US20060289731A1 - Optical scanning device and image forming apparatus using the same - Google Patents
Optical scanning device and image forming apparatus using the same Download PDFInfo
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- US20060289731A1 US20060289731A1 US11/469,540 US46954006A US2006289731A1 US 20060289731 A1 US20060289731 A1 US 20060289731A1 US 46954006 A US46954006 A US 46954006A US 2006289731 A1 US2006289731 A1 US 2006289731A1
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
- G02B27/42—Diffraction optics, i.e. systems including a diffractive element being designed for providing a diffractive effect
- G02B27/4205—Diffraction optics, i.e. systems including a diffractive element being designed for providing a diffractive effect having a diffractive optical element [DOE] contributing to image formation, e.g. whereby modulation transfer function MTF or optical aberrations are relevant
- G02B27/4211—Diffraction optics, i.e. systems including a diffractive element being designed for providing a diffractive effect having a diffractive optical element [DOE] contributing to image formation, e.g. whereby modulation transfer function MTF or optical aberrations are relevant correcting chromatic aberrations
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
- G02B27/0025—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00 for optical correction, e.g. distorsion, aberration
- G02B27/0031—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00 for optical correction, e.g. distorsion, aberration for scanning purposes
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
- G02B27/0025—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00 for optical correction, e.g. distorsion, aberration
- G02B27/0037—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00 for optical correction, e.g. distorsion, aberration with diffracting elements
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
- G02B27/42—Diffraction optics, i.e. systems including a diffractive element being designed for providing a diffractive effect
- G02B27/4205—Diffraction optics, i.e. systems including a diffractive element being designed for providing a diffractive effect having a diffractive optical element [DOE] contributing to image formation, e.g. whereby modulation transfer function MTF or optical aberrations are relevant
- G02B27/4227—Diffraction optics, i.e. systems including a diffractive element being designed for providing a diffractive effect having a diffractive optical element [DOE] contributing to image formation, e.g. whereby modulation transfer function MTF or optical aberrations are relevant in image scanning systems
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
- G02B27/42—Diffraction optics, i.e. systems including a diffractive element being designed for providing a diffractive effect
- G02B27/4283—Diffraction optics, i.e. systems including a diffractive element being designed for providing a diffractive effect having a diffractive element with major temperature dependent properties
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
- G02B27/42—Diffraction optics, i.e. systems including a diffractive element being designed for providing a diffractive effect
- G02B27/4294—Diffraction optics, i.e. systems including a diffractive element being designed for providing a diffractive effect in multispectral systems, e.g. UV and visible
-
- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B5/00—Optical elements other than lenses
- G02B5/18—Diffraction gratings
- G02B5/1847—Manufacturing methods
- G02B5/1852—Manufacturing methods using mechanical means, e.g. ruling with diamond tool, moulding
Definitions
- the present invention relates to an optical scanning device and an image forming apparatus using the same.
- the present invention relates to an optical scanning device that is suitably used for an apparatus such as a laser beam printer or a digital copying machine having an electrophotographic process, in which a light flux optically modulated and emitted from light source means is reflected and deflected on a polygon mirror serving as optical deflection means and then a surface to be scanned is scanned with the light flux through a scanning optical system to record image information.
- the present invention relates to an optical scanning device capable of providing a satisfactory image by adopting a diffraction grid for correction of chromatic aberration of magnification or temperature compensation and to an image forming apparatus using the same.
- the present invention relates to a color image forming apparatus which uses a plurality of optical scanning devices and is composed of a plurality of image bearing members corresponding to respective colors.
- the light flux which is deflected on the deflection surface 5 a of the deflecting means 5 is guided onto a photosensitive drum surface 8 serving as a surface to be scanned through a scanning optical system 6 having an f ⁇ characteristic.
- a scanning optical system 6 having an f ⁇ characteristic.
- FIG. 14 is a schematic view showing a main part of a multi-beam optical scanning device. Two light fluxes emitted from light sources 81 and 82 are converted into parallel light fluxes by collimator lenses 83 and 84 and then synthesized into one by a synthesizing optical element 85 .
- the synthesized light flux forms a linear image extended in the main scanning direction near a deflection surface of a polygon mirror 87 by the action of a cylindrical lens 86 and then forms a light spot on a photosensitive drum 89 by a scanning optical system 88 . Therefore, the two scanning lines can be formed by performing optical scanning once, so that extremely high speed scanning can be achieved as compared with a conventional optical scanning device.
- a multi-beam light source other than one using the above-mentioned synthesizing optical element a monolithic multi-beam laser in which a large number of light emitting points exist has been produced. In the case where the monolithic multi-beam laser is used, it is unnecessary to use the synthesizing optical element. Thus, it is possible to simplify the optical system and the optical adjustment.
- any countermeasure such as the appropriate selection of the light sources has been taken so as to minimize the wavelength difference between the light sources.
- the jitter caused by the wavelength difference between the light sources chromatic aberration of magnification
- a plurality of lenses having different dispersion characteristics are required.
- the number of lenses generally increases to cause an increase in cost. There is a limitation with respect to the range of selection of the wavelengths of the light sources.
- a semiconductor laser used as a conventional light source is an infrared laser (780 nm) or a visible laser (675 nm).
- an optical scanning device in which a minute spot shape is obtained by using a short wavelength laser having an oscillating wavelength of 500 nm or less is under way.
- the advantage of the use of the short wavelength laser is that a minute spot size which is about half of a conventional spot size can be achieved while an exit F number of the scanning optical system is kept equal to a conventional one.
- a focal depth is proportional to a wavelength of a used light source and to the square of the exit F number of the scanning optical system. Therefore, to obtain the same spot size, the focal depth in the infrared laser becomes equal to or smaller than about 1 ⁇ 2 of the focal depth in the short wavelength laser.
- a (temperature compensation) optical scanning device using a diffraction optical element for a scanning optical system has been proposed to reduce a focal variation on a surface to be scanned due to the correction of the chromatic aberration of magnification and an environmental variation.
- a dispersion characteristic of a material used for a scanning lens is large. Therefore, the chromatic aberration of magnification becomes six times or seven times larger than that in a conventional infrared laser.
- the jitter is significantly caused in the main scanning direction to reduce the image quality.
- a phase function ⁇ for determining a grid shape is inversely proportional to the wavelength. Therefore, when a diffraction grid having the same power is designed, a grid pitch of the short wavelength laser having 500 nm or less becomes smaller than that of the conventional infrared laser.
- a grid shape formed on a mold by mold forming is transferred to an optical resin member or an optical glass member.
- the transferred grid shape is separated from the mold to produce the diffraction optical element.
- a method of dropping a small amount of ultraviolet curable resin on a glass substrate (lens can be also used) serving as a base and curing the grid shape similarly formed on the mold by ultraviolet light is used.
- an optical scanning device includes:
- light source means for emitting at least one light flux having a wavelength of 500 nm or less
- deflecting means for deflecting the at least one light flux emitted from the light source means
- a first optical system for guiding the light flux emitted from the light source means to the deflecting means
- a second optical system for imaging the light flux deflected by the deflecting means on a surface to be scanned
- one of the first optical system and the second optical system includes a diffraction grid and a design order of the diffraction grid is set to a diffraction order equal to or larger than a second order.
- an optical scanning device includes:
- light source means for emitting at least one light flux having a wavelength of 500 nm or less
- deflecting means for deflecting the at least one light flux emitted from the light source means
- a first optical system for guiding the light flux emitted from the light source means to the deflecting means
- a second optical system for imaging the light flux deflected by the deflecting means on a surface to be scanned in which:
- one of the first optical system and the second optical system includes a diffraction grid
- a diffraction order of the diffraction grid is represented by k
- the wavelength of the light flux is represented by ⁇ ( ⁇ m)
- a refraction index of a material composing the diffraction grid is represented by n
- FIG. 1 is a main scanning sectional view showing an optical scanning device according to a first embodiment of the present invention
- FIG. 2 shows a diffraction optical element in the first embodiment of the present invention
- FIG. 3 is a sectional view showing a diffraction grid at a minimum pitch in the case using secondary diffraction light
- FIGS. 5A and 5B are explanatory views showing mold processing for the diffraction optical element
- FIG. 6 is a graph showing a chromatic aberration of magnification of a scanning optical system in a main scanning direction in the first embodiment of the present invention
- FIG. 7 is a graph showing a focal variation of the scanning optical system in a sub scanning direction in the first embodiment of the present invention.
