JP2006235069A - Optical scanner and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce fluctuation of a beam spot diameter caused by change not only in temperature but also in oscillation wavelength through mode hopping, using a power diffraction face. <P>SOLUTION: The optical scanner performs optical scanning by converting an optical beam from a semiconductor laser to a desired type of optical beam with a coupling lens, guiding it to an optical deflector through an anamorphic optical element, and converging the deflected optical beam on a surface to be scanned by a scanning optical system to form an optical spot. The scanning optical system contains one or more resin-made lens. The anamorphic optical element 4 is a resin-made lens one side of which is a rotation symmetric face having a concentric power diffraction face and the other side of which is a face including a power diffraction face parallel to the main scanning direction and convergent only in the subscanning direction, wherein a power in each power diffraction face is set in a manner making a beam waist positional change come to nearly zero in the main scanning direction and/or subscanning direction caused by mode hopping or temperature change in the semiconductor laser. <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. There is a need for an optical scanning device capable of forming a clear image. Also, as an image forming apparatus using an optical scanning device, a plurality of photoconductors are optically scanned, the formed electrostatic latent images are visualized with different color toners, and the resulting different color toner images are combined to form a color image. There is also known a method for obtaining the above (Patent Document 1).

  When the various lenses used in the optical scanning device are made of resin material, the resin lens is lightweight and can be formed at low cost, and it is easy to form special surface shapes typified by aspherical surfaces. By adopting a special surface for the manufactured 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. However, on the other hand, as is well known, resin lenses change shape and refractive index with 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 of canceling out “changes in optical characteristics due to environmental changes” that occur in a manufactured lens is well known.

  Further, 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 also a wavelength change due to “mode hop”. . 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.

  Patent Documents 2 to 4 describe optical scanning devices (laser scanning devices) that employ a power diffractive surface to stabilize optical properties in consideration of changes in optical properties associated with temperature changes and wavelength changes in the light source. It has been known.

  In Patent Document 2, a light source optical system that condenses 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 no rotational symmetry axis. Disclosed is 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, and a comparative example "An optical scanning device in which 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. Is disclosed. The “power diffractive surface” is a diffractive surface having lens power by diffraction.

  In Patent Document 3, a diffractive surface is disposed on the light source side of the optical deflector, and a diffractive surface is also disposed in a scanning optical system that converges the deflected light beam toward the scanned surface, and the change in the diffraction angle due to the wavelength variation. Discloses a method for reducing or compensating for fluctuations in the “condensing position of the light beam for scanning the surface to be scanned”.

  In Patent Document 4, a lens surface of a collimating lens that converts a divergent laser beam emitted from a semiconductor laser into a parallel beam is used as a power diffractive surface. A method of correcting the “variation in the condensing position of the laser beam” is disclosed.

  According to Patent Document 2, “one optical element having one or more reflecting surfaces that do not have a rotational symmetry axis and two transmissive surfaces, a power diffractive surface provided on the transmissive surface, and made of resin” The light source optical system has to form a transmission surface and a reflection surface in one optical element, and is not always easy to manufacture because it includes a curved reflection surface.

  A collimator lens for converting a laser beam from a semiconductor laser into a parallel beam is generally a “lens having the strongest power among optical elements used in an optical scanning device”, and is disclosed in Patent Document 2 as a comparative example. When a power diffractive surface is used for the collimator lens as in the case of “What is” or Patent Document 4, “deterioration of wavefront aberration of collimated light beam” is a concern 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.

  In addition, the optical scanning device generally includes “anamorphic optical elements having different imaging effects in the main scanning direction and the sub-scanning direction” among the optical elements arranged between the light source and the surface to be scanned. In general, the image forming action in the optical scanning device is different between the main scanning direction and the sub-scanning direction. In the “method for forming a power diffractive surface on a collimating lens” described in Patent Document 2 and Patent Document 4 described above, since the formed power diffractive surface is rotationally symmetric with respect to the optical axis, the influence of fluctuations in the emission wavelength of the semiconductor laser is mainly used. Correction cannot be performed independently in the scanning direction and the sub-scanning direction.

  In the case of the method described in Patent Document 3, the “scanning optical system that converges the deflected light beam toward the scanning surface” that forms the power diffraction surface is generally a large lens that is long in the main scanning direction. If an attempt is made to form a diffractive surface in the effective area of a large lens, the diffractive surface becomes large, and it takes time to process the diffractive surface, resulting in low manufacturing efficiency of the lens having the diffractive surface, resulting in an increase in cost. easy.

