JPS63210818A - Recorder - Google Patents

Abstract

PURPOSE:To reduce the size and cost of a device by forming the light incident face and exit face of a 2nd lens respectively to aspherical shapes and setting the distance between the light exit face and image plane of the 2nd ftheta lens at f/22-f/3 when the focal length of the ftheta lens system is designated as f. CONSTITUTION:The light incident face 124a and light exit face 124b of the 2nd ftheta lens 124 are respectively formed to the aspherical shapes. The distance (d) from the light exit face 124b of the 2nd ftheta lens 124 to a photosensitive drum 54 which is the image plane is set in a d=f/22-f/3 range when the combined focal length of the 1st and 2nd ftheta lenses 102, 124 is designated as f. The ftheta lens characteristics are thus satisfied even at a wide scanning angle of about + or -45 deg. and the optical path from a deflection scanning means to a recording medium is shortened. The size and cost of the device are thereby reduced and the correction of the curvature of the image plane created by the luminous flux within the section orthogonal with the plane of polarization is permitted.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a recording device such as a laser printer.

(Prior Art) For example, in a laser printer, as shown in FIG. 21, a laser beam L generated by a laser generator (not shown) is deflected and scanned by a rotating polygon mirror 2, and the deflected and scanned laser Beam L is first fθ lens 4, first mirror 6, second mirror 8, second fθ
The lenses 10 are sequentially guided onto the photosensitive drum 12.

As a result, the photoreceptor drum 12, which has been charged in advance by the charger 14, is exposed to light, and an electrostatic latent image is formed. In the figure, 16 is a developing device, 18 is a transfer charger, 2
0 is a fixing device, 22 is a paper output tray, 24 is a cleaning device, 26 is a static elimination lamp, 28 is a paper feed cassette, 30
is the manual paper feed guide.

(Problems to be Solved by the Invention) However, the conventional first fθ lens 4 is a spherical lens, and the second fθ lens 10 has either a light entrance surface or a light exit surface that rotates an arc. Since it is a toric lens, the scanning angle of the rotating polygon mirror 2 that can satisfy the fθ lens characteristics is ±3 at maximum.
It is about 0 degrees. Therefore, the optical path from the rotating polygon mirror 2 to the photosensitive drum 12 becomes relatively long, which poses a problem in that the device becomes larger and more expensive. Moreover, in such a configuration, the closer the light exit surface of the second fθ lens is to the image plane, the thicker the lens becomes, which poses a problem in design. It is not possible to sufficiently correct the curvature of the field created by the light beam.

The present invention has been made based on the above-mentioned circumstances, and an object of the present invention is to reduce the size and cost of the device, and to reduce the luminous flux within a cross section perpendicular to the plane of polarization without causing any problems in design. The object of the present invention is to provide a recording device that can sufficiently correct the curvature of the image plane created by the above.

[Structure of the Invention] (Means for Solving the Problems) In order to solve the above-mentioned problems, the present invention provides a light generation means for generating light modulated according to information, and a method for generating light generated by the light generation means. The fθ lens system includes a deflection scanning means for deflecting and scanning the deflected light, and an fθ lens system for forming an image on a recording medium of the light deflected and scanned by the deflection scanning means, and the fθ lens system is configured to perform the deflection scanning and the recording. A first lens and a second lens are arranged in order from the deflection scanning means side on the optical path of the light guided onto the medium, and the light entrance surface and the light exit surface of the second lens are arranged within the polarization plane, respectively. When the lens surface shape of is an aspherical shape other than a circle, and the focal length of the fθ lens system is f, the distance between the light exit surface of the second fθ lens and the image plane is f/22 to f/ 3.

