WO2009010043A2 - Dispositif à déphasage et cavité laser pour la production d'un rayonnement laser à polarisation radiale ou azimutale - Google Patents

Dispositif à déphasage et cavité laser pour la production d'un rayonnement laser à polarisation radiale ou azimutale Download PDF

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WO2009010043A2
WO2009010043A2 PCT/DE2008/001136 DE2008001136W WO2009010043A2 WO 2009010043 A2 WO2009010043 A2 WO 2009010043A2 DE 2008001136 W DE2008001136 W DE 2008001136W WO 2009010043 A2 WO2009010043 A2 WO 2009010043A2
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laser
grating
phase
laser radiation
laser resonator
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WO2009010043A8 (fr
WO2009010043A3 (fr
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Joachim Schulz
Andreas Voss
Marwan Abdou Ahmed
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Universitaet Stuttgart
Trumpf Laser und Systemtechnik GmbH
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Universitaet Stuttgart
Trumpf Laser und Systemtechnik GmbH
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    • 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
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/13—Stabilisation of laser output parameters, e.g. frequency or amplitude
    • H01S3/1308—Stabilisation of the polarisation
    • 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
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08—Construction or shape of optical resonators or components thereof
    • H01S3/08059—Constructional details of the reflector, e.g. shape
    • H01S3/08068—Holes; Stepped surface; Special cross-section
    • 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
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08—Construction or shape of optical resonators or components thereof
    • H01S3/081—Construction or shape of optical resonators or components thereof comprising three or more reflectors
    • H01S3/0811—Construction or shape of optical resonators or components thereof comprising three or more reflectors incorporating a dispersive element, e.g. a prism for wavelength selection
    • H01S3/0812—Construction or shape of optical resonators or components thereof comprising three or more reflectors incorporating a dispersive element, e.g. a prism for wavelength selection using a diffraction grating
    • 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
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/06—Construction or shape of active medium
    • H01S3/07—Construction or shape of active medium consisting of a plurality of parts, e.g. segments
    • H01S3/073—Gas lasers comprising separate discharge sections in one cavity, e.g. hybrid lasers
    • H01S3/076—Folded-path lasers
    • 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
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08—Construction or shape of optical resonators or components thereof
    • H01S3/08004—Construction or shape of optical resonators or components thereof incorporating a dispersive element, e.g. a prism for wavelength selection
    • H01S3/08009—Construction or shape of optical resonators or components thereof incorporating a dispersive element, e.g. a prism for wavelength selection using a diffraction grating
    • 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
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08—Construction or shape of optical resonators or components thereof
    • H01S3/081—Construction or shape of optical resonators or components thereof comprising three or more reflectors
    • H01S3/0813—Configuration of resonator
    • H01S3/0816—Configuration of resonator having 4 reflectors, e.g. Z-shaped resonators
    • 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
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/13—Stabilisation of laser output parameters, e.g. frequency or amplitude
    • H01S3/1307—Stabilisation of the phase

Definitions

  • Phase shift device and laser resonator for generating radially or azimuthally polarized laser radiation
  • the present invention relates to a phase shift device for stabilizing radially or azimuthally polarized laser radiation within a laser resonator, the laser resonator comprising a polarizer device for generating radially or azimuthally polarized laser radiation. It further relates to a laser resonator for generating and stabilizing radially or azimuthally polarized laser radiation with a polarizer device for generating radially or azimuthally polarized laser radiation.
  • a laser resonator with a polarizer device for generating radially or azimuthally polarized laser radiation is described by the article "Optical Elements of a Laser Cavity for the Production of a Beam with Axial Symmetry Polarization” by Goncharskii et al., Optics and Spectroscopy Vol. 89, no 1, 2000, pages 146 to 149.
  • the article describes axially symmetrical gratings as polarizer devices for generating radially and / or azimuthally polarized laser radiation in a laser resonator, wherein a distinction is made between two types of grating: the first, star-shaped grating type
  • the grating lines extend radially outward from a common center point, so that the distance between two adjacent grating lines and thus the grating period increases with increasing distance from the center point, resulting in radially polarized laser radiation in this grating type only with a fraction of incorrectly polarized radiation received n can.
  • the grid lines are annular, so that the grid has a constant grid period over the entire radial area.
  • the generation of radially or azimuthally polarized laser radiation in a laser resonator can also be carried out by means of polarizer devices, such as, for example, in US Pat. No. 6,680,799 B1, DE 10 2004 042 748 A1 or US Pat. No. 6,191,890 B1, which by way of reference be made to the content of this application. It is understood that in each case the reflectance difference between radially and azimuthally polarized laser radiation per revolution in the laser resonator must be greater than the depolarization for radially / azimuthally polarized radiation per revolution in the laser resonator must be selected. Depolarization is understood to mean the proportion of power in the undesired direction of polarization (e.g., azimuthal polarization) relative to the total power.
