JPH05307151A - Method and device for deflection scanning - Google Patents

Abstract

PURPOSE:To provide the deflection scanning device which has a high resolution and is inexpensive. CONSTITUTION:The laser light L1 emitted by a light source 1 becomes a Bessel beam L2 which has a focus position on a plane A through a Bessel beam generating device 2 and the beam is converged on the reflecting surface of a rotary polygon mirror 4 through a condenser lens 3. The Bessel beam L3 after deflection scanning by the rotary polygon mirror 4 form a beam spot on the surface of a rotary drum D through an image forming lens system 7 consisting of spherical lenses 5 and a toric lens 6. The beam spot has an intensity distribution which is almost proportional to the square of a Bessel function of the 1st class and degree 0 and the depth of focus is large even when the diameter is made small, so the possibility of out-of-forcus is eliminated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a deflection scanning method and apparatus used in laser printers and laser facsimiles.

[0002]

2. Description of the Related Art An example of a deflection scanning device used in a laser printer or a laser facsimile will be described with reference to FIG. The laser light L ₀ generated from the light source 101 is collimated by the collimator lens 102, linearly condensed by the cylindrical lens 103, and irradiated on the reflecting surface of the rotary polygon mirror 104, which is composed of a plurality of mirror surfaces 104 a to 104 d. It Each mirror surface 104a to 1 of the rotary polygon mirror 104
The laser beam L ₀ deflected and scanned by 04d passes through an image forming lens system 107 having a spherical lens 105 and a toric lens 106 for correcting the field curvature, and then the rotary drum D.
And it reaches the photosensitive member on ₀ (not shown). The laser beam L ₀ reaching the photoconductor forms an electrostatic latent image on the photoconductor by the main scanning by the rotation of the rotary polygon mirror 104 and the sub-scanning by the rotation of the rotary drum D ₀ .

Conventionally, the intensity distribution of a point image formed on the surface of a rotary drum by such an optical system is approximate to a Gaussian distribution, and the diameter d of the point image is expressed as follows. (See Optics, Vol. 10, No. 5 (1981) p. 306.) d = 1.27λF (1) where λ: wavelength of laser light F: F number of imaging beam With the progress of development of higher performance laser printers and laser facsimiles, there is a demand for deflection scanning devices with higher resolution and lower manufacturing cost.

[0004]

However, according to the above-mentioned conventional technique, as described above, the intensity distribution of the point image formed on the surface of the rotating drum is approximate to a Gaussian distribution, and the point image In order to reduce the diameter and increase the resolution, as can be seen from the equation (1), it is necessary to reduce the F number of the imaging beam, but an optical system with a small F number has a complicated design, and Since the number of lenses used for this purpose is large, the manufacturing cost is high. In addition, if the diameter of the point image is reduced, the depth of focus decreases in proportion to the square of the diameter. Therefore, the spherical lens and the toric lens that correct the blur of the point image due to the curvature of field must have higher accuracy. As a result, the design of the imaging lens system becomes more complicated, it is not easy to improve the resolution, and the manufacturing cost further increases.

The present invention has been made in view of the above-mentioned unsolved problems of the prior art, and a deflection scanning method and apparatus capable of improving the resolution without increasing the precision of the imaging lens system. It is intended to provide.

[0006]

In order to achieve the above-mentioned object, the method of the present invention is such that the laser light applied to the rotary polygon mirror is deflected and scanned by the rotary polygon mirror to correct the curvature of field. A deflection scanning method for forming an image on a photosensitive surface by an image lens system, wherein a Bessel beam for forming a point image in which the image forming laser light has an intensity distribution substantially proportional to the square of the 0th-order Bessel function of the first type. It is characterized by being.

Further, the apparatus uses a Bessel beam generating means for converting a laser beam into a Bessel beam for forming a point image having an intensity distribution substantially proportional to the square of the 0th-order Bessel function of the first kind, and the Bessel beam. A condenser lens that collects light,
It is composed of a rotary polygon mirror that deflects and scans the focused Bessel beam, and an image forming lens system that forms an image of the deflected and scanned Bessel beam on a photosensitive surface. The image forming lens system corrects curvature of field. It is characterized by

[0008]

With the Bessel beam, even if the diameter of the point image formed on the photosensitive surface is reduced, the point image will not be blurred because the depth of focus is large.

