This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-270896, filed Sep. 24, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a multi-beam optical scanning device performing scanning with a plurality of beams for scanning, which may be used for a color printer, a color copying machine, a high-speed laser printer, or a digital copying machine, and the like; and an image forming device using the multi-beam optical scanning device.
The image forming device, such as the color printer, and the copying machine uses an optical scanning device (laser exposing device) providing image data corresponding to color components, that is, a plurality of laser beams to a plurality of image formation parts forming each image of color components obtained through color separation with subtractive primaries, that is, Yellow (Y) image, Magenta (M) image, Cyan (C) image, and, Black (B) image to enhance the ray and shade, and to the image formation parts.
There have been known two types of examples as the above image forming device, one is a plurality of optical scanning devices are arranged corresponding to each image formation part, and the other is a multi-beam optical scanning device is arranged so as to provide a plurality of laser beams.
The optical scanning device includes, a plurality of semiconductor laser devices (laser diodes) as light source, a first group of lenses (a before deflection optical system) which narrows section diameters of a plurality of laser beams emitted from each laser diode to a predetermined diameter, an optical deflection device which continuously reflects a plurality of laser beams narrowed by the first group of lenses in a direction perpendicular to a carrying direction of a recording medium, and a second group of lenses (an after deflection optical system) by which laser beams after deflection by the optical deflection device are imaged to a predetermined position of the recording medium. In many cases, the direction in which the laser beams are deflected by the optical deflection device is denoted as a main-scanning direction, and the carrying direction of the recording medium, that is, a direction perpendicular to the main-scanning direction as a sub-scanning direction.
A plurality of laser beams passing through the first group of lenses are continuously reflected (deflected) by a reflection plane of polygon mirror of the optical deflection device rotating at predetermined rotation speed for deflection in the main-scanning direction. Each deflected laser beam is imaged at a predetermined position of the recording medium by the second group of lenses.
The second group of lenses gives approximately the same section diameter of the laser beams on the recording medium to be scanned with polygon mirror of the optical deflection device to give different degrees of convergence depending on a point of reflection by the polygon mirrors.
Between the second group of lenses and an imaging position on the recording medium (photosensitive drum), there is a horizontal synchronization detector to detect that the laser beams reach a predetermined position before an area where an electrostatic latent image is recorded. When each laser beam enters the above detector, horizontal synchronization signals are generated.
Based on the above horizontal synchronization signal, the laser beams intensity-modulated according to the images, that is, image data to be recorded are irradiated to the photosensitive drum at a predetermined timing.
Generally, in the above optical scanning device, it is difficult to correct changes in image sizes (caused at any position in the main-scanning direction of the imaging position) according to the main-scanning position, and there is easily generated sidelobe. Generation of the sidelobe causes reduction in distribution of energy of the main beam in the intensity of the laser beams, and density especially in the lower density part is reduced by the changes in the distribution of energy, when latent images are formed on the photosensitive drum. Therefore, there is caused a problem that striped images are obtained. And, in the color image forming device, there is a problem that color balance is lost, even when there is caused only a density change in any one of colors due to overlapping four color images. Then, the generation of the sidelobe causes a serious problem for the color image forming device.
On the other hand, when plastic lenses are used for the after deflection optical system (the second group of lenses), it is well known that sidelobe is consequently caused, as the plastic lenses are usually made by injection molding to easily cause form errors. Therefore, when the plastic lenses are used for the color image forming device, there is a problem that there is easily caused a change in the density as previously described.
In molding using an insert die for making plastic lenses, a process to reduce surface roughness by polishing after molding is required. But, when plastic lenses without a symmetry axis of rotation are used, it is difficult to maintain a predetermined curved surface by polishing, and actually, to perform polishing. Therefore, there is larger sidelobe to cause easier change in the image density as previously described.
When, other than the above, a set of lenses are used for the after deflection optical system in the sub-scanning direction, it is required to have larger incidence angle in the sub-scanning direction, as there are laser beams passing through parts away from the center of the lenses in the sub-scanning direction. Therefore, optical aberration is increased to cause larger sidelobe.
