JP2007248670A - Optical scanner and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical scanner and an image forming apparatus using the same, while being facilitated to mold a diffracting surface employed for an optical element at low cost, by making optical scans with a stable beam spot diameter, even without making the shape accuracy of the diffracting surface high. <P>SOLUTION: The optical scanner is equipped with a first optical element 2, which shapes the sectional form of a light beam from a semiconductor laser 1, a second optical element 4 which guides a light beam transmitted through the first optical element 2 to an optical deflector 5, and a third optical element 6, which converges a light beam deflected by the optical deflector on a scanned surface 8 to form a light spot and optically scan the scanned surface. At least one among the first, second, and third optical elements is a resin-made lens, and at least one of which is a diffracting optical element. The diffraction optical element has an anamorphic power diffracting surface, formed on a plane substrate as one surface and a rotationally symmetric surface as the surface on the reverse side. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical scanning device used as an image writing device in an image forming apparatus such as a digital copying machine or an optical printer, and the above-described image forming apparatus using the same.

An optical scanning device has been widely known as an image writing device in an image forming apparatus such as an optical printer, a digital copying machine, or an optical plotter. What can form a high-quality image is desired.
The optical scanning device includes optical components such as various lenses. When various lenses used in the optical scanning device are formed of a resin material, the resin lens is lightweight, can be formed at low cost, and it is easy to form a special surface shape represented by an aspherical surface. By adopting a special surface for the resin lens, the optical characteristics can be improved and the number of lenses constituting the optical system can be reduced. In other words, the use of a resin lens greatly contributes to the reduction in size, weight, and cost of an optical scanning device.

  However, as is well known, the resin lens changes its shape with changes in the environment, especially with changes in temperature, and its refractive index changes. Therefore, the optical characteristics, especially power (refractive index), are designed values. There is a problem that the “beam spot diameter”, which is the diameter of the light spot on the surface to be scanned, varies due to environmental fluctuations. The power fluctuation of the resin lens due to the temperature change occurs in the opposite direction between the positive lens and the negative lens. Therefore, the positive and negative resin lenses are arranged in the optical system of the optical scanning device. There is a well-known method for canceling changes in optical characteristics caused by environmental changes occurring in resin lenses.

In addition, a general semiconductor laser as a light source of an optical scanning device has a property that the emission wavelength shifts to a longer wavelength side when the temperature rises (this property is called “wavelength change due to temperature change”), and “mode hop”. There is also a wavelength change due to. A wavelength change in the light source causes a characteristic change due to chromatic aberration of an optical system used in the optical scanning apparatus, and this characteristic change also causes a beam spot diameter fluctuation.
Therefore, in an optical scanning device that includes a resin lens in the optical system and uses a semiconductor laser as the light source, an optical design that takes into account changes in the optical characteristics accompanying changes in the wavelength of the light source as well as changes in the optical characteristics accompanying changes in temperature. There is a need to do.

  There is known an optical scanning device (laser scanning device) that adopts a power diffractive surface and stabilizes optical properties in consideration of changes in optical characteristics due to temperature changes and wavelength changes in a light source (for example, Patent Documents). 1). In Patent Document 1, a light source optical system for condensing laser light emitted from a laser light source into parallel light in the main scanning direction and in the sub-scanning direction in the vicinity of the deflecting reflection surface of the optical deflector has a rotational symmetry axis. There is disclosed an optical scanning device having one or more reflecting surfaces and two transmitting surfaces, a power diffractive surface provided on the transmitting surface, and a single optical element made of resin. As a comparative example, a power diffractive surface is provided on each of a resin collimator lens that collimates a light beam from a semiconductor laser and a resin cylinder lens that focuses the collimated light beam in the sub-scanning direction. An optical scanning device provided is disclosed. The “power diffractive surface” is a diffractive surface having lens power by diffraction.

JP 2002-287062 A

  Patent Document 1 discloses “one optical element having one or more reflecting surfaces not having a rotational symmetry axis and two transmitting surfaces, provided with a power diffractive surface on the transmitting surface, and made of resin. The light source optical system according to the method requires the formation of a transmission surface and a reflection surface in one optical element, and includes a curved reflection surface, so that it is not always easy to manufacture and the cost of the optical scanning device is low. There is still room for improvement in terms of conversion. In general, in order to form a power diffractive surface, a fine processing technique is required, and an extremely high accuracy is required. For example, when a power diffractive surface having a power equivalent to that of a spherical lens as shown in FIG. 6B is illustrated, the shape is as shown in FIG. That is, the spherical surface of the spherical lens is divided along contour lines, and the divided spherical surface is arranged on a flat base so as to have a uniform height, thereby forming a corrugated cross section. In other words, it is a shape in which only inclined surfaces on which light is refracted are arranged concentrically. As is clear from FIG. 6B, the rising angle of the inclined surface that draws an arc increases as the power diffractive surface moves away from the optical axis, and the interval between the grooves, that is, the interval between adjacent inclined surfaces decreases. In particular, the greater the power to be applied to the power diffractive surface, the greater the tendency. The problem here is the mutual eccentricity between the entrance surface and the exit surface. In particular, the eccentricity of the rotation with respect to the center of the optical axis significantly deteriorates the wavefront aberration and causes the beam spot diameter on the surface to be scanned to be thickened.