- FIGS. 10A and 10B are graphs showing a spherical aberration and an on-axis chromatic aberration in the third embodiment of the present invention.
- FIG. 11 is a main part schematic diagram showing an image forming apparatus according to the present invention.
- FIG. 12 is a main part schematic diagram showing a color image forming apparatus according to the present invention.
- a divergent light flux emitted from a single beam semiconductor laser 1 serving as light source means is limited by a diaphragm 2 so as to reduce a width of the light flux, thereby obtaining a desirable spot size. Then, the light flux is converted into a substantially parallel light flux by a collimator lens 3 .
- the substantially parallel light flux is imaged as a linear image extended in the main scanning direction onto the vicinity of a deflection surface 5 a of a deflecting means 5 described later by a cylindrical lens 4 having predetermined refractive power in only the sub scanning direction.
- the deflecting means 5 is composed of, for example, a polygon mirror having four surfaces (rotating polygonal mirror) and rotated in a direction indicated by an arrow “A” in FIG.
- the scanning optical system composed of the deflection optical element (lens) 6 and the diffraction optical element (lens) 7 having f ⁇ characteristics, the deflection light flux which is reflected and deflected on the deflecting means 5 is imaged onto a photosensitive drum surface 8 serving as a surface to be scanned.
- a tangle error of the deflection surface 5 a of the deflecting means 5 is corrected by the scanning optical systems.
- a shape of each generating line of the incident and exit surfaces of the deflection optical element (lens) 6 and the incident surface of the diffraction optical element (lens) 7 is based on an aspherical shape which can be indicated as a function of up to the tenth order.
- Table 1 shows design parameters of the optical system of the optical scanning apparatus shown in FIG. 1 .
- the lens 6 is the defection optical element and the lens 7 is the diffraction optical element in which the diffraction grid is provided on the deflection base surface.
- TABLE 1 Design data Wavelength, Refractive index Lens 6 Figures Use wavelength ⁇ (nm) 405 First plane Second plane First plane Second plane Lens 7 Figures Lens 6 Refractive index n 405 1.5466198 R ⁇ 8.74966E+01 ⁇ 4.71453E+01 R ⁇ 4.04355E+02 ⁇ K 5.71925E ⁇ 01 ⁇ 1.04001E+00 K ⁇ 3.69841E+01 Lens 7 Refractive index n 405 1.5466198 B4 1.40393E ⁇ 06 9.40875E ⁇ 08 B4 2.07946E ⁇ 07 B6 1.26075E ⁇ 09 5.42879E ⁇ 10 B6 ⁇ 1.81403E ⁇ 11 B8 ⁇ 1.16558E ⁇ 12 4.08844E ⁇ 13 B8 8.5
- Table 2 shows phase coefficients, the total number of grids m within an effective range L, and a minimum pitch P of grids.
- Table 2 shows phase coefficients, the total number of grids m within an effective range L, and a minimum pitch P of grids.
- FIG. 2 is a schematic view showing the diffraction optical element 7 , in which the diffraction grid 7 a is formed in a saw-tooth shape (blazed type) in the main scanning direction and the sub scanning direction.
- a saw-tooth shape blazed type
- a spot diameter in the main scanning direction becomes 27 ⁇ m and a spot diameter in the sub scanning direction becomes 35 ⁇ m.
- the spot diameter indicates a diameter of a region which is obtained by slicing at 1/e2 of a peak light amount.
- FIG. 3 shows a grid shape at a minimum pitch in the case where the diffraction grid is designed using secondary diffraction light in this embodiment.
- FIG. 4 shows a grid shape at a minimum pitch in the case where the diffraction grid is designed using primary diffraction light.
- a grid height becomes lower than 1 ⁇ m and the minimum pitch P becomes a half of that in the case using the secondary diffraction light.
- the total number of grids of the diffraction grid m becomes two times larger than that in the case using the secondary diffraction light.
- the diffraction optical element 7 is designed using the secondary diffraction light. Therefore, it is possible to obtain the grid pitch which is about two times larger than that in the case using the primary diffraction light and the total number of grids which is about a half of that in the case using the primary diffraction light.
- the diffraction order equal to or larger than the second order is used for diffraction light for forming an image on the surface to be scanned.
- FIGS. 5A and 5B are explanatory views showing a mold cutting process for the diffraction optical element 7 .
- the central portion is cut plural times using an end portion of a cutting tool which is shaded ( FIG. 5A ).
- the grid pitch in a peripheral portion of the diffraction optical element 7 is small, the peripheral portion is cut one time for each grid to obtain the grids each having an oblique surface ( FIG. 5B ).
- Mold processing is always performed on the diffraction optical element while the end portion of the cutting tool is damaged. The cutting tool is gradually worn during cutting. Therefore, a problem that a target grid shape cannot be obtained occurs.
- the effective range of the diffraction optical element is represented by L (mm)
- the wavelength of the light source is represented by ⁇ ( ⁇ m)
- a grid shape suitable to the wavelength of the used light source can be obtained.
- the design order of the diffraction grid is the second order to the fifth order.
- the diffraction order is the first order, as described above, the total number of grids is too large.
- the diffraction order is equal to or larger than the sixth order, the grid depth becomes too deep, so that a cutting resistance of the cutting tool increases.
- significant influence is caused by a variation in diffraction efficiency due to a variation in grid depth.
- the diffraction order k is 2, ⁇ is 0.405 ( ⁇ m), and a refraction index n 405 of an optical resin composing the lens 7 is 1.54662. Therefore, k ⁇ /(n ⁇ 1) becomes 1.4818 ( ⁇ m), so that Expression 3 is satisfied.
- Table 3 shows refraction indexes of optical resins used in the first to third embodiments. A chromatic aberration of magnification and a focal variation during an increase in temperature are calculated based on values shown in Table 3.
- the second diffraction order is used as the design order of the diffraction grid. Even when the design order equal to or larger than the third order is used, a diffraction grid which is easily produced without reducing a grid size can be provided. Even when the short wavelength light source having a wavelength of 500 nm or less is used, it is possible to provide an optical scanning device having functions such as a chromatic aberration correction and a temperature compensation, and an image forming apparatus using the optical scanning device.
- the design order equal to or larger than the third order may be used as the design order of the diffraction grid.
- FIG. 6 is a graph showing a chromatic aberration of magnification in the main scanning direction, which is one of effects in this embodiment (design data shown in Tables 1 to 3 are used).
- a refraction optical element composed of only refraction surfaces without using the diffraction optical element 7 it is apparent that the diffraction optical element 7 has a function for sufficiently correcting the chromatic aberration of magnification.
- the optical scanning device using the multi-beam light source for emitting the plurality of light fluxes it is possible to obtain an effect that a chromatic aberration of magnification in the main scanning direction due to a wavelength difference in a multi laser and a focal variation on the surface to be scanned which is caused with an increase in temperature are minimized.
- a change ratio of the refraction index of the used optical resin (dn/dt) is set to ⁇ 7.988E-05 and a temperature characteristic of the wavelength of the laser (d ⁇ /dt) is set to 0.04 nm/° C.
- Table 4 shows curvatures of the cylinder lens 4 , phase coefficients of the diffraction grid 4 a , and the like.
- the diffraction grid 4 a is constructed using the second design order as in the first embodiment.
- Table 4 shows design data in this embodiment.
- the diffraction grid 4 a is used for temperature compensation in the sub scanning direction.
- a position of a focal line near the polygon mirror 5 is shifted to the scanning lens side by 0.17356 mm at an increased temperature of up to 25° C.
- the diffraction grid is used for temperature compensation in this embodiment to completely prevent the focal variation.
- FIG. 9 is an explanatory view showing a part of an optical scanning device according to a third embodiment of the present invention, which shows an embodiment in which a diffraction grid is formed in the collimator lens 3 .
- a diffraction grid 3 a is formed on an incident surface side of the collimator lens 3 . Because of a concentric diffraction grid, although the single collimator lens is used, it becomes an optical system in which an on-axis chromatic aberration is corrected.
- Table 5 shows curvatures of the collimator lens 3 , phase coefficients of the diffraction grid 3 a , and the like.
- the diffraction grid 3 a is constructed using the second design order as in the first and second embodiments.
- the diffraction grid is used to correct the on-axis chromatic aberration.
- Table 5 shows design data in this embodiment.
- FIGS. 10A and 10B are graphs showing a spherical aberration and an on-axis chromatic aberration of the collimator lens in the case of reverse tracing from the exit surface side (parallel light side) of the collimator lens.
- FIG. 10A shows the case where the diffraction grid 3 a is formed on the incident surface of the collimator lens.
