JPH11202242A - Optical scanning device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attain improvement of uniformity of the beam diameter and uniformity of the quantity of light on a surface to be scanned of a light beam for scanning the surface of a photosensible body. SOLUTION: This device is constituted by providing a deflector 16 arranged between a light source 10 and a photosensible body 20 so as to receive a beam from a light source 10 over a range wider than a deflecting area in scanning direction, to deflect and scan it onto the photosensible body while having plural deflecting planes, and the incident angle of main scanning direction on the said deflecting plane is set excepting 0 deg.. In this case, both a phase shift function and a diffracting function are provided between the light source 10 and the deflector 16, and an optical element 30 is provided while having characteristics making both a phase shift amount and diffraction efficiency asymmetric to the optical axis.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical scanning apparatus, and more particularly, to an optical scanning apparatus used in an image recording apparatus such as a laser printer or a digital copying machine. The present invention relates to a so-called overfilled type optical scanning device in which the width of a light beam incident on the rotary polygon mirror is larger than the width.

[0002]

2. Description of the Related Art As an optical scanning device that deflects a light beam in a main scanning direction by a rotary polygon mirror and scans a surface to be scanned, the width of the deflection surface of the rotary polygon mirror, that is, the width of the reflection surface along the rotation direction (hereinafter, referred to as the width) , Surface width) is larger than the width of the light beam incident on the rotating polygonal mirror along the main scanning direction.
So-called underfilled
Type is common.

[0003] In image recording apparatuses such as laser printers and digital copiers, there is a constant demand for faster image recording and higher resolution. In the underfilled optical scanning device as described above, a method of increasing the rotation speed of the rotating polygon mirror in order to respond to a demand for high speed,
There is a method of increasing the number of deflection surfaces of the rotary polygon mirror.

[0004] However, it is not easy to increase the rotation speed of the motor that drives the rotary polygon mirror. Increasing the number of deflecting surfaces of the rotating polygonal mirror while maintaining the surface width of the rotating polygonal mirror increases the diameter of the rotating polygonal mirror, thereby increasing the size thereof, and rotating and driving the enlarged rotating polygonal mirror. Since the load on the motor increases, it is difficult to rotate the rotary polygon mirror.

Therefore, in order to prevent the rotary polygon mirror from being enlarged and to increase the number of deflecting surfaces, the surface width of each deflecting surface is made smaller than the width of the light beam, so-called overfilled type optics. Scanning devices are known. An overfilled type optical scanning device is disclosed in Japanese Patent Application Laid-Open No. 50-93719.

In an overfilled type optical scanning device, a part of a light beam incident on a rotary polygon mirror is used as a scanning light beam, so that a scanning angle (scanning position) is used.
Scan luminous flux width changes and the entrance pupil (F number)
Changes, the uniformity of the beam diameter of the light beam on the scanning surface deteriorates, and the uniformity of the light beam amount on the scanning surface deteriorates.

An optical scanning device for solving these problems is disclosed in Japanese Patent Application Laid-open No.
No. -171069 and Japanese Patent Application Laid-Open No. 8-171070. In the optical scanning device described in Japanese Patent Application Laid-Open No. 8-17069, the light amount is increased by setting the width of the incident light beam to the width of the deflecting surface of the polygon mirror to 1.5 to 4 times and the scanning angle to 30 ° to 60 °. Can be about 84%.

In the optical scanning device described in Japanese Patent Application Laid-Open No. 8-171070, the beam diameter is adjusted by setting the relationship between the maximum deflection angle α and the angle of incidence β to the polygon mirror under the condition shown in the following equation (1). Is suppressed to a range of 60 to 90 μm.

[0009]

(1) COS ((β−α) / 2) / COS ((β−α) / 2) ≧ 0.75 (1) Further, as a means for improving the uniformity of the light amount, the distance between the light source and the polygon mirror is set. A technique of providing a filter using a metal film in the optical path is disclosed in JP-A-8-160338.

[0010]

SUMMARY OF THE INVENTION However, Japanese Patent Laid-Open No.
The optical scanning devices described in JP-A-171069 and JP-A-8-171070 do not use an optical element for changing the beam diameter. It is difficult to further improve the uniformity of the diameter.

Also, in the optical scanning device described in Japanese Patent Application Laid-Open No. 8-160338, it is necessary to manufacture an optical filter of a metal film with high accuracy in order to obtain a uniform light amount, which increases the cost of the device. There is.

Further, there has been no means for simultaneously improving the uniformity of the beam diameter and the uniformity of the light amount on the scanning surface.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide an optical scanning device which improves the uniformity of a beam diameter of a light beam for scanning a photosensitive member on a surface to be scanned. This is the first purpose.