In view of the above-described problems of the prior art, the present invention has not only beam spot diameter fluctuation due to temperature fluctuation but also beam spot diameter fluctuation due to oscillation wavelength change due to mode hop in an optical scanning device using a power diffraction surface. An object of the present invention is to realize an optical scanning device that can perform optical scanning with a reduced and more stable beam spot diameter, and to realize an image forming apparatus using such an optical scanning device.
The present invention is also capable of performing optical scanning with a stable beam spot diameter without increasing the shape accuracy of a diffraction surface employed in an optical element mounted on an optical scanning device or an image forming apparatus, The object is to make the diffractive surface inexpensive and easy to mold.

According to the first aspect of the present invention, the first optical element for converting the cross-sectional form of the light beam from the semiconductor laser into a desired form, and the light beam transmitted through the first optical element is optically deflected. A second optical element that guides light to the scanning device, and a third optical element that condenses the light beam deflected by the optical deflector onto the surface to be scanned to form a light spot and optically scans the surface to be scanned. The optical scanning device is configured as follows.
At least one of the first, second, and third optical elements includes a resin lens.
At least one of the resin lenses is a diffractive optical element.
The diffractive optical element is an anamorphic power diffractive surface in which one surface shape is formed on a flat substrate, and the opposite surface shape is a rotationally symmetric surface.

According to a second aspect of the present invention, in the optical scanning device according to the first aspect, one surface of the diffractive optical element is a power diffractive surface in which a plurality of parallel linear grooves are formed on a flat substrate, and the diffractive optical element The opposite surface is a flat surface.
According to a third aspect of the present invention, in the optical scanning device according to the second aspect, the diffractive optical element is employed as the second optical element, and has a positive power at least in the sub-scanning direction. In the vicinity of the deflecting reflection surface of the optical deflector as a long line image in the main scanning direction.

According to a fourth aspect of the present invention, in the optical scanning device according to any one of the first to third aspects, the power diffractive surface has a main scanning direction and / or a sub-scanning direction caused by a mode hop or temperature change in the semiconductor laser. The variation of the beam waist position is set to be substantially zero.
As described above, the “power diffractive surface” is a diffractive surface having a diffraction function equivalent to a lens action. “Due to mode hop and / or temperature change” means “Due to mode hop and / or temperature change”.

According to a fifth aspect of the present invention, in the optical scanning device according to any one of the first to fourth aspects, the diffractive optical element can be adjusted along the optical axis direction.
In the invention according to claim 5, the first optical element converts a light beam from a semiconductor laser as a light source into a light beam having a desired cross-sectional shape. In the invention according to claim 5, the first optical element It is preferable that this action is a collimating action. Furthermore, it is preferable that the second optical element has no power in the main scanning direction. If the second optical element does not have power in the main scanning direction, a processing error at the time of initial assembly of the optical system, a beam waist position variation in the sub-scanning direction when an assembly error occurs, By displacing the optical element in the optical axis direction, adjustment can be performed without affecting the optical characteristics in the main scanning direction. Therefore, even if the shape accuracy of the diffractive surface employed in the second optical element is not required at a high level, fluctuations in power caused by processing errors are absorbed by adjustment of the second optical element in the optical axis direction. (Reducing). In particular, if adjustment can be performed without affecting the optical characteristics in the main scanning direction, the first optical element can be adjusted independently in the main scanning direction, and the second optical element can be adjusted independently in the sub scanning direction. Adjustment work is greatly simplified. The “power of the second optical element” is a power obtained by combining the power of the refractive surface and the power of the power diffraction surface.

According to a sixth aspect of the present invention, in the optical scanning device according to any one of the first to fifth aspects, the diffractive optical element can be adjusted around the optical axis.
The invention according to claim 7 is the optical scanning device according to any one of claims 1 to 6, wherein all of the first, second and third optical elements are resin lenses.

According to an eighth aspect of the present invention, in the optical scanning device according to any one of the first to seventh aspects, the semiconductor laser is a semiconductor laser array having a plurality of light emitting portions.
The semiconductor laser used as the light source can be configured to perform a single beam scanning method using one normal laser, but as in the invention according to claim 8, a semiconductor laser array having a plurality of light emitting portions By using the above, or by using a plurality of semiconductor lasers, a multi-beam scanning optical scanning device can be configured. The semiconductor laser array has better assembly stability than using multiple semiconductor lasers, and the light beam is incident on the anamorphic optical element in almost the same manner, so the optical characteristics vary among the multiple light beams. Can be reduced. Therefore, the optical scanning device according to claim 8 is a preferable mode.

