JP3367246B2 - fθ lens system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [0001] The present invention relates to an fθ lens system,
In particular, optical scanning devices such as laser printers and digital copiers
Fθ lens system used in the above. [0002] 2. Description of the Related Art Conventional laser printers and digital copiers
Optical scanning device applied to the present invention will be described with reference to FIG.
I do. As shown in FIG. 21, this type of optical scanning device
Emits a laser beam modulated according to the image signal
Semiconductor laser 22 and a dustproof housing (not shown)
A cylindrical photosensitive drum 4 that is stored and sensitizes a laser beam
0. Emitted from the semiconductor laser 22
The laser beam is substantially collimated by the collimator lens 24.
On the outer periphery of the cylinder of the photosensitive drum 40 by the slit 26.
A focusing state is defined. After passing through this slit 26,
The laser beam is reflected by the cylindrical lens 28
Near the side surface of the rotary polygon mirror 32 via the mirror 30, a linear
Image. The rotating polygon mirror 32 is shown rotating at a high speed.
Image of a linear laser beam rotated by the motor
The beam is reflected and deflection scanning is performed. This optical scanning device is composed of a rotating polygon mirror 32.
Corrects the scanning speed of the laser beam deflected and scanned by
A predetermined surface on the outer periphery of the cylinder of the drum 40 (hereinafter, referred to as a scanned surface)
U. ) Having an fθ lens system 20 for forming an image on H
I have. Laser beam passing through this fθ lens system 20
Is a reflecting mirror 34, one of which has a cylindrical shape and a rotating polygon mirror 32.
The scanning direction of the laser beam (hereinafter referred to as main scanning)
It is called direction. ) (Hereinafter referred to as the sub-scanning direction)
U. ) And a photosensitive drum 36 for correcting shake
Through a window 38 mounted on a housing surrounding the drum 40
Then, an image is formed on the scanning surface H. In addition, in front of the reflection mirror 34
The area not used for recording at the scanning start end of the
A reflection mirror 42 for reflecting the beam in a predetermined direction is provided.
And a beam arranged at a position facing the reflection mirror 42.
The photoelectric conversion by the position detection sensor 44
It is used as a synchronizing signal. The laser emitted from the semiconductor laser 22
The beam has very good monochromaticity (temporal coherence)
Lens system, the image height at a specific wavelength
(Y) = focal length of fθ lens system (f) × scan angle of view
(Θ) should be satisfied, and correction for chromatic aberration is considered
I didn't have to. However, in the conventional fθ lens system,
Temperature change (for example, temperature change or rotating polygon mirror
Semiconductor laser 22 generated by the
Chromatic Aberration of Laser Beam Emitted from Laser
Could not be corrected, so try to increase the resolution
It was difficult. In the conventional optical scanning device, a single laser is used.
Since the beam was used, it was not possible to speed up the scanning
It was difficult. To solve this problem, multiple
A technology for simultaneously scanning the scanned surface with the beam is disclosed.
(Japanese Patent Application Laid-Open No. Sho 51-100742,
166916), but in these disclosed technologies,
If there is a wavelength difference between the number of laser beams,
The focal length (f), ie the image height (Y)
U. For this reason, as shown in FIG.
The irradiation point of the laser beam shifts in the main scanning direction,
Image quality is worse than scanning with one laser beam
Problem. [0008] To solve these problems, chromatic aberration is compensated.
Corrected fθ lens (hereinafter referred to as achromatic fθ lens)
(JP-A-56-123509,
JP-A-59-7918, JP-A-59-17081
0, JP-A-62-254110, JP-A-6-254110
JP-A-2-262812, JP-A-62-299927
Gazette, JP-A-63-249119, JP-A-2-1
No. 6520, Japanese Unexamined Patent Application Publication No. 3-65917,
JP-A-3-84509, JP-A-3-177808
Report, JP-A-4-90505, JP-A-4-1744
No. 13, JP-A-4-340519). [0009] SUMMARY OF THE INVENTION
Chromatic aberration correction capability of most designed achromatic fθ lenses
Indicates that the surface to be scanned has a laser beam wavelength change of 1 nm.
The shift of the incident position is about 2 to 4 μm. others
Therefore, if the wavelength difference of the laser beam is 5 nm, 0.1 m
A high-resolution optical scanning device
Position shift that cannot be ignored for
There was a problem of inviting. Also, the number of lenses constituting the fθ lens system
Lens with high refractive index and high dispersion glass
Required, so the cost of the optical scanning device will be high.
There was a problem. Further, in recent years, images desired for digital copying machines and the like have been developed.
Since the quality is higher than before, the field curvature and fθ
There is a demand for an fθ lens system with good performance in terms of characteristics.
