JP4381116B2 - Scanning optical system and printer - Google Patents

Description

  The present invention relates to a scanning optical system for forming an electrostatic latent image on the surface of a photosensitive drum, and a printer in which such a scanning optical system is incorporated.

  As is well known, a scanning optical system is incorporated in a printing apparatus such as a laser beam printer, a facsimile machine, or a copier. The scanning optical system dynamically deflects the laser beam modulated according to the image information by the rotating polygon mirror, and converges the dynamically deflected laser beam on the surface of the photosensitive drum by the imaging optical system, Scan the photosensitive drum. A plurality of dots are drawn as an electrostatic latent image on the surface of the scanned photosensitive drum (surface to be scanned).

In general, the intensity distribution of the laser beam incident on the scanning target surface is not a perfect Gaussian distribution, and it is around the main beam due to the diffraction phenomenon at the aperture (aperture) provided in the optical path of the laser beam. It is known to have several light rings (side lobes) that have a lower light intensity than the main beam. It is also known that when the intensity of the side lobe exceeds about 6% of the center intensity of the main beam, the side lobe sensitizes the photosensitive drum to cause printing defects called black stripes during halftone printing. (See Patent Document 1). However, when the imaging optical system is in an ideal state, the side lobe intensity is about 4% of the center intensity of the main beam, and therefore no black streak occurs.
JP 09-080333 A

  However, if the optical surface of the imaging optical system has microscopic undulations, the intensity of the side lobe changes when the laser beam passes through the undulations. If the intensity of the side lobe exceeds the threshold due to the change, there is a problem that black streaks occur during halftone printing.

  Accordingly, an object of the present invention is to provide a scanning optical that can suppress the side lobe intensity from exceeding a threshold value as much as possible even when a certain degree of microscopic undulation occurs on the optical surface of the imaging optical system. A system and printer is to be provided.

  The scanning optical system invented to solve the above problems and the scanning optical system incorporated in the printer invented to solve the above problems are both configured as described below. ing.

  That is, the scanning optical system according to the present invention dynamically deflects a laser beam emitted from a light source by a deflector and uses the dynamically deflected laser beam as a spot light on a scanning target surface by an imaging optical system. A scanning optical system that scans the spot light along the main scanning direction on the surface to be scanned by converging, and includes a collimating lens on an optical path between the light source and the deflector, Any one of the optical surfaces of the collimating lens transmits a central region of the laser beam emitted from the light source and a central region that transmits the light beam in the vicinity thereof and a part of the light beam that is incident on the outside of the central region. And a first outer region acting so as to give a predetermined first phase difference between the transmitted light beam and the light beam transmitted through the central region, A part of the light beam excluding the light beam incident on the central region and the first outer region is transmitted, and a predetermined second phase difference is given to the transmitted light beam and the light beam transmitted through the central region. The first phase difference does not include a phase difference zero, and the second phase difference includes a phase difference zero, and the first phase difference Are different values.

  If comprised in this way, the 1st and 2nd phase difference of the light beam after each passing 1st and 2nd outer area | region will be set suitably, and the magnitude | size of the 1st and 2nd outer area | region will be set. If appropriately selected, the intensity of the side lobe of the laser beam incident on the scanning target surface can be suppressed to 2% of the center intensity of the main beam. Therefore, even if the intensity of the side lobe increases by about several percent due to the microscopic undulation of the optical surface of the imaging optical system, the threshold value is not exceeded. As a result, the occurrence of black streaks during halftone printing is suppressed. Further, by providing the phase shift structure in which the first and second phase differences are set on the lens surface of the collimating lens instead of a separate phase shift element, adjustment of the phase shift element becomes unnecessary, and the beam spot is symmetric. It is possible to prevent the characteristics from breaking.

In the scanning optical system and the printer according to the present invention, the first outer region has a specific phase difference θ [0] that does not include zero phase difference between the laser beam transmitted through itself and the laser beam transmitted through the central region. rad], where θ is the following conditional expressions (1) and (2),
cosθ ≦ 0 --- (1)
0 <θ <10π --- (2)
Or the following conditional expressions (6) and (7),
cosθ ≦ 0 --- (6)
−10π <θ <0 --- (7)
Is preferably satisfied. In both the former and the latter cases, when the integer is N, the sidelobe reduction effect is more effective as the first phase difference θ is closer to π × (2N−1) [rad]. Also, if the upper limit of conditional expressions (1) and (6) is exceeded, the effect of reducing side lobes will be diminished. Furthermore, if the upper limit of conditional expression (2) is exceeded, the lens thickness in the first outer region is too small compared to the lens thickness in the central region. If the lower limit of conditional expression (7) is not reached, the lens thickness in the first outer region becomes too large compared to the lens thickness in the central region.

Furthermore, when the first outer region gives the first phase difference θ [rad] that satisfies the conditional expressions (1) and (2) to the laser beam transmitted through the first outer region, A specific phase difference θ ′ [rad] including a phase difference of zero is given to the laser beam transmitted through itself and the laser beam transmitted through the central region, and θ ′ is the following conditional expression: (3) to (5),
0.9 ≦ cosθ '--- (3)
0 ≦ θ ′ <10π --- (4)
θ <θ '--- (5)
Is preferably satisfied. In addition, when the first outer region gives the first phase difference θ [rad] that satisfies the conditional expressions (6) and (7) to the laser beam that passes through the first outer region, A specific phase difference θ ′ [rad] including a phase difference of zero is given to the laser beam transmitted through itself and the laser beam transmitted through the central region, and θ ′ is the following conditional expression: (8) to (10),
0.9 ≦ cosθ '--- (8)
−10π <θ ′ ≦ 0 --- (9)
θ '<θ --- (10)
It is preferable to provide the second phase difference θ ′ [rad] that satisfies the above. In either case of the former or the latter, when the integer is M, the decrease in the center intensity can be suppressed as the second phase difference θ ′ approaches 2π × M [rad]. If the lower limit of conditional expressions (3) and (8) is not reached, the effect of reducing the side lobes will be diminished, and the amount of decrease in the center strength will be large. When the upper limit of conditional expression (4) is exceeded, the lens thickness in the second outer region is too small compared to the lens thickness in the central region. If the lower limit of conditional expression (9) is not reached, the lens thickness in the second outer region becomes too large compared to the lens thickness in the central region. Further, by satisfying the conditional expressions (5) and (10), the projecting direction of the first and second outer regions is the same with respect to the central region, so that the collimating lens mold can be easily processed.