The invention according to claim 9 has an image forming section for forming a latent image by performing optical scanning by the optical scanning means on the photosensitive image carrier and visualizing the latent image by the developing means to obtain an image. An image forming apparatus, wherein the optical scanning unit is the optical scanning apparatus according to any one of claims 1 to 8.
It is possible to form a monochrome image with a single image forming unit, or to form an image forming apparatus so that two or more image forming units form a two-color image, a multicolor image, or a color image. It can also be configured. When two or more image forming units are provided, the optical scanning device that performs optical scanning in each image forming unit may be separate for each image forming unit. For example, according to Japanese Patent Laid-Open No. 2004-280056 As is known, a part of the optical element, for example, a part of the optical deflector or the scanning optical system may be shared by the plurality of scanning optical systems.
When two or more image forming units are arranged, two or more image forming units can be arranged at different positions with respect to the same image carrier, or in a predetermined direction as in a so-called tandem color image forming apparatus. An image forming unit can be individually provided for each of the plurality of image carriers arranged in the above.

  According to a tenth aspect of the present invention, there is provided an image forming unit for forming a latent image by performing optical scanning by a light scanning unit on a photosensitive image carrier and visualizing the latent image by a developing unit to obtain an image. 9. An image forming apparatus, wherein a plurality of image carriers are arranged, and the optical scanning unit scans with a light beam corresponding to each image carrier according to any one of claims 1 to 8. Each light beam is modulated with an image signal corresponding to the color component, whereby a latent image corresponding to the color component is formed on each image carrier, and the developing unit converts each latent image with a toner of a color corresponding to the latent image. It is a color-compatible image forming apparatus that is visualized.

Here, in the case where a resin lens is included in the optical system of the optical scanning device, the beam waist position variation of the light beam collected toward the scanned surface with respect to environmental variation, for example, temperature variation or wavelength variation. Consider briefly. First, the cause of beam waist position fluctuations due to temperature fluctuations is as follows. Changes in the refractive index of the resin lens itself,
2. Plastic lens shape change,
3. Refractive index change (chromatic aberration) of resin lens due to wavelength change of semiconductor laser,
Can be considered.
1. The change in the refractive index of the resin lens itself appears as a phenomenon in which the refractive index decreases due to the resin expanding and decreasing in density as the temperature rises. 2. This change in the shape of the resin lens appears as a phenomenon in which the resin expands as the temperature rises and the curvature of the lens surface decreases. 3. The change in the emission wavelength of the semiconductor laser generally appears as a phenomenon that shifts to the longer wavelength side as the temperature rises. When the wavelength shifts to the longer wavelength side, the refractive index of the resin lens generally shifts to the decreasing side. In other words, regardless of whether the resin lens is a positive lens or a negative lens, the absolute value of the power changes as the temperature rises.

  On the other hand, since the diffraction angle of the power diffracting surface is proportional to the wavelength, the power of the diffracting portion of the power diffractive surface is positive or negative. The absolute value of tends to increase as the wavelength increases. Therefore, for example, when the combined power of the resin lens in the optical system of the optical scanning device is positive (or negative), the power of the “diffractive part” of the power diffractive surface is positive (or negative), It is possible to cancel the power change accompanying the temperature fluctuation in the resin lens with the power change accompanying the temperature fluctuation in the “diffractive part” of the power diffractive surface.

  Here, the “diffractive portion” of the power diffractive surface means that the power diffractive surface of the anamorphic optical element in the present invention is not necessarily formed on a flat surface, but formed on a spherical surface or a cylindrical surface. Therefore, there is a case where the portion that hits the substrate on which the diffractive surface is formed also has power. Therefore, in the present specification, this is called a “diffractive portion” of the power diffractive surface in the meaning of the power of only the diffractive surface excluding the power of the portion that hits the substrate.

For the sake of more specific explanation, let us consider a case where the environmental temperature rises when the power of the resin lens included in the optical system and the power of the “diffractive part” of the power diffractive surface are both positive.
Beam waist position fluctuation amount due to change in refractive index of resin lens: A
Beam waist position variation due to resin lens shape change: B
Beam waist position fluctuation amount due to change in refractive index of resin lens due to change in emission wavelength of semiconductor laser: C
Beam waist position fluctuation amount due to power change of “diffractive part” of power diffractive surface due to change in emission wavelength of semiconductor laser: D
If the change in the direction away from the optical deflector is positive,
A> 0, B> 0, C> 0, D <0
It is. And the total amount of beam waist position fluctuation | variation accompanying this temperature change is A + B + C-D. A to C are determined when the optical system including the resin lens is determined. Therefore, the power of the “diffractive portion” of the power diffraction surface is set so as to satisfy the condition that the beam waist position fluctuation amount is 0: A + B + C−D = 0. By setting, it is possible to satisfactorily correct the beam waist position fluctuation accompanying the temperature change.

