DE102019108084A1 - zoom optics - Google Patents

Abstract

The invention relates to an optical system for laser radiation and provides an optical system for imaging laser radiation with an adjustable magnification, comprising four lenses, which are arranged in three lens groups, wherein the arranged in the beam direction of the laser radiation first lens group comprises a first lens with a positive focal length, the second lens group comprises second and third negative focal length lenses, and the third lens group comprises a fourth positive focal length lens, and the second lens has a plano-concave shape to a meniscus, and the third lens is biconcave.

Description

Field of the invention

The invention relates to an optic for laser radiation.

BACKGROUND OF THE INVENTION

High power lasers are used for material processing. The emerging from a light guide laser beam is collimated in a mounted at the end of the laser head by means of laser optics with corresponding lenses and then focused.

With zoom collimators, reflections at high angles of incidence on optical surfaces lead to energy inputs on opto-mechanically moving parts. These parts must be stored friction-free due to dynamics and contamination. As a result, they are usually automatically thermally decoupled well, which then the housing of the optics can not be used for cooling and dissipation of heat produced.

Various measures for reducing the reflection of laser radiation are known from the prior art. This is how the US patent describes US Pat. No. 7,450,301 B2 an EUV lithography system, in which the different angles of incidence then result in uneven illumination of the exit pupil due to varying reflectivities over a mirror surface. This leads to losses in the resolution. This document proposes, as a technical solution, to provide the mirror with a coating whose thickness varies over the mirror surface, so that locally each location has an adapted coating design depending on the prevailing angle of incidence of the radiation. A disadvantage of this solution is that the coating process is expensive and expensive.

From the WO 2009/024164 A For example, a lithography system is known in which the coating thickness of the mirror has been adapted to the angle of incidence of the steel, which then also requires "free-form coatings" for free-form mirrors. In particular, the reflectivity is locally reduced in order to achieve a uniform illumination of the exit pupil at the expense of the transmission. However, this technical solution can not be transferred to applications using high laser powers.

The published Chinese utility model number CN 206 696 535 U refers to a laser with two wavelengths. The structure of the device comprises five lenses, the first lens being a biconvex positive lens, the second lens being a negative concave lens, the third and fourth lenses each being a negative biconcave lens, and the fifth lens being a plano-convex positive lens. The utility model discloses a dual wave band laser capable of providing two wavelengths simultaneously, 532nm and 1064nm.

The published German patent application DE 10 2011 117607 A1 discloses an optical system for imaging laser radiation of adjustable magnification, comprising a collimating lens and a focusing lens disposed after the collimating lens for imaging the collimated beam into a beam focus, the collimating lens being composed of three lens groups, the first Lens group has a positive focal length, the second lens group has a negative focal length and the third lens group has a positive focal length, wherein the collimating lens of a laser beam source is disposed downstream with a beam waist, and arranged the second lens group of the collimating lens axially displaceable within an adjustment e is, wherein the second lens group has within its adjustment range a position in which the divergence or convergence angle of the beam before and after this lens group is the same, ie the magnification of this second lens group pe is equal to -1 in this position, where a12 = 2f2 + f1 / (1-f1 / s01) for the distance a12 between the first and second lens groups and this must lie within the distance range s12 defined by the adjustment range e and the ratio of Focal length f1 of the first lens group to the distance s01 of Beam source to the object-side main plane of the first lens group is between 1/4 and 3/4, so 0.25 ≤ f1 / s01 ≤ 0.75 and the focal length f2 of the second, adjustable lens group in a range 0>f2> [(e / 2 ) - f1 / (1 - f1 / s01)] / 2, where f3> -2f2 + e / 2 for the relationship between the focal length of the second lens group f2 and the focal length of the third lens group f3, where e = Adjustment range of the second lens group of the collimating objective, a12 = distance between first and second lens group, f1 = focal length first lens group, f2 = focal length second lens group, f3 = focal length third lens group, s01 = distance of the beam source to the object side main plane of the first lens group and s12 = distance between the image-side main plane of the first lens group and the object-side main plane of the second lens group.

The published German patent application DE 10 2016 005376 A1 relates to an imaging optics for material processing with laser radiation, with a collimator optics for collimating a divergent working laser beam and a focusing optics for focusing a working laser beam on a workpiece to be machined, the collimator optics a first movable lens or lens group with positive focal length for imaging a working laser beam source in a virtual intermediate focus and a second movable lens or lens group having a negative focal length for imaging the virtual intermediate focus to infinity. Furthermore, a laser processing head is provided with a housing through which a working laser beam can be passed. An imaging optics according to the invention serves to generate a working focus.

