Scanning optical system

Abstract: A scanning optical system images a light beam from a light source (10) as a line spread function image in an auxiliary scanning plane before the light beam is made incident to a scanning deflector (30) and in which a static deflector (20) is disposed in a line spread function image position in order to guide a light beam from a light source to a scanning deflector. The static deflector has a slit mirror (21) for reflecting a light beam from the light source or a mirror with a slit for permitting a light beam to transmit therethrough.

Description: [BACKGROUND OF THE INVENTION] 1. Field of the Invention This invention relates to a scanning optical system for scanning a laser beam on a scanning surface to form a pattern thereon, and particularly to an apparatus in which a pattern to be formed is high in accuracy. 2. Description of the Prior Art In a conventional scanning optical system, as shown in Fig. 20, an intensively modulated laser beam is made incident to a polygon mirror 1 and the light beam reflected and deflected by the polygon mirror 1 is focused by an fϑ lens 2 thereby to form a spot on a surface to be scanned. In general, the angle formed by an optical axis Ax1 of the light beam made incident to the polygon mirror 1 and an axis Ax2 of an fϑ lens is approximately 50° ∼90°. In order to obtain an image drawing with high accuracy, the spot must be reduced. As the diameter of a spot becomes shorter, a focal depth becomes shallow. Therefore, in order to perform an image drawing with high accuracy. the curvature of field must be restricted to be small. However, with the above-mentioned construction, the point of deflection on the polygon mirror, as shown in Fig. 21, is changed asymmetrically on both sides of the optical axis Ax2 of the fϑ lens according to rotation of the polygon mirror, Therefore, there was such a problem as that the curvature of field becomes asymmetric on the surface to be scanned and correction can not be made when a symmetric lens with respect to the optical axis is used. Fig. 22 shows a curvature of field for the construction of Fig, 20. The effective scanning field is 600mm of an image height -300∼+300mm here. In order to reduce the curvature of field, it is effective to make the change of the point of deflection symmetric with respect to the optical axis Ax2. Therefore, as is shown in Fig. 23, it can be considered such a construction as that a laser beam is made incident to a polygon mirror along the optical axis Ax2 of the fϑ lens. However, in order to employ such construction, it is required that a static deflector for separating a light beam directed from a light source to the polygon mirror from a light source from a light beam incoming to an fϑ lens is disposed in the optical path. When a laser beam is made incident to the polygon mirror along the optical axis Ax2 of the fϑ lens, the curvature of field becomes as shown in Fig. 24. As the way of appearance of the curvature of field becomes symmetric, correction can be made using a symmetric lens with respect to the optical axis and the value becomes very small. By the way, JP-A-60-233616 discloses a construction in that a light beam from a light source is made incident to a polygon mirror along the optical axis of a scanning lens using a lens and a static deflector comprising a polarizing beam splitter and a 1/4 wavelength plate, with the static deflector being adapted to reflect the beam from the light source to the scanning deflector and to transmit the reflected beam from the polygon mirror to the scanning lens. However, according to the construction of the above-mentioned Publication, the static deflector becomes complicated in structure and cost increase is induced. Furthermore, because the angle or incidence of the light beam to a polarized beam splitter is changed in accordance with rotation of the polygon mirror, the transmittance of the polarized beam splitter is gradually changed as it goes toward the peripheral portion from the central portion and irregularity of light quantity occurs on the surface to be scanned. Fig. 25 shows one example of the change of transmittance of a polarized beam splitter caused by the change of angle of incidence. It is noted that in the construction of JP-A-60-233616, a laser beam is temporarily imaged on a reflecting surface of the polygon mirror in an auxiliary scanning plane in order to correct adverse affection caused by inclination of the reflecting surface of the polygon mirror with respect to a rotational axis thereof. Therefore, as the optical path in the auxiliary scanning plane becomes the same both for the incident light beam to the polygon mirror and the reflected light beam from the polygon mirror, the optical path can not be separated by a total reflection mirror, but a polarized beam splitter or a half mirror must be used. EP-A-123 038 discloses an optical scanner. Incoming radiation is reflected off a framing mirror being turnable for a vertical scan and passes through an imaging lens system to form a linear trace on a