DE102017116477B4 - Polarization converter with reflective axicon optics, laser optics, and method for manufacturing a polarization converter - Google Patents

Abstract

Polarization converter (P), comprising - a reflective axicon optics (1) with a first hollow lexicon (2) with a straight frustoconical recess (2.1) and a first inner axicon (3) arranged concentrically therein and to an optical axis (X), the conical recess (2.1) and the inner axicon (3) have the same cone angle (β) and are provided on the spaced lateral surfaces (2.1, 3.1) with a polarization-dependent reflective coating (HR) and with an optically transparent, perpendicular to the optical axis (X) arranged holding plate (4) with a first plane surface (4.1) on which the base surfaces (2.3, 3.3) of the axicons (2, 3) are arranged flush, with the tip of the frustoconical recess (2.1) concentric to the optical axis (X) an entry hatch (2.2) is arranged, - a first half-wave plate (5) arranged on the exit side of the reflective axicon optics (1) perpendicular to the optical axis (X), the reflective coating (HR) is chosen so that with a collimated to the optical axis (X) entering beam (S) through at least one reflection on each lateral surface (2.1, 3.1) of the axicons (2, 3) on the exit side a phase difference totaling Δφ = N · π + π / 2 is effected between the fast and slow light propagation direction, where N · π indicates an integral multiple of π.

Description

The invention relates to a polarization converter for changing the polarization state of collimated light.

Laser sources known from the prior art emit laser light with a linear polarization that is not rotationally symmetrical with respect to the beam direction of the laser light. It is known that linearly polarized laser light, due to the different degree of absorption of the components aligned in parallel and the components aligned vertically, causes material removal in material processing that is dependent on the cutting direction and which impairs the quality of material processing.

Devices for transforming linearly polarized laser light into circularly polarized laser light are known with which, although a cutting direction-independent, but overall reduced amount of material can be achieved, which thus forces a reduced process speed.

It is also known that laser light with radial or azimuthal polarization enables both a material removal independent of the cutting direction and a high process speed. It is also known that radially polarized light has improved focusability, in particular a beam diameter in the focus that is reduced compared to linear polarization, which leads to improved applications of laser light in stimulated emission depletion (STED) or multiphoton microscopy, in optical tweezers, in lithography, the Confocal microscopy, optical data storage or optical particle traps. There is thus a need for a polarization converter for transforming linearly polarized or circularly polarized light into radially or azimuthally polarized light.

Devices suitable for this transformation are known from the prior art, which include an axicon shaped as a hollow cone, the inner and outer circumferential surfaces of which are shaped concentrically tapering at the same cone angle. In polarization converters of this type, the optical path for the rotation of the polarization state runs in an optically dense medium, usually in glass, preferably in quartz glass. This is accompanied by a partial absorption of the coupled-in laser radiation and its conversion into heat, which limits the radiant energy that can be passed through. In addition, because of the dispersion, such refractive polarization converters bring about a generally undesirable broadening of ultrashort laser pulses. In addition, such polarization converters have a reduced or no polarization-rotating effect and a reduced radiation density on the exit side in the central area around the optical axis. This prevents or makes it more difficult to use such polarization converters for laser processing methods, since laser sources with a Gaussian beam density distribution arranged on the inlet side and with a Gaussian beam density distribution couple in the majority of the radiant energy near the optical axis.

Furthermore, wave plates segmented into azimuth angle ranges are known from the prior art, with which a polarization rotation dependent on the azimuth angle in coarse angular steps of approximately 30 ° can be achieved. This makes it possible to reduce, but not eliminate, the differences in the material removal that are dependent on the cutting direction. In the manufacture of such segmented corrugated plates, adhesive points are required along the segment boundaries, the manufacture of which is complex and which increase the risk of breakage.

In addition, nematic liquid crystals and nano-structured retardation plates are known with which linearly polarized light can be transformed into radially or azimuthally polarized light.

The document US 2012/0320458 A1 describes a polarizer and polarizing beam splitter, comprising at least one pair of axicons, which are set up to split an incoming beam into a radially polarized component and a tangentially or azimuthally polarized component. A second pair of axicons can be provided to bring the azimuthally polarized components together to create a more uniform radiance profile. The radially polarized component can be reflected or deflected so that the radially and / or the azimuthally polarized component can each be used individually.

The document US 2015/0002934 A1 describes an axially symmetrical polarization converter that converts incident light into axially symmetrically polarized light. The polarization converter comprises a reflecting surface with a shape which is formed by rotating a cross section of a Fresnel parallelepiped aligned along the optical axis about an axis parallel to the optical axis. The polarization converter generates axially symmetrically polarized light through two Fresnel reflections on the reflective surface.

The document DE 103 21 598 A1 describes an illumination system for a microlithography projection exposure system with an axicon module for generating an illumination distribution with a central intensity minimum. The axicon Module has a first axicon element with a first axicon surface and an associated second axicon element with a second axicon surface. Furthermore, the lighting system has a first polarization-influencing optical element which is arranged in the direction of light in front of the first axicon element and is constructed in such a way that rays hitting the axicon surfaces are polarized approximately perpendicularly or approximately parallel to the respective plane of incidence of the rays.

The document DE 10 2014 215 952 A1 describes a lighting module for an optical sensor for measuring a surface of a workpiece by means of a coordinate measuring machine, comprising at least one light source and optical elements for guiding the light. An optical polarization unit is arranged between the light source and the workpiece.

The document WO 2010/016028 A1 describes a laser with a laser beam path for a laser beam with a portion of an azimuthal polarization state in a laser resonator. In the laser beam path, a beam-focusing optical component is arranged between two beam-widening optical components, which has mirror surfaces on which laser light strikes and is reflected at an angle at or near the Brewster angle, the proportion of the azimuthal polarization state being maximized in the reflection.

