JP5009574B2 - Diffractive optical element, scanning optical system, optical scanning apparatus, and image forming apparatus - Google Patents

Description

  The present invention relates to a diffractive optical element, a scanning optical system, an optical scanning device, and an image forming apparatus.

  The light beam emitted from the light source is guided to the optical deflector by the first optical system and deflected by the optical deflector, and the deflected light beam is formed as a light spot on the surface to be scanned by the second optical system. 2. Description of the Related Art Optical scanning devices that perform surface optical scanning are widely known in connection with image forming apparatuses such as digital copying machines, facsimile machines, optical printers, and optical plotters.

  A semiconductor laser or LED generally used as a light source of such an image forming apparatus is a substantially monochromatic light source, but its emission wavelength varies depending on the type of the semiconductor laser or LED. The photoconductor that is optically scanned by the optical scanning device has different spectral characteristics of photosensitivity depending on the type, and the light source generally has a light emission wavelength suitable for the photosensitivity of the photoconductor.

  On the other hand, in an optical scanning device, a scanning optical system that condenses light emitted from a light source as a light spot on a photosensitive member includes a lens that is a refractive optical system, but the refractive index of the lens material is a dispersion inherent in the material. And varies depending on the wavelength. For this reason, conventionally, the scanning optical system is designed on the assumption of the emission wavelength of the light source used, and the optical element used in the scanning optical system is not compatible with the light source wavelength.

  It is known that "diffractive optical element" having a diffraction function is used for the optical element used in the scanning optical system, and that "variation due to temperature change in focus position of the scanning optical system" is reduced by combining with other lenses. (Patent Document 1) does not improve “compatibility of diffractive optical elements” with respect to the light source wavelength.

JP 2006-085487 A

  In view of the above, the present invention realizes a diffractive optical element included in a scanning optical system, which is compatible with two or more light sources having different emission wavelengths, and scans using such a diffractive optical element. An object is to realize an optical system, an optical scanning device, and an image forming apparatus.

  According to the diffractive optical element of the present invention, the light beam emitted from the light source is guided to the optical deflector by the first optical system and deflected by the optical deflector, and the deflected light beam is projected onto the surface to be scanned by the second optical system. A light-transmitting diffractive optical element formed as a light spot and used in the first and / or second optical system in an optical scanning device that performs optical scanning of the surface to be scanned, and is characterized by the following points (Claim 1).

That is, the diffractive optical element has a “diffractive surface due to a step”.
A “diffractive surface” is a surface that causes a diffractive action on a light beam incident on this surface.
The step of the diffractive surface: d is greater than or equal to two wavelengths: λ _i (i = 1, 2,...) And the refractive index of the element material for these wavelengths: λ _i : n (λ _i )
λ _i / {n (λ _i ) −1}
Is set to be substantially equal to the common multiple of.

The “surface shape of the diffractive surface” of the diffractive optical element according to claim 1 can be set so that “the power of the diffractive part cancels the power of the refracting part” (Claim 2). The shape of the diffractive surface is preferably “multi-step type”.
In other words, the diffractive surface in claim 2 has a “surface shape in which a diffractive portion due to a step is superimposed on a refracting portion having power due to refraction”, and this surface shape is expressed as “power of the diffracting portion and power due to refraction of the refracting portion” Is set to cancel.
The diffractive surface in claim 3 has a “surface shape formed by superimposing a diffractive part due to a step on a refracting part having power by refraction”, and the shape is a multi-step type.
Preferably, the diffractive surface of the diffractive optical element according to any one of claims 1 to 3 is formed by a “linear groove shape parallel to the main scanning direction and / or the sub-scanning direction”. ).

The step of the diffractive surface in the diffractive optical element according to any one of claims 1 to 4, wherein d is two or more wavelengths: λ _i (i = 1, 2,...), And elements for these wavelengths λ _i For the refractive index of the material: n (λ _i )
λ _i / {n (λ _i ) −1}
Preferably, it is set to be substantially equal to the least common multiple of (Claim 5).
The “maximum pitch on the diffractive surface” of the diffractive optical element according to any one of claims 1 to 5 is smaller than a width of a light beam incident on the diffractive surface (claim 6).
The diffractive optical element according to any one of claims 1 to 6 can be formed as "a coupling lens for coupling a light beam from a light source" (claim 7).
The diffractive surface in the diffractive optical element according to any one of claims 1 to 7 can have a "function to compensate for a change in optical function of the first and / or second optical system due to temperature fluctuation" (claim). Item 8).

  According to the scanning optical system of the present invention, the light beam emitted from the light source is guided to the optical deflector by the first optical system and deflected by the optical deflector, and the deflected light beam is projected on the surface to be scanned by the second optical system. In an optical scanning device that is formed as a light spot and performs optical scanning of a surface to be scanned, it is a scanning optical system composed of first and second optical systems ”, and the first and / or second optical systems are One or more diffractive optical elements according to any one of claims 1 to 8 are included (claim 9).

The scanning optical system according to claim 9, wherein the two or more wavelength relative lambda _i, have substantially the same optical characteristics, wavelength: "scanning for two or more light sources that emit light of lambda _i It can be shared as an optical system ”(claim 10).

The optical scanning device according to the present invention is directed to “a light beam emitted from a light source is guided to an optical deflector by a first optical system and deflected by the optical deflector, and the deflected light beam is projected onto a surface to be scanned by a second optical system. 11. An optical scanning device that forms an optical spot and performs optical scanning of a surface to be scanned ”, and has the scanning optical system according to claim 9 or 10 as a scanning optical system constituted by the first and second optical systems. And a light source having a wavelength of “two or more wavelengths: λ _i (i = 1, 2,...)” (Claim 11).

