JPS6156316A - Laser printer - Google Patents

Abstract

PURPOSE:To obtain a clear image with high resolution by a simple constitution, even in case an optical path length is shortened, by synthesizing plural optical beams to an optical beam of one direction by using an optical synthesizing element, so that it is made incident on a polygon mirror. CONSTITUTION:Plural optical beams B1-B4 emitted from the respective independent laser light sources 11-14 are synthesized by using an optical synthesiz- ing element such as half mirrors 91, 92 and a polarizing prism 10, etc., made adjacent to the extent of an interval of a laser beam spot for projecting an optical axis of each optical beam onto a photosensitive drum 8, and thereafter, made incident on a polygon mirror 6. Also, a shape of the laser beam spot is made elliptical, it is arranged so that a long axis direction of its ellipse vertical to the main scanning direction, and also a lighting time of the laser light sources 11-14 or an optical output is controlled within a range of the time corresponding to one dot in relation to a scan position of the laser beam spot, therefore, a non-linear error in the scanning line can be reduced.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention horizontally scans a light beam spot emitted from a laser light source on a photoreceptor to form an image to be recorded in dots (
This relates to a laser printer that displays and records information using a set of pixels (pixels).

(Prior art) Conventionally, in such laser printers, it is common to scan a photoreceptor with a single light beam, but in order to increase the recording speed, multiple light beams are scanned at the same time. Multi-beam type laser printers that perform scanning have also been put into practical use.

FIG. 9 is a block diagram showing an example of a conventional multi-beam laser printer. The laser printer shown in the figure is configured to scan four light beams at once. That is, a laser light source 1 such as an Ic-He laser
The light beam emitted from mirror 2. ~2. and beam splitter 3. ~3. The beam is divided into four beams with equal brightness by the focusing lens 4. ~Change through 44!
IJ device5. ~5. guided by. Change 1 JII device 5. ~5.
The modulator 5. controls the on/off of the light beam according to the image information to be recorded. 54 is reflected by the polygon mirror 6, projected onto the photosensitive drum 8 via the optical path adjusting section 7, and horizontally scans (main scans) the photosensitive drum 8 as a laser beam spot. The photosensitive drum 8 rotates in a direction perpendicular to the horizontal scanning direction of the laser beam spot (this is called sub-scanning), and an electrostatic latent image corresponding to image information is formed on the photosensitive drum 8 by a collection of dots. It is formed. Although this electrostatic latent image is not shown,
After a development process and a transfer process, a recorded image is obtained on recording paper.

Here, the interval between the laser beam spots obtained on the photosensitive drum 8 is determined according to the resolution of the image, and the modulator 5. .about.54 is positioned such that the laser beam spots are aligned at predetermined intervals and perpendicularly to the main scanning direction.

[Problem that the invention seeks to solve]

However, in order to accurately project a plurality of light beams having different optical axes as shown in the figure onto the photosensitive drum 8, the modulator 5. High accuracy is required for positioning, such as ~54. Further, even if it is attempted to adjust these positions iJ1 by mechanical processing, very high processing accuracy is required, as in the above case.

Furthermore, if there is a difference in the angle of incidence of each light beam on the polygon mirror 6, the scanning line of the laser beam spot projected onto the photosensitive drum 8 will not be uniformly straight;
This results in non-linear errors.

The illustrated device has a long optical path length to reduce the difference in the angle of incidence between the light beams and reduce non-linear errors. If the length is shortened, there is a limit to miniaturization of the modulators 5. to 54, and it becomes impossible to bring each light beam close to each other and reduce non-linear errors.

Further, in the laser printer having such a configuration, the focusing lens 4. ~4. focuses each light beam so that the laser beam spot is focused, for example, on the center point Pc of the photosensitive drum 8 (so that the spot diameter is minimized); QP, and near both ends of the photosensitive drum 8, it comes off the photosensitive surface, resulting in a decrease in resolution.

In order to eliminate such drawbacks, an optical system such as an imaging lens may be installed between the polygon mirror 6 and the photosensitive drum 8, but this lens and other optical systems are expensive and have a complicated structure. It becomes complicated.

The present invention eliminates the drawbacks of the conventional device as described above, and even when the optical path length is shortened, the nonlinear error of the scanning line does not become large, and multiple laser beam spots can be arranged at regular intervals. The object of the present invention is to realize a laser printer with a simple configuration that can perform scanning and obtain clear images with high resolution.