The document U.S. 5,607,730 A describes a method for applying paints and other layers by means of a laser beam, into which a particle cloud of coating particles is blown under pressure with an inert gas.

The document US 2013/0272653 A1 describes a method for producing an axicon, in which a structure is provided with an axicon-shaped recess into which a polymeric substance is introduced and an axicon is formed therefrom.

The invention is based on the object of specifying polarization converters for transforming collimated light with circular or linear polarization into light with radial polarization or in light with azimuthal polarization. In particular, the invention is based on the object of specifying such polarization converters which are suitable for the transformation of laser light with very short pulsed lasers with a pulse duration of at most 150 femtoseconds and with a high radiation energy in continuous wave operation of up to 7 kilowatts. Furthermore, the invention is based on the object of specifying such polarization converters which are suitable for a large wavelength range from 250 nanometers to 2500 nanometers, preferably from 400 nanometers to 1200 nanometers.

Another object of the invention is to provide a method for producing such polarization converters.

Another object of the invention is to provide laser optics for the scanning movement of a laser beam transformed by means of such a polarization converter.

With regard to the polarization converter, the object is achieved according to the invention by the features of claim 1. With regard to the production method for a polarization converter, the object is achieved according to the invention by the features of claim 11. With regard to the laser optics, the object is achieved according to the invention by the features of claim 8.

Advantageous refinements of the invention are the subject matter of the subclaims.

A polarization converter comprises reflective axicon optics with a first hollow axicon and a first inner axicon arranged concentrically therein. The hollow lexicon has a recess in the shape of a straight truncated cone. The inner axicon has the shape of a straight cone and is arranged concentrically in the hollow axicon, the cone tip pointing to the tip of the frustoconical recess and the jacket surface of the frustoconical recess being spaced from the jacket surface of the inner axicon.

The inner axicon and the frustoconical recess have the same cone angle. The outer surfaces are provided with a reflective coating. The ring-shaped base of the hollow axicon and the base of the inner axicon are arranged flush and concentrically on a first plane surface of an optically transparent holding plate, so that the axicons are perpendicular to the holding plate.

At the tip of the truncated cone of the recess of the hollow axicon, an entry hatch is arranged concentrically to the longitudinal axis of the hollow axicon, which coincides with the longitudinal axis of the inner axicon and forms an optical axis. The exit hatch of the reflective axicon optics is formed by the annular gap located concentrically between the base surfaces of the axicons.

Furthermore, the polarization converter comprises a first half-wave plate which is oriented perpendicular to this optical axis and is arranged at the exit of the reflective axicon optics, which retards the slow relative to the fast Causes light propagation by half a wavelength. Correspondingly, a half-wave plate causes the direction of polarization to be reflected on the crystallographic axis of the first half-wave plate.

The reflective coating of the outer surfaces of the axicons has a polarizing effect and has a fast axis and a slow axis. With each reflection on the reflective coating, a phase difference is created between the component of the reflected light oriented along the fast axis and the component of the reflected light oriented along the slow axis. Such polarizing reflective coatings are known from the prior art. It is also known how the phase difference between the fast and slow axes can be controlled by forming the reflective coating, for example by choosing the coating thickness.

The reflective coating of the outer surfaces of the axicons is chosen so that, depending on the cone angle of the inner axicon and thus also on the cone angle of the frustoconical recess of the hollow axicon, there is a total phase difference in the case of reflections on the outer surface of the inner axicon and on the outer surface of the first hollow axicon Δφ = N · π + π / 2 or N · 180 ° + 90 ° between the fast and slow light propagation direction is effected, where N is a positive integer. In other words: the reflective coating of the lateral surfaces causes a phase difference of π / 2 plus an integral multiple of π.

The cone angle of the inner axicon is preferably selected to be 45 ° in order to enable the shortest possible overall length of a reflective axicon optic.

Such a reflective axicon optics, as a quarter-wave plate, thus effects a change in the direction of polarization dependent on the azimuth angle and enables an incident, collimated, axially-oriented bundle of light with circular polarization to be transformed into light emerging in the form of a ring.

This light is converted into an azimuthally inhomogeneous linear polarization by means of the first half-wave plate, this first half-wave plate being arranged in such a way that the crystal axis is rotated by π / 8 or 22.5 ° relative to the azimuth angle of the plane of entry of the incoming collimated light.

Compared to refractive axicon optics known from the prior art, the reflective axicon optics according to the invention has the advantage that the tapering tip of the conical inner axicon can be manufactured more precisely, more easily and in particular with a smaller tip diameter than the tip of the conical recess with the inner lateral surface of a conical one Recess in a known refractive axicon. Due to the limitations of the tools that can be used when forming this conical recess, for example by means of diamond turning, a dead zone inevitably arises around the optical axis in which the inner lateral surface is flattened or rounded off with respect to the cone angle. Light coupled in within this dead zone is therefore lost for the polarization rotation and is largely converted into heat and scattered light, which is difficult or impossible to dissipate.

Another advantage of the reflective axicon optics is that the incoming light essentially propagates in the air and is only reflected at two highly reflective boundary surfaces, the lateral surfaces provided with the reflective coatings. Optical losses can thus be reduced compared to the propagation in an optically dense medium such as glass of the optically dense medium to air in a refractive axicon optics according to the prior art. This also increases the non-destructively transparent light output due to the reduced absorption. The reflective axicon optics is therefore particularly advantageous for the polarization rotation of light from powerful and short- or ultra-short pulsed laser sources.

