WO2009019006A2

WO2009019006A2 - Device and method for reducing speckle in the field of laser applications

Info

Publication number: WO2009019006A2
Application number: PCT/EP2008/006472
Authority: WO
Inventors: Dieter Grebner; Rico Fuchs; Alexander Kalies
Original assignee: Carl Zeiss Meditec Ag
Priority date: 2007-08-06
Filing date: 2008-08-06
Publication date: 2009-02-12
Also published as: WO2009019006A3; DE102008035898A1

Abstract

The present invention relates to a device for improving the laser beam quality for machining applications, comprising a laser (1) having an asymmetrical beam profile, comprising a first, short beam axis (20), a second, long beam axis (30), wherein the laser (1) comprises at least one optical element (10) and/or one resonator mirror (11) having a cylindrical mirror surface (13) for reducing spatial coherence and/or increasing the divergence in the short beam axis.

Description

Apparatus and method for reducing speckling in the field of

laser applications

The invention relates to an apparatus and a method in the field of laser applications for reducing speckle.

When using a laser beam in lithography, micromachining and, in particular, refractive surgery, an occurrence of speckles in the

Processing level undesirable. The speckle pattern of the laser beam is formed on the working plane and leads to inaccuracies. To avoid speckles and the associated production of a smooth Strkahlprofils following solutions are known.

"The Laser Guidbook" (Author: Jeff Hecht, Copy Rights: McGraw-HiIl Ins., ISBN: 0071359672) describes a use of spherical HR stable resonators to reduce coherence, increase divergences and thus avoid speckles Another disadvantage is the lack of adjustment of the beam axes to each other and the associated different divergence and coherence in the axes. In addition, pulse shape and pulse duration can change in this solution.

WO 2004021529 A1 describes a resonator which uses spherical and aspherical hemispherical mirrors to equalize the divergence and coherence of the beam. Furthermore, it is described herein that diffusers mounted on or between the mirrors and acting only in the critical axis can solve the problem.

In WO 1996016455 Al unstable resonators are described with convex cylindrical mirror surface which change the divergence and thus coherence axis-dependent. The coherence is increased and the divergence is reduced. Continue described the possibility of achieving this effect by lying in the resonator cylindrical lenses.

In US 5946337 an unstable resonator with diverging (convex) reflective cylindrical mirror surface and dispersive wavelength selector is described to obtain a beam with low bandwidth and yet high energy yield. Low bandwidth leads to an increase in temporal coherence.

In DE 4225781 an unstable resonator with cylindrical mirrors is described with which a Divergenzanpassung can be done. The coherence is increased and the divergence is reduced.

The known devices are not able to reduce the speckle occurring in the working plane, without having to accept energy losses, pulse duration changes or pulse shape changes.

The object of the invention is to reduce the speckle effect in the working plane and to provide a smooth beam profile.

The present invention solves this problem by the device according to the invention and the method according to the independent claims. Further advantageous embodiments of the invention are given in the dependent claims.

According to a first aspect of the present invention there is provided a laser beam quality improvement apparatus for laser applications, comprising a laser (1) having an asymmetric beam profile and a first, short beam axis (20) and a second, long beam axis (30) and a beam propagation axis ( 35) and one

Resonator (60), wherein the laser (1) at least one optical element (10) and a resonator mirror (11) with cylindrical mirror surface (13), for reducing the spatial coherence and / or increasing the divergence, in particular in the short, ie generally coherent beam axis comprises.

Laser is preferably referred to as sources which emit coherent, hardly divergent electromagnetic radiation.

Gas lasers, particularly preferably discharge lasers of any kind, are preferably used according to the invention. The gas laser most preferably used is the excimer laser.

Laser applications are preferably ablative or exposing laser application. This is preferably understood to mean applications which preferably interact with material in a working plane by means of pulsed jet delivery, more preferably removing material. This is preferably done in one point (spot). In addition to punctiformly focused beams (spots), large-area, homogenized beams in the working plane are also preferably understood. The resulting profile in the working plane is called the removal profile.

An Abtragsprofil can preferably by the known methods of Spotscannings particularly preferred by

Variable aperture processes, most preferably also produced by mask exposure techniques.

