# Patent

## Title
F-theta objective

## Abstract

An F-theta objective is provided that has five individual lenses with a first individual lens as biconcave lens with a focal length, a second individual lens as meniscus with a focal length, a third individual lens as meniscus with a focal length, a fourth individual lens as a biconvex lens with a focal length and a fifth individual lens as a plano-convex lens with a focal length and a total focal length f of the F-theta objective. A ratio between the focal lengths to the total focal length satisfies predetermined conditions.

## Background
This nonprovisional application claims priority under 35 U.S.C. §119(a) to German Patent Application Nos. DE 20 2013 006 369.6, which was filed in Germany on Jul. 16, 2013; DE 20 2013 009 184.3, which was filed in Germany on Oct. 17, 2013, and to DE 20 2014 002 322.0, which was filed in Germany on Mar. 14, 2014, all of which are herein incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an F-theta objective as can be used in a scan apparatus for laser material processing.
2. Description of the Background Art
An F-theta objective focuses a laser beam, which is incident in a manner such that it scans over a scan-angle region +/−θ with respect to the optical axis of the F-theta objective, into a flat image field, wherein, within this scan-angle region, the ratio of scan angle and distance of the point of incidence of the laser beam from the optical axis in the image field follows a linear function. That is to say that a laser beam scanning at a constant angular velocity generates a focal point in the image field, which focal point moves at a constant speed. Here, the size of the focal point should be constant at each location in the image field.
The size of the focal point is determined in dependence on the purpose of the laser material processing, e.g. writing, coating removal or cutting.
Owing to the wavelength-dependent refraction of the laser beam as it passes through the F-theta objective, F-theta objectives are corrected, in order to achieve a high focal point quality, to the wavelength of the processing laser beam used, that is to say the objective is calculated such that it has, for an image field of prespecified size within a permissible temperature tolerance for a prespecified wavelength and a prespecified laser beam diameter, no or only very slight optical aberrations which result in a distinct change in the size of the focal point. In particular for use in laser material processing, F-theta objectives have a large image field and a large total focal length.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide an F-theta objective for high-power lasers with a wavelength of 355 nm and a total focal length of between 90 mm and 110 mm, in particular 100 mm-105 mm, which enables imaging of the laser radiation without retroreflection and should be of a telecentric configuration. It is thus possible, for example, for holes to be inserted perpendicularly into a workpiece and/or to achieve identical diameters in the X- and Y-scanning directions [otherwise ellipse in X or Y]. At the same time, a uniform burr formation is intended to be achieved thereby during material processing. The aim is therefore to obtain a higher energy density with a circular focus.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.



BRIEF DESCRIPTION OF THE DRAWING
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawing which is given by way of illustration only, and thus, is not limitive of the present invention, wherein the sole FIGURE illustrates a geometric optics diagram of an objective according to an embodiment of the invention.



DETAILED DESCRIPTION
An F-theta objective according to an embodiment of the invention has, arranged at a distance upstream of the F-theta objective, an entrance pupil EP which can be arranged in a plane in which a scanner mirror, or if two scanner mirrors are used a substitute plane that is calculated for this, is located, and comprises five individual lenses L1-L5 which are arranged on a common optical axis A. The five individual lenses L1-L5 are configured such that they form a “negative-positive-positive-positive-positive” lens sequence. That is to say that the first individual lens L1 has a negative focal length, whereas the second to fifth lenses L2-L5 have a positive focal length.
The first lens L1 here is a biconcave lens, the second lens L2 is a meniscus, the third lens L3 is also a meniscus, the fourth lens L4 is a biconvex lens, and the fifth lens L5 is a plano-convex lens.
The focal lengths of the five individual lenses should satisfy the following requirement:
The focal length ratio of the focal length f1 of the first lens L1 to the total focal length is f: −1.0<f1/f<−0.4.
The focal length ratio of the focal length f2 of the second lens L2 to the total focal length is f: +3.6<f2/f<+4.2.
The focal length ratio of the focal length f3 of the third lens L3 to the total focal length is f: +2.0<f3/f<+2.8.
The focal length ratio of the focal length f4 of the fourth lens L4 to the total focal length is f: +1.4<f4/f<+1.8.
The focal length ratios of the focal length f5 of the fifth lens L5 to the total focal length is f: +2.7<f5/f<+3.1.
A protective glass SG can be arranged downstream of the five individual lenses L1-L5.
The useful effect achieved by an F-theta objective according to the invention is that with five individual lenses L1-L5 of quartz glass, an F-theta objective with a total focal length of between 90 mm and 110 mm, which is suitable for a high-power laser and is corrected for a wavelength of 355 nm, results. The F-theta objective is configured with its parameters such that retroreflections, which are produced on the optically active faces of the lenses L1-L5 and, if appropriate, the protective glass SG, are not focused onto the optical surfaces of the lenses L1-L5 or onto the mirrors arranged around the entrance pupil EP. The configuration is adapted such that no retroreflections occur in the material of the lenses and no retroreflections occur at the sites of the scanner positions.
The specific structure and the parameters of an exemplary embodiment for such an F-theta objective are described below.
The entrance pupil EP of the F-theta objective is located at a distance d1 in front of the front vertex point of the first lens L1, a biconcave lens with a thickness d2, the front surface of which has a radius r1 and the back surface of which has a radius r2. This first lens L1 is followed, with an air gap d3, by the second lens L2, a positive meniscus having a thickness d4, the front surface of which has a radius r3 and the back surface of which has a radius r4.
This is followed, with an air gap d5, as third lens L3, likewise by a positive meniscus having a thickness d6, the front surface of which has a radius r5 and the back surface of which has a radius r6. This is followed, with an air gap d7, by the fourth lens L4, a biconvex lens having a thickness d8, the front surface of which has a radius r7 and the back surface of which has a radius r8. This is followed, with an air gap d9, by a plano-convex lens having a thickness of d10, the front surface of which has a radius r9 and the back surface of which has a radius r10. In the exemplary embodiment, this is followed, with an air gap d11, by a plane-parallel protective glass SG having a thickness of d12. The image field BE is produced at a distance of d13 from the protective glass SG. Quartz glass with a refractive index ne was selected as the material for all of the individual lenses L1-L5 and the protective glass SG.
The radii of the individual lenses L1-L5, and the thicknesses and distances d thereof are given in the following table 1.














