JPH03154018A - Achromatic laser scanning optical system - Google Patents

Abstract

PURPOSE:To always form a stable spot by constituting a collimator lens and a cylindrical lens of an optical system, of two pieces, respectively, and correcting an image surface position caused by a wavelength fluctuation as the whole optical system. CONSTITUTION:A collimator lens 7 for correcting the movement of the image surface in the main scanning surface is constituted of two pieces of concave and convex lenses 7a, 7b, and on the other hand, a cylindrical lens 8 for correcting the movement of the image surface of the sub-scanning surface is constituted of two pieces of concave and convex cylindrical lenses 8a, 8b having power only in the sub-scanning direction. In such a state, by selecting appropriately radiuses of curvature r1-r8, lens surface intervals d1-d7 and refractive indexes of the lens n1-n7 of these lenses 7a, 7b, 8a and 8b, an image surface positional fluctuation caused by the wavelength fluctuation of a laser beam source becomes small in both the main scanning surface and the sub-scanning surface, and a stable spot can always be formed.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to laser scanning optical systems used in laser beam printers, etc., and in particular to achromatic lasers that use semiconductor lasers as light sources and aim for high resolution. It relates to a scanning optical system.

[Prior art 1] FIGS. 6(a) and 6(b) show the general configuration of a laser scanning optical system. In the figure, a laser beam emitted from a semiconductor laser 1 is collimated by a collimator lens 2. ,
This parallel light flux is in the sub-scanning direction (a direction perpendicular to the main scanning plane formed by the scanning beam over time, and here, it is shown in FIG. 6(a).
), a linear image is formed on the reflective surface of a polygon mirror 4 by a cylindrical lens 3 having power only in the direction perpendicular to the plane of the drawing, and then deflected by a scanning lens system 5 including a toric lens. Photosensitive drum 6
A spot image is formed on top.

By the way, as for the properties of semiconductor laser l.

It is known that the wavelength of oscillated light fluctuates due to environmental changes such as temperature. For example, in a semiconductor laser with a wavelength of 780 nm, it varies by ±20 nm in the temperature range of 40°C to 60°C.
There are variations in degree. As the wavelength changes, the image plane position of the laser scanning optical system also changes due to chromatic aberration.In a laser scanning optical system where the imaging spot diameter is not very small and the F number of the scanning lens system 5 is large, it is difficult to obtain a good spot. Since the range (depth of focus) that can be captured is wide, the movement of the image plane due to fluctuations in the laser wavelength is negligible.

However, in a laser scanning optical system designed to form a finer spot, the F number of the scanning lens 5 is as small as 35 or less. 5 will be off and the desired spot will not be obtained.1 For example, if the wavelength is 780n
In a laser scanning optical system having a scanning lens 15 as shown in FIG. 7, which aims at a spot diameter of 50 um or less when using a semiconductor laser of The wavelength of l is ±20μm
If it fluctuates, the image plane position will move by about ±0.8 mm.

This situation can be investigated by combining the scanning lens 15 shown in FIG. 7 shown in Table 6 below with the cylindrical lens 13 consisting of a single lens shown in FIG. 8 shown in Table 7 (Table 6, For the codes in Table 7, please refer to the explanation of the codes in Table 1 (described later)). The surface position fluctuates considerably.

Therefore, in order to ensure that one echo due to wavelength fluctuation is within the depth of focus, it is necessary to attach the laser scanning optical system to the photosensitive drum 6 with a positional accuracy of ±0.2 mm, which makes processing precision more difficult. The cost will be very high,
Therefore, in order to reduce the influence of the wavelength fluctuation mentioned above, the image plane position is adjusted by driving a part of the collimator lens system based on the signal from the sensor that detects the focusing state of the laser spot (autofocus). , collimator lens 2
A method has been proposed in which the chromatic aberration of the entire scanning system is corrected so that the chromatic aberration of the scanning lens 5 and the chromatic aberration of the scanning lens 5 cancel each other out.

[Problems to be Solved by the Invention] However, the autofocus method described above has a complicated configuration and the entire device becomes expensive, while the latter example does not have these drawbacks but is similar to the first-order method. There's a problem.