An apparatus for forming a special laser beam from a laser beam emitted from a He-Ne laser light source to form a point image having an intensity distribution substantially proportional to the square of the 0th-order Bessel function of the first kind is known. Many laser beams have been developed, and the laser beam formed by these devices is usually called a non-diffractive beam or Bessel beam (hereinafter referred to as "Bessel beam"), and it is known that the depth of focus is particularly deep. Has been.

Theoretically, it is possible to form a Bessel beam which forms a point image having an intensity distribution I ₀ (r) proportional to the square of the 0th-order Bessel function of the first kind (Durni).
n: J. Opt. Soc. Am. A, vol. 4, N
o. 4, p. 651, (1987)). The intensity distribution I ₀ (r) is expressed as follows.

I ₀ (r) = A · J ₀ ² (αr) (2) where r: distance from the optical axis A: constant α: constant Bessel beam formed by an actual device It is known that the intensity distribution I ₁ (r) of the point image of is approximated by multiplying the square of the 0th-order Bessel function of the first kind by the Gaussian function (Gori et al., Optics Communic).
ations, vol. 64, No. 6, p. 491,
(1987)).

That is, I ₁ (r) = AJ ₀ ² (αr) exp [-2 (r / W ₀ ) ² ] ... (3) FIG. 11 shows that the parameter W ₀ is relatively large. The intensity distribution of the point image of the Bessel beam represented by the equation (3) is shown.
Such a Bessel beam has an extremely large depth of focus as compared with a laser beam (hereinafter, referred to as a “Gaussian beam”) that is formed into a point image having an intensity distribution similar to a Gaussian distribution, and has a spot diameter and a luminous flux diameter. Depending on the example, the depth of focus is about 30 times that of a Gaussian beam.

[0013]

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

FIG. 1 is an explanatory view for explaining the first embodiment. A deflection scanning device E ₁ of this embodiment comprises a light source 1 for generating a laser beam L ₁ and a laser beam emitted from the light source 1. L ₁
The Bessel beam generator 2 is Bessel beam generating means to Bessel beam L ₂ to the plane A focal position across the light path, for focusing the Bessel beam L ₂ which diverge after passing through the plane A Condenser lens 3
A rotary polygonal mirror 4 for deflecting and scanning the Bessel beam L ₂ focused by the condenser lens 3 toward the surface of the rotary drum D, which is a photosensitive surface, and between the rotary polygonal mirror 4 and the rotary drum D. And an imaging lens system 7 having a spherical lens 5 for correcting field curvature and a spherical lens 5'provided in
The rotary polygon mirror 4 has a plurality of mirror surfaces 4a-4d on its outer peripheral surface.
And the distance between the plane A and the condenser lens 3 and the distance at which the optical axis portion of the Bessel beam L ₂ is incident on the reflection surface of the condenser lens 3 and the rotary polygon mirror 4 are both. Is equal to the focal length of the condenser lens 3. Further, the imaging lens system 7 corrects the field curvature of the Bessel beam L ₃ deflected and scanned by the rotary polygon mirror 4 and forms an image on the surface of the rotating drum D to form a beam spot which is a point image. Is designed. That is, the plane A and the rotary drum D
The surface of is optically conjugated. In addition, the condenser lens 3
Has the same positive refractive power in the main scanning direction and the sub scanning direction.

Laser light L ₁ emitted from the light source ₁
Becomes a Bessel beam L ₂ with the plane A as the focal position by the Bessel beam generating means 2, is condensed on the plane A, and then diverges. The diverging Bessel beam L ₂ is deflected and scanned by the rotary polygon mirror 4 through the condenser lens 3, and the deflected and scanned Bessel beam L ₃ is imaged on the surface of the rotating drum D by the imaging lens system 7, and the surface of the surface is rotated. An electrostatic latent image is formed on a photoconductor (not shown). The focal length of the condenser lens 3 is f ₀ , and the focal length of the imaging lens system 7 is f _1.
Then, the diameter of the beam spot formed on the surface of the rotating drum D is f ₁ / f ₀ times the diameter of the beam spot formed on the plane A, and the diameter of the beam spot formed on the surface of the rotating drum D is The depth of focus is (f ₁ / f ₀ ) ² times the depth of focus of the beam spot formed on the plane A.

