JP2008310257A - Scanning optical system and optical scanner and image forming apparatus equipped with the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve both of reduction in the size of a scanning optical system and correction of an fθ error and curvature of an image plane with high accuracy. <P>SOLUTION: The scanning optical system 10 includes a polygon mirror 6 that has a deflection faces 6b and rotates, and deflects a beam B from a light source 1 on the deflection faces 6b of the polygon mirror 6 to scan a surface 8 to be scanned with the beam. The optical system is equipped with: a first imaging optical system placed between the light source 1 and the polygon mirror 6 for focusing the beam B from the light source 1 onto the deflection face 6b of the polygon mirror 6; and a second imaging optical system placed between the polygon mirror 6 and the surface 8 for focusing the beam B deflected by the polygon mirror 6 onto the surface 8. The first imaging optical system contains a second reflection mirror 5 and a control unit 50 for correcting a fθ error. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a scanning optical system, an optical scanning device including the same, and an image forming apparatus.

  An image forming apparatus such as a laser beam printer generally includes a semiconductor laser as a light source, a collimator lens and a cylindrical lens as a first imaging optical system for focusing a laser beam from the semiconductor laser, and the collimator lens. And a polygon mirror as a deflector for deflecting the laser beam focused by the cylindrical lens, and a second imaging optical system for focusing the laser beam deflected by the polygon mirror on the photosensitive drum surface as the scanning surface As a scanning lens.

  In order to form a highly accurate image in this type of image forming apparatus, it is necessary to satisfactorily correct the fθ error and the field curvature. Here, the fθ error is a property that satisfies Y = f · θ when the image height at which the parallel beam incident on the scanning lens having the focal length f at the angle of view θ is focused on the surface to be scanned is Y ( This means the amount of deviation from the fθ characteristic.

  For example, in the image forming apparatus according to Patent Document 1, the second imaging optical system includes a first scanning lens and a second scanning lens, and the first scanning lens mainly corrects field curvature. At the same time, the second scanning lens mainly corrects the fθ error. As described above, the second imaging optical system is configured by two scanning lenses, and the correction of the curvature of field and the correction of the fθ error are assigned to each scanning lens, so that the fθ error and the curvature of field can be accurately performed. It is corrected.

On the other hand, in order to configure the image forming apparatus at a low cost and in a small size, it is also known that the second imaging optical system is composed of one scanning lens. For example, in the image forming apparatus according to Patent Document 2, the second imaging optical system is configured by one scanning lens, and the fθ error and the curvature of field are corrected by the scanning lens.
JP 2006-154845 A JP 09-179017 A

  However, in the configuration in which the fθ error and the curvature of field are corrected by a single scanning lens as in the image forming apparatus according to Patent Document 2, both of them are traded off due to the trade-off between the fθ error correction capability and the field curvature correction capability. The scanning lens is designed to balance this or at the expense of any performance. As a result, the correction capability is inferior to that in which the fθ error and the field curvature are separately corrected by the two scanning lenses as in the image forming apparatus according to Patent Document 1.

  On the other hand, the image forming apparatus according to Patent Document 1 can correct the fθ error and the curvature of field with high accuracy, but the second imaging optical system includes a scanning lens that mainly corrects the curvature of field and the fθ error. It is necessary to comprise at least two scanning lenses including a scanning lens that mainly corrects the above, and the apparatus becomes large.

  The present invention has been made in view of such a point, and an object of the present invention is to achieve both the downsizing of the scanning optical system and the highly accurate correction of the fθ error and the field curvature.

  In the present invention, the fθ error is corrected by the first imaging optical system.

  Specifically, the scanning optical system according to the present invention is disposed between the light source and the deflector, and focuses the light beam from the light source on the deflection surface of the deflector; A second imaging optical system disposed between the deflector and the surface to be scanned, and focusing the light beam deflected by the deflector on the surface to be scanned; , Fθ error correction means for correcting the fθ error is included.