  As described above, the change in the emission wavelength of the semiconductor laser as the light source is not only due to temperature change, but also due to mode hopping. The change in the emission wavelength due to the mode hop is caused by a microscopic physical phenomenon, which is very difficult to predict. The change in the emission wavelength due to the mode hop is irrelevant to the change in temperature, and if the change in the emission wavelength due to the mode hop occurs in the state where there is no temperature change from the reference temperature, A and B are 0. Is not corrected because CD <0, and the beam waist position changes greatly. As described above, when the power diffractive surface is adopted in the optical scanning device, not only the correction of the beam waist position fluctuation due to the temperature fluctuation but also the reduction of the beam waist position fluctuation due to the emission wavelength change due to the mode hop is always performed. A stable beam spot diameter cannot be obtained.

  In addition to correcting beam waist position fluctuations due to temperature fluctuations, in order to reduce beam waist position fluctuations due to emission wavelength changes due to mode hops, it is necessary to appropriately set the power applied to the “diffractive part” of the power diffractive surface. . If too much power is given to the “diffractive part” of the power diffractive surface, the beam waist position fluctuation due to the emission wavelength change due to the mode hop increases.

  In view of the problems of the prior art described above, in the optical scanning device of the present invention, the variation in the beam waist position in the main scanning direction and / or the sub-scanning direction due to the mode hop in the semiconductor laser and the temperature change is substantially zero. Thus, the power of the “diffractive part” of the power diffraction surface is set.

  The power diffractive surface to which the power is set in this way can generally take various shapes, but as described above, the formation of the power diffractive surface requires a fine processing technique. Furthermore, the thing with the extremely high precision is requested | required. If this accuracy cannot be ensured, a variety of undesirable phenomena such as a decrease in diffraction efficiency, deterioration of wavefront aberration, and generation of scattered light occur. Moreover, in order to ensure such high accuracy, a very good measurement technique is indispensable. However, even a power diffractive surface with a spherical surface as a basic shape is difficult to measure, and it is a fact that a high quality power diffractive surface cannot be obtained.

Therefore, the power diffractive surface of the diffractive optical element used in the present invention is a power diffractive surface by forming a flat substrate shape and forming an anamorphic groove shape thereon. Further, the greatest feature is that the surface shape on the opposite side of the diffractive optical element is a rotationally symmetric shape. If the substrate shape is a flat surface, the depth of all the grooves is invariant, so the apex height of the grooves will be uniform, and not only will it be easy to measure the shape, it will also be very easy to process. .
Furthermore, since the surface shape on the opposite side is rotationally symmetric with respect to the optical axis, even if there is an eccentricity with the power diffractive surface, the effect of this is extremely small, so a high degree of machining accuracy is required. Nevertheless, it is possible to reduce the beam spot diameter.

  As described above, according to the optical scanning device of the present invention, the fluctuation of the beam waist position in the main scanning direction and / or the sub-scanning direction due to the mode hop and temperature change in the semiconductor laser as the light source is substantially zero. As described above, since the power of the power diffractive surface is set, the beam waist position variation is effectively corrected not only for temperature variation but also for emission wavelength variation due to mode hopping, and optical scanning is always performed with a stable beam spot diameter. be able to. In addition, according to the image forming apparatus of the present invention using this optical scanning device, stable image formation is possible.

  Embodiments of an optical scanning device and an image forming apparatus according to the present invention will be described below with reference to the drawings.

  FIG. 1 shows an optical arrangement of an embodiment of an optical scanning device. In FIG. 1, reference numeral 1 is a semiconductor laser as a light source, reference numeral 2 is a coupling lens as a first optical element, reference numeral 3 is an aperture, reference numeral 4 is an anamorphic optical element as a second optical element, and reference numeral 5 is light. A polygon mirror (rotating polygonal mirror) which is a deflector, reference numeral 6 denotes a scanning lens as a third optical element, and reference numeral 8 denotes a surface to be scanned. Reference numeral G1 indicates a soundproof glass that closes a window of a soundproof housing (not shown) that houses the polygon mirror 5, and reference numeral G2 indicates the emission of the deflected light beam of the housing that houses the optical system of the optical scanning device shown in FIG. The dustproof glass provided in the part is shown.

  The divergent light beam emitted from the semiconductor laser 1 is converted into a light beam having a desired cross-sectional shape by the coupling lens 2, is shaped by the aperture 3, and enters the anamorphic optical element 4. The light beam that has passed through the anamorphic optical element 4 passes through the soundproof glass G1 while being focused in the sub-scanning direction, and forms a long line image in the vicinity of the deflecting reflection surface of the polygon mirror 5 in the main scanning direction. The light beam is reflected by the deflection reflection surface of the polygon mirror 5, passes through the soundproof glass G 1, and enters the scanning lens 6. The scanning lens 6 is composed of a single lens, and the light beam that has passed through the lens 6 passes through the dust-proof glass G2 and enters the surface to be scanned 8, and the light spot on the surface to be scanned 8 by the action of the scanning lens 6 Form.

