JPH02157809A - High resolution scanning optical system - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention is a high-resolution laser beam printer that can be used in high-resolution laser beam printers that deflect and scan a light beam from a light source by rotating or vibrating a light deflector about one axis. Related to resolution scanning optical system.

[Prior Art] In recent years, it has been desired to improve the recording density of image recording devices such as laser beam printers. However, in order to realize this, a high-resolution scanning optical system is required that can reduce the spot diameter of the light beam projected onto the surface to be scanned.

Generally, when designing a scanning optical system that only requires ordinary resolving power, it is often sufficient to correct only the three aberrations of astigmatism, field curvature, and distortion. When designing the system, it is necessary to satisfactorily correct all five aberrations, which are the three aberrations plus spherical aberration and coma aberration.

An example of such a high-resolution scanning optical system is an f/theta lens (the ideal image height is a focal length f and the angle of incidence θ).

[Problem to be solved by the invention] However, in these conventional high-resolution scanning optical systems,
Tilt correction (even if the deflection reflection surface of the light deflection means tilts in the direction in the sub-scanning direction (sub-scanning direction) perpendicular to the main-scanning direction plane formed by the light beam to be deflected and scanned, the beam on the scanned surface Since there is no consideration given to the fact that the imaging position in the sub-scanning direction of the image does not change much, when performing deflection scanning using a rotating polygon mirror or vibrating mirror that rotates or vibrates around one axis, the sub-scanning position of the deflection reflecting surface is The tolerance for inclination in the scanning direction becomes very strict. Therefore, an object of the present invention is to provide a scanning optical system having both high resolution and tilt correction function.

[Means for Solving the Problem] In order to achieve the above object, in the scanning optical system of the present invention, a small part of the lens surface of the lens constituting the optical system (
In both cases, the shape of one lens surface deviates from a circle with respect to a cross section in the sub-scanning direction perpendicular to the main-scanning direction plane. That is, an aspherical component is introduced into the cross-sectional shape of one lens surface in the sub-scanning direction.

[Function] In a high-resolution scanning optical system with a tilt correction function configured as described above, spherical aberration in the sub-scanning direction tends to occur due to the refractive power in the sub-scanning direction which is considerably larger than the refractive power in the main scanning direction. is reduced by a small aspherical component introduced into the cross-sectional shape of one lens surface in the sub-scanning direction, and satisfies the high-resolution specification with a small F-number.

[Embodiment] FIG. 1 is a sectional view in the main scanning direction of an embodiment of the scanning optical system of the present invention, and FIG. 2(a) is a sectional view in the sub-scanning direction including the optical axis.

In the figure, 1 is a linear imaging cylindrical lens 2 which has refracting power only in the sub-scanning direction, and 3 is a rotating polygonal mirror that rotates around an axis O perpendicular to the main scanning direction plane. The f/θ lens 4, which is composed of two spherical lenses 3a and 3b and one toric lens 3C, is a surface to be scanned.

In this embodiment, the cross-sectional shape in the sub-scanning direction of the lens surface As ph on the incident side of the cylindrical lens 1 is as shown in FIG.
) is deviated from the circular shape shown by the broken line. This deviation is
In the above configuration, in which the absolute value of the radius of curvature increases from the center to the periphery of the lens surface Asph, the parallel beam formed by the laser light source and the collimating optical system (not shown) is transmitted through the cylindrical lens 1.
The light is refracted only in the sub-scanning direction, and is reflected in the main-scanning direction onto the deflecting reflection surface (almost perpendicular to the main-scanning direction surface) of the polygon mirror 2, which is placed near the focal position of the lens 1 in the sub-scanning direction. Connect the line images that extend linearly.

This line image is reflected by the deflection reflection surface and enters the f/theta lens 3, where it is refracted and forms a spot on the surface to be scanned 4. Since the toric lens 3C has different refractive powers in the main scanning direction and the sub-scanning direction, the f/theta lens 3 that includes it also has different refractive powers in both directions, and therefore the beam imaging position in both directions of the line image extending in the main scanning direction can be made to coincide with the surface to be scanned 4. In this way, since the deflection reflection surface of the rotating polygon mirror 2 and the scanned surface 4 have a conjugate relationship in the sub-scanning direction, even if the deflection reflection surface of the polygon mirror 2 falls in the sub-scanning direction, the deflection reflection surface of the polygon mirror 2 is tilted in the sub-scanning direction. The imaging position of the beam in the sub-scanning direction hardly changes. That is, the tilt correction function can be achieved.