As removal of material is preferably called the evaporation or Ablatieren of material in this point or

Spot. By the local juxtaposition of preferably punctual interventions, is preferably a machined

Area created. Preferably, this forms

Cross-sectional profile of the spot at the point of engagement. According to the invention, preference is given to a Gaussian, smooth

Cross-sectional profile of the spot. It can also plateau-shaped

Cross-sectional profiles of the spot, depending on the application, preferred become. Laser lithography, most preferably laser micromachining, most preferably refractive surgery or other type of laser treatment are examples of ablative laser applications.

The laser beam quality is defined in the laser spot in this invention. The laser spot is the point where the collimated laser beam hits the working plane and exerts its effect, i. Material removes. The working plane is understood to be the plane in which the laser beam interacts with matter during the laser application. Is in the profile section through the spot the profile of the laser beam, equal to the profile of the theoretically desired beam, the Laserstrahlσualität is good = According to the invention, a Gaussian profile section in the spot is preferred. Top hat or other distributions are also preferred.

The long beam axis is preferably the direction from electrode to electrode. Along this axis, the discharge preferably proceeds in the case of discharge-pumped gas lasers. The beam profile in this direction is preferably top hat-shaped. The number of transverse modes in this direction is large depending on the electrode gap. The coherence length is correspondingly low and usually uncritical with regard to the interference effect on microoptical elements (for example beam guides). The divergence is correspondingly large.

The short beam axis is at right angles to the long beam axis. In accordance with the propagation of the discharge, in the case of discharge-pumped lasers, the beam profile in this direction is preferably Gaussian. The excitation concentrates on the center of the discharge according to the distribution of the discharge excitation Gauss. The formation of transversal modes in this direction is correspondingly limited and their number is lower. This results in a greater coherence length in this direction, which is has a critical effect on the interference effect when using micro-optical elements. The divergence is correspondingly lower in this direction.

Short and long beam axes are preferably formed in their properties in cooperation with micro-optical components in the working plane. The interference effect and thus speckle formation preferably occurs in the critical short beam axis. The short beam axis is therefore usually the more coherent beam axis when compared to the long beam axis.

Short and long beam axes are also preferably used for orientation on the working plane. Particularly preferably, they span the working plane as coordinate axes. As a working plane, for example, the convex cornea is called.

The beam propagation axis is preferably the axis along which the laser beam propagates. The

Beam propagation axis is preferably perpendicular to the short and long beam axis.

The resonator of the laser is an optical resonator, which serves to reflect emitted light as often as possible back and forth. Due to interference, a standing wave then forms in the resonator when the optical path length of the resonator is a multiple of the wavelength of the incident light. According to the invention, stable resonators are preferably used.

Particularly preferred in this invention apply laser with asymmetric beam profile application. The beam profile preferably also forms in the removal profile, depending on the material properties of the material being processed.

According to the definition of long and short beam axis, the beam profile is considered to be asymmetric if the Beam shape (energy-power distribution over the surface) is different in both directions. This results in preferably different properties of the beam, such as coherence and divergence, along these beam directions. A beam profile is asymmetric if beam shape, coherence and the resulting divergence preferably differ in the beam axes of the machining planes.

An optical element is preferably an element which preferably reflects at least one electromagnetic beam, particularly preferably breaks, most preferably influences the phase or amplitude of the beam.

By imaging optics is preferably understood optics, - with which the beam, preferably in the course of a

Beam shaping is imaged on the processing plane to preferably a predetermined shape and size of the intensity distribution to obtain.

Spatial coherence refers to the ability of a light source to produce stationary interference phenomena at two different locations, but at the same time. It is the statement of the correlation of the phase of the signal at spatially separated points at the same time.

Time coherence refers to the ability of a light source to still produce stationary interference phenomena at a fixed location with light that has left the light source at two different times. It is the statement of the correlation of the phase of the signal at different times in the same place.

Divergence is the property of a beam that diverges from a center perpendicular to the propagation direction. The angle of divergence is the angle that passes through the pair of straight lines which asymptotically represents the envelope of the increasing beam dimension.

