JP2559501B2 - Laser beam shaping optics - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a laser beam shaping optical system, and in particular, an acousto-optic element is used to divide a laser beam into a plurality of beams, and scanning exposure is performed by the plurality of laser beams. The present invention relates to a laser beam shaping optical system for making a plurality of laser beams emitted from an acousto-optic element of an optical system incident on a polygon mirror surface.

[Problems to be Solved by Prior Art and Invention]

A laser beam printer, an L-COM (laser computer output microfilmer), and the like are provided with a scanning optical system for two-dimensionally scanning a laser beam on a recording material. This scanning optical system consists of a polygon mirror (rotating polygonal mirror) for sweeping the laser beam oscillated from the laser oscillator in the main scanning direction, and a potentiometer mirror or resonant scan for sweeping the laser beam in the sub scanning direction. It is composed of a sub-scanning system such as a trunk and a scanning lens. The central axis of the polycon mirror is directly connected to the axis of the motor and is rotated at a constant rotation speed. When the laser beam is incident on the mirror surface of the polygon mirror, the reflected light is swung by twice the rotation angle of the mirror, and thereafter the laser beam is swept onto the recording material through the scanning lens as the polygon mirror rotates. Become.

However, in reality, the distance between the straight lines drawn on the recording material in the sub-scanning direction is not constant because of the amount of surface tilt of the polygon mirror. Therefore, conventionally, as shown in FIG. 4, the first cylindrical lens 30 and the second cylindrical lens 30 having the lens power only in the sub-scanning direction, in which the inclination of each surface of the polygon mirror before and after the polygon mirror 20, poses a problem. By arranging the cylindrical lens 34, the inclination of each surface of the polygon mirror is corrected. First cylindrical lens 30
Is a collimator lens (not shown) and a polygon mirror 20.
It is arranged in the optical path between and, and narrows the laser beam only in the sub-scanning direction to form an image on the mirror surface of the polygon mirror 20. The second cylindrical lens 34 is an fθ lens.
It is arranged in the optical path between 32 and the recording surface 36. The laser beam reflected by the polygon mirror 20 is fθ lens 32.
And the second cylindrical lens 34 forms an image on the recording surface 36. At this time, if the fθ lens 32 and the second cylindrical lens 34 are considered as one combined lens,
For this combined lens, the image on the polygon mirror surface becomes the object point and the image on the recording surface becomes the image point, and the relationship becomes conjugate. Therefore, all the laser beams emitted from the object point are always focused on the image point, so that the beam position on the recording surface does not change in the sub-scanning direction even if the polygon mirror surface is tilted. As a result, the correction due to the inclination of the polygon mirror surface (surface tilt correction) can be performed.

In a general laser beam printer, a laser beam in which the intensity distribution of a laser beam emitted from a laser oscillator is a normal distribution (Gaussian distribution) is used, and such a Gaussian laser beam follows a Gaussian beam propagation side, Diversify uniformly from the beam waist. As a characteristic of this Gaussian beam, the intensity distribution is a normal distribution at any position in the propagation optical path,
When using the surface tilt correction optical system shown in FIG. 4, it is not necessary to consider the relay of the laser beam.

By using an acousto-optic modulation deflector (hereinafter, referred to as multi-frequency AOM) that modulates and deflects the incident laser beam in a direction according to the frequency of the incident ultrasonic wave, a plurality of elements arranged in the sub-scanning direction by deflection are used. When the laser beam is modulated for scanning exposure, the laser beam is split into a plurality of planes only on the Fourier transform plane of the diffraction plane of the AOM.
On the other hand, when the surface tilt correction optical system shown in FIG. 4 is used, it is necessary to form a conjugate relationship so that the object point on the polygon mirror surface is imaged on the recording surface at a predetermined magnification. For this reason, in order to perform surface tilt correction using a multi-frequency AOM and obtain spots of the laser beam split into a plurality of beams on the recording surface receiver, on the polygon mirror surface, the beam diameter on the recording surface is set to the above value. It is necessary to make a laser beam having a diameter divided by a predetermined magnification incident in a properly separated state.

Further, in the main scanning direction, in order to prevent the influence of dust adhering to the polygon mirror surface, and because of the design problem of the scanning lens, a considerably large beam diameter is required as compared with the sub scanning direction.

The present invention has been made to meet the above demands,
It is an object of the present invention to provide a laser beam shaping optical system that simultaneously satisfies the requirements for a surface tilt correction optical system using a polygon mirror and the requirements for a Fourier transform surface relay optical system using a multi-frequency AOM. To do.