In this way, since the scanning optical system equipped with the tilt correction function includes the f/θ lens 3 as described above, the refractive power is different in the main scanning direction and the sub-scanning direction, and the latter is several times as large as the former. . Therefore, in addition to the spherical aberration of the cylindrical lens 1 in the sub-scanning direction, a large spherical aberration in the sub-scanning direction tends to occur as a whole. This is F in the conventional low resolution specification.
Since the f-number is large and the depth of focus is large, and the occurrence of spherical aberration itself is small, it was not a particular problem. Since the spherical aberration itself also occurs significantly, it is difficult to meet the specifications without reducing these spherical aberrations in the sub-scanning direction.The present invention solves these problems by This is solved by introducing an aspherical component into the cross-sectional shape of the lens in the scanning direction to generate spherical aberration in a direction that cancels out the spherical aberration in the sub-scanning direction.

The configuration and surface shape of each lens of this example will be explained in detail below.

A cross section in the sub-scanning direction of the lens surface on the incident side of the line image forming cylindrical lens 1 shown in FIG. 2(a) is expressed by the following equation.

Z”/r.

X=L++Zr
s) 2+DZ'+EZ'+FZ'
+GZ"・ ・ ・ ・ ・ ・ ・ (1) However, X and Z are the coordinates of the optical axis direction A and the sub-scanning direction B, respectively, with the center point (apex) of the lens surface as the origin, r8
and K are the radius of curvature and conic coefficient in the sub-scanning direction, D,
E, F, and G are 4th-order, 6th-order, 8th-order, and 10th-order aspheric coefficients, respectively.

This cross-sectional shape in the sub-scanning direction is shown in FIG.

As shown in FIG. 2(a), the line image forming cylindrical lens l is composed of one lens, and the surface on the light exit side is flat. This is because it is easy to manufacture. As mentioned above, the cross-sectional shape in the sub-scanning direction of the lens surface Asph into which the aspherical component is introduced is such that the absolute value of the radius of curvature increases from the center to the periphery. This is to generate an over spherical aberration on this surface in order to correct the under spherical aberration in the sub-scanning direction that would occur if no aspherical component was introduced.In this way, it is possible to correct the spherical aberration well.

Further, the reason why an aspherical component is introduced into the cylindrical surface is that in an anamorphic lens surface such as a cylindrical surface or a toric surface, the curvature in the main scanning direction and the curvature in the sub-scanning direction can be set independently. That is, by introducing an aspherical component in the sub-scanning direction of such an anamorphic surface, aberrations only in the sub-scanning direction can be controlled without affecting aberrations in the main-scanning direction at all.

Furthermore, since an aspherical component is introduced into the cylindrical surface on the incident side of the cylindrical lens l, there is no need to process the surface of the rotating polygon mirror 2 on the light source side that receives the light beam that does not have a certain angle into an aspherical surface. . Therefore, the area to be aspherically processed is small (and manufacturing is easy).

Next, the f/theta lens 3 has a three-element configuration as described above, but this is because the high-resolution f/theta lens 3 needs to properly correct all five aberrations, and the two-element configuration requires only a few degrees of freedom. This is because there is a shortage of anamorphic lenses, and to simplify manufacturing by minimizing the number of anamorphic lenses.

In addition, a toric lens 3c constituting the f/θ lens 3
The lens surface on the polygon mirror 2 side is a cylindrical surface with an infinite radius of curvature in the main scanning direction and having refractive power only in the sub-scanning direction (FIG. 2(a) ff). This is to correct field curvature and facilitate manufacturing.

Furthermore, lenses 3a, 3b, 3 constituting the f/θ lens 3
The center of curvature of each surface c is on the polygon mirror 2 side in both the main scanning direction and the sub-scanning direction, except that the radius of curvature of the incident side surface of the toric lens 3C in the main scanning direction is infinite.

