JP2009080319A - Laser scanning optical device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser scanning optical unit which can correct the relative light amount difference of the beams in a simple configuration and also alleviate drooping when forming images in a multi-beam system using two semiconductor lasers. <P>SOLUTION: This is a laser scanning optical unit having a light source 2A composed of two semiconductor lasers 3a, 3b, a half-wavelength plate 4, a beam splitter 5, a collimator lens 6, and a polarizing filter 7. The polarizing filter 7 is disposed at the downstream side of the collimator lens 6 in the beam traveling direction x and adjusts the rotation angles of the beams α, β to equalize their transmissivities. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a laser scanning optical device, and more particularly to a laser scanning optical device mounted as a print head in an image forming apparatus such as an electrophotographic copying machine or printer.

  In recent years, in copiers and printers, a multi-beam system that uses a semiconductor laser as a light source and scans the photoreceptor with multiple beams at predetermined intervals in the sub-scanning direction based on the demand for high-speed and high-definition drawing Has become mainstream.

  In the multi-beam method, first, it is necessary to correct the relative difference in the amount of beam light due to variations in the characteristics of the respective semiconductor lasers. Furthermore, the semiconductor laser has a droop characteristic, which is a cause of image quality degradation. The droop characteristic means that the amount of light of a semiconductor laser tends to decrease as the temperature rises, and when it is driven to light at a constant current, the amount of light decreases slightly due to its own heat generation as the driving time elapses. Image degradation due to droop characteristics appears more significantly in the multi-beam method than in the single-beam method.

With regard to correcting the relative difference in the amount of beam light due to individual differences between semiconductor lasers, Patent Document 1 describes that a transmittance adjusting element is arranged only on one or only one of the beams emitted from two semiconductor lasers. Yes. However, it is difficult to install the transmittance adjusting element for each semiconductor laser in a limited space of the light source unit. Also, if a transmittance adjusting element is provided for only one beam, the amount of light on the photoconductor is lower than that of the other beam. It must be raised and is not practical. Further, Patent Document 1 does not mention any measures for preventing image degradation due to droop characteristics.
JP 2004-45840 A

  Accordingly, an object of the present invention is to correct the relative light quantity difference between each beam with a simple configuration and to reduce the droop characteristic when drawing an image with a multi-beam using two semiconductor lasers. It is an object of the present invention to provide a laser scanning optical device capable of suppressing the above.

In order to achieve the above object, a laser scanning optical device according to the present invention includes:
Two semiconductor lasers,
A polarizing element that changes a polarization direction of a beam emitted from at least one of the semiconductor lasers;
A combining element that combines beams emitted from the two semiconductor lasers in substantially the same direction;
A condensing element for condensing the beams emitted from the two semiconductor lasers;
A single transmittance adjusting element that is arranged downstream of the combining element in the beam traveling direction and adjusts the transmittance of the beams emitted from the two semiconductor lasers;
It is provided with.

  In the laser scanning optical device according to the present invention, the respective beams emitted from the two semiconductor lasers are synthesized in substantially the same direction by the synthesis element, and the transmittance is adjusted by a single transmittance adjustment element, The amount of light irradiated on the photoreceptor is made uniform. Since there is only one transmittance adjusting element, it is advantageous in terms of space, and the output of each semiconductor laser is increased by the amount that is transmitted through the transmittance adjusting element, so that the reduction in the amount of light due to the droop characteristic is alleviated. As a result, image degradation can be suppressed. The semiconductor laser may have one or more light sources.

  In the laser scanning optical apparatus according to the present invention, it is preferable that the transmittance adjusting element is disposed on the downstream side of the light converging element in the beam traveling direction. Since the beam that has passed through the condensing element is condensed into substantially parallel light, the position of the transmittance adjusting element can be set relatively freely.

  The polarizing element may be a half-wave plate, the transmittance adjusting element may be a polarizing filter, and the polarizing filter may be disposed so as to be rotatable about the optical axis. The beams emitted from the two semiconductor lasers by the half-wave plate are made into linearly polarized light whose polarization direction is rotated by 90 °, and the transmittance is increased or decreased by a predetermined ratio by the rotation of the polarization filter. If the initial value of the rotation angle about the optical axis is set, this polarizing filter can be easily adjusted and the adjustment time can be shortened.

  Further, the ND filter may be disposed downstream of the transmittance adjusting element in the beam traveling direction. By providing the ND filter, the semiconductor laser can be used at a higher output, and the influence of the droop characteristic can be further alleviated. A holding unit to which a beam intensity measuring sensor is attached and detached may be provided between the ND filter and the transmittance adjusting element. The beam intensity can be measured before the amount of light is reduced by the ND filter, and the influence of measurement errors can be suppressed. Moreover, the measurement workability is improved by providing a holding portion for the measurement sensor.

  Embodiments of a laser scanning optical apparatus according to the present invention will be described below with reference to the accompanying drawings.