(Function) The fθ lens characteristics are satisfied even for a wide scanning angle of approximately ±45°, and the optical path from the polarization scanning means to the recording medium is shortened.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 2 shows a laser printer as a recording device according to the present invention, and numeral 52 in this figure is a housing. A photosensitive drum (recording medium) 54 that is rotated in the direction of the arrow is provided in the center of the housing 52.
An exposure device 56 is provided below 4. The exposure device 56 exposes a predetermined portion on the photoreceptor drum 54 based on image information, and sequentially from this exposed portion along the rotational direction of the photoreceptor drum 54, the developing device 58, the transfer roller 60, the static eliminator, etc. lamp 62 and charger 6
4 is placed. In addition, the bottom of the housing 52 has a removable
A paper feed cassette 66 and a fixing device 68 are provided at the upper part of the housing 52, respectively. Paper cassette 66
The paper p inside is first fed by a paper feed roller 70, and then aligned by an aligning roller 72.

Next, the aligned paper p passes through the transfer section between the photosensitive drum 54 and the transfer roller 60 and the fixing device 68 in order, and is sent to a pair of paper ejection rollers 74 . The paper is discharged onto a paper discharge tray 76 at the top of the housing 52. Note that 78 is a control device disposed between the paper feed cassette 66 and the paper discharge tray 76.

When forming an image, the surface of the photosensitive drum 54 is first uniformly charged by the charger 64.

Next, this charged photoreceptor drum 54 is transferred to the exposure device 5.
Exposure scanning is performed by 6.

As a result, an electrostatic latent image is formed on the photoreceptor drum 54''. Next, a developer is supplied to this electrostatic latent image by the developing device 58, whereby the electrostatic latent image is visualized and a developer image is formed. Next, this developer image is transferred onto the paper p by a transfer roller 60 in a transfer section. The developer image transferred onto the paper p is fixed onto the paper p by a qualitative device 68, and then ejected onto a paper ejection tray 76 by a paper ejection roller 74. On the other hand, after the developer image has been transferred onto the paper p, the surface of the photosensitive drum 54 is neutralized by the static eliminating lamp 62, and is prepared for the next cycle.

As shown in FIGS. 3 and 4, a hole 80 is provided at the bottom of the housing 52, and the exposure device 56 is attached and detached through this hole 80. L-shaped holes 84 are provided on both sides of the case 82 of the exposure device 56.
Additionally, four portions 86 are provided on one side other than these. On the other hand, the housing 52 is provided with a pair of pins 88 that engage with the grooves 84 and a convex portion 90 that fits into the "-2" recess 86. When attaching the exposure device 56 to the housing 52, insert the exposure device 56 from the corner of the housing 52 by aligning the bin 84 with the bin 88, and then slide it horizontally.
When the bottle 88 is located at the end of the full length 84, the convex portion 90 of the housing 52 and the concave portion 86 of the case 82 fit together. As a result, the exposure device 56 is restricted from moving vertically and in the rotational direction, and is positioned relative to the photoreceptor drum 54.

The exposure device 56 is fixed to the housing 52 with screws 92 in this positioned state. As a result, the exposure device 56
can be attached and detached independently, and is held in place even when the screws 92 are removed, making it easy to assemble, positioning, and 2! No adjustment is required.

As shown in FIGS. 5 and 6, a laser unit 94 is provided on the upper surface of the case 82 of the exposure device 56. As shown in FIGS. This laser unit 94 includes a semiconductor laser 96.
and a collimator lens 98 are integrally set. This semiconductor laser 96 generates a laser beam L modulated according to image information, and this generated laser beam L is collimated by a collimator lens 98. Further, this laser unit 94 has an optical axis (
That is, the optical axis of the laser beam L collimated by the collimating lens 98) is the rotation axis.

Further, the laser unit 94 includes a semiconductor laser 96
It is attached to the case 82 so that the optical axis of the laser beam L emitted from the mirror is substantially perpendicular to the optical scanning plane between the rotating polygon mirror 100 and the first fθ lens 102, which will be described later.

As shown in FIG. 5, a prism unit 104 is provided inside the case 82. This prism unit 104 has two triangular prisms 106, and both ends of these triangular prisms 106 are held by a holding member 110 by one leaf spring 108, respectively, so that their apex angles are in the same direction. Been name. The laser beam L collimated by the collimator lens 98 is
These two triangular prisms 106 allow the photosensitive drum 5 to
4, the cross-sectional shape is reduced in only one direction so as to have an appropriate spot size, and at the same time, it is deflected in a substantially perpendicular direction.

