JP3073129B2 - Optical scanning device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical scanning device used for exposing a photosensitive member in a laser printer or the like.

[0002]

2. Description of the Related Art For example, a laser printer uniformly charges the surface of a photosensitive drum and then scans the surface of the photosensitive drum with a scanning laser beam from an optical scanning device to form an electrostatic latent image on the photosensitive drum. After the toner image is formed, the electrostatic latent image is developed with toner and visualized, the toner image is transferred to a transfer sheet, and printing is performed by thermally fixing the toner of the transfer sheet.

[0003] As an optical scanning device used in such an apparatus, for example, one disclosed in Japanese Patent Publication No. 5-15339 is known. In this publication, a semiconductor laser beam is deflected and scanned by a polygon mirror, and an optical system in which a scanning speed is increased from the scanning center to an end on an image forming surface is used, and a data clock is supplied to the optical system by a voltage controlled oscillator. The fθ characteristic is electrically corrected by controlling so that the frequency becomes higher from the scanning center to the end, and the decrease in the exposure energy at the end caused by this is determined by the power of the semiconductor laser light. Is increased so that the apparent print density becomes uniform at the center and the end of the scan.

That is, as the scanning speed increases, the exposure energy on the photosensitive drum decreases at the end, and a phenomenon occurs in which the print density decreases from the center to the end. The light quantity is controlled so that the emission energy of the semiconductor laser light is increased step by step to achieve uniform printing density.

For this reason, in the publication, a circuit capable of changing the power of the semiconductor laser light stepwise is added to the drive circuit of the semiconductor laser.

[0006]

As described above, in the conventional apparatus, it is necessary to add a circuit capable of changing the power of the semiconductor laser light in a stepwise manner to the drive circuit of the semiconductor laser. Was.

Accordingly, the present invention provides an optical scanning device which can make the print density from the center to the end of the scan substantially uniform with a simple configuration.

[0008]

The present invention SUMMARY OF] deflects the scanning light beam in rotary reflector, an optical scanning device to form an image on a flat image plane of the scanning beam in the scanning direction, the scanning Bee
Focuses the image on the image forming surface through the correction lens
Is <br/> to increase the beam diameter of the scanning beam to be imaged on the imaging surface in accordance with the scanning angle of the scanning beam increases.

[0009]

In the present invention having such a configuration, as the scanning beam from the rotating reflector passes through the correction lens , the beam diameter on the image forming surface increases as the scanning angle of the scanning beam increases.

[0010]

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

FIG. 1 is a plan view showing a partial configuration of the optical scanning device, and FIG. 2 is a partially sectional side view showing the entire configuration of the optical scanning device.

In FIG. 1, reference numeral 1 denotes a base, on which a scanner motor 2 is fixed. The scanner motor 2 drives the polygon mirror 3 to rotate.

A housing 4 is fixed on the base 1 by screwing. At the top of the housing 4,
A semiconductor laser element 6 fixed to a heat sink 5, a collimator barrel 7 to which a collimator lens for adjusting a laser beam (light beam) from the semiconductor laser element 6 is adhered and fixed, and the collimator barrel 7 is slidably mounted in one dimension. A light emitting unit 9 including a collimator barrel holding member 8 is fixed.

A cylindrical lens 10 and a convergent lens (plano-convex lens) 1 are located in front of the light emitting unit 9.
1. Image forming unit 13 in which folding mirror 12 is integrated
Is fixed to the upper lower surface of the housing 4 by screwing.

In the image forming unit 13, the cylindrical lens 10 is fixed to a flat lens holder 19, and the lens holder 19 is slidably mounted on an internal frame. Further, a rectangular opening hole 20 communicating with the lens holder 19 is formed in the housing 4 to enable the slide adjustment of the lens holder 19 from outside.

