JP3216836B2 - Multi-beam laser recorder - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser recording apparatus used in a digital copying machine, a laser beam printer, and the like, and more particularly, to a multi-beam laser recording apparatus for recording information by simultaneously scanning a photosensitive member surface with a plurality of laser beams. It is about.

[0002]

2. Description of the Related Art In a laser beam printer, for example, a laser beam modulated by an image signal is applied to a high-speed rotating polygon scanner, and the reflected light from the polygon scanner scans the surface of the photoconductor, and an electrostatic latent image is formed on the surface of the photoconductor. Form an image. The electrostatic latent image is developed into a toner image, and the toner image is transferred and fixed on plain paper to form an image.

The most important problem in increasing the definition and speed of the laser beam printer is that the rotation speed of the polygon scanner is limited. In order to solve this problem, a multi-beam scanning method in which the surface of the photosensitive member is scanned at once by a plurality of laser beams is already known. In this method, it is needless to say that a plurality of laser beam spots need not be brought sufficiently close to a direction perpendicular to a scanning direction (hereinafter, referred to as a main scanning direction) by a polygon scanner (hereinafter, referred to as a sub-scanning direction). Not be. For this reason, efforts have been made to manufacture a plurality of semiconductor lasers in close proximity, and a semiconductor laser array in which a plurality of semiconductor lasers are brought close to each other at an interval of 10 μm is currently being prototyped (for example, JP-A-2-39583 and RL). .T
Hornton et al. , "Property
s of closedly spaced indep
endly addressable race
rsfabricated by impurity-
induced disordering ”, App
l. Phys. Lett. 56 (17), 1623-1
625 (1990)).

However, even if the semiconductor lasers are brought close to each other at intervals of 10 μm, it is insufficient to arrange the spots sufficiently densely in the sub-scanning direction. For this reason, Japanese Patent Publication No. 60-330
As disclosed in Japanese Patent Publication No. 19, it is conceivable to employ a tilt type multi-beam laser printer that tilts a semiconductor laser array to effectively reduce the laser beam spot interval in the sub-scanning direction. If the semiconductor laser array is inclined, the interval p between the laser spots in the sub-scanning direction, that is, the interval between the scanning lines I can be reduced while the center interval between the two laser spots S1 and S2 is wide as shown in FIG. In the figure, X indicates the moving direction of the laser beam. If semiconductor laser arrays at intervals of 10 μm are used, it is calculated that the angle ω formed by the arrangement direction of laser spots with respect to the main scanning direction should be inclined to ω ≒ 22 ° as described later. FIG. 5 shows a case where beam shaping is performed as described later.

Incidentally, in a laser beam printer, the diameter of the laser spot S on the photosensitive member surface is often set to be longer in the sub-scanning direction and shorter in the main scanning direction as shown in FIG. This is because the laser spot S moves in the main scanning direction indicated by the arrow X, so that when a circular laser spot is used, the spot recorded as an electrostatic latent image becomes longer in the main scanning direction. . Normally, the laser spot diameter in the main scanning direction is set to approximately half of the laser spot diameter r ₁ in the sub-scanning direction, and, by performing the modulation of the duty ratio of 50% to the laser beam, a main spot to be recorded The length in the scanning direction is prevented.

As shown in FIG. 7, the laser beam emitted from the semiconductor laser 21 is 1 in the direction parallel to the bonding surface.
It has a relatively small divergence angle θ ₁ (1 / e ² value) of about 0 °, and a relatively large divergence angle θ ₂ (1 / e ² value) of about 30 ° to 40 ° in the direction perpendicular to the joint surface. Have. That is, the ellipticity of the laser beam is about 3 to 4. For this reason, when an image of a laser beam having an elliptical profile (shown by hatching) as shown in FIG. 7 is formed without performing beam shaping as it is, an elliptical shape in which the spot diameter corresponding to the direction perpendicular to the bonding surface is the shorter diameter. Is obtained. This is explained as follows.