Further, this holding member 110 is attached to a hole 11 provided in the case.
2, and is fixed at a desired position by screws 114. Therefore, by finely adjusting the rotational position of the holding member 110 using this screw 114, the light emitted from the triangular prism 106 can be finely adjusted in a desired direction.

As shown in FIG. 6, a deflection scanning device (deflection scanning means) 116 is provided inside the case 82. This deflection scanning device 116 has a motor 118, and this motor 1
18 is attached to the upper wall of the case 82. A shaft 120 of this motor 118 projects downwardly, and a rotating polygon mirror 100 is attached to this shaft 120. This rotating polygon mirror 100 deflects and scans the laser beam L deflected by the triangular prisms 106 and 106 by a width corresponding to the recording area on the photosensitive drum 54. Here, the rotating polygon mirror 100 is ±45
A hexagonal columnar structure having six reflective surfaces 122 on its side surfaces is used so that wide-angle scanning of about .degree. can be performed.

That is, the distance from the rotating polygon mirror 100 to the photosensitive drum 54 is 258 io+, whereas it is only 261 am in the conventional device shown in FIG. In addition, the conventional device
Since it is for 4 sizes, if a scanned area with a width of 210m + 11 is scanned with an fθ lens with a focal length of 1vf - 215cm,
θ-(1/2) X (210/f)(rad)
-(1/2)X (210/215)X (18
0/π) - ±28", and an octagonal prism-shaped rotating polygon mirror with eight reflective surfaces on the side is used, but since the device in this example is for A4 size, it is not effective. Assuming that the focal length f is approximately the same as 209io+ and a scanned area with a width of 297a+a+ is to be scanned, θ~(1/2)X
An angle of view of (297/209)X (180/π)−±41° is required.

For this reason, a hexagonal column is used.

Also, inside the case 82 are first and second fθ lenses 1.
An f-theta lens system 126 consisting of 02, 124 and first and second mirrors 128, 130 are provided. The second fθ lens 124 is provided at the top, and the second fθ lens 124
0 is located near the motor 118. The laser beam L deflected and scanned by the rotating polygon mirror 100 passes through the first fθ lens 102 and is sequentially reflected by the first and second mirrors 128 and 130.

The laser beam L reflected by the second mirror 130 is
After passing between the rotating polygon mirror 100 and the first fθ lens 102 and further passing through the second fθ lens 124, the light is focused on the photoreceptor drum 54'2 and is directed in the width direction (main direction) of the photoreceptor drum 54. scan direction). Here, the first f
The width of the θ lens 102 is such that the effective scanning angle is, for example, ±40°.
It is set so that First fθ lens 102
Since the lens has a refractive index of 1F, the width of the second fθ lens 84 is set so that the effective scanning angle is, for example, ±32°. A third mirror 132 is provided on one end side of the second fθ lens 124 (see FIG. 4),
This third mirror 132 is configured such that a laser beam L having a scanning angle of +32' or more is reflected in a direction perpendicular to the shaft 120 of the motor 118, and a part of the laser beam L is detected by a photoelectric converter 134. , is located.

The photoelectric conversion probe 134 obtains a signal for controlling the recording start position.

The light entrance surfaces 102a, 124a and the light exit surface 10 of the first and second fθ lenses 102° 124, respectively.
2b and 124b are each formed into an aspherical shape.

That is, as shown in FIG.
Reference numeral 2 is a lens having a positive refractive power, with a light entrance surface 102a being a concave surface and a light exit surface 102b being a convex surface. The second fθ lens 124 is a lens having a concave optical axis portion 124al and convex portions 124a2 on both sides of the optical axis portion 124al of both the light entrance surface 124a and the light exit surface 124b, and which has negative refractive power at least on the optical axis “2”. It is.

