JP2000292691A - Scanning imaging lens, and optical scanner - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively reduce an influence of fluctuation of emission wavelength in a semiconductor laser, in an optical scanner using the semiconductor laser as a light source. SOLUTION: This scanner has an fθ function as to a direction corresponding to main scanning, and a function for forming an imaging position of a line image and a scanned surface 9 into a geometrically conjugated relation as to a direction corresponding to sub-scanning, is constituted to make a light deflector 3 side serve as an incident side, and is arranged with a first - a fourth groups in order toward a scanned surface 9 side. The first - the fourth groups are composed of four groups of four sheets, the first group 5 is a lens with positive refractive power, the second group 6 is an anamorphic lens with negative refractive power having a cylinder face, the third group 7 is a positive meniscus lens of a spherical single lens with a concaved face directed toward the incident side, and the fourth group 8 is an anamorphic lens having a toric face of positive refractive power. Respective Abbe numbers νd2, νd4 of the second group 6 and the fourth group 8 satisfy the condition: 1.0<νd4/νd2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art A light beam from a semiconductor laser is formed into a long line image in a direction corresponding to the main scanning, and the light beam is deflected at an equal angular velocity by an optical deflector having a deflecting / reflecting surface near an image forming position of the line image. An optical scanning device that condenses a light beam as a light spot on a surface to be scanned by a scanning image forming lens and performs constant-speed optical scanning of the surface to be scanned is related to a laser printer, a digital copying device, a plate making device, and the like. Widely known. The main scanning direction is
A direction corresponding to the main scanning direction on the scanned surface on the optical path from the semiconductor laser as the light source to the scanned surface, and similarly,
The sub-scanning corresponding direction is a direction corresponding to the sub-scanning direction on the scanned surface on the optical path from the semiconductor laser to the scanned surface.
The scanning image forming lens in the optical scanning device as described above has an fθ function of converging a deflected light beam deflected at a constant angular velocity as a light spot on a surface to be scanned and making the light scanning speed of the light spot uniform. Surface deflection that corrects the effect of the tilting of the deflecting / reflecting surface of the optical deflector by making the position of the deflecting / reflecting surface of the optical deflector and the position of the scanned surface geometrically conjugate in the sub-scanning corresponding direction. Has a correction function.
As is well known, a semiconductor laser used as a light source in the above-described optical scanning device has a property that "the emission wavelength fluctuates due to a temperature change". For this reason, if the chromatic aberration of the scanning imaging lens is not corrected for the above-mentioned wavelength variation, the spot diameter of the light spot formed on the surface to be scanned fluctuates, or the image of the light spot is affected by off-axis chromatic aberration. The height may fluctuate in the main scanning direction, or the surface tilt correction function may not function sufficiently, and the image height of the light spot in the sub-scanning direction may fluctuate, which may adversely affect optical scanning. In recent years, in optical scanning devices, the scanning density has been increasing, and the diameter of the light spot formed on the surface to be scanned has been required to be reduced.To form a good light spot having a small diameter, It is necessary that coma is well corrected.

[0003]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical scanning device using a semiconductor laser as a light source, in which the effect of fluctuations in the emission wavelength of the semiconductor laser is effectively reduced. It is another object of the present invention to form a good light spot having a small diameter.

[0004]

According to the present invention, there is provided a scanning image forming lens which forms a line image which is emitted from a semiconductor laser and is long in a direction corresponding to the main scanning, and has a deflecting reflection surface near the image forming position of the line image. An imaging optical system for converging a light beam deflected at an equal angular velocity by an optical deflector having a light spot on a surface to be scanned as a light spot and performing a constant speed optical scanning of the surface to be scanned. '' ,
In addition to having an fθ function in the main scanning corresponding direction, it has a function of making the image forming position of the line image and the scanned surface geometrically conjugate in the sub-scanning corresponding direction. The scanning image forming optical system is configured by sequentially arranging first to fourth groups toward the surface to be scanned, with the optical deflector side being the incident side.

The first group is a lens having a positive refractive power, the second group is an anamorphic lens having a cylinder surface and a negative refractive power, and the third group is a concave lens on the incident side. And the fourth unit is an anamorphic lens having a toric surface having a positive refractive power. Therefore, the scanning image forming lens has a total of four elements in four groups.

When the Abbe numbers of the second group and the fourth group are respectively ν _d2 and ν _d4 , they satisfy the following condition: (1) 1.0 <ν _d4 / ν _d2 Condition (1) is a condition for correcting chromatic aberration so that the light spot position does not fluctuate even when the emission wavelength of the semiconductor laser of the light source fluctuates due to a temperature change or the like. The fluctuation of the light spot position cannot be sufficiently suppressed. In the ideal optical scanning, the “intersection point between the optical axis of the scanning imaging lens and the deflecting reflection surface” when the image height of the light spot is 0 is referred to as a “start point of deflection”. Is imaged near the origin of the deflection.