Another solution to improve the performance of zoom optics when using high laser powers would be to cool the optical elements, such as lens frames. However, this is structurally very complex and also requires a higher effort in the maintenance of the optics.

Object of the present invention is therefore to provide a solution that increases the performance of zoom optics by reduced heat input and at the same time ensures a small space and a minimum number of optical elements.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides an optical system for imaging laser radiation with adjustable magnification comprising a source for generating a laser radiation with a power of more than 4 kW, and further four lenses available, wherein the lenses are arranged in three lens groups, wherein in the beam direction the laser radiation arranged first lens group comprises a first lens with positive focal length, the second lens group comprises a second and a third lens with negative focal length and the third lens group at least a fourth lens with positive focal length and second lens has a plano-concave shape to a meniscus and the third lens is biconcave.

In a further embodiment, it is provided that the radii of curvature of the second lens from side 1 to side 2 in the beam direction are between 20 mm / 10 mm to -100 mm / 17 mm.

Furthermore, the radii of curvature of the third lens from side 1 to side 2 in the beam direction can be between -20 mm / 20 mm and -50 mm / 17 mm.

It can also be provided according to the invention that no area of one of the first to fourth lenses has a local angle of incidence of> 25 ° at maximum focal length of the optical system.

For the second and third lenses, a maximum local focal length of the optical system can be a local angle of incidence> 20 °.

According to the invention, each lens group can have at least one aspherical surface each.

For the optical system, it can also be provided that the second lens has an aspherical surface on the first side.

In a further embodiment of the invention, in the second lens group, one lens may be aspherical and the other lens may be spherical.

The maximum length of the virtual fiber tip to the last collimating lens can be 195 mm.

The distance from the virtual fiber tip to the surface of the first lens can be at least 50 mm.

According to the invention, it is provided that the numerical aperture is greater than 0.1. Furthermore, the numerical aperture can be 0.125.

The optical system according to one of the preceding claims, wherein at least one of the lenses of the second group consists of synthetic quartz glass.

Another object of the invention relates to the use of an optical system as described above for imaging of laser radiation with an adjustable magnification.

It is also provided according to the invention a use of the optical system described above in combination with a semiconductor solid-state laser, a disc laser or a fiber laser.

Other aspects, features, and advantages of the present invention will become more readily apparent from the following detailed description, which illustrates simply preferred embodiments and implementations. The present invention may be embodied in other and different embodiments, and its various details may be modified in various, obvious aspects without departing from the spirit and scope of the present invention. Accordingly, the drawings and descriptions are to be considered illustrative and not restrictive. Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention

list of figures

• 1 Reflektivität in Abhängigkeit vom Einfallswinkel bei einer Wellenlänge für unterschiedlich polarisiertes Licht.
• 2 Vergleich der spektralen Reflektivität einer Beschichtung bei unterschiedlichen Einfallswinkeln
• 3 Schematische Darstellung von vier Linsen in drei Gruppen

The invention is illustrated in more detail below with reference to figures. It is obvious to the person skilled in the art that these are only possible embodiments, without the invention being restricted to the embodiments shown. It shows:

• 1 Reflectivity as a function of the angle of incidence at a wavelength for differently polarized light.
• 2 Comparison of the spectral reflectivity of a coating at different angles of incidence
• 3 Schematic representation of four lenses in three groups

DETAILED DESCRIPTION OF THE INVENTION

The object of the invention is achieved by the features of the independent claims. The present invention solves the problem of always keeping local angles of incidence at the edge of a lens below defined critical angles. This reduces the heating of the entire look in the interior.

Even a relatively small change in magnification can lead to a significant increase in the temperature in the interior of the optics. This applies in particular to lasers with a capacity of more than 4 kW. Investigations of the inventors of the present application have shown that this is related to the curvatures of the lenses, so that in peripheral areas then the angles of incidence at a small magnification massively increase and the concomitant increase in reflectivity is reflected in a temperature increase. The low-friction mounting of movable components leads to their thermal decoupling of the components against the housing, so that the housing can not dissipate the heat and thereby contribute to cooling.

For multi-kilowatt laser power fiber optic cables, quartz glass blocks are often spliced to the end of the fiber to reduce the power density at the interface with air. Due to the refraction at this interface, the cone of light which emerges from the glass fiber appears to come from a position shifted to the real position. This image of the real fiber tip generated by the quartz block is also called a virtual fiber tip, and is closer to the optic than the real fiber tip due to the refraction. If no quartz block is used to terminate the fiber, then, according to this definition, the virtual fiber tip is identical to the real fiber tip.