Here, the light entrance surfaces 102a, 124a and the light exit surfaces 102b, 124b of the first and second fθ lenses 102° 124 respectively have the following three-dimensional coordinates:
If the deflection surface is the y-z plane and the optical axis is the two axes, then the relationship between the height 2 of the lens surface and the distance y on the y-z plane is % Formula % However, RD is the radius of curvature, AD, AE, A.F.

AG are 4th, 6th, 8th, and 10th aspheric coefficients, respectively, and are 1ADl+1AEl+1AFl+...≠0.

The first fθ lens 102 has a light entrance surface 102a.
The curvature of is 1/lRD+l, and the curvature of the light exit surface 102b is 1
/ l RD2 When 1, the following relationship holds true. Note that when the lens surface is concave with respect to the object side (semiconductor laser 96 side), RD is negative.

Further, the second fθ lens 124 has a curvature of 1/1RD31 of the light entrance surface 124a and a curvature of 1/1RDal of the light exit surface 124b, then RL
13 relationships are established.

Furthermore, the light entrance surface 102a and the light exit surface 102b of the first fθ lens]02, and the light entrance surface 124a of the second fθ lens 124 are arranged so that the curve 2 expressed by the above relational expression is aligned with the optical axis (two axes). It is formed in a rotated shape.

Furthermore, the light exit surface 124b of the second fθ lens 124 is
It is formed into a toric surface obtained by rotating curve 2 expressed by the above relational expression about a straight line parallel to the y-axis and at a distance of Z-CVX from the lens surface on the optical axis (z-axis) as the rotation axis. There is.

Here, the ranges of AD, AE, AF, and AG are: l AD l <100/(AP)', I
AE l < 100ha^P)6゜l AFI < 1
00barΔP)8. ! AG l < too/(
AP)' range. This range is a range in which the coefficient is not too large for the order and the lens does not have an ungainly shape.

In addition, the equation representing the above-mentioned toric surface is the above-mentioned curve 2, assuming that the intersection point of curve 2 and the optical axis (two axes) is z-0 and the position of the rotation axis is z-1/cvx in FIG. and x2+ (1/cvx -ZL) 2- (1/cv
x −z) and 1/cvx −(1/CVX −Zl) 2
+x2. Further, although the curve 2 is within the yz plane, that is, within the beam deflection plane, it may be slightly shifted from the yz plane if stray light from the photoreceptor drum 54 is taken into consideration.

Further, a light entrance surface 102a of the first fθ lens 102,
The distance from the reflection point of the laser beam L on the rotating polygon mirror 100 is 1.5, where f is the combined focal length of the first and second fθ lenses 102 and 124. ff-f/15-f
/3 range. For example, this distance is 30
aun, and if f=209+am, then i/f-30/20917 in this case.

In addition, the first fθ lens 102 has a distortion factor (y−fθ)/y [y is the beam position on the image plane] of its fθ characteristic of 1.
It is set to be within 0%.

Further, the distance d from the light exit surface 124b of the second fθ lens 124 to the image plane (photoreceptor drum 54) is d if the combined focal length of the first and second fθ lenses 102°124 is f. - It is set in the range of f/22 to f/3. For example, if this distance d is 30Il111 and f-209+am, then in this case d/f-
30/209-1/7.

Further, the second fθ lens 124 is set so that the amount of correction of the field curvature created by the light beam within the deflection plane is approximately 0 to 30I1m.

The first and second f-theta lenses 102 and 124 are made of a plastic material such as acrylic and have a rectangular shape. Further, the second fθ lens 124 is shown in FIGS.
As shown in FIG. 11, a portion (used portion) 140 excluding the peripheral edge portions of two opposing surfaces is formed to have a concave cross-section in a direction along the optical axis. A light entrance surface 124a and a light exit surface 124b are formed in this use portion 140, and the peripheral portion thereof serves as a reinforcing portion 142. Furthermore, a thinned portion 144 having a concave cross section is formed on the surface where the light entrance surface 124a and the light exit surface 124b are not formed.

According to the above configuration, the following effects can be obtained.

