JP2006154701A - Optical scanner and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce not only the variation in the beam spot diameter due to a temperature variation but also the variation in the beam spot diameter due to the variation in the oscillation wavelength caused by a mode hopping in an optical scanner using a power diffraction face. <P>SOLUTION: A scanning optical system 6 includes one or more resin lenses 6-1 and 6-2, an anamorphic optical element 4 is an anamorphic resin lens having an anamorphic refraction face and an oval power diffraction face of which the axis is in a main scanning direction, and the power of a power diffraction face is so set that the variation in a beam waist position in the main scanning direction and/or the subscanning direction due to the mode hopping or temperature variation becomes substantially zero in a semiconductor laser 1. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical scanning device and an image forming apparatus.

Conventionally, optical scanning devices are widely known in connection with image forming apparatuses such as optical printers, digital copying machines, and optical plotters. What can form an image is required.
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 the optical scanning device. On the other hand, as is well known, resin lenses change shape and refractive index due to environmental changes, especially temperature changes, so optical characteristics, especially power, change from design values. There is a problem that the “beam spot diameter” which is the diameter of the light spot on the surface to be scanned fluctuates due to environmental fluctuations.

Since the power fluctuation of the resin lens accompanying the temperature change occurs in the opposite direction between the positive lens and the negative lens, the positive and negative resins are included in the optical system of the optical scanning device. A method for canceling out “optical property changes caused by environmental changes” occurring in a lens manufactured is well known.

In addition, a general semiconductor laser as a light source of an optical scanning device has a property that an emission wavelength shifts to a longer wavelength side when a temperature rises (“wavelength change due to temperature change”), and wavelength change due to “mode hop” also occurs. is there. 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.

As an optical scanning device (laser scanning device) that employs a power diffractive surface and stabilizes optical properties in consideration of changes in optical properties due to temperature changes and wavelength changes in the light source, the one described in Patent Document 1 is disclosed. Are known.

In Patent Document 1, a light source optical system that condenses laser light emitted from a laser light source into parallel light in the main scanning direction and near the deflecting reflection surface of the optical deflector in the sub scanning direction has a “rotationally symmetric axis. An optical scanning device having one or more reflecting surfaces and two transmissive surfaces, a power diffractive surface provided on the transmissive surface, and a single optical element made of resin is disclosed and compared. As an example, “optical scanning with a power diffractive surface 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 apparatus "is disclosed. The “power diffractive surface” is a diffractive surface having lens power by diffraction.

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 no room for improvement in terms of conversion.

Further, in the “disclosed as a comparative example” in Patent Document 1, a power diffractive surface is also formed on the collimator lens. However, the collimator lens is generally “the strongest optical element used in the optical scanning device”. When a power diffractive surface is used for the collimator lens, there is a concern that “deterioration of the wavefront aberration of the collimated light beam” may occur as a side effect. Deterioration of the wavefront aberration of the light beam has the effect of increasing the beam spot diameter, and therefore becomes a serious problem when a very small beam spot diameter is required to form a high-definition image.

JP 2002-287062

In view of the circumstances described above, the present invention is more stable in an optical scanning device using a power diffraction surface by reducing not only beam spot diameter fluctuation due to temperature fluctuation but also beam spot diameter fluctuation due to oscillation wavelength change due to mode hopping. It is an object of the present invention to realize an optical scanning device capable of performing optical scanning with the beam spot diameter, and to realize an image forming apparatus using such an optical scanning device.

The optical scanning device of the present invention “converts a light beam from a semiconductor laser into a light beam having a desired beam shape by a coupling lens, then guides the light beam to an optical deflector through an anamorphic optical element, and deflects it by the optical deflector. An optical scanning device that condenses the light beam on the surface to be scanned by the scanning optical system to form a light spot and optically scans the surface to be scanned ”, and has the following characteristics. ).

That is, the “scanning optical system” includes one or more resin lenses.
The “anamorphic optical element is an anamorphic resin lens having an elliptical power diffractive surface whose one surface is an anamorphic refractive surface and the other surface has an axis in the main scanning direction”.
Then, 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 or temperature change in the semiconductor laser is “substantially zero”. As described above, the “power diffractive surface” is a diffractive surface having a diffractive function equivalent to the lens action, and that it is elliptical means that “the shape of the grating grooves constituting the power diffractive surface is elliptical”. means. The above “because of mode hop and / or temperature change” means “because of mode hop and / or temperature change”.