1 shows the reflectivity as a function of the angle of incidence at one wavelength for differently polarized light. The reflectivity increases with light of a wavelength with increasing angle of incidence. This relationship applies regardless of the polarization of the light of a wavelength, which split at high wavelengths, the polarizations. The reflectivity is thus absolutely worse at very large angles of incidence and can not be easily optimized by shifting the central wavelength of the coating.

2 shows the wavelength dependence of the reflectivity of a typical coating at different angles of incidence. It can be seen that both the spectral dependence of the reflectivity and the absolute reflectivity at an incident angle of 0 ° and at an incident angle of 20 ° are very similar. In this range of angles of incidence, a good antireflection effect can be achieved with a simply constructed and thus low-absorption coating by adapting the design wavelength of the coating. At larger angles of incidence, on the other hand, the reflectivities are worse in absolute terms over the entire wavelength range.

In the case of compact zoom collimators, very high local incidence angles of the laser light on the lenses can occur, in particular with large focal length settings. These occur mainly in the edge region of the lenses, and above all on lenses with negative focal lengths. According to the invention it is therefore proposed to limit the angles of incidence on the lenses in the edge region. For this purpose, according to the invention, the groups with negative focal length are split into two lenses, and in addition the curvatures on the negative lenses are selected so that minimal angles of incidence are achieved.

It is therefore proposed according to the invention a design with 4 lenses in 3 modules, wherein the group 1 have a lens with positive refraction, group 2 two lenses with negative refraction and group 3 a lens with positive refraction.

According to the present invention, it has proven to be structurally advantageous to firmly install the two central negative lenses and to be able to move the two outer positive lenses by means of drives.

3 schematically shows the arrangement and shape of the lenses of the three modules. The lenses are numbered from left to right with L1, L2, L3 and L4 in the beam direction. In this case, L2 and L3 form the lenses with negative refractive power of the middle group. It is intended for L2 a biconcave shape to a meniscus, with a convex shape in the direction of the fiber. L3 is biconcave according to the invention.

$$\text{L}\left(\text{S1}/\text{S2}\right)=-20/20\left(\text{exact bikonkav}\right)\text{ bis}-50/17$$

The curvatures for L2 and L3 can be given as follows, where S1 and S2 indicate the sides of the lenses in the beam direction and the values indicate the respective radius of curvature in mm: $$\text{L2}\left(\text{S1}/\text{S2}\right)=20/10\left(\text{meniscus}\right)\text{up to}-100/17\left(\text{almost plankonkav}\right)$$

$$\text{L}\left(\text{S1}/\text{S2}\right)=-20/20\left(\text{exact biconcave}\right)\text{to}-50/17$$

• - Keine Fläche hat bei maximaler Brennweite des Zooms (=maximale Ausleuchtung der Linsen) einen lokalen Einfallswinkel > 25°;
• - Speziell für die Linsen der Negativgruppe soll der lokalen Einfallswinkel nicht > 20° sein;
• - Beugungsbegrenzte Abbildung in allen Positionen des Zoomkollimators möglich;
• - Für die Bauform von L2 nahe plankonkav kann bei konkaver Krümmungen der ersten Fläche auch eine asphärische Fläche verwendet werden

The bounds for the bends were calculated by defining the first area of the L2 and then experimentally optimizing the remaining bends in the system. This was done under the following conditions:

• - No area has a local angle of incidence> 25 ° at maximum focal length of the zoom (= maximum illumination of the lenses);
• - Especially for the lenses of the negative group, the local angle of incidence should not be> 20 °;
• - Diffraction-limited imaging possible in all positions of the zoom collimator;
• - For the design of L2 near plankonkav can be used in concave curvatures of the first surface and an aspherical surface

A local angle of incidence of less than 20 ° has proved to be advantageous because it avoids back reflections from the negative group to the first positive lens and its socket. The first positive lens is indeed moved by means of a drive and is therefore thermally insulated due to their storage and thus difficult to cool.

This is not the case for the positive lenses and the back-reflexes of the positive groups primarily affect stationary optical elements: L1 meets the protective glass or the diaphragm, L4 meets L2 / 3. But the stationary, so immobile elements can be cooled well, therefore, with these lenses, a higher angle of incidence without disadvantages is possible.

It is also provided according to the invention that each lens group each has an aspherical surface. This would be preferred in the negative group L2 / S1, as it is heavier, a concave aspheric surface This may even be impossible with the high curvatures, as the machining tool at the tip can not become narrow enough.

In the split negative group, one lens may be aspherical for cost reasons, while the other lens is spherical.