(1) Since the light entrance surfaces 102a, 124a and the light exit surfaces 102b, 124b of the first and second fθ lenses 102°124 are aspherical, ±45°
The fθ characteristic can be satisfied for a relatively wide scanning angle, and the curvature of field can be corrected. This allows the device to be made smaller and less expensive.

That is, the modulation of the semiconductor laser 96 is normally performed in proportion to time. On the other hand, the rotating polygon mirror 100 rotates at a constant angular velocity. If the case where the collimated laser beam L is incident on the rotating polygon mirror 100 is considered as an object point at infinity, the height of the object point is proportional to Lan θ with respect to the incident angle θ of the rotating polygon mirror 100. However, the reflected laser beam L is transmitted through the first and second fθ lenses 102.・1
24, and an image height y proportional to the incident angle θ is obtained. This image 1'jIY is expressed as y-fθ (rad). Here, the first and second fθ are applied to an aspheric surface such that f is constant not as a focal length but as a proportionality coefficient.
A lens is formed. Therefore, sufficient fθ characteristics can be satisfied even for a wide scanning angle.

In this case, the first fθ lens 102 has a light entrance surface 102
The lens a is a concave surface, and the light exit surface 102b is a convex surface, which is an aspherical lens having positive refractive power, exhibits a θ characteristic as shown in FIG. 12, and also produces curvature of field. However, in the second fθ lens 124, the optical axis portion 124al of both the light entrance surface 124a and the light exit surface 124b is concave, and both side portions 124a2 thereof are convex, and the refractive power of the member is white on the optical axis. Since it is an aspherical lens, it has negative refractive power along the optical axis, but as it moves away from the optical axis, it temporarily loses positive refractive power. Therefore, the curvature of the image plane created by the fθ characteristic of the first fθ lens 102 and the light beam in the yz plane can be corrected. That is, when the refractive power of the first fθ lens 102 is positive, the refractive power of the second fθ lens 124 is negative.
The Petzval curvature can be approximated to 0 as much as possible,
As a result, it is possible to satisfy fθ characteristics even for a wide scanning angle of about ±45 degrees, and it is also possible to prevent curvature of the image plane caused by the light beam in the yz plane.

Therefore, since the laser beam L can be scanned over a wide scanning angle, the optical path from the rotating polygon mirror 100 to the photosensitive drum 54 can be shortened, thereby making it possible to downsize the apparatus. Further, since the parallel laser beam L can be focused on the photosensitive drum 54 on the YZ plane, the resolution can be improved.

In addition, since the light exit surface 124a of the second fθ lens 124 is a toric surface, the image created by the light beam in the cross section perpendicular to the xz plane can be corrected separately from the field curvature correction on the yz plane. Curvature of the surface, that is, surface tilt in the X-axis direction (sub-scanning direction) can be corrected. That is, even if the reflecting surface 122 of the rotating polygon mirror 100 is tilted due to machining accuracy or the like, the laser beam L can be focused on the yz plane. That is, the surface tilt correction image plane in the sub-scanning direction can be made to coincide with the focal point of the parallel laser beam on the yz plane, that is, the image plane.

Note that as the rotating polygon mirror 100 rotates, the reflection point on the reflection surface 122 also moves, for example, by about 2 cm, but since this movement of the reflection point is along the optical axis of the incident laser beam L, the reflection is If it is perpendicular to the pupil, it can be treated as a movement of the pupil, and it is considered that the position of the image is not affected. However, the depth of focus corresponding to the diameter of the incident laser beam L is required for the surface tilt correction optical system. Here, the depth of focus becomes deeper when the light exit surface 124b of the second fθ lens 124, which is a toric surface, is brought as close to the image side, that is, as close as possible to the photoreceptor drum 54, because the vertical magnification becomes smaller. This is advantageous for surface tilt correction. The depth of focus is, for example, beam diameter ±2.5 mm.
degree is required.

(2) The light entrance surface 102a and the light exit surface 102b of the first f.theta. Distance Ml from the reflection point of
Since it is set to no f/15 to f/3, the rotating polygon mirror 100
and the first fθ lens 102, the second mirror 13
The interval can be set such that the laser beam L reflected at 0 can pass between the rotating polygon mirror 100 and the first fθ lens 102.

