JPH01177512A - Post-objective type optical scanner - Google Patents

Abstract

PURPOSE:To obtain the title scanner having a high performance, and also, consisting of an inexpensive and simple structure by combining the correction effects of the reflecting surface of a rotary reflecting mirror, and the incident surface and the emitting surface of a correction lens and executing a correction of an ftheta characteristic and a correction of the curvature of the sagittal image face and the meridional image face. CONSTITUTION:By forming the reflecting surface of a rotary reflecting mirror 7 to the curved surface on which the curvature is different in each part in its reflecting surface, the curvature of the image face (sagittal image face) in the scanning direction can be corrected more effectively than the spherical surface or the cylindrical surface whose curvature is constant. Also, since the incident surface 14 of a correction lens 9 has been formed to the rotation symmetrical curved surface 17 having a rotation symmetrical axis being parallel to the scanning direction, power in the sub-scanning direction being vertical to the scanning direction can be varied, and the curvature of the image face (meridional image face) in the sub-scanning direction can be corrected. Moreover, since the emitting surface 15 of the correction lens 9 has been formed to the curved surface having power in the scanning direction, a correction effect of an ftheta characteristic and a correction effect of the sagittal image of a higher level are obtained by this curved surface. In such a way, said scanner having a high performance, and also, consisting of an inexpensive and simple structure can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a post-objective optical scanning device used in laser printers and the like.

Conventional technology Conventionally, optical scanning devices used in laser printers and the like include pre-objective type optical scanning devices in which the light beam is deflected and scanned by a rotating polygon mirror and then passes through a converging lens, and a pre-objective type optical scanning device in which the light beam is deflected and scanned by a converging lens. There are two main types of optical scanning devices: post-objective type optical scanning devices, in which the optical beam is input to a rotating reflecting mirror after the beam is reflected.

First, pre-objective optical scanning devices have been widely used in recent years because it is easy to correct field curvature and fθ characteristics using a converging lens, and it is also easy to set the convergence position to a flat surface.

However, since this converging lens must be configured as a wide-angle lens that covers the deflection angle, there is a problem that the configuration is complicated and expensive.

In addition, post-objective type optical scanning devices have simple converging lens configurations, making it easy to make the devices smaller and cheaper; however, the convergence point of the light beam is on a curved surface, and
Since the scanning speed is also not constant, it is necessary to correct the curvature of the field and the fθ characteristics. In order to make such a correction, it is possible to correct the curvature of field by making the reflecting surface of the rotating reflecting mirror a spherical or cylindrical surface, as disclosed in Japanese Patent Application Laid-Open No. 156020/1982. Proposed. As a result, the curvature of field can be reduced to a level that poses no problem in practice, but nonlinearity of about 10% to 15% remains in the fθ characteristic. For this reason, in laser printers that require high-quality printing, this nonlinearity can be reduced to 2% or more.
Since it is required to suppress the nonlinearity to about 3%, in the conventional technology, nonlinearity is corrected by electrical means. That is, by changing the clock of the print signal continuously or stepwise in response to the nonlinearity of the fθ characteristic, the correction for the nonlinearity of the fθ characteristic is suppressed to 2% to 3%.

Problems to be Solved by the Invention However, conventional methods of correcting the nonlinearity of fθ characteristics are performed by electrical means.

Therefore, in laser printers and the like, there is a problem that the electric circuit becomes complicated and expensive.

Means for solving the problem In order to solve this problem, the reflecting surface of the rotating reflector is formed as a curved surface with different curvature at each part within the reflecting surface, and the reflecting surface and scanning surface are An arcuate rotationally symmetrical curved surface drawn in the sub-scanning direction with a rotationally symmetrical axis parallel to the scanning direction as the center is on the optical path between □, which includes the rotational axis of the rotary reflecting mirror and is perpendicular to the rotationally symmetrical axis. an entrance surface that is formed to have a plane of symmetry and has power in the scanning direction and the sub-scanning direction; and an exit surface that is located on the opposite side of this entrance surface and is formed symmetrically with respect to the scanning direction and has power in the scanning direction. A correction lens consisting of a surface is provided.

Therefore, by making the reflecting surface of the rotating reflecting mirror a curved surface with different curvatures at different parts within the reflecting surface, the image plane in the scanning direction (sagittal image plane) can be more effectively formed than a spherical or cylindrical surface with constant curvature. curvature can be corrected. In addition, since the entrance surface of the correction lens is a rotationally symmetric curved surface with an axis of rotational symmetry parallel to the scanning direction, it is possible to change the power in the sub-scanning direction perpendicular to the scanning direction. It is possible to correct the curvature of (meridional image plane) (in this case, since the entrance plane also has power in the scanning direction, f
This will also affect the θ characteristics and the sagittal image plane,
Sufficient correction can be made by the sagittal image plane correction effect of the rotating reflecting mirror, the fθ characteristic at the exit surface of the correction lens, and the sagittal image plane correction effect, which will be described later.)
. Furthermore, since the exit surface of the correction lens is a curved surface having power in the scanning direction, it is possible to obtain an effect of correcting the fθ characteristic and a more advanced correction effect of the sagittal image plane.

