Patent Application
20210237199
The invention concerns the field of adaptive beam shaping of coherent or partially-coherent radiation, in view of parallel direct structuring, micro structuring and beam splitting, as well as for an industrial use in machining material, whereby a phase-modulating Spatial Light Modulator (SLM) is used for beam shaping.
A number of publications are presented hereinafter to describe the background of the invention.
Methods for adaptive beam shaping of laser radiation by means of Spatial Light Modulators (SLM) are described only relatively seldom in professional and scientific literature. One of the most remarkable publications in this area is the paper [SIL13] by Matti Silvennoinen published in 2014. The method of proceeding described therein has the following properties:
The disclosed manner of proceeding requires that the output distribution, or primary laser beam profile is known at the position of the SLM display. The measurement thereof requires at least a supplementary effort in time and a change of the optical set-up of the beam path, which is a drawback for production.
The disclosed set-up lacks a monolithic character, i.e., the partial extraction of the beam for the measurement by a camera, which is essential for the CL, is part of the final optical group and occurs at the last tilted mirror. For this reason, the optical set-up and its manner of operation are rather inflexible. The adaptive beam shaping may only be realized for the entire micro-structuring system, but not for the beam by itself. Unfortunately, this property is a real problem for an industrial use, which may be overcome only with difficulty, because the concept of the system must be adapted specifically for each machining task, and production is stopped during this period.
In addition, the system makes no use of any transformation lens to produce an intermediate distribution, which in turn may be projected in a reduced scale. The intermediate distribution is obtained by means of spherical phase terms that are added to the original phase distribution reproduced on the SLM, and acts as a virtual lens. This has the advantage that non-diffracted beam components will not aggregate in the zero order, but instead spread over the entire output distribution and thereby would be outshone—at least this is the case for most of the intensity distributions. A disadvantage of this method is that some information may be lost in the phase representation; see FIG. 3A in [SIL13]. This happens because the spatial frequencies in the phase representation increase towards the borders due to the spherical summand, until they get higher than the resolution of the SLM display. This also sets a lower bound for the focal length of such spherical summands, which is at around 500 mm for a 20×20 μm2 pixel size. Up to this lower bound of focal length, the SLM display does not comprise any useful phase information and thereby increases the output in the zero-order content. This is the reason why it is contrary to the initially mentioned advantage. Furthermore, this loss of information towards the border may also account for the fact that a non-ordered diffraction of the laser radiation occurs, whereby radiation is diffracted in angles, which are not caused by the initial grating period and may not be corrected anymore by the CL.
A further development is disclosed in [BEC11], which also presents a method based on a CL to achieve beam splitting. The method is called a “modified iterative Fourier-transformation Algorithm” (MIFTA). The method may be described as follows:
In the presented set-up, there is only use of nanosecond laser radiation. The publication fails to mention whether the disclosed manner of proceeding is also suitable for ultra-short pulsed laser radiation. Ultra-short pulsed laser radiation, i.e., pico- and femtosecond pulsed laser radiation, may behave differently at optical gratings because of the shorter time-dependent length of coherence and the greater bandwidth during diffraction at optical gratings.
The display of the utilized SLM is controlled in a binary fashion, which has the consequence that the liquid crystals oscillate around their rotation axis and thereby, the phase position of the laser varies in time, which in turn has an effect on the machining result. The solution to this problem as presented in this publication comprises to make a plurality of exposures and to shift the phase position between the exposures perpendicularly to the direction of propagation of the laser beam. This solution will significantly increase the processing time since the SLM frame-change frequency is in the range of only several 10 Hz and therefore strongly restricts the context of industrial applicability because the structuring time is increased by several orders of magnitude, rendering structuring by means of radiation impossible.
The presented set-up may only be used to obtain real intermediary distributions because there is only one transformation lens. A physical separation of each of a plurality of partial main beams is not possible because of the one transformation lens. If the one transformation lens were removed, the simultaneous monitoring of the produced output distribution could not be realized anymore.