The laser beam from the semiconductor laser element 6 is reflected by a return mirror 12 through a collimator lens, a cylindrical lens 10 and a converging lens 11,
The reflected laser light is reflected by the reflecting surface of the polygon mirror 3 and is deflected and scanned. And the polygon mirror 3
The scanning light (scanning beam) passes through a correction lens 14 attached to a through-hole on the front surface of the housing 4 and is reflected by folding mirrors 16 and 17 fixed to a folding unit 15 to form a photosensitive image forming surface. Irradiation is performed on the photosensitive surface of the body drum 18. The folding unit 15 is fixed to the outside of the front surface of the housing 4 by screwing.

As shown in FIG. 3, the correction lens 14 has an entrance surface a1 which is an aspherical surface whose rotation axis is in the main scanning direction and whose envelope is expressed by an even-order higher-order polynomial, and whose exit surface a2 has an optical axis. An envelope having a rotation axis in the direction is an aspheric surface represented by an even-order higher-order polynomial. In assembling the optical scanning device having such a configuration, first, the light emitting unit 9, the imaging unit 13, and the correction lens 14 are fixed to the housing 4. At this time, the alignment of the collimator lens of the light emitting unit 9 and the semiconductor laser element 6 is separately performed using an autocollimator or the like.

A two-dimensional area sensor or the like is arranged at a position corresponding to the reflection surface of the polygon mirror 3 to roughly adjust the collimator in one direction in the optical axis direction and to adjust the cylindrical lens 1.
Zero position adjustment in the sub-scanning direction is performed.

Next, the collimator is tightly adjusted in the optical axis direction while looking at the beam diameter evaluation device connected to the scanner motor 2 and arranged at a position corresponding to the focal plane.

The folding unit 15 is attached to the housing 4.
Install. By doing so, it is possible to deflect and scan in a plane almost perpendicular to the rotation axis of the scanner motor 2 without being affected by the accuracy and dirt of the mirrors 16 and 17 of the folding unit 15, and the optical scanning device and the beam diameter evaluation device Setting becomes easy.

Next, a description will be given of the design values of the optical scanning device in which the fθ correction is not performed optically, in other words, the correction is required electrically.

FIG. 4 shows a simulation result.
(a) is the fθ error (the deviation of the actual scanning position from the normal scanning position), (b) is the main scanning field curvature, (c) is the sub-scanning field curvature,
(d) shows the scanning line curvature. The scanning position is a position from the center to 110 mm to one side from the center, and the graph shows an error amount at each position.

In this simulation, the off-axis amount of the correction lens 14 is -1.434 mm, the skew angle of the incident beam is 3.406 °, and the virtual convergence point y coordinate is -30.4 mm.
012 mm, the radius of the inscribed circle of the polygon mirror 3 is 14 mm,
The scanning angle of the polygon mirror 3 for scanning of 20 mm is 42
°, the elliptical radius of the reflecting surface of the polygon mirror 3 is 120.2
97 mm, the thickness of the correction lens 14 is 5 mm, and the correction lens 14
The center radius of the incident surface is 12.1366 mm, and the correction lens 14
Was performed with the vertex y coordinate of 38.2 mm.

That is, the polygon mirror 3 has an inscribed circle radius of 14 mm, a scanning angle of 42 °, and an (elliptical) circle radius of 12 on the reflection surface.
A main scanning component is condensed on a 0.3042 mm (y-axis oblique component to a plane (xy plane) perpendicular to the polygon mirror rotation axis) from the rotation center with a skew angle of 3.406 ° at 0.297 mm. It is configured so that: The sub-scanning light condensing position is a polygon reflection surface.

The correction lens 14 is a polygon mirror 3
Is a lens that forms an image of the scanning light deflected by the scanning and corrects the surface tilt, but the incident surface has a rotating body shape with a rotationally symmetric axis as a straight line parallel to the main scanning direction, and scanning at a scanning angle of 0 °. It is symmetrical with respect to the plane position, and the cross-sectional shape in the main scanning direction is represented by the following even-numbered high-order polynomial. The coefficients at this time are as follows: the incident surface second order coefficient is 0.007361, the incident surface fourth order coefficient is 6.732446 × 10 ⁻⁷ , and the incident surface sixth order coefficient is −.
6.848163 × 10 ⁻¹⁰ , the eighth-order coefficient of incidence plane is 4.3
83077 × 10 ^-13 .

