VI compound semiconductor is very high, the current is effectively closed in the optical resonator. In addition, since the refractive index of the embedded layer 32 differs from that of the Al-Ga-As semiconductor layer, the beam which travels in the optical resonator in the direction exactly just or substantially perpendicular to the surface of the device substrate 22 is totally reflected at the interface with the embedded layer 32. Thus, the beam is efficiently closed in the optical resonator. Therefore, when such a semiconductor laser array 21 is used, the laser oscillation starts with a very small amount of current in comparison with the conventional laser array. In other words, the threshold value of the semiconductor laser array 21 is lower than that of the conventional one. Thus, even if a plurality of light emitting portions 21a are disposed on the single device substrate 22, the amount of lost heat is small and thereby high optical power or a large number of light emitting portions 21a can be obtained. In addition, since the sectional area (near field pattern) of the laser beam emitting portions (light emitting portions) 21a of the surface light emitting type semiconductor laser array 21 is larger than that of the conventional edge emitting type semiconductor laser array, the spread angles of the laser beams are small. Although the value of the spread angle is determined by 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. Moreover, the ratio of the length and width of the spread angle of a laser beam, namely, the ratio of the longer diameter and shorter diameter of a beam whose cross section is in an elliptic shape can be freely set with the shape of the light emitting window. For example, when the shape of the window is a perfect circle, a laser beam with a circular cross section or an isotropic spread angle can be obtained. Thus, the astigmatic difference on the cross section in the direction of the optical axis of a beam is small. However, in a conventional laser beam printer, the image spot of a laser beam on the image bearing member is sometimes in an elliptic shape where the scanning direction of the laser beam accords with the minor axis thereof. Since the spot moves in the scanning direction for the lighting time of the laser beam and thereby the image is expanded, the image should be corrected. Thus, the shape of the cross section of the laser beam which enters the image forming optical system is preferably elliptic. As described above, in the surface light emitting type semiconductor laser array 21, since the elliptic ratio of each emitted laser beam can be freely controlled, the laser beam with a cross section whose major axis accords with the scanning plane and whose ratio of the major axis and the minor axis is appropriate can be introduced into the image forming optical system. Moreover, as described above, in the surface light emitting type semiconductor laser array 21, the spread angle of the laser beams from respective light emitting portions 21a can be made uniform. Thus, the diameter of each laser beam introduced into the collimator lens or the image forming optical system can be kept nearly constant. As a result, the size of each image spot on the image bearing member can be kept constant. As the sectional area of the optical resonator increases on the surface of the device substrate 22 of the surface light emitting type semiconductor laser array 21 becomes large, laser oscillations of high order modes along with zero-th mode start. The distribution of the amount of light of the image spot has several peaks. Thus, this situation is not suitable for forming a static latent image on the image bearing member 5. To prevent this situation, a plurality of small optical resonators are closely disposed, and the phases of the laser beams oscillated are synchronized. As a result, the light emitting portions 21a which are large in size and oscillated in the zero-th mode can be obtained. Fig. 4 shows a partial sectional view of a light emitting portion 21a of the phase-lock surface-light-emitting type semiconductor laser array 21. In this semiconductor laser array 21, a plurality of optical resonators are disposed at very short intervals. The bottom of the embedded layer 32 does not reach the active layer 25. Thus, rays which leak out from the adjacent optical resonators adversely affect each other through the clad layer 26 below the embedded layer 32. Thus, laser beams are oscillated with the same phase. As a result, the plurality of adjacent optical resonators operate as a single optical resonator. Since the wave faces of beams emitted from the optical resonators are matched, these optical resonators operate as a surface laser emitting source. Thus, the apparent area of the light emitting portions becomes large. The spread angles of laser beams are as small as 2 degrees or less in full width at half maximum. As described above, in the phase-lock surface-light-emitting type semiconductor laser array 21, the spread angles of the laser beams are small in comparison with the conventional semiconductor laser. With respect to this characteristic, the relation between the embodiment of the present invention and the embodiment of the related art will be described. When the spread angle of a laser beam is 2 degrees in full width at half maximum and the laser beam is introduced into an image forming optical system with a beam diameter of 2 mm, the focal length fc of the collimator lens becomes approximately 35 mm. Since the focal length fc of the collimator lens 2 can become long, the adjustment allowance of the distance of the collimator lens 2 to the semiconductor laser array 21 is increased. In addition, when the distance of each adjacent laser beam