1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus using the same. In particular, the optical scanning apparatus is suitable for an image forming apparatus in which a polygon mirror as an optical deflector reflects and deflects a light beam emitted from a light source, and image information is recorded by optical scanning on a surface to be scanned with a light beam through a scanning optical system, such as a laser beam printer (LBP), a digital copying machine, or a multi-function printer, which employs, for example, an electrophotographic process.
2. Related Background Art
Conventionally, in an image forming apparatus such as a laser beam printer or a digital copying machine, a light beam which is optically modulated according to an image signal by a light source composed of, for example, a semiconductor laser and emitted therefrom is periodically deflected by an optical deflector composed of, for example, a rotating polygonal mirror (polygon mirror). The deflected light beam is converged in a spot shape onto the surface of a photosensitive recording medium (photosensitive drum) by a scanning optical system (scanning lens system) having an fθ characteristic. The surface of the recording medium is optically scanned with the light beam to perform image recording.
FIG. 15 is a main part sectional view of an optical scanning apparatus used for such a conventional image forming apparatus in a main scanning direction (main scanning sectional view).
In FIG. 15, a parallel light beam emitted from a laser unit 91 including a semiconductor laser is incident on a cylindrical lens (condensing lens) 92 having predetermined optical power only in a sub scanning direction. The parallel light beam which is incident on the cylindrical lens 92 exits therefrom without changing a parallel light beam state within a main scanning section.
On the other hand, the parallel light beam is condensed within a sub scanning section and imaged as a linear image extended in the main scanning direction near a deflection surface 93a of an optical deflector 93 composed of a rotating polygonal mirror. The light beam which is reflected and deflected on the deflection surface 93a of the optical deflector 93 is imaged as a light spot onto the surface of a photosensitive drum 95 serving as a surface to be scanned through a scanning optical system (fθ lens system) 94 having an fθ characteristic. The surface of the photosensitive drum 95 is repeatedly scanned with the light spot. The scanning optical system 94 is composed of a spherical lens 94a and a toric lens 94b. 
In the optical scanning apparatus, a beam detector (BD) sensor 98 serving as an optical detector is provided to adjust a timing of starting an image formation on the surface of the photosensitive drum 95 before the surface of the photosensitive drum 95 is scanned with the light spot. The BD sensor 98 receives a BD light beam which is a part of the light beam which is reflected and deflected on the optical deflector 93, that is, a light beam with which a region other than an image forming region on the surface of the photosensitive drum 95 is being scanned before the image forming region is scanned. The BD light beam is reflected on a BD mirror 96, condensed by a BD lens (condensing lens) 97, and incident on the BD sensor 98. A BD signal (synchronous signal) is detected from an output signal of the BD sensor 98 and a timing to start image recording on the surface of the photosensitive drum 95 is adjusted based on the BD signal.
The photosensitive drum 95 rotates at constant speed in synchronization with a drive signal of the semiconductor laser in the laser unit 91 and the surface of the photosensitive drum 95 is moved in the sub scanning direction with respect to the light spot for scanning.
Thus, an electrostatic latent image is formed on the photosensitive drum 95. The electrostatic latent image is developed by a known electrophotographic process and transferred to a transfer material, such as a paper to obtain a visualized image.
According to a multi-image forming apparatus using a scanning optical system, images having different colors are generally formed by a plurality of image forming portions. Paper is transferred by a conveying member such as a conveyor belt. The plurality of images are superimposed on the paper to perform the image formation. In particular, even if a slight superimposition displacement occurs in a multi-color development, an obtained full color image deteriorates. For example, even if a superimposition displacement of a fraction of one pixel, 63.5 μm, occurs in the case of 400 dpi, it appears as a color misregistration, thereby significantly deteriorating the image.
To cope with it, the color development has been conventionally performed by the use of the same scanning optical system. That is, optical scanning has been performed with the same optical characteristic to reduce an image displacement. However, this method has contained a problem that it takes time to output a multi-image or a full color image. In order to solve the problem, there is a method of forming images by the use of separate optical scanning apparatus to obtain respective color images and then superimposing the images on a paper transferred by a conveying portion.
In such a method, there may be a concern over a color misregistration when the images are superimposed. It is an effective method against the color drift to detect the positions of the images and to control an image forming portion so as to correct the images based on the detection signals (For example, see Japanese Patent Publication No. H01-281468).
In an image forming apparatus in which a plurality of photosensitive members are scanned with a beam, the scanning optical systems as many as the photosensitive members are generally used to form latent images on the plurality of the photosensitive members. In the image forming apparatus, since there needs optical parts as many as the scanning optical systems, and in particular, the optical deflector (polygon mirror) is expensive, there is a problem that a cost of the image forming apparatus may rise. In the case of a high resolution scanning optical system operated at high speed, since the optical deflector becomes larger in size, the optical deflector is required to have the ability of deflecting light at high speed. Therefore, the problem is serious.
In order to overcome the problem, an optical scanning apparatus in which a plurality of beams are deflected by a common optical deflector has been proposed. In an optical scanning apparatus in which a photosensitive member in the sub scanning direction is scanned by a common optical deflector, it is necessary to provide a mechanism for shifting a beam drawing position in the sub scanning direction in order to improve the precision of superimposition of the images in the sub scanning direction. In the method, the superimposition has been adjusted by the shift of the drawing position line by line in the sub scanning direction, by selecting the deflection surface of the optical deflector with which the drawing of a beam in the sub scanning direction starts.
Since a compact, low-cost, and high-quality full color image forming apparatus has been required recently, a system of scanning a plurality of beams using a single common polygon mirror has been proposed as a method that satisfies the requirement to reduce the number of parts, thereby lowering a cost of the image forming apparatus.
When the common polygon mirror is used so as to guide the plurality of beams to different surfaces to be scanned respectively, it is necessary to separate optical paths. Therefore, the beams need to be apart from each other in the sub scanning direction. As a result, there is a problem that the polygon mirror becomes larger in thickness to increase a cost of the image forming apparatus.
Even when a first optical element (scanning lens) of the scanning optical system is commonly used to lower the cost of the image forming apparatus, light beam transmitting positions of the lens are apart from one another in the sub scanning direction, so that a height of the lens in the sub scanning direction is increased. As a result, there is a problem that a cost reduction effect is small.