Another advantage of reflective axicon optics is that the cone angle of the inner axicon can be selected independently of the critical angle of total reflection in the case of an optically dense medium. The geometry of the reflective axicon optics can thus be designed independently of the difference in the refractive index between the optically dense medium and the surrounding medium, and thus also independently of the wavelength of the light fed in. The cone angle of the inner axicon, and thus also the cone angle of the hollow axicon, can preferably be selected in such a way that production is particularly easy.

Another advantage of reflective axicon optics is that highly reflective coatings can be produced for a wide range of wavelengths and can therefore be used more widely than dispersion-limited refractive axicon optics according to the prior art, in which the critical angle of total reflection must also be taken into account for the selection of the cone angle. For example, it is thereby possible to use the same polarization converter for a laser with an emitted fundamental wavelength as for a frequency-doubled laser source based on this laser.

Very short laser pulses, for example laser pulses with a pulse duration of no more than 150 Femtoseconds have very broad spectra and are consequently broadened in transmissive optics due to dispersion. This undesired pulse lengthening is advantageously avoided in the reflective axicon optics according to the invention, since the optical path here, with the exception of the holding plate, runs predominantly in air.

In one embodiment of a polarization converter with paired axicons, a second hollow lexicon is shaped identically and mirror-symmetrically to the first hollow lexicon on a second flat surface of the holding plate of the reflective axicon optics opposite the first flat surface. In this second hollow axicon, a second inner axicon is arranged mirror-symmetrically to the first inner axicon. At the tip of the frustoconical recess of the second hollow icon, an exit hatch is arranged centrally and thus concentrically to the optical axis. The reflective coating and the cone angles of the axicons are chosen to be the same.

In this embodiment of a polarization converter, the light emerges from the reflective axicon optics with paired axicons from the central exit hatch of the second outer hollow axicon and thus in a beam profile arranged close to the optical axis. In this case, the coating of the outer surfaces of the paired axicons is chosen so that a total phase difference of Δφ = N · π + π / 2 or N · 180 ° + 90 ° is brought about between the fast and slow light propagation direction, where N is a is positive integer. In other words: the reflective coating of the lateral surfaces causes a total phase difference of π / 2 plus an integral multiple of π. By means of the downstream first half-wave plate, light with azimuthally inhomogeneous linear polarization is thus also generated in this embodiment, this first half-wave plate also being arranged in this embodiment in such a way that the crystal axis is π / 8 or 22.5 ° relative to the azimuth angle of the entry plane of the incoming collimated light is rotated.

The beam guidance reflected on the holding plate is particularly suitable for laser light that is to be used in a bundled manner with a high energy density, for example for material removal. Another advantage of this embodiment is that the diameter of the entering collimated light beam remains unchanged when passing through such a polarization converter with paired axicons. Installation of such a polarization converter in the beam path of an existing optical system therefore does not require any further optical components to be adapted.

Such a reflected beam path can also be achieved with a paired refractive axicon optics from the prior art. In this embodiment, however, the reflective axicon optics has the additional advantage of a particularly small optical dead zone around the apex of the cone of the inner axicon. As a result, the emerging beam path in the central area around the optical axis, which is particularly interesting for these applications, is significantly less affected in terms of the polarization state and the radiation density and an optical dead zone in the central area around the optical axis with a diameter of less than 100 micrometers is achieved. In refractive axicon optics, the diameter of the optical dead zone is about 5 millimeters and is therefore considerably larger because of the unavoidable blunting of the cone tip during the production of the conically recessed inner jacket surface.

In one embodiment of a polarization converter, a second half-wave plate is arranged on the exit side after and parallel to the first half-wave plate. By means of the additional second half-wave plate, light emerging after the first half-wave plate with azimuthally inhomogeneous linear polarization is transformed into radially polarized light, the crystal axis of the second half-wave plate being arranged at the azimuth angle of the entry plane of the entering collimated light to the optical axis or, in other words, in the the exit side extended entry plane of the linearly polarized light lies. In other words: in this embodiment of the invention, the crystal axis of the second half-wave plate is rotated by -π / 8 or -22.5 ° relative to the crystal axis of the first half-wave plate, a negative angle of rotation corresponding to a rotation counter to the mathematically positive direction of rotation and thus clockwise .

If, in another embodiment, the crystal axis of the second half-wave is rotated at an angle of π / 4 or 45 ° relative to the azimuth angle of the entry plane of the entering collimated light, exiting light is generated with azimuthal polarization. In other words: in this embodiment of the invention, the crystal axis of the second half-wave plate is rotated by π / 8 or 22.5 ° relative to the crystal axis of the first half-wave plate, a positive angle of rotation corresponding to a rotation in the mathematically positive direction of rotation and thus counterclockwise.

Thus, by rotating the second half-wave plate by π / 4 or 45 °, it is possible to switch between radial and azimuthal polarization at the output of the polarizer.

In one embodiment of a polarization converter, the reflective coating of the lateral surfaces is designed as a highly reflective coating with a reflectivity of at least 99.7%. As a result, a particularly low transmission loss along the optical path is achieved through the reflective axicon optics and the absorbed radiant energy, which is converted into heat and has to be dissipated, is reduced. This also enables the polarization converter to be used for laser sources with high radiation energy emitted in continuous wave operation. In addition, the unavoidable broadening of ultrashort laser pulses in refractive axicon optics according to the prior art because of the dispersion is avoided. This enables the use of a polarization converter according to the invention, in particular for methods and devices for laser processing, for example for cutting, drilling or welding workpieces, in which high energy densities are required.

The highly reflective coating is preferably designed for wavelengths from 400 nanometers to 1200 nanometers, particularly preferably for wavelengths from 1030 nanometers to 1070 nanometers. This enables the use of reflective axicon optics for various applications and laser sources, in particular also for frequency-doubled laser sources. In particular, this enables a modularized construction of polarization converters, since the reflective axicon optics can be constructed independently of the wavelength and since half-wave plates that can be adapted to the respective wavelength are available in a standardized manner for many wavelengths and are easily exchangeable in an optical system.

