JP2022138795A - optical element - Google Patents

Abstract

To provide an optical element for an optical scanner that can prevent a deterioration in printing performance while maintaining a reduced size of the device.SOLUTION: An optical element 901 according to the present invention is used for an optical scanner 100 including a deflector 704 that deflects a light beam to scan a scanning target surface 706 in a main scanning direction, and an imaging optical system 709 that guides the light beam deflected by the deflector 704 to the scanning target surface 706. The optical element 901 has an internal reflection surface 904 that reflects the light beam deflected by the deflector 704, and at least one of optical surfaces of the optical element 901 is a diffraction surface.SELECTED DRAWING: Figure 1

Description

The present invention relates to an optical element, and is suitable for an optical scanning device mounted in an image forming apparatus such as a laser beam printer (LBP), a digital copier, or a multifunction printer.

Conventionally, it is known that an optical scanning device is provided with a folding mirror that reflects a light beam deflected by a deflector toward a surface to be scanned, in order to improve the degree of freedom in layout and reduce the size of the device.
In such a folding mirror, the angle of incidence of the light beam on the folding mirror in the main scanning cross section differs between the central portion and the end portion, and as a result, the reflectance with respect to the light beam differs. It is also known that density unevenness occurs in

Japanese Patent Application Laid-Open No. 2002-200002 attempts to suppress the occurrence of such density unevenness by providing an optical element having an internal reflecting surface that totally reflects a light beam deflected by a deflector toward a surface to be scanned instead of using a folding mirror. An optical scanning device is disclosed.

JP 2018-36438 A

On the other hand, in the optical scanning device, the optical path length of the imaging optical system is shortened by configuring the imaging optical system with only one imaging optical element and arranging the imaging optical element close to the deflector. By doing so, it is known that the size of the device can be reduced.
In such an optical scanning device, it is difficult to provide the imaging optical element with the fθ characteristic while maintaining the optical performance of the device. It is also known to

If the imaging optical element is constructed so as to have a non-uniform velocity property in the optical scanning device, a partial magnification shift occurs between the on-axis image height and the off-axis image height. This leads to deterioration of the image formed on the surface, that is, deterioration of the printing performance.
At this time, in an optical scanning device such as that disclosed in Patent Document 1, an optical element having an internal reflection surface is also provided in the optical path of the imaging optical system. It is conceivable to suppress deterioration of performance.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an optical element for an optical scanning device capable of suppressing deterioration in printing performance while maintaining the miniaturization of the device.

An optical element according to the present invention includes a deflector that deflects a light beam to scan a surface to be scanned in the main scanning direction, and an imaging optical system that guides the light beam deflected by the deflector to the surface to be scanned. An optical element used in a scanning device, wherein the optical element has an internal reflecting surface for reflecting a light beam deflected by a deflector, and at least one of the optical surfaces of the optical element is a diffractive surface. Characterized by

According to the present invention, it is possible to provide an optical element for an optical scanning device capable of suppressing deterioration of printing performance while maintaining the miniaturization of the device.

1A and 1B are a schematic main-scanning sectional view and a partially enlarged schematic sub-scanning sectional view of an optical scanning device according to an embodiment; FIG. 2 is a schematic sub-scanning cross-sectional view of the internal reflection element according to the present embodiment; FIG. 5 is a diagram showing partial magnification deviation at each image height in the optical scanning device according to the embodiment; 4A and 4B are diagrams showing the LSF spot diameter and the LSF depth center position at each image height in the optical scanning device according to the embodiment; FIG. 4 is a diagram showing main scanning direction position dependency of sagittal beam tilt amount on the internal reflection surface of the internal reflection element according to the present embodiment. FIG. 5 is a diagram showing sub-scanning direction irradiation positions at respective image heights, astigmatism in the 45-degree direction, and spot intensity distribution in the optical scanning device according to the embodiment; FIG. 2 is a sub-scanning cross-sectional view of a main part of the monochrome image forming apparatus according to the embodiment;

An optical scanning device including an internal reflection element, which is an optical element according to this embodiment, will be described in detail below with reference to the accompanying drawings. Note that the drawings shown below may be drawn on a scale different from the actual scale in order to facilitate understanding of the present embodiment.
In the following description, the main scanning direction is the direction perpendicular to the rotation axis of the deflection means and the optical axis of the optical system (the direction in which the light beam is deflected and scanned by the deflection means). The sub-scanning direction is a direction parallel to the axis of rotation of the deflection means. A main scanning section is a section perpendicular to the sub-scanning direction. A sub-scanning section is a section perpendicular to the main scanning direction.
Therefore, in the following description, it should be noted that the main scanning direction and the sub-scanning cross section are different between the incident optical system and the imaging optical system.

