JPH0377506B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a light beam exposure apparatus, and more particularly to a light beam exposure apparatus that scans a light beam on a photoreceptor to form an image.

A light beam scanning type printing device includes a light beam scanning device, a light beam modulation device, a photoreceptor that forms an electrical, magnetic, or other latent image using a light beam, and various devices for realizing these. It consists of a computer output terminal, facsimile,
Applied to copying machines, etc.

An example is shown in FIG.

Unit A is a light beam scanning device, which is composed of a rotating polygon mirror 2 and an f.theta. lens 3, and scans a light beam from a laser at a constant speed in the longitudinal direction of a photosensitive drum 4, which will be described later. Unit B is a laser and laser modulation device that outputs information from a computer, image signals from a facsimile line,
It converts the output from a photoelectric conversion element such as a CCD into a predetermined modulation signal, modulates the laser output with the modulation signal, and generates the intensity of the light beam. Unit C is a device for forming an electrostatic latent image on the photoreceptor drum 4, and in this example, a so-called CdS/binder with a transparent insulating layer is used for the photoreceptor.
Obtain an electrostatic image using the NP method. That is, 5 is a pre-static eliminator for leveling the surface potential of the photoreceptor, 6 is a primary charger, 7 is a secondary static eliminator which also has a slit for laser beam exposure, and 8 is a front exposure lamp. be. With the above-described device, a latent image is formed on the photosensitive drum 4 at a lower potential in the area irradiated with the laser beam than in the area not irradiated with the laser beam.
If negative polarity toner is used, the developing device 9 will
The areas not irradiated by the beam can be visualized in black.

The image obtained in this way is transferred to the transfer charger 1
1, the image is transferred to paper, fixed by a fixing device 12, and discharged to a paper discharge stacker 14 as a permanent image. Toner remaining on the drum without being transferred is cleaned by a cleaner 10 for the next process.

Such a device has the advantage that very good images can be obtained at high speed and on plain paper. Furthermore, since it is a non-impact type, it is useful for reducing noise.

However, lasers are generally large in shape;
When incorporated into a device, the device itself inevitably becomes larger. Using semiconductor lasers can significantly downsize the device, but the oscillation wavelength of semiconductor lasers is mainly infrared, so they must be used where the sensitivity of the photosensitive drum is extremely low. The junction temperature of a semiconductor laser changes depending on the semiconductor laser drive current, and this appears as output fluctuation.
Unless a certain amount of drive current is applied to the semiconductor laser, pure frame single-mode oscillation will not occur, and the spot diameter of the beam on the photosensitive drum surface will widen, resulting in deterioration of the picture area. When a semiconductor laser is used in this way, the formation of a latent image becomes unstable.

By the way, in a device that performs recording by scanning a laser beam from a semiconductor laser, a detector such as a photo sensor receives the light beam after being deflected by a scanning means to detect a synchronization signal for light beam modulation. Masu. In order to detect such synchronization signals well, it is desirable to increase the amount of light that enters the detector, and to do so, it is necessary to increase the power of the semiconductor laser that is the light source. However, if the power of the semiconductor laser serving as the light source is increased more than necessary, an excessive amount of light reaches the photoreceptor, making it impossible to form a stable image on the photoreceptor. The present invention has been made in view of the above circumstances, and an object thereof is to provide a device that can output a stable image without adversely affecting the photoreceptor, and has high detection precision of a synchronization signal and is highly reliable. do.

FIG. 2a shows an example of a configuration according to the present invention. 15 is a half mirror provided on the back side of the exposure slit of the secondary charger 4. The beam intensity is attenuated to a fraction by this half mirror. The transmittance of the half mirror depends on the sensitivity of the photosensitive drum to the laser wavelength,
Determined by the most stable output value of the laser.

