JP5105250B2 - Optical scanning apparatus and image forming apparatus - Google Patents

Description

  The present invention relates to an optical scanning device provided in an electrophotographic image forming apparatus, and an image of a laser printer, a laser plotter, a digital copying machine, a plain paper facsimile, or a composite machine of these equipped with the optical scanning device. The present invention relates to a forming apparatus.

  In electrophotographic image forming apparatuses used in laser printers, laser plotters, digital copying machines, plain paper facsimiles, or multi-function machines of these types, in recent years, colorization and high speed have been advanced, and photoconductors that are image carriers. An image forming apparatus compatible with a tandem method having a plurality of (usually four) is widely used. In this tandem type image forming apparatus, for example, four photoconductors are arranged in parallel along a conveyance belt (or intermediate transfer belt) that conveys a sheet-like recording medium (recording material such as recording paper, postcards, and OHP sheets). After each photoconductor is charged by the charging means, a latent image is formed on each photoconductor by the writing unit, and the latent image on each photoconductor is developed with a developer having a different color from the developing means (for example, yellow, magenta, cyan). , Each color toner) is developed into a visible image, and each color image is transferred onto a recording material (or an intermediate transfer belt) conveyed by a conveyance belt to form a color image.

  The electrophotographic color image forming apparatus has only one photoconductor, rotates the photoconductor by the number of colors, and sequentially superimposes and transfers the image onto the intermediate transfer member. There is also a so-called one-drum-intermediate transfer method in which the image is formed and transferred onto the recording material in a lump. In this case, if there are four colors and one drum, the photoconductor is formed every time one image is formed. Must be rotated four times, and productivity is inferior compared to the tandem method.

As described above, the tandem image forming apparatus can increase the speed and improve the productivity of color image formation as compared with the one-drum-intermediate transfer system. However, in the case of the tandem image forming apparatus, optical scanning is possible. In order to perform optical writing on a plurality of photoconductors by a writing unit using the apparatus, the number of light sources of the optical scanning device inevitably increases (for example, when there are four photoconductors, four light sources are usually required). As a result, problems such as an increase in the number of components, a color shift due to a wavelength difference between a plurality of light sources, and an increase in cost occur.
Further, deterioration of the semiconductor laser is cited as a cause of the failure of the writing unit. For this reason, if the number of light sources increases, the probability of failure increases and the recyclability deteriorates.

Therefore, in order to achieve the following objects (A) to (C), “a beam from a common light source is divided, and the beams are incident on different stages of reflecting mirrors. Has been proposed (see, for example, Patent Document 1).
(A) An optical scanning device that enables high-speed image output while reducing the number of light sources is provided.
(B) Along with (A), the number of parts is reduced and the cost is reduced.
(C) Along with (A), the failure rate of the entire unit is reduced and the recyclability is improved.

The above-described method includes means for dividing a beam from a common light source. For example, the light beam is divided by the following method.
(A) A method using a half mirror prism.
(B) A method combining a half mirror and a mirror.
(C) A method of spatially dividing the emitted beam by providing a plurality of openings.

However, since both of the methods (a) and (b) use the mirror as the separating means, the beam spot diameter is likely to be deteriorated due to the influence of the variation in the surface accuracy of the mirror and the influence of the arrangement error. Further, the half mirror prism (a) is very expensive, which increases the cost.
The method of combining the half mirror and the mirror in (b) is difficult to layout and has an opening angle in the deflection rotation plane, so that the optical characteristics such as the beam spot diameter deteriorate.
In the method (c) of dividing the light beam by the plurality of openings, the peripheral portion of the beam from the light source is used, so that the light amount is insufficient and the beam spot diameter is increased.

  Accordingly, the applicant of the present application is based on the premise of the method of “dividing a beam from a common light source, causing the beam to enter a reflecting mirror at a different stage, and scanning different surfaces to be scanned” (A) It enables high-speed image output while reducing the number of light sources, (B) reduces the number of parts, reduces costs, and (C) reduces the overall unit failure rate and improves recyclability. An optical scanning device that can solve the above-described problems while maintaining the merit is proposed (see Patent Document 2).

The method disclosed in Patent Document 2 includes means for splitting a beam from a common light source, and splits a light beam by the following method.
(1) A volume phase type diffraction grating is used and divided into zero-order transmitted light and first-order terms.
(2) A diffractive grating having a blazed (sawtooth) surface shape or a blazed (sawtooth) surface shape approximated in a staircase shape is used to divide into zero-order transmitted light and primary light.
(3) A rectangular convex / concave diffraction grating with a continuously changing duty factor is used to divide into zero-order transmitted light and first-order terms.