strip mirror, which reflects the radiation toward a concave reflector of a horizontally driven scanning disk. Radiation emerges from the reflector as a collimated beam which travels past the strip mirror to a detector. Such a scanner is not capable for precise image generation and high resolution because optical distortion is not prevented. A continuous tone recording system is disclosed in EP-A-179 554. With this system light from a laser diode is focused by a cylindrical lens onto a rotating polygon mirror via a plane mirror. The light reflected from the polygon mirror passes through a toroidal lens to be incident on a F-theta lens. Similarly to the prior art discussed above with such an arrangement the point of deflection on the polygon mirror is changed asymetrically on both sides of the optical axis of the F-theta lens according to rotation of the polygon mirror providing the already described problems. SUMMARY OF THE INVENTION The present invention has been accomplished in view of the above-mentioned problems. It is therefore an object of the inyention to provide a scanning optical system which is comparatively simple in structure and yet capable of preventing deterioration of wave aberration on a scanning plane and drawing an image with high accuracy. A scanning optical system according to the present invention focuses a light beam from a light source in one dimension, so that the focused beam forms a line, in the following referred to as a line spread function image in an auxiliary scanning plane before the light beam is made incident to a scanning deflector, and in which a static deflector in the form of a narrow elongated mirror or a narrow elongated slit in a mirror is disposed in a line spread function image position in order to make a light beam from a light source incident to a scanning deflector. The static deflector has a slit-shaped narrow elongated mirror for reflecting a light beam from the light source or a mirror with a slit for permitting a light beam to transmit therethrough. The term "slit-shaped mirror" when used herein refers to an elongated mirror, while the term "mirror with a slit" refers to a mirror having an elongated slit (for permitting a light beam to transmit therethrough) formed in the center thereof. In the case the slit-shaped mirror is used, most of the reflected light by the scanning deflector passes nearby the slit-shaped mirror and forms a spot on the scanning surface through a scanning lens. In the case the mirror with a slit is used, most of the reflected light by the scanning deflector is reflected by the mirror around the slit. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a scanning optical system explaining art inclusive construction of first and second embodiments of the present invention. Fig. 2 is a diagram showing the principal scanning plane according to a first embodiment. Fig. 3 is a diagram showing the auxiliary scanning plane according to the first embodiment. Fig. 4 is an enlarged view of the static deflector portion according to the first embodiment. Fig. 5 is a diagram of the principal scanning plane for explaining numeric values of the apparatus according to the first embodiment. Fig. 6 is a diagram of the auxiliary scanning plane for explaining numeric values according to the first embodiment. Fig. 7 is an aberration diagram according to the construction of a numeric example 1 of the first embodiment. Fig. 8 is an aberration diagram according to the construction of a numeric example 2 of the first embodiment. Fig. 9 is a perspective view of a scanning optical system according to a second embodiment of the present invention. Fig. 10 is a diagram for explaining the change of the point of deflection and the function of a prism block. Fig. 11 is a diagram showing the principal scanning plane for explaining the numeric values of the apparatus of Fig. 9. Fig. 12 is a diagram showing the auxiliary scanning plane for explaining the numeric values of the apparatus of Fig. 9. Fig. 13 is an aberration diagram according to the construction of a numeric example 3 of the second embodiment. Fig. 14 is an aberration diagram according to the construction of a numeric example 4 of the second embodiment. Fig. 15 is a diagram showing a principal scanning plane of a scanning optical system according to a third embodiment of the present invention. Fig. 16 is a diagram of a principal scanning plane of an optical system showing a modified embodiment of the apparatus of Fig. 15. Fig. 17 is a diagram of a principal scanning plane of a scanning optical system according to a fourth embodiment of the present invention. Fig. 18 is a diagram of an auxiliary scanning plane of the optical system shown in Fig. 17. Fig. 19 is an enlarged view of a static deflector portion of the apparatus shown in Fig. 17. Fig. 20 is a diagram of a principal scanning plane showing an optical system of a scanning optical system for causing a light beam to be made incident to a polygon mirror from outside the scanning field. Fig. 21 is a diagram for explaining the change of the