A semiconductor laser 2 as shown in FIG.
Considering a laser optical system that forms an image of the laser beam from No. 1 on the photoreceptor surface 25 through a collimator 22, an aperture 23 and an image forming lens 24, it is known that the image forming spot diameter d is given by the following equation. I have.

[0008]

(Equation 2) In the equation (1), λ is the wavelength of the laser beam, θ _b is the convergence angle of the laser beam on the image plane, π is the circular constant, and A is the apodization coefficient determined by the aperture of the optical system. A is a function of the truncation ratio T = a / D.
a is the aperture of the optical system (aperture diameter: see FIG. 8), D
Is the beam diameter when converted into a parallel beam (1 / e ² value:
FIG. 8). Although a> D in FIG. 8, this is only an example. FIG. 9 shows the relationship between the truncation ratio T and the apodization coefficient A. In FIG. 9, P is a passing power ratio, which indicates a ratio of energy passing through the aperture. If the aperture of the optical system is sufficiently large (if T ≒ ∞), A ≒ 1. The details of such an imaging relationship are described in the specification of Japanese Patent Application No. 3-158608 filed by the present applicant.
Since the lateral magnification β of the optical system in FIG. 8 is f ₂ / f ₁ , θ _a
≒ the βθ _b. This is substituted into equation (1) to obtain the following equation.

[0009]

(Equation 3) Equation (2) means that the imaging spot diameter decreases as the divergence angle of the laser beam from the light source increases.
Therefore, the spot diameter corresponding to the direction perpendicular to the bonding surface of the semiconductor laser having a large divergence angle is smaller.

The ratio of the divergence angle of the laser beam emitted from the semiconductor laser in the direction parallel to and perpendicular to the bonding surface (ellipticity of the laser beam) is as large as about 3 to 4, as described above. Therefore, in the optical system of the conventional laser beam printer, the ellipticity of the laser beam is reduced to about 2 by using a prism, a cylindrical lens, or the like. Changing the ellipticity of the laser beam in this way is called beam shaping. The ellipticity of a laser beam is the ratio of the major axis to the minor axis of the laser beam diameter, and the divergence angle of the laser beam and the beam diameter have an inversely proportional relationship as shown in the above equation (2). Therefore, the ratio of the spread angle can also be expressed as the ellipticity. Note, however, that the denominator and numerator are opposite.

Note that in the above equation (2), θ _a = θ ₁ = 12
°, β = 10, A ≒ 1.0 (meaning that the aperture is sufficiently large), the optical imaging spot diameter d ≒ 5
0 μm is obtained. Also, θ _a = θ ₂ = 36 °, β = 10
Is obtained, d 17 μm. What is formed as an image after development is different from this imaging spot diameter d.
This is a value obtained by dividing d by a certain coefficient k. This coefficient k is called a spot correction coefficient, which is usually in the range of 1.4 ≦ k ≦ 1.8, and a typical value of the spot correction coefficient k is k =
1.5 (for example, Ota, Ito, Tatsuoka, "Element spacing of LD array for interlaced scanning LBP", Applied Physics Society of Japan, Fall 1991, 11p-ZM-19 (1991), or filed by the present applicant. Japanese Patent Application No. 3-1586
No. 08). Therefore, an image having a minor axis of 33 μm is 1
It will be formed at a distance of 00 μm. From this,
The angle ω = sin ⁻¹ (33/100) ≒ 22 ° between the arrangement direction of the laser spots on the photoconductor surface and the main scanning direction is calculated.