JP 2005-92129 A JP 2007-279670 A

However, the method described in Patent Document 2 also has the following problems.
Since the volume phase type diffraction grating of (1) is prepared by using a chemical reaction, it is not suitable for mass production and the cost is high.
(2) and (3) have a finer structure in the grating period, which requires high-precision jigs and time to process the mold, which increases costs, and is too fine for injection molding to transfer. There are also concerns that cannot be made. Therefore, it is impossible to make an object whose grating period is less than the wavelength.
In addition, since the emission angle of the primary light can be increased as the grating period becomes finer, in (2) and (3), a large emission angle cannot be obtained. It is necessary to take a long space between the optical elements, and the apparatus becomes large.

  The present invention has been made in view of the above circumstances, and can provide a compact and low-cost optical scanning device that can be mass-produced, and also enables high-speed image output while reducing the number of light sources. The purpose is to provide an optical scanning device. Further, the optical scanning device can be provided, the number of parts can be reduced and the cost can be reduced, the failure rate of the entire unit can be reduced, and the recyclability can be improved. It is an object of the present invention to provide an image forming apparatus that can be used.

In order to achieve the above object, the present invention employs the following solutions.
The first means of the present invention includes a light source, an optical deflector provided with a plurality of deflection reflection surfaces in the sub-scanning direction, and a light beam from the light source into a plurality of light beams incident on each of the plurality of deflection reflection surfaces. A first diffractive optical element to be split; a second diffractive optical element deflected in the sub-scanning direction opposite to the direction deflected by the first diffractive optical element; and the light beam deflected by the optical deflector A scanning optical system for condensing the light on the surface to be scanned, and a plurality of light beams incident on the optical deflector scan on different surfaces to be scanned,
In the first and second diffractive optical elements, the surface relief type duty factor has a constant binary diffractive surface regardless of the location, and the grating periods of the first and second diffractive optical elements are equal, The wavelength of the first diffractive optical element is less than the wavelength, and the depth of the grating of the second diffractive optical element is twice that of the first diffractive optical element. and the lamination, and the first and second diffractive optical element are parallel, and wherein the light beam is split by the first diffractive optical element 0 is order diffracted light and 1-order diffracted light, and the first diffraction The light beam is obliquely incident on the optical element.

According to a second means of the present invention, in the optical scanning device of the first means, the first and second diffractive optical elements are made of a thermoplastic resin.
The third aspect of the present invention is the optical scanning device of the first hand stage, with respect to the first diffractive optical element, a grating pitch lambda, the wavelength of the light beam lambda, the incidence angle is θ When the conditional expression:
2Λ sinθ = λ
Is substantially satisfied .

A fourth means of the present invention is the optical scanning device of the third hand stage, the first and second diffractive optical element is characterized in that it is made of thermoplastic resin.
According to a fifth means of the present invention, in the optical scanning device of the third means, when the grating pitch is Λ and the wavelength of the light beam is λ with respect to the first diffractive optical element, the conditional expression:
Λ ≦ 0.707 × λ
It is characterized by satisfying.
Further, a sixth means of the present invention is the optical scanning device of the fifth means, wherein the first and second diffractive optical elements are made of a thermoplastic resin.

According to a seventh means of the present invention, an electrostatic latent image is individually formed on a plurality of image carriers by optical scanning, the electrostatic latent images are developed and visualized, and the obtained image is directly or intermediately formed. In the tandem type image forming apparatus that performs image formation synthetically by transferring onto the same sheet-like recording medium via a transfer body, first to thirty-first optical scanning means for optically scanning the plurality of image carriers The optical scanning device according to any one of the sixth means is provided.
Further, according to an eighth means of the present invention, in the image forming apparatus of the seventh means, the image carrier is a photoconductive photoconductor.

In the optical scanning device of the first means, the first and second diffractive optical elements have a surface relief type duty factor and have a constant binary diffractive surface regardless of the location, so that mass production is possible and low cost. An optical scanning device can be realized, and since the grating periods of the first and second diffractive optical elements are equal and less than or equal to the light source wavelength, the separation angle can be increased, so that a compact optical scanning device can be realized, and said first and second diffractive optical elements Ri alignment easy der so parallel, and, since the depth of the grating of the second diffractive optical element is twice the first diffractive optical element, By superimposing two first diffractive optical elements, a second diffractive optical element can be obtained, and the pitch of the first and second diffractive optical elements can be completely matched and divided into two parallel light beams. The second diffractive optical element is the first diffractive optical element. Since the diffractive optical elements are bonded together, the pitch between the two elements is completely coincident and only one mold is required, so that a highly reliable and low-cost optical scanning device can be realized.

In the optical scanning device of the second means, in addition to the configuration and effects of the first means, the first and second diffractive optical elements are made of thermoplastic resin, so that mass production is possible and low-cost light is provided. A scanning device can be realized .