In the scanning image forming lens according to the first aspect, the distance on the optical axis from the starting point of deflection to the entrance side of the first group is D ₀ , and the thickness and the refractive index of the material of the first group are D and D, respectively. ₁ and N ₁ , the air gap between the first and second groups is D ₂ , the radius of curvature of the entrance side of the second group is R ₃ , the radius of curvature of the exit side of the fourth group in the main scanning direction is R _8X , and the deflection is Assuming that the distance from the starting point of the fourth group to the entrance side surface of the fourth group is D _A1 , and the focal length of the first group is f ₁ , these conditions are: (2) -1.7 <R ₃ / [{(D ₀ · f ₁ ) / (f ₁ -D ₀ )｝ + (D ₁ /
N ₁ ) + D ₂ ] <− 1.0 (3) It is preferable to satisfy −1.5 <R _8X / D _A1 <−1.1 (claim 2). Conditions (2) and (3) are conditions for suppressing coma and improving imaging performance in order to reduce the diameter of the light spot. The coma aberration occurs in the under when the lower limit of the conditions (2) and (3) is exceeded, and the over occurs when the upper limit is exceeded, and it is difficult to form a good light spot having a small diameter. Become. The scanning imaging lens according to claim 1, wherein
The second group may be an "anamorphic lens having a concave spherical surface on the entrance side and a concave cylinder surface on the exit side having power only in the sub-scanning direction" (claim 3). Further, in the scanning image forming lens according to claim 1, the fourth lens group includes a triangular lens having a positive refractive power whose curvature in the sub-scanning direction is stronger than that in the main scanning direction. An anamorphic lens having a surface and having a stronger positive refractive power in the sub-scanning corresponding direction ”(claim 4). The scanning imaging lens according to any one of claims 1 to 4, wherein the first group is a "convex plano lens having a convex spherical surface on the incident side and a flat planar surface on the exit side", and has a complicated aspherical surface in the entire system. (Claim 5). The optical scanning device according to the present invention is configured such that "a light beam from a semiconductor laser is formed into a long line image in the main scanning direction, and the light beam is deflected at an equal angular velocity by an optical deflector having a deflecting / reflecting surface near an image forming position of the line image. An optical scanning device that performs a constant-speed optical scanning of the scanned surface by converging the deflected light beam as a light spot on the scanned surface by the scanning image forming lens. A scanning imaging lens according to any one of 1 to 5 is used (claim 6).

[0008]

FIG. 1 shows an embodiment of an optical scanning device according to the present invention. A substantially parallel light beam is emitted from a light source device 1 that is a combination of a semiconductor laser as a light source and a collimator lens, and the parallel light beam is actuated by a cylinder lens 2 in a sub-scanning corresponding direction (a direction orthogonal to the drawing). And is formed as a long "line image" in the direction corresponding to the main scanning near the position of the deflecting reflection surface 4 of the rotary polygon mirror 3, which is an optical deflector. The reflected light beam reflected by the deflecting reflection surface 4 is a parallel light beam with respect to the main scanning corresponding direction.
In the sub-scanning corresponding direction, the light enters the scanning imaging lens as a divergent light beam. The scanning imaging lens, as shown in the figure,
The first group 5, the second group 6, the third group 7, and the fourth group 8 are arranged in this order from the deflecting / reflecting surface 4 side to the scanned surface 9 side. In practice, the light is focused as a light spot on, for example, a photosensitive surface of a photoconductive photoconductor.
That is, the scanning imaging lens connects the position near the deflection reflection surface 4 and the position of the surface 9 to be scanned to a geometric conjugate relationship in the sub-scanning corresponding direction. Accordingly, in the sub-scanning corresponding direction, since the light spot is formed with the line image as an object point, the light spot does not substantially fluctuate in the sub-scanning direction even if the deflecting and reflecting surface is tilted. When the rotary polygon mirror 3 rotates at a constant speed, the light spot on the surface 9 to be scanned moves in the main scanning direction to perform optical scanning. At this time, the moving speed of the light spot is made uniform by the fθ function of the scanning imaging lens.
As the scanning imaging lens, the one described in any one of claims 1 to 5 is used.

Therefore, the optical scanning apparatus shown in FIG. 1 forms the light beam from the semiconductor laser into a long line image in the main scanning direction, and the deflecting surface 4 near the image forming position of the line image.
Is deflected at a constant angular velocity by the optical deflector 3 having the above, and the deflected light beam is condensed as a light spot on the surface 9 to be scanned by the scanning imaging lenses 5, 6, 7, and 8, and An optical scanning device for performing an optical scanning, wherein the scanning imaging lens according to any one of claims 1 to 5 is used as the scanning imaging lens (claim 6).