It is a second object of the present invention to provide an optical scanning device which improves the uniformity of the light amount of the light beam on the surface to be scanned by the light beam for scanning the photosensitive member.

Further, a third object of the present invention is to provide an optical scanning apparatus which improves the uniformity of the beam diameter and the light quantity of the light beam on the surface to be scanned by the light beam for scanning the photosensitive member. And

[0016]

Means for Solving the Problems To achieve the first object, the invention according to claim 1 is arranged between a light source and a photosensitive member and has a plurality of deflection surfaces on a peripheral surface, A deflector that receives a beam from the light source in a range wider than the width of the deflecting surface in the scanning direction and deflects and scans the photosensitive member; and sets the angle of incidence on the deflecting surface in the main scanning direction to a value other than 0 °. An optical scanning device according to claim 1, wherein an optical element having a characteristic that a phase shift amount is asymmetric with respect to an optical axis is provided between the light source and the deflector.

According to the first aspect of the present invention, since the optical element whose phase shift amount is asymmetric with respect to the optical axis, that is, the phase shift element is provided between the light source and the deflector, the beam waist is generated by the phase shift. Is adjusted, and the beam diameter distribution of the beam diameter distribution on the photoreceptor is corrected so that the diameter is small or thick, and the beam diameter distribution with less variation can be obtained.
Therefore, even in an overfilled optical system in which the F-number changes according to the scanning angle, it is possible to ensure uniformity of the beam diameter distribution on the photoconductor.

In order to achieve the second object, the invention according to claim 2 is arranged between a light source and a photoreceptor and has a plurality of deflecting surfaces on a peripheral surface, and scans a beam from the light source in a scanning direction. An optical scanning device comprising a deflector that receives a wider range than the deflection surface width and deflects and scans on the photoreceptor, and an incident angle in the main scanning direction on the deflection surface is set to other than 0 °. An optical element is provided between the light source and the deflector, the optical element having a characteristic that the diffraction efficiency of the zero-order light is asymmetric with respect to the optical axis.

According to the second aspect of the present invention, since an optical element whose diffraction efficiency is asymmetric with respect to the optical axis, that is, a diffractive optical element is provided between the light source and the deflector, the light amount distribution on the photosensitive member is provided. It is possible to correct a high-value portion to obtain a light amount distribution with less variation. Therefore, even in an overfilled optical system in which the F-number changes according to the scanning angle, it is possible to ensure uniformity of the light amount distribution on the photoconductor.

According to a third aspect of the present invention, there is provided an image forming apparatus, comprising: a light source; a photosensitive member; a plurality of deflecting surfaces on a peripheral surface; An optical scanning device comprising a deflector that receives a wider range than the deflection surface width and deflects and scans on the photoreceptor, and an incident angle in the main scanning direction on the deflection surface is set to other than 0 °. An optical element having both a phase shift function and a diffraction function between the light source and the deflector, and having a characteristic that both the phase shift amount and the diffraction efficiency of the zero-order light are asymmetric with respect to the optical axis. Special features.

According to the third aspect of the present invention, an optical element having both phase shift and diffraction functions between a light source and a deflector, and having both a phase shift amount and a diffraction efficiency asymmetric with respect to an optical axis. The beam waist position is adjusted by the phase shift, the beam diameter distribution on the photoreceptor is corrected to make the diameter small and thick, and the beam diameter distribution is reduced with less variation. It is possible to correct a portion having a high distribution to obtain a light amount distribution with little variation. Therefore, even in an overfilled optical system in which the F-number changes according to the scanning angle, it is possible to ensure uniformity of the beam diameter distribution and uniformity of the light amount distribution on the photoconductor.

[0022] The invention described in claim 4 provides claims 1 to 3
Wherein the optical element such as the phase shift element, the diffractive optical element, and the optical element having both the phase shift function and the diffraction function are manufactured by resin molding or glass molding. And

According to the invention described in claim 4, according to claim 1,
In the optical scanning device according to any one of (1) to (3), optical elements such as a phase shift element, a diffractive optical element, and an optical element having both a phase shift function and a diffraction function are manufactured by resin molding or glass molding. It is easy to manufacture and advantageous in cost.

According to a fifth aspect of the present invention, in the optical scanning device according to the first, third or fourth aspect, the phase shift amount is a variation rate of a beam diameter in a scanning direction on a scanning surface in a scanning direction. Is within 10%.

According to the fifth aspect of the present invention, in the optical scanning device according to the first, third or fourth aspect, the phase shift amount is a variation rate of the beam diameter in the scanning direction on the scanning surface within the scanning area. The shape of the phase shift element and the optical element is formed so as to have a distribution characteristic such that is within 10%, so that high image quality can be achieved.