  The polygon mirror 5 is driven to rotate at a constant speed by a motor. When the polygon mirror 5 rotates at a constant speed, the light beam reflected by the deflecting reflecting surface is deflected at a constant angular velocity. The scanning lens 6 has an fθ characteristic that allows a light spot by a light beam incident while being deflected at a constant angular velocity to move at a constant speed on the scanned surface 8 in the main scanning direction (vertical direction in FIG. 1). The light spot optically scans the scanned surface 8 at a constant speed. The scanning lens 6 is an anamorphic optical element, and in the sub-scanning direction, the position of the deflecting reflection surface of the polygon mirror 5 and the position of the surface 8 to be scanned are in a geometric optical conjugate relationship. The tilting of the reflective surface is corrected. The scanned surface 8 is essentially a photosensitive surface of a photosensitive medium such as a photosensitive drum.

Next, specific numerical examples of the optical components constituting the above embodiment will be given. Table 1 shows data of the glass material (referred to as “glass 1”) and the resin material (referred to as “resin 1”) used in the above examples.
Table 1

  In Table 1, “median” is the refractive index with respect to the wavelength used at the reference temperature: 25 ° C., and “wavelength jump” is the refractive index when the wavelength jump occurs due to mode hops, and “temperature fluctuation”. This is the refractive index when the temperature rises by 20 degrees from the reference temperature. “Wavelength skip” due to mode hopping assumes a wavelength change of 0.8 nm with a margin.

Table 2 shows optical system data arranged after the optical deflector.
Table 2

In Table 2, R _m is “paraxial curvature in the main scanning direction”, R _s is “paraxial curvature in the sub-scanning direction”, and expressed as D _x , D _y (“X”, “Y” in Table 2). ) Represents “relative distance from the origin of each optical element to the origin of the next optical element”. The unit is mm. For example, regarding D _x and D _y of the optical system subsequent to the optical deflector, the origin of the incident surface of the scanning lens 6 (the optical axis of the incident side surface) when viewed from the rotation axis of the optical deflector (polygon mirror 5). The position is 42.99 mm away in the optical axis direction (X direction, left-right direction in FIG. 1) and 6.91 mm away in the main scanning direction (Y direction, up-down direction in FIG. 1). The thickness of the scanning lens 6 on the optical axis is 13.5 mm, and the distance from the scanning lens 6 to the scanned surface 8 is 176 mm. In addition, between the scanning lens 6 and the to-be-scanned surface 8, as shown in FIG. 1, the dust-proof glass G2 of thickness 1.9mm which uses the glass 1 as a material is arrange | positioned. Each surface of the scanning lens 6 is an aspherical surface, and both surfaces have a non-arc shape given by “Expression 1” described later in the main scanning direction, and are sub-scanning sections (virtual parallel to the optical axis and the sub-scanning direction). This is a special surface in which the curvature in the cross section changes in the main scanning direction according to “Equation 2” described later.

The “non-arc shape” is as follows.
Paraxial radius of curvature in main scanning section: R _m , distance in main scanning direction from optical axis: Y, conic constant: K, higher order coefficients: A ₁ , A ₂ , A ₃ , A ₄ , A ₅ , ... Depth in the optical axis direction: X is expressed by the following formula 1.
Formula 1

The “change in curvature in the sub-scanning section” is as follows.
Assuming that the coordinate in the main scanning direction with the optical axis position as the origin is Y, the expression expressing the state of curvature C _s (Y) in the sub scanning section changes in the main scanning direction is the sub scanning section including the optical axis. The radius of curvature is: R _s (0), B ₁ , B ₂ , B ₃ ,...
Formula 2

Table 3 lists the coefficients of the incident side surface (special surface) of the scanning lens 6.
Table 3

Table 4 lists the coefficients of the exit side surface (special surface) of the scanning lens 6.
Table 4

  Next, an example in which the power diffractive surface which is a characteristic configuration of the present invention is adopted for the anamorphic optical element 4 as the second optical element will be described. The anamorphic optical element 4 is a resin lens, and one surface is a power diffractive surface formed by forming a plurality of linear grooves parallel to each other on a flat substrate, and the opposite surface is a flat surface. It is.

