JPH10206773A - Optical scanning device for beams - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the optical scanning device which can convert print dot density to specific values by a laser beam printer using a multi-beam scan. SOLUTION: Of the optical scanning device having light beam generating means which can impose light intensity modulation independently, a rotary polygon mirror 9 which deflects and scans light beams emitted by the generating means together, an Fθ lens 10 which converges each beam to a specific spot diameter on a scanning surface, and a 1st optical system 5 which guides the light beams on the scanning surface to nearby a reflecting surface of the rotary polygon mirror 9, at least one lens of the 1st optical system 5 is arranged movably on the optical axis, the focal length of the 1st optical system 5 is varied, and the spot diameters and intervals of the light beams on the scanning surface are set to specific values.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical scanning device used for a laser beam printer, a copying machine, and the like, and more particularly to an optical system for scanning a plurality of beams.

[0002]

2. Description of the Related Art In order to realize high speed printing and high dot density printing with a laser beam printer, it is necessary to increase the number of scans per unit time. Further, in order to diversify output patterns in recent years, it is increasingly necessary to change the print dot density with the same printer device. When a single beam is used, the number of repetitive scanning lines can be increased to some extent by an increase in the number of rotations of the rotating polygon mirror or an increase in the number of mirror surfaces, but there is a limit. Therefore, it has been well known that a multiple beam scanning method for scanning a large number of laser beams at once is effective. For example, a method in which a single laser light source is separated into a plurality of laser beams, each of which is modulated by an output pattern signal, and simultaneously scans a scanning surface via a single rotating polygon mirror and an Fθ lens (Japanese Patent Laid-Open No. 62-239119) or arraying a plurality of individually modulatable semiconductor laser elements in an array, making each outgoing light parallel by a single collimator lens, and scanning on a scanning surface via a rotating polygon mirror and an Fθ lens. Is simultaneously scanned with a plurality of laser beams (Japanese Patent Publication No. 60-33019).

[0003]

However, in these multi-beam scanning optical systems, no consideration is given to changing the print dot density. On the other hand, a two-beam scanning optical system having a function of changing the interval between two beams according to a predetermined dot density has been proposed (US Patent).
5,006,705). However, in this case, since the beam spot diameter is constant, an appropriate change range of the dot density is limited, and it is not suitable for a wide range of changes in the print dot density.

An object of the present invention is to provide an optical system for a plurality of beams,
An object of the present invention is to realize a simple and highly reliable printing dot density changing method so as to be able to cope with various printing patterns. For this purpose, it is necessary that the spot diameters of the plurality of beams on the scanning surface and the intervals between the beams can be converted into appropriate values in accordance with the print dot density.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a plurality of light beam generating means capable of independently modulating light intensity, and a rotary polygon mirror for deflecting and scanning a plurality of light beams emitted from the generating means collectively. An optical scanning device comprising: an Fθ lens that converges each beam to a predetermined spot diameter on a scanning surface; and a first optical system that guides the light beam to a vicinity of a reflecting surface of a rotary polygon mirror on the scanning surface. At least one lens in the first optical system is disposed so as to be movable on the optical axis, and by moving this lens on the optical axis, the focal length of the first optical system is changed to perform scanning. This can be achieved by setting the spot diameter and interval of the light beam on the surface to a plurality of predetermined values.