As mentioned above, f
It is possible to correct the θ characteristic and the curvature of the sagittal image plane and meridional image plane, and as a result, it is possible to correct the fθ characteristic, which has been a problem in the past, by optical means instead of electrical means. Because it can do this, it has high performance, and can also be made inexpensive and simple in structure.

Embodiment A first embodiment of the present invention will be explained based on FIGS. 1 and 2. A semiconductor laser 1 as a light source, a collimator lens 2 that converts light into parallel light, a cylindrical lens 3, and a converging lens 4 are sequentially provided on the same optical path.

Further, on the optical path, a polygon mirror 7 as a rotating reflecting mirror has a plurality of reflecting surfaces 6 fixed on the motor 5.
is provided. The reflective surface 6 is a curved surface 8 having a different curvature at each portion within the reflective surface.

A correction lens 9 is provided on the optical path reflected by the reflection surface 6, and a cylindrical photoreceptor having a scanning surface 10 on which the light beam transmitted through the correction lens 9 is irradiated. A photosensitive drum 11 is provided.

The correction lens 9 has power (this power refers to the refractive power or imaging power of the optical surface) in the scanning direction (arrow direction) and the sub-scanning direction (direction perpendicular to the scanning direction), and is capable of scanning. An entrance surface 14 that is a rotationally symmetrical curved surface 13 drawn in the sub-scanning direction about a rotationally symmetrical axis 12 parallel to the direction.
and an exit surface 1 located on the opposite side to the entrance surface 14, formed symmetrically with respect to the scanning direction, and having power in the scanning direction.
It consists of 5. This exit surface 15 is the entrance surface 1
The rotationally symmetrical curved surface 17 has a rotationally symmetrical axis 16 at the center point 02 of a shape that is orthogonal to the rotationally symmetrical axis 12 of No. 4 and is formed symmetrically with respect to the scanning direction.

By the way, as shown in FIG.
They are arranged so that the distance to 0 is A. Also,
The correction lens 9 is connected to the rotation axis 1 of the polygon mirror 7.
The distance from the incident surface 14 to the incident surface 14 is B.

Specific numerical values of these distances A and B will be described later.

Further, the semiconductor laser 1 and the collimator lens 2
, the cylindrical lens 3, and the convergent lens 4.
is disposed at a position inclined downward by θ=3.4 degrees with respect to a plane perpendicular to the reflective surface 6 of the polygon mirror 7, and conversely, θ=3 degrees is inclined upwardly. The correction lens 9 and the photosensitive drum 11 are arranged at a position inclined by .4 degrees.

In such a configuration, the light beam emitted by the semiconductor laser 1 according to the print signal is transmitted through the collimator lens 2.
The light is converted into parallel light, passes through a cylindrical lens 3 having power in the sub-scanning direction, and further passes through a converging lens 4 before being irradiated onto a reflective surface 6 of a polygon mirror 7. The light beam deflected and scanned as the polygon mirror 7 rotates passes through the correction lens 9 and is scanned in the scanning direction on the scanning surface 10 of the photosensitive drum 11 to perform recording.

Next, the shape of the reflective surface 6 of the polygon mirror 7 is changed to an elliptical cylindrical surface 1
9 will be explained based on FIGS. 4 and 5. If the two elliptical radii from the center point 0 of the elliptical cylindrical surface 19 are a and b, the reflective surface 6 of the polygon mirror 7 is located on the locus of one radius a. Further, the rotation axis 18 of the polygon mirror 7 is set at the center point of an inscribed circle with a radius c (=16 mm), and this rotation axis 18 is set at the center point O.
parallel to the axis passing through.

Further, the cylindrical lens 3 and the converging lens 4 allow the light beam transmitted through them to reach the rotation axis 18 of the polygon mirror 7.
The light beam is arranged so that it converges at a point S when projected in the direction of , and converges on the reflective surface 6 of the polygon mirror 7 when projected in the scanning direction. in this case,
The distance between the rotation axis 18 of the polygon mirror 7 and the point S is d. Note that specific numerical values for the radii a, b, and distance d will be described later.

Next, the shape of the correction lens 9 will be explained.

FIG. 6 shows a cross-sectional shape of the correction lens 9 when cut into a plane parallel to the scanning direction.

The entrance surface 14 is a rotationally symmetric curved surface 1 having a rotationally symmetrical axis 12.
3, and the radius at the center point 01 is e.