  According to the present invention, by correcting the fθ error by the fθ error correcting means of the first imaging optical system, in the second imaging optical system, the fθ error is corrected by the first imaging optical system. It is possible to focus on suppression (or correction) of field curvature. As a result, the first imaging optical system can be designed to mainly correct the fθ error, and the second imaging optical system can be designed to mainly correct the field curvature, and the fθ error and the field curvature can be corrected with high accuracy. can do. In addition, since it is only necessary to mainly correct the field curvature in the second imaging optical system, the second imaging optical system is compared with the case where the fθ error and the field curvature are corrected in the second imaging optical system. Can be simplified, and the scanning optical system can be miniaturized.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows an optical scanning device 10 according to the present embodiment. The optical scanning device 10 includes a light source 1 and a scanning optical system 20 and optically scans a surface to be scanned 6. The light source 1 can be constituted by, for example, a semiconductor laser.

  The scanning optical system 20 includes a collimator lens 2, a cylindrical lens 3, a first reflecting mirror 4, a second reflecting mirror 5, a polygon mirror 6, and a scanning lens 7. In this scanning optical system 20, a collimator lens 2, a cylindrical lens 3, a first reflecting mirror 4 and a second reflecting mirror 5 constitute a first imaging optical system, and the scanning lens 7 constitutes a second imaging optical system. ing. Each of the first and second imaging optical systems is an anamorphic optical system having different optical powers in the main scanning direction and the sub-scanning direction orthogonal to the main scanning direction.

  The collimator lens 2 is for making the divergent light beam B emitted from the light source 1 into a substantially parallel light beam. In the case where the light beam B emitted from the light source 1 is a substantially parallel light beam, the collimator lens 2 is not necessarily an essential component.

  The cylindrical lens 3 has no optical power in the main scanning direction, has positive optical power only in the sub-scanning direction, and is formed on the cylindrical surface formed on the light source 1 side and the polygon mirror 6 side. A lens having a flat surface. The light beam B incident on the cylindrical lens 3 is reflected by the first reflecting mirror 4 and the second reflecting mirror 5 and then formed as a line image extending in the main scanning direction in the vicinity of the deflection surface of the polygon mirror 6.

  The first reflecting mirror 4 and the second reflecting mirror 5 are for reflecting the light beam B incident from the cylindrical lens 3 and guiding it to the polygon mirror 6. The first reflecting mirror 4 is a reflecting mirror having a reflecting surface fixed, while the second reflecting mirror 5 is configured such that the shape of the reflecting surface is variable.

  Specifically, the second reflecting mirror 5 adjusts the incident angle of the light beam B on the deflection surface 6b of the polygon mirror 6. The second reflecting mirror 5 is a plurality of micromirrors supported by the substrate in a state of being separated from each other, and each of the plurality of micromirrors is independently driven, thereby arranging each micromirror and the substrate. A deformable mirror including a plurality of drive units for controlling the relationship. That is, the second reflecting mirror 5 is composed of a plurality of MEMS (Micro Electro Mechanical System) mirrors arranged in an array. The MEMS mirror is composed of a MEMS device including a minute mirror and an actuator that drives the mirror. MEMS is a minute electromechanical system in which electrons and mechanical mechanisms are fused using semiconductor process technology (such as photolithography technology). In the present embodiment, each MEMS mirror is configured to tilt independently, and the second reflecting mirror 5 changes the angle of the reflecting surface by uniformly tilting each MEMS mirror. The MEMS mirror may be configured to move linearly in the height direction (normal direction) in addition to the tilting operation.