[Means for solving the problem]

In order to achieve the above-mentioned object, the invention according to claim (1) is such that a plurality of lasers emitted from an acousto-optic device that deflects the incident laser beam in a direction according to the frequency of the incident ultrasonic wave and emits it. A laser beam shaping optical system that shapes a beam.The Fraunhofer diffraction image is formed at the final focus position by an odd number of lens powers where the focus positions overlap each other in the arrangement direction of multiple laser beams. The laser beams are made parallel by an even lens power with respect to the direction intersecting the arrangement direction of the plurality of laser beams.

Further, the invention according to claim (2) shapes a plurality of laser beams emitted from an acousto-optic device that deflects the incident laser beam in a direction according to the frequency of the incident ultrasonic wave and emits the laser beam. A laser beam shaping optical system that is incident on the polygon mirror surface, and the Fraunhofer diffraction of the deflection position of the acousto-optic element on the polygon mirror surface by the odd lens powers in which the focal points overlap each other in the sub-scanning direction. An image is formed and the laser beam is made parallel to the main scanning direction by an even lens power.

[Action]

Hereinafter, the operation of the present invention will be described. The acousto-optic element deflects the incident laser beam in a direction according to the frequency of the incident ultrasonic wave and emits a plurality of laser beams. The laser beam shaping optical system forms a Fraunhofer diffraction image of the deflection position of the acousto-optic element at the final focus position by the odd lens power where the focus positions overlap each other in the arrangement direction of the multiple laser beams. To do. For this reason, after the laser beam is deflected by the acousto-optic device, it is correctly relayed by its Fourier transform plane to form a Fraunhofer diffraction image at the final position, that is, a diffraction image in which the laser beam is separated into a plurality of images. To do. Therefore, the polygonal mirrors are arranged so that the arrangement direction of the plurality of laser beams is directed in the sub-scanning direction and the Fraunhofer diffraction image of the deflection position is formed on the polygon mirror surface, and the laser beam is arranged on the exit side of the polygon mirror. By disposing a part of the surface tilt correction optical system including the scanning lens and the second cylindrical lens shown in FIG.
Trouble correction can be performed.

Further, the laser beam shaping optical system makes the laser beams parallel to each other by an even lens power with respect to the direction intersecting the arrangement direction of the plurality of laser beams. Therefore, when the polygon mirror is arranged so that the direction intersecting the arrangement direction of the plurality of laser beams faces the main scanning direction, a laser beam having a large diameter in the main scanning direction can be obtained.

〔The invention's effect〕

As described above, according to the present invention, since the laser beam is correctly relayed on the Fourier transform plane and the laser beam having a large diameter in one direction is obtained, it is troublesome to apply it to the scanning optical system equipped with the polygon mirror. It is possible to obtain an effect that it is possible to efficiently prevent the influence of the correction of the dust and the dust.

〔Example〕

Hereinafter, examples of the present invention will be described in detail. In this embodiment, a laser beam having a large diameter in the main scanning direction and a plurality of separated laser beams in the sub scanning direction is formed on the mirror surface 22 of the polygon mirror 20. As shown in FIG. 1, the deflection surface 24 of the AOM 10 is located on the laser beam emitting side of the AOM 10.
Is located on the first focal plane, the converging lens 12 having the focal point distance fa is arranged. On the laser beam emission side of the converging lens 12, the sub-scanning direction beam shaping cylindrical having the focal point distance fbf and having the lens power only in the sub-scanning direction so that the first focal plane overlaps with the second focal plane of the converging lens 12. A lens 14 is arranged. Sub-scanning direction beam shaping The laser beam emitting side of the cylindrical lens 14 has a lens power only in the main scanning direction so that the first focus overlaps with the second focus of the converging lens 12 A shaping cylindrical lens 16 is arranged.
Further, on the laser beam emission side of the main scanning direction beam shaping cylindrical lens 16, a lens unit having a lens power only in the sub scanning direction so that the first focal plane overlaps the second focal plane of the sub scanning direction beam shaping cylindrical lens 14. Point distance is
The fcy tilt correction cylindrical lens 18 is arranged. The polygon mirror 20 is arranged so that the second focal plane of the tilt correction cylindrical lens 18 and the mirror surface 22 overlap each other. Therefore, the mirror surface 22, the AOM polarization surface 24,
The focal planes 26 and 28 are beam waist planes.

The operation of this embodiment will be described below. In the main scanning direction, as shown in FIG. 2, only the converging lens 12 and the main scanning direction beam shaping cylindrical lens 16 have lens power. Therefore, a beam expander is configured by the converging lens 12 and the main scanning direction beam shaping cylindrical lens 16, and the laser beam emitted from the cylindrical lens 16 is converted into a parallel laser beam expanded in the main scanning direction to be mirror surface 22. Is irradiated.

In the sub-scanning direction, as shown in FIG. 3, a converging lens 12 and a beam-shaping cylindrical lens 14 in the sub-scanning direction.
And the tilt correction cylindrical lens 18 has lens power. Since the focal planes of the converging lens 12, the sub-scanning direction beam shaping cylindrical lens 14 and the tilt correction cylindrical lens 18 are overlapped, the focusing lens 12 to the cylindrical lens 14 and the cylindrical lens 14 to the cylindrical lens 16 are relayed in order on the Fourier transform surface. A Fraunhofer diffraction image of the deflection surface 24 of the AOM 10 is formed on the mirror surface 22. This makes the mirror surface 22
A plurality of laser beams separated in the sub-scanning direction are imaged on the top. The image of the focusing surface 26 becomes a Franhofer diffraction image, and the image of the focusing surface 28 becomes one ellipse.

As a result, a laser beam having a diameter in the main scanning direction larger than that in the sub scanning direction and separated into a plurality of laser beams in the sub scanning direction is formed on the mirror surface 22. Therefore,
Between the polygon mirror 20 and the recording surface 36, f shown in FIG.
By arranging an optical system including the θ lens 32 and the second cylindrical lens 34, surface tilt correction can be performed. Also, since the beam diameter in the main scanning direction is considerably larger than the beam diameter in the sub-scanning direction, recording can be performed without being affected by dust adhering to the polygon mirror surface, and the scanning lens can be designed freely. The degree improves.

The beam shaping cylindrical lens 16 in the main scanning direction
The mirror surface 22 of the polygon mirror 20 is arranged so as to overlap the second focal plane of the polygon mirror 20. The focal length of the beam shaping cylindrical lens 16 in the main scanning direction is the focal length f _bs shown in FIG.
It may be larger or smaller.
Further, although the example in which the cylindrical lens is used as the lens other than the converging lens has been described above, the cylindrical lens having power in the main scanning direction and the sub-scanning direction may be formed of a spherical lens or a toric lens. Further, the converging lens 12 may be composed of two, that is, a cylindrical lens having a lens power in the sub scanning direction and a cylindrical lens having a lens power in the main scanning direction.
In addition, the cylindrical lens 16 is replaced with a cylindrical lens.
Although it is arranged between 14 and 18, it may be arranged at any position as long as it constitutes the beam expander. Furthermore, the lens power in the main scanning direction does not have to be two and may be an even number, and the lens power in the sub scanning direction does not have to be three and may be an odd number.

[Brief description of drawings]

 FIG. 1 is a perspective view showing an optical system of an embodiment of the present invention, FIG. 2 is a diagram showing an optical path in the main scanning direction of this embodiment, and FIG. 3 is an optical path of a sub scanning direction in this embodiment. A diagram and FIG. 4 are side views of a conventional surface tilt correction optical system.

Claims (2)

(57) [Claims] 1. A laser beam shaping optical system for shaping a plurality of laser beams emitted from an acousto-optical element for deflecting an incident laser beam in a direction according to a frequency of an incident ultrasonic wave and emitting the deflected laser beam. , The Fraunhofer diffraction image of the deflection position of the acousto-optic device is formed at the final focus position by an odd number of lens powers in which the focus positions overlap with each other in the arrangement direction of the multiple laser beams. A laser beam shaping optical system that collimates a laser beam with an even lens power in a direction intersecting the arrangement direction of. 2. A laser for shaping a plurality of laser beams emitted from an acousto-optic device for deflecting the incident laser beam in a direction according to the frequency of the incident ultrasonic wave and for emitting the incident laser beam to a polygon mirror surface. This is a beam shaping optical system, in which the Fraunhofer diffraction image of the deflection position of the acousto-optic element is formed on the polygon mirror surface by an odd number of lens powers in which the focal positions overlap each other in the sub-scanning direction. A laser beam shaping optical system that collimates a laser beam with an even lens power in the scanning direction.