JPH0743467B2 - Scanning optics - Google Patents

Description

TECHNICAL FIELD The present invention relates to a scanning optical system, such as a laser beam printer, in which scanning irregularities are removed.

b. Prior Art and its Problems A scanning optical system using a semiconductor laser as a light source, such as a laser beam printer, has a semiconductor laser 1 that emits laser light and a semiconductor laser 1 that emits laser light, as shown in FIG. It comprises a collimator lens 2 for converting the emitted laser light into a substantially parallel light beam, a deflector 3 such as a rotating polygon mirror for deflecting the light beam, and a scanning lens system 4 for collecting the deflected light beam at a predetermined position. , Spots are formed on the scanning surface 5.

Hereinafter, the scanning direction on the scanning surface 5 will be referred to as the main scanning direction, and the direction perpendicular to the scanning direction will be referred to as the sub-scanning direction.

Where the spot diameter S is D, the focal length of the scanning lens system 4 is f, and the wavelength is λ, Given in. Here, K is a constant, an amount determined by the shape of the light flux, and It corresponds to the F number determined by the luminous flux diameter.

The scanning lens system 4 often uses an f.theta. Lens so that the deflection speed is proportional to the scanning speed.

In the f.theta. lens, the angle (incident angle) .theta. formed with the lens optical axis of the incident beam and the distance (image height) y from the optical axis of the spot on the scanning surface 5 are y = f, where f is the focal length.・ This is a lens with θ, and this characteristic is that the scanning lens system has distortion.
As a Dist, It is obtained by designing to have a negative value.

Further, a semiconductor laser as a light source emits light differently in a direction parallel to the bonding surface (hereinafter, simply referred to as a parallel direction) and a direction perpendicular to the bonding surface (hereinafter, simply referred to as a vertical direction). That is, when the spread angle in the parallel direction is θ ₂ and the spread angle in the vertical direction is θ ₁ as shown in FIG. 8, θ ₂ <θ ₁ .

Therefore, in the scanning optical system including the semiconductor laser, the collimator lens, the deflector and the scanning lens system as described above,
Since the divergence angle of the laser light emitted from the semiconductor laser is different, the beam diameter D _{A in} the parallel direction after exiting the collimating lens and the beam diameter D _{B in the} vertical direction are also different, and D _A = 2 · fc · sin (θ _{2/2) <2 · fc} · sin (θ 1/2) = D B ( where, because fc is the focal length) of the collimating lens, Supotsuto diameter also (Here, S ₂ is the spot diameter in the parallel direction and S ₁ is the spot diameter in the vertical direction), so a circular spot cannot be obtained.

As a method of solving the above-mentioned drawbacks caused by using a semiconductor laser as a light source, that is, the drawback that a circular spot cannot be obtained, the collimator lens 2 may be made small in diameter or a slit may be provided to provide a vertical light beam. The optical beam diameter is adjusted by arranging the optical beam diameters, or by arranging the afocal anamorphic optical system 10 between the collimator lens 2 and the deflector 3 as shown in FIG. However, the former method reduces the energy efficiency and the latter method complicates the optical system.

Further, although a rotary polygon mirror, a hologram scanner, etc. are used for the deflector 3, due to processing accuracy, sub-scanning of the deflection surface due to tilt error of each surface of the rotary polygon mirror, error between elements of the hologram scanner, etc. A directional error occurs, which causes uneven scanning pitch.

As a method for correcting this, conventionally, as shown in FIG. 10, a first anamorphic optical system 13 acting in the sub-scanning direction plane between the collimator lens 2 and the deflector 3 is used.
To form a line image on the deflecting surface, and to add a second anamorphic optical system 14 to the scanning lens system 4, or to make the scanning lens system 4 itself an anamorphic structure.
A method of making the deflecting surface and the scanning surface conjugate in the sub-scanning direction plane to eliminate the image point error in the sub-scanning direction caused by the surface tilt, etc. are shown. In either method, the optical system after the deflector is used. However, there is a drawback that the curvature of the image surface to be scanned becomes large due to an increase in the positive refracting power in the sub-scanning direction. As this method, Japanese Patent Publication Nos. 52-28666 and 57-144515 are disclosed. And JP-A-58-93021. In particular,
In JP-A-58-93021, a cylinder lens is used as the second anamorphic optical system.
In order to reduce the field curvature, the cylinder lens must be placed close to the scan plane, resulting in a long cylinder lens.

