JPH04328511A - Laser beam scanning optical system - Google Patents

Abstract

PURPOSE:To constitute a photosensor and a condenser lens for detecting an image write start position of each scan as a small-sized and, lightweight unit. CONSTITUTION:A photodiode 35 and a Fresnel lens 36 which utilizes a refraction phenomenon and a diffraction phenomenon are stored in a case 31 to form the sensor unit 30. The photodiode 35 is arranged almost at the focus position of the Fresnel lens 36 and part of a laser beam which is deflected and scanned by a polygon mirror is converged by the Fresnel lens 36 to irradiate the photodiode 35. The photodetection signal of the photodiode 35 is used as a signal for controlling the image write start position of each scan.

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). In order to detect the image writing start position for each scan, the scanning start portion of the laser beam deflected and scanned by the deflector is configured to irradiate the photosensor.

[0003] Conventionally, this type of photosensor was incorporated into an optical system housing together with a detection circuit board, and a cylindrical lens was separately installed in front of it to improve light condensation (Japanese Patent Laid-Open No. 57-68866). (see official bulletin). However, if the photosensor and the cylindrical lens are installed independently, the positions of both must be adjusted when they are assembled into the housing, which is complicated. A unit that combines both in one package is also available, but since the cylindrical lens is large, the unit is also large, and it is difficult to install it in the housing of the optical system in terms of space.

0004

[Object, structure, and operation of the invention] Therefore, the object of the present invention is to
To provide a laser beam scanning optical system in which a photosensor and its condensing lens are formed into a compact unit, and the unit can be installed as a sensor for detecting an image writing start position for each scan without adjustment.

In order to achieve the above object, the laser beam scanning optical system according to the present invention includes a photosensor and a condensing lens having a diffraction effect, and the photosensor is arranged approximately at the focal point of the condensing lens. This unit is configured as a unit, and this unit is placed at a position where it receives the scanning start portion of the laser beam deflected and scanned by the deflector, and is used as a sensor for detecting the image writing start position for each scan.

[0006] The condensing lens has a thin flat plate shape and has a focal length of about 1 to 10 mm, and is smaller and lighter than a conventional cylindrical lens, so that a compact sensor unit is constructed. If both are accurately positioned and made into a unit, it becomes possible to incorporate them into the housing of an optical system, etc., without having to adjust the positions of the photosensor and the condensing lens one by one.

[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. [First embodiment, see FIGS. 1 to 3] FIG. 1 shows a laser beam scanning optical system as a first embodiment. This optical system 10
are a light source unit 11, a cylindrical lens 20, a polygon mirror 21, an fθ lens 22, a plane mirror 23,
A sensor unit 30 for detecting an image writing start position and mirrors 28 and 29 for guiding a laser beam to the sensor unit 30 are attached to a housing 40.

The light source unit 11 is a combination of a laser diode and a condensing lens, and the diffused light emitted from the laser diode is condensed into parallel light by the condensing lens. The laser beam emitted from the light source unit 11 is
By passing through the cylindrical lens 20, the light 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 sensor unit 30 will be explained. As shown in FIG. 2, the sensor unit 30 includes a photodiode 35 provided on the inner surface of the rear wall of a case 31, and a Fresnel lens 36 provided in the opening 32 of the front wall. The Fresnel lens 36 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 36 has a refraction effect and a diffraction effect, and light is bent at each part of the grating. When parallel light is incident, it is converged to one point (focal point), and the diffused light emitted from the focal point is made into parallel light (see FIG. 3).

Therefore, by placing the light receiving part of the photodiode 35 at the focal point of the Fresnel lens 36, the laser beam (parallel light) guided by the mirrors 28 and 29 passes through the Fresnel lens 36 and is directed to the photodiode 35.
The image is formed on the light-receiving section. Here, the Fresnel lens 36 used is made of polycarbonate and has a wavelength of 780
It is designed to be compatible with nm laser beams. Further, the Fresnel lens 36 is attached to the case 31 with the grating forming surface facing inside. This is to protect the lattice forming surface.