In the optical scanning device of the third means, in addition to the structure and effects of the first hand stage, with respect to the first diffractive optical element, a grating pitch lambda, the wavelength of the light beam lambda, the incidence angle is θ When the conditional expression:
2Λ sinθ = λ
Therefore, the light beam can be separated with little loss, the light source power is suppressed, and the energy is saved.
In the optical scanning device of the fourth means, in addition to the configuration and effects of the third means, the first and second diffractive optical elements are made of thermoplastic resin, so that mass production is possible and low cost. An optical scanning device can be realized.
Further, in the optical scanning device of the fifth means, in addition to the configuration and effects of the third means, the condition is satisfied when the grating pitch is Λ and the wavelength of the light beam is λ with respect to the first diffractive optical element. formula:
Λ ≦ 0.707 × λ
Therefore, the beam separation angle is 90 ° or more, and a compact optical scanning device is possible.
Further in the optical scanning device of the sixth means, in addition to the configuration and effects of the fifth hand stage, the first and second diffractive optical elements are the thermoplastic resin, can be mass-produced, low An inexpensive optical scanning device can be realized.

  In the image forming apparatus of the seventh and eighth means, the optical scanning of any one of the first to sixth means is used as the optical scanning means for performing optical scanning on a plurality of image carriers (photoconductive photosensitive members). Equipped with an optical scanner that can reduce the number of light sources, but enables high-speed image output, reduce the number of parts and reduce costs, and reduce the failure rate of the entire unit. Therefore, it is possible to realize an image forming apparatus capable of improving recyclability.

Embodiments of an optical scanning device, an image forming apparatus, a beam splitting diffractive optical element, and a diffractive optical element according to the present invention will be described below in detail with reference to the drawings.
In the following description, the direction in which the optical scanning device optically scans the surface to be scanned is defined as the main scanning direction, and the direction orthogonal to the main scanning direction is defined as the sub-scanning direction.

[Configuration example of optical scanning device]
FIG. 1 is an optical layout diagram showing a basic configuration example of an optical scanning device in which the present invention is implemented. In FIG. 1, reference numerals 1 and 1 'are semiconductor lasers as light sources, 2 is a base for holding the semiconductor lasers 1 and 1', 3, 3 'are coupling lenses, 4 is a half mirror prism as light beam separating means, and 5a. , 5b are cylindrical lenses, and 7 is an optical deflector having two-stage polygon mirrors (rotating polygon mirrors) 7a, 7b. Reference numeral 6 denotes a soundproof glass provided in a window of a soundproof housing (not shown) of the optical deflector 7.

The semiconductor lasers 1 and 1 ′ are modulated and driven based on image signals, and each emits one divergent light beam. Each light beam emitted from the semiconductor laser 1 or 1 ′ is coupled into a beam form (parallel light beam or weakly divergent or weakly convergent light beam) suitable for the subsequent optical system by the coupling lenses 3 and 3 ′. The
Each light beam that has passed through the coupling lenses 3 and 3 ′ passes through the opening of the aperture 12 that regulates the light beam width, is shaped into a beam, and then enters the half mirror prism 4. Divided into two in the sub-scanning direction by the action of the prism 4.

Thus, one light beam from the semiconductor laser 1 is divided into two light beams by the half mirror prism 4, and similarly, one light beam emitted from the semiconductor laser 1 'is converted into a half mirror prism. Divided by 4 into two light beams.
The four light beams that have passed through the half mirror prism 4 are incident on the cylindrical lenses 5a and 5b, and are condensed in the sub-scanning direction by the action of the cylindrical lenses 5a and 5b. Rotating polygon mirrors 7a and 7b are formed as line images that are long in the main scanning direction on or near the deflecting reflecting surfaces.
The light beam that has passed through the half mirror prism 4 enters the light deflector 7 through the soundproof glass 6.

  The polygon mirrors 7a and 7b constituting the optical deflector 7 are arranged in two upper and lower stages in the sub-scanning direction, and are rotationally driven at a constant speed by a drive motor (not shown). Two light beams emitted from the semiconductor laser 1 and divided by the half mirror prism 4 are incident on the upper polygon mirror 7a. Two light beams emitted from the semiconductor laser 1 ′ and divided by the half mirror prism 4 are incident on the lower polygon mirror 7 b.

  Here, the upper polygon mirror 7a and the lower polygon mirror 7b are both of the same shape having four deflection reflection surfaces, but the deflection reflection surface of the lower polygon mirror 7b is the same as that of the upper polygon mirror 7a. A predetermined angle: θ (= 45 °) is deviated in the rotation direction with respect to the deflecting reflecting surface. The upper polygon mirror 7a and the lower polygon mirror 7b may be integrally formed or may be assembled separately.