c. Object The present invention has an object to obtain a scanning optical system capable of more surely correcting the surface tilt of the deflector.

d. Summary of the Invention The present invention is directed to a semiconductor laser, a collimator lens for making a laser beam emitted from the semiconductor laser a substantially parallel light beam, a deflector for deflecting the light beam emitted from the collimator lens, and this deflector. In a scanning optical system including a scanning lens system for condensing a deflected light beam on a scanning surface, a light beam emitted from the collimator lens is narrowed in a sub-scanning cross section between a collimator lens and a deflector. A convex lens which is shaped and made incident on the deflector is disposed, and the scanning lens system has a surface on the most deflector side that has a strong negative power in the sub-scanning direction rather than the main scanning direction. An anamorphic lens system in which the surface away from the main scanning direction has a strong positive power in the sub-scanning direction, the focal length of the convex lens is f _p , and the rear principal point of the convex lens and the deflecting surface of the deflector are Is d and the focal length of the cross section of the scanning lens system in the main scanning direction is f ₁ , and 0.5 <(f _p −d) / f ₁ <2.5 (1) is satisfied.

That is, according to the present invention, the image forming position by the convex lens on the light source side of the deflector is intentionally shifted by (f _p −d) from the deflector so that dust and scratches on the deflecting surface of the deflector are not projected. In order to match the focus position in the scanning direction with the focus position in the main scanning direction, the position of the principal point in the sub-scanning direction is shifted in the main scanning image plane direction. The surface on the device side is a surface having a negative power stronger in the sub-scanning direction than the main scanning direction, and the surface farthest from the deflector is a surface having a positive power stronger in the sub-scanning direction than the main scanning direction. It uses a retrofocus anamorphic lens system. Conditional expression (1) shows the relationship between the focal length of the convex lens and the shift amount of the image forming position.

e. Embodiment of the Invention An embodiment of the present invention will be described below with reference to the drawings. First
The figure is a diagram showing the basic contents of a scanning optical system such as a laser beam printer according to the present invention, the vertical direction of the semiconductor laser corresponds to the main scanning direction of the laser beam printer, the parallel direction of the semiconductor laser is a laser beam It matches the sub-scanning direction of the printer or the like. The scanning optical system includes a semiconductor laser from the object side, a collimator lens 2 for converting a laser beam emitted from the semiconductor laser into a substantially parallel light beam, and a convex cylinder lens having power in the sub-scanning direction.
11, a deflector for deflecting a light beam that passes through the cylinder lens 11 and becomes convergent light in the sub-scanning direction, and has a cross section that is thinner in the sub-scanning direction than when the collimator lens 2 is emitted.
3. The anamorphic scanning lens system 20 is constructed in this order. However, the semiconductor laser, the collimator lens 2 and the deflector 3 are publicly known, so the description thereof will be simplified and the convex cylinder lens 11 and the anamorphic scanning lens system 20 will be described. I will explain mainly.

Hf ₁ and Hb ₁ are front and rear principal point positions in the main scanning direction cross section of the scanning lens 20, and Hf ₂ and Hb ₂ are front and rear principal point positions in the sub scanning direction cross section. Further, d is the distance between the rear principal point Hp of the convex cylinder lens 11 and the deflecting surface 3, and e is the front principal point Hf ₂ in the sub-scanning direction cross section of the deflecting surface 3 and the scanning lens 20.
Indicates the distance to.

(1) Regarding the error in the sub-scanning direction of the deflecting surface When, for example, an f.theta. Lens is used in the conventional scanning lens system 4, since y = f.theta. As described above, the focal length f is It is determined to be a certain value depending on the scanning width and scanning angle of a laser beam printer or the like.