(Refer 1st Example and FIGS. 1-6)
FIG. 1 shows a schematic configuration of a laser scanning optical apparatus 1 according to the first embodiment. The apparatus 1 includes a light source unit 2A, a polygon mirror 10 that is rotationally driven at a predetermined speed, scanning lenses 11 and 12 having an fθ function, a horizontal synchronization sensor 14, and a beam condensed on the sensor 14. The condenser lens 15 is housed in the housing 20.

  As shown in FIGS. 2 and 3, the light source unit 2A includes semiconductor lasers 3a and 3b having a single light source, and a polarizing element (specifically, a polarization element that changes the polarization direction of the beam β emitted from the semiconductor laser 3b). , A half-wave plate) 4 and a synthesizing element (specifically, a beam splitter made of a prism) 5 for synthesizing the beams α and β emitted from the semiconductor lasers 3a and 3b in the same direction. A collimator lens 6 that collects the beams α and β, a transmittance adjusting element (specifically, a polarization filter) 7 that adjusts the transmittance of the beams α and β, and a cylindrical lens 8 for correcting the surface tilt (FIG. 1). Reference).

  The beam α emitted from the semiconductor laser 3a is polarized in the vertical direction shown in FIG. The beam β emitted from the semiconductor laser 3 b is polarized by 90 ° by the half-wave plate 4. These beams α and β are combined in the same direction by the beam splitter 5, converted into parallel light by the collimator lens 6, and transmitted through the polarization filter 7, so that the light intensity is uniformly adjusted as described below. . Thereafter, the beams α and β are condensed almost parallel to the sub-scanning direction z by the cylindrical lens 8 and guided to the polygon mirror 10.

  The beams α and β are deflected at a constant angular velocity in the main scanning direction y based on the rotation of the polygon mirror 10, and necessary aberrations are corrected by passing through the scanning lenses 11 and 12, thereby forming an image on the photosensitive drum 25. . The beams α and β are scanned on the photosensitive drum 25 in the main scanning direction y. 1 to 3, the beams α and β are drawn as if they were synthesized on the same optical axis. However, in reality, the beams α and β are separated at a predetermined interval in the sub-scanning direction z, and 2 in one scan. Draw a line image.

  Here, the transmittance adjustment of the beams α and β by the polarizing filter 7 will be described. The polarization directions of the beams α and β are different by 90 ° as shown in FIG. 4, and the polarizing filter 7 is disposed at an angle of 45 ° with respect to any polarization direction. In this state, the transmittances of the beams α and β are equal to 50%. FIG. 6 shows the transmittances of the beams α and β with respect to the rotation angle of the polarizing filter 7. When the rotation angle is decreased from 45 °, the transmittance of the beam α increases and the transmittance of the beam β decreases. On the other hand, when the rotation angle is increased beyond 45 °, the transmittance of the beam α decreases and the transmittance of the beam β increases.

  If the intensity of the beam α is larger than that of the beam β, the relative difference between the light amounts of the beams α and β can be corrected by rotating the polarizing filter 7 to an angle larger than 45 °. Conversely, if the intensity of the beam α is smaller than that of the beam β, the polarizing filter 7 may be rotated to an angle smaller than 45 °.

  As shown in FIG. 5, the polarizing filter 7 is set in the recess 21 of the housing 20 so as to be rotatable about the optical axis while being held by the holder 71. The polarizing filter 7 is set to an initial value of 45 ° by an adjustment screw 73 attached to a bracket 72 provided on the side of the holder 71 coming into contact with the seat portion 22. With the light source unit 2A alone or in the completed state as the laser scanning optical device 1 incorporating the light source unit 2A, the intensities of the beams α and β are measured, and the adjustment screw 73 is rotated so that the light intensities are uniform. Then, the rotation angle of the polarizing filter 7 is adjusted to correct the relative difference in the amount of light.

  In the first embodiment, the amount of light irradiated on the photosensitive drum 25 is adjusted in order to adjust the transmittance of the beams α and β emitted from the two semiconductor lasers 3 a and 3 b by the single polarizing filter 7. Of course, the number of polarizing filters 7 is one, which is advantageous in terms of space. Further, the output of each of the semiconductor lasers 3a and 3b is increased by the amount that passes through the polarizing filter 7, and the reduction in the light amount due to the droop characteristic is alleviated. As a result, image degradation can be suppressed.

  Further, the polarizing filter 7 is disposed downstream of the collimator lens 6 in the beam traveling direction x. Since the beams α and β transmitted through the collimator lens 6 are condensed into parallel light, the distance L1 between the collimator lens 6 and the polarizing filter 7 can be freely set as shown in FIG. On the other hand, when the polarizing filter 7 is arranged upstream of the collimator lens 6 in the beam traveling direction x, the distance L2 from the semiconductor laser 3a to the collimator lens 6 is determined by the focal length of the collimator lens 6 (usually about 10 mm). It is very difficult to arrange the polarizing filter 7 within the distance L2.

  In the first embodiment, the half-wave plate 4 is used to cause the beams α and β to enter the polarizing filter 7 with the polarization directions having a difference of 90 °. Therefore, the light amount difference can be easily corrected by adjusting the rotation angle of the polarizing filter 7. Moreover, since the rotation angle of the polarizing filter 7 is initially set to 45 °, the adjustment time can be shortened.

