JPH112769A - Optical scanning device - Google Patents

Abstract

(57) [Problem] To provide an optical scanning device having a good beam spot shape, a stable beam hippot size and optical power, and a high optical power utilization efficiency. SOLUTION: In a sub-scanning direction, a real image 63 of an aperture 61 is placed on a deflector side of a surface 17 to be scanned, and a real image 5
3 is arranged on the opposite side of the deflector from the surface to be scanned 17, the surface to be scanned 17 is arranged at a substantially beam waist position in the sub-scanning direction, and the aperture 61 has a center intensity of the light beam. Is set so as to block the portion outside the sub-scanning direction from the position of 25% or less, and the fluctuation of the beam spot size due to the defocus of the scanned surface 17 can be minimized. It can be used, and the utilization efficiency of optical power is high.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical scanning device used for a laser beam printer or the like, and more particularly, to an optical scanning device having an aperture in a shaping optical system for entering a light beam into a deflector. The present invention relates to an optical scanning device that reduces fluctuations in beam spot size due to light emission and has high optical power utilization efficiency.

[0002]

2. Description of the Related Art Conventionally, an image recording device such as a laser beam printer and an optical scanning device used for various image reading and measuring devices generally form a light beam emitted from a light source such as a semiconductor laser through a shaping optical system. The beam is deflected by a deflector such as a rotating polygon mirror as a deflecting means, and the deflected light beam is scanned by forming a beam spot on a surface to be scanned by an imaging lens system as an f · θ lens. ing.

In these apparatuses, a light beam is repeatedly scanned on a surface to be scanned in a straight line or a curved line.
The two-dimensional scanning is performed by relatively moving the medium to be scanned located on the surface to be scanned in a direction substantially orthogonal to the above-described scanning direction. The scanning direction of the former optical scanning device is referred to as a main scanning direction, and the latter direction of relative movement of the medium to be scanned is referred to as a sub-scanning direction.

In such an optical scanning device, an aperture is provided in a shaping optical system for making a light beam incident on a deflector, a real image is formed near an incident side of a surface to be scanned, and a real image of a light source is formed. Japanese Patent Application Laid-Open No. Hei 4-229819 proposes a method in which an image is formed behind the scanning surface to stabilize the beam spot size by defocusing on the surface to be scanned.

In the above-described optical scanning apparatus, a light beam emitted from a light source is incident on a rotary polygon mirror with a very small diameter in the main scanning direction in order to improve resolution and processing speed. Japanese Patent Application Laid-Open No. Sho 51-32340 proposes a high-speed optical scanning device in which a deflected light beam is again incident on a rotary polygon mirror via a transmission optical system. In the optical scanning device in which the light beam is incident twice on the rotating polygon mirror, the diameter of the light beam in the main scanning direction when the light beam first enters the rotating polygon mirror is extremely smaller than when the light beam enters the second time. In addition, the transmission optical system is configured such that the light beam incident on the rotating polygon mirror for the second time follows the center point of the rotating reflecting surface in the main scanning direction.

On the other hand, in an optical scanning apparatus, a light beam is incident at an angle to a scanning plane perpendicular to the rotation axis of a rotary polygon mirror to deflect the light beam.
No. 2 and the like.

In the present specification, the case where the light is incident twice on different reflecting surfaces of the rotary polygon mirror as described above is referred to as "double incidence". Further, letting the light beam enter the scanning plane of the rotating polygon mirror at an angle will be referred to as “oblique incidence”.

[0008]

In an optical scanning device having an aperture in a shaping optical system for making a light beam incident on a deflector, a real image of the aperture is located near a surface to be scanned. When the real image and the surface to be scanned coincide with each other, there is a problem that the beam spot size largely fluctuates due to defocus due to the influence of aperture diffraction.

In Japanese Patent Application Laid-Open No. 4-229819, the beam spot diameter in the sub-scanning direction before and after the surface to be scanned is as shown in FIG. The position where the defocus Z on the horizontal axis of FIG.
The actual image of the aperture is
The light source side (position A) with respect to the surface to be scanned, the real image of the light source is opposite to the light source with respect to the surface to be scanned (position B), and the beam waist (the position where the beam spot size is minimum) is the aperture of the aperture. It is located between the real image (position A) and the real image of the light source (position B) and on the light source side (position C) with respect to the surface to be scanned.

In Japanese Unexamined Patent Publication No. 4-229819, the aperture is blocked by a portion of at least 40% (in the concrete embodiment, at least 71%) of the central intensity of the light beam. Yes, since the shielding portion is large, there are the following two problems. (1) As is clear from FIG. 16, since the surface to be scanned is arranged at the position where the curve indicating the beam spot size is inclined (the position where the defocus is 0), the fluctuation of the beam spot size due to the defocus is still large. . (2) optical power loss due to light blocking of the aperture is large,
Poor utilization of optical power.