By the way, each first outer region imparts a first phase difference satisfying conditional expressions (1) and (2) to the laser beam, and each second outer region has conditional expressions (3) to (5). When the second phase difference satisfying () is given to the laser beam, or each first outer region gives the first phase difference satisfying conditional expressions (6) and (7) to the laser beam. In addition, when each second outer region gives the laser beam a second phase difference that satisfies the conditional expressions (8) to (10), the total sum of the areas of the first outer regions is set appropriately. It is desirable that For example, when the sum of the areas of the regions where the laser beam is incident in each of the first outer regions is S ′ and the area of the cross section perpendicular to the beam central axis in the laser beam is S, the following conditional expression (11 ),
0.03 <S '/ S <0.3 --- (11)
Can be set to satisfy. In such a setting condition, the effect of reducing the side lobe is reduced if the lower limit is exceeded, and conversely, if the upper limit is exceeded, the amount of decrease in the center intensity of the main beam is increased.

  In the scanning optical system and the printer according to the present invention, the collimating lens may include one set of the first and second outer regions or a plurality of sets of the first and second outer regions. When the number becomes large, the level difference increases on the optical surface and the amount of light is lost. Therefore, it is preferable that the first and second outer regions provided in the collimating lens are limited to about two sets. When a plurality of sets are provided, it is desirable that the first and second outer regions are alternately arranged in the direction away from the central region, and the second outer region is arranged on the outermost side.

  In the scanning optical system and the printer according to the present invention, the collimating lens may integrally include a shielding portion that functions as an aperture stop. In this case, it is possible to prevent the symmetry of the beam spot from being lost due to the positional deviation between the collimating lens and the aperture stop.

  In the scanning optical system according to the present invention, the imaging optical system may be a transmissive type using an fθ lens or the like, or a reflective type using an fθ mirror or the like. Further, the deflector may be a rotary polygon mirror or a galvanometer mirror.

  As described above, according to the present invention, even when a certain degree of microscopic undulation occurs on the optical surface of the imaging optical system, it is possible to suppress the side lobe from exceeding the threshold as much as possible. it can.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described with reference to the accompanying drawings. The first to third embodiments described below show examples in which the scanning optical system according to the present invention is applied to a laser beam printer.

Embodiment 1

<Schematic configuration of laser beam printer>
First, a schematic configuration of this laser beam printer will be described based on a side configuration diagram of FIG. The laser beam printer is used by being connected to an external personal computer or the like, and prints print data (including image data) transmitted from the personal computer or the like on continuous paper (fanfold paper) P. Is.

  In FIG. 1, a charging unit 13, a reflection mirror 11, a developing unit 14, and a transfer unit 15 are sequentially provided around the photosensitive drum 12 in the clockwise direction. When the photosensitive drum 12 rotates clockwise in the drawing, first, the charging unit 13 charges the surface of the photosensitive drum 12. Next, the reflection mirror 11 reflects the scanning light (modulated light) emitted from the laser scanning unit (LSU) 10 according to the print data toward the photosensitive drum 12, and electrostatically reflects on the surface of the photosensitive drum 12. A latent image is formed. Next, the developing unit 14 attaches toner to the electrostatic latent image and visualizes it as a toner image. Next, the transfer unit 15 transfers this toner image onto the fanfold paper P.

  The fanfold paper P is a continuous paper drawn from the supply port A to the discharge port B of the laser beam printer, and feed holes (not shown) are opened at a constant pitch on both side edges. The tractor 16 is a belt conveyor in which a large number of protrusions 16 a that fit into the feed holes are formed. The fan fold paper P is conveyed at the same speed as the rotational peripheral speed of the photosensitive drum 12 by the protrusions 16 a.

  On the downstream side of the fanfold paper P conveyed by the tractor 16, a heat roll 17 and a press roll 18 are provided that sandwich the fanfold paper P from both sides and press-contact it. The heat roll 17 incorporates a heat-generating halogen lamp 19 therein, and is rotated by a motor (not shown) at the same rotational peripheral speed as the conveyance speed of the fanfold paper P. On the other hand, the press roll 18 is in pressure contact with the heat roll 17 at a constant pressure, and is driven to rotate by the rotation of the heat roll 17. Accordingly, when the portion of the fanfold paper P onto which the toner image is transferred passes between the heat roll 17 and the press roll 18, the toner is crushed by heat and pressure and is fused onto the fanfold paper P. The toner image is fixed.

<Optical configuration of LSU>
Next, the scanning optical system built in the LSU 10 will be described. FIG. 2 is a schematic optical configuration diagram of the scanning optical system. As shown in FIG. 2, the scanning optical system includes a laser light source 1, a cover glass 2, a collimating lens 3, an aperture stop 4, a cylindrical lens 5, a polygon mirror 6, and an fθ lens group 7.

  The laser beam emitted as divergent light from the laser light source 1 is converted into parallel light having an elliptical cross section by passing through the cover glass 2 and then passing through the collimating lens 3. The laser beam converted into parallel light passes through the aperture stop 4 and the cylindrical lens 5 in order, and is dynamically deflected by the reflecting surface of the polygon mirror 6 that rotates at a constant angular velocity. The laser beam deflected by the polygon mirror 6 is sequentially transmitted through the first to third lenses 7a to 7c constituting the fθ lens group 7 (focal length 135.5 mm) which is an imaging optical system, so that the surface to be scanned is scanned. The light is converged as spot light for exposure on S, and the surface (scanning target surface) S of the photosensitive drum 12 is scanned at a constant speed along the main scanning direction as the polygon mirror 6 rotates. The spot light draws a linear trajectory (scanning line) on the scanning target surface S, but the scanning target surface S itself is moved at a constant speed in the sub-scanning direction orthogonal to the main scanning direction. On S, a plurality of scanning lines are formed at equal intervals. Further, the laser beam repeatedly scanned on the scanning target surface S in this way is on-off modulated in accordance with image information by a modulator (or the laser light source 1 itself) (not shown). A two-dimensional image composed of a plurality of dots is drawn.