The scanning lens 5 includes an anamorphic optical system such as a toric lens, and a tilt correction optical system (even if a deflection reflecting surface such as a polygon mirror that is a deflector is tilted, the scanning light beam is imaged on the same scanning line on the irradiated object) (optical system), the chromatic aberration is different on the main scanning plane and the sub-scanning plane (the plane that includes the optical axis of the lens and is perpendicular to the main scanning plane), so the chromatic aberration on both planes cannot be removed at the same time. For example, by combining the scanning lens 15 (Fig. 7) in Table 6 and the cylindrical lens 13 (Fig. 8) consisting of a single lens in Table 7, and using the collimator lens (3 Figures 10(a) and 10(b) show how the image plane position changes due to wavelength fluctuation when using Figure 10(a). Although there is almost no movement, it can be seen that in the sub-scanning plane (FIG. 1O(b)), the movement of the image plane is not sufficiently corrected.

Therefore, in view of the above-mentioned problems, it is an object of the present invention to provide an achromatic laser scanning optical system having a configuration capable of correcting variations in image plane position on both the main scanning plane and the sub-scanning plane due to wavelength variations. It is in.

[Means for Solving the Problems] In the present invention to achieve the above object, the entire scanning optical system including a laser light source, a collimator lens, a cylindrical lens having power in the sub-scanning direction, and a scanning lens is Fluctuations in the image plane position on the main scanning plane and fluctuations in the image plane position on the sub-scanning plane due to variations in the wavelength of the light are corrected.

More specifically, two or more cylindrical lenses are used between the collimator lens and the deflector such as a polygon mirror.

[Operation J] In the laser scanning optical system according to the present invention having the above configuration, the entire scanning optical system including the cylindrical lens in addition to the collimator lens and the scanning lens attempts to correct the fluctuation in the image plane position due to wavelength fluctuation. , it is possible to sufficiently correct variations in the image plane position on both the main scanning plane and the sub-scanning plane.

As for the configuration of a laser scanning optical system that corrects the movement of the image plane due to wavelength fluctuations on the main scanning plane, it is preferable to devise a collimator lens between the laser light source and the cylindrical lens. As for the configuration of the laser scanning optical system that corrects the movement of the surface, it is good to devise a cylindrical lens between the collimator lens and the deflector [Example] Figure 1 has the same basic configuration as Figure 6. A collimator lens 7 and a cylindrical lens 8 used in a scanning optical system having a configuration are shown.

FIG. 1(a) shows the collimator lens 7, and FIG. 1(b) shows the cross section of the cylindrical lens 8 in the sub-scanning plane. The right side in the figure is the laser light source side.

The collimator lens 7 is composed of two concave and convex lenses 7a and 7b, and the cylindrical lens 8 is composed of two concave and convex cylindrical lenses 8a and 8b, both of which have power only in the sub-scanning direction.

With the configuration shown in Figures (a) and (b), the scanning lens 15 shown in Table 6
Tables 1 and 2 show examples of the collimator lens 7 and cylindrical lens 8 designed in accordance with (FIG. 7).

In each table, s r + b d + + nl is the radius of curvature of the i-th lens surface counting from the light source side, the distance between the surfaces from the i-th lens surface to the i-th + 1st lens surface,
The refractive index of the medium on the rear side of the i-th lens surface is shown.

Topsoil wavelength 780 nm r l = 132, 18062 d+ = 2.00 nl = l, 51072 r z
= 4, 97103 dz =0-1 ns = 1 (air) r 3
= 4, 97468 ds =5.00 n-=1.76591r, =-6
, 77860 f (focal length) = 23.2 F, o (F number) = 4 wavelength 780 nm r above = -16,967 Or, , : ■ di = 2.
00 ns = l, 51072r top a =
-5,67111r z a=ωaa=on
5=1 (air) r on y = -5,67111r t=■d, =2.
00 nt = 1. 76591r top @ =-9
, 30761r ++ a=(1) (However, on r, the radius of curvature r11 on the sub-scanning surface is the radius of curvature on the main scanning surface.) The design examples in Table 1 and Table 6 were used together with the scanning lens 15 in Table 6 (Figure 7). In this case, Fig. 2(a) shows the variation in the position of the image plane when the wavelength of the laser beam varies.
2(b) shows the state in the main scanning plane, and FIG. 2(b) shows the state in the sub-scanning plane.

In this case, the collimator lens 7 is composed of two lenses, and the cylindrical lens 8 between the collimator lens 7 and the polygon mirror 4 is also composed of two lenses. In both cases, the influence of wavelength fluctuations on image plane position fluctuations is small.

