JPS58100117A - Laser beam scanning method - Google Patents

Abstract

PURPOSE:To correct a curved scan to a linear scan by varying control over an angle of beam incidence for the inclination correction of a rotary polygon mirror during a scanning period. CONSTITUTION:The deflection angle of the mirror 22 of an electrostrictive strain device 2 which performs deflection by the output of a controller 8 corresponding to the angle of inclination of each mirror surface of a rotary polygon mirror 3 corresponding to the optical axis 91 of an f.theta lens 9 is corrected for every scanning line by an extent corresponding to the amount of curvature DELTA=2phi.f.{cos(alpha-theta)-cosalpha}/cos2theta, thus correcting the angle of inclination. In practice, the correction is made enough in the form of DELTA=ktheta approximately (k: constant depending upon (f), phi, and alpha). Instead of the electrostrictive strain device, an acoustooptic element may be used to correct the angle of beam incidence to the rotary polygon mirror.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention can be used when performing laser recording by injecting a laser beam into a rotating polygon mirror and irradiating the reflected beam onto an image carrier, or when performing laser recording by irradiating one beam onto a document. Regarding a laser beam scanning method that automatically corrects scanning line pip unevenness caused by the inclination angle of a rotating polygon mirror when performing laser reading with The present invention relates to a laser beam scanning Ah method that can correct the resulting curved scan into a straight scan.

For example, in a beam scanning laser printer, due to the angle error of the rotating polygon mirror (the error in the angle between each reflecting mirror surface and the rotation axis, and the angle rlL error caused by the inclination of the rotation axis),
A deviation of the beam spot in a direction perpendicular to the main scanning direction causes so-called scanning line pitch unevenness. As a device for suppressing this pitch unevenness to the extent that it does not adversely affect images, the applicant of the present application developed a laser beam scanning device that corrects the tilt angle of a rotating polygon mirror as shown in FIG. has applied. In FIG. 1, 1 is a semiconductor laser device having a laser diode inside, and 2 is an electrostrictive device. This electrostrictive device [2 uses a piezo bimorph element 21 that bends in the direction of the arrow in the figure according to the applied voltage,
A mirror 22 is provided on the side surface of the tip, and the lower end is fixed to a holding member 23. Reference numeral 3 denotes a rotating polygon mirror, which rotates at a constant speed in the direction of the arrow in the figure in order to scan the laser beam in the main scanning direction X. 4 is a cylindrical image holder (photosensitive drum) that rotates in the sub-scanning direction Y;
After the cylindrical surface is uniformly charged by the charger 5, an electrostatic latent image is formed by laser beam irradiation. This electrostatic latent image is converted into a hard copy with the help of a developing device and a heat fixing device (not shown). Reference numeral 7 denotes a laser diode driver having various functions such as providing a modulated drive current to the semiconductor laser device 1 based on image information. Reference numeral 6 denotes a photodetector that detects the laser beam at the scanning start end and supplies its output signal to the laser diode drive device 7 as a synchronization signal in the main scanning direction. This synchronization signal is a signal useful for aligning the starting points of image information in each main scan. Reference numeral 8 denotes a control device a which generates an electric signal (1@moon) corresponding to the inclination angle of each mirror surface of the rotating polygon mirror 3 and drives the electrostrictive device@2. Reference numeral 9 denotes an f.theta. lens for focusing the laser beam on the surface of the image holder 4. Note that 12 is a lens system disposed on the optical path of the laser beam 11 traveling from the semiconductor laser device [1 to the mirror 22.

In such a configuration, the laser beam 11 emitted from the semiconductor laser device 1 passes through the lens system 12, enters the mirror 22 of the electrostrictive device 2, is reflected there, and then enters the rotating polygon 13.

The reflected beam from the multifaceted surface 11!3 passes through the f·θ lens 9 and forms an image on the surface of the image carrier 4. This imaging spot is modulated ON/OFF according to the image information, and this ON/OFF! The f-tuned beam is scanned in the X direction, and an electrostatic latent image is formed on the image carrier 4 based on image information. In this case, the control device 8 operates the electrostrictive device 2 at a constant voltage during the scanning period so that the mirror 22 of the electrostrictive device 2 is tilted at the same angle as the inclination angle φ of the scanning mirror surface 31 as shown in FIG. Drive. Therefore, the reflected beam at the scanning mirror surface 31 is
The light enters the .theta. lens 9 parallel to the optical axis, and is imaged on the scanning @L parallel to the axis of the image carrier 4.

However, with respect to the rotation of the rotating polygon mirror 3, the position of the mirror 22 within one surface of the polygon mirror is fixed;
On the image holder 4, the scanning beam is not parallel to the main scanning direction as shown in FIG.
Dotted line L'). For example, assume that the inclination angle of the mirror 22 is determined so that the scanning beam at a certain rotational position of the rotating polygon 1i3 coincides with the optical axis 91 of the [.theta. lens 9. Then, if the inclination angle of the scanning mirror surface 31 is φ, the angle between the beam incident on the rotating polygon mirror 3 and the optical axis 91 of the f/θ lens 9 is α, and the focal length of the f/θ lens 9 is f, then The amount of deviation Δ of the beam spot from the scanning line when the rotating polygon 113 is rotated by a rotation angle θ from the rotational position is expressed as follows.

Δ−2・f・φ (COS(α−0> −cos
α)/cos 2 θ・・・・・・ (1) Generally, due to problems in the arrangement of optical system components, α is usually set to 90 degrees, so for example, α-90', f-25
If it is 0 mm, the deviation amount Δ (μm) with respect to the rotation angle θ (degrees) in the case of φ...10 seconds, 120 seconds, and 30 seconds is as shown in FIG. This deviation amount Δ is too large to be ignored compared to the beam spot diameter (for example, about 100 μm fog), and there is a problem that the recorded image is distorted.

The present invention has been made in view of these points, and its purpose is to scan the beam spot 1i1.
An object of the present invention is to provide a laser beam scanning method that can eliminate the amount of deviation from L.

The method of the present invention that achieves this objective includes changing the incident angle of the laser beam incident on the scanning mirror surface appropriately during the scanning period, so that the laser beam scans the image carrier in a straight line. This is a characteristic feature.

The present invention will be described in detail below with reference to the drawings.

For example, the rotating polygon 1 with respect to the optical axis 91 of the t/θ lens 9
It is assumed that the angles of inclination of each of the 13 mirror surfaces are as shown in FIG. 5(A).

According to the device shown in FIG. 1, the deflection angle (inclination angle) of the mirror 22 of the electrostrictive device 12 that is deflected by the output of the Ill III device 8 is as shown in FIG. (
It is driven in a one-to-one correspondence with the inclination angle of the mirror surface in (b). The conceptual configuration of the device for carrying out the method of the present invention is the same as that shown in FIG. It differs from the one shown in FIG. 1 in that it is configured to generate a voltage. With the output voltage of such a control device, the electrostrictive device 2
As shown in FIG. 5C, the mirror 22 follows the rotation of the rotating polygon 113 for each scan, that is, the deflection angle changes in accordance with the rotation angle θ. The laser beam is deflected in such a way that the laser beam is reduced, resulting in a linear scan of the laser beam. At the first position of the scanning line L' shown in FIGS. 3 and 4, the magnitude of the deflection angle decreases, but depending on the optical system, the magnitude of the deflection angle increases as the mirror surface rotates. Sometimes it is controlled.

Incidentally, the shift collapse with respect to the rotation angle θ in equation (1) is actually nonlinear, and in order to generate a nonlinear output voltage corresponding to this, the control device requires an extremely complicated configuration. In practice, instead of formula (1), Δakθ (where k is f, φ, α
You may use an approximation formula of the constant determined by . FIG. 6 shows the tl for generating the voltage corresponding to the amount of deviation Δ of the beam spot from the main scanning line expressed by such an approximation formula.
lJ control device output stage circuit configuration diagram d In the same figure,
61 is an integrator, and 63 is a summing amplifier.The integrator 61 integrates the voltage E1 selected corresponding to the inclination angle of the scanning mirror surface at the start of scanning, and calculates the integrated voltage E1- (-Ei /
CR)·t (where CR is the time constant of the integrator and t is the integration time). Further, the output voltage of the integrator 61 is reset by a switch 62 every time each scan is completed. The summing amplifier 63 adds and amplifies the integrated voltage E1 and the voltage Ei, and by setting the constant of each resistor to be the same, the output voltage EO expressed as Eo - Ei (1-t /CR) is obtained. This is what you get.

This voltage Eo is a voltage whose absolute value decreases linearly from the initial value Ei for each scan, and by driving the electrostrictive device l12 with such a voltage, the deviation Δ is corrected and the laser beam can be scanned in a straight line.

In addition, in the example, a case was shown in which an electrostrictive device was used as a means for correcting the inclination angle of the rotating polygon mirror. You can also correct the angle. Naturally, the method of the present invention can also be applied to such cases.

Further, although the above explanation was about a laser printer using a photosensitive drum, the method of the present invention can also be applied to other types of laser recording or laser reading. Therefore,
The surface to be scanned in the method of the present invention is made of a photoconductive insulator (selenium, 0()c, 7nO, amorphous silicon, etc.)
(1) Not limited to photoreceptors, silver salt photosensitive materials, photosensitive resins.

Various materials such as thermosensitive recording materials such as thermoplastic materials will be formed.

As explained above, according to the present invention, it is possible to easily correct the curved scanning of the laser beam that results when correcting the angle of the incident beam on the rotating polygon mirror to convert it into linear scanning. .

[Brief explanation of the drawing]

Fig. 1 is a configuration diagram of a laser printer including a laser beam scanning device that corrects the inclination angle of a rotating polygon mirror, Fig. 2 is an explanatory diagram of a method for correcting the inclination angle of a scanning mirror surface, and Fig. 3
Figure 4 is an explanatory diagram showing the scanning mode of the laser beam in the device, Figure 4 is an explanatory diagram showing the deviation amount Δ, Figure 5 is an explanatory diagram showing the inclination angle of the scanning mirror surface and the deflection angle of the mirror, and Figure 6 is an explanatory diagram showing the inclination angle of the scanning mirror surface and the deflection angle of the mirror. FIG. 2 is a configuration diagram of a circuit for generating a voltage that changes in accordance with the rotation of a rotating polygon mirror. 2... Electrostrictive device I 22... Mirror 3...
Rotating polygon 14...image holder 8...control device 9...f/θ lens 61...
...Integrator 63...Summing amplifier patent applicant
Konishi Roku Photo Industry Co., Ltd. Representative Patent Attorney Osamu Ijima Fuji'p51 Atsushi Figure 5 Figure 6 Figure 1

Claims (2)

[Claims] (1) A laser beam is incident on a rotating polygon mirror, and the
In a laser beam scanning method that performs scanning with an incident beam and performs tilt quotient correction by controlling the incident angle of the laser beam on each surface of the rotating polygon mirror, the incident angle is changed during the main scanning period, and the laser A laser beam scanning method characterized in that the beam scans in a straight line on a surface to be scanned. (2) A laser beam scanning method according to claim 1, characterized in that the incident angle is changed to decrease or increase following the rotation of a rotating polygon mirror.