The resonator mirror is preferably one of the two outermost mirrors in a resonator, which enclose the active medium (gain medium) with mutually directed mirror surfaces. This resonator mirror is preferably an HR mirror on the non-outcoupling side, particularly preferably an OC mirror on the outcoupling side of the resonator. The highly reflective mirror preferably has the highest possible degree of reflection.

The resonator mirrors are preferably parallel to the mirror surfaces. In various embodiments, the invention preferably has the following configurations (the form of the resonator mirror on the outcoupling side being called first, and then the shape of the resonator mirror on the noncoupling side): concave-plane, plano-concave, concave-concave.

The mirror surface of the concave resonator mirror is preferably cylindrical. The concave mirror surface is preferably a segment of a cylinder. The radius of curvature of the cylindrical inner surface is preferably 1 mm to 1000 m, more preferably 50 mm to 200 m, most preferably 10 m to 10 m. The best possible adjusted radius of curvature is preferably dependent on the resonator length, the coherence and the size of the individual elements of a microoptical component.

Optical elements are also preferably diffractive elements such as gratings or refractive elements such as lenses, which may affect spatial coherence. Particularly preferably, step structures can shift the beams temporally relative to one another and thus reduce the temporal coherence. Thus, the interference effect on micro-optical elements can preferably be reduced. In a further preferred embodiment, the cylindrical mirror surface (13) of the concave resonator mirror (11) is arranged so that the curved axis of the mirror is perpendicular to the long beam axis (30) and / or perpendicular to the beam propagation axis (35) of the laser (1) ,

The curved axis of the mirror is the center axis of the imaginary cylinder.

By the preferred arrangement of this direction of curvature perpendicular to the long beam axis, i. along the short, usually more coherent beam axis and perpendicular to the beam propagation axis, the coherence of the coherent short axis is reduced and the divergence in that direction is increased. The energy yield remains the same, because despite the focusing by the Gaussian excitation distribution in the short axis sufficient emission is produced. For the same reason, the pulse duration and shape are retained.

In a preferably stable resonator, the divergence of the beam outside the resonator preferably increases with increasing curvature of the concave mirror. This is particularly preferred in the case of cylindrical mirrors in the direction of curvature. The increase in divergence is equivalent to a reduction in spatial coherence.

In a preferred embodiment, the laser (1) is an excimer laser.

The excimer laser is an ultraviolet gas laser whose resonator is filled with gas. The excimer laser is preferably used according to the invention since it provides an asymmetric beam profile. The gases used in the resonator are preferably Fe 2 or Xe or ArF or KrF or XeBr or XeCl or XeF. Preferably, the excimer laser provides high UV pulse energies.

In a preferred embodiment, the optical element (10) is a homogenizer (42) and / or an integrator and / or a beam shaper.

A homogenizer (beam homogenizer) is preferably an optical element that converts an incident intensity distribution into a changed uniform intensity distribution.

A beam shaper (beam shaper, beam integrator) is preferably a homogenizer, with which an incident intensity distribution is converted into an intensity distribution with a laterally predetermined shape.

In a further preferred embodiment, the optical element (10) is an imaging optics (44).

An imaging optics is preferably at least one optical lens, more preferably parallel in-line lenses, which image the laser beam in one point.

The invention may preferably be with a micro-optical

Element, particularly preferably also be used with an enlarging or reducing acting imaging optics. Most preferably, a refractively effective beam shaper and a telescope are used to reduce image size.

In a further preferred embodiment, the optical element (10) is a microoptical element (40).

A micro-optical element is an element which preferably lies outside the laser apparatus, particularly preferably behind the exit opening of the laser, particularly preferably in the axis of the laser beam. On the properties of the Micro-optical element is preferably aligned, the exiting laser beam. The adjustment of the laser beam preferably reduces the speckle effect which would occur without adaptation of the laser beam to the microoptical element. By adapting the laser beam to the optical properties of the micro-optical elements, the spatial coherence is preferably reduced and the divergence of a preferred excimer laser beam in the short beam direction is increased while the pulse properties (pulse energy, pulse duration, pulse shape) remain the same. As a result, the disturbing speckle effect in the working plane is preferably reduced.

Micro-optical elements are preferably optical elements. whose geometrical dimensions are only a few orders of magnitude above the wavelength of the light transmitted through them. Due to these size ratios, the wave characteristic of the light strongly comes to the fore.