This is to satisfactorily correct all five aberrations.

Next, FIG. 2(b) shows an embodiment in which the cross-sectional shape in the sub-scanning direction of the toric surface Asph on the light exit side of the toric lens 3C constituting the f/θ lens 3 deviates from a circular shape.

In this embodiment as well, the second
What has been said regarding the embodiment of figure (a) applies.

Finally, Examples 1 to 1O, which are numerical examples of the present invention, will be listed. In each numerical example, fo is the focal length of the line image forming cylindrical lens 1, and f2 is f・θ
The focal length of the lens 3 in the main scanning direction (M indicates the meridional plane), f, f is the focal length of the lens 3 in the sub-scanning direction (S indicates the sagittal plane), F so, v and F
so and s are image-side effective F numbers of the f-theta lens 3 in the main scanning direction and sub-scanning direction, respectively, ω is a half angle of view, and e is a wavelength.

The surfaces are numbered in the order in which the light rays advance, and the first to second surfaces are each surface of the line image forming cylindrical lens 1, the third surface is the mirror surface (diaphragm) of the rotating polygon mirror 2, and the fourth surface is the mirror surface (aperture) of the rotating polygon mirror 2. to the ninth surface represent each surface of the f/θ lens 3. ri is the radius of curvature of the first surface, d,' is the distance between the surfaces from the first surface to the i+1th surface,
n' is the refractive index of the medium behind the first surface, r4 and r,9
are the radius of curvature of the first surface in the main scanning direction (main scanning direction surface, i.e., meridional plane direction) and the sub-scanning direction (sub-scanning direction plane, i.e., sagittal plane direction), K, , D,
, Ei, F, and G are aspherical coefficients (shown in equation (1)) of the first surface in the sub-scanning direction, respectively.

Note that the refractive index of the part where the medium is air (n' = 1) is n'
The description of is omitted.

4 to 13 are examples 1 to 10, respectively.
It is a figure which shows the imaging performance of.

In each figure, (a) represents the achievable recording density,
(b) represents astigmatism and field curvature, 6M represents the image plane in the main scanning direction, ΔS represents the image surface in the sub-scanning direction, and (c) represents distortion (f·θ characteristics). In the figure (a), the spot diameter (diameter within a value of 1/e2 of the peak value) is calculated for each angle of view by simulation, and the result is converted into a realizable recording density and displayed. The vertical axis is the achievable recording density (dpi, number of dots per inch), and the horizontal axis is the scan angle (θ, half-width).

As shown in FIG. 14, this recording density was calculated assuming that the spots are arranged densely without overlapping each other in the main scanning direction, and are recorded by half overlapping each other in the sub-scanning direction.

As can be seen from FIGS. 4 to 13, in numerical examples with good performance, the entire surface of the F6 plate can be recorded at a recording density of 1000 dpi or more.