  The polygon mirror 6 is a rotating polygon mirror that is driven to rotate by a driving means such as a motor (not shown) around a rotating shaft 6a. In this embodiment, the polygon mirror 6 has six deflecting surfaces 6b, 6b,... And is rotationally driven at a constant speed in the direction of arrow A in the figure. The polygon mirror 6 performs deflection scanning by deflecting the incident light beam B by each deflection surface 6b that rotates about the rotation axis 6a. The number of deflection surfaces 6b, 6b,... Of the polygon mirror 6 is not limited to six, and any number of surfaces can be employed. The polygon mirror 6 has an optical axis of the light beam B incident on the polygon mirror 6 and a scanning center in the main scanning direction of the light beam B deflected by the deflecting surface 6b (the light beam B is centered on the image height on the scanned surface 8). The optical axis when passing through (hereinafter simply referred to as the optical axis L) is arranged at a predetermined finite angle.

  The scanning lens 7 focuses the light beam B deflected and scanned by the polygon mirror 6 onto the scanned surface 8. The scanning lens 7 is an anamorphic lens (which differs from a cylindrical lens and has optical power in the sub-scanning direction) having different optical powers in the main scanning direction and the sub-scanning direction. The scanning lens 7 has a function of correcting at least the field curvature in the main scanning direction on the scanned surface 8. The anamorphic surface of the scanning lens 7 is preferably a free curved surface, and particularly preferably a free curved surface in which a line connecting the centers of curvature in the sub-scanning direction becomes a curved non-arc. By doing so, the curvature of field can be corrected satisfactorily.

  In the second imaging optical system constituted by the scanning lens 7, as shown in FIG. 2, the deflection surface 6b of the polygon mirror 6 and the scanned surface 8 are in an optically conjugate relationship in the sub-scan section. . By doing so, the second imaging optical system has a so-called surface tilt correction function, and even if the deflection surface 6b of the polygon mirror 6 is inclined with respect to the rotation axis 6a, the second imaging optical system is on the surface to be scanned 8. The focus position in the sub-scanning direction of the formed spot-like image does not change.

  The second reflecting mirror 5 is driven and controlled by the control unit 50 as shown in FIG. Specifically, the detection signal is input from the start point sensor 51 to the control unit 50, and the light emission control signal and the tilt control signal are respectively transmitted to the light source driver 52 that drives the light source 1 and the MEMS controller 53 that drives the second reflecting mirror 5. Is configured to output.

  The start point sensor 51 is provided in the vicinity of the outer side of the end of the scanning surface 8 in the main scanning direction on the side where the scanning of the light beam B is started (the upper side in FIG. 1). The start point sensor 51 is configured to detect the light beam B, and outputs a detection signal to the control unit 50 when the light beam B is detected. That is, it is possible to detect that the scanning of the light beam B onto the scanned surface 8 is started by outputting a detection signal from the start point sensor 51.

  The light source driver 52 drives the light source 1 in accordance with a light emission control signal from the control unit 50.

  The MEMS controller 53 is for tilting the reflecting surface of the second reflecting mirror 5. A memory 54 is connected to the MEMS controller 53, and the tilt angle α of the second reflecting mirror 5 corresponding to the rotation angle θp of the polygon mirror 6 is stored in the memory 54. Based on the tilt control signal input from the control unit 50, the MEMS controller 53 reads the tilt angle α of the second reflecting mirror 5 corresponding to the rotation angle θp of the polygon mirror 6 from the memory 54, and the read tilt angle α and The second reflecting mirror 5 is tilted and driven so as to be.

  The control of the control unit 50 configured as described above will be described in more detail. When the detection signal from the start point sensor 51 is input, the control unit 50 is a light source driver based on image information formed on the scanned surface 8. The light emission control signal is output to 52, and image formation on the scanned surface 8 is started. At this time, the control unit 50 calculates the rotation angle θp of the polygon mirror 6 based on the elapsed time since the detection signal was input. Then, the control unit 50 calculates the focus position of the light beam B on the scanned surface 8 from the rotation angle θp of the polygon mirror 6 and outputs a light emission control signal to the light source driver 52 according to each focus position. At the same time, a tilt control signal is output to the MEMS controller 53. That is, the control unit 50 adjusts the focusing angle of the light beam B on the scanned surface 8 by adjusting the tilt angle α of the second reflecting mirror 5 according to the rotation angle θp of the polygon mirror 6. An image is formed.

  Here, the adjustment of the focus position by the second reflecting mirror 5 will be described in more detail.

  In general, in a scanning optical system, since a spot-like image formed on the surface to be scanned 8 has an fθ error, the interval between the images in the main scanning direction is not constant. Therefore, the control unit 50 controls the tilt angle α of the second reflecting mirror 5 so that the distance between the spot-like images formed on the scanned surface 8 in the main scanning direction is constant. Yes. That is, the control unit 50 adjusts the focus position of the light beam B on the scanned surface 8 by adjusting the incident angle of the light beam B incident on the deflecting surface 6b of the polygon mirror 6 from the second reflecting mirror 5. ing.

  Specifically, when the focus position is adjusted (moved) to the positive side (upper side in FIG. 1) in the main scanning direction, the incident angle β of the light beam B to the deflecting surface 6b is small as shown in FIG. Thus, the tilt angle α of the second reflecting mirror 5 is controlled. By doing so, the light beam B deflected by the deflecting surface 6b of the polygon mirror 6 is returned to the negative side in the main scanning direction, and the in-focus position of the light beam B on the scanned surface 8 is made negative in the main scanning direction. Can be adjusted to the side.

  On the other hand, when the focusing position of the image is adjusted (moved) to the positive side (lower side in FIG. 1) in the main scanning direction, the incident angle β of the light beam B to the deflecting surface 6b is opposite to FIG. The tilt angle α of the second reflecting mirror 5 is controlled so as to increase (that is, the tilt angle α is controlled in the opposite direction to the case where the focus position is adjusted to the negative side in the main scanning direction). ). By doing so, the light beam B deflected by the deflecting surface 6b of the polygon mirror 6 is advanced to the positive side in the main scanning direction as compared with before correction, and the focusing position of the light beam B on the scanned surface 8 is reached. Can be adjusted to the positive side in the main scanning direction.

  In this way, the control unit 10 corrects the in-focus position of the light beam B so that the main scanning direction interval between the image formed on the scanned surface 8 is equal, thereby correcting the fθ error. is doing.

  Here, if the maximum value of the tilt angle α of the second reflecting mirror 5 is αmax and the rotation angle θp of the polygon mirror 6 at that time is θpmax (θpmax ≠ 0),

Is preferably satisfied. However, the tilt angle α and the rotation angle θp are values based on when the light beam B passes through the optical axis L.

  By satisfying the above formula (1), the fθ error can be corrected satisfactorily.

  The relationship between the maximum change amount αmax of the tilt angle α of the second reflecting mirror 5 and the rotation angle θpmax of the polygon mirror 6 at that time is as follows:

Is more preferable. By satisfying the above mathematical formula (2), the above-described effects can be achieved more remarkably.

  In addition, it is desirable that the above α is expressed by a polynomial of θp, and the degree of the polynomial is a higher order polynomial of 4th order or more, preferably 6th order or more. By expressing α as a polynomial of θp, it is possible to satisfactorily correct the fθ error.

In the present embodiment, if the magnification in the sub-scanning direction in the second imaging optical system is m,
2.5 <m (3)
Designed to satisfy.

  By satisfying the above formula (3), the scanning lens 7 is installed near the polygon mirror 6, and the second imaging optical system, and hence the scanning optical system 20, can be miniaturized. In addition, by satisfying Equation (3), the amount of jitter generated on the scanned surface 8 due to the vibration of the polygon mirror 6 can be suppressed. In the present embodiment, the fθ error can be sufficiently corrected by the second reflecting mirror 5 and the field curvature can be sufficiently corrected by the scanning lens 7, so that the fθ error is deteriorated even when the mathematical formula (3) is satisfied. It is possible to prevent the field curvature from being increased.

The magnification m is
3.5 <m (4)
Is preferably satisfied.

Furthermore, the magnification m is
4.7 <m (5)
Is more preferable. By satisfying the formula (4) or the formula (5), the above-described effect can be achieved more remarkably.