This elliptical shape has “an axis parallel to the main scanning direction”. For this reason, the elliptical “other axis” of the power diffraction surface is parallel to the sub-scanning direction. The axis in the main (sub) scan direction can be a long (short) axis or a short (long) axis.

As described above, the “coupling lens” converts a light beam from a semiconductor laser into a “light beam in a desired beam form”. The “light beam in a desired beam form” referred to here is a “parallel beam”. It can be a “weakly divergent or weakly convergent light beam”. Depending on what kind of light beam is converted by the coupling lens, the properties of the optical system closer to the image side than the coupling lens are adjusted.

The light spot formed on the surface to be scanned by the optical scanning device is an image of the light emitting part of the semiconductor laser that is the light source, but the power of the optical system disposed between the light source and the surface to be scanned is generally the main scanning. Since the direction and the sub-scanning direction are different, the beam waist position needs to be considered separately in the main scanning direction and the sub-scanning direction.

The “anamorphic optical element (anamorphic resin lens)” of the optical scanning device according to claim 1 may have positive power in the sub-scanning direction without power in the main scanning direction (claim). Item 2). In this case, the coupling action of the coupling lens is preferably a “collimating action”. The “power of the anamorphic optical system” is a power obtained by combining the power of the refractive surface and the power of the power diffraction surface. Further, since the power diffractive surface has an elliptical shape with an axis in the main / sub-scanning direction, the power by the power diffractive surface is different between the main scanning direction and the sub-scanning direction. If the anamorphic optical element is set so that it does not have power in the main scanning direction, the processing error during the initial assembly of the optical system and the variation in the beam waist position in the sub-scanning direction when an assembly error occurs will be anamorphic. By displacing the optical element in the optical axis direction, adjustment can be performed “without affecting the optical characteristics in the main scanning direction”.

The refracting surface of the anamorphic optical element in the optical scanning device according to claim 1 or 2 is preferably “a surface having an absolute value of a radius of curvature larger in the main scanning direction than in the sub scanning direction”.

In the optical scanning device according to any one of claims 1 to 3, the “power diffractive surface of the anamorphic optical element” can be “an elliptical power diffractive surface formed on a plane” (claim 4). Of course, the power diffractive surface may be formed on a curved surface other than a flat surface, for example, a spherical surface, a cylinder surface, a toric surface, or the like. It is easy to form a power diffractive surface.

The coupling lens in the optical scanning device according to any one of claims 1 to 4 is preferably a “glass lens” (claim 5). Since glass lenses are not easily affected by environmental fluctuations, the use of glass coupling lenses facilitates the design of other optical elements.

To add a little about the optical scanning device of the present invention, the semiconductor laser used as the light source can be configured to perform a single beam scanning method using one normal laser,
A well-known “multi-beam scanning method” can be implemented by using a semiconductor laser array or two or more semiconductor lasers.

The image forming apparatus according to the present invention includes: “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; What is claimed is: 1. An image forming apparatus having one or more of the optical scanning apparatuses according to any one of claims 1 to 5, wherein the optical scanning apparatus performs optical scanning of an image carrier. 6).

Since the number of image forming units is one or more, it is possible to form a monochrome image by using one image forming unit. Alternatively, two or more image forming units can be used to form a two-color image, a multicolor image, or a color image. The image forming apparatus can also be configured to form an image. In this case, the optical scanning device that performs optical scanning in each image forming unit may be separate for each image forming unit. For example, as known from JP-A-2004-280056, the optical scanning device A part, for example, a part of the optical deflector and the scanning optical system may be shared by a plurality of scanning optical systems.

When there are two or more image forming units, the two or more image forming units can be set at different positions with respect to the same image carrier, or arranged in the front-rear direction as in a so-called tandem color image forming apparatus. An individual image forming unit can be set for each of the image bearing members.

Here, when a resin lens is included in the optical system of the optical scanning device, the fluctuation of the beam waist position of the “light beam condensed toward the scanned surface” can be easily changed with respect to environmental fluctuations and wavelength changes. Consider.

First, the cause of beam waist position fluctuations due to temperature fluctuations is “change in refractive index of resin lens itself”, “change in shape of resin lens”, and “resin made by change in wavelength of semiconductor laser”. The change in the refractive index of the lens (chromatic aberration) is considered.

