JPS62296113A - Laser scanning device equipped with means for correcting chromatic aberration of different-wavelength laser beam - Google Patents

Abstract

PURPOSE:To perform highly accurate matching color, by correcting the color blurring of a scanning beam caused on an image forming plane due to a difference in color by deflecting a laser beam by a minute angle by means of a chromatic aberration correcting means. CONSTITUTION:An He-Ne laser 1, Ar laser 2, and He-Cd laser 3 are respectively used for red, green, and blue. The optical beams emitted from these lasers 1, 2, and 3 are sent to a rotating polygon mirror 7 after they are synthesized to one optical beam by means of a synthesizing optical system using dichroic mirrors 4-6. The polygon mirror 7 scan the inputted optical beam and causes the optical beam to form an image on an image forming plane 9 through an ftheta lens 8. In this case, the color blurring quantity DELTAftheta an ideal image forming location is proportional to the deflecting angle of the reflecting surface of the polygon mirror 7. Therefore, the color blurring quantity DELTAftheta is corrected by regarding the reflecting surface of the mirror 7 and deflecting surface of a chromatic aberration correcting means 11 as equivalent to each other by using the means 11 and convex lenses 12 and 13. The means 11 is constituted in such a way that the laser beam can be deflected by a minute angle so that the refractive index of the medium can be changed and the color shifting can be corrected.

Description

[Detailed Description of the Invention] 3. Detailed Description of the Invention (Field of Industrial Application) This invention relates to a laser scanning device having laser beams of two or more different wavelengths (Background of the Invention) Different wavelengths A laser scanning device using a laser beam is applied to, for example, a color laser printer.

In this case, multiple laser beams are superimposed into one,
There is one that makes the light incident on an imaging lens (fθ lens) and uses this imaging lens to main scan and form an image on a transfer master drum or the like.

For example, as shown in FIG. 4, the light source lasers include a helium neon (He-Ne) laser (ti length 632.8 nm) 1 for red color and an argon ion (Ar) laser ([length 514 nm) for line color. 5nm>2, helium cadmium (He-Cd) laser (wavelength 441.6n) for blue color.
m) Take one bottle of each of 3. The light beams emitted from these three lasers are sent to the scanning system as a single thick light beam by a synthetic optical system using dichroic mirrors 4, 5, and 6. The incident light beam is scanned by a rotating polygon mirror 7 of the scanning system, and an image is formed on an image forming surface 9 of a transfer master drum or the like via an fθ lens 8.

(Problem to be Solved by the Invention) By the way, due to the difference in the refractive index of the fθ lens 8 due to the difference in wavelength, due to chromatic aberration, the image is not formed into a single spot on the imaging plane 9, but at different positions depending on the wavelength. However, color shift occurs.

That is, FIG. 5 shows the rotating polygon iJ! An example of the relationship between the deflection angle θ of No. 7 and the deviation Δfθ from the ideal imaging position fQ is shown.

For a wavelength of λ=632.8 nm, Δfθ=0,
It can be seen that the image is formed at the ideal position over the entire travel width.

However, for the light beams of λ=514.5 nm and λ=441.6 nm, a deviation Δfθ from the ideal imaging position occurs depending on the deflection angle θ. For this reason, the three laser beam spots do not converge on one point, resulting in color shift and deterioration of image quality.

In order to correct such chromatic aberration, we usually use glass with low chromatic dispersion or arrange multiple f-theta lenses.
Although efforts have been made to reduce color shift by devising the configuration, this increases cost due to larger and more complex f-theta lenses, and it is technically difficult to completely correct color shift. . Even if it were theoretically possible, complete correction is almost impossible due to errors in lens design and mounting errors. For this reason, the device becomes expensive, and some inexpensive devices allow color misregistration to some extent.