That is, if I-i/15-14 (am), the outer end of the rotating polygon mirror 100 and the first fθ lens 102
and the light incident surface 102a is 2■, less than this ()
<1/15), the laser beam L reflected by the second mirror 130 cannot pass through. vice versa,
If the fθ lens 102 is made large, the first fθ lens 102 will hit the first mirror 128, and
Since the f.theta. lenses 102 and 124 must be made long and thick, a large number of lenses are required and a large amount of molding time is required, resulting in high costs. For such design reasons, it is desirable that f/151<f/15) be the maximum.

In addition, the first fθ lens 102 has a distortion rate (y−fθ)/y [y is the beam position on the image plane] of its fθ special characteristic of 1.
Since it was made to be within 0%, the second fθ lens 12
Both the distortion at 4 and the curvature of the field created by the light beam within the deflection plane can be largely corrected.

In other words, the first fθ lens 102 exhibits its original performance when combined with the second fθ lens 124, but unless the distortion is corrected to a considerable extent by the first fθ lens 102, the second fθ lens 124 Both distortion and field curvature cannot be significantly corrected with an fθ lens. Therefore, by correcting distortion to a considerable extent with the first fθ lens 102, the second fθ lens 124 can be used mainly for correcting field curvature. The distortion of the first fθ lens 102 used here is, for example, the first
As shown in Figure 2, the maximum is (y-fθ) /Y-8,7
/164-5.3%.

(3) The light entrance surface 124a and the light exit surface 124b of the second fθ lens 124 have an aspherical shape such that the lens surface on the yz, jli plane has a shape represented by curve 2 in 1-, and The distance d from the light exit surface 124b of the second fθ lens 124 to the photosensitive drum 54 is d−f
/22 to f/3, sufficient surface tilt correction (correction of the curvature of the image plane created by the light beam in the cross section perpendicular to the deflection surface) can be performed and there is no problem in design.

That is, the second fθ lens 124 is connected to the photoreceptor drum 54.
If it is too far away, problems such as hitting the motor 118 will occur. Furthermore, the refractive power of the toric surface-shaped light exit surface becomes weak, and the beam diameter in the sub-scanning direction (direction along the x-2 plane) becomes large. Furthermore, if it is df/3 or more, sufficient surface tilt correction cannot be obtained. On the contrary, d
As the lens size is made smaller, the thickness of the lens must be increased, which is disadvantageous in manufacturing. For example, when df/22 or less, the lens thickness becomes 25a++a or more. Therefore, d-
In terms of design, it is desirable to set it to f/22 to f/3.

Further, since the second fθ lens 124 is configured such that the amount of correction of the curvature of the field created by the light beam within the deflection plane is approximately 0 to 30a++a, it is possible to perform even more sufficient tilt correction.

That is, as described above, since the first fθ lens 102 mainly has fθ characteristics, the curvature of field cannot be reduced. Therefore, the field curvature correction ability of the second fθ lens 124 needs to be relatively large. However, the second fθ
The lens 124 slightly corrects the fθ characteristic and also corrects the tilt, so the amount of correction is quite large.
! However, the first fθ lens 102 can reduce the field curvature to within 30IiII1. Therefore, the correction m of the field curvature of only the second fθ lens 124 may be about 0 to 30 mm.

Here, FIG. 13 shows the curvature of field when only the first fθ lens 102 is used. The curvature of the field created by the light beam within the deflection plane is about 7II1m, which is
This is corrected by the fθ lens 124. In addition, the curvature (surface tilt) of the image plane created by the light beam in the cross section perpendicular to the deflection plane is about 25II1m, but this is mainly due to the second fθ
The light is corrected by the light exit surface 124b of the lens 124 having a toric surface shape.

(4) The first and second fθ lenses 102° 124 are made of plastic and molded into aspherical shapes, so they can be provided with high precision and at low cost.