Reference numeral 8 (8a, 8b) is a first scanning lens, 10a, 10b are second scanning lenses, 9a, 9b are optical path bending mirrors, and 11a, 11b are optical scanning surfaces (image-conducting photoconductors). Is shown.
The first scanning lens 8a, the second scanning lens 10a, and the optical path bending mirror 9a constitute a set of scanning imaging optical system, and two light beams deflected by the upper polygon mirror 7a are subjected to corresponding optical scanning. Two light beam spots that are guided on the surface of the photoreceptor 11a and separated in the sub-scanning direction are formed.
The first scanning lens 8b, the second scanning lens 10b, and the optical path bending mirror 9b constitute a pair of scanning imaging optical systems, and correspond to the two light beams deflected by the lower polygon mirror 7b. Two light beam spots are formed on the photosensitive member 11b, which is the light scanning surface, and separated in the sub-scanning direction.

  The optical arrangement of the light beams emitted from the semiconductor lasers 1 and 1 ′ is determined so that the principal rays intersect in the vicinity of the position of the deflecting reflection surface when viewed from the rotation axis direction of the optical deflector 7. Therefore, the two light beams forming a pair incident on the deflecting / reflecting surface are orthogonal to each other when the opening angle, that is, when viewing the light source side from the deflecting / reflecting surface side, is orthogonal to the rotation axes of the two light beams. The angle formed by the projection onto the surface to be formed. Due to this opening angle, the two light beam spots formed on the photoreceptors 11a and 11b are also separated in the main scanning direction. Therefore, it is possible to individually detect two light beams that optically scan each of the photoconductors 11a and 11b, and to synchronize the start of optical scanning for each light beam.

  As described above, the two light beams deflected by the upper polygon mirror 7a of the optical deflector 7 are used to perform multi-beam scanning on the photosensitive member 11a with the two light beams. Further, the multi-beam scanning of the photoconductor 11b is performed by the two light beams by the two light beams deflected by the lower polygon mirror 7b of the optical deflector 7.

  The deflection reflecting surfaces of the upper polygon mirror 7a and the lower polygon mirror 7b of the optical deflector 7 are shifted from each other by 45 degrees in the rotational direction. Therefore, when the light beam deflected by the upper polygon mirror 7a scans the photoconductor 11a, the light beam deflected by the lower polygon mirror 7b is not guided to the photoconductor 11b. When the deflected light beam from the upper polygon mirror 7b scans the photoconductor 11b, the deflected light beam from the lower polygon mirror 7a is not guided to the photoconductor 11a. That is, the optical scanning of the photoconductors 11a and 11b is performed "alternatingly shifted in time".

FIG. 7 is a diagram for explaining this situation, and is a diagram for explaining a time lag in optical scanning by the upper and lower polygon mirrors 7a and 7b. Although there are actually four light beams incident on the optical deflector 7, they are depicted as one incident light, and the light beams deflected by the upper and lower polygon mirrors 7a and 7b are respectively deflected light a. , Shown as deflected light b.
FIG. 7A shows a situation when incident light is incident on the optical deflector 7 and the deflected light a reflected and deflected by the upper polygon mirror 7a is guided to the optical scanning surface (photoreceptor 11a). Is shown. At this time, the deflected light b from the lower polygon mirror 7b does not go to the optical scanning surface (photosensitive member 11b).
FIG. 7B shows the case where incident light is incident on the optical deflector 7 and the deflected light b reflected and deflected by the lower polygon mirror 7b is guided to the optical scanning surface (photoconductor 11b). Shows the situation. At this time, the deflected light a from the upper polygon mirror 7a does not travel toward the optical scanning surface (photosensitive member 11a).

In addition, as shown in FIG. 7, light shielding means is provided at an appropriate position so that the deflected light from the other polygon mirror does not act as ghost light while the deflected light from one polygon mirror is guided to the optical scanning position. (For example, a light shielding member SD) may be arranged so that the deflected light that is not guided to the optical scanning surface is shielded by the light shielding means (light shielding member SD).
Also, when the light source modulation drive timing is shifted between the upper and lower stages and the photoconductor 11a corresponding to the upper stage is scanned, the light source is modulated and driven based on image information of the color corresponding to the upper stage (for example, black). When the photoconductor 11b corresponding to the lower stage is scanned, the light source may be modulated based on the image information of the color (for example, magenta) corresponding to the lower stage.

  FIG. 8 shows a case where a black image and a magenta image are written by a common light source, for example, the semiconductor lasers 1 and 1 ′ shown in FIG. A time chart is shown (in the figure, a solid line indicates a portion corresponding to writing of a black image, and a broken line indicates a portion corresponding to writing of a magenta image). The main scanning timing for writing out the black image and the magenta image is optically scanned by a well-known synchronization detecting means (not shown in FIG. 1; normally a photodiode is used) arranged outside the effective scanning area. It is determined by detecting the light beam toward the position.

  In FIG. 8, the light emission intensity of the light source is set to be the same in the time domain in which the black image and the magenta image are written, but actually, in each optical path from the light source to the photoconductors 11a and 11b, When there is a relative difference in transmittance and reflectance, the amount of light beam reaching each photoconductor is different. Therefore, as shown in FIG. 9, when scanning each photoconductor surface corresponding to each color, the light intensity reaching the different photoconductor surfaces can be made equal by changing the light emission intensity (light emission amount) in the light source. .