  The image forming apparatus according to the present invention is an “image forming apparatus that forms an image by optical scanning”, and has at least one optical scanning device according to claim 11 as an optical scanning device that performs optical scanning. Item 12). In other words, the number of optical scanning devices of the image forming apparatus according to the twelfth aspect may be one or plural. In the case of having a plurality of optical scanning devices, the optical deflector in the plurality of optical scanning devices may be referred to as a “polygon mirror”, and the optical deflector based on the polygon mirror may be “shared with multiple optical scanning devices”. it can.

  The image forming apparatus according to a twelfth aspect of the present invention may be “having a plurality of light sources and performing image formation by multi-beam scanning” (claim 13). Also in this case, the optical scanning device that performs multi-beam scanning may be single or plural.

  The image forming apparatus according to claim 12 or 13 includes a plurality of light sources, performs optical scanning on a plurality of optical scanning targets, and forms an image by superimposing images formed on the respective optical scanning targets. (Claim 14). Such an image forming apparatus can be implemented as, for example, a “tandem color image forming apparatus”.

  Supplementing the description, various types of conventionally known optical systems can be used as the first and second optical systems. For example, as the simplest configuration of the first optical system, the light beam from the light source is configured only by a coupling lens that converts a light beam from a light source into a parallel beam, a weak divergent beam, or a weak convergent beam, and the coupled light beam is an optical deflector. It can be made to guide to.

  Alternatively, a coupling lens that converts the light beam from the light source into a parallel light beam, a weak divergent light beam, or a weak convergent light beam, and the coupled light beam as a line image that is long in the main scanning direction at the position of the deflecting reflection surface of the optical deflector. It can be constituted by a line image imaging optical system such as a cylindrical lens for imaging.

The second optical system is a known fθ lens composed of one or more lenses, or an fθ mirror with a reflecting surface provided with an fθ function, or one that realizes desired optical characteristics with one or more lenses or mirrors. It can be used appropriately.
As the optical deflector, a known appropriate one such as a polygon mirror, a rotating dihedral mirror, and a rotating single mirror can be used.

  There are various types of “diffractive optical elements”, but like a Fresnel lens, a transmission type (phase type) diffraction in which a ring zone divided by steps becomes narrower as it goes from the optical axis to the periphery. An optical element will be described as an example. Formulas used in the following explanation are based on “scalar diffraction theory”.

When a light beam having a wavelength of λ is incident on a diffractive optical element having a diffractive surface having a step difference of d (condensing light), if the diffractive surface shape satisfies the following formula (1) for a natural number: m, The luminous flux transmitted through each annular zone divided by the steps is phase-matched with “a phase difference for m wavelengths” and “collected as m-order diffracted light”.
(1) d = mλ / {n (λ) −1}
In the formula (1), “n (λ)” is the refractive index of the element material with respect to the wavelength: λ.

Such focal length of the "m-th order diffracted light diffractive optical element light-collecting condensing the": f is the focal length for the zero-order light: given as follows with respect to f _0.
(2) f = [{n (λ) −1} d / λm] f ₀
According to the equation (1), [{n (λ) −1} d / λm] in the right side of the equation (2) is equal to 1, so that the focal length of the diffractive optical element with respect to the mth order diffracted light is equal to _{f 0:} the focal length.

Here, two types of wavelengths: λ _j and λ _k are considered, and a case where the step: d of the diffractive optical element is set so as to be phase-matched to these wavelengths: λ _j and λ _k is considered. That is, with respect to the step: d, the light of wavelength: λ _j is phase matched with “phase difference for m _j wavelengths”, and the light of wavelength: λ _k is phase matched with “phase difference for m _k wavelengths”. Shall.

In order for this condition to hold,
(3) d = m _j λ _j / {n (λ _j ) −1} = m _k λ _k / {n (λ _k ) −1}
May be the Naritate, the focal length of the time the diffractive optical element, wavelength: For the lambda _j light,
(4-1) f _j = [{n (λ _j ) −1} d / λ _j m _j ] f ₀
Wavelength: For light of λ _k
(4-1) f _k = [{n (λ _k ) −1} d / λ _k m _k ] f ₀
It becomes.

That is, when the step of the diffractive surface: d satisfies the expression (3), the focal length of the diffractive optical element is the same for the two wavelengths: λ _j and λ _k , and the same for these two wavelengths. It will perform the optical function.

The condition that the expression (3) is satisfied is that the step: d is the wavelength: λ _j , λ _k .
λ _i / {n (λ _i ) -1}, λ _k / {n (λ _k ) -1}
The result is a common multiple of.

That is, when “two types of optical scanning devices” having different light emission wavelengths: λ _j and λ _k are considered, among the first and / or second optical systems of these optical scanning devices, (3) If a diffractive optical element satisfying the equation is used, this diffractive optical element is an optical device having the same optical function for any of two types of optical scanning devices having “one of light sources having different emission wavelengths”. It can be used as an element. In other words, the diffractive optical element is compatible with these two types of optical scanning devices.

The above explanation can be generalized: "The light beam emitted from the light source is guided to the optical deflector by the first optical system and deflected by the optical deflector, and the deflected light beam is scanned by the second optical system. A light-transmitting diffractive optical element formed as a light spot on the surface and used in the first and / or second optical system in the optical scanning device that scans the surface to be scanned forms a diffractive surface due to a step. The step: d is greater than or equal to two wavelengths: λ _i (i = 1, 2,...) And the refractive index of the element material for these wavelengths: λ _i : n (λ _i )
λ _i / {n (λ _i ) −1}
If the diffractive optical element is set to be substantially equal to a common multiple of λ, the diffractive optical element is compatible with two or more light sources having wavelengths λ _i (i = 1, 2,...).