In a further preferred embodiment, at least one micro-optical element 40 is a beam shaper 41 or a beam homogenizer 42 or a beam integrator 43 or an imaging optics 44.

Preferably, the micro-optical element is a beamshaper to preferably form the radiation distribution. As a result, the setting of erosive laser beams is preferably possible.

The micro-optical element is preferably one

Beam homogenizer with which irregularities in the laser beam profile are compensated and preferably a uniform beam is formed.

Preferably, the micro-optical element is a beam integrator to preferably produce relatively flat intensity profiles of the laser beam. Micro-optical devices (eg, beam shaper or homogenizer or integrator) preferably have the following specifications.

The micro-optical component is preferably a refractive-effective micro-optical element. The microlenses have a preferred diameter of 0, lμm to 2mm, more preferably a diameter of lμm to lmm and most preferably a diameter in the range of lOμm to 600μm.

The microoptical component is furthermore preferably a diffractive microoptical element with a lattice spacing of 0.1 μm to 1 mm, particularly preferably 1 μm to 500 μm, and most preferably in a range of 2 μm to 200 μm.

The device of claims 1 to 5, wherein the resonator (60) is an anamorphic stable resonator.

Preferably, the resonator is an anamorphic stable resonator. An "anamorphic stable resonator" is a stable resonator with beam axis differential effects in terms of coherence and divergence, ie, axis-dependent optimization of coherence and thus reduction of speckle in the critical, short beam axis.

The object of the invention is further achieved by a method for providing a smooth beam profile, ie by reducing the speckle in the working plane, wherein a laser beam with asymmetric beam profile comprising a first, short beam axis (20) and a second, long beam axis (30) provided and the spatial coherence is reduced and / or the divergence of the short beam axis of the laser beam is increased. Preferably, laser beams are optimized by this method so that they form a smooth beam profile. As a result, a precise material removal is preferably made possible during refractive surgery.

A smooth beam profile preferably has no "outliers" in the profile section of the laser spot, but preferably a slight deviation in shape and roughness with respect to the desired beam profile The profile section is most preferably preferably Gaussian, and profile shapes such as top hat are also preferred ,

Smooth beam profiles are particularly preferred in refractive corneal surgery. Preferably, smooth steel profiles are used in lithography or micromachining.

Preferably, lasers are used in this invention, which are preferably pumped by two opposing electrodes and / or form an asymmetric beam profile.

It prefers to form on the short beam axis of the laser greater coherence and smaller divergence than on the long beam axis.

The long beam axis is the distance from electrode to electrode, the short beam axis is at right angles to the long beam axis.

The reduction of the spatial coherence and / or increase in the divergence of the short beam axis of the laser beam is preferably generated by a concave cylindrical resonator mirror. As a result, the laser beams of the short beam axis are preferably collimated. Due to the preferred Gaussian excitation distribution in this axis is sufficient Emission and only a slight reduction in energy yield.

The reduction in spatial coherence and / or increase in divergence is due to the increase in the number of transverse modes due to the changed resonator arrangement.

In the working plane, the focused laser beams preferably strike one point. This point is called a spot. The spot is the site of intervention, for example during an operation. Here, material is preferably removed.

In a further preferred embodiment, a method is provided, wherein the laser beams with an imaging optical system (44) is imaged on a working plane (45), an examination of the existing speckle pattern is carried out, an adaptation of the radii of curvature of the resonator mirror (11) takes place, the spatial Coherence and / or increase in the divergence of the short beam axis (20) of the laser beam is further reduced, rechecking of the speckle pattern is performed.

Imaging optics is preferably understood to mean an optic with which the beam, preferably in the course of beam shaping, is imaged onto the working plane in order to preferably obtain a predetermined shape and size of the intensity distribution.

On the working plane, the beam is preferably imaged in the focus. The test surface is a surface that allows the beam profile to be examined. Preferably, from the generated spot on the test surface, a profile section through the spot can preferably be represented visually, particularly preferably calculated. Preferably, a beam observation system is positioned in the working plane. The examination of the incident laser beam in the working plane and thus the analysis of the speckle pattern can preferably be accomplished automatically with known methods and systems.