point of deflection caused by rotation of the polygon mirror. Fig. 22 is a diagram of the curvature of field caused by the apparatus of Fig. 20. Fig. 23 is a diagram of a principal scanning plane showing an optical system of a scanning optical system for causing a light beam to be made incident to the polygon mirror along the optical axis of an fϑ lens. Fig. 24 is a diagram of the curvature of field caused by the apparatus of Fig. 23. Fig. 25 is a graph showing the change of transmission of a polarized beam splitter caused by change of the angle of incidence. DETAILED DESCRIPTION OF THE EMBODIMENTS Fig. 1 shows an inclusive construction which coveres first and second embodiments below. The illustrated optical system includes a semiconductor laser 10 as a light source, a collimate lens 11 for collimating a divergent light which is emitted by the semiconductor laser 10, a cylindrical lens 12 acting as an focusing lens for focusing the collimated light beam as a line spread function image this is as a light beam being focused in one dimension to form a focus line, a slit-shaped narrow elongated mirror 21 acting as a static deflector disposed in such a manner as to be coincident with the line spread function image, a polygon mirror 30 acting as a scanning deflector for reflecting and deflecting a light beam reflected by the slit mirror 21, and an anamorphic fϑ lens 40 acting as a scanning lens for focusing a reflected light beam by the polygon mirror 30 on a scanning surface in order to form a spot thereon. For the purpose of easy understanding, in the description to follow, a plane on which a light beam is scanned by the polygon mirror 30 is referred to as principal scanning plane", while a plane perpendicular to the principal scanning plane and including an optical axis of the scanning lens is referred to as "auxiliary scanning plane". FIRST EMBODIMENT Figs. 2 through 8 show the first embodiment of a scanning optical system according to the present invention. Fig. 2 is a diagram of the principal scanning plane of an optical scanning system of the first embodiment and Fig. 3 is a diagram of the auxiliary scanning plane. This apparatus is constructed such that the optical axis of a light beam made incident to the narrow elongated slit-shaped mirror 21 from the semiconductor laser 10 is generally perpendicular to the principal scanning plane, and a light beam made incident to the polygon mirror 30 from the narrow elongated mirror 21 passes the optical axis Ax2 of the fϑ lens 40. Furthermore, a prism block 20 in which is formed the narrow elongated mirror 21, as shown in Fig. 4, has a rectangular parallelepiped shape formed of two triangle pole prisms 22 and 23 cemented together at slanted surfaces thereof and such cemented surfaces are evaporated with a narrow elongated mirror 21 acting as a total reflection mirror. The angle of the narrow elongated mirror 21 with respect to the principal scanning plane is approximately 45° and a crossing line formed when the narrow elongated mirror crosses the principal scanning plane is perpendicular to the auxiliary scanning plane. The narrow elongated mirror 21 as illustrated may be provided to the entire area of the scanning width or otherwise it may be provided only to the central portion to which the light beam from the semiconductor laser 10 is made incident. The divergent light emitted from the semiconductor laser 10 is collimated and then focused as a line spread function image perpendicular to the auxiliary scanning plane by the cylindrical lens 12. As the narrow elongated mirror 21 is disposed in such a manner as to be coincident with this line spread function image or in other words focus line, the light beam from the light source is focused on the narrow elongated mirror 21 and at the same time, the total quantity of the light is reflected by this reflecting surface and made incident to the polygon mirror 30 along the optical axis Ax2 of the fϑ lens 40. The light beam reflected and deflected by the polygon mirror 30 reaches the prism block 20 with a certain degree of spreading. Most of the light beam is caused to be incident to the fϑ lens 40 after passing through the prism block 20 in the circumambient portion of the narrow elongated slit-shaped mirror 21 and form a spot on a scanning surface (not shown). The prism block 20, different from the conventional beam splitter, has no change of transmittance caused by change of angle or rotation of the polygon mirror 30 and has no change of spot intensity caused by the height of an image on the scanning surface. The reason why the light beam from the light source can be reflected by the narrow elongated slit-shaped mirror 21 as described above is that the light beam is temporarily imaged as a line spread function image at this side of the polygon mirror 30. If it is designed such that the light beam from the light source is imaged on the reflecting surface of the polygon mirror as in the above-mentioned prior art, an optical path leading to the polygon mirror from the static deflector becomes the same to