This invention was made in view of the above circumstances, and in order to overcome the difficulty of color correction, it is an object of the present invention to provide a laser scanning device that can perform color matching with a high degree of aperture without performing color correction on the imaging lens. The purpose is

(Means for Solving the Problems) In order to solve the above-mentioned problems, the present invention provides a laser scanning device having laser beams of two or more different wavelengths, in which scanning beams on an imaging plane due to differences in wavelength are used. The present invention is characterized in that it is equipped with a chromatic aberration correcting means for correcting the color shift, and that the laser beam is shot at a minute angle so as to correct the color shift.

(Operation) In the present invention, two or more laser beams of different wavelengths are scanned, and the scanning beams are imaged through an imaging lens. At this time, the color shift of the scanning beam that occurs on the imaging plane due to the difference in wavelength is corrected by swinging the laser beam by a small angle in the lower chromatic aberration correction stage.

(Example) Hereinafter, an example of the present invention will be described in detail based on the accompanying drawings.

In Figure 1, the light source laser is a helium neon (He-Ne) laser (wavelength: 632.8 nm) for red color.
) 1. Argon ion (Ar) laser (wavelength 5
14.5nm), helium cadmium (He-
Cd) laser (wavelength 441.6 nm) 3 to 1 each
Use this book.

The light beams emitted from these three lasers are sent to the scanning system as one light beam by a combining optical system using dichroic mirrors 4, 5, and 6. dichroic mirror 4
reflects the red light beam, and the dichroic mirror 5 transmits the red light beam and reflects the green light beam. Also,
Dichroic mirror 6 transmits the red and green light beams and reflects the blue light beam.

The scanning thread rotating polygon mirror 7 scans the incident light beam, and forms an image on the imaging surface 9 via the fθ lens 8 .

Ar laser (wavelength 514.5 nm) 2, He-Cd
The optical system of the laser (wavelength: 441.-6 nm) 3 is composed of a chromatic aberration correcting means 11 and lenses 12 and 13, respectively, and the lens 13 is placed in a composite optical system and is used also.

λ=632.8nm, λ=514.5nm, λ=441
．． When using the fθ lens 8 with the characteristics shown in FIG. 5 for a wavelength of 6 nm, the color shift Δfθ from the ideal imaging position is due to the deflection on the reflecting surface of the one-rotation polygon mirror 7. It is proportional to the angle θ.

Here, the fθ characteristic is satisfied for the laser beam with λ=632.8 nm, and Δfθ=0 for the total deflection angle θ.
It becomes.

Correction of color shift will be explained using a laser beam of this wavelength as a reference beam.

Regarding the Ar, He-Cd laser 2.3, if these scanning beams are swung by a minute angle θ on the reflecting surface of the rotating polygon mirror 7 so as to cancel the value of Δfθ shown in the graph of FIG. good.

For example, in FIG. 5, when the scanning beam of the He-Cd laser 3 has a deflection angle θl, it is assumed that a color shift occurs by Δfθ1 with respect to the reference beam (He-Ne laser 1). To correct this, the scanning beam from the He-Cd laser 3 is directed to the reflecting surface of the rotating polygon mirror 7.
It is only necessary to shift the color shift by Δθ1=Δfθl/f in the direction of canceling the color shift.

In the center of the running width, the beam may be the same as the reference beam because no color shift occurs. In this way, you can find the amount of color shift over the entire width of the capital run and swing it in a direction to cancel it out.

If the color shift △fθ is in a proportional relationship to the deflection angle θ of the scanning beam as shown in Figure 2, it is best to simply find the deflection angle m necessary for correction, or if there is no constant proportional relationship, However, by determining the color shift characteristics of the fθ lens 8, it is possible to determine the angle of view necessary for correction.

Further, correction can be made in the same manner for λ=514.5 nm.

Since this correction cannot control the amplitude of the beam on the reflecting surface of the rotating polygon mirror 7, a navigation chromatic aberration correcting means 11 and two convex lenses 12 and 13 are used to correct the beam amplitude on the reflecting surface of the rotating polygon mirror 7. and the deflection surface of the chromatic aberration correcting means 11 are controlled to be equivalent.