Note that, for two or more wavelengths: λ _i (i = 1, 2,...) And the refractive index of the element material for these wavelengths: λ _i : n (λ _i ), the step: d is set to λ _i / {n (λ _i ) −1}
However, it is not always technically easy to make the step: d exactly match the above-mentioned “common multiple”. However, the diffractive optical element is substantially the same for the plurality of wavelengths. Since it is only necessary to have an optical function, the “step and common multiple” need only match within a range in which “substantially the same optical function for a plurality of wavelengths” can be realized from such a practical viewpoint. . In claim 1, “step difference: d is set to be substantially equal to a common multiple” means this.

  A characteristic regarding the collection of diffracted light is that its focal length is inversely proportional to the wavelength, that is, the power is proportional to the wavelength. In the emission wavelength region of a semiconductor laser or LED used as a light source of an optical scanning device, the “power fluctuation due to wavelength fluctuation” of the diffractive optical element is overwhelmingly larger than the power fluctuation due to dispersion of the refractive index of the material.

  That is, even if the expression (3) is not strictly established due to the dispersion of the refractive index of the material, the power difference is very small compared to the power difference due to the initial wavelength difference, so the diffractive optical element according to claim 1 is different. It can be used as an optical element having compatibility with a light source having an emission wavelength. Also, the drop in diffraction efficiency can be reduced for the same reason in terms of light quantity.

  Thus, since the diffractive optical element according to claim 1 is compatible with a plurality of types of optical scanning devices using light sources having different emission wavelengths, the diffractive optical element is a component for implementing a plurality of types of optical scanning devices. This can reduce the number of points and reduce the price.

Next, in terms of the surface shape of the diffractive surface, the surface shape of the diffractive surface is preferably set such that the power of the diffractive portion and the power of the refracting portion cancel each other (claim 2).
Taking the case of the aforementioned Fresnel lens as an example, the diffractive part of the diffractive surface is “a shape obtained by folding the surface shape of the refracting part with an appropriate step and pitch” as shown in FIG. Therefore, the shape and power of the diffractive surface is “a combination of the diffractive part and the refractive part”. Normally, the pitch of the step of the diffractive part becomes finer as it goes to the lens peripheral part as illustrated in the left figure of FIG. 5, so when forming the diffractive part by molding, the diffractive part is formed. Tends to be difficult.

In such a case, as described in claim 2, the power (positive power) of the diffractive part (left figure of FIG. 5) and the power (negative power) of the refracting part (middle figure of FIG. 5) cancel each other. If set to “Yes”, the folded portion of the diffractive portion becomes obtuse as in the shape of the diffractive surface in the right diagram of FIG.
That is, a “refractive surface having negative power” as shown in the center diagram of FIG. 5 is used as a refracting portion, and a “diffractive portion having positive power” as shown in the left diagram of FIG. ”Are overlapped to form a“ diffractive surface ”. Then, the powers of the refracting unit and the diffractive unit are set so that the powers opposite to each other of the superimposed refracting unit and diffracting unit cancel each other.

In the example shown in the right diagram of FIG. 5, the diffraction surface shape of the diffractive surface obtained by superimposing the diffractive portion on the refracting portion is “multi-step type (Claim 3)”, and the “angle of the folded portion” is a right angle. It has a stepped shape symmetrical to the optical axis. Such a multi-step type diffractive surface is easier to mold. Optically, the 0th-order light and the 1st-order or higher-order diffracted light are the same and equivalent to a non-powered surface, and are optical with respect to decentration. Performance is unlikely to deteriorate.

  The power of the diffractive part of the diffractive surface can be set “independently in the main / sub-scanning direction”. When the diffractive parts set independently in the main / sub-scanning direction are provided on the same surface in this way, When the diffractive surface is viewed from the axial direction, the folded portion of the diffractive portion becomes a concentric ellipse or a concentric circle. However, when concentric elliptical or concentric diffractive surfaces are formed by mold forming, it is necessary to devise the effort to put out the axis and "the escape of the bite".

  In such a case, as described in claim 4, one diffractive surface has a “shape in which the power of the diffraction part acts only in a certain direction (main scanning direction or sub-scanning direction)”, that is, “main scanning direction and By forming with “/ a linear groove shape parallel to the sub-scanning direction”, the mold can be easily manufactured.

  That is, the shape of the diffractive surface is realized by cutting the mold, but in the case of a straight groove shape, the cutting tool can be cut only by running in one direction, and there is no problem in letting the tool escape. . Further, as in the case of the third aspect, if the stepped shape is symmetric with respect to the optical axis, the angle for hitting the cutting tool becomes almost a right angle, so that the mold can be manufactured more easily.

  If the step of the diffractive surface of the diffractive optical element becomes too large, "geometrical vignetting" may occur at the step, or large spherical aberration may occur when a light beam that is not parallel light is incident. .

From the viewpoint of avoiding such a problem, it is preferable to minimize the size of the step. Specifically, as described in claim 5, the step of the diffractive surface: d is set to “2 or more wavelengths: λ. _i (i = 1, 2,...) and the refractive index of the element material for these wavelengths λ _i : n (λ _i )
λ _i / {n (λ _i ) −1}
Should be set substantially equal to the least common multiple of ".

  Further, when the level difference of the diffractive surface increases, the maximum pitch increases accordingly. However, if the maximum pitch exceeds the width of the light beam incident on the diffractive surface, the diffractive optical element is equal to a simple refracting surface for the light beam, and the effect peculiar to diffraction cannot be exhibited. Accordingly, the magnitude relationship between the incident light beam diameter and the “maximum pitch of the diffractive surface” is adjusted so that “the maximum pitch on the diffractive surface is smaller than the width of the light beam incident on the diffractive surface”.

  When the diffractive optical element is compatible with a plurality of wavelengths having different emission wavelengths, or the scanning optical system is shared by two or more wavelengths as in the scanning optical system according to claims 9 and 10 However, since the lens always has dispersion characteristics, the geometrical optical focusing position inevitably changes when the wavelength changes. In order to avoid this problem, if the optical element included in the scanning optical system includes an element that can adjust the set position, the geometric optical converging position shifts due to replacement of light sources having different wavelengths. Can be dealt with.