[Means for solving problems]

The laser printer of the present invention combines a plurality of light beams emitted from independent laser light sources into a unidirectional light beam using a light combining element such as a half mirror or a polarizing prism, and the optical axis of each light beam is The laser beams are brought close to each other at the same distance as the laser beam spots projected onto the photosensitive drum, and then made incident on the polygon mirror.

Further, in the laser printer of the present invention, the shape of the laser beam spot incident on the polygon mirror from each laser light source is elliptical, and each laser beam spot is arranged so that the long axis direction of the ellipse is perpendicular to the main scanning direction. The lighting time or optical output of the laser light source is controlled within the time range corresponding to one dot in relation to the scanning position of the laser beam spot.

[For production]

In this way, by combining multiple light beams using a light combining element such as a half mirror or polarizing prism, the optical axis of each light beam can be brought close to the distance between the laser beam spots on the photosensitive drum. It is possible,
Nonlinear errors in scanning lines can be reduced.

Furthermore, by making the shape of the laser beam spot elliptical and controlling the exposure time according to the scanning position, the shape of the dot formed on the light drum can be made circular at any scanning position. , high resolution and clear images can be obtained.

〔Example〕

FIG. 1 is a configuration diagram showing an embodiment of a laser printer of the present invention. In the figure, the same parts as in FIG. 9 are designated by the same reference numerals. 1□ to 14 are laser light sources such as semiconductor lasers, and 9. ．． 9. A half mirror 110 is a polarizing prism, and 11 is a λ/2 plate. Laser light source 1. The light beam B1 emitted from the focusing lens 4. After being focused to a desired beam diameter, the beam enters a half mirror 91. On the other hand, the light beam B2 emitted from the laser beam R13 is transmitted through the focusing lens 4. After being focused to a desired beam diameter, the beam enters a half mirror 9. Here, half mirror 9,
transmits or reflects the light beams Bl, B2 depending on its angle of incidence? In this case, the light beam B1 passes through the half mirror 91, and the two light beams B1 and B2 are combined by the half mirror 9 into a unidirectional light beam with close optical axes. Similarly, laser light source 1. , 1. A light beam 83 emitted from
The light beams 84 are also combined into a unidirectional light beam with close optical axes by the half mirror 9.

In addition, the light beam (
Bl, B2) is mirror 2. Polarizing prism 1 through
The light beams (B3 and B4) incident on the polarizing prism 10 are incident on the polarizing prism 10 via the λ/2 plate 11 and are combined by the half mirror 9. Since the polarizing prism 10 transmits or reflects the incident light beam depending on its polarization direction, here as well, one of the light beams B3. B4 passes through the polarizing prism 10, and the four light beams 81 to B4 are combined into a unidirectional light beam.

Here, the λ/2 plate 1 placed after the half mirror 93
1 is for rotating the polarization direction of the light beams B3 and B4 without changing the elliptical direction of the laser beam spot, and transmitting the polarizing prism 10. In the figure, an ellipse in light beams 81 to B4
! and its polarization direction.

The light beams 81 to B4 thus combined are reflected by the polygon mirror 6 and projected onto the photosensitive drum 8. At this time, the optical axes of the four light beams 81 to B4 combined by the half mirrors 9, 9 and the polarizing prism 10 are close to each other at the desired laser beam spot spacing, and the laser beam spot is located within the ellipse. They are arranged in a line with their long axes perpendicular to the main scanning direction.

For this reason, each of the light beams 81 to 81 to the polygon mirror 6
There is no difference in the incident angle of B4 (therefore, non-linear errors will not occur in the scanning line of each laser beam spot).

Furthermore, in general, a light beam emitted by a semiconductor laser has a different radiation angle in a direction parallel to the junction surface and in a direction perpendicular to the junction surface, so that the far-field image becomes an ellipse. For this reason, the laser light source 1. The orientation of the ellipse in the laser beam spot can be aligned by simply changing the arrangement direction of the elements 1 to 14. Note that in order to combine the light beams 81 to B4 using the polarizing prism 10 while keeping the orientation of the ellipses aligned, the λ/2 plate 11 is required as described above.

Next, the laser beam spot is made into an ellipse, and each laser light source 1. ~1. The operation when controlling the lighting time or light output will be explained.

FIG. 2 is a configuration diagram showing an example of a control system in a laser printer of the present invention. In the figure, laser light source 1. Illustrated only. Laser light source 1. The light beam B1 emitted from the half mirror 91 and the mirror 2. The light enters the polygon mirror 6 through the polygon mirror 6 and is projected onto the photosensitive drum 8.

