Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B17/00—Systems with reflecting surfaces, with or without refracting elements
  • G02B17/08—Catadioptric systems
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B19/00—Condensers, e.g. light collectors or similar non-imaging optics
  • G02B19/0004—Condensers, e.g. light collectors or similar non-imaging optics characterised by the optical means employed
  • G02B19/0028—Condensers, e.g. light collectors or similar non-imaging optics characterised by the optical means employed refractive and reflective surfaces, e.g. non-imaging catadioptric systems
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B17/00—Systems with reflecting surfaces, with or without refracting elements
  • G02B17/08—Catadioptric systems
  • G02B17/0804—Catadioptric systems using two curved mirrors
  • G02B17/0812—Catadioptric systems using two curved mirrors off-axis or unobscured systems in which all of the mirrors share a common axis of rotational symmetry
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B19/00—Condensers, e.g. light collectors or similar non-imaging optics
  • G02B19/0033—Condensers, e.g. light collectors or similar non-imaging optics characterised by the use
  • G02B19/0095—Condensers, e.g. light collectors or similar non-imaging optics characterised by the use for use with ultraviolet radiation
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
  • G02B27/09—Beam shaping, e.g. changing the cross-sectional area, not otherwise provided for
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
  • G02B27/09—Beam shaping, e.g. changing the cross-sectional area, not otherwise provided for
  • G02B27/0927—Systems for changing the beam intensity distribution, e.g. Gaussian to top-hat
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
  • G02B27/09—Beam shaping, e.g. changing the cross-sectional area, not otherwise provided for
  • G02B27/0938—Using specific optical elements
  • G02B27/0977—Reflective elements

Definitions

• the present invention relates to a device for expanding a laser beam, in particular a UV laser beam, and an associated method for
• UV lasers ie lasers which generate laser radiation in the UV wavelength range (with wavelengths below the visible wavelength range), are becoming increasingly important, especially in micromachining. This increases the demand for UV lasers of high and medium power.
• Ultraviolet laser radiation often occurs by frequency conversion of an infrared laser beam in nonlinear crystals.
• this frequency conversion requires high infrared intensities and low beam divergence in the nonlinear crystals in order to achieve efficient conversion from the infrared wavelength range into the
• the intensity - in particular that of the UV radiation produced - must not be too high to destroy the crystal or a rapid degradation of its optical
• These conditions may include, but are not limited to
• Galilean lens 1 for beam expansion of (nearly collimated) laser beams, devices for beam expansion, for example in the form of a Galilean lens 1 (see FIG.
• Such an objective 1 has at least two lenses 2, 3 in order to convert an incoming, collimated laser beam 4 into a (also: 6-fold) expanded, likewise collimated laser beam 5.
• the Galilei lens 1 is a beam expansion optics of simple design and can be designed with a short overall length and typically small space using axially arranged spherical lenses 2, 3 so that they very low beam deformations with high tolerance to the input beam position ( ie tilting or displacement of the input beam) lead.
• the input-side lens 2 of the Galilean lens 1 is itself exposed to one of the high UV intensity of the not yet expanded laser beam 4, so that it is particularly when using a dielectric
• Degradation can come from their transmission properties.
• the Galileo lens 1 with an uncoated Input optics are operated, ie without anti-reflection coating.
• Fresnel reflections on the uncoated lens 2 produce power losses.
• the problem of radiation-related remains
• FIG. 2 shows a telescope arrangement 6 with two spherical folding mirrors 7, 8 as reflecting optical elements in a Z-fold for (here: 6-fold) conversion of an incoming collimated laser beam 4 into an expanded collimated laser beam 5.
• the distance between the folding mirrors 7 , 8 is in this case about 150 mm, for both folding levels 7, 8 identical folding angle 2 ⁇ is about 20 °.
• a non-axial mirror system for beam expansion as shown by way of example in FIG.
• Intensity distribution is assumed, in which the 1 / e 2 diameter (ie the diameter at which the intensity has dropped to the 1 / e 2- fold of the maximum value is) 0.83 mm, ie the emerging laser beam 5 should ideally have a 1 / e 2 diameter of 5 mm at the 6-fold magnification shown here.
• the pupil of the simulated mirror telescope 6 was chosen here so that the calculated radius (of approximately 42 prad) of the Airy disk 9 for a UV wavelength of 343 nm is approximately equal to the divergence angle of a laser beam with Gaussian intensity distribution and 1 / e 2 diameter of 5 mm corresponds.
• each three additional beams were calculated with 0.2 ° deviation from the input axis of the laser beam, which are shown in the upper right and lower left and lower right.
