JPS6145804B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to improvements in optical systems used in optical laser scanning. As is well known, scanners that use laser beams have recently expanded their application fields and are making remarkable progress, not only in computer-related applications such as memory and printers, but also in areas such as laser processing machines. . By the way, the most important thing for commonly used laser scanners is that when the scanning speed of the laser beam on the object side is constant, the image plane scanning speed must also be correspondingly constant. now,
When the incident angle of the laser beam to this optical system is θ, the combined focal length of the optical system is f, and the spot position on the image plane is y, measured from the optical axis, the following relationship is established: y=f・F[θ] There is. If the image plane scanning speed is v, then from v=dy/dt, v=f·dF/dθ·dθ/dt. Since the scanning speed of the incident laser beam is constant, dθ/dt=const. Therefore, v=A·f·dF/dθ. However, A is a constant. From this, in order to keep v constant, dF/dθ=const. There are two methods that have traditionally been used to achieve this goal. One of them is electrical correction means and the other is optical means. By the way, in the above-mentioned former method of electric correction means, the lens system uses a normal mapping optical system with y=f・tanθ, so v=A・f・sec ² θ, and the image plane scanning speed v is not constant. Therefore, the effect of sec ² θ is canceled by giving an electrical nonlinear effect to the incident beam angle, and the image plane scanning speed v
is kept constant. When such an electric correction means system is used, the structure of the apparatus itself inevitably becomes complicated and expensive. On the other hand, the latter method of optical correction means is as follows:
The lens system is given a special mapping relationship, that is, y=f·θ. In this way, v=A・f (where A is a constant), and the image plane scanning speed v becomes constant, so there is no need for a complicated electrical system like in the former case, and the problem can be solved simply with a lens system. . Therefore, the latter optical correction means has a great advantage over the former electrical correction means in that the device itself has a simpler configuration and can be provided at a lower cost. However, as a problem with this latter optical correction means, the following three performances are required of this f/θ lens in its use. These three performances are: a) It must be possible to scan a large screen, and the lens system must be compact, that is, it must have as wide an angle as possible. b. The spot diameter of the laser beam on the image plane should be close to the diffraction limit in order to improve the energy collection rate. c. The deviation (distortion aberration) between the ideal f-θ mapping point and the actual mapping point is small. It is. This is because if the angle of view you are currently using is narrow,
The application is limited to small screen scanning and lacks versatility, and when the beam spot diameter becomes large, the energy collection speed becomes poor in the case of fine laser processing, resulting in a significant deterioration in processing efficiency, etc., and large distortion aberrations. In this case, either the f or θ mapping point becomes large, and high-precision scanning becomes impossible. Prior art regarding this type of f/theta lens includes techniques described in US patent documents and techniques that can be detected through actual products that have been commercialized.
As an example, the angle of view of the f/θ lens described in the patent specification of U.S. Patent No. 3,773,404 is about ±6°, and at this angle of view, y=f・tanθ≒f・θ holds true. , does not need to be an f·θ mapping. Also, some commercialized products have angles of view of ±
Some have a wide angle of view of 28.7°,
The distortion aberration is large, the spot diameter on the image plane is large, and other factors such as f.
Those equipped with a θ mapping lens have a large number of lenses and have a relatively low angle of view. In view of the above-mentioned current situation, the present invention provides y=f・θ
In an imaging system that can be considered as such, it is possible to reduce the number of constituent lenses as much as possible to make it compact and widen the angle of view, and it is possible to correct distortion to a sufficiently small value, and the beam spot on the image plane is close to the diffraction limit. The configuration of the optical system for optical laser scanning according to the present invention will be explained in detail below with reference to the embodiment shown in FIGS. 1 and 2. . In the figure, viewed from the object side, the first group L ₁
In order to have a negative focal length, one or two concave lenses l ₁ or l ₁ ′ and l ₁ ″ were arranged, and by making the whole retro type, a wide angle was achieved. Two convex lenses _L2 and _L3 are arranged in the second group _L2 to have a positive focal length, and in order to properly correct various aberrations as a whole, the total length of this optical system is The combined focal length of the system is f, the focal length of the first group is f ₁ , the focal length of the second group is f ₂ , and the focal length of the second group is f 2 .
When the refractive indices of the two convex lenses l ₂ and l ₃ in the group are na and nb, 0.105f≦l≦0.1544f …(1) 0.9≦｜f ₁ |/f ₂ ≦1.3 …(2) This optical system is unique in that it satisfies the following conditions: na≧1.7, nb≧1.7 (3). The reason why the optical system for optical laser scanning according to the present invention requires the above conditions will be described below. That is, the lens configuration of the optical system according to the present invention is a concave-convex type optical system, and various aberrations, especially astigmatism and distortion caused by the concave group of the first group, are eliminated from the second lens group.
The overall performance is good due to cancellation by the convex group of the group, but the total length l of this optical system is the condition (1)
If it becomes smaller than the lower limit, correction of distortion becomes extremely insufficient, coma flare occurs, and the spot diameter increases. On the other hand, if the upper limit is exceeded, the total length of this optical system becomes large, and not only distortion aberration becomes excessively corrected, but also astigmatism becomes large. Also, condition (2) is the same as condition (1) and is necessary to obtain good performance, and the absolute value of the focal length of the first group |f ₁ | and the focal length of the second group f ₂ Ratio is condition 2
If it becomes smaller than the lower limit, the spherical image plane and meridional image plane will be extremely overcorrected, and conversely, if the upper limit of this condition (2) is exceeded, each of the above image planes will become extremely overcorrected. There will be insufficient correction. Furthermore, when condition (3) is not satisfied, if an attempt is made to suppress the spot diameter within the diffraction limit, the Petzval sum increases, and as a result, the field curvature increases. In order to keep this field curvature constant and to prevent the Petzval sum from increasing, each radius of curvature must be made stronger. If this happens, coma flare will occur, and the spot diameter on the image plane will expand beyond the diffraction limit. Note that the second embodiment illustrated in FIG.
In order to correct the insufficient correction of distortion aberration that occurred in the optical system of the first embodiment shown in the figure, one concave lens was added to the first group. It is within the limit. Next, specific examples of the optical system according to the present invention will be shown.