Incidentally, the above-mentioned Japanese Patent Application Laid-Open No. S51-32340.
In a twice-incident optical scanning device, the beam spot size in the main scanning direction on the first reflecting surface is extremely small, and the light source, the first reflecting surface, the second reflecting surface, and the surface to be scanned are conjugated. When the surface tilt correction is performed, the beam spot size on the first reflecting surface is extremely small even in the sub-scanning direction. Therefore,
The cross-sectional area of the light beam on the first reflecting surface is extremely small, and scratches on the reflecting surface and dust adhering to the reflecting surface adversely affect the imaging characteristics, and the beam spot shape on the surface to be scanned is distorted. There is a problem that the power of the light beam is reduced due to the interruption of the beam.

SUMMARY OF THE INVENTION The present invention has been made in view of such a point of the prior art, and has as its object to provide a beam spot in an optical scanning device having an aperture in a shaping optical system for making a light beam incident on a deflector. An object of the present invention is to provide an optical scanning device which has a good shape, a stable beam hipot size and optical power, and a high optical power utilization efficiency.

[0013]

In order to achieve the above object, the present invention provides an optical scanning apparatus comprising: a light source for generating a light beam; and a shaping optical system for converting the light beam from the light source into a shaped light beam having predetermined characteristics. An aperture provided in the shaping optical system; a deflector having a plurality of reflection surfaces for reflecting and deflecting the shaping light beam; and a light beam reflected and deflected by the reflection surface of the deflector on a surface to be scanned. A scanning optical system for forming and scanning a beam spot in the sub-scanning direction, wherein the real image of the aperture is on the deflector side of the surface to be scanned, and the real image of the light source is The deflector is located on the opposite side of the surface, and in the effective scanning range, the surface to be scanned is arranged at a substantially beam waist position in the sub-scanning direction; 25 with respect to central intensity of the light beam
% Is set so as to block the portion outside the sub-scanning direction from the position of% or less.

In this case, the deflector comprises a rotating polygon mirror,
A transmission optical system for transmitting and entering the light beam reflected and deflected by the first reflection surface of the rotary polygon mirror to the second reflection surface of the rotary polygon mirror, and scanning the light beam reflected and deflected by the second reflection surface It is desirable that the system is configured to scan by forming a beam spot on the surface to be scanned.
The transmission optical system has a substantially conjugate relationship between the first reflection surface and the second reflection surface in the sub-scanning direction, and the scanning optical system has a substantially conjugate relationship between the second reflection surface and the surface to be scanned in the sub-scanning direction. Is desirable. The optical axis of the shaping optical system,
The optical axis of the transmission optical system and the optical axis of the scanning optical system may be arranged in a common sub-scanning plane including the rotation axis of the rotary polygon mirror.

In the present invention, in the sub-scanning direction,
The real image of the aperture is located on the deflector side of the surface to be scanned, and the real image of the light source is located on the opposite side of the deflector on the surface to be scanned. In the effective scanning range, the surface to be scanned is substantially a beam in the sub-scanning direction. Since it is arranged at the waist position and the aperture is set so as to block the portion outside the sub-scanning direction from a position of 25% or less with respect to the center intensity of the light beam, the defocus of the scanning surface is performed. The variation of the beam spot size due to the above can be minimized, the portion having the smallest beam spot size can be used, and the light power utilization efficiency is high. In addition, by applying the present invention to a twice-incident optical scanning device, a beam having a good shape and a stable optical power on the surface to be scanned is not adversely affected by scratches and dust on the first reflecting surface. You can get a spot.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an optical scanning device according to the present invention will be described in detail with reference to the drawings. First, an embodiment of the optical scanning device of the present invention will be described. FIG. 1 is a plan view showing the configuration of the optical scanning device of the present embodiment, FIG. 2 is a side view thereof, FIG. 3 is a perspective view of a main part thereof, FIG.
FIG. 3 is a diagram of the aperture as viewed from the semiconductor laser side. Hereinafter, in the present invention, at an arbitrary position of the optical system, a plane including the optical axis of the optical system at that position and being parallel to the rotation axis 41 of the rotary polygon mirror 4, which is a deflector, is defined as a sub-scanning plane. And a plane perpendicular to the sub-scanning plane is defined as a main scanning plane. Further, a direction perpendicular to the optical axis in the main scanning plane is defined as a main scanning direction, and a direction perpendicular to the optical axis in the sub-scanning plane is defined as a sub-scanning direction.

