JP3467089B2 - Scanning optical system - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention is used in optical scanning systems,
The present invention relates to a scanning optical system with chromatic aberration corrected.

[0002]

2. Description of the Related Art Semiconductor lasers have the property that the oscillation wavelength changes due to mode hopping or the like. Therefore, when a semiconductor laser is used as the light source of the optical scanning system, it may be necessary to correct the chromatic aberration of the optical system in consideration of the change in the wavelength. For example, as in the case of drawing directly on the polymer liquid crystal cell with laser light,
Especially when high-definition drawing is required, this wavelength change becomes a problem.

On the other hand, in order to achieve high definition, it is necessary to reduce the diameter of the imaging spot formed by the optical system, and for that purpose the F number of the optical system must be reduced.

That is, when attempting to achieve high-definition drawing by using a semiconductor laser, it is necessary to reduce the F number and also consider the correction of chromatic aberration, which doubles the design difficulty.

Conventionally, fθ corrected for such chromatic aberration
Several applications have been made regarding lens systems. For example, in JP-A-56-125309, fθ in which chromatic aberration is corrected for two wavelengths of 632.8 nm and 1064 nm.
A lens system has been proposed, and JP-A-5-80251 discloses an fθ lens system in which chromatic aberration is corrected in the wavelength range of 780 nm to 890 nm.

However, the former has a large F number of about 12 despite the large number of lens elements, seven.
Moreover, in order to correct chromatic aberration, a glass material having an ultra-low dispersion is used for some lenses, which is expensive. On the other hand, the latter has a small F number of 6 and does not use expensive glass materials, but has a large number of lens components of 8 and a scanning angle of about ± 14 degrees.

Further, regarding an fθ lens system having a small F number, JP-A-60-123815 and JP-A-2-
No.-93511 discloses a lens system having an F number of about 5.5 to 8 and a number of constituent elements of 5 to 6, but none of them considers chromatic aberration, and is a gas laser with a stable oscillation wavelength. Can be used for
Not suitable for semiconductor lasers.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an fθ lens system having a small number of components, that is, a low cost, a small F number, a wide scanning angle, and a small chromatic aberration.

[0009]

SUMMARY OF THE INVENTION The scanning optical system of the present invention comprises a first lens group consisting of one negative lens; a second lens group having a positive power; a third lens of a meniscus lens having a convex surface facing the light source side. The second lens group includes at least one negative lens and at least three positive lenses.
Only, the following conditional expressions (1), (2) and (3) are satisfied . _{(1) -0.75 <f 1 /f<-0.40} (2) 0.30 <f 2 /f<0.70 (3) -0.30 <f / f 3 <0.30 where, f _{1 is} the focal length of the first lens group, f _{2 is} the focal length of the second lens group, f _{3 is} the focal length of the third lens group, and f is the focal length of the entire lens system . When the second lens group is composed of only one negative lens and three positive lenses, an fθ lens system having a small number of constituent elements can be obtained.

[0010]

The scanning optical system of the present invention preferably further satisfies the following conditional expressions (4), (5) and (6). _{(4) 0.33 <R L1 /f<3.00} (5) n r> 1.6 (6) -1.00 <R 1 /L<-0.45 However, R _L1: the third lens group Radius of curvature of the surface of the meniscus lens on the light source side, n _r : average value of the refractive index of the positive lens in the second lens group, R ₁ : radius of curvature of the surface of the first lens on the light source side, L: from the entrance pupil The distance to one lens.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The scanning optical system (f.theta. Lens system) of the present invention.
In the above, if the first lens group is composed of one negative lens, the incident height of the light beam to the second lens group having a positive power is increased, and a large negative distortion aberration is given to the entire lens system. It is effective to achieve the fθ property, and it is also advantageous to reduce the cost by using only one sheet.

The second lens group is composed of at least one negative lens and at least three positive lenses having a positive power as a whole. In order to correct chromatic aberration including magnification chromatic aberration, it is necessary to have at least two negative lenses in the entire lens system. The negative lens of the first lens group also contributes to correction of chromatic aberration, but in order to correct chromatic aberration of magnification in particular, at least another lens in the second lens group located farther from the pupil is used. Placing a negative lens is effective.

