Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
  • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
  • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
  • G02F1/135—Liquid crystal cells structurally associated with a photoconducting or a ferro-electric layer, the properties of which can be optically or electrically varied
• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
  • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
  • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
  • G02F1/13306—Circuit arrangements or driving methods for the control of single liquid crystal cells
  • G02F1/13318—Circuits comprising a photodetector
• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F2203/00—Function characteristic
  • G02F2203/12—Function characteristic spatial light modulator
• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F2203/00—Function characteristic
  • G02F2203/18—Function characteristic adaptive optics, e.g. wavefront correction
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping

Definitions

• the invention relates to a method and a device for shaping the intensity profile of a laser beam, in particular for producing a homogeneous intensity profile, and to the use of an optically addressable spatial resolution modulator (OASLM) for shaping the intensity profile of a laser beam.
• OFAM optically addressable spatial resolution modulator
• Laser light is coherent and can be generated as a light beam with a small, albeit finite, opening angle. This tight bundling is particularly advantageous for lighting and imaging purposes, since the wave fronts of the laser light come close to the ideal of plane waves. They allow particularly easy conversion into spherical wave fronts and can be used for high-resolution diffraction-limited focusing.
• a disadvantage of the laser beam is its Gaussian character, which is due to the type of light generation in the resonator.
• the intensity distribution of the light across the beam is in the form of a Gaussian bell curve, i.e. H. the intensity is maximum in the middle of the beam and then falls exponentially towards the edge.
• optical filters so-called "bull-eye” filters, which weaken the laser beam more in the middle than at the edge, so that the bell-shaped course of the beam profile is flattened to an almost rectangular profile.
• These filters essentially consist of a transparent plate, e.g. a glass plate, which depending on the location is covered with a more or less reflective coating, for example a metal.
• the desired beam profile is generated by a suitable choice of the location-dependent optical density or the local transmission and reflection properties.
• These filters are static and can therefore only be used for a specific laser with a fixed, known intensity profile. When the laser changes its profile, e.g.
• the filters change the profile in an undesired form, since they are no longer adapted to the data of the laser.
• the reflective filters of this type also have the disadvantage that the unevenly reflected laser light reacts on the laser and can impair its stability.
• the use of holographic filters for beam shaping is also known (I. Gur et al .: Diffraction limited domain flat-top generator; Opt. Communications 145, 237 (1998)). These filters are also static and cannot react to changes in the laser beam profile over time. Another problem is that the rectangular profile is only generated in the imaging plane of the holographic element.
• the invention is therefore based on the object, while avoiding the disadvantages of the prior art, of forming a predetermined, in particular as homogeneous as possible, rectangular beam profile from any initial intensity profiles, in particular from a Gaussian beam profile, the shaped beam profile being sensitive to fluctuations in the incident Intensity profile and the light intensity should be largely stable. Disclosure of the Invention:
• the object is achieved by a method for shaping the intensity profile of a laser beam, in particular for producing a homogeneous intensity profile, the laser beam striking an optically addressable spatial light modulator (OASLM), the local transmission or reflection properties of which local lighting intensity in a non-linear manner.
• OFAM optically addressable spatial light modulator
• a device for shaping the intensity profile of a laser beam, in particular for producing a homogeneous intensity profile, consists of an optically addressable spatial resolution light modulator (optical addressable spatial light modulator, OASLM), the local transmission or reflection properties of which depend in a non-linear manner on the local illumination intensity, and at least one Teiescope imaging system, which is able to spatially expand the laser beam.
• OASLM optical addressable spatial light modulator
• OASLM optically addressable spatial light modulator
• the invention provides a beam shaper or a method for beam shaping, in which the shaped intensity profile is contrasted by the use of an active or adaptive optical element with optical properties that are dependent on the local illumination intensity
• Changes in the original intensity profile is stabilized.
• an optical element is used, the local transparency of which changes with the local intensity of illumination.
• a specific beam profile is generated automatically without additional controlling influence from the outside, which is almost independent of fluctuations in the initial intensity distribution.
• the method or the beam former can therefore be used for any laser system and only has to be adapted to the current situation to a small extent.
• the optically nonlinear element is an optically addressable, spatially resolving light modulator (English optical addressable spatial light modulator, OASLM, or liquid crystal light valve).
• OASLM optical addressable spatial light modulator
• liquid crystal light valve To be a homogeneous To generate (rectangular) beam profile of a laser, it is preferably operated in the saturation range; the local transmitted intensity is then independent of the local lighting intensity.
• Optically addressable, spatially resolving optical modulators are e.g. from "Spatial Light Modulators; OSA - Technial Digest ISBN 155752-494-7 Washington 1997” and consist of a photoconductive layer and an electro-optical layer that is sensitive to voltage. With local exposure, the voltage in the photoconductor locally breaks down and is transferred to the electro-optical layer, which is thereby local in its transmission or
• the photoconductive layer must be sensitive to the striking illumination wavelength.