JP2004-280056 JP 2002-287062 Japanese Patent No. 3397683 JP 2000-171741 A

  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.
In addition, the “anamorphic optical element” that guides the light beam from the coupling lens side to the optical deflector is “a rotationally symmetric surface having a concentric power diffractive surface” on one side and “the main surface”. This is a resin lens having a linear power diffractive surface parallel to the scanning direction and having a condensing function only in the sub-scanning direction.
Then, the power of each of the power diffractive surfaces 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 that of the lens function, and that it is concentric is that “the shape of the grating grooves constituting the power diffractive surface is concentric”. This means that this is a straight line parallel to the main scanning direction, which means that “the shape of the grating grooves constituting the power diffraction surface is a straight line parallel to the main scanning direction”. The above “because of mode hop and / or temperature change” means “because of mode hop and / or temperature change”.

  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 has one surface “forms a concentric power diffractive surface on a spherical surface” and the other surface “on a cylindrical surface. A linear power diffractive surface can be formed "(Claim 2). “A concentric power diffractive surface is formed on a spherical surface.” Means that the surface on which the concentric power diffractive surface is formed is a spherical surface, and “a linear power on a cylindrical surface. “A diffractive surface is formed” means that the surface on which the linear power diffractive surface is formed is a cylindrical surface.

  When the power diffractive surface is formed on a spherical surface or a cylindrical surface in this way, the power of the power diffractive surface and the “optical power of the curved surface on which the power diffractive surface is formed” can be combined as power.

The anamorphic optical element in the optical scanning device according to claim 2 is configured to “cancel the power of the concentric power diffractive surface and the power of the spherical surface on which the concentric power diffractive surface is formed with respect to the reference wavelength”. Thus, “an optical element having no power in the main scanning direction and having a positive power in the sub scanning direction with respect to the reference wavelength” can be configured.
The “reference wavelength” is the emission wavelength of the semiconductor laser in the optical system design.
In this case, the coupling action of the coupling lens is preferably a “collimating action”.

  If the anamorphic optical element is `` no power in the main scanning direction '', the beam waist position fluctuations in the sub-scanning direction due to processing errors and assembly errors during the initial assembly of the optical system will be reduced. By adjusting the displacement in the optical axis direction, adjustment can be performed “without affecting the optical characteristics in the main scanning direction”.

  Further, if one surface is a “concentric power diffractive surface formed on a spherical surface” as in the case of claim 2, this surface contributes to the deterioration of the optical performance due to the rotation about the optical axis. Therefore, it is easy to align the entrance surface and the exit surface.

  The coupling lens in the optical scanning device according to any one of claims 1 to 3 is preferably a “glass lens” (claim 4). 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, a semiconductor laser used as a light source can be configured to perform a single beam scanning method using one normal laser, but a semiconductor laser array or “two or more semiconductors” By using a light source of “method of combining light beams from lasers”, a well-known “multi-beam scanning method” can be executed.

  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 4, wherein the optical scanning apparatus performs optical scanning of an image carrier. 5).

  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 Patent Document 1 or the like, “part of optical elements” 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 plurality of image carriers.

  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 of all, the causes of beam waist position fluctuations due to temperature fluctuations are “change in refractive index of resin lens itself”, “change in shape of resin lens”, “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” decreases as the density decreases 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.
That is, regardless of whether the lens made of resin is a positive lens or a negative lens, the “absolute value of power with respect to light from the semiconductor laser” decreases 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. There is a tendency that the absolute value of increases as the wavelength increases.

  Therefore, for example, when the “composite 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). Thus, the “power change due to temperature fluctuation” in the resin lens can be canceled out by the “power change accompanying temperature fluctuation” in the “diffractive part” of the power diffraction surface.

  As described above, the power diffraction surface of the anamorphic optical element used in the optical scanning device of the present invention includes not only a flat surface but also a spherical surface or a cylindrical surface, and forms a diffraction surface. There is a case where the portion that hits the substrate is also powered. Thus, the portion excluding the power of the “portion that hits the substrate”, that is, the portion that generates only the power of the diffractive surface is referred to as the “diffractive portion” of the power diffractive surface as described above.

For the sake of specific explanation, consider a case where the environmental temperature rises when both the power of the resin lens contained in the optical system and the power of the “diffractive part” of the power diffraction surface are 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 change in “diffractive part” of power diffractive surface due to change in emission wavelength of semiconductor laser: D
Then, A> 0, B> 0, C> 0, and D <0 (change in the direction away from the optical deflector is positive).