The laser beam transmitted through the cylindrical lens 5 is reflected by the polygon mirror 6 as a parallel light beam in the main scanning direction and converged on the scanning target surface S by the convergence power of the fθ lens group 7. In the scanning direction, the light is converged once in the vicinity of the reflection surface of the polygon mirror 6 by the convergence power of the cylindrical lens 5, is incident on the fθ lens group 7 as divergent light, and is again incident on the scanning target surface S by the convergence power of the fθ lens group 7. Converged. At this time, since the vicinity of the reflection surface of the polygon mirror 6 and the scanning target surface S are optically conjugate in the sub-scanning direction by the fθ lens group 7, a slight inclination of each reflection surface of the polygon mirror 6 (so-called “so-called”). A shift in the sub-scanning direction of the scanning position on the scanning target surface S due to “surface tilt”) is corrected.

<Collimating lens>
Next, the collimating lens 3 will be described. FIG. 3 is a cross-sectional view showing the optical path of the laser beam that passes through the collimating lens 3. Table 1 shows specific numerical configurations on the paraxial axis of the cover glass 2 and the collimating lens 3.

この表１において、記号NOは面番号を示す。具体的には、第１面及び第２面はカバーガラス２を、第３面及び第４面はコリメートレンズ３を、それぞれ示す。なお、記号Ｐは、光学面ではなく、レーザーの発光点を示す。また、記号Ｒは、光軸上での光学面の曲率半径（単位は[mm]）であり、記号ｄは、次の光学面までの光軸上での距離（単位は[mm]）であり、記号ｎは、設計波長780nmでの各レンズの屈折率である。また、コリメートレンズ３の焦点距離は、９．００ｍｍであり、開口数ＮＡは、０．３０である。

In Table 1, the symbol NO indicates a surface number. Specifically, the first surface and the second surface indicate the cover glass 2, and the third surface and the fourth surface indicate the collimating lens 3, respectively. The symbol P indicates not the optical surface but the laser emission point. Symbol R is the radius of curvature of the optical surface on the optical axis (unit: [mm]), and symbol d is the distance on the optical axis to the next optical surface (unit: [mm]). The symbol n represents the refractive index of each lens at the design wavelength of 780 nm. The focal length of the collimating lens 3 is 9.00 mm, and the numerical aperture NA is 0.30.

  The optical surface on the incident side of the collimating lens 3 having such a numerical configuration is formed as a rotationally symmetric aspherical surface.

The rotationally symmetric aspherical surface is high when the direction parallel to the optical axis is the x direction and the distance from the optical axis in the direction perpendicular to the optical axis is the height h (= √ (y ² + z ² )). Sag amount X (h) in the x direction from the tangential plane on the optical axis at the point h
X (h) = Ch ² / [1 + √ [1- (1 + κ) C ² h ² ]] + A ₄ h ⁴ + A ₆ h ⁶ + A ₈ h ⁸ +… --- (12)
It is expressed by. In the above equation (12), C is a curvature (the reciprocal of the paraxial radius of curvature R), κ is a conical coefficient, and A ₄ , A ₆ , A ₈ ,. 6th, 8th,... Aspherical coefficients.

  Table 2 shows specific paraxial radii of curvature, conical coefficients, and aspherical coefficients when the surface shape on the incident side of the collimating lens 3 is expressed by Expression (12). In addition, the aspheric coefficient which is not shown is zero.

一方、コリメートレンズ３の射出側の面形状は、以下の通りである。図４は、コリメートレンズ３をその射出側から見たときの背面図であり、図５は、図４のＡ−Ａ線に沿って半分に切断されたコリメートレンズ３の一部の断面図である。但し、図５において、光軸方向（図５の紙面内における左右方向）の長さが誇張されるように、その方向の縮尺が変えられている。

On the other hand, the surface shape on the exit side of the collimating lens 3 is as follows. 4 is a rear view of the collimating lens 3 when viewed from the exit side, and FIG. 5 is a cross-sectional view of a part of the collimating lens 3 cut in half along the line AA in FIG. is there. However, in FIG. 5, the scale of the direction is changed so that the length in the optical axis direction (left and right direction in the plane of FIG. 5) is exaggerated.

  As shown in FIG. 4, the optical surface on the exit side of the collimator lens 3 has a circular central region 3a located at the center thereof, a ring-shaped first region 3b in which the central region 3a is inscribed, and the first optical surface. The area 3b is divided into a ring-shaped second area 3c inscribed therein. The first region corresponds to the first outer region, and the second region corresponds to the second outer region. As shown in FIG. 5, the first region 3b protrudes from the central region 3a on the exit side, and the second region 3c projects from the first region 3b to the exit side. However, since the protruding amounts of the first region 3b and the second region 3c are very small as will be described later, the optical surface on the exit side of the collimating lens 3 is macroscopically as shown in FIG. It is a smooth curved surface.

  Of the regions 3a to 3c, the central region 3a is formed in a rotationally symmetric aspherical shape, and the first and second regions 3b and 3c are also rotationally symmetric aspherical surfaces, respectively. It is formed in a shape equivalent to shifting part of it in the optical axis direction. Table 3 shows specific paraxial radii of curvature, conical coefficients, and aspherical coefficients when the surface shapes in these regions 3a to 3c are expressed by the equation (12). In addition, the aspheric coefficient which is not shown is zero.

