JP4891146B2 - 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 copying machines, and optical plotters are required to have high definition (high image quality), high speed, low price, and high stability against temperature changes. . Further, these image forming apparatuses include an optical scanning device that scans a 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, the method (1) has a disadvantage that (a) the light source device becomes complicated, (b) the number of parts increases, and (c) the adjustment becomes complicated because a plurality of elements are mounted. It was. 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. 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, when various lens materials are made of resin, (1) weight reduction is possible, (2) low-cost molding is possible, (3) specially shaped optical surface (hereinafter also referred to as “special surface” 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 temperature change using negative dispersion of the diffraction surface. 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 that is corrected by the power change of the diffraction section due to the 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. An optical scanning device is disclosed.

  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 A scanning optical device using a surface emitting semiconductor laser as a light source is disclosed.

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

  By the way, VCSEL has an extremely small wavelength transition amount due to temperature fluctuations as compared with a normal edge-emitting semiconductor laser device because of its characteristic of single longitudinal mode oscillation. In addition, since the plurality of light emitting portions of the VCSEL array are formed at the same time, variation in oscillation wavelength among the light emitting portions is extremely small.

  However, variation in oscillation wavelength between VCSEL arrays is not necessarily small due to variations in various conditions in the manufacturing process between lots. That is, the oscillation wavelength of the VCSEL array is not necessarily the same as the design wavelength. The diffractive surface provided to ensure stability against temperature changes is very sensitive to the wavelength of the incident light beam. In particular, when a VCSEL array is used as the light source, the power of the diffractive surface necessary for temperature compensation becomes large, and thus the deviation of the oscillation wavelength from the design wavelength may affect the magnification of the optical system.

  The present invention has been made under such circumstances, and a first object thereof is to provide an optical scanning device capable of stably ensuring desired scanning characteristics without incurring an increase in cost. is there.

  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, there is provided an optical scanning device that scans a surface to be scanned with a light beam emitted from a light source and deflected by a deflector via a pre-deflector optical system. The light source includes a plurality of light emitting units. Has a two-dimensionally arrayed surface emitting laser array, the pre-deflector optical system has a diffractive surface on at least one surface, and the deviation Δλ _d of the oscillation wavelength of the surface emitting laser array from the design wavelength Using the deviation ΔS (Δλ _d ) of the focused position of the light beam in the sub-scanning direction and the deviation ΔS (ΔT) of the focused position of the light flux in the sub-scanning direction caused by the change in environmental temperature ΔT, The diffractive surface is formed so that ΔS (Δλ _d ) × ΔS (ΔT) <0 is satisfied , and the pre-deflector optical system is a first optical system for coupling the light beam emitted from the light source And the light beam that has passed through the first optical system is A diffractive surface provided in the second optical system, formed with a concentric elliptical groove, and a light beam from a central light emitting part among the plurality of light emitting parts. Is an optical scanning device that enters a region surrounded by a step closest to the center on the diffraction surface with a light beam diameter smaller than the diameter of the region .

According to this, it is possible to stably ensure desired scanning characteristics 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 (scanning 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 “interval between the light emitting portions” refers to the distance between the centers of the 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.

As an example, each light emitting unit has an oscillation wavelength of 790 nm, 10 nm larger than the design oscillation wavelength (here, λ ₀ = 780 nm), and a temperature of 1 ° C. with respect to the reference temperature (T ₀ = 25 ° C.). When it rises, the oscillation wavelength is shifted to the long wavelength side by 0.062 nm.

  For the sake of convenience, the light emitting part closest to the −S side is referred to as the light emitting part v1, and the light emitting part closest to the + S side is referred to as the light emitting part v40.

  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.

  The incident surface of the line image forming lens 4 is a cylindrical surface having 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 118.5 mm.

  As shown in FIG. 4 as an example, the exit surface of the line image forming lens 4 is formed with a concentric elliptical groove having a minor axis in the direction corresponding to the main scanning direction and a major axis in the direction corresponding to the sub-scanning direction. It is a diffracted surface. The direction of the long axis can be freely selected according to the characteristics of the optical scanning device to be mounted.

  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 = −0.001999851 and C2 = −0.002183171.

  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. 5, the shape 13 of the first surface is a shape obtained by folding back the shape 12 of the predetermined refracting surface with an appropriate step and pitch. Accordingly, the first surface has power.

  The shape of the second surface is a surface shape that cancels the power of the first surface. Here, the shape of the second surface is the shape of a toric surface having a radius of curvature of 131 mm in the direction corresponding to the main scan and a radius of curvature of 120 mm in the direction corresponding to the sub-scan. Thereby, the diffractive surface of the line image forming lens 4 becomes non-power in both the direction corresponding to the main scanning direction and the direction corresponding to the sub-scanning direction.

  Here, since the power of the first surface and the power of the second surface are equal in absolute value, the radius of curvature of the toric surface (hereinafter also referred to as “substrate curvature radius”) is used as an index of the power of the first surface. ) Values can be used.