FIG. 2b shows an enlarged view of the half mirror portion. Half mirrors are made by depositing silver on glass, and are sand-chilled with glass to ensure durability. Further, the beam is tilted to prevent the secondary beam reflected by the photosensitive drum from being further reflected onto the photosensitive member. The transmittance can be changed depending on this slope. It is also possible to adjust at the time of manufacture. It is also possible to vary the amount of light by changing the inclination using a servo motor or the like (not shown).

An ND filter can also be used as the light intensity attenuation means. In this case, it is easy to obtain changes in light intensity using a wedge-shaped ND filter.

In these cases, there is no problem with changes in the optical path length due to passing through the glass, vapor deposited layer, or ND filter because the depth of focus of the focusing system such as the fθ lens exists on the order of mm. If an extremely short focal length focusing system is required, or if mechanical precision needs to be corrected, a glass plate or other device for changing the optical path length should be designed in advance to complement the optical attenuation means. You can compensate for it.

The effects and advantages of the present invention will be explained in detail below with reference to Examples.

FIG. 3 shows the sensitivity versus wavelength of a typical photoreceptor, where the horizontal axis is the wavelength and the vertical axis is the relative sensitivity, where the sensitivity of the photoreceptor at a wavelength of 800 nm is set to 1. was sensitized with Te
Se is the sensitivity curve of the OdS/resin binder system sensitized with In. Both photoconductors are 800n
The sensitivity decreases sharply from m to 900 nm. When measured at a paper feeding process speed of 100 mm/sec, the required laser output is 800 nm in wavelength.
So, it was 1.05mW at , and 0.35mW at .

On the other hand, semiconductor lasers generally oscillate at infrared wavelengths and have characteristics as shown in FIG.

While the current flowing through the laser is low, it emits light in LED mode, but when a certain threshold is exceeded, it begins to oscillate. For a current near the threshold that starts laser oscillation, the LED mode and laser mode are mixed, and the oscillation waveform is not single. In other words, the laser beam will be spread out more than necessary. This means that the latent image formed on the photoreceptor becomes blurred, which is undesirable because the expressibility of thin lines becomes extremely poor. On the other hand, if an excessive current is applied, the laser itself will be destroyed, or even if it is not destroyed, oscillations other than the reference mode will occur and the beam will spread, which is undesirable. FIG. 4 shows a drive current-beam output intensity characteristic diagram of the semiconductor laser.
The GaAlAs semiconductor laser shown in Figure 4 is approximately
Although it has a wavelength of 800 nm, the S/N is the best when the output is 5 to 10-odd mW, and the lifespan is not shortened.

As already mentioned, the amount of light required by the photoreceptor is 1
Since it is approximately mW, if this embodiment is used, both the photoreceptor and the laser can be used under optimal conditions, and a good image with excellent fine line expressibility can be obtained.

In addition, if the drive current of the laser drive circuit changes, the single mode property of the laser is excellent and the beam spreads, which not only deteriorates the image quality but also causes a shift in the latent image potential between bright and dark areas due to changes in the amount of light. This will make the output image unstable, but if this embodiment is used, the laser oscillation waveform will hardly change as described above, and the change in light intensity itself will decrease in proportion to the amount of attenuation of the beam light intensity. Therefore, the output image becomes extremely stable.

Furthermore, photoreceptors and semiconductor lasers are subject to considerable variation during manufacturing. Even in such a case, by changing the amount of attenuation of the beam light quantity, an optimal latent structure can be formed without changing the charger conditions or the laser drive current, so the burden on device manufacturing is greatly reduced. In addition, by applying feedback to the amount of attenuation of the beam light amount using the potential center (by rotating the half mirror with a servo motor or the like), image stability can be further improved.

Additionally, it is generally known to use a beam detector to achieve good horizontal synchronization of images. A photodiode or PiN diode is used for the beam detector, but since the laser beam has a spread and the photo sensor has a light-receiving area, there is a difference in the timing at which the detection signal is generated.

