JP5364968B2 - Optical scanning device, multi-beam optical scanning device, image forming apparatus - Google Patents

Description

  The present invention includes a coupling optical system for coupling a light beam emitted from a light source device, a first optical system for condensing the light beam from the coupling optical system in a substantially long line in the main scanning direction, The present invention relates to a rotary polygon mirror as a deflecting unit that deflects and scans a light beam from an optical system, and an optical scanning device that includes a scanning optical system that focuses a light beam from a light source on a surface to be scanned.

  In recent years, colorization of laser printers and copiers is rapidly progressing. For this reason, optical scanning devices used in these devices are also required to be able to form a plurality of scanning lines at a time for a plurality of photosensitive members.

  Several methods are conceivable as methods that satisfy such a requirement. For example, there is a tandem method in which four photoconductors corresponding to CMYK are arranged.

  As a low-cost scanning optical system suitable for the tandem system, there is an oblique incidence optical system that makes an incident with an angle in the sub-scanning direction with respect to the normal line of the deflecting reflection surface of the optical deflector. By making a plurality of light beams obliquely incident toward the center of the sub-scanning section of the deflection surface, it is possible to employ a low-cost optical deflector in which the height of the deflection surface in the sub-scanning direction is reduced.

  However, the oblique incidence optical system has a problem that “scanning line bending” is large. The amount of bending of the scanning line varies depending on the oblique incident angle of each light beam in the sub-scanning direction, and color misregistration occurs when the latent images drawn by the respective light beams are superimposed and visualized with the respective color toners. Appears. In addition, the oblique incident light causes the light beam to be twisted and incident on the scanning lens, thereby increasing the wavefront aberration. Particularly, the optical performance is significantly deteriorated at the peripheral image height, the beam spot diameter is increased, and the image quality is improved. It becomes a factor to prevent.

  In the oblique incidence optical system, when the light source is arranged at a position overlapping with the optical axis of the scanning lens in the main scanning direction so that the light beam from the light source side is incident on the rotation axis of the polygon mirror, interference with the scanning lens is caused. In order to avoid this, the oblique incident angle increases.

  As a method of correcting the “large scanning line bending” inherent to the oblique incidence method, the scanning imaging optical system “changes the inherent inclination of the lens surface in the sub-scan section in the main scanning direction so as to correct the scanning line bending. In the scanning imaging optical system, the “inherent inclination of the reflecting surface in the sub-scanning section is changed in the main scanning direction so as to correct the scanning line curvature”. A method of including a “corrected reflecting surface having a reflected surface” (Patent Document 2) has been proposed.

  Further, in Patent Document 3, the obliquely incident light beam passes off the axis of the scanning lens, and the position of the scanning line is aligned using a surface that changes the aspherical amount of the child line of the scanning lens along the main scanning direction. A method has been proposed. In this publication, an example in which correction is performed with one scanning lens is given, and the scanning line bending can be corrected. However, the beam spot diameter deterioration due to an increase in wavefront aberration described below is described. Not.

  Further, in a scanning optical system in which a plurality of light beams incident on the deflector are incident obliquely at different angles with respect to the normal line of the deflection surface in the sub-scan section, the power in the sub-scan section of the commonly used lens is substantially zero. Patent Document 4 “Optical scanning device and image forming apparatus using the same” is disclosed.

  Although it is possible to suppress the occurrence of scanning line bending by setting the power in the sub-scan section of the commonly used lens to be substantially zero, the deterioration of the beam spot diameter due to the increase of wavefront aberration described below will be described. Not.

  Another problem with the oblique incidence method is that a large wavefront aberration is likely to occur at the peripheral image height (near both ends of the scanning line) due to the light beam skew. When such wavefront aberration occurs, the spot diameter of the light spot increases at the peripheral image height. If this problem cannot be solved, “high-density optical scanning”, which has been strongly demanded recently, cannot be realized. In the optical scanning device described in the above publication, a large scanning line curve peculiar to the oblique incidence method is corrected extremely well, but the correction of the wavefront aberration is not sufficient.

  As an optical scanning device that can satisfactorily correct the above-mentioned “scan line bending and wavefront aberration degradation” which can be said to be a problem with the oblique incidence method, the scanning imaging optical system includes a plurality of rotationally asymmetric lenses, and the lens surfaces of these rotationally asymmetric lenses. There has been proposed one in which the shape of a bus connecting the child line vertices is curved in the sub-scanning direction (Patent Document 5).

  However, the lens having the “lens surface curved in the sub-scanning direction of the generatrix connecting the child vertexes” solves various problems by curving the generatrix, and an individual scanning lens corresponding to the incident light beam Therefore, when applied to a tandem scanning optical system, the number of scanning lenses increases.

  When a common lens is used on the deflector to reduce the number of scanning lenses and multiple light beams traveling toward different scanning surfaces are incident, the problem is solved for one light beam by curving the shape of the generatrix. However, it is difficult to reduce scanning line bending and wavefront aberration for the other light flux.

  Also, since it has a curvature in the sub-scanning direction, if the light beam incident on the lens shifts in the sub-scanning direction due to the effects of assembly errors, processing errors, environmental fluctuations, etc., the influence of the refractive power of the lens in the sub-scanning direction Therefore, there is a problem in that the shape of the scanning line curve changes, the effect of suppressing the initial (or design) color shift in the color image cannot be obtained, and color shift occurs.

  Further, Patent Document 6 discloses an optical system in which a light beam perpendicularly incident on a deflecting surface and a light beam obliquely incident are deflected by a single deflector to form images on different scanned surfaces.

However, since there are two light beams that are perpendicularly incident on the deflection surface, a polygon mirror having a two-stage deflection surface is required for separating the light beams, which is problematic in terms of cost reduction. In addition, since the deflection surface has two stages, power consumption is increased due to the influence of windage loss, and there is a problem in terms of energy saving.
Japanese Patent Laid-Open No. 11-14932 Japanese Patent Laid-Open No. 11-38348 JP 2004-70109 A JP 2005-62834 A Japanese Patent Laid-Open No. 10-73778 JP-A-2005-92148

  In view of these circumstances, in the present invention, a coupling optical system that couples a light beam emitted from the light source device, and a first light beam that converges the light beam from the coupling optical system on a substantially long line in the main scanning direction. In an optical scanning device having an optical system, a rotating polygon mirror as a deflecting means for deflecting and scanning a light beam from the first optical system, and a scanning optical system for condensing the light beam from a light source on a scanned surface, Optical scanning device that effectively corrects scanning line bending and wavefront aberration degradation in an oblique incidence type optical scanning device having an angle in the sub-scanning direction with respect to the normal line of the deflecting reflection surface of the optical deflector. Is the first purpose.