[0012]

Considering the above, considering the optical system of a tilt type multi-beam laser printer, beam shaping is not easy with a tilt type multi-beam laser printer as described below. It turns out that there is a problem. If the image spot on the surface of the photoreceptor may be circular, beam shaping may be performed so that the ellipticity of the laser beam is 1, and in this case, a plurality of image spots on the surface of the photoreceptor are as shown in FIG. Is imaged. However, a problem arises when trying to obtain an elliptical imaging spot. That is, when the arrangement direction of the imaging spots is simply tilted using the conventional beam shaping method, the major axes of the imaging spots S1 and S2 are formed with respect to the moving direction X of the laser beam as shown in FIG. An image is formed in a state where the angle ω coincides with the arrangement direction of the plurality of imaging spots S1 and S2, and a desired vertically long imaging spot cannot be obtained. When the major axes of the imaging spots S1 and S2 are tilted in this manner, when the multi-beam semiconductor laser serving as a light source is rotated for adjusting the pitch between the imaging spots, the size of the imaging spot in the sub-scanning direction is reduced. Is changed.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical system for forming an image by inclining a plurality of elliptical spots having a minor axis in the main scanning direction with respect to the main scanning direction.

[0014]

In order to solve the above-mentioned problems, a multi-beam laser recording apparatus according to the present invention comprises a multi-beam laser recording apparatus in which a plurality of laser spots are arranged on a photoreceptor surface while being inclined with respect to a main scanning direction. In the apparatus, a scanning optical system of the multi-beam laser recording apparatus includes:
At least, a laser array that emits a plurality of laser beams, a collimator optical system that is irradiated with the plurality of laser beams from the laser array, and a laser beam from the collimator optical system that forms a laser beam having a perfectly circular cross section. A first beam shaper to be converted, and a laser beam from the first beam shaper, the operation axis of which is set to a main scanning direction.
And a second beam shaper that converts the laser beam into a laser beam having an elliptical cross section , which is fixed so as to coincide with the direction .

The laser array, the collimator optical system, and the first beam shaper may be provided integrally, and the integrated portion may be rotatable with respect to the second beam shaper. it can.

When the angle between the array direction of the laser array and the main scanning direction is ω, it is preferable that the angle ω be determined by the following equation.

[0017]

(Equation 4) Here, λ is the wavelength of the laser beam, A is the apodization coefficient determined by the aperture of the optical system, r is the distance between the laser arrays, k is the spot correction coefficient, and 1.4 ≦ k ≦
1.8, π is the pi, θ ₁ is the spread angle of the laser beam in the array arrangement direction.

[0018]

The laser beam from the laser array is once converted into a perfect circular beam by the collimator optical system and the first beam shaper, and then shaped by the second beam shaper. The optical axis of the elliptical beam can be set to an arbitrary angle with respect to the arrangement direction. Therefore, the arrangement direction of the plurality of imaging spots is inclined with respect to the main scanning direction, and the minor axis direction of each elliptical laser spot is the main scanning direction.
Each laser spot can be imaged on the photoreceptor surface. Further, by selecting the angle ω between the array direction of the laser array of the light source and the main scanning direction so as to satisfy the expression (3), it is possible to obtain good image reproduction.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The features of the present invention will be specifically described below based on embodiments with reference to the drawings.

FIG. 1 shows an embodiment of an optical system of a multi-beam laser recording apparatus according to the present invention. This optical system includes a semiconductor laser array 1, a light source unit 11 including a first beam shaper 2 also serving as a collimator, a second beam shaper 12 including cylindrical lenses 3 and 4, an aperture 20, a polygon scanner 7, It is composed of a main imaging lens 8 called an f-θ lens, and a surface tilt correction optical system including cylindrical lenses 5 and 9.

[0021] Semiconductor laser laser beam emitted from the array 1 scan lines I ₁ by scanning the photosensitive member 10, I ₂ is formed, an electrostatic latent image formed along the scanning lines I _1, I ₂ is The image is formed by an electrophotographic process (not shown). The photoconductor 10 is driven at a constant speed in the direction of arrow 14. Also, the laser beam is a polygon scanner 7
Scans the photosensitive member 10 in the direction of arrow 13.
The direction of arrow 13 corresponds to the main scanning direction, and the direction of arrow 14 corresponds to the sub-scanning direction. Further, η is the operating axis of the first beam shaper 2, which is the semiconductor laser array 1
In the array direction. ε is the working axis of the second beam shaper 12, which coincides with the main scanning direction. ω is the angle between the action axis η and the action axis ε. In addition,
The working axis is the direction in which the beam shaper changes the beam diameter (divergence angle).