According to a sixth aspect of the present invention, in the optical scanning device according to the second, third, or fourth aspect, the diffraction efficiency is such that a fluctuation rate of a beam light amount on a scanning surface in a scanning area is 1 unit.
It is characterized by having a distribution characteristic within 0%.

According to the sixth aspect of the present invention, in the optical scanning device according to the second, third or fourth aspect, the efficiency of the diffracted light is such that the variation rate of the light beam amount on the scanning surface in the scanning area is 10%. Since the shapes of the diffractive optical element and the optical element are formed so as to have a distribution characteristic within the range, high image quality can be achieved.

In the fifth and sixth aspects, the variation rate of the beam diameter in the scanning direction on the scanning surface within the scanning area is within 10%, and the variation rate of the beam light amount on the scanning surface in the scanning area is 10%. Phase shift elements,
The shapes of the diffractive optical element and the optical element are specified because, in order to obtain high image quality in the optical scanning device, the variation of the beam diameter of the light beam on the scanning surface is within 10%, and the variation of the light amount is within 10%. It is known that

[0029]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 shows a schematic configuration of an optical system of an optical scanning device according to an embodiment of the present invention. The optical scanning device according to the embodiment of the present invention is used for a laser printer, a digital copying machine, and the like. In FIG. 1, a semiconductor laser 10 that emits a light beam having a substantially Gaussian distribution is provided as a light source. A collimator lens 12 having a function of turning a light beam having a different divergence angle in the vertical and horizontal directions from a focal position thereof into substantially parallel light on the emission side of the light beam of the semiconductor laser 10, the incident light beam Is rotated at substantially constant angular velocity in the direction of arrow P by a driving means such as a cylinder lens 14 and a motor (not shown) that converges in the sub-scanning direction near the deflection surface of the polygon mirror 16 that becomes the main scanning surface. A polygon mirror 16 for deflecting the light beam incident on the deflecting surface in the main scanning direction on the photoconductor 20 is provided. An fθ lens 18 having a lens power only in the main scanning direction is provided between the polygon mirror 16 and the photoconductor 18.

Further, an optical element between the cylinder lens 14 and the polygon mirror 16 for correcting one or both of a beam diameter distribution and a light amount distribution of a scanning light beam cut by a deflection surface serving as a main scanning surface of the polygon mirror 16. 3
0 is provided.

In the above configuration, the light beam emitted from the semiconductor laser 10 is made substantially collimated by the collimator lens 12, and then converged in the sub-scanning direction by the cylinder lens 14 in the vicinity of the main scanning surface of the polygon mirror 16 near the deflection surface. I do.

The light beam incident on the polygon mirror 16 is corrected by the optical element 30 for one or both of the beam diameter distribution and the light amount distribution of the scanning light beam cut off by the deflection surface serving as the main scanning surface. The light is deflected by the polygon mirror 16 in the main scanning direction on the photosensitive member 20 which is the surface to be scanned, and scans the photosensitive member 20. Although the polygon mirror 16 is driven to rotate at a constant angular velocity, fθ
The light beam deflected by the lens 18 is repeatedly scanned on the photoconductor 20 at a substantially constant speed.

The beam diameter distribution and light quantity distribution of the light beam passing through the optical element 30 around the incident optical axis I and the positional relationship between the optical element 30 and the photosensitive member 20 are determined by the coordinates shown in FIG. In FIG. 1, the symbols correspond to the symbols + and-near the optical element 30 and the photoconductor 20 as shown by the symbols + and-in the system.

FIG. 3 shows the relationship between the scanning angle α of the polygon mirror 16 and the incident angle β in the main scanning direction. In the present embodiment, the polygon mirror 16 has 12 deflecting surfaces, and the center line CL of the optical scanning device (which coincides with the optical axis of the fθ lens 18) and the reflected optical axis R1 on the main scanning start side or the main scanning end. The scanning angle α (absolute value), which is the angle formed by the reflection optical axis R2 on the side, is α = 2
The incident angle β (absolute value) between the incident optical axis I and the center line CL with respect to the polygon mirror 16 with respect to the polygon mirror 16 is 7 °, and β = 45 °.