  In addition, the realization of the optical scanning device considering the environment, such as the miniaturization of the optical deflector and the reduction of the power consumption due to the decrease in the rotational speed of the rotary polygon mirror, which is an optical deflector by multi-beams, and the purpose of the above description are achieved. The second object is to realize the image forming apparatus.

An aspect of the present invention that achieves the above object includes a plurality of light source devices that emit light beams, a light deflector that deflects and scans a plurality of light beams emitted from the light source devices on the same deflection surface, and deflection by the light deflector. A plurality of light beams formed on different scanning surfaces, and the plurality of light beams emitted from the light source device are sub-scanned with respect to the normal line of the deflecting reflection surface of the optical deflector. A first lens group that is incident at a predetermined angle in the direction and through which all light beams deflected and scanned by the imaging optical system on the same deflecting surface pass, and a second lens group provided for each scanned surface; In the optical scanning device, the reflection point on the optical deflector and the incident point on the lens closest to the deflector of the first lens group are arranged on opposite sides with respect to a reference plane, and the first lens A group is a sub-scan section including an optical surface reference axis. And the absolute value of the power in the sub-scan section parallel to the optical surface reference axis is changed by changing the curvature of the exit surface in the sub-scan section along the distance from the optical surface reference axis. made from a single larger lens, wherein Ri a strong negative power in the sub-scanning cross-section with increasing distance from the optical surface reference axis, said reference plane is the plane containing the optical surface reference axis perpendicular to the rotation axis of the deflector der is Rukoto relates to an optical scanning apparatus according to claim.

  The curvature in the sub-scanning direction in the sub-scanning section including the optical surface reference axis of the first lens group is zero or substantially zero.

  Further, at least one surface of the second lens group is configured by a surface having no curvature in the sub-scanning direction, and a tilt eccentric angle in the sub-scanning direction changes according to a distance from the optical surface reference axis. It has at least one special tilt eccentric surface. The decentering amount of the special tilt eccentric surface of the second lens group is zero in the sub-scan section including the optical surface reference axis.

  The imaging optical system is a reduction optical system in the sub-scan section.

  Another aspect of the present invention relates to a multi-beam optical scanning device using the optical scanning device as an optical scanning unit.

  Another aspect of the present invention relates to an image forming apparatus using the optical scanning device as an electrophotographic writing unit.

  In the optical scanning device of the present invention, the first lens group that is commonly used in the imaging optical system is one lens whose power in the sub-scan section including the optical surface reference axis is zero or close to zero, and Since the curvature of the exit surface changes in the sub-scanning section according to the image height, the wavefront aberration is less deteriorated due to the twist of the light beam incident on the lens surface near the optical surface reference axis, resulting in manufacturing errors and eccentricity during assembly. Reduce performance fluctuations. Therefore, even with the oblique incidence method, it is possible to effectively correct scanning line bending and wavefront aberration degradation.

  In addition, by using a light beam having an angle in the sub-scanning direction, it is possible to reduce the cost and reduce the power consumption and noise because there are no multiple stages of optical deflectors with high cost ratios in the components constituting the optical scanning device. It is possible to realize an optical scanning device that can be reduced and considers the environment.

  Hereinafter, the best mode for carrying out the optical scanning device of the present invention will be described. In the description, the drawings submitted at the same time as this specification will be referred to as appropriate.

  An embodiment of the optical scanning device of this embodiment will be described with reference to FIG. FIGS. 1B and 1C are cross-sectional views in the main scanning section and the sub-scanning direction in a state where the folding mirror is expanded and an appropriate virtual mirror is inserted in the one imaging optical system of FIG. It is.

  A divergent light beam emitted from a semiconductor laser as a light source is converted into a light beam shape suitable for a later oblique incidence optical system by a coupling lens. The form of the light beam converted by the coupling lens can be a parallel light beam or a light beam with weak divergence or weak convergence.

  The light beam from the coupling lens is condensed in the sub-scanning direction by the cylinder lens, and is incident on the deflecting / reflecting surface of the rotary polygon mirror (optical deflector) that rotates the polygon mirror. As shown in the figure, the light beam from the light source side is incident with an inclination with respect to a plane orthogonal to the rotation axis of the deflection reflection surface of the polygon mirror. Therefore, the light beam reflected by the deflecting reflecting surface is also inclined with respect to the plane. The light beam having an angle with respect to the plane orthogonal to the rotation axis of the rotary polygon mirror may be arranged by tilting the light source device, the coupling optical system, and the first optical system at a desired angle, or by using a folding mirror. An angle may be added. Further, the optical beam toward the deflecting / reflecting surface may be angled by shifting the optical axis of the first optical system in the sub-scanning direction.

  Here, the light beam incident on the deflection surface includes a plane parallel to the main scanning direction (including the rotation axis of the deflector and the optical surface reference axis of the first lens group (L1)). A total of four light beams are incident from both sides at two different incident angles with respect to the vertical plane. In this embodiment, they are ± 1.46 deg and ± 3.30 deg. Since the imaging optical system is plane symmetric with respect to a plane including the optical surface reference axis of the first lens group (L1) and parallel to the main scanning direction, only one side will be described in the following description.

  The light beam reflected by the deflecting reflecting surface is deflected at a constant angular velocity along with the constant speed rotation of the polygon mirror, passes through the scanning optical system including the second lens group (L2), and is condensed on the surface to be scanned. Thereby, the deflected light beam forms a light spot on the surface to be scanned, and performs optical scanning of the surface to be scanned.

  In the horizontal-incidence single-side scanning optical scanning device, as shown in FIG. 2A, the polygon mirror sub-scanning direction is used to obtain an interval Z necessary to separate the light beams traveling toward the corresponding scanned surfaces. Will become very thick. Although a method using a four-stage polygon can be considered, it is not suitable for speeding up and cost reduction.