And, laser beams irradiated from a semiconductor laser device have smaller angle of divergence in a parallel direction to the bonding surface of a laser oscillation chip and larger angle of divergence in a perpendicular direction larger than the smaller angle of divergence. Then, it is required that the beam section diameter in the main-scanning direction is smaller than the diameter of the sub-scanning direction, as a section diameter of the laser beams required of the optical scanning device. But, in order to secure luminous energy, considering the opposite luminescence characteristics, the beam section of the laser beam from the laser device is rotated to cause the smaller angle of divergence in the main-scanning direction. Accordingly, the beam section in the main-scanning direction becomes larger, that is, there is a problem to cause lower resolution.
On the other hand, vertical incidence causes the largest transmittance of lenses, and the transmittance becomes smaller in the case of a larger angle (incidence angle) with the normal line of a plane of incidence. Therefore, fxcex8 lenses have larger incidence angle at the end part to reduce the transmittance. That is, there is a problem to cause larger difference in the image density, as there is larger difference in the luminous energy at the center part and the end part. There is an example to provide the surface of the lenses with evaporated film to reduce reflection in order to prevent decrease in the luminous energy at the end part. But, when the fxcex8 lens is made of plastic, there are worse optical characteristics due to thermal deformation effected by evaporated film, in some cases. Then, it is difficult to obtain required performance, as a shape different from the final shape is required to be given at manufacturing lenses, considering the thermal deformation.
In the optical scanning device to form color images, when a set of lenses are used for the after deflection optical system in the sub-scanning direction, the problem of the transmittance as mentioned above may be also applied to the sub-scanning direction. Then, there is easily caused a problem that each color has each density.
As previously described, in plastic lenses with a lens surface having no symmetry axis of rotation, polishing after injection molding, and polishing of metal molds are difficult, and it is impossible to reduce the surface roughness. Therefore, color components scattered on the lens surface are generated to reduce the transmittance, and especially, the transmittance at the end part are reduced to cause a problem that there is an irregular density in the images.
An object of the present invention is to provide an optical scanning device capable of reducing an irregular density in images, using the after deflection optical system comprising lenses made of plastic.
The present invention is to provide an optical device comprising: at least one of lens to convert divergent rays from a laser diode to parallel or convergent rays; a slit to regulate luminous energy of the parallel or convergent rays; a before deflection optical system having a lens or lenses with different power for main-scanning and sub-scanning; optical scanning means to scan objects to be scanned with the parallel or convergent rays; and an imaging optical system for imaging on the objects to be scanned by passing the scanned rays with the optical scanning means, wherein the slit of the before deflection optical system having an opening with a larger width than a 13.5% diameter of the optical intensity of the rays in at least one direction of the main-scanning and the sub-scanning directions.
The present invention is to provide an optical device comprising: at least one of lens or lenses to convert divergent rays from a laser diode to parallel or convergent rays; a before deflection optical system includes at least one of lens with different power for main-scanning and sub-scanning; optical scanning means to scan objects to be scanned with the parallel or convergent rays; and an imaging optical system for imaging on the objects to be scanned by passing the scanned rays with the optical scanning means, wherein an angle xcex8 between at least one of active surface layer and polarization direction of the laser diode, and a direction perpendicular to a plane which the rays scan by the optical scanning means is 10xc2x0xe2x89xa6xcex8xe2x89xa680xc2x0.
The present invention is to provide an optical device comprising: at least one of lens or lenses to convert divergent rays from a laser diode to parallel or convergent rays; a slit to regulate luminous energy of the parallel or convergent rays; a before deflection optical system having at least one of lens with different power for main-scanning and sub-scanning; optical scanning means to scan objects to be scanned with the parallel or convergent rays; and an imaging optical system for imaging on the objects to be scanned by passing the scanned rays with the optical scanning means, wherein a polarization device to change a polarization direction of the parallel or convergent rays is provided.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.