この表３において、記号hmaxは、光軸に直交する径方向に沿った各領域３ａ〜３ｃの最大有効半径（単位は[mm]）を示す。また、記号ΔＤは、説明図である図６に示されるように、中央領域３ａ内の光学面と光軸との交点を原点としたとき、各領域３ｂ，３ｃ内の光学面を延長した面（図６内の破線部分）と光軸との交点の原点からのずれ量（単位は[mm]）を示し、厚みが増す方向を正とする。

In Table 3, the symbol hmax indicates the maximum effective radius (unit: [mm]) of each of the regions 3a to 3c along the radial direction orthogonal to the optical axis. Further, as shown in FIG. 6, which is an explanatory diagram, the symbol ΔD is a surface obtained by extending the optical surfaces in the respective regions 3b and 3c when the origin is the intersection of the optical surface in the central region 3a and the optical axis. This indicates the amount of deviation from the origin of the intersection of the intersection of the optical axis (the broken line portion in FIG. 6) (unit: [mm]), and the direction in which the thickness increases is positive.

  Note that the position of the collimating lens 3 is adjusted so that the beam central axis of the laser beam incident from the laser light source 1 through the cover glass 2 and the light beam in the vicinity thereof pass through the central region 3a. . As shown in FIG. 4 and Table 3, the maximum effective radius of the second region 3c is 2.7 mm, the maximum effective radius of the central region 3a is 2.0925 mm, and the radial direction of the first region 3b The width is 0.135 mm. On the other hand, the cross section of the laser beam after passing through the collimating lens 3 is shaped into an elliptical shape with the major axis directed in the main scanning direction. In the first embodiment, the radius of the major axis is 2. The radius of the short axis is 1.8 mm (see the broken line in FIG. 4). For this reason, most of the laser beam passes through the central region 3a, and a part thereof passes through the first and second regions 3b and 3c.

  As described above, since the first and second regions 3b and 3c protrude in the optical axis direction from the central region 3a, each of the regions 3b and 3c has a central region with respect to the light beam passing through the first and second regions 3b and 3c. It acts so as to give a predetermined phase difference between the light beam passing through 3a.

  More specifically, the light beam after passing through the first region 3b is −π [corresponding to the optical path length difference of half wavelength (λ / 2 [nm]) with respect to the light beam after passing through the central region 3a. rad] and the light flux after passing through the second region 3c has a phase difference θ ′ of −2π [rad]. Accordingly, the light beam after passing through the second region 3c has the same phase as that of the central region 3a. As a result, among the laser beams that have passed through the collimating lens 3, the light beams that have passed through the central region 3a and the second region 3c travel in phase with each other, and only the light beams that have passed through the first region 3b A phase difference of −π [rad] is given to the light beam passing through 3a. Note that the phase differences θ and θ ′ imparted to the light beams in the first and second regions 3b and 3c are θ = −π and θ ′ = − 2π as described above, so that cos θ = −1.0 and cos θ. Since '= 1.0, all the conditional expressions (6) to (10) are satisfied.

<Functions of First Embodiment>
Hereinafter, when the first and second regions 3a and 3b are not formed, the intensity distribution of the laser beam scanned on the scanning target surface S by the scanning optical system of the first embodiment configured as described above is used. Comparison will be made between the case where the optical surface on the exit side of the collimating lens 3 is configured only by the surface shape of the central region 3a and the case where the first and second regions 3a and 3b are formed. .

  7 and 8 are graphs showing the intensity distribution of the laser beam incident on the scanning target surface S in the range from the beam central axis to 0.25 mm in the main scanning direction. 7 shows the case where the first and second regions 3b and 3c are not formed, and FIG. 8 shows the case where the first and second regions 3b and 3c are formed. In these graphs, the intensity is displayed as a ratio to the center intensity of the beam. 9 and 10 are graphs showing the intensity ratio in the graphs of FIGS. 7 and 8 in an enlarged manner from 0% to 10%.

  As shown in FIGS. 7 and 9, when the first and second regions 3b and 3c are not formed, the intensity of each side lobe gradually decreases as the distance from the main beam increases. The strength of the adjacent side lobe is a little over 4%.

  On the other hand, when the first and second regions 3b and 3c are formed, as shown in FIGS. 8 and 10, the strength of each side lobe is less than 2% at any radial position. It has become.

  Therefore, even if there is some microscopic undulation on the lens surfaces of the lenses 7a to 7c of the fθ lens group 7, even if the intensity of any one or a plurality of side lobes increases by about several percent, the photosensitive drum 12 does not exceed the threshold of the intensity of light exposure.

  By the way, the area S ′ of the first region 3b when the collimating lens 3 is viewed from the back side (FIG. 4) can be set appropriately with respect to the area S of the cross section of the laser beam after passing through the collimating lens 3. desirable. In the first embodiment, since S ′ / S is 0.06, the conditional expression (11) is satisfied.

  In the above description, the collimating lens 3 and the aperture stop 4 are separated from each other. However, they may be integrally formed. For example, a film (or coating) having a transmittance of 0% in which an opening functioning as the aperture stop 4 is formed may be attached to the optical surface on the emission side of the collimating lens 3. FIG. 11 shows a rear view of the collimating lens 3 to which such a film is attached.

  In the above description, an example in which the present invention is applied to a so-called transmission type scanning optical system having the fθ lens group 7 as an imaging optical system has been shown. However, an fθ mirror 7 ′ as shown in FIG. The present invention can also be applied to a so-called reflection-type scanning optical system having an image forming optical system. Note that, in the reflection type scanning optical system, the amount of increase in side lobe intensity due to microscopic undulations of the optical surface of the imaging optical system is larger than in the transmission type. Therefore, in the reflection type scanning optical system, black streaks are more likely to occur during halftone printing. Therefore, by applying the present invention to a reflective scanning optical system, side lobes can be reduced and black streaks can be reduced.