The “refractive index of the resin lens itself” decreases due to the low density due to the expansion due to the temperature rise.
In the “resin lens shape”, the curvature of the lens surface decreases due to the expansion due to the temperature rise.
“The emission wavelength of the semiconductor laser” generally 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 power” changes as the temperature rises.

On the other hand, since the power of the power diffractive surface is proportional to the wavelength, the power of the power diffractive surface is positive or negative. Has a tendency to become.

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 power diffractive surface is made positive (or negative), so that the resin lens It is possible to cancel the “power change due to temperature fluctuation” in “1” with the “power change due to temperature fluctuation” on the power diffraction surface.

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 power diffraction surface are both positive. At this time, 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 diffractive surface power change caused by change in emission wavelength of semiconductor laser: If D, A> 0, B> 0, C> 0, and D <0 (in a direction away from the optical deflector) Change is positive.)

And the total beam waist position fluctuation amount accompanying this temperature change is A + B + C-D.
It is. Since A to C are determined when an optical system including a resin lens is determined, by setting the power of the power diffraction surface so as to satisfy the condition that the beam waist position fluctuation amount is 0: A + B + C−D = 0, It is possible to satisfactorily correct the beam waist position fluctuation accompanying the temperature change.

By the way, 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 temperature change, and if the emission wavelength change due to the mode hop occurs in the “state where there is no temperature change from the reference temperature”, A and B are 0. The fluctuation amount 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 the beam waist position variation due to temperature variation, it is necessary to appropriately set the power applied to the power diffraction surface in order to reduce the beam waist position variation due to the emission wavelength variation due to the mode hop. If too much power is applied to the power diffractive surface, the beam waist position variation due to the emission wavelength change due to the mode hop is increased.

In view of the above, in the optical scanning device of the present invention, the power 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 or temperature change in the semiconductor laser is “substantially zero”. The power of the diffractive surface is set.

The power (in the main scanning direction and / or sub-scanning direction) power Pm (main scanning direction) and Ps (sub-scanning direction) of the power diffractive surface set in this way are the power of the coupling lens: Pcm.
(Main scanning direction) and Pcs (Sub scanning direction)
(1) 4 <Pcm / Pm <26
(2) 0.5 <Pcs / Ps <26
It is preferable that it is the range of these.

Condition (1) parameters: Pcm / Pm on the horizontal axis, the amount of fluctuation of the beam waist position in the main scanning direction due to the change in emission wavelength due to mode hops on the vertical axis, and the relationship between them is examined. Variation in beam waist position in the main scanning direction due to changes "
Increases linearly with increasing parameter: Pcm / Pm.

“Changes in beam waist position in the main scanning direction due to changes in emission wavelength due to mode hops”
Is preferably suppressed to 0.5 mm or less. In the relationship of the linear increase described above, “variation in beam waist position in the main scanning direction due to change in emission wavelength due to mode hop: 0.5 mm”
The parameter corresponding to: Pcm / Pm has a value of 26. Therefore, the upper limit value of the parameter of condition (1) is given as 26.

Condition (1) parameters: Pcm / Pm is plotted on the horizontal axis, the amount of fluctuation in the beam waist position in the main scanning direction due to temperature change is plotted on the vertical axis, and the relationship between the two is examined. The “positional variation” decreases linearly as the parameter: Pcm / Pm increases.

It is preferable that the “variation in beam waist position in the main scanning direction due to temperature change” is also suppressed to 0.5 mm or less. In the linear reduction relationship, the parameter Pcm / Pm corresponding to “variation in beam waist position in the main scanning direction due to temperature change: 0.5 mm” is 4. Therefore, the lower limit value of the parameter of condition (1) is given as 4.

The same applies to condition (2). The parameter: Condition (2): Pcs / Ps is taken on the horizontal axis, and the amount of fluctuation of the beam waist position in the sub-scanning direction due to the change in emission wavelength due to mode hops is taken on the vertical axis. The “amount of fluctuation in the beam waist position in the sub-scanning direction due to the change in the emission wavelength due to mode hop” increases linearly as the parameter: Pcs / Ps increases.