FIG. 2 shows the light source unit 11 and the second beam shaper 1.
2 is shown enlarged. A semiconductor laser array 1 and a first collimator serving as a collimator are placed on a support 15 that can rotate around the optical axis and can be fixed at a desired rotational position.
And the beam shaper 2 are fixed. The support 15 is held at an angle of ω with respect to the main scanning direction by an angle adjusting mechanism (not shown) (see FIG. 1). In the first beam shaper 2 also serving as a collimator, both surfaces of the lens are cylindrical lenses 2a and 2b, respectively. The directions of the axes of the cylindrical lenses 2a and 2b are orthogonal to each other. The second beam shaper 12 is an inverted telescope optical system including the cylindrical lenses 3 and 4. Since the second beam shaper 12 operates only in the main scanning direction, the laser beam diameter in the sub-scanning direction is expanded, and as a result, an elliptical spot whose main scanning direction is shorter in the main scanning direction is formed on the photoconductor surface. I do. The reduction magnification of the second beam shaper 12 is γ
far. Usually, γ is selected around 2.

If there is no second beam shaper 12, a perfect circular spot having a diameter r ₂ ′ is formed as shown in FIG. 5, but if there is a second beam shaper 12, as shown in FIG. 3, the main scanning direction of the minor axis r ₂ elliptical spot (in the sub-scanning direction size remains _{r 2 '(= r 1)} )
Is imaged. Further, the spot interval changes from L ′ (see FIG. 5) to L. In the second beam shaping, the reduction ratio γ = 2
Then, a reduction projection optical system having a magnification of 1/2 (= 1 / γ) is obtained, so that r ₂ = r ₂ ′ / γ = r ₂ ′ / 2 and r ₁ =
r ₂ ′, L = L ′ / γ = L ′ / 2 (in this case,
γ matches the ellipticity). As the spot interval changes, the angle between the arrangement direction of the plurality of spots and the main scanning direction changes from ω in FIG. 5 to ψ in FIG. For example, in the above example, when γ = 2, ω = 22 ° changes to ψ = 40 °. The details of this relationship will be described later.

In the above-mentioned optical system, as shown in FIG. 3, regardless of the angle of the light source unit 11 with respect to the main scanning direction, the spot formed on the photoreceptor surface is an ellipse having a short diameter in the main scanning direction. This is because the laser beam emitted from the light source unit 11 is substantially circular, so that the second
This is because there is no change in the shape of the laser beam incident on the beam shaper 12 of FIG. Since the working axis ε of the second beam shaper is fixed so as to coincide with the main scanning direction, even if the angle of tilting the light source unit 11 for adjustment is changed,
The imaging spot on the photoreceptor surface remains an ellipse having a short diameter in the main scanning direction.

By the way, as described in the section of the prior art,
There is a predetermined relationship between the divergence angle of the laser beam and the diameter of the laser spot to be imaged. Therefore, unless the optical system is designed taking this fact into consideration, good image reproduction cannot be obtained. The following describes how to determine the angle ω between the semiconductor laser array and the main scanning direction.

First, consider the case as shown in FIG.
This corresponds to the case where the second beam shaper 12 is not provided in the embodiment of FIG. The diameter r ₂ ′ of the circular spot in FIG.
, The size of the image finally developed is r ₂ ′ / k (k is a spot correction coefficient) as described in the section of the prior art. Therefore, the scanning line pitch p needs to satisfy the following expression in order to prevent a gap or overlap from occurring between the scanning lines after development.

P = r ₂ '/ k (4) Here, the laser spot arrangement direction in FIG. 5 (η in FIG. 5)
(Axis). By substituting θ _a = θ ₁ , r ₁ = d, and β = β ₁ into equation (2), the scanning line pitch p is expressed by the following equation.