- FIG. 10B shows the case of a single lens in which the incident surface and the exit surface each are a refraction surface.
- an on-axis chromatic aberration of 0.029 nm is caused at a wavelength difference of 5 nm.
- the diffraction grid 3 a is used to correct the chromatic aberration in this embodiment. Therefore, although the wavelength difference of 5 nm is provided, the on-axis chromatic aberration is not caused at all.
- the diffraction grid may be provided for each of the collimator lens 3 , the lens 7 in the scanning optical system, and the cylindrical lens 4 .
- the diffraction grid may be provided for two of the collimator lens 3 , the lens 7 in the scanning optical system, and the cylindrical lens 4 .
- FIG. 11 is a main part sectional view in the sub scanning direction, showing an image forming apparatus using the optical scanning device according to the first, second, or third embodiment.
- reference numeral 104 denotes an image forming apparatus.
- Code data Dc is inputted from an external device 117 such as a personal computer to the image forming apparatus 104 .
- the code data Dc is converted into image data (dot data) Di by a printer controller 111 in the image forming apparatus 104 .
- the image data Di is inputted to an optical scanning unit 100 having the structure indicated in the first, second, or third embodiment.
- a light beam 103 modulated according to the image data Di is emitted from the optical scanning unit 100 .
- a photosensitive surface of a photosensitive drum 101 is scanned with the light beam 103 in the main scanning direction.
- the photosensitive drum 101 serving as an electrostatic latent image bearing member (photosensitive member) is rotated clockwise by a motor 115 . According to the rotation, the photosensitive surface of the photosensitive drum 101 is moved in the sub scanning direction orthogonal to the main scanning direction with respect to the light beam 103 .
- a charging roller 102 for uniformly charging the surface of the photosensitive drum 101 is provided on an upper part of the photosensitive drum 101 so as to be contact with the surface thereof.
- the surface of the photosensitive drum 101 which is charged by the charging roller 102 is irradiated with the light beam 103 scanned by the optical scanning unit 100 .
- the toner image developed by the developing device 107 is transferred onto a sheet 112 serving as a transfer material by a transfer roller 108 provided below the photosensitive drum 101 so as to oppose the photosensitive drum 101 .
- the sheet 112 is contained in a sheet cassette 109 located in the front (right side in FIG. 11 ) of the photosensitive drum 101 . Manual sheet feeding is also possible.
- a feed roller 110 provided at the end portion of the sheet cassette 109 serves to feed the sheet 112 in the sheet cassette 109 to a transport path.
- the sheet 112 to which an unfixed toner image is transferred is further transported to a fixing device located in the rear (left side in FIG. 11 ) of the photosensitive drum 101 .
- the fixing device is composed of a fixing roller 113 having a fixing heater (not shown) therein and a pressure roller 114 provided so as to press the fixing roller 113 .
- the sheet 112 transported from a transferring part is heated while it is pressurized by a press-contacting part between the fixing roller 113 and the pressure roller 114 , so that the unfixed toner image on the sheet 112 is fixed.
- a delivery roller 116 is provided in the rear of the fixing roller 113 and the fixed sheet 112 is delivered to the outside of the image forming apparatus 104 by the delivery roller 116 .
- the printer controller 111 conducts not only data conversion described earlier but also control of each part of the image forming apparatus 104 , which is represented by the motor 115 , control of a polygon motor in the optical scanning unit as described later, and the like.
- FIG. 12 is a main part schematic diagram showing a color image forming apparatus according to the first, second, or third embodiment of the present invention.
- This is a tandem type color image forming apparatus in which four optical scanning devices are arranged to record image information in parallel on the surfaces of the photosensitive drums serving as the image bearing members.
- reference numeral 60 denotes a color image forming apparatus
- 11 , 12 , 13 , and 14 each denote the optical scanning device having the structure described in the first embodiment
- 21 , 22 , 23 , and 24 each denote the photosensitive drums serving as the image bearing members
- 31 , 32 , 33 , and 34 each denote a developing device
- 51 denotes a transport belt.
- respective color signals of R (red), G (green), and B (blue) are inputted from an external device 52 such as a personal computer to the color image forming apparatus 60 .
- the color signals are converted into respective image data (dot data) of C (cyan), M (magenta), Y (yellow), and B (black) by a printer controller 53 in the color image forming apparatus 60 .
- image data are separately inputted to the optical scanning devices 11 , 12 , 13 , and 14 .
- Light beams 41 , 42 , 43 , and 44 modulated according to the respective image data are emitted from the optical scanning devices 11 , 12 , 13 , and 14 .
- the photosensitive surfaces of the photosensitive drums 21 , 22 , 23 and 24 are scanned with the light beams in the main scanning direction.
- the four optical scanning devices ( 11 , 12 , 13 , and 14 ) are arranged, each device corresponding to C (cyan), M (magenta), Y (yellow), and B (black), and the image signals (image information) are recorded in parallel on the surfaces of the photosensitive drums 21 , 22 , 23 and 24 by the optical scanning devices, thus printing a color image at high speed.
- the grid shape can be easily produced by selecting a suitable design order of the diffraction grid.
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Abstract
There is provided a diffraction grid which is easily produced without reducing a grid size. There is provided an optical scanning device having effects such as a chromatic aberration correction and a temperature compensation even when a short wavelength light source having a wavelength of 500 nm or less is used, and an image forming apparatus using the optical scanning device. In the optical scanning device having the diffraction grid, for which the short wavelength light source having a wavelength of 500 nm or less is used, a design order of the diffraction grid is set to a diffraction order equal to or larger than a second order to obtain a grid shape which is easily formed.
Description
- 1. Field of the Invention
- The present invention relates to an optical scanning device and an image forming apparatus using the same. In particular, the present invention relates to an optical scanning device that is suitably used for an apparatus such as a laser beam printer or a digital copying machine having an electrophotographic process, in which a light flux optically modulated and emitted from light source means is reflected and deflected on a polygon mirror serving as optical deflection means and then a surface to be scanned is scanned with the light flux through a scanning optical system to record image information. The present invention relates to an optical scanning device capable of providing a satisfactory image by adopting a diffraction grid for correction of chromatic aberration of magnification or temperature compensation and to an image forming apparatus using the same. In addition, the present invention relates to a color image forming apparatus which uses a plurality of optical scanning devices and is composed of a plurality of image bearing members corresponding to respective colors.
- 2. Related Background Art
- Up to now, in an optical scanning device used for a laser beam printer (LBP) or the like, a light flux optically modulated according to an image signal and emitted from light source means is periodically deflected by, for example, an optical deflector composed of a rotating polygonal mirror (polygon mirror). The deflected light flux is converged to form a spot shape on a photosensitive recording medium (photosensitive drum) by a scanning optical system having an fθ characteristic. The surface of the recording medium is scanned with the light flux to perform image recording.
-
FIG. 13 is a schematic view showing a main part of a conventional optical scanning device. - In
FIG. 13 , a divergent light flux emitted from a light source means 1 is converted into a substantially parallel light flux by acollimator lens 3. The substantially parallel light flux is limited by adiaphragm 2 and incident into acylindrical lens 4 having predetermined refractive power only in the sub scanning direction. Of the substantially parallel light fluxes incident into thecylindrical lens 4, the light flux within a main scanning section outgoes therefrom without changing an optical state. The light flux within a sub scanning section is converged and imaged as a substantial linear image onto a deflection surface (reflection surface) 5 a of a deflecting means 5 composed of a polygon mirror. - The light flux which is deflected on the
deflection surface 5 a of thedeflecting means 5 is guided onto aphotosensitive drum surface 8 serving as a surface to be scanned through a scanning optical system 6 having an fθ characteristic. By rotating the deflecting means 5 in a direction indicated by an arrow “A”, thephotosensitive drum surface 8 is scanned with the light flux in a direction indicated by an arrow “B” to record image information. - Further, in order to achieve high speed scanning, a multi-beam optical scanning device that simultaneously forms a plurality of scanning lines by light fluxes from a plurality of light sources has been proposed and commercially available from various companies.