A light beam emitted from a semiconductor laser 1 as a light source passes through an aperture 61, a first shaping lens 2, and a second shaping lens 3 and is shaped, and a first reflecting surface of a rotary polygon mirror 4 as a deflector. 5 and is deflected for the first time. At this time, since the light beam is incident on the first reflection surface 5 at an angle with respect to a plane perpendicular to the rotation axis 41 of the rotary polygon mirror 4, the incident light beam and the reflected light beam do not interfere with each other. The light beam reflected by the first reflection surface 5 is transmitted to the first transmission lens 7, the second transmission lens 8, the third transmission lens 9
And is reflected by the first transmission mirror 10, transmitted through the fourth transmission lens 11 and the fifth transmission lens 12, reflected by the second transmission mirror 13, and again incident on the second reflection surface 6 of the rotary polygon mirror 4. Then, the second deflection is performed. Also at this time, since the light beam is incident on the second reflecting surface 6 at an angle with respect to the plane perpendicular to the rotation axis 41 of the rotary polygon mirror 4, the incident light beam and the reflected light beam do not interfere with each other. .

The light beam reflected by the second reflecting surface 6 is formed as a light beam spot on a surface 17 to be scanned by a first scanning lens 14, a second scanning lens 15, and a third scanning lens 16 and scanned. You. The number of surfaces of the rotary polygon mirror 4 is 12 (even number). The third scanning lens 16 is decentered in the sub-scanning direction, which is the direction of the arrow in FIG.
The reason why the third scanning lens 16 is decentered in this manner is that the light beam reflected and deflected by the second reflecting surface 6 of the rotary polygon mirror 4 draws a conical trajectory, and the coordinate system of the cross section of the light beam is deflected. This is because the image formation spot on the scanned surface 17 is distorted due to the rotation depending on the angle, but the decentering of the third scanning lens 16 can prevent the dislocation.

By the way, the optical system between the semiconductor laser 1 and the first reflecting surface 5 is formed by the shaping optical system 21 and the first reflecting surface 5.
The optical system between the second reflecting surface 6 and the transmission optical system 22,
If the optical system between the reflecting surface 6 and the surface to be scanned 17 is called a scanning optical system 23, the first reflecting surface 5 of the rotary polygon mirror 4
And the second reflective surface 6 are mutually parallel reflective surfaces facing each other across the rotation axis 41, and the optical axes of the shaping optical system 21, the transmission optical system 22, and the scanning optical system 23 include the rotation axis 41. Are arranged in the sub-scanning plane. Therefore, this optical scanning device has a symmetrical configuration with respect to the sub-scanning surface, while being incident twice and obliquely incident. With such an arrangement, the optical axes of the shaping optical system 21, the transmission optical system 22, and the scanning optical system 23 are arranged in a straight line when viewed in the main scanning plane. Each element constituting the optical system can be arranged with high accuracy on a plane. In addition, since the optical axis of the transmission optical system 22 partially overlaps the optical axes of the shaping optical system 21 and the scanning optical system 23 when viewed on the main scanning plane, the optical axis can be arranged in a small space, and the installation area of the optical scanning device can be reduced. The size of the device can be reduced. With such an arrangement, it is possible to prevent a position change of the scanning line in the sub-scanning direction due to the eccentricity of the rotating shaft 41 of the rotating polygon mirror 4.

FIG. 6 shows an optical path diagram (a) in the main scanning direction of the shaping optical system 21 and an optical path diagram (b) in the sub-scanning direction. The light beam b (FIG. 5) emitted from the semiconductor laser 1 having a bonding surface perpendicular to the main scanning surface and having a bonding surface parallel to the sub-scanning surface and having a cover glass so as to spread more in the main scanning direction than in the sub-scanning direction.
), The peripheral part is shielded by an aperture 61 having a rectangular opening arranged at the position of the incident surface of the first shaping lens 2,
The light is converted into a parallel light beam by the first shaping lens 2 constituting the aspherical collimator lens. The second shaping lens 2 is a positive cylindrical lens having a positive refractive power only in the sub-scanning direction. Therefore, the light beam transmitted through the second shaping lens 2 is converted into a parallel light beam on the main scanning surface by the first light beam.
The light enters the reflecting surface 5 and forms an image (convergence) near the first reflecting surface 5 on the sub-scanning surface.

FIG. 7 shows an optical path diagram (a) in the main scanning direction of the transmission optical system 22 and an optical path diagram (b) in the sub-scanning direction. Each of the first transmission lens 7, the second transmission lens 8, and the third transmission lens 9 is a cylindrical lens having a refractive power only in the main scanning direction, and the first transmission lens 7 and the second transmission lens 8 are positive cylindrical lenses. The third transmission lens 9 is a negative cylindrical lens, and these three lenses form a main scanning direction positive refractive power transmission lens group 24. The fourth transmission lens 11 is a positive cylindrical lens having a positive refractive power only in the sub-scanning direction, and the fifth transmission lens 12 is a spherical lens having a positive refractive power. In these operations, the light beam reflected by the first reflection surface 5 is once formed on the main scanning surface by the main scanning direction positive refractive power transmission lens group 24. The image-side focal point of the transmission lens group 24 and the object-side focal point of the fifth transmission lens 12 coincide, and constitute an afocal optical system on the main scanning plane. Therefore, the light beam is converted again into a parallel light beam by the fifth transmission lens 12 and enters the second reflection surface 6. On the sub-scanning surface, the first reflection surface 5 and the second reflection surface 6 are in a conjugate relationship due to the combined positive refracting power of the fourth transmission lens 11 and the fifth transmission lens 12, and the vicinity of the first reflection surface 5 Is formed again in the vicinity of the second reflecting surface 6.

