JP2678495B2 - Scanning optical system such as laser printer - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an improvement in a compact and low-cost scanning optical system such as a laser beam printer using a semiconductor laser as a light source.

[Prior Art] A scanning optical system such as a laser beam printer basically has a light source unit that emits a light beam, a deflector that deflects the light beam, and a deflected light beam at a position according to a deflection angle. It is composed of a scanning lens system for emitting light, and a small-sized semiconductor laser capable of direct modulation is often used for the light source section. Since the light from the semiconductor laser is diverged,
Usually, it is used together with a collimating lens that makes this a parallel light flux.

Here, the divergence angle of the laser light emitted from the semiconductor laser is a direction parallel to the bonding surface of the semiconductor laser (hereinafter,
Unlike the parallel direction) and the vertical direction (hereinafter referred to as the vertical direction), since the divergence angle is larger in the vertical direction, the light beam diameter after passing through the collimator lens is larger in the vertical direction than in the parallel direction. As a result, the F number of the light beam finally condensed on the scanning surface by the scanning lens system is smaller in the vertical direction, and the spot diameter proportional to the F number of the light beam is in the parallel direction. Grows larger.

In the past, in order to improve this, the collimator lens aperture was reduced to cut the vertical light flux, and the light flux was made uniform at the expense of energy efficiency, or an anamorphic optical system such as a prism was used. He took measures such as beam shaping.

A deflector such as a rotary polygon mirror has an error in a direction in which light is scanned (hereinafter, referred to as a main scanning direction) and a direction perpendicular to the scanning direction (hereinafter, referred to as a sub-scanning direction), so-called plane tilt. Since there is an error called "," uneven pitch of the scanning lines occurs.

In order to correct this, an anamorphic optical system is placed in front of the deflector, the laser beam is imaged on the deflecting surface within the cross section of the scanning optical system cut in the sub-scanning direction, and a scanning lens system with an anamorphic structure is installed. The scanning surface and the deflection surface are made conjugate with each other, and the laser light imaged on the deflection surface is re-imaged on the scanning surface to eliminate the influence of surface tilt, and an anamorphic optical system and scanning lens system. There is known a method in which the focal length and magnification in the sub-scanning direction are reduced by using and to reduce the influence of surface tilt.

[Problems to be Solved by the Invention] However, in the former case, since the laser light is linearly imaged on the deflection surface, it is vulnerable to an image on the deflection surface and dust, and is strongly affected by the change of the deflection point of the rotary polygon mirror. It is difficult to maintain the performance over the entire scanning width. In the latter case, the optical system for beam shaping becomes complicated, and the effect of correcting the tilt is insufficient, so high precision is required for the rotary polygon mirror,
It led to an increase in cost.

Further, a so-called f.theta. Lens in which the incident angle and the image height are proportional to each other is generally used in a scanning lens system for converging the deflected light beam at a position corresponding to the deflection angle on the scanning surface.
The f.theta. Lens has a proportional relationship between the incident angle and the image height (hereinafter,
To have a strong negative distortion,
The error in the proportional relationship between the incident angle and the image height must be very small.

[Object of the Invention] The present invention has been made in view of the above problems,
An object of the present invention is to provide a scanning optical system such as a laser beam printer which has a large side tilt correction effect and a beam shaping effect and which is good in performance and low in cost.

[Means for Solving the Problems] A scanning optical system according to claim 1 is a cylinder lens that forms an image of laser light in a sub-scanning section before a deflector,
An anamorphic scanning lens system for condensing the light beam deflected by the deflector on the scanning surface, the scanning lens system being a negative lens having a concave toric having a stronger curvature in the sub-scan section. A lens and a second lens group, which is a positive lens having a convex toric surface having a stronger curvature in the sub-scan section, are arranged in order from the deflector side, and the focal point in the main-scan section of the scanning lens system is formed. When the distance is f and the distance from the laser light image forming position by the cylinder lens to the deflection point of the laser light by the deflector is 1, the condition of 0.015f <l <0.160f.

The formula is a condition related to the distance from the image formation position by the cylinder lens to the deflection surface, and when l is smaller than the lower limit, the cross-sectional area of the laser beam on the deflection surface becomes small, and it becomes vulnerable to scratches and dust. The influence of the point change becomes large, and it may not be possible to guarantee good performance over the entire scanning angle. On the other hand, when l is larger than the upper limit, the effect of correcting the surface tilt becomes small, and there is a possibility that uneven pitch occurs.

A second aspect of the invention is characterized in that the surface of the first lens group on the scanning surface side is a concave toric surface.

According to a third aspect of the present invention, the surface of the first lens group on the deflector side is a concave toric surface.

A fourth aspect of the invention is characterized in that both surfaces of the first lens group are concave toric surfaces.

A fifth aspect of the invention is characterized in that the surface of the first lens group on the deflector side is a convex toric surface.

A sixth aspect of the invention is characterized in that the deflector side surface of the second lens group is a flat surface.