  2A and 2B are explanatory views of the anamorphic optical element 4. FIG. 2A is a front view, FIG. 2B is a cross-sectional view of a virtual cut surface parallel to the sub-scanning direction and the optical axis direction, and FIG. FIG. 6 is a cross-sectional view in a virtual cut parallel to the optical axis direction. In FIG. 2, the horizontal direction is the main scanning direction, and the vertical direction is the sub-scanning direction. FIG. 2A shows a state in which the anamorphic optical element 4 is viewed from the optical axis direction. As shown in FIGS. 2A and 2B, a power diffractive surface is formed on one surface of the anamorphic optical element 4 by forming a plurality of linear grooves parallel to each other on the flat substrate. As shown in FIGS. 2A and 2C, a “plane” is formed on the surface. Thus, one surface of the anamorphic optical element 4 is a power diffractive surface formed by forming a plurality of linear grooves parallel to the planar substrate, and the other surface is a flat lens. Yes. The power of the power diffractive surface is set so that the fluctuation of the beam waist position in the main scanning direction and / or the sub-scanning direction due to the mode hop and temperature change in the semiconductor laser 1 is substantially zero.

The specific configuration of each element constituting the optical system of the optical scanning device according to the embodiment shown in FIG. 1 is as follows.
"light source"
The semiconductor laser 1 which is a light source has a design emission wavelength of 785 nm, and when the temperature rises by 1 ° C. with respect to the standard temperature of 25 ° C., the emission wavelength shifts to the long wavelength side by 0.25 nm. The mode hop assumes a wavelength change of 0.8 nm as described above.
"Coupling lens"
The coupling lens 2 as the first optical element is a glass lens, and has a focal length of 14.5 mm and is arranged to have a function of converting into a weakly divergent light beam. The surface on one side of the coupling lens 2 is aspherical, and the wavefront aberration of the coupled light beam is sufficiently corrected by the aspherical surface.
Both 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 7.0 × 10 ⁻⁵ . The incident surface of the coupling lens 2 is a plane, and the exit surface is a rotationally symmetric aspherical surface.
"Aperture"
The aperture 3 has a rectangular opening with an opening diameter of 2.76 mm in the main scanning direction and an opening diameter of 2.34 mm in the sub-scanning direction, and has a predetermined cross-sectional shape of the light beam coupled by the coupling lens 2. Shape to the shape of

"Anamorphic optics"
In the anamorphic optical element 4 as the second optical element, the incident side surface is a power diffractive surface formed by forming a plurality of linear grooves parallel to each other on a flat substrate, and the exit side surface is a flat surface. It has become. Power diffractive surface of the incident surface side, the phase function: w _in
w _in = C _z · Z ²
It is represented by Coefficient: C _z is C _z = −1.3287 × 10 ⁻² .

"Optical deflector"
The polygon mirror 5 as an optical deflector has six reflecting surfaces and an inscribed circle radius of 13 mm.
The soundproof glass G1 is made of the glass 1, has a thickness of 1.9 mm, and an inclination angle α in the y direction (vertical direction in FIG. 1) is 12 degrees. Further, an angle 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 deflecting / reflecting surface of the polygon mirror 5: θ Is 68 degrees.

Table 6 shows beam waist position fluctuations in the main scanning direction and the sub-scanning direction in the embodiment.
Table 6

これらの表からわかるとおり、アナモフィック光学素子４のパワー回折面の効果で、特に副走査方向のビームウエスト位置変動が低減されていることがわかる。 On the other hand, if a power diffraction surface is not employed in the anamorphic optical element 4, the beam waist position fluctuation is as shown in Table 7.
Table 7

As can be seen from these tables, the effect of the power diffractive surface of the anamorphic optical element 4 reduces the fluctuation in the beam waist position particularly in the sub-scanning direction.

  Next, the case where the power diffraction surface in the above embodiment has a processing error will be considered. The groove spacing of the power diffractive surface formed by forming a plurality of linear grooves in parallel on the flat substrate, which is employed on the incident surface side of the anamorphic optical element 4 of the embodiment, is determined from the optical axis. Although gradually shortening as the distance increases, the groove interval is about 100 μm as a minimum value in this embodiment. On the other hand, for example, it is assumed that there are processing errors of 2 μm, 4 μm, and 6 μm. This processing error causes the power of the anamorphic optical element 4 to fluctuate greatly. If the anamorphic optical element 4 is mounted on the optical scanning device as it is, the condensing point of the light beam is greatly deviated from the surface to be scanned 8, and the beam spot diameter is reduced. It gets bigger. When such an optical scanning device is developed especially in a color optical printer or the like, color reproducibility deteriorates and gradation is lost. However, since the deviation of the condensing point of the light beam is substantially the same over the entire image height, it can be absorbed (reduced) by shifting the anamorphic optical element 4 in the optical axis direction.