The exit surface has a rotationally symmetric structure with respect to the optical axis rotation center when the scanning angle is 0 °, and the cross-sectional shape in the main scanning direction is represented by the following even-numbered high-order polynomial. The coefficient at this time is as follows: the outgoing surface secondary coefficient is 0.008121, the outgoing surface 4
The order coefficient is 1.409573 × 10 ⁻⁶ , the sixth-order coefficient of the exit surface is −4.242289 × 10 ⁻¹⁰ , and the eighth-order coefficient of the exit surface is 4.
043859 × 10 ^-14 .

The central axes of the entrance plane and the exit plane are shown in FIG.
(a) As shown in (b), they are not in the same plane. The off-axis amount is -1.434 mm, and the exit surface axis passes through the optical axis. The general formula of the aspherical surface is represented by the following formula. When the quadratic term of the aspherical surface coefficient is converted into R, A2 = １ ／ R. However, R of the incident surface
Is 67.925 mm, R of the exit surface is 61.57 mm, and k is −1 for both the entrance surface and the exit surface.

[0028]

(Equation 1) Next, the fθ error (scanning speed for each scanning position) of the optical scanning device will be described.

Although the scanning speed increases from the center to the end, the deviation from the ideal scanning position to the actual scanning position is shown in FIG. And the maximum is about 65 mm at about 65 mm from the center. When indicating the ratio of the scanning speed v _x of the scanning position with respect to the center of the scanning velocity v ₀ is as shown in FIG. That is, at a position 110 mm from the center, the magnification becomes about 1.37 times.

In such an optical scanning apparatus, in this embodiment, the print density is made uniform by increasing the beam diameter in accordance with the scanning speed ratio without adding a circuit for modulating the power of the semiconductor laser element 6. That is, this is realized by gradually increasing the beam diameter of the laser light from the center to the end.

There are two types of patterns: a pattern in which only the main scanning beam diameter increases toward the end, and a pattern in which both the main scanning and the sub-scanning increase toward the end.

FIG. 8 is a graph showing a pattern in which only the main scanning beam diameter increases toward the end. The ratio between the beam diameter w _{0 at} the center of scanning and the beam diameter w _x at each scanning position is as shown in FIG. It is as shown in the graph g _1, as shown in the graph g ₂ in FIG. 8 is a sub-scan. That is, the beam diameter in the main scanning is increased substantially in accordance with the scanning speed ratio, and the beam diameter in the sub-scanning is kept constant.

A graph showing a pattern in which both the main scanning and the sub-scanning become larger toward the end is shown by a graph. The ratio of the beam diameter w _{0 at} the center of scanning to the beam diameter w _x at each scanning position is determined by the following _equation . as it is shown in the graph g ₃ of FIG. 8 in the sub-scanning both. That is, both the main scanning and the sub-scanning are increased so as to substantially coincide with the square root of the scanning speed ratio.

Specifically, as an example, the change in the beam diameter in the main scanning is set as shown in FIG. 9, and the change in the beam diameter in the sub-scanning is set as shown in FIG. That is, FIGS. 9 and 10 show the measured values (1 / e ² ) of the main scanning and sub-scanning beam diameters at each scanning position when the collimator focus adjustment is performed so that the beam diameter at the center is minimized. In both the main scanning and the sub-scanning, the square root of the scanning speed ratio is thickened.

FIG. 11 shows the relationship between the process threshold value (Pth) (the limit value of whether toner is attached or not) and the printing diameter when the beam diameter changes while the power of the laser beam is constant. become. Here, Pth1 indicates the process threshold value for the center position, and Pth1 'indicates the process threshold value for the peripheral position. Further, w1 to w4 are the beam diameter at the process threshold value Pth1 when the beam diameter is changed,
That is, it indicates the printing diameter. Also, x = v (x) · t
Indicates a main scanning position.