1A and 1B respectively show a schematic main scanning sectional view and a partially enlarged schematic sub-scanning sectional view of an optical scanning device 100 including an internal reflection element 901 (optical element) according to this embodiment. ing.

The optical scanning device 100 includes a light source 701, an anamorphic lens 702, a sub-scanning diaphragm 703s, a main scanning diaphragm 703m, a deflection means 704 (deflector), a first imaging optical element 705 (imaging optical element), and an internal reflection element 901. I have.
The light source 701 can use, for example, a semiconductor laser having at least one light emitting point, and emits a light beam toward the sub-scanning stop 703s.
The sub-scanning diaphragm 703s has a rectangular opening and shapes the diameter of the light beam emitted from the light source 701 in the sub-scanning direction. In the main scanning direction, a rough aperture wider than the beam diameter actually used is used.

The anamorphic lens 702 has a positive refractive power within the main scanning section, and converts the light flux that has passed through the sub-scanning diaphragm 703s into a parallel light flux within the main scanning section. Here, the parallel light flux includes not only strictly parallel light flux but also approximately parallel light flux such as weakly diverging light flux and weakly converging light flux.
In the optical scanning device 100, the anamorphic lens 702 converts the light flux that has passed through the sub-scanning diaphragm 703s into a weakly convergent light flux within the main scanning section, thereby reducing the refractive power required for the imaging optical system 709. FIG.
The anamorphic lens 702 has a positive refractive power within the sub-scanning section, and forms a long line image in the main scanning direction by condensing the incident light flux near the deflecting surface 704a of the deflecting means 704. is doing.

The main scanning diaphragm 703m has a rectangular opening, and shapes the diameter of the light beam that has passed through the anamorphic lens 702 in the main scanning direction. In the sub-scanning direction, a rough aperture wider than the beam diameter actually used is used.
In the optical scanning device 100, the incident optical system 708 is composed of the above-described sub-scanning diaphragm 703s, anamorphic lens 702, and main scanning diaphragm 703m.

A light beam emitted from the light source 701 and passed through the incident optical system 708 is deflected by the deflecting surface 704 a of the deflecting means 704 and then enters the first imaging optical element 705 .
The first imaging optical element 705 has two optical surfaces (lens surfaces), an entrance surface and an exit surface. It is configured to scan with desired scanning characteristics in a direction.
In the sub-scanning section, the first imaging optical element 705 conjugates the vicinity of the deflecting surface 704a of the deflecting means 704 and the vicinity of the scanned surface 706 to each other to compensate for surface tilt. This reduces the scanning position deviation in the sub-scanning direction on the surface to be scanned 706 when the scanning surface 706 falls.

After passing through the first imaging optical element 705 , the light flux is guided onto the surface to be scanned (photosensitive surface) 706 after being reflected by an internal reflection element 901 which will be described later in detail.
At this time, the first imaging optical element 705 and the internal reflection element 901 form spot-like images in the vicinity of the scanned surface 706 in both the main scanning section and the sub-scanning section.
In the optical scanning device 100, an imaging optical system 709 is configured by the first imaging optical element 705 and the internal reflection element 901 described above.

In the optical scanning device 100, the scanning surface 706 is scanned in the direction of arrow C in the main scanning direction by rotating the deflection means 704 at a constant speed in the direction of arrow B in the figure by a drive unit (not shown). , an electrostatic latent image is formed on the surface 706 to be scanned.
The light flux deflected at a predetermined deflection angle by the deflection means 704 is guided to a synchronization detection sensor (not shown) by a synchronization detection optical system (not shown), thereby obtaining a synchronization detection signal.
Based on the acquired synchronization detection signal, the rotation of the deflection means 704 is controlled at a constant speed, and the light emission timing of the light source 701 is controlled.

In the optical scanning device 100, the anamorphic lens 702 is used in the incident optical system 708. Alternatively, the optical function may be divided into a plurality of optical elements such as a coupling lens and a cylindrical lens.
Further, in the optical scanning device 100, a rotating polygon mirror (polygon mirror) having four deflection surfaces 704a is used as the deflection means 704; may

In addition, in the optical scanning device 100, the anamorphic lens 702 and the first imaging optical element 705 use plastic mold lenses formed by injection molding, but the invention is not limited to this, and glass mold lenses may be used.
At this time, the molded lens can be easily molded into an aspherical shape and is suitable for mass production. can be improved.

FIG. 2 shows a schematic sub-scanning sectional view of the internal reflection element 901 according to this embodiment.

As shown in FIG. 2, the internal reflection element 901 has an entrance surface 903 (first transmission surface), an internal reflection surface 904 and an exit surface 905 (second transmission surface).
In the internal reflection element 901, the total reflection condition is satisfied by setting the angle of incidence of the scanning light beam on the internal reflection surface 904 to be large. The scanning light beam can be totally reflected without depositing a reflective film on the surface 904 .
Thereby, cost reduction of the internal reflection element 901 can be achieved.