• a cylindrical convex mirror which may be inclined to the laser beam by 45 °, and uses a convex cylindrical lens. Due to the inclination of the convex mirror by 45 °, the energy density on the mirror surface should be reduced by the factor root 2 so that the mirror itself does not suffer any damage.
• the cylinder lens is designed to divide the divergent laser beam into one Transfer parallel beam path with enlarged beam cross section. But this only offers the possibility to expand the laser beam in one direction.
• Embodiment of DE 10 2007 009 318 A1 proposed, a device for beam expansion 10 (see Fig. 4) with a spherical convex mirror 7 for beam expansion and two with their cylinder axes orthogonal to each other oriented cylindrical lenses 11, 12 for the conversion of the convex
• Concave mirror 7 divergently expanded laser beam in an exiting
• the device 10 is very much
• FIG. 5 shows a representation analogous to FIG. 3, wherein the properties of the entering laser beam 4 and the pupil correspond to those of FIG. 3, but the scale size S 2 is smaller and lies at 200.
• the spot representation of the far field shown on the top right in FIG. 3 all rays or spots are located within the Airy disk 9, ie the root-mean-square (RMS) radius of the beam distribution is significantly smaller than the radius (approx. 42 rad) of the Airy disk 9, so that the optics shown in Fig. 4 for the expanded laser beam 5 diffraction-limited, if the input beam is axially aligned.
• RMS root-mean-square
• FIG. 6 shows a representation of a far field spot diagram analogous to FIG. 3 and FIG. 5, in which both the properties of the incident laser beam 4 and the pupil (and the scale size S 2 ) are selected as shown in FIG. 5, top left were, but one of the cylindrical lenses 1 1, 12 was tilted by 0.2 ° to the beam direction. As can be clearly seen, a slight misalignment of a respective leads
• Cylindrical lens 1 1, 12 to a strong astigmatism below about 45 °, which leads that the far-field distribution is not diffraction-limited, since a plurality of spots outside the Airy disk 9 (radius about 37 Mrad) come to rest.
• a device for expanding a laser beam comprising: a telescope arrangement with two spherical folding mirrors for expanding the incident laser beam, and a lens arranged in the divergent beam path after the telescope arrangement with a spherical lens surface for collimation of the
• the first folding mirror in the beam path is a convex curved spherical folding mirror and wherein the second folding mirror in the beam path is a concave curved spherical folding mirror.
• the inventors have recognized that by using a combination of two spherical mirrors and a spherical lens as divergenzkorrigierendes output element in the already expanded laser beam beam expansions with very small beam deformation or aberrations can be achieved well below the diffraction limit, wherein the folding angle at the folding mirrors on a relatively large range are freely selectable and the device can be realized with a relatively short design. Since the lens is arranged in the already expanded laser beam and thus exposed to a reduced laser intensity, eliminates the problem of degradation of the transmissive optics, as shown in connection with the Galilean lens of Fig. 1.
• the Device for expanding the laser beam high tolerance to errors in the input beam position and in the orientation of the individual optical
• the production of high-precision spherical optics is simpler and cheaper than the production of cylindrical optics or lenses with aspherical lens surfaces.
• the lens may have a spherical lens surface (in combination with a plane surface) or even two spherical lens surfaces, i. it can be completely dispensed with the provision of an aspherical lens surface.
• the telescope arrangement, in particular the second folding mirror in the beam path can already take over part of the collimation in addition to the expansion, so that the refractive power of the lens which collimates
• Laser beam generated can be reduced.
• the device comprises a displacement device for displacing the lens in the beam direction of the collimated laser beam. Possible radii errors or a collimation error of the input beam can in this way for the collimation of the output beam by simply moving the
• Output lens can be compensated in the axial direction, without causing a lateral beam offset or other optics must be realigned.
• the first, convexly curved folding mirror in the beam path takes over the actual beam expansion in the telescope arrangement.
• the second folding mirror in the beam path is a concavely curved spherical folding mirror, which acts together with the lens as converging optics. In this way, the required refractive power of the collecting optics in the (partially) expanded beam is distributed to two optical elements. This has the advantage that small imaging errors occur and in particular beam position errors or optical adjustment errors have only a slight effect on the imaging properties.
• a majority of the optical aberrations (essentially astigmatism and coma) arising at the first convex mirror disposed at an angle to the direction of incidence of the laser beam may be at the second, also at an angle to the direction of incidence of the (partially) expanded one Laser beam standing folding mirror can be compensated.
• the lens is typically a plano-convex lens, the convex spherical lens surface of which faces away from the telescope assembly, so that the lens can be used to collimate the laser radiation.