  To add a little about an image forming apparatus that forms an image by optical scanning, various types of photoconductors that are optical scanning targets that are optically scanned by the optical scanning apparatus can be used. For example, a silver salt film can be used as the photoreceptor, and in this case, a latent image formed by writing by optical scanning can be visualized by processing by 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.

  As the photoreceptor, a coloring medium (positive photographic paper) that develops color by the thermal energy of the beam spot during optical scanning can be used. In this case, a visible image can be directly formed by optical scanning. A “photoconductive photoreceptor” can also be used as the photoreceptor. As the photoconductive photoreceptor, a sheet-like one such as zinc oxide paper can be used, or a drum-type or belt-like one such as a selenium photoreceptor or an organic optical semiconductor can be used. it can.

  When a photoconductive photoconductor is used, an electrostatic latent image is formed by uniform charging of the photoconductor and optical scanning by an optical scanning device. The electrostatic latent image is visualized as a toner image by development. The toner image is directly fixed on the photoconductor when the photoconductor is in the form of a sheet such as zinc oxide paper, and transfer paper or an OHP sheet when the photoconductor can be used repeatedly. It is transferred and fixed on a sheet-like recording medium such as (plastic sheet for overhead projector).

  The transfer of the toner image from the photoconductive photosensitive member to the sheet-like recording medium may be directly transferred from the photosensitive member to the sheet-like recording medium (direct transfer method), or may be temporarily transferred from the photosensitive member to an intermediate transfer belt or the like. After transfer to the intermediate transfer medium, transfer from the intermediate transfer medium to a sheet-like recording medium (intermediate transfer method) may be performed. Such an image forming apparatus can be implemented as an optical printer, an optical plotter, a digital copying apparatus, or the like.

  According to another aspect of the present invention, there is provided an image forming apparatus including a plurality of the photoconductive photoconductors arranged along a conveyance path of a sheet-like recording medium, and an electrostatic latent image for each photoconductor using a plurality of optical scanning devices. Can be implemented as a tandem type image forming apparatus that synthetically obtains a color image or a multicolor image by transferring and fixing a toner image obtained by visualizing the toner image to the same sheet-like recording medium.

  As described above, according to the present invention, a diffractive optical element having compatibility with two or more light sources having different emission wavelengths, a scanning optical system using the diffractive optical element, an optical scanning apparatus, and an image forming apparatus are provided. Can be realized.

Embodiments of the invention will be described below.
FIG. 1 is a diagram for explaining one embodiment of an optical scanning device.
The light source denoted by reference numeral 1 is a “surface emitting semiconductor laser”. The light beam emitted from the light source 1 is coupled to the subsequent optical system by the coupling lens 2. When the light beam that has passed through the coupling lens 2 passes through the opening of the aperture 3, the periphery of the light beam is blocked and shaped, and enters the line image forming lens 4 that is a line image forming optical system. The line image forming lens 4 is a cylindrical lens having a power-less direction in the main scanning direction and a positive power in the sub-scanning direction, and converges an incident light beam in the sub-scanning direction. Is condensed as a long line image in the main scanning direction in the vicinity of the deflection reflection surface of the polygon mirror 5.
That is, the coupling lens 2 and the line image forming lens 4 constitute a “first optical system”.

  The light beam reflected by the deflecting and reflecting surface is deflected at a constant angular velocity as the polygon mirror 5 rotates at a constant speed, and is transmitted through the scanning lens 6 constituting the second optical system to guide the light beam to the surface to be scanned. The optical path is bent by the bending mirror 7 for focusing, and the light is condensed as a light spot on the photoconductive photosensitive member 8 forming the substance of the surface to be scanned, and the surface to be scanned is optically scanned.

  In the embodiment of FIG. 1, the second optical system is configured by a single scanning lens 6, but of course, the second optical system may be configured by a plurality of lenses.

  Prior to the optical scanning of the photoconductor 8, the deflected light beam by the polygon mirror 5 is reflected by the synchronization mirror 9 and condensed by the synchronization lens 10 on the synchronization detection unit 11 in the main scanning direction. Based on the output of the synchronization detector 11, the write start timing of optical scanning is determined.

In other words, in this specification, the “spot diameter of the light spot” is defined as “1 / e ² intensity in the line spread function of the light intensity distribution” in the light spot on the surface to be scanned.

The “line spread function” is a light spot light intensity distribution: f (Y, Z) based on coordinates Y, Z in the main scanning direction and the sub-scanning direction with reference to the center coordinates of the light spot formed on the surface to be scanned. Is determined, the line spread function in the Z direction: LSZ is
LSZ (Z) = ∫f (Y, Z) dY
(Integration is performed for the full width of the beam spot in the Y direction) and the line spread function in the Y direction: LSY is
LSY (Y) = ∫f (Y, Z) dZ
(Integration is performed for the full width of the beam spot in the Z direction).

These line spread functions: LSZ (Z), LSY (Y) are generally of a Gaussian distribution type, and the spot diameters in the Y direction and Z direction are determined by the line spread functions: LSZ (Z), LSY (Y ) Is given by the width in the Y and Z directions of a region that is 1 / e ² or more of the maximum value.

  The spot diameter defined above by the line spread function can be easily measured by scanning the light 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. There are also commercially available devices for performing such measurements.