the optical path leading to the static deflector from the polygon mirror and the total reflection mirror can not be provided as in the present embodiment. As the optical system of the present invention is set to have a large positive power in the auxiliary scanning direction in order to temporarily image the light beam on the slit-shaped mirror, the curvature of field tends to become under at the peripheral portion of the scanning surface, In order to correct the curvature of field, there can be considered that the cylinder surface or toric surface having a negative curvature in the auxiliary scanning plane is used to other lenses. However, if the negative power in the auxiliary scanning direction is made strong, it is possible indeed to favorably correct the curvature of field but the wave aberration in the skew direction is deteriorated, thus resulting in a degraded spot shape. A scanning optical system used in a laser printer, etc. has an F number of about 1:50∼1:70 and a spot diameter of about 60∼100µm. Therefore, the degraded spot shape hardly affects adversely to the quality of an image. However, in such an apparatus as laser processing or the like which requires such high accuracy in dot density as 1000dpi or more, a lens system having an F number of about 1:25∼1:35 is required in order to reduce the spot diameter to about 30µm and deterioration of the quality of an image caused by wave aberration in the skew direction becomes significant. Particularly, as for laser processing devices used for printing posters, etc. , there are some which have a scanning extent of 600mm or more. As the focal length of the lens system becomes long, it was very difficult to favorably correct the wave aberration which is expressed by wavelength unit and to correct the curvature of field. In view of the above, in this embodiment, by means of a provision the prism block 20 acts as a focusing point changing element. This element changes a focusing point in the auxiliary scanning plane so that the focusing point in a peripheral portion of the scanning surface is shifted for the beam traveling direction as compared with the focusing point in the central portion, the curvature of field is corrected utilizing the change of the point of deflection of the polygon mirror 30 and the movement of the focusing point caused by the prism block. Next, the principle concerning the change of the deflecting point of the polygon mirror 30 and the shift of the focusing point caused by the prism block 20 will be described with reference to Fig. 10. When a reflected light beam from the polygon mirror 30 has a scanning angle ϑ with respect to the optical axis of the fϑ lens, the polygon mirror is rotated by ϑ/2 with reference to a state where the reflecting surface of the polygon mirror is perpendicular to the optical axis. At this time, the point of intersection between the reflecting surface and the incident beam is shifted from a reference deflection point in a state where the reflecting surface is perpendicular to the optical axis, by the following amount Δ ; wherein r is the inscribed circle radius, The object distance becomes short in the optical axis direction by the following amount S₁ comparing with a case where the deflection point is not changed; S₁=Δ·(1 + cos ϑ) The light beam toward the fϑ lens from the polygon mirror is a parallel light in the principal scanning plane but a divergent light beam in the auxiliary scanning plane. Because of the foregoing reason, the prism block 20 located in the optical path does not act on the light beam in the principal scanning plane and acts on the light beam in such a manner as to move the focal point depending on the angle of incidence in the auxiliary scanning plane. An optical path length of a plane parallel plate having a refractive index n and a thickness d is able to convert a distance d/n in an air. Therefore, there occurs a movement of the focal point by (d·(n-1))/n between a case where the plane parallel plate is inserted in an optical path of a converging or divergent light and a case where the plane parallel plate is not inserted. Similarly, when the prism block 20 acting as a plane parallel plate having a thickness d F in the optical axis direction is disposed between the polygon mirror and the fϑ lens, an air converted distance of the beam passing through the prism block 20 is d F /n F (wherein n F is the refractive index of the prism block) with reference to the light beam on the axis. On the other band, as the light beam outside the axis is a sagittal light beam, an air converted distance in the prism block is changed by portion equal to the refractive index. An air converted distance of the refracted beam in the prism block becomes d F /(n F ·cos ϑ') wherein the refractive angle is ϑ'. n F ·sin ϑ' = sin ϑ here. The beam which is made incident to the prism block at an angle ϑ travels the same distance at an angle ϑ as shown by the broken line if the prism block is not provided there. When the beam shown by the broken lines is projected to the optical axis, the length