Embodiment 2

  The second embodiment includes four regions for imparting a phase difference to the light beam, and uses a collimating lens having an optical surface that protrudes toward the exit side with respect to the central region. It has the same configuration as the embodiment. Therefore, only the differences from the first embodiment will be described below.

  The specific numerical configuration on the paraxial axis of the collimating lens 8 of the second embodiment is as shown in Table 1 above. Further, the surface shape on the incident side of the collimating lens 8 is a rotationally symmetric aspherical surface having the numerical configuration shown in Table 2 above. On the other hand, the surface shape on the exit side of the collimating lens 3 is as follows.

  FIG. 13 is a rear view of the collimating lens 8 of the second embodiment, and FIG. 14 is a cross-sectional view of a part of the collimating lens 8 cut in half along the line BB in FIG. However, in FIG. 14, the scale of the direction is changed so that the length in the optical axis direction (left-right direction in the plane of FIG. 14) is exaggerated.

  As shown in FIG. 13, the optical surface on the exit side of the collimator lens 8 has a circular central region 8a located at the center thereof, a ring-shaped first region 8b in which the central region 8a is inscribed, and the first optical surface. A zone-shaped second region 8c in which the region 8b is inscribed, a ring-shaped third region 8d in which the second region 8c is inscribed, and a ring-shaped fourth region 8e in which the third region 8d is inscribed Has been. The first and third regions 8b and 8d correspond to the first outer region, and the second and fourth regions 8c and 8e correspond to the second outer region.

  And as FIG. 14 shows, the 1st area | region 8b has protruded to the injection | emission side with respect to the center area | region 8a, the 2nd area | region 8c has protruded to the injection | emission side with respect to the 1st area | region 8b, The third region 8c projects to the exit side with respect to the second region 8b, and the fourth region 8d projects to the exit side with respect to the third region 8e. However, since the protruding amounts of the first to fourth regions 8b to 8e are very small as will be described later, the optical surface on the emission side of the collimating lens 8 is macroscopically a smooth curved surface.

  Of the regions 8a to 8e, the central region 8a is formed in a rotationally symmetric aspherical shape, and the first to fourth regions 8b to 8d are also rotationally symmetric aspherical surfaces, respectively. It is formed in a shape equivalent to shifting part of it in the optical axis direction. Table 4 shows specific paraxial radii of curvature, conical coefficients, and aspherical coefficients when the surface shapes in these regions 8a to 8e are expressed by Expression (12). In addition, the aspheric coefficient which is not shown is zero. In Table 4, the meaning of each symbol is the same as that in Table 2.

なお、コリメートレンズ８の位置は、レーザー光源１からカバーガラス２を介して入射してくるレーザービームのうちのビーム中心軸及びその近傍の光束が中央領域８ａを透過するように、調整されている。また、図１３及び表４に示されるように、第４領域８ｅの最大有効半径は２．７ｍｍであり、中央領域８ａの最大有効半径は１．８４５ｍｍであり、第１領域８ｂの径方向の幅は０．０９ｍｍである。また、第２領域８ｃの最大有効半径は２．３８５ｍｍであり、第３領域８ｄの径方向における幅は０．０９ｍｍである。これに対し、このコリメートレンズ８を透過した後のレーザービームの断面は、長軸が主走査方向に向けられた楕円形状に整形され、第２の実施形態では、その長軸の半径は２．７ｍｍであり、その短軸の半径は１．８ｍｍである（図１３の破線参照）。このため、レーザービームの大部分は、中央領域８ａを通過し、その一部が、第１乃至第４領域８ｂ〜８ｅを通過する。

Note that the position of the collimating lens 8 is adjusted so that the beam central axis of the laser beam incident from the laser light source 1 through the cover glass 2 and the light beam in the vicinity thereof pass through the central region 8a. . Further, as shown in FIG. 13 and Table 4, the maximum effective radius of the fourth region 8e is 2.7 mm, the maximum effective radius of the central region 8a is 1.845 mm, and the radial direction of the first region 8b The width is 0.09 mm. The maximum effective radius of the second region 8c is 2.385 mm, and the width in the radial direction of the third region 8d is 0.09 mm. On the other hand, the cross section of the laser beam after passing through the collimating lens 8 is shaped into an ellipse with the major axis directed in the main scanning direction. In the second embodiment, the radius of the major axis is 2. The radius of the short axis is 1.8 mm (see the broken line in FIG. 13). For this reason, most of the laser beam passes through the central region 8a, and a part thereof passes through the first to fourth regions 8b to 8e.

  And as above-mentioned, since the 1st thru | or 4th area | regions 8b-8e have protruded to the injection | emission side rather than the center area | region 8a, these each area | region 8b-8e is center area | region 8a with respect to the light beam which permeate | transmits itself. Acts so as to have a predetermined phase difference between the light beam passing through the light beam.

  More specifically, the light beam after passing through the first region 8b corresponds to the optical path length difference of 2/3 wavelength (2λ / 3 [nm]) with respect to the light beam after passing through the central region 8a − The light beam having a phase difference θ of 4π / 3 [rad] and having transmitted through the second region 8c has a phase difference θ ′ of −2π [rad]. The light beam after passing through the third region 8d has a phase difference θ of −5 / 2π [rad] corresponding to the optical path length difference of 5/4 wavelength (5λ / 4 [nm]). The light beam after passing through the four regions 8e has a phase difference θ ′ of −4π [rad]. Therefore, the light beam after passing through the second and fourth regions 8c and 8e has the same phase as that of the central region 8a. As a result, among the laser beams that have passed through the collimating lens 8, the light beams that have passed through the central region 8a and the second and fourth regions 8c and 8e travel in phase with each other, and the first and third regions 8b, A phase difference of −4π / 3 [rad] and −5 / 2π [rad] is given to the light beam that has passed through 8d with respect to the light beam that has passed through the central region 8a. Note that the phase differences θ and θ ′ imparted to the light beams in the first and second regions 8b and 8c are θ = −4π / 3 and θ ′ = − 2π as described above, and therefore cos θ = −0.5. Since cos θ ′ = 1.0, all the conditional expressions (6) to (10) are satisfied. In addition, since the phase differences θ and θ ′ imparted to the light flux in the third and fourth regions 8d and 8e are θ = −5 / 2π and θ ′ = − 4π as described above, cos θ = 0, Since cos θ ′ = 1.0, all the conditional expressions (6) to (10) are satisfied.