FIG. 14 is a schematic view showing a main part of a multi-beam optical scanning device. Two light fluxes emitted fromlight sources collimator lenses optical element 85. The synthesized light flux forms a linear image extended in the main scanning direction near a deflection surface of apolygon mirror 87 by the action of acylindrical lens 86 and then forms a light spot on aphotosensitive drum 89 by a scanningoptical system 88. Therefore, the two scanning lines can be formed by performing optical scanning once, so that extremely high speed scanning can be achieved as compared with a conventional optical scanning device. With respect to a multi-beam light source other than one using the above-mentioned synthesizing optical element, a monolithic multi-beam laser in which a large number of light emitting points exist has been produced. In the case where the monolithic multi-beam laser is used, it is unnecessary to use the synthesizing optical element. Thus, it is possible to simplify the optical system and the optical adjustment. - In an optical scanning device using a multi-beam light source, in order to eliminate a jitter caused by a wavelength difference between a plurality of light sources (variation in interval between scanning lines on the photosensitive drum surface in the main scanning direction), any countermeasure such as the appropriate selection of the light sources has been taken so as to minimize the wavelength difference between the light sources. When the jitter caused by the wavelength difference between the light sources (chromatic aberration of magnification) is corrected by the scanning optical system, a plurality of lenses having different dispersion characteristics are required. As compared with a scanning optical system that does not correct the chromatic aberration of magnification, the number of lenses generally increases to cause an increase in cost. There is a limitation with respect to the range of selection of the wavelengths of the light sources. Therefore, it is difficult that the wavelengths are made completely equal to one another. There is a problem with respect to a cost for the selection of the wavelengths. When a semiconductor laser is activated, an image quality is deteriorated by a variation in wavelength, which is called mode hopping. Thus, even in an optical scanning device other than the optical scanning device using the multi-beam light source, in order to improve the stability of the image quality, it is necessary to minimize the jitter caused by the variation in wavelength.
- A semiconductor laser used as a conventional light source (as disclosed in, for example, Japanese Patent Application Laid-Open No. H10-197820 and Japanese Patent Application Laid-Open No. H10-068903) is an infrared laser (780 nm) or a visible laser (675 nm). However, in order to realize a high resolution, the development of an optical scanning device in which a minute spot shape is obtained by using a short wavelength laser having an oscillating wavelength of 500 nm or less is under way. The advantage of the use of the short wavelength laser is that a minute spot size which is about half of a conventional spot size can be achieved while an exit F number of the scanning optical system is kept equal to a conventional one. In the case where a spot size is reduced to half of the conventional spot size while using the infrared laser, it is necessary to increase the intensity of the scanning optical system to an intensity about two times larger than that in a conventional case. A focal depth is proportional to a wavelength of a used light source and to the square of the exit F number of the scanning optical system. Therefore, to obtain the same spot size, the focal depth in the infrared laser becomes equal to or smaller than about ½ of the focal depth in the short wavelength laser.
- In such an optical scanning device, in order to record image information with high precision, it is necessary to preferably correct a curvature of field over the entire surface to be scanned, to have a distortion characteristic (fθ characteristic) with a constant speed, between an angle of view θ and an image height Y, and to make spot sizes on the image plane uniform at respective image heights. Various optical scanning devices or various scanning optical systems that satisfy the optical characteristics like those have been proposed up to now.
- According to Japanese Patent Application Laid-Open No. H10-197820 and Japanese Patent Application Laid-Open No. H10-068903 as described above, a (temperature compensation) optical scanning device using a diffraction optical element for a scanning optical system has been proposed to reduce a focal variation on a surface to be scanned due to the correction of the chromatic aberration of magnification and an environmental variation.
- In particular, in an optical scanning device using a short wavelength light source having a wavelength of 500 nm or less, a dispersion characteristic of a material used for a scanning lens is large. Therefore, the chromatic aberration of magnification becomes six times or seven times larger than that in a conventional infrared laser. Thus, in the optical scanning device using the multi-beam laser, the jitter is significantly caused in the main scanning direction to reduce the image quality.
- When a blue-violet laser which is made of a material such as gallium nitride and oscillated at the wavelength of 405 nm is used to obtain a spot size which is about half of a spot size of the infrared laser, as described above, the focal depth is proportional to the wavelength. Therefore, only a depth about half the conventional depth can be allowed. Thus, along with the improvement of precision of respective parts composing the optical scanning device, a (temperature compensation) optical scanning device in which the focal variation is not caused even in the case of the environmental variation is desired.
- As described below, a phase function φ for determining a grid shape is inversely proportional to the wavelength. Therefore, when a diffraction grid having the same power is designed, a grid pitch of the short wavelength laser having 500 nm or less becomes smaller than that of the conventional infrared laser.
- For the above-mentioned reason, when a light flux having a short wavelength of 500 nm or less is imaged on the surface to be scanned in the optical scanning device using the short wavelength light source having a wavelength of 500 nm or less, there is a problem in that a grid size becomes smaller.
- In general, as shown in
FIG. 15A , a grid shape formed on a mold by mold forming is transferred to an optical resin member or an optical glass member. After that, as shown inFIG. 15B , the transferred grid shape is separated from the mold to produce the diffraction optical element. Alternatively, a method of dropping a small amount of ultraviolet curable resin on a glass substrate (lens can be also used) serving as a base and curing the grid shape similarly formed on the mold by ultraviolet light is used. - However, as the grid size becomes smaller, the following problems are caused:
- it is hard to produce the mold by a cutting tool;
- transfer property of the grid shape formed on the mold deteriorates; and
- along with the deterioration of the transfer property, an imaging performance deteriorates and a diffraction efficiency reduces.
- An object of the present invention is to provide a diffraction grid which can be used for an optical scanning device and easily produced by selecting a suitable design order of the diffraction grid. Another object of the present invention is to provide a diffraction grid that attains a preferable performance even when a short wavelength light source having a wavelength of 500 nm or less is used. Still another object of the present invention is to provide an optical scanning device using the diffraction grid, which has functions such as a chromatic aberration correction and a temperature compensation, and an image forming apparatus using the optical scanning device.
- According to one aspect of the present invention, an optical scanning device includes:
- light source means for emitting at least one light flux having a wavelength of 500 nm or less;
- deflecting means for deflecting the at least one light flux emitted from the light source means;
- a first optical system for guiding the light flux emitted from the light source means to the deflecting means; and
- a second optical system for imaging the light flux deflected by the deflecting means on a surface to be scanned,
- in which one of the first optical system and the second optical system includes a diffraction grid and a design order of the diffraction grid is set to a diffraction order equal to or larger than a second order.