  On the other hand, in the oblique incidence optical system as in this embodiment, it is not necessary to provide a plurality of light beams with a predetermined interval in the sub-scanning direction on the deflection reflection surface of the polygon mirror. That is, as shown in FIG. 2B, a pair of light beams from a plurality of light source devices having different angles in the sub-scanning direction with respect to the normal line of the reflection surface of the polygon mirror is a plane perpendicular to the rotation axis of the deflector. By making the light incident on the same reflective surface from the top and bottom, the polyhedron that forms the deflecting and reflecting surface of the polygon mirror can be formed in a single stage and the thickness in the sub-scanning direction can be reduced, and the inertia as a rotating body can be reduced and the startup time can be reduced. Can be shortened.

  As described above, in the optical system corresponding to four different scanned surfaces on one side, all the light beams, that is, all the light beams directed to the four different scanned surfaces are made normal to the reflecting surface of the optical deflector. An environment that can reduce the power consumption and noise by reducing the cost of the optical deflector with a high cost ratio with the components that make up the optical scanning device by using a light beam with an angle to the sub-scanning direction It is possible to provide an optical scanning device in consideration of the above.

  It is well known that, in the conventional method of oblique incidence in the sub-scanning direction with respect to horizontal incidence, the amount of various aberrations increases and the optical performance deteriorates by entering the scanning lens at an angle in the sub-scanning direction.

  On the other hand, in Patent Document 6, in a shared lens that passes both horizontally incident and obliquely incident light beams, a wavefront aberration is improved by using a special surface in which only a region through which obliquely incident light beams pass is tilted decentered. It has been corrected. However, in this embodiment, since there are two horizontally incident light beams, it is necessary to use a two-stage polygon mirror (or a thick polygon mirror in the sub-scanning direction), which is expensive. In order to decenter a part of the plane, a certain distance is required between the light beams in the sub-scan section because of processing requirements. For this reason, when all the light beams are obliquely incident to use a low-cost polygon mirror, the distance from the deflector to the shared lens is increased in order to ensure the distance necessary for the light beam separation, This will increase the size of the device.

  Therefore, in this embodiment, in order to suppress the height of the lens, the sub-scanning section of the common lens has an arc shape, and the curvature in the sub-scanning direction changes according to the image height (hereinafter referred to as “special toroidal surface”). ) Is used to correct the deterioration of the optical performance. As a result, the light beams having different oblique incident angles are corrected with the same shape surface, so that the light beams can be more adjacent to each other. Therefore, it is possible to reduce the angle with respect to the normal line of the deflecting reflection surface of the polygon mirror (the angle at which the light is obliquely incident in the sub-scanning direction), and to suppress the deterioration of the optical performance, thereby realizing good optical performance. As a result, a stable beam spot diameter can be obtained, which is advantageous for improving the image quality by reducing the beam spot diameter.

  In the case of the one-side scanning method, as shown in FIG. 3B, the conventional optical scanning device in which all the light beams are horizontal with respect to the normal line of the deflecting reflection surface of the polygon mirror can obtain good optical performance. On the other hand, the distance between the light beams from each light source device, that is, the light beams guided to different scanning surfaces, is an interval necessary for separating each light beam (Δd in the figure), usually 3 mm to 5 mm. It is necessary to have an interval. For this reason, the height (height in the sub-scanning direction) h of the deflecting means (polygon mirror) is increased, the contact area with the air is increased, power consumption is increased due to the influence of windage loss, noise is increased, and costs are increased. The problem was occurring. In particular, the cost ratio occupied by the deflecting means in the component parts of the optical scanning device is high, and the problem in cost is large.

  In that respect, according to the above-described embodiment of the optical scanning device according to the present embodiment, the light beams from the plurality of light source devices reflected by the deflecting / reflecting surfaces of the polygon mirror as the deflecting means are deflected / reflecting surfaces of the polygon mirror. As shown in FIG. 3C, the height h of the polygon mirror is significantly reduced by making the light beam having an angle (with an angle in the sub-scanning direction) incident on the scanning lens. As in the description of the opposed scanning method, the polyhedron that forms the deflecting and reflecting surface of the polygon mirror can be formed in a single stage, the thickness in the sub-scanning direction can be reduced, the inertia as a rotating body can be reduced, and the startup time can be reduced. Can be shortened. Further, the cost can be reduced with respect to the two-stage polygon mirror in the conventional counter scanning system.

  In order to set the oblique incidence angle to be the smallest in the one-side scanning method, a combination of horizontal incidence and oblique incidence as shown in FIG. 3 (a) can be considered. However, the embodiment (c) is the most compact and can solve various problems.

  In this method in which the oblique incidence is made in the sub-scanning direction with respect to the horizontal incidence method, there is a problem that “scanning line bending” is large. The amount of bending of the scanning line varies depending on the oblique incident angle of each light beam in the sub-scanning direction, and color misregistration occurs when the latent images drawn by the respective light beams are superimposed and visualized with the respective color toners. Appears. In addition, the oblique incident light causes the light beam to be twisted and incident on the scanning lens, thereby increasing the wavefront aberration. Particularly, the optical performance is significantly deteriorated at the peripheral image height, the beam spot diameter is increased, and the image quality is improved. It becomes a factor to prevent.

  The occurrence of scanning line bending due to oblique incidence will be described. For example, the shape of the scanning lens constituting the scanning optical system, particularly the scanning lens having a strong refractive power in the sub-scanning direction (second scanning imaging lens in FIG. 1) in the main scanning direction is the light beam of the deflecting reflecting surface. Unless the arc shape is centered on the reflection point, the distance from the deflecting reflection surface of the optical deflector to the incident surface of the scanning lens differs depending on the lens height in the main scanning direction. Usually, it is difficult to make the scanning lens in the above shape in order to maintain optical performance. That is, as shown in FIG. 1, a normal light beam is deflected and scanned by an optical deflector, and does not enter the lens surface perpendicularly at each image height in the main scanning section, but in the main scanning direction. Is incident.

