JP4979444B2 - Optical scanning apparatus and image forming apparatus - Google Patents

Description

  The present invention relates to an optical scanning apparatus and an image forming apparatus, and more particularly to an optical scanning apparatus that scans a surface to be scanned with a light beam and an image forming apparatus including the optical scanning apparatus.

  In recent years, image forming apparatuses such as optical printers, digital copiers, and optical plotters are required to be low-cost, highly stable against temperature changes, high definition (high image quality), and high speed. . These image forming apparatuses also include an optical scanning device that scans the surface to be scanned with a light beam from a light source.

  A method using a plurality of light fluxes has been implemented for high definition and high speed of the image forming apparatus. There are mainly two methods for forming a plurality of luminous fluxes: (1) a method of combining a plurality of edge-emitting semiconductor laser elements, and (2) a method of using an edge-emitting semiconductor laser array in which a plurality of light emitting portions are formed on one substrate. . However, in the method (1), since a plurality of elements are mounted, the light source device is complicated, the number of parts is increased, and adjustment is complicated. Further, the method (2) has a disadvantage in that there are variations in wavelengths among a plurality of light emitting units, and adjustment cannot be made individually.

  By the way, recently, a light source using a surface emitting semiconductor laser element (Vertical Cavity Surface Emitting Laser, VCSEL) has been proposed. In this light source, it is easy to form several tens of light-emitting portions two-dimensionally on one substrate. Further, since each light emitting unit emits light by stable laser oscillation by single longitudinal mode oscillation, the wavelength variation between the light emitting units is extremely small. Hereinafter, a surface emitting semiconductor laser element in which a plurality of light emitting portions are formed on one substrate is also referred to as a “VCSEL array” for convenience.

  As a typical method for reducing the price of an image forming apparatus, there is resinization of an optical element used in an optical scanning apparatus. For example, if the materials of various lenses are resin materials, (1) weight reduction is possible, (2) low-cost molding is possible, and (3) specially shaped optical surfaces (hereinafter referred to as “special surfaces” for convenience) There is an advantage that it is easy to form. In particular, when a resin lens having a special surface is employed, the optical characteristics of the lens can be improved. As one of such special surfaces, there is a diffractive surface having a shape obtained by folding the shape of the refracting surface at an appropriate pitch. Since this diffractive surface can add more power than the refractive surface to the lens, the number of lenses constituting the optical system of the optical scanning device can be reduced.

  In addition, in order to realize an image forming apparatus that is highly stable with respect to temperature changes, in the optical scanning device, (A) a plurality of lenses having opposite powers are combined to correct optical characteristic deterioration due to temperature changes. And (B) a method for correcting deterioration of optical characteristics due to a temperature change using negative dispersion of a diffraction surface (for example, see Patent Documents 2 to 5). In particular, the method using a diffractive surface is an effective method for realizing an optical scanning device with a small number of parts, a low cost, and a high stability with respect to a temperature change as the molding technology of a resin material is advanced. .

  For example, Patent Document 1 discloses a first optical system for coupling a light beam from a light source, a second optical system for collecting the light beam from the first optical system in a substantially linear shape that is long in the main scanning direction, A deflected light beam deflected by an optical deflector having a deflecting / reflecting surface in the vicinity of the substantially linear condensing portion and deflecting the light beam by the deflecting / reflecting surface is condensed toward the surface to be scanned, and A third optical system that forms a light spot for optical scanning on the scanning surface, the third optical system includes one or more resin imaging elements, and the second optical system includes: It has one or more resin imaging elements and one or more glass imaging elements, and at least one surface of the second optical system is independently shaped in the main scanning direction and the sub-scanning direction. A sub-non-arc surface having a non-arc shape in the sub-scanning cross section and having at least one sub-non-arc surface. However, among the imaging elements of the second optical system, the scanning imaging optics characterized in that the imaging optical element through which the light beam from the first optical system side passes with the maximum light beam diameter in the sub-scanning direction is formed A system is disclosed. In this scanning imaging optical system, focus position variation due to temperature is corrected by combining at least three lenses in the optical system before the deflector without using a diffraction surface.

  In Patent Document 2, a plurality of scanning optical devices having a light source means including a semiconductor laser and an optical element including a refracting portion and a diffractive portion are provided, and a plurality of light beams emitted from the respective scanning optical devices are simply used. A color image forming apparatus that guides light to different areas on the surface of one image carrier and scans the surface of the image carrier with the plurality of light beams to form a color image. An aberration change in the main scanning direction on the surface of the image carrier due to an environmental change of the scanning optical device is corrected by a power change between the refractive part and the diffractive part of the optical element and a wavelength change of the semiconductor laser. A color image forming apparatus is disclosed.

  In Patent Document 3, a light beam emitted from a light source means made of a semiconductor laser is guided to a deflecting surface of a deflecting element via an anamorphic optical element having a diffractive portion, and the light beam deflected by the deflecting element is refraction unit. A scanning optical device that forms an image on a surface to be scanned through a scanning optical element and scans the surface to be scanned, and the focus change in the main scanning direction due to the temperature variation of the refractive portion of the scanning optical device. There is disclosed a scanning optical device which is corrected by a power change of a diffraction part due to a wavelength variation of the light source means due to the temperature variation.

