JPH04336519A - Laser beam scanning optical system - Google Patents

Abstract

PURPOSE:To reduce the defocus on an image surface based on the variation of the focus distance of a Fresnel lens as small as possible by using a small- sized and lightweight Fresnel lens as the light collecting lens for the laser beam which is radiated from a laser diode, in a laser beam scanning optical system. CONSTITUTION:A light source unit 11 is constituted by accommodating a laser diode 15, Fresnel lens 16, and a slit plate 17 into a case 12. In assembly, the focus adjustment for the beam is carried out in the state where the light quantity is reduced by the slit 17a, and the laser diode driving electric current at this time is made nearly equal to the driving electric current in the actual operation.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser beam scanning optical system, and more particularly to a laser beam scanning optical system used as an image writing head of an image forming apparatus such as an electrophotographic copying machine, a laser printer, or a facsimile machine.

0002

[Prior Art] Generally, a laser beam scanning optical system incorporated in an image forming apparatus such as a laser printer or a facsimile uses a deflector (polygon mirror) to deflect a laser beam emitted from a laser diode onto one plane at a constant angular velocity. After scanning, the scanning speed is corrected using an fθ lens system or an fθ mirror system, and an image is formed on a scanning line (photoreceptor). Since the laser beam emitted from the laser diode is a diverging beam having a certain spread angle, a condensing lens (collimator lens) is provided in front of the laser diode to condense the beam into substantially parallel light. Furthermore, a slit for shaping the collimated beam into a predetermined shape is installed between the collimator lens and the polygon scanner. Each component of this type is constructed separately, and first, the position of the laser diode and the collimator lens is adjusted (focus adjustment), and then the slit is installed.

On the other hand, a Fresnel lens has been developed which is a set of lattice-like concentric circular patterns having a period on the order of microns and whose cross section is sawtooth-like. This Fresnel lens uses refraction and diffraction phenomena, and when parallel light enters it, the light bends at each part of the grating, converging the incident light onto a single point. Conversely, the diverging light emitted from the focal point is collimated at each part of the grating.

[0004] Therefore, it is conceivable to construct a laser light source unit by replacing the Fresnel lens with a conventional collimator lens. However, the biggest problem here is defocus caused by changes in the oscillation wavelength of the laser diode. If a slit is provided after adjusting the positions of the laser diode and the condensing lens, the amount of light will decrease. If a normal collimator lens is used, the driving current of the laser diode may be increased during actual operation. However, when the Fresnel lens is used, the amount of heat generated by the laser diode increases due to an increase in drive current to compensate for the decrease in light intensity, and the oscillation wavelength shifts. A Fresnel lens that utilizes a diffraction phenomenon is unstable with respect to changes in wavelength, and its focal length changes sensitively to slight changes in wavelength. In a laser beam scanning optical system, the slight change in focal length is magnified hundreds of times through the scanning optical system.
This causes defocus on the image plane (photoreceptor).

[0005]

[Object, structure, and operation of the invention] Therefore, the object of the present invention is to
The Fresnel lens is used as a condensing lens for the laser beam to make the light source unit more compact, and it also minimizes fluctuations in focal length, which is one of the problems with Fresnel lenses, and reduces defocus on the image plane. An object of the present invention is to provide a laser beam scanning optical system that can be made small.

In order to achieve the above object, a laser beam scanning optical system according to the present invention includes a condensing lens having a diffraction effect that condenses diverging light emitted from a laser diode into approximately parallel light, It is characterized by being integrated into a unit with a slit that shapes the beam shape emitted from the lens. The condensing lens has a thin flat plate shape with a focal length of about 1 to 10 mm, and is smaller and lighter than conventional collimator lenses, and includes a light source unit that is densely packaged together with a laser diode and a slit in one package. can get. In addition, the light amount is adjusted at the same time as the focus adjustment during assembly, and the laser diode is driven with substantially the same drive current as in actual operation, making it possible to adjust the focus in anticipation of changes in the focal length of the condenser lens.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Examples of the laser beam scanning optical system according to the present invention will be described below with reference to the accompanying drawings. Figure 1 shows
1 shows a laser beam scanning optical system as an embodiment of the present invention. This optical system 10 includes a light source unit 11, a cylindrical lens 20, a polygon mirror 21, and an fθ lens 22.
, a flat mirror 23, a sensor unit 30 for detecting the image writing start position, and this sensor unit 30.
The mirrors 28 and 29 that guide the laser beam to the housing 4
It is attached to 0.

Light source unit 11 (its configuration will be described later)
The laser beam emitted from the polygon mirror 21 passes through the cylindrical lens 20 and is converged near the reflective surface of the polygon mirror 21 in a straight line that coincides with its deflection surface. The polygon mirror 21 is driven to rotate at a constant speed in the direction of arrow a, and continuously deflects and scans the laser beam at a constant angular speed. The scanned laser beam passes through the fθ lens 22, is reflected by the plane mirror 23, and forms an image on a photoreceptor drum (not shown) through the slit 41 of the housing 40. At this time, the laser beam is scanned at a constant speed in the axial direction of the photoreceptor drum, and this is called main scanning. Further, scanning based on the rotation of the photoreceptor drum is referred to as sub-scanning.