Referring now to [KUA08], [KUA09], [KUA09_2] and [JIN15], researchers working with Zhen Kuang and Walter Perrie present various uses of phase-modulating SLMs for parallel micro structuring. In these papers, the authors utilize:
The described examples always make use of phase gratings that produce beam-splitting ratios smaller than 1:200. This could be tied to the fact that for ultra-short pulsed radiation the power distribution in the partial beams in the real output distribution, which is obtained by means of Liquid Crystals on Silicon (LCoS) SLMs, differs from the theoretical distribution. This causes a loss of precision in the removal in depth among the regions that are machined in parallel. The origins of this behavior have not been studied to date. It is suspected that there may be an overlap of phase information or crosstalk between the pixels of the SLM display as well as a particular coherence behavior of the laser radiation being used. Since these errors are not yet fully known, they cannot be compensated for virtually at the time of calculating the phase values. In case it is sought to have higher splitting proportions and more reliable removal rates between the beams, it is necessary to use a control loop that utilizes the measured beam profile as input value for control.
Laser micro machining may be used for fast and flexible machining of masks, masters and specific surface modifications. In addition, it is possible to achieve a focused machining of certain solids by pico- and femtosecond pulsed laser radiation, i.e., so-called ultra-short pulsed radiation. A further advantage of ultra-short pulses is the wide spectrum of materials that can be machined. Hence, it is possible to produce micrometer and sub-micrometer structures particularly in glasses, metals, metalloids, hard coatings, and plastics.
For smallest possible non-self-organizing structures that are to be structured by a laser radiation, in general a single, sharply focused beam having a comparatively large divergence will be utilized. The resulting comparatively high peak intensities in the focal plane allow obtaining such structures with pulse-energies in the range of nJ up to the lower μJ-range. In order to use this technology at industry relevant machining speeds, the usual approach has been to increase the repetition rate and feeding speed of the beam guiding systems. Alternatively, it is possible to apply adaptive beam shaping to utilize high pulse energies; SLMs reveal themselves as efficient for this purpose. The phase gratings applied to the display split the beam into hundreds or thousands of partial beams and hence allow parallel machining ([SIL13]). However, it has been shown that there are a number of factors influencing the quality of the produced hologram that go beyond the mere quality of the phase calculation algorithm. This is for example the case for the dispersion and deviation in the optical projection with LCoS displays, caused by the high bandwidth when using fs-lasers ([KUA09]; [JES10]; [HAS07]; [RON12]). Again, this has a limiting effect in the machining speeds that may be achieved practically and depending on the set-up, may also account for the fact that the full energy of the laser pulses may not be made available and/or the achievable precisions of parallel machining is inferior compared to single-focus, sequential machining.
Further, to Date Unsolved Problems Relating to Micro Structuring with Ultra-Short Pulsed Radiation
A possibility to increase the area rate with high pulse energies is to use masks and a homogenized, flattop beam profile and to project this resulting distribution in reduced scale. However, it often happens that beam profiles may not allow to be homogenized at all or homogenized in the long term. For this, it would be required to have an optical homogenizer that requires a stable and determined beam profile. However, real-life beam profiles of ultra-short lasers with high pulse energies are unstable over time and very often deviate too much from the ideal Gaussian profile. Furthermore, the mask production and the change of mask during the structuring require additional time.
A simple possibility to accelerate the mask change is offered by SLMs. Phase-modulating SLMs also offer the possibility to influence the beam profile of the laser. This enables to make the beam profile programmable and may be switched in fractions of seconds. However, the produced output distributions often differ from the theoretically predicted ones. By using a control loop that measures the produced output beam profile, it is possible to deal with the phase error and produce beam profiles that may be used for a precise parallel micro structuring.
Most of the set-ups of this kind known to date have a strongly limiting effect on the functionality of the laser system and often only fulfill one specific type of machining. In addition, there are still no devices commercially available that can overcome the error in the output distribution caused by the erroneous phase representation and by primary laser beams of low quality. Also, it is necessary in most of the cases to provide the actual input profile of the beam as boundary condition to the calculation methods of such set-ups, despite the fact that this has been shown not to be necessary. In such an optimized setup, if any error should occur in the output distribution that is caused by an erroneous input distribution, it would be eliminated automatically by the closed-loop algorithm. This fact may be taken into account for a monolithic black box set-up that may implement various beam splitting/shaping functions for any type of primary beam and independently of the laser source.
