JPS6120848B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical scanning device using a linearly polarized laser used in electrophotographic recording devices and the like.

Laser recording devices, etc. that use a laser beam to write an image corresponding to an input image signal on a photosensitive layer,
In general, an optical modulator modulates a laser beam emitted from a laser tube according to an input signal, a rotating polygon mirror that sweeps the modulated light in the horizontal direction, and a rotating polygon mirror that focuses the light onto the photosensitive layer to be written, and one side. It consists of a -θ lens, etc. that moves the spot light from one spot to the other at the same speed. However, some of the components that make up these laser scanning optical systems, such as optical modulators or rotating polygon mirrors, have characteristics that change depending on the polarization direction of the incident light.
Optical modulators that use crystals such as PbM ₀ O ₄ and TeO ₂ as ultrasonic media have diffraction efficiency that changes depending on the polarization direction of the incident light.
A normal rotating polygon mirror formed with a protective film has a characteristic that its reflectance changes depending on the incident angle and polarization direction of light. Therefore, when a non-polarized gas laser is used, the polarization direction changes randomly over time, and the diffraction efficiency of the optical modulator or the reflectance of the rotating polygon mirror changes over time, making it stable. Conventional optical scanning devices have used linearly polarized gas lasers because of the inability to perform detailed writing, but even with such gas lasers,
Due to the angular dependence of the reflectance of a rotating polygon mirror, the reflection intensity changes depending on the polarization angle of light scanning, so it suppresses the occurrence of exposure unevenness called the shading phenomenon, especially when intermediate tones must be produced. I couldn't do it.

The present invention provides a rotating polygon mirror for S-polarized light in which the polarization vibration direction of a linearly polarized laser oscillates in a plane parallel to the rotation axis of the rotating polygon mirror, and P-polarized light in which the polarization vibration direction of a linearly polarized laser oscillates in a plane perpendicular to the rotation axis. Focusing on the fact that the reflectance changes depending on the angle of incidence, which is the cause of the shedding phenomenon, the purpose of this project is to propose a new optical scanning method that can solve the above-mentioned problems with a simple device. Its feature is that the polarization vibration direction of the linearly polarized laser is set at approximately 45 degrees with respect to the rotation axis of the rotating polygon mirror.

That is, as shown in Fig. 1, the polarization vibration directions of the linearly polarized laser emitted from the gas laser tube a are S-polarized light that vibrates in a plane parallel to the rotation axis of the rotating polygon b, and S-polarized light that vibrates in a plane perpendicular to the rotation axis. P vibrating in the plane
The reflectance of rotating polygon mirror b, which has a protective film of SiO on the Al evaporation surface, for polarized light varies slightly depending on the Al film thickness, SiO film thickness, SiO evaporation conditions, etc. As seen in the figure, both increase or decrease as the reflection angle increases.
However, the reflectance of the rotating polygon b for P+S polarized light vibrating in a plane making an angle of 45 degrees with the rotation axis of the rotating polygon b is located between the above-mentioned S polarized light and P polarized light, regardless of the incident angle. It was found that the value was almost constant. In Figure 2, the P+S polarization tends to increase slightly depending on the incident angle, but this is thought to be due to the fact that the polarization is not set accurately in the 45° direction. According to the experimental results of
It can be said that a direction at an angle of 45° with respect to the axis of rotation is best.

As described above, by setting the polarization vibration direction of the linearly polarized laser to approximately 45 degrees with respect to the rotation axis of the rotating polygon mirror, the S-polarized light whose reflectance increases as the incident angle increases, and the P-polarized light whose reflectance decreases as the incident angle increases. A substantially intermediate value of polarization can be obtained, and changes in reflectance can be kept to a minimum regardless of the incident angle, making it possible to form good images without any shedding phenomenon.

[Brief explanation of the drawing]

FIG. 1 is a diagram illustrating the direction of polarization vibration of a linearly polarized laser with respect to a rotating polygon mirror, and FIG. 2 is a diagram showing reflectance angle characteristics of the rotating polygon mirror with respect to various types of polarized light. a...Gas laser tube, b...Rotating polygon mirror.

Claims (1)

[Claims] 1. In an optical scanning device using a linearly polarized laser that modulates the linearly polarized laser according to an input signal and sweeps and writes the modulated light onto a recording medium using a rotating polygon mirror, the linearly polarized laser is An optical scanning method using a linearly polarized laser, characterized in that the direction of polarized light vibration is set at approximately 45 degrees with respect to the rotation axis of the rotating polygon mirror.