<Function of Second Embodiment>
Hereinafter, when the first to fourth regions 8b to 8e are not formed, the intensity distribution of the laser beam scanned on the scanning target surface S by the scanning optical system of the second embodiment configured as described above. Comparison will be made between the case where the surface shape on the exit side of the collimator lens 8 is configured only by the surface shape of the central region 8a and the case where the first to fourth regions 8b to 8e are formed. .

  FIG. 15 is a graph showing the intensity distribution of the laser beam incident on the scanning target surface S in a range from the beam central axis to 0.25 mm in the main scanning direction. FIG. 15 shows the time when the first to fourth regions 8b to 8e are formed, but the graph showing the time when the first to second regions 8b to 8e are not formed is shown in FIG. It is shown. FIG. 16 is an enlarged graph showing the range of the intensity ratio from 0% to 10% in the graph of FIG. Furthermore, the graph in which the intensity ratio in the graph of FIG. 7 is enlarged and shown in the range from 0% to 10% is shown in FIG. 9 as described above.

  As shown in FIGS. 7 and 9, when the first to fourth regions 8b to 8e are not formed, the intensity of each side lobe gradually decreases as the distance from the main beam increases. The strength of the adjacent side lobe is a little over 4%.

  On the other hand, when the first to fourth regions 8b to 8e are formed, as shown in FIGS. 15 and 16, the intensity of each side lobe is 2 as the intensity of the side lobe adjacent to the main beam. The other side lobe is less than 2%, although it is slightly over%.

  Therefore, even if there is some microscopic undulation on the lens surfaces of the lenses 7a to 7c of the fθ lens group 7, even if the intensity of any one or a plurality of side lobes increases by about several percent, the photosensitive drum 12 does not exceed the threshold of the intensity of light exposure.

  By the way, also in the second embodiment, the sum S ′ of the area of the first region 8b and the area of the third region 8d when the collimating lens 8 is viewed from the back surface (FIG. 13) is obtained after passing through the collimating lens 3. It is desirable to set appropriately for the cross-sectional area S of the laser beam. In the second embodiment, since S ′ / S is 0.08, the conditional expression (11) is satisfied.

  The collimating lens 8 in FIG. 13 may also be configured integrally with the aperture stop 4 in the same manner as the collimating lens 3 in FIG. Further, the collimating lens 8 of FIG. 13 may be used in a reflection type scanning optical system as shown in FIG.

Embodiment 3

  The third embodiment includes four regions for imparting a phase difference to the light beam, and uses a collimating lens having an optical surface in which these regions are drawn toward the incident side with respect to the central region. It has the same configuration as the embodiment. Therefore, only the differences from the first embodiment will be described below.

  The specific numerical configuration on the paraxial axis of the collimating lens 9 of the third embodiment is as shown in Table 1 above. Further, the surface shape on the incident side of the collimating lens 9 is a rotationally symmetric aspherical surface having the numerical configuration shown in Table 2 above. On the other hand, the surface shape on the exit side of the collimating lens 9 is as follows.

  FIG. 17 is a rear view of the collimating lens 9 of the third embodiment, and FIG. 18 is a cross-sectional view of a part of the collimating lens 9 cut in half along the line CC in FIG. However, in FIG. 18, the scale of the direction is changed so that the length in the optical axis direction (the left-right direction in the plane of FIG. 18) is exaggerated.

  As shown in FIG. 17, the optical surface on the exit side of the collimating lens 9 has a circular central region 9a located at the center thereof, a ring-shaped first region 9b in which the central region 9a is inscribed, and the first optical surface. A zone-shaped second region 9c inscribed in the region 9b, a ring-shaped third region 9d inscribed in the second region 9c, and a ring-shaped fourth region 9e inscribed in the third region 9d are divided. Has been. The first and third regions 9b and 9d correspond to the first outer region, and the second and fourth regions 9c and 9e correspond to the second outer region.

  As shown in FIG. 18, the first region 9b is drawn to the incident side with respect to the central region 9a, the second region 9c is drawn to the incident side with respect to the first region 9b, The third region 9c is drawn to the incident side with respect to the second region 9b, and the fourth region 9d is drawn to the incident side with respect to the third region 9e. However, since the pull-in amounts of the first to fourth regions 9b to 9e are very small as will be described later, the optical surface on the exit side of the collimator lens 9 is macroscopically a smooth curved surface.

  Of the regions 9a to 9e, the central region 9a is formed in a rotationally symmetric aspherical shape, and the first to fourth regions 9b to 9d are also rotationally symmetric aspherical surfaces. It is formed in a shape equivalent to shifting part of it in the optical axis direction. Table 5 shows specific paraxial radii of curvature, conical coefficients, and aspherical coefficients when the surface shapes in these regions 9a to 9e are expressed by Expression (12). In addition, the aspheric coefficient which is not shown is zero. In Table 5, the meaning of each symbol is the same as in Table 2.