emitted from the semiconductor laser array 21 is d, the distance d' of each adjacent image spot can be given by the following formula. d' = fi fc × d = 200 35 × 0.1 = 0.57 (mm) When the distance of each adjacent scanning line is 84.7 µm which is the same as that of the related art, the angle α made of each spot and the scanning plane is given by the following formula. α = sin -1 P d' = 8.55 (deg) Thus, the angle α of the semiconductor laser array 21 is much larger than that of the conventional semiconductor laser array (see Formula (3)). Therefore, the mounting adjustment in the direction of the optical axis of the semiconductor laser array 21 can be easily performed. In addition, depending on the machining tolerance of each part, the part can be mounted without necessity of the adjustment of the angle α. Moreover, since the image spots and the laser beams are closely disposed, the effective diameter of the optical system becomes small as shown in the figure. Further, by excessively decreasing the spread angle of each laser beam, after the laser beam travels from the semiconductor laser array 21 to the rotating polygon mirror 3 to the image forming lens 4, the laser beam is not largely spread. Thus, even on the incidence plane of the image forming lens 4, the size of the laser beam can be decreased enough to obtain a predetermined diameter of an image spot. In other words, a collimator lens which collimates a laser beam with a predetermined collimate diameter is not required in contrast to the conventional laser scanning optical system. However, according to the deflection angle of the rotating polygon mirror 3, the length of the optical path varies, so does the size of the laser beam introduced into the image forming lens 4. Thus, there should be provided an optical system which corrects the size of the laser beam. Nevertheless, the image forming lens 4 can be easily provided with such an optical function. Thus, the number of constituent parts of the overall optical system can be decreased. Moreover, in the surface light emitting type semiconductor laser array 21, provided that beams of the light emitting portions 21a do not interfere with each other, these light emitting portions 21a can be disposed at any positions of the device substrate 22. Thus, the light emitting portions 21a can be two-dimensionally disposed on the device substrate 22. Now, an exposure system which scans an image bearing member 5 with four laser beams like the optical system of the embodiment of the prior art as shown in Fig. 8 will be considered. When four laser beams are disposed as shown in Fig. 5 (a), the mutual angles and distances of each of the adjacent laser beams can be decreased in comparison with the case where four laser beams are disposed in line as shown in Fig. 5 (b). As a result, the size of the optical system can be accordingly decreased. 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 21a can be freely disposed on the semiconductor laser array 21 so that the spots 6 are disposed at the closest positions on the image bearing member 5. As a result, more significant effects can be obtained than by the above described arrangements. In Fig. 5 (c), an example of the arrangement of image spots 6 to scanning lines 9 in the case of eight laser beams is shown. The relative positions of image spots are not always similar to the positions of the light emitting portions 21a on the semiconductor laser array 21. For example, like the above-mentioned optical system for compensating tilt angle of the rotating polygon mirror 3, if an optical element wherein the optical characteristics in the scanning direction differ from those in the perpendicular direction thereof is disposed midway in the optical path of laser beams, the angle and distance of each adjacent laser beam in the scanning direction sometimes differ from those in the perpendicular direction thereof. However, in such a situation, according to the conventional edge emitting laser array, the light emitting portions disposed in line at most allow image spots in line to be disposed on the image bearing member. In contrast, according to the present invention, the above-mentioned effect where the light emitting portions 21a are disposed two-dimensionally on the device substrate 22 can be likewise accomplished. In the surface light emitting type semiconductor laser array 21, generally the emitted laser beams are linearly polarized. The direction of the polarization depends on the shape of the plane of the optical resonators on the device substrate 22. The plane of polarization substantially accords with the longitudinal direction of the shape of the plane of each optical resonator. For example, when the optical resonator is in an elliptic shape, the direction of the major axis thereof becomes the plane of polarization. As described above, in the phase synchronizing type semiconductor laser array 21, one light emitting portion is composed of a plurality of phase synchronizing optical resonators. The number of optical resonators is for example four. At this point, the shape of the cross section of the laser beams emitted is a composite shape of laser beams emitted from all the optical resonators. Thus, in accordance with the arrangement of the optical resonators, the shape of the cross section of the laser beams emitted can be freely set. In this case, the orientation of the plane of polarization depends on the shape of the plane of each optical resonator. Thus, in the case where laser beams in a composite elliptic shape are obtained, the major axis and the direction of the plane of polarization can be independently set. Fig. 6 (a) schematically shows this situation. This figure is a plan view of one light emitting portion 42 on a semiconductor