JP2004191967A - Scanning optical system and printer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a scanning optical system and a printer in which a side lobe exceeding a threshold is suppressed as much as possible even when a microscopic waviness exists on the optical face of an image formation optical system. <P>SOLUTION: A scanning optical system is so composed that a collimate lens 3, which has a first region 3b which acts to give a predetermined phase difference to a part of a luminous flux other than the luminous flux which passes through the beam axis and its vicinity of a laser beam on an optical face on the emitting side, is arranged on the optical path of the laser beam between a laser light source 1 and a polygon mirror 6. Further, a laser beam printer in which such a scanning optical system is included is composed. <P>COPYRIGHT: (C)2004,JPO&NCIPI

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 having such a scanning optical system incorporated therein.

  As is well known, a scanning optical system is incorporated in a printing apparatus such as a laser beam printer, a facsimile or a copier. The scanning optical system dynamically deflects the laser beam modulated according to the image information by the rotating polygon mirror, and focuses 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 a laser beam incident on a scanning target surface is not completely Gaussian distribution, and the intensity distribution around a main beam is caused by a diffraction phenomenon at an aperture (aperture) provided in an optical path of the laser beam. It is known to have several rings of light (side lobes) that are lower in 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 exposes the photosensitive drum, causing a printing defect called black streak during halftone printing. (See Patent Document 1). However, when the imaging optical system is in an ideal state, the intensity of the side lobe is about 4% of the center intensity of the main beam, and thus no black streak occurs.
JP 09-080333 A

  However, if there is microscopic undulation on the optical surface of the imaging optical system, the intensity of the side lobe changes when the laser beam passes through the undulation. If the intensity of the side lobe exceeds the threshold value due to the change, there is a problem that black stripes are generated during halftone printing.

  Therefore, an object of the present invention is to provide a scanning optical system capable of suppressing the intensity of a side lobe from exceeding a threshold value as much as possible even when a certain degree of microscopic undulation occurs on an optical surface of an imaging optical system. System and a printer.

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

  That is, the scanning optical system according to the present invention dynamically deflects the laser beam emitted from the light source by the deflector, and converts the dynamically deflected laser beam into spot light on the surface to be scanned by the imaging optical system. By converging, a scanning optical system that scans the spot light on the surface to be scanned along the main scanning direction, comprising a collimating lens on an optical path between the light source and the deflector, One of the optical surfaces of the collimator lens transmits a central region of the laser beam emitted from the light source and transmits a light beam near the central axis of the laser beam and a portion of the light beam incident outside the central region. And a first outer region acting to impart 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 provided between the transmitted light beam and the light beam transmitted through the central region. And the first phase difference does not include zero phase difference, and the second phase difference includes zero phase difference, and the first phase difference and Are different values.

  With this configuration, the first and second phase differences of the light beam after passing through the first and second outer regions are appropriately set, and the size of the first and second outer regions is reduced. If properly selected, the intensity of the side lobe of the laser beam incident on the surface to be scanned can be suppressed to just over 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 microscopic undulation of the optical surface of the imaging optical system, it does not exceed the threshold value. 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 collimator lens instead of a separate phase shift element, the adjustment of the phase shift element becomes unnecessary, and the beam spot becomes symmetric. The property can be prevented from being lost.

Further, in the scanning optical system and the printer according to the present invention, the first outer region has a specific phase difference θ [not including zero phase difference between the laser beam transmitted through the first outer region and the laser beam transmitted through the central region. rad], and θ is the following conditional expression (1) and (2):
cosθ ≦ 0 --- (1)
0 <θ <10π --- (2)
Is preferable, or the following conditional expressions (6) and (7):
cosθ ≦ 0 --- (6)
-10π <θ <0 --- (7)
Is preferably satisfied. In any of the former and latter cases, assuming that an integer is N, the closer the first phase difference θ is to π × (2N−1) [rad], the more the side lobe is reduced. If the upper limits of conditional expressions (1) and (6) are exceeded, the effect of reducing side lobes will be reduced. When the value exceeds the upper limit of the conditional expression (2), the lens thickness in the first outer region becomes too small as compared with the lens thickness in the central region. When the value goes below the lower limit of conditional expression (7), the lens thickness in the first outer region becomes too large as compared with the lens thickness in the central region.