  Since the light beam is obliquely incident and has an angle in the sub-scanning direction, the light beam deflected and reflected by the optical deflector has a distance from the deflecting reflection surface of the optical deflector to the scanning lens incident surface depending on the image height. In contrast, the incident height in the sub-scanning direction to the scanning lens is incident on a position higher or lower than the center as it goes to the periphery (depending on the direction of the angle of the light beam in the sub-scanning direction). As a result, when passing through a surface having refracting power in the sub-scanning direction, the refracting power received in the sub-scanning direction differs and scanning line bending occurs. In the case of normal horizontal incidence, even if the distance from the deflecting / reflecting surface to the scanning lens incidence surface is different, the light beam travels horizontally with respect to the scanning lens, so the incident position in the sub-scanning direction on the scanning lens is different. There is no occurrence of bending of the scanning line.

  Wavefront aberration deterioration due to oblique incidence will be described. As described above, the deflection of the optical deflector depends on the image height unless the shape in the main scanning direction of the entrance surface of the scanning lens constituting the scanning optical system is an arc shape centered on the reflection point of the light beam on the deflection reflection surface. The distance from the reflecting surface to the scanning lens entrance surface is different. Usually, it is difficult to make the scanning lens in the above shape in order to maintain optical performance. That is, a normal light beam is deflected and scanned by an optical deflector, and does not enter the lens surface perpendicularly at each image height in the main scanning section, but with an incident angle in the main scanning direction.

  The light beam deflected and reflected by the optical deflector has a certain width in the main scanning direction, and the light beams at both ends in the main scanning direction within the light beam are deflected from the deflection reflecting surface of the optical deflector to the scanning lens incident surface. Since the angle is different and the angle is obliquely incident, there is an angle in the sub-scanning direction, so that the light enters the scanning lens in a twisted state (see FIG. 4). As a result, the wavefront aberration is significantly degraded and the beam spot diameter is increased. The incident angle in the main scanning direction becomes tighter toward the peripheral image height, the twist of the light beam increases, and the beam spot diameter increases due to the deterioration of the wavefront aberration toward the periphery.

  In this embodiment, a special toroidal surface is adopted to correct wavefront aberration and scanning line bending. Further, the correction of the scanning line bending can also be performed by tilting the lens surface in the sub-scanning direction. By balancing the scanning position in the sub-scanning direction between image heights and the amount of deteriorated wavefront aberration, the scanning position and wavefront aberration at each image height are corrected, and the scanning line curve on the scanned surface is corrected. The beam spot diameter due to the wavefront aberration deterioration is corrected.

  However, the amount of degradation of wavefront aberration due to the twist (skew) of the light beam incident on the lens surface, the amount of change in the sub-scanning direction of the object point between the image heights due to oblique incidence on the rotating polygon mirror, the lens from the deflecting reflecting surface Since the distance to the surface varies between image heights, it is not possible to completely correct wavefront aberrations or scan line bending.

  Therefore, in the present embodiment, the first lens group (L1) that is commonly used in the imaging optical system is one lens whose power in the sub-scan section including the optical surface reference axis is zero or close to zero, and The wavefront aberration is corrected by making the exit surface a special toroidal surface whose curvature in the sub-scan section changes according to the image height.

  Here, the power in the sub-scanning section is set to substantially zero because there is little deterioration of wavefront aberration due to torsion (skew) of the light beam incident on the lens surface in the vicinity of the optical surface reference axis, as well as manufacturing errors and assembling. This is to reduce fluctuations in performance due to eccentricity. The “optical surface reference axis” refers to a line connecting the origins of the expressions expressing the lens shape. Further, for a light beam traveling toward the peripheral image height, as a lens whose power increases in the sub-scan section as it moves away from the optical surface reference axis, the higher the image height, the light beam is transmitted to the optical surface of the first lens group (L1). Wavefront aberration correction is performed by jumping up away from a plane including the reference axis and parallel to the main scanning direction (hereinafter referred to as “optical reference plane”).

  In addition, in the oblique incidence optical system, when the reflected light from the lens surface returns to the deflecting surface, the light beam may reach a photoconductor different from the original one, which may cause deterioration in image quality. In this embodiment, only the exit surface has a change in the curvature of the reflected light of the lens surface, so that the reflected light from the entrance surface of the first lens group (L1) is reflected away from the optical reference surface, and the light beam is deflected. I try not to return to the surface again.

  More preferably, the lens surface is disposed closer to the optical deflector than the lens having the largest refractive power in the sub-scanning direction.

  Deterioration of wavefront aberration is greatly caused by twisting of the light beam when entering a scanning lens having a strong refractive power especially in the sub-scanning direction. Therefore, in order to correct the wavefront aberration, a strong refractive power in the sub-scanning direction is used. Therefore, it is necessary to correct the incident height to the scanning lens having a focal point so that the light is condensed at one point on the surface to be scanned.

  When correcting wavefront aberration with a special toroidal surface, the incident height to the second lens group (L2) is increased, and the light beams at both ends of the main scanning direction in the light beam are also strongly refracted in the sub-scanning direction as they go to the periphery. Correction can be made by increasing the incident height in the sub-scanning direction to the second lens group (L2) having a force.

  That is, the optical surface for the light beam having an angle in the sub-scanning direction with respect to the normal of the deflecting reflection surface of the optical deflector to the scanning lens closer to the optical deflector than the scanning lens having the strongest refractive power in the sub-scanning direction A special toroidal surface is formed so that the power in the sub-scanning section increases as the distance from the reference axis increases, and the wavefront is adjusted by adjusting the incident position in the sub-scanning direction to the scanning lens having strong refractive power in the sub-scanning direction. Aberration deterioration can be corrected. For this reason, it is desirable to provide the special toroidal surface used for correcting the wavefront aberration on the lens on the optical deflector side rather than the scanning lens having the strongest refractive power in the sub-scanning direction.

  More preferably, in the second lens group (L2), the shape in the sub-scanning direction is a planar shape having no curvature, and the short direction of the lens according to the lens height in the lens longitudinal direction (main scanning direction). A special tilt eccentric surface having a different eccentric angle (tilt amount) in the (sub-scanning direction) may be provided to correct scanning line bending. The tilt amount (eccentric angle) of the special tilt eccentric surface refers to a tilt angle in the short direction on the optical surface of the optical element. When the tilt amount is 0, there is no tilt, that is, the same state as a normal lens.