For example, in a full-color digital copying machine using halftone,
The position where the laser beam on the scanned surface becomes the narrowest
Beam waist. ) Beam diameter is about 40μm
Very small ones are required. Usually print
The required beam diameter variation over the entire width is less than 20%
is there. When the beam diameter is 40 μm, the beam waist
1 mm away from it, the beam diameter increases by 20% to 48 μm.
You. Therefore, in order to guarantee a beam diameter of 40 μm,
The curvature must be less than ± 1 mm. Also, beam diameter
Is less than 20% when the beam diameter is 50 μm.
Field curvature must be at least ± 1.5 mm
Need to be The curvature of field in the sub-scanning direction is described above.
Can be corrected by the cylindrical reflecting mirror 36
The curvature of field to be corrected by the θ lens system mainly depends on the main scanning direction.
(Hereinafter, the field curvature is shown only in the main scanning direction.
You. ). On the other hand, from the fθ characteristic, that is, from the ideal value Y of the image height,
Of the actual image height Y '(distortion)
Very small, less than 0.1 mm over the entire width
A value has been requested. [0012] In view of the above facts, the present invention has a small number of lenses.
No need for high refractive index or high dispersion glass
± 1.5 mm or less in field curvature and ± 0.1 in fθ characteristics
mm and chromatic aberration correction capability of 1 μm / nm or less
The objective is to obtain a satisfactory fθ lens system. [0013] [MEANS FOR SOLVING THE PROBLEMS] To achieve the above object
The present invention provides a biconcave first lens in order from the light incident side,
Biconvex second lens and plano-convex third lens with positive power
The lens comprises three lenses, the first lens and the
Adhered to the second lens, further satisfies the following relationship
Fθ lens system. [0014]   (1) -4.0 ≦ f (L₁+ L_Two) / F (L_Three) ≦ −1.0   (2) −0.02 <(n_d1+ N_d3) / 2-n_d2≤0.235   (3) (ν_Two+ Ν_Three) / 2-ν₁> 14 However, ν_i: Abbe number of the i-th lens n_di: Refractive index of d-line of the i-th lens f (L_Three): Focal length of the third lens f (L₁+ L_Two): Composite focus of the first lens and the second lens
Point distance [0015] In the fθ lens system according to the present invention, the expression (1) represents the field curvature.
Under the condition of lens power to make it less than ± 1.5mm
is there. If this upper limit is exceeded or less than the lower limit,
The song gets worse. Equation (2) is to reduce the fθ characteristic to ± 0.1 mm or less.
This is the condition of the refractive index of the lens to be used. This upper limit
If it exceeds or falls below the lower limit, the fθ characteristics deteriorate. Equation (3) shows that the chromatic aberration correction ability is 1 μm / nm.
This is the condition of the Abbe number of the lens for the following. this
When the value falls below the lower limit, the scanning start side
Chromatic aberration exceeds 1 μm / nm in both directions,
The difference correction ability deteriorates. [0018] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The fθ lens system according to the present invention is applied below.
An embodiment of the optical scanning device will be described with reference to the drawings.
You. First, the configuration of the first embodiment will be described.
You. As shown in FIG. 20, the fθ lens system of the optical scanning device
Reference numeral 10 denotes a biconcave first lens 12 and a biconvex first lens 12 in the order of light incidence.
Second lens 14 and third lens having plano-convex and positive power
It consists of 16 three lenses. These three records
As shown below, high refractive index and high dispersion glass
Is not used. The rotating polygon mirror 32 is a regular octagon.
And the distance between the facing surfaces is 55 mm. this
Laser beam incident on rotating polygon mirror 32 and fθ lens system
The angle between the ten optical axes is 60 °. Fθ lens system 10 at a wavelength of 785 nm
Is 290 mm, and the rotating polygon mirror 32 is
Between the emitted laser beam and the optical axis of the fθ lens system 10.
The maximum scanning angle of view is ± 29.5 °. R_iOf the fθ lens system 10 in the order of light incidence.
Radius of curvature of the surface, d_iBetween the i-th surface and the (i + 1) -th surface
Distance. Also, d₀To the side 18A of the rotating polygon mirror.
Distance from one surface, d_FiveFrom the fifth surface of the fθ lens system.
Assuming that the distance to the scanning plane is the fθ lens system of the first embodiment,
10 is as follows. [0022]                   d₀      32.4    r₁   -29.5 d₁       4.0 n_d1    1.75000 ν_d1    27.6000    r_Two   439.1 d_Two      17.4 n_d2    1.597347 ν_d2    61.5257    r_Three   -40.3 d_Three      20.0    r_Four      ∞ d_Four      19.1 n_d3    1.583400 ν_d3    62.3550    r_Five   -95.0 d_Five     440.8 f (L₁ + L_Two) / f (L_Three) = -1.05 (n_d1+ N_d3) / 2- n_d2    = 0.07 (ν_d2+ Ν_d3) / 2-ν_d1  = 34.3 Note that the configuration of the optical scanning device according to the present embodiment is the same as the fθ lens system 1.