Next, the optical element 30 provided between the semiconductor laser 10 as a light source and the polygon mirror 16 will be described in detail. As described above, the light beam emitted from the semiconductor laser 10 in FIG. 1 is converted into parallel light by the collimator lens 12 and converged by the cylinder lens 14 only in the sub-scanning direction of the deflecting surface serving as the main scanning surface of the polygon mirror 16. . Here, the optical element 30 is the semiconductor laser 1
If the laser beam is not provided in the optical path between the polygon mirror 16 and the polygon mirror 16, the light intensity of the laser beam has a Gaussian distribution, and as shown in FIG.
Is used so that a part of the light beam of the laser beam incident on the polygon mirror 16 is cut off as a scanning light beam by a deflecting surface serving as a main scanning surface. Scanning position). In FIG. 2A, as shown in FIG. 4, the width of the deflecting surface Q serving as the main scanning surface of the polygon mirror 16 is set to 1, the position of the optical axis I incident on the polygon mirror 16 is set to the origin (0), and scanning is performed. In the coordinate system with the direction as the X axis, the projection plane of the deflection surface Q in the X axis direction moves along with the rotation of the polygon mirror 16 (A → B → C). As the movement moves from B to C, the projected area of the deflection surface Q in the X-axis direction decreases. Since the intensity distribution of the beam incident on the polygon mirror 16 is substantially Gaussian distribution,
As the scanning position of the deflecting surface Q changes, the intensity distribution of the light beam cut by the deflecting surface Q at each scanning position is shown in FIG.
(B) to (D) show the solid lines.

As described above, as the polygon mirror 16 rotates, the width of the scanning light beam cut by the main scanning plane changes depending on the scanning angle (scanning position). The uniformity of the beam diameter of the light beam deteriorates, and the uniformity of the light amount of the light beam on the photoconductor 20 deteriorates.

In the present invention, the problem is solved by providing a phase shift element, a diffractive optical element, or an optical element having both a phase shift function and a diffraction function between the light source and the deflector (polygon mirror) as the optical element 30.

FIG. 5 shows the moving state of the main scanning surface (deflection surface) of the polygon mirror 16 and the cross-sectional shapes of various phase shift elements when a phase shift element is used as the optical element 30 in FIG.

FIG. 5A shows a state (A → B → C) in which the projection plane of the deflection surface Q in the X-axis direction moves with the rotation of the polygon mirror 16 similarly to FIG. 2 (A). Since the projected area of the deflecting surface Q in the X-axis direction decreases as the deflecting surface Q moves from A to B to C, the width of the scanning light beam cut by the deflecting surface decreases, and the entrance pupil (F number) changes. You can see.

In order to correct the change in the beam diameter distribution of the light beam on the photosensitive member 20 due to the change in the F number, the refractive index n
A phase shift element having a phase distribution P (x) = 2π (x−0.4) ² as shown in FIG. The phase shift element shown in FIG. 5B is located on the left side (−) of the polygon mirror 16 from the vicinity of the central portion which is arranged so as to coincide with the incident optical axis I.
Side), specifically, in the section of −1.1 ≦ X ≦ 0.4, the thickness of the cross-sectional shape is formed so that the phase distribution P (x) changes continuously according to the curve of 2π (x−0.4) ^2. Accordingly, the amount of phase shift continuously changes from 4π to 0. When X ≧ 0.4, the thickness of the sectional shape is formed to be constant so that the phase shift amount becomes zero. In this phase shift element, X ≦ −1.
In the region 1, the phase changes so as to generate a phase difference of 4π.

The phase shift element shown in FIG.
From the vicinity of the center to the polygon mirror 16 on the left side (−
Side), specifically, in the region of −1.1 ≦ X ≦ −0.6, the thickness of the cross-sectional shape is formed so that the phase distribution P (x) changes continuously according to the curve of 2π (x−0.4) ^2. Is 2
continuously changes from π to 0, then the area −0.6 ≦ X ≦
At 0.4, the thickness of the cross-sectional shape is formed such that the phase distribution P (x) changes continuously according to the curve of 2π (x−0.4) ² , and the phase shift amount changes continuously from 2π to 0 with this. I do.

As described above, the thickness of the cross-sectional shape of the phase shift element shown in FIG. 5C is changed so that the phase distribution P (x) changes continuously based on the phase distribution characteristic shown in FIG.
It is formed so as to be repeated in two stages so that a phase difference equivalent to 2π occurs in a region where the phase shift amount is 0.

Further, the phase shift element shown in FIG. 5D is based on binary optics, and is located on the left side (negative side) of the polygon mirror 16 from near the center, specifically, -1.1≤X≤-0.6. In the region, the phase distribution P (x) is 2
The thickness of the cross-sectional shape is formed so as to change stepwise by approximating by the curve of π (x−0.4) ² , and the phase changes stepwise from 2π to 0 according to this. Then the area −0.6 ≦
When X ≦ 0.4, the phase distribution P (x) is formed so that the thickness of the cross-sectional shape changes continuously according to the curve of 2π (x−0.4) ² , and the phase shift amount is stepwise from 2π to 0 accordingly. Change.