In one embodiment of a polarization converter, the reflective axicon optics are made in one piece, the axicons being made from quartz glass and connected to a holding plate made from quartz glass. In an advantageous manner, a particularly temperature-stable reflective axicon optic is made possible, which can non-destructively transmit a large amount of radiation energy, preferably radiation energy of over 7 kilowatts, corresponding to a power density of 10 joules per square centimeter.

In one embodiment of a polarization converter, the reflective axicon optics is held in a mount that can be cooled with a coolant. This enables improved cooling of the axicon optics and thus a non-destructive transmission of large radiation energies. This embodiment of the invention is particularly advantageous for use in laser processing methods with short-pulsed lasers of high laser power.

Laser optics comprise a polarization converter with reflective axicon optics and a laser source arranged on the entry side of the polarization converter, the laser source being set up to emit circularly polarized laser light collimated to the optical axis of the polarization converter. Such laser sources, comprising a laser which emits linearly polarized light, and a quarter-wave plate arranged downstream in the beam path, the crystal axis of which is rotated by π / 4 or 45 ° with respect to the polarization direction of the linearly polarized light emitted by the laser, are known from the prior art.

In one embodiment, it is also possible to use a laser source which is set up to emit linearly polarized laser light collimated to the optical axis of the polarization converter. In this embodiment, a quarter-wave plate is arranged on the polarization converter on the entry side, the crystal axis of which is rotated by π / 4 or 45 ° with respect to the polarization direction of the laser light. In this embodiment, it is advantageous to arrange a device for receiving a quarter-wave plate, rotatable about the optical axis and releasably connectable, on the inlet side of the polarization converter. Such receiving devices for quarter-wave plates are known from the prior art.

By means of the polarization converter, laser light from the laser source is transformed into radially or azimuthally polarized laser light, thus enabling material to be removed regardless of the cutting direction when the radially or azimuthally polarized laser light strikes a workpiece.

Here, the crystal axis of the first half-wave plate is rotated by π / 8 or 22.5 ° relative to the azimuth angle of the plane of entry of the linearly polarized light emitted by the laser source. For the generation of radially polarized light, the crystal axis of the second half-wave plate lies in the azimuth angle of the entrance plane. To generate azimuthally polarized light, however, the crystal axis of the second half-wave plate is rotated by π / 4 or 45 ° relative to the azimuth angle of the plane of entry of the linearly polarized light emitted by the laser source. Thus, the crystal axes of both half-wave plates are rotated by π / 4 or -π / 4 relative to one another, with a positive sign of the angle of rotation indicating a rotation in the mathematically positive direction of rotation.

Such laser optics are advantageously suitable for laser processing methods with a particularly high processing quality.

In one embodiment of laser optics, there is a polarization converter on the exit side polarization-maintaining scanning optics are arranged, which are set up for pivoting a beam collimated to the optical axis of the polarization converter and emerging from the polarization converter along at least one scanning direction.

In one embodiment of laser optics, a beam shaper for adapting the diameter of the collimated beam emerging from the polarization converter to the polarization-maintaining scanning optics is arranged on the entry side and / or exit side of the polarization converter. A polarization converter for scanning optics with different input apertures and / or different swivel ranges can thus be adapted in an advantageous manner.

In a method for producing a polarization converter with reflective axicon optics, the hollow truncated cone-shaped recess of the at least one hollow axicon and the at least one inner axicon are formed by diamond turning with an angular deviation of the cone angle of at most 0.01 ° and to a surface deviation of at most a Fresnel ring at 546 nanometers , forming a cone tip with a tip diameter of 800 micrometers or less.

It is also possible to shape the hollow truncated cone-shaped recess of the at least one hollow axicon and the at least one inner axicon by polishing. The polishing movements on an axicon cone are controlled in such a way that they do not lead over the apex of the axicon cone and a cone tip with a tip diameter of at most 800 micrometers is polished.

Then the axicons are permanently connected, preferably glued, to the holding plate to form a one-piece reflective axicon optic by means of alignment putty. This one-piece reflective axicon optic is held in a mount with the at least one half-wave plate. The socket can provide a device for releasably connecting a second half-wave plate at the outlet. This makes it easy to generate radially polarized light with a polarization converter without a second half-wave plate or to generate azimuthally polarized light by adding a second half-wave plate.

In this way, it is advantageously possible to reduce the optical dead zone about the optical axis within which there is no reflection on the at least one inner axicon. In addition, the adjustment of the polarization converter is simplified by cementing the axicons and the holding plate to form a one-piece reflective axicon optic.

Embodiments of the invention are explained in more detail below with reference to drawings.

• 1 schematisch einen Polarisationswandler mit refraktiver Axikonoptik nach dem Stand der Technik,
• 2 schematisch die Transformation von Polarisationszuständen mittels Halbwellenplatten nach dem Stand der Technik,
• 3 schematisch einen Polarisationswandler mit reflektierender Axikonoptik zur Erzeugung eines ringförmigen Strahldichteprofils,
• 4 schematisch einen Polarisationswandler mit reflektierender Axikonoptik zur Erzeugung eines scheibenförmigen Strahldichteprofils sowie
• 5 schematisch eine Laseroptik mit einem gefassten Polarisationswandler.

Show in it:

• 1 schematically a polarization converter with refractive axicon optics according to the prior art,
• 2 schematically the transformation of polarization states by means of half-wave plates according to the state of the art,
• 3 schematically a polarization converter with reflective axicon optics for generating an annular radiance profile,
• 4th schematically a polarization converter with reflective axicon optics for generating a disk-shaped radiance profile as well
• 5 schematically a laser optics with a mounted polarization converter.