laser array, the light emitting portion 42 being seen from the beam emitting side. As shown in Fig. 6 (a), one light emitting portion 42 is composed of four optical resonators 41 which phase-synchronously oscillate. The plane of polarization 43 of a laser beam emitted from each optical resonator 41 is inclined by 45 degrees as shown in Fig. 6 (a). The major axis of the composite elliptic laser beam accords with the vertical direction of the light emitting portion. In addition, as shown in Fig. 6 (b), when the planes of polarization 43 of the laser beams emitted from the optical resonators 41 are oriented at different angles, the composite laser beam nearly becomes circularly polarized light. As described above, in the conventional laser beam printer, an image spot 6 on the image bearing member is often in an elliptic shape where the minor axis thereof accords with the scanning direction. When the semiconductor laser array 21 is disposed in such a way that the orientation of the plane of polarization of a laser beam from each optical resonator is inclined to the direction of the major axis of the elliptic cross section of the composite laser beam by 45 degrees and the major axis of the composite beam is matched with the scanning direction, the plane of polarization of the laser beam from each optical resonator is inclined to the beam scanning plane by 45 degrees. As a result, the plane of polarization is inclined to the rotating axis of the rotating polygon mirror 3 by 45 degrees. Thus, as shown in Fig. 10, the difference of reflectances of laser beams according to incidence angles thereof to the rotating polygon mirror becomes small. This situation also applies to a laser beam with a cross section of an elliptic shape, the beam being nearly circularly polarized light. In some optical systems, the minor axis of a laser beam emitted from the semiconductor laser array 21 is matched with the scanning direction. In this situation, the same effects can be obtained. It should be appreciated that the above-mentioned embodiment is an example of the present invention. For example, by using a Galvano mirror or a hologram disk instead of the rotating polygon mirror 3, the same effects can be obtained. In addition, regardless of whether the collimator lens, the tilt angle compensation lens, and/or the image forming lens is used or not, the same effects can be obtained. Moreover, even if the construction and/or relative positions of these lenses are changed, the same effects of the present invention can be likewise achieved. Furthermore, the image forming apparatus of the present invention can be used for facsimile machines, display units, and so forth as well as printing units such as printers and copy machine. 1-3 Effects As described above, in the image forming apparatus of the present invention, since the exposing technique using the semiconductor laser array which scans the image bearing member with a plurality of laser beams, a high-speed and high-resolution scanning unit can be accomplished with a low scanning frequency and a short length of the optical path. Thus, the size and cost of the apparatus can be reduced. In addition, by using a surface light emitting type semiconductor laser array for the above-mentioned semiconductor laser array, the following results can be obtained. (1) Since the spread angle of each leaser beam is small and the distance between the collimator lens and the semiconductor laser array is large, the adjustment allowance in the direction of the optical axis of the collimator lens is increased and thereby the producibility of the apparatus is improved. Moreover, without influences of aged deterioration and temperature fluctuation in operation, an image can be exposed with a predetermined spot diameter. As a result, the image quality is improved. (2) When the light emitting portions are disposed in an array shape, the spread angle of a beam from each light emitting portion less deviates, so does the diameter of each image spot. In addition, the angle between each adjacent beam and the distance of each adjacent image spot can be decreased. As a result, the construction of the optical system can be simplified and the effective areas of each lens and deflecting unit can be decreased. Therefore, this feature contributes to reducing the cost of the apparatus. (3) Since the light emitting portions of the surface light emitting type semiconductor laser array can be disposed two-dimensionally, the effective areas of each lens and deflecting unit can be further reduced. (4) Since the distance of each adjacent image spot is not larger than that of each adjacent scanning line, the allowance of the mounting angle of the optical axis of the semiconductor laser array becomes large. Thus, this mounting angle can be easily adjusted. The deviation of the distance of each adjacent scanning line becomes small. As a result, the image quality can be improved. (5) Since the astigmatic difference of the surface light emitting type semiconductor laser array is low as a characteristic thereof, the elliptic shape of the cross section of each beam (ratio of major axis and minor axis) can be freely set. Thus, without necessity of an optical system which compensates the astigmatic difference, each beam can be precisely shaped. (6) Since a group II-VI compound semiconductor is used for the surface light emitting type semiconductor laser array as an embedded layer, the laser oscillation can be accomplished with a low threshold current value. Thus, the bad influence of heat generated by the device against the laser characteristics can be reduced. As a result, a large number of light emitting portions can be integrally disposed. (7) When a phase-lock surface-light-emitting type