A seventh aspect of the invention is characterized in that the refractive index n ₂ of the second lens group satisfies n ₂ <1.6.

Embodiments Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows the entire scanning optical system of a laser beam printer or the like according to an embodiment of the present invention, and FIG.
FIG. 1 is a cross-sectional view of the scanning optical system taken in the main scanning direction (hereinafter, simply referred to as main scanning cross section). FIG. 1B shows a cross section taken in the sub scanning direction of the scanning optical system (hereinafter , A sub-scan section).

Such a scanning optical system includes a semiconductor laser 1, a collimator lens 2 for converting a laser beam emitted from the semiconductor laser 1 into a substantially parallel light beam, a curvature in a sub-scan section, and an image of the laser beam once formed in the section. A cylinder lens 3 to be driven, a deflector 4 arranged behind the imaging position F ₁ where the laser light is imaged in the sub-scan section by the cylinder lens 3, and a light beam deflected by the deflector 4 is scanned. It is composed of an anamorphic scanning lens system 5 that collects light on a surface 6.

In the figure, Hf and Hb indicate the front and rear principal points in the main scanning section, and H'f and H'b indicate the front and rear principal points in the sub scanning section. ing.

In the above-mentioned scanning optical system, as shown in FIG. 2, when the polygon mirror P is used as the deflector 4, it is natural that the optical axis O is folded back at the deflection point M and the polygon mirror P is used. P is arranged so that the deflection point on the optical axis is located between the inscribed circle P ₁ and the circumscribed circle P ₂ .

Further, the scanning lens system 5 used in the present invention includes, as shown in FIG. 3, a negative first lens group L ₁ having a concave toric surface having a stronger curvature in the scanning cross section,
This is a two-group two-element configuration in which the second group lens L ₂ having a convex toric surface having a stronger curvature in the sub-scan section and the deflector 4 side are arranged in order.

The scanning lens system 5 has a focal length shorter than that in the main scanning direction in the sub-scanning section, and is a finite image formation with the image formation position F ₁ by the cylinder lens 3 as an object point.
The tilt effect of the deflector 4 which is between the imaging position F ₁ and the scanning lens system 5 can be increased relaxation.

Further, the focal length of the entire system in the sub-scanning direction is changed by changing the focal length of the cylinder lens 3 while keeping the image forming position F ₁ imaged in the sub-scanning section by the cylinder lens 3 constant. The beam shaping rate can be adjusted. Therefore, even with respect to the incident beam shape that varies variously depending on the ellipticity of the semiconductor laser and the NA of the collimating lens, optimum beam shaping can be performed and an imaging spot having a desired shape can be obtained.

When the image forming position F ₁ and the deflector 4 are made to coincide with each other, a so-called conjugate type is used, and an infinite correction magnification is obtained. However, when the polygon mirror is used, the deflection point changes and the processing error of the toric lens which is difficult to process. As a result, the conjugate point of the image plane shifts, and the surface tilt correction rate changes significantly.

Therefore, in the present invention, the image forming position F ₁ and the deflection point are shifted so that the deviation of the conjugate point of the image plane due to the processing error does not influence.

For this reason, the light beam has an advantage that it is incident on the deflecting surface not as a line image but as an area and is not easily affected by scratches or dust on the deflecting surface.

In addition, the first lens group L ₁ of the scanning lens system 5 has a negative power in the main scanning section, so that it is the second lens which is a positive lens.
The spherical aberration and the coma aberration generated in the group lens L ₂ are corrected, and the light flux entering the second group lens L ₂ is made incident at a position away from the optical axis to generate a strong negative distortion f.
It is intended to improve the linearity between the incident angle of the θ lens and the image height.

Further, the second group lens L ₂ generates a strong negative distortion aberration on the deflector side to obtain the linearity of the f · θ lens and at the same time divides the light beam on the scanning surface 6 into the positive power on the scanning surface side. It has the function of forming an image.

On the other hand, in the sub-scan section, the light flux incident on the scanning lens system 5 is divergent light and requires a power higher than that in the main-scan section, but the surface of the second group lens L _{2 on} the scanning surface 6 side is the sub A large positive power can be obtained because the toric surface has a curvature larger in the scanning section than in the main scanning section. Further, since this surface is a positive toric surface and the first group lens L ₁ has a negative toric surface, the power distribution of these surfaces can favorably correct the field curvature in the sub-scanning direction.

Furthermore, in the present invention, since the concave toric surface and the convex toric surface are combined, it is possible to satisfactorily correct the field curvature and suppress the deterioration of off-axis wavefront aberration. Therefore, even if the incident light is shifted in the sub-scanning direction, the aberrations cancel each other out due to the action of the concave toric surface and the convex toric surface, so that good performance can be maintained.

When using glass materials having high refractive index in the second group lens L _2, together with the combined field curvature and effect of concave toric can be satisfactorily corrected, and with minimal spherical aberration, the scanning angle of ±
A wide angle of 48 ° is possible, and the second lens group L ₂
It is also possible to improve the workability of the convex toric lens by setting the surface on the deflector side as a plane.

Further, due to the action of the concave toric of the first group lens L ₁ , the second group lens L ₂ does not necessarily have a high refractive index as described above, and thus a glass material or a plastic material having a low refractive index satisfying the formula is satisfied. Can also be used, in which case the glass material can be reduced.

Next, seven preferred examples of the scanning lens system 5 constituting a part of the scanning optical system of the present invention will be described.

Symbols in each table indicate the radius of curvature of the main scanning cross section of the i-th surface from the deflector side to ri, the radius of curvature of the sub-scanning cross section to r′i, the lens thickness between the i-th surface and the (i + 1) th surface, or the air. The distance is di,
The refractive index at the used wavelength of the lens between the i-th surface and the (i + 1) -th surface is ni, the distance between the deflection point M and the first surface is e, the imaging position F ₁ in the sub-scan section and the deflection point Let l be the distance from M, fb be the distance between the scanning lens system and the scanning surface, and f be the focal length in the main scanning direction.

Note that FIG. 4 shows aberrations of the scanning optical system having the configuration shown in Table 1, and FIGS. 5 to 10 show aberrations having the configurations shown in Tables 2 to 7 below.

The respective examples have the following relationships with respect to the conditions of claims 2 to 7.

That is, it is the first, second, third, fourth, fifth that satisfies the condition of claim 2.
Table example, the conditions of claim 3 meet the conditions of tables 6 and 7, the conditions of claim 4 meet the example of table 7, the conditions of claim 5 meet the conditions of tables 2 and 4. , The conditions of claim 6 are satisfied in the examples of Tables 5, 6, and 7, and the conditions of claim 7 are satisfied in the examples of Tables 1, 2, 3, and 4.

[Effect] As described above, since the scanning lens system of the present invention has a focal length shorter than that in the main scanning direction in the sub-scanning section and is a finite image formation with the image formation position by the cylinder lens as the object point, The influence of the tilt of the deflector between the image forming position and the scanning lens system can be greatly reduced. Therefore, it is possible to obtain a scanning optical system that exerts a large surface tilt correction effect and a beam shaping effect, and is not easily affected by scratches or dust on the deflecting surface.

Further, in the scanning lens system of the present invention, since the concave toric surface and the convex toric surface are combined, the field curvature can be satisfactorily corrected and the deterioration of the off-axis wavefront aberration is small. Can be suppressed.

Further, when a glass material having a high refractive index is used for the second lens group, the workability of the convex toric lens can be improved by making the deflector side surface of the second lens group a plane.

Since the toric lens, which is the second lens group, does not necessarily have a high refractive index due to the action of the concave toric lens of the first lens group, an inexpensive glass material or plastic can be used, and the cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing the entire scanning optical system of the present invention, in which (a) shows a main scanning section and (b) shows a sub scanning section. FIG. 2 is an explanatory diagram when a polygon mirror is used as a deflector. FIG. 3 is a configuration diagram showing a scanning lens system. 4 to 10 are aberration diagrams of the scanning lens system shown in the examples. 1 ... semiconductor laser 2 ... collimator lens 3 ... cylinder lens 4 ... deflector 5 ... scanning lens system 6 ... scanning surface

Claims (7)

(57) [Claims] 1. A cylinder lens for forming an image of laser light in a sub-scanning cross section in front of a deflector, and an anamorphic scanning lens system for condensing a light beam deflected by the deflector on a scanning surface, The scanning lens system is a first lens group, which is a negative lens having a concave toric surface having a stronger curvature in the sub-scan section, and a positive lens having a convex toric surface having a stronger curvature in the sub-scan section. A second group lens is arranged in order from the deflector side, the focal length in the main scanning cross section of the scanning lens system is f, and the laser from the deflector to the laser beam from the image forming position of the laser beam from the cylinder lens. A scanning optical system such as a laser beam printer, which satisfies the condition of 0.015f <l <0.160f, where l is the distance to the deflection point of light. 2. A scanning optical system for a laser beam printer or the like according to claim 1, wherein the surface of the first group lens on the scanning surface side is a concave toric surface. 3. A scanning optical system for a laser beam printer or the like according to claim 1, wherein the surface of the first lens group on the deflector side is a concave toric surface. 4. A scanning optical system for a laser beam printer or the like according to claim 1, wherein both surfaces of the first lens group are concave toric surfaces. 5. A scanning optical system for a laser beam printer or the like according to claim 1, wherein the surface of the first lens group on the deflector side is a convex toric surface. 6. A scanning optical system for a laser beam printer or the like according to claim 1, wherein a surface of the second group lens on the deflector side is a flat surface. 7. A scanning optical system for a laser beam printer or the like according to claim 1, wherein the refractive index n ₂ of the second lens group satisfies n ₂ <1.6.