As described above, the thickness of the cross-sectional shape of the phase shift element shown in FIG. 5D is changed so that the phase distribution changes stepwise based on the phase distribution characteristic shown in FIG. Are formed in two steps in a stepwise manner so that a phase difference equivalent to 2π is generated in the region.

FIG. 6 shows the effect when the phase shift element is provided between the semiconductor laser 10 as the light source and the polygon mirror 16. In the figure, the beam diameter distribution is represented by the ratio of the beam diameter of the light beam in each part to the beam diameter of the light beam on the optical axis (center) incident on the polygon mirror 16 as 1.0. In the embodiment of the present invention, the phase shift amount is changed in accordance with the beam diameter on the left side (negative side) of the polygon mirror 16 from the vicinity of the center where the phase shift element is arranged so as to coincide with the incident optical axis I. As a result, the position of the beam waist is adjusted, and the correction is performed so that the narrow portion of the beam diameter becomes thick, and the phase shift amount is not corrected on the right side (+ side) from the center. FIG. 6 shows that the beam diameter distribution does not include the phase shift element (shown by a dotted line).
It can be seen that what was 20% or more was reduced to less than 10% in the present embodiment (shown by the solid line).

The case where a diffractive optical element is used as the optical element 30 in FIG. 1 will be described with reference to FIGS. FIG. 7 shows the function of the diffractive optical element. The diffractive optical element used in this example is a sinusoidal phase grating with a period c. In the figure, the incident plane wave has a period c.
0th order light, 1st order light, -1
It is emitted as a diffracted wave of the next light. Here, higher-order diffracted waves are omitted. 0th order light and 1st order light of the diffracted wave, -1
The angle θ with the next light is θ = λ / c.

FIG. 8 shows the zero-order light diffraction efficiency of the diffractive optical element. Among the phase gratings, a sine wave phase grating and a triangular wave phase grating will be described. FIG. 8 shows the relationship between the groove depth of the grating when the wavelength λ of light used in the optical scanning device is λ = 780 nm and the refractive index n of the grating material (for example, transparent resin or glass is used) is 1.5. It shows the zero-order light diffraction efficiency,
The zero-order light diffraction efficiency can be controlled by the groove depth of the grating. In the figure, a curve m1 indicates the zero-order light diffraction efficiency of the sine wave phase grating, and a curve m2 indicates the zero-order light diffraction efficiency of the triangular wave phase grating.
Is shown.

FIG. 9 shows a mounting position of the diffractive optical element 30 in the optical scanning device. In the figure, the wavelength λ of light used in the optical scanning device is λ = 780 nm, the refractive index of the grating material n = 1.5, the period of the phase grating c = 26 μm, and the distance from the diffractive optical element 30 to the polygon mirror 16. Assuming that L1 is 100 mm, the distance from the optical axis of the 0th-order light to the optical axes of the primary light and the −1st-order light on the deflection surface of the polygon mirror 16 is 3 mm. Here, the thickness of the polygon mirror 16 is 3 mm, and zero-order light is incident on the center thereof. Therefore, the primary light and the primary light both pass outside the deflection plane of the polygon mirror 16.

FIG. 10 shows a triangular wave phase grating as an example of the shape of the diffractive optical element 30. As shown in the figure, the diffraction grating and the plane have a left-right asymmetric shape combined.
Specifically, in the region of X <0.4, the depth corresponding to the zero-order light diffraction efficiency of 75% from the characteristic curve shown in FIG.
00 nm. ) Has a triangular groove with a period of 26μ.
m, a large number of triangular wave phase diffraction gratings 30A are provided in parallel.
Are formed, and the zero-order light diffraction efficiency is 10 in the region of X ≧ 0.4.
A flat surface portion 30B having a flat surface is formed so as to be 0%, and these are synthesized and formed of transparent resin or glass.

The reason why the diffractive optical element 30 is configured in this way is to increase the light amount as the projected area of the main scanning surface of the polygon mirror 16 in the X-axis direction decreases. That is, the light amount of the light beam incident on the left side in the figure from the center of the diffractive optical element 30 is reduced, and the light amount of the light beam incident on the right side is increased. The connecting portion 30C between the diffraction grating 30A and the plane portion 30B in the diffractive optical element 30 has the same height between the center of the groove depth of the diffraction grating 30A and the plane portion 30B so that there is no step in the phase of the wavefront of the zero-order light. It is.