The sensor unit 30 having the above configuration uses the Fresnel lens 36 which is thin, lightweight, and has a short focal length, so that it can be compactly stored in the case 31 together with the photodiode 35. Conventionally, the cylindrical lens was incorporated into the housing independently of the photodiode, but according to this embodiment, there is no need to adjust the positions of the photodiode and Fresnel lens relative to each other when they are incorporated into the housing. Further, Fresnel lenses have the advantage that they can be mass-produced by a molding method and do not require a polishing process.

Furthermore, in the first embodiment, the sensor unit 30 is arranged adjacent to the light source unit 11. With this, the drive circuit of the light source unit 11 and the detection circuit of the sensor unit 30 can be integrated on one circuit board 15, improving the space efficiency of the optical system. Moreover, conventionally, noise from the outside world invaded the harness that connects the two boards, but in this embodiment, the harness is not needed, so there is no noise intrusion.

[Second Embodiment, See FIG. 4] FIG. 4 shows a sensor unit 50 used in a laser beam scanning optical system as a second embodiment. This sensor unit 50 has a photodiode 55 and a Fresnel lens 56 attached to a base 51 and sealed with a metal cover 52. On the front surface of the cover 52, an oval window 53 is formed parallel to the scanning direction of the laser beam, and a protective glass 54 is installed. Further, the notch 51a formed in the base 51 is for regulating the mounting direction of the unit 50.

The Fresnel lens 56 is of a type in which a grating is formed in parallel, and the grating is placed parallel to the scanning direction of the laser beam. Therefore, the incident laser beam is focused in the sub-scanning direction. Although not shown, the configuration of the scanning optical system is the same as that shown in FIG.
It is similar to Furthermore, in the second embodiment, since the entrance portion is formed into an oval window portion 53 along the scanning direction of the laser beam, the beam irradiation time can be set for a long time.

[Third Embodiment, See FIG. 5] FIG. 5 shows a sensor unit 50' used in a laser beam scanning optical system as a third embodiment. This sensor unit 50' basically has the same configuration as the sensor unit 50 shown in FIG. 4, and the same members are given the same reference numerals as in FIG. 4, and their explanation will be omitted. The difference is that the Fresnel lens 56' is shown in Figure 2.
The lattice shown in 1 is formed into a concentric circular pattern and installed with the lattice forming surface facing outward.

[Other Embodiments] The laser beam scanning optical system according to the present invention is not limited to the embodiments described above, and can be variously modified within the scope of the invention. For example, in the embodiment described above, parallel light is emitted from the light source unit 11 and the image is formed on the photoreceptor through the fθ lens 22, but convergent light may be emitted from the light source unit 11. Alternatively, an fθ mirror may be used in place of the fθ lens 22.

Furthermore, the sensor unit does not necessarily need to be provided in the housing of the scanning optical system, but may be provided at a predetermined position inside the printer. Further, as the deflector, a galvano mirror may be used instead of a polygon mirror.

[0019]

[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 photodiode, a compact and lightweight sensor unit can be constructed, and scanning can be performed without adjustment. Can be incorporated into optical systems.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing a laser beam scanning optical system as a first embodiment.

FIG. 2 is a partially cutaway perspective view of a sensor unit used in the scanning optical system shown in FIG. 1;

FIG. 3 is a perspective view showing the light focusing effect of the Fresnel lens shown in FIG. 2;

FIG. 4 is a partially cutaway perspective view of a sensor unit constituting a second embodiment.

FIG. 5 is a partially cutaway perspective view of a sensor unit constituting a third embodiment.

[Explanation of symbols]

10...Laser beam scanning optical system 11...Light source unit 21...Polygon mirror 30, 50, 50'...sensor unit 35, 55...Photodiode 36, 56, 56'...Fresnel lens

Claims (1)

[Claims] 1. A laser beam scanning optical system that scans a recording medium with a laser beam emitted from a laser light source according to image information via a deflector and an optical element, comprising a photosensor and a condenser lens having a diffraction effect. is configured as one unit by arranging the photosensor at approximately the focal point position of the condensing lens, and this unit is arranged at a position to receive the scanning start portion of the laser beam deflected and scanned by the deflector, and one scanning is performed. A laser beam scanning optical system characterized in that it is used as a sensor for detecting the starting position of image writing.