FIG. 8 shows an optical path diagram (a) in the main scanning direction of the scanning optical system 23 and an optical path diagram (b) in the sub-scanning direction. The first scanning lens 14 is a spherical lens having a positive refractive power. The second scanning lens 15 is a prism having a refracting action only in the sub-scanning direction, and the third scanning lens 16 is a long lens made of resin and long in the main scanning direction. The entrance surface of the third scanning lens 16 has a concave shape with a large radius of curvature in the main scanning direction and a convex shape with a small radius of curvature in the sub-scanning surface direction. This is a surface formed by rotating around an axis parallel to the main scanning direction, which is located closer to the scanned surface 17 than the incident surface. Such a surface is also called a saddle-shaped toric surface. The exit surface of the third scanning lens 16 has a convex non-arc shape having a large radius of curvature in the main scanning direction, and has a linear cross section in the sub-scanning direction and has no refractive power. Scanning optical system 23 having such a configuration
Makes the second reflection surface 6 and the surface to be scanned 17 conjugate in the sub-scanning surface, and forms an image of a convergence point near the second reflection surface 6 near the surface to be scanned 17. In the main scanning plane, the second
The parallel light beam reflected from the reflecting surface 6 is scanned by the scanning surface 17.
An image is formed in the vicinity.

Next, the operation of the transmission optical system 22 will be described. FIG. 9 is a sectional development view of the main scanning surface of the transmission optical system 22. The main scanning direction positive refractive power transmitting lens group 24 composed of the first transmitting lens 7, the second transmitting lens 8, and the third transmitting lens 9 is simplified and shown as a single lens. The fourth transmission lens 11 has no refractive power in the main scanning direction, and is not shown. FIGS. 9A and 9B show the state of the light beam when the rotary polygon mirror 4 rotates. by the way,
As shown in FIGS. 1 to 4 and the like, the optical path of the transmission optical system 22 is
The light is reflected twice by the transmission mirrors 10 and 13. That is, the light is reflected an even number of times. In FIG. 9, since the development is performed for these even-numbered reflections, the rotation directions of the first reflection surface 5 and the second reflection surface 6 are the same as in FIG. 9B.

The diameter of the parallel light beam incident on the first reflecting surface 5 is w _i . Since the transmission optical system 22 forms an afocal optical system in the main scanning plane, the light beam incident on the second reflection surface 6 is also parallel, and the diameter of the light beam is w _o.
It is. Assuming that the focal length of the transmission lens group 24 is f ₁ and the focal length of the fifth transmission lens 12 is f ₂ , the ratio of the diameter of the light beam obtained by dividing w _o by w _i is f ₂ divided by f ₁ . Is equal to

As shown in FIG. 9B, when the rotating polygon mirror 4 rotates by the angle θ ₁ , the light beam is deflected by the first reflection surface 5 by the angle 2θ ₁ . The deflected light beam passes through the transmission lens group 24 and the fifth transmission lens 12 and has an angle θ ₂
Is only deflected. This light beam intersects the optical axis at point Q. On the second reflecting surface 6, the distance between the deflected light beam and the optical axis is d, the rotary polygon mirror 4 is rotated by an angle theta _1, so that also the second reflecting surface 6 moves by the same distance d Is set to a proper positional relationship. Therefore, the movement amount of the light beam and the movement amount of the second reflection surface 6 match, and the light beam does not protrude from the second reflection surface 6.

At this time, the deflected light beam is deflected to the side where the incident angle increases with respect to the second reflection surface 6 by an angle θ _2, so that the light beam reflected by the second reflection surface 6 is scanned. The angle θ _s is represented as θ _s = 2θ ₁ + θ ₂ .

Since the transmission optical system 22 of this embodiment is an afocal optical system on the main scanning plane, its optical magnification β is a value obtained by dividing the focal length f ₂ by the focal length f ₁ . It is also equal to the light beam diameter ratio w _o / w _i . Further, since the deflection angle changes the light beam transmitted through the transmission optical system 22 from the angle 2 [Theta] ₁ in the angle theta _2, the optical magnification β is 2 [Theta]
It can also be expressed as ₁ / θ ₂ . Therefore, the optical magnification β
Is represented by the following equation.

