JPS635736B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a projection lens called an f-theta lens, which has constant velocity scanning performance and is used in a recording and reading device that performs optical scanning using a laser beam that has undergone brightness modulation. It's about lenses.

Conventional configuration and problems The fθ lens has a light focusing function, and the distance between the optical axis and the imaging position on the recording imaging plane perpendicular to the optical axis is proportional to the deflection angle with a constant angular velocity. This is a projection lens designed to have barrel distortion aberration so as to always obtain a uniform scanning speed, and is used in an optical system such as the one shown in Fig. 1.

In the figure, a beam emitted from a laser light source 1 is modulated by a modulator 2, expanded through a beam expander 3, reflected by a rotating polyhedron 4, and focused onto a recording medium 6 via an fθ lens 5. scanned at high speed.

This fθ lens requires the following basic characteristics.

In other words, in a normal lens system with well-corrected distortion aberration, the relationship between the incident angle θ of the light ray and the image height Y is expressed as Y=f _tao θ, where f is the focal length of the lens system, so it is equal to When scanning a laser beam with a constant speed angle, dθ/dt, the rate of change in image height, that is, the scanning speed of the imaging spot dY/dt
is a function f·sec ² θ·dθ/dt of θ, and the scanning speed dY/dt changes as θ changes.

Therefore, in order to keep the scanning speed of the imaging spot constant, it is necessary to establish a proportional relationship between the image height and the angle of incidence as follows: Y=f·θ. In order to have this characteristic, the distortion D of the lens must have the following characteristic: D=f·θ−ftanθ/ftanθ×100 (%).

f-theta lenses with the above basic characteristics are considered to be wide-angle types (1) modified Gaussian type (2) orthometer type, but (1) has a lot of residual comatic aberration at intermediate angles of view. This is a drawback, (2)
The wide-angle type is characterized by the fact that a flat image surface can be obtained in a wide-angle range, that is, the Petzval sum is small. This allows the lens system to be designed more compactly.

Heretofore, several orthomometer type fθ lenses have been known (for example, see Japanese Patent Application Laid-open No. 53308/1983). A typical example is shown in Figure 2, which has good astigmatism, coma, and spherical aberration, and can obtain a geometric spot shape that is almost below the diffraction limit.
It has a large number of lenses, 5 elements in 5 groups.

Furthermore, as shown by the broken line in FIG. 2, the beam reflected by the lens surface is reflected by the deflection surface, forming similar spots near the recording and reading sections.
Hereinafter, the beam reflected by this lens surface will be referred to as a ghost beam.

This ghost beam deteriorates the drawing condition on the recording surface, and also has the disadvantage of increasing the noise of the detection signal in the reading section.

Purpose of the Invention The present invention solves the above-mentioned drawbacks.
Spherical aberration, astigmatism, and coma aberration are small, and good spot imaging can be obtained, and ghost beams are prevented from converging near the reading and recording surfaces.
It is an object of the present invention to provide a projection lens having characteristics such that distortion aberration satisfies y=f·θ.

Structure of the Invention The present invention achieves the above object, and is composed of four lenses in four groups, including three positive meniscus lenses and one negative meniscus lens. A positive meniscus lens with a concave surface facing each side is used, and the second group is a negative meniscus lens with a concave surface facing the light beam incident direction, thereby providing a projection lens that satisfies the following conditions.

1.00＜｜r ₂ /r ₃ ｜＜1.08 −0.48f＜r ₆ ＜−0.35f −1.60f＜r ₇ ＜−1.44f where r ₂ is the opposite surface of the first lens of the first group from the light incident side. , r ₃ is the radius of curvature of the second lens in the second group on the light incident side, r ₆ is the radius of curvature of the surface opposite to the light incident side of the third lens in the third group, r ₇ is the fourth lens in the fourth group. The radius of curvature of the lens on the light incident side, f, is the combined focal length of the entire system.

DESCRIPTION OF EMBODIMENTS The following embodiments of the present invention will be described with reference to the drawings.

FIG. 3 shows a sectional view of a projection lens in an embodiment of the present invention.