In a first aspect, the invention provides a method for adaptively splitting an at least partially coherent primary light beam by means of a Spatial Light Modulator (SLM). The method comprises providing the partially coherent primary light beam by means of a collimated laser beam from one of the list comprising a nanosecond laser, a picosecond laser, a femtosecond laser, producing a desired far-field distribution which is not dependent on the profile of the coherent primary light beam by phase modulating the primary light beam with the Spatial Light Modulator (SLM), the primary coherent light beam being directed to reflect on a display element of the spatial light modulator, thereby avoiding any moving element to shape the primary coherent light beam, extracting from the primary light beam, after it has passed the spatial light modulator, a monitoring beam and a main beam, directing a far-field distribution produced by the SLM in the monitoring beam onto a sensor surface of a camera and measuring the monitoring beam with the camera. In a first option, the method comprises guiding the primary beam through a first focusing element that is configured to generate the far-field distribution in the main beam onto a focusing plane of the first focusing element as a real output distribution, as well as projecting the far-field distribution in the monitoring beam onto the sensor surface by means of the first focusing element. In a second option, the method comprises guiding the monitoring beam through a second focusing element that is configured to project the far-field distribution only onto the sensor surface. For either the first or the second option the method further comprises to match a dynamic range of the camera by using a variable intensity regulator to control the intensity of the incoming monitoring beam as a function of the desired far-field distribution, independent of the average power of the main beam, and configuring a closed loop to enable a phase calculation for the display element of the spatial light modulator, whereby an output signal from the camera is input into the closed loop for a plurality of iterations of a phase-calculation algorithm performed by a controller. Further, in the first option, the first focusing element is used, excluding the second focusing element, and in the second option, the second focusing element is used, excluding the first focusing element.
In a preferred embodiment, the method further comprises structuring a solid surface with the main beam and controlling the structuring by setting the spatial light modulator to obtain a determined profile for the primary light beam.
In a further preferred embodiment, using the first option, the method further comprises adjusting the focusing plane of the first focusing element for the step of structuring the solid surface, such that the focusing plane corresponds to the solid surface.
In a further preferred embodiment, using the first option, the method further comprises adjusting the focusing plane of the first focusing element to an intermediate plane, and imaging the intermediate plane by means of a projecting optical set-up in a reduced scale onto the solid surface.
In a further preferred embodiment, using the second option, the step of structuring involves applying the far-field distribution, for obtaining the structures on the solid surface, by projecting the far-field distribution by means of a focusing optical set-up onto the solid surface.
In a further preferred embodiment, the step of extracting involves a beam-splitting element. The method further comprises integrating the beam-splitting element, the first and the second focusing elements, the spatial light modulator, the variable intensity regulator, and the camera into a single compact enclosure, whereby the single compact enclosure is configured to be an exclusive component to be placed in the primary coherent light beam, switching between the first option and the second option by means of a controller device, and adjusting the variable intensity regulator by means of the controller device.
In a further preferred embodiment, using the second option, the method further comprises splitting the primary light beam into a plurality of partial main beams by means of the spatial light modulator, individually adjusting a number of the plurality of partial main beams, angles of separation between each of the plurality of partial main beams after the plurality of partial main beams leaves the display element of the spatial light modulator, and intensities of each of the plurality of partial main beams among each other by phase control. The intensities of the plurality of partial main beams among each other is controlled by the controller that is further configured to control an individual generation for each of the plurality of partial main beams, the spatial light modulator and the primary light beam. The method also comprises structuring the solid surface with the plurality of partial main beams.
In a further preferred embodiment, the solid surface comprises any one of the list comprising metal, diamond, sapphire, glass, plastic, composite materials, leather.
In a further preferred embodiment, the solid surface is a part of a working tool.
In a further preferred embodiment, the working tool is any one of the list comprising an embossing roller, a stamping device, a metal tool, a wristwatch component, a jewelry part, a packaging component.