• the electro-optical layer is e.g. B. a liquid crystal that has optical modulator properties in wide spectral ranges. Certain materials combine the properties of the photosensitive and tension-sensitive layers, such as. B. photorefractive crystals or polymers (Spatial Light Modulators; OSA - Technial Digest ISBN 155752-494-7 Washington 1997; M. Petrov et al .: Photorefractive Crystals, Berlin 1991).
• liquid crystals which have nonlinear optical properties are used in OASLMs.
• the OASLM is based, for example, on nematic or helical smectic liquid crystals, the latter having an operating frequency of 10 2 to 10 3 Hz and thus enabling a faster reaction to changes in the initial profile compared to elements based on nematic liquid crystals (switching time in the range 10 ⁇ 2 s).
• the modulation properties of these liquid crystals depend in a nonlinear manner on the voltage applied and thus on the local illumination intensity I on the photoconductor.
• a typical transmission characteristic of such an OALSM consists of a linear relationship between illuminating and transmitted intensity for low illuminating intensities and a transition into a saturation range in which the transmitted intensity is almost independent of the illuminating intensity. For higher intensities, the transmitted intensity can again depend more on the lighting intensity.
• the properties of an OASLM described above make it possible in particular to attenuate high light intensities more than low ones. This suppresses the intensity of a Gaussian beam in the center opposite the peripheral areas and the transmitted intensity approaches the rectangular shape with a flat plateau in the center.
• the layers of the OASLM are structured, preferably broken down into individual zones, in particular optical points (pixels), which can preferably be controlled individually. This reduces crosstalk between nearby pixels.
• additional pixel-by-pixel intervention can thus be made in the modulator, in particular in order to specifically adapt the local transmission properties to the initial intensity profile.
• the control of the individual zones can advantageously be regulated by measuring the shaped beam profile and detecting deviations from a desired shape, in particular from the
• Rectangular shape is examined.
• the size of the local deviations is then used via a feedback branch to adapt the transmission properties of the zones or image points of the OASLM.
• the intensity of the laser beam to be shaped is preferably adapted to the saturation range of the OASLM by expanding the beam and / or by means of optical filters.
• an imaging optical system is preferably inserted into the beam path, within which the OASLM is located.
• the imaging optics preferably comprise two telescope imaging systems, which are preferably designed as mechanically or electrically adjustable or controllable zoom systems. The beam expansion is thus variable, so that changes in intensity due to fluctuations in the intensity of the laser or after a replacement of the laser can always be compensated for.
• FIG. 1 shows a typical transmission characteristic of an OASLM
• FIG. 1 schematically shows a typical transmission characteristic of an OASLM, as is used according to the invention, the on the x-axis incident and the transmitted intensity is plotted on the y-axis.
• the OASLM has an essentially linear transmission characteristic, for example it is essentially transparent to the incident radiation.
• the transmitted intensity is essentially independent of the incident one
• this saturation range is chosen as the working range for beam shaping.
• the intensity of the incident laser beam to be shaped is adapted to this work area by means of filters or by beam expansion. Beam expansion has the advantage that the light can be focused again after passing through the OASLM and thus there is less loss of the overall intensity.
• FIGS. 2a-c show three arrangements for beam shaping according to the invention using an OASLM.
• FIG. 2a shows an arrangement in which the OASLM is inserted between two coupler telescopes 1, 2 in the beam path of a laser.
• the telescopes 1, 2 each consist of two lenses with focal lengths f ,, f 2 and f, ', f,', which are arranged at a distance f, + f 2 and f 1 '+ f 2 '.
• the telescopes serve to expand the beam in order to reduce the intensity in the center of the laser beam to such an extent that it falls within the plateau range of the characteristic according to FIG.
• the laser beam with a flattened beam profile leaves the second telescope 2 on the right. If the beam is to be expanded at the same time, the right telescope 2 must have a lower magnification than the left telescope 1.
• the intensity of the laser can already be optimally adapted to the OASLM.
• the light can then be guided directly onto the OASLM without prior expansion, as shown in FIG. 2b.
• the light from fiber 3 falls directly on the beam former OASLM, which can be in optical contact with fiber 3.
• reflection losses during the transition to the OASLM can be kept small by adjusting the refractive index with an oil.
• An advantage of this embodiment is that the OASLM can be made particularly small.
• the invention can be advantageously used commercially in a variety of fields of application in which the most uniform possible illumination of surfaces with laser light is important, in particular in image processing and projection technology, in interferometry and in material processing by means of lasers.

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• Physics & Mathematics (AREA)
• Nonlinear Science (AREA)
• Mathematical Physics (AREA)
• Chemical & Material Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Laser Beam Processing (AREA)
• Liquid Crystal (AREA)
• Lasers (AREA)
• Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)

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Publications (1)

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Cited By (4)

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Citations (4)

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• 1999
  • 1999-07-09 DE DE19931989A patent/DE19931989A1/de not_active Withdrawn
• 2000
  • 2000-06-10 US US09/786,837 patent/US6952296B1/en not_active Expired - Fee Related
  • 2000-06-10 DE DE50001853T patent/DE50001853D1/de not_active Expired - Lifetime
  • 2000-06-10 JP JP2001510036A patent/JP4546684B2/ja not_active Expired - Fee Related
  • 2000-06-10 AT AT00942048T patent/ATE238571T1/de not_active IP Right Cessation
  • 2000-06-10 EP EP00942048A patent/EP1116064B1/de not_active Expired - Lifetime
  • 2000-06-10 WO PCT/EP2000/005367 patent/WO2001004685A1/de not_active Ceased

Patent Citations (4)

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