Semiconductor laser 23 emitting zero and multiple laser beams
21 is the same as the conventional technology shown in FIG. Next, the operation of the first embodiment will be described. Semi-conduct
The plurality of laser beams emitted from the body laser 23 are
Remeter lens 24, slit 26, cylindrical lens
Lens 28 and a reflecting mirror 30 to rotate
Is scanned in the opposite direction. The deflection-scanned laser beams are f
θ lens system 10, reflection mirror 34, cylindrical reflection mirror 36,
An image is formed on the surface to be scanned via the window 38. FIG. 1A shows the field curvature of the first embodiment.
Show. FIG. 1A shows an ideal scanning surface (so-called pen-shape) on the horizontal axis.
(Zebar surface, unit mm), Petzval on the vertical axis
The aberration (unit: mm) from the surface is obtained. 1st real
FIG. 1B shows the fθ characteristics of the embodiment. FIG. 1 (B) is horizontal
The axis represents the main scanning direction (unit: mm), and the vertical axis represents the focal length f.
× Ideal image height represented by scanning angle θ and fθ of the first embodiment
The difference (unit: mm) from the actual image height of the lens system 10 was calculated.
Things. Note that the field curvature and the fθ characteristic have a wavelength of 785n.
m. FIG. 1 shows the chromatic aberration of the first embodiment.
It is shown in (C). FIG. 1C shows the main scanning direction on the horizontal axis (unit).
mm), the fθ characteristics at wavelengths of 785 nm and 795 nm
The sex difference is plotted on the vertical axis (unit: μm). In addition,
1A to 1C, the fθ lens system 10
The optical axis of is set to the origin 0, and-from the scanning start side to the optical axis is set to-.
(Hereinafter referred to as image surface bays in the second to nineteenth embodiments)
The figures showing the tune, fθ characteristic and chromatic aberration are shown in the first embodiment.
Is the same as Field curvature of the fθ lens system 10 of the first embodiment
Is ± 1.2 mm (see FIG. 1A), and the fθ characteristic is ± 0.2 mm.
1 mm (see FIG. 1B), chromatic aberration 2.8 μm / 10 n
m (see FIG. 1C). Next, a second embodiment will be described. The second implementation
The configuration of the example is the same as the configuration of the first embodiment except for the following points.
It is. [0027]                   d₀      31.0    r₁   -28.2 d₁       4.2 n_d1   1.755000 ν_d1    27.6000    r_Two   412.9 d_Two      17.4 n_d2   1.591326 ν_d2    61.8762    r_Three   -38.9 d_Three      20.0    r_Four      ∞ d_Four      19.3 n_d3   1.576062 ν_d3    62.8176    r_Five   -91.8 d_Five     450.9 f (L₁ + L_Two) / f (L_Three) = -1.0 (n_d1+ N_d3) / 2- n_d2    = 0.07 (ν_d2+ Ν_d3) / 2-ν_d1  = 34.7 The field curvature of the second embodiment is ± 1.5 mm, which is the required performance.
It is very close (see FIG. 2A). On the other hand, fθ
The characteristics are ± 0.1 mm and the chromatic aberration is 4.0 μm / 10 nm.
Good (see FIGS. 2B and 2C). Next, a third embodiment will be described. The third implementation
The configuration of the example is the same as the configuration of the first embodiment except for the following points.
It is. [0029]                   d₀      29.7    r₁   -26.9 d₁       4.2 n_d1   1.755000 ν_d1    27.6000    r_Two   349.0 d_Two      17.5 n_d2   1.584218 ν_d2    62.3046    r_Three   -37.5 d_Three      20.0    r_Four      ∞ d_Four      19.9 n_d3   1.561974 ν_d3    63.7584    r_Five   -87.3 d_Five     461.9 f (L₁ + L_Two) / f (L_Three) = -0.95 (n_d1+ N_d3) / 2- n_d2    = 0.07 (ν_d2+ Ν_d3) / 2-ν_d1  = 35.4 The field curvature of the third embodiment is ± 2 mm or more, which is required performance.
A certain ± 1.5 mm cannot be achieved (FIG. 3
(A)). On the other hand, the fθ characteristic is ± 0.1 mm,
Is as good as 5.0 μm / 10 nm (FIG. 3 (B),
(C)). Next, a fourth embodiment will be described. The fourth implementation
The configuration of the example is the same as the configuration of the first embodiment except for the following points.