“Changes in beam waist position in the sub-scanning direction due to changes in emission wavelength due to mode hops”
Is preferably suppressed to 0.5 mm or less. In the above-described linear increase relationship, “variation in beam waist position in the sub-scanning direction due to emission wavelength change due to mode hop: 0.5 mm”
The parameter corresponding to: Pcs / Ps has a value of 26. Therefore, the upper limit value of the parameter of condition (2) is given as 26.

Condition (2) parameters: Pcs / Ps is plotted on the horizontal axis, and the amount of fluctuation of the beam waist position in the sub-scanning direction due to temperature change is plotted on the vertical axis. The “position fluctuation amount” decreases linearly as the parameter: Pcs / Ps increases.

It is preferable that the “variation in beam waist position in the sub-scanning direction due to temperature change” is also suppressed to 0.5 mm or less. In the above linear reduction relationship, the parameter corresponding to “variation in beam waist position in the sub-scanning direction due to temperature change: 0.5 mm”: the value of Pcs / Ps is 0.
5. Therefore, the lower limit value of the parameter of condition (2) is given as 0.5.

As described above, in the optical scanning device of the present invention, the power waist position fluctuations in the main scanning direction and / or the sub-scanning direction due to mode hops and temperature changes in the semiconductor laser are set to “substantially zero”. Since the power of the diffractive surface is set, the beam waist position variation is effectively corrected not only for temperature variations but also for emission wavelength variations due to mode hops, and optical scanning can always be performed with a stable beam spot diameter. By using the optical scanning device, the image forming apparatus of the present invention can form a stable image.

Embodiments of the invention will be described below.
FIG. 1 shows an optical arrangement of an embodiment of an optical scanning device.
Reference numeral 1 is a semiconductor laser as a light source, reference numeral 2 is a coupling lens, reference numeral 3 is an aperture, reference numeral 4 is an anamorphic optical element, reference numeral 5 is a polygon mirror of a rotating polygon mirror as an optical deflector, reference numeral 6 is a scanning optical system, Reference numeral 8 denotes a surface to be scanned. Reference numeral G1 denotes a soundproof glass that closes a window of a soundproof housing (not shown) that houses the polygon mirror 5, and reference numeral G2 is provided at the exit portion of the deflected light beam of the housing that houses the optical system of FIG. The dustproof glass is shown.

The divergent light beam emitted from the semiconductor laser 1 is converted into a substantially parallel light beam by the coupling lens 2, is shaped by the aperture 3, and enters the anamorphic optical element 4. The light beam transmitted through the anamorphic optical element 4 passes through the soundproof glass G1 while being focused in the sub-scanning direction, and is formed in the vicinity of the deflection reflection surface of the polygon mirror 5 as a “line image long in the main scanning direction”. When reflected by the surface, the light passes through the dust-proof glass G 1 and enters the scanning optical system 6.

The scanning optical system 6 includes two lenses 6-1 and 6-2, and these lenses 6-1 and 6 are included.
-2 is incident on the scanned surface 8 through the dust-proof glass G2, and forms a light spot on the scanned surface 8 by the action of the scanning optical system 6.

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 optical system 6 has an fθ characteristic that allows a light spot generated by an incident light beam while being deflected at a constant angular velocity to move at a constant speed in the main scanning direction (vertical direction in the figure) on the surface to be scanned. The light spot scans the scanned surface 8 at a constant speed.

The scanning optical system 6 is also an anamorphic optical element, and in the sub-scanning direction, the deflection reflection surface position of the polygon mirror 5 and the scanned surface position are in a geometric optical conjugate relationship, thereby correcting the surface tilt of the polygon mirror. is doing. The scanned surface 8 is essentially a “photosensitive surface of a photosensitive medium”.

The anamorphic optical element 4 is an “anamorphic resin lens having an elliptical power diffractive surface with one surface being an anamorphic refractive surface and the other surface having an axis in the main scanning direction”.

FIG. 2 illustrates the anamorphic optical element 4 in an explanatory manner. The horizontal direction in the figure is the main scanning direction, and the vertical direction is the sub-scanning direction. In FIG. 2, the portion indicating the anamorphic optical element by reference numeral 4 is viewed from the optical axis direction, and an “elliptical power diffractive surface” is formed on one surface as shown in FIG. Has been.

