JPH03189635A - Light beam diameter conversion device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a light beam diameter conversion device used for optical writing in laser printers, digital copying machines, laser facsimiles, and the like.

(Prior Art) In general, a laser beam printer converges a laser beam with coherent characteristics emitted from a laser light source using a condensing lens to form a minute spot on a light receiving surface, and forms a minute spot on a photoreceptor surface that is charged in a negative manner. It scans and exposes the image. By controlling the laser beam using pixel signals, an electrostatic latent image is formed on the surface of the photoreceptor and developed with fine powder toner.

In such a laser beam printer, the diameter of the light beam is fixed according to the pixel density, and when expressing gradation, an area gradation method using a matrix such as a dither method is often used.

(Problem to be solved by the invention) However, since there is a problem that the resolution decreases when gradation is expressed using a matrix, for example, the diameter of the light beam in the sub-scanning direction is made variable, and the laser exposure time is The size of the dot can be varied in units of one dot, such as by changing the dot size (Japanese Patent Laid-Open No. 62-275214), or by performing both intensity modulation and time modulation of the laser beam (Japanese Patent Laid-Open No. 61-124921). Various means have been proposed for expressing gradation without reducing resolution.

These conventional means are complicated and uneconomical.

(Object of the Invention) The present invention has been made in view of the above-mentioned circumstances, and provides a light beam that can economically vary the size of dots in units of one dot in an arbitrary direction using extremely simple means. The object of the present invention is to provide a diameter conversion device.

(Structure and operation) In order to achieve the above object, the present invention includes a beam compressor, an electro-optic crystal having electrodes on both end faces, and a beam expander, and the focus of the beam compressor and the beam expander is The refractive index of the electro-optic crystal is changed by the voltage applied to the electrodes on both end faces of the electro-optic crystal, which are arranged to coincide with the central axis of the electro-optic crystal, thereby making the diameter of the light beam variable in any one direction. Features.

In the present invention, a beam compressor and a beam expander converge laser light in one direction, for example.

Consisting of a cylindrical lens, an electro-optic crystal changes its refractive index due to the electro-optic effect when a voltage is applied to both end electrodes, and this is used to vary the diameter of the emitted beam of one dot. That is, it can be performed with an extremely simple configuration and is economical.

(Embodiment) FIG. 1 shows the configuration of an embodiment of the present invention, and in the figure,
1 is a beam compressor, 2 is a beam expander, 3
4 indicates an electro-optic crystal having a semi-cylindrical shape, and 4 indicates electrodes provided on both end surfaces of the electro-optic crystal 3. Also,
f2 is the beam compressor 1. d indicates the focal length of each beam expander 2, and d indicates the thickness between the electrodes at both ends of the electro-optic crystal 3.

Beam compressor 1. The beam expander 2° electro-optic crystal 3 is connected to the beam compressor 1. The beam expander 2 is arranged so that its focal point coincides with the central axis of the electro-optic crystal 3. Here, beam compressor 1. The beam expander 2 is, for example, a cylindrical lens that converges the laser beam in one direction. The electro-optic crystal 3 is a crystal whose refractive index changes due to the electro-optic effect when a voltage is applied.
The refractive index of the electro-optic crystal 3 can be changed by applying an arbitrary voltage to .

Next, the principle of operation of the beam diameter conversion device of the present invention will be explained based on FIG. FIG. 2 schematically shows the configuration of the bead 11 diameter converting device of the present invention.

Now, when the linearly polarized light beam L (incident beam) is made incident on the electro-optic crystal 3 via the beam compressor 1,
At the incident surface, it is refracted according to the refractive index of the electro-optic crystal 3, and at the exit surface, the exit angle is always 90 degrees with respect to the crystal surface.
Because it forms an angle of 100°, no refraction occurs and it passes through as is.

In FIG. 2, the optical path when the voltage V is not applied to the electro-optic crystal 3 is shown by a solid line, and the optical path when the voltage V is applied to the electro-optic crystal 3 is shown by a broken line.

The refractive index and refraction angle when no voltage V is applied to the electro-optic crystal 3 are no and ψ, respectively. , the refractive index and refraction angle when voltage V is applied are n7 and φ7, respectively, the incident angle of light beam L (incident beam) is θ, and the difference in radius of output beam L' caused by applying voltage V is If ΔD, then ΔDv=f, (tanψ, −tanψ.)...
- The relationship (2) holds true.

That is, by applying a voltage to the electro-optic crystal 3, the refraction angle changes, and the diameter of the light beam can be changed by changing the applied voltage.

Furthermore, the relationship between the voltage V applied to boat fishing and the refractive index nv is
The electro-optic effect exhibited by the electro-optic crystal 3 is called Pockels.
It depends on whether there is an effect or a K err effect. In the above case, the refractive index changes in linear proportion to the applied electric field, and in the latter case, it changes in proportion to the second order of the applied electric field. The electric field here is the applied voltage V divided by the thickness d of the crystal in the direction in which this voltage is applied.

(Effects of the Invention) As described above, the present invention can economically change the diameter of a light beam by applying a voltage to an electro-optic crystal with an extremely simple configuration. Therefore, when the light beam diameter conversion device of the present invention is applied to an optical writing section of a laser beam printer or the like, fine gradation can be expressed without reducing resolution, and high-quality images can be formed.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, and FIG. 2 is a schematic diagram of a light beam diameter conversion device for explaining the operating principle of the present invention. 1... Beam compressor, 2... Beam expander, 3... Electro-optic crystal, 4...
·electrode.

Claims (1)

[Claims] It has a beam compressor, an electro-optic crystal having electrodes on both end faces, and a beam expander, arranged so that the focal points of the beam compressor and the beam expander coincide with the central axis of the electro-optic crystal, and the electric A light beam diameter conversion device characterized in that the refractive index of an electro-optic crystal is changed by a voltage applied to both end face electrodes of the optical crystal, thereby making the light beam diameter variable in any one direction.