FIG. 2 shows a characteristic curve of the imaging lens system 7 and shows a change in the amount of curvature of field B on the rotary drum D according to the scanning angle θ by the rotary polygon mirror 4. In the figure, a curve M represents a change in the amount of curvature of field in the main scanning direction, and a curve S represents a change in the amount of curvature of field in the sub scanning direction. Further, the deviation of the image plane position calculated from the depth of focus of the Gaussian beam, that is, the allowable value of the defocus amount is about 0.3 mm,
In this figure, it is represented by the distance R between the straight lines R ₁ and R ₂ . Further, the allowable value of the defocus amount calculated from the depth of focus of the Bessel beam is about 10 mm, which is represented by the distance T between the straight lines T ₁ and T ₂ in this figure. Said curves M and S,
By comparing the straight lines R ₁ , R ₂ , T ₁ , and T ₂ , the field curvature in the main scanning direction exceeds the allowable value R of the defocus amount of the Gaussian beam as the scanning angle θ increases. However, it is understood that there is no possibility of exceeding the allowable value T of the defocus amount of the Bessel beam. That is, the beam spot formed by the Bessel beam on the rotary drum D has a large depth of focus and therefore does not blur over the entire scanning angle.

FIG. 3 shows the results of an experiment in which the intensity distribution of the beam spot imaged on the rotating drum is measured by changing the defocus amount. FIG. 3A shows the defocus amount. The intensity distribution of the beam spot when Z is 0, and (b) and (c) show the intensity distribution of the beam spot when the defocus amount Z is 5 mm and 10 mm, respectively. As is clear from FIG. 3, if the defocus amount is within 10 mm, the intensity distribution of the beam spot with a diameter of about 20 μm hardly changes. Also, the same result as this could be obtained by the calculation using the above-mentioned formula (3).

According to the present embodiment, since the Bessel beam has a deep depth of focus, the beam spot does not blur, and therefore, a high precision imaging lens system is not used,
A deflection scanning device having an extremely high resolution can be realized.

Next, various devices used as the Bessel beam generator of this embodiment will be described.

FIG. 4 shows a narrow ring-shaped slit opening 21.
And a lens 22 to generate a Bessel beam (Durnin et al .: Physical Review
Letters, vol. 58, No. 15, p. 1
499, (1987)), the slit aperture 21 is arranged at the front (front) focus position on the object side of the lens 22, and the laser light L ₄ collimated by a known means is collected by the lens 22 via the slit aperture 21. A beam spot having an intensity distribution proportional to the square of the 0th-order Bessel function of the first kind is formed on the plane A ₄ which is the rear (rear) focal position of the image side illuminated.

FIG. 5 shows that the aperture of the resonator in the laser light source is made into a narrow ring-shaped slit and a Bessel beam is generated by using the ring-shaped reflecting mirror 23, lens 24 and output mirror 25 (Uehara). : Applied Physics, vol.5
9, No. 6, p. 746 (1990)), reflecting mirror 2
3 and the output mirror 25 are arranged at the front focus position and the rear focus position of the lens 24, respectively. This apparatus can generate a high intensity Bessel beam by using a laser medium such as an argon laser tube.

FIG. 6 shows a Bessel beam generated by using a conical prism 26 (Kawada, Arimoto: Proceedings of Spring Society of Applied Physics, pp. 829, 30p-A-4, (19).
91), laser beam L ₆ collimated by a known means
Forms a Bessel beam having a vertex 26a of the conical prism 26 as one end and being converged in a shaded area in the drawing and having a plane A ₆ as a focal position. This device is also suitable for generating a high intensity Bessel beam.

FIG. 7 shows a Bessel beam generated using a phase / amplitude filter 61 for changing the phase and amplitude of a light wave. The phase / amplitude filter 61 has an amplitude transmissivity approximated to a 0th-order Bessel function of the first kind. Hold. The laser beam L ₇ collimated by a known means passes through the phase / amplitude filter 61 to become a Bessel beam having the phase / amplitude filter 61 as a focal position. Also, the phase,
Instead of the amplitude filter, a phase filter that changes only the phase of the light wave and an amplitude filter that changes only the amplitude can be used.

The phase and amplitude transmittance C of the amplitude filter 61 changes as shown in FIG. 8 according to the distance r from the optical axis. If the intensity distribution of the laser light L ₇ is uniform, the following equation is established. To do.

C (r) = J ₀ (αr) where α: a constant Therefore, the intensity distribution of the laser light transmitted through the phase / amplitude filter 61 at the focus position is J ₀ ² (αr). However, in practice, it is difficult to make the intensity distribution of the laser beam L ₇ completely uniform, and the intensity distribution of the laser beam L ₇ approximates to the one expressed by the above-mentioned equation (3).