Β = w _o / w _i = f ₂ / f ₁ = 2θ ₁ / θ _{2 In} this embodiment, the optical magnification β is set to 1 <β <20.

The optical scanning device in which the light beam is deflected twice by the rotary polygon mirror 4 as in this embodiment can increase the scanning speed compared to the conventional optical scanning device in which the light beam is deflected only once. it can. This will be described below.

In a conventional optical scanning device that deflects only once, the reflection surface moves when the rotating polygon mirror rotates.
In order to always put the entire light beam on the same reflecting surface in each scanning, the size of the reflecting surface must be larger than the size of the light beam incident on the rotary polygon mirror in the main scanning direction. Therefore, the number of reflecting surfaces of the rotary polygon mirror cannot be increased so much.

In this embodiment, a light beam parallel to the first reflecting surface 5 is incident on the main scanning surface. Since β> 1, the diameter w _i of the light beam in the main scanning direction on the first reflecting surface 5 is smaller than the diameter w _o of the light beam in the main scanning direction on the second reflecting surface 6. Therefore, even if the size of the first reflecting surface 5 is smaller than that of the conventional optical scanning device,
In each scan, the entire light beam can always enter the same reflecting surface. The smaller the value of w _i , the smaller the size of the first reflecting surface 5 can be. Also,
In the second deflection, the amount of movement of the light beam when the rotary polygon mirror 4 rotates and the amount of movement of the second reflecting surface 6 match.
The size of the second reflection surface 6 in the main scanning direction only needs to be at least as large as the size of the incident light beam.

Therefore, in comparison with the conventional optical scanning device that deflects only once, the optical scanning device that deflects twice in the present embodiment has the diameter w in the main scanning direction of the light beam on the second reflecting surface 6. _{On the} other hand, by reducing the diameter w _i of the light beam on the first reflecting surface 5 in the main scanning direction, the reflecting surface of the rotary polygon mirror 4 can be reduced, so that the number of reflecting surfaces is increased. And the scanning speed can be increased accordingly.

Table 1 shows specific examples of numerical values of the optical scanning device thus configured. In Table 1, the radius of curvature of the cylindrical surface and the toric surface in the sub-scanning direction and the main scanning direction are _rix and _riy (i indicates the surface number from the light source 1 to the surface 17 to be scanned). For the aspherical surface, the radius of curvature indicates a value on the optical axis. In addition,
The unit of the length is mm.

[0034] Note) S _i: the surface of the surface number i, r _i: curvature of the surface number i radius, d _i: surface distance between the surface of number i and i + 1, n _i: wavelength 780nm of the medium between the surface number i and i + 1 Is the refractive index of

Second shaping lens 2 and third scanning lens 16
The expression representing the aspherical surface of is given by z _i = (y ² / r _i ) / [1+ ｛1− (K _i +1) (y
/ R _i) ^{2} 1/2]} a ^{_{+ A i y 4 + B i}} y 6 + C i y 8, showing the aspherical coefficients in the following Table 2.

[0036] Note) S ₄ : Aspherical surface coefficient of surface number 4, S _26y : Aspherical surface coefficient of surface number 26 in the main scanning direction.

In this specific example, the third scanning lens 16
Of the incident surface S ₂₅ is a toric surface formed by an arc of r _25y = -1475.39378 rotated at r _25x = 37.95675. The second scanning lens 15 and the third
When the optical path is refracted, such as when passing through the scanning lens 16, the optical axis is refracted in the same manner as the principal ray. , Always coincides with the principal ray of the beam that scans the scanning center.

The number of faces of the rotary polygon mirror 4 is 12, and the diameter of the inscribed circle is 38.64 mm.
The incident angle of the light beam on the reflecting surface 5 and the second reflecting surface 6 in the sub-scanning direction is 6 °, and the incident angle of the light beam on the first transmitting mirror 10 and the second transmitting mirror 13 in the sub-scanning direction. Is 3 °. Also, the exit surface S ₂₄ of the second scanning lens 15
Is inclined by 13 ° in the sub-scan section, and the incident surface S ₂₅ of the third scanning lens 16 is 8.7503 in the sub-scan section.
The exit surface S ₂₆ of the third scanning lens 16 is inclined by 2.875374 ° in the sub-scan section. See FIGS. 2 and 4 for the directions of these inclination angles.

Further, in accordance with the first shaping lens incident surface S ₃ , the main scanning direction is 0.7154 mm, and the sub-scanning direction is 1.05 mm.
A 26 mm rectangular aperture 61 is arranged. Then, in the sub-scanning direction, the light emitting point 1 and the first reflecting surface 5 of the rotary polygon mirror 4 are out of the geometrical conjugate relationship. However, since the first reflecting surface 5, the second reflecting surface 6, and the scanned surface 17 of the rotary polygon mirror 4 are all in a conjugate relationship with each other, the tilting of the rotary polygon mirror 4 is corrected. . Therefore, the light emitting point 1 and the scanned surface 17 are out of the conjugate relationship. However, the position where the light beam becomes minimum (beam waist) is shifted from the geometrical optical imaging point due to the influence of diffraction, and the scanned surface 17 is located at the position where the light beam becomes substantially minimum (beam waist). Are located. This will be described later.