Corresponding parts are provided with the same reference symbols in all figures.

1 shows schematically a polarization converter P. with a refractive axicon optic 100 to change the polarization state of collimated light. The axicon optics 100 has the outer shape of a truncated cone with an outer lateral surface 100.2 on, which is rotationally symmetrical to an optical axis X lies. The truncated cone is with a flat surface 100.3 truncated that is perpendicular to the optical axis X to entry E. shows. Exit side of the refractive axicon optic 100 is a first half-wave plate 5 arranged.

In the axicon optics 100 is a conical recess with an inner jacket surface 100.1 incorporated. The opening angle of the inner lateral surface 100.1 and the opening angle of the outer lateral surface 100.2 match and form the cone angle β. The one perpendicular to the optical axis X to exit A. pointing footprint 100.4 the axicon optics 100 is thus ring-shaped.

A bundle to the optical axis X collimated rays S. occurs perpendicular to the plane surface 100.3 one and perpendicular to the annular base 100.4 the end. Along the beam path in the axicon optics 100 hit the rays S. at the angle of incidence α = 90 ° - β / 2 on the lateral surfaces, with a sufficient difference in the refractive index n of the axicon optics 100 a total reflection takes place in relation to the refractive index n _air = 1 of the surrounding medium.

erzeugt wird.It is known from the prior art that, in each of the two total reflections, a phase difference δ between the perpendicularly polarized and the parallel polarized component of $$\delta =2\text{atan}\left(\frac{\text{cos}\alpha \cdot \sqrt{{\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="DE102017116477B4\_0004.png,DE102017116477B4\_0005.png"\; state="\{\{state\}\}">n</figure-callout>}}^2{\text{<figure-callout id="2" label="sin" filenames="DE102017116477B4\_0004.png,DE102017116477B4\_0005.png" state="{{state}}">sin</figure-callout>}}^2\alpha -1}}{\mathrm{<figure-callout\; id="2"\; label="n\; sin"\; filenames="DE102017116477B4\_0004.png,DE102017116477B4\_0005.png"\; state="\{\{state\}\}">n</figure-callout>}{\text{sin}}^{}{}^2\alpha }\right)$$

is produced.

A desired phase difference δ can thus be achieved by choosing the cone angle β = 180 ° -2α as a function of the refractive index n des for the axicon optics 100 glass used.

Refractive axicon optics are, for example, from the prior art 100 for δ = 45 ° known, with which a total phase difference of 90 ° along the optical path through the axicon optics 100 can be generated, the orientation of the polarization being dependent on the azimuth angle of the plane of incidence. Such a refractive axicon optic 100 thus acts as a quarter-wave plate with an azimuth angle-dependent polarization orientation and makes it possible to transform an incident, collimated bundle of light close to the axis into light emerging in the form of a ring. Has the incident light circular polarization P1 on, so it has from the first half-wave plate 5 outgoing light azimuthally inhomogeneous linear polarization P2 on.

The incident light becomes with circular polarization P1 generated from linearly polarized light which is passed through a quarter-wave plate rotated by π / 4 to the polarization direction of the linearly polarized light, the crystal axis is the first half-wave plate 5 to be arranged rotated in relation to the polarization direction of the linearly polarized light in an angle of rotation of π / 8.

It is also known from the prior art that by means of the axicon optics 100 and the downstream first half-wave plate 5 generated light with an azimuthally inhomogeneous linear polarization P2 by means of a downstream second half-plate 6th optionally in a radial polarization P3 or in an azimuthal polarization P4 is transformable, as in 2 shown schematically.

To generate a radial polarization P3 is the crystal axis of the second half-wave plate 6th to be arranged pointing in the direction of polarization of the linearly polarized light. In other words: to generate a radial polarization P3 is the crystal axis of the second half-wave plate 6th rotated by -π / 8 with respect to the crystal axis of the first half-wave plate 5, the negative sign indicating a rotation opposite to the mathematically positive direction of rotation.

To generate an azimuthal polarization P4 the crystal axis of the second half-wave plate 6 is to be rotated by π / 4 or 45 ° with respect to the polarization direction of the linearly polarized light. In other words: to generate an azimuthal polarization P4 is the crystal axis of the second half-wave plate 6th rotated by π / 8 with respect to the crystal axis of the first half-wave plate 5, the positive sign indicating a rotation in the mathematically positive direction of rotation.

3 shows schematically a polarization converter P. with a reflective axicon optic according to the invention 1 and a downstream half-wave plate 5 . The reflective axicon optics 1 comprises a first outer hollow lexicon 2 and one therein coaxial with the optical axis X arranged first inner axicon 3 .

The hollow lexicon 2 has a frustoconical recess with a lateral surface 2.1 on that at a taper angle β converges. At the tapering end of the lateral surface 2.1 is a centric entry hatch 2.2 arranged. At the opposite diverging end of the lateral surface 2.1 is the hollow lexicon 2 through an annular base 2.3 closed.

The inner axicon 3 is as a cone with the same cone angle β shaped like the frustoconical recess of the hollow ax 2 and has a lateral surface 3.1 and a base 3.3 on.

The base areas 2.3 , 3.3 the axicons 2 , 3 are on a first plane surface facing them 4.1 an optically transparent retaining plate 4th flush and concentric to the optical axis X attached, the lateral surfaces 2.1 , 3.1 are spaced. Because of the same taper angle β of both axicons 2 , 3 are the lateral surfaces 2.1 , 3.1 parallel to each other and coaxial to the optical axis X .