thereof becomes as follows; (d F ·cos ϑ)/(n F ·cos ϑ') Therefore, an optical axis difference S₂ with respect to the beam on the optical axis becomes as follows; S₂=(d F /n F )(1-(cos ϑ/cos ϑ')) and the object distance of the peripheral portion is short by S₂. That is, this indicates that the focusing point formed by the beam having the scanning angle ϑ is shifted by S₁ + S₂ for the beam traveling direction as compared with the focusing point formed by the beam on the optical axis Ax2. Therefore, the peripheral curvature of field is improved by approximately; mZ²(S₁+S₂) wherein the power of the fϑ lens system in the auxiliary scanning plane is mz. In order to obtain the effect of correction of the curvature of field caused by the change of deflection point of the polygon mirror, it is desirable to satisfy the condition r>0.05f, wherein r is the inscribed circle radius of the polygon mirror, and f is the focal length in the principal scanning direction of the fϑ lens, If r is smaller than 0.05f, the effect owing to the change of the deflection point is small. Thus, even if the focusing point changing element is inserted in the optical path, a sufficient effect of correction of the curvature of field is unobtainable. Also, in order to favorably correct the curvature of field, it is desirable to satisfy the following condition; 0.05f<mz²(S₁+S₂)<0.15f wherein mz is a power of the fϑ lens in the auxiliary scanning direction, S₁ is the change of the object distance in the auxiliary scanning direction caused by the polygon mirror, and S₂ is the change of the object distance in the auxiliary scanning direction caused by the focusing point changing element. If it is lower than the lower limit, the change of the object distance caused by the polygon mirror and the focusing point changing element is small and a sufficient correcting effect of the curvature of field is unobtainable. If the shortage of correction is compensated by increasing the negative power in the auxiliary scanning direction, the wave aberration is deteriorated. On the contrary, if it exceeds the highest limit, the effect caused by the polygon mirror and the focusing point changing element becomes excessive and the polygon mirror and the focusing point changing element become impracticably large. Next, two concrete numeric examples of the first embodiment will be described. Figs, 5 and 6 show the principal and auxiliary scanning plane for explaining numeric values. [NUMERIC EXAMPLE 1] Table 1 and table 2 show the numeric example 1 of the first embodiment, table 1 shows the construction of the cylinder lens and table 2 shows the construction of the fϑ lens. This example is suitable for a case where the semiconductor laser of a wavelength λ =780nm is used. In Fig. 3, the cylindrical lens is single for the convenient purpose but actually the cylindrical lens is a two piece construction consisting of a positive lens element and a negative lens element. In the tables, the symbolic characters ry denote the radius of curvature in the principal scanning direction, rz the radius of curvature in the auxiliary scanning direction, fc the focal length in the auxiliary scanning direction of the cylinder lens, fy the focal length in the principal scanning direction of the fϑ lens. and mz the lateral magnification in the auxiliary scanning direction of the fϑ lens. The aberration due to the construction of the numeric example 1 is as shown in Fig. 7. In this optical system, in order to balance the aberration, the actual image point is displaced from the paraxial image point in the principal and auxiliary scanning directions. And in the spherical aberration diagram of Fig. 7, numeric values including such displacement of the image point are shown. This is likewise applicable to the following numeric examples. Distance from the final surface to the line spread function image L1=80.12mm Air converted distance from the line spread function image to the reference deflection point : L2=29.96 Inscribed radius of the polygon mirror : r=75 Focusing point movement caused by the polygon mirror : S₁=4.26 Prism block Thickness dF=30.0 Refractive index nF=1.76591 Focusing point movement caused by the prism : S₂=1.41 Distance from the reference deflection point to the first surface of the fϑ lens including the prism block : e=118.35 Distance from the final surface to the scanning surface : fb=751.26 [NUMERIC EXAMPLE 2] Table 3 and table 4 show the numeric example 2 of the first embodiment, table 3 shows the construction of the cylinder lens and table 4 shows the construction of the fϑ lens. This example is suitable for a case where & He-Ne laser of a wavelength λ =632.8nm is used as a the light source instead of the semiconductor laser. The aberration caused by the construction of the numeric example 2 is as shown in Fig. 8. Distance from the final surface to the line spread function image L1=79.28. Air converted distance from the line spread function Image to the reference deflection point : L2=29.96 Inscribed radius of the polygon mirror : r=75 Focusing point