The optical magnification β in the main scanning direction of the transmission optical system 22 in the above specific example is 8.24, the optical magnification β _t in the sub-scanning direction is 1.12, and the optical magnification β of the scanning optical system 23 in the sub-scanning direction. β _s is 0.406.

Here, as described above, the second scanning lens 15 is a prism having a refraction function only in the sub-scanning direction. The operation of the prism will be described. The light beam reflected and deflected by the reflecting surface 6 of the rotary polygon mirror 4 draws a conical trajectory, and when the prism of the second scanning lens 15 is not disposed, the beam is curved on the long lens of the third scanning lens 16. It becomes a trajectory. This prism 16 has the function of converting the trajectory of the conical light beam a into a linear beam trajectory A on the incident surface of the third scanning lens 16, as schematically shown in FIG.

FIG. 11 is a diagram showing the beam locus on the incident surface of the third scanning lens 16 in the above specific example, and the beam locus is shown by a solid line. In the drawing, the Y direction indicates the main scanning direction, and the X direction indicates the sub scanning direction. For comparison, the third reflection surface 6 to the third reflection surface 6
The trajectory of the beam on the incident surface of the third scanning lens 16 when only the second scanning lens 15 is removed without changing the distance to the scanning lens 16 is indicated by a broken line. From FIG.
It can be seen that the prism function of the second scanning lens 15 has the function of correcting the beam trajectory into a straight line.

FIG. 12 shows the sub-scanning cross section of the third scanning lens 16 at several positions (five positions) in the main scanning direction, and shows the error of the measured value with respect to the design value of the cross-sectional shape. Things. In the drawing, X, Y, and Z are the sub-scanning direction, the main scanning direction, and the optical axis direction, respectively. As shown in FIG. 12, the shape error of a lens having a saddle-shaped toric surface such as the third scanning lens 16 shows substantially the same state regardless of the position in the main scanning direction, but changes periodically in the sub-scanning direction. There are features. As described above, by the prism action of the second scanning lens 15, the beam trajectories on the third scanning lens 16 linearly A, and the beam is always shape error of point regardless of the position in the main scanning direction B ₁ .about.B ₅ is It is incident on the convex part. At any position in the main scanning direction, when the light beam is incident on a portion where the shape error of the third scanning lens 16 is convex, the imaging position in the sub-scanning direction is shifted forward from the designed position, but the entire scanning region , The third scanning lens 1
Correction can be made by adjusting the optical system such as by adjusting the position of 6, and such adjustment does not cause curvature of field.

Now, the arrangement position of the scanned surface 17 and the size of the aperture 61 will be examined. The exact optical path diagram in the sub-scanning direction of the optical scanning device according to the embodiment of the present invention shown in FIG. 1 and the like is as shown in FIG. Here, the shaping optical system 21, the transmission optical system 22, and the scanning optical system 23 are each simplified and shown as a single lens. Semiconductor laser 1
Are located near the first reflection surface 5, the second reflection surface 6, and the surface 17 to be scanned, respectively, and on the opposite side of the semiconductor laser 1 with respect to those surfaces. The real images 62 and 63 of the aperture 61 are located near the second reflection surface 6 and the surface to be scanned 17 and on the semiconductor laser 1 side with respect to those surfaces. In addition, the first reflection surface 5, the second reflection surface
The reflecting surface 6 and the surface to be scanned 17 have a geometrical optical conjugate relationship with each other.

FIG. 14 shows the results obtained by simulating the beam spot diameter in the vicinity of the surface 17 to be scanned by using specific numerical examples of the optical scanning device of the above embodiment.
The divergence angle of the semiconductor laser 1 in the sub-scanning direction is 10 ° at full width at half maximum, and is shielded by the aperture 61 at a position of 10% with respect to the center intensity. The scanned surface 17 is located at Z = 0, and the real image 53 of the semiconductor laser 1 is at Z = +
4.1 mm, the real image 63 of the aperture 61 is Z =
-7.6 mm. Note that the beam spot diameter is a diameter at a position where the intensity becomes 1 / e ² with respect to the center intensity. As shown in FIG. 14, the beam waist at which the beam spot size becomes the minimum is located at Z = 0 before (on the side of the semiconductor laser 1) the position of the real image 53 of the semiconductor laser 1. This phenomenon is also disclosed in the above-mentioned JP-A-4-229819.