The chromatic aberration correcting means 11 is configured to change the refractive index of the medium and swing the laser beam by a small angle so as to correct color shift. In this example, an acousto-optic optical deflection-3 (AOD) ultrasonic optical deflector is used as the chromatic aberration correction means 11.

Alternatively, a mirror may be attached to a vibrator capable of high-speed response such as a piezoresistive effect element, and by vibrating the mirror, the scanned beam may be swung by a small angle.

Figure 2 shows this principle, and shows the chromatic aberration correction means 1.
1 and the reflective surface of the rotating polygon mirror 7 are arranged so as to be substantially conjugate. The chromatic aberration correcting means 11 is placed at the front focal length of the lens 12, and the reflective surface of the rotating polygon mirror 7 is placed at the rear focal length of the lens 13. , the two lenses 12.13 do not necessarily have to be the same, but the focal length! When l1fl = f2, the deflection angle in the first chromatic aberration correcting means and the deflection angle on the rotating polygon m70 reflecting surface become equal. If the focal lengths of the two lenses 12, 13 are different, it is necessary to adjust the deflection angle in the chromatic aberration correcting means 11 according to the ratio.

The chromatic aberration correcting means 11 controls the deflection angle by the frequency of the ultrasonic wave applied to the element as described above, and the deflection angle is expressed by Fλ/V. Here, V is the propagation speed of the ultrasound, F is the frequency of the ultrasound, and λ is the wavelength of the laser beam.

For example, ・Focal length of fθ lens! 1f is 250mm・Scanning beam deflection angle θ1 is maximum 20゛・Color shift Δfθl=0.5mm at θl=20− (He−Ne vs. He−Ne)
Cd color shift) ・ fl = f2 ・ Ultrasonic propagation speed is 3.63 x 103 m ・ λ = 44
1. Assuming that the wavelength is 6 nm, the control frequency of the small angle deflection element 11 will be determined.

If the amount of correction required on the reflective surface of the rotating polygon ii7 is Δθl, then ΔθI=Δfθl/f=0.5/250=2 mrad.

Therefore, color correction can be performed by swinging the laser beam to be corrected by a small angle proportional to the scanning angle with respect to the J[beam by ±2 mrad on the reflective surface of the rotating polygon ti7.

In the above example, since fl = f2, the deflection angle at the chromatic aberration correcting means 11 becomes the deflection angle on the reflective surface of the rotating polygon mirror 7. Therefore, it is sufficient to use the chromatic aberration correction means 11 to swing the laser beam by ±2 mrad.

Since the deflection angle (deflection angle) of the chromatic aberration correcting means 11 is Fλ/V, when the reference frequency is set to 80 MH2, the frequency of the ultrasonic waves applied to the chromatic aberration correcting means 11 is changed from 96.44 MHz to 63.56 MHz for each scan. It's good to change it. The situation is shown in Figure 3.

Although this embodiment has been described using two lenses 12 and 13 to make it easier to understand the principle, it is not necessary to use these lenses. Any other method may be used as long as the scanning beam can be swung by a small angle on the reflecting surface of the rotating polygon mirror 7.

Furthermore, in principle, it is necessary to swing on the reflective surface of the rotating polygon m7, but some deviation can be tolerated. This tolerance depends on the design of the fθ lens.

In addition, although the amount of color shift △fθ of the fθ lens has been explained as being proportional to the scanning angle of the scanning beam, this is also because it is easy to understand the principle, and the fθ lens has the characteristics described above.
This is not something that can only be applied to lenses. However, if the relationship between the deflection angle θ and the color shift Δfθ becomes complicated, the control of the frequency applied to the chromatic aberration correcting means 11 becomes correspondingly more complicated.