Each of the lateral surfaces 2.1 , 3.1 is with a highly reflective coating MR provided which has a reflectivity of at least 99.7% in a wavelength range of 250 nanometers to 2500 nanometers, preferably from 400 nanometers to 1200 nanometers.

The highly reflective coating MR has a polarizing effect and has a fast axis and a slow axis. With every reflection on the highly reflective coating MR a phase difference is created between the component of the reflected light aligned along the fast axis and the component of the reflected light aligned along the slow axis. Such polarizing highly reflective coatings MR are known from the prior art. It is also known how the phase difference between the fast and slow axes is caused by the formation of the highly reflective coating MR , for example through the choice of the coating thickness, can be controlled.

In addition, the highly reflective coating MR designed so that it depends on the cone angle β of both axicons 2 , 3 with reflections on the lateral surfaces 2.1 , 3.1 the axicons 2 , 3 a phase difference of a total of Δφ = N · π + π / 2 or N · 180 ° + 90 ° is caused between the fast and slow light propagation direction, where N is a positive integer that represents the number of reflections on a respective lateral surface 2.1 , 3.1 describes.

This is how the reflective axicon optics works 1 as a quarter-wave plate with an azimuth angle-dependent polarization orientation and enables an incident collimated near-axis bundle of light with circular polarization P1 to transform into ring-shaped emerging light. The one between the base areas 2.3 , 3.3 the axicons 2 , 3 remaining, to the optical axis X concentric ring surface forms the exit hatch for this embodiment of a reflective axicon optic 1 .

Through the downstream half-wave plate 5 becomes an azimuthal inhomogeneous linear polarization P2 generated. In one embodiment of the invention, there is a further second half-wave plate, not shown in any more detail here 6th downstream, with which the azimuthal inhomogeneous linear polarization P2 into a radial polarization P3 or in an azimuthal polarization P4 is transferable.

Compared to the refractive axicon optics known from the prior art 100 exhibits the reflective axicon optics according to the invention 1 the advantage that the tapering tip of the conical inner axicon 3 can be manufactured more precisely, more easily and in particular with a smaller tip diameter than the tip of the conical recess with the inner lateral surface 100.1 in refractive axicon optics 100 . The limitations of the tools that can be used for diamond turning this conical recess inevitably result in a dead zone around the optical axis X , in which the inner lateral surface 100.1 compared to the cone angle β is flattened or rounded. Light coupled in within this dead zone is therefore lost for the polarization rotation and is largely converted into heat that is difficult or impossible to dissipate.

Another advantage of the reflective axicon optics 1 consists in the fact that the incoming light essentially propagates in air and only at two highly reflective interfaces, the one with the reflective coatings MR provided lateral surfaces 2.1 , 3.1 , is reflected. In this way, optical losses compared to propagation in glass and total reflection at the glass-air interface can be avoided in refractive axicon optics 100 according to the state of the art. This also increases the non-destructively transparent light output due to the reduced absorption. The reflective axicon optics is therefore particularly advantageous for the polarization rotation of light from powerful, short- or ultra-short pulsed laser sources.

Another advantage of the reflective axicon optics 1 is that highly reflective coatings MR can be produced for a wide range of wavelengths and can therefore be used more widely than dispersion-limited refractive axicon optics 100 According to the state of the art. For example, it is thereby possible to use the same polarization converter for a laser with an emitted fundamental wavelength P. to be used as for a frequency-doubled laser source based on this laser.

Very short laser pulses, for example laser pulses with a pulse duration of at most 150 femtoseconds, have very broad spectra and are consequently broadened in transmissive optics as a result of dispersion. This unwanted pulse lengthening is used in the reflective axicon optics according to the invention 1 advantageously avoided, since the optical path here with the exception of the retaining plate 4th predominantly runs in air.

4th shows a further embodiment of the invention with reflective axicon optics 1 with paired axicons, in which a first and an identically formed second hollow lexicon 2 , 2 ' as well as a first and an identically designed second inner axicon 3 , 3 ' along the optical axis X in each case opposite one another on the holding plate 4th are arranged.

Along the course of the rays S. through this embodiment of the reflective axicon optics 1 takes place on each of the with a highly reflective coating MR coated outer surfaces 3.1 , 2.1 , 2'.1 , 3'.1 at least one reflection each, which in each case generates a phase difference between the vertically polarized and the parallel polarized component.

The highly reflective coating MR is designed so that it depends on the cone angle β the axicons 2 , 2 ' , 3 , 3 ' in the case of reflections on their lateral surfaces 2.1 , 2'.1 , 3.1 , 3'.1 a phase difference totaling Δφ = N. π + π / 2 or N · 180 ° + 90 ° between the fast and slow light propagation direction, where N is a positive integer that represents the number of reflections on a respective lateral surface 2.1 , 2'.1 , 3.1 , 3'.1 describes.

This is how the reflective axicon optics works 1 as a quarter-wave plate with an azimuth angle-dependent polarization orientation and, together with the downstream half-wave plate, enables 5 , an incident collimated near-axis bundle of light with circular polarization P1 in outgoing light with azimuthally inhomogeneous linear polarization P2 to transform.

In one embodiment of the invention, there is a further second half-wave plate, not shown in any more detail here 6th downstream, with which the azimuthal inhomogeneous linear polarization P2 into a radial polarization P3 or in an azimuthal polarization P4 is transferable.

In contrast to the in 3 illustrated embodiment of the invention, the light emerges from the reflective axicon optics 1 with paired axicons from a centrally arranged exit hatch 2'.2 of the second outer hollow lexicon 2 ' and thus in one close to the optical axis X arranged beam profile.