However, in the f.theta. Scanning lens system composed of the spherical lens system, the sub-scanning direction also has the same focal length as the main scanning direction, so that the light beam is emitted in the sub-scanning direction due to the surface tilt error of the deflecting surface 3 or the like. When the spot is swung by Δθs, the spot is also scanned at a position displaced from the reference position by f · Δθs in the sub-scanning direction, resulting in uneven pitch.

In the present invention, a convex cylinder lens 11 is provided between the collimator lens 2 and the deflector 3, and the scanning lens system
Since 20 is an anamorphic structure, if the focal length in the main scanning direction is f ₁ , the focal length in the sub scanning direction is f ₂ , and the focal length of the cylinder lens 11 is fp, then f ₁ > f ₂ can be obtained. , And the image point error in the sub-scanning direction is Therefore, the sensitivity to the deflection surface error in the sub-scanning direction is
Compared to the case where the scanning lens consists of a spherical lens system with a focal length f ₁ , Is reduced to.

In this method, unlike the method of forming a line image on the deflecting surface, complete correction is not performed, but since the light beam impinges on the deflecting surface and is reflected, noise such as scratches and dust on the deflecting surface is generated. It has the advantage of being strong.

(2) Beam shaping effect In the present invention, in the present invention, the vertical direction with a wide divergence angle and the main scanning direction with a long focal length of the scanning lens system 20, the parallel direction with a narrow divergence angle and the sub-scanning direction with a short focal length. And have a substantial beam shaping effect. That is, since the divergence angle of the semiconductor laser is θ ₁ > θ ₂ as described above, the beam diameter after passing through the collimating lens 2 is the beam diameter in the main scanning direction D ₁ and in the sub scanning direction. When the beam diameter is _{D 2, D 1 = 2 ·} fc sin (θ 1/2)> 2 · fc sin (θ 2/2) = D 2 , and the focus of the sub-scanning direction, including the cylindrical lens 11 where The distance f ₂ ′ is And the spot diameters S ₁ and S ₁ in the main scanning direction and the sub scanning direction.
₂ is Therefore, the ratio of the spot diameters in the main scanning direction and the sub scanning direction is Becomes That is, The beam shaping effect is produced by the ratio of.

Therefore, Then, a spot having a substantially circular shape can be obtained. By the way, when there is no convex cylinder lens 11, namely fp = ∞
In the case of, f ′ ₂ = f ₂ and the ratio of the sub-scanning direction error correction effect of the deflection surface and the ratio of the beam shaping effect are both f ₂ / f _1. However, in the present invention, the deflection surface Since the ratio of the error correction effect in the sub-scanning direction and the ratio of the beam shaping effect are given by the above formulas and formulas, by appropriately setting fp, d, and e, there is a degree of freedom in the ratio of both effects.

That is, in the present invention, the presence of the convex cylinder lens 11 makes it possible to adjust the ratio of the error correction in the sub-scanning direction of the deflecting surface and the ratio of the beam shaping effect. Can be obtained, and the effect of correcting the error in the sub-scanning direction of the deflecting surface can be further increased.

In addition, when the NA of the collimator lens 2 cannot be set sufficiently large and the light in the vertical direction is cut off due to manufacturing problems,
Even if the above formula is satisfied, S ₁ > S ₂ will be satisfied, so it is possible to limit the luminous flux by further inserting slits in the parallel direction to make the spot diameters uniform, but in the present invention, the collimator lens 2 and the scanning are used. By inserting a convex cylinder lens 11 having power in the sub-scanning direction between the lens system 20 and the lens system 20,
Since the light flux that has passed through becomes a convergent light in the sub-scanning direction and has the effect of making the light flux incident on the scanning lens system 20 into a light flux having a cross section that is thinner in the sub-scanning direction than when it exits the collimator lens 2, the slit Even if it is inserted, the restriction of the luminous flux can be loosened, and the loss of the light quantity can be reduced. Also,
In the present invention, since the light flux passing through the scanning lens 20 is thinned in the sub-scanning direction by the convex cylinder lens 11, it is advantageous in terms of surface accuracy tolerance of the toric surface which is difficult to correct aberrations and is difficult to process.

Next, a specific configuration of the scanning optical system according to the present invention will be described.