[0010]

EXAMPLES Three specific examples will be described below. Embodiments 1 to 3 are designed with the optical arrangement as shown in FIG. The semiconductor laser used as a light source in the light source device 1 has a reference emission wavelength (used wavelength of the optical scanning device): 800
nm. The rotating polygon mirror 3 has a number of deflection reflecting surfaces:
8, the radius of the inscribed circle: 32.335 mm, and the angle α between the direction of the light beam incident on the deflection reflecting surface 4 from the light source device side 1 and the optical axis of the scanning imaging lens: 65 degrees. The distance h shown in FIG. 1 (the distance between the rotation axis of the rotary polygon mirror 4 and the optical axis of the scanning imaging lens) is 17.52 mm in the first embodiment, 17.53 mm in the second embodiment, and 17.53 mm in the third embodiment. 17.5
4 mm. The plane shown in FIG. 1 coincides with the plane drawn by the principal ray of the deflected light beam ideally deflected by the rotary polygon mirror 3, and this plane is called the "deflection plane" and is the optical axis of the scanning imaging lens. And a plane orthogonal to the deflection plane is referred to as a “deflection orthogonal plane”. In FIG. 1, R _xi (i = 1 to 8) is a radius of curvature of the i-th lens surface in the deflection surface (the radius of curvature of the lens surface in the main scanning corresponding direction) counted from the side of the deflecting reflection surface 4. R
_yi (i = 1 to 8) represents the radius of curvature of the i-th lens surface in the plane orthogonal to the deflection (the radius of curvature of the lens surface in the sub-scanning corresponding direction). In D _i (i = ₀ , _{1 to} 8), D ₀ is on the optical axis from the deflecting reflecting surface 4 to the entrance lens surface of the first group 5 when the image height of the light spot is 0. distance,
D _{1 to} D ₇ are the distance between the optical axis upper surfaces of the i-th and (i + 1) -th lens surfaces counted from the deflecting / reflecting surface 4 side, and D ₈ is the exit-side lens surface of the fourth unit 8 and the scanned surface 9 Represents the distance on the optical axis between Also, N _j (j = 1 to 4) and v _dj (j = 1 to
4) represents the refractive index and the dispersion value (Abbe number) at the d-line of the j-th lens counted from the side of the deflecting reflection surface 4 with respect to light having a wavelength of 800 nm. Further, fm and fs respectively represent the combined focal lengths of the entire scanning imaging lens system in the main scanning corresponding direction and the sub scanning corresponding direction, and 2θ represents the deflection angle.

K1, K2 and K3 represent the parameters of the conditional expressions (1), (2) and (3), respectively. Note that the focal length: fm in the main scanning corresponding direction is standardized to 100. Example 1 fm = 100, fs = 29.443,2θ = 50.8 ° _{i R xi R yi D i j} N j ν dj 0 20.447 1 177.819 177.819 2.216 1 1.51032 64.2 2 ∞ ∞ 2.840 3 -39.446 -39.446 1.733 2 1.78439 25.5 4 ∞ 25.035 2.560 5 -187.836 -187.836 5.064 3 1.51032 64.2 6 -41.650 -41.650 0.714 7 ∞ 174 6.174 4 1.79171 46.6 8 -56.351 -16.517 113.749 Value of conditional expression: K1 = 1.827 K2 = -1. 515 K3 = -1.350.

[0012] Example 2 fm = 100, fs = 30.173,2θ = 50.8 ° _{i R xi R yi D i j} N j ν dj 0 20.456 1 182.809 182.809 2.188 1 1.51032 64.2 2 ∞ ∞ 3.035 3 -35.011 -35.011 1.377 2 1.67405 31.2 4 ∞ 21.970 3.401 5 -126.714 -126.714 5.261 3 1.51032 64.2 6 -40.859 -40.859 0.676 7 ∞ ∞ 6.522 4 1.79171 46.6 8 -55.776 -17.354 115.490 Value of the conditional expression: K1 = 1.494 K2 -1.337 K3 = -1.300.

[0013] Example 3 fm = 100, fs = -52.038,2θ = 50.8 ° _{i R xi R yi D i j} N j ν dj 0 20.339 1 130.850 130.850 2.264 1 1.51032 64.2 2 ∞ ∞ 2.697 3 - 36.438 -36.438 1.196 2 1.82347 23.8 4 ∞ 29.073 2.484 5 -214.282 -214.282 5.727 3 1.51032 64.2 6 -37.009 -37.009 0.351 7 ∞ ∞ 6.310 4 1.71941 54.7 8 -53.458 -15.636 114.166 Value of the conditional expression: K1 = 2.298 K = -1.386 K3 = -1.292.