FIG. 2 is a diagram for explaining another embodiment. In order to avoid complications, the same reference numerals as those in FIG.
The embodiment shown in FIG. 2 shows an optical arrangement of an optical scanning device that performs “multi-beam scanning” in a planar manner. Reference numerals 1 and 1 ′ are light sources that are surface-emitting semiconductor lasers, reference numerals 2 and 2 ′ are coupling lenses, reference numerals 3 and 3 ′ are apertures, reference numeral 4A is a cylindrical lens that is a line image imaging optical system, and reference numeral DM is Reference numeral 5 denotes a polygon mirror, reference numerals 61 and 62 denote scanning lenses constituting a “second optical system”, and reference numeral 8 denotes a surface to be scanned (actually, a photosensitive surface of a photosensitive member). Reference numeral G1 denotes a soundproof glass of the housing that seals the polygon mirror 5, and reference numeral G2 denotes a dustproof glass of the housing of the optical scanning device. Although not shown in FIG. 1, the soundproof glass G1 and the dustproof glass G2 are also used in the embodiment shown in FIG.

  The luminous fluxes emitted from the light sources 1 and 1 ′ are respectively coupled by the coupling lenses 2 and 2 ′, beam-formed by the apertures 3 and 3 ′, transmitted through the cylindrical lens 4A, reflected by the mirror DM, and polygon mirror 5 Is formed as a long line image in the main scanning direction in the vicinity of the deflection reflection surface. These line images are separated by a predetermined interval in the sub-scanning direction.

  The two light beams reflected by the polygon mirror 5 are deflected at a constant angular velocity along with the constant speed rotation of the polygon mirror 5, pass through the scanning lenses 61 and 62 and the dust-proof glass G 2, and are “in the sub-scanning direction” Two separated light spots "are formed, and the scanned surface 8 is optically scanned for two lines simultaneously.

  Although not shown in FIG. 2, the write start timing of optical scanning by each light spot is determined by the same method as in the embodiment of FIG.

An embodiment of the image forming apparatus will be described with reference to FIG.
This image forming apparatus is a “laser printer”.
The laser printer 100 includes a “photoconductive photosensitive member formed in a cylindrical shape” as a photosensitive image carrier (optical scanning target) 111. Around the image carrier 111, a charging roller 112 as a charging unit, a developing device 113, a transfer roller 114, and a cleaning device 115 are provided. A “corona charger” can also be used as the charging means.

  An optical scanning device 117 that performs optical scanning with the laser beam LB is provided, and “exposure by optical writing” is performed between the charging roller 112 and the developing device 113.

  In FIG. 6, reference numeral 116 denotes a fixing device, reference numeral 118 denotes a cassette, reference numeral 119 denotes a registration roller pair, reference numeral 120 denotes a paper feed roller, reference numeral 121 denotes a conveyance path, reference numeral 122 denotes a discharge roller pair, reference numeral 123 denotes a tray, reference numeral P Indicates transfer paper as a sheet-like recording medium.

  When image formation is performed, the image carrier 111 that is a photoconductive photosensitive member rotates at a constant speed in the clockwise direction, and the surface thereof is uniformly charged by the charging roller 112, so that the optical writing of the laser beam LB of the optical scanning device 117 is performed. An electrostatic latent image is formed upon exposure to the image. The formed electrostatic latent image is a so-called “negative latent image”, and the image portion is exposed.

  This electrostatic latent image is reversely developed by the developing device 113, and a toner image is formed on the image carrier 111. The cassette 118 storing the transfer paper P is detachable from the main body of the image forming apparatus 100. When the transfer paper P is mounted as shown in the drawing, the uppermost sheet of the stored transfer paper P is fed by the paper supply roller 120. The transferred transfer paper P is fed to the registration roller pair 119 at the leading end. The registration roller pair 119 feeds the transfer paper P to the transfer unit at the timing when the toner image on the image carrier 111 moves to the transfer position.

The transferred transfer paper P is superimposed on the toner image at the transfer portion, and the toner image is electrostatically transferred by the action of the transfer roller 114. The transfer paper P to which the toner image is transferred is sent to the fixing device 116, where the toner image is fixed by the fixing device 116, passes through the conveyance path 121, and is discharged onto the tray 123 by the discharge roller pair 122.
The surface of the image carrier 111 after the toner image has been transferred is cleaned by a cleaning device 115 to remove residual toner, paper dust, and the like.

By using the optical scanning device of the present invention as the optical scanning device 117, an image forming apparatus compatible with a plurality of types of light sources can be obtained.
FIG. 7 shows only the main part of another embodiment of the image forming apparatus.
This image forming apparatus is a tandem type color image forming apparatus. In the reference numerals in the figure, Y indicates yellow, M indicates magenta, C indicates cyan, and K indicates black. The photoconductors 11Y, 11M, 11C, and 11K, which are optical scanning targets, can rotate clockwise. Around these photoconductors, the chargers TY, TM, TC, TK, the developing units GY, GM, GC, GK, transfer units 15Y, 15M, 15C, 15K and cleaning means BY, BM, BC, BK are provided.

  Optical scanning is performed by the optical scanning device 20 on the surface of the photosensitive member between the chargers TY, TM, TC, TK and the developing units GY, GM, GC, GK, and an electrostatic latent image is formed on each photosensitive member. It is like that. Then, the electrostatic latent image on each photoconductor is developed by a corresponding developing device, and Y, M, C, and K color toner images are formed on the photoconductor surface.

  While the transfer belt 17 rotates counterclockwise, the sheet-like recording medium is electrostatically adsorbed and conveyed on the upper peripheral surface thereof. The yellow toner image on the photoreceptor 11Y is transferred onto the sheet-like recording medium by the transfer device 15Y. The color toner images on the photoconductors 11M, 11C, and 11K are sequentially transferred onto the sheet-like recording medium by the transfer units 15M, 15C, and 15K. In this way, four color toner images are superimposed and transferred onto the sheet-like recording medium, and a color image is formed. This color image is fixed on the sheet-like recording medium by the fixing device 19.