It is. [0031]                   d₀      61.2    r₁   -61.1 d₁       2.0 n_d1   1.755000 ν_d1    27.6000    r_Two   464.8 d_Two      21.4 n_d2   1.687999 ν_d2    49.9603    r_Three   -72.9 d_Three      20.0    r_Four      ∞ d_Four      17.2 n_d3   1.698278 ν_d3    48.8364    r_Five  -174.6 d_Five     349.2 f (L₁ + L_Two) / f (L_Three) = -3.0 (n_d1+ N_d3) / 2- n_d2    = 0.04 (ν_d2+ Ν_d3) / 2-ν_d1  = 21.8 The field curvature of the fourth embodiment is ± 1.1 mm, and the fθ characteristic is
± 0.1mm, good chromatic aberration of 4.1μm / 10nm
(See FIG. 4). Next, a fifth embodiment will be described. The fifth implementation
The configuration of the example is the same as the configuration of the first embodiment except for the following points.
It is. [0033]                   d₀      65.4    r₁   -66.7 d₁       2.0 n_d1   1.755000 ν_d1    27.6000    r_Two   322.0 d_Two      23.0 n_d2   1.710403 ν_d2    47.6127    r_Three   -79.5 d_Three      20.0    r_Four      ∞ d_Four      16.8 n_d3   1.722533 ν_d3    46.4866    r_Five  -194.3 d_Five     344.0 f (L₁ + L_Two) / f (L_Three) = -4.0 (n_d1+ N_d3) / 2- n_d2    = 0.03 (ν_d2+ Ν_d3) / 2-ν_d1  = 19.4 The field curvature of the fifth embodiment is ± 1.5 mm, which is the required performance.
It is very close (see FIG. 5A). On the other hand, fθ
The characteristics are ± 0.1 mm, and the chromatic aberration is 4.6 μm / 10 nm.
Good (see FIGS. 5B and 5C). Next, a sixth embodiment will be described. The sixth implementation
The configuration of the example is the same as the configuration of the first embodiment except for the following points.
It is. [0035]                   d₀      68.1    r₁   -70.5 d₁       2.0 n_d1   1.755000 ν_d1    27.6000    r_Two   273.6 d_Two      24.2 n_d2   1.720185 ν_d2    46.6975    r_Three   -83.6 d_Three      20.0    r_Four      ∞ d_Four      16.5 n_d3   1.744000 ν_d3    44.7000    r_Five  -208.8 d_Five     341.5 f (L₁+ L_Two) / f (L_Three) = -5.0 (n_d1+ N_d3) / 2- n_d2    = 0.03 (ν_d2+ Ν_d3) / 2-ν_d1  = 18.1 The field curvature of the sixth embodiment is ± 1.6 mm, which is the required performance.
A certain ± 1.5 mm cannot be achieved (FIG. 6).
(A)). On the other hand, the fθ characteristic is ± 0.1 mm,
Is as good as 5.0 μm / 10 nm (FIG. 6 (B),
(C)). Next, a seventh embodiment will be described. The seventh implementation
The configuration of the example is the same as the configuration of the first embodiment except for the following points.
It is. [0037]                   d₀      42.1    r₁   -40.5 d₁       2.0 n_d1   1.755000 ν_d1    27.6000    r_Two   460.3 d_Two      17.2 n_d2   1.666542 ν_d2    48.3128    r_Three   -51.2 d_Three      20.0    r_Four      ∞ d_Four      16.3 n_d3   1.604085 ν_d3    61.1463    r_Five  -123.1 d_Five     375.3 f (L₁+ L_Two) / f (L_Three) = -1.72 (n_d1+ N_d3) / 2- n_d2    = 0.013 (ν_d2+ Ν_d3) / 2-ν_d1  = 27.1 The field curvature of the seventh embodiment is ± 0.9 mm, and the fθ characteristic is
Good ± 0.1 mm, chromatic aberration 2.0 μm / 10 nm
(See FIG. 7). Next, an eighth embodiment will be described. The eighth implementation
The configuration of the example is the same as the configuration of the first embodiment except for the following points.
It is. [0039]                   d₀      39.7    r₁   -39.0 d₁       2.0 n_d1   1.755000 ν_d1    27.6000    r_Two   281.1 d_Two      16.9 n_d2   1.685748 ν_d2    48.8450    r_Three   -50.4 d_Three      20.2    r_Four      ∞ d_Four      15.9 n_d3   1.581650 ν_d3    62.4635    r_Five  -118.9 d_Five     375.9 f (L₁+ L_Two) / f (L_Three) = -1.7 (n_d1+ N_d3) / 2- n_d2    = -0.017 (ν_d2+ Ν_d3) / 2-ν_d1  = 28.1 The fθ characteristic of the eighth embodiment is close to the lower limit, and
Does not reduce surface curvature to ± 1.5 mm, the limit of required performance
(If not bad), required performance of fθ characteristics ± 0.1mm
The following cannot be achieved (see FIGS. 8A and 8B).
See). On the other hand, the chromatic aberration is as good as 3.0 μm / 10 nm.
(See FIG. 8C). Next, a ninth embodiment will be described. The ninth implementation
The configuration of the example is the same as the configuration of the first embodiment except for the following points.
It is. [0041]                   d₀      40.2    r₁   -39.3 d₁       2.0 n_d1   1.755000 ν_d1    27.6000    r_Two   311.0 d_Two      16.8 n_d2   1.685929 ν_d2    46.6942    r_Three   -50.7 d_Three      15.5    r_Four      ∞ d_Four      15.3 n_d3   1.560900 ν_d3    63.8332    r_Five  -113.4 d_Five     370.3 f (L₁+ L_Two) / f (L_Three) = -1.74 (n_d1+ N_d3) / 2- n_d2    = -0.028 (ν_d2+ Ν_d3) / 2-ν_d1  = 27.7 In the ninth embodiment, the required field curvature is ± 1.4 mm.
Fθ characteristic is only ± 0.22mm
Therefore, the required performance cannot be satisfied (see FIG. 9).
(See (A) and (B)). On the other hand, chromatic aberration is 2.8 μm / 1
It is as good as 0 nm (see FIG. 9C). Next, a tenth embodiment will be described. The tenth
The configuration of the embodiment is the same as that of the first embodiment except for the following points.
Is the same. [0043]                   d₀      64.1    r₁   -63.0 d₁       2.0 n_d1   1.755000 ν_d1    27.6000    r_Two   546.1 d_Two      22.2 n_d2   1.519500 ν_d2    67.1213    r_Three   -75.6 d_Three       0.1    r_Four      ∞ d_Four      20.5 n_d3   1.744000 ν_d3    44.7000    r_Five  -121.4 d_Five     358.6 f (L₁+ L_Two) / f (L_Three) = -1.50 (n_d1+ N_d3) / 2- n_d2    = 0.23 (ν_d2+ Ν_d3) / 2-ν_d1  = 28.3 The curvature of field of the tenth embodiment is ± 1.3 mm or less, fθ
Characteristics are 0.1 mm or less, chromatic aberration is 5.0 μm / 10 nm
(See FIG. 10). Next, an eleventh embodiment will be described. The eleventh
The configuration of the embodiment is the same as that of the first embodiment except for the following points.
Is the same. [0045]                   d₀      62.6    r₁   -62.4 d₁       2.0 n_d1   1.755000 ν_d1    27.6000    r_Two   586.5 d_Two      21.5 n_d2   1.514500 ν_d2    67.5797    r_Three   -74.2 d_Three       0.1    r_Four      ∞ d_Four      19.9 n_d3   1.744000 ν_d3    44.7000    r_Five  -120.2 d_Five     357.4 f (L₁+ L_Two) / f (L_Three) = -1.50 (n_d1+ N_d3) / 2- n_d2    = 0.235 (ν_d2+ Ν_d3) / 2-ν_d1  = 28.5 In the eleventh embodiment, the field curvature is required to be ± 1.4 mm.
Required performance of fθ characteristics unless dropped to near the limit of performance ±
0.1 mm cannot be achieved (FIG. 11 (A),
(B)). On the other hand, the chromatic aberration is 5.0 μm / 10 nm.
Good (see FIG. 11C). Next, a twelfth embodiment will be described. The twelfth
The configuration of the embodiment is the same as that of the first embodiment except for the following points.
Is the same. [0047]                   d₀      58.7    r₁   -60.2 d₁       2.0 n_d1   1.755000 ν_d1    27.6000    r_Two -2802.8 d_Two      19.4 n_d2   1.509500 ν_d2    68.0535    r_Three   -66.0 d_Three       0.1    r_Four      ∞ d_Four      16.2 n_d3   1.744000 ν_d3    44.7000    r_Five  -131.3 d_Five     346.5 f (L₁+ L_Two) / f (L_Three) = -1.73 (n_d1+ N_d3) / 2- n_d2    = 0.240 (ν_d2+ Ν_d3) / 2-ν_d1  = 28.8 In the twelfth embodiment, the field curvature is reduced to ± 1.4 mm.
However, the fθ characteristic becomes 0.25 mm or more (FIG. 12).
(A) and (B)), the radius of curvature of the second surface is also negative.
Become. On the other hand, the chromatic aberration is as good as 4.8 μm / 10 nm.
You. Next, a thirteenth embodiment will be described. The thirteenth
The configuration of the embodiment is the same as that of the first embodiment except for the following points.
Is the same. [0049]                   d₀      72.8    r₁   -73.7 d₁       2.0 n_d1   1.755000 ν_d1    27.6000    r_Two   361.7 d_Two      24.2 n_d2   1.702838 ν_d2    47.9388    r_Three   -88.8 d_Three       3.4    r_Four      ∞ d_Four      15.8 n_d3   1.747500 ν_d3    37.2784    r_Five  -188.7 d_Five     337.9 f (L₁+ L_Two) / f (L_Three) = -3.52 (n_d1+ N_d3) / 2- n_d2    = 0.048 (ν_d2+ Ν_d3) / 2-ν_d1  = 15.0 The field curvature of the thirteenth embodiment is ± 1.5 mm, fθ characteristics
Is 0.1 mm or less, and the chromatic aberration is 8.1 μm / 10 nm.
Good (see FIG. 10). Next, a fourteenth embodiment will be described. The 14th
The configuration of the embodiment is the same as that of the first embodiment except for the following points.
Is the same. [0051]                   d₀      71.7    r₁   -72.6 d₁       2.0 n_d1   1.755000 ν_d1    27.6000    r_Two   374.6 d_Two      23.8 n_d2   1.701205 ν_d2    48.5313    r_Three   -87.3 d_Three       4.0    r_Four      ∞ d_Four      15.7 n_d3   1.749097 ν_d3    34.6687    r_Five  -187.8 d_Five     338.3 f (L₁+ L_Two) / f (L_Three) = -3.45 (n_d1+ N_d3) / 2- n_d2    = 0.05 (ν_d2+ Ν_d3) / 2-ν_d1  = 14.0 The curvature of field of the fourteenth embodiment is ± 1.4 mm or less, fθ
The characteristics are 0.1 mm or less (see FIGS. 14A and 14B).
See). On the other hand, chromatic aberration is required to be 10.8μm / 10nm
The capacity slightly exceeds the limit (see FIG. 14C). Next, a fifteenth embodiment will be described. The fifteenth
The configuration of the embodiment is the same as that of the first embodiment except for the following points.
Is the same. [0053]                   d₀      71.2    r₁   -71.9 d₁       2.0 n_d1   1.755000 ν_d1    27.6000    r_Two   401.9 d_Two      23.5 n_d2   1.697949 ν_d2    48.8710    r_Three   -86.3 d_Three       4.7    r_Four      ∞ d_Four      15.7 n_d3   1.750447 ν_d3    32.7395    r_Five  -187.1 d_Five     338.5 f (L₁+ L_Two) / f (L_Three) = -3.39 (n_d1+ N_d3) / 2- n_d2    = 0.05 (ν_d2+ Ν_d3) / 2-ν_d1  = 13.2 In the fifteenth embodiment, the curvature of field is ± 1.4 mm or less,
Although the fθ characteristic satisfies the required performance of 0.1 mm or less,
(See FIGS. 15A and 15B), chromatic aberration is 14.0 μm
/ 10 nm, which greatly exceeds the required performance limit (see FIG. 1).
5 (C)). According to the first to fifteenth embodiments, the image
F (L satisfying surface curvature ± 1.5 mm or less₁+ L_Two) / f (L_Three)of
Satisfies the range and fθ characteristic of 0.1 mm or less (n_d1+ N_d3) /
2- n _d2And the chromatic aberration of 1 μm / nm is satisfied (ν_d2
+ Ν_d3) / 2-ν_d1Is determined as follows. To satisfy field curvature of ± 1.5 mm or less
As is apparent from the first to third embodiments,
f (L₁+ L_Two) / f (L_Three) Must not exceed -1.0
No. Further, it is clear from the fourth to sixth embodiments.
F (L₁+ L_Two) / f (L_ThreeThe lower limit of) is not less than -4.0.
I have to. Therefore, the curvature of field should be within ± 1.5 mm.
The satisfying range is given by the following equation (1).
Then, the field curvature deteriorates. [0056]             -4.0 ≦ f (L₁+ L_Two) / f (L_Three) ≦ −1.0 (1) In order to satisfy the fθ characteristic of 0.1 mm or less,
As is clear from the seventh to ninth embodiments, (n_d1+
n_d3) / 2- n_d2Must be greater than -0.02
No. Further, it is apparent from the tenth to twelfth embodiments.
Like kana, (n_d1+ N_d3) / 2- n_d2Is 0.235
Must be: Therefore, out of the range of the following equation (2)
, The fθ characteristic deteriorates. [0057]             −0.02 <(n_d1+ N_d3) / 2- n_d2≦ 0.235 (2) Further, in order to satisfy the chromatic aberration of 1 μm / nm, the thirteenth
As is clear from the embodiments to the fifteenth embodiment, the following equation (3)
Out of the range, the chromatic aberration correction ability can be satisfied.
I can't. [0058]                (ν_d2+ Ν_d3) / 2-ν_d1> 14 ・ ・ ・ (3) Thus, the fθ value within the range of the above equations (1) to (3) is obtained.
According to the lens system, changes in the wavelength of the laser beam and
Laser on the surface to be scanned due to the difference in the wavelength of the laser beam
Minimize displacement of laser beam irradiation point in main scanning direction
be able to. This enables high-speed, high-resolution optical scanning equipment.
Lens system suitable for the position can be provided. Further, the fθ lens system of the above embodiment is constituted.
The number of lenses is three, high refractive index and high dispersion glass
Is not used, reducing the cost of the optical scanning device.
Can be Next, field curvature, fθ characteristics and chromatic aberration correction
F (L₁+ L_Two) / f
(L_Three), (N_d1+ N_d3) / 2- n_d2And (ν_d2+ Ν_d3) / 2-ν_d1of
A more preferable range will be described. The configuration of the following sixteenth embodiment is different from the following in the following points.
The configuration is the same as that of the first embodiment. [0062]                   d₀      39.1    r₁   -35.3 d₁       3.8 n_d1   1.755000 ν_d1    27.6000    r_Two   470.0 d_Two      18.7 n_d2   1.601828 ν_d2    61.2719    r_Three   -46.6 d_Three      20.0    r_Four      ∞ d_Four      19.8 n_d3   1.606952 ν_d3    60.9887    r_Five  -106.3 d_Five     419.9 f (L₁+ L_Two) / f (L_Three) = -1.2 (n_d1+ N_d3) / 2- n_d2    = 0.08 (ν_d2+ Ν_d3) / 2-ν_d1  = 33.5 In the sixteenth embodiment, the field curvature is ± 1 mm or less, fθ
Characteristics are ± 0.1mm or less, chromatic aberration 1.2μm / 10nm
(See FIG. 16). On the other hand, in the fourth embodiment described above,
± 1.1 mm for field curvature, ± 0.1 mm for fθ characteristics, color
The aberration is 4.1 μm / 10 nm, and the value of each equation is
It is on the street. [0063] f (L₁+ L_Two) / f (L_Three) = -3.0 (n_d1+ N_d3) / 2- n_d2    = 0.04 (ν_d2+ Ν_d3) / 2-ν_d1  = 21.8 Thus, the range of the above equation (1) is the range of the following equation (4).
In some cases, the curvature of field is ± 1 mm or less,
Become. [0064]           -3.0 <f (L₁+ L_Two) / f (L_Three) ≦ −1.2 (4) Next, a seventeenth embodiment will be described. The configuration of the seventeenth embodiment is as follows.
Except for the following points, the configuration is the same as that of the first embodiment. [0065]                   d₀      48.9    r₁   -46.3 d₁       2.0 n_d1   1.755000 ν_d1    27.6000    r_Two   603.4 d_Two      18.6 n_d2   1.652181 ν_d2    54.1357    r_Three   -57.2 d_Three      20.0    r_Four      ∞ d_Four      17.3 n_d3   1.649362 ν_d3    55.0844    r_Five  -136.1 d_Five     369.8 f (L₁+ L_Two) / f (L_Three) = -1.8 (n_d1+ N_d3) / 2- n_d2    = 0.05 (ν_d2+ Ν_d3) / 2-ν_d1  = 27.0 In Example 17, the field curvature is ± 0.7 mm or less, and f
θ characteristic is ± 0.06 mm or less, chromatic aberration is 1.5 μm / 10
nm (see FIG. 17). Next, an eighteenth embodiment will be described. 18th embodiment
Is the same as the configuration of the first embodiment except for the following points.
is there. [0067]                   d₀      68.8    r₁   -62.5 d₁       2.0 n_d1   1.755000 ν_d1    27.6000    r_Two   350.7 d_Two      26.5 n_d2   1.573633 ν_d2    62.9740    r_Three   -73.9 d_Three      20.0    r_Four      ∞ d_Four      28.2 n_d3   1.672266 ν_d3    51.8577    r_Five  -138.5 d_Five     384.1 f (L₁+ L_Two) / f (L_Three) = -1.6 (n_d1+ N_d3) / 2- n_d2    = 0.14 (ν_d2+ Ν_d3) / 2-ν_d1  = 29.8 In Example 18, the field curvature is ± 1.5 mm or less, and f
θ characteristic is ± 0.06 mm or less, chromatic aberration is 5 μm / 10 nm
(See FIG. 18). From the seventeenth embodiment and the eighteenth embodiment,
If the range of Expression (2) is within the range of Expression (5), the fθ characteristic
Is further improved to ± 0.06 mm. [0069]           + 0.05 ≦ (n_d1+ N_d3) / 2- n_d2<0.15 ... (5) From the first to eighteenth examples, the value of equation (3) and chromatic aberration
Comparing the correction abilities, the range of the above equation (3) is
Within the range of (6), the chromatic aberration correction ability is 5 μm / 10
nm or less, which is even better. [0070]            (ν_d2+ Ν_d3) / 2-ν_d1> 18 ... (6) Note that the above equation (4) is within the range of the equation (1).
(5) is within the range of equation (2), and equation (6) is equivalent to equation (3).
Within the range. Next, the most preferred embodiment is the nineteenth embodiment.
Shown in The configuration of the eighteenth embodiment is similar to that of the first embodiment except for the following points.
The configuration is the same as that of the embodiment. [0072]                   d₀      52.7    r₁   -49.3 d₁       2.0 n_d1   1.755000 ν_d1    27.6000    r_Two   669.9 d_Two      19.6 n_d2   1.630354 ν_d2    58.2949    r_Three   -60.0 d_Three      20.0    r_Four      ∞ d_Four      18.8 n_d3   1.664333 ν_d3    52.9070    r_Five  -137.4 d_Five     371.7 f (L₁+ L_Two) / f (L_Three) = -1.78 (n_d1+ N_d3) / 2- n_d2    = 0.08 (ν_d2+ Ν_d3) / 2-ν_d1  = 28.0 According to the nineteenth embodiment, the field curvature is ± 0.4 mm or less.
Below, fθ characteristic is ± 0.5 mm or less, chromatic aberration is 1.4 μm
/ 10 nm, which is very high performance.
Is included in the more preferable range described above. This 19th
In the fθ lens system of the embodiment, the beam diameter of the beam waist is 4
It is possible to sufficiently cope with a thickness of 0 μm. [0073] According to the present invention, as described above,
Change in laser beam wavelength or difference in wavelength of multiple laser beams
Deviation of the irradiation position of each beam on the scanned surface due to
To improve the field curvature and fθ performance.
Can improve the image quality of the optical scanning device.
Can be obtained. Further, a glass having a high refractive index and a high dispersion is required.
Fθ lens system can be constructed,
That the cost of the inspection device can be reduced
Can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing field curvature, fθ characteristics, and chromatic aberration of a first embodiment. FIG. 2 is a diagram illustrating field curvature, fθ characteristics, and chromatic aberration of a second embodiment. FIG. 3 is a diagram illustrating field curvature, fθ characteristics, and chromatic aberration of a third example. FIG. 4 is a diagram showing the field curvature, fθ characteristics, and chromatic aberration of a fourth embodiment. FIG. 5 is a diagram showing the field curvature, fθ characteristics, and chromatic aberration of the fifth embodiment. FIG. 6 is a diagram illustrating field curvature, fθ characteristics, and chromatic aberration in a sixth example. FIG. 7 is a diagram showing the field curvature, fθ characteristics, and chromatic aberration of the seventh embodiment. FIG. 8 is a diagram illustrating field curvature, fθ characteristics, and chromatic aberration of an eighth embodiment. FIG. 9 is a diagram showing the field curvature, fθ characteristics, and chromatic aberration of the ninth embodiment. FIG. 10 is a diagram showing field curvature, fθ characteristics, and chromatic aberration of a tenth example. FIG. 11 is a diagram illustrating field curvature, fθ characteristics, and chromatic aberration of an eleventh example. FIG. 12 is a diagram showing the field curvature, fθ characteristics, and chromatic aberration of the twelfth embodiment. FIG. 13 is a diagram illustrating field curvature, fθ characteristics, and chromatic aberration in a thirteenth embodiment. FIG. 14 is a diagram showing the field curvature, fθ characteristics, and chromatic aberration of the fourteenth embodiment. FIG. 15 is a diagram showing the field curvature, fθ characteristics, and chromatic aberration of the fifteenth embodiment. FIG. 16 is a diagram showing the field curvature, fθ characteristics, and chromatic aberration of the sixteenth embodiment. FIG. 17 is a diagram showing the field curvature, fθ characteristics, and chromatic aberration of the seventeenth embodiment. FIG. 18 is a diagram illustrating field curvature, fθ characteristics, and chromatic aberration in an eighteenth example. FIG. 19 is a diagram illustrating a curvature of field, fθ characteristics, and chromatic aberration in a nineteenth example. FIG. 20 is a sectional view showing a section of the fθ lens system according to the first to nineteenth embodiments. FIG. 21 is a schematic perspective view schematically showing a conventional scanning optical device. FIG. 22 is a conceptual diagram illustrating an adverse effect on image quality when there is a difference between the wavelengths of a plurality of laser beams. [Description of Signs] 10 fθ lens system 12 First lens 14 Second lens 16 Third lens

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) G02B 9/00-17/08 G02B 21/02-21/04 G02B 25/00-25/04

Claims (1)

(57) [Claims 1] A bi-concave first lens, in order from the light incident side,
Biconvex second lens and plano-convex third lens with positive power
An fθ lens system including three lenses, wherein the first lens and the second lens are bonded to each other, and further satisfies the following relationship. (1) -4.0 ≦ f (L 1 + L 2) / f (L 3) ≦ -1.0 (2) -0.02 <(n d1 + n d3) / 2-n d2 ≦ 0.235 ( 3) (ν ₂ + ν ₃ ) / 2−ν ₁ > 14 where, ν _i : Abbe number of the i-th lens n _di : refractive index of the i-th lens with respect to d-line f (L ₃ ): focal length of the third lens f (L ₁ + L ₂ ): composite focal length of the first lens and the second lens