Preferably, the method of moving slit is used. In this case, a slit diaphragm with the smallest possible slit width (with respect to the beam size) in the working plane is moved through the laser beam step by step, and the energy measured after the iris with an energy detector. The aperture positioning can preferably be ensured via automatic translation stages with stepper motor. The energy is thus split into small location-dependent parts, and one obtains the spatial fluence distribution of the

Laser beam, the beam profile, as well as the speckle pattern. The beam profile with recognizable speckle pattern is preferably stored in digital form.

The method of beam testing and / or speckle pattern testing by means of a beam camera is particularly preferably used. In this method, preferably the beam is e.g. via a beam splitter in a suitable direction coupled to the beam camera. Depending on the laser wavelength it may be necessary to make a frequency conversion to make the beam visible to the camera chip. For a 193nm laser beam is preferred for a

Fluorescence disc used. The image of the laser beam must lie in the working plane. The fluorescent light is imaged by a lens on the camera chip (usually CCD or CMOS) and can be stored digitally.

The digitized beam profiles and / or speckle patterns of both preferred measurement methods are preferably mathematically analyzed. In the beam profile, the roughness is preferably determined as the difference of a fit function with the measured data. The roughness is then preferably a measure of the strength of the speckle effect and can preferably be reduced iteratively in the process according to the invention.

Preferably, the beam is thus visually inspected for speckle. By the beam observation system, preferably also checking other laser parameters such as energy, pulse duration, pulse shape can be monitored. This ensures preferably that the bacon effect is minimized with only minimal influence on the other laser parameters.

The beam profile preferably offers the possibility of representing the possible speckle pattern of the beam, if the beam profile or the spot profile is not smooth, a speckle pattern is formed.

By adapting the resonator elements, preferably by changing the radius of curvature of one or both resonator mirrors, it is possible to influence the spatial coherence and / or the increase in the divergence of the short beam axis of the laser beam. This is preferably carried out iteratively until the speckles no longer occur. As a result, the beam or the spot profile is preferably optimized.

Another test, identical to the first test described above, checks the beam quality again.

In a further preferred embodiment, a method is provided which images the laser beam with a wavelength of 193 nm onto the cornea, scans the laser beam over the cornea with a pulse duration of 4 to 15 ns, removes refraction-improved profiles with reduced roughness and optimized form fidelity and with energy in the Range from 0.5 mJ to 1.5 mJ. In this method, preferably lasers with a wavelength of 193 nm are used, which depart from the cornea of the eye.

Preferably, a spot scanning method is used for scanning the cornea, wherein the laser beam preferably used is reduced in size in the working plane (cornea). The reduced beam image is preferably referred to as a spot. The laser beam with a preferred pulse duration of 4 to 15 ns is preferably moved across the cornea by means of lateral beam deflecting optics (preferably Galvoscanner).

In this preferred method, the spot is preferably the location where the laser beam interacts with the matter (the corneal tissue). The preferred crossing of a minimum energy surface density (Schwellfluence) leads to the ablation of tissue. The energy range is preferably 0.5 mJ - 1.5 mJ.

Ablation is preferably referred to as non-thermal molecule decomposition, in which the material is removed. By distributing many such spots over the cornea, this can be changed by ablation

Form of curvature and thus a change in the refaction experience.

In a second method of imaging, the size of the beam during the treatment is preferably varied via diaphragms and variable diaphragms in order to obtain a removal which alters the refractive properties of the cornea.

These methods are preferably used for the treatment of refractive disorders such as myopia, hyperopia, astigmatism and their mixed forms, as well as for the patient-adapted treatment of higher aberrations. The method according to the invention preferably leads to smoother, more faithful removal profiles when used in refractive corneal surgery. Improved smoothness may cause the formation of haze (clouding) as a result of

Reduce surgery. Due to the improved beam shape, the removal can be better predicted, which stabilizes the generation of a target refraction.

In a further preferred embodiment, the

Invention used in a method for micromachining. Particularly preferred among these methods are applications such as patterning, cutting, drilling and structuring. In this embodiment, materials such as ceramics, glass and polymers are preferably processed. The laser beam in the working plane (spot) interacts with these materials. If the laser exceeds a minimum energy surface density in the working plane, non-thermal ablation of the materials occurs. Preferred are with these methods

Inkjet heads, masks and fiber structures generated. By the method according to the invention, the roughness can be significantly reduced and the dimensional accuracy of the structures produced can be improved.