This embodiment includes a first lens 7 of the first group having radii of curvature r ₁ and r ₂ and an axial distance d ₁ between the lens surfaces, and a radius of curvature of
A second lens 8 in a second group having r ₃ , r ₄ and an axial distance d ₃ between lens surfaces, and a third lens 8 in a third group having radii of curvature r ₅ , r ₆ and an axial distance d ₅ between lens surfaces. The lens 9 and the fourth group third lens 10 having radii of curvature r ₇ and r ₈ and an axial distance d ₇ between the lens surfaces are arranged between the first lens 7 of the first group and the second lens 8 of the second group. The axial distance is d ₂ , and the axial distance between the second lens 8 of the second group and the third lens 9 of the third group is
d ₄ , third lens of third group 9 and fourth lens of fourth group 10
The axial _distance between
Group 8 is composed of a negative meniscus lens with a concave surface facing the direction of incidence of the light beam, and furthermore, 1.00<|r ₂ /r ₃ |<1.08 −0.48f<r ₆ <−0.35f −1.60f<r ₇ <−1.44f It is configured so that

Here, n ₁ to _{n 4} are the refractive index of each lens at a wavelength of 488 nm, f is the focal length of the entire system, F is the F number, 2ω is the entire scanning angle of view, and d ₀ is the incidence in front of the r ₁ plane and the r ₁ plane. The distance between the pupil position is the first, third, fourth
The lens group uses a glass material with a refractive index of 1.7 or higher to reduce the Petzval sum.

In the case of this example, the first condition is |r ₂ /r ₃ |
If it exceeds 1.08, spherical aberration will be overcorrected, meridional narcoma will be large, and distortion will also be excessive. When |r ₂ /r ₃ | is less than 1.00, spherical aberration increases, coma in the sagittal direction becomes large, and distortion becomes insufficient.

Next, if r ₆ , which is the second condition, exceeds -0.35f, the meridional image plane will collapse in the positive direction, and distortion will become larger than necessary, and if r ₆ becomes -0.48f or less, the meridional image plane will tilt in the negative direction. The lens collapses, and the distortion becomes less than the corrected value.

Furthermore, when the third condition, _r7 , exceeds -1.44f, the astigmatism curve falls in the negative direction, and _r7 exceeds -1.60f.
If it is below, astigmatism will be overcorrected and r ₇
The reflected light is reflected from the deflection mirror, and the ghost beam approaches the actual beam focusing position, causing the spot diameter of the reflected light at the actual beam position to be less than φ2 mm, which affects the reading operation. I end up giving.

Examples are given below.

Here, r ₁ to r ₈ are the radius of curvature of each lens surface, d ₁ to d ₇ are the axial distances between each lens surface, and n ₁ to n ₄ are the refractive index of each lens at a wavelength of 488 nm.
f is the focal length of the entire system, F is the F number, 2ω is the entire scanning angle of view, and d ₀ is the entrance pupil position in front of the _r1 plane.

In this example, since the number of lenses is four, in order to reduce the Petzval sum, the first, third, and fourth group lenses, which are positive lenses, are made of glass material with a refractive index of 1.7 or more. This has been achieved.

Example 1 In this example, |r ₂ /r ₃ |=1.028, r ₆ =-0.4060f, r ₇ =- in the projection lens having the structure shown in FIG.
In the case of 1.5520f, it has the following parameters,
The aberration curve diagram is shown in FIG.

r ₁ = −0.2536 d ₁ = 0.03588 n ₁ = 1.75925 r ₂ = −0.1373 _{d 2 = 0.0038 r 3 = −0.1336 d 3 = 0.02944} n ₂ = 1.70425 r ₄ = −1.4520 d ₄ = 0.0192 r ₅ = −1.9000 d ₅ = 0.0320 n ₃ = 1.73419 r ₆ = −0.4060 d ₆ = 0.003 r ₇ = −1.5520 d ₇ = 0.0384 n ₄ = 1.80560 r ₈ = −0.4764 f = 1.0 F/3 0.7 2ω = 60° d ₀ = 0.1 λ =488nm Example 2 In this example, |r ₂ /r ₃ |=1.028, r ₆ =-0.4060f, r ₇ =- in the projection lens having the structure shown in FIG.
In the case of 1.5520f, it has the following parameters,
The aberration curve diagram is shown in FIG.