(Refer to the second embodiment, FIGS. 7 and 8)
FIG. 7 shows a light source unit 2B of the laser scanning optical apparatus according to the second embodiment. In the light source unit 2B, the ND filter 9 is disposed downstream of the polarizing filter 7 in the beam traveling direction x. Other configurations of the laser scanning optical device are the same as those of the first embodiment shown in FIG.

  As is well known, the ND filter 9 has a function of reducing the amount of transmitted light without changing the component of incident light without performing selective absorption. By providing such an ND filter 9, the semiconductor lasers 3a and 3b can be used with higher output, and the influence of the droop characteristic can be further alleviated.

  When measuring the intensity of the beams α and β, the beam intensity measuring sensor 91 (see FIG. 8) is mounted between the ND filter 9 and the polarizing filter 7. A holder 23 detachably attaching the holder 92 of the sensor 91 is provided in a part of the housing 20, and the holder 23 is installed between the ND filter 9 and the polarizing filter 7. When the ND filter 9 is provided, the influence of the measurement error can be suppressed by measuring the beam intensity before the light amount is reduced by the ND filter 9. Further, by providing the housing 20 with the holding portion 23 of the measurement sensor 91, the measurement workability is improved.

(Refer to the third embodiment, FIG. 9)
FIG. 9 shows a light source unit 2C of the laser scanning optical apparatus according to the third embodiment. The light source section 2C is configured by disposing collimator lenses 6a and 6b immediately after the semiconductor lasers 3a and 3b, and the other configuration is the same as that of the first embodiment shown in FIG.

(Refer to the fourth embodiment, FIG. 10)
FIG. 10 shows a light source unit 2D of the laser scanning optical apparatus according to the fourth embodiment. This light source section 2D is provided with two semiconductor lasers 3a ′ and 3B ′ each having two light emitting sources, and the other configuration is the same as that of the first embodiment shown in FIG. A total of four beams emitted from each light source are separated at a predetermined interval in the sub-scanning direction z and irradiated onto the photosensitive drum 25 to draw an image of four lines in one scan.

(Other examples)
The laser scanning optical device according to the present invention is not limited to the above-described embodiments, and can be variously modified within the scope of the gist thereof.

  For example, the image forming apparatus on which the laser scanning optical device is mounted is arbitrary, and may be a monochrome image forming apparatus or a color image forming apparatus, and may be a printer, a copier, a facsimile machine, or a complex machine thereof. Either may be sufficient. Of course, the configuration of the optical system downstream of the polygon mirror is also arbitrary.

It is a top view which shows 1st Example of the laser scanning optical apparatus based on this invention. It is a perspective view which shows the light source part of the said 1st Example. It is a top view which shows the said light source part. It is explanatory drawing of a polarizing filter. It is an elevation view which shows the attachment state of the said polarizing filter. It is a graph which shows the change of the transmittance | permeability of the beam with respect to the rotation angle of a polarizing filter. It is a top view which shows the light source part of 2nd Example of the laser scanning optical apparatus based on this invention. It is a perspective view which shows the sensor for a measurement in the said 2nd Example, and its holding | maintenance part. It is a perspective view which shows the light source part of 3rd Example of the laser scanning optical apparatus based on this invention. It is a top view which shows the light source part of 4th Example of the laser scanning optical apparatus based on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Laser scanning optical apparatus 2A, 2B, 2C, 2D ... Light source part 3a, 3b, 3a ', 3b' ... Semiconductor laser 4 ... 1/2 wavelength plate 5 ... Beam splitter 6, 6a, 6b ... Collimator lens 7 ... Polarization Filter 9 ... ND filter 20 ... Housing 23 ... Holding part 91 ... Sensor for beam intensity measurement α, β ... Beam

Claims (6)

Two semiconductor lasers,
A polarizing element that changes a polarization direction of a beam emitted from at least one of the semiconductor lasers;
A combining element that combines beams emitted from the two semiconductor lasers in substantially the same direction;
A condensing element for condensing the beams emitted from the two semiconductor lasers;
A single transmittance adjusting element that is arranged downstream of the combining element in the beam traveling direction and adjusts the transmittance of the beams emitted from the two semiconductor lasers;
A laser scanning optical device comprising:   The laser scanning optical apparatus according to claim 1, wherein the transmittance adjusting element is disposed downstream of the light collecting element in a beam traveling direction.   The polarizing element is a half-wave plate, the transmittance adjusting element is a polarizing filter, and the polarizing filter is disposed so as to be rotatable about the optical axis. 3. A laser scanning optical device according to 2.   4. The laser scanning optical apparatus according to claim 3, wherein an initial value of a rotation angle about the optical axis of the polarizing filter is set.   5. The laser scanning optical apparatus according to claim 1, wherein an ND filter is disposed downstream of the transmittance adjusting element in a beam traveling direction.   6. The laser scanning optical device according to claim 5, further comprising a holding unit to which a beam intensity measuring sensor is attached and detached between the transmittance adjusting element and the ND filter.

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