Further, in the present lens system having a positive power as a whole, most of the positive power is taken care of by the second lens group. Therefore, in order to reduce the F number to about 7, spherical aberration and coma aberration are required. Need to be corrected sufficiently.
Therefore, at least three positive lenses are required.

In order to achieve the object of the present invention with as few lenses as possible in view of cost, it is preferable that the second lens group be composed of only one negative lens and three positive lenses.

The third lens group is composed of a meniscus lens having a convex surface on the light source side. As the F-number is reduced, the depth becomes shallower in proportion to the square of the F-number. Therefore, in order to obtain uniform imaging performance over the entire image surface, it is necessary to sufficiently correct the field curvature. is there. Placing the meniscus lens near the image plane is effective for reducing Petzval sum, that is, for correcting field curvature.

In order to make the object of the present invention more effective, first, it is desirable that the above three conditional expressions (1) to (3) are satisfied.

Conditional expression (1) relates to the power of the first lens group. As described above, a major object of the present invention is to obtain an fθ lens having a small F number and a large scanning angle, in which chromatic aberration is corrected. For that purpose, it is necessary to reduce spherical aberration and chromatic aberration first, and then sufficiently correct field curvature and astigmatism. The negative lens of the first lens group plays a large role in achieving this purpose. That is, since the entire lens system has positive power, spherical aberration and chromatic aberration tend to be under-prone, but relatively strong negative power as given by this conditional expression should be given to this first lens group. As a result, over spherical aberration and chromatic aberration are generated, and the aberrations in the entire lens system can be balanced. Further, by imparting a strong negative power to the first lens group, the Petzval sum can be reduced, and the field curvature can be sufficiently corrected.

If the lower limit of conditional expression (1) is exceeded, the power of the first lens unit will become too weak, and the above effect will not be exhibited. On the other hand, when the value exceeds the upper limit, the power becomes too strong and high-order aberrations are generated, and in particular, high-order components of spherical aberration occur and the RMS value of the wavefront aberration becomes large. Astigmatism is unfavorable because of the disadvantage that the imaging performance cannot be balanced on the entire image plane.

Conditional expression (2) relates to the power of the second lens group. The second lens group has a strong positive power, and the entire lens system is a positive lens system. If the upper limit of the conditional expression (2) is exceeded, the power becomes small, the positive distortion aberration necessary for obtaining the fθ property does not sufficiently occur, and the basic specifications of the fθ lens cannot be satisfied. If the value goes below the lower limit, the power becomes too strong, spherical aberration and chromatic aberration become too low, and astigmatism increases, making it difficult to balance the performance on the entire image surface.

Conditional expression (3) relates to the power of the third lens group. The meniscus lens of the third lens group has a relatively weak power, has almost no influence on aberrations other than field curvature, and has a role of controlling field curvature. If the upper limit or the lower limit of conditional expression (3) is exceeded,
Positive or negative power becomes too strong and astigmatism occurs,
The RMS value of the wavefront aberration with respect to off-axis light becomes large, and it becomes impossible to obtain a small imaging spot diameter.

In order to make conditional expression (3) more effective, it is desirable that the radius of curvature of the surface on the light source side of the third lens group meniscus lens satisfies conditional expression (4). By satisfying this conditional expression (4), the effect of reducing the Petzval sum by the third lens group is further enhanced.

Further, in order to maintain good and uniform performance over the entire image plane, it is necessary to highly correct the field curvature and astigmatism. Therefore, it is desirable to satisfy conditional expression (5). Conditional expression (5) defines the average value of the refractive indices of the positive lenses in the second lens group. In order to keep Petzval sum in the entire lens system small and highly correct the field curvature, it is necessary to set the refractive index of the positive lens in the second lens group to be high as a whole. Conditional expression (5)
If it deviates from the range, the Petzval sum cannot be reduced, or it is necessary to increase the power of each lens group, especially the power of the first lens, as another means of reducing the Petzval sum. However, a high-order component of astigmatism is generated, and it becomes impossible to obtain a good and small imaging spot.