Example 1 f o = l O0, 00m m f, = 170.26mm fs = 41.95mfI F, 011 = 12 F, ), 1 = 252ω = 4
2.2° E = 780nm Cylindrical lens part rtv = co a' l = 10.00n
, = 1.51072 r+5 = 51.072 K + = O D, = -1,1626xlO"-' E, = F, = G, = O r 2 = (1) f・θ lens part r3 = ■ (Aperture ) d,' = 30.0Or
, =-38,430d4'=2.70n4' =1
．． 51072 rs ” 249. 127 ds ′
=2.90r6 = 1 33. 003
ds'=9.550,' =1. 5107
2 rt =-48,522d,' = 3. 0Or
8mm” da = 1
0. OOna' = 1. 78569
ras = 90.242 r, 14 = -103,098 r9s = -26,200 Example 2 fo = 101.59 mm f, = 169.89 mm fs = 42.67 mm F, O,, I = 12 F,,, ,=252ω=
42.2" E=780nm cylindrical lens part r,,=Q) al'= 10.00n'=
1.51072 rt5 = 51.884, = O D, = -1,1165 (Aperture) d,'=30.0゜r
4=-37,016d4'=2.70n,'=1.5
1072rs = -281,306d-' =3.59r
,""-143,466d,'=9.55ns'
=l, 51072 r, =-48,364d,' =3.0Or8M=■
d,′=10.00na′”
1. 78569 1",, knee 95, 609 r, Il"-101,083 r+as= 26.902 Example 3 fo = 100.00mm f, = 1 69. 95mm f==42. 92mm F,,,=12F,,,1=252c+,
+=42. 2' λ==780nm cylindrical lens part r, :Oo dl'= 10.00n+'
=1.51072 rt5=51.072, =OD, =-1,1392XlO-' E, =-8,3921x 10-" F, =G, =O rz=ω f・θ lens part r3=oo (Aperture) d,' = 30.00
r −” 36, 292 d < ′= 2
, 70n4' = 1, 51072 rs = 258.793 ds' = 3.70r
s ” 141.381 da'=9.55n6
'=1.51072 rt=47. 1 96 dt
=3. 0 Or, &1=co
da = 1 0. 0 On-' = 1.
78569 re-= 98.387 rsM= 1 03. 085 res=-27,327 Example 4 f,=102.09mm f,a=169.92mm fs=47i! , 86 m mF NO,M
= 12 F No, -= 252ω=42.
2' Circle=780nm cylindrical lens part r 1w=ω d,'=10.0On,'
= 1.51072 rt, = 52.137 to + = O D, = -1,1024 =c' (aperture) d,' =30.00
r, =-36,214d, =2.70n4' =1
．． 51072 rs = 255.852 d-' = 3.65r,
=-141.021 d6'"9.55ns '=1
．． 51072 r, =-47,010d,'=3.00d8' =lQ
,o.

r aM=■ na' ”1. 78569 r8g=-97,966 rev=-103,366 rom=-27,285 Example 5 f,=lO0,00mm f,,=169.93mm fs=42.62mm F , o, = 12 Fso, 5 = 252 ω = 42.
2' E=780nm Cylindrical lens part r++a:Q) cL'= 10.00n
,' =1.51072 r+s=51.072 to 1=O DI =-1,3253X 10-' El =3.4202X 10-9 F, =G, =O rz=(1) f・θ Lenz part rs ”■(Aperture) ds ' = 30.0O
r,=-38,470d,'=2.70n4'=1.
51072 rs = 249.396 ds' = 2.91
r6 = 133.327 d6' = 9.55ns
' = 1.51072 ry = 48.536 d7' = 3.00r
sb+=”d a ′= 10, 0
Ons' ” 1.78569 ras” 107.579 r,, = -103,143 r9s = 27.500 Example 6 f o = l O2, 16mm f, = 170.26mm fs = 42.63mm F No, v = 12 F −o, s= 252
ω=42.2° n=780nm Cylindrical lens part r,,=CX) al ′= 10.00n+
'=1.51072 r+5=52.176 K=0 DI=-1,0713XIO-'E, mF,
=G, =O r 2 =■ f・θ sin 1 part r, =co (aperture) ci3' =30.0
Or4=-38,430d4'=2.70n,'=
1.51072 rs = -249,127ds ”2. 90r
6 =-133,003ds' =9. 55n
s' ” 1- 51072 rt = 48. 522 d,'
=3. 001”,,=(f)
da ′= l O, OOn s ′ = 1
, 78569ras=-106,904 r9. =-to3. 098 r 911 = 27, 46 1 Example 7 fo = l O2, 15 mm f, = 169.91 mm f s = 42, 60 mm F so, -” 12 F, 4o, s = 2
52ω=42.2° N=780nm Cylindrical lens part r, M=co d+ ′=l O,0On
,' =1.51072 r l! l:" 52, 168, =0 D+ =-1,0712X10-' E, =-5,5509X 1 0-"F+ =
G, = O ra = circle f・0121 part r3 = ω (aperture) d,' = 30.0 Or
-=-38,494d,'=2.70n4'=1.5
1072 r5= 250.711 ds'=2.90ra
=-134,930d,"'9.55no = 1
．． 51072 rt = 48.534 dt' = 3.00r
sM=ωd, =10.0Ono
=l, 78569 r a9” I O6, 851 r9v= 103°551 r,, I knee 27.474 Example 8 fu =170.00mm Fso, m= 15.0 Fso, s=38.92
(,1=36° E=780nm cylindrical lens part r,,=OQ dt'=10.00n
r = 1, 63552 r+5 = 94.391 r 2 = f・0121 part r, = co (aperture) d,' = 35.56 r
4=-58,929d4'=3.00n4'=1.5
1072 ra'= 177.082 ds'=2.00r
s = 133.754 ds' = 12.61
ns' = 1.51072 ry = -62,212dt' = 0.50r8. =■
da'=12.90na'=1.7
6591 rs, knee 75.472 r, M=-1:31510 rss=-26,987, =-0,89074 D, =E, =F9 =G, =0 Example 9 f11=169.91mm F, . ,=15. OFNo, 9=24.32ω=36°
E=780nm Cylindrical lens part r+v:01) dt' =10.00
n+ ' = 1.63552 r + 9 = 57.905 r 2 = ■ f・0121 part ri = (X) (aperture) d, ' = 35.0 Or
, =-76,366d-'=3.00n,' =1.
51072 r s = 156°117 d, i'=2. o.