That is, the first and second fθ lenses 102.12
As shown in FIGS. 14 to 16, the aberration characteristics of lens No. 4 are very well corrected at room temperature (25° C.). Changes caused by temperature, mainly due to changes in the refractive index of plastic materials, are still within the usable range.

In addition, since both the first and second fθ lenses 102° 124 are formed in a short Ill shape, the occupied volume is small, thereby shortening the molding time and reducing costs.

In addition, the second fθ lens 124 has a portion (used portion) 140 of the two opposing surfaces excluding the peripheral edge portions formed to have a concave cross section in the direction along the optical axis, and this used portion 140 has a light incident surface. Since the light entrance surface 124a and the light exit surface 124b are formed, the light entrance surface 124a and the light exit surface 124b can be formed individually, and accuracy can be improved.

Further, on the surface where the light entrance surface 124a and the light exit surface 124b are not formed, a thinned portion 1 having a concave cross section is formed.
44, the molding time can be shortened without reducing the strength.

In other words, precision plastic lens molding usually requires 1
■It takes 1 minute per thickness. Therefore, the thinner the lens is, the more advantageous it is in terms of accuracy and cost. However, simply making the lens thinner will cause it to warp. In addition, the lens usage part 140 is located at the central part.
This is the part about +a+width. Therefore, by providing the cut-out portion 144, unnecessary molding can be omitted and molding time can be shortened, and the strength can be reduced.

(5) The first and second mirrors 128, 1 are arranged so that the laser beam L reflected by the second mirror 130 passes between the rotating polygon mirror 100 and the first fθ lens 102.
30, the first fθ lens 1
02 and the first mirror 128 (the second
Shown in Figure 1. ), the distance between the rotating polygon mirror 100 and the first mirror 128 is shorter, and the entire optical system can be made more compact.

(6) Since the second mirror 128 is placed near the rotating polygon mirror 100, the first mirror 128 and the rotating polygon mirror 10
The distance from zero is shortened, and the entire optical system can be made smaller.

Moreover, the motor 118 is attached to the upper wall of the case 82,
A rotating polygon mirror 100 is attached to the shaft 120 of this motor 118.
Since the motor 118 is mounted diagonally downward, when the second mirror 130 is placed near the rotating polygon mirror 100, the motor 118
does not get in the way.

FIG. 17 shows a modification of the second fθ lens. Light exit surface 1 of second fθ lens 152 in this modification
52b is formed into a toric shape whose cut surface has a constant curvature when cut along a plane parallel to the xz plane, that is, as shown in FIG. 18, the curvature is constant regardless of the distance My. This toric equation is +A from the equation representing curve 2 written in J- and (z2-z) 2+x2-1/ cvx2.
Dy' +^By6 +APy8 +AGylO+
...becomes... Note that the light incidence surface 152a is
It has the same configuration as the light entrance surface 124a of the fθ lens 124.

According to such a configuration, processing is easy when making a female mold for plastic molding, especially when polishing.

FIG. 19 shows another modification of the second fθ lens. The second fθ lens 162 in this modification has a light entrance surface 162a as a concave surface, and a light exit surface 162b as the optical axis portion 162.
2 old is concave, and both side portions 162b2 are convex,
This lens has negative refractive power a on the optical axis.

Here, the light entrance surface 162a and the light exit surface 162b of the second fθ lens 162 are respectively on the yz plane in three-dimensional coordinates, assuming that the deflection surface is the yz plane and the optical axis is the two axes. Relational expression between the height 2 of the lens and the distance my % Formula % However, RD is the radius of curvature, AD, AE, AP.

AC are 4th, 6th, 8th, and 10th aspheric coefficients, respectively, and are 1ADl+1AEl+IAFl+...≠0.

Also, the curvature of the light entrance surface 162a is 1/1RD31, and the curvature of the light exit surface 162b is 1/1RD31.
When al, the following relationship holds true.

Furthermore, the light entrance surface 162a of the second fθ lens 162 is a toric surface formed when the curve 2 expressed by the above equation is rotated around a constant rotation axis that is parallel to the y-axis and intersects with the two axes. It is formed.