  • a sub-wavelength grating for generating radially polarized radiation is known, whose grating period is smaller than the wavelength of the incident laser radiation.
  • the sub-wavelength grating is applied in one embodiment to a dielectric multilayer system disposed on a metallic or dielectric substrate.
  • the polarization selectivity of the grating depends on whether the laser beam is incident from the substrate side or the grating side: in the first case, by coupling in the undesired ones Polarization component in a waveguide mode of a cover layer of the multilayer system, in the second case by absorbing the unwanted polarization component in a metallic substrate or, when using a dielectric substrate in that the unwanted polarization component at angles that are in the range of total reflection, in leaky modes in the Substrate is coupled.
  • the bandwidth of the coupling is very narrow, so that sub-wavelength grids are usually finished with tight tolerances.
  • Grating period is greater than the laser wavelength, so next to the zeroth
  • Diffraction order also higher diffraction orders occur.
  • the grating period and the grating shape are chosen such that with respect to the laser wavelength
  • Reflectance of the TM component in the one used diffraction order is greater than the reflectance of the TE component in this diffraction order.
  • Laser radiation is not thrown back into the one used diffraction order, but depending on the polarization, more or less in others
  • No. 6,191,890 B1 describes an arrangement in which a dielectric layer which has a periodic variation of the dielectric properties parallel to the layer and whose period length is smaller than the wavelength of the laser radiation in the second dielectric medium is disposed between a first and a second dielectric medium is.
  • a dielectric layer which has a periodic variation of the dielectric properties parallel to the layer and whose period length is smaller than the wavelength of the laser radiation in the second dielectric medium is disposed between a first and a second dielectric medium is.
  • Linearly polarized laser radiation 2 has an identical orientation of the electric field strength vector, which is referred to below as the polarization direction Ey, over the entire beam cross-section 1, as shown in FIG. 1a.
  • the laser radiation 2 has no component of the electric field strength vector in a polarization direction E x perpendicular thereto.
  • the laser radiation 3a, 3b in a laser resonator is azimuthally or radially polarized, as shown in Fig. 1b and Fig. 1c.
  • the laser radiation 3a, 3b is locally linearly polarized, but has an inhomogeneous distribution over the beam cross-section 1, ie the electric field strength vectors E ⁇ , E r extend at each point in the azimuthal or in the radial direction the center of the beam cross-section is a zero and the electric field strength vector E ⁇ or E r disappears at this point.
  • FIG. 2 shows a square-folded laser resonator 4 of a CO 2 gas laser with eight discharge paths.
  • a partially transmissive Auskoppelapt 5 and a highly reflective end mirror 6 define the laser resonator 4.
  • the laser beam is folded over eight folding mirrors 7a-c, 8a-b, 9a-c in two superimposed planes square, wherein four folding mirrors 7a-c, 8a in the upper Level and four folding mirrors 8b, 9a-c are arranged in the lower level.
  • the laser beam is deflected by the end mirror 6 at the first three folding mirrors 7a, 7b and 7c in the upper plane by 90 °.
  • the laser beam which is deflected from the third folding mirror 7c to the fourth folding mirror 8a in the upper plane, is also deflected by this and then by the first folding mirror 8b in the lower plane by 90 °, but in a direction perpendicular thereto, and passes from the upper level to the lower level.
  • the three further folding mirrors 9a, 9b and 9c deflect the laser beam by 90 ° in the lower plane.
  • the laser beam impinges on the partially transmitting outcoupling mirror 5, at which part of the laser beam is coupled out of the laser resonator 4 and inserted other part is reflected back in itself.
  • the reflected laser beam strikes the eight folding mirrors 7a-c, 8a-b, 9a-c in the opposite direction and traverses all discharge paths between the outcoupling mirror 5 and the end mirror 6.
  • any polarization state of a laser beam (e.g., radial or azimuthal) after reflection from an optical element can be represented by the superposition of a perpendicular and a parallel polarized component.
  • the perpendicularly polarized (s-polarized) component is oriented perpendicular to the plane of incidence formed by the incident and reflected laser beam and the parallel polarized (p-polarized) component parallel to the plane of incidence.
  • Optical materials (coated and uncoated mirrors) have different reflectivities R as a function of the angle of incidence ⁇ for the s and p-polarized components of a laser beam, as shown in FIG. 3a.