As is apparent from the graph of FIG. 14, since the curve is inclined near the real image 53 of the semiconductor laser 1, the beam spot size greatly changes due to defocus, and the scanned surface 17 is made to coincide with the real image 53. That is undesirable. Therefore, in the above-described embodiment of the present invention, the scanned surface 17 is made to substantially coincide with the beam waist at which the beam spot diameter becomes smallest. Since this position is at the bottom of the curve, the fluctuation of the beam spot size due to defocus is small, and the smallest beam spot can be used.

In the embodiment of Japanese Patent Application Laid-Open No. 4-229819, since the light beam blocked by the aperture is as large as 40% or more of the central intensity, as shown in FIG. Are asymmetric, and the beam spot size increases sharply when defocusing to the light source side. Therefore, the beam waist is arranged closer to the optical system than the surface to be scanned, and the change in the beam spot size is suppressed.

On the other hand, in the present invention, the shielding by the aperture is small, and in the above numerical example, the light beam is shielded at a position 10% of the center intensity of the light beam. In such a case, as shown in FIG. 14, the variation of the beam spot size becomes substantially symmetrical with respect to the beam waist position, and by making the scanned surface 17 substantially coincide with the beam waist, the portion having the smallest beam spot size is used. In addition, the fluctuation of the beam spot size due to defocus can be minimized.

In a specific numerical example of the above optical system,
FIG. 15 shows a change in the beam spot diameter in consideration of the variation in the spread angle of the semiconductor laser 1. In general,
The divergence angle of the semiconductor laser 1 in the sub-scanning direction has a variation of about 7 ° to 13 ° in full width at half maximum, and the divergence angle is 7 °.
Fig. 1 shows the beam spot diameter at 10 °, 10 ° and 13 °.
It is shown in FIG. When the divergence angles are 7 °, 10 °, and 13 °, the positions where the aperture 61 blocks are 1%, 10%, and 25% with respect to the center intensity, respectively. At any divergence angle, the beam spot size is substantially symmetrical with respect to the beam waist. Therefore, by making the scanned surface 17 substantially coincide with the beam waist, it is possible to minimize the fluctuation of the beam spot size due to defocus. it can. Defocusing occurs at about ± 1 mm due to lens manufacturing accuracy, lens mounting accuracy, and temperature fluctuations. In this range, the beam spot diameter is 75 ± 13 μm, which is a practically sufficient level. The variation of the divergence angle is 7 ° to 13 ° and there is a variation of ± 30%, whereas the beam spot diameter is 75 ± 13 μm, which can be reduced to ± 17%.

On the other hand, when a portion larger than 25% of the center intensity of the light beam is blocked by the aperture as in Japanese Patent Application Laid-Open No. 4-229819, the portion blocked by the aperture becomes too large, and the beam spot size varies. There is a problem that the beam waist becomes asymmetrical and a small beam spot diameter cannot be stably obtained.

Further, as in the present invention, when the position blocked by the aperture 61 is 25% or less with respect to the center intensity of the light beam, the loss of the optical power due to the aperture is 9.6%.
Thus, the efficiency of using the optical power can be increased.

Further, in the optical scanning device in which the light is incident twice on the different reflecting surfaces of the rotary polygon mirror 4 as described above,
The beam spot diameter in the main scanning direction on the first reflection surface 5 is extremely small. However, as in the present invention, the scanned surface 1
When the beam spot 7 substantially coincides with the beam waist, the real image 51 of the semiconductor laser 1 is formed at a position distant from the first reflection surface 5 in the sub-scanning direction as shown in FIG. Then, the light beam has a certain size. Therefore, even if the first reflecting surface 5 is damaged or dust adheres to the first reflecting surface 5, the image forming characteristics are adversely affected,
The light beam is not interrupted by the scratches and dust, and a beam spot having a good shape and a stable optical power can be obtained on the surface 17 to be scanned.

In the above-described embodiment, the description has been given of the case where the rotary polygon mirror is used as the deflector. However, in addition to the rotary polygon mirror, a galvanometer mirror which performs a sine oscillation about the rotation axis, a rotary 2 Similar effects can be achieved with a plane mirror. In addition, the present invention can be applied not only to a device which is reflected twice by the reflecting surface of the rotary polygon mirror but also to an optical scanning device which is incident only once and is reflected and deflected. As described above, the optical scanning device of the present invention has been described based on the embodiments, but the present invention is not limited to these, and various modifications are possible.

[0054]

As is apparent from the above description, according to the optical scanning device of the present invention, in the sub-scanning direction, the real image of the aperture is on the deflector side of the surface to be scanned, and the real image of the light source is on the surface of the surface to be scanned. In the effective scanning range, the surface to be scanned is arranged substantially at the beam waist position in the sub-scanning direction, and the aperture is set at 25% with respect to the center intensity of the light beam. %, The outer portion in the sub-scanning direction is set to be shielded, so that the fluctuation of the beam spot size due to the defocus of the scanned surface can be minimized, and the portion having the smallest beam spot size can be reduced. It can be used, and the utilization efficiency of optical power is high. In addition, by applying the present invention to a twice-incident optical scanning device, a beam having a good shape and a stable optical power on the surface to be scanned is not adversely affected by scratches and dust on the first reflecting surface. You can get a spot.