FIG. 2 is a block diagram showing the entire scanning optical system according to the present invention, in which 1 is a semiconductor laser, 2 is a collimating lens,
11 is a convex cylindrical lens having a curvature in the sub-scanning direction, 3
Is a deflector, and 20 is an anamorphic scanning lens system composed of two groups. In the present invention, the above scanning lens system
Since 20 is an anamorphic, f ₂ can be made smaller than f ₁ , so that the effect of correcting the error in the sub-scanning direction of the deflecting surface and the beam shaping effect adjusted by the convex cylinder lens 11 can be obtained. Further, the scanning lens system 20 includes a concave cylinder surface having a negative refractive power on the deflector side and a convex toric surface having a positive refractive power on the scanning surface side in a cross section in the sub-scanning direction. Since a focus configuration can be adopted, the image-side principal point position in the sub-scanning direction is largely shifted to the scanning surface side, and the image points in the main-scanning direction and the sub-scanning direction coincide with each other in the optical system including the convex cylinder lens 11. Can be made. However, the image point on the axis may be slightly shifted in consideration of the curvature of the image plane and the shift of the best image plane.

In the parallel direction due to the structure of the semiconductor laser, the light source is not ideal because of the transverse mode of oscillation and the like, and as a result, the spot becomes thick. Even in such a case, the present invention is advantageous because the scanning lens has a short focal length in the sub-scanning direction and the projection magnification of the light source is small.

Further, the scanning lens system 20 has a surface having a negative refracting power and a surface having a positive refracting power in the sub-scanning direction surface after the deflecting surface, and these surfaces are stronger than in the scanning direction section. Since they have refracting power, by appropriately adjusting these, it is possible to satisfactorily correct the curvature of field with a compact configuration.

Here, when the scanning lens system 20 is composed of an f.theta. Lens, the f.theta. Lens has a first surface a formed on a concave cylinder surface having a curvature in a cross section in the sub-scanning direction from the deflector 3 side. The first group lens 21 having a concave spherical surface on the two sides and the third surface C are formed on a flat surface, and the fourth surface D is formed on a toric surface having a greater curvature in the sub-scanning direction cross section than in the scanning direction cross section. The second lens group 22 and the second lens group 22 can be used as the second lens group, and the scanning lens system 20 according to the present invention having good performance can be configured with the minimum number of lenses.

That is, according to the f · θ lens, in the scanning direction cross section,
From the incident side, the first lens group 21 is a plano-concave lens, and the second lens group
Since 22 is plano-convex and the first lens group has a negative refractive power, it is advantageous for correction of spherical aberration and coma.

Further, since the second surface (b) is concave and the refracting power is strong against the off-axis light flux, the height of incidence on the third surface (c) is increased, and the negative distortion aberration generated on this surface can be increased, so that good f・ Theta characteristics can be obtained.

Further, in the cross section in the sub-scanning direction, the first lens group 21 is a biconcave lens and the second lens group 22 is a plano-convex lens, and the first surface (a) has a strong curved concave surface and a strong negative refractive power. Since the four-sided d is a convex surface with a strong curvature and has a strong positive refractive power,
With the so-called retro focus structure, it is possible to shorten the focal length in the sub-scanning direction and bring the image-side principal point position closer to the image plane.

Further, in the f / θ lens, the first surface a is a cylinder surface,
The fourth surface D is a toric surface, and the cylindrical lens 11 is included in front of the deflecting surface 3 as well, so that it has a parameter independent of the scanning direction to correct the curvature of field in the sub-scanning direction cross section. Is advantageous.

In this f.theta. Lens, the first lens group 21 includes a cylinder surface, and the second lens group 22 includes a toric surface, but the cylinder surface is relatively easy to process, and the second lens surface includes the toric surface. The group lens 22 is also advantageous in manufacturing because one surface is flat. Further, the first lens group 21 and the second lens group 22 respectively have a cylinder surface and a toric surface, but even if these surfaces cause a processing error,
The focus position in the main scanning direction and the sub scanning direction can be adjusted by the distance between the first group lens 21 and the second group lens 22.

This is because the refracting powers of the first group lens 21 and the second group lens 22 are stronger in the sub-scanning direction, and the sensitivity of the focal position change with respect to the interval change is also different.