In a second aspect, the invention provides a device configured for an adaptive splitting of an intended at least partially coherent primary light beam by means of a Spatial Light Modulator (SLM), whereby the intended partially coherent primary light beam is from a collimated laser beam from one of the list comprising a nanosecond laser, a picosecond laser, a femtosecond laser. The device comprises the spatial light modulator configured to produce a desired far-field distribution which is not dependent on the profile of the coherent primary light beam by phase modulation of the intended primary light beam, whereby the spatial light modulator is further configured to obtain the primary light beam from a display element of the spatial light modulator, thereby avoiding any moving elements to shape the primary light beam; a beam splitting element positioned in the intended primary light beam after it has passed the spatial light modulator, and configured to extract from the primary light beam, an intended monitoring beam and an intended main beam; and a camera positioned in the intended monitoring beam and further comprising a sensor surface configured to measure a far-field distribution produced by the spatial light modulator in the monitoring beam. In a first configuration option, the device comprises a first focusing element configured to project the far-field distribution produced by the spatial light modulator in the monitoring beam on the sensor surface, and further configured to guide the primary beam and generate the far-field distribution in the main beam onto a focusing plane of the first focusing element as a real output distribution. In a second configuration option, the device comprises a second focusing element positioned in the intended monitoring beam and configured to project the far-field distribution produced by the spatial light modulator only onto the sensor surface. For either the first or the second configuration option, the device further comprises a variable intensity regulator positioned in the intended monitoring beam in front of the camera, and configured to match a dynamic range of the camera by control of the intensity of the incoming intended monitoring beam as a function of the desired far field distribution, independent of the average power of the main beam; a closed-loop set-up configured to enable a phase calculation for the display element of the spatial light modulator, comprising a controller connected to the camera to receive an output signal, and configured to use the output signal for a plurality of iterations of a phase-calculation algorithm performed in the controller. The device further comprises switching means configured to switch between the first configuration option and the second configuration option. In the first configuration option, the first focusing element is used, excluding the second focusing element, and in the second configuration option, the second focusing element is used, excluding the first focusing element.
In a third aspect, the invention provides a device configured for a machining of an intended solid surface, which comprises the device configured for an adaptive splitting of an intended at least partially coherent primary light beam by means of a Spatial Light Modulator (SLM). The device configured for the machining of the intended solid surface further is configured to position the intended solid surface in the main beam, whereby the machining of the intended solid surface results from at least a setting of the spatial light modulator to the effect of producing a determined profile for the primary light beam.
In a further preferred embodiment, wherein the device configured for an adaptive splitting is configured in the first configuration option, and further wherein the focusing plane of the first focusing element (L1) is configured to correspond to the intended solid surface.
In a further preferred embodiment, wherein the device configured for an adaptive splitting is configured in the first configuration option, the device further comprises a imaging optical set-up, the first focusing element being positioned such that its focusing plane corresponds to an intermediate plane, and the imaging optical set-up being configured such that it images the intermediate plane in a reduced scale onto the solid surface.
In a further preferred embodiment, wherein the device configured for an adaptive splitting is configured in the second configuration option, the device further comprises a focusing optical set-up, and the focusing optical set-up being configured such that it projects the desired far field distribution of the main beam onto the solid surface for the machining.
In a further preferred embodiment, the device further comprises a single compact enclosure configured to integrate the beam-splitting element, the first and second focusing elements, the spatial light modulator, the variable intensity regulator, and the camera, whereby the single compact enclosure is configured to be an exclusive component to be placed into the primary light beam; further wherein the controller is configured to control the switching means to switch between the first configuration option and the second configuration option, and to adjust the variable intensity regulator.
In a further preferred embodiment, the spatial light modulator is enabled to split the primary light beam into a plurality of partial main beams, and to individually adjust a number of the plurality of partial main beams, angles of separation between each of the plurality of partial main beams after the plurality of partial main beams leaves the display element of the spatial light modulator, and intensities of the plurality of partial main beams among each other by phase control, wherein the controller is further configured to control the intensities of each of the plurality of partial main beams among each other, by control of spatial physical properties for each of the plurality of partial main beams, whereby the spatial physical properties are at least one of phase and amplitude, the spatial light modulator and the primary light beam. The device is further configured to position the intended solid surface in the plurality of main beams.