FIG. 3 shows a second embodiment. FIG. 3(a) shows the collimator lens 9, and FIG. 3(b) shows a cross section of the cylindrical lens 10 in the sub-scanning plane. In the figure, the right side is the laser light source side.

The collimator lens consists of three lenses 9a, 9b, which are concave and convex.
The cylindrical lens IO is the same as in the first embodiment (convex and convex lenses 10a, 1obll configuration), which can effectively correct spherical aberration because an aspherical lens is used for one of the convex lenses 9a and 9C. With this lens configuration, the image plane position on the main scanning plane can be adjusted by moving the concave-convex lenses 9b and 9c of the collimator lens 9 in the optical axis direction, and the cylindrical lens 10 can be moved in the optical axis direction. By doing so, the image plane position on the sub-scanning plane can be adjusted.Therefore, the positional processing accuracy of the scanning optical system is not so strict.

With the configuration shown in FIGS. 2(a) and (b), the scanning lens 15 shown in Table 6
(Fig. 7) is shown in Table 3 and Table 4. When the collimator lens 9 and cylindrical lens 10 in Table 4 are used together with the scanning lens 15 in Table 6, the wavelength changes. FIG. 4 shows the fluctuation of the image plane position at this time.

In this case as well, the main scanning plane (Fig. 4(a)) and the sub-scanning plane (
In both cases (FIG. 4(b)), the influence of wavelength fluctuations on image plane position fluctuations is small.

Wavelength: 780 nm r+=ω d, =2° r2=Aspherical surface da =6. oo n,=t (air)r,=-12
,84291 d,=2.00 n,=1.7659150 n
t=l, 57645r,=17. 52189 da = on4 = t (air) rs
=17. 52189 d, =3-00 ns =l, 576
45ra ”-9,37369 f = 23. 2mm Fno=4 Aspheric data is x = (h” /R) / [1+ (l-(h/R)
2) +zt ] +Ah"+Bh" (h: distance from optical axis, X: deviation jI from spherical surface) R = -11,8399, A = -7,9792x101,
B=4.05317xlO-' wavelength 780nm rly=-16,9670rz t=ωd-=2.
00 nt = 1. 51072rLa =-5
-67111rz s=ωd@ =Ons = 1 (
air) r soil e = -5,67111rth*””) d-=2.
0On= =1.76591rSat,. =-9, 307
61r 11+. = ω (where r is the radius of curvature on the sub-scanning surface.

(rzl is the radius of curvature in the main scanning plane) FIG. 5 shows the third embodiment, FIG. 5(a) shows the collimator lens 11, and FIG. 5(b) shows the cross section of the cylindrical lens 12 in the sub scanning plane. The collimator lens 11 shown in FIG.
In particular, an aspherical lens is used for one of the convex lenses 11a and 11c as in the second embodiment. The cylindrical lens 12 has a concave and convex lens configuration 12a, 12b.

In the third embodiment as well, by moving the concave-convex laminated lenses 11b and 11c of the collimator lens 11 in the optical axis direction and the cylindrical lens 12 in the optical axis direction, the image plane position on the main scanning plane and the sub-scanning can be adjusted. The image plane position can be adjusted independently.

With the configuration shown in FIGS. 5(a) and 5(b), the scanning lens 15 (
Examples designed in accordance with FIG. 7) are shown in Tables 3 and 5 above.

In this case, both the main scanning plane and the sub scanning plane.

The influence of wavelength fluctuations on image plane position fluctuations is small.

wavelength 780 nm rly =8.36557 r r=ωd, =2.
0On? =1.76591rJLs =6.0981
4 rll , = C0da = Qna = t (air) r on * = 6.09814 r, 1 s = ωdo =
2.00 ns = 1.51072r above + o = 9.9
2218 r z +a = O' (where rl is the radius of curvature in the sub-scanning plane.

In the laser scanning optical system of the present invention described above, the optical system from the laser light source to the deflector, including the collimator lens and the cylindrical lens, is connected to the main scanning surface and the sub-scanning surface. In this case, the chromatic aberration of the optical system itself is made to have opposite characteristics to the chromatic aberration of the scanning lens, or the chromatic aberration of the optical system is overcorrected in the main scanning plane and the sub-scanning plane. It has a configuration that allows for

Explain the meaning of overcorrection. In the case of a convex lens made of glass, if there is no chromatic aberration, the focal length will also change as the wavelength changes towards the longer side.0 When the chromatic aberration is corrected, even if the wavelength changes towards the longer side, the focal length will also change towards the longer side. When zero chromatic aberration, whose focal length does not change, is overcorrected, when the wavelength changes toward a longer wavelength, the focal length changes toward a shorter wavelength.