Let us now consider the case of 12 pixels/mm, which can be easily achieved with a device having this configuration. This value can separate 4 to 6 lines/mm on the output image, which is sufficient to obtain a fairly sharp image quality. At a process speed of 100 mm/sec, 1200 scans are required per second. If a 12-sided rotating polyhedron is used, the required rotation speed is 6000 rpm, which is a practical value. When the speed of the scanning beam on the drum was measured under these conditions, it was 420 m/sec.
Furthermore, in order to realize 12 pixels/mm, a beam spot size of 83 μm is required, which is also an achievable value. Therefore, the time required for one pixel is
83μm÷420m/sec = 0.2μsec. If the light receiving width of the detector is 2 mm, the ideal detect signal will be as shown by the solid line in FIG. However, when the inventors conducted experiments, only the output signal shown by the dotted line in FIG. 5, for example, could be obtained. This is because the sensitivity of the photo sensor is not uniform over its light-receiving area, and the energy of the light beam itself is small. If only the detect signal shown by the dotted line is obtained, the tilt of each surface of the rotating polygon mirror is not zero, so the beams reflected by each surface will not be irradiated on the same part of the photo sensor. Therefore, the timing of the horizontal synchronization signal will be shifted. According to the data obtained by the present inventors, there was a horizontal synchronization shift of about 0.5 mm on the image. This makes the image look very bad. Some general photo sensors use a condensing lens to increase the amount of light received, but this widens the light receiving surface and produces worse results. In order to prevent such a situation, the second embodiment is changed to the sixth embodiment.
As shown in the figure.

In FIG. 6, parts having the same functions as those in FIG. 2 are given the same numbers. In the figure, 21 is a photo sensor for beam detection, and a slit 20 is provided in front of the light receiving surface to narrow the irradiation beam width.

22 is a delay circuit which sends a signal instructing a control circuit 23 that controls modulation of the semiconductor laser 1 to start modulation.

As is clear from the figure, the photo sensor is irradiated with the amount of beam light without being attenuated, and the portion of the photoreceptor 4 where the latent image is formed is irradiated with the amount of beer light attenuated by the half mirror 15. With this configuration, the S/N of the beam detect signal can be improved and good horizontal synchronization can be achieved.
Since the laser output does not change and only the beam incident on the photoreceptor 4 is attenuated, the laser output does not fluctuate due to changes in the junction temperature of the semiconductor laser due to momentary excessive inrush current. , it is possible to provide the photoreceptor with an optimal amount of light.

As described above, according to the present invention, a laser can be used in the most stable state with a simple configuration, so that stable images can be obtained and the reliability of the apparatus can be improved. Furthermore, by removing at least the light beam that is irradiated to the beam detection means that detects the light beam, the precision of the beam detection signal is further increased, and an image with horizontal synchronization can be obtained.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing a conventional printing device using a light beam, FIG. 2a is a schematic diagram showing a printing device using a light beam according to the present embodiment, and FIG. An enlarged view of the mirror part, Fig. 3 is a characteristic diagram showing the photoreceptor's sensitivity to wavelength, Fig. 4 is a semiconductor laser drive current vs. beam output intensity characteristic diagram, Fig. 5 is an output waveform diagram of the photo sensor, and Fig. 6 is the second
FIG. In the figure, 1 is a laser and a modulator, 2 is a rotating polygon mirror, 4 is a photoreceptor, 15 is a half mirror, 20
21 represents a slit, and 21 represents a photo sensor.

Claims (1)

[Claims] 1. A semiconductor laser, a deflection means for deflecting a light beam from the semiconductor laser to scan the photoreceptor, and a detection means for detecting the light beam after being deflected by the deflection means. and a light beam exposure apparatus having a control means for controlling the start of modulation of the light beam based on the output of the detection means, wherein a light intensity attenuation means is provided on the optical path of the light beam after being deflected by the deflection means, A light beam exposure apparatus characterized in that the light beam reaching the photoreceptor from the deflection means passes through the light intensity attenuation means, and the light beam reaching the detection means from the deflection means does not pass through the light intensity attenuation means.