  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.

  By the way, as described above, the change in the emission wavelength of the semiconductor laser, which is a light source, is not only due to a temperature change, but also due to a mode hop. 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 light emission wavelength change due to the mode hop occurs independently of the temperature change, and when the light 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. Changes excessively, the beam waist position fluctuation amount becomes CD <0 and is not corrected, 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 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 part” of the diffraction surface is set.

The power (in the main scanning direction and / or the sub-scanning direction) of the “diffractive part” of the power diffractive surface set in this way: Pm (main scanning direction) and Ps (sub-scanning direction) are the power of the coupling lens: For 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. In the present invention, since the power diffractive surfaces are formed on both the incident side surface and the exit side surface of the anamorphic optical element, Pm and Ps mentioned here indicate the power of the power diffractive surface of each surface of the anamorphic optical element as “the main and sub scanning directions respectively. "Synthesized power".

  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. The “variation in the beam waist position in the main scanning direction due to the change” increases linearly as the parameter: Pcm / Pm increases.

  The “variation in the beam waist position in the main scanning direction due to the change in the emission wavelength due to the mode hop” is preferably suppressed to 0.5 mm or less. In the above-described linear increase relationship, the value of the parameter: Pcm / Pm corresponding to “variation in beam waist position in the main scanning direction due to emission wavelength change due to mode hop: 0.5 mm” is 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 of the beam waist position in the sub-scanning direction due to the emission wavelength change due to the mode hop” increases linearly as the parameter: Pcm / Pm increases.

  It is preferable that “the amount of fluctuation of the beam waist position in the sub-scanning direction due to the change in the emission wavelength due to the mode hop” is also suppressed to 0.5 mm or less. The value of the parameter: Pcs / Ps corresponding to “the variation amount of the beam waist position: 0.5 mm” is 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: Pcs / Ps corresponding to “variation in beam waist position in the sub-scanning direction due to temperature change: 0.5 mm” 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 as a “long line image in the main scanning direction” in the vicinity of the deflection reflection surface of the polygon mirror 5 to be deflected and reflected. When reflected by the surface, the light passes through the soundproof glass G 1 and enters the scanning optical system 6.

  The scanning optical system 6 is composed of two lenses 6-1 and 6-2, and the light beam transmitted through these lenses 6-1 and 6-2 is incident on the surface to be scanned 8 through the dust-proof glass G2, and is scanned. A light spot is formed on the scanned surface 8 by the action of the 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 by a light beam incident while being deflected at a uniform 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 anamorphic, and in the sub-scanning direction, the deflecting / reflecting surface position of the polygon mirror 5 and the scanned surface position are in a geometric optical conjugate relationship, thereby correcting the tilting of the polygon mirror. The scanned surface 8 is essentially a “photosensitive surface of a photosensitive medium”.

  The anamorphic optical element 4 is an anamorphic optical element in which “one surface is a surface on which a concentric power diffraction surface is formed on a spherical surface and the other surface is a surface on which a linear power diffraction surface is formed on a cylindrical surface”. It is a resin lens.

FIG. 2 illustrates the anamorphic optical element 4 in an explanatory manner.
In FIG. 2, (2A) shows a “view of the incident side surface of the anamorphic optical element 4 from the optical axis direction”, and (2B) shows “the exit side surface of the anamorphic optical element 4 in the optical axis direction”. It is the figure seen from ". In these drawings, the horizontal direction is the main scanning direction, and the vertical direction is the sub-scanning direction. FIG. 2C is an explanatory view of “a cross-sectional shape on a virtual cross section including the optical axis and parallel to the main scanning direction” of the anamorphic optical element 4, and FIG. 2D is a diagram illustrating the “optical axis of the anamorphic optical element 4. The cross-sectional shape on a virtual cross section parallel to the sub-scanning direction is illustrated.

  As shown in FIG. 2A, the incident side surface 4A of the anamorphic optical element 4 has a “power diffractive surface formed by a set of concentric grooves” when viewed from the optical axis direction. As shown in FIGS. 2C and 2D, it is formed on a “concave spherical surface”. The “concentric power diffractive surface” of the power diffractive surface 4AR on the incident side has a positive power, but the concave spherical surface formed with this power diffractive surface has a negative power. The positive power due to the concentric power diffractive surface and the negative power due to the concave spherical surface cancel each other with respect to the reference wavelength.