12 is a light source drive circuit for the laser light source 1□, and the laser light a1
．． For example, it is turned on and off depending on the image information to be recorded.

Reference numeral 13 denotes a buffer memory in which image information for one raster or one sheet is sequentially stored as a set of dots. 1
4 is a counter that specifies the read address of the buffer memory 13; 15 is a clock oscillator whose output is applied to the counter 14; 16 is a laser light source 1. This is a lighting time control circuit that controls the lighting time or light output of the light source, and the output from this circuit is applied to the light source drive circuit 12. 1
Reference numeral 7 denotes a timing control circuit to which a timing signal from the light beam scanning position detection means 18 is applied and which controls the drive motor 1.
Polygon mirror 6 and photosensitive drum 8 via 9 and 20
It controls the rotational movement of.

Figure 3 shows laser light source 1. FIG. 3 is an explanatory diagram for explaining the spot shape of a light beam B1 emitted from the .

Laser light source 1. The imaged spot shape of the light beam B1 emitted from the lens is elliptical at the imaged position, as shown in FIG. 3 (A). In addition, in this light beam B1, the major axis diameter h of the elliptical spot (in the direction of arrow X)
The depth of focus is deep as shown in Figure 3 (b), and the depth of focus of the short axis diameter wo (direction of arrow X) of the elliptical spot is shallow as shown in Figure 3 (b). In the apparatus of the present invention, such a spot-shaped light beam is used and the long axis of the spot is projected perpendicular to the main scanning direction.The light beam emitted from the laser light source is If the spot shape is not a desired ellipse, a spot shape setting optical system including, for example, a cylindrical lens and a focusing lens is inserted after the laser light source.

FIG. 4 shows the light beam 81 in FIGS. 1 and 2.
It is a top view which shows the scanning state of -B4. In 22, the laser light source 1. A case is shown in which the imaging positions (indicated by solid lines q) of the light beams 81 to 84 from 14 to 14 coincide with the surface of the photosensitive drum 8 at two points P, , P,.

FIG. 5 shows a spot shape obtained on the photosensitive drum 8 when the laser beam spot is simply scanned in the optical system as shown in FIG. 4. In this case, a point P on the photosensitive drum 8.

At Pl, since the imaging positions match, the spot shape becomes an ellipse, with a central part near point Pc and both ends P,.

In the vicinity of 24 points, the spot shape becomes close to a circle because the spot deviates from the imaging position. Therefore, in the present invention,
Laser light sources 11 to 1. lighting time t
By controlling α, including the vicinity of the point P,,P,,
A substantially circular spot shape can be obtained at any position on the photosensitive drum 8.

FIG. 6 shows the shape of a scanning spot corresponding to one dot in the vicinity of points P, , P, on the photosensitive drum 8. In the present invention, the main scanning direction (arrow
laser light source 1. ~1. By controlling the lighting time (meeting time) tα according to the signal Sα from the lighting time control circuit 16, the shape of the scanning spot can be changed to Qo+ as shown in the figure.
It is controlled so that it becomes wo. Here, Qo is the scanning length corresponding to one dot, and if the long axis diameter of the elliptical spot is controlled to be Qo + womh, the shape of the scanning spot will be changed around the point P, , P, It can also be approximately circular.

FIG. 7 is an operational waveform diagram for explaining the operation of the apparatus shown in FIG. 2. In the figure, (a) is the clock signal from the clock oscillator 15, and (b) is the buffer memory 1.
3 is an example of a write signal output from 3. here,
Exposure is represented by a signal of °I I 11, and non-exposure is represented by a signal of "0" °. Further, (c) is the control signal Sα from the lighting time control circuit 16, in which a pulse width signal corresponding to the period of the clock signal is used, and this pulse width α
is adapted to change in relation to the scanning position on the photosensitive drum 8. , the light source drive circuit 12 has a buffer memory 13
and a control signal Sα from the lighting time control circuit 16, and outputs a laser beam according to both signals.

14 and controls its lighting time tα. (D) is the laser light source 1□ to 1. The lighting time t is determined by the pulse width α of the control signal Sα.
α is controlled and varies within a time range T corresponding to one dot.

FIG. 8 shows the scanning position of the laser beam spot on the photosensitive drum 8 and the laser light source 1. It is a figure showing the relationship with lighting time tα of -14. The lighting time control circuit 16 generates a signal related to the scanning position from lighting time control data stored in advance in a ROM, etc., and changes the lighting time tα to P as shown in the figure.
, , P, is longest near the point, P * , P c
-P Control so that it is the shortest around 4 points.