[Brief description of the drawings]

FIG. 1 is a plan view illustrating a configuration of an optical scanning device according to an embodiment of the present invention.

FIG. 2 is a side view of the optical scanning device of FIG.

FIG. 3 is a perspective view of a main part of the optical scanning device of FIG. 1;

FIG. 4 is a side view of a main part of the optical scanning device of FIG.

FIG. 5 is a diagram of an aperture of the optical scanning device of FIG. 1 as viewed from a semiconductor laser side.

6 is an optical path diagram of a shaping optical system of the optical scanning device in FIG. 1 in a main scanning direction and a sub scanning direction.

7 is an optical path diagram of a transmission optical system of the optical scanning device of FIG. 1 in a main scanning direction and a sub scanning direction.

8 is an optical path diagram of a scanning optical system of the optical scanning device of FIG. 1 in a main scanning direction and a sub-scanning direction.

FIG. 9 is a developed cross-sectional view of the main scanning plane for explaining the operation of the transmission optical system.

FIG. 10 is a diagram for explaining a correcting operation of a refractive prism.

FIG. 11 is a diagram showing a beam trajectory on an incident surface of a third scanning lens according to one embodiment of the present invention.

FIG. 12 is a diagram for explaining the reason why field curvature does not occur in one specific example of the present invention.

13 is a strict optical path diagram in the sub-scanning direction of the optical scanning device of FIG.

FIG. 14 is a diagram showing a result obtained by simulating a beam spot diameter near a surface to be scanned according to one specific example of the present invention.

FIG. 15 is a diagram showing a change in a beam spot diameter in a case where variation in a spread angle of a semiconductor laser according to one specific example of the present invention is considered.

FIG. 16 is a diagram showing a beam spot diameter in a sub-scanning direction before and after a surface to be scanned in a conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Semiconductor laser (light source) 2 ... 1st shaping lens 3 ... 2nd shaping lens 4 ... Rotation polygon mirror 5 ... 1st reflection surface of rotation polygon mirror 6 ... 2nd reflection surface of rotation polygon mirror 7 ... 1st transmission lens 8 Second transmission lens 9 Third transmission lens 10 First transmission mirror 11 Fourth transmission lens 12 Fifth transmission lens 13 Second transmission mirror 14 First scanning lens 15 Second scanning lens (prism) 16: Third scanning lens (long lens) 17: Scanned surface 21: Shaping optical system 22: Transmission optical system 23: Scanning optical system 24: Main scanning direction positive refractive power transmission lens group 41: Rotation of the rotating polygon mirror Axis 51, 52, 53 ... Real image of semiconductor laser 61 ... Aperture 62, 63 ... Real image of aperture

Claims (4)

[Claims] A light source for generating a light beam; a shaping optical system for converting the light beam from the light source into a shaped light beam having predetermined characteristics; an aperture provided in the shaping optical system; Light having a deflector having a plurality of reflecting surfaces for reflecting and deflecting a beam, and a scanning optical system for forming a beam spot on a surface to be scanned with the light beam reflected and deflected by the reflecting surface of the deflector and scanning the light beam. In the scanning device, in the sub-scanning direction, the real image of the aperture is arranged on the deflector side of the surface to be scanned, the real image of the light source is located on the opposite side of the deflector of the surface to be scanned, In the effective scanning range, the surface to be scanned is disposed substantially at the beam waist position in the sub-scanning direction, and the aperture is sub-scanned from a position of 25% or less of the center intensity of the light beam. Optical scanning apparatus characterized by being configured to shield the outer part of the direction. 2. The deflector comprises a rotary polygon mirror, and has a transmission optical system for transmitting a light beam reflected and deflected by a first reflection surface of the rotary polygon mirror to a second reflection surface of the rotary polygon mirror. 2. The light according to claim 1, wherein the light beam reflected and deflected by the second reflection surface is configured to scan by forming a beam spot on the surface to be scanned by the scanning optical system. Scanning device. 3. The transmission optical system has a substantially conjugate relationship between the first reflection surface and the second reflection surface in a sub-scanning direction, and the scanning optical system has the second reflection surface and the second reflection surface in a sub-scanning direction. 3. The optical scanning device according to claim 2, wherein the surface to be scanned has a substantially conjugate relationship. 4. An optical axis of the shaping optical system, an optical axis of the transmission optical system, and an optical axis of the scanning optical system are arranged in a common sub-scanning plane including a rotation axis of the rotary polygon mirror. 4. The optical scanning device according to claim 2, wherein: