JP4429255B2 - Multi-beam scanning optical apparatus and image forming apparatus using the same - Google Patents

Description

  The present invention relates to a multi-beam scanning optical apparatus and an image forming apparatus using the same, and in particular, a plurality of scanning lines (dots) are simultaneously formed (written) on a photoreceptor surface, thereby enabling high-speed and high-quality printing. For example, it is suitable for a laser beam printer or a digital copying machine.

  2. Description of the Related Art Conventionally, in an image forming apparatus such as a laser beam printer or a digital copying machine, a multi-beam scanning optical apparatus capable of simultaneously writing a plurality of scanning lines on a photoreceptor surface is used in order to perform high-speed printing.

  In this multi-beam scanning optical device, when the oscillation wavelengths of the laser light sources are different from each other, a plurality of scanning lines formed by a laser beam emitted from each laser light source due to chromatic aberration of the scanning lens is dot-dotted in the main scanning direction for each scanning line. There is a problem that the position shifts.

Therefore, conventionally, for example, by limiting the relative difference between the oscillation wavelengths of the laser light sources, the amount of dot shift is suppressed to ½ or less of the size of one pixel (Patent Document 1).
JP 2000-11820 A

  However, in a color image forming apparatus that outputs a halftone image, such as a color laser printer, for example, an allowable amount of dot misregistration is not sufficient if it is 1/2 of the size of one pixel.

  12 and 13 are diagrams showing an example of a pattern of a halftone image that is generally used for forming a color image. FIG. 12 shows a case where there is no positional deviation of dots in the main scanning direction. Reference numeral 13 denotes a case where a positional deviation of dots occurs in the main scanning direction.

  12 and 13 each show a case where two laser beams are used, and a horizontal line drawn by a solid line indicates a scanning line formed by one laser beam. A horizontal line drawn by a broken line indicates a scanning line formed by the other laser beam, and a circle on the horizontal line indicates a dot DOT drawn by each scanning line.

  In FIG. 13, it can be seen that wide portions (width Wa) and narrow portions (width Wb) of dots are alternately generated in an oblique direction. When this state is viewed in an actual image, a portion where the dot interval is narrow is dark and a portion where the dot interval is wide appears thin, so that it appears that an oblique stripe pattern is formed on the entire screen. A color laser printer or the like uses several kinds of such patterns, and some patterns are very sensitive. Therefore, an allowable amount of dot displacement (Wa-Wb) in the main scanning direction is very small. It is necessary to suppress it to 1/4 or less of the pixel size.

  In this case, as a cause of the positional deviation of the dots in the main scanning direction, there is an optical path length difference described below in addition to the relative difference in oscillation wavelength described in the conventional example.

  Next, the positional deviation of dots due to the difference in optical path length will be described with reference to FIGS.

  14 is a sectional view (main scanning sectional view) of the principal part in the main scanning direction of the multi-beam scanning optical apparatus, and FIG. 15 is a sectional view of the principal part (sub scanning sectional view) in the sub scanning direction of FIG.

  14 and 15, reference numeral 100 denotes a light source means, which has two light sources (laser light sources) 101 and 102 made of, for example, a semiconductor laser. Reference numeral 103 denotes a collimator lens that converts two laser light beams emitted from the light source means 100 into substantially parallel light beams. A cylindrical lens 104 has a predetermined refractive power only in the sub-scanning direction. Reference numeral 108 denotes an aperture stop, which shapes the laser beam emitted from the cylindrical lens 104 into a desired optimum beam shape. Each element such as the collimator lens 103, the cylindrical lens 104, and the aperture stop 108 constitutes one element of the incident optical means 114.

  Reference numeral 105 denotes an optical deflector serving as a deflecting unit, which is composed of, for example, a rotating polygon mirror (polygon mirror), and is rotated at a constant speed in the direction of arrow A in the figure by a driving unit (not shown) such as a motor. Reference numeral 106 denotes scanning optical means having fθ characteristics, and has first and second fθ lenses 106a and 106b. Two laser beams deflected by the optical deflector 105 are imaged in a spot shape on a photosensitive member (recording medium) 107 to form two scanning lines S101 and S102. The scanning optical means 106 has a tilt correction function by providing a conjugate relationship between the vicinity of the deflecting surface 105a of the optical deflector 105 and the vicinity of the photoreceptor surface 107 in the sub-scan section.

  Reference numeral 107 denotes a photosensitive member (photosensitive drum) having a substantially cylindrical shape as a recording medium.

  In the figure, two laser light beams B101 and B102 which are light-modulated and emitted from the light source means 100 in accordance with image information are converted into substantially parallel light beams by the collimator lens 103 and enter the cylindrical lens 104. The laser beam incident on the cylindrical lens 104 is emitted as it is in the main scanning section and passes through the aperture stop 108 (partly shielded). In the sub-scan section, the light beam converges and passes through the aperture stop 108 (partially shielded) to form an almost line image (a line image elongated in the main scanning direction) on the deflection surface 105a of the optical deflector 105. The two laser beams B101 and B102 reflected and deflected by the deflecting surface 105a of the optical deflector 105 are imaged in a spot shape on the surface of the photosensitive member 107 by the scanning optical means 106, and the optical deflector 105 is moved in the direction of arrow A. Rotate. As a result, the surface of the photoconductor 107 is optically scanned in the direction of arrow B (main scanning direction) at a constant speed. As a result, an image is recorded on the surface of the photoreceptor 107 as a recording medium.

  14 and 15, S101 and S102 are scanning lines formed by laser beams B101 and B102 emitted from the laser light sources 101 and 102, respectively. L101 and L102 are paths of the laser beams B101 and B102 when forming the scanning lines S101 and S102, respectively.

  Here, as shown in FIG. 15, the laser beams B101 and B102 need to be incident on positions shifted in the sub-scanning direction from the apex T of the surface of the photosensitive member 107. The reason for this is that when a laser beam is incident on the apex T of the surface of the photoconductor 107, the laser beam near the center of the image height is reflected on the surface of the photoconductor 107 and reversely follows the same path. It will return to the light sources 101 and 102. As a result, the optical output fluctuates due to the noise phenomenon of the return light of the semiconductor laser, and appears as density unevenness in the printed image.

  However, when the laser beam is incident on the position shifted in the sub-scanning direction from the apex T on the surface of the photosensitive member 107 as described above, the optical path lengths of the two laser beams B101 and B102 are changed by ΔB. In this case, as apparent from FIG. 15, the lengths of the two scanning lines S101 and S102 also change. This is the mechanism of the positional deviation of the dots in the main scanning direction due to the optical path length difference described above, and this amount cannot be ignored.

  In the present invention, the positional deviation of dots caused by the wavelength difference of each laser light source in the main scanning direction and the positional deviation of dots caused by the optical path length difference of each laser beam are mutually canceled. Accordingly, it is possible to provide a multi-beam scanning optical apparatus capable of high-quality print output and an image forming apparatus using the same by correcting the overall magnification difference and suppressing the amount of dot displacement in the main scanning direction to be small. Objective.

The multi-beam scanning optical device according to the first aspect of the present invention includes a plurality of light sources, deflection means for deflecting the plurality of laser beams emitted from the plurality of light sources, and a plurality of laser beams deflected by the deflection means. Scanning optical means having a refracting optical element and a diffractive optical element to form an image on the top, and each of the plurality of laser light beams forms a predetermined angle with a normal line of the photoreceptor surface in a sub-scanning section. In the multi-beam scanning optical device incident on the surface of the photoreceptor,
A plurality of laser light beams deflected by the deflecting unit are guided to a synchronization detecting element by a synchronization detecting lens without passing through the scanning optical unit, and a plurality of laser beams on the surface of the photosensitive member are used by using signals from the synchronization detecting element. Synchronization position detection means for controlling the timing of the scanning start position of image recording of the laser beam,
The synchronization detection lens is disposed in a face-to-face relationship with the optical path from the deflection means to the synchronization detection element; and
The scanning optical means is set so that lateral chromatic aberration is overcorrected using the refractive optical element and the diffractive optical element, and a plurality of laser light beams incident on the photoconductor surface and a normal line of the photoconductor surface The oscillation wavelength of the light source that emits the laser beam having the smallest angle in the sub-scanning direction is shorter than the oscillation wavelength of the light source that emits the largest laser beam.

  According to a second aspect of the present invention, in the first aspect of the invention, the laser beam is three or more laser beams.

  According to a third aspect of the present invention, there is provided an image forming apparatus comprising: the multi-beam scanning optical apparatus according to the first or second aspect; and an electrostatic latent image formed on the surface of the photosensitive member by a light beam scanned by the multi-beam scanning optical apparatus. Development means for developing the toner image as a toner image, transfer means for transferring the developed toner image to a transfer material, and fixing means for fixing the transferred toner image to the transfer material. It is said.

  According to a fourth aspect of the present invention, there is provided an image forming apparatus comprising: the multi-beam scanning optical apparatus according to the first or second aspect; And a controller.

  According to the present invention, as described above, the positional deviation of the dots in the main scanning direction caused by the optical path length difference of each laser beam and the positional deviation of the dots in the main scanning direction caused by the wavelength difference of each laser light source cancel each other. By doing so, a multi-beam scanning optical device capable of realizing a high-quality print output by correcting the overall magnification difference and thereby suppressing the amount of positional deviation of dots in the main scanning direction, and an image forming apparatus using the same Can be achieved.

[Embodiment 1]
FIG. 1 is a sectional view (main scanning sectional view) of main parts in the main scanning direction of the multi-beam scanning optical apparatus according to the first embodiment of the present invention. FIG. 2 is a sectional view of main parts in the sub scanning direction (sub scanning sectional view) in FIG. ).

  In this specification, the surface formed by the optical axis of the scanning optical means and the light beam deflected by the optical deflector is the main scanning section, and the surface including the optical axis of the scanning optical means and perpendicular to the main scanning section is sub-scanned. It is defined as a cross section.