The process threshold value is set to Pth3
(> Pth1 '), that is, if the beam diameter is largely changed, as shown in FIG. 13, the change is small, and if the beam diameter changes more than a certain level, printing becomes impossible.

On the other hand, when the process threshold value is set to Pth2 (approximately Pth1 '), that is, low, the printing diameter becomes significantly larger as the beam diameter changes greatly, as shown in FIG.

As described above, the process threshold value Pth is equal to or smaller than the half value of the beam diameter at the center position, and the printing with respect to the increase in the beam diameter is caused by the increase in the process threshold value accompanying the increase in the scanning speed in the periphery. It is desirable to set the position to be canceled by the reduction in diameter.

As a means for increasing the beam diameter toward the periphery, that is, for increasing the beam diameter, the correction lens 14 is used.
What is necessary is just to make the curvature of field in the main scanning direction and the sub-scanning direction gradually increase toward the periphery. In other words, this can be realized by setting the field curvature to 0 at the center and preventing the field curvature curve from becoming zero on the axis in the middle of the scanning area.

In this manner, the scanning speed increases as the scanning goes from the center to the end, and the process threshold value increases with the occurrence of a phenomenon that the printing diameter becomes narrower. Since the beam diameter increases as the scanning goes from the center to the end so as to compensate for the thinning, the printing diameter can be made substantially uniform over the entire area from the center to the end of the scanning, so that the printing density can be made substantially uniform over the entire area. .

Furthermore, the configuration can be realized only by using the correction lens 14, and the configuration is simple because there is no need to incorporate a special circuit.

[0042]

As described above, according to the present invention, the scanning beam is changed by the optical means so that the beam diameter on the image forming surface becomes larger as the scanning angle of the scanning beam becomes larger. It is possible to provide an optical scanning device that can make the print density from the center to the end of the scanning substantially uniform.

[Brief description of the drawings]

FIG. 1 is a plan view showing a partial configuration of an optical scanning device according to an embodiment of the present invention.

FIG. 2 is a partially sectional side view showing the entire configuration of the optical scanning device of the embodiment.

FIG. 3 is a diagram showing a configuration of a correction lens according to the embodiment.

FIG. 4 is a diagram showing simulation results of fθ error, main scanning field curvature, sub-scanning field curvature, and scanning line curvature of the embodiment.

FIG. 5 is a diagram showing a relationship between a central axis of an entrance surface and an exit surface of the correction lens of the embodiment.

FIG. 6 is a graph showing a change in a scanning speed ratio with respect to a main scanning position in the embodiment.

FIG. 7 is a graph showing a scanning position accumulated error with respect to a main scanning position in the embodiment.

FIG. 8 is a graph showing a change in a beam diameter ratio with respect to a main scanning position in the embodiment.

FIG. 9 is a graph showing an example of increasing the beam diameter in the main scanning direction with respect to the scanning position in the embodiment.

FIG. 10 is a graph showing an example of increasing the beam diameter in the sub-scanning direction with respect to the scanning position in the embodiment.

FIG. 11 is a diagram showing a relationship between a process threshold value and a printing diameter when the beam diameter changes while the power of the laser beam is constant.

FIG. 12 is a diagram illustrating a relationship between a laser beam power and a printing diameter when a process threshold value is set low when a beam diameter changes with a constant power.

FIG. 13 is a diagram illustrating a relationship between a laser beam power and a printing diameter when a process threshold value is set high when a beam diameter changes with a constant power.

[Explanation of symbols]

 2 scanner motor 3 polygon mirror 6 semiconductor laser element 14 correction lens 18 photosensitive drum (image forming surface)

Claims (1)

(57) [Claims] 1. A deflected light beam scanning a rotating reflector, the optical scanning device to form an image on a flat image plane of the scanning beam in the scanning direction, before through a scanned beam correcting lens
The correction lens forms an image on the image forming surface, and the correction lens forms a scan on the image forming surface as the scanning angle of the scanning beam increases.
Optical scanning apparatus characterized by increasing the beam diameter of the beam.