FIG. 9 is an explanatory view for explaining the second embodiment, in which the deflection scanning device E ₂ of this embodiment is replaced with the condenser lens 3 of the first embodiment in the main scanning direction and the sub scanning direction. The anamorphic lens 33 having different positive refracting power is used, and the anamorphic image forming lens system 7'comprising the spherical lens 5 and the toric lens 6 is also used as the image forming lens system. The other members or means are the same as those in the first embodiment, so they are designated by the same reference numerals and the description thereof is omitted.

By using the anamorphic lens 33, the plane A and the surface of the rotary drum D are conjugate with each other in the main scanning direction, and as shown in FIG. 10, the plane A and the rotary polyhedral surface are in the sub scanning direction. An optical system is formed in which the position where the Bessel beam L ₂ is incident on the mirror 4 and the surface of the rotating drum D are conjugate with each other. In such an optical system, the beam spot of the Bessel beam formed on the plane A can be expanded or contracted on the surface of the rotating drum D, and the Bessel beam is condensed on the rotating polygon mirror 4. It is also possible to correct the surface tilt of the beam spot in the sub-scanning direction due to the inclination of the reflecting surface of the rotary polygon mirror 4.

Needless to say, an optical system similar to the above optical system can be made by using a plurality of anamorphic lenses. If the slit opening 21 of the Bessel beam generator of FIG. 4 is formed into an elliptical ring shape to generate an elliptical beam spot on the plane A, the rotating drum D
An elliptical beam spot can also be obtained above. In addition, the conical prism 26 of the Bessel beam generator shown in FIG.
The same applies when is replaced by an elliptic cone prism.

Further, as the photosensitive member used in the first and second embodiments, not only the electrophotographic photosensitive material but also a silver salt photosensitive material may be used.

[0030]

Since the present invention is configured as described above, it has the following effects.

The diameter of the point image formed on the photosensitive surface can be reduced without improving the accuracy of the imaging lens system. Therefore,
The resolution can be improved without increasing the manufacturing cost of the deflection scanning device. As a result, it is possible to realize a low cost, high performance laser facsimile or laser printer.

[Brief description of drawings]

FIG. 1 is an explanatory diagram illustrating a first embodiment.

FIG. 2 is a diagram showing characteristics of an imaging lens system of a deflection scanning device.

FIG. 3 is a graph showing a result of measuring an intensity distribution of a beam spot on a rotating drum by changing a defocus amount.

FIG. 4 is a diagram illustrating an example of a Bessel beam generator.

FIG. 5 is a diagram illustrating another example of the Bessel beam generator.

FIG. 6 is a diagram illustrating another example of the Bessel beam generator.

FIG. 7 is a diagram illustrating another example of the Bessel beam generator.

8 is a graph showing a change in amplitude transmittance of the device of FIG.

FIG. 9 is an explanatory diagram illustrating a second embodiment.

10 is a cross-sectional view of the device of FIG. 9 taken along line XX.

FIG. 11 is a graph showing the intensity distribution of a Bessel beam point image.

FIG. 12 is an explanatory diagram illustrating a conventional example.

[Explanation of symbols]

L ₁ laser light L ₂ , L ₃ Bessel beam D Rotating drum 1 Light source 2 Bessel beam generating means 3 Condensing lens 4 Rotating polygonal mirror 5, 5'Spherical lens 6 Toric lens 7, 7 'Imaging lens

Claims (3)

[Claims] 1. A deflection scanning method in which a laser beam applied to a rotary polygonal mirror is deflected and scanned by the rotary polygonal mirror, and an image is formed on a photosensitive surface by an imaging lens system that corrects curvature of field. A deflection scanning method, wherein the imaging laser light is a Bessel beam that forms a point image having an intensity distribution substantially proportional to the square of the 0th-order Bessel function of the first kind. 2. Laser light of the first-order zero-order Bessel function 2
Bessel beam generating means for forming a Bessel beam for forming a point image having an intensity distribution substantially proportional to the power, a condenser lens for condensing the Bessel beam, and a rotary polygon mirror for deflecting and scanning the condensed Bessel beam. And a focusing lens system for focusing the deflected and scanned Bessel beam on a photosensitive surface, and the focusing lens system corrects field curvature. 3. The deflection scanning device according to claim 2, wherein the condenser lens is an anamorphic lens.