  In the figure, reference numeral 1 denotes light source means, which has two light sources (laser light sources) 1a and 1b made of, for example, semiconductor lasers. The two laser light sources 1a and 1b are arranged apart from each other in the main scanning direction and the sub-scanning direction.

  A collimator lens 2 converts the two laser light beams 14a and 14b emitted from the light source means 1 into substantially parallel light beams. A cylindrical lens 3 has a predetermined refractive power only in the sub-scanning direction. Reference numeral 4 denotes an aperture stop, which shapes the two laser light beams 14a and 14b emitted from the cylindrical lens 3 into a desired optimum beam shape.

  Each element such as the collimator lens 2, the cylindrical lens 3, and the aperture stop 4 constitutes one element of the incident optical means 21.

  An optical deflector 5 serving as a deflecting unit is composed of, for example, a rotary polygon mirror (polygon mirror), and is rotated at a constant speed in the direction of arrow A in the figure by a driving unit (not shown) such as a motor.

  Reference numeral 22 denotes scanning optical means having fθ characteristics, which includes a toric lens 6 and a diffractive optical element 7. Two laser beams 14a and 14b deflected by the optical deflector 5 are imaged in a spot shape on the surface of the photosensitive member 8 (scanned surface) to form two scanning lines 15a and 15b. The scanning optical means 6 has a tilt correction function by providing a conjugate relationship between the vicinity of the deflecting surface 5a of the optical deflector 5 and the vicinity of the surface of the photosensitive member 8 in the sub-scan section.

  Reference numeral 8 denotes a substantially cylindrical photosensitive member (photosensitive drum) as a recording medium.

  Reference numeral 9 denotes a synchronization detection lens for synchronization detection, which focuses (condenses) two synchronization signal detection light beams (BD light beams) on a slit 11 surface provided in the vicinity of a synchronization detection element 13 described later. . The synchronization detection lens 9 is disposed facing the optical path from the polygon mirror 5 to a BD sensor 13 described later. Note that the synchronization detection lens 9 in the present embodiment may be formed integrally with the toric lens 6 constituting one element of the scanning optical means.

  Reference numeral 10 denotes a folding mirror (hereinafter referred to as “BD mirror”), which reflects two BD light beams for adjusting the timing of the scanning start position on the surface of the photoconductor 8 toward the synchronization detecting element 13 described later. Yes.

  Reference numeral 11 denotes a slit for synchronization detection (hereinafter referred to as “BD slit”), which is disposed at a position optically equivalent to the surface of the photosensitive member 8 and determines the image writing position.

  Reference numeral 12 denotes a correction lens for synchronization detection (hereinafter referred to as “BD lens”), which is used to make the BD mirror 10 and the synchronization detection element 13 conjugate to each other. It has been corrected.

  Reference numeral 13 denotes an optical sensor (hereinafter referred to as “BD sensor”) as a synchronization detection element. In this embodiment, a synchronization signal (BD signal) obtained by detecting an output signal from the BD sensor 13 is used. Thus, the timing of the scanning start position for image recording on the surface of the photosensitive member 8 is adjusted.

  Each element such as the synchronization detection lens 9, the BD mirror 10, the BD slit 11, the BD lens 12, and the BD sensor 13 constitutes one element of the synchronization position detection means (BD optical system).

  Reference numerals 14a and 14b denote laser beams emitted from the corresponding laser light sources 1a and 1b, respectively. Reference numerals 15a and 15b denote scanning lines formed by the corresponding laser beams 14a and 14b, and reference numerals 16a and 16b denote paths of the laser beams 14a and 14b when forming the scanning lines 15a and 15b, respectively.

  In the present embodiment, the two laser light beams 14 a and 14 b that are light-modulated and emitted from the light source means 1 according to image information are converted into substantially parallel light beams by the collimator lens 2 and are incident on the cylindrical lens 3. The laser beam incident on the cylindrical lens 3 is emitted as it is in the main scanning section and passes through the aperture stop 4 (partially shielded). In the sub-scan section, the light beam converges and passes through the aperture stop 4 (partially shielded) to form an almost linear image (a linear image longitudinal in the main scanning direction) on the deflecting surface 5a of the optical deflector 5. The two laser light beams 14a and 14b reflected and deflected by the deflecting surface 5a of the optical deflector 5 are each imaged in a spot shape on the surface of the photosensitive member 8 by the scanning optical means 22. Then, by rotating the optical deflector 5 in the direction of arrow A, the surface of the photoconductor 8 is optically scanned in the direction of arrow B (main scanning direction) at a constant speed. As a result, two scanning lines 15a and 15b are simultaneously formed on the surface of the photoconductor 8 as a recording medium to perform image recording.

  At this time, in order to adjust the timing of the scanning start position on the surface of the photoconductor 8 before optically scanning the surface of the photoconductor 8, a part of the two laser light beams 14a and 14b reflected and deflected by the optical deflector 5 is used. After being condensed on the surface of the BD slit 11 via the BD mirror 10 by the synchronization detection lens 9, the light is guided to the BD sensor 13 via the BD lens 12. Then, the timing of the scanning start position of image recording on the surface of the photosensitive member 8 is adjusted for each BD light beam using a synchronization signal (BD signal) obtained by detecting the output signal from the BD sensor 13.