The example of the basic configuration and operation of the optical scanning device according to the present invention has been described above. However, in the optical scanning device having the configuration shown in FIG. Instead, a light beam separating means comprising two diffractive optical elements 13 and 14 as shown in FIG. 2 is used.
FIG. 2 is a sub-scan sectional view of the optical system before the optical deflector 7 (two-stage rotary polygon mirrors 7a and 7b), showing the main part of the light beam separating means according to the present invention.

  In FIG. 2, the light beam separating means is composed of first and second diffractive optical elements 13 and 14, and the first and second diffractive optical elements 13 and 14 have a constant surface relief type duty factor regardless of location. The first and second diffractive optical elements 13 and 14 have the same grating period, are equal to or shorter than the light source wavelength, and the first and second diffractive optical elements 13 and 14 are parallel to each other. In addition, the light beam divided by the first diffractive optical element 13 is a 0th-order diffracted light and a first-order diffracted light, and the light beam is obliquely incident on the first diffractive optical element 13. Hereinafter, specific examples of the first and second diffractive optical elements 13 and 14 will be described.

[Example 1]
First, the 1st Example of this invention is shown.
Light emitted from a light source 1 such as a semiconductor laser (LD) is converted into a parallel beam by a coupling lens (for example, a collimating lens) 3 and is incident obliquely on the first diffractive optical element 13 and travels in the same direction as the incident light. Next-order diffracted light and first-order diffracted light (herein referred to as “−1st-order diffracted light”) diffracted upward in the drawing in the figure are generated. The 0th-order diffracted light is incident on the deflection reflection surface of the lower polygon mirror 7b. On the other hand, the −1st order diffracted light enters the second diffractive optical element 14, is diffracted again by the second diffractive optical element 14, and enters the upper polygon mirror 7 a. Between the first diffractive optical element 13 and the second diffractive optical element 14 and the rotary polygon mirror 7, an aperture 12 and cylindrical lenses 5a and 5b for defining an aperture are arranged.

  The light beam incident on the upper polygon mirror 7 a is re-diffracted by the second diffractive optical element 14 and then deflected in the direction opposite to the sub-scanning direction with respect to the direction deflected by the first diffractive optical element 13. To do. By doing so, it is possible to reduce the change in the incident angle to the optical deflector (rotating polygonal mirror) 7 when the wavelength change of the semiconductor laser 1 occurs, so that the sub-scanning direction of the light beam directed to different scanning lines The beam spot position fluctuation can be reduced, and the scanning line bending fluctuation can be reduced.

More preferably, the light beam incident on the upper polygon mirror 7a should be parallel to the incident light on the first diffractive optical element 13. At this time, the incident angle in the sub-scanning direction to the optical deflector (rotating polygonal mirror) 7 is the same in both the upper and lower stages, and preferably incident perpendicularly to the rotation axis of the optical deflector (rotating polygonal mirror) 7. To. Thereby, when the wavelength of the semiconductor laser 1 is changed, the change in the incident angle to the optical deflector (rotating polygon mirror) 7 can be reduced to zero, and the beam in the sub-scanning direction of the beam toward the different scanning line. Spot position fluctuation can be reduced, and scanning line bending fluctuation can be reduced. This can be realized by setting the grating pitches of the first diffractive optical element 13 and the second diffractive optical element 14 to be equal and the diffractive surfaces to be parallel to each other.
The first diffractive optical element 13 and the second diffractive optical element 14 are both made of a thermoplastic resin, and can be mass-produced at low cost by injection molding or the like.

The configuration and optical characteristics of the first diffractive optical element 13 at this time will be described with reference to FIG.
As an example, wavelength λ = 0.655 μm, polarization direction is sub-scanning direction, substrate refractive index N = 1.525, grating period Λ = 0.575 μm of diffraction surface, fill factor f = 0.565, grating depth D = 0.72 μm.
Here, FIG. 4 shows the incident angle θ dependency of the diffraction efficiency of the first diffractive optical element 13 calculated by the Fourier mode method based on the vector diffraction theory.
As is clear from FIG. 4, the diffraction efficiency of -1st order diffracted light is highest at θ = 34.72 °, and this is defined as the incident angle.
Further, 2Λsinθ = 0.655 μm, which matches the wavelength λ.

When the diffraction angle of the first diffractive optical element 13 is obtained based on the grating equation, the 0th-order diffracted light is transmitted in the same direction as the incident light, and θ ₋₁ = 21.93 ° and θ ′ = This is 34.72 °, which is about 70 ° with respect to the incident light. The reason why the exit angle of the −1st order diffracted light is about 70 ° is that Λ <λ.
Thus, since the angle is high, a sufficient light beam separation interval can be obtained even if the distance between the semiconductor laser and the polygon mirror is narrow.
The reason why the diffraction efficiency of the −1st order diffracted light is larger than that of the 0th order diffracted light is that it is designed in consideration of the loss of the −1st order diffracted light in the second diffractive optical element 14.