The upper diagram of the anamorphic optical element 4 is an end view of the “virtual cutting end surface parallel to the main scanning direction and the optical axis direction” of the anamorphic optical element 4, and the left side diagram is “ FIG. 11 is an end view of a “virtual cut end face parallel to the sub-scanning direction and the optical axis direction”. As shown in these end views, the surface opposite to where the power diffractive surface was formed is
An anamorphic refractive surface.

The cross-sectional shape of the power diffractive surface in the main scanning direction (upper view in FIG. 2) 4BM and the cross-sectional shape in the sub-scanning direction (left side view in FIG. 2) 4BS have different grating widths due to the grooves. The lens itself has an anamorphic lens action with different powers in the main scanning direction and the sub-scanning direction.

Further, as shown in FIG. 2, the anamorphic refracting surface of the anamorphic optical element 4 has a different radius of curvature of the end surface shape 4AM in the main scanning direction and a radius of curvature of the end surface shape 4AS in the sub scanning direction. In the embodiment being described, the “anamorphic refractive surface power” of the anamorphic optical element 4 is negative, and the “power diffractive surface power” is positive.

The light beam (parallel light beam) incident on the anamorphic optical system 4 from the light source side is given a divergence tendency in both the main and sub scanning directions by the negative power of the anamorphic refracting surface, and then the action of the power of the power diffractive surface. receive. Since the elliptical shape of the power diffractive surface has the main scanning direction as the major axis direction, the positive power of the power diffractive surface is greater in the sub-scanning direction than in the main scanning direction.

The positive power of the power diffractive surface in the main scanning direction cancels the divergence given in the main scanning direction by the anamorphic refractive surface, and “parallelizes the transmitted light beam in the main scanning direction”. The positive power in the sub-scanning direction of the power diffractive surface surpasses the divergence given in the sub-scanning direction by the anamorphic refracting surface, and the transmitted light beam becomes a “convergent light beam in the sub-scanning direction”.

In this way, the light beam transmitted through the anamorphic optical element 4 has a beam form that is parallel to the main scanning direction and converged in the sub-scanning direction.

The power in the main / sub-scanning direction 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 mode hopping and temperature change in the semiconductor laser 1 is substantially zero. The

Specific examples relating to the above embodiment will be given below.
Table 1 shows data of glass materials (referred to as glass 1 and glass 2) and resin materials (referred to as resins) used in Examples and Comparative Examples described later.

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.
Each element of the optical system is as follows.

"light source"
The semiconductor laser 1 as the light source has a design emission wavelength of 655 nm, and when the temperature rises by 1 ° C. with respect to the standard temperature of 25 ° C., the emission wavelength is shifted to 0.2 nm and the longer wavelength side. The mode hop assumes a wavelength change of 0.8 nm as described above.

"Coupling lens"
The coupling lens 2 is a glass lens made of the glass 1 and has a focal length:
The front principal point is 27 from the light emitting part of the semiconductor laser 1 so as to have a collimating action at 27 mm.
mm is arranged so as to be located at a distant position. An aspherical surface is used for the coupling lens 2, and the wavefront aberration of the collimated light beam is sufficiently corrected by the aspherical surface.

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 .

"Aperture"
The aperture 3 has an opening diameter in the main scanning direction: 8.1 mm and an opening diameter in the sub-scanning direction: 2.92.
The light beam collimated by the coupling lens 2 is shaped with a “rectangular opening” of mm.

"Anamorphic optics"
The anamorphic optical element 4 has an anamorphic refracting surface on the incident side, an arc-shaped “normal toroidal surface” in both the main scanning direction and the sub-scanning direction, and an exit side formed an “ellipsoidal power diffractive surface on the plane”. Is.

The power diffraction surface has the following second-order phase function: W in both the main scanning direction and the sub-scanning direction.
W = Cy · Y ² + Cz · Z ²
It is represented by Y is a coordinate in the main scanning direction with the optical axis as the origin, Z is a coordinate in the sub-scanning direction with the optical axis as the origin, and coefficients: Cy and Cz are Cy = −1.06506 × 10 ⁻³ , Cz =
-9.06664 × 10 ⁻³ .

"Optical deflector"
The polygon mirror 5 of the optical deflector has the number of reflection surfaces: 5 and the inscribed circle radius: 18 mm.
The distance between the exit side surface (surface on which the power diffraction surface is formed) of the anamorphic optical element 4 and the rotation axis of the polygon mirror 5 is “distance in the horizontal direction: x” and “distance in the vertical direction in the arrangement of FIG. : Y ”is set to x = 82.97 mm and y = 112.77 mm.