r a ” 83.589 do ’ = 12.0
0ns' =1.51072 r7=-63,501d7' =0.50rsv=■
da' = 13.00 na' =
1.76591 rss=-78,429 "g work=-126,538 r9s= 26.560 Ks=-0,79061 D9 =E, =F, =G, =0 Example IO f, =170.05mm FNo , m= 12 , 8 F, lo, -= 16
．． 32ω=366 λ=780nm Cylindrical lens part r 1M= (X) d t' =
10.00n1'=1.63552 r1s=57,905 r2=(1) f・0121 part r,,=a:+(aperture) d3'=35.
00r, =-42,831d,'=3.00n4'
=1.51072 r's = 54.891 ds'=0. 1
Or6 = 100.053 da'=16-
00ns ' 1. 51072 r, = 61. 281 d, ' = 13.
00 r ev = ω d,
=16.0Ona =l, 76591 rss=-64,435 r9. = -190,540 rss = -29,369 to 9 = -0,74999 D9 '' Es = F9 = Go = 0 [Effect] As explained above, according to the present invention, high resolution with tilt correction function is achieved. A scanning optical system can be realized, and its performance is capable of achieving a recording density of 1000 dpi or more over the entire 6th printing plate.

[Brief explanation of the drawing]

FIG. 1 is a sectional view in the main scanning direction of the scanning optical system of the present invention, and FIG.
Figures (a) and (b) are cross-sectional views in the sub-scanning direction of two embodiments of this scanning optical system, Fig. 3 is a view showing the aspherical shape of the anamorphic lens surface in the sub-scanning direction, and Figs. 13
The figures are diagrams showing the imaging performance of Examples 1 to 10, respectively, and FIG. 14 is an explanatory diagram for calculating recording density. 1... Cylindrical lens for line image formation 2... Rotating polygon mirror, 3... f/θ lens, 3a, 3b... Spherical lens, 3C... Toric lens, 4...・
・Scanned surface, Asph...Aspheric lens surface

Claims (1)

[Scope of Claims] 1. In a scanning optical system having a tilt correction function and used in a device that deflects and scans a light beam from a light source by rotation or vibration about one axis of a deflecting means, the scanning optical system is configured. A high-resolution scanning optical system in which the shape of at least one of the lens surfaces of the lens deviates from a circle with respect to a cross-section in the sub-scanning direction perpendicular to the main-scanning direction plane formed by the deflected and scanned light beam. 2. The height according to claim 1, wherein the cross-sectional shape in the sub-scanning direction of the lens surface into which the aspherical component deviating from the circle is introduced is such that the absolute value of the radius of curvature increases from the center toward the periphery. Resolution scanning optical system. 3. The high-resolution scanning optical system according to claim 1, wherein the lens surface into which the aspherical component deviating from the circle is introduced is an anamorphic lens surface of an anamorphic lens. 4. The high-resolution scanning optical system according to claim 3, wherein the anamorphic lens surface is a cylindrical surface of a lens disposed between the light source and the deflection means. 5. The high-resolution scanning optical system according to claim 3, wherein the anamorphic lens surface is a toric surface of a lens disposed between the deflection means and the surface to be scanned.