In this case, the X1 axis passes through the center point 01 and is parallel to the scanning direction,
If the Y1 axis is taken perpendicular to this to create an X101Y□ coordinate system, the cross-sectional shape of the entrance surface 14 can be expressed by an 8th-order polynomial as follows.

Y1=a,Xl"+ α4X1'+ α6X,s+aI
lX,! (1) The exit surface 15 is symmetrical to the left and right (vertically symmetrical on the paper) with respect to the Y□, and is a rotationally symmetrical curved surface 17 with the Y1 axis as the axis of rotational symmetry 16.

In this case, take the X2 axis passing through the center point 02 and parallel to the scanning direction, take the Y2 axis that coincides with Y1, and
When the Y2 coordinate system is created, the cross-sectional shape of the entrance surface 14 can be expressed by an 8th order polynomial as follows.

Y2=β2X2”+ β, X2'+ β., x, 'ten/
11. X2' (2) Note that the correction lens 9 was made of acrylic resin with a refractive index of 1.48. Also, the radius e and the coefficient α2 in equation (1)
．． α4. α6. α8, coefficient β2 in equation (2). β4
．． β6. The specific value of β8 will be described later.

Next, we performed ray tracing using a computer simulation to correct the fθ characteristics, the curvature of the sagittal and meridional image planes, and the curvature of the scanning line on the photosensitive drum 11 to the extent that there are no practical problems. A specific numerical example of the parameter is shown below. However, the bending of the scanning line on the photosensitive drum 11 causes the light beam to be directed at an angle that is not perpendicular to the reflective surface 6 of the polygon mirror 7 (
Skew is particularly likely to occur in human-injection optical systems in which the light is incident at an angle of θ), so the design was designed with this in mind.

Specific figures are shown below.

A = 208.3 mm B = 160.8 mm a = 116.15 mm b = 125.52 mm d = 33.76 mm m = 16.33 mm a2 = 8.610 X 10-'mu-"α,
= 4.336X10-'mm-"a, =-5,
509X10-13 mm-5α, = -3,071X
10-17 Group 17β2 Knee 4.389 X 10
-' mm-1β, = 1.616X10-7
+n+a-3βs=-6,542X 10-"+
n+n-”βm=-2,689×10-” mm-
'However, the effective scanning length is 220 mm, and the corresponding rotation angle of the polygon mirror 7 is 36''.

Here, various characteristics of the fθ error, the curvature of the sagittal image plane and the meridional image plane, and the bending of the scanning line are shown in FIG. The vertical axis in the figure is taken along the scanning direction, and the horizontal axis corresponds to each characteristic.

fθ error = (Beam incident position on the photosensitive drum) - (Linear incident position) ... (3) (Linear incident position) = (Polygon mirror rotation angle) x (2
20 mm/36') However, the rotation angle (degrees) of the polygon mirror 7 is set to 0 degrees in the state shown in FIG.

Next, a second embodiment of the present invention will be described.

The configuration and operation of this embodiment are the same as those of the first embodiment, so only the numerical values of each parameter are shown here.

Specific figures are shown below.

A=210. '7 mm B=158.2 mm m=68.84 mm b=9'1. OOmm d= 29.62mm m= 17.63mm a2= 9. 'l' 37 ×10-' mm-1
a, = 7.725X10-9 mm-3a6=-
5,725X 10-13mm7'αg' ==-L4
．． 355 x 10-"mm-'β, = -3.586
xlO-' mm-1β4= 1.474 X 1
0-' mm-3βG= 4.439 X 10-
13mm-'β++=-1,469X 10-" m
m-7 However, in this example as well, the effective scanning length is 220 m.
m, and the corresponding rotation angle of the polygon mirror 7 is 36°.

Next, the first embodiment and the second embodiment described above, as well as the curvature and fθ characteristics of the sagittal image plane and meridional image plane in the optical scanning device of Japanese Patent Application Laid-Open No. 61-15602OJ described in the prior art. Table 1 shows the characteristics of the linearity and the amount of bending of the scanning line.Furthermore, the characteristics of the first embodiment are shown in FIG.

Note that ■, ■, and ■ indicate the values of the conventional example, ■ indicates the first embodiment of the present invention, and ■ indicates the second embodiment of the present invention. ■、■
The linearity in is shown as the maximum value for the scanning speed at the center of the scanning line.

From the values in Table 1 mentioned above, this example has the following characteristics compared to the conventional example:
It can be seen that the curvature of the sagittal image plane can be improved by about twice, the curvature of the meridional image plane can be improved by the same or more, and the linearity can be improved by about 4 to 5 times. In particular, regarding linearity, the value in this embodiment can be set to a value of 2 to 3% or less, which does not require additional correction means such as electrical correction means. Furthermore, the bending of the scanning line can be corrected to the extent that there is no problem in practical use.

In the first and second embodiments of the present invention, the correction lens 9 is composed of one lens, but it is not limited to this, and may be composed of a plurality of lenses in consideration of various circumstances such as processing and molding. Good too. For example, if we consider a two-lens correction lens 9 and designate it as a first lens and a second lens, the first lens' entrance surface is the same shape as the one-lens entrance surface, its exit surface is a spherical surface, and the second lens is The entrance surface of the lens may be a spherical surface, and the exit surface thereof may have the same shape as the exit surface in the case of a single lens configuration.

Effects The present invention forms the reflective surface of a rotating reflective mirror as a curved surface with different curvatures in each part of the reflective surface, and creates a rotational symmetry axis parallel to the scanning direction on the optical path between the reflective surface and the scanning surface. A circular arc-shaped rotationally symmetrical curved surface drawn in the sub-scanning direction as a center includes a rotational axis of the rotary reflecting mirror and has a symmetrical plane perpendicular to the rotational symmetrical axis. A correction lens is provided, which consists of an entrance surface having a power of
By making the reflecting surface of the rotating reflector a curved surface with different curvatures in each part of the reflecting surface, the curvature of the image plane (sagittal image plane) in the scanning direction can be more effectively reduced compared to spherical or cylindrical surfaces with constant curvature. Can be corrected. In addition, since the entrance surface of the correction lens is a rotationally symmetric curved surface with an axis of rotational symmetry parallel to the scanning direction, it is possible to change the power in the sub-scanning direction perpendicular to the scanning direction. (meridional image plane) can be corrected. Furthermore, since the exit surface of the correction lens is a curved surface having power in the scanning direction, it is possible to obtain an effect of correcting the fθ characteristic and a more advanced correction effect of the sagittal image plane.

Therefore, by combining the correction effects of the reflection surface of the rotating reflector and the entrance and exit surfaces of the correction lens, it is possible to correct the fθ characteristic and the curvature of the sagittal and meridional image surfaces. Since correction of the fθ characteristic, which has hitherto been considered a problem, can be performed by optical means rather than electrical means, the present invention has high performance, and can be constructed at low cost and simple.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing a first embodiment of the present invention, FIG. 2 is a plan view thereof, FIG. 3 is a side view of FIG. 1, and FIGS.
The figure is an explanatory diagram when the reflective surface of the polygon mirror is a cylindrical surface, FIG. 6 is an explanatory diagram showing the cross-sectional shape when the correction lens is cut along a plane parallel to the scanning direction, and FIG. 2 is an explanatory diagram showing various characteristics of the optical device in FIG. 1. FIG. DESCRIPTION OF SYMBOLS 1... Semiconductor laser, 6... Reflecting surface, 7... Rotating reflecting mirror, 8... Curved surface, 9... Correction lens, 10...
・Scanning surface, 11... Photoreceptor, 12... Rotational symmetry axis,
13...Rotationally symmetric curved surface, 14...Incidence surface, 15...
- Output surface, 16... Rotationally symmetrical axis, 17... Rotationally symmetrical curved surface, 18... Rotation axis, 02... Center point Applicant Tokyo Electric Co., Ltd. J U diagram

Claims (1)

[Claims] 1. Scanning the light beam by irradiating a light beam from a semiconductor laser onto a reflecting surface of a rotating reflecting mirror, and irradiating the scanning surface of a photoreceptor with the light beam deflected and scanned by the reflecting surface. In the post-objective type optical beam scanning device, the reflecting surface of the rotating reflecting mirror is formed as a curved surface having a different curvature at each part within the reflecting surface, and the optical path between the reflecting surface and the scanning surface is , an arc-shaped rotationally symmetric curved surface drawn in the sub-scanning direction about a rotationally symmetrical axis parallel to the scanning direction includes a rotational axis of the rotary reflecting mirror and has a symmetrical plane perpendicular to the rotationally symmetrical axis. A correction lens consisting of an entrance surface that is formed and has power in the scanning direction and the sub-scanning direction, and an exit surface that is located on the opposite side of the entrance surface and is formed bilaterally symmetrically with respect to the scanning direction and has power in the scanning direction. A post-objective optical scanning device characterized by the following: 2. Claim 1, wherein the entrance surface of the correction lens has a shape that can be expressed by an even-order polynomial within an arc-shaped rotationally symmetric curved surface drawn in the sub-scanning direction with the rotationally symmetric axis as the center.
The post-objective optical scanning device described above. 3. The exit surface of the correction lens is a rotationally symmetrical curved surface whose axis of rotational symmetry is the center point of a shape that is orthogonal to the axis of rotational symmetry of the entrance surface and symmetrical with respect to the scanning direction. 2. The post-objective optical scanning device according to item 1. 4. The post-objective optical scanning device according to claim 1, wherein the exit surface of the correction lens has a shape that can be expressed by an even-order polynomial within a rotationally symmetric curved surface that includes the rotationally symmetric axis of the exit surface. .