The soundproof glass G1 is made of glass 1, has a thickness of 1.9 mm, and an inclination angle α from the y direction (vertical direction in the drawing): 16 degrees.
Also, an angle θ between the traveling direction of the light beam incident from the light source side and the “traveling direction of the light beam reflected toward the position where the image height is 0 on the scanned surface 8” by the deflection reflecting surface is 58 degrees. It is. Table 2 shows what was described above.

In the above notation, Rm is the radius of curvature in the main scanning direction, Rs is the radius of curvature in the sub-scanning direction, D is the surface spacing, and the unit is mm.

  Table 3 gives optical system data after the optical deflector.

In the above notation, Rm is “paraxial curvature in the main scanning direction”, and Rs is “paraxial curvature in the sub-scanning direction”.
And Dx and Dy represent “relative distances from the origin of each optical element to the origin of the next optical element”. The unit is mm.
For example, regarding Dx and Dy for the optical deflector, the optical deflector (polygon mirror 5)
, The origin of the incident surface of the lens 6-1 of the scanning optical system 6 (the optical axis position of the incident side surface).
Is 79.75 mm away in the optical axis direction (x direction, left and right direction in FIG. 1), and the main scanning direction (y direction,
It is 8.8 mm apart in the vertical direction in FIG.

The thickness of the lens 6-1 on the optical axis is 22.6 mm, the surface interval between the lenses 6-1 and 6-2 is 75.85 mm, and the thickness of the lens 6-2 on the optical axis is 4. 9 mm, and the distance from the lens 6-2 to the surface to be scanned is 158.71 mm. Note that, between the lens 6-2 of the scanning optical system 6 and the surface to be scanned, a dust-proof glass G2 having a thickness of 1.9 mm made of glass 1 as shown in FIG.
Is placed.

Each surface of the lenses 6-1 and 6-2 of the scanning optical system 6 is aspheric.
The incident side surface of the lens 6-1 and the incident side surface and the exit side surface of the lens 6-2 have a “non-arc shape given by Formula 1” in the main scanning direction, and are in the sub-scanning section (parallel to the optical axis and the sub-scanning direction). This is a special surface in which the curvature in the (virtual cross section) “changes according to Equation 2” in the main scanning direction.

  In addition, the exit side surface of the lens 6-1 is “a coaxial aspheric surface expressed by Expression 3.”

"Non-arc shape"
Paraxial radius of curvature in the main scanning direction: Rm, distance in the main scanning direction from the optical axis: Y, conic constant: Km
, 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 equation.

X = (Y ² / Rm) / [1 + √ {1- (1 + Km) (Y / Rm) ² }] + A ₁ Y + A ₂ Y ² + A ₃ Y ³ + A ₄ Y ⁴ + A ₅ Y ⁵ + ・ ・ (Formula 1)
"Change of curvature in sub-scan section"
An expression expressing a state in which the curvature in the sub-scanning section: Cs (Y) (Y: coordinates in the main scanning direction with the optical axis position as the origin) changes in the main scanning direction is expressed in the sub-scanning section including the optical axis. Radius of curvature: Rs (0),
B ₁ , B ₂ , B ₃ ,... Are coefficients as follows.
Cs (Y) = {1 / Rs (0)} + B ₁ Y + B ₂ Y ² + B ₃ Y ³ + B ₄ Y ⁴ +. (Formula 2)
"Coaxial aspheric surface"
Paraxial radius of curvature: R, distance from optical axis: H, 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 equation.
X = (H ² / R) / [1 + √ {1- (1 + K) (H / R) ² }] + A ₁ Y + A ₂ Y ² + A ₃ Y ³ + A ₄ Y ⁴ + A ₅ Y ⁵ + ・ ・ （Formula 3）
Table 4 lists the coefficients of the incident side surface (special surface) of the lens 6-1.

  Table 5 lists the coefficients of the exit side surface (coaxial aspheric surface) of the lens 6-1.

  Table 6 lists the coefficients of the incident side surface (special surface) of the lens 6-2.

  Table 7 lists the coefficients of the exit side surface (special surface) of the lens 6-2.