FIG. 11A shows a case where the width of the deflecting surface Q serving as the main scanning surface of the polygon mirror 16 is 1, as shown in FIG.
The position of the optical axis I incident on the polygon mirror 16 is defined as the origin (0).
And a state in which the projection surface of the deflection surface Q in the X-axis direction moves along with the rotation of the polygon mirror 16 in the coordinate system with the scanning direction being the X-axis (A → B → C). It becomes smaller as the projection plane of Q in the X-axis direction moves from A to B to C.

FIG. 11B shows the intensity distribution of the beam incident on the polygon mirror 16 shown by the curve r1 (broken line) and the zero-order light diffraction efficiency characteristic of the diffractive optical element 30 shown by the curve r2 (dashed line). Are shown. As described above, the intensity distribution of the light beam incident on the diffractive optical element 30 is substantially Gaussian distribution.
As the scanning position of the deflection surface Q changes, the intensity distribution of the light beam cut off by the deflection surface Q at each scanning position becomes as shown by the solid lines in FIGS. 11C to 11E due to the action of the diffractive optical element. . 11 (C), (D) and (E) show FIG.
0 represents the 0th-order light diffraction efficiency distribution of the diffractive optical element 30 as x
The polygon mirror 16 when the 0th-order light diffraction efficiency is set to 75% for <0.4 and the 0th-order light diffraction efficiency is set to 100% for x ≧ 0.4.
Is a scanning light beam cut off by the deflecting surface. FIG. 12 shows the effect of correcting the light amount distribution by providing this diffractive optical element in the optical path between the semiconductor laser 10 as the light source and the polygon mirror 16. As shown in the drawing, the light amount distribution is represented by the ratio of the light amount of the light beam at each part with the light amount of the light beam at the incident optical axis (center) to the polygon mirror 16 being 1.0. It can be seen from the figure that the variation in the light amount distribution of the light beam is 20% or more when the diffractive optical element is not provided (shown by a dotted line), but can be reduced to less than 10% in the present embodiment (shown by a solid line). .

In this embodiment, the diffraction efficiency is set in two steps, but may be changed in multiple steps or continuously.

The phase shift element and the diffractive optical element can be easily manufactured by molding glass or resin using a mold obtained by cutting metal. It can also be produced by a binary optics technique.

In the optical scanning apparatus shown in FIG. 1, it is desirable to use an optical element having both functions of a phase shift element and a diffractive optical element as the optical element 30. Although not shown, the phase shift element and the diffractive optical element are separately provided. It may be incorporated, or one of the phase shift element and the diffractive optical element may be used.

Another example of the optical element is shown in FIGS. The optical elements 40 and 50 shown in FIGS. 13 and 14 are obtained by adding a function of a phase shift element and a function of a diffractive optical element to a cylinder lens, and the optical element 40 shown in FIG. 13 scans the cylinder surface of the cylinder lens. A diffractive optical element is formed on the back side with a phase shift function in the direction.

The optical element 50 shown in FIG. 14 has the function of a diffractive optical element and the function of a phase shift element by forming a diffractive optical element on the back surface of the cylinder surface and making the surface on which the diffractive optical element is formed a curved surface. I'm making it.

In the above, the transmission type phase shift element, the diffractive optical element, and the optical element having both the phase shift function and the diffraction function have been described. However, these may be of the reflection type.

It is known that conditions for obtaining high image quality in the optical scanning device are such that the variation of the beam diameter of the light beam on the scanning surface is within 10% and the variation of the light amount is within 10%. Therefore, in order to obtain high image quality,
The shapes of the phase shift element and the diffractive optical element satisfy the following conditions. First, in the coordinate system shown in FIG. 3 where the optical axis I incident on the polygon mirror and the point in contact with the deflection surface of the polygon mirror 16 are the origin (0) and the scanning direction is the X axis, the light beam incident on the polygon mirror 16 is The intensity distribution is I (x)
Then, the complex amplitude distribution U ₀ (x) can be expressed as the following equation (2).

[0061]

U ₀ (x) = √I (x) (2) The window function W (θ) of the deflection surface width that cuts the light beam incident on the deflection surface serving as the main scanning surface of the polygon mirror 16 is a polygon mirror. 16 diameters and number of deflection surfaces, polygon mirror 1
6 and the incident angle β of the deflection surface, and the incident optical axis I of the light beam incident on the polygon mirror 16.
As shown in FIG. 4, the boundary of the deflection surface at the scan angle θ is x1,
x2. Therefore, the window function of the deflection surface width W (θ)
Becomes as shown in Expression (3).