- Also, according to another aspect of the present invention, an optical scanning device includes:
- light source means for emitting at least one light flux having a wavelength of 500 nm or less;
- deflecting means for deflecting the at least one light flux emitted from the light source means;
- a first optical system for guiding the light flux emitted from the light source means to the deflecting means; and
- a second optical system for imaging the light flux deflected by the deflecting means on a surface to be scanned, in which:
- one of the first optical system and the second optical system includes a diffraction grid; and
- provided that the total number of grids of the diffraction grid is represented by m, an effective range of the diffraction grid is represented by L (mm), a minimum grid pitch of the diffraction grid is represented by P (μm), and the wavelength of the light flux is represented by λ (μm), the following expressions are satisfied:
m·L·λ<300000 (1)
30<P<λ (2) - In further aspect of the optical scanning device, provided that a diffraction order of the diffraction grid is represented by k, the wavelength of the light flux is represented by λ (μm), and a refraction index of a material composing the diffraction grid is represented by n, the following expression may be satisfied:
1<k·λ/(n−1)<5 (3) -
FIG. 1 is a main scanning sectional view showing an optical scanning device according to a first embodiment of the present invention; -
FIG. 2 shows a diffraction optical element in the first embodiment of the present invention; -
FIG. 3 is a sectional view showing a diffraction grid at a minimum pitch in the case using secondary diffraction light; -
FIG. 4 is a sectional view showing a diffraction grid at a minimum pitch in the case using primary diffraction light; -
FIGS. 5A and 5B are explanatory views showing mold processing for the diffraction optical element; -
FIG. 6 is a graph showing a chromatic aberration of magnification of a scanning optical system in a main scanning direction in the first embodiment of the present invention; -
FIG. 7 is a graph showing a focal variation of the scanning optical system in a sub scanning direction in the first embodiment of the present invention; -
FIG. 8 is a sub scanning sectional view showing an optical scanning device according to a second embodiment of the present invention; -
FIG. 9 is a sectional view showing a part of an optical scanning device according to a third embodiment of the present invention; -
FIGS. 10A and 10B are graphs showing a spherical aberration and an on-axis chromatic aberration in the third embodiment of the present invention; -
FIG. 11 is a main part schematic diagram showing an image forming apparatus according to the present invention; -
FIG. 12 is a main part schematic diagram showing a color image forming apparatus according to the present invention; -
FIG. 13 is a perspective view showing a conventional optical scanning device; -
FIG. 14 is a perspective view showing a conventional optical scanning device using a multi-beam light source; and -
FIGS. 15A and 15B are explanatory views showing a process for forming a diffraction optical element. -
FIG. 1 is a main scanning sectional view showing an optical scanning device according to a first embodiment of the present invention. - Here, a main scanning direction indicates a direction perpendicular to the rotational axis of deflecting means. A sub scanning direction indicates a direction parallel to the rotational axis of the deflecting means. In addition, a main scanning section indicates a plane which is parallel to the main scanning direction and includes the optical axes of a deflection optical element (lens) 6 and a diffraction optical element (lens) 7 having a
diffraction grid 7 a formed on an exit surface of a diffraction part, which compose a scanning optical system. - A divergent light flux emitted from a single
beam semiconductor laser 1 serving as light source means is limited by adiaphragm 2 so as to reduce a width of the light flux, thereby obtaining a desirable spot size. Then, the light flux is converted into a substantially parallel light flux by acollimator lens 3. The substantially parallel light flux is imaged as a linear image extended in the main scanning direction onto the vicinity of adeflection surface 5 a of a deflecting means 5 described later by acylindrical lens 4 having predetermined refractive power in only the sub scanning direction. The deflecting means 5 is composed of, for example, a polygon mirror having four surfaces (rotating polygonal mirror) and rotated in a direction indicated by an arrow “A” inFIG. 1 at a constant rate by drive means such as a motor (not shown). With the scanning optical system composed of the deflection optical element (lens) 6 and the diffraction optical element (lens) 7 having fθ characteristics, the deflection light flux which is reflected and deflected on the deflecting means 5 is imaged onto aphotosensitive drum surface 8 serving as a surface to be scanned. In addition, a tangle error of thedeflection surface 5 a of the deflecting means 5 is corrected by the scanning optical systems. At this time, the deflection light flux which is reflected and deflected on thedeflection surface 5 a of the deflecting means 5 is guided onto thephotosensitive drum surface 8 through the deflection optical element (lens) 6 and the diffraction optical element (lens) 7. When thepolygon mirror 5 is rotated in a direction indicated by an arrow “A”, thephotosensitive drum surface 8 is optically scanned in a direction indicated by an arrow “B”. Therefore, scanning lines are formed on thephotosensitive drum surface 8, thereby performing image recording. - Here, an optical arrangement and figures in this embodiment are shown in Table 2.
- A shape of each generating line of the incident and exit surfaces of the deflection optical element (lens) 6 and the incident surface of the diffraction optical element (lens) 7 is based on an aspherical shape which can be indicated as a function of up to the tenth order. For example, in the case where an intersection point between the deflection optical element (lens) 6 and the optical axis is set to an origin, the optical axis direction is set to an X-axis, and an axis perpendicular to the optical axis within the main scanning section is set to an Y-axis, a generating line direction corresponding to the main scanning direction is indicated by the expression,
(where R is a curvature radius of a generating line and K, B4, B6, B8, and B10 are aspherical coefficients). - In addition, a meridian direction corresponding to the sub scanning direction is indicated by the expression,
S indicates a shape of a meridian line which includes a normal of the generating line at each position in the generating direction and is defined within a plane perpendicular to the main scanning surface. - Here, a curvature radius in the sub scanning direction (meridian line curvature radius) Rs* at a position away from the optical axis by Y in the main scanning direction is indicated by the expression,
Rs*=Rs×(1+D2×Y 2 +D4×Y 4 +D6×Y 6 +D8×Y 8 +D10×Y 10)
(where Rs is the meridian line curvature radius on the optical axis and D2, D4, D6, D8, and D10 are meridian line change coefficients). - Note that the figure is defined by the above-mentioned expressions in this embodiment. However, the scope of the present invention is not limited to this.
- In the diffraction
optical element 7, thediffraction grid 7 a is formed on the exit surface side of the refraction portion having the fθ characteristic and the grid shape thereof is defined by the following phase function. That is, the grid shape is expressed by the following expression related to the diffraction surface indicated by a phase function of up to a tenth order in the main scanning direction and a phase function of a second order in the sub-scanning direction, which is changed according to a position in the main scanning direction,
φ=2πk/λ{b2Y 2 +b4Y 4 +b6Y 6 +b8Y 8 +b10Y 10+(d0+d1Y+d2Y 2 +d3Y 3 +d4Y 4 +d5Y 5 +d6Y 6)Z 2}
(where φ is a phase function, k is a diffraction order, λ is a design wavelength (405 nm), Y is a height from a lens optical axis, and b2, b4, b6, b8, b10, d0, d1, d2, d3, d4, d5, and d6 each is a phase coefficient, secondary diffraction light being used in this embodiment). - Table 1 shows design parameters of the optical system of the optical scanning apparatus shown in
FIG. 1 . The lens 6 is the defection optical element and thelens 7 is the diffraction optical element in which the diffraction grid is provided on the deflection base surface.TABLE 1 Design data Wavelength, Refractive index Lens 6 Figures Use wavelength λ (nm) 405 First plane Second plane First plane Second plane Lens 7 Figures Lens 6 Refractive index n405 1.5466198 R −8.74966E+01 −4.71453E+01 R −4.04355E+02 ∞ K 5.71925E−01 −1.04001E+00 K −3.69841E+01 Lens 7 Refractive index n405 1.5466198 B4 1.40393E−06 9.40875E−08 B4 2.07946E−07 B6 1.26075E−09 5.42879E−10 B6 −1.81403E−11 B8 −1.16558E−12 4.08844E−13 B8 8.57936E−16 Light beam angle B10 1.96978E−16 −4.10320E−16 B10 −1.82040E−20 Incident angle to polygon θp −70 r0 1.43515E+02 −3.21543E+01 Diffraction grid, Phase coefficient Maximal exit angle on θe 45 D2u 2.46277E−04 b2 −1.01974E−04 polygon D4u −1.56775E−07 b4 2.09153E−09 Arrangement D6u 7.89931E−11 b6 −3.45669E−13 Polygon surface to lens 6 e1 30 D2l 1.86868E04 b8 2.59139E−17 Central thickness of lens 6 e2 11 D4l −1.07713E−07 b10 −8.18565E−22 Lens 6 to lens 7 e3 75 D6l 5.27788E−11 d0 −3.37934E−03 Central thickness of lens 7 e4 5 Suffios u: Opposite Laser Side d1 4.58488E−07 Lens 7 to surface to be Sk 111 Suffios l: Laser Side d2 8.43600E−08 scanned Polygon axis to surface to L 238.4 Y-Axis Sign +: Laser Side d3 −1.80357E−11 be scanned Effective scanning width W 297 d4 −2.74067E−12 d5 0.00000E+00 d6 4.41097E−17 - Table 2 shows phase coefficients, the total number of grids m within an effective range L, and a minimum pitch P of grids.
TABLE 2 Primary Secondary Coefficient Order diffraction light diffraction light Main scanning phase coefficient & Grid pitch b2 y2 −2.039480E−04 −1.01974.E−04 b4 y4 4.183050E−09 2.09153.E−09 b6 y6 −6.913380E−13 −3.45669.E−13 b8 y8 5.182780E−17 2.59139.E−17 b10 y10 −1.637130E−21 −8.18565.E−22 Total grid number m 5680 2840 Effective width (mm) 216 216 Minimum pitch (mm) 0.0089 0.0178 Sub scanning phase coefficient & Grid pitch d0 z2 −6.75868E−03 −3.379340.E−03 d1 z2y 9.16976E−07 4.584880.E−07 d2 z2y2 1.68720E−07 8.436000.E−08 d3 z2y3 −3.60713E−11 −1.803565.E−11 d4 z2y4 −5.48134E−12 −2.740670.E−12 d5 z2y6 8.82193E−17 4.410965.E−17 Total grid number m 150 75 Effective width (mm) 6 6 Minimum pitch (mm) 0.0100 0.0201 -
FIG. 2 is a schematic view showing the diffractionoptical element 7, in which thediffraction grid 7 a is formed in a saw-tooth shape (blazed type) in the main scanning direction and the sub scanning direction. - In this embodiment, a short wavelength light source having λ=405 nm is used. In the entire scanning region, it is achieved that a spot diameter in the main scanning direction becomes 27 μm and a spot diameter in the sub scanning direction becomes 35 μm. Here, the spot diameter indicates a diameter of a region which is obtained by slicing at 1/e2 of a peak light amount.