  FIG. 5 is a diagram illustrating a configuration of an image forming apparatus 9 using the optical scanning device 10 described in the present embodiment.

  The image forming apparatus 9 includes an optical scanning device 10, a primary charger 90, a developing device 92, a transfer charger 93, a cleaner 94, a fixing device 95, a paper feed cassette 96, and a photosensitive drum 97. ing.

  Here, the surface of the photosensitive drum 97 corresponds to the surface to be scanned 8 in FIG. 1, and the optical scanning device 10 focuses a light beam based on predetermined image information on the surface of the photosensitive drum 97. The optical scanning device 10 is focused, and the photosensitive drum 97 is rotated relative to the optical scanning device 10 so that the light beam is sequentially focused on the surface of the photosensitive drum 97. .

  The primary charger 90 is disposed on the upstream side of the exposure position of the optical scanning device 10 in the rotation direction of the photosensitive drum 97 so as to face the photosensitive drum 97, and uniformly charges the surface of the photosensitive drum 97. It is. The optical scanning device 10 scans and exposes the photosensitive drum 97, and an electrostatic latent image is formed by scanning exposure by the optical scanning device 10. The developing device 92 is for visualizing the electrostatic latent image formed on the photosensitive drum 97 with toner. The paper supply cassette 96 supplies paper, and the transfer charger 93 transfers toner onto the paper supplied from the paper supply cassette 96. Specifically, the paper is fed to a position facing the surface of the photosensitive drum 97 to which the toner is attached. The transfer charger 93 is disposed so as to face the photosensitive drum 97 with the fed sheet interposed therebetween. The transfer charger 93 is for charging the sheet fed to a polarity opposite to that of the toner, thereby sucking the toner adsorbed on the photosensitive drum 97 and attaching it to the sheet. The fixing device 95 is for fixing the transferred toner onto the paper. Specifically, the fixing device 95 fixes the toner on the paper on which the toner has been transferred by heating and softening the toner. The cleaner 94 is for removing the toner remaining on the photosensitive drum 97 after transfer onto the paper.

  Next, the operation of the image forming apparatus 9 will be described. First, the photosensitive drum 97 is uniformly charged by the primary charger 90. Next, the surface of the photosensitive drum 97 is exposed and scanned by the optical scanning device 10, and an electrostatic latent image is formed on the photosensitive drum 97. Toner adheres to the electrostatic latent image formed on the photosensitive drum 97 by the developing device 92. The adhered toner is transferred onto a sheet supplied from the sheet feeding cassette 96 by the transfer charger 93. The toner transferred onto the paper is fixed on the paper by the fixing device 95 to form a predetermined image.

  Therefore, according to the above-described embodiment, the second reflecting mirror 5 for correcting the fθ error is provided in the first imaging optical system, so that the scanning lens 7 of the second imaging optical system mainly performs only field curvature. It will be sufficient to correct. As a result, the scanning lens 7 has only to be designed specifically for correction of field curvature, and the field curvature is corrected with higher accuracy than a configuration in which a single scanning lens corrects fθ error and field curvature. can do. Similarly, the fθ error can also be corrected with the second reflecting mirror 5, and therefore, the fθ error can be made with higher accuracy than the configuration in which the fθ error and the curvature of field are corrected with one scanning lens. It can be corrected. Further, when it is only necessary to correct only the curvature of field in the second imaging optical system, even the single scanning lens 7 can sufficiently correct the curvature of field, so that the second imaging optical system has There is no need to provide a plurality of scanning lenses, and the second imaging optical system, and thus the scanning optical system 20, can be configured compactly. That is, according to the present embodiment, it is possible to achieve both the downsizing of the scanning optical system 20 and the highly accurate correction of the fθ error and the field curvature.

  Further, according to the present embodiment, the incident angle of the light beam B incident on the deflecting surface 6b of the polygon mirror 6 is adjusted based on the rotation angle θp of the polygon mirror 6 by the second reflecting mirror 5 and the control unit 50. Thus, the fθ error can be corrected. In other words, when the incident angle of the light beam B on the deflecting surface 6b of the polygon mirror 6 changes, the focal position of the light beam B on the scanned surface 8 also changes, so that the image formed on the scanned surface 8 is image-to-image. The fθ error can be corrected by adjusting the incident angle of the light beam B to the deflecting surface 6b so that the main scanning direction interval is equal.

  The incident angle of the light beam B on the deflecting surface 6b of the polygon mirror 6 is changed by changing the tilt angle α of each MEMS mirror of the second reflecting mirror 5, that is, the light beam B reflected by the second reflecting mirror 5. It is possible to adjust by changing the reflection angle. In addition, since the MEMS mirror has high responsiveness, the second reflecting mirror 5 is constituted by a plurality of MEMS mirrors, so that the refracting surface 6b of the polygon mirror 6 is highly responsive to the rotation of the polygon mirror 6 by following the rotation of the polygon mirror 6. The incident angle of the light beam B can be adjusted.

  The scanning lens 7 is not limited to correcting only the curvature of field. That is, the scanning lens 7 only needs to mainly correct the field curvature, and may have a function of correcting an fθ error and other aberrations in addition to the field curvature. Further, the number of scanning lenses provided in the second imaging optical system is not limited to one. In other words, the second imaging optical system may be provided with a plurality of scanning lenses. Even in such a configuration, by correcting the fθ error with the first imaging optical system, the burden of correcting the fθ error with the second imaging optical system can be reduced, and the curvature of field is accordingly increased. In addition, it is possible to design a scanning lens specialized in correcting aberrations and other aberrations, and to reduce the number of scanning lenses.

  Furthermore, in the said embodiment, although the 2nd reflective mirror 5 was comprised with several MEMS mirror, it is not restricted to this. Any optical element such as a galvanometer mirror that can change the tilt of its reflecting surface can be employed as the second reflecting mirror. However, as described above, the second reflecting mirror 5 is preferably a MEMS mirror from the viewpoint of high-speed response.

  Moreover, in the said embodiment, although the angle of each MEMS mirror which comprises the 2nd reflective mirror 5 is changed uniformly, it is not restricted to this. For example, the reflection surface shape of the second reflecting mirror 5 may be changed by changing the tilt angle of each MEMS mirror to a different value. By doing so, aberrations other than the fθ error can be corrected by the second reflecting mirror 5. For example, the start point sensor 51 is not a sensor that simply detects the light beam B, but a sensor that can detect a spot shape to be focused, so that the second point is formed based on the spot shape formed on the scanned surface 8. Each aberration can be corrected by feedback control of the shape (including the tilt angle) of the reflecting surface of the reflecting mirror 5.

  Furthermore, in the said embodiment, although the deflector was comprised with the polygon mirror 6, it is not restricted to this. For example, a galvanometer mirror having a rotating deflection surface may be employed.

  Hereinafter, the scanning optical system embodying the present invention will be described more specifically with reference to construction data and various aberration diagrams. Numerical examples described here are numerical examples corresponding to the above-described embodiment. The lens configuration diagram (FIG. 1) representing the above embodiment shows the lens configuration of the present numerical example.

  The base aspherical shape is given by the following mathematical formulas (6) to (8).

here,
x: Coordinate in the sub-scanning direction with the vertex of the surface as the origin,
y: main scanning direction coordinates with the vertex of the surface as the origin,
z: Sag amount from the vertex (origin)
P (y): an expression representing a non-arc in the main scanning direction at the origin,
RDy: radius of curvature in the main scanning direction at the origin,
RDs: radius of curvature in the sub-scanning plane at the origin,
RDx: radius of curvature in the sub-scanning direction at the y position,
K: conical constant contributing to the main scanning direction,
AD, AE, AF, AG: even-order coefficients that contribute to the main scanning direction,
AOD, AOE, AOF, AOG: are odd-order constants that contribute to the main scanning direction,
BC, BD, BE, BF, BG: odd-order coefficients that determine the radius of curvature in the sub-scanning direction at the y position;
BOC, BOD, BOE, BOF, BOG: Even-order coefficients that determine the radius of curvature in the sub-scanning direction at the y position.

  Next, construction data of this numerical example is shown. S0 represents the deflection surface 6b of the polygon mirror 6, S1 represents the entrance surface of the scanning lens 7, and S2 represents the exit surface of the scanning lens 7. The design wavelength is 780 nm, the maximum deflection angle of the light beam B deflected in the main scanning direction by the polygon mirror 6 is 81.32 °, and the magnification m in the sub-scanning direction of the second imaging optical system is 4. 7.

  The data shown in Table 2 is the aspheric coefficient of each lens surface.

  FIG. 6 shows the fθ error on the surface to be scanned 8 when the second reflecting mirror 5 does not correct the fθ error in this numerical example. The vertical axis represents the image height on the surface to be scanned 8, and the horizontal axis represents the positional deviation (fθ error) in the main scanning direction between the actual focal position of the spot focused on the scanned surface 8 and the target focal position. Represents. The horizontal axis is positive when the actual in-focus position is shifted to the side where the image height is higher than the target in-focus position, and is negative when the image is shifted toward the side where the image height is lower.

  When the fθ error is not corrected by the second reflecting mirror 5, the fθ error becomes a curve as shown in FIG. This fθ error depends on the scanning lens 7.

  Therefore, in this numerical embodiment, the fθ error in FIG. 6 is corrected by tilting the reflecting surface of the second reflecting mirror 5 and adjusting the incident angle of the light beam B incident on the deflecting surface 6 b of the polygon mirror 4. ing. Specifically, as described above, when the in-focus position is adjusted to the negative side in the main scanning direction, the tilt angle α of the second reflecting mirror 5 is set so that the incident angle β of the light beam B to the deflecting surface 6b becomes large. Control (see FIG. 4). On the other hand, when the in-focus position of the image is adjusted to the positive side in the main scanning direction, the tilt angle α of the second reflecting mirror 5 is controlled so that the incident angle β of the light beam B on the deflecting surface 6b becomes small.

  Specifically, the tilt angle α of the reflecting surface of the second reflecting mirror 5 with respect to the rotation angle θp of the polygon mirror 4 in the main scanning plane is given by the following formula (9).

  However, the rotation angle θp is an angle based on when the light beam B is focused on the center of the image height on the scanned surface 8, and the light beam B is focused on the positive side in the main scanning direction from the image height center. The angle at which the luminous flux B is focused on the negative side in the main scanning direction with respect to the image height center is positive. The tilt angle α is an angle based on when the light beam B is focused on the center of the image height on the scanned surface 8, and the light beam B deflected by the polygon mirror 6 is adjusted to the positive side in the main scanning direction. The angle at the time (counterclockwise in FIG. 4) is positive, and the angle when adjusting to the negative side in the main scanning direction (clockwise in FIG. 4) is negative.

  Table 3 shows the ratio of the tilt angle α to the tilt angle α and the rotation angle θp given by Equation (9).

  As a result, the tilt angle α is greatest when the rotation angle θp = 20.331 and −20.331, and the ratio of the tilt angle α to the rotation angle θp at that time is 0.066.

  FIGS. 7 and 8 show aberration diagrams of the present numerical example when the fθ error is corrected based on the above formula (9) (or Table 3). FIG. 7 shows the fθ error on the scanned surface 8, and the vertical and horizontal axes are the same as those in FIG. FIG. 8 shows field curvature on the scanned surface 8. The vertical axis represents the image height on the scanned surface 8, and the horizontal axis represents the positional deviation in the optical axis L direction between the actual focused position of the spot focused on the scanned surface 8 and the target focused position. ing. The horizontal axis is positive when the actual in-focus position is shifted from the ideal image plane toward the front side in the traveling direction of the light beam B, and negative when the actual focusing position is shifted toward the far side in the traveling direction.