Further, when the first outer region gives the laser beam passing through the first outer region a first phase difference θ [rad] that satisfies the conditional expressions (1) and (2), the second outer region includes: A specific phase difference θ ′ [rad] including zero phase difference is given to the laser beam transmitted through itself and the laser beam transmitted through the central region, and the θ ′ is given by the following conditional expression. (3) to (5),
0.9 ≦ cosθ '--- (3)
0 ≦ θ ′ <10π --- (4)
θ <θ '--- (5)
Is preferably satisfied. When the first outer region imparts a first phase difference θ [rad] that satisfies the conditional expressions (6) and (7) to the laser beam transmitted through the second outer region, A specific phase difference θ ′ [rad] including zero phase difference is given to the laser beam transmitted through itself and the laser beam transmitted through the central region, and the θ ′ is given by the following conditional expression. (8) to (10),
0.9 ≦ cosθ '--- (8)
−10π <θ ′ ≦ 0 --- (9)
θ '<θ --- (10)
It is preferable to provide a second phase difference θ ′ [rad] satisfying the following condition. In any of the former case and the latter case, assuming that an integer is M, as the second phase difference θ ′ approaches 2π × M [rad], a decrease in center intensity can be suppressed. If the lower limits of conditional expressions (3) and (8) are not reached, the effect of reducing the side lobe will be reduced, and the amount of decrease in the center intensity will also increase. When the value exceeds the upper limit of conditional expression (4), the lens thickness in the second outer region becomes too small as compared with the lens thickness in the central region. When the value goes below the lower limit of conditional expression (9), the lens thickness in the second outer region becomes too large as compared with the lens thickness in the central region. Further, by satisfying conditional expressions (5) and (10), the direction in which the first and second outer regions project from the central region becomes the same direction, so that the mold of the collimator lens is easily processed.

By the way, each first outer region gives the laser beam a first phase difference satisfying the conditional expressions (1) and (2), and each second outer region has the conditional expressions (3) to (5). When the second phase difference that satisfies the conditions (6) and (7) is given to the laser beam, or the first phase difference satisfies the conditional expressions (6) and (7) for 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 sum of the areas of the first outer regions is appropriately set. It is desirable to be done. For example, assuming that the sum of the areas of the first outer regions where the laser beam is incident is S ′ and the area of a cross section of the laser beam orthogonal to the beam center axis is S, the following conditional expression (11) ),
0.03 <S '/ S <0.3 --- (11)
Can be set so as to satisfy In such a setting condition, when the value is below the lower limit, the effect of reducing the side lobe is reduced. On the contrary, when the value exceeds the upper limit, 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 may include a plurality of sets. As the number increases, the level difference on the optical surface increases and the light amount is lost. Therefore, it is preferable that the number of the first and second outer regions included in the collimating lens is up to about two sets. When a plurality of sets are provided, it is preferable that the first and second outer regions are alternately arranged in a direction away from the central region, and the second outer region is arranged at the outermost side.

  Further, in the scanning optical system and the printer according to the present invention, the collimating lens may be integrally provided with a shielding portion functioning 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 shift between the collimator lens and the aperture stop.

  In the scanning optical system according to the present invention, the imaging optical system may be a transmission type using an fθ lens or the like, or a reflection 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.

  Hereinafter, an embodiment 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, the schematic configuration of the laser beam printer will be described with reference to the side view of FIG. This 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. Things.

  In FIG. 1, around the photosensitive drum 12, a charging unit 13, a reflection mirror 11, a developing unit 14, and a transfer unit 15 are sequentially provided clockwise. 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 reflecting mirror 11 reflects the scanning light (modulated light) emitted from the laser scanning unit (LSU) 10 in accordance with the print data toward the photosensitive drum 12, and the surface of the photosensitive drum 12 is electrostatically reflected. Form a latent image. Next, the developing unit 14 causes toner to adhere to the electrostatic latent image and visualizes it as a toner image. Next, the transfer unit 15 transfers the toner image onto the fanfold paper P.

  The fanfold paper P is a continuous paper drawn from a supply port A to a discharge port B of a laser beam printer, and has feed holes (not shown) at a constant pitch on both side edges. The tractor 16 is a belt conveyor having a large number of protrusions 16a fitted into the feed holes. The tractor 16 conveys the fanfold paper P at the same speed as the rotational speed of the photosensitive drum 12 by the protrusions 16a.