  In this way, wavefront aberration correction is performed on the special toroidal surface of the scanning lens close to the optical deflector (at least on the optical deflector side of the scanning lens having a strong refractive power in the sub-scanning direction), and scanning close to the surface to be scanned is performed. The beam spot diameter is further reduced and the scanning line is separated by separating each correction function so that the scanning line bending correction is performed on the special tilt eccentric surface of the lens (scanning lens having a strong refractive power in the sub-scanning direction). Reduction in bending can be achieved. Of course, it is not necessary to completely separate the functions, and each special tilt eccentric surface may be responsible for part of wavefront aberration correction and part of scanning line bending correction.

  FIG. 5 shows an optical path diagram after wavefront aberration correction and scanning line bending correction according to this embodiment. The light rays shown in the figure are two light rays at the center of the aperture in the sub-scanning direction and at both ends in the main scanning direction after passing through the coupling lens. A lens having a strong refractive power in the sub-scanning direction is the second lens group (L2).

  Furthermore, the “virtual surface” in the figure is a surface that does not actually exist, and is a virtual mirror surface for arranging the second lens group (L2) horizontally with the first lens group (L1) in the figure. . A special toroidal surface is adopted as the second surface of the first lens group (L1) to correct wavefront aberration. Incident height to the second lens group (L2) is increased, and the light beams at both ends in the main scanning direction in the light flux are also directed to the second lens group (L2) having a strong refractive power in the sub-scanning direction toward the periphery. The incident height in the sub-scanning direction is increased.

  Normally, it is difficult to form a lens surface so that the scanning lens is concentric in the main scanning direction with the deflection reflection point of the polygon mirror as the center in order to ensure desired optical performance.

  For this reason, the light beam deflected and reflected by the polygon mirror as the light deflector is higher in the direction having an angle in the sub-scanning direction with respect to the normal line of the deflecting reflection surface of the light deflector, as it goes to the periphery. Is incident on. That is, as shown in the optical path diagram of FIG. 5, when the light beam is bounced up on the deflecting reflecting surface, the upper surface of the scanning lens (second lens group ( When the light beam passage position at the image height of 0 is used as a reference in L2), the light beam passes through the +110 image height at the height in the sub-scanning direction plus side).

  When correcting wavefront aberration with a special toroidal surface, the incident height to the second lens group (L2) is increased, and the light beams at both ends of the main scanning direction in the light beam are also strongly refracted in the sub-scanning direction as they go to the periphery. Correction can be made by increasing the incident height in the sub-scanning direction to the second lens group (L2) having a force. In other words, the scanning lens having the strongest refractive power in the sub-scanning direction is closer to the scanning lens on the optical deflector side than the light beam having an angle in the sub-scanning direction with respect to the normal of the deflecting reflection surface of the optical deflector. A special toroidal surface is formed so that the angle with respect to the normal line is larger than the heading direction, and the incident position in the sub-scanning direction to the scanning lens having a strong refractive power in the sub-scanning direction is adjusted to correct the wavefront aberration deterioration. It becomes possible.

但し、Cm = 1/Ry ,Cs(Y) = 1/Rz とする。 The special tilt eccentric surface will be described with reference to FIG. The surface shape of the special tilt eccentric surface is according to the following shape formula (Equation 1). However, the content of the present invention is not limited to the following shape formula, and the same surface shape can be specified using another shape formula. The paraxial radius of curvature in the `` main scanning section '', which is a flat section parallel to the main scanning direction including the optical axis, is Ry, the distance from the optical axis in the main scanning direction is Y, and the higher order coefficient is Ai (i = 1, 2,..., And Rz is the paraxial radius of curvature in the “sub-scanning section” orthogonal to the main scanning section.

However, Cm = 1 / Ry and Cs (Y) = 1 / Rz.

  (F0 + F1 * Y + F2 * Y ^ 2 + F3 * Y ^ 3 + F4 * Y ^ 4 + ...) Z is the part that represents the tilt amount. When there is no tilt amount, F0, F1, F2, ... are all zero. When F1, F2,... Are not 0, the tilt amount changes in the main scanning direction.

The results when various numerical values are input to the above coefficients are summarized in Table 1 as Numerical Example 1 and in Table 2 as Numerical Example 2.

  Further, the reason why the shape of the special tilt eccentric surface in the sub-scanning direction is a planar shape having no curvature will be described.

  When curvature is added in the sub-scanning direction, the shape in the main-scanning direction changes greatly for each height in the sub-scanning direction, and the incident position of the light beam shifts in the sub-scanning direction due to temperature fluctuations and optical element assembly errors In the color machine, the beam spot position is shifted between the colors and color shift occurs. Therefore, the surface shape of the special tilt eccentric surface in the sub-scanning direction is a plane shape having no curvature, so that the shape error in the main scanning direction can be reduced for each height in the sub-scanning direction. A variation in magnification error when the incident position of the light beam is shifted in the scanning direction can be reduced, and the occurrence of color shift can be suppressed.

  Actually, by using a special tilt eccentric surface, the main scanning shape changes depending on the height in the sub-scanning direction, but the amount is slight, and the main scanning shape changes compared to the case where the curvature is added in the sub-scanning direction. Can be reduced. As a result, the difference in magnification fluctuation between the light beams due to the temperature distribution can be reduced, and by synchronizing, the color shift at the intermediate image height when the writing position and the writing end position are matched with each light beam is reduced. it can.

  Further, as shown in FIG. 7B, when the incident light beam is shifted in the sub-scanning direction, the special tilt eccentric surface does not have refractive power, so only the traveling direction of the light beam is shifted, and the change in the direction is small. . On a surface having a curvature in the sub-scanning direction, that is, having a refractive power, when the incident light beam is shifted in the sub-scanning direction as shown in FIG. 7A, the traveling direction of the light beam is changed by changing the refractive power. If the amount of change in the advancing direction is different at each image height, the scanning line is greatly bent. For the above reasons, the shape in the sub-scanning direction on the special tilt eccentric surface needs to be a planar shape having no curvature.