Tables 1, 2 and 3 below show the specification values of the optical scanning device 100 .
In Table 2, the surface distances between the entrance surface 903 and the internal reflection surface 904 of the internal reflection element 901 and between the internal reflection surface 904 and the exit surface 905 are calculated from respective coordinates.
Also, in Table 3, each aspheric coefficient is a value when the traveling direction of light rays is positive.

As shown in Table 3, the incident surface of the anamorphic lens 702 provided in the optical scanning device 100 is a diffraction surface on which a diffraction grating is formed.
As a result, the anamorphic lens 702, which is formed by injection molding using a plastic material, compensates for changes in refractive power due to environmental changes by changes in diffraction power accompanying changes in the wavelength of the light flux emitted from the semiconductor laser. It can be an adaptive optics system.
Further, the internal reflection surface 904 of the internal reflection element 901 is also a diffraction surface on which a diffraction grating is formed.

ここで、φは位相関数、Ｍは回折次数、λは設計波長である。なお、光走査装置１００では、１次回折光（すなわち、回折次数Ｍは１）を用いており、設計波長λは７９０ｎｍとなっている。 The diffraction surface formed on the entrance surface of the anamorphic lens 702 and the internal reflection surface 904 of the internal reflection element 901 provided in the optical scanning device 100 has a phase function expressed by the following equation (1): defined by

where φ is the phase function, M is the diffraction order, and λ is the design wavelength. The optical scanning device 100 uses first-order diffracted light (that is, the diffraction order M is 1), and the design wavelength λ is 790 nm.

ここで、Ｒは母線曲率半径、Ｋ、Ｂ_４、Ｂ_６、Ｂ_８、Ｂ_１０及びＢ_１２は非球面係数である。
なお、母線形状が主走査方向において非対称である場合には、非球面係数Ｂ_４、Ｂ_６、Ｂ_８、Ｂ_１０及びＢ_１２はそれぞれ、光源７０１が配置されていない側（Ｙ≧０）ではＢ_４Ｕ、Ｂ_６Ｕ、Ｂ_８Ｕ、Ｂ_１０Ｕ及びＢ_１２Ｕ、光源７０１が配置されている側（Ｙ≦０）ではＢ_４Ｌ、Ｂ_６Ｌ、Ｂ_８Ｌ、Ｂ_１０Ｌ及びＢ_１２Ｌとし、数値を互いに異ならせればよい。 In addition, the shape (generatrix shape) of the entrance surface and the exit surface of the first imaging optical element 705 provided in the optical scanning device 100 in the main scanning cross section can be expressed as a function of Y up to the 12th order. It is composed of a spherical shape.
Specifically, on each of the entrance surface and the exit surface of the first imaging optical element 705, the intersection with the optical axis of the first imaging optical element 705 is the origin, the optical axis is the X axis, and the light beam Assuming that the axis perpendicular to the axis is the Y-axis, the generatrix shape is defined by the following formula (2).

where R is the radius of curvature of the generatrices and K, B ₄ , B ₆ , B ₈ , B ₁₀ and B ₁₂ are the aspheric coefficients.
Note that when the generatrix shape is asymmetric in the main scanning direction, the aspheric coefficients B ₄ , B ₆ , B ₈ , B ₁₀ and B ₁₂ are respectively B _4U , B _6U , B _8U , B _10U and B _12U , B _4L , B _6L , B _8L , B _10L and B _12L on the side where the light source 701 is arranged (Y≦0), and the numerical values are different from each other. Just do it.

ここで、Ｓは母線方向の各位置における母線の法線を含み主走査断面に垂直な面内において定義される子線形状である。 Further, the shape (sagittal line shape) of the incident surface and the exit surface of the first imaging optical element 705 provided in the optical scanning device 100 in the sub-scanning cross section is defined by the following formula (3). .

Here, S is a sagittal line shape defined in a plane that includes the normal to the generatrix at each position in the generatrix direction and is perpendicular to the main scanning section.

Further, the coefficient _mjk is the coefficient of each term of the polynomial included in the formula (3) expressing that the optical surface is curved with respect to the sagittal line shape. For example, _mj1 expresses the sagittal line tilt amount. .
That is, in the optical scanning device 100, as shown in Table 3, the internal reflecting surface 904 has a sagittal tilt angle (the normal of the sagittal on the optical axis to the main scanning cross section) along the main scanning direction. It is a shape in which the angle formed, the sagittal line tilt amount) changes, that is, the sagittal line tilt change surface.