  Patent Document 4 discloses a light source unit, an optical unit that guides a light beam from the light source unit to a light deflecting unit, and an imaging optical system that guides a light beam from the light deflecting unit to a scanned surface. In the optical scanning device that optically scans the surface to be scanned based on the rotation operation of the light deflecting means, the optical means has a diffractive portion on one or more surfaces and satisfies a predetermined condition. Optical scanning device characterized in that

  Patent Document 5 discloses a scanning optical device in which a laser beam emitted from a semiconductor laser is deflected by a deflector, and the beam is scanned on an image carrier by a refracting element and a diffractive element to form an image. The change in the imaging position in the main scanning direction on the image carrier surface due to the environment fluctuation of the apparatus is corrected by the power change between the refractive element and the diffraction element in the scanning optical apparatus and the wavelength fluctuation of the semiconductor laser, and the light flux There is disclosed a scanning optical device using a surface emitting semiconductor laser as the light source.

Japanese Patent No. 3484141 Japanese Patent Application Laid-Open No. 11-223783 JP 2003-337295 A JP 2004-126192 A JP 2005-215188 A

  However, even if environmental fluctuations are taken into consideration as in Patent Documents 1 to 5, the image quality may be deteriorated when used for a long time.

  The present invention has been made under such circumstances, and a first object of the present invention is to provide an optical scanning device capable of performing stable optical scanning without increasing the cost.

  A second object of the present invention is to provide an image forming apparatus capable of stably forming a high-quality image without incurring an increase in cost.

According to a first aspect of the present invention, a light source having a surface emitting laser array in which a plurality of light emitting units are two-dimensionally arranged, and a deflector disposed on an optical path of a plurality of light beams emitted from the light source. A pre-optical system, a deflector for deflecting a plurality of light beams from the pre-deflector optical system, and a scanning optical system for guiding the plurality of light beams deflected by the deflector to a surface to be scanned. optics have at least one diffractive surface linear groove is formed extending in a direction deviation of the wavelength is larger in the plurality of light beams incident, said at least one diffractive surface, the wavelength of the plurality of light beams incident In a direction in which the deviation of the light source is small, the power change due to the environmental change of the optical surface and the change due to the environmental change of the oscillation wavelength of the light source in all optical systems including the pre-deflector optical system and the scanning optical system By The generated optical waist position correction on the scanned surface is corrected, and the entire optical system has a lateral magnification in a direction in which the wavelength deviation is large in the incident light beams, and a lateral magnification in a direction in which the wavelength deviation is small. It is an optical scanning device characterized by being smaller than the above.

According to this, stable optical scanning can be performed without increasing the cost.

  According to a second aspect of the present invention, there is provided at least one image carrier; and at least one optical scanning device according to the invention that scans a light beam including image information on the at least one image carrier. An image forming apparatus provided.

  According to this, since at least one optical scanning device according to the present invention is provided, as a result, it is possible to stably form a high-quality image without increasing the cost.

  Hereinafter, an embodiment of the present invention will be described with reference to FIGS. FIG. 1 shows a schematic configuration of a laser printer 500 as an image forming apparatus according to an embodiment of the present invention.

  The laser printer 500 includes a photosensitive drum 511, a charging roller 512, a developing device 513, a transfer roller 514, a cleaning device 515, a fixing device 516, an optical scanning device 900, a cassette 518, a registration roller pair 519, a paper feed roller 520, a discharge roller A paper roller pair 522 and a tray 523 are provided.

  The photosensitive drum 511 is an image carrier, and a photosensitive layer having photoconductivity is formed on the surface thereof. That is, the surface of the photosensitive drum 511 is a surface to be scanned.

  The charging roller 512, the developing device 513, the transfer roller 514, and the cleaning device 515 are each disposed near the surface of the photosensitive drum 511. The charging roller 512, the developing device 513, the transfer roller 514, and the cleaning device 515 are arranged in this order along the rotation direction of the photosensitive drum 511 (the arrow direction in FIG. 1).

  The charging roller 512 is a charging unit that uniformly charges the surface of the photosensitive drum 511. A “corona charger” can also be used as the charging means.

  The optical scanning device 900 scans the surface of the photosensitive drum 511 charged by the charging roller 512 with light LB modulated based on image information from a host device (for example, a personal computer). By the optical scanning by the optical scanning device 900, the charge disappears only on the surface irradiated with light on the surface of the photosensitive drum 511, and a latent image (electrostatic latent image) corresponding to the image information is formed. This latent image is a so-called negative latent image and moves in the direction of the developing device 513 as the photosensitive drum 511 rotates. The configuration of the optical scanning device 900 will be described later.

  The developing device 513 includes a toner cartridge in which toner is stored, and causes the toner to adhere only to a portion irradiated with light on the surface of the photosensitive drum 511. That is, the developing device 513 makes the image information visible by attaching toner to the latent image formed on the surface of the photosensitive drum 511. Here, the latent image to which the toner is attached (hereinafter also referred to as “toner image” for convenience) moves in the direction of the transfer roller 514 as the photosensitive drum 511 rotates.

  The cassette 518 can be attached to and detached from the main body of the laser printer 500, and the transfer paper P as a transfer object is accommodated therein. A paper feed roller 520 is disposed in the vicinity of the cassette 518, and the paper feed roller 520 takes out the uppermost sheet of the transfer paper P stored in the cassette 518.

  The registration roller pair 519 is disposed in the vicinity of the transfer roller 514 and captures the leading end portion of the transfer paper taken out by the paper supply roller 520. The registration roller pair 519 feeds the transfer paper into the gap between the transfer roller 514 and the photosensitive drum 511 in accordance with the timing at which the toner image on the photosensitive drum 511 moves to the transfer position. The transferred transfer paper is superimposed on the toner image by the transfer roller 514, and the toner image is electrostatically transferred.

  The transfer paper onto which the toner image has been transferred is sent to the fixing device 516, where the toner image is fixed by the fixing device 516, passes through the conveyance path 521, and is discharged onto the tray 523 by the paper discharge roller pair 522.

  The surface of the photosensitive drum 511 after the toner image is transferred is cleaned by a cleaning device 515 to remove residual toner, paper dust, and the like.

  Next, the configuration of the optical scanning device 900 will be described.

  As shown in FIG. 2, the optical scanning device 900 includes a light source 1, a coupling lens 2, an aperture plate 3, a line image forming lens 4, a polygon mirror 5, two scanning lenses (6 a and 6 b), and a bending mirror 7. , A synchronization mirror 9, a synchronization lens 10, a synchronization detection sensor 11, and the like.

  As shown in FIG. 3 as an example, the light source 1 includes a VCSEL array 100 in which 40 light emitting units are formed on one substrate.

  The VCSEL array 100 has an inclination angle from a direction corresponding to the main scanning direction (hereinafter also referred to as “M direction” for convenience) to a direction corresponding to the sub-scanning direction (hereinafter also referred to as “S direction” for convenience). There are four light emitting part rows in which ten light emitting parts are arranged at equal intervals along the direction forming α (hereinafter referred to as “T direction” for convenience). These four light emitting unit rows are arranged at equal intervals in the S direction. That is, the 40 light emitting units are two-dimensionally arranged along the T direction and the S direction. In the present specification, the “light emitting portion interval” refers to the distance between the centers of two light emitting portions. The traveling direction of the light beam that is orthogonal to both the M direction and the S direction and travels from the light source 1 toward the polygon mirror 5 is hereinafter referred to as a “W direction” for convenience.

  Here, as an example, in the light emitting part row, the distance d1 between the light emitting parts in the T direction is 24.0 μm, the distance ds2 between the light emitting part rows in the S direction is 24.0 μm, and the inclination angle α is 5.74 °. . Therefore, the intervals between the light emitting portions when the 40 light emitting portions are orthogonally projected onto the virtual line extending in the S direction are equal intervals, and the value ds1 is 2.4 μm. And the space | interval dm of the light emission part regarding M direction is 23.9 micrometers.

  The interval between the two light emitting portions located at both ends in the M direction is 23.9 × 9 = 25.1 μm, and the interval between the two light emitting portions located at both ends in the S direction is 2.4 × 39 = 93.6 μm. is there. That is, in the plurality of light emitting units, the interval between the two light emitting units located at both ends in the S direction is smaller than the interval between the two light emitting units located at both ends in the M direction.

  As an example, each light emitting unit has a design oscillation wavelength of 780 nm, and when the temperature rises by 1 ° C. with respect to the reference temperature (25 ° C.), the oscillation wavelength shifts to the longer wavelength side by 0.062 nm. is doing.

  By the way, generally, when the VCSEL array continues to emit light for a long time, a non-uniform temperature distribution is generated in the VCSEL array even if the environmental temperature is constant. And the wavelength of the light beam emitted from each light emitting part varies. In the present specification, the degree of variation in the wavelength of the light beam emitted from each light emitting unit is referred to as “wavelength deviation”. Here, as an example, in the VCSEL array 100 when used for a long time, the wavelength deviation in the M direction is 4 nm, and the wavelength deviation in the S direction is 1 nm.

  The coupling lens 2 has a focal length of about 45 mm, and the light beam emitted from the light source 1 is substantially parallel light.

For example, the coupling lens 2 includes an incident surface having a paraxial radius of curvature of 98.97 mm in the direction corresponding to the main scanning direction and a paraxial radius of curvature of 98.97 mm in the direction corresponding to the sub-scanning direction, and the main scanning direction. Is a glass lens having an exit surface with a paraxial radius of curvature of -31.07 mm in the direction corresponding to, and a paraxial radius of curvature in the direction corresponding to the sub-scanning direction of -31.07 mm. As an example, this glass has a refractive index of 1.511913 at a reference temperature for light of 780 nm, a refractive index of 1.511934 when the temperature rises by 20 ° C. from the reference temperature, and a linear expansion coefficient of 7. It is a glass having physical properties of 5 × 10 ⁻⁶ / K.

  The coupling lens 2 has both an entrance surface and an exit surface that are aspheric, and sufficiently corrects the wavefront aberration of the coupled light beam.

The light source 1 and the coupling lens 2 are held by a holding member (not shown) using a material having a linear expansion coefficient of 2.3 × 10 ⁻⁵ / K.

  As an example, the aperture plate 3 has a rectangular aperture having a width in the direction corresponding to the main scanning direction of 6.4 mm and a width in the direction corresponding to the sub-scanning direction of 1.18 mm. The beam flux is shaped to determine the beam spot diameter on the photosensitive drum 511.

  In this specification, the “beam spot diameter” is defined using a line spread function in the light intensity distribution of the beam spot. If the light intensity distribution of the beam spot at the coordinate Y in the direction corresponding to the main scanning direction and the coordinate Z in the direction corresponding to the sub-scanning direction is defined as f (Y, Z) with the center of the beam spot as a reference, The line spread function LSZ (Z) in the corresponding direction is expressed by the following equation (1). The integration is performed for the entire width of the beam spot in the direction corresponding to the main scanning direction.

  The line spread function LSY (Y) in the direction corresponding to the main scanning direction is expressed by the following equation (2). The integration is performed for the entire width of the beam spot in the direction corresponding to the sub-scanning direction.

The line spread function LSZ (Z) and the line spread function LSY (Y) are generally of a Gaussian distribution type, and the beam spot diameters in the direction corresponding to the main scanning direction and the direction corresponding to the sub-scanning direction are The line spread function is given by the width in the direction corresponding to the main scanning direction and the direction corresponding to the sub-scanning direction of an area that is 1 / e ² or more of the maximum value.

  The beam spot diameter defined above by the line spread function is easily measured by scanning the beam spot at a constant speed with a slit, receiving the light passing through the slit with a photodetector, and integrating the amount of light received. Devices that perform such measurements are also commercially available.

  The line image forming lens 4 forms an image of the light beam that has passed through the opening of the aperture plate 3 in the vicinity of the deflection reflection surface of the polygon mirror 5 in the sub-scanning direction.

The line image forming lens 4 is a resin lens. As an example, this resin has a refractive index of 1.523961 at a reference temperature with respect to light of 780 nm, a refractive index of 1.522188 when the temperature rises by 20 ° C. from the reference temperature, and a linear expansion coefficient of 7. It is a resin having physical properties of 0 × 10 ⁻⁵ / K.

  As shown in FIG. 4 as an example, the incident surface of the line image forming lens 4 has a paraxial radius of curvature in the direction corresponding to the main scanning direction ∞ and a paraxial radius of curvature in the direction corresponding to the sub-scanning direction of 118. It is a 5 mm cylindrical surface. That is, the incident surface of the line image forming lens 4 is a refractive surface.

  As shown in FIG. 5 as an example, the exit surface of the line image forming lens 4 is a so-called diffractive surface in which a linear groove substantially parallel to a direction corresponding to the main scanning direction is formed. That is, a linear groove extending in the direction in which the wavelength deviation is large is formed.

  The phase function φ (Y, Z) of this diffractive surface is expressed by the following equation (3).

φ (Y, Z) = C1 · Y ² + C2 · Z ² (3)

  Here, C1 = 0 and C2 = −0.014084973.

  The diffractive surface has a surface shape in which a surface having a diffraction effect (hereinafter also referred to as “first surface” for convenience) and a surface having a refraction effect (hereinafter also referred to as “second surface” for convenience) are combined. is doing.

  As an example, as shown in FIG. 6, the shape 13 of the first surface is a shape obtained by folding back the shape 12 of the surface having a refractive effect with an appropriate step and pitch. Accordingly, the first surface has power.

  The shape of the second surface is a shape that cancels the power of the first surface. That is, the diffraction surface of the line image forming lens 4 is non-powered in both the direction corresponding to the main scanning direction and the direction corresponding to the sub-scanning direction.

  Here, as an example, the shape of the first surface is a shape in which the shape of the cylindrical surface having a paraxial radius of curvature of 18.6 mm in the direction corresponding to the sub-scan is turned back with an appropriate step and pitch, The shape of the surface is that of a cylindrical surface having a paraxial radius of curvature of −18.6 mm in the direction corresponding to sub-scanning.

  As a result, the plurality of diffraction grooves on the diffraction surface have a multi-step shape.

  By the way, if the diffractive surface is formed by folding the surface shape of the refracting surface with an appropriate step and pitch, the pitch gradually decreases toward the periphery of the lens, so the mold for forming the diffractive surface is manufactured. Is difficult. However, if a diffractive surface is created by combining the first surface and the second surface having opposite powers, the folded portion on the diffractive surface becomes obtuse, which is advantageous for mold manufacture. In particular, as in this embodiment, when the surface shape of the diffractive surface is a multi-step shape, as shown in FIG. 7 as an example, the angle of the folded portion becomes a right angle, a step-like shape symmetrical to the optical axis, Convenience in mold manufacture is further improved.

  The optical system arranged on the optical path between the light source 1 and the polygon mirror 5 is also called a pre-deflector optical system. In the present embodiment, the pre-deflector optical system includes a coupling lens 2, an aperture plate 3, and a line image forming lens 4.

  Returning to FIG. 2, the polygon mirror 5 includes, for example, a tetrahedral mirror having an inscribed circle radius of 7 mm, and each mirror is a deflecting reflection surface. The polygon mirror 5 is surrounded by soundproof glass (not shown) having a thickness of 1.9 mm. The glass of the soundproof glass is the same as the glass of the coupling lens 2.

  The scanning lens 6a has an incident surface having a paraxial radius of curvature of −110.142 mm in the main scanning direction and a paraxial radius of curvature of −472.788 mm in the sub-scanning direction, and a paraxial radius of curvature in the main scanning direction of −57.939 mm. The exit surface has a paraxial radius of curvature in the sub-scanning direction of −500 mm.

  The scanning lens 6b has an incident surface with a paraxial radius of curvature in the main scanning direction of −5000 mm and a paraxial radius of curvature in the sub scanning direction of 93.8 mm, a paraxial radius of curvature in the main scanning direction of 724.16 mm, and the sub scanning direction. Has an exit surface with a paraxial radius of curvature of −60.71 mm.

  The scanning lens 6a and the scanning lens 6b are both resin lenses. This resin is a resin having the same physical properties as the material of the line image forming lens 4.

  Further, each surface of the scanning lens 6a and the scanning lens 6b is an aspheric surface, and each surface has a non-arc shape expressed by the following equation (4) in the main scanning direction, and the optical axis and the sub-axis. This is a surface in which the curvature of a section parallel to a virtual plane including the scanning direction (hereinafter referred to as “sub-scanning section”) changes in the main scanning direction according to the following equation (5).

Here, X is the depth in the optical axis direction, R _m is the paraxial radius of curvature in the main scanning direction, Y is the distance in the main scanning direction from the optical axis, K is the conic constant, A ₁ , A ₂ , A ₃ , A ₄ , A ₅ ,... _Are coefficients, R _s0 is the paraxial radius of curvature in the sub-scanning direction when Y = 0, and B ₁ , B ₂ , B ₃ ,.

  Table 1 shows conical constants and coefficients of the incident surface of the scanning lens 6a.

  Table 2 shows the conic constant and coefficient of the exit surface of the scanning lens 6a.

  Table 3 shows conical constants and coefficients of the incident surface of the scanning lens 6b.

  Table 4 shows the conical constants and coefficients of the exit surface of the scanning lens 6b.

  The bending mirror 7 bends the optical path of the light beam through the scanning lens 6 b toward the surface of the photosensitive drum 511. Thereby, a beam spot is formed on the surface of the photosensitive drum 511. This beam spot moves in the longitudinal direction of the photosensitive drum 511 as the polygon mirror 5 rotates. That is, the photosensitive drum 511 is scanned. The moving direction of the beam spot at this time is the main scanning direction.

  An optical system disposed on the optical path between the polygon mirror 5 and the photosensitive drum 511 is also called a scanning optical system. In the present embodiment, the scanning optical system includes a scanning lens 6a, a scanning lens 6b, and a bending mirror 7. A dust-proof glass (not shown) having a thickness of 1.9 mm is disposed between the bending mirror 7 and the photosensitive drum 511. The dustproof glass is made of glass having the same physical properties as the soundproof glass.

  By the way, the positional relationship of each optical element is shown in FIG. Table 5 shows an example of specific values (unit: mm) of the symbols d1 to d11 in FIG.

  Further, an angle formed by the T direction and the traveling direction of the light beam reflected toward the position of the image height 0 on the surface of the photosensitive drum 511 (position p0 in FIG. 8) by the deflection reflection surface of the polygon mirror 5. (Θ in FIG. 8) is 59 degrees.

  Returning to FIG. 2, a part of the light beam deflected by the polygon mirror 5 and going outside the effective scanning area is received by the synchronization detection sensor 11 via the synchronization mirror 9 and the synchronization lens 10. The synchronization detection sensor 11 outputs a signal (photoelectric conversion signal) corresponding to the amount of received light. Based on the output signal of the synchronization detection sensor 11, the timing for starting scanning is determined.

  In the laser printer 500 according to this embodiment, the deviation amount of the beam waist position in the main scanning direction when used for a long time is 0.407 mm, and the deviation amount of the beam waist position in the sub scanning direction is 0.343 mm. Met.

  When the wavelength deviation is small and the environmental temperature is + 20 ° C. with respect to the reference temperature, the deviation amount of the beam waist position in the main scanning direction is 0.152 mm, and the deviation amount of the beam waist position in the sub scanning direction is 0.038 mm.

  By the way, when a line image forming lens having a diffractive surface in which circular or elliptical grooves are formed instead of the line image forming lens 4, the beam waist position in the main scanning direction when used for a long time is used. The amount of deviation was -1.075 mm. On the other hand, when the wavelength deviation was small and the environmental temperature was + 20 ° C. with respect to the reference temperature, the deviation amount of the beam waist position in the main scanning direction was −0.007 mm.

  As described above, in the main scanning direction, by using the line image forming lens 4, the deviation amount of the beam waist position due to the wavelength deviation is greatly reduced while keeping the deviation amount of the beam waist position due to the environmental temperature low. be able to.

  When a line image forming lens having no diffractive surface is used in place of the line image forming lens 4, the deviation amount of the beam waist position in the sub-scanning direction when used for a long time is 0.139 mm. . In this case, when the wavelength deviation was small and the environmental temperature was + 20 ° C. with respect to the reference temperature, the deviation amount of the beam waist position in the sub-scanning direction was 1.414 mm.

  As described above, in the sub-scanning direction, by using the line image forming lens 4, the beam waist position shift due to the environmental temperature is suppressed while keeping the beam waist position shift amount due to the wavelength deviation within an allowable range. The amount can be reduced. That is, the optical characteristic which absorbs a temperature change can be provided to the optical system, and the robustness of the optical system is improved.

  As described above, by using the line image forming lens 4, the deviation amount of the beam waist position due to the environmental temperature and the deviation amount of the beam waist position caused by the wavelength deviation can be suppressed within a predetermined allowable range. it can.

  As described above, the optical scanning apparatus 900 according to the present embodiment includes the light source 1, the coupling lens 2, the line image forming lens 4, the polygon mirror 5, and the scanning optical system, and the line image forming lens 4 is emitted. The surface is a diffractive surface in which a linear groove extending in a direction in which the wavelength deviation is large is formed. Thereby, it is possible to selectively give a diffraction effect that reduces both the influence of the environmental temperature and the influence of the wavelength deviation on the beam waist position, and the beam can be applied in both the main scanning direction and the sub-scanning direction. The amount of deviation of the waist position can be reduced. Therefore, as a result, stable optical scanning can be performed without increasing the cost.

  By the way, when the diffractive surface is formed of resin, a molding die (or metal piece) in which the shape of the diffractive surface and the concavity and convexity (hereinafter referred to as “transfer shape” for convenience) are formed by cutting. Is used. According to the present embodiment, the power of the first surface and the power of the second surface are offset from each other on the diffractive surface of the line image forming lens 4. As a result, the folded portion of the diffractive surface has an obtuse angle, and it becomes easy to manufacture a mold (or metal piece) for forming the diffractive surface.

  Further, according to the present embodiment, the shape of the diffractive surface of the line image forming lens 4 is a linear groove shape, and as an example, as shown in FIG. The transfer shape can be formed on the mold (or the metal piece), and there is no problem in releasing the cutting tool. If an attempt is made to have a diffraction effect in both the direction corresponding to the main scanning direction and the direction corresponding to the sub-scanning direction, an elliptical groove centered on the optical axis is inevitably considered in consideration of the difference in magnification. However, it takes time and effort to put out the shaft, and it is also necessary to devise a tool for the escape of the cutting tool, so that high machining accuracy is hardly desired (see FIG. 10).

  Further, according to the present embodiment, the line image forming lens 4 has a refracting surface and a diffractive surface, the surface on the light source 1 side is a refracting surface, and the surface on the polygon mirror 5 side is a diffractive surface. Thereby, scattered light, reflected light, and unnecessary-order diffracted light are prevented from returning to the light source. If the diffractive surface is provided on the light source 1 side, it has an incident surface perpendicular to the optical axis of the incident light beam, so that the incident light beam is strongly reflected on the incident surface, which returns to the light source. , May induce interference.

  Further, according to the present embodiment, since the light source 1 has a plurality of light emitting units, a plurality of scans can be performed simultaneously. Therefore, stable and high-density optical scanning can be performed without increasing the cost.

  Further, according to the present embodiment, a highly stable optical scanning device can be realized without increasing the number of parts. For this reason, it is not necessary to increase the amount of material used for the production of the optical scanning device, and as a result, it is possible to suppress an increase in environmental load with respect to the resource mining amount and the plastic waste discharge amount.

  In addition, the laser printer 500 according to the present embodiment includes the optical scanning device 900 that can perform stable optical scanning without incurring high costs. As a result, high quality without incurring high costs. An image can be formed stably.

  In addition, according to the present embodiment, since the light source 1 has a plurality of light emitting units, a plurality of scans can be performed simultaneously, and the speed of image formation can be increased.

  In the above embodiment, the case where the line image forming lens 4 does not have a compensation function for fluctuations in environmental temperature has been described. However, for example, when the temperature compensation in another optical system is not sufficient, FIG. 11, the shape of the incident surface of the line image forming lens 4 may be concave with respect to the direction corresponding to the main scanning direction, and a temperature compensation function may be added. In this case, the incident surface is a so-called toric surface in which a concave surface is added to the cylindrical surface. Further, instead of the line image forming lens 4, a convex lens and a concave lens may be combined to add a temperature compensation function.

  Moreover, although the said embodiment demonstrated the case where the space | interval of the two light emission parts located in both ends regarding S direction was smaller than the space | interval of the two light emission parts located in both ends regarding M direction, on the contrary, as an example As shown in FIG. 12, the interval between the two light emitting units located at both ends in the M direction may be smaller than the interval between the two light emitting units located at both ends in the S direction. In this case, if the wavelength deviation in the S direction is larger than the wavelength deviation in the M direction, a linear groove substantially parallel to the direction corresponding to the sub-scanning direction is formed instead of the line image forming lens 4. A line image forming lens having a diffractive surface formed is used.

  In the above embodiment, the case where the scanning optical system has two scanning lenses has been described. However, the present invention is not limited to this, and the scanning optical system may have only one scanning lens. There may be three or more scanning lenses.

  In the above-described embodiment, the case where the line image forming lens 4 has a diffractive surface in which a linear groove extending in a direction in which the wavelength deviation is large is described. However, the present invention is not limited to this. The optical system disposed on the optical path between the light source 1 and the polygon mirror 5 only needs to have a diffractive surface in which a linear groove extending in a direction in which the wavelength deviation is large is formed. In general, the error of the coupling optical system included in the pre-deflector optical system is the largest due to the magnification and affects the beam spot on the scanned surface. Therefore, when the error of the coupling optical system is large, it is preferable that the light beam after coupling is incident on the diffraction surface.

  In the above embodiment, the case of the direct transfer method in which the transfer of the toner image from the photosensitive drum 511 to the transfer paper is directly performed from the photosensitive drum 511 to the transfer paper has been described. An intermediate transfer method may be used in which the image is once transferred to an intermediate transfer medium such as an intermediate transfer belt and then transferred from the intermediate transfer medium to a transfer sheet.

  In the above-described embodiment, the case where the image carrier has a drum shape has been described. However, the image carrier is not limited to this and may be a sheet shape or a belt shape. For example, zinc oxide paper may be used as the sheet-like photoconductive photoreceptor.

  In the above embodiment, the case of the laser printer 500 as the image forming apparatus has been described. However, the present invention is not limited to this, and may be, for example, an optical plotter or a digital copying apparatus.

  Further, an image forming apparatus using a silver salt film as the image carrier may be used. In this case, a latent image is formed on the silver salt film by optical scanning, and this latent image can be visualized by a process equivalent to a developing process in a normal silver salt photographic process. Then, it can be transferred to a photographic paper as a transfer object by a process equivalent to a printing process in a normal silver salt photographic process. Such an image forming apparatus can be implemented as an optical plate making apparatus or an optical drawing apparatus that draws a CT scan image or the like.

  Further, an image forming apparatus using a color developing medium (positive photographic paper) that develops color by the heat energy of a beam spot as an image carrier may be used. In this case, a visible image can be directly formed on the image carrier by optical scanning.

  In short, the image forming apparatus provided with the optical scanning device 900 can stably form a high-quality image without increasing the cost as a result.

  Further, even in an image forming apparatus that forms a color image, it is possible to stably form a high-quality image without incurring high costs by using an optical scanning device corresponding to the color image. Become.

  As an example, a tandem color machine capable of forming a color image is shown in FIG. This tandem color machine includes a black photosensitive drum K1, a charging device K2, a developing device K4, a cleaning device K5, a transfer charging device K6, a cyan photosensitive drum C1, a charging device C2, a developing device C4, Cleaning means C5, transfer charging means C6, magenta photosensitive drum M1, charging device M2, developing device M4, cleaning means M5, transfer charging means M6, yellow photosensitive drum Y1, charging device Y2, a developing unit Y4, a cleaning unit Y5, a transfer charging unit Y6, an optical scanning device 900A, a transfer belt 80, a fixing unit 30 and the like.

  The optical scanning device 900A has a pre-deflector optical system similar to the pre-deflector optical system of the optical scanning device 900.

  Corresponding chargers, developing units, transfer charging units, and cleaning units are arranged around the photosensitive drums along the rotation direction of the photosensitive drums (in the direction of the arrows in FIG. 13). Each charger uniformly charges the surface of the corresponding photosensitive drum. The surface of the photosensitive drum charged by the charger is irradiated with a light beam by the optical scanning device 900A, and an electrostatic latent image is formed on the photosensitive drum. Then, a toner image is formed on the surface of the photosensitive drum by the corresponding developing device. Further, the toner images of the respective colors are transferred onto the recording paper by the corresponding transfer charging means, and finally the image is fixed on the recording paper by the fixing means 30. That is, a color image or a multicolor image can be synthetically obtained by transferring and fixing toner images of respective colors onto the same sheet-like recording medium.

  For example, when the optical scanning device 900A has a light source for black (light source K), a light source for cyan (light source C), a light source for magenta (light source M), and a light source for yellow (light source Y), the polygon mirror May be common to all colors. As a result, the number of relatively expensive polygon mirrors can be reduced, and cost reduction can be promoted. Further, since the polygon mirror is the largest heat source in the optical scanning device, the temperature rise of the optical scanning device can be suppressed by reducing the number of polygon mirrors. Further, by suppressing the temperature rise of the optical scanning device, it is possible to reduce unwanted mode hops in the surface emitting laser. In this case, the configuration of the optical scanning device includes a method in which the light beams of each color enter substantially symmetrically with respect to the sub-scan section including the rotation axis of the polygon mirror (so-called “opposite scanning method”), There is a method of deflecting by the same reflecting surface of the mirror (so-called “one-side scanning method”).

  FIG. 14 shows a configuration example of the counter scanning method using the polygon mirror 5 having two deflection reflection surfaces. Here, the light beam from the light source K and the light beam from the light source Y are respectively deflected by the upper deflection reflection surface, and the light beam from the light source C and the light beam from the light source M are respectively deflected by the lower deflection reflection surface. . In FIG. 14, reference numeral 6K is a scanning lens for black, reference numeral 6C is a scanning lens for cyan, reference numeral 6M is a scanning lens for magenta, and reference numeral 6Y is a scanning lens for yellow.

  Further, the scanning method is not limited to the form in which the light beam from the light source is incident in parallel to the normal line of the deflection reflection surface of the polygon mirror. The light beam from the light source may be incident at an angle with respect to the normal line of the deflection reflection surface of the polygon mirror. In this specification, this method is referred to as “oblique incidence method”. FIG. 15 shows a configuration example of a grazing incidence method using a counter scanning method. Here, the light beam from the light source K is deflected by the polygon mirror 5 and condensed on the photosensitive drum K1 via the scanning lenses 6a1 and 6bK. The light beam from the light source C is deflected by the polygon mirror 5 and condensed on the photosensitive drum C1 through the scanning lenses 6a1 and 6bC. The light beam from the light source M is deflected by the polygon mirror 5 and condensed on the photosensitive drum M1 through the scanning lenses 6a2 and 6bM. The light beam from the light source Y is deflected by the polygon mirror 5 and condensed on the photosensitive drum Y1 through the scanning lenses 6a2 and 6bY. The advantage of the oblique incidence method is that the deflecting light can be deflected from four light sources with a polygon mirror having a single stage of deflecting reflection surface, so that the cost can be further reduced. However, when the oblique incidence method is applied, a scanning lens corresponding to the oblique incidence method needs to be used because the scanning line is bent and the wavefront aberration is deteriorated.

  As described above, the optical scanning device of the present invention is suitable for performing optical scanning stably without increasing the cost. The image forming apparatus according to the present invention is suitable for stably forming a high-quality image without increasing the cost.

It is a figure for demonstrating schematic structure of the laser printer which concerns on one Embodiment of this invention. It is a perspective view which shows schematic structure of the optical scanning device in FIG. It is a figure for demonstrating the VCSEL array of the light source in FIG. It is a figure for demonstrating the output surface of the line image formation lens in FIG. It is a figure for demonstrating the entrance plane of the line image formation lens in FIG. It is FIG. (1) for demonstrating a diffraction surface. It is FIG. (2) for demonstrating a diffraction surface. It is a figure for demonstrating the positional relationship of the principal part in the optical scanning device of FIG. It is a figure for demonstrating the manufacturing method of the metal mold | die (or metal piece) for shape | molding a diffraction surface. It is a figure for demonstrating the manufacturing method (comparative example) of the metal mold | die (or metal piece) for shape | molding a diffraction surface. It is a figure for demonstrating the modification of the entrance plane of the line image formation lens in FIG. It is a figure for demonstrating the modification of the VCSEL array of the light source in FIG. It is a figure for demonstrating schematic structure of a tandem color machine. It is a figure for demonstrating the structural example 1 of the optical scanning device in FIG. It is a figure for demonstrating the structural example 2 of the optical scanning device in FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Light source, 2 ... Coupling lens (1st optical system), 3 ... Aperture plate, 4 ... Line image formation lens (2nd optical system), 5 ... Polygon mirror (deflector), 6a ... Scanning lens (scanning optics) Part of the system), 6b ... Scanning lens (part of the scanning optical system), 500 ... Laser printer (image forming apparatus), 511 ... Photosensitive drum (image carrier), 900 ... Optical scanning apparatus, 900A ... Optical scanning Apparatus, K1, C1, M1, Y1... Photosensitive drum (image carrier).

Claims (9)

A light source having a surface emitting laser array in which a plurality of light emitting units are two-dimensionally arranged;
A pre-deflector optical system disposed on an optical path of a plurality of light beams emitted from the light source;
A deflector for deflecting a plurality of light beams from the pre-deflector optical system;
A scanning optical system for guiding a plurality of light beams deflected by the deflector to a surface to be scanned,
The pre-deflector optical system is to have at least one diffractive surface linear groove is formed extending in a direction deviation of the wavelength is larger in the plurality of light beams incident,
The at least one diffractive surface has a power due to an environmental change of an optical surface in an entire optical system including the pre-deflector optical system and the scanning optical system in a direction in which a wavelength deviation among a plurality of incident light beams is small. Correcting the deviation of the beam waist position on the scanned surface caused by the change and the change due to the environmental change of the oscillation wavelength of the light source,
The all-optical system is characterized in that the lateral magnification in the direction in which the wavelength deviation is large in the plurality of incident light beams is smaller than the lateral magnification in the direction in which the wavelength deviation is small . The diffractive surface is a surface in which a first surface having a diffractive effect and a second surface having a refracting effect are combined, and the power of the first surface and the power of the second surface cancel each other. The optical scanning device according to claim 1 , wherein: The pre-deflector optical system includes a first optical system that couples a light beam emitted from the light source, and a second optical system that condenses the light beam that has passed through the first optical system in the vicinity of the deflector. Have
It said second optical system, an optical scanning device according to claim 1 or 2, characterized in that it has the diffraction surface. The optical element having the diffractive surface in the pre-deflector optical system,
The deflector-side is the surface of the diffraction surface, the optical scanning apparatus according to any one of claims 1 to 3, the surface of the light source side is characterized by a refracting surface. In the plurality of light emitting units, the interval between the two light emitting units located at both ends with respect to the second direction orthogonal to the first direction is larger than the interval between the two light emitting units located at both ends with respect to the first direction. small, the direction of the grooves in the diffraction surface, the optical scanning apparatus according to any one of claims 1 to 4, characterized in that it is substantially parallel to the first direction. The optical scanning device according to claim 5 , wherein the first direction is a direction corresponding to a main scanning direction. The optical scanning device according to claim 5 , wherein the first direction is a direction corresponding to a sub-scanning direction. At least one image carrier;
Image forming apparatus comprising; said the optical scanning apparatus according to any one of the at least one of claims 1 to 7 for scanning a light beam that contains image information for at least one image bearing member. The image forming apparatus according to claim 8 , wherein the image information is color image information.