  According to this embodiment, it is possible to correct the direction of the scanning beam in the sub-scanning direction of the light beam directed to each image height by the special tilt eccentric surface by optimally providing a different tilt amount in the main scanning direction of the scanning lens. .

  At this time, the lens having the special tilt eccentric surface is tilted in the sub-scan section so that the light beam traveling toward the central image height passes in the vicinity of the optical surface reference axis. Note that the optical surface reference axes on both sides coincide. For this reason, in the vicinity of the optical axis, there is almost no light beam skew or scanning line bending with respect to the scanning lens. For this reason, in the special tilt eccentric surface in this embodiment, the amount of eccentricity on the optical surface reference axis can be made zero.

  In order to further reduce the size and cost of the optical scanning device of this embodiment, the reflection point on the optical deflector and the incident point on the lens closest to the deflector are orthogonal to the rotation axis of the deflector. And it is good to arrange | position on the mutually opposite side with respect to the plane (henceforth a "reference plane") containing the optical-surface reference axis of the said lens. FIG. 8A shows a trajectory (broken line) in the sub-scanning direction of the conventional oblique incidence optical system and a trajectory in the sub-scanning direction (solid line) according to this embodiment. In the conventional oblique incidence optical system, the reflection point on the deflector is matched with the reference plane. In this embodiment, the height of the first scanning imaging lens in the sub-scanning direction can be reduced because Zr is shifted to the opposite side of the incident point of the first scanning imaging lens with respect to the sub-scanning direction. It becomes possible to reduce the height of the entire apparatus. Although the height of the deflector in the sub-scanning direction also slightly increases, there is no problem because the amount of reduction in the height of the first scanning imaging lens is even larger.

  In order to further reduce the size, as shown in FIG. 8B, it is preferable that a plurality of light beams are incident on the same reflection point at different oblique incident angles. Further, as shown in FIG. 8C, it is desirable to arrange a plurality of light sources so as to be plane-symmetric with respect to the reference plane. The mirror symmetry here refers to a state in which all the folding mirrors after being deflected and reflected by the polygon mirror are omitted, and a plurality of the mirrors reflected and deflected by the polygon mirror are horizontal to the normal line of the reflecting surface of the polygon mirror. This is for the plane including the center of the light beam in the sub-scanning direction.

  In order to suppress the occurrence of scanning line bending, it is preferable to use a reduction optical system in which the imaging magnification in the sub-scan section of the imaging optical system is 1.0 times or less. By doing so, even if a manufacturing error and assembly error of the scanning optical system occur, it is possible to suppress the influence of the change in the scanning line bending.

  FIG. 9 shows the effect of scanning line bending correction according to this embodiment. The two curves in the figure show the scanning line bending of the optical system having the special tilt eccentric surface and the optical system not using the special tilt eccentric surface according to this embodiment. As is apparent from the figure, the scan line bending of the oblique incidence optical system is well corrected by the special tilt eccentric surface.

  In order to make the scanning optical system an optical scanning device that is more resistant to decentration and environmental fluctuations, the first lens group (L1) has a surface whose sub-scanning cross-sectional shape is flat at all lens heights. It is good to make it. The first lens group (L1) has power in the sub-scan section as it moves away from the optical surface reference axis. However, the first lens group (L1) is integrated into one surface and the other surface has a planar shape, thereby decentering the lens. It becomes possible to reduce the influence of.

  More desirably, a surface having a flat sub-scanning sectional shape at all lens heights may be provided on the incident surface side of the first lens group (L1). When the reflected light on the lens surface reaches the image surface or the light source, the image quality deteriorates. By doing so, the reflected light on the incident surface of the first lens group (L1) is incident on the deflector again and the image surface or the light source. Will not reach. At this time, the power of the lens in the sub-scan section not including the optical surface reference axis of the first lens group (L1) has a negative power (see FIG. 10).

  The special tilt eccentric surface described above is optimally set for each light beam directed to a different surface to be scanned, that is, for each angle (oblique incidence angle) in the sub-scanning direction with respect to the normal line of the reflection surface of the optical deflector. This makes it possible to correct the wavefront aberration and to correct the scanning line bending. In this case, even if the oblique incident angles are different, this special tilt eccentric surface can be used by changing the coefficient of the shape formula and designing optimally.

  Furthermore, as shown in FIG. 1, the incident angle in the sub-scanning direction can be reduced by causing the light beam incident on the deflecting / reflecting surface of the rotary polygon mirror to enter the main scanning direction so as not to interfere with the scanning lens. Can be set small. As described above, if the angle of oblique incidence in the sub-scanning direction is large, the optical performance is greatly deteriorated, so that good correction becomes difficult. For this reason, it is desirable that the light beam incident on the deflecting / reflecting surface of the rotary polygon mirror is incident at an angle in the main scanning direction.

  In order to further increase the speed of the optical scanning device of this embodiment, it is preferable to use at least one surface having a curvature different in the sub-scanning direction in accordance with the image height for the lens closest to the surface to be scanned. With such a configuration, the magnification deviation between image heights can be sufficiently reduced. In addition, an effect of better correcting the curvature of field in the sub-scanning direction can be expected.

  More preferably, the curvature in the sub-scanning direction is changed asymmetrically in the main scanning direction around the reference axis. In the optical scanning device of this embodiment, the light beam is incident on the optical deflector at an angle in the main scanning direction. As a result, the occurrence of “optical sag” by the rotating polygon mirror does not occur symmetrically in the main scanning direction with respect to the reference axis of the scanning lens. In other words, since the difference in optical path length that causes various aberrations is not symmetrical with respect to the center, the occurrence of various aberrations also occurs asymmetrically, so this configuration enables effective aberration correction. Become.

  In order to further reduce the cost in the optical scanning device according to the present embodiment, the scanning imaging lens is preferably a plastic lens. By making the scanning imaging lens plastic, the degree of freedom of the surface shape is increased, and the effect of achieving better optical performance can be expected.