In the above configuration, an image (electrostatic latent image) is formed on the photosensitive drum by turning on and off the laser beam from the light source unit 11 and the main scanning and sub-scanning. The fθ lens 22 corrects (distortion aberration) so that the scanning speed of the laser beam in the main scanning direction is equalized from the center to both ends of the scanning area. The cylindrical lens 20 works together with the fθ lens 22 to correct the surface tilt error of the polygon mirror 21.

On the other hand, a part of the laser beam deflected and scanned by the polygon mirror 21 enters the sensor unit 30 through mirrors 28 and 29, and the image writing start position for each line is controlled based on the detection signal. Here, the light source unit 11 will be explained. As shown in FIG. 2, the light source unit 11 has a metal protective cover 19 fixed to the front part of a resin case 12, and the protective cover 19 has a beam exit window 19a formed therein. The case 12 is integrally molded from resin, and houses a laser diode 15, a condensing lens 16, and a slit plate 17. The slit plate 17 has a slit 17a, and is fixed with adhesive after adjustment so that the center of the Fresnel lens 16 and the center of the slit 17a coincide with each other. The laser diode 15 emits diverging light from its junction surface by supplying a predetermined current. The Fresnel lens 16 is a collection of lattice-like concentric circular patterns having a period on the order of microns, and has a sawtooth cross section. This Fresnel lens 16 has a refraction effect and a diffraction effect, and light is bent at each part of the grating. When parallel light enters, one point (
The diverging light emitted from the focal point is made into parallel light (see FIG. 4).

Therefore, by placing the light emitting part of the laser diode 15 at the focal point of the Fresnel lens 16, the diverging light emitted from the laser diode 15 is condensed into parallel light by the Fresnel lens 16, and the light source unit 11 emits the cylindrical light. The light is emitted toward the lens 20. In addition, the light emitting part of the laser diode 15 is connected to a Fresnel lens 16.
When set at a position slightly farther than the focal point of the light source unit 11, convergent light is emitted from the light source unit 11.

[0012] Here, the Fresnel lens 1 used
6 is made of polycarbonate and is designed to correspond to a laser beam with a wavelength of 780 nm. The Fresnel lens 16 is extremely small and lightweight, and the laser diode 15
etc. can be mounted in a single package with high density. Conventionally, a glass-molded single aspherical lens was used as a collimator lens, but compared to this, the light source section has become smaller, and the positions of the laser diode and Fresnel lens must be adjusted relative to each other when incorporated into the optical system housing. There will be no need. Further, Fresnel lenses have the advantage that they can be mass-produced by a molding method and do not require a polishing process.

Furthermore, nowadays, as the speed of laser printers continues to decrease, the sensitivity of photoreceptors has improved, and the amount of light required on the image plane is sometimes sufficient to be about 0.2 mW. In this case, since the light transmittance of a normal optical system is about 25 to 30%, the output of the laser diode is about 0.8 mW. However, in this case, the laser diode only has a threshold output in the region where LED light emission is switched to LD light emission, resulting in poor response. However, a Fresnel lens with a light transmission efficiency of 50% or less can be manufactured, and a laser diode can be driven in the LD emission region to improve responsiveness.

On the other hand, as shown in FIGS. 5 and 6, the divergence angle of the beam emitted from the laser diode 15 is large in the direction perpendicular to the junction surface 15a and small in the parallel direction. In this embodiment, the laser diode 15 is installed so that its junction surface 15a coincides with the sub-scanning direction V. That is, the beam is emitted such that the beam with a larger divergence angle coincides with the main scanning direction. The slit 17a restricts such a laser beam in the sub-scanning direction. This is done so that the beam spot on the image plane approaches a predetermined shape.
This is to shape the beam.

In order to shape the beam, it is conceivable to cut a Fresnel lens into the same shape as the slit 17a and attach it onto a transparent substrate. However, this causes a problem that the beam that is not focused is scattered inside the housing 40. Light source unit 1 consisting of the above configuration
1, the laser diode 15 is accurately positioned and fixed relative to the case 12. fresnel lens 16
is integrated into the slit plate 17, so when performing focus adjustment or centering adjustment of the light source unit 11, it is only necessary to position the slit plate 17 with respect to the case 12. Once the positioning is completed, the slit plate 17
By pouring adhesive between the case 12 and the case 12, both are fixed. The laser diode 15 has a characteristic that as the driving current increases, the amount of heat generated increases and the oscillation wavelength changes. A Fresnel lens that utilizes the diffraction effect is unstable with respect to changes in wavelength, and its focal length changes sensitively to slight changes in wavelength. Considering the laser beam scanning optical system as a whole, a slight change in focal length is magnified several hundred times through the aforementioned optical elements 20, 21, 22, and 23, and defocusing on the image plane (photoreceptor drum surface) is prevented. generate.