  In FIG. 7, the optical path portion below the optical deflector 50 of the optical scanning device 20 is depicted. Although not shown in FIG. 7, the optical scanning device 20 has four types of light sources for the photoconductors 11Y to 11K, and light beams from these light sources are respectively guided to the optical deflector 50 by the first optical system. The The first optical system corresponding to each light source is the same as that described with reference to FIG. 1 or 2, and includes a coupling lens, a cylindrical lens, an aperture, and the like.

The optical deflector 50 is a polygon mirror that is long in the axial direction, and deflects the light beam guided from each light source by the first optical system as shown in the figure.
As an example, a description will be given of a light beam that optically scans the photoconductor 11M. This light beam is intensity-modulated by image information of a magenta color component of a color image to be imaged and scanned when deflected by the optical deflector 50. The scanning lens 8M that passes through the lens 8M, is sequentially reflected by the folding mirrors mM1, mM2, and mM3 to bend the optical path, passes through the scanning lens 10M, and is guided onto the photoconductor 11M, thereby constituting the second optical system. A light spot is formed on the photoconductor 11M by the action of 10M. Optical scanning of other photoconductors is performed in the same manner.

  By using the optical scanning device of the present invention as the optical scanning device 20, an image forming apparatus compatible with a plurality of types of light sources can be obtained.

A specific embodiment will be described below in accordance with the optical scanning device described with reference to FIG.
As described above, although not shown in FIG. 1, the optical scanning device of FIG. 1 has a soundproof glass for a housing that seals the polygon mirror 5 and a dustproof glass for the housing of the optical scanning device. . Both the soundproof glass and the dustproof glass have a thickness of 1.9 mm, and the soundproof glass is inclined by 10 degrees with respect to the “direction parallel to the main scanning direction” in a plane perpendicular to the rotation axis of the polygon mirror 5. Yes.

  The scanning optical system in the optical scanning device of the embodiment in the description is made common to two types of light sources having emission wavelengths of 655 nm and 785 nm. The emission wavelength is at 25 ° C.

  The refractive index with respect to the wavelength: 655 nm and 785 nm, the temperature fluctuation, and the linear expansion coefficient of the “glass” which is the material of the above soundproof glass / dustproof glass are shown below.

Glass Median Temperature fluctuation Linear expansion coefficient Wavelength: 655 nm 1.514350 1.514290 7.5 × 10 ⁻⁶
Wavelength: 785 nm 1.511076 1.511027 7.5 × 10 ⁻⁶
“Median” is the refractive index at the reference temperature: 25 ° C., and “temperature fluctuation” is the refractive index for each wavelength when the temperature is raised by 20 degrees from the reference temperature.

  The two types of light sources are referred to as light source 1 and light source 2. The emission wavelength at 25 ° C. for the light sources 1 and 2 and the temperature transition amount (temperature change: wavelength change amount per 1 ° C.) are shown in Table 1.

  The coupling lens 2, the cylindrical lens 4 and the scanning lens 6 constituting the first optical system are all made of “the same resin material”. Hereinafter, this resin material is simply referred to as “resin”.

  Table 2 shows the refractive indexes of the “resins” with respect to the two types of emission wavelengths.

The linear expansion coefficient of “resin” is “7.0 × 10 ⁻⁵ ”.

In Table 2, “wavelength” and “median” are the refractive index at the reference temperature: 25 ° C., and “temperature fluctuation” is the refractive index for each wavelength when the temperature rises 20 degrees from the reference temperature. It is.
The coupling lens 2 and the cylindrical lens 4 constituting the first optical system and the aperture 3 disposed between them will be described.

"Coupling lens"
The coupling lens 2 is formed of the “resin” and has a focal length of about 5 mm and is disposed so as to have a function of converting a divergent light beam from a light source into a “weak divergent light beam”.
In the coupling lens 2, a diffractive surface is used as an incident surface, and an exit surface has an aspherical shape. The aspherical shape is set so that “the wavefront aberration of the coupled light beam is sufficiently corrected”.

The diffractive surface on the incident side of the coupling lens 2 has a phase function: φ (R), where C is a coefficient.
φ (R) = C ・ R ²
It is represented by

  Table 3 shows values of the coefficient C for the light emission wavelengths of the light sources 1 and 2.

  Since there is a difference in refractive index due to material dispersion when the wavelength is different, the phase function (and hence the coefficient: C) differs depending on the wavelength even in the same shape.

The semiconductor laser 1 and the coupling lens 2 are fixedly held by a holding member made of a material having a linear expansion coefficient of 1.7 × 10 ⁻⁵ .

Diffraction surface is an incident surface of the coupling lens 2 is superimposed "parabolic constant coefficient 0.051546391 (refracting portion)", "the diffractive portion diffraction grooves of concentric circles" on "stepped surface shape ". In this case, power on the incident surface of the coupling lens 2, the main scanning and sub-scanning direction with P1 (power of the diffractive portion) = - P2 (power paraboloid forming the refraction portion), and the resulting diffraction surface " It becomes a multi-step type having a “stepped surface shape” . The power on the incident side surface of the coupling lens 2 is non-power in both main scanning and sub-scanning.
The “step difference: d” of the diffraction surface is 7496.38 nm.

The refractive index: n (λ) of the “resin” that is the material of the coupling lens 2 is 1.523859 for the wavelength: 785 nm and 1.527235 for the wavelength: 655 nm. Therefore,
λ / {n (λ) −1}
The values are 1.499 μm for the wavelength: 785 nm and 1.249 for the wavelength: 655 nm.

At this time,
1.499 × 5 = 7.495 μm
1.249 × 6 = 7.494 μm
So, the above step:
d = 7496.38 nm≈7.496 μm
Is substantially equal to the least common multiple of λ / {n (λ) −1} for wavelengths: 785 nm and 655 nm.

  That is, the diffractive surface of the embodiment has a shape optimized for fifth-order diffracted light for light with a wavelength of 785 nm and for sixth-order diffracted light for light with a wavelength of 655 nm.