-
FIG. 3 shows a grid shape at a minimum pitch in the case where the diffraction grid is designed using secondary diffraction light in this embodiment. For comparison withFIG. 3 ,FIG. 4 shows a grid shape at a minimum pitch in the case where the diffraction grid is designed using primary diffraction light. As is apparent fromFIG. 4 , in the case using the conventional primary diffraction light, a grid height becomes lower than 1 μm and the minimum pitch P becomes a half of that in the case using the secondary diffraction light. As is read from table 2, the total number of grids of the diffraction grid m becomes two times larger than that in the case using the secondary diffraction light. - As described above, the diffraction
optical element 7 is designed using the secondary diffraction light. Therefore, it is possible to obtain the grid pitch which is about two times larger than that in the case using the primary diffraction light and the total number of grids which is about a half of that in the case using the primary diffraction light. - In other words, in this embodiment, with respect to a light flux having a wavelength of 500 nm or less, the diffraction order equal to or larger than the second order is used for diffraction light for forming an image on the surface to be scanned.
-
FIGS. 5A and 5B are explanatory views showing a mold cutting process for the diffractionoptical element 7. With respect to a grid shape around a central portion of the diffractionoptical element 7, because the grid pitch is relatively large, the central portion is cut plural times using an end portion of a cutting tool which is shaded (FIG. 5A ). On the other hand, because the grid pitch in a peripheral portion of the diffractionoptical element 7 is small, the peripheral portion is cut one time for each grid to obtain the grids each having an oblique surface (FIG. 5B ). Mold processing is always performed on the diffraction optical element while the end portion of the cutting tool is damaged. The cutting tool is gradually worn during cutting. Therefore, a problem that a target grid shape cannot be obtained occurs. - As the number of grids and the effective width of the diffraction optical element become larger, the amount of wear of the cutting tool increases.
- Therefore, in the case where the total number of grids of the diffraction optical element is represented by m, the effective range of the diffraction optical element is represented by L (mm), and the wavelength of the light source is represented by λ (μm), when
m·L·λ<300000 (1)
is satisfied, a grid shape suitable to the wavelength of the used light source can be obtained. - Even when the grid pitch is small, a damage to the cutting tool becomes larger. Therefore, when the minimum pitch of the diffraction optical element is represented by P (μm), it is preferable to satisfy
30<P/λ (2). - In this embodiment, m·L·λ=248443 and P/λ=43.95. Therefore, both
Expression 1 andExpression 2 are satisfied. - When the diffraction order of the diffraction optical element is represented by k and a refraction index of a material composing the diffraction optical element is represented by n, a grid depth h necessary to set a diffraction efficiency to 100% at a use wavelength is expressed to be h=k·λ/(n−1).
- When the grid depth is too shallow, processing precision is hard to improve. When the grid depth is too deep, the damage to the cutting tool becomes larger. Therefore, the diffraction order and the material are selected so as to satisfy
1 <k·λ/(n−1)<5 (3).
Thus, a grid shape suitable to the wavelength of the used light source can be obtained. - In the case of the diffraction grid using the short wavelength light source having a wavelength of 500 nm or less, it is desirable that the design order of the diffraction grid is the second order to the fifth order. When the diffraction order is the first order, as described above, the total number of grids is too large. When the diffraction order is equal to or larger than the sixth order, the grid depth becomes too deep, so that a cutting resistance of the cutting tool increases. In addition, when the diffraction grid has the diffraction order equal to or larger than the sixth order, significant influence is caused by a variation in diffraction efficiency due to a variation in grid depth.
- In this embodiment, the diffraction order k is 2, λ is 0.405 (μm), and a refraction index n405 of an optical resin composing the
lens 7 is 1.54662. Therefore, k·λ/(n−1) becomes 1.4818 (μm), so thatExpression 3 is satisfied. - Table 3 shows refraction indexes of optical resins used in the first to third embodiments. A chromatic aberration of magnification and a focal variation during an increase in temperature are calculated based on values shown in Table 3.
- In this embodiment, the second diffraction order is used as the design order of the diffraction grid. Even when the design order equal to or larger than the third order is used, a diffraction grid which is easily produced without reducing a grid size can be provided. Even when the short wavelength light source having a wavelength of 500 nm or less is used, it is possible to provide an optical scanning device having functions such as a chromatic aberration correction and a temperature compensation, and an image forming apparatus using the optical scanning device.
- Even in the second and third embodiments described below, the design order equal to or larger than the third order may be used as the design order of the diffraction grid.
TABLE 3 Wavelength (nm) 405 406 410 Refractive During normal 1.5466198 1.5464526 1.5457982 index temperature (t = 25° C.) During increase 1.5446228 1.5444540 1.5437929 in temperature (t = 55° C.) -
FIG. 6 is a graph showing a chromatic aberration of magnification in the main scanning direction, which is one of effects in this embodiment (design data shown in Tables 1 to 3 are used). InFIG. 6 , a difference between imaging positions in the main scanning direction in the case where a wavelength difference ΔA is set to 5 nm (λ=410 nm) with respect to a reference wavelength (λ=405 nm) is plotted. As compared with a refraction optical element composed of only refraction surfaces without using the diffractionoptical element 7, it is apparent that the diffractionoptical element 7 has a function for sufficiently correcting the chromatic aberration of magnification. - As described above, although the single
beam semiconductor laser 1 is used as the light source means, the chromatic aberration of magnification is corrected. Therefore, even in an optical scanning device using a multi-beam light source for emitting a plurality of light fluxes (for example, three or four light fluxes), a jitter caused in the main scanning direction is reduced, so that a preferable image quality can be achieved. - In the case of the optical scanning device using the multi-beam light source for emitting the plurality of light fluxes, it is possible to obtain an effect that a chromatic aberration of magnification in the main scanning direction due to a wavelength difference in a multi laser and a focal variation on the surface to be scanned which is caused with an increase in temperature are minimized.
-
FIG. 7 is a graph showing an effect obtained by only the scanning optical system with respect to a temperature compensation in the sub scanning direction, which is one of effects in this embodiment (design data shown in Tables 1 to 3 are used). With respect to the temperature compensation, as disclosed in Japanese Patent Application Laid-Open No. H10-197820, a focal variation due to a temperature change of an optical resin (reduction in refraction index of the optical resin, or the like) in the optical scanning device is corrected by a change in power of the diffraction portion resulting from a variation in wavelength of the light source due to the temperature change. In the calculation, a change ratio of the refraction index of the used optical resin (dn/dt) is set to −7.988E-05 and a temperature characteristic of the wavelength of the laser (dλ/dt) is set to 0.04 nm/° C. As compared with a refraction optical element composed of only refraction surfaces without using the diffractionoptical element 7, it is apparent fromFIG. 7 that the diffractionoptical element 7 has a sufficient temperature compensation effect. - As described above, the diffraction order, the number of grids, the effective range, the minimum pitch, the grid depth, and the like in the diffraction optical element are suitably set in the optical scanning device using the short wavelength light source having a wavelength of 500 nm or less. Therefore, it is possible to provide a diffraction optical element in which the grid shape is prevented from by being distorted by the mold cutting process and the mold forming process to reduce the degree of difficulty of manufacturing.
- When the chromatic aberration of magnification is corrected using the diffraction optical element and the focal variation due to the temperature change is reduced using the diffraction optical element, it is possible to provide an optical scanning device capable of obtaining a preferable image all the time.
- In this embodiment, the diffraction grid is provided on the surface of the diffraction portion in the scanning optical system. The diffraction grid may be provided on a plane. The diffraction grid may be provided on the surfaces of the plurality of lenses in the scanning optical system.
-
FIG. 8 is an explanatory view showing an optical scanning device according to a second embodiment of the present invention, which shows an embodiment in which a diffraction grid is formed in thecylinder lens 4. In this embodiment, adiffraction grid 4 a is formed on an exit surface side of thecylinder lens 4 and has power in only the sub scanning direction. Thecylinder lens 4 itself is made of an optical resin and can be produced at low cost by injection molding. - Table 4 shows curvatures of the
cylinder lens 4, phase coefficients of thediffraction grid 4 a, and the like. - The phase function used here is the same as in the first embodiment.