Next, the configuration and optical characteristics of the second diffractive optical element 14 will be described with reference to FIG.
The refractive index N of the substrate was 1.525, the grating period Λ = 0.575 μm of the diffraction surface, the fill factor f = 0.565, and the grating depth D = 1.44 μm. θ ′ = 34.72, θ ″ = 21.93 °, and the −1st order diffracted light is emitted in the same direction as the incident light. The diffraction efficiency of the −1st order diffracted light is 93.8%.

[Example 2]
Next, a second embodiment of the present invention will be shown.
This embodiment is the same as the first embodiment except for the configuration of the second diffractive optical element 14.
In the first embodiment, the grating depth of the second diffractive optical element 14 is twice the depth of the first diffractive optical element 13.
Therefore, as shown in FIG. 6, the same effect can be obtained by bonding two first diffractive optical elements 13 in place of the second diffractive optical element 14.
Furthermore, by using the diffractive optical element of FIG. 6, all can be made with the same mold, so that good performance can be obtained without a shift in the period Λ.

[Example 3]
Next, a third embodiment of the present invention will be shown.
In the third embodiment of the present invention, both the grating period of the first diffractive optical element 13 and the grating period of the second diffractive optical element 14 are set to a period Λ = 0.463 μm (= 0.707 × λ). The grating depth of one diffractive optical element 13 is D = 0.94 μm, the incident angle θ = 45 °, the grating depth of the second diffractive optical element 14 is D = 1.88 μm, and the others are the first embodiment. Was the same. In this case, θ ₋₁ = 27.63 ° and θ ′ = 45 °, and the light emitted from the first diffractive optical element 13 becomes 90 ° with respect to the incident light, and a narrow space regardless of the light beam separation interval. With this, the light beam can be separated.

As shown in the first to third embodiments, in the optical scanning device according to the present invention, the first and second diffractive optical elements 13 and 14 have constant binary diffraction regardless of where the surface relief type duty factor is located. Since it has a surface, mass production is possible, a low-cost optical scanning device can be realized, and the grating periods Λ of the first and second diffractive optical elements 13 and 14 are equal and are equal to or less than the light source wavelength λ. Since the separation angle can be increased, a compact optical scanning device can be realized, and the first and second diffractive optical elements 13 and 14 are parallel, so that alignment is easy.
In the optical scanning device according to the present invention, since the first and second diffractive optical elements 13 and 14 are made of thermoplastic resin, mass production is possible and a low-cost optical scanning device can be realized.
Furthermore, in the optical scanning device according to the present invention, since the grating depth of the second diffractive optical element 14 is twice that of the first diffractive optical element 13, as shown in FIG. By superimposing two elements 13, the second diffractive optical element is obtained, and the grating pitch Λ of the first and second diffractive optical elements can be perfectly matched and divided into two parallel light beams.

In the optical scanning device according to the present invention, with respect to the first diffractive optical element 13, when the grating pitch is Λ, the wavelength of the light beam is λ, and the incident angle is θ, the conditional expression:
2Λ sinθ = λ
Therefore, the light beam can be separated with little loss, the light source power is suppressed, and the energy is saved.
Further, with respect to the first diffractive optical element 13, when the grating pitch is Λ and the wavelength of the light beam is λ, the conditional expression:
Λ ≦ 0.707 × λ
Therefore, the beam separation angle is 90 ° or more, and a compact optical scanning device is possible.
Further, in the optical scanning device according to the present invention, the second diffractive optical element 14 is configured by bonding the first diffractive optical element 13 so that the grating pitch Λ of the two elements is completely matched and the mold is also formed. Since only one is required, a highly reliable and low-cost optical scanning device is possible.

[Configuration example of image forming apparatus]
Next, an embodiment of an image forming apparatus provided with the above-described optical scanning device of the present invention will be described.
FIG. 10 is a schematic configuration diagram of a multicolor image forming apparatus showing an embodiment of the present invention, and shows an example of a tandem laser printer advantageous for high-speed output of a color image.
This image forming apparatus includes an optical scanning device 33 including scanning optical systems corresponding to yellow (Y), magenta (M), cyan (C), and black (K), and a photoconductor 31X (corresponding to each scanning optical system). X: Y, M, C, K, and so on)), a conveyance belt 37 stretched between a driving roller 38a and a driven roller 38b, a fixing device 39, and a paper feed cassette 40 having recording paper as a sheet-like recording medium. And a paper discharge tray (not shown).
Above the transport belt 37, a photoconductive photosensitive member 31 </ b> X formed in a cylindrical shape as an image carrier on which an electrostatic latent image is formed by exposure by the optical scanning device 33 is upstream in the moving direction of the transport belt 37. From the side, yellow (31Y), magenta (31M), cyan (31C), and black (31K) are arranged in this order. The diameters of the photoreceptors 1X are all the same.