  FIG. 3 shows a state of phase matching at the step of the diffractive surface. A portion indicated by reference numeral 30 is a resin portion. Since the diffractive surface is formed on the incident side surface of the coupling lens, light enters from the right side of the figure.

FIG. 3A shows a state of phase matching with respect to the wavelength: 785 nm. Phase matching occurs at the 10th wavelength propagating through the air portion and the 15th wavelength propagating through the resin portion, and fifth-order diffracted light is generated.
FIG. 3B shows a state of phase matching with respect to the wavelength: 655 nm. Phase matching occurs between the 12th wavelength propagating through the air portion and the 18th wavelength propagating through the resin portion, and sixth-order diffracted light is generated.

  If the step difference d is within the coherence length of the laser light emitted from the light sources 1 and 2, a diffractive optical element using any m-th order diffracted light can be designed. The coherence length also indicates “the fundamental limit of the diffraction function” in the diffractive optical element of the present invention. In general, the spectrum of a semiconductor laser has a full width at half maximum of 1 nm or less, and the coherence length is said to range from several mm to several m. Therefore, if the step is about several μm, sufficient diffracted light can be obtained.

The diffractive surface having the step: d can be used in common for light sources having a wavelength of 785 nm and a wavelength of 655 nm.
The difference between the geometric optical condensing positions of the two diffracted lights (5th order diffracted light and 6th order diffracted light) by the diffractive surface is only “a minute difference due only to the influence of material dispersion”. In order to absorb this geometrical difference, the position of the coupling lens 2 can be adjusted in the direction of the optical axis, and “adhesive layer for bonding the coupling lens 2 to the housing” is selected according to the emission wavelength of the light source. The thickness of “is adjusted.
As described above, the surface shape of the diffractive surface is set so that “the power of the diffractive part: P1 cancels the power of the refracting part: −P2” (Claim 2), and is a multi-step type (Claim). 3).

  Further, in this embodiment, “the power for correcting the fluctuation of the power of the entire optical system due to the temperature fluctuation by the negative dispersion characteristic of the diffraction surface” is provided on the diffraction surface (claim 8).

"aperture"
The aperture 3 disposed between the cylindrical lens 4 and the coupling lens 2 determines a beam waist diameter (light spot diameter). The aperture 3 is a “rectangular aperture” having an aperture diameter of 2.72 mm in the main scanning direction and an aperture diameter of 2.28 mm in the sub-scanning direction, and shapes the light beam coupled by the coupling lens 2. Of course, the aperture diameter is determined depending on the design of the optical system aimed at a desired spot diameter on the image plane.

"Cylindrical lens"
The cylindrical lens 4 is made of the “resin”, and the incident surface is a cylindrical surface having a sub-scanning radius of curvature of 19.72 mm, and the exit surface is a flat lens surface. The line image forming lens can also have a diffractive surface to which the present invention is applied. The design value of the cylindrical lens 4 is determined by the layout of the optical system.
Hereinafter, the optical system below the optical deflector will be described.

"Optical deflector"
The polygon mirror 5 which is an optical deflector has a number of reflecting surfaces: 6 and an inscribed circle radius of 13 mm. The angle θ between the traveling direction of the light beam incident from the light source side and the “traveling direction of the light beam reflected toward the position of the image height 0 on the scanned surface 8” by the deflection reflecting surface is 68 degrees. .

  Table 4 below shows data of the optical system on the optical path from the optical deflector to the surface to be scanned.

In Table 4, Rm is “paraxial curvature in the main scanning direction”, Rs is “paraxial curvature in the sub-scanning direction”, and Dx and Dy are “relative from the origin of each optical element to the origin of the next optical element”. Distance ". The unit is “mm”.
For example, regarding Dx and Dy with respect to the optical deflector, when viewed from the rotation axis of the optical deflector (polygon mirror 5), the origin of the incident surface of the scanning lens 6 (the optical axis position of the incident side surface) is in the optical axis direction. 43.0 mm away and 6.7 mm away in the main scanning direction. As described above, the soundproof glass is disposed between the optical deflector 5 and the scanning lens 6, and the dustproof glass is disposed between the scanning lens 6 and the scanned surface 8.

  Each surface of the scanning lens 6 is aspheric. Each surface is a special surface having a “non-arc shape” in the main scanning direction and “a curvature in the sub-scanning section (a virtual section parallel to the optical axis and the sub-scanning direction) changes in the main scanning direction”.

"Non-arc shape"
Paraxial radius of curvature in the main scanning section (virtual section including the optical axis and parallel to the main scanning direction): Rm, distance in the main scanning direction from the optical axis: Y, conic constant: Km, higher order coefficient: A ₁ , A ₂ , A ₃ , A ₄ ,..., Depth in the optical axis direction: X is expressed by the following well-known expression.

X = (Y ² / Rm) / [1 + √ {1- (1 + Km) (Y / Rm) ² }]
+ A ₁ Y + A ₂ Y ² + A ₃ Y ³ + A ₄ Y ⁴ + A ₅ Y ⁵ + A ₆ Y ⁶ +.

"Change in curvature in the sub-scan section"
An expression expressing a state in which the curvature in the sub-scanning section: Cs (Y) (Y: coordinates in the main scanning direction with the optical axis position as the origin) changes in the main scanning direction is expressed in the sub-scanning section including the optical axis. Radius of curvature: Rs (0), coefficients: B ₁ , B ₂ , B ₃ ,...

Cs (Y) = {1 / Rs (0)} + B 1 Y + B 2 Y 2 + B 3 Y 3 + B 4 Y 4 + B 5 Y 5 + B 6 Y 6 + ··.

  Table 5 gives the coefficient of the incident side surface (special surface) of the scanning lens 6.

  The coefficients of the exit side surface (special surface) of the scanning lens 6 are given in Table 6.