In a further preferred embodiment, the intended solid surface is a surface of an intended embossing roller and the controller is further configured to position the surface of the intended embossing roller in the main beam.
In a further preferred embodiment, the intended solid surface comprises any one of the list comprising metal, diamond, sapphire, glass, plastic, composite material, leather.
In a further preferred embodiment, the intended solid surface is a part of an intended working tool.
In a further preferred embodiment, the intended working tool is any one of the list comprising an embossing roller, a stamping device, a metal tool, a wristwatch component, a jewelry part, a packaging component.
The invention will be understood better through the detailed description of preferred embodiments, and in reference to the figures, wherein
It is a goal of the invention to make it easier for a user to obtain beam shaping, and more particularly beam splitting of laser radiation. The invention, herein after named the “SLM module”, in a further aim, provides a device and a method that may be integrated in any optical path, and offers a comparatively high degree of compactness and individual functionality, independent of the place of use. Another aim of the invention is to allow a splitting of any type of collimated primary beam (laser source) with a comparatively high precision, independent of beam profile, phase position, bandwidth/pulse duration and laser power.
One advantage that derives from the invention is that any limitation of the SLM module may be anticipated through the understanding of the properties of its core element, the SLM display as such. Another advantage lies in the fact that the beam splitting may be implemented with very little losses only. Hence, the incoming radiation should be purely phase modulated, although a simultaneous or sole amplitude modulation would also produce this type of functionality. A further advantage is that the invention enables to switch back and forth between a real output distribution in a focal plane of a lens, i.e., the Fourier plane and a far field distribution, i.e., the diffraction image that occurs at infinite distance. A further advantage is that a comparatively high degree of precision may be achieved in the produced output distribution as well as the beam-power distribution from one partial beam to another, because the invention utilizes a closed-loop approach for the calculation of the phase values that is not dependent on the input profile or the primary beam profile of the laser source, respectively.
The inventive SLM module allows a more efficient use of the laser power by enabling parallel micro machining and machining of solid surfaces with a plurality of beams, too. This may be at a working station by means of an output optics used in common, or at a plurality of systems simultaneously, whereby each of the plurality of systems makes use of a split primary beam that is shared by all machining systems. The parallel machining may reduce the duration of the process for machining larger surface and volumes, linearly with the number of partial beams. Mien using a shared output optics for all partial beams, it becomes possible to produce groves, grating structure and other highly repetitive patterns with increased speed. The SLM module according to the invention enables to achieve a simultaneous and precise machining with a large number of partial beams, for example 1000 beams or more.
Based on the above-mentioned goals and properties of the SLM module according the invention, the production performance of a laser micro machining/structuring device may be increased drastically while still keeping a compact footprint and highly stable operation condition. Therefore, this will allow for industrially applicable reductions in production cost and lead time.
The primary laser beam used throughout this invention must respond to a number of criteria in order to enable the functionality of the SLM module. A first criterion is that the primary beam at the input of the SLM module is collimated or parallelized. The person skilled in the art is aware of the fact that the beam for wave-optics elements and optical gratings must have a coherence length in the order of the grating period. Since ultra-short pulsed radiation is used in the context of this invention, the time-dependent coherence length may also be very short, such as for example only a few micrometers.
The person skilled in the art is further also aware, that when a SLM display of the SLM module comprises an element based on liquid crystals on Silicon (LCoS) technology, the input radiation must inherently obey to determined polarization states relative to an orientation of the SLM display. This can be adjusted and taken care of by means of commercially available structural elements positioned in front of the SLM module. This is the reason why this will not be discussed further in the present document. It is also generally known that LCoS technology has a strongly wavelength-dependent behavior.
Also generally known is the fact that a user of the SLM module may select and adjust the beam-diameter, the mean optical power or a pulse energy, as long as the user remains below physical destruction limit of any component found in the SLM module.