3A and 3B show the relationship between the beam spot diameters in the main scanning direction and the sub-scanning direction and “the beam waist position is defocused with respect to the scanning surface” in the first embodiment.
These figures show the relationship when the reference temperature is 25 ° C. (“room temperature”), the relationship when there is a temperature increase of 20 ° C. relative to the room temperature (“temperature fluctuation”), and the emission wavelength due to the mode hop. 0.8n
The relationship (“wavelength skip”) when m is changed is shown.

FIG. 3A shows the beam spot diameter in the main scanning direction, and FIG. 3B shows the beam spot diameter in the sub-scanning direction, both of them when “image height of light spot: 0”. As is apparent from FIG. 3, in the optical scanning apparatus of the first embodiment, the relationship between the beam spot diameter and the defocus amount is substantially “in normal temperature state, temperature fluctuation state, or wavelength skip state” in both the main and sub scanning directions. Does not change. This means that the beam waist position in the main scanning direction and the sub-scanning direction does not substantially change regardless of temperature fluctuations and mode hops.

Incidentally, in Example 1, the power of the power diffraction surface in the main / sub-scanning direction: Pm (main scanning direction), Ps (sub-scanning direction), and the coupling lens (in the main scanning direction and / or the sub-scanning direction). Power: Ratio to Pcm (main scanning direction), Pcs (sub scanning direction): Pcm / P
The values of m and Pcs / Ps are respectively
Pcm / Pm = 9.2, Pcs / Ps = 1.1
And the above-mentioned conditions (1) and (2) are satisfied.

That is, the optical scanning device of the first embodiment converts the light beam from the semiconductor laser 1 into a light beam having a desired beam shape by the coupling lens 2 and then guides it to the optical deflector 5 through the anamorphic optical element 4. An optical scanning device that irradiates and deflects a light beam deflected by an optical deflector onto a surface to be scanned 8 by a scanning optical system 6 to form a light spot, and optically scans the surface to be scanned 8. The scanning optical system 6 includes one or more resin lenses 6-1 and 6-2, and the anamorphic optical element 4 has an elliptical power with one surface being an anamorphic refractive surface and the other surface having an axis in the main scanning direction. It is an anamorphic resin lens having a diffractive surface, and changes in beam waist position in the main scanning direction and / or sub-scanning direction due to mode hopping and temperature change in the semiconductor laser 1 are made substantially zero. It is obtained by setting the power of the power diffraction plane (claim 1).

The anamorphic optical element 4 does not have power in the main scanning direction but has positive power in the sub-scanning direction (Claim 2), and the refractive surface has an absolute value of the radius of curvature in the main scanning direction. The surface is larger than that in the sub-scanning direction. The power diffractive surface of the anamorphic optical element 4 is “an elliptical power diffractive surface formed on a plane” (Claim 4), and the coupling lens 2 is a glass lens (Claim 5).

A comparative example is given below.
"Comparative example"
In the comparative example, the aperture diameter of the aperture of the aperture 3 is changed to 7.85 mm in the main scanning direction and 3 mm in the sub scanning direction in the first embodiment, and a cylinder lens made of glass 2 is used as the anamorphic optical element 4. It was. Also, for the optical arrangement after the optical deflector,
In order to make the conditions the same as in Example 1, the positional relationship between the cylinder lens and the optical deflector was changed.
Others are the same as the first embodiment.

  The data on the light source side of the comparative example is shown in Table 8 following Table 2.

FIG. 4 shows the relationship when the beam spot diameter and the beam waist position in the main scanning direction and the sub-scanning direction are defocused with respect to the surface to be scanned in the optical scanning device of the comparative example.
This is shown following (b). These figures show the relationship when the reference temperature is 25 ° C ("room temperature")
And a relationship when there is a temperature increase of 20 ° C. with respect to normal temperature (“temperature fluctuation”).

As apparent from FIG. 4, since the power diffractive surface is not used in the comparative example, when the temperature rises, the beam waist position fluctuates greatly in both the main scanning direction (upper view of FIG. 4) and the sub-scanning direction (lower view of FIG. 4). It can be seen that, in order to perform high-definition image writing, it is necessary to take measures to suppress the beam waist position fluctuation due to the environmental fluctuation as much as possible.