In the description of the figures further preferred embodiments are shown. The figures show:

Fig. 1 is a schematic representation of a laser;

Fig. 2a, 2b a laser spot with profile section without

Device according to the invention; and

Fig. 3a, 3b a laser spot with profile section with inventive device. 4a, 4b, 4c are three-dimensional views of

Embodiments of the resonator according to the invention.

Fig. 5a, 5b Two-dimensional views of embodiment

4a of the resonator according to the invention.

6a, 6b is a schematic representation of the pulse curve with a curved resonator mirror.

FIG. 1 shows a schematic representation of a laser. The excimer laser 1.1 comprises a resonator 60 filled with excimer gas. In the resonator are two parallel resonator 11.1 and 11.2 opposite, wherein the resonator mirror 11.2 on the output side and

Resonator mirror 11.1 is on the non-decoupling side. Resonator mirror 11.2 is a planar resonator mirror. Resonator mirror 11.1 is a concave highly reflective mirror 12 with a cylindrical mirror surface 13. The side of the resonator 60 are the electrodes 70.1 and 70.2. 70.1 and 70.2 are arranged so that they are parallel to each other and perpendicular to the direction of curvature of the cylinder mirror. Outside the laser 1, the micro-optical element 41 is in the beam direction in front of the imaging optics 44, in front of the processing plane 45. 41,44 and 45 are arranged so that the emerging laser beam is imaged on the processing plane 45 in the desired manner.

For micro-material processing and corneal surgery, different physical quantities apply to the laser beam.

In refractive corneal surgery, output energies at the laser source of not greater than 50 μm, more preferably 10 μm and very particularly preferably less than 2 μm, and machining energies (in the working plane) are preferred 10μJ to 15mJ, more preferably from 0, ImJ to 5mJ and most preferably from 0.5mJ to 1.5mJ. A wavelength in the UV range is used, preferably 150 nm to 250 nm, particularly preferably 180 nm to 200 nm. It is particularly preferred ArF used as Excimergas. It will be one

Pulse duration in the range of preferably less than 2μs, more preferably used 0.1ns to 50ns, most preferably in the range of 3ns to 8ns. It is preferably used a Gaussian beam shape to make the removal of composite spots smoother. It is preferably a

Beam size used in FWHM (Fill Width at Half Maximum) range of less than 5mm, more preferably in the range of lOμm to 2.5mm, most preferably from 0.5 to 1.5mm. It is a pulse repetition rate in the range of preferably IHz to 5kHz, more preferably from 5Hz to IkHz, most preferably from 10Hz to 500Hz used.

In micromachining, a pulse repetition rate in the range of 1 Hz to 10 kHz is particularly preferred in the range of 500 Hz to 5 kHz, very particularly preferably in the range of 1 kHz to 4 kHz. A wavelength in the UV range is used, preferably 150 nm to 250 nm, particularly preferably 180 nm to 200 nm. And most preferably in the range of 240nm to 260nm. The excimer laser gas is preferably ArF, and more preferably KrF. A pulse duration in the range of preferably less than 2 μs, more preferably 0.1 ns to 50 ns, very particularly preferably in the range of 5 ns to 25 ns is used. Energies of 0.1mJ / cm_ to 10J / cm_, more preferably 0, lJ / cm_ to 5J / cm_ are used.

The ignition voltage for the gas discharge is generated by the electrodes 70.1 and 70.2. The gas discharge, provoked by the two opposite electrodes 70.1 and 70.2, leads to the development of an asymmetric beam profile. The short beam axis 20 of the laser 1 has a greater coherence and a smaller divergence than the long beam axis 30. The curvature of the resonator mirror 12 lies along the short beam axis 20 of the laser 1. Thus, the coherence of the coherent short axis 20 is reduced and the divergence in this direction is increased. The energy yield and pulse duration / shape remain the same because, despite focusing by the Gaussian excitation distribution in the short axis, sufficient emission is produced.