movement caused by the polygon mirror : S₁=4.26 Prism block Thickness : dF=30.0 Refractive index : nF=1.77861 Focusing paint movement caused by the prism : S₂=1.40 Distance from the reference deflection point to the first surface of the fϑ lens including the prism block : e=116.43 Distance from the final surface to the scanning surface : fb=752.86 SECOND EMBODIMENT Fig. 9 shows the second embodiment of the present invention. The illustrated optical system includes a semiconductor laser 10 as a light source, a collimate lens 11 for collimating a divergent light which is emitted by the semiconductor laser 10, a mirror 13 for reflecting such collimated light, a cylindrical lens 12 for imaging the collimated light beam as a line spread function image, a prism block 20 having a slit-shaped mirror 21 disposed in such a manner as to be coincident with the line spread function image of the light beam, a polygon mirror 30 reflecting and deflecting a light beam reflected by the narrow elongated slit-shaped mirror 21, and an fϑ lens 40 for focusing a reflected light beam by the polygon mirror 30 on a scanning surface in order to form a spot thereon. The prism block 20 has a rectangular parallelepiped shape formed of a triangular pole prism 22 and a trapezoidal prism 23 cemented together and such cemented surfaces are evaporated with a narrow elongated mirror 21 acting as a total reflection mirror. The angle of the narrow elongated mirror 21 with respect to the principal scanning plane is approximately 45° In this embodiment, although there is provided a prism block as a focusing point changing element having a plane perpendicular to the optical axis at both incident and outgoing sides thereof. it may be a lens each surface of which is a curved surface so that a part of the power of the fϑ lens can be born by it. Next, two concrete numeric examples will be described. Fig. 11 shows the principal scanning plane and Fig. 12 shows the auxiliary scanning plane. [NUMERIC EXAMPLE 3] Table 5 and table 6 shows the numeric example 3 of tile second embodiment, Table 5 shows the construction of the cylinder lens and Table 6 shows the construction of the fϑ lens. This example is suitable for a case where the semiconductor laser of a wavelength λ =780nm is used. Aberration caused by the construction of the numeric example 3 is as shown in Fig. 13. Distance from the final surface to the line spread function image : L1=74.03mm Air converted distance from the line spread function image to the reference deflection point : L2-28.05 Inscribed radius of the polygon mirror : r=49 Focusing point movement caused by the polygon mirror : S₁=2.78 Prism block Thickness dF=83.6 Refractive index : nF=1.51072 Focusing point movement caused by the prism : S₂=3.87 Distance from the reference deflection point to the first surface of the fϑ lens including the prism block : e=129.59 Distance from the final surface to the scanning surface : fb=763.23 [NUMERIC EXAMPLE 4] Table 7 and table 8 show the numeric example 4 of the second embodiment, table 7 shows the construction of the cylindrical lens and table 8 shows the construction of the f ϑ lens. This example is suitable for a case where an He-Ne laser of a wavelength λ =632.8nm is used as a light source. Aberration caused by the construction of the numeric example 4 is as shown in Fig. 14. Distance from the final surface to the line spread function image L1=73.15mm Air converted distance from the line spread function image to the reference deflection point : L2=27.42 Inscribed radius of the polygon mirror : r=49 Focusing point movement caused by the polygon mirror : S₁=2.78 Prism block Thickness : dF=83.6 Refractive index : nF=1.51432 Focusing point movement caused by the prism : S₂=3.87 Distance from the reference deflection point to the first surface of the fϑ lens including the prism block : e=130.72 Distance from the final surface to the scanning surface : fb=764.47 THIRD EMBODIMENT Fig. 15 is a principal scanning plane diagram showing the third embodiment of the present invention. In this example, the optical axis of the light beam made incident to a static deflector from the semiconductor laser 10 is generally included within the principal scanning plane. The static deflector is the narrow elongated mirror 21 having the longitudinal direction in the principal scanning plane. This narrow elongated mirror 21 is located in the imaging or focus position of the light beam caused by the cylindrical lens 12. Also, the narrow elongated mirror 21 is disposed in such a manner as to be perpendicular to the principal scanning plane and inclined with respect to the optical axis of the fϑ lens so that the light beam from the semiconductor laser 10 is be made incident to the polygon mirror 30 along the optical axis Ax2 of the fϑ lens 40. Accordingly, the crossing line formed by crossing the narrow elongated mirror 21 with the auxiliary scanning plane is perpendicular to the principal scanning plane. With the above-mentioned construction, the light beam from the semiconductor laser 10 is made incident