FIG. 5 schematically shows one embodiment of the image forming apparatus.
This image forming apparatus is a “tandem type full-color optical printer”.
A transport belt 32 that transports transfer paper (not shown) fed from a paper feed cassette 30 disposed in the horizontal direction is provided on the lower side of the apparatus. Above the conveyor belt 32, a yellow Y photoconductor 7Y, a magenta M photoconductor 7M, a cyan C photoconductor 7C, and a black K photoconductor 7K are arranged in order from the upstream side at equal intervals. Has been. In the following,
Yellow, magenta, cyan, and black are distinguished by Y, M, C, and K in the code.

The photoreceptors 7Y, 7M, 7C, and 7K are all formed to have the same diameter, and process members are sequentially disposed around the photoreceptors according to an electrophotographic process. 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. 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 scanning surfaces set for the respective colors, and a pair of optical scanning devices 50Y, 50M, 50C, and 50K is provided for each. 1 correspondence relationship.

As these optical scanning devices, those having an optical arrangement as shown in FIG. 1 can be used independently. For example, as disclosed in JP 2004-280056 A, for example, The optical deflector (rotating polygon mirror) is shared, and the lens 6-1 of the scanning optical system in each optical scanning device is shared for optical scanning of the photoconductors 7M and 7Y, and the photoconductors 7K and 7C.
It can also be shared for optical scanning.

Around the conveyance belt 32, a registration roller 9 and a belt charging charger 10 are provided on the upstream side of the photoconductor 7Y, and a belt separation charger 11 and a charge removal charger are provided on the downstream side of the photoconductor 7K. 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, each of the photoconductors 7Y and 7M.
, 7C and 7K, based on the image signals of Y, M, C and K colors, the respective optical scanning devices 50Y and 50
An electrostatic latent image is formed by optical scanning with M, 50C, and 50K. These electrostatic latent images are developed with corresponding color toners to form toner images, which are superposed by being sequentially transferred onto transfer paper that is electrostatically adsorbed onto the conveyance belt 32 and conveyed. After being fixed as a full-color image, it is discharged onto a discharge tray 15.

By using the optical scanning device described in the embodiment for such an image forming apparatus, it is possible to always obtain a stable beam spot diameter, and to realize an image forming apparatus suitable for high-definition printing in a compact and inexpensive manner.

It is a figure for demonstrating one Embodiment of an optical scanning device. It is a figure for demonstrating an anamorphic optical element. It is a figure for demonstrating the characteristic of an Example. It is a figure for demonstrating the characteristic of the comparative example which does not use a power diffraction surface. It is a figure for demonstrating one Embodiment of an image forming apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Semiconductor laser 2 Coupling lens 3 Aperture 4 Anamorphic optical element 5 Optical deflector 6 Scanning optical system 8 Surface to be scanned

Claims (6)

A light beam from a semiconductor laser is converted into a light beam having a desired beam shape by a coupling lens, and then guided to an optical deflector through an anamorphic optical element, and the light beam deflected by the optical deflector is scanned. A light spot is formed by condensing on the surface to be scanned by the optical system,
An optical scanning device that optically scans the surface to be scanned,
The scanning optical system includes one or more resin lenses,
The anamorphic optical element is an anamorphic resin lens having an elliptical power diffractive surface having an anamorphic refractive surface on one side and an axis in the main scanning direction on the other side,
Light characterized in that 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 mode hopping and temperature change in the semiconductor laser becomes substantially zero. Scanning device. The optical scanning device according to claim 1,
An anamorphic optical element has no power in the main scanning direction and has a positive power in the sub-scanning direction. The optical scanning device according to claim 1 or 2,
An optical scanning device characterized in that the refracting surface of the anamorphic optical element is a surface in which the absolute value of the radius of curvature is larger in the main scanning direction than in the sub scanning direction. The optical scanning device according to any one of claims 1 to 3,
The optical diffractive surface of the anamorphic optical element is an elliptical power diffractive surface formed on a flat surface. The optical scanning device according to any one of claims 1 to 4,
An optical scanning device, wherein the coupling lens is a glass lens. In an image forming apparatus having one or more image forming units that perform light scanning by a light scanning unit on a photosensitive image carrier to form a latent image, and visualize the latent image with a developing unit to obtain an image.
An image forming apparatus using at least one optical scanning device according to any one of claims 1 to 5 as optical scanning means for optically scanning an image carrier.

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