Therefore, in order to avoid the occurrence of the defocus, it is preferable that the laser diode drive current during focus and centering adjustment be performed under the same conditions as during actual operation, and in this embodiment, the slit 17a is provided. Since the focus adjustment is performed while the image plane (
defocus on the photoconductor) can be made small.

Next, the light source unit 11 is attached to the housing 40.
The structure and attachment method for attaching to will be explained with reference to FIGS. 7 to 9. The case 12 is composed of a cylindrical portion 13 and a pedestal 14, and a notch 14a is formed in one side of the pedestal 14. On the other hand, a guide hole 43 is formed in the wall portion 42 of the housing 40, and the cylindrical portion 13 can engage with the guide hole 43. The upper portion of the guide hole 43 is formed with stepped surfaces 43a, 43a and a groove portion 43b having a substantially semicircular arc shape.

A fixture 50 is also provided. This fixture 50 is made of a metal material having spring properties, and is approximately V
It has protrusions 50a, 50a which are opened in a letter shape. To attach the light source unit 11 to the wall portion 42, first, the mounting tool 50 is set in the groove portion 43b. At this time, the protrusion 5
0a and 50a elastically sandwich the stepped surfaces 43a and 43a. Next, the cylindrical portion 13 is inserted into the guide hole 43 from the outside, with the protective case 19 at the beginning, until the pedestal 14 abuts against the wall portion 42 . The mounting tool 50 has an inclined portion that is connected to the groove portion 4.
3b and is restricted from moving upward, and the bent portion 50b presses the cylinder portion 13. The light source unit 11 is positioned in the main scanning direction and the sub-scanning direction by the outer peripheral surface 13a of the cylinder part engaging with the guide hole 43, and the light source unit 11 is positioned by the pedestal planes 14b and 14b coming into contact with the outside of the wall part 42. axially positioned. Furthermore, the notch 14a of the pedestal 14 engages with a guide pin 49 protruding from the wall 42, thereby regulating the mounting direction of the light source unit 11 and preventing its rotation.

The light source unit 11 is attached to the wall 42 as described above, and after making necessary adjustments, it is attached to the wall 42 with adhesive.
is fixed to. [Other Embodiments] The laser beam scanning optical system according to the present invention is not limited to the embodiments described above, and can be modified in various ways within the scope of the gist.

For example, in the above embodiment, the Fresnel lens 1
6 and the slit plate 17 are constructed separately, however, a light-shielding paint may be applied to a portion of the Fresnel lens 16 other than the portion corresponding to the slit 17a. Furthermore, a galvanometer mirror may be used as the deflector in place of the polygon mirror, and the configuration of the scanning optical system is also arbitrary.

[0021]

[Effects of the Invention] As is clear from the above explanation, according to the present invention, since a condensing lens having a diffraction effect is used in combination with a laser diode, a compact and lightweight light source unit can be constructed, and positional adjustment is possible. Can be integrated into the housing without any Furthermore, since a slit for shaping the beam is integrated into the light source unit, when adjusting the focus of the light source unit, the current supplied to the laser diode can be set to approximately the same conditions as during actual operation, and this eliminates fluctuations in the drive current value. The resulting fluctuation in the focal length of the condenser lens can be suppressed as much as possible, and ultimately defocus on the image plane can be reduced.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing an embodiment of a laser beam scanning optical system according to the present invention.

FIG. 2 is a sectional view of a light source unit used in the scanning optical system shown in FIG. 1.

FIG. 3 is a front view of the light source unit shown in FIG. 2 with the cover removed.

FIG. 4 is a perspective view showing the light focusing effect of a Fresnel lens.

FIG. 5 is a perspective view showing a beam shape (convergent light) emitted from a light source unit.

FIG. 6 is a perspective view showing a beam shape (parallel light) emitted from a light source unit.

FIG. 7 is a perspective view showing a light source unit, a housing wall, and a fixture.

FIG. 8 is a sectional view showing a state in which the light source unit is attached to a wall.

FIG. 9 is a right side view of FIG. 8.

[Explanation of symbols]

10...Laser beam scanning optical system 11...Light source unit 12...Case 15...Laser diode 16...Fresnel lens 17...Slit plate 17a...Slit

Claims (1)

[Claims] Claim 1: In a laser beam scanning optical system that scans a recording medium with a laser beam emitted from a laser diode according to image information via a deflector and an optical element, the diverging light emitted from the laser diode is A laser beam scanning optical system is characterized in that a condenser lens having a diffraction effect that condenses parallel light and a slit that shapes the shape of a beam emitted from the condenser lens are integrated into a unit.