Scanning exposure unit, semiconductor laser driving circuit...

Details Scanning exposure unit, semiconductor laser driving circuit... Scanning exposure unit, semiconductor laser driving circuit...

2000-07-31

2002-05-14

Brase, Sandra (Department: 2852)

Incremental printing of symbolic information

Electric marking apparatus or processes

Electrostatic

C347S247000

Reexamination Certificate

active

06388689

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a scanning exposure unit, a semiconductor laser driving circuit and an image forming apparatus, and more particularly to a scanning exposure unit adapted to turn on a semiconductor laser on the basis of image information and scan and expose a photosensitive body with a laser beam outputted from a semiconductor laser; a semiconductor laser driving circuit; and an image forming apparatus.
2. Description of the Related Art
An image forming apparatus of an electrostatic system utilizing a laser, including a laser printer is being spread at present. In this image forming apparatus utilizing a laser (mainly, a semiconductor laser, which will hereinafter be referred to as “LD”), a scanning exposure unit adapted to scan and expose a photosensitive body with a laser beam is used.
To be more exact, an electrostatic latent image is formed on a photosensitive body a surface of which is uniformly charged with the scanning exposure unit, by scanning the same surface with a laser beam modulated on the basis of image data. After this electrostatic latent image has been developed with a toner supplied thereto, transfer paper is superposed on a developed toner image, and the toner image is transferred onto the transfer paper by having the toner electrostatically adsorbed to an outer surface of the transfer paper. The transferred toner image is then fixed by applying heat or a pressure to the transfer paper, to form an image.
A LD driving method used in the scanning exposure unit will now be described. In the case where an electrostatic latent image is formed by using a laser beam, the condition of formation of an electrostatic latent image differs with an optical intensity (quantity) of the laser beam, so that it is necessary to drive the LD so as to obtain a laser beam of a predetermined optical intensity (a predetermined quantity of output light). As shown in
FIG. 12
, the LD has the characteristics of outputting coherent light when a driving current therefor has attained a predetermined level (which will hereinafter be referred to as “threshold current”) Ith.
In order to form an electrostatic latent image on a photosensitive body, it is necessary to output a laser beam modulated on the basis of an image signal (video signal) based on image data, i.e., on the basis of an ON/OFF signal indicating the turning on and off of the LD.
Therefore, as shown in
FIG. 13
, a LD driving circuit
400
was provided with a current source
404
for supplying a current corresponding to a desired intensity of light to a LD
402
, and a switching circuit
406
for modulating a laser beam outputted from LD
402
on the basis of image data. The current source
404
is adapted to supply an electric current a level of which corresponds to that of a set voltage to LD
402
through the switching circuit
406
, and the switching circuit
406
is adapted to supply the electric current to and stop the electric current flowing to LD
402
, on the basis of the video signal. This enables a laser beam having a desired optical intensity and modulated on the basis of image data to be outputted. Such a modulation method is generally called a pulse width modulation (PWM) system.
The modulation methods positively utilizing the fact that the condition of formation of a latent image differs with a quantity of output light of a laser include a pulse amplitude modulation (PAM) system. In the formation of an image by this PAM system, an image is formed by varying an emission intensity (i.e. a quantity of output light) of LD on the basis of the image data.
Japanese Patent Laid-Open No. 206366/1989 discloses the techniques concerning the driving of LD by the PAM system. According to the techniques, some of plural LD driving current sources are selected on the basis of an intensity setting signal from the outside, and the sum of the currents from the selected current sources is supplied to the LD, whereby the intensity of light (quantity of output light) of LD can be varied. In general, when the optical intensity increases, the range in which a latent image is formed increases, and a dot image developed becomes large as compared with that developed when the optical intensity is low.
In an image forming apparatus, an image besides ordinary letters is printed in some cases, and it is known that the quality of an image, especially, the reproducibility of an intermediate color (which will hereinafter be referred to as “halftone”) receives the influence of transitional characteristics of LD being modulated.
For example, Japanese Patent Laid-Open No. 2051:83/1989 discloses that, when LD is turned on and off by a modulation signal (pulse signal), fluctuation occurs in an optical output in a transitional period, which causes nonuniformity of the density of an image formed, failure in the formation of a beautiful (rectangular) pulse waveform corresponding to the modulation signal, and failure in the faithful reproduction of a halftone of the image. Japanese Patent Laid-Open No 2878/1991 discloses that, when distortion occurs in a driving current of LD, a waveform of an optical output from the LD is also distorted to cause turbulence to occur in a dot image formed, and therefore a decrease in the quality of the image.
FIG. 14
shows waveforms of an optical output from LD. Referring to
FIG. 14
, a curve {circle around (
1
)} shows an ideal rectangular waveform of optical output, and a curve {circle around (
2
)} shows a waveform of optical output having a rounded rising edge, and a curve {circle around (
2
)} shows a waveform of optical output having a rippled rising edge.
FIG. 15
shows the relation (output characteristics), which corresponds to each of the output waveforms of
FIG. 14
, between a pulse width and an average quantity of optical output in the PWM system.
In
FIG. 15
, a line {circle around (
1
)} shows ideal output characteristics corresponding to the waveform of optical output of the curve {circle around (
1
)} in FIG.
14
and generally called output characteristics having a lineality. Namely, an average quantity of optical output increases or decreases in proportion to an increase and a decrease in the duty of a pulse width.
In the case of the waveform (the curve {circle around (
2
)} in
FIG. 14
) having a rounded rising edge, the characteristics of an optical output become as shown the curve {circle around (
2
)} in FIG.
15
. In this case, when the duty is large, the output characteristics do not differ greatly from those in the ideal case of the line {circle around (
1
)} in
FIG. 15
but, when the duty is decreased, the average quantity of output light drops suddenly by a level corresponding to the rounded portion of the waveform. As a result, a minute image displayed by reducing the duty disappears and cannot be reproduced, and a low density (highlight) of a halftone becomes lower than a regular level and unable to be reproduced.
In the case of the waveform (the curve {circle around (
3
)} in
FIG. 14
) having a rippled rising edge, the characteristics of an optical output become as shown the curve {circle around (
3
)} in FIG.
15
. In this case, when the duty is large, the output characteristics do not differ greatly, either, from those in the ideal case of the line {circle around (
1
)} in
FIG. 15
but, when the duty is reduced, an average quantity of optical output becomes large contrariwise as compared with that of optical output in the case of the curve {circle around (
2
)} in FIG.
14
. As a result, a minute image is crushed and cannot be reproduced, and the highlight becomes denser than a regular level and unable to be reproduced.
For these reasons, it has been necessary to set a waveform of an optical output being modulated to a turbulence-free rectangular waveform identical with a waveform of a modulation signal. To meet the requirements, the waveform of an optical output being modulated is corrected by using the differentiation circuit disclosed in Japanese Patent Laid-Open No. 205183/1989, or by using a more regular snubber circui

Affiliated with

Also associated with

Rating

Scanning exposure unit, semiconductor laser driving circuit... does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Scanning exposure unit, semiconductor laser driving circuit..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Scanning exposure unit, semiconductor laser driving circuit... will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2835396