Furthermore, the spherical aberration generated by the first lens,
It is desirable to satisfy the conditional expression (6) in order to suppress coma aberration as much as possible and prevent a high-order aberration component from remaining in the entire lens. Conditional expression (6) defines the ratio between the radius of curvature of the surface of the first lens on the light source side and the distance from the entrance pupil to the first lens. By satisfying this conditional expression (6), it is possible to appropriately share the aberration generated in the first lens by the first surface and the second surface. If either the upper limit or the lower limit is exceeded, the aberration is biased on one of the surfaces, and higher-order components remain.

Further, in order to highly correct the axial chromatic aberration and the chromatic aberration of magnification, the negative lens of the first lens group and the second lens
The Abbe number of the glass material used for the negative lens in the lens group is preferably 45 or less. Further, it is desirable that the average Abbe number of the glass material used for the positive lens of the second lens group is 45 or more.

Next, specific examples will be shown. Example is 1
In each of the lens configurations, the first group is composed of a single lens of the negative lens 11, and the second lens group 20 is composed of the first positive lens 21, the second positive lens 22,
The negative lens 23 and the third positive lens 24 have a four-lens structure, and the third lens group includes a single meniscus lens 31 having a convex surface facing the light source. E indicates the position of the entrance pupil.

[Embodiment 1] FIG. 1 is a lens configuration diagram of a first embodiment of the present invention. The negative lens 23 and the third positive lens 24 in the second lens group 20 are cemented lenses. Table 1 shows specific numerical data of this lens system, and FIG. 2 shows various aberrations. In the various aberration diagrams, SA is spherical aberration, SC is sine condition, 785 nm, 770 nm and 800 nm are chromatic aberration and magnification chromatic aberration indicated by spherical aberration, S is sagittal and M is meridional at respective wavelengths.

In the table and drawings, F _NO is the F number, f is the focal length, ω is the half angle of view, f _B is the back focus, R is the radius of curvature of each lens surface, D is the lens thickness or lens interval, and N ( 785) is the refractive index for light with a wavelength of 785 nm, and N _d is d
Refractive index for the line, ν _d is the Abbe number for the d line.

[0029]

[Table 1] F _NO = 1: 7.0 f = 90.17 ω = 14.0 ° f _B = 78.29 Entrance pupil position; Front of first surface of first lens 48.15 surface NO R D N (785) N _d ν _d 1 -28.167 4.20 1.62779 1.63980 34.5 2 -248.873 4.11---3 -106.796 10.42 1.76185 1.77250 49.6 4 -42.704 0.20---5 192.186 12.10 1.71991 1.72916 54.7 6 -71.865 20.85---7 -53.924 6.00 1.70189 1.71736 29.5 8 79.596 15.00 1.71991 1.72 54.7 9 -79.596 0.20---10 97.525 15.00 1.50444 1.51009 63.7 11 118.054----

[Second Embodiment] FIG. 3 is a lens configuration diagram of a second embodiment of the scanning optical system of the present invention. Laminated lenses are not included. Table 2 shows specific numerical data of this lens system, and FIG. 4 shows various aberrations thereof.

[0031]

[Table 2] F _NO = 1: 7.0 f = 90.21 ω = 14.0 ° f _B = 82.83 Entrance pupil position; in front of the first surface of the first lens 59.88 NO R D N (785) N _d ν _d 1 -37.954 4.80 1.62779 1.63980 34.5 2 725.569 5.26---3 -142.456 9.35 1.76185 1.77250 49.6 4 -52.402 0.20---5 152.125 22.00 1.76185 1.77250 49.6 6 -64.303 3.31---7 -57.899 6.70 1.76556 1.78472 25.7 8 ∞ 0.20---9 241.013 12.52 1.71991 1.72916 54.7 10 -104.325 0.20---11 50.463 16.00 1.50444 1.51009 63.7 12 37.282----

[Third Embodiment] FIG. 5 is a lens configuration diagram of a third embodiment of the scanning optical system of the present invention. The second positive lens 22 and the negative lens 23 in the second lens group 20 are cemented lenses. Table 3 shows specific numerical data of this lens system, and FIG. 6 shows various aberrations thereof.