  1 and 2 each show a two-beam scanning optical apparatus using two laser light sources for the sake of simplicity of explanation, three or more laser light sources may be used. Further, all the laser light fluxes depicted in FIGS. 1 and 2 are drawn only for the main light flux for easy understanding.

[Dot misalignment in the main scanning direction due to optical path length difference]
Here, the positional deviation of the dots in the main scanning direction due to the optical path length difference between the two laser light beams 14a and 14b forming the two scanning lines 15a and 15b will be described.

  The two laser light beams 14a and 14b forming the two scanning lines 15a and 15b are respectively deflected by the polygon mirror 5, and then follow the paths 16a and 16b as shown in FIG. The

  Here, the distance d in the sub-scanning direction of the two laser beams 14a and 14b on the surface of the photosensitive member 8 is determined by the size of one pixel of the image forming apparatus on which the present apparatus is mounted. For example, in the case of a resolution of 600 dpi. d = 42.3 μm. The two laser beams 14a and 14b need to be incident with a certain shift in the sub-scanning direction from the apex T of the surface of the photoconductor 8 having a substantially cylindrical shape. This is to prevent fluctuations in the optical output due to the noise phenomenon of the return light of the semiconductor laser as described above.

  However, in this case, as apparent from FIG. 2, an optical path length difference ΔL consisting of the following (formula 1) occurs between the laser beam 14a and the laser beam 14b.

However,
R: radius of the photoreceptor,
S: A shift amount from the vertex T of the incident position of the laser beam 14a on the surface of the photosensitive member 8 (distance in the sub-scanning direction from the vertex T on the surface of the photosensitive member 8 to the scanning line 15a)
d: scanning line interval between two scanning lines 15a and 15b (sub-scanning interval between two scanning lines 15a and 15b on the surface of the photosensitive member 8)
In the present embodiment, the above parameters are set as follows.

Photoconductor radius R = 15mm
Scan line interval d = 42.3 μm (600 dpi)
Laser beam shift amount S = 2.5mm
FIG. 3 is an explanatory diagram showing the positional deviation of dots due to the optical path length difference in the present embodiment. In the figure, the solid line is the amount of positional deviation of the dots of the other scanning line 15b with respect to the scanning line 15a.

  As shown in the figure, the magnification in the main scanning direction is larger for the scanning line 15b than for the scanning line 15a. As a result, a dot position shift occurs in the image drawn between the two scanning lines.

Further, the shift amount at one end between both scanning lines is ΔY _L , and the shift amount at the other end is ΔY _R. Then, the deviation amounts ΔY _L and ΔY _R are as shown in the following (Equation 2), and the relationship between the image height and the dot position deviation amount becomes 0 at the center of the image height as shown in FIG. As it grows larger, it becomes maximum at both ends.

However,
α _L and α _R are the normal line of the surface of the photoconductor 8 at both ends of the image height, the laser beam 14a,
The angle formed by 14b in the main scanning direction In FIG. 3, the code on the vertical axis indicates whether the dot formed by the scanning line 15b is on the left or the right with respect to the dot formed by the scanning line 15a. It is.

[Position displacement of dots in the main scanning direction due to wavelength difference]
Next, the positional deviation of dots in the main scanning direction due to the relative difference between the oscillation wavelengths of the two laser light sources 1a and 1b will be described.

  In this embodiment, in order to correct the positional deviation of the dots in the main scanning direction due to the optical path length difference of the laser beam, the toric lens 6 and the diffractive optical element 7 are used as scanning optical elements, and the chromatic aberration of magnification due to both 6, 7 Is set to be overcorrected. Here, the overcorrection of the lateral chromatic aberration means that the magnification is reduced when the wavelength is changed to the longer wavelength side with respect to the reference wavelength, which means that it is opposite to the chromatic aberration generated by a normal single convex lens.

  FIG. 4 is an explanatory diagram showing dot misregistration per unit wavelength (1 nm) of laser light due to chromatic aberration of magnification of the scanning optical means 22 in this embodiment.

  In the figure, the solid line is the amount of dot misalignment when the wavelength is shifted by 1 nm to the long wavelength side with respect to the reference wavelength (780 nm). This change causes no dot misalignment in the center of the image and the image height increases. The dot position shift becomes maximum at the outermost part.

  In this embodiment, when the wavelength changes, the dot position does not shift in the center of the image because the synchronization detection lens 9 is arranged facing the optical path, so that the center of the image and the BD optical system This is because both lateral chromatic aberration is eliminated.

In the present embodiment, the dot positional deviation in the main scanning direction caused by the optical path length difference of the laser light beam described above is corrected by the positional deviation of the dot in the main scanning direction caused by the relative wavelength difference. The oscillation wavelength of the light source 1a that emits the laser beam 14a having the smallest angle in the sub-scanning direction formed by the plurality of laser beams incident on the surface of the photoconductor 8 and the normal line of the surface of the photoconductor 8 is specified. The oscillation wavelength of the light source 1a at this time is set to be shorter than the oscillation wavelength of the light source 1b that emits the laser beam 14b having the largest angle. More specifically,
Oscillation wavelength of laser light source 1a: 780 nm
Oscillation wavelength of laser light source 1b: 781 nm
It is set to. At this time, since the dot position shift caused by the optical path length difference and the dot position shift caused by the wavelength difference of the laser light source have different signs and the absolute values are almost equal, they almost cancel each other as shown in FIG. As a result, no dot displacement occurs in the entire image area.

  Table 1 shows optical design parameters in the present embodiment. Note that the following expression is used as an expression of the shape of the optical element.

  Refractive surface. . . The origin is the intersection of the aspherical shape that can be expressed by a function up to the 10th order in the main scanning direction and the optical axis. When the optical axis direction is the x axis, the axis orthogonal to the optical axis in the main scanning plane is the y axis, and the axis orthogonal to the optical axis in the sub scanning plane is the z axis, the generatrix direction corresponding to the main scanning direction is

(Where R is the radius of curvature, and K, B4, B6, B8, and B10 are aspheric coefficients)
The sub-scanning direction (the direction including the optical axis and orthogonal to the main scanning direction) and the sub-line direction are

Where r ′ = r ₀ (1 + D ₂ Y ² + D ₄ Y ⁴ + D ₆ Y ⁶ + D ₈ Y ⁸ + D ₁₀ Y ¹⁰ )
(Where r ₀ is the radius of curvature on the optical axis on the optical axis, and D ₂ , D ₄ , D ₆ , D ₈ and D ₁₀ are coefficients)
Diffraction surface . . Diffraction surface represented by second-order phase function with main scanning direction up to sixth order and sub-scanning direction depending on position in main scanning direction φ = mλ = b2Y2 + b4Y4 + b6Y6 + (d ₀ + d ₁ Y + d ₂ Y ² + d ₃ Y ³ + d ₄ Y ⁴ ) Z ² (Formula 5)
(Where m is the diffraction order: + 1st order diffracted light is used in the embodiment)

  As described above, in the present embodiment, as described above, the positional deviation of the dots in the main scanning direction due to the optical path length difference of each laser beam and the positional deviation of the dots in the main scanning direction due to the wavelength difference of each laser light source cancel each other. Make a relationship. Thereby, the amount of dot misregistration can be almost eliminated over the entire area of the image, thereby obtaining a very high quality multi-beam scanning optical apparatus.

[Embodiment 2]
Next, a second embodiment of the present invention will be described.

  In the present embodiment, the difference from the first embodiment described above is that the relative difference between the wavelengths of the two laser light sources 1a and 1b is suppressed so as to suppress the amount of positional deviation of the dots in the main scanning direction over the entire image height to the extent that it cannot be visually recognized. It is limited. Other configurations and optical actions are substantially the same as those of the first embodiment, and the same effects are obtained.

  If the oscillation wavelengths of the two light sources 1a and 1b are set as in the first embodiment, a very high quality multi-beam scanning optical device can be achieved.

  However, since the oscillation wavelength of a semiconductor laser as a laser light source generally has a manufacturing error, it is difficult to always manufacture in the best state. The oscillation wavelength of a semiconductor laser is determined by its composition, but varies depending on its manufacturing error. It has been found that a difference of about 2 nm at maximum occurs even at adjacent light emitting points such as a so-called multi-beam laser in which a plurality of light emitting points are provided on one chip.

  Therefore, in the present embodiment, the relative difference between the oscillation wavelengths of the two laser light sources 1a and 1b is limited so as to suppress the amount of positional deviation of the dots in the main scanning direction over the entire image height to a level that cannot be visually confirmed. A multi-beam scanning optical device is achieved. The scanning optical system used in this embodiment is the same as that in the first embodiment.

  That is, in a color image forming apparatus that outputs a halftone image, such as a color laser printer, if the allowable amount of dot misregistration is less than 1/4 of the size of one pixel, the image quality cannot be determined visually. There is no loss. Therefore, in this embodiment, each element is set so as to satisfy the following (Formula 6).

However,
ΔY ₁ : ΔY _L or ΔY _R in (Equation 2), whichever is greater
ΔY ₂ : Dot shift amount at the outermost part of the image height per unit wavelength
λ _a , λ _b : oscillation wavelengths of the laser light sources 1a and 1b
D: Size of one pixel (42.3 μm at 600 dpi)
When the design parameters of this embodiment are applied, D / 4 = 10 μm. Therefore, the oscillation wavelengths λ _a and λ _b of the laser light sources 1a and 1b are
−1 ≦ λ _b −λ _a ≦ 3 (unit: nm) (Formula 7)
If so, (Equation 6) can be satisfied.

  The description so far has been made using a two-beam laser scanning optical apparatus using two laser light sources. Next, consider an n-beam laser scanning optical apparatus that uses n laser light sources to form n scanning lines. In this case, it is easily understood that the relationship of (Equation 6) should be established for each of the n laser beams.

Therefore, in the n-beam laser scanning optical device, the oscillation wavelength of the light source that emits the laser beam that forms the first scanning line and the light source that emits the laser beam that forms the m-th (1 <m ≦ n) scanning line. Are λ ₁ and λ _m , respectively. At this time, the above (Formula 1), (Formula 2), and (Formula 6) can be rewritten as the following (Formula 8).

  As shown in FIG. 2, the first scanning line is formed at a position separated from the apex T on the surface of the photoconductor 8 by a distance S in the sub-scanning direction, and the mth (1 <m ≦ n) scanning line is It is formed at a position separated from the apex T on the surface of the photoconductor 8 by a distance (S + d) in the sub-scanning direction.

However,
ΔY ₂ : Dot shift amount at the outermost image height per unit wavelength,
D: the size of one pixel,
α: The laser beam incident on the photoreceptor surface and the normal to the photoreceptor surface are in the main scanning direction
The maximum angle to make,
R: radius of the photoreceptor Furthermore, (Equation 7) be replaced by the lambda ₂ to lambda _m,
−1 ≦ λ _m −λ ₁ ≦ 3 (unit: nm) (Formula 9)
It becomes. Each parameter R, S, d in the above (formula 8) has been described in the above (formula 1).

  In the present embodiment, by satisfying (Equation 8) and (Equation 9), in the n-beam laser scanning optical device, the allowable amount of dot misregistration is set to D / 4 of one pixel without impairing image quality. It is as follows.

  Thus, in this embodiment, each element is set so as to satisfy the conditional expressions (Expression 8) and (Expression 9) as described above. In other words, the dot positional deviation in the main scanning direction due to the optical path length difference of each laser beam and the positional deviation of the dot in the main scanning direction due to the wavelength difference between the laser light sources cancel each other. As a result, the amount of dot misregistration can be suppressed to the extent that it cannot be visually recognized over the entire image area, thereby obtaining a high-quality multi-beam scanning optical apparatus.

[ Reference Example 1 ]
6 is a sectional view (main scanning sectional view) of the principal part in the main scanning direction of the multi-beam scanning optical apparatus of Reference Example 1 of the present invention, and FIG. 7 is a sectional view of principal part in the sub scanning direction (sub scanning sectional view) in FIG. ). 6 and 7, the same elements as those shown in FIGS. 1 and 2 are denoted by the same reference numerals.

In the present reference example , the difference from the first embodiment is that the scanning optical means 31 is composed of first and second toric lenses (refractive optical elements) 61 and 62. Further, the lateral chromatic aberration caused by both the first and second toric lenses 61 and 62 is set to be uncorrected (undercorrected). Further, the oscillation wavelength of the light source that emits the laser beam that minimizes the angle in the sub-scanning direction formed by the plurality of laser beams incident on the photoreceptor surface and the normal line of the photoreceptor surface is the light source that emits the largest laser beam. This is a point set longer than the oscillation wavelength. Other configurations and optical actions are substantially the same as those of the first embodiment, and the same effects are obtained.

That is, in the figure, reference numeral 31 denotes scanning optical means having an fθ characteristic, which has first and second toric lenses 61 and 62. In this reference example , the chromatic aberration of magnification due to the two toric lenses 61 and 62 is set to be uncorrected (undercorrected). Here, the uncorrected lateral chromatic aberration means a state in which the magnification increases when the wavelength changes to the longer wavelength side with respect to the reference wavelength, and means that it is the same as the chromatic aberration that occurs in one normal convex lens.

FIG. 8 is an explanatory diagram showing the positional deviation of dots due to the optical path length difference in this reference example , and FIG. 9 shows the positional deviation of dots per unit wavelength (1 nm) due to the chromatic aberration of magnification of the scanning optical means 31 in this embodiment. It is explanatory drawing shown.

In this reference example , the positional deviation of the dots in the main scanning direction due to the optical path length difference of the laser beam is corrected by the positional deviation of the dots in the main scanning direction due to the relative wavelength difference. For this reason, the oscillation wavelength of the light source 41a that emits the laser beam 14a with the smallest angle in the sub-scanning direction formed by the plurality of laser beams incident on the surface of the photoconductor 8 and the normal line of the surface of the photoconductor 8 is specified. At this time, the oscillation wavelength of the light source 41a is set to be longer than the oscillation wavelength of the light source 41b that emits the laser beam 14b having the largest angle. More specifically,
Oscillation wavelength of laser light source 41a: 781 nm
Oscillation wavelength of laser light source 41b: 780 nm
It is set to. At this time, since the dot position shift due to the optical path length difference and the dot position shift due to the wavelength difference of the laser light source have different signs and the absolute values are almost equal, they almost cancel each other as shown in FIG. As a result, no dot displacement occurs in the entire image area.

In this reference example , the design parameter of (Equation 1) is
Photoconductor radius R = 15mm
Scan line spacing d = 21.2μm (1200dpi)
Laser beam shift amount S = 2.5mm
It is set to.

Table 2 shows the optical design parameters in this reference example . The expression of the shape of the optical element is the same as that in the first embodiment.

Thus, in the present reference example , the scanning optical means 31 is constituted by the two toric lenses 61 and 62 as described above. The dot positional deviation in the main scanning direction caused by the optical path length difference between the laser beams 14a and 14b and the dot positional deviation in the main scanning direction caused by the wavelength difference between the laser light sources 41a and 41b cancel each other. thing. As a result, the amount of dot misregistration can be almost eliminated over the entire image area, thereby obtaining a very high quality multi-beam scanning optical device.

[ Reference Example 2 ]
Next, Reference Example 2 of the present invention will be described.

In this reference example , the difference from the reference example 1 described above is that the relative difference between the wavelengths of the two laser light sources 41a and 41b is set so as to suppress the amount of dot misregistration in the main scanning direction over the entire image height to the extent that it cannot be visually recognized. It is limited. Other configurations and optical actions are substantially the same as those of Reference Example 1, and the same effects are obtained.

That is, in this reference example , the oscillation wavelength of the laser light source has a manufacturing error as in the second embodiment, and it is difficult to always manufacture in the best state. The oscillation wavelength of a semiconductor laser is determined by its composition, but varies depending on its manufacturing error.

Therefore, in this reference example , the relative difference between the oscillation wavelengths of the two laser light sources 41a and 41b is limited so that the amount of positional deviation of the dots in the main scanning direction over the entire image height is suppressed to a level that cannot be visually confirmed. A multi-beam scanning optical device is achieved.

That is, in this reference example , each element is set so as to satisfy the relational expression (Expression 10) for the dot displacement amount corresponding to the (Expression 8) being within the allowable range.

  The relative difference between the oscillation wavelengths of the two laser light sources 41a and 41b is set so as to satisfy the following (Formula 11).

−1 ≦ λ ₁ −λ _m ≦ 3 (unit: nm) (Formula 11)
Thus, in this reference example , each element is set so as to satisfy (Expression 10) and (Expression 11) as described above. Thus, the dot positional deviation in the main scanning direction due to the optical path length difference between the laser beams 14a and 14b and the dot positional deviation in the main scanning direction due to the wavelength difference between the laser light sources 41a and 41b cancel each other. Make a relationship. As a result, the amount of dot misregistration can be suppressed to the extent that it cannot be visually recognized over the entire image area, thereby obtaining a high-quality multi-beam scanning optical apparatus.

In each of the embodiments and the reference examples , the scanning optical unit is configured by a combination of a refractive optical element (toric lens) and a diffractive optical element, or two refractive optical elements, but is not limited thereto. For example, it may be composed of one or three or more refractive optical elements or three or more optical elements including a diffractive optical element.

[Image forming apparatus]
Next, an image forming apparatus applied to the present invention will be described.

FIG. 11 is a cross-sectional view of a main part in the sub-scanning direction showing an embodiment of an image forming apparatus (electrophotographic printer) using the multi-beam scanning optical device according to any one of Embodiments 1 and 2 described above.

  In FIG. 11, reference numeral 204 denotes an image forming apparatus. Code data Dc is input to the image forming apparatus 204 from an external device 117 such as a personal computer. The code data Dc is converted into image data (dot data) Di by a printer controller 211 in the apparatus. This image data Di is input to the optical scanning unit 200. The light scanning unit (multi-beam scanning optical device) 200 emits a light beam (light beam) 203 modulated in accordance with the image data Di, and the light beam 203 causes the photosensitive surface of the photosensitive member 201 to perform main scanning. Scanned in the direction.

  A photosensitive member 201 as an electrostatic latent image carrier (photosensitive member) is rotated clockwise by a motor 215. With this rotation, the photosensitive surface of the photoconductor 201 moves in the sub-scanning direction perpendicular to the main scanning direction with respect to the light beam 203. Above the photosensitive member 201, a charging roller 202 for uniformly charging the surface of the photosensitive member 201 is provided so as to contact the surface. The surface of the photoconductor 201 charged by the charging roller 202 is irradiated with a light beam 203 scanned by the light scanning unit 200.

  As described above, the light beam 203 is modulated based on the image data Di, and an electrostatic latent image is formed on the surface of the photoconductor 201 by irradiating the light beam 203. This electrostatic latent image is developed as a toner image by a developing device 207 disposed so as to be in contact with the photoconductor 201 further downstream in the rotation direction of the photoconductor 201 than the irradiation position of the light beam 203.

  The toner image developed by the developing device 207 is transferred onto a sheet 212 as a transfer material by a transfer roller 208 disposed below the photoconductor 201 so as to face the photoconductor 201. The paper 212 is stored in a paper cassette 209 in front of the photoconductor 201 (on the right side in FIG. 11), but can be fed manually. A paper feed roller 210 is disposed at the end of the paper cassette 209 and feeds the paper 212 in the paper cassette 209 to the transport path.

  As described above, the sheet 212 onto which the unfixed toner image has been transferred is further conveyed to a fixing device behind the photoconductor 201 (left side in FIG. 11). The fixing device includes a fixing roller 213 having a fixing heater (not shown) therein and a pressure roller 214 disposed so as to be in pressure contact with the fixing roller 213. Then, the unfixed toner image on the sheet 212 is fixed by heating the sheet 212 fed from the transfer unit while being pressed by the pressing portion between the fixing roller 213 and the pressure roller 214. Further, a paper discharge roller 216 is disposed behind the fixing roller 213 and discharges the fixed sheet 212 to the outside of the image forming apparatus.

  Although not shown in FIG. 11, the print controller 211 controls not only the data conversion described above but also each part in the image forming apparatus including the motor 215 and the polygon motor in the optical scanning unit 200. Do.

1 is a cross-sectional view of main parts in a main scanning direction of the multi-beam scanning optical apparatus according to Embodiment 1 of the present invention (main scanning cross-sectional view) FIG. 1 is a cross-sectional view of the main part in the sub-scanning direction (sub-scanning cross-sectional view). The figure explaining the positional offset amount of the dot resulting from the optical path length difference of Embodiment 1 of this invention. FIG. 6 is a diagram for explaining the amount of positional deviation of dots due to a wavelength difference according to the first embodiment of the present invention. The figure explaining the residual of the positional offset amount of the dot of Embodiment 1 of this invention. Sectional view in the main scanning direction of the multi-beam scanning optical apparatus of Reference Example 1 of the present invention (main scanning sectional view) FIG. 6 is a cross-sectional view of the main part in the sub-scanning direction (sub-scanning cross-sectional view). The figure explaining the positional offset amount of the dot resulting from the optical path length difference of the reference example 1 of this invention The figure explaining the positional offset amount of the dot resulting from the wavelength difference of the reference example 1 of this invention The figure explaining the residual of the positional offset amount of the dot of the reference example 1 of this invention Schematic diagram of main parts of the image forming apparatus of the present invention The figure explaining the influence on the image of the position shift of the dot of the main scanning direction The figure explaining the influence on the image of the position shift of the dot of the main scanning direction Main scanning sectional view of a conventional multi-beam scanning optical device Sub-scan sectional view of conventional multi-beam scanning optical device

Explanation of symbols

1,41 Light source means (semiconductor laser)
1a, 1b Laser light source 41a, 41b Laser light source 2 Collimator-lens 3 Cylindrical lens 4 Aperture stop 5 Deflection means (polygon mirror)
Reference Signs List 6 toric lens 7 diffractive optical element 8 photoconductor 9 synchronization detection lens 10 BD mirror 11 BD slit 12 BD lens 13 BD sensor 21 incident optical means 22, 31 scanning optical means 100 multi-beam scanning optical device 101 photoconductor 102 charging roller 103 light Beam 104 Image forming device 107 Developing device 108 Transfer roller 109 Paper cassette 110 Paper feed roller 112 Transfer material (paper)
113 Fixing roller 114 Pressure roller 116 Paper discharge roller

Claims (4)

A plurality of light sources, deflecting means for deflecting a plurality of laser light beams emitted from the plurality of light sources, and a refractive optical element and a diffractive optical element for imaging the plurality of laser light beams deflected by the deflecting means on a photosensitive member surface A multi-beam scanning optical device, wherein each of the plurality of laser light beams is incident on the photoconductor surface at a predetermined angle with a normal line of the photoconductor surface in a sub-scan section. ,
A plurality of laser light beams deflected by the deflecting unit are guided to a synchronization detecting element by a synchronization detecting lens without passing through the scanning optical unit, and a plurality of laser beams on the surface of the photosensitive member are used by using signals from the synchronization detecting element. Synchronization position detection means for controlling the timing of the scanning start position of image recording of the laser beam,
The synchronization detection lens is disposed in a face-to-face relationship with the optical path from the deflection means to the synchronization detection element; and
The scanning optical means is set so that lateral chromatic aberration is overcorrected using the refractive optical element and the diffractive optical element, and a plurality of laser light beams incident on the photoconductor surface and a normal line of the photoconductor surface A multi-beam scanning optical apparatus, wherein an oscillation wavelength of a light source that emits a laser beam having the smallest angle in the sub-scanning direction is shorter than an oscillation wavelength of a light source that emits the largest laser beam. 2. The multi-beam scanning optical apparatus according to claim 1, wherein the laser beam is three or more laser beams. The multi-beam scanning optical apparatus according to claim 1 or 2, and a developing unit that develops, as a toner image, an electrostatic latent image formed on the surface of the photosensitive member by a light beam scanned by the multi-beam scanning optical apparatus; An image forming apparatus comprising: transfer means for transferring a developed toner image to a transfer material; and fixing means for fixing the transferred toner image to the transfer material. 3. A multi-beam scanning optical apparatus according to claim 1; and a printer controller that converts code data input from an external device into image data and inputs the image data to the multi-beam scanning optical apparatus. Image forming apparatus.