  FIG. 3 is a schematic diagram showing a mechanism for absorbing the focal point shift of the light beam. In FIG. 6, reference numeral 100 denotes an anamorphic optical element, and 101 denotes a holder for fixing the anamorphic optical element. The holder 101 is positioned by being pressed against an abutting reference pin 102 and a gear 103 provided on the housing by a biasing force of a spring (not shown). A rack-like gear is provided on the side surface of the holder 101 at a portion in contact with the gear 103, and the rack-like gear is engaged with the gear 103. Therefore, when the gear 103 is rotated, the holder 102 is moved along the optical axis direction accordingly. With such a configuration, it is possible to shift the anamorphic optical element 4 in the optical axis direction. Therefore, even if the power diffraction surface employed in the anamorphic optical element 4 has a processing error, it is desired. Can be formed on the scanned surface 8.

  As a premise of this method, a desired diffraction effect must be obtained even if the anamorphic optical element 4 has a processing error. However, even if there is a processing error of 2 μm, 4 μm, 6 μm, for example, in the distance between the grooves constituting the power diffractive surface of the anamorphic optical element 4, a diffraction effect that is completely different from the design median can be expected. FIG. 4 illustrates this. 4 (a) and 4 (b) show cases where the specifications of the optical system are different, and FIGS. 4 (a) and 4 (b) show graphs when the processing errors of the grooves are 2 μm, 4 μm, and 6 μm, respectively. However, each graph almost overlaps and looks like a single line. As can be seen from the graph shown in FIG. 4, when the atmospheric temperature of the anamorphic optical element is changed from 25 ° C. to 10 ° C. and 45 ° C., it can be understood that the focal length variation of the anamorphic optical element is exactly the same.

  FIG. 3 shows an example of adjustment of the anamorphic optical element by a mechanical mechanism. However, a method of adjusting the position when fixing the anamorphic optical element to the optical scanning device and fixing the anamorphic optical element to the adjustment position with an adhesive is also possible. is there. This eliminates the adjustment mechanism and is advantageous in that no unnecessary parts are left in the optical scanning device as a function of the optical scanning device thereafter. In this adjustment, if the adjustment around the optical axis is also performed, the predetermined beam spot diameter can be set by bonding and fixing the anamorphic optical element in a state where the thickness of the beam spot diameter due to the attachment error of the anamorphic optical element is reduced. It becomes possible to obtain stably.

  FIG. 5 schematically shows an embodiment of an image forming apparatus using the optical scanning device according to the present invention. The image forming apparatus according to this embodiment is a tandem type full-color optical printer. In FIG. 5, a conveying belt 32 that conveys transfer paper (not shown) fed from a paper feeding cassette 30 arranged in the horizontal direction in a substantially horizontal direction is provided at the lower part of the apparatus. On the upper surface side of the conveyance belt 32, a photosensitive member 7Y for yellow Y, a photosensitive member 7M for magenta M, a photosensitive member 7C for cyan C, and a photosensitive member 7K for black K are formed on the transfer paper by the conveying belt 32. They are arranged at equal intervals in the above order from the upstream side in the transport direction. Hereinafter, yellow, magenta, cyan, and black are distinguished by Y, M, C, and K added to the reference numerals. The photoreceptors 7Y, 7M, 7C, and 7K also function as image carriers, and are all formed to have the same diameter, and process members for executing an image forming process according to an electrophotographic process are sequentially disposed around the photoreceptors. . Taking the photoconductor 3Y as an example, a charging charger 40Y, an optical scanning device 50Y, a developing device 60Y, a transfer charger 30Y, a cleaning device 80Y, and the like are sequentially arranged in the clockwise direction that is the rotation direction of the photoconductor. The same applies to the other photoconductors 3M, 3C, and 3K. That is, this image forming apparatus uses the photoconductors 7Y, 7M, 7C, and 7K as the scanning surfaces set for the respective color components, and one optical scanning device 50Y, 50M, 50C, and 50K is provided for each. They are provided in a one-to-one correspondence.

  As the optical scanning devices 50Y, 50M, 50C, and 50K, optical scanning devices each having an optical arrangement as shown in FIG. 1 can be used independently. Further, for example, as conventionally known from Japanese Patent Application Laid-Open No. 2004-280056 and the like, the optical deflector (rotating polygonal mirror) is shared, and the lens 6-1 of the scanning optical system in each optical scanning device (FIG. 3) can be shared for optical scanning of the photoconductors 7M and 7Y, and can be shared for optical scanning of the photoconductors 7K and 7C.

  Around the conveyance belt 32, a registration roller 9 and a belt charging charger 10 are provided upstream of the photoreceptor 7Y in the transfer paper conveyance direction, and a belt separation charger is disposed downstream of the photoreceptor 7K. 11, a static elimination charger 12, a cleaning device 13, and the like are provided. A fixing device 14 is provided downstream of the belt separation charger 11 in the transport direction, and is connected to a paper discharge tray 15 by a paper discharge roller 16.