  In the optical scanning device according to the present embodiment, the light source is, for example, a semiconductor laser array having a plurality of light emission points, or a multi-beam light source device using a single light emission point or a plurality of light sources having a plurality of light emission points, and a plurality of light sources. The beam may be scanned on the surface of the photoconductor at the same time. By doing so, it is possible to configure an optical scanning device and an image forming apparatus that are increased in speed and density, and even when such an optical scanning device and an image forming apparatus are configured, the same effects as described above are obtained. The effect of can be obtained. FIG. 11 shows an example of a light source unit constituting the multi-beam light source device.

  In FIG. 11A, the semiconductor lasers 403 and 404 are individually fitted in fitting holes 405-1 and 405-2 (not shown) formed on the back side of the base member 405, respectively. The fitting holes 405-1 and 405-2 are inclined at a predetermined angle in the main scanning direction, in the embodiment, about 1.5 °, and the semiconductor lasers 403 and 404 fitted in the fitting holes are also main. It is inclined about 1.5 ° in the scanning direction. The semiconductor lasers 403 and 404 have notches formed in the cylindrical heat sink portions 403-1 and 404-1, and the protrusions 406-1 and 407-1 formed in the center round holes of the pressing members 406 and 407 are provided. The alignment direction of the light emitting sources is adjusted by matching the notch portion of the heat sink portion. The holding members 406 and 407 are fixed to the base member 405 with screws 412 from the back side thereof, so that the semiconductor lasers 403 and 404 are fixed to the base member 405. Further, the collimating lenses 408 and 409 are adjusted in the optical axis direction so that the outer circumference thereof is aligned with the semicircular mounting guide surfaces 405-4 and 405-5 of the base member 405. Positioned and bonded so as to be a parallel light beam.

  In the above-described embodiment, since the light beams from the respective semiconductor lasers are set so as to intersect within the main scanning plane, the fitting holes 405-1 and 405-2 and the semicircular mounting guide are disposed along the light beam direction. The surfaces 405-4 and 405-5 are formed to be inclined. By engaging the cylindrical engagement portion 405-3 of the base member 405 with the holder member 410 and passing the screw 413 through the through hole 410-2 and screwing into the screw holes 405-6 and 405-7, the base member Reference numeral 405 denotes a light source unit that is fixed to the holder member 410.

  The holder member 410 of the light source unit has a cylindrical portion 410-1 fitted into a reference hole 411-1 provided in the mounting wall 411 of the optical housing, and a spring 611 is inserted from the front side of the mounting wall 411 to stop the stopper member 612. Is held in close contact with the back side of the mounting wall 411, thereby holding the light source unit. One end of the spring 611 is hooked on the protrusion 411-2 of the mounting wall 411, and the other end of the spring 611 is hooked on the light source unit, thereby generating a rotational force about the center of the cylindrical portion in the light source unit. An adjustment screw 613 provided to lock the rotational force of the light source unit is provided, and the adjustment screw 613 can rotate the entire unit in the θ direction around the optical axis to adjust the pitch. It is configured as follows. An aperture 415 is disposed in front of the light source unit, and the aperture 415 is provided with a slit corresponding to each semiconductor laser, and is configured to be attached to an optical housing to define an emission diameter of the light beam.

  FIG. 11B shows a second embodiment of the light source unit. In FIG. 11B, each light beam from the semiconductor laser 703 having four light emitting sources is configured to be combined using beam combining means. Reference numeral 706 denotes a pressing member, 705 denotes a base member, 708 denotes a collimating lens, and 710 denotes a holder member. This embodiment is different from the embodiment shown in FIG. 11A in that there is one semiconductor laser 703 as a light source and there is one pressing member 706 according to this, and other configurations are as follows. Basically the same.

  FIG. 11C shows a configuration similar to the example shown in FIG. 11B, in which the light beams from the semiconductor laser array 801 having four light emitting sources are combined using beam combining means. Is shown. Since the basic components are the same as those shown in FIGS. 11A and 11B, description thereof is omitted here.

  Next, an embodiment of an image forming apparatus using the optical scanning device according to the present embodiment will be described with reference to FIG. This embodiment is an example in which the optical scanning device according to this embodiment is applied to a tandem full-color laser printer. In FIG. 12, a transport belt 17 for transporting a recording material (for example, transfer paper) S fed from a paper feed cassette 13 disposed in the horizontal direction is provided on the lower side in the apparatus. A yellow (Y) photosensitive member 7Y, a magenta (M) photosensitive member 7M, a cyan (C) photosensitive member 7C, and a black (K) photosensitive member 7K are transferred onto the transfer belt 17. S is arranged at equal intervals in order from the upstream side in the transport direction to the downstream side. Hereinafter, subscripts Y, M, C, and K are appropriately added to the reference numerals for distinction. These photoreceptors 7Y, 7M, 7C, and 7K are all formed to have the same diameter, and process members that perform each process in accordance with the electrophotographic process are sequentially disposed around the photoreceptors. Taking the photoconductor 7Y as an example, a charging charger 8Y, an optical scanning optical system 6Y of the optical scanning device 9, a developing device 10Y, a transfer charger 11Y, a cleaning device 12Y, and the like are sequentially arranged. The same applies to the other photoconductors 7M, 7C, and 7K.

  In the present embodiment, the surfaces of the photoconductors 7Y, 7M, 7C, and 7K are used as scan surfaces (or irradiated surfaces) that are set for the respective colors, and the photoconductors 7Y, 7M, 7C, and 7K are provided on the respective photoconductors 7Y, 7M, 7C, and 7K. On the other hand, the optical scanning optical systems 6Y, 6M, 6C, and 6K of the optical scanning device 9 are provided in a one-to-one correspondence relationship. However, as in FIG. 1, the optical deflector 5 and the first lens unit L1 on the side close to the optical deflector 5 are commonly used by the four optical scanning optical systems 6Y, 6M, 6C, and 6K. The second lens group L2 on the side close to the photoreceptors (scanned surfaces) 7Y, 7M, 7C, and 7K is provided in each optical system. Note that illustration of a pre-deflector optical system such as a plurality of light source devices, coupling lenses, apertures, and cylindrical lenses is omitted.