r ₁ = −0.2463 d ₁ = 0.03484 n ₁ = 1.75925 r _{2 = −0.1330 d 2} = 0.00354 r ₃ = −−0.1304 d ₃ = 0.02743 _{n 2 =} 1.70425 r ₄ = −1.4347 d ₄ = 0.0178 9 r ₅ = −1.7888 d ₅ = 0.02981 n ₃ = 1.73419 r ₆ = −0.3950 d ₆ = 0.00279 r ₇ = −1.4906 d ₇ = 0.03578 n ₄ = 1.80560 r ₈ = −0.4658 f = 1.0 F/3 0.7 2ω = 60゜d ₀ = 0.09316 λ=488nm Example 3 In this example, |r ₂ /r ₃ |=1.043, r ₆ =-0.4658f, r ₇ =- in the projection lens having the structure shown in FIG.
In the case of 1.5280f, the parameters are shown below,
The aberration curve diagram is shown in FIG.

r ₁ = −0.2614 d ₁ = 0.03443 n ₁ = 1.75925 r _{2 = −0.1361 d 2} = 0.00368 r ₃ = −0.13045 d ₃ = 0.02874 _{n 2 =} 1.70425 r ₄ = −1.3617 d ₄ = 0.0173 1 r ₅ = −1.4728 d ₅ = 0.02725 n ₃ = 1.73419 r ₆ = -0.4658 d ₆ = 0.002762 r ₇ = -1.5280 d ₇ = 0.03535 n ₄ = 1.80560 r ₈ = -0.3977 f = 1.0 F/3 0.7 2ω = 60゜d ₀ =0.9205λ =488 These Examples 1 to 3 have small spherical aberration, as shown in FIGS. 4 to 6, (a) spherical aberration, (b) astigmatism, and (c) fθ characteristics, and the astigmatism curve shows sagittal The intermediate value between the curve and the meridional curve is almost constant over all angles of view, and a stable spot can be obtained over the entire scanning width with the same amount of defocus, preventing ghost beams from converging near the reading and recording surface, and making it easier for the fθ lens to It is suitable as

Effects of the Invention In summary, the present invention provides a positive meniscus lens 3
The first, third, and fourth groups are positive meniscus lenses each having a concave surface facing the light incident direction, and the second group is a positive meniscus lens with a concave surface facing the light incident direction. It is a negative meniscus lens with a concave surface facing , where _r2 is the radius of curvature of the surface opposite to the light incident side of the first lens of the first group, _r3 is the radius of curvature of the second lens of the second group on the light incident side, and r ₆ is the radius of curvature of the surface opposite to the light incident side of the third lens in the third group, _r7 is the radius of curvature of the fourth lens in the fourth group on the light incident side, and f is the combined focal length of the entire system, then 1.00 ＜｜r ₂ /r ₃ ｜＜1.08 −0.48f＜r ₆ ＜−0.35f −160f＜r ₇ ＜−1.44f. , has the advantage of having small coma aberration and preventing ghost beams from converging near the reading and recording surfaces.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of an optical system using an fθ lens.
FIG. 2 is a sectional view of a conventional projection lens, FIG. 3 is a sectional view showing the basic configuration of a projection lens in an embodiment of the present invention, and FIGS. 4 to 6 are a sectional view of a projection lens in each embodiment of the present invention. FIG. 3 is an aberration curve diagram showing characteristics. 7...First lens of the first group, 8...Second lens of the second group, 9...Third lens of the third group, 10...Fourth lens of the fourth group.

Claims (1)

[Claims] 1. Has a four-element configuration in four groups consisting of three positive meniscus lenses and one negative meniscus lens;
The third and fourth groups are positive meniscus lenses each having a concave surface facing the light incident direction, and the second group is a negative meniscus lens having a concave surface facing the light incident direction, and is characterized by satisfying the following conditions. projection lens. 1.00＜｜r ₂ /r ₃ ｜＜1.08 −0.48f＜r ₆ ＜−0.35f −1.60f＜r ₇ ＜−1.44f However, r ₂ is the surface opposite to the light incident side of the first lens of the first group. , r ₃ is the radius of curvature of the second lens in the second group on the light incident side, r ₆ is the radius of curvature of the surface opposite to the light incident side of the third lens in the third group, and r ₇ is the radius of curvature of the fourth lens in the fourth group. The radius of curvature of the lens on the light incident side, f, is the combined focal length of the entire system.