  As shown in FIG. 2B, the exit side surface 4B of the anamorphic optical element 4 is formed with a linear power diffraction surface 4BR based on “a set of linear grooves” when viewed from the optical axis direction. The power diffractive surface 4BR is formed on a “concave cylindrical surface” as shown in FIG. 2D. The “linear power diffractive surface” of the power diffractive surface 4BR on the exit side has a positive power in the sub-scanning direction, but the concave cylindrical surface on which the power diffractive surface is formed has a negative power. The positive power in the sub-scanning direction due to the linear power diffraction surface surpasses the negative power in the sub-scanning direction due to the concave cylindrical surface with respect to the reference wavelength.

  Therefore, in the anamorphic optical element 4, the power of the concentric power diffractive surface and the power of the concave spherical surface on which the power diffractive surface is formed cancel each other with respect to the reference wavelength, and the sub-scanning by the linear power diffractive surface is performed. The positive power in the direction exceeds the negative power in the sub-scanning direction due to the concave cylindrical surface with respect to the reference wavelength. The optical element has the power of “.

  Therefore, when the light beam (parallel light beam) incident on the anamorphic optical system 4 from the light source side is transmitted through the anamorphic optical element 4, the light beam is parallel to the main scanning direction and converges 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, the “median” is the refractive index with respect to the wavelength used (reference wavelength) at the reference temperature: 25 ° C., and the “wavelength jump” is the refractive index when the wavelength jump is caused by the mode hop, “Temperature fluctuation” 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 as a reference wavelength, and when the temperature rises by 1 ° C. with respect to the standard temperature of 25 ° C., the emission wavelength shifts to the longer wavelength side by 0.2 nm. The mode hop assumes a wavelength change of 0.8 nm as described above.

"Coupling lens"
The coupling lens 2 is a glass lens using the glass 1 as a material, and is arranged so that the front principal point is located 27 mm away from the light emitting portion of the semiconductor laser 1 so as to have a collimating function at a focal length of 27 mm. Is done. 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 a “rectangular opening” having an aperture diameter of 8.14 mm in the main scanning direction and an aperture diameter of 2.96 mm in the sub-scanning direction, and shapes the light beam collimated by the coupling lens 2. .

"Anamorphic optics"
In the anamorphic optical element 4, the incident side surface is a “concentric power diffractive surface formed on a concave spherical surface”, and the emission side surface is a “linear power diffractive surface formed on a concave cylindrical surface”. .

Power diffractive surface of the incident side, the phase _{function: W in} is,
W _in = C ₀ · r ²
The power diffractive surface on the exit side has a phase function: w _out ,
W _out = C _z · Z ²
It is represented by In addition, r is the coordinate in the main scanning direction as Y and the coordinate in the sub scanning direction as Z (both are the optical axis position as the origin).
r ² = Y ² + Z ²
It is represented by

The above coefficients: C ₀ and C _z are
C ₀ = −1.07033 × 10 ⁻³ , C _z = −7.8825 × 10 ⁻³
It is.

"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 of the anamorphic optical element 4 and the rotation axis of the polygon mirror 5 is “displacement in the horizontal direction: x” and “distance in the vertical direction: y” in the arrangement of FIG. y is set to 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 the “optical system data from the light source to the optical deflector” described above.

In notation above, R _m in the main scanning direction of the radius of curvature, R _s is the sub-scanning direction of the curvature radius, L is a plane interval, the unit is in mm.

  Table 3 gives optical system data after the optical deflector.

In the above notation, R _m is “paraxial curvature in the main scanning direction”, R _s is “paraxial curvature in the sub scanning direction”, and D _x and D _y are “from the origin of each optical element to the next optical element. "Relative distance to the origin". The unit is mm.
For example, regarding D _x and D _{y with} respect to the optical deflector, the origin of the incident surface of the lens 6-1 of the scanning optical system 6 (the optical axis of the incident side surface) is viewed from the rotational axis of the optical deflector (polygon mirror 5). The position is 79.75 mm away in the optical axis direction (x direction, left-right direction in FIG. 1) and 8.8 mm away in the main scanning direction (y direction, up-down direction in FIG. 1).

  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. In addition, 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 of Table 1” is disposed as shown in FIG. The

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 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 well-known formula 1.