However, the frequency given to the chromatic aberration correcting means 11 does not need to be one that completely corrects color shift, and even if it is approximate, there is almost no visual problem.

Also, although the imaging lens has been explained as an fθ lens,
A similar correction is possible with a general ftanθ lens.

Furthermore, although a system using lasers with wavelengths of three colors has been described, it is also possible to use two colors or three or more colors.

In addition, in the description of this embodiment, the color shift in the main scanning direction is 1+
Although M is positive, it can also be used for color misregistration in the sub-scanning direction. In this case, the tilt angle error of the rotating polygon mirror 7 may be corrected at the same time. At this time, the signal applied to the chromatic aberration correcting means differs for each surface of the rotating polygon mirror 7, and remains constant for - times of scanning.

Furthermore, an acousto-optic optical deflector (
When using an acousto-optic optical deflector (AOD), use an acousto-optic optical deflector (AOD).
In other words, by changing the frequency of the ultrasonic waves, the deflection of the scanning beam can be controlled by changing the ultrasonic frequency.
The scanning beam can be modulated by changing the imaging width of the ultrasonic waves. This eliminates the need to use a new element for small angle deflection.

In another example, a laser beam before entering an acousto-optic deflector or an acousto-optic modulator is split by a beam splitter, etc., used as monitor light, and fed back to the acousto-optic deflector or modulator. Accordingly, it can also be used for stabilizing the amount of light. Furthermore, by monitoring the scanning beam emitted from the acousto-optic light deflector or modulator and feeding it back to the acousto-optic light deflector or modulator, it is possible to Changes in the diffraction efficiency of can also be compensated for. Furthermore, by monitoring the laser beam after scanning and feeding it back to the acousto-optic optical deflector or modulator, compensation including shape ink correction is possible.

(Effects of the Invention) As described above, in this invention, since the chromatic aberration correcting means cuts the laser beam at a minute angle to correct color shift, glass with low chromatic dispersion is used to correct chromatic aberration, Color misregistration can be corrected with a simple configuration in which the scanning beam is swung by a small angle by using a means such as arranging a plurality of fθ lenses, and the device is inexpensive and highly accurate color matching is possible.

[Brief explanation of drawings]

Fig. 1 is a schematic configuration diagram of a laser scanning device that can be used in this invention, Fig. 2 is a principle diagram explaining this invention, Fig. 3 is a diagram showing the frequency of ultrasonic waves used in this invention, and Fig. 4 is a conventional one. FIG. 5 is a schematic configuration diagram of the laser scanning device shown in FIG. 5, and is a diagram showing the relationship between the scanning beam and color shift. 1 ・=He-N e laser 2...Ar laser 3...He-Cd laser 4.5.6...Dichroic mirror 7...Rotating polygon mirror 8...Fθ lens 9...Conclusion Image plane 11--monochromatic aberration correction means 12.13--lens No. 1, Fig. 2, Fig. 3

Claims (6)

[Claims] (1) In a laser scanning device that has laser beams of two or more different wavelengths, a chromatic aberration correction means is provided to correct the color shift of the scanning beam on the imaging plane due to the difference in wavelength, and the laser beam corrects the color shift. A laser scanning device that is characterized by swinging a beam at a small angle. (2) The laser scanning device according to claim 1, wherein the laser beam is oscillated by a small angle in accordance with the scanning angle of the scanning beam. (3) The laser scanning device according to claim 1, wherein the laser beam is swung by a small angle depending on the reflection surface of a rotating polygon mirror being scanned. (4) The laser scanning device according to claim 1, wherein the chromatic aberration correcting means is an ultrasonic light deflector. (5) The laser scanning device according to claim 1, wherein the chromatic aberration correcting means includes a mirror and an element to which the mirror can be attached, which changes when an electric field is applied thereto. (6) The laser scanning device according to claim 4 or 5, wherein the chromatic aberration correcting means performs deflection and modulation simultaneously.