[0062]

W (θ) = 1 x1 ≦ x ≦ x2 = 0 x <x1, x> x2 (3) Here, the optical element 30 shown in FIG. 1 is an element having both a phase shift function and a diffraction function, or In the case where the phase shift element and the diffractive optical element are combined, assuming that the phase distribution of the phase shift element is P (x) and the zero-order light diffraction efficiency distribution of the diffractive optical element is E (x), they are synthesized. The complex amplitude distribution U ₁ (X, θ) is as follows.

[0063]

U ₁ (x, θ) = U ₀ (x) × W (θ) × P (x) × E (x) (4) An image is formed on the photoconductor 20 by the fθ lens 18 having the focal length f. The complex amplitude distribution of the light beam is expressed by U ₂ shown in the following equation (5).
(U, θ).

[0064]

U ₂ (u, θ) = Fourier [U ₁ (x, θ)] (5) However, in equation (5), Fourier [U ₁ (x, θ)
θ)] is the Fourier transform of U ₁ (x, θ), and u = x
/ Λf.

The intensity distribution I of the light beam on the photoreceptor 20
₂ (u, θ) is

[0066]

I ₂ (u, θ) = U ₂ (u, θ) × U ₂ ^* (u, θ) (6) where U ^* is a conjugate complex number of U.

The intensity distribution In (u, u, θ) of the light beam on the photosensitive member 20 normalized by the maximum value I max (θ) of I ₂ (u, θ)
θ) is

[0068]

In (u, θ) = I ₂ (u, θ) / Imax (θ) (7) Beam diameter of light beam at scanning angle θψ (θ)
If the relative intensity 1 / e ² is defined as the beam diameter, In
It is obtained from the value of u that satisfies (u, θ) = 1 / e ² .

The light amount P of the light beam on the photosensitive member 20
(Θ) is

[0070]

P (θ) = ∫U ₁ (x, θ) × U ₁ ^* (u, θ) dx (8) Here, the symbol に お け る in the equation (8) changes from −∞ to + ∞
It means to integrate up to.

From the above relationship, the phase shift element has a variation rate of the beam diameter ψ (θ) in the scanning direction on the surface to be scanned of the photoreceptor 20 within a scanning area (−α ≦ θ ≦ α) within 10%. The diffractive optical element has a phase distribution P (x) such that the variation rate of the beam light amount P (θ) on the scanning surface of the photoconductor 20 in the scanning region (1α ≦ θ ≦ α) is 10 %, So that the variation of the beam diameter distribution and the light quantity distribution on the surface to be scanned of the light beam that scans the photoreceptor is reduced to within 10% by providing the 0th-order light diffraction efficiency distribution E (x) within the range of 10%. Can be.

[0072]

According to the first aspect of the present invention, an optical element whose phase shift amount is asymmetric with respect to the optical axis, that is, a phase shift element is provided between the light source and the deflector. It is possible to adjust the position of the beam waist and correct a narrow or thick portion of the beam diameter distribution on the photosensitive member to obtain a beam diameter distribution with less variation. Therefore, even in an overfilled optical system in which the F-number changes according to the scanning angle, it is possible to ensure uniformity of the beam diameter distribution on the photoconductor.

According to the second aspect of the present invention, since the optical element whose diffraction efficiency is asymmetric with respect to the optical axis, that is, the diffractive optical element is provided between the light source and the deflector, the light amount distribution on the photosensitive member is provided. It is possible to correct a high-value portion to obtain a light amount distribution with less variation. Therefore, even in an overfilled optical system in which the F-number changes according to the scanning angle, it is possible to ensure uniformity of the light amount distribution on the photoconductor.

According to the third aspect of the present invention, an optical element having both phase shift and diffraction functions between a light source and a deflector, and having both a phase shift amount and diffraction efficiency asymmetric with respect to the optical axis. The beam waist position is adjusted by the phase shift, the beam diameter distribution on the photoreceptor is corrected to make the diameter small and thick, and the beam diameter distribution is reduced with less variation. It is possible to correct a portion having a high distribution to obtain a light amount distribution with little variation. Therefore, even in an overfilled optical system in which the F-number changes according to the scanning angle, it is possible to ensure uniformity of the beam diameter distribution and uniformity of the light amount distribution on the photoconductor.

According to the invention set forth in claim 4, according to claim 1,
4. In the optical scanning device according to any one of to 3, the optical element such as a phase shift element, a diffractive optical element, and an optical element having both a phase shift function and a diffraction function is manufactured by resin molding or glass molding. Can be easily manufactured, which is advantageous in cost.

According to the fifth aspect of the present invention, in the optical scanning device according to the first, third or fourth aspect, the phase shift amount is a variation rate of the beam diameter in the scanning direction on the scanning surface within the scanning area. The shape of the phase shift element and the optical element is formed so as to have a distribution characteristic such that is within 10%, so that high image quality can be achieved.