As will become apparent from the following description, the inventive method and device enable the user, starting from a primary input beam that fulfills the above-mentioned criteria, to split this primary input beam into an arbitrary number of output beams, which may be put to use at the user's choice. The output beams may be controlled in their spatial and intensity distribution in a time-dependent manner by the user or a control process. The invention further enables to use either a real intermediate distribution or merely an angular distribution, depending on the desired machining task. The invention enables to achieve this without any precise knowledge of the profile of the primary input beam or its coherence length. The invention requires no other prerequisites for the primary input beam than those described herein above. Any deviations occurring in the output main beam due to the lack of information may be compensated by means of the closed-loop algorithm, which is used to optimize the output distribution.
The invention enables a so-called “Black-Box-Optics” that may be integrated into any kind of optical paths for micro machining.
While in the present detailed description the SLM head 101 is used in a mode in which it reflects the radiation from the primary beam, in a further preferred embodiment it is possible to have a SLM head which works as a transmitter for the primary beam.
A picture recorded by the CCD camera 104 must always be the Fourier transformation of the input distribution. For this reason, the sensor surface (not represented in
The CCD camera 104 enables to obtain a measure of the output distribution, which in turn may be used as an input into an iterative Fourier-Transformation-Algorithm (IFTA)-based closed-loop control to optimize the phase distribution generated by the SLM head 101. As mentioned before, this requires measuring the output distribution with the CCD camera 104 and feed the algorithm executed on the controller CPU 106 as part of the boundary conditions of the IFTA. The IFTA algorithm will not be discussed here in more detail, because it may be found in dedicated literature from prior art. The calculated phase values are applied subsequently to the SLM display.
The SLM module may adopt any one of two hardware configurations to enable a switch between angular distribution and real intermediate distribution. A switching between both of these configurations, as well as a change of filter settings for the variable intensity regulator may be realized manually or with automated switching means.
In a first hardware configuration, called configuration 1 herein, and as represented in
Now looking at
The invention may be realized through a variety of alternative constructions that all provide the same functionality. Each of the variety of alternative constructions may be considered to possess various disadvantages and advantages as compared to configurations 1 and 2.
One of the advantages of the inventive SLM module is to enable a beam shaping with virtually no losses. Consequently, no absorption masks or scatter masks need to be used to obtain the desired output distribution. The beam shaping is done exclusively by means of the phase distribution applied to the SLM head. However, the person skilled in the art will acknowledge that for real life optical paths, there will inevitably occur small losses to a laser beam.
When using ideal optical elements, there will only be scattering losses at the diffraction grating of the SLM, and this only for higher diffraction orders, since the sought-after target distribution is located exclusively in the first order of diffraction.
The inventive method and device may be integrated into any optical path, as long as there is enough physical space to do so and the afore-mentioned conditions for the laser-beam are fulfilled.
The increased number of focal points has the beneficial effect that machining speeds of the laser process that are based on focused beams increase in proportion to the number of partial main output beams. In addition, it also allows making a more optimal use of the laser power. The switching between the partial beam distribution and the focal distribution may occur at a speed corresponding with the display refresh rate of the SLM head 101 and involves no moving part at all. As the person skilled in the art is aware of, phase modulation also allows correcting Zernike modes and in turn, this will increase the focus quality. If the erroneous phase position of the primary beam is also known, it is possible to improve the beam quality of all partial main beams by means of the SLM module 800. The SLM module 800 does not influence imaging scaling factors as well as beam diameters otherwise.
Parallel micro structuring by means of many partial main beams with individually controllable pulse energy or beam power offers similar possibilities of structuring as with individual beams.
A further possibility to work with such a short pulse-offset is a so-called surface removal, as shown in
A further potential approach in parallel micro structuring consists in spacing the partial beams by a distance of dx, as shown in
An even further possibility is to slant a periodic output beam distribution as compared to the feed by an angle alpha as illustrated in
Making use of a pulsed beam for working, a structuring along the partial beam may allow attributing a periodic function f(x) to the removal-depth or illumination intensity. To this end the feed, the number of partial main beams along the line and their distance must have a fixed ratio.
| Number | Date | Country | Kind |
|---|---|---|---|
| 18193276.5 | Sep 2018 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2019/057532 | 9/6/2019 | WO | 00 |
