Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2001-03-05
2002-12-17
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S205100, C359S207110, C359S215100, C359S216100
Reexamination Certificate
active
06496293
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an optical scanning device, a scanning optical system, an optical scanning method and an image forming apparatus.
2. Description of the Related Art
Optical scanning devices have been widely used in image forming apparatuses such as a digital copier, an optical printer, a facsimile apparatus and so forth. Recently, a request for image quality in images formed by the image forming apparatuses have become severe. Thereby, improvement in performance of the optical scanning devices has been demanded.
‘A writing density’ is one factor which directly affects image quality of images formed through optical writing by the optical scanning device. As the writing density is increased, the resolution of images formed increases, and, thereby, it is possible to form clear and smooth images.
In order to increase the writing density (dpi), it is necessary to reduce the diameter of a beam spot formed on a surface to be scanned by the optical scanning device.
Ideally, the spot diameter is a beam spot diameter of a deflected light beam. However, when a curvature of field occurs, it is not possible that the image surface of the deflected light beam coincides with the surface to be scanned completely, and, thereby, the spot diameter varies as the image height varies. Accordingly, in order to render ‘a stable beam spot’ having a small variation in spot diameter, it is necessary to well correct the curvature of field of the optical scanning system. In the related art, there are many scanning optical systems in which the curvature of field is well corrected.
When an optical system of the optical scanning device is assembled, an error in precision of assembly inevitably occurs. Accordingly, even when the curvature of field is well corrected in a design stage, it may not be possible that the image surface of the scanning optical system coincides with the surface to be scanned according to the design. When the image surface is separate from the surface to be scanned due to an influence of assembling error or the like, the spot diameter of the beam spot formed on the surface to be scanned becomes larger than the spot diameter according to the design.
Accordingly, when the scanning optical system of the optical scanning device is designed, ‘a certain degree of separation’ of the image surface from the surface to be scanned due to assembling error or working error is assumed, and, a design is made such that, even when the image surface is separate from the surface to be scanned, the variation in the spot diameter of the beam spot on the surface to be scanned should fall within ‘an allowable range’.
A position of a beam waist of the scanning beam is located on the image surface. Accordingly, a difference between the position of the beam waist and the surface to be scanned is called ‘defocus’.
A range in defocus such that the variation in spot diameter due to the defocus falls within the allowable range is called ‘allowable depth’, and, a characteristic curve indicating a relationship between th defocus and spot diameter is called ‘depth curve’.
A practical ‘satisfactory scanning optical system’ is an optical system having a satisfactory optical performance of design, and, also, having a proper allowable depth such that ‘an excessive precision’ is not requested to assembling and working.
Japanese Laid-Open Patent Application (Kokai) No. Hei 10-232358 discloses an optical system rendering a beam spot having a small diameter. This optical system is such that a scanning optical system condensing a deflected beam toward a surface to be scanned includes three or four lenses, and the beam spot having a very small diameter of 30 &mgr;m for a wavelength of 780 nm used is rendered, and an allowable depth of 1 mm is rendered.
It is considered that it is very difficult to reduce the diameter of a beam spot while the allowable depth is secured to be more than approximately 1 mm for a wavelength more than 700 nm used.
Employing a beam having a short wavelength is effective for reducing the diameter of a beam spot as is well known. In principle, it is possible to reduce the diameter of a beam spot in proportion to a wavelength used. For a semiconductor laser which is generally used as a light source of the optical scanning device, a wavelength of light emitted therefrom is reduced, and, ‘a short wavelength equal to or shorter than 400 nm’ is being rendered. For an excimer laser, a wavelength of 200 nm has been already rendered.
In principle, it is possible to reduce a diameter of a beam spot by employing a light source of a short wavelength. However, when simply reducing a diameter of a beam spot, the allowable depth becomes narrower accordingly.
In the related art, it is not known to secure ‘a necessary allowable depth’ while reducing a diameter of a beam spot employing a light source of a wavelength shorter than 600 nm, that is, ‘an art such as to render both reduction of diameter of beam spot employing a short wavelength and satisfactory allowable depth’.
SUMMARY OF THE INVENTION
An object of the present invention is to reduce a diameter of a beam spot employing a light source for optical scanning of a short wavelength, and, also, to secure a necessary allowable depth.
An optical scanning device according to the present invention, comprises:
an aperture shaping a beam from a light source;
a light deflector deflecting the beam; and
a scanning imaging optical system condensing the deflected beam toward a surface to be scanned so as to form a beam spot on the surface to be scanned,
wherein:
a wavelength &lgr; of the beam emitted by the light source satisfies:
350 (nm)≦&lgr;≦600 (nm) (1)
and,
a desired diameter &phgr; of the beam spot and the wavelength &lgr; satisfy:
0.5 (mm)≦&phgr;
2
/&lgr;≦6 (mm) (2)
The above-mentioned spot diameter &phgr; means a diameter of an area through which the light intensity is equal to or higher than 1/e
2
where the intensity distribution of the beams spot is normalized so that the maximum value in the intensity distribution becomes 1. Based on the above-mentioned conditional formula (2), the range of the spot diameter &phgr; is 17.3 through 60 &mgr;m when the wavelength &lgr;=600 (nm); and the range of the spot diameter &phgr; is 13.2 through 46 &mgr;m when the wavelength &lgr;=350 (nm). It is possible to render the beam spot having the spot diameter in this range with a practical allowable depth.
Further, the following conditions (the range of the &lgr; is further limited, and the desired spot diameter &phgr; and wavelength &lgr; are further limited) may be satisfied:
350 (nm)≦&lgr;≦500 (nm) (3)
0.5 (mm)≦&phgr;
2
/&lgr;≦3 (mm) (4)
and,
a root-mean-square value RMS(wavefront aberration) of wavefront aberrations on a surface of an exit pupil may satisfy:
RMS(wavefront aberration)≦0.2 (5)
Thereby, it is possible to render the satisfactory beam spot having a smaller diameter.
There, the above-mentioned ‘exit pupil’ means an image of the aperture (image of the opening thereof) formed through the optical system disposed on the surface-to-be-scanned side of the aperture.
The above-mentioned root-mean-square value RMS(wavefront aberration) of the wavefront aberrations on the surface of the exit pupil is calculated as follows:
The wave surface of the beam on the surface of the exit pupil is divided into N area elements having the minute areas same as each other. Then, the root-mean-square value is calculated from the maximum wavefront aberrations Wi (i=1, 2, 3, . . . , N) for the respective i-th area elements according to the well-known definition of root mean squire. The method of dividing the wave surface into area elements may be a method such that squares are obtained, a method such that concentric circular areas or concentric elliptical areas are obtained, or the like. The number N of the thus-obtained area elements is preferably equal to or larger than 100. When the number N is as large as
Phan James
Ricoh & Company, Ltd.
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