According to the sixth aspect of the present invention, in the optical scanning device according to the second, third or fourth aspect, the efficiency of the diffracted light is such that the variation rate of the light beam amount on the scanning surface in the scanning region is 10%. Since the shapes of the diffractive optical element and the optical element are formed so as to have a distribution characteristic within the range, high image quality can be achieved.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a schematic configuration of an optical system of an optical scanning device according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a state in which a main scanning surface cuts a part of a light beam of a laser beam incident on a polygon mirror as a scanning light east in the overfilled optical scanning device.

FIG. 3 is an explanatory diagram showing a relationship between a scanning angle α of a light beam by a polygon mirror in the optical scanning device shown in FIG. 1 and a deflection surface incident angle β.

FIG. 4 is an explanatory diagram showing a coordinate system in which the surface width of a deflecting surface of a polygon mirror is 1, an intersection point between an optical axis incident on the polygon mirror and the deflecting surface is an origin, and a scanning direction is an X axis.

FIG. 5 is an explanatory diagram showing a relationship between a shape of a phase shift element and a phase shift amount.

FIG. 6 is an explanatory diagram showing a result of correcting a beam diameter distribution of a light beam on a scanned surface of a photoconductor by a phase shift element, in comparison with a case where a phase shift element is not used.

FIG. 7 is an explanatory diagram showing a diffraction function of the diffractive optical element.

FIG. 8 is a characteristic diagram showing the relationship between the groove depth of the diffractive optical element and the zero-order light diffraction efficiency.

FIG. 9 is an explanatory diagram showing a mounting position of the diffractive optical element in the optical scanning device.

FIG. 10 is a perspective view showing an example of a diffractive optical element.

FIG. 11 is an explanatory diagram showing a relationship between a scanning position and a scanning light beam cut by the deflecting surface as the main scanning surface of the polygon mirror moves when the diffractive optical element is provided.

FIG. 12 is an explanatory diagram showing a result of correcting a light amount distribution of a light beam on a scanned surface of a photoconductor by a diffractive optical element, in comparison with a case where no diffractive optical element is used.

FIG. 13 is a perspective view showing an example of an optical element having both a phase shift function and a diffraction function.

FIG. 14 is a perspective view showing another example of an optical element having both a phase shift function and a diffraction function.

[Explanation of symbols]

 Reference Signs List 10 semiconductor laser 12 collimator lens 14 cylinder lens 16 polygon mirror 18 fθ lens 20 photoconductor 30 optical element 40 optical element 50 optical element

Claims (6)

[Claims] 1. A photoreceptor which is disposed between a light source and a photoreceptor and has a plurality of deflecting surfaces on a peripheral surface, and receives a beam from the light source in a range wider than a width of the deflecting surface in a scanning direction. An optical scanning device configured to include a deflector for deflecting and scanning, wherein an incident angle in the main scanning direction on the deflection surface is set to a value other than 0 °, wherein a phase shift amount is set between the light source and the deflector. An optical scanning device comprising an optical element having characteristics that are asymmetric with respect to an optical axis. 2. A light source, comprising: a plurality of deflecting surfaces disposed on a peripheral surface between a light source and a photoreceptor; receiving a beam from the light source in a range wider than a deflecting surface width in a scanning direction; An optical scanning device comprising a deflector for scanning, wherein an incidence angle in the main scanning direction on the deflection surface is set to a value other than 0 °, wherein a diffraction efficiency of zero-order light is provided between the light source and the deflector. An optical scanning device provided with an optical element having an asymmetric characteristic with respect to an optical axis. 3. A light source is provided between the light source and the photosensitive member and has a plurality of deflecting surfaces on a peripheral surface thereof. An optical scanning device comprising a deflector for scanning, wherein an incident angle in the main scanning direction on the deflection surface is set to a value other than 0 °, wherein a phase shift function and a diffraction function are provided between the light source and the deflector. And an optical scanning device having an optical element having characteristics that both the phase shift amount and the diffraction efficiency of zero-order light are asymmetric with respect to the optical axis. 4. The optical scanning device according to claim 1, wherein the optical element is manufactured by resin molding or glass molding. 5. The phase shift amount has a distribution characteristic such that a variation rate of a beam diameter in a scanning direction on a scanning surface within a scanning area is within 10%. Or the optical scanning device according to 4. 6. The diffraction efficiency according to claim 2, wherein the diffraction efficiency has a distribution characteristic such that a variation rate of a beam light amount on a scanning surface within a scanning area is within 10%. Optical scanning device.