  Around each photoreceptor 31X, an electrostatic latent image is formed by using charging means (for example, charging roller) 32X, developing means (one-component developer (each color toner) or two-component developer (each color toner and carrier)). A process according to an electrophotographic system (electrophotographic process) such as a developing device 34X for visualizing with toner, a transfer means (transfer charger or transfer roller) 36X, and a cleaning means (cleaning device comprising a cleaning blade, a cleaning brush, etc.) 35X. The members are arranged in order. As a charging unit, a corona charger can be used instead of the charging roller.

Around the conveyance belt 37, a registration roller 45 and a belt charging charger (not shown) are arranged on the upstream side of the recording paper conveyance path from the photoreceptor 31X, and further downstream of the recording paper conveyance path from the photoreceptor 31X. On the side, a belt separation charger (not shown), a static elimination charger (not shown), a cleaning device (not shown), and the like are sequentially arranged.
As described above, in the image forming apparatus, the photoconductors 31Y, 31M, 31C, and 31K are optical scanning surfaces set for the respective colors, and the scanning optical systems are provided in a one-to-one correspondence relationship with respect to each. However, the light deflector is shared by each color.

The optical scanning device 33 is an optical writing device that performs optical writing on the photosensitive member 31X, and performs an exposure process of an electrophotographic process. The optical scanning device 33 scans the surface of the photosensitive member 31X uniformly charged by the charging unit 32X. Scan to form an electrostatic latent 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 means 34X, and a toner image is formed on the photoreceptor 31X.
As the optical scanning device 33, the optical scanning device according to the present invention described above (the basic configuration is the same as that in FIG. Is used).

  The uppermost sheet of the sheet-like recording medium (for example, regular recording paper) S stored in the paper feeding cassette 40 is fed by the paper feeding roller 41, and the fed recording paper S is separated by the separation roller 42, conveyed. The roller is transferred to the registration roller 45 through the rollers 43 and 44, and the leading end thereof is caught by the registration roller 45. The registration roller 45 sends the recording paper S to the transfer unit in accordance with the timing at which the toner image on the photoconductor 31X moves to the transfer position. The fed recording paper S is superimposed on the toner image at the transfer portion, and the toner image is electrostatically transferred by the action of the transfer means 36X.

  The recording sheet S to which the toner image is transferred is sent to the fixing device 39, where the toner image is fixed by the fixing device 39, passes through a conveyance path (not shown), and is discharged to a discharge tray (not shown) by a discharge roller (not shown). Discharged. The surface of the photoreceptor 31X after the toner image is transferred is cleaned by the cleaning device 35X, and residual toner, paper dust, and the like are removed.

  In the tandem-type image forming apparatus configured as described above, for example, when the multi-color mode (full color mode) is selected, the optical scanning device 33 corresponding to the image signal of the corresponding color for each photoconductor 31X. As a result of this exposure scanning, an electrostatic latent image is formed on each photoconductor. These electrostatic latent images are developed with corresponding color toners of the respective developing devices 34X to become toner images, and are sequentially transferred onto the recording paper S that is electrostatically attracted onto the transport belt 37 and transported. So that they are superimposed. Then, after being fixed as a color image by the fixing device 39, the recording sheet S is discharged to a discharge tray.

  When the single color mode is selected, the photosensitive members and process members of other colors are in a non-operating state as a certain color S (any one of Y, M, C, and K). Here, only on the photoreceptor 31S, an electrostatic latent image is formed by exposure scanning of the optical scanning device 33, and is developed with toner of a certain color S to become a toner image, which is electrostatically attracted onto the conveyance belt 37. Then, it is transferred onto the recording paper S that is conveyed. Then, after being fixed as a single color image by the fixing device 39, the recording sheet S is discharged to a discharge tray.

  In the optical scanning device shown in FIG. 1, only the figures corresponding to the two photoconductors 11a and 11b are shown. However, the optical scanning device 33 of the image forming apparatus shown in FIG. In addition to the configuration similar to that of the scanning device, a scanning optical system similar to the optical system shown in the figure is disposed at a symmetrical position with the optical deflector 7 interposed therebetween, and the four photosensitive members 31Y described above are arranged. , 31M, 31C, and 31K can be scanned.

  In the image forming apparatus of the present embodiment described above, the optical scanning device 33 having the configuration described in the above-described embodiment is provided as an optical scanning unit that performs optical scanning on the plurality of photoconductors 31Y, 31M, 31C, and 31K. Using an optical scanning device that enables high-speed image output while reducing the number of light sources, the number of parts can be reduced and the cost can be reduced, the failure rate of the entire unit can be reduced, and recyclability can be improved. be able to.