  The arrangement of the optical elements in the first optical system before the deflector and the second optical system after the deflector is such that “the imaging positions in the main scanning direction / sub-scanning direction of the entire optical system are in the vicinity of the scanned surface”. Properly arranged.

  In this embodiment, when the coupling lens 2 does not have a diffractive surface, when the temperature changes by 20 degrees, the variation in the beam waist position in the main scanning direction becomes as shown in Table 7.

  On the other hand, in this embodiment in which the coupling lens 2 has a diffractive surface, the beam waist position fluctuation in the main scanning direction when the temperature changes by 20 degrees is as shown in Table 8.

  By using a diffractive surface as described above for the coupling lens 2, the “optical scanning device that is robust against fluctuations in the beam waist position in the main scanning direction due to temperature fluctuations” can be applied to both the light sources 1 and 2 having different emission wavelengths. Has also been realized.

  In the embodiment, the coupling lens 2 has the function of weakening the divergence of the divergent light beam from the light source. However, the present invention is not limited to this, and the coupling lens 2 has a function of converting the light beam from the light source into a parallel light beam. A light source having a function of converting a light beam from a light source into a “weak focused light beam” or a “weak divergent light beam” can be used.

  The diffractive surface can be used not only for a coupling lens but also for a line image imaging optical system and a scanning lens. As the first optical system and the second optical system, well-known appropriate ones can be used in addition to those described above, and as the image forming apparatus described above, well-known “appropriately structured” Can be used.

It is a figure for demonstrating one Embodiment of an optical scanning device. It is a figure for demonstrating one Embodiment of the optical scanning device of a multi-beam scanning system. It is a figure for demonstrating the phase matching by a diffraction surface. It is a figure for demonstrating one example of a diffraction surface shape. It is a figure for demonstrating invention of Claim 2, 3. It is a figure for demonstrating one Embodiment of an image forming apparatus. It is a figure for demonstrating one Embodiment of an image forming apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light source 2 Coupling lens 3 Aperture 4 Cylindrical lens 5 Polygon mirror 6 Scan lens 8 Scanned surface

Claims (14)

The light beam emitted from the light source is guided to the optical deflector by the first optical system and deflected by the optical deflector, and the deflected light beam is formed as a light spot on the scanned surface by the second optical system, A light-transmitting diffractive optical element used in the first and / or second optical system in an optical scanning device that performs optical scanning of a surface to be scanned,
A diffractive surface is formed by steps,
The step of the diffraction surface: d is 2 or more wavelengths: λ _i (i = 1, 2,...), And the refractive index of the element material for these wavelengths: λ _i : n (λ _i )
λ _i / {n (λ _i ) −1}
A diffractive optical element characterized by being set to be substantially equal to a common multiple of. The diffractive optical element according to claim 1,
The diffractive surface has a surface shape in which a diffractive part due to a step is superimposed on a refracted part having power by refraction,
The above-sectional shape of the diffraction surface, the diffraction optical element, characterized in that the power by refraction power and the bent portion of the diffraction section is configured so as to offset. The diffractive optical element according to claim 2,
A diffractive optical element characterized in that the surface shape of a diffractive surface in which a diffractive part due to a step is superimposed on a refracting part having power by refraction is a multi-step type. The diffractive optical element according to any one of claims 1 to 3,
A diffractive optical element, wherein the diffractive surface is formed in a linear groove shape parallel to the main scanning direction and / or the sub-scanning direction. The diffractive optical element according to any one of claims 1 to 4,
With respect to the step of the diffraction surface: d is 2 or more, the wavelength: λ _i (i = 1, 2,...) And the refractive index of the element material for these wavelengths λ _i : n (λ _i )
λ _i / {n (λ _i ) −1}
A diffractive optical element characterized in that it is set to be substantially equal to the least common multiple. The diffractive optical element according to any one of claims 1 to 5,
A diffractive optical element, wherein a maximum pitch on the diffractive surface is smaller than a width of a light beam incident on the diffractive surface. The diffractive optical element according to any one of claims 1 to 6,
A diffractive optical element formed as a coupling lens for coupling a light beam from a light source. The diffractive optical element according to any one of claims 1 to 7,
A diffractive optical element, wherein the diffractive surface has a function of compensating for a change in optical function of the first and / or second optical system due to temperature fluctuation. The light beam emitted from the light source is guided to the optical deflector by the first optical system and deflected by the optical deflector, and the deflected light beam is formed as a light spot on the scanned surface by the second optical system, In an optical scanning device that performs optical scanning of a surface to be scanned, a scanning optical system configured by the first and second optical systems,
A scanning optical system, wherein the first and / or second optical system includes one or more diffractive optical elements according to any one of claims 1 to 8. The scanning optical system according to claim 9, wherein
2 or more wavelength relative lambda _i, have substantially the same optical characteristics, wavelength scanning optical, characterized in that it is common to two or more light sources that emit light of lambda _i system. The light beam emitted from the light source is guided to the optical deflector by the first optical system and deflected by the optical deflector, and the deflected light beam is formed as a light spot on the scanned surface by the second optical system, In an optical scanning device that performs optical scanning of a surface to be scanned,
As a scanning optical system constituted by the first and second optical systems, the scanning optical system according to claim 9 or 10,
An optical scanning device having a light source that emits any one of two or more wavelengths: λ _i (i = 1, 2,...). In an image forming apparatus that forms an image by optical scanning,
An image forming apparatus comprising at least one optical scanning device according to claim 11 as an optical scanning device for performing optical scanning. The image forming apparatus according to claim 12.
An image forming apparatus having a plurality of light sources and performing image formation by multi-beam scanning. The image forming apparatus according to claim 12 or 13,
An image forming apparatus having a plurality of light sources, performing optical scanning on a plurality of optical scanning targets, and performing image formation by superimposing images formed on the respective optical scanning targets.

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