Such a reflected beam path is also possible with a paired refractive axicon optics 100 achievable from the state of the art. In this embodiment, the reflective axicon optics 1 in contrast, the additional advantage of a particularly small optical dead zone around the apex of the cone of the inner axicons 3 , 3 ' on. As a result, the emerging beam path is in the central area around the optical axis that is particularly interesting for these applications X significantly less impaired in terms of the polarization state and the radiance. The optical dead zone in the central area around the optical axis X is with a refractive axicon optic 100 because of the conically recessed inner lateral surface during manufacture 100.1 unavoidable blunting of the cone tip larger.

5 shows schematically a laser processing system with a polarization converter P. with a mounted reflective axicon optic 1 in the embodiment as a paired axicon optics. On both sides of the retaining plate 4th made of optically transparent material, there are two mirror-symmetrical inner axicons 3 , 3 ' and two outer axicons 2 , 2 ' each with the base area 2.3 , 2'.3 , 3.3 , 3'.3 to the retaining plate 4th arranged pointing, with an inner axicon 3 , 3 ' each concentric to the optical axis X in the frustoconical recess of the outer axicon 2 , 2 ' is arranged. All axicons 2 , 2 ' , 3 , 3 ' have the same cone angle β on. The centric openings of the outer axicons 2 , 2 ' act as an entry hatch 2.2 or as an exit hatch 2'.2

In the hatch 2.2 becomes the optical axis X collimated, linearly polarized light from a laser source 8th coupled. The inlet side is on the polarization converter P. a quarter wave plate 11 arranged whose crystal axis is rotated by π / 4 or 45 ° with respect to the polarization direction of the laser light. The quarter wave plate 11 can be fastened by means of a holding device, not shown, which allows rotation of the quarter-wave plate 11 around the optical axis X enables. That behind the quarter-wave plate 11 into the hollow lexicon on the entry side 2 coupled light has a circular polarization P1 on. From the exit hatch 2'.2 of the exit-side outer axicon 2 ' collimated light emerges close to the axis and is released by a first half-wave plate 5 in azimuthal inhomogeneous linear polarization P2 radial polarization P3 and further from a second half-wave plate 6th in azimuthal polarization P4 transferred, the crystal axis of the first half-wave plate 5 by π / 8 or 22.5 ° with respect to the polarization direction of the laser source 8th emitted linearly polarized laser light is rotated.

By means of the downstream second half-wave plate 6th this azimuthally linearly polarized light is further transformed into light that has a radial polarization P3 or an azimuthal polarization P4 having. For generating radial polarization P3 is the crystal axis of the second half-wave plate 6th in the direction of polarization of the laser source 8th emitted linearly polarized laser light and thus aligned by -π / 8 with respect to the crystal axis of the first half-wave plate 5 turned. For generating azimuthal polarization P4 is the crystal axis of the second half-wave plate 6 by π / 4 with respect to the polarization direction of the laser source 8th emitted linearly polarized laser light rotated and thus rotated by π / 8 with respect to the crystal axis of the first half-wave plate 5 turned.

That from the polarization converter P. emerging, azimuthally polarized light is captured by polarization-maintaining scanning optics 10 on a workpiece W. steered. In the beam path between the exit A. of the polarization converter P. and the scanning optics 10 is a beam shaper 9 arranged with which the beam diameter of the polarization converter P. exiting beam to the scanning optics 10 can be customized. In another embodiment of a laser processing system, an alternative or an additional beam shaper is in the beam path between the laser source 8th and entry E. of the polarization converter P. arranged.

The reflective axicon optics 1 and the half-wave plates 5 , 6th are in a tubular socket 7th kept the cooling fins on their outer surface 7.1 having. The cooling fins 7.1 can be hollow and a cooling liquid can flow through them. It is also possible to have a fin structure made of cooling fins that can be cooled with air 7.1 to train. With the cooling fins 7.1 an improved removal of heat is guaranteed, which is caused by optical losses during reflection on the coatings MR as well as the transmission through the retaining plate 4th and the half-wave plates 5 , 6th arises. This makes it possible to process workpieces as well W. suitable light from pulsed laser sources 8th with high energy and non-destructive thanks to the polarization converter P. to conduct and into a radial or azimuthal polarization that is particularly advantageous for material processing P3 , P4 to transform.

By means of the frame 7th and the retaining plate 4th are polarization-changing components of the polarization converter P. , especially the reflective axicon optics 1 and the half-wave plates 5 , 6th , fixed to each other. A polarization converter is thus advantageously used P. can be integrated particularly easily, in particular with little adjustment effort, into a laser processing system.

It is also possible to use the second half-wave plate 6th outside of the frame 7th and to be releasably connected to this. The second half-wave plate 6th can be arranged rotatable and lockable in fixed angular steps. This makes it possible to use a polarization converter P. to be provided both for the generation of radially polarized light and for the generation of azimuthally polarized light.

List of reference symbols

1

reflective axicon optics

2, 2 '

Hohlaxikon, Axikon

2.1, 2'.1

frustoconical recess, lateral surface

2.2

Entry hatch

2'.2

Exit hatch

2.3, 2'.3

Floor space

3, 3 '

inner axicon, axicon

3.1, 3'.1

Outer surface

3.3, 3'.3

Floor space

4th

Retaining plate

4.1

first plane surface

4.2

second plane surface

5

first half-wave plate

6th

second half-wave plate

7th

Version

7.1

Cooling fins

8th

Laser source

9

Beam shaper

10

Scanning optics

11

Quarter wave plate

100

refractive axicon optics

100.1

inner jacket surface, conical recess

100.2

outer jacket surface

100.3

Flat surface

100.4

Floor space

A.

exit

E.

entry

MR

reflective coating

P.