In addition, the cylindrical lens is placed right in the sub-scanning plane so that the chromatic aberration of the cylindrical lens itself has opposite characteristics to the chromatic aberration of the scanning lens.
In this way, the chromatic aberration of the cylindrical lens is overcorrected in the sub-scanning plane.
The optical system as a whole, which includes a cylindrical lens in addition to the collimator lens and the scanning lens, corrects fluctuations in the image plane position due to wavelength fluctuations, so it can sufficiently compensate for fluctuations in the image plane position on both the main scanning plane and the sub-scanning plane. It can be corrected to

In addition, in the second and third embodiments, it is also possible to incorporate an autofocus device using a bonded lens in the collimator lens. ■ Wavelength 780 nm R, = -31,90404 D, = 4.7 N, =1.51072R,=-1
56,20703 D,=2. 09475 Nz =1R,=-1
07,65086 D,=16.7 N,=1.76591R,=-5
2,70210 D, =1. 00 N, =1R11s=c
X3 on R = -157,46Ds =16.
I N,=1. 78569Rz-=-131,
55R soils = -3g.

08 (However, R11 is the radius of curvature on the main scanning plane, and R is the radius of curvature on the sub-scanning plane.) f = 170.4 m m s half angle of view Cω/2) = 37
．． 5° Fso”29 艮yu rls =55.22 rl,, =■ds =7
．． 00 nl = 1.76591r 6 = ■ (where rl is the radius of curvature on the sub-scanning surface, rll is the radius of curvature on the main scanning surface) [Effects of the Invention] As explained above, according to the present invention, the scanning optical system can be With an appropriate configuration, the image plane position remains constant regardless of wavelength fluctuations of a light source such as a laser, so even if you are aiming for a minute spot and have a 4° resolution, you can always obtain a stable spot with a simple configuration. It will be done.

Furthermore, since variations in the position of the image plane due to wavelength variations can be ignored, the actual depth of focus of the image plane can be regarded as wide, which allows the positional processing accuracy of the scanning lens system to be relaxed and processing costs to be reduced.

[Brief explanation of the drawing]

Fig. 1(a) is a diagram of the collimator lens of the first embodiment, Fig. 1(b) is a diagram of the cylindrical lens of the first embodiment, and Fig. 2 is an image plane due to wavelength fluctuation in the first embodiment. 3(a) is a diagram showing the collimator lens of the second embodiment, FIG. 3(b) is a diagram showing the cylindrical lens of the second embodiment, and FIG. 4 is a diagram showing the cylindrical lens of the second embodiment. FIG. 5(a) is a diagram showing the collimator lens of the third embodiment, FIG. 5(b) is a diagram showing the cylindrical lens of the third embodiment, and FIG. is a diagram for explaining a conventional laser scanning optical system, FIG. 7 is a diagram showing an example of a scanning lens, FIG. 8 is a diagram showing an example of a cylindrical lens,
FIGS. 9 and 10 are diagrams showing image plane position fluctuations due to wavelength fluctuations in a conventional example. l...Semiconductor laser, 4...Polygon mirror%6...Photosensitive drum, 7.9.11...
Collimator lens, 810.12... Cylindrical lens, 15... Scanning lens

Claims (1)

[Scope of Claims] 1. In a laser scanning optical system including a laser light source, a collimator lens, a cylindrical lens having power in the sub-scanning direction, and a scanning lens, the scanning optical system as a whole can be controlled by wavelength fluctuations of light from the laser light source. An achromatic laser scanning optical system characterized by correcting variations in image plane position on a main scanning plane and variations in image plane position on a sub-scanning plane. 2. The laser scanning optical system according to claim 1, wherein the cylindrical lens is composed of at least two lenses. 3. The laser scanning optical system according to claim 1 or 2, wherein the collimator lens is composed of at least two lenses. 4. The laser scanning optical system according to claim 1, wherein a part of the collimator lens and the cylindrical lens are movable in the optical axis direction.