φ=2 πk/λ·d0·Z 2 (k=2 and λ=405 nm) - In this embodiment, the
diffraction grid 4 a is constructed using the second design order as in the first embodiment. - With respect to the diffraction grid used here, m·L·λ=111 and P/λ=56.35. Therefore, both
Expression 1 andExpression 2 are satisfied. When the present invention is applied to the cylinder lens, there is no problem with respect to limitations to the number of grids and the effective width. With respect to the minimum grid pitch, it is necessary to carefully design the diffraction grid as in the first embodiment. Because the grid depth is determined according to the wavelength of the light source, the refraction index of the material, and the diffraction order, there is no relationship with the number of grids and the grid pitch. Therefore, also in view of the grid depth, when the diffraction grid is constructed using the secondary diffraction light, the degree of difficulty of manufacturing is reduced. In this embodiment as well, the grid depth is 1.4818 (μm) as in the first embodiment, so thatExpression 3 is satisfied. - Table 4 shows design data in this embodiment.
TABLE 4 Second Comparative Example Embodiment (refraction system) Incident surface side R1s 64.93016 24.59789 curvature radius Exit surface side R2s ∞ ∞ curvature radius Exit surface side d0 −3.56742E−03 phase coefficient Incident surface to e1 6.00 6.00 Exit surface Focal distance in sub fs 45.00 45.00 scanning direction Focal variation amount Δs 0 0.17356 at increased temperature of up to 25° C. Effective width of L 5.00 5.00 diffraction grid Total grid number in m 55 diffraction grid Minimum pitch of P 0.02282 diffraction grid - In this embodiment, the
diffraction grid 4 a is used for temperature compensation in the sub scanning direction. When the cylinder lens is composed of only refraction surfaces, a position of a focal line near thepolygon mirror 5 is shifted to the scanning lens side by 0.17356 mm at an increased temperature of up to 25° C. In contrast to this, the diffraction grid is used for temperature compensation in this embodiment to completely prevent the focal variation. -
FIG. 9 is an explanatory view showing a part of an optical scanning device according to a third embodiment of the present invention, which shows an embodiment in which a diffraction grid is formed in thecollimator lens 3. In this embodiment, adiffraction grid 3 a is formed on an incident surface side of thecollimator lens 3. Because of a concentric diffraction grid, although the single collimator lens is used, it becomes an optical system in which an on-axis chromatic aberration is corrected. - Table 5 shows curvatures of the
collimator lens 3, phase coefficients of thediffraction grid 3 a, and the like. - The phase function used here is the same as in the first embodiment.
φ=2πk/λ·c0·H 2 , H 2 =Y 2 +Z 2 (k=2 and λ=405 nm) - In this embodiment, the
diffraction grid 3 a is constructed using the second design order as in the first and second embodiments. - With respect to the
diffraction grid 3 a used here, m·L·λ=47 and P/λ=129.60. Therefore, bothExpression 1 andExpression 2 are satisfied. When the present invention is applied to the collimator lens, there is no problem with respect to limitations to the number of grids and the effective width. Even in the minimum grid pitch, the conditional expression is sufficiently satisfied. Because the grid depth is determined according to the wavelength of the light source, the refraction index of the material, and the diffraction order, there is no relationship with the number of grids and the grid pitch. Therefore, also in view of the grid depth, when the diffraction grid is constructed using the secondary diffraction light, the degree of difficulty of manufacturing is reduced. In this embodiment as well, the grid depth is 1.4818 (μm) as in the first and second embodiments, so thatExpression 3 is satisfied. - In this embodiment, the diffraction grid is used to correct the on-axis chromatic aberration.
- Table 5 shows design data in this embodiment.
TABLE 5 Reverse tracing from exit surface side (parallel light side) Third Comparative Example Embodiment (refraction system) Exit surface side R2 12.34417 10.9324 curvature radius Incident surface side R1 ∞ ∞ curvature radius Incident surface side c0 −1.56392E−03 phase coefficient Exit surface to e1 3.00 3.00 Incident surface Focal distance f 20.00 20.00 On-axis chromatic Δm 0 0.02907 aberration (Δλ = 5 nm) Effective width of L 5.00 5.00 diffraction grid Total grid number in m 23 diffraction grid Minimum pitch of P 0.05249 diffraction grid -
FIGS. 10A and 10B are graphs showing a spherical aberration and an on-axis chromatic aberration of the collimator lens in the case of reverse tracing from the exit surface side (parallel light side) of the collimator lens. -
FIG. 10A shows the case where thediffraction grid 3 a is formed on the incident surface of the collimator lens.FIG. 10B shows the case of a single lens in which the incident surface and the exit surface each are a refraction surface. When thecollimator lens 3 has only refraction surfaces, an on-axis chromatic aberration of 0.029 nm is caused at a wavelength difference of 5 nm. In contrast to this, thediffraction grid 3 a is used to correct the chromatic aberration in this embodiment. Therefore, although the wavelength difference of 5 nm is provided, the on-axis chromatic aberration is not caused at all. - Hereinafter, a modified example will be described.
- The diffraction grid may be provided for each of the
collimator lens 3, thelens 7 in the scanning optical system, and thecylindrical lens 4. The diffraction grid may be provided for two of thecollimator lens 3, thelens 7 in the scanning optical system, and thecylindrical lens 4. -
FIG. 11 is a main part sectional view in the sub scanning direction, showing an image forming apparatus using the optical scanning device according to the first, second, or third embodiment. InFIG. 11 ,reference numeral 104 denotes an image forming apparatus. Code data Dc is inputted from anexternal device 117 such as a personal computer to theimage forming apparatus 104. The code data Dc is converted into image data (dot data) Di by aprinter controller 111 in theimage forming apparatus 104. The image data Di is inputted to anoptical scanning unit 100 having the structure indicated in the first, second, or third embodiment. Alight beam 103 modulated according to the image data Di is emitted from theoptical scanning unit 100. A photosensitive surface of aphotosensitive drum 101 is scanned with thelight beam 103 in the main scanning direction. - The
photosensitive drum 101 serving as an electrostatic latent image bearing member (photosensitive member) is rotated clockwise by amotor 115. According to the rotation, the photosensitive surface of thephotosensitive drum 101 is moved in the sub scanning direction orthogonal to the main scanning direction with respect to thelight beam 103. A chargingroller 102 for uniformly charging the surface of thephotosensitive drum 101 is provided on an upper part of thephotosensitive drum 101 so as to be contact with the surface thereof. The surface of thephotosensitive drum 101 which is charged by the chargingroller 102 is irradiated with thelight beam 103 scanned by theoptical scanning unit 100. - As described earlier, the
light beam 103 is modulated according to the image data Di. The surface of thephotosensitive drum 101 is irradiated with thelight beam 103 to form an electrostatic latent image thereon. The electrostatic latent image is developed as a toner image by a developingdevice 107 provided in the downstream side from the irradiation position of thelight beam 103 in the rotational direction of thephotosensitive drum 101 so as to be in contact with thephotosensitive drum 101. - The toner image developed by the developing
device 107 is transferred onto a sheet 112 serving as a transfer material by atransfer roller 108 provided below thephotosensitive drum 101 so as to oppose thephotosensitive drum 101. The sheet 112 is contained in asheet cassette 109 located in the front (right side inFIG. 11 ) of thephotosensitive drum 101. Manual sheet feeding is also possible. Afeed roller 110 provided at the end portion of thesheet cassette 109 serves to feed the sheet 112 in thesheet cassette 109 to a transport path. - By the above operation, the sheet 112 to which an unfixed toner image is transferred is further transported to a fixing device located in the rear (left side in
FIG. 11 ) of thephotosensitive drum 101. The fixing device is composed of a fixingroller 113 having a fixing heater (not shown) therein and apressure roller 114 provided so as to press the fixingroller 113. The sheet 112 transported from a transferring part is heated while it is pressurized by a press-contacting part between the fixingroller 113 and thepressure roller 114, so that the unfixed toner image on the sheet 112 is fixed. Further, adelivery roller 116 is provided in the rear of the fixingroller 113 and the fixed sheet 112 is delivered to the outside of theimage forming apparatus 104 by thedelivery roller 116. - Although not shown in
FIG. 11 , theprinter controller 111 conducts not only data conversion described earlier but also control of each part of theimage forming apparatus 104, which is represented by themotor 115, control of a polygon motor in the optical scanning unit as described later, and the like. -
FIG. 12 is a main part schematic diagram showing a color image forming apparatus according to the first, second, or third embodiment of the present invention. This is a tandem type color image forming apparatus in which four optical scanning devices are arranged to record image information in parallel on the surfaces of the photosensitive drums serving as the image bearing members. InFIG. 12 ,reference numeral 60 denotes a color image forming apparatus; 11, 12, 13, and 14 each denote the optical scanning device having the structure described in the first embodiment; 21, 22, 23, and 24 each denote the photosensitive drums serving as the image bearing members; 31, 32, 33, and 34 each denote a developing device, and 51 denotes a transport belt. - In
FIG. 12 , respective color signals of R (red), G (green), and B (blue) are inputted from anexternal device 52 such as a personal computer to the colorimage forming apparatus 60. The color signals are converted into respective image data (dot data) of C (cyan), M (magenta), Y (yellow), and B (black) by aprinter controller 53 in the colorimage forming apparatus 60. These image data are separately inputted to theoptical scanning devices optical scanning devices photosensitive drums - With the color image forming apparatus according to this embodiment, the four optical scanning devices (11, 12, 13, and 14) are arranged, each device corresponding to C (cyan), M (magenta), Y (yellow), and B (black), and the image signals (image information) are recorded in parallel on the surfaces of the
photosensitive drums - With the color image forming apparatus according to this embodiment, as described above, the latent images of the respective colors are formed on the corresponding surfaces of the
photosensitive drums optical devices - For example, a color image reading apparatus including a CCD sensor may be used as the
external device 52. In this case, the color image reading apparatus and the colorimage forming apparatus 60 compose a color digital copying machine. - According to the present invention, in the optical scanning device having the diffraction grid, for which the short wavelength light source having a wavelength of 500 nm or less is used, the grid shape can be easily produced by selecting a suitable design order of the diffraction grid.