In this way, the spot shape of the light beams 81 to B4 is made into an ellipse, and the laser light source 1. ~14 lighting time 1
If the laser beam spot is controlled in relation to the scanning position within the time range corresponding to one dot, the shape of the laser beam spot obtained on the photosensitive drum 8 can be made almost circular at any scanning position, and the resolution It is possible to realize a laser printer that can obtain clear images with high brightness.

In the above explanation, a half mirror and a polarizing prism are exemplified as a light combining element that combines light beams, but the light combining element is not limited to these, and can reflect or transmit an incident light beam and combine multiple light beams. Any light combining element may be used as long as it combines the light beams of 1 to 1 into a unidirectional light beam with adjacent optical axes. In addition, in the embodiment shown in Fig. 1, a half mirror and a polarizing prism are used together, but this is to reduce the power loss of the light beam as much as possible. The type can be selected arbitrarily. By the way, there is a half mirror in the optical path.
The loss due to installation is approximately 1/2. On the other hand, if only a polarizing prism is used, the loss will be very small, but the equipment will be expensive...! Further, in the above description, the case where four light beams are combined is illustrated, but the number of light beams to be combined is not limited to four.

〔Effect of the invention〕

As explained above, in the laser printer of the present invention, multiple light beams emitted from independent laser light sources are combined using a light combining element such as a half mirror or a polarizing prism, and the optical axis of each light beam is The laser beams are made to approach the same distance as the laser beam spots projected onto the photosensitive drum, and are incident on the polygon mirror, and the shape of the laser beam spot incident on the polygon mirror from each laser light source is made into an elliptical shape. The spots are arranged so that the long axis direction of the ellipse is perpendicular to the main scanning direction, and the lighting time or optical output of the laser light source is set to the time range corresponding to one dot in relation to the scanning position of the laser beam spot. Since the laser beam spot is controlled within a certain range, it is possible to reduce nonlinear errors in the scanning line, and the shape of the laser beam spot scanning the photosensitive drum can always be circular regardless of the scanning position. Even when the optical path length is shortened, the nonlinear error of the scanning line does not become large, and multiple laser beam spots can be scanned at regular intervals, and high resolution and clear images can be obtained. A laser printer capable of producing images can be realized with a simple configuration.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the laser printer of the present invention, FIG. 2 is a block diagram showing an example of a control system in the laser printer of the present invention, and FIG. 3 is a block diagram of the apparatus shown in FIG. 2. An explanatory diagram of the shape of the laser beam spot, FIG. 4 is a plan view showing the arrangement of the optical system in the apparatus of FIG. 2, P! FIG. 45 is an explanatory diagram for explaining the shape of the scanning spot on the photosensitive drum, FIG. ε is an explanatory diagram for explaining the shape of the scanning spot on the photosensitive drum in the apparatus of FIG. 2, and FIG. Fig. 8 is an explanatory diagram showing the relationship between the scanning position of the laser beam spot and the lighting time of the laser light source, and Fig. 9 is a configuration diagram showing an example of a conventional multi-beam laser printer. be. 1.1. ~1. ...Laser light source, 2, ~2. ... Mirror, 31-31 ... Beam splitter, 41-44
...Focusing lens, 51-54...Modulator, 6...
Polygon mirror, 7... Optical path adjustment @[;, 8... Photosensitive drum, 9. ,9. ...half mirror-110...
- Polarizing prism, 11... λ/2 plate, 12... Light source drive circuit, 13... Buffer memory, 14... Counter, 15... Clock oscillator, 16... Lighting time control circuit, 17...timing control circuit, 18...
Scanning position detection means, 19, 20... drive motor. Figure 1 Hand Figure 2 Figure 3 (a) W^ Figure 4 Figure 6

Claims (1)

[Claims] A laser printer that simultaneously scans a photoreceptor with a plurality of light beams includes a plurality of laser light sources that each emit a light beam having an elliptical spot shape, and a laser printer that reflects or transmits an incident light beam to A light combining element that combines a plurality of light beams into a unidirectional light beam with adjacent optical axes, and a polygon mirror that reflects the light beam combined by the light combining element and projects it onto a photoreceptor, A laser printer characterized in that the lighting time or optical output of a laser light source is controlled within a time range corresponding to one dot in relation to the scanning position of a laser beam spot scanning the photoreceptor.