• the radius of curvature of the spherical lens surface is hereby preferably matched to the radii of curvature of the folding mirrors such that aberrations of the folding mirrors and aberrations of the lens largely compensate each other and the emerging, collimated laser beam has the desired widening and is collimated as desired.
• a desired expansion eg, by a factor of 1, 42, 2, 3, 6, 10, 20, etc.
• the distances between the first folding mirror, the second folding mirror, and the lens and the folding angles can be suitably selected .
• the requirements with respect to folding angles and overall lengths are far lower than with comparable expansion optics with two spherical mirrors.
• a beam direction of the laser beam entering the telescope arrangement and a beam direction of the laser beam emerging from the telescope arrangement run parallel to one another.
• the folding angles can be freely selected over a wide range and the device can be of a relatively short design be realized, aberrations can be compensated particularly well.
• the folding angle at the two folding mirrors can be chosen to be the same size, but not necessarily the same size must be chosen.
• the beam path of the laser beam entering the telescope arrangement overlaps the beam path of the laser beam emerging from the telescope arrangement. This also called X-folding
• the angle between the laser beam entering the telescope arrangement and the laser beam emerging from the telescope arrangement can be 90 °.
• the device further comprises: a
• Frequency conversion device for frequency conversion of the laser beam from a wavelength in the IR range to a wavelength in the UV range.
• Frequency conversion means may be used for this purpose e.g. have non-linear crystals, which allow the generation of a collimated laser beam with a small diameter.
• the device may also comprise a laser for generating the laser beam, which may have a wavelength in the IR range and in this case is typically converted with the aid of the frequency conversion device into a laser beam having a wavelength in the UV range.
• the IR laser may be an Nd: YV0 4 laser which generates laser radiation with a wavelength of 1064 nm, so that its third harmonic lies at approximately 355 nm and thus in the ultraviolet wavelength range.
• the laser may also be a Yb: YAG laser which generates laser radiation having a wavelength of 1030 nm, so that the third harmonic lies at 343 nm.
• the device has a tilting device for tilting the lens relative to the beam direction of the expanded, divergent laser beam.
• a tilting device for tilting the lens relative to the beam direction of the expanded, divergent laser beam.
• the invention also relates to a method for expanding a laser beam, in particular a UV laser beam, by means of a telescope arrangement with two spherical folding mirrors, wherein the first folding mirror in the beam path and wherein the second folding mirror in the beam path is a concavely curved spherical folding mirror, the method comprises: expanding an incident collimated laser beam at the first folding mirror in the beam path, and collimating the expanded laser beam at the second folding mirror in the beam path and at a divergent beam path after the telescope arrangement arranged lens, which has a spherical lens surface.
• FIG. 1 shows a schematic representation of a device for expanding a
• Fig. 2 is a schematic representation of a device for expanding a
• FIG. 4 shows a representation of a device for beam expansion with a different angle of incidence of the inlet beam
• FIG. 5 representations of the far field of the device of Fig. 4 at
• FIG. 6 representations of the far field of the device of Fig. 4 at a
• Fig. 7 is an illustration of an embodiment of an inventive
• FIG. 9 is an illustration of the device of FIG. 7 with a
• FIG. 10 is an illustration of the far field of the device of FIG. 7
• FIG. 7 shows a device 11 for beam widening of an incoming, collimated laser beam 4, which is connected to a first, convex folding mirror 7 of FIG
• Telescope assembly 6 expanded and converted to a second, concave folding mirror 8 of the telescope assembly 6 and a arranged in the divergent beam path to the second folding mirror 8 lens 3 in a koliim investigating, exiting laser beam 5, which is expanded 6-fold in the present example.
• the folding mirrors 6, 7 are spherical mirrors, the lens 3 is a plano-convex lens, facing away from the second folding mirror 8
• Lens surface 3a is spherically curved.
• the arrangement of the folding mirrors 7, 8 shown in FIG. 7 permits a parallel alignment of the entrance-side laser beam 4 to the exit-side laser beam 5 along a common beam axis (X-direction), which is also referred to as Z-folding.