  In such a configuration, for example, in the full color mode, Y, M, and C are applied to the surfaces (that is, the scanned surfaces) of the photoreceptors 7Y, 7M, 7C, and 7K that are uniformly charged by the charger. When the optical scanning is performed by the optical scanning devices 50Y, 50M, 50C, and 50K based on the image signals of the respective colors K, electrostatic latent images are formed on the surfaces of the respective photoreceptors. These electrostatic latent images are developed with a corresponding color toner supplied from a corresponding developing device, and become a toner image. The toner images of these colors are superimposed on the transfer paper that is electrostatically attracted and conveyed on the conveyance belt 32 to form a full color image, which is fixed by the fixing device 14 and then discharged. The paper is discharged onto the tray 15.

  By using the optical scanning device according to the above-described embodiment as an optical scanning device that executes the exposure process of such an image forming apparatus, a stable beam spot diameter can be obtained at all times, which is suitable for high-definition printing. The image forming apparatus can be realized in a compact and inexpensive manner.

The image forming apparatus according to the present invention can also be configured as a monochrome image forming apparatus including one photoconductor as an image carrier and a corresponding optical scanning device.
At least one of the first, second, and third optical elements may be a resin lens, and a power diffraction surface may be formed on at least one of the resin lenses.

It is an optical arrangement | positioning figure which shows schematically the Example of the optical scanning device concerning this invention. (A) which shows the example of the lens which employ | adopted the power diffractive surface in the said Example, (b) Subscanning direction sectional drawing, (c) is a main scanning direction sectional drawing. It is a perspective view which shows typically the example of the mechanism for reducing the condensing point shift | offset | difference of the light beam applicable to this invention. It is a graph which shows the difference of the diffraction effect in case an anamorphic optical element has a processing error and a processing error differs. 1 is a front view showing an embodiment of an image forming apparatus according to the present invention. It is a figure for demonstrating regarding a power diffraction surface, and is an optical path diagram which shows that the power diffraction surface shown to (a) and (b) is equivalent.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Semiconductor laser as light source 2 Coupling lens which is 1st optical element 3 Aperture 4 Anamorphic optical element which is 2nd optical element 5 Polygon mirror as optical deflector 6 3rd optical element 7Y As image carrier Photoconductor 7M Photoconductor as image carrier 7CY Photoconductor as image carrier 7K Photoconductor as image carrier 8 Scanned surface

Claims (10)

A first optical element that converts a cross-sectional form of a light beam from a semiconductor laser into a desired form, a second optical element that guides a light beam that has passed through the first optical element to an optical deflector, and an optical deflector An optical scanning device comprising a third optical element that focuses a deflected light beam on a surface to be scanned to form a light spot and optically scans the surface to be scanned,
At least one of the first, second, and third optical elements includes a resin lens,
At least one of the resin lenses is a diffractive optical element,
The diffractive optical element is an optical scanning device in which one surface shape is an anamorphic power diffractive surface formed on a flat substrate, and the opposite surface shape is a rotationally symmetric surface.   2. The optical scanning device according to claim 1, wherein one surface of the diffractive optical element is a power diffractive surface in which a plurality of parallel linear grooves are formed on a flat substrate, and the opposite surface of the diffractive optical element is a flat surface. An optical scanning device.   3. The optical scanning device according to claim 2, wherein the diffractive optical element is employed as the second optical element, has a positive power at least in the sub-scanning direction, and transmits the light beam to the deflecting / reflecting surface of the optical deflector. An optical scanning device having a function of forming a long line image in the vicinity in the main scanning direction.   4. The optical scanning device according to claim 1, wherein the power diffractive surface substantially eliminates fluctuations in the beam waist position in the main scanning direction and / or the sub-scanning direction due to mode hopping and temperature change in the semiconductor laser. An optical scanning device set to zero.   5. The optical scanning device according to claim 1, wherein the diffractive optical element is adjustable along the optical axis direction.   6. The optical scanning device according to claim 1, wherein the diffractive optical element is adjustable around the optical axis.   7. The optical scanning device according to claim 1, wherein all of the first, second, and third optical elements are resin lenses.   8. The optical scanning device according to claim 1, wherein the semiconductor laser is a semiconductor laser array having a plurality of light emitting units. An image forming apparatus having an image forming unit that forms a latent image by performing optical scanning with a light scanning unit on a photosensitive image carrier, and visualizes the latent image with a developing unit to obtain an image.
9. The image forming apparatus according to claim 1, wherein the optical scanning unit is the optical scanning device according to any one of claims 1 to 8. An image forming apparatus having an image forming unit that forms a latent image by performing optical scanning with a light scanning unit on a photosensitive image carrier, and visualizes the latent image with a developing unit to obtain an image.
A plurality of image carriers are arranged,
The optical scanning unit according to any one of claims 1 to 8, wherein the optical scanning unit can scan with a light beam corresponding to each image carrier,
Each light beam is modulated with an image signal corresponding to a color component, whereby a latent image corresponding to the color component is formed on each image carrier,
A developing unit visualizes each latent image with toner of a color corresponding to the latent image, and a color-compatible image forming apparatus.

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