The thus changed laser beam now exits through the OC mirror 11.2 and hits first the micro-optical element 41, then the imaging optics 44 and then the processing plane 45. The task of the micro-optical element is to homogenize the beam in the working plane and / or to provide the desired beam shape. The imaging optics has the task of scaling the beam in the working plane to the desired size.

FIG. 2 a shows a laser spot 2 of a laser beam on a working plane 45 according to the prior art. You can see a spot 2 with a non-homogeneous border. That in the focused laser beam interference occur which lead to a speckle effect in the spot 2. This structure would be mapped on the machined material during removal.

2b shows a profile section 3 of the "rough" laser beam in spot 2, from FIGURE 2a. The profile section clearly shows an inhomogeneous, non-Gaussian distribution.

This effect occurs when using devices according to the prior art.

In FIG. 3 a, a laser spot 2 of a laser beam bundle according to a device according to the invention is placed on a laser beam

Processing level 45 shown. You can see a spot 2 with a homogeneous border. That is, in the laser beam is no See speckle pattern. In this embodiment, the laser beam was generated and imaged with the device according to the invention.

In Figure 3b, the smooth beam profile 3 of the laser beam in the spot 2 is shown. The profile section clearly shows a largely homogeneous Gaussian distribution. This effect is achieved by the use of the device according to the invention.

FIG. 4 a shows an embodiment of the resonator according to the invention. The resonator mirrors 11.1 as a highly reflecting mirror and 11.2 as a coupling-out mirror face each other with the reflecting surfaces and enclose the active medium 80. 11.1 is a concave mirror 12 with a cylindrical mirror surface 13. 11.2 is a flat surface. The axis 30 is the axis between the electrodes that is the direction of the discharge. The axis 30 is the long beam axis. The axis 20 is the short beam axis and is perpendicular to 30. The axes 30 and 20 span a plane on which the axis 35 is perpendicular. The axis 35 is the optical axis of the beam.

In the direction of the axis, the discharge voltage discharges, producing a top-hat shaped excitation and beam profile. Perpendicular to the discharge direction, i. in the direction of axis 20, a Gaussian similar excitation and beam profile is formed. In the direction of axis 35, the laser beam propagates.

FIG. 4b shows an embodiment of the resonator according to the invention. The resonator mirrors 11.1 as a highly reflecting mirror and 11.2 as a coupling-out mirror face each other with the reflecting surfaces and enclose the active medium 80. 11.1 is a mirror with a flat surface. 11.2 is a domed mirror with a cylindrical mirror surface. The Axis 30 is the axis between the electrodes that is the direction of discharge. The axis 30 is the long beam axis. The axis y is the short beam axis and is perpendicular to 30. The axes 30 and 20 span a plane on which the axis 35 is perpendicular. The axis 35 is the optical axis of the beam.

In the direction of the axis 30 discharges the discharge voltage with a top hat-shaped excitation and beam profile is formed. Perpendicular to the discharge direction, i. in the direction of axis 20, a Gaussian similar excitation and beam profile is formed. In the direction of axis 35, the laser beam propagates.

FIG. 4c shows an embodiment of the resonator according to the invention. The resonator mirrors 11.1 as a highly reflecting mirror and 11.2 as a coupling-out mirror face each other with the reflecting surfaces and enclose the active medium 80. In this case, 11.1 is a concave mirror 12 with a cylindrical inner surface 13. Mirror 11.2 is also curved outward and has a mirror surface 13. The axis 30 is the axis between the electrodes so the direction of discharge. The axis 30 is the long beam axis. The axis 20 is the short beam axis and is perpendicular to 30. The axes 30 and 20 span a plane on which the axis 35 is perpendicular. The axis 35 is the optical axis of the beam.

In FIG. 5a, a resonator 60, viewed from the direction of the axis 20, is shown. Furthermore, the figure shows an HR Mirror 11.1 and a coupling-out mirror 11.2. The long beam axis 30 lies along the direction of discharge between the electrodes. The concave cylindrical mirror is not curved according to the device according to the invention in this direction.

The intensity distribution of the laser beam in the direction of the axis 30 is top-hat-shaped according to the discharge distribution. Axis 35 is the propagation direction of the laser beam, i. the optical axis.