Investigations have shown that the aforementioned parameters for the construction of the lens groups with a maximum length of 195 mm from the virtual fiber tip to the last collimating lens are advantageous. The distance of the virtual fiber tip to the surface of L1 is preferably> 50 mm. With the structure shown, if the third positive lens group is movable along the optical axis, the position of the main plane of the optic can also be shifted to the virtual fiber tip. Together with a focusing optics, a change of the z position of the focus relative to the focusing lens is then possible in addition to the change in magnification. In this case, studies have shown that the aforementioned parameters for the construction of the lens groups with a maximum length of 195 mm from the fiber tip to the last Kollimationslinse are advantageous to achieve minimal heat input at maximum adjustment range in magnification and z-position adjustment. In the case of a limitation to the pure zoom functionality, the investigations have shown that the parameters mentioned are advantageous with a maximum length of 170 mm from the virtual fiber tip to the last collimating lens.

According to the invention, a numerical aperture greater than 0.1 is provided, which may also be equal to 0.125. Basically, it is important to exclude heavily curved lenses, as they are no longer illuminated to the edge.

This makes it possible to produce a coating without a complex lateral layer thickness profile, which nevertheless has a low reflectivity over the entire surface. It is advantageous that the present invention manages with the simplest possible structure of the coatings, thereby reducing costs and ensure low Absorptions.

With regard to the published Chinese utility model number CN 206 696 535 U It should be noted that the device shown therein is not usable for high laser powers like the present invention. The person skilled in the art would therefore not fall back on the teaching of this document in the context of high laser powers, since materials for the lenses of the lenses are used which can not be used in high-energy lasers.

The object of the present invention is to reduce back-reflexes and warm-up. For this reason, the present application is also based on the condition that the angles of incidence remain minimized. For this purpose, it is explicitly accepted that the imaging performance can be more sensitive to production-related tolerances of the lenses. Only if one disregards this condition, which represents a significant contribution to the invention, can a solution result, for example, from the documents quoted above in the prior art, which provide manufacturing technology more stable imaging performance, but just for that more energy is reflected at the edge , Exactly this problem solves the present invention. None of the trembled documents are suitable for contributing to the solution of the problem of the present invention, as completely different technical problems are addressed.

The foregoing description of the preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiment has been chosen and described to illustrate the principles of the invention and its practical application to enable those skilled in the art to use the invention in various forms suitable for the particular intended use. It is intended that the scope of the invention be defined by the appended claims and their equivalents. The entirety of each of the above documents is incorporated herein by reference.

LIST OF REFERENCE NUMBERS

1

L1

5

L2

10

L3

15

L4

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 7450301 B2 [0004]
• WO 2009/024164 A [0005]
• CN 206696535 U [0006, 0045]
• DE 102011117607 A1 [0007]
• DE 102016005376 A1 [0008]

Claims (15)

An optical system for imaging laser radiation with an adjustable magnification, comprising a source for generating a laser radiation with a power of more than 4 kW, and further four lenses, which are arranged in three lens groups, wherein the arranged in the beam direction of the laser radiation first lens group a first The lens has a positive focal length lens, the second lens group includes second and third negative focal length lenses, and the third lens group includes at least a fourth positive focal length lens and the second lens has a plano-concave shape to a meniscus and the third lens is biconcave. The optical system after Claim 1 , wherein the radii of curvature of the second lens from side 1 to side 2 in the beam direction are between 20 mm / 10 mm to -100 mm / 17 mm. The optical system according to one of Claims 1 or 2 in which the radii of curvature of the third lens from side 1 to side 2 in the beam direction are between -20 mm / 20 mm and -50 mm / 17 mm. The optical system according to one of Claims 1 to 3 , wherein no area of one of the first to fourth lens at maximum focal length of the optical system has a local angle of incidence of> 25 °. The optical system according to one of Claims 1 to 4 , wherein the second and third lens at maximum focal length of the optical system have a local angle of incidence <20 °. The optical system according to one of Claims 1 to 5 wherein each lens group has at least one aspheric surface each. The optical system after Claim 6 wherein the second lens has an aspherical surface on the first side. The optical system according to one of Claims 6 or 7 In the second lens group, one lens is aspherical and the other lens is spherical. The optical system according to one of Claims 1 to 8th , wherein the maximum length of virtual fiber tip to the last collimating lens is 195 mm. The optical system according to one of Claims 1 to 9 , wherein the distance from the virtual fiber tip to the surface of the first lens is a maximum of 50 mm. The optical system according to one of Claims 1 to 10 , where the numerical aperture is greater than 0.1. The optical system according to one of Claims 1 to 11 with a numerical aperture of 0.125. The optical system according to one of the preceding claims, wherein at least one of the lenses of the second group consists of synthetic quartz glass. A use of an optical system according to one of Claims 1 to 13 for imaging laser radiation with adjustable magnification A use of an optical system according to one of Claims 1 to 13 in combination with a semiconductor solid-state laser, a disk laser or a fiber laser.

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