ここで、ｒは光軸上における子線曲率半径、Ｅ_１、Ｅ_２、Ｅ_４、Ｅ_６、Ｅ_８、Ｅ_１０及びＥ_１２は子線変化係数である。 In addition, the radius of curvature in the sub-scanning cross section at a position spaced apart by Y in the main scanning direction from the optical axis of the first imaging optical element 705 on each of the entrance surface and the exit surface of the first imaging optical element 705, that is, the sagittal line The radius of curvature r' is defined as in Equation (4) below.

Here, r is the sagittal curvature radius on the optical axis, and E ₁ , E ₂ , E ₄ , E ₆ , E ₈ , E ₁₀ and E ₁₂ are sagittal variation coefficients.

As with the meridional shape described above, when the sagittal shape is asymmetrical in the main scanning direction, the sagittal line variation coefficients E1 to E12 are _{respectively on} the side where the light source 701 is not arranged (Y≧0). Then E _1U to E _12U , and E _1L to E _12L on the side where the light source 701 is arranged (Y≦0), so that the values are different from each other.
Further, in the optical scanning device 100, the shape of the optical surface is defined by the formula shown above, but the scope of the rights of the present invention is not limited to this.

As described above, equation (2) indicates the shape (generatrix shape) of the optical surface in the main scanning cross section (XY cross section), and equation (3) is the sub scanning cross section (ZX cross section) of the optical surface (sagittal line shape).
Then, at this time, the radius of curvature r' of the sagittal line shape of the optical surface changes according to the value of Y, as shown in Equation (4).

In the optical scanning device 100, each coefficient in the equations (2) and (4) is set to upper when Y≧0, and set to lower when Y≦0.
In Table 3, the upper, that is, the aspherical coefficients in the case of Y≧0, is suffixed with u, and the lower, that is, the aspherical coefficients in the case of Y≦0, is suffixed with l. .

In the optical scanning device 100, as shown in Table 1, the optical path length O51 of the imaging optical system 709 is extremely short.
In addition, in order to suppress interference with other components (not shown) included in the optical scanning device 100, the arrangement of the first imaging optical element 705 is also restricted. difficult to meet.

That is, in an imaging optical system with a short optical path length, such as that provided in the optical scanning device 100, the partial magnification in the main scanning direction at the off-axis image height becomes larger than the on-axis image height, resulting in non-uniform scanning. will be performed.
Along with this, the diameter of the spot in the main scanning direction on the surface to be scanned also increases proportionally.

When an optical scanning device having such an imaging optical system is mounted in an image forming apparatus, the width of the grid becomes uneven when forming a grid pattern image. In addition, when a half-pattern image is formed, the density distribution becomes non-uniform, and it becomes difficult to reproduce a 1-dot image at the maximum off-axis image height. do.
Further, it is possible to electrically correct the deviation of the partial magnification in the main scanning direction between the on-axis image height and the off-axis image height. It also has the disadvantage of increasing.

In addition, in the optical scanning device 100, an internal reflection element 901 is provided in the imaging optical system 709 in order to reduce the cost and size of the device.
If the internal reflection element 901 is given a curvature in the main scanning direction, there is a possibility that the above non-uniform velocity can be improved.

However, if the internal reflection element 901 is given a curvature in the main scanning direction, the ridgeline of the internal reflection element 901 is curved, and the mold part for molding the internal reflection element 901 is curved.
Therefore, it becomes difficult to maintain molding accuracy in the mold for molding the internal reflection element 901, and as a result, the relative eccentricity error between the optical surfaces of the molded internal reflection element 901 becomes large.

Further, in order to sufficiently correct the distortion in the imaging optical system 709 by an optical element spaced from the deflecting means 704, the optical element should have a strong refractive power. It is conceivable to bend it greatly.
However, the internal reflection element 901 provided apart from the deflecting means 704 in the imaging optical system 709 of the optical scanning device 100 has a prism shape in the sub-scanning section in order to internally bend the optical path.
Therefore, even if the optical performance is satisfied in the optical simulation by designing the internal reflection element 901 with a shape in which the generatrix is greatly curved, the three-dimensional shape of the internal reflection element 901 including such a shape is not actually realized. It is difficult to mold.

Therefore, in the optical scanning device 100, the diffraction power is applied to the internal reflection element 901 to correct the distortion while maintaining the generatrix shape substantially straight.
By designing the generatrix shape of the internal reflection element 901 to be substantially straight, the mold part becomes a straight line, and the internal reflection element 901 with high relative eccentricity accuracy between the optical surfaces can be formed. It can improve performance.

Specifically, as shown in Table 3, by forming the inner reflecting surface 904 of the inner reflecting element 901 as a diffractive surface, diffraction power is imparted in the main scanning direction (within the main scanning cross section). .
Further, in the optical scanning device 100 according to the present embodiment, the internal reflection element 901 is formed by injection molding so that the three optical surfaces 903, 904, and 905 in the sub-scanning cross section are non-parallel prism shapes. ing.

When the internal reflection element 901 is injection-molded into such a prism shape, one of the optical surfaces 903, 904 and 905 is non-parallel to the mold opening direction.
At this time, if a diffractive surface having diffraction power in the sub-scanning direction (within the sub-scanning cross section) is provided in the internal reflection element 901, an undercut area is generated in the structural portion for forming the diffractive surface depending on the structure of the mold. . Therefore, it becomes difficult to transfer the diffraction structure to the internal reflection element 901 satisfactorily.

In addition, an increase in mold release resistance when molding the internal reflection element 901 also causes a problem of deterioration of molding stability.
Therefore, in the optical scanning device 100, as shown in Table 3, the internal reflection element 901 is provided with a diffraction surface having diffraction power only in the main scanning direction, thereby solving the above problem.

FIG. 3 shows partial magnification deviation at each image height in the optical scanning device 100 having the internal reflection element 901 according to this embodiment.
The term "partial magnification deviation" as used herein refers to, for example, the partial magnification in the scanning characteristics when the scanning characteristics of the imaging optical system 709 having non-uniform velocity as described above are expressed as Y=f(θ). It means the amount of deviation with respect to the partial magnification in the scanning characteristic Y=KK.[theta] when it has speed.

As shown in FIG. 3, the partial magnification deviation can be satisfactorily corrected to ±5% or less over the entire image height. It can be seen that characteristics of substantially constant speed scanning are obtained.
Accordingly, when the optical scanning device 100 is installed in an image forming apparatus, it is possible to obtain good image quality over the entire image.

FIG. 4A shows LSF (Line Spread Function) spot diameters in the main scanning direction and the sub-scanning direction at each image height in the optical scanning device 100 having the internal reflection element 901 according to this embodiment.
Here, the LSF spot diameter in the main scanning direction is the width obtained by slicing the light amount profile obtained by integrating the spot profile in the sub-scanning direction at each image height at a position 13.5% of the maximum value. point to

In addition, the LSF spot diameter in the sub-scanning direction is the width of the light amount profile obtained by integrating the spot profile in the main scanning direction at each image height and slicing it at a position 13.5% of the maximum value. Point.
As shown in FIG. 4A, in the optical scanning device 100, the LSF spot diameter is substantially constant over the entire image height of the surface to be scanned 706, and when mounted on the image forming apparatus, the image is satisfactorily image quality can be obtained.

4B and 4C respectively show the LSF depth center positions in the main scanning direction and the sub-scanning direction at each image height in the optical scanning device 100. FIG.
Here, the LSF depth center position means that the LSF spot diameter becomes a predetermined size (slice level) or less when defocusing is performed in the optical axis direction of the imaging optical system 709 in the vicinity of the scanned surface 706. It refers to the central position of the area between the front focus permissible position and the rear focus permissible position.

In the optical scanning device 100, the slice level is set to 118 μm or less in the main scanning direction and 114 μm or less in the sub-scanning direction.

As shown in FIGS. 4B and 4C, in the optical scanning device 100, the LSF depth center position is within ±2 mm over the entire image height in each of the main scanning direction and the sub-scanning direction. It can be seen that excellent image plane performance can be achieved.
That is, as shown in FIGS. 4B and 4C, in the optical scanning device 100, the LSF depth center position is set to be near zero defocus over the entire image height in each of the main scanning direction and the sub-scanning direction. is well corrected for

Further, in the optical scanning device 100, the angle of each optical surface with respect to the scanning light flux incident on the internal reflection element 901 is set so that the internal reflection surface 904 satisfies the total reflection condition.
As described above, in the internal reflection element 901 , the incident scanning light flux is totally reflected on the internal reflection surface 904 , thereby eliminating the need to form a reflection film on the internal reflection surface 904 by vapor deposition or the like. Manufacturing costs can be reduced.

Further, in the optical scanning device 100, as shown in Table 3, the internal reflection surface 904 of the internal reflection element 901 is formed into a diffraction grating shape expressed by the phase coefficients of the 2nd, 4th, and 6th orders of Y. ing.
In the optical scanning device 100, as shown in Table 3, on the inner reflecting surface 904, the diffraction power in the main scanning direction is positive at the center of the optical axis, and is positive toward the end along the main scanning direction. Each phase coefficient is set so that the diffraction power of is strengthened.

In the optical scanning device 100, by designing the inner reflecting surface 904 of the inner reflecting element 901 as a diffraction grating as described above, diffraction that bends the incident scanning light beam toward the optical axis toward the maximum off-axis image height is performed. increasing power.
This makes it possible to correct the distortion that cannot be corrected in the imaging optical system 709, that is, the tendency of the partial magnification to increase toward the most off-axis image height, and also improve the curvature of field.

Further, in the optical scanning device 100, in order to reduce the cost of forming a reflective film on the resin when forming the internal reflection element 901, the incident angle of the scanning light beam to the internal reflection surface 904 within the sub-scanning cross section is set to an obtuse angle. It is designed so that the conditions for total reflection are satisfied by setting it.
On the other hand, when the scanning light beam is incident at an angle within the sub-scanning cross section on the reflecting surface or transmitting surface to which the refractive power or diffraction power is imparted in the main scanning direction, the illumination at the off-axis image height on the scanned surface 706 The position deviates greatly in the sub-scanning direction, or the astigmatism in the 45-degree direction becomes worse.

As a result, a large scanning line curvature occurs and the spot diameter increases at the off-axis image height, making it difficult to obtain good image quality.
Therefore, in the optical scanning device 100, the internal reflection surface 904 of the internal reflection element 901 is set as a sagittal line tilt changing surface in which the sagittal line tilt amount changes along the main scanning direction.

FIG. 5 shows the main scanning direction position dependency of the sagittal beam tilt amount on the internal reflection surface 904 of the internal reflection element 901 according to this embodiment.

As shown in FIG. 5, in the optical scanning device 100, the sagittal line tilt amount on the inner reflecting surface 904 of the inner reflecting element 901 is set to increase negatively from the center to the end in the main scanning direction. is doing.
That is, in the optical scanning device 100, the sagittal beam tilt amount on the inner reflecting surface 904 is defined as the angle of reflection in the sub-scanning cross section for the light beam incident on the inner reflecting surface 904 from the deflection means 704 going from the center to the end in the main scanning direction. It is changed so that it becomes larger with time.
This makes it possible to satisfactorily correct scanning line curvature and astigmatism in the 45-degree direction, as described below.

FIG. 6A shows sub-scanning direction irradiation positions at respective image heights on the surface to be scanned 706 in the optical scanning device 100 having the internal reflection element 901 according to this embodiment.
FIG. 6B shows 45-degree direction astigmatism at each image height on the scanned surface 706 in the optical scanning device 100 including the internal reflection element 901 according to this embodiment.

FIG. 6C shows the spot intensity distribution at each image height on the scanned surface 706 in the optical scanning device 100 having the internal reflection element 901 according to this embodiment.
Specifically, in FIG. 6C, when the light intensity at the position where the light intensity is maximum is 1, the light intensities are 0.5, 0.368, 0.135 and 0.05, respectively. The position is drawn with contour lines.

As shown in FIG. 6A, in the optical scanning device 100, the misalignment of the irradiation position in the sub-scanning direction, which is worsened by setting the internal reflecting surface 904 of the internal reflecting element 901 as a diffractive surface, is corrected by the sagittal line tilt change. It can be seen that the curvature of the scanning line is sufficiently reduced by correcting it by setting it on the plane.
Further, as shown in FIG. 6B, in the optical scanning device 100, the twist of the wavefront aberration caused by setting the internal reflection surface 904 of the internal reflection element 901 as a diffractive surface is applied to the sagittal tilt change surface. It can be seen that the astigmatism in the direction of 45 degrees is sufficiently reduced by correcting so as to cancel out by setting.
By correcting the astigmatism in the direction of 45 degrees, a good spot shape is obtained over the entire image height, as shown in FIG. 6(c).

As described above, in the optical scanning device 100, by providing the sagittal line tilt changing surface in the internal reflection element 901, the unique aberration caused by providing the internal reflection element 901 with the diffraction shape in the main scanning direction can be corrected satisfactorily. can do.
Accordingly, a good image can be obtained when the optical scanning device 100 is installed in an image forming apparatus.

It should be noted that the astigmatism in the direction of 45 degrees and the scanning line curvature caused by setting the internal reflecting surface 904 as a diffractive surface can be corrected by an optical surface other than the internal reflecting surface 904, that is, the entrance surface 903 or the exit surface 905. In this case, it is difficult to correct both aberrations with only one surface.
Therefore, in the optical scanning device 100, the inner reflection surface 904 itself, which generates the above aberration, is set as a sagittal tilt changing surface. Aberrations are well corrected.

The optical scanning device 100 is designed to satisfy the following conditional expression (5) where Wm and Ws are the widths in the main scanning direction and the sub-scanning direction of the effective area of the internal reflection surface 904 of the internal reflection element 901. It is
10<Wm/Ws<50 (5)
The effective area of the internal reflection surface 904 here corresponds to the area between the positions on the internal reflection surface 904 where the light beams reaching the most off-axis image heights on both sides of the scanned surface 706 are reflected. do.

If the internal reflection element 901 is too long to exceed the upper limit of conditional expression (5), the rigidity of the internal reflection element 901 becomes insufficient and the internal reflection element 901 easily vibrates or deforms under its own weight. It becomes difficult to obtain good image quality.
On the other hand, if the width of the internal reflection surface 904 in the main scanning direction becomes too narrow as the lower limit value of conditional expression (5) is exceeded, the light beams directed to the respective image heights on the internal reflection surface 904 are not sufficiently separated from each other. It becomes difficult to correct the characteristics.

Alternatively, if the internal reflection element 901 has a prism shape with an extremely large wall thickness as the lower limit of conditional expression (5) is exceeded, heat distribution during injection molding increases, resulting in increased internal strain.
As a result, the refractive index distribution and birefringence of the internal reflection element 901 become too large, making it difficult to obtain good imaging performance.

In the optical scanning device 100, since Wm=123 mm and Ws=4 mm, Wm/Ws=30.75, which satisfies conditional expression (5).

Further, in the optical scanning device 100, in order to reduce the sub-scanning magnification and reduce the sensitivity to deterioration of imaging performance due to tolerance, sub-scanning is performed on the entrance surface 903 and the exit surface 905 of the internal reflection element 901. It gives refractive power within the cross section.
That is, in the optical scanning device 100, the entrance surface 903 and the exit surface 905 of the internal reflection element 901 are formed to be concave or convex in the sub-scanning direction.

When the internal reflection element 901 is injection-molded, the sub-scanning direction on the internal reflection surface 904 is non-parallel to the die-cutting direction.
Therefore, if a diffractive structure is given to the inner reflecting surface 904 in the sub-scanning direction, an undercut area is generated, which lowers molding stability.

Therefore, in the optical scanning device 100, the internal reflecting surface 904 of the internal reflecting element 901 is designed to have a diffraction grating shape in the main scanning direction, thereby imparting diffraction power within the main scanning cross section.
On the other hand, the entrance surface 903 and the exit surface 905 are designed to be concave and convex in the sub-scanning direction, thereby imparting refractive power within the sub-scanning section. have power.

In the optical scanning device 100, the imaging optical system 709 is composed of only one first imaging optical element 705 and the internal reflection element 901 in order to reduce the cost, but the configuration is not limited to this.
That is, in the optical scanning device 100, even if a second imaging optical element and a folding mirror are further provided in the optical path of the imaging optical system 709, the effects of this embodiment can be sufficiently obtained.

Further, in the optical scanning device 100, the internal reflection element 901 is formed by injection molding a resin material, which makes it possible to reduce costs compared to the case of using a glass material, and to easily provide a sub-scanning cylinder surface. be able to.
However, the internal reflection element 901 may be formed using cut glass, glass mold, or the like.

In the optical scanning device 100, the internal reflecting surface 904 is set as a diffractive surface, while the entrance surface 903 and the exit surface 905 are set as surfaces to which refractive power is imparted within the sub-scanning section.
However, not limited to this, for example, the incident surface 903 or the exit surface 905 may be set as a diffractive surface.
In other words, in the optical scanning device 100, at least one of the optical surfaces of the internal reflection element 901 should be a diffractive surface.

Further, in the optical scanning device 100, the internal reflecting surface 904 is set as a sagittal tilt changing surface, but this is not restrictive. You can get morphological effects.

Further, in the optical scanning device 100, in order to suppress the occurrence of an undercut area when the internal reflection element 901 is injection molded, the internal reflection surface 904 is set as a diffractive surface, while the entrance surface 903 and the exit surface 905 are set as secondary surfaces. It is set to a surface to which refractive power is given in the scanning section.
However, not limited to this, for example, the incident surface 903 or the exit surface 905 may be set as a diffractive surface.

As described above, according to the optical scanning device 100, it is possible to achieve cost reduction and miniaturization while ensuring good imaging performance and printing performance.

[Image forming apparatus]
FIG. 7 shows a sub-scanning cross-sectional view of a main part of a monochrome image forming apparatus 104 equipped with an optical scanning unit 100 having an internal reflection element 901 according to this embodiment.

Code data Dc output from an external device 117 such as a personal computer is input to the monochrome image forming apparatus 104 . This code data Dc is converted into image data (dot data) Di by the printer controller 111 in the image forming apparatus 104 . This image data Di is input to the optical scanning unit 100 . The optical scanning unit 100 emits a light beam 103 modulated according to the image data Di, and the light beam 103 scans the photosensitive surface of the photosensitive drum 101 in the main scanning direction.

A photosensitive drum 101 as an electrostatic latent image bearing member (photosensitive member) is rotated clockwise by a motor 115 . Along with this rotation, the photosensitive surface of the photosensitive drum 101 moves relative to the light beam 103 in the sub-scanning direction perpendicular to the main scanning direction. A charging roller 102 for uniformly charging the surface of the photosensitive drum 101 is provided above the photosensitive drum 101 so as to contact the surface. The surface of the photosensitive drum 101 charged by the charging roller 102 is irradiated with the light beam 103 scanned by the optical scanning unit 100 .

As described above, the light beam 103 is modulated based on the image data Di, and the irradiation of the light beam 103 forms an electrostatic latent image on the surface of the photosensitive drum 101 . This electrostatic latent image is developed into a toner image by a developing device 107 arranged so as to be in contact with the photosensitive drum 101 further downstream in the rotational direction of the photosensitive drum 101 than the irradiation position of the light beam 103 .

The toner image developed by the developing device 107 is transferred onto a sheet of paper 112 as a transfer material by a transfer roller (transfer device) 108 arranged below the photosensitive drum 101 so as to face the photosensitive drum 101 . The paper 112 is stored in the paper cassette 109 in front of the photosensitive drum 101 (on the right side in FIG. 7), but it can also be manually fed. A paper feed roller 110 is arranged at the end of the paper cassette 109, and feeds the paper 112 in the paper cassette 109 to the transport path.

As described above, the paper 112 onto which the unfixed toner image has been transferred is further conveyed to the fixing device 150 behind the photosensitive drum 101 (left side in FIG. 7). The fixing device 150 is composed of a fixing roller 113 having a fixing heater (not shown) therein and a pressure roller 114 arranged to press against the fixing roller 113 . The unfixed toner image on the paper 112 is fixed by heating the paper 112 conveyed from the transfer section while pressurizing it at the pressure contact portion between the fixing roller 113 and the pressure roller 114 . Further, a paper discharge roller 116 is arranged behind the fixing device 150 to discharge the fixed paper 112 to the outside of the monochrome image forming apparatus 104 .

The printer controller 111 not only converts data, but also controls the motor 115 and other components in the monochrome image forming apparatus 104 and the polygon motor in the optical scanning unit 100 .
Further, the optical scanning unit 100 including the internal reflection element 901 according to this embodiment can be used not only in the monochrome image forming apparatus 104 but also in a color image forming apparatus.

100 optical scanning device 704 deflection means (deflector)
706 surface to be scanned 709 imaging optical system 901 internal reflection element (optical element)
904 inner reflective surface

Claims (16)

Optical used in an optical scanning device including a deflector that deflects a light beam to scan a surface to be scanned in the main scanning direction, and an imaging optical system that guides the light beam deflected by the deflector to the surface to be scanned an element,
the optical element has an internal reflecting surface that reflects the light beam deflected by the deflector;
An optical element, wherein at least one of the optical surfaces of the optical element is a diffractive surface. 2. The optical element according to claim 1, wherein at least one of the optical surfaces of said optical element is a sagittal tilt changing surface. 3. The optical element according to claim 2, wherein the internal reflection surface is the sagittal tilt changing surface. The sagittal line tilt angle on the sagittal line tilt changing surface is such that the angle of reflection in the sub-scanning cross section of the light beam incident on the sagittal line tilt changing surface from the deflector increases from the center to the end in the main scanning direction. 4. An optical element according to claim 2 or 3, characterized in that it varies. 5. The optical element according to any one of claims 1 to 4, wherein the internal reflection surface is the diffraction surface. 6. The optical element according to any one of claims 1 to 5, wherein the diffraction surface has diffraction power in a main scanning cross section. 7. The optical element according to claim 6, wherein the sign of said diffraction power is positive. 8. The optical element according to any one of claims 1 to 7, wherein the internal reflection surface is a total reflection surface. When the widths of the effective area of the internal reflection surface in the main scanning direction and the sub-scanning direction are respectively Wm and Ws,
10<Wm/Ws<50
9. The optical element according to any one of claims 1 to 8, wherein the following condition is satisfied. 10. The optical element according to any one of claims 1 to 9, wherein the optical element has refractive power in a sub-scanning cross section. 11. The optical element according to any one of claims 1 to 10, wherein the diffraction surface has no diffraction power in the sub-scanning cross section. An optical scanning device, comprising: the imaging optical system including the optical element according to claim 1 ; and the deflector. 13. The optical scanning device according to claim 12, wherein the imaging optical system is configured such that partial magnifications in the main scanning direction are different between the on-axis image height and the maximum off-axis image height. 14. The optical scanning device according to claim 12, wherein said imaging optical system comprises said optical element and an imaging optical element. 15. The optical scanning device according to claim 12, a developing device for developing an electrostatic latent image formed on the surface to be scanned by the optical scanning device as a toner image, and the developed toner. An image forming apparatus comprising: a transfer device for transferring an image onto a transfer material; and a fixing device for fixing the transferred toner image to the transfer material. 15. An image comprising: the optical scanning device according to claim 12; and a printer controller that converts a signal output from an external device into image data and inputs the image data to the optical scanning device. forming device.

Images