In FIG. 5b, a resonator 60, viewed from the direction of the axis 30, is shown. Furthermore, the figure shows an HR mirror 11.1 and a coupling-out mirror 11.2. The short beam axis 20 is perpendicular to the direction of discharge between the electrodes 70.1 and 70.2. The concave cylindrical mirror 11.1 is curved according to the device according to the invention in this direction.

The intensity distribution of the laser beam is similar according to the discharge distribution Gauss. Axis 35 is the propagation direction of the laser beam, i. the optical axis.

FIG. 6 a shows a representation of the pulse progression 100 when using a curved resonator mirror 11. 1 in the long beam axis 30.

The possible effects of a curved mirror on the top hat-shaped excitation distribution in the long beam axis 30 can be seen. With mirror curvature in this beam direction, the region of the feedback comprises only a part of the excited volume. Therefore, pulse duration and pulse shape changes in pulse progression 100 can occur. FIG. 6b shows a representation of the pulse progression 100 when using a curved resonator mirror 11.1 in the short beam axis 20.

It can be seen that the limitation of the range of the feedback by the curved mirror 11.1 has less influence in the Gaussian discharge distribution. Since the majority of the population inversion is used, in this case, no change in the pulse duration and shape of the pulse course 100 is induced. This effect is used in the device according to the invention.

Figures 6a and b show the effect of curved resonator mirrors on the beam axes. When used before. Spherical mirrors would cause a pulse change according to FIG. 6a. By the cylindrical, concave mirror of the device according to the invention (curved only in the short axis and effective) a pulse duration change can be avoided.

LIST OF REFERENCE NUMBERS

1 laser

2 laser spot

3 Profile cross section of the spot

10 optical element

11 resonator mirror

11.1 Highly reflective mirror

11.2 Output mirror

12 concave HR mirrors

13 Cylindrical mirror surface

20 First, short jet axis

30 Second, long beam axis

35 axis of propagation direction

40 micro-optical element

41 beamshaper

42 beam homogenizer

43 beam integrator

44 imaging optics

45 working plane

60 resonator

70 electrode

80 Active medium

100 pulse course

Claims

claims

An apparatus for improving laser beam quality for laser applications, comprising a laser (1) having an asymmetric beam profile and a first, short one

Beam axis (20) and a second, long beam axis (30) and a beam propagation axis (35) and a resonator (60), characterized in that the laser (1) at least one optical element (10) and a resonator mirror (11) with cylindrical Mirror surface (13), to reduce the spatial coherence and / or increase the divergence in the short beam axis (20) υmfasst.

2. Device according to claim 1, wherein the cylindrical mirror surface (13) of the concave resonator mirror (11) is arranged so that the curved axis of the mirror perpendicular to the long beam axis (30) and / or perpendicular to the beam propagation axis (35) of the laser (1 ) lies.

3. Device according to claim 1 or 2, wherein the laser (1) is an excimer laser.

4. Apparatus according to claim 1 to 3, wherein the optical element (10) is a homogenizer (42) and / or an integrator (43) and / or a beam shaper (41).

5. Apparatus according to claim 1 to 3, wherein the optical element (10) is a micro-optical element (40).

6. Apparatus according to claim 1 to 5, wherein the resonator (60) is an anamorphic stable resonator.

7. A method of providing a smooth beam profile, ie, reducing speckle in the working plane, comprising the following steps: Providing a laser beam having an asymmetrical beam profile, comprising a first, short beam axis (20) and a second, long beam axis (30), reducing the spatial coherence and / or increasing the divergence of the short beam axis (20) of the laser beam.

8. Method according to claim 7, comprising the additional following steps: imaging the laser beam with the imaging optics (44) on a working plane (45), checking the existing speckle pattern, adapting the radii of curvature of the resonator mirrors (11) further reducing the spatial coherence and / or magnification the divergence of the short beam axis (20) of the laser beam, retesting the speckle pattern.

9. The method according to claim 7 or 8, comprising the additional following steps:

Imaging the laser beam at 193 nm wavelength on the cornea,

Scanning of the laser beam with a pulse duration of 4 ns to 15 ns via the cornea, removal of refraction-improving profiles with reduced roughness and optimized form fidelity, ablation with an energy in the range of 0.5 mJ to 1.5 mJ.