Polarization converter

P1

circular polarization

P2

azimuthal inhomogeneous linear polarization

P3

radial polarization

P4

azimuthal polarization

S.

beam

W.

workpiece

X

optical axis

α

Angle of incidence

β

Cone angle

Claims (13)

Polarization converter (P), comprising - A reflective axicon optics (1) with a first hollow lexicon (2) with a straight frustoconical recess (2.1) and a first inner axicon (3) arranged concentrically therein and to an optical axis (X), the conical recess (2.1) and the inner axicon (3) having the same cone angle (β) and on the spaced-apart lateral surfaces (2.1, 3.1) are provided with a polarization-direction-dependent reflective coating (HR) and with an optically transparent holding plate (4) arranged perpendicular to the optical axis (X) and having a first plane surface (4.1) on which the base surfaces (2.3, 3.3) of the axicons (2, 3) are arranged flush, with an entry hatch (2.2) being arranged at the tip of the frustoconical recess (2.1) concentric to the optical axis (X), - A first half-wave plate (5) arranged on the exit side of the reflective axicon optics (1) perpendicular to the optical axis (X), the reflective coating (HR) being selected so that when a beam (S) enters collimated to the optical axis (X) at least one reflection on each lateral surface (2.1, 3.1) of the axicons (2, 3) on the exit side causes a phase difference totaling Δφ = N π + π / 2 between the fast and slow light propagation direction, where N π is an integral multiple of π indicates. Polarization converter (P) according to Claim 1 , additionally comprising a second half-wave plate (6) arranged on the outlet side of the first half-wave plate (5), the crystal axis of the second half-wave plate (6) relative to the crystal axis of the first half-wave plate (5) by an angle of rotation of -π / 8 or an angle of rotation of π / 8 is rotated. Polarization converter (P) according to Claim 1 or 2 , characterized in that on a second planar surface (4.2) of the holding plate (4) of the reflective axicon optics (1) opposite the first planar surface (4.1) a second hollow lexicon (2 ') mirror-symmetrical to the first hollow lexicon (2) and therein a second inner axicon (3 ') are arranged mirror-symmetrically to the first inner axicon (3), with an exit hatch (2'.2) at the tip of the frustoconical recess (2'.1) of the second hollow axicon (2') concentric to the optical axis (X) is arranged and where the cone angles (β) of the axicons (2, 2 ', 3, 3') are chosen to be the same and the reflective coating (HR) is chosen so that when a beam (S ) through at least one reflection on each lateral surface (2.1, 2'.1, 3.1, 3'.1) of the axicons (2, 2 ', 3, 3') on the exit side, a phase difference totaling Δφ = N · π + π / 2 between the fast and slow light propagation direction, where N · π is an integer is a multiple of π. Polarization converter (P) according to one of the preceding claims, characterized in that the reflective coating (HR) of the lateral surfaces (2.1, 2'.1, 3.1, 3'.1) as a highly reflective coating with a reflectivity of at least 99.7% in is carried out in a wavelength range. Polarization converter (P) according to one of the preceding claims, characterized in that the reflective axicon optics (1) is made in one piece, the axicons (2, 2 ', 3, 3') made of quartz glass and with a holding plate (4 ) are connected. Polarization converter (P) according to one of the preceding claims, characterized in that the reflectivity of the reflective coating (HR) is chosen so that the laser-induced damage threshold of the reflective axicon optics (1) is above 10 joules per square centimeter. Polarization converter (P) according to one of the preceding claims, characterized in that at least the reflective axicon optics (1) is held in a mount (7) which can be cooled by means of air or a cooling liquid. Laser optics, comprising a polarization converter (P) according to one of the preceding claims and a laser source (8) arranged on the inlet side of the polarization converter (P), comprising a laser, the laser collimating to the optical axis (X) of the polarization converter (P) linearly polarized laser light is set up and wherein a quarter-wave plate (11) is arranged in the beam path between the laser and the polarization converter (P), the crystal axis of which is rotated by π / 4 with respect to the polarization direction of the laser light and the crystal axis of the first half-wave plate (5) π / 8 is rotated with respect to the polarization direction of the linearly polarized laser light. Laser optics after Claim 8 , characterized in that a polarization-maintaining optical scanning system (10) is arranged on the exit side of the polarization converter (P) and is set up for pivoting a beam collimated to the optical axis (X) of the polarization converter (P) and emerging from the polarization converter (P) along at least one scanning direction . Laser optics after Claim 9 , characterized in that a beam shaper (9) for adapting the diameter of the collimated beam exiting from the polarization converter (P) to the polarization-maintaining scanning optics (10) is arranged on the entry side and / or exit side of the polarization converter (P). Process for the production of a polarization converter (P) according to one of the Claims 1 until 6th , characterized in that the at least one hollow axicon (2, 2 ') and the at least one inner axicon (3, 3') are measured with an angular deviation of the cone angle (β) of at most 0.01 ° and a surface deviation of at most one Fresnel ring at a wavelength of 546 nanometers, are glued to the holding plate (4) by means of alignment putty to form a one-piece reflective axicon optic (1) and these are held in a mount (7) with the at least one half-wave plate (5, 6) or the wave plate whereby a cone tip with a tip diameter of at most 800 µm is formed. Procedure according to Claim 11 , characterized in that the at least one hollow axicon (2, 2 ') and the at least one inner axicon (3, 3') are formed by means of diamond turning. Procedure according to Claim 12 , characterized in that the at least one hollow lexicon (2, 2 ') and the at least one inner axicon (3, 3') are formed by polishing, with polishing movements on an inner axicon (3, 3 ') being controlled so that they do not lead over the tip of the cone of the inner axicon (3, 3 ') and a Conical tip is polished out with a tip diameter of no more than 800 µm.

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