  6 and 7 show that the fθ error is effectively corrected by providing the second reflecting mirror 5. Further, since the fθ error is corrected by the second reflecting mirror 5 in this way, the scanning lens 7 may be designed so as to mainly correct the field curvature, and as a result, the field curvature is accurate as shown in FIG. You can see that it has been corrected.

  As described above, the present invention is useful for laser printers, laser facsimiles, digital copying machines, and the like because it can correct fθ error and field curvature with high accuracy and can be made compact.

1 is a cross-sectional view in the main scanning direction showing a configuration of an optical scanning device according to an embodiment of the present invention. It is sectional drawing of the subscanning direction which shows the 2nd imaging optical system of an optical scanning device. It is a block diagram which shows the control part of an optical scanning device. It is a schematic sectional drawing which shows the structure of the polygon mirror periphery of an optical scanning device. 1 is a schematic explanatory diagram illustrating a main configuration of an image forming apparatus using an optical scanning device. It is a figure which shows f (theta) error of the main scanning direction in the to-be-scanned surface when there is no 2nd reflective mirror in Numerical Example 1. FIG. It is a figure which shows f (theta) error in the to-be-scanned surface which concerns on the numerical example 1. It is a figure which shows the curvature of field of the main scanning direction in the to-be-scanned surface which concerns on the numerical example 1. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light source 10 Optical scanning device 20 Scanning optical system 2 Collimator lens (1st imaging optical system)
3 Cylindrical lens (first imaging optical system)
4 First reflector (first imaging optical system)
5 Second reflecting mirror (first imaging optical system, fθ error correction means)
50 control unit (fθ error correction means)
6 Polygon mirror (deflector)
7 Scanning lens (second imaging optical system)
8 Scanned surface 9 Image forming apparatus

Claims (9)

A scanning optical system comprising a deflector having a deflecting surface and rotating, and deflecting the light beam from the light source by the deflecting surface of the deflector and scanning the scanned surface,
A first imaging optical system that is disposed between the light source and the deflector and focuses a light beam from the light source on a deflection surface of the deflector;
A second imaging optical system that is disposed between the deflector and the surface to be scanned and focuses the light beam deflected by the deflector onto the surface to be scanned;
The first imaging optical system is a scanning optical system including fθ error correction means for correcting an fθ error. The scanning optical system according to claim 1,
The fθ error correction means is a scanning optical system that corrects an fθ error by adjusting an incident angle of a light beam incident on a deflecting surface of the deflector. The scanning optical system according to claim 2,
The fθ error correction means includes a reflecting mirror that reflects the light beam from the light source toward the deflector, and adjusts the reflection angle of the light beam from the reflecting mirror based on the rotation angle of the deflector. A scanning optical system that adjusts the incident angle of the light beam incident on the deflector. The scanning optical system according to claim 3,
The above-mentioned reflecting mirror is composed of a plurality of MEMS mirrors arranged in an array, and adjusts the reflection angle of the light beam by tilting each MEMS mirror. The scanning optical system according to claim 4, wherein
A scanning optical system satisfying the following formula (1);

However,
αmax: Maximum value of the tilt angle of the reflecting mirror when the light beam is focused on the center of the image height on the scanned surface θpmax: The deflector of the deflector when the tilt angle of the reflecting mirror is maximum A rotation angle based on when the light beam is focused on the center of the image height on the surface to be scanned, θpmax ≠ 0
It is. The scanning optical system according to any one of claims 1 to 5,
A scanning optical system that satisfies the following formula (2);
2.5 <m (2)
However,
m: a magnification in the sub-scanning direction of the second imaging optical system. The scanning optical system according to any one of claims 1 to 6,
The second imaging optical system is a scanning optical system configured by a single scanning lens.   An optical scanning device using the scanning optical system according to claim 1.   An image forming apparatus using the scanning optical system according to claim 1.

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