[0028]

(Equation 5) Here, β ₁ is the longitudinal magnification in the η-axis direction in FIG. Therefore, the interval between adjacent laser spots on the photoconductor surface is β ₁ r, and the relationship of sin ω = p / β ₁ r is established. From this, ω is given by the following equation.

[0029]

(Equation 6) Equation (3) shows that the wavelength λ of the laser beam, the apodization coefficient A determined by the aperture of the optical system, the spacing r between the semiconductor laser arrays, the spot correction coefficient k, and the divergence angle θ ₁ of the laser beam in the array direction of the semiconductor laser array. This indicates that the angle ω between the semiconductor laser array and the main scanning direction is determined.

Desirable values of the above parameters are as follows. The wavelength λ of the laser beam is desirably 800 nm or less. Apodization coefficient A is 1.00 ≦ A
≦ 1.97 is desirable, especially 1.34 ≦ A ≦ 1.
A range of 97 is preferred. Specifically, the value of A is changed by changing the diameter of the aperture. The interval r between the semiconductor laser arrays is preferably in the range of 10 μm ≦ r ≦ 100 μm. The spot correction coefficient k is desirably in the range of 1.4 ≦ k ≦ 1.8, and when reversal development is used, 1.4 ≦ k ≦
1.6 and when forward development is used, 1.6 ≦ k ≦ 1.8 is desirable, and particularly, k = about 1.5 is preferred. The divergence angle θ ₁ of the laser beam is 3 ° ≦ θ ₁ ≦
A range of 20 ° is desirable.

Next, the relationship between ω and ψ will be described.
In the embodiment of FIG. 1, since the second beam shaper is provided, the inclination angle ω of the semiconductor laser array with respect to the main scanning direction does not coincide with the inclination angle の of the laser spot array on the photosensitive member surface. The relationship between ω and ψ is as shown in FIG. FIG.
(A) shows a case without the second beam shaper 12, and (b) shows a case with the second beam shaper 12.
That is, the distance in the main scanning direction between adjacent spots is 1 /
γ, but the distance in the sub-scanning direction does not change. Therefore, ψ is expressed by the following equation.

[0032]

(Equation 7) λ = 780 nm, A = 1.00, r = 10 μm, k =
Assuming that 1.5, θ ₁ = 12 °, ω よ り 22 ° from the equation (3).
Is obtained. Next, by substituting ω ≒ 22 ° and γ = 2 into the equation (6), ψ ≒ 40 ° is obtained. This calculated value is as described above. The aperture limit of the optical system cannot be ignored, for example, A = 1.34
Then, ω ≒ 31 ° and ψ ≒ 52 °.

When the above relationships are arranged, first, λ, A,
ω is determined from the parameters such as r, k, and θ1 according to the equation (3), and ψ is determined from the ω and γ according to the equation (6). In other words, the value of ω does not change with γ. That is, Expression (3) is a relational expression that holds regardless of the presence or absence of the second beam shaper 12, and is also applicable to the case where the photosensitive member is scanned with a circular beam. If expression (3) is not satisfied, image formation can be performed,
Since the overlapping of the scanning lines in the sub-scanning direction becomes inappropriate, it is difficult to obtain good image reproduction.

In the above multi-beam laser recording apparatus, it is desirable to design the array interval and the magnification of the optical system so that the image spot interval L in the sub-scanning direction is an integral multiple of the scanning line interval p. This is because the control of the image signal is facilitated as disclosed in Japanese Patent Application Laid-Open No. 56-67277.

[0035]

According to the present invention, even if the angle of inclination of the light source unit is changed for adjustment, the image forming spot on the photoreceptor surface remains an ellipse having a short diameter in the main scanning direction. This has the effect of making it easier. Further, if the inclination angle of the light source unit is selected so as to satisfy a specific relational expression, good image reproduction can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic perspective view showing one embodiment of a multi-beam laser recording apparatus of the present invention.

FIG. 2 is an enlarged schematic perspective view of a light source unit and a second beam shaper in the laser recording apparatus shown in FIG.