  The conveying belt 17 is supported by a driving roller 18 and a driven roller 19 and rotated in the direction of the arrow in the figure. Around the periphery thereof, a registration roller 16 and a belt charging charger 20 are positioned upstream of the photoreceptor 7Y. And a belt separation charger 21, a belt neutralization charger 22, a belt cleaning device 23, and the like are provided in this order so as to be positioned downstream of the photoreceptor 7K in the rotation direction of the belt 17. A fixing device 24 including a heating roller 24 a and a pressure roller 24 b is provided downstream of the belt separation charger 21 in the transfer paper conveyance direction, and is connected to a paper discharge tray 26 by a paper discharge roller 25.

  In the laser printer having such a schematic configuration, for example, in the full color mode (multiple color mode), each of the photoconductors 7Y, 7M, 7C, and 7K is charged by the charging chargers 8Y, 8M, 8C, and 8K, Light beams from the optical scanning optical systems 6Y, 6M, 6C, and 6K of the optical scanning device 9 based on image signals of colors Y, M, C, and K for the photoreceptors 7Y, 7M, 7C, and 7K. By scanning, an electrostatic latent image corresponding to each color signal is formed on the surface of each photoconductor. These electrostatic latent images are developed with toners of respective colors Y, M, C, and K by the corresponding developing devices 10Y, 10M, 10C, and 10K to form toner images. The transfer paper S in the paper feed cassette 13 is fed by the paper feed roller 14 and the transport roller 15 in synchronization with this image forming process, and is sent out to the transport belt 17 by the registration roller 16. The transfer sheet S fed to the transport belt 17 is electrostatically attracted to the transport belt 17 by the action of the belt charger 20 and transported toward the photoconductors 7Y, 7M, 7C, 7K. , 7M, 7C, and 7K are sequentially transferred onto the transfer paper S so that they are superimposed, and a full-color image is formed on the transfer paper S. The transfer sheet S on which the full-color image has been transferred is separated from the transport belt 17 by the belt separation charger 21 and transported to the fixing device 24. After the full-color image is fixed on the transfer paper S by the fixing device 34, the discharge roller 25 As a result, the paper is discharged to the paper discharge tray 26.

  By using the optical scanning optical systems 6Y, 6M, 6C, and 6K of the image forming apparatus as the optical scanning apparatus according to the above-described embodiment, the scanning line bending and the wavefront aberration can be effectively corrected, and there is no color shift. Thus, an image forming apparatus that can ensure high-quality image reproducibility can be realized.

  The above-described embodiment is the best for implementing the optical scanning device of the present embodiment, but is not intended to be limited to such an embodiment. Therefore, various modifications can be made to the implementation form without departing from the scope of the present embodiment.

  Development of a digital copying machine, a laser printer, a laser facsimile, etc. is desired for the optical scanning device of this embodiment.

2 illustrates the configuration of an imaging optical system of an optical scanning device of an oblique incidence optical system according to the present embodiment. The shape (a) of the polygon mirror in the optical scanning device of the horizontal incidence one side scanning system and the shape (b) of the polygon mirror in the optical scanning device of the oblique incidence optical system are illustrated. The relationship between the height h of the polygon mirror in the sub-scanning direction and the separation interval Δd of the light beam is illustrated, (a) a combination of horizontal incidence and oblique incidence, (b) due to horizontal incidence, c) Due to oblique incidence. The light beam having an angle in the sub-scanning direction because it is obliquely incident is shown in a state of being incident on the first scanning imaging lens in a twisted state. The optical path diagrams (a) to (c) after correction of wavefront aberration and scanning line bending according to this embodiment are shown. The relationship between the lens height and the tilt is illustrated for the special tilt eccentric surface. (A) and (b) show how the traveling direction of the light beam is shifted when the incident light beam is shifted in the sub-scanning direction due to the shape of the special tilt eccentric surface. (A)-(c) which showed the locus | trajectory of the sub-scanning direction of the oblique incidence optical system of a light beam. It is a graph for demonstrating the effect of a scanning line curvature correction. It is a graph regarding the power of the lens in the sub-scan section of the first lens group (L1). 1A to 1C illustrate light source units constituting a multi-beam light source device. 1 illustrates a configuration of an optical scanning device according to the present embodiment applied to a tandem type full color laser printer.

Explanation of symbols

L1 First lens group L2 Second lens group

Claims (7)

A plurality of light source devices that emit light beams, a light deflector that deflects and scans a plurality of light beams emitted from the light source device on the same deflection surface, and a plurality of light beams deflected by the light deflector are scanned differently. A plurality of light beams emitted from the light source device are incident at a predetermined angle in a sub-scanning direction with respect to the normal line of the deflection reflection surface of the optical deflector, and In an optical scanning device comprising a first lens group through which all light beams deflected and scanned by the image optical system on the same deflection surface, and a second lens group provided for each scanned surface,
The reflection point at the optical deflector and the incident point to the lens closest to the deflector of the first lens group are arranged on opposite sides with respect to the reference plane,
The first lens group has zero or substantially zero power in the sub-scan section including the optical surface reference axis, and the curvature of the exit surface in the sub-scan section changes as the distance from the optical surface reference axis changes. It consists of a single lens that increases the absolute value of power in the sub-scan section parallel to the optical surface reference axis,
Ri a strong negative power in the sub-scanning cross-section with increasing distance from the optical surface reference axis,
The reference plane is perpendicular to the rotation axis of the deflector optical scanning device comprising a planar der Rukoto including the optical surface reference axis.   2. The optical scanning device according to claim 1, wherein a curvature in a sub-scanning direction in a sub-scanning section including an optical surface reference axis of the first lens group is zero or substantially zero.   At least one surface of the second lens group is a surface having no curvature in the sub-scanning direction, and a special tilt in which the tilt eccentric angle in the sub-scanning direction changes according to the distance from the optical surface reference axis. The optical scanning device according to claim 1, wherein the optical scanning device has at least one eccentric surface.   4. The optical scanning device according to claim 3, wherein an eccentric amount of the special tilt eccentric surface of the second lens group is zero in a sub-scan section including the optical surface reference axis.   5. The optical scanning device according to claim 1, wherein the imaging optical system is a reduction optical system in a sub-scan section.   6. A multi-beam optical scanning device using the optical scanning device according to claim 1 as optical scanning means.   An image forming apparatus using the optical scanning device according to claim 1 as an electrophotographic writing unit.

Images