JPH0550908B2 - - Google Patents

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 (I _th ). Therefore, the excitation current based on the gray level information of the input video signal is
By applying a range of (I _th ) to (I _nax ) to the semiconductor laser, a laser beam with an intensity commensurate with the density can be obtained, and by imaging and scanning this laser beam on the 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 _th ) and lose linearity. The region used for specific modulation was limited to the range above the threshold current. In other words,
The optical output is limited to the range between (P _th ) and (P _nax ). And this (P _th ) and (P _nax )
The ratio (P _nax /P _th ) is about 30 to 50 in a high-output semiconductor laser, which corresponds to the density ratio of an image of about 1.5 to 1.7. 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 includes: a laser beam modulating means for applying an excitation current according to an input video signal to a semiconductor laser to oscillate intensity-modulated laser beam; An optical filter is installed in the optical path of the laser beam between the photoreceptor and the photoreceptor on which the image is scanned and an image is formed on the photoreceptor. This is because we have established

[Effect]

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 and dashed lines in FIG. 3
As shown by the solid line in the figure, the spectrum becomes sharper as it moves to a longer wavelength band. Moreover, the spectral band and sharpness change significantly after the threshold current (I _th ).

Therefore, based on the above characteristics, by blocking light with a wavelength lower than the wavelength of the oscillating laser light at the threshold current with an optical filter, it is possible to generate an oscillating laser with an excitation current below the threshold current. As shown in Figure 4, the optical output is reduced and the actual output characteristics are improved over the entire output range of the semiconductor laser, including the range that could not be used for intensity modulation of the oscillated laser light. can be made almost linear.

〔Example〕

Embodiments of the present invention will be described 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. Laser light modulation means M
As a result, laser light (B _p ) is oscillated from the semiconductor laser 1 to which an excitation current I is applied in accordance with the input video signal. Here, the semiconductor laser 1 has an optical output P characteristic with respect to the excitation current I as shown in FIG. 2, and an oscillation wavelength λ characteristic with the excitation current I as a parameter as shown in FIG. The laser beam (B _p ) is made into parallel light by the collimator lens 2 and reflected by one mirror surface of the polygon mirror 3 that rotates at high speed.

In the optical path of this laser beam (B _p ), an interference filter 4 having a laminated structure of a plurality of dielectric thin films is provided as an example of an optical filter. This filter 4 is a sharp cut type in which the transmittance τ changes rapidly near a certain wavelength (λ _p ) and blocks light in the low wavelength band, as shown by the solid line in FIG. This wavelength (λ _p ) is set approximately equal to the wavelength of the oscillated laser light (hereinafter referred to as threshold wavelength) at the laser light oscillation threshold voltage (I _th ). Therefore, the output characteristics of the laser light that has passed through this filter 4 are substantially linear over almost the entire output range of the semiconductor laser 1, as shown in FIG.

Further, 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 the threshold current (I _th ) can be corrected by fixing the filter 4 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 Fig. 1, with the rotation of the polygon mirror 3 that reflects the laser beam (B _p ) mentioned earlier, the inclination of the mirror surface with respect to the laser beam (B _p ) changes, and accordingly, the angle of the reflected laser beam (B p ) changes. The laser beam (B _R ) is scanned in the longitudinal direction of the photoreceptor drum 5, which is an example of a photoreceptor. This laser light (B _R ) is focused by the 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, reference numeral 7 denotes a laser beam detection optical sensor for aligning each scanning 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. By heating, the toner is melted and fixed on the paper, producing an output image. Therefore, as shown in FIG.
The excitation of the semiconductor laser 1 is within the range from the threshold current (I′ _th ) in the actual output characteristics of the semiconductor laser 1 to the excitation current (I _nax ) at which the maximum output (P _nax ) of the oscillated laser light is obtained. By changing the 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 changed over a wide range, and In contrast, an output image with high gradation reproducibility can be obtained.

Instead of the optical filter 4 provided in the optical path of the laser beam (B _p ) having the characteristics shown in FIG. 5, for example, the optical filter 4 having opposite light transmission band characteristics as shown in FIG. 6 may be used. It may be something you have. In other words,
In this case, the optical filter 4 acts as a reflecting mirror on the incident surface of the laser beam, and there is no need to transmit light in the lower wavelength region, so there is no need to polish one side when creating the optical filter 4. It happens. Further, although the resolution is slightly lower than that of an interference type filter, an optical filter, a prism, or the like 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 providing 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 the drawing]

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

Claims (1)

[Claims] 1 Laser light modulation means that applies an excitation current according to an input video signal to a semiconductor laser to oscillate intensity-modulated laser light, and optical scanning that scans the laser light to form an image on a photoreceptor. In the laser scanning image forming apparatus equipped with the device, 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. Scanning image forming device.