Especially when the semiconductor laser has an astigmatic difference as a light source,
Although the image points deviate in the optical axis direction between the main scanning direction and the sub scanning direction, this method can correct the image points. Further, the adjustment of the focus position in the main scanning direction and the sub-scanning direction can also be performed by changing the curvature or position of the convex cylinder lens 11, and in this case, the image point in the main scanning direction does not move and the scannability is improved. Has the advantage that it does not change.

Further, in the scanning optical system, the focal length of the convex cylinder lens 11 having a curvature in the sub-scanning direction is fp, the distance between the rear principal point of the convex cylinder lens and the deflection surface 3 is d, and the main scanning of the scanning lens system 20 is performed. F _{1 is} the focal length of the cross section in the directional direction, f _{2 is} the focal length of the cross section in the sub-scanning direction, r ₁ ′ is the radius of curvature of the cross section in the sub-scanning direction of the first surface a of scanning lens system 20, and the sub-scanning direction of the fourth surface Let the radius of curvature of the cross section be r ₄ ′, If the above conditions are satisfied, a scanning optical system with good performance is constructed.

The condition (1) relates to the focal length of the convex cylinder lens 11, Is smaller than the left side, the effect of making the light beam incident as a narrow light beam in the cross section in the sub-scanning direction becomes large, but in order to make the focal point in the sub-scanning direction coincide with the main scanning direction, the radius of curvature of the cylinder surface of the first surface b. Becomes too small and causes processing difficulties.

On the other hand, if it exceeds the right side, the above effect becomes small, and the radius of curvature of the convex cylinder becomes large, so that the machining becomes difficult.

The condition (2) is a condition regarding the ratio of the focal lengths of the scanning lens system 20 in the sub-scanning direction and the main scanning direction, and balances the correction effect of the sub-scanning direction error of the deflecting surface and the beam shaping effect.

Is smaller than the left side, the effect of correcting the error in the sub-scanning direction becomes large, but the beam shaping effect becomes excessive. Also, Is greater than the right side, the effect of correcting the error in the sub-scanning direction of the deflecting surface and the effect of beam shaping are insufficient.

The condition (3) is a condition for the radius of curvature of the cylinder surface. If | r ₁ ′ | / f ₂ is smaller than the left side, the divergence of the light beam in the sub-scanning direction surface on the first surface becomes too large. , It becomes difficult to correct the field curvature by well balancing with r ₄ ′. If | r ₁ ′ | / f ₂ exceeds the right side,
The effect of increasing the principal point spacing in the sub-scanning direction surface weakens,
In order to bring the image-side principal point position closer to the image side, the entire lens system becomes large, and the correction of the under-field curvature that occurs on the toric surface is insufficient.

Condition (4) is a condition for the radius of curvature in the surface of the Torrick surface in the sub-scanning direction. If | r ₄ ′ | / f ₁ is smaller than the left side, the image-side principal point does not approach the image surface, and under The occurrence of field curvature becomes large.

If | r ₄ ′ | / f ₁ exceeds the right side, the focal length in the sub-scanning direction cannot be made sufficiently shorter than that in the scanning direction, and the tilt correction effect and the beam shaping effect become thin.

In order to keep the Petzval's sum small and perform good aberration correction, n ₁ <n ₂ where n _{1 is} the refractive index of the _first lens group and n ₂ is the refractive index of the second lens group. Things are desirable.

Hereinafter, the third specific example of the scanning optical system according to the present invention will be described.
This will be shown with reference to FIGS. 3 and 4 show the first embodiment, and FIGS. 5 and 6 show the second embodiment.

Here, the focal length of the collimator lens 2 is fc, and the diameter of the light beam emitted from the collimator lens 2 is the main scanning direction.
D ₁ , in the sub scanning direction, D ₂ , the focal length of the convex cylinder lens 11 is fp, the distance between the rear principal point of the convex cylinder lens 11 and the deflecting surface 3 is d, the deflecting surface 3 and the front principal of the scanning lens system 20. The distance from the point is e, and the focal length of the scanning lens system 20 is in the scanning direction.
Let f _{1 be} f _{2 in} the sub-scanning direction, and f ₂ ′ be the focal length in the sub-scanning direction that includes the convex cylinder lens 11.