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"
The expression expressing the state in which the curvature in the sub-scanning section: C _s (Y) (Y: coordinates in the main scanning direction with the optical axis position as the origin) changes in the main scanning direction is in the sub-scanning section including the optical axis. The curvature radii of: R _s (0), B ₁ , B ₂ , B ₃ ,.
Cs (Y) = {1 / Rs (0)} + B ₁ Y + B ₂ Y ² + B ₃ Y ³ + B ₄ Y ⁴ +. (Formula 2)
"Rotationally symmetric aspheric surface"
Paraxial radius of curvature: R, distance from optical axis: H, conic constant: K, higher order coefficients as A ₁ , A ₂ , A ₃ , A ₄ , A ₅ ,..., Depth in optical axis direction: X It is expressed by the following formula.

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 room temperature (“temperature fluctuation”), and the emission wavelength due to mode hopping. The relationship (“wavelength skip”) when changed by 0.8 nm 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: Pcm (main scanning direction), Pcs (sub-scanning direction) ratio: Pcm / Pm, Pcs / Ps values are respectively
Pcm / Pm = 9.2, Pcs / Ps = 1.1
And the above-mentioned conditions (1) and (2) are satisfied. Pm is the power in the main scanning direction of the diffractive portion of the power diffractive surface (concentric power diffractive surface) on the incident side surface, and Ps is the power in the sub-scanning direction of the diffractive portion of the power diffractive surface on the incident side surface. The power in the sub-scanning direction of the diffractive portion of the power diffractive surface on the exit side is combined.

  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. The anamorphic optical element 4 is a rotationally symmetric surface having a concentric power diffractive surface on one side, and the other surface is the main surface. This is a resin lens that is a surface having a power diffractive surface that is parallel to the scanning direction and has a condensing function only in the sub-scanning direction, and is caused by mode hopping and temperature change in the semiconductor laser 1 in the main scanning direction and the sub-scanning direction. Beamow Variations in strike position to substantially zero, is obtained by setting the power of each power diffractive surface (claim 1).

  The anamorphic optical element 4 has one surface formed with a concentric power diffractive surface on a spherical surface, and the other surface formed with a linear power diffractive surface on a cylindrical surface. ) The power of the concentric power diffractive surface and the power of the spherical surface on which the power diffractive surface is formed cancel each other with respect to the reference wavelength, so that there is no power in the main scanning direction with respect to the reference wavelength. And has a positive power in the sub-scanning direction. The coupling lens 2 is a glass lens.

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

  The data from the light source of the comparative example to the optical deflector is shown in Table 8 following Table 2.

  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. It is shown following b). These figures show the relationship when the reference temperature is 25 ° C. (“room temperature”) and the relationship when the temperature rises by 20 ° C. relative to the room temperature (“temperature fluctuation”).

  As apparent from FIG. 4, since the power diffraction surface is not used in the comparative example, the beam waist position in both the main scanning direction (FIG. 4A) and the sub-scanning direction (FIG. 4B) when the temperature rises. It can be seen that in order to perform high-definition image writing with large fluctuations, it is necessary to take measures to minimize the beam waist position fluctuation due to environmental fluctuations.

  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, optical deflectors such as those conventionally known from JP-A-2004-280056 are used. (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 is shared for optical scanning of the photoconductors 7K and 7C. You can also

  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, the optical scanning devices 50Y, 50M, and 50C are applied to the photosensitive members 7Y, 7M, 7C, and 7K based on the image signals of Y, M, C, and K colors. , An electrostatic latent image is formed by optical scanning at 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, a stable beam spot diameter can be obtained at all times, and an image forming apparatus suitable for high-definition printing can be realized in a compact and inexpensive manner ( Claim 5).

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 (5)

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. An optical scanning device that focuses light on a surface to be scanned by an optical system to form a light spot and optically scans the surface to be scanned,
The scanning optical system includes one or more resin lenses,
The anamorphic optical element is a rotationally symmetric surface with one side having a concentric power diffractive surface, and the other surface having a power diffractive surface parallel to the main scanning direction and having a condensing function only in the sub-scanning direction. A resin lens,
The power of each of the power diffractive surfaces 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 is set to approximately zero. Optical scanning device. The optical scanning device according to claim 1,
An anamorphic optical element is characterized in that one side has a concentric power diffractive surface formed on a spherical surface, and the other surface has a linear power diffractive surface formed on a cylindrical surface. apparatus. The optical scanning device according to claim 2.
The anamorphic optical element has the power of the concentric power diffractive surface and the power of the spherical surface on which the power diffractive surface is formed cancel each other with respect to the reference wavelength. An optical scanning device characterized by having no power and positive power in the sub-scanning direction. The optical scanning device according to any one of claims 1 to 3,
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 4 as optical scanning means for optically scanning an image carrier.

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