[0033]

[Table 3] F _NO = 1: 7.0 f = 90.27 ω = 14.0 ° f _B = 102.04 Entrance pupil position; Front 44.20 surface of the first surface of the first lens NO R D N (785) N _d ν _d 1 -37.633 4.00 1.62779 1.63980 34.5 2 ∞ 4.26---3 -93.861 16.96 1.76185 1.77250 49.6 4 -48.911 0.20---5 69.576 16.44 1.71991 1.72916 54.7 6 -181.763 6.00 1.70189 1.71736 29.5 7 61.153 4.77---8 99.834 10.65 1.71991 1.72916 54.7 9 -206.690 0.20---10 109.670 10.00 1.50444 1.51009 63.7 11 167.761----

[Fourth Embodiment] FIG. 7 is a lens configuration diagram of a fourth embodiment of the scanning optical system of the present invention. Laminated lenses are not included. Table 4 shows specific numerical data of this example, and FIG. 8 shows various aberrations thereof.

[0035]

[Table 4] F _NO = 1: 7.0 f = 90.19 ω = 14.0 ° f _B = 84.60 Entrance pupil position; Front of first surface of first lens 45.00 surface NO R D N (785) N _d ν _d 1 -39.916 4.00 1.62779 1.63980 34.5 2 331.705 5.38---3 -129.251 17.17 1.76185 1.77250 49.6 4 -52.298 0.20---5 73.212 14.30 1.74506 1.75500 52.3 6 -230.739 14.20---7 -131.285 6.00 1.74374 1.76182 26.5 8 63.340 4.26--- 9 89.830 12.27 1.71991 1.72916 54.7 10 -127.419 0.20---11 172.423 15.00 1.50444 1.51009 63.7 12 268.918----

[Fifth Embodiment] FIG. 9 is a lens configuration diagram of a fifth embodiment of the scanning optical system of the present invention. The negative lens 23 and the third positive lens 24 in the second lens group 20 are cemented lenses. Table 5 shows specific numerical data of this lens system, and FIG. 10 shows various aberrations thereof.

[0037]

[Table 5] F _NO = 1: 7.0 f = 90.10 ω = 14.0 ° f _B = 79.10 Entrance pupil position; Front of first surface of first lens 46.34 surface NOR D N (785) N _d ν _d 1 -28.279 4.20 1.62779 1.63980 34.5 2 -273.852 3.90---3 -114.342 9.80 1.76185 1.77250 49.6 4 -43.053 0.20---5 201.919 11.55 1.74506 1.75500 52.3 6 -69.779 18.40---7 -53.390 6.00 1.70189 1.71736 29.6 8 79.998 14.15 1.71991 1.72916 54.5 9 -79.998 0.20---10 111.379 15.00 1.50444 1.51009 63.7 11 132.354----

[Sixth Embodiment] FIG. 11 is a lens configuration diagram of a sixth embodiment of the scanning optical system of the present invention. Second lens group 20
The negative lens 23 and the third positive lens 24 inside are cemented lenses. Table 6 shows specific numerical data of this lens system.
The various aberrations are shown in FIG.

[0039]

[Table 6] F _NO = 1: 7.0 f = 90.01 ω = 7.3 ° f _B = 96.23 Entrance pupil position; Front 56.67 surface of the first surface of the first lens NO RD N (785) N _d ν _d 1 -35.310 4.00 1.63532 1.64769 33.8 2 144.000 4.80---3 -3898.795 8.76 1.71991 1.72916 54.7 4 -45.821 0.20---5 109.125 8.40 1.78683 1.79952 42.2 6 -164.013 6.15---7 -62.490 4.00 1.65925 1.67270 32.1 8 62.490 11.00 1.68805 1.69680 5 9 -62.490 0.50---10 46.500 5.00 1.51062 1.51633 64.1 11 36.800----

Table 7 shows the values corresponding to the conditional expressions of Examples 1 to 6.