The movable lens of the first optical system includes two lenses having the same refractive index and a sum of focal lengths of 0, and the two lenses are closely contacted on the optical axis. The separation can be achieved by simultaneously changing the spot diameter and the interval of the light beam on the scanning surface and setting a plurality of predetermined values.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, for comparison, a detailed description will be given using a multi-beam optical scanning apparatus shown in FIG. This optical system configuration shows an optical path only for the optical axis of a beam for four-beam scanning. In FIG. 2, light emitted from a light source 1 such as a laser is divided into a plurality of elements 2 such as a grating.
Are divided into a plurality of beams, and then the light is condensed by a condenser lens 3 to a modulator 4 having a modulation element corresponding to each beam. When the modulated light comes out of the modulator 4, each light is converted into a parallel light by the first optical system 5, and then a plurality of beams are turned on the photosensitive member by a rotating element 6 such as a Dove prism. Rotate by a predetermined amount so as to provide an interval. Thereafter, the light passes through the cylinder lens 8 and passes through the rotating polygon mirror 9
Only the sub-scanning direction is narrowed upward. Here, the configuration is such that deflection scanning is performed by the rotating polygon mirror 9 and an image is formed on the photoconductor 11 by the Fθ lens 10. The plurality of beams are collectively scanned on the photoconductor while being arranged obliquely to the scanning direction. In FIG. 2, the spots on the photoconductor 11 and the pitches thereof when the focal length of the first optical system 5 changes are calculated. FIG. 3 is a view of a certain beam of the optical scanning device of FIG. 2 viewed from a cross section. FIG. 4 is a plan view of the same. First calculate the spot diameter. Let the spot diameter at the modulator 4 be the sub-scanning direction α and the scanning direction β. It is assumed that the first optical system 5, the cylinder lens 8, and the Fθ lens 10 are set so that the spot diameter in the sub-scanning direction on the photoconductor 11 becomes α, that is, at a magnification of 1. Further, the focal length of the first optical system 5 and the focal length of the Fθ lens 10 are determined so that the spot diameter in the scanning direction on the photoconductor 11 becomes β. At this time, the focal length of the first optical system 5 is g.
When h is a ratio for converting the print dot density, the spot diameter in the sub-scanning direction when the focal length of the first optical system 5 changes to g × h is α / h. Also,
The spot diameter in the scanning direction is β / h.

Next, a beam pitch is calculated. Modulator 4
Let R be the beam pitch at. At this time, assuming that the magnification is 1 as in the calculation of the spot diameter, the photosensitive member 1
The pitch above 1 is also R. This is shown in FIG. In this drawing, a case where a plurality of beams are collectively scanned while being inclined by an angle θ with respect to the scanning direction is described. Here, assuming that the focal length of the first optical system 5 changes from g to g × h, the beam pitch on the photoconductor 11 becomes R / h. This is shown in FIG.

The above is summarized. FIG. 5 shows a spot when the scanning pitch is set to P in accordance with the print dot density of P. When the rotating elements 6 such as a Dove prism arrange the spots at an angle θ, a relationship of P = R sin θ is established so that a predetermined pitch P is obtained. When the focal length of the first optical system 5 is changed from g to g × h, it changes as shown in FIG. That is, both the spot diameter and the pitch are 1 / h times. At this time, the scanning line pitch interval between the multiple beams is 1 / h times without changing θ. Now, assuming that the moving speed of the photoreceptor is constant and the number of rotations of the rotary polygon mirror 9 is increased by h times, the print dot density becomes equal to the scanning line pitch interval over the entire printing surface.
Conversion can be performed h times uniformly. Further, by changing the modulation frequency to (h) ² times, the print dot density can be converted.

In the above description, for the sake of simplicity, the case where the spot diameter and the pitch at the modulator 4 are the same on the photosensitive member 11, that is, when the magnification = 1, is shown. it is obvious. It is also apparent that a large number of print dot density conversions can be performed by providing a large number of conversion stages for the first optical system.

An embodiment of the present invention will be described below. FIG. 1 is a diagram showing a first embodiment according to the present invention. The entire basic optical system is the same as that described with reference to FIG. 2, and the description of the common components will be omitted. In the present embodiment, the first optical system has a three-lens configuration including a lens 12, a lens 13, and a lens 5. Since the lenses 12 and 13 are made of a material having the same refractive index and the sum of the focal lengths is set to 0, the radii of curvature of the respective lenses are the same. Therefore, the two lenses can be brought into close contact with each other. FIG.
Thus, only the lens 13 can be moved on the optical axis by the lens driving mechanism 17. If the lens 13 moves on the optical axis and comes into close contact with the lens 12, the lens effect of the combination of the two lenses disappears.
The focal length of the lens set of 2, lens 13, and lens 5 is the same as that of the lens 5. On the other hand, when the same three lenses are used and the lens 13 is arranged at a position on a predetermined optical axis, the combined focal length of the three lenses can be changed to another new focal length. The present invention utilizes this property to change the focal length.

Hereinafter, this will be described in detail. FIG. 7 shows the state of the modulator 4 and the first optical system 5 before the print dot density is changed. The lens 12 and the lens 13 are in close contact with each other. A plano-convex lens 12 having the same refractive index and a focal length a
And the plano-concave lens 13 having the focal length b (= -a) have the same radius of curvature, so that they can be brought into close contact with each other. Therefore, in the case of FIG. 7, the state is the same as that in which one lens 5 having a focal length of g is arranged.

FIG. 8 shows a graph for changing the print dot density.
This is a state in which a lens 12 having a focal length a and a lens 13 having a focal length b (= −a) are arranged separately between the modulator 4 and the first optical system 5 having a focal length g. 1
3 is at a predetermined position. This position will be described below. The distance in the figure is the distance from the lens principal point position.
For example, the print dot density is changed from D dots / inch to h ×
Consider the case of changing to D dots / inch. For this purpose, the lens 12 and the lens 13 may be separated from each other and combined with the lens 5 to form a new first optical system so that the beam width of light coming out of the lens 5 becomes h × L.
Assuming that the beam width at the lens 13 is y as shown in FIG. 8, y is obtained from h, d, and g, and is represented by Expression (1).

Y = hL−hLd / g (1) On the other hand, the beam diameter at the lens 12 is obtained as cL / g, as shown in FIG. Further, since the focal length of the lens 12 is a and the light at the modulator 4 is imaged at the point B in FIG. 9 by the lens 12, if the distance between the lens 12 and the point B is X, X There will be a relationship such as equation (2).

A ² = (c−a) × (X−a) (2) When this equation is modified, the following equation (3) is obtained.

X = ca / (ca) (3) From the above, each arrangement can be represented as shown in FIG. When y in FIG. 9 is represented by a, c, g, and d, equation (4) is obtained.
Can be expressed as

Y = L × (ca) × {ca / (ca) −g + d + c} / ag (4) Since the focal length of the lens 13 is b (= −a), the expression (5) is Holds.

B ² = (g−d + b) × {ca / (ca) −g + d + c + b} (5) Expression (1) and expression (4) are combined to obtain expression (6).

A = c × (c + d−g) / ((1−h) × (d−g)) (6) Therefore, for given g and h, Expressions (5) and (6)
The focal lengths and positions of a, b (= -a), c, and d should be set such that

As described above, at the same time as changing the focal length of the first optical system, according to the present invention, both the number of rotations and the modulation frequency of the rotary polygon mirror can be externally changed according to the print dot density. When the print dot density is to be changed, the lens 13 that has been electrically in close contact with the lens 12 is moved to a predetermined position by the drive mechanism 17 through the control unit 19 and combined with the lens 5 to change the focal length and rotate. By changing the rotation frequency and modulation frequency of the polygon mirror, a predetermined print dot density can be obtained. This is the same in the second embodiment described below.

A study will be made by actually entering numerical values. Calculate when to change from 600 dots / inch to 480 dots / inch. At this time, h = 480/600 and g = 472.9.
5 mm. In this case, a = 198.6 mm, b (= − a) = − 198.6 mm,
Equations (5) and (6) are satisfied with c = 380.5 mm and d = 47.07 mm.

The number of rotations of the rotary polygon mirror is the same in both the first embodiment and the second embodiment described below.
What is necessary is just to rotate at 480/600 times at 0 dots / inch, that is, at 0.8 times. For example, the number of rotations of the rotating polygon mirror at 600 dots / inch is 19800 rpm
If 480 dots / inch, 480/6
It may be set to 15840 rpm which is 00 times.

The modulation frequency is, this case of the first embodiment, the same applies in the second embodiment shown below, when the 600 dots / inch (480/600) ^twice, i.e. 0.64 You only need to rotate it twice. For example, if the modulation frequency at 600 dots / inch is 38.6 MHZ, then at 480 dots / inch, (480/600)
^The modulation may be performed at 24.7 MHz, which is twice as high.

As shown in FIG. 10, the lens 12 is positioned at an appropriate position (c).
= 380.5 mm) and when the lens 13 is in close contact with it, it is 600 dots / inch, and FIG.
When the lens 13 is moved to the position of d = 47.07 millimeters as shown in FIG. Conversely, the same is true even if the lens 13 is fixed to an appropriate place as shown in FIG. The lens 12 and the lens 13
It is apparent that the same applies to the case where the sheets are simultaneously moved on the optical axis and brought into close contact with each other or are arranged at a predetermined position.

Next, as a second embodiment, a method of changing the focal length of the first optical system by independently moving the two lenses 12 and 13 on the optical axis will be described. In this case, the print dot density that can be changed is stepless, and an arbitrary print dot density can be selected from, for example, 600 dots / inch to 480 dots / inch. First
Consider a case in which the lens 12, the lens 13, and the lens 5 having the same focal length as in the embodiment are used. At this time, h = 0.
The lens arrangement from 7 to 0.9 can be represented as shown in the graph of FIG. The value of c changes from 360 millimeters to around 400 millimeters, and the value of d changes from 15 millimeters to 30 millimeters.
c and d are the same as the distances shown in FIG. If the lens 12 and the lens 13 can be moved independently on the optical axis according to the graph of FIG. 13, h can be arbitrarily selected. For example, h
If 0.9 is set to 600 dots / inch, the print dot density can be freely selected according to h as shown in FIG. At this time, the number of rotations of the rotating polygon mirror and the modulation frequency of light may be changed according to the print dot density, as in the first embodiment. In the second embodiment, the same lens as in the first embodiment is used, but the lens 12 and the lens 1 are used.
Obviously, the same is true even if the sum of the focal lengths does not become zero.

[0026]

As described above, according to the present invention, the scanning spot diameter and the scanning beam interval can be changed by a simple configuration according to the print dot density in a plurality of beam scanning, and the number of rotations of the rotary polygon mirror and the number of rotations can be reduced. By changing the light intensity modulation frequency, it becomes possible to easily change the printing dot density,
A variety of printing can be supported.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing a first embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of the present invention.

FIG. 3 is a cross-sectional view of an optical path of the optical scanning device illustrating the principle of the present invention.

FIG. 4 is a plan view of an optical path of the optical scanning device illustrating the principle of the present invention.

FIG. 5 is a schematic diagram showing a state of a spot of the first optical system of the present invention.

FIG. 6 shows the focal length of the first optical system of the present invention changed from g to g ×
It is a schematic diagram which shows the state of the spot when changing to h.

FIG. 7 is a schematic diagram illustrating a first optical system according to a first example of the present invention.

FIG. 8 is a schematic diagram illustrating a first optical system according to a first example of the present invention.

FIG. 9 is a schematic diagram illustrating a first optical system according to a first example of the present invention.

FIG. 10 is a schematic diagram showing a specific configuration example of a first optical system in the first embodiment.

FIG. 11 is a schematic diagram showing a specific configuration example of a first optical system in the first embodiment.

FIG. 12 is a schematic diagram showing a specific configuration example of a first optical system in the first embodiment.

FIG. 13 is a graph showing a change in lens arrangement according to the second embodiment.

FIG. 14 is a graph showing a relationship between a conversion ratio and a print dot density in the second embodiment.

[Explanation of symbols]

1 is a light source such as a laser, 2 is an element for dividing light into a plurality,
Reference numeral 3 denotes a condenser lens, 4 denotes a modulator, 5 denotes a first optical system, 6 denotes a rotating element, 7 denotes a mirror, 8 denotes a cylinder lens, 9 denotes a rotating polygon mirror, 10 denotes an Fθ lens, 11 denotes a photoconductor, 12 , 13 are lenses, 15 is a motor for rotating the rotary polygon mirror, 16 is a modulation circuit, 17 is a lens driving mechanism, 18 is a rotary polygon mirror driver, and 19 is a control unit.

Claims (3)

[Claims] 1. A plurality of light beam generating means capable of independently modulating light intensity, a rotary polygon mirror for collectively deflecting and scanning a plurality of light beams emitted from the generating means, and each beam on a scanning surface. An Fθ lens that converges to a predetermined spot diameter,
A first optical system for guiding the light beam to a position near a reflecting surface of a rotary polygon mirror on a scanning surface, wherein at least one lens in the first optical system is moved back and forth on the optical axis. Moving the lens on the optical axis, thereby changing the focal length of the first optical system, and setting the spot diameter and interval of the light beam on the scanning surface to a plurality of predetermined values. A multi-beam optical scanning device characterized by the above-mentioned. 2. The movable lens of the first optical system includes two lenses having the same refractive index and a sum of focal lengths of 0, and the two lenses are closely contacted or separated on an optical axis. The light beam spot diameter and the interval of the light beam on the scanning surface are changed simultaneously, and are set to a plurality of predetermined values. 3. The focal length of the first optical system is D2 / D.
1 times, the number of rotations of the rotary polygon mirror to (D2 / D1) times,
Change the light intensity modulation clock frequency to (D2 / D1) ² times and change the print dot density from D1 (dot / inch) to D
3. The multi-beam optical scanning device according to claim 1, wherein the number is changed to 2 (dots / inch).