FIGS. 2 to 4 sequentially show field curvatures (solid lines in the sub-scanning direction, dotted lines in the main scanning direction) relating to the first to third embodiments.
FIG. 3 shows fθ characteristics, and a diagram of coma aberration and spherical aberration in a main scanning direction. In the figures of coma and spherical aberration, WL1 =
800 nm, WL2 = 790 nm, WL3 = 810 nm
Is expressed with respect to WL1, and represents magnification and axial chromatic aberration. In each embodiment, fm normalized to 100
Is 399.3 mm in all examples.

[0015]

As described above, according to the present invention, it is possible to provide a novel scanning imaging lens in which chromatic aberration or chromatic aberration and coma are well corrected, and an optical scanning device using the same. This scanning imaging lens has good field curvature and fθ characteristics, and has a very small variation in the light spot diameter, despite having a large angle of view as shown in each embodiment. Therefore,
An optical scanning device using this scanning imaging lens can realize high-density optical scanning. Further, since the scanning image forming lens according to the fifth aspect does not include a complicated "aspherical surface" in the entire system, it can be manufactured easily and inexpensively.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining an embodiment of an optical scanning device according to the present invention.

FIG. 2 is a diagram illustrating aberration and fθ characteristics according to the first embodiment.

FIG. 3 is a diagram illustrating aberration and fθ characteristics according to a second embodiment.

FIG. 4 is a diagram illustrating aberration and fθ characteristics according to a third embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Light source device 2 Cylinder lens 3 Optical deflector 4 Deflection / reflection surface 5 First group 6 Second group 7 Third group 8 Fourth group 9 Scanned surface

Claims (6)

[Claims] 1. A light beam emitted from a semiconductor laser is formed into a long line image in a main scanning corresponding direction, and is deflected at a constant angular velocity by an optical deflector having a deflecting reflection surface near an image forming position of the line image. An image forming optical system for converging a light beam as a light spot on a surface to be scanned and performing uniform speed optical scanning of the surface to be scanned. With respect to the corresponding direction, it has a function of making the image forming position of the line image and the surface to be scanned a geometrically conjugate relationship, the light deflector side as an incident side, and sequentially from the first to the surface to be scanned. A fourth group is arranged, a first group is a lens having a positive refractive power, a second group is an anamorphic lens having a cylinder surface and a negative refractive power, and a third group is an incident side. Is a spherical single lens with a concave surface facing the positive meniscus lens, and the fourth unit has a positive refractive power. - a 4 four-group configuration is anamorphic lens having Rick surface, [nu _d2 and a second group the fourth group of Abbe number, respectively, when the [nu _d4, these conditions: (1) 1.0 <scanning image forming lens satisfies the ν _d4 / ν _d2. 2. The scanning imaging lens according to claim 1, wherein the distance on the optical axis from the starting point of deflection to the incident side of the first group is D ₀ , and the thickness and the refractive index of the material of the first group are respectively D ₁
And N ₁ , the air spacing between the first and second groups is D ₂ , the radius of curvature of the entrance side of the second group is R ₃ , the radius of curvature of the exit side of the fourth group in the main scanning direction is R _8X , When the distance from the starting point to the entrance side surface of the fourth group is D _A1 , and the focal length of the first group is f ₁ , these conditions are: (2) −1.7 <R ₃ / [{(D ₀ · f ₁ ) / (f ₁ -D ₀ )｝ + (D ₁ /
N ₁ ) + D ₂ ] <− 1.0 (3) A scanning imaging lens satisfying −1.5 <R _8X / D _A1 <−1.1. 3. The scanning image forming lens according to claim 1, wherein the second group has a concave spherical surface on the entrance side and a concave cylinder surface on the exit side having power only in the sub-scanning corresponding direction. A scanning imaging lens characterized by being an anamorphic lens. 4. The scanning image forming lens according to claim 1, wherein the fourth group has a positive refractive power on the exit side, wherein the curvature in the sub-scanning corresponding direction is stronger than the curvature in the main scanning corresponding direction. A anamorphic lens having a toric surface and a stronger positive refractive power in the sub-scanning corresponding direction. 5. The scanning image forming lens according to claim 1, wherein the first group is a convex and flat lens having a convex spherical surface on an entrance side and a flat surface on an exit side. A scanning imaging lens characterized in that it does not include any aspheric surface. 6. A light beam from a semiconductor laser is formed into a long line image in a direction corresponding to the main scanning, and the light beam is deflected at an equal angular velocity by an optical deflector having a deflecting / reflecting surface near an image forming position of the line image. An optical scanning device for converging a light beam as a light spot on a surface to be scanned by a scanning image forming lens and performing uniform scanning of the surface to be scanned, wherein the scanning image forming lens is used as a scanning image forming lens. An optical scanning device using the scanning imaging lens according to any one of the above.