[Table 7]               Conditional expression (1) Conditional expression (2) Conditional expression (3) Example 1 -0.565 0.528 0.101 Example 2 -0.635 0.534 -0.189 Example 3 -0.664 0.603 0.152 Example 4-0.627 0.573 0.100 Example 5 -0.561 0.515 0.080 Example 6 -0.492 0.432 -0.215               Conditional expression (4) Conditional expression (5) Conditional expression (6) Example 1 1.081 1.7339 -0.585 Example 2 0.559 1.7479 -0.746 Example 3 1.215 1.7339 -0.851 Example 4 1.912 1.7423 -0.887 Example 5 1.236 1.7423 -0.653 Example 6 0.517 1.7316 -0.623

As is apparent from Table 7, the numerical values of Examples 1 to 6 all satisfy the conditional expressions (1) to (6). Further, in the scanning optical system of the present invention, as shown in various aberration diagrams, chromatic aberration is corrected, and other aberrations are relatively well corrected.

[0042]

According to the present invention, the F
A scanning optical system having a small number and a wide scanning angle and a small chromatic aberration can be obtained.

[Brief description of drawings]

FIG. 1 is a lens configuration diagram showing a first embodiment of a scanning optical system according to the present invention.

FIG. 2 is a diagram of various types of aberration of the lens system in FIG.

FIG. 3 is a lens configuration diagram showing a second embodiment of the scanning optical system according to the present invention.

FIG. 4 is a diagram of various types of aberration of the lens system in FIG.

FIG. 5 is a lens configuration diagram showing a third embodiment of the scanning optical system according to the present invention.

FIG. 6 is a diagram of various types of aberration of the lens system in FIG.

FIG. 7 is a lens configuration diagram showing a fourth embodiment of the scanning optical system according to the present invention.

FIG. 8 is a diagram of various types of aberration of the lens system in FIG.

FIG. 9 is a lens configuration diagram showing a fifth embodiment of the scanning optical system according to the present invention.

FIG. 10 is a diagram of various types of aberration of the lens system in FIG.

FIG. 11 is a lens configuration diagram showing a sixth embodiment of the scanning optical system according to the present invention.

12 is a diagram of various types of aberration in the lens system of FIG.

[Explanation of symbols]

11 First lens group 20 Second lens group 31 Third lens group

Continuation of front page (56) Reference JP-A-2-93511 (JP, A) JP-A-2-52307 (JP, A) JP-A-7-120670 (JP, A) JP-A-62-254112 (JP , A) JP 62-254111 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G02B 13/00 G02B 26/10

Claims (3)

(57) [Claims] 1. A first lens group consisting of one negative lens; a second lens group having a positive power; a third lens group of a meniscus lens having a convex surface facing the light source side; The two-lens group includes at least one negative lens and at least three positive lenses, and satisfies the following conditional expressions (1), (2), and (3).
Scanning optical system characterized by. _{(1) -0.75 <f 1 /f<-0.40} (2) 0.30 <f 2 /f<0.70 (3) -0.30 <f / f 3 <0.30 where, f ₁ : focal length of the first lens group, f ₂ : focal length of the second lens group, f ₃ : focal length of the third lens group, f: focal length of the entire lens system. 2. The scanning optical system according to claim 1, wherein the second lens group includes one negative lens and three positive lenses. 3. The scanning optical system according to claim 2, further satisfying the following conditional expressions (4), (5) and (6). (4) 0.33 <R _{L1 /} f <3.00 (5) n _r > 1.6 (6) -1.00 <R ₁ /L<-0.45 where R _{L1 is} the third lens group. Radius of curvature of the surface of the meniscus lens on the light source side, n _r : Average value of the refractive index of the positive lens in the second lens group, R ₁ : Radius of curvature of the surface of the first lens on the light source side, L: From the entrance pupil Distance to one lens.