  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 the present embodiment, when the surface shape of the diffraction surface is made into a multi-step shape, as shown in FIG. 6 as an example, the angle of the folded portion becomes a right angle and becomes a stepped shape symmetrical to the optical axis, Convenience in mold manufacture is further improved.

The diffractive surface is designed so that the following expression (4) is satisfied. Here, ΔS (Δλ _d ) is a distance between the focus position of the scanning light in the sub-scanning direction and the photosensitive drum 511 (hereinafter, referred to as “Δλ _d”) caused by the deviation Δλ _d of the oscillation wavelength λ of the VCSEL array from the design wavelength λ _0. For the sake of convenience, it is also referred to as “sub-scanning focus position deviation”), and ΔS (ΔT) is a sub-scanning focus position deviation caused by the change ΔT of the environmental temperature T with respect to the reference temperature T ₀ .

ΔS (Δλ _d ) × ΔS (ΔT) <0 (4)

Here, ΔS (ΔT) and ΔS (Δλ _d ) will be described with reference to FIGS. 7A to 8B.

FIGS. 7A and 7B show the sub-scanning focus position shift in the case of Δλ _d = 0 and T = T ₀ + ΔT. In FIG. 7A, reference numeral 1 ′ denotes a light source having a VCSEL array with an oscillation wavelength of λ ₀ , reference numeral 4 ′ denotes a line image forming lens on which no diffraction surface is formed, and reference numeral 6 denotes a scanning lens. It is. When the environmental temperature changes, the wavelength λ of the light beam emitted from the light source 1 ′ changes. Here, the wavelength change due to the change of the environmental temperature and [Delta] [lambda] _t. It is assumed that the wavelength changes due to self-heating of the VCSEL array is also included in the [Delta] [lambda] _t. Further, when the environmental temperature changes, the power of the entire optical system also changes. The power change due to this environmental temperature change is represented by Δx. That is, when the environmental temperature changes, the oscillation wavelength and the power of the entire optical system change simultaneously. Therefore, the sub-scanning focus position shift due to the wavelength change Δλ _t is ΔS (Δλ _t ), and the sub-scan focus position shift due to the power change Δx is ΔS. If (Δx), it can be written as ΔS (ΔT) = ΔS (Δλ _t ) + ΔS (Δx) (see FIG. 7B). The power change of the entire optical system is due to (1) thermal expansion of the optical element shape and optical element spacing, (2) dispersion of the optical element, and (3) refractive index change of the optical element due to temperature change. It is. Therefore, for example, when a diffractive surface is provided in order to compensate for a change in environmental temperature, if the diffractive surface shape is designed so that ΔS (Δλ _t ) = − ΔS (Δx), ΔS (ΔT) = 0 and the temperature Compensation will be realized.

FIGS. 8A and 8B show the sub-scanning focus position deviation in the case of λ = λ ₀ + Δλ _d and T = T ₀ . Reference numeral 1 '' in FIG. 8 (A) is a light source whose oscillation wavelength lambda has a [Delta] [lambda] _d greater VCSEL array than the design wavelength lambda _0. Since the change in the oscillation wavelength has little influence on the optical element having no diffraction surface, the power change Δx ′ in this case is almost zero. Therefore, ΔS (Δλ _d ) is dominant in the sub-scanning focus position deviation in this case (see FIG. 8B).

In practice, ΔS (Δλ _d ) is overwhelmingly larger than ΔS (Δλ _t ). Therefore, ΔS (Δλ _d ) degrades the optical performance before considering the change in environmental temperature. Even if focus adjustment is performed at the time of assembly, since focus and magnification cannot be achieved at the same time, it can be said to be fatal for a VCSEL-equipped optical scanning device that requires high-precision magnification stability.

  FIGS. 9A and 9B show an example in which the above equation (4) is not satisfied. In the case of FIG. 9A, it is not necessary to consider the deviation of the oscillation wavelength from the design wavelength, and the deterioration of the optical performance due to the change of the environmental temperature is enhanced. In the case of FIG. 9B, significant optical performance degradation occurs due to the deviation of the oscillation wavelength from the design wavelength.

  Therefore, by using a diffractive surface that satisfies the above expression (4), it is possible to balance temperature compensation and oscillation wavelength deviation compensation.

  In the present embodiment, the substrate curvature radius is 120 mm, and the above equation (4) is satisfied as shown in FIG. Then, the positional deviation in the sub-scanning direction of the outermost light spot (here, the light spot due to the light beam from the light emitting part v1 or the light emitting part v40) among the plurality of light spots on the surface of the photosensitive drum 511 (for convenience, “the outermost light spot”). The “outer beam sub-scanning position shift” is also 2 μm or less with respect to both temperature change and oscillation wavelength shift. That is, in the present embodiment, since the above expression (4) is satisfied, it is possible to suppress both the sub-scanning focus position shift due to the environmental temperature change and the magnification change in the sub-scanning direction due to the oscillation wavelength shift. It becomes possible. If the above equation (4) is not satisfied, the optical performance deteriorates with respect to both the temperature change and the oscillation wavelength shift. Note that when the substrate radius of curvature is 18.6 mm, ΔS (ΔT) is almost 0, so it is called a temperature compensation solution.

  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 aspherical surface, and each surface has a noncircular arc shape expressed by the following equation (5) 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 (6).

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.

  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, the angle formed by the W 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 (the position indicated by the symbol p0 in FIG. 11) by the deflecting and reflecting surface of the polygon mirror 5. (Θ in FIG. 11) 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.

  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 designed so as to satisfy the above expression (4). Thereby, both the sub-scanning focus position shift due to the environmental temperature change and the magnification change in the sub-scanning direction due to the oscillation wavelength shift can be suppressed. Therefore, as a result, it is possible to stably secure desired scanning characteristics 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 are formed by cutting is used. According to the present embodiment, the plurality of diffraction grooves on the diffraction surface of the line image forming lens 4 are multi-stepped. This facilitates the manufacture of a mold (or metal piece) for forming the diffractive surface (see FIG. 12).

  Further, according to the present embodiment, the diffraction surface of the line image forming lens 4 is formed with concentric elliptical grooves having a minor axis in the direction corresponding to the main scanning direction and a major axis in the direction corresponding to the sub-scanning direction. It is a diffractive surface. Thereby, the effect of a diffractive surface can be expressed independently with respect to the major axis direction and the minor axis direction.

  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, high-speed and high-density optical scanning is possible.

  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 stably secure desired scanning characteristics without causing an increase in cost, resulting in an increase in cost. High quality images can be stably formed without incurring.

  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 has a diffractive surface has been described. However, the present invention is not limited to this. It is sufficient that the pre-deflector optical system has a diffractive surface. In general, the error of the coupling optical system included in the pre-deflector optical system has the greatest influence on the beam spot on the surface to be scanned. 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 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 embodiment, the case where the diameter of the light beam incident on the 0th annular zone, which is the region surrounded by the step closest to the center of the diffraction surface, is larger than the diameter of the 0th annular zone, is described as an example. As shown in FIG. 13, the diameter of the light beam incident on the 0th annular zone may be smaller than the diameter of the 0th annular zone. For example, when the substrate curvature radius is 150 mm, the diameter of the light beam incident on the 0th annular zone is smaller than the diameter of the 0th annular zone. In this case, since the light beam diameter of 4a is smaller than the diameter of the 0th annular zone, the light beam 4b can receive only an action equivalent to that of the refractive lens, but the light beam 4b can receive the action of the diffraction surface. In other words, the diffraction effect can be selectively imparted to the outermost light flux emitted from the VCSEL array 100.

  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. 14). 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. 15 shows an example of the configuration of the opposed 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. 15, 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. 16 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 stably securing desired scanning characteristics 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 diffraction surface 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. FIGS. 7A and 7B are diagrams for explaining sub-scanning focus position shifts due to changes in the environmental temperature. FIGS. 8A and 8B are diagrams for explaining sub-scanning focus position shifts due to oscillation wavelength shifts. FIG. 9A and FIG. 9B are diagrams for explaining examples in which the above equation (4) is not satisfied. It is a figure for demonstrating the relationship between a substrate curvature radius, subscanning focus position shift, and outermost beam subscanning position shift. 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 light beam which injects into a diffraction surface. 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 system), 6b ... Scanning lens (part of scanning optical system), 100 ... VCSEL array (surface emitting laser array), 500 ... Laser printer (image forming apparatus), 511 ... Photosensitive drum (image carrier) 900, optical scanning device, 900A, optical scanning device, K1, C1, M1, Y1, photosensitive drum (image carrier).

Claims (4)

In an optical scanning device that scans a surface to be scanned with a light beam emitted from a light source and deflected by a deflector via a pre-deflector optical system,
The light source includes a surface emitting laser array in which a plurality of light emitting units are two-dimensionally arranged.
The pre-deflector optical system has a diffractive surface on at least one surface;
A deviation ΔS (Δλ _d ) of the in-focus position of the light beam with respect to the sub-scanning direction due to the deviation Δλ _d from the design wavelength of the oscillation wavelength of the surface emitting laser array, and a sub-scanning direction due to the change ΔT in the environmental temperature. The diffractive surface is formed so that ΔS (Δλ _d ) × ΔS (ΔT) <0 is satisfied using the in-focus position shift ΔS (ΔT) of the light beam ,
The pre-deflector optical system has a first optical system for coupling a light beam emitted from the light source, and a second optical system for guiding the light beam via the first optical system to the deflector, The diffractive surface is provided in the second optical system, and a concentric elliptical groove is formed.
An optical scanning device in which a light beam from a central light emitting unit among the plurality of light emitting units is incident on a region surrounded by a step closest to the center of the diffraction surface with a light beam diameter smaller than the diameter of the region . The optical scanning device according to claim 1 , wherein the shape of the diffraction surface is a multi-step shape. At least one image carrier;
Wherein at least an optical scanning device according to at least one of claims 1 or 2 for scanning a light beam that contains image information for one image bearing member; image forming apparatus provided with. The image forming apparatus according to claim 3 , wherein the image information is color image information.