to the narrow elongated mirror 21 at angles in the principal scanning plane. Therefore, the peripheral portion of the light beam which is imaged on the narrow elongated mirror 21 is out of focus and the width in the auxiliary scanning direction of the slit mirror 21 can not be made too narrow. However, the entire path of the light beam can be set in the principal scanning plane, and the construction of the apparatus can be simplified. Furthermore, it can be designed such that a ghost light caused by reflection between the polygon mirror 30 and the narrow elongated mirror 21 is allowed to escape outside the area where the image is drawn. Fig. 16 is a principal scanning plane diagram showing a modified embodiment of the third embodiment. The construction of this embodiment is the same to that of Fig. 15 only except that the set angle of the narrow elongated mirror 21 with respect to the optical axis Ax2 of the fϑ lens 40 is different. With the above-mentioned construction, as the angle formed between the narrow elongated mirror 21 and the optical path of the light beam, made incident from the light source becomes large, the length of the narrow elongated slit-shaped mirror 21 can be reduced as compared with the construction of Fig. 15. Although the light beam traveling toward the polygon mirror 30 from the narrow elongated mirror 21 is made incident from outside the optical axis of the fϑ lens, deterioration of quality caused by this large angle is minor because the angle formed between the narrow elongated mirror 21 and the optical axis Ax2 is small when compared with the prior art as shown in Fig. 20. 4TH EMBODIMENT Figs. 17 through 18 show the fourth embodiment of the present invention. In this embodiment, contrary to the first to the third embodiments, the light beam from the semiconductor laser 10 is permitted to transmit a prism block 50 toward the polygon mirror 30, and the reflected light by the polygon mirror 30 is deflected by a mirror 52 as the static deflector and made incident to the fϑ lens 40. Furthermore, it is designed such that the optical axis of the light beam emitted from the semiconductor laser 10 is perpendicular to the optical axis Ax2 of the fϑ lens 40. The prism block 50, as shown in its enlarged scale in Fig. 19, includes a narrow elongated slit 51 which is located in the imaging position of the light beam caused by the cylindrical lens 12 and permits the light beam from the semiconductor laser 10 to be transmitted therethrough toward the polygon mirror 30 side, and a mirror 52 disposed in the periphery of the slit 51 and adapted to reflect the reflected light by the polygon mirror 30 toward the fϑ lens 40 side. The mirror 52 is disposed on the cemented surface of the triangle pole prisms 53 and 54. It may be designed such that the slit 51 is arranged on the entire area of the scanning extent, or otherwise it may be arranged only at the central portion to which the light beam from the semiconductor laser 10 is made incident. In the above embodiment, the polygon mirror was used as the scanning deflector. However, this is not material to the present invention and a galvano mirror may be employed as the scanning deflector. Furthermore, by integrally assembling the whole component parts from the semiconductor laser 10 to the static deflector into one unit, assembling accuracy can be increased and displacement between the imaging position and the slit mirror 21 can be prevented.

Claims: A scanning optical system comprising in the order of the beam path: a light source (10) for emitting a light beam, a cylindrical lens (12) focusing the light beam in the form of a line, a narrow elongated mirror (21) positioned so that the focused line produced by the cylindrical lens (12) falls on this narrow elongated mirror (21), a scanning deflector (30) which reflects the light from said mirror (21) back towards said mirror (21), where the part of the light beam not falling on the mirror (21) can pass by said mirror (21), a scanning lens (40) which focuses said part of the light beam passing by said mirror (21) on a surface to be scanned. A scanning optical system comprising in the order of the beam path: a light source (10) for emitting a light beam, a cylindrical lens (12) focusing the light beam in the form of a line, a mirror (52) with a narrow elongated slit (51) positioned so that the focused line produced by the cylindrical lens (12) falls on said slit (51), whereby the light beam passes through said slit (51), a scanning deflector (30) which reflects the light beam from said slit (51) back towards said mirror (52), where the part of the light beam not falling on the slit (51) is reflected by the mirror (52), a scanning lens (40) which focuses the light beam reflected by the mirror (52) on a surface to be scanned. A scanning optical system according to one of the preceding claims, characterized in that said cylindrical lens (12) is adapted to focus said light beam from said light source (10) in a position between said scanning deflector (30) and said scanning lens (40). A scanning optical system according to claim 3, characterized in that said narrow elongated mirror (21) respectively said narrow elongated slit (51) is located on said focus position. A scanning optical system according to one of the preceding claims, characterized in that said cylindrical lens (12) has a power at least in an auxiliary scanning plane which includes an optical axis (Ax2) of the scanning lens (40) and is perpendicular to a principal scanning plane on which a light beam is scanned by the scanning deflector (30). A scanning optical system according to claim 5, characterized in that a crossing line formed when said narrow elongated mirror (21) respectively said slit (51) crosses said principal scanning plane is perpendicular to said auxiliary scanning plane. A scanning optical system according to claim 5, characterized in that a crossing line formed when said narrow elongated mirror (21) respectively said slit (51) crosses said auxiliary scanning plane is perpendicular to said principal scanning plane. A scanning optical system according to one of the preceding claims, characterized in that said scanning deflector (30) is a rotational polygon mirror. A scanning optical system according to claim 8, characterized in that it further comprises a focusing point changing element (20; 50) which is adapted to change a focusing point in an auxiliary scanning plane which includes an optical axis (Ax2) of the scanning lens (40) and is perpendicular to a principal scanning plane on which a light beam is scanned by the rotational polygon mirror (30). A scanning optical system according to claim 9, characterized in that said focusing point changing element (20; 50) is disposed between said rotational polygon mirror (30) and said scanning lens (40) and is adapted to change a focusing point in said auxiliary scanning plane so that the focusing point in a peripheral portion of said scanning surface is shifted for the beam travelling direction as compared with the focusing point in the central portion of said scanning surface. Scanning optical system according to one of claims 9 or 10, characterized in that said focusing point changing element (20; 50) has an incident plane to which a light beam from said polygon mirror (30) is made incident and an outgoing plane from which a light beam is outgone to said scanning lens (40). Scanning optical system according to claim 11, characterized in that said outgoing plane is generally in a plane parallel plate relation with said incident plane. A scanning optical system according to one of claims 9 to 12, characterized in that said narrow elongated mirror (21) respectively said mirror (52) with said narrow elongated slit (51) is disposed within said focusing point changing element (20 resp. 50). A scanning optical system according to claim 1 and one of claims 3 to 13, characterized in that said narrow elongated mirror (21) is disposed between said scanning deflector (30) and said scanning lens (40), so that said narrow elongated mirror (21) causes the light beam from said light source (10) to be made incident to said scanning deflector (30) along the optical axis (Ax2) of said scanning lens (40). A scanning optical system according to one of the preceding claims, characterized in that said scanning lens (40) is an anamorphic optical system. A scanning optical system according to one of the preceding claims, characterized in that said scanning lens (40) is a fϑ lens which focuses the deflected beam on a scanning surface to form a spot thereon. A scanning optical system according to claim 8 and claim 16, characterized in that it satisfies the following relation: r > 0.05 f wherein r is the radius of an inscribed circle of said polygon mirror (30) and f is a focal length of said fϑ lens (40) in the principal scanning direction. A scanning optical system according to claim 8 and claim 16 or 17, characterized in that it satisfies the following relation: 0.05 f < mz² (S₁ + S₂) < 0.15 f wherein f is a focal length of said fϑ lens (40) in the principal scanning direction, mz is a power of said fϑ lens (40) in the auxiliary scanning direction, S₁ is the change of an object distance in the auxiliary scanning direction caused by change of a deflecting point of said polygon mirror (30) and S₂ is the change of an object distance in the auxiliary scanning direction caused by said focusing point changing element (20; 50). A scanning optical system according to one of the preceding claims, characterized in that said narrow elongated mirror (21) respectively said mirror (52) with said narrow elongated slit (51) is located in a center portion of a light scanning extent. A scanning optical system according to one of the preceding claims, characterized in that said scanning lens (40) focuses the deflected beam on a scanning surface to form a spot thereon.

Inventors: REI MORIMOTO C O ASAHI KOGAKU, REI, MORIMOTO, C/O ASAHI KOGAKU KOGYO K.K.
Applicants: ASAHI OPTICAL CO LTD, ASAHI KOGAKU KOGYO KABUSHIKI KAISHA
Application year: 1990
Main classification: G02B26
IPC codes: G02B26, G02B13