なお、コリメートレンズ９の位置は、レーザー光源１からカバーガラス２を介して入射してくるレーザービームのうちのビーム中心軸及びその近傍の光束が中央領域９ａを透過するように、調整されている。また、図１７及び表５に示されるように、第４領域９ｅの最大有効半径は２．７ｍｍであり、中央領域９ａの最大有効半径は１．８２３ｍｍであり、第１領域９ｂの径方向の幅は０．１１２ｍｍである。また、第２領域８ｃの最大有効半径は２．３６３ｍｍであり、第３領域９ｄの径方向における幅は０．１３５ｍｍである。これに対し、このコリメートレンズ９を透過した後のレーザービームの断面は、長軸が主走査方向に向けられた楕円形状に整形され、第３の実施形態では、その長軸の半径は２．７ｍｍであり、その短軸の半径は１．８ｍｍである（図１７の破線参照）。このため、レーザービームの大部分は、中央領域９ａを通過し、その一部が、第１乃至第４領域９ｂ〜９ｅを通過する。

Note that the position of the collimating lens 9 is adjusted so that the beam central axis of the laser beam incident from the laser light source 1 through the cover glass 2 and the light beam in the vicinity thereof pass through the central region 9a. . Further, as shown in FIG. 17 and Table 5, the maximum effective radius of the fourth region 9e is 2.7 mm, the maximum effective radius of the central region 9a is 1.823 mm, and the radial direction of the first region 9b The width is 0.112 mm. The maximum effective radius of the second region 8c is 2.363 mm, and the width in the radial direction of the third region 9d is 0.135 mm. On the other hand, the cross section of the laser beam after passing through the collimating lens 9 is shaped into an elliptical shape with the major axis directed in the main scanning direction. In the third embodiment, the radius of the major axis is 2. The radius of the short axis is 1.8 mm (see the broken line in FIG. 17). For this reason, most of the laser beam passes through the central region 9a, and a part thereof passes through the first to fourth regions 9b to 9e.

  And as above-mentioned, since the 1st thru | or 4th area | regions 9b-9e are drawn in to the entrance side rather than the center area | region 9a, these each area | region 9b-9e is center area | region 9a with respect to the light beam which permeate | transmits itself. Acts so as to have a predetermined phase difference between the light beam passing through the light beam.

  More specifically, the light beam after passing through the first region 9b is 19π corresponding to the optical path length difference of 19/12 wavelength (19λ / 12 [nm]) with respect to the light beam after passing through the central region 9a. The light flux after having passed through the second region 9c has a phase difference θ ′ of 4π [rad]. The light beam after passing through the third region 9d has a phase difference θ of 5π [rad] corresponding to the optical path length difference of 5/2 wavelength (5λ / 2 [nm]), and the fourth region 9e. The light flux after passing through has a phase difference θ ′ of 8π [rad]. Accordingly, the light beam after passing through the second and fourth regions 9c and 9e has the same phase as that of the central region 9a. As a result, among the laser beams that have passed through the collimating lens 9, the light beams that have passed through the central region 9a and the second and fourth regions 9c, 9e travel in phase with each other, and the first and third regions 9b, 9b, A phase difference of 19π / 6 [rad] and 5π [rad] is imparted to the light beam transmitted through 9d with respect to the light beam transmitted through the central region 9a. Note that the phase differences θ and θ ′ imparted to the light beams in the first and second regions 9b and 9c are θ = 19π / 6 and θ ′ = 4π as described above, so that cos θ = −0.87 and cos θ. Since '= 1.0, all the conditional expressions (1) to (5) are satisfied. Further, since the phase differences θ and θ ′ imparted to the light flux in the third and fourth regions 9d and 9e are θ = 5π and θ ′ = 8π as described above, cos θ = −1.0 and cos θ ′ = Since 1.0, all the conditional expressions (1) to (5) are satisfied.

<Function of Third Embodiment>
Hereinafter, when the first to fourth regions 9b to 9e are not formed, the intensity distribution of the laser beam scanned on the scanning target surface S by the scanning optical system of the third embodiment configured as described above. Comparison will be made between the case where the surface shape on the exit side of the collimating lens 9 is composed only of the surface shape of the central region 9a and the case where the first to fourth regions 9b to 9e are formed. .

  FIG. 19 is a graph showing the intensity distribution of the laser beam incident on the scanning target surface S in a range from the beam central axis to 0.25 mm in the main scanning direction. FIG. 19 shows the case where the first to fourth regions 9b to 9e are formed, but the graph showing the case where the first to second regions 8b to 8e are not formed is shown in FIG. It is shown. FIG. 20 is a graph showing the intensity ratio in the graph of FIG. 19 in an enlarged manner from 0% to 10%. Furthermore, the graph in which the intensity ratio in the graph of FIG. 7 is enlarged and shown in the range from 0% to 10% is shown in FIG. 9 as described above.

  As shown in FIGS. 7 and 9, when the first to fourth regions 9b to 9e are not formed, the intensity of each side lobe gradually decreases as the distance from the main beam increases. The strength of the adjacent side lobe is a little over 4%.

  On the other hand, when the first to fourth regions 9b to 9e are formed, as shown in FIGS. 19 and 20, the intensity of each side lobe is 1 as the intensity of the side lobe adjacent to the main beam. The other side lobe is less than 1%, although it is slightly over%.

  Therefore, even if there is some microscopic undulation on the lens surfaces of the lenses 7a to 7c of the fθ lens group 7, even if the intensity of any one or a plurality of side lobes increases by about several percent, the photosensitive drum 12 does not exceed the threshold of the intensity of light exposure.

  By the way, also in the third embodiment, the sum S ′ of the area of the first region 9b and the area of the third region 9d when the collimator lens 9 is viewed from the back surface (FIG. 17) is obtained after passing through the collimator lens 9. It is desirable to set appropriately for the cross-sectional area S of the laser beam. In the second embodiment, since S ′ / S is 0.10, the conditional expression (11) is satisfied.

  Note that the collimating lens 9 in FIG. 17 may also be configured integrally with the aperture stop 4 in the same manner as the collimating lens 3 in FIG. Further, the collimating lens 9 of FIG. 17 may be used in a reflection type scanning optical system as shown in FIG.