semiconductor laser array with a plurality of optical resonators which emit phase-locked laser beams is used as the surface light emitting type semiconductor laser, the spread angle of each laser beam can be further decreased. In some cases, the collimator lens can be omitted. Thus, the construction of the optical system can be further simplified. (8) Since one light emitting portion can be composed of a plurality of optical resonators whose plane of polarization can be freely set, when a composite laser beam with an elliptic cross section is used, the direction of the plane of polarization of each laser beam can be set freely and independent from the direction of the major axis of the cross section of the composite laser beam. As a result, the fluctuation of the amount of light according to the position in the scanning direction of the laser beam due to the difference of incident angle of the laser beam to the polygon mirror can be minimized. 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: a image bearing member (5) for forming a static latent image thereon; a charging unit (52) for charging the surface of said image bearing member (5); a laser beam scanning unit (53) for scanning a plurality of laser beams on the surface of said image bearing member (5) which is charged; and a developing unit (55) for causing a developing agent to adhere on the surface of said image bearing member (5) scanned with the laser beams; wherein said laser beam scanning unit (53) comprises: a semiconductor laser array (21) having a plurality of light emitting portions (21a) for emitting laser beams, said light emitting portions (21a) being formed on a device substrate (22); and a deflecting unit (3) for deflecting laser beams emitted from said light emitting portions (21a) to the surface of said image bearing member (5); characterized in that said light emitting portions (21a) are disposed two-dimensionally on a surface of said semiconductor laser array (21); and wherein the lighting and the amount of light of each of said light emitting portions (21a) are discretely controlled. The image forming apparatus as set forth in claim 1, wherein each of said light emitting portions (21a) of said semiconductor laser array (21) has an optical axis substantially perpendicular to the surface of said device substrate (22). The image forming apparatus as set forth in claim 2, wherein each of said light emitting portions of said semiconductor laser array (21) comprises: a pair of reflecting mirrors (23 and 28) with different reflectances disposed on the surface of said device substrate (22); an optical resonator disposed midway between said reflecting mirrors (23 and 28) and having a plurality of semiconductor layers including a column shape clad layer (26); and a group II-VI compound semiconductor epitaxial layer (32) embedded in the periphery of said column shape clad layer (26). The image forming apparatus as set forth in claim 3, wherein said group II-VI compound semiconductor epitaxial layer (32) 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. The image forming apparatus as set forth in claim 3, wherein the lattice constant of said group II-VI compound semiconductor epitaxial layer (32) accords with the lattice constant of each of said semiconductor layers (24, 25, and 26). The image forming apparatus as set forth in claim 3, wherein said column shape clad layer (26) is formed by a separation groove disposed in said semiconductor layers; and wherein said group II-VI compound semiconductor epitaxial layer (32) is embedded in said separation groove, said multiple semiconductor layers having an active layer (25) disposed below said separation groove, so as to synchronize the phase of light of each of said optical resonators. The image forming apparatus as set forth in claim 6, wherein a laser beam emitted from each of said light emitting portions (21a) of said semiconductor laser array (21) is linearly polarized light, the cross section perpendicular to the optical axis of said laser beam being in an elliptic shape, the direction of the plane of polarization of said laser beam being different from both the directions of the major axis and minor axis of the ellipse. The image forming apparatus as set forth in claim 6, wherein each of said light emitting portions (21a) of said semiconductor laser array (21) comprises a plurality of optical resonators with different planes of polarization. The image forming apparatus as set forth in claim 1, wherein the spread angle of a laser beam emitted from each of said light emitting portions (21a) of said semiconductor laser array (21) is 20 degrees or less in full width at half maximum. The image forming apparatus as set forth in claim 1, wherein a laser beam emitted from each of said light emitting portions (21a) of said semiconductor array (21) is entered into said deflecting unit (3), said laser beam being not collimated. A laser beam scanning unit, comprising: a semiconductor laser array (21) having a plurality of light emitting portions (21a) for emitting laser beams, said light emitting portions (21a) being formed on a device substrate; and a deflecting unit (3) for deflecting the laser beams emitted from said light emitting portions (21a); characterized in that said light emitting portions (21a) are disposed two-dimensionally on a surface of said semiconductor laser array (21); and wherein the lighting and the amount of light of each of said light emitting portions (21a) are discretely controlled.

Inventors: INOUE NOZOMU, KOGA YOSHIRO, KURIHARA HAJIME, TAKADA KYU, INOUE, NOZOMU, KOGA, YOSHIRO, KURIHARA, HAJIME, TAKADA, KYU
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1992
Main classification: G03G15
IPC codes: H04N1, G06K15, G02B26, B41J2