Further, the main scanning lens system 20 has a two-group structure having f.theta. Characteristics in the scanning direction. From the deflector side, the first surface is a cylinder surface having a curvature in the sub-scanning direction, the second surface is a concave surface, and the second surface is a concave surface. The third surface is a flat surface, and the fourth surface is a convex surface having a strong curvature in the sub-scanning direction.

The radius of curvature of the cross section in the scanning direction of the i-th surface is r _i , the radius of curvature of the cross section in the sub-scanning direction is r _i ′, the center thickness of the first group lens is d ₁ , the lens interval is d ₂ , and the center thickness of the second group lens is Is d ₃ and the wavelength used is 78
The refractive index of the first lens group at 0 mm is n ₁ and the refractive index of the second lens group at 0 mm is n ₂ . The scanning angle is ± ω °.
The scanning lens system 20 is arranged so that its entrance pupil position EnP. Coincides with the deflecting surface.

Example 1 Overall configuration Scan lens system configuration 2ω = 96.5 ° Entrance pupil position Forward from one plane 15.17 Example 2 Overall configuration Scan lens system configuration 2ω = 96.5 ° Entrance pupil position 1 in front of plane 16.0 f. Effect of the invention The scanning optical system of the present invention uses a convex lens on the light source side of the deflector so that dust and scratches are not projected on the deflecting surface of the deflector. an imaging position shifting from (f _p -d) only deflector, whereas, for aligning the sub-scanning direction of the focus position in the focus position of the main scanning direction, the scanning lens system, the surface of its most deflector-side main scanning A retrofocus anamorphic lens system having a surface having a stronger negative power in the sub-scanning direction than the main direction and a surface having the strongest positive power in the sub-scanning direction than the main scanning direction. ,
Furthermore, the relationship between the focal length of the convex lens and the shift amount of the image forming position is defined by the conditional expression (1). Therefore, it is possible to more surely correct the surface tilt of the deflector.

[Brief description of drawings]

FIG. 1 is a diagram showing the basic contents of a scanning optical system according to the present invention, FIG. 2 is a configuration diagram showing the entire scanning optical system according to the present invention, and FIG. 3 is a scanning lens system of Example 1. FIG. 4 is a diagram showing the aberration of the entire system of Example 1, FIG. 5 is a diagram showing the configuration of the scanning lens system of Example 2, and FIG. 6 is a diagram of the entire system of Example 2. FIG. 7 is an aberration diagram, FIG. 7 is an overall view showing a scanning optical system of a conventional laser beam printer and the like, and FIG. 8 is different in how a semiconductor laser emits light in a direction parallel to a bonding surface and a direction perpendicular to the bonding surface. FIG. 9 and FIG. 10 are views showing a conventional scanning optical system. 1 ... Semiconductor laser, 2 ... Collimating lens, 3 ...
... Deflector, 4 ... Scanning lens system, 5 ... Scanning surface, 11 ...
Convex cylinder lens, 20 ... Scanning lens system, 21 ... First
Group lens, 22 ... Second group lens

Claims (1)

[Claims] 1. A semiconductor laser, a collimator lens for converting a laser beam emitted from the semiconductor laser into a substantially parallel light beam, a deflector for deflecting the light beam emitted from the collimator lens, and a light beam deflected by the deflector. In a scanning optical system including a scanning lens system for condensing light on a scanning surface, a light beam emitted from the collimator lens is formed between the collimator lens and the deflector so that a diameter of a sub-scanning cross section thereof is reduced. And a convex lens to be incident on the deflector is disposed, and the scanning lens system has a surface on the most deflector side which has a negative power stronger in the sub-scanning direction than in the main scanning direction. distant plane from is an anamorphic lens system consisting of a surface having strong positive power than the main scanning direction in the sub-polarized focal length of the convex lens f _p, the rear principal point of the convex lens The distance between the deflecting surface of the vessel d, a focal length in the main scanning cross section of the scanning lens system as f _1, and satisfies a _{0.5 <(f p -d) /} f 1 <2.5 (1) Scanning optics.