  • the reflectance R of the s-polarized component (curve S) is usually greater than the reflectance R of the p-polarized component (curve P).
  • a phase difference ⁇ dependent on the angle of incidence ⁇ forms between the vertically and parallel-polarized components during reflection, as shown in FIG. 3b.
  • the first three folding mirrors 7a-c of the upper level and the last three folding mirrors 9a-c of the lower level have the same deflection plane (first deflection plane), whereas the deflection plane of the two remaining folding mirrors 8a and 8b is oriented perpendicular thereto (second deflection plane).
  • first deflection plane the deflection plane of the two remaining folding mirrors 8a and 8b is oriented perpendicular thereto
  • second deflection plane the vertically polarized component of the first deflection plane becomes the parallel-polarized component of the second, perpendicular deflection plane.
  • parallel polarized component of the first deflection plane the parallel polarized component of the first deflection plane.
  • MMR Maximum Metal Reflector
  • MMR mirrors have a dielectric coating and produce reflectivities> 99.7% at 10.6 ⁇ m.
  • Manufacturing inaccuracies, for example, in the layer thickness or in the refractive indices of the individual layers result in each folding mirror having individual reflectance and phase differences ⁇ Rj, ⁇ j between the s and p polarized components of the laser beam.
  • the optics manufacturer H-Vl specifies for its folding mirrors tolerances for the reflectance difference of 0.2% and for the phase difference ⁇ 2 °.
  • a measurement of the reflectance and phase differences of a plurality of folding mirrors has shown that typical folding mirrors for CO 2 high-power lasers with a reflection at an angle of incidence of 45 ° have a reflectance difference ⁇ Rj of approximately 0.2% and a phase difference ⁇ j of approximately 1 ° between the perpendicular and parallel polarized components of the laser beam per folding mirror.
  • the production of folding mirrors with narrower manufacturing tolerances would mean a considerable additional expense in terms of production costs.
  • phase difference ⁇ between the s- and p-polarized laser beam components can lead to a situation other than the desired radial or azimuthal polarization state better fulfilling the boundary conditions of the laser resonator and settles in the laser resonator, so that the radial or azimuthal polarization is destroyed.
  • the folding mirrors are selected and arranged in the laser resonator so that the sums of the
  • phase differences between the perpendicular and parallel polarized components of the laser beam can be completely solved neither by more accurate manufacturing process nor by the selection and pairing of the folding mirror.
  • aging effects occur which cause the reflectance and phase differences ⁇ Rj, ⁇ , of the individual folding mirrors to change during the lifetime of a folding mirror.
  • the reflectance and phase differences ⁇ Rj, ⁇ are temperature-dependent, so that the values change due to the absorption of laser radiation in the folding mirror.
  • phase shift device of the type mentioned, which is designed such that they per revolution in the laser resonator, a phase difference between about 30 ° and about 330 °, preferably between about 70 ° and about 290 °, more preferably between about 160 ° and about 200 °, in particular of about 180 ° between the radial and the azimuthal polarization direction of the laser radiation generated and has an axial symmetry.
  • the inventors have recognized that radially or azimuthally polarized laser radiation in the laser resonator can be better controlled by providing means for producing a phase difference between the radial and azimuthal polarization directions, especially when additional phase-shifting optical elements, e.g. Folding mirror or beam splitter are arranged in the laser resonator. Since the unwanted phase difference is usually generated at non-axisymmetric optical elements such as the planar folding mirrors, a phase shifting device having the desired axial symmetry is advantageous for compensating for this undesired phase difference.
  • additional phase-shifting optical elements e.g. Folding mirror or beam splitter
  • the phase-shifting device is designed in such a way that the undesired phase difference that occurs over the entire beam cross-section and varies locally in accordance with the symmetry of the folding mirror is outperformed by a magnitude-wise, axially symmetric distributed phase difference.
  • This phase difference can be constant over the entire field of the laser radiation or vary radially or azimuthally over the field in the abovementioned limits. A constant over the entire field phase difference of about 180 ° has proved to be particularly advantageous.
  • the phase shift device is formed as a grid, wherein the grating period and preferably also the grating shape are selected such that the grating generates the desired phase difference between the radial and azimuthal polarization direction.
  • the axially symmetrical grating may additionally serve as a polarizer device and generate a reflectance difference or a transmittance difference between the radial and azimuthal polarization directions, depending on whether the grating is designed as a reflective or as a transmissive optical element.
  • the phase shift device is designed as a transmissive optical element.