- This application claims priority from Japanese Patent Application No. 2003-208957 filed on Aug. 27, 2003, which is hereby incorporated by reference herein.
Claims (6)
1. An optical scanning device, comprising:
light source means for emitting at least one light flux having a single wavelength of 500 nm or less;
deflecting means for deflecting the at least one light flux emitted from the light source means;
a first optical system for guiding the light flux emitted from the light source means to the deflecting means; and
a second optical system for imaging the light flux deflected by the deflecting means on a surface to be scanned,
wherein one of the first optical system and the second optical system includes a diffraction grating, and
wherein a design order of the diffraction grating is the second order to the fifth order.
2-16. (canceled)
17. An image forming apparatus, comprising:
the optical scanning device according to claim 1;
a photosensitive member that is located on the surface to be scanned;
a developing device for developing, as a toner image, an electrostatic latent image that is formed on the photosensitive member scanned with the light flux by the optical scanning device;
a transfer device for transferring the developed toner image to a transfer material; and
a fixing device for fixing the transferred toner image to the transfer material.
18. An image forming apparatus, comprising:
the optical scanning device according to claim 1; and
a printer controller that converts code data inputted from an external device into an image signal and outputs the image signal to the optical scanning device.
19. A color image forming apparatus, comprising:
a plurality of optical scanning devices, each of which is an optical scanning device according to claim 1; and
a plurality of image bearing members for forming images having colors different from one another, each of which is located on a surface to be scanned in a respective one of the plurality of optical scanning devices.
20. A color image forming apparatus according to claim 19 , further comprising a printer controller that converts a color signal inputted from an external device into image data in different colors and outputs the image data to the plurality of optical scanning devices.
Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US11/469,540 US20060289731A1 (en) | 2003-08-27 | 2006-09-01 | Optical scanning device and image forming apparatus using the same |
Applications Claiming Priority (4)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
JP2003208957A JP2005070124A (en) | 2003-08-27 | 2003-08-27 | Optical scanner and image forming apparatus using same |
JP2003-208957 | 2003-08-27 | ||
US10/921,853 US7126625B2 (en) | 2003-08-27 | 2004-08-20 | Optical scanning device and image forming apparatus using the same |
US11/469,540 US20060289731A1 (en) | 2003-08-27 | 2006-09-01 | Optical scanning device and image forming apparatus using the same |
Related Parent Applications (1)
Application Number | Title | Priority Date | Filing Date |
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US10/921,853 Division US7126625B2 (en) | 2003-08-27 | 2004-08-20 | Optical scanning device and image forming apparatus using the same |
Publications (1)
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US20060289731A1 true US20060289731A1 (en) | 2006-12-28 |
Family
ID=34209008
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US10/921,853 Expired - Fee Related US7126625B2 (en) | 2003-08-27 | 2004-08-20 | Optical scanning device and image forming apparatus using the same |
US11/469,540 Abandoned US20060289731A1 (en) | 2003-08-27 | 2006-09-01 | Optical scanning device and image forming apparatus using the same |
Family Applications Before (1)
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US10/921,853 Expired - Fee Related US7126625B2 (en) | 2003-08-27 | 2004-08-20 | Optical scanning device and image forming apparatus using the same |
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US (2) | US7126625B2 (en) |
JP (1) | JP2005070124A (en) |
Cited By (3)
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US20090002792A1 (en) * | 2006-03-13 | 2009-01-01 | Kohji Sakai | Optical scanning device and image forming apparatus |
US20090059337A1 (en) * | 2007-08-27 | 2009-03-05 | Kenichiroh Saisho | Optical scanning device and image forming apparatus |
US20110013284A1 (en) * | 2009-07-14 | 2011-01-20 | Canon Kabushiki Kaisha | Diffractive optical element containing grating surface and grating wall surface, and optical system having the same |
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JP2005258392A (en) * | 2004-02-12 | 2005-09-22 | Ricoh Co Ltd | Optical scanner and image forming apparatus |
JP4455308B2 (en) * | 2004-12-16 | 2010-04-21 | キヤノン株式会社 | Optical scanning device and image forming apparatus using the same |
JP2006301299A (en) * | 2005-04-20 | 2006-11-02 | Ricoh Co Ltd | Laser scanning apparatus, image forming apparatus, method of laser scanning and method of forming image |
JP4677277B2 (en) * | 2005-05-09 | 2011-04-27 | キヤノン株式会社 | Optical scanning device and image forming apparatus using the same |
US7253937B2 (en) * | 2005-10-20 | 2007-08-07 | Ricoh Company, Ltd. | Multi-beam optical scanning device and image forming apparatus |
JP4841268B2 (en) * | 2006-02-28 | 2011-12-21 | 株式会社リコー | Optical scanning apparatus and image forming apparatus |
GB0617945D0 (en) | 2006-09-12 | 2006-10-18 | Ucl Business Plc | Imaging apparatus and methods |
US7688491B2 (en) | 2006-09-15 | 2010-03-30 | Ricoh Company, Ltd. | Diffractive-optical element, scanning optical system, optical scanner, and image forming apparatus |
JP5151118B2 (en) * | 2006-11-15 | 2013-02-27 | 株式会社リコー | A diffractive optical element, an optical scanning device, and an image forming apparatus. |
JP4976092B2 (en) * | 2006-09-19 | 2012-07-18 | 株式会社リコー | Optical scanning device and image forming apparatus using the same |
GB201006679D0 (en) | 2010-04-21 | 2010-06-09 | Ucl Business Plc | Methods and apparatus to control acousto-optic deflectors |
JP5731811B2 (en) | 2010-12-15 | 2015-06-10 | キヤノン株式会社 | Method for manufacturing blazed diffraction grating and method for manufacturing mold for the same |
GB201106787D0 (en) | 2011-04-20 | 2011-06-01 | Ucl Business Plc | Methods and apparatus to control acousto-optic deflectors |
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2006
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US6807019B2 (en) * | 2001-03-30 | 2004-10-19 | Pentax Corporation | Objective lens having diffractive structure for optical pick-up |
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US20090002792A1 (en) * | 2006-03-13 | 2009-01-01 | Kohji Sakai | Optical scanning device and image forming apparatus |
US7554708B2 (en) * | 2006-03-13 | 2009-06-30 | Ricoh Company, Ltd. | Optical scanning device and image forming apparatus |
US20090059337A1 (en) * | 2007-08-27 | 2009-03-05 | Kenichiroh Saisho | Optical scanning device and image forming apparatus |
US8213067B2 (en) * | 2007-08-27 | 2012-07-03 | Ricoh Company, Limited | Optical scanning device and image forming apparatus |
US20110013284A1 (en) * | 2009-07-14 | 2011-01-20 | Canon Kabushiki Kaisha | Diffractive optical element containing grating surface and grating wall surface, and optical system having the same |
US8681424B2 (en) | 2009-07-14 | 2014-03-25 | Canon Kabushiki Kaisha | Diffractive optical element containing grating surface and grating wall surface, and optical system having the same |
Also Published As
Publication number | Publication date |
---|---|
JP2005070124A (en) | 2005-03-17 |
US7126625B2 (en) | 2006-10-24 |
US20050045814A1 (en) | 2005-03-03 |
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