JPS61126865A - Laser scanning type image forming device - Google Patents

Abstract

PURPOSE:To elevate tone reproducibility of image by equipping an optical filter to intercept the light of lower wavelength side than that of oscillation laser light in laser oscillation threshold electric current in an optical path of laser light. CONSTITUTION:An interference filter 4 composed of the laminated layers of plural pieces of dielectric films is equipped in the optical path of laser light B0. The filter is a sharp cut type one whose transmittivity tau is suddenly changed on the near border of a certain wavelength lambda0 to intercept the light on the band side of low wavelength. The wavelength lambda0 is designed to be almost equal to wavelength of oscillation laser light in the oscillation threshold current Ith of laser light. Therefore, the output character of laser light passed through the filter 4 becomes linear over almost all the parts of output range of a semiconductor laser 1. Thus, as compared with the conventional one, modulation of strength of oscillation laser light can be performed, and image of higher tone reproducibility can be obtained in wider range.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a laser scanning image forming apparatus such as a laser beam printer using a semiconductor laser as a light source.

More specifically, a laser light modulation means provides an excitation current according to an input video signal to a semiconductor laser to oscillate intensity-modulated laser light, thereby making an image with density gradation characteristic; The present invention relates to a laser scanning type image forming apparatus including an optical scanning device that forms an image by scanning the laser beam onto a photoreceptor.

[Conventional technology]

Semiconductor lasers are small, highly efficient, fast, and stable, and have the great advantage of being able to be easily directly modulated during laser beam oscillation, so in recent years they have been widely used as light sources for laser scanning image forming devices such as laser beam printers. ing.
Conventionally, in the above-described laser scanning image forming apparatus, the intensity of the oscillated laser light is modulated by changing the excitation current to the semiconductor laser according to the input video signal. In other words, the current-optical output characteristic of the semiconductor laser is as shown in FIG. 2, and exhibits an almost linear optical output characteristic for a current equal to or higher than the laser oscillation threshold current (Ita). Therefore, by applying an excitation current based on the gradation information of the input video signal to the semiconductor laser number in the range of (■i) to (I, , , ), laser light with an intensity commensurate with the gradation can be obtained. By scanning a laser beam onto a photoreceptor, an image with gradation can be obtained.

[Problem that the invention seeks to solve]

However, as can be seen from the graph in Figure 2, the output characteristics of a semiconductor laser change greatly in the region below the threshold current (I) and lose linearity. The region used for sexual modulation was only the range above the threshold current. In other words, the optical output is (P) and (P□X)
It is limited to the range of And this (pth)
The ratio (P-x/Pzh) between tp and tp is about 30 to 50 in a high-power semiconductor laser, which
This corresponds to a density ratio of about 1.5 to 1.7 as an image. Therefore, the gradation reproducibility as an image cannot necessarily be said to be high.

In view of the above-mentioned circumstances, an object of the present invention is to improve the gradation reproducibility of images by effectively utilizing the characteristics of semiconductor lasers.

[Means for solving problems]

The characteristic configuration of the laser scanning image forming apparatus according to the present invention is as follows:
A laser light modulator that applies an excitation current according to a human video signal to a semiconductor laser to oscillate intensity-modulated laser light, and a photosensitive device that scans the laser light to form an image using an optical scanning device to form an image thereon. This is because an optical filter is provided in the optical path of the laser beam between the body and the laser oscillation threshold current to block light having a wavelength lower than the wavelength of the dimly oscillated laser beam.

[For production]

That is, as can be seen from the graph in FIG. 3, the spectrum of the laser light emitted from the semiconductor laser changes using the input excitation current as a parameter. That is, when the input excitation current is small, the spectrum is biased toward the low wavelength band and relatively broad, as shown by the dashed line and dashed line in FIG. As shown by the solid line in the middle, the spectrum becomes sharper as it moves to the longer wavelength band, but the band and sharpness of the spectrum change markedly when the threshold current (I) is reached. .

Therefore, based on the above characteristics, by blocking light with a wavelength lower than the wavelength of the oscillated laser beam at the threshold current with an optical filter, the output of the oscillated laser beam with an excitation current below the threshold current is achieved. As shown in FIG.
This allows the semiconductor laser to be substantially linear over the entire output range.

〔Example〕

The present invention will be described in detail below based on the drawings.

FIG. 1 shows a schematic configuration of an optical scanning device (S) in a laser beam printer as an example of a laser scanning image forming device. By the laser light modulation means (M),
A laser beam (Bo) is oscillated from a semiconductor laser (1) to which an excitation current (I) corresponding to an input video signal is applied.
Here, the semiconductor laser (1) has optical output (P) characteristics with respect to excitation current (I) as shown in FIG. 2, and oscillation wavelength (with the excitation current (() as a parameter) as shown in FIG.
λ) characteristics. The laser beam (Bo) is
It is made into parallel light by a collimator lens (2) and reflected by one mirror surface of a polygon mirror (3) that rotates at high speed.