1 is an optical layout diagram illustrating a basic configuration example of an optical scanning device in which the present invention is implemented. It is a sub-scan sectional view of the optical system before the optical deflector (two-stage rotary polygon mirror), showing the main part of the light beam separating means according to the present invention. It is a schematic principal part sectional drawing of a 1st diffractive optical element. It is a figure which shows the incident angle dependence of the diffraction efficiency of a 1st diffractive optical element. It is a schematic principal part sectional drawing of a 2nd diffractive optical element. It is a schematic principal part sectional drawing of the 2nd diffractive optical element formed by bonding together two 1st diffractive optical elements. It is a figure for demonstrating the optical scanning by the optical deflector (2-stage polygon mirror) of the optical scanning apparatus shown in FIG. It is a figure which shows the timing chart of the exposure for multiple colors. It is a figure which shows the timing chart for varying the exposure amount according to a color. 1 is a diagram showing an embodiment of an image forming apparatus using an optical scanning device according to the present invention, and is a schematic configuration diagram of a tandem type laser printer.

Explanation of symbols

1, 1 ': Light source (semiconductor laser)
2: Base 3, 3 ': Coupling lens 4: Light beam separation means 5a, 5b: Cylindrical lens 6: Soundproof glass 7: Optical deflector 7a, 7b: Polygon mirror (rotating polygon mirror)
8 (8a, 8b): first scanning lenses 9a, 9b: folding mirrors 10a, 10b: second scanning lenses 11a, 11b: photoconductor as an optical scanning surface 12: aperture stop (aperture)
13: First diffractive optical element 14: Second diffractive optical element 31Y, 31M, 31C, 31K: Photoconductor (image carrier)
32Y, 32M, 32C, 32K: Charging means 33: Optical scanning device 34Y, 34M, 34C, 34K: Developing means 35Y, 35M, 35C, 35K: Cleaning means 36Y, 36M, 36C, 36K: Transfer means 37: Conveying belt 38a : Driving roller 38b: driven roller 39: fixing device 40: paper feeding cassette 41: paper feeding roller 42: separation roller 43, 44: conveying roller 45: registration roller S: recording paper (sheet-like recording medium)

Claims (8)

A light source, an optical deflector provided with a plurality of deflecting and reflecting surfaces in the sub-scanning direction, and a first diffractive optical element that splits a light beam from the light source into a plurality of light beams incident on each of the plurality of deflecting and reflecting surfaces; The second diffractive optical element that deflects in the reverse direction in the sub-scanning direction with respect to the direction deflected by the first diffractive optical element and the light beam deflected by the optical deflector are condensed on the surface to be scanned. In an optical scanning device comprising a scanning optical system, wherein a plurality of light beams incident on the optical deflector scan different scanning surfaces, respectively.
In the first and second diffractive optical elements, the surface relief type duty factor has a constant binary diffractive surface regardless of the location, and the grating periods of the first and second diffractive optical elements are equal, The wavelength of the first diffractive optical element is less than the wavelength, and the depth of the grating of the second diffractive optical element is twice that of the first diffractive optical element. and the lamination, and the first and second diffractive optical element are parallel, and wherein the light beam is split by the first diffractive optical element 0 is order diffracted light and 1-order diffracted light, and the first diffraction An optical scanning device characterized in that a light beam is obliquely incident on an optical element. 2. The optical scanning device according to claim 1, wherein the first and second diffractive optical elements are made of a thermoplastic resin. The optical scanning apparatus according to claim 1 Symbol placement, with respect to the first diffractive optical element, a grating pitch lambda, the wavelength of the light beam lambda, when the incident angle is theta, Condition:
2Λ sinθ = λ
An optical scanning device characterized by substantially satisfying The optical scanning apparatus according to claim 3 Symbol mounting, the first and second diffractive optical element includes an optical scanning device which is a thermoplastic resin. 4. The optical scanning device according to claim 3 , wherein for the first diffractive optical element, when the grating pitch is Λ and the wavelength of the light beam is λ, the conditional expression:
Λ ≦ 0.707 × λ
An optical scanning device characterized by satisfying the above. The optical scanning apparatus according to claim 5 Symbol mounting, said first and second diffractive optical element includes an optical scanning device which is a thermoplastic resin. An electrostatic latent image is individually formed on a plurality of image carriers by optical scanning, the electrostatic latent images are developed and visualized, and the obtained image is directly or via an intermediate transfer member in the same sheet form In a tandem type image forming apparatus that performs image formation synthetically by transferring onto a recording medium,
An image forming apparatus comprising the optical scanning device according to claim 1 as an optical scanning unit that performs optical scanning on the plurality of image carriers. The image forming apparatus according to claim 7.
The image forming apparatus, wherein the image carrier is a photoconductive photoconductor.

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