  On the downstream side of the fanfold paper P conveyed by the tractor 16, a heat roll 17 and a press roll 18 that sandwich and press the fanfold paper P from both sides are provided. The heat roll 17 incorporates a halogen lamp 19 for heat generation therein, and is rotated by a motor (not shown) at the same peripheral speed as the transport speed of the fanfold paper P. On the other hand, the press roll 18 is pressed against the heat roll 17 at a constant pressure, and is driven to rotate by the rotation of the heat roll 17. Therefore, when the portion of the fanfold paper P on 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 fused on the fanfold paper P. Thus, 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 from the laser light source 1 as divergent light passes through the cover glass 2 and then passes through the collimating lens 3 to be converted into parallel light having an elliptical cross section. 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 reflection surface of the polygon mirror 6 rotating at a constant angular velocity. The laser beam deflected by the polygon mirror 6 sequentially passes 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 exposing on S, and scans the surface (scanning target surface) S of the photosensitive drum 12 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. The laser beam that is repeatedly scanned on the scanning target surface S is modulated on and off according to 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 is converged on the scanning target surface S by the convergence power of the fθ lens group 7. In the scanning direction, the light is once converged in the vicinity of the reflection surface of the polygon mirror 6 by the converging power of the cylindrical lens 5, enters the fθ lens group 7 as divergent light, and is again focused on the scanning target surface S by the converging 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 conjugated 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 “slant”) is performed. The displacement of the scanning position on the scanning target surface S in the sub-scanning direction due to “surface tilt” is corrected.

<Collimate lens>
Next, the collimating lens 3 will be described. FIG. 3 is a cross-sectional view illustrating an optical path of a laser beam transmitted through the collimator lens 3. Table 1 shows a specific numerical configuration of the cover glass 2 and the collimator lens 3 on the paraxial axis.

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 a laser light emitting point, not an optical surface. The symbol R is the radius of curvature of the optical surface on the optical axis (unit is [mm]), and the symbol d is the distance on the optical axis to the next optical surface (unit is [mm]). The symbol n is 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 collimator lens 3 having such a numerical configuration is formed as a rotationally symmetric aspherical surface.

The rotationally symmetric aspheric surface has a height h (= √ (y ² + z ² )) where 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 height h (= √ (y ² + z ² )). Sag amount X (h) in the x direction from the tangent plane on the optical axis at the point of 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)
Is represented by In the above equation (12), C is the curvature (the reciprocal of the paraxial radius of curvature R), κ is the conic coefficient, and A ₄ , A ₆ , A ₈ ,. 6th, 8th,... Aspherical coefficients.

  Table 2 shows specific paraxial curvature radii, conic coefficients, and aspheric coefficients when the incident-side surface shape of the collimator lens 3 is expressed by Expression (12). Note that the aspheric coefficient whose display is omitted 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 of the exit side of the collimator lens 3 is as follows. FIG. 4 is a rear view of the collimating lens 3 as 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 in that direction is changed so that the length in the optical axis direction (the left-right direction in the paper plane of FIG. 5) is exaggerated.

  As shown in FIG. 4, the exit-side optical surface 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, The region 3b is divided into a ring-shaped second region 3c inscribed therein. The first region corresponds to a first outside region, and the second region corresponds to a second outside region. Then, as shown in FIG. 5, the first region 3b projects toward the emission side with respect to the central region 3a, and the second region 3c projects toward the emission side with respect to the first region 3b. However, since the protrusion amounts of the first region 3b and the second region 3c are very small as described later, the exit-side optical surface of the collimator lens 3 is macroscopically, as shown in FIG. It is a smooth curved surface.

  In each 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 symmetrical aspherical surfaces. It is formed in a shape equivalent to a part shifted in the optical axis direction. Table 3 shows specific paraxial curvature radii, conical coefficients, and aspherical coefficients when the surface shapes in these regions 3a to 3c are expressed by Expression (12). Note that the aspheric coefficient whose display is omitted 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 surface in each of the regions 3b and 3c when the intersection point between the optical surface in the central region 3a and the optical axis is set as the origin. The amount of deviation (unit: [mm]) of the intersection of the optical axis (indicated by the broken line in FIG. 6) and the optical axis is shown, and the direction in which the thickness increases is positive.

  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 via the cover glass 2 and a light beam in the vicinity thereof pass through the central region 3a. . In addition, 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 is 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 ellipse whose major axis is directed in the main scanning direction. In the first embodiment, the radius of the major axis is 2. 7 mm, and the radius of the short axis is 1.8 mm (see the broken line in FIG. 4). Therefore, most of the laser beam passes through the central region 3a, and part of the laser beam passes through the first and second regions 3b and 3c.

  As described above, since the first and second regions 3b and 3c protrude more in the optical axis direction than the central region 3a, each of the regions 3b and 3c has a central region It works so as to have a predetermined phase difference with the light beam transmitted through 3a.

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

<Function of First Embodiment>
Hereinafter, 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 will be described when the first and second regions 3a and 3b are not formed. (When the exit-side optical surface of the collimating lens 3 is constituted only by the surface shape of the central region 3a) and when the first and second regions 3a and 3b are formed will be described in comparison. .

  7 and 8 are graphs showing the intensity distribution of the laser beam incident on the scanning target surface S in a range from the beam center axis to 0.25 mm in the main scanning direction. FIG. 7 shows a case where the first and second regions 3b and 3c are not formed, and FIG. 8 shows a case where the first and second regions 3b and 3c are formed. In these graphs, the intensity is indicated by the ratio of the intensity to the center intensity of the beam. 9 and 10 are graphs in which the intensity ratio in the graphs of FIGS. 7 and 8 is enlarged 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, and the intensity of each side lobe decreases. The intensity of the adjacent side lobe is slightly more than 4%.

  On the other hand, when the first and second regions 3b and 3c are formed, as shown in FIGS. 8 and 10, the intensity of each side lobe is less than 2% at any radial position. 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%, the photosensitive drum The threshold value of the intensity exposed to T.12 is rarely exceeded.

  Incidentally, the area S ′ of the first region 3b when the collimator lens 3 is viewed from the back (FIG. 4) may be appropriately set with respect to the cross-sectional area S of the laser beam after passing through the collimator lens 3. desirable. In the first embodiment, S ′ / S is 0.06, which satisfies the conditional expression (11).

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

  Further, 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 described. However, the fθ mirror 7 ′ as shown in FIG. The present invention can also be applied to a so-called reflection-type scanning optical system having the above as an imaging optical system. It should be noted that the reflection type scanning optical system has a larger increase in side lobe intensity due to microscopic waviness of the optical surface of the imaging optical system than the transmission type scanning optical system. For this reason, in the reflective scanning optical system, black stripes are more likely to occur during halftone printing. Thus, by applying the present invention to a reflection type scanning optical system, side lobes can be reduced, and the occurrence of black streaks can be further reduced.

Embodiment 2

  The second embodiment is different from the first embodiment in that a collimating lens including four regions that impart a phase difference to a light beam and having an optical surface that projects toward the emission side with respect to the central region is used. 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. The surface shape on the incident side of the collimating lens 8 is a rotationally symmetric aspheric surface having a numerical configuration shown in Table 2 above. On the other hand, the surface shape of the exit side of the collimator lens 3 is as follows.

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

  As shown in FIG. 13, the optical surface on the exit side of the collimating 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, An annular second region 8c in which the region 8b is inscribed, a third annular region 8d in which the second region 8c is inscribed, and a fourth annular region 8e in which the third region 8d is inscribed. Have been. The first and third regions 8b and 8d correspond to a first outer region, and the second and fourth regions 8c and 8e correspond to a second outer region.

  Then, as shown in FIG. 14, the first region 8b projects toward the emission side with respect to the central region 8a, and the second region 8c projects toward the emission side with respect to the first region 8b. The third region 8c projects toward the emission side with respect to the second region 8b, and the fourth region 8d projects toward the emission side with respect to the third region 8e. However, since the projection amounts of the first to fourth regions 8b to 8e are very small as described later, the optical surface on the exit side of the collimating lens 8 is a macroscopically smooth curved surface.

  In each of the regions 8a to 8e, the center region 8a is formed in a rotationally symmetric aspherical shape, and the first to fourth regions 8b to 8d are also rotationally symmetrical aspherical surfaces. It is formed in a shape equivalent to a part shifted in the optical axis direction. Table 4 shows specific paraxial curvature radii, conic coefficients, and aspheric coefficients when the surface shapes in these regions 8a to 8e are expressed by Expression (12). Note that the aspheric coefficient whose display is omitted 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。

The position of the collimating lens 8 is adjusted so that the beam center axis of the laser beam incident from the laser light source 1 via the cover glass 2 and the light beam near the beam center axis pass through the central region 8a. . In addition, 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 is The width is 0.09 mm. The maximum effective radius of the second region 8c is 2.385 mm, and the width of the third region 8d in the radial direction 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 whose major axis is directed in the main scanning direction. In the second embodiment, the radius of the major axis is 2. 7 mm, and the radius of the short axis is 1.8 mm (see the broken line in FIG. 13). Therefore, most of the laser beam passes through the central region 8a, and part of the laser beam passes through the first to fourth regions 8b to 8e.

  And, as described above, since the first to fourth regions 8b to 8e project more toward the emission side than the central region 8a, each of the regions 8b to 8e receives the central region 8a with respect to the light transmitted therethrough. So that there is a predetermined phase difference between the light beam and the light beam that transmits the light beam.

  More specifically, the light beam transmitted through the first region 8b corresponds to the light path length difference of 、 ２ wavelength (2λ / 3 [nm]) with respect to the light beam transmitted through the central region 8a. The luminous flux after passing through the second region 8c has a phase difference θ of 4π / 3 [rad] and a phase difference θ ′ of -2π [rad]. The light beam transmitted through the third region 8d has a phase difference θ of −5 / 2π [rad] corresponding to an optical path length difference of 5/4 wavelength (5λ / 4 [nm]). The light beam transmitted through the four regions 8e has a phase difference θ ′ of −4π [rad]. Therefore, the luminous flux 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, of the laser beam transmitted through the collimating lens 8, the light flux transmitted through the central region 8a and the second and fourth regions 8c and 8e travels in phase with each other, and the first and third regions 8b and 8b. The light beam transmitted through 8d is given a phase difference of -4π / 3 [rad] and -5 / 2π [rad] with respect to the light beam transmitted through central region 8a, respectively. Since the phase differences θ and θ ′ given to the light flux in the first and second regions 8b and 8c are θ = −4π / 3 and θ ′ = − 2π as described above, cos θ = −0.5. , Cos θ ′ = 1.0, so that all of the conditional expressions (6) to (10) are satisfied. Further, as described above, since the phase differences θ and θ ′ given to the light flux in the third and fourth regions 8 d and 8 e are θ = −5 / 2π and θ ′ = − 4π, cos θ = 0, Since cos θ ′ = 1.0, all the conditional expressions (6) to (10) are satisfied.

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

  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 center axis to 0.25 mm in the main scanning direction. FIG. 15 shows the case where the first to fourth regions 8b to 8e are formed, but the graph showing the case where the first and second regions 8b to 8e are not formed is shown in FIG. It is shown. FIG. 16 is a graph showing an enlarged range of the intensity ratio from 0% to 10% in the graph of FIG. Further, the graph in which the range of the intensity ratio in the graph of FIG. 7 from 0% to 10% is enlarged 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 intensity of the adjacent side lobe is slightly more than 4%.

  On the other hand, when the first to fourth regions 8b to 8e are formed, as shown in FIG. 15 and FIG. 16, the intensity of each side lobe is 2%. The other side lobes are less than 2%, though slightly more than 10%.

  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%, the photosensitive drum The threshold value of the intensity exposed to T.12 is rarely exceeded.

  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 collimator lens 8 is viewed from the back (FIG. 13) is the value after transmission through the collimator lens 3. It is desirable that the area be appropriately set with respect to the area S of the cross section of the laser beam. In the second embodiment, S '/ S is 0.08, which satisfies the conditional expression (11).

  Note that the collimating lens 8 in FIG. 13 may be integrally formed with the aperture stop 4 as in 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 is different from the first embodiment in that a collimating lens including four regions that impart a phase difference to a light beam and having an optical surface that is drawn toward the incident side with respect to the central region is used. 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 collimator lens 9 of the third embodiment is as shown in Table 1 above. The incident-side surface shape of the collimating lens 9 is a rotationally symmetric aspheric surface having a numerical configuration shown in Table 2 above. On the other hand, the surface shape of the exit side of the collimator lens 9 is as follows.

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

  As shown in FIG. 17, the exit-side optical surface of the collimator lens 9 has a circular central region 9a located at the center thereof, a ring-shaped first region 9b inscribed by the central region 9a, and a first annular region 9b. An annular second region 9c in which the region 9b is inscribed, a third annular region 9d in which the second region 9c is inscribed, and a fourth annular region 9e in which the third region 9d is inscribed. Have been. The first and third regions 9b and 9d correspond to a first outer region, and the second and fourth regions 9c and 9e correspond to a second outer region.

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

  In each 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 symmetrical aspherical surfaces. It is formed in a shape equivalent to a part shifted in the optical axis direction. Table 5 shows specific paraxial curvature radii, conic coefficients, and aspheric coefficients when the surface shapes in these regions 9a to 9e are expressed by Expression (12). Note that the aspheric coefficient whose display is omitted is zero. In Table 5, the meanings of the symbols are the same as those 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。

The position of the collimating lens 9 is adjusted so that the beam center axis of the laser beam incident from the laser light source 1 via the cover glass 2 and a light beam in the vicinity thereof pass through the central region 9a. . 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 is The width is 0.112 mm. The maximum effective radius of the second region 8c is 2.363 mm, and the radial width 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 ellipse whose major axis is directed in the main scanning direction. In the third embodiment, the radius of the major axis is 2. 7 mm, and the radius of its short axis is 1.8 mm (see the broken line in FIG. 17). Therefore, most of the laser beam passes through the central region 9a, and part of the laser beam passes through the first to fourth regions 9b to 9e.

  As described above, since the first to fourth regions 9b to 9e are more drawn toward the incident side than the central region 9a, each of the regions 9b to 9e receives the central region 9a with respect to the light transmitted therethrough. So that there is a predetermined phase difference between the light beam and the light beam that transmits the light beam.

  More specifically, the light beam transmitted through the first region 9b is different from the light beam transmitted through the central region 9a by 19π corresponding to the optical path length difference of 19/12 wavelength (19λ / 12 [nm]). The luminous flux after passing through the second region 9c has a phase difference θ of 4π [rad]. The luminous flux transmitted 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 Has a phase difference θ ′ of 8π [rad]. Therefore, the luminous flux 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, of the laser beam transmitted through the collimating lens 9, the light flux transmitted through the central region 9a and the second and fourth regions 9c and 9e travels in phase with each other, and the first and third regions 9b and 9b. The light beam transmitted through 9d is given a phase difference of 19π / 6 [rad] and 5π [rad] to the light beam transmitted through central region 9a, respectively. Since the phase differences θ and θ ′ given to the light flux in the first and second regions 9b and 9c are θ = 19π / 6 and θ ′ = 4π as described above, cos θ = −0.87 and cos θ Since '= 1.0, all the conditional expressions (1) to (5) are satisfied. Further, as described above, since the phase differences θ and θ ′ given to the light flux in the third and fourth regions 9d and 9e are θ = 5π and θ ′ = 8π, cos θ = −1.0 and cos θ ′ = Since this is 1.0, all the conditional expressions (1) to (5) are satisfied.

<Function of Third Embodiment>
Hereinafter, the intensity distribution of the laser beam scanned on the scanning target surface S by the scanning optical system according to the third embodiment configured as described above, when the first to fourth regions 9b to 9e are not formed. (When the surface shape of the exit side of the collimator lens 9 is constituted only by the surface shape of the central region 9a) and when the first to fourth regions 9b to 9e are formed will be described in comparison. .

  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 center axis to 0.25 mm in the main scanning direction. Note that FIG. 19 shows a case where the first to fourth regions 9b to 9e are formed, but a graph showing a case where the first and second regions 8b to 8e are not formed is shown in FIG. It is shown. FIG. 20 is an enlarged graph showing the range of the intensity ratio from 0% to 10% in the graph of FIG. Further, the graph in which the range of the intensity ratio in the graph of FIG. 7 from 0% to 10% is enlarged 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, and the intensity of each side lobe decreases. The intensity of the adjacent side lobe is slightly more than 4%.

  On the other hand, when the first to fourth regions 9b to 9e are formed, as shown in FIG. 19 and FIG. 20, the intensity of each side lobe is 1%. Other side lobes are less than 1%, though slightly more than 10%.

  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%, the photosensitive drum The threshold value of the intensity exposed to T.12 is rarely exceeded.

  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 rear surface (FIG. 17) is the value after transmission through the collimator lens 9. It is desirable that the area be appropriately set with respect to the area S of the cross section of the laser beam. In the second embodiment, S '/ S is 0.10, which satisfies the conditional expression (11).

  Note that the collimator lens 9 in FIG. 17 may be integrally formed with the aperture stop 4 as in the case of the collimator lens 3 in FIG. Further, the collimating lens 9 in 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 the scanning optical system Optical path diagram of collimating lens and its vicinity Rear view of the collimating lens according to the first embodiment FIG. 4 is a cross-sectional view of a part of the collimating lens along the line AA in FIG. 4; Explanatory drawing of the amount of deviation from the origin at the intersection of the surface shape of each area and the optical axis 4 is a graph showing the intensity distribution of a laser beam when there are no first and second regions. 4 is a graph showing the intensity distribution of the laser beam according to the first embodiment. The graph which expanded a part of the graph of FIG. The graph which expanded a part of the graph of FIG. Rear view when the collimating lens and the aperture stop are integrally formed Perspective view of a reflective scanning optical system incorporating a collimating lens Rear view of the collimating lens according to the second embodiment Sectional view of a part of the collimator lens along the line BB in FIG. 4 is a graph showing an intensity distribution of a laser beam according to the second embodiment. The graph which expanded a part of graph of FIG. Rear view of the collimating lens according to the third embodiment FIG. 17 is a cross-sectional view of a part of the collimator lens along the line CC in FIG. 17. 4 is a graph showing an intensity distribution of a laser beam according to a third embodiment. The graph which expanded a part of graph of FIG.

Explanation of reference numerals

Reference Signs List 1 laser light source 2 cover glass 3 collimating lens 3a central area 3b first area 3c second area 4 aperture stop 5 cylindrical lens 6 polygon mirror 7 fθ lens group 8 collimating lens 8a central area 8b first area 8c second area 8d third Area 8e Fourth area 9 Collimating lens 9a Central area 9b First area 9c Second area 9d Third area 9e Fourth area

Claims (14)

The laser beam emitted from the 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, so that the spot light is A scanning optical system for scanning along the main scanning direction on the surface to be scanned,
A collimating lens is provided on an optical path between the light source and the deflector,
One of the optical surfaces of the collimating lens,
A central region that transmits a beam central axis of a laser beam emitted from the light source and a light beam in the vicinity thereof,
A first element which transmits a part of the light beam incident on the outside of the central region and acts so as to give a predetermined first phase difference between the transmitted light beam and the light beam transmitted through the central region. An outer area;
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 provided between the transmitted light beam and the light beam transmitted through the central region. And a second outer region that acts as
The first phase difference does not include zero phase difference,
The scanning optical system according to claim 1, wherein the second phase difference includes a phase difference of zero, and has a value different from the first phase difference. When the polarity of the first phase difference is positive when the optical surface of the first outer region is shifted in the direction of decreasing the lens thickness with respect to the optical surface of the central region, the first Phase difference, 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] is satisfied. When the polarity of the second phase difference is positive when 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 surface The phase difference is determined 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] is satisfied. When the polarity of the first phase difference is positive when the optical surface of the first outer region is shifted in the direction of decreasing the lens thickness with respect to the optical surface of the central region, the first Phase difference, 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] is satisfied. When the polarity of the second phase difference is positive when 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 surface Phase difference, 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] is satisfied. The 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. The scanning optical system according to claim 3, wherein the first outer region closest to the beam center 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 closest to the beam center axis is adjacent to the first outer region outside the first outer region. 10. . 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 among the first outer regions is S ′, and the beam center axis of the laser beam passing through the range indicated by the effective aperture of the optical surface having the respective regions 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 following is satisfied. The scanning optical system according to claim 1, wherein the collimating lens has a blocking portion as an aperture stop, and the opening portion has the central region and the outer regions. . The scanning optical system according to claim 1, wherein the imaging optical system is an optical system including a reflection surface. The laser beam emitted from the light source is dynamically deflected by a deflector, and the dynamically deflected laser beam is converged as spot light on a scanning target surface by an imaging optical system, so that the spot light is scanned. A printer including a scanning optical system that scans the target surface along the main scanning direction,
A collimating lens is provided on an optical path between the light source and the deflector,
One of the optical surfaces of the collimating lens,
A central region that transmits a beam central axis of a laser beam emitted from the light source and a light beam in the vicinity thereof,
A first element which transmits a part of the light beam incident on the outside of the central region and acts so as to give a predetermined first phase difference between the transmitted light beam and the light beam transmitted through the central region. An outer area;
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 provided between the transmitted light beam and the light beam transmitted through the central region. And a second outer region that acts as
The first phase difference does not include zero phase difference,
The printer, wherein the second phase difference includes a phase difference of zero and has a value different from the first phase difference.

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