An interference filter (4), which is an example of an optical filter, is provided in the optical path of this laser beam (Bo), which is made up of a plurality of laminated dielectric thin films.

As shown by the solid line in Figure 5, this filter (4) has a sharp cut whose transmittance (τ) changes rapidly around a certain wavelength (λ) and blocks light in the low wavelength band. This wavelength (λ) is equal to the wavelength (λ) of the oscillated laser light at the laser oscillation threshold current (Ith).
(hereinafter referred to as the threshold wavelength). Therefore, the output characteristic of the laser light that has passed through this filter (4) becomes substantially linear over almost the entire output range of the semiconductor laser (1), as shown in FIG.

In addition, as shown in Fig. 1, this filter (4) is attached so as to be rotatable and fixed around an axis (P) with respect to the traveling direction of the laser beam, and is used to maintain the threshold between individual semiconductor lasers. This filter (4
) can be corrected by fixing it at an appropriate angle with respect to the laser beam. In other words, as a characteristic of an interference filter, if the incident angle of light changes, the optical transmission band characteristic changes as shown by the broken line in Figure 5, so the cutoff wavelength should always be set almost equal to the threshold wavelength. This is possible.

As shown in Figure 1, the rotation of the polygon mirror (3) that reflects the laser beam (Bo) as described above changes the inclination of the mirror surface with respect to the laser beam (Bo), and accordingly, the The laser beam (BII) is scanned in the longitudinal direction of a photoreceptor drum (5), which is an example of a photoreceptor. This laser beam (B,) is focused by an fθ lens (6) and forms an image on the photosensitive drum (5) whose surface is uniformly charged, thereby reducing the charged potential at that position. By repeating the above operations, an electrostatic latent image is formed on the photoreceptor drum (5). In the figure, (7) is a laser beam detection optical sensor for aligning each scan start position with respect to the rotational direction of the drum (5).

Thereafter, although not shown, toner, which is a colored pigment, is selectively attached to this electrostatic latent image area and developed, and the output paper is brought into contact with the toner image surface to transfer the toner image onto the paper surface. The heat melts the toner and fixes it on the paper, creating an output image. Therefore, as shown in FIG. 4, from the threshold current (I') in the substantial output characteristics of the semiconductor laser (1), the excitation that provides the maximum output (P, , ) of the oscillated laser light The excitation current (I
) in accordance with the input video signal, the exposure amount of each dot constituting the electrostatic latent image on the photoreceptor drum (5) is linearly varied over a wide range, and As a result, an output image with high gradation reproducibility can be obtained.

An optical filter (4) provided in the optical path of the laser beam (Bo)
) may have the opposite light transmission band characteristics as shown in FIG. 6, for example, instead of having the characteristics as shown in FIG.

In other words, in this case, the optical filter (4) will act as a reflecting mirror on the incident surface of the laser beam, and since the light in the lower wavelength region to be transmitted is not required, one side is removed when creating the optical filter (4). It is not necessary to polish the filter. Also, although the resolution is slightly lower than that of an interference type filter, an optical filter, a prism, etc. may be used, and the type thereof can be changed as appropriate.

〔Effect of the invention〕

As described above, the laser scanning image forming apparatus according to the present invention includes an optical filter in the optical path of the laser beam that blocks light having a wavelength lower than the wavelength of the oscillated laser beam at the laser beam oscillation threshold current. This makes it possible to extend the substantially linear range of the output characteristics of the semiconductor laser by simply modifying it by simply adding an optical filter. This makes it possible to modulate the intensity of the oscillated laser light over a wider range than before, making it possible to obtain images with higher gradation reproducibility.

[Brief explanation of drawings]

The drawings show an embodiment of the laser scanning image forming apparatus according to the present invention, in which FIG. 1 is a schematic diagram of the optical scanning device, FIG. 2 is a graph showing the current-output characteristics of a semiconductor laser, and FIG. 3 is a graph showing the current-output characteristics of a semiconductor laser. Figure 4 is a graph showing the actual output characteristics of a semiconductor laser, Figure 5 is a graph showing the light transmission band characteristics of an optical filter, and Figure 6 is a graph showing the light transmission band of another optical filter. It is a graph showing characteristics. (1)... Semiconductor laser, (4)...
Optical filter, (5)...Photoreceptor, (S)...
...Optical scanning device, (M) ...Laser light modulation means, (I) - Old - Excitation current, (I) ...
Laser light oscillation threshold current.

Claims (1)

[Claims] A laser light modulator that applies an excitation current according to an input video signal to a semiconductor laser to oscillate intensity-modulated laser light, and an optical scanning device that scans the laser light to form an image on a photoreceptor. In the laser scanning image forming apparatus, an optical filter is provided in the optical path of the laser beam to block light having a wavelength lower than the wavelength of the oscillated laser beam at the laser beam oscillation threshold current. type image forming device.