Schematic configuration diagram of the laser beam printer of the first embodiment Schematic optical configuration diagram of scanning optical system Collimating lens and its optical path diagram Rear view of the collimating lens of the first embodiment Sectional drawing of a part of collimating lens along the AA line of FIG. Explanatory drawing of deviation amount from origin of intersection of surface shape and optical axis of each area Graph showing intensity distribution of laser beam when there is no first and second region The graph which shows intensity distribution of the laser beam of 1st Embodiment A graph obtained by enlarging a part of the graph of FIG. A graph obtained by enlarging a part of the graph of FIG. Rear view when collimating lens and aperture stop are integrally formed A perspective view of a reflective scanning optical system incorporating a collimating lens Rear view of the collimating lens of the second embodiment Sectional drawing of a part of collimating lens along the BB line of FIG. The graph which shows intensity distribution of the laser beam of 2nd Embodiment The graph which expanded a part of graph of FIG. Rear view of the collimating lens of the third embodiment Sectional drawing of a part of collimating lens along CC line of FIG. The graph which shows intensity distribution of the laser beam of 3rd Embodiment The graph which expanded a part of graph of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Laser light source 2 Cover glass 3 Collimating lens 3a Central area | region 3b 1st area | region 3c 2nd area | region 4 Aperture stop 5 Cylindrical lens 6 Polygon mirror 7 f (theta) lens group 8 Collimating lens 8a Central area | region 8b 1st area | region 8c 2nd area | region 8d 3rd Region 8e Fourth region 9 Collimating lens 9a Central region 9b First region 9c Second region 9d Third region 9e Fourth region

Claims (14)

A laser beam emitted from a light source is dynamically deflected by a deflector, and the dynamically deflected laser beam is converged as a spot light on a scanning target surface by an imaging optical system. A scanning optical system that scans along the main scanning direction on the surface to be scanned,
A collimating lens is provided on the optical path between the light source and the deflector,
One of the optical surfaces of the collimating lens is
A central region for transmitting a beam central axis of the laser beam emitted from the light source and a light beam in the vicinity thereof; and
A first function that transmits a part of the light beam incident on the outside of the central region and gives a predetermined first phase difference between the transmitted light beam and the light beam transmitted through the central region. An outer region,
A part of the light beam excluding the light beam incident on the central region and the first outer region is transmitted, and a predetermined second phase difference is given to the transmitted light beam and the light beam transmitted through the central region. A second outer region that acts as
The first phase difference does not include zero phase difference,
The scanning optical system, wherein the second phase difference includes a phase difference of zero and has a value different from the first phase difference. When the optical surface of the first outer region is shifted in the direction in which the lens thickness is thinner than the optical surface of the central region, the first phase difference has a positive polarity. The phase difference is expressed by the following conditional expressions (1) and (2),
cosθ ≦ 0 --- (1)
0 <θ <10π --- (2)
The scanning optical system according to claim 1, wherein θ [rad] satisfies the following. When the polarity of the second phase difference in the case where the optical surface of the second outer region is shifted in the direction of decreasing the lens thickness with respect to the optical surface of the central region, the second phase difference is positive. The phase difference is expressed by the following conditional expressions (3) to (5),
0.9 ≦ cosθ '--- (3)
0 ≦ θ ′ <10π --- (4)
θ <θ '--- (5)
The scanning optical system according to claim 2, wherein θ ′ [rad] satisfies the following. When the optical surface of the first outer region is shifted in the direction in which the lens thickness is thinner than the optical surface of the central region, the first phase difference has a positive polarity. The phase difference is expressed by the following conditional expressions (6) and (7),
cosθ ≦ 0 --- (6)
−10π <θ <0 --- (7)
The scanning optical system according to claim 1, wherein θ [rad] satisfies the following. When the polarity of the second phase difference in the case where the optical surface of the second outer region is shifted in the direction of decreasing the lens thickness with respect to the optical surface of the central region, the second phase difference is positive. The phase difference is expressed by the following conditional expressions (8) to (10),
0.9 ≦ cosθ '--- (8)
−10π <θ ′ ≦ 0 --- (9)
θ '<θ --- (10)
The scanning optical system according to claim 4, wherein θ ′ [rad] satisfies the following. 6. A scanning optical system according to claim 3, wherein the collimating lens includes a plurality of sets of the first and second outer regions. The scanning optical system according to claim 6, wherein the collimating lens includes two sets of the first and second outer regions. 8. The scanning optical system according to claim 3, wherein the first outer region located closest to the beam central axis is adjacent to the first outer region outside the central region. 9. The scanning optical system according to claim 3, wherein the second outer region located closest to the beam central axis is adjacent to the second outer region outside the first outer region. . The scanning optical system according to any one of claims 6 to 9, wherein the first and second outer regions are alternately arranged in a direction away from the central region. The sum of the areas of the regions where the laser beam is incident in each of the first outer regions is S ′, and the center axis of the laser beam passing through the range indicated by the effective aperture of the optical surface having each region is When the area of the orthogonal cross section is S, the following conditional expression (11),
0.03 <(S '/ S) <0.3 --- (11)
The scanning optical system according to claim 8, 9 or 10, wherein: The scanning optical system according to claim 1, wherein the collimating lens has a shielding part as an aperture stop, and the central part and each outer region are provided in the opening part. . The scanning optical system according to claim 1, wherein the imaging optical system is an optical system including a reflecting surface. A laser beam emitted from a light source is dynamically deflected by a deflector and the laser beam deflected dynamically is converged as a spot light on a scanning target surface by an imaging optical system, thereby scanning the spot light. A printer comprising a scanning optical system that scans along a main scanning direction on a target surface,
A collimating lens is provided on the optical path between the light source and the deflector,
One of the optical surfaces of the collimating lens is
A central region for transmitting a beam central axis of the laser beam emitted from the light source and a light beam in the vicinity thereof; and
A first function that transmits a part of the light beam incident on the outside of the central region and gives a predetermined first phase difference between the transmitted light beam and the light beam transmitted through the central region. An outer region,
A part of the light beam excluding the light beam incident on the central region and the first outer region is transmitted, and a predetermined second phase difference is given to the transmitted light beam and the light beam transmitted through the central region. A second outer region that acts as
The first phase difference does not include zero phase difference,
The printer according to claim 1, wherein the second phase difference includes a phase difference zero and is a value different from the first phase difference.

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