• X-direction a common beam axis
• the spherical lens 3 as divergenzkorrigierendem output element in the already expanded, divergent beam path can at relatively large, over a wide range freely selectable folding angles 2a, 2ß at the two folding mirrors 7, 8 and in a relatively short design beam expansions are achieved with very small beam deformation or aberrations significantly below the diffraction limit. Since the lens 3 is arranged in the widened Strahiengang and therefore the laser radiation impinges on this with reduced intensity, damage to the lens 3 can be avoided or
• the lens material e.g. Quartz glass
• the required widening of the entering laser beam 4 is first determined (eg 1, 42-fold, 2-fold, 3-fold, 6-fold, 10-fold, 20-fold Etc.). If the expansion is fixed (here: 6-fold), a suitably dimensioned installation space for the device 11 is defined and a suitable positioning of the folding mirrors 7, 8 in the installation space is selected. There is comparatively great freedom in choosing the folding angles 2 a, 2 ⁇ and the distances between the optical elements (i.e., between the folding mirrors 7, 8 and the lens 3).
• the spherical curvature of the folding mirrors 7, 8 and the spherical lens surface 3a of the lens 3 is determined such that on the one hand the desired widening is achieved and on the other hand the aberrations of the folding mirrors 7, 8 or the aberrations of the folding mirrors 7 , 8 and the lens 3 largely compensate, and the exiting laser beam 5 is collimated as desired.
• the folding angle 2 ⁇ (here: approx. 20 °) at the first
• Folding mirrors 8 can, but do not have to be chosen the same.
• the distance between the folding mirrors 7, 8 is in the present case about 150 mm, but it is understood that this can also be chosen larger or smaller.
• the folding mirrors 7, 8 are arranged in a Z-fold
• the folding angle 2 ⁇ here: about 20 °
• the folding angle 2 ß here: about 70 °
• the folding mirrors 7, 8 consist on their surface of a material which is highly reflective for UV laser radiation, e.g. from dielectric layer systems on quartz glass. It is understood that others too
• FIG. 9 shows the device 1 of FIG. 7 on an optical module 14 defining the installation space. On the optical module 14 are next to those in FIG. 7
• a tilting device 16 can also be attached to the lens 3 in order to tilt it relative to the X direction or to the divergent laser beam. By tilting at a suitable angle, the optical errors can be further reduced and / or larger folding angles at the folding mirrors 7, 8, smaller
• Curvature radii of the folding mirror 7, 8 or smaller lengths can be realized. However, it is understood that a tilting of the lens 3 is not mandatory is necessary and even without tilting a diffraction-limited image can be achieved.
• the radii of curvature of the folding mirrors 7, 8 and of the spherical lens surface 3 a are approximately 3 to approximately 6 times widening, typically around 100 to 300 mm for the first folding mirror 7, approximately 1000 to 2000 mm for The second folding mirror 8, or at about 200 - 400 mm for the lens 3, in order to achieve the required refractive power for the expansion.
• the distance between the second folding mirror 8 and the lens 3 may be, for example, in the range between approximately 100 mm and 150 mm. It is understood that it is possible to deviate from the value ranges specified above, in particular if an expansion is to take place which lies outside the specified range (3-fold to 6-fold).
• 9 also shows a frequency conversion device 17 arranged on the module 14 for frequency conversion of an irradiated laser beam 18 whose wavelength lies in the infrared spectral range into a wavelength in the ultraviolet spectral range.
• the frequency conversion device 17 comprises non-linear crystals to achieve the frequency conversion in a manner familiar to those skilled in the art.
• 9 also shows an infrared laser 19, in the present example in the form of a Yb: YAG laser, which generates IR laser radiation with a wavelength of 1030 nm, which in the frequency conversion device 17 into a UV laser beam with high radiation intensity and a wavelength of about 343 nm.
• a Nd: YV0 4 laser can be used to generate the IR laser beam 18.
• the laser wavelength is 1064 nm and the third harmonic generated in the frequency converter 17 is about 355 nm.
• the spots are far in the example shown within the Airy disk 9, ie the RMS radius of the beam distribution is significantly smaller than the Airy radius (about 42 prad), so that the optics shown in Fig. 9 for the laser beam 4, 5 and for the widening imaging diffraction-limited.
• Even with non-axial incidence of the laser beam 4 can thus be achieved with the aid of the device 1 1 a diffraction-limited imaging.
• an expansion of the laser beam 4 while avoiding the degradation of the optics 3, 7, 8 used can be achieved by intensive laser irradiation, the aberrations of the individual optical elements 3, 7 being compensated by mutual compensation. 8 overall, the aberrations kept small and so a diffraction-limited image can be obtained.

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• 2011
  • 2011-03-21 DE DE102011005835A patent/DE102011005835A1/de not_active Ceased
• 2012
  • 2012-03-15 JP JP2014500331A patent/JP6049683B2/ja active Active
  • 2012-03-15 KR KR1020137026820A patent/KR101837232B1/ko active Active
  • 2012-03-15 EP EP12712604.3A patent/EP2689285B1/de active Active
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