【table】

[Table] Atsube number of lenses for

[Table] Atsube numbers of lenses for _ν4

[Brief explanation of the drawing]

1 and 2 are arrangement diagrams of lenses constituting a first embodiment and a second embodiment of an optical system for optical laser scanning according to the present invention, and FIGS. It is each aberration curve diagram of 2nd Example. In FIG. 1, L ₁ ; first group, L ₂ ; second group,
l: Total length of the entire lens system, l ₁ : Lenses for forming the first group, l ₂ and l ₃ ; Lenses forming the second group, r ₁ , r ₂ ,...,
r ₆ ; radius of curvature of the constituent lens counted sequentially from the object side; d ₁ , d ₃ , d ₅ ; axial thickness of the constituent lens counted sequentially from the object side; d ₂ , d ₄ ; counted sequentially from the object side. On-axis air spacing of the lens for construction. In FIG. 2, L ₁ ; first group, L ₂ ; second group,
l: Total length of the entire lens system, l ₁ ′+l ₁ ″; Lens for 1st group configuration, l ₂ +l ₃ ; Lens for 2nd group configuration, r ₁ ,
r ₂ ,...r ₈ ; Radius of curvature of the constituent lens counted sequentially from the object side, d ₁ , d ₃ , d ₅ , d ₇ ; Axial thickness of the constituent lens counted sequentially from the object side, d ₂ , d ₄ , d ₆ ; Axial air spacing of the constituent lenses counted sequentially from the object side.

Claims (1)

[Claims] 1. When viewed from the object plane, the first group has a negative focal length and is made up of one or two concave lenses, and the second group has a positive focal length and is made up of two convex lenses. In the constructed optical system, the total length of this optical system is l,
The combined focal length of the entire system is f, the optical system of the first group is f ₁ ,
When the focal length of the second group is f ₂ and the refractive index of the two convex lenses are na and nb, 0.105f≦l≦0.1544f ...(1) 0.9≦｜f ₁ |/f ₂ ≦1.3 ... (2) na≧1.7, nb≧1.7 ...(3) An optical system for optical laser scanning that satisfies the following conditions.