FIG. 3 is an explanatory diagram illustrating a state of a laser spot that forms an image on a photoconductor surface in the optical system of the laser recording apparatus illustrated in FIG. 1;

FIG. 4 is a diagram showing that the inclination of the laser spot array on the photoconductor surface with respect to the main scanning direction is changed by a second beam shaper.

FIG. 5 is a view showing a state of a laser spot formed on a photosensitive member surface in a multi-beam laser recording apparatus using a perfect circular beam.

FIG. 6 is a view showing a state of a laser spot formed on a photosensitive member surface in a single-beam laser recording apparatus using an elliptical beam.

FIG. 7 is a schematic diagram showing a beam shape of a laser beam emitted from a semiconductor laser.

FIG. 8 is a diagram illustrating a laser imaging optical system.

FIG. 9 is a graph showing a relationship between a truncation ratio and an apodization coefficient and a passing power coefficient.

FIG. 10 is a view showing a state of a laser spot which forms an image on a photosensitive member surface when a semiconductor laser is attached at an angle in an optical system of a conventional multi-beam laser recording apparatus.

[Explanation of symbols]

1. Semiconductor laser array, 2. First beam shaper,
3, 4, 5: cylindrical lens, 7: polygon scanner, 8: main imaging lens, 9: cylindrical lens, 10: photoconductor, 11: light source unit, 12: second beam shaper, 13: main scanning direction , 14 ... sub-scanning direction, 15 ...
Support for light source unit, 20: aperture, 21: semiconductor laser, 22: collimator, 23: aperture, 2
4 imaging lens, 25 photoreceptor surface, I ₁ , I ₂ scanning line, θ ₁ divergence angle of laser beam in horizontal direction to semiconductor laser junction surface, θ ₂ vertical direction to semiconductor laser junction surface The divergence angle of the laser beam, θ _a … the divergence angle of the laser beam light source, θ _b … the convergence angle of the laser beam on the image plane, f ₁ … the focal length of the collimator lens, f
₂ : focal length of the imaging lens, D: diameter of the laser beam, d: diameter of the imaging spot, η: working axis of the first beam shaper (array direction of the semiconductor laser array),
ε: working axis (main scanning direction) of the second beam shaper, ω:
The angle between the multi-beam semiconductor laser and the main scanning direction (η
And the angle of the epsilon), r ₁ ... imaging major axis of the spot, the short diameter of r ₂ ... imaging spot, r 2 _'... imaging spots of a diameter which is shaped into a true circle, when there is a L ... beam expander Focusing spot spacing, L ': Focusing spot spacing without beam expander

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) B41J 2/44

Claims (3)

(57) [Claims] 1. A multi-beam laser recording apparatus in which a plurality of laser spots are arranged on a photoreceptor surface so as to be inclined with respect to a main scanning direction, the scanning optical system of the multi-beam laser recording apparatus includes at least a plurality of laser beams. Array that emits light, a collimator optical system that is irradiated with a plurality of laser beams from the laser array, and a first beam that converts the laser beam from the collimator optical system into a laser beam having a perfectly circular cross section A shaper and a laser beam from the first beam shaper;
Its working axis is fixed so that it coincides with the main scanning direction.
A second beam shaper for converting the laser beam into a laser beam having an elliptical cross section. 2. The laser array, the collimator optical system, and the first beam shaper are integrally provided, and the integrated portion is rotatable with respect to the second beam shaper. The multi-beam laser recording apparatus according to claim 1, wherein: 3. The multi-beam laser recording according to claim 1, wherein the angle ω is defined by the following equation, where ω is the angle between the array direction of the laser array and the main scanning direction. Apparatus (Equation 1) Here, λ is the wavelength of the laser beam, A is an apodization coefficient determined by the aperture of the optical system and is a real number of A ≧ 1, r is the distance between laser arrays, k is a spot correction coefficient, 1.4 ≦ k ≦ 1.8, π Is the pi, and θ1 is the spread angle of the laser beam in the array direction.