Medium
Radius (mm)

Thickness (mm)
ne




















air

EP
d1
35.0



L1
r1
38.8
d2
3.0
1.46


L1
r2
243.6


air


d3
5.5
1


L2
r3
118.8
d4
15.0
1.46


L2
r4
76.5


air


d5
2.2
1


L3
r5
285.8
d6
20
1.46


L3
r6
86.0


air


d7
0.2


L4
r7
1695.9
d8
25
1.46


L4
r8
90.2


air


d9
0.2


L5
r9
138.0
d10
15
1.46


L5
r10
□


air


d11
5


SG
r11
□


SG
r12
□









The distances and thicknesses are all designated d and are numbered according to their sequence along the optical axis A of the F-theta objective in the beam passage direction and indicated as d1-d13 in FIG. 1.
The terms “front” and “back” surface can refer to the beam passage direction. The radii r1-r12 can be associated clearly with reference to the relevant lenses L1-L5 and the protective glass SG and are therefore not indicated in FIG. 1 for the sake of clarity.
In dependence on the material-dependent refractive indices ne of the individual lenses L1-L5, the thicknesses d2, d4, d6, d8, d10 of the individual lenses L1-L5 determine, in connection with the radii of curvature r1-r10 of the individual lenses, in each case the focal lengths f1-f5 of the individual lenses L1-L5. The focal lengths f1-f5, which in each case describe the distance of a focal point from a main plane of an individual lens L1-L5, are not shown in FIG. 1, because the main planes of the individual lenses L1-L5 are not indicated for the sake of clarity. The total focal length f, which describes the distance of the image field BE from a substitute main plane for the F-theta objective, is likewise not indicated.
For an F-theta objective with the parameters indicated here, the result is a focal length f1 with respect to the total focal length f of −0.7 for the first lens L1, a focal length f2 with respect to the total focal length f of +3.9 for the second lens L2, a focal length f3 with respect to the total focal length f of +2.4 for the third lens L3, a focal length f4 with respect to the total focal length f of +1.6 for the fourth lens L4 and a focal length f5 with respect to the total focal length f of +2.9 for the fifth lens L5.
The result of the arrangement of the lenses L1-L5 successively in conjunction with their air gaps d3, d5, d7, d9 is the total focal length f of 103 mm. The F-theta objective is corrected for a wavelength of 355 nm.
The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.

## Claims

1. An F-theta objective, comprising five individual lenses sequentially arranged, comprising:
a first individual lens L1 as a biconcave lens with a focal length f1,
a second individual lens L2 as a meniscus with a focal length f2;
a third individual lens L3 as a meniscus L3 with a focal length f3;
a fourth individual lens L4 as a biconvex lens with a focal length f4; and
a fifth individual lens L5 as a plano-convex lens with a focal length f5,
wherein the F-theta objective has a total focal length f, and
wherein the ratio between the focal lengths f1-f5 to the total focal length f satisfies the following conditions:


−1.0<f1/f<−0.4



+3.6<f2/f<+4.2



+2.0<f3/f<+2.8



+1.4<f4/f<+1.8



+2.7<f5/f<+3.1.

2. The F-theta objective as claimed in claim 1, wherein f1/f=−0.7, f2/f=+3.9, f3/f=+2.4, f4/f=+1.6, f5/f=+2.9.

3. The F-theta objective as claimed in claim 2, wherein an entrance pupil EP of the F-theta objective is located in front of the first individual lens L1 at a distance d1 of 35.0 mm.

4. The F-theta objective as claimed in claim 1, wherein a scanned beam is aligned telecentrically with respect to an optical axis.