Here, as shown in FIG. 20, when the light exit surface 162b is a toric surface, the beam diameter in the sub-scanning direction tends to become small at a large angle of view;
When 2a is a toric surface, on the contrary, it tends to become large at large angles of view. Since the amount of beam light decreases as the angle of view increases, it is better to make the light incident surface 162a a toric surface in order to make the size of the image uniform on the photoreceptor drum 5:4.

[Effects of the Invention] As explained above, according to the present invention, the fθ lens characteristics can be satisfied even for a wide scanning angle of about ±45°, and the optical path from the deflection scanning means to the recording medium can be shortened. The device can be made smaller and have a lower 41ti, and it also has excellent effects such as being able to correct the curvature of the field created by the beam in the cross section perpendicular to the deflector without causing any design problems. play.

[Brief explanation of the drawing]

The drawings show one embodiment of the present invention; FIG. 1 is a diagram schematically showing the configuration of an optical system of an exposure device, FIG. 2 is a cross-sectional view schematically showing a laser printer, and FIG. 3 is an exposure device. FIG. 4 is a cross-sectional plan view showing how the exposure device is installed; FIG. 5 is a view showing the l/-zer unit and prism unit; FIG. 6 is a vertical cross-sectional view showing the exposure device;
FIG. 7 is a diagram for explaining the shape of the lens, FIG. 8 is a front view showing the second fθ lens, and FIG. 9 is a diagram for explaining the shape of the lens.
10 is a sectional view of the θ lens taken along line A-A, FIG. 10 is a sectional view taken along line BB of the fθ lens shown in FIG. 8, and FIG. 11 is a sectional view taken along line C-C of the fθ lens shown in FIG. Cross-sectional view along, 12th
The figure shows the fθ characteristics of the first fθ lens, FIG. 13 shows the relationship between the curvature of field and the scanning angle due to the first fθ lens, and FIG. 14 shows the fθ characteristics of the first fθ lens. Figure 15 is a diagram showing the relationship between the curvature of the field created by the light flux in the deflection plane and the scanning angle, and Figure 16 shows the curvature of the field created by the light flux in the cross section perpendicular to the deflection plane. A diagram showing the relationship with the scanning angle, FIG. 17 is a diagram showing a modification of the second fθ lens, FIG. 18 is a diagram for explaining the shape of the second fθ lens in the modification, and FIG. 19 is the second fθ
FIG. 20 is a diagram showing the relationship between the scanning angle and spot size, and FIG. 21 is a diagram showing a conventional example. 96...Light generating means (semiconductor laser), 116...
- Deflection scanning means (deflection scanning device), 54... Recording medium (photosensitive drum), 102... First fθ lens, 1
24.152,162...second fθ lens, 124
a, 152a, 162a...light incidence surface, 124b, 1
52b, 162b...light exit surface, 126...fθ lens system. Applicant's representative Patent attorney Takehiko Suzue Figure 1 Figure 3 Figure 4 Kaya 1 f-theta lens! = Waving Elephant Bay III (mm) 1st
3 Figure 4i ``Within 1 plane'' was created with a request and was sent to Wangsho (
mm) Partial l1 FIfrl = dir intersecting assertive light coming l
Futagami 77th work Five statues of Bay Sho
) Figure 18・E

Claims (2)

[Claims] (1) A light generating means that generates light modulated according to information, a deflection scanning means that deflects and scans the light generated by the light generating means, and a recording medium that transmits the light deflected and scanned by the deflection scanning means. an fθ lens system that forms an image on the
This fθ lens system has a first and a second lens arranged in order from the deflection scanning means side on the optical path of the light deflected and guided onto the recording medium, and the light of the second lens is The entrance surface and the light exit surface each have a non-circular lens surface shape in the polarization plane, and the above f
When the focal length of the θ lens system is f, the second fθ
A recording device characterized in that the distance between the light exit surface of the lens and the image surface is f/22 to f/3. (2) The recording apparatus according to claim 1, wherein the second fθ lens corrects the curvature of the field created by the light beam in the polarization plane by 0 to 30 mm.