  • a transmissive optical element can, for example, within the laser resonator between a Auskoppelapt and a End mirror are arranged and is passed through by the laser radiation during a revolution in the laser resonator twice, so that this compared to a phase shift device, which is designed as a reflective optical element, only half the phase difference must be generated.
  • the transmissive optical element is designed as a form-birefringent element.
  • a transmissive optical element can be produced on the basis of the shape birefringence Delay between "faster” and “slower” polarization direction and its local orientation of the polarization axes by the type of microstructuring (eg shape and orientation of the individual structures) can be suitably controlled to a radial / azimuthal (or radially symmetric) - birefringent retardation plate ( preferably with approximately 90 ° phase difference in single pass).
  • microstructures it is not absolutely necessary for the microstructures to form a grid, but it is also possible to use microstructures in which structures are provided locally which are not related at the macroscopic level.
  • Shape birefringent gratings as phase shifting devices are e.g. in the following publications: "Broadband form birefringent quarter-wave plate for the mid-infrared wavelength region": Gregory P. Nordin and Panfilo C. Deguzman (11 October 1999 / Vol. 5, No. 8 / OPTICS EXPRESS 163), as well as Lin Pang, Maziar Nezhad, Uriel Levy, Chia-Ho Tsai, and Yeshaiahu Fainman (APPLIED OPTICS / Vol. 44, No. 12), "Form-birefringence structure fabrication in GaAs by use of SU-8 as a dry etching mask” / 20 Apr. 2005), which are incorporated herein by reference for the purposes of this application.
  • the phase shift device is designed as a reflective optical element.
  • the reflective optical element is designed as an end mirror or as Auskoppelapt.
  • These optical elements can additionally be designed as a polarizer device and therefore in addition to the desired phase difference and a reflectance difference generate, by which the radially or azimuthally polarized laser radiation is generated in the laser resonator. Both the generation of the radially or azimuthally polarized laser radiation and their stabilization can therefore be carried out on one and the same optical element.
  • one of the folding mirrors present in the laser resonator may have a grating which is designed as a phase shifting device and generates a phase difference between the radial and azimuthal polarization directions.
  • the reflective optical element comprises a multilayer system which is arranged on a substrate.
  • the multiple-layer system can be designed as a reflection-enhancing coating and for this purpose has a number of individual layers whose refractive indices and thicknesses are matched to one another in such a way that the proportion of radiation reflected by the optical element is as great as possible due to interference effects.
  • Structures may be provided on or in the multilayer system or on the substrate, which produce the desired phase difference.
  • the phase shift device is formed as a grid, in which the grating period, the structure of the multilayer system, the material of the substrate and preferably also the grid shape are selected such that the radial or azimuthal polarization direction of the laser radiation in at least one leaky mode coupled to the multilayer system and the substrate and the phase difference between the radial and the azimuthal polarization direction (E r , E ⁇ ) of the laser radiation is generated.
  • the structure of the multilayer system is determined by the number of individual layers, the sequence of the individual layers and by their refractive indices and geometric layer thicknesses.
  • the grating may in principle be designed like the gratings described in US Pat. No.
  • 6,680,799 B1 which are designed to produce a reflectance difference between the radially and azimuthally polarized laser beam components, wherein in addition the abovementioned parameters still have to be coordinated with one another in such a way that the desired Sets phase difference between the radial and azimuthal polarization direction.
  • the phase-shifting device is designed as a periodic or quasi-periodic, concentric or spiral grating in which the grating period and preferably also the grating shape are selected such that with respect to the laser wavelength, the TM reflectance of the grating into the one diffraction order used is greater or smaller than the TE reflectance of the grating in this diffraction order and the phase difference between the radial and the azimuthal polarization direction of the laser radiation is generated.
  • the grating can basically be designed like the gratings described in the aforementioned DE 10 2004 042 748 A1, whereby in this case too, the abovementioned parameters must be coordinated with one another such that a phase difference between the radially and azimuthally polarized ones occurs.
  • Sets laser beam components which outbid the unwanted, caused for example by the folding mirrors in the laser resonator phase difference.
  • the reflective optical element is formed as a grating at an interface between a first dielectric medium having a first refractive index and a second dielectric medium having a second, smaller refractive index, the grating period being smaller than the wavelength of the laser radiation in the second dielectric Medium is, and wherein the grating period and preferably also the grid shape are selected such that an average refractive index of the grating is between the first refractive index and the second refractive index.
  • the laser radiation propagates in the first medium and is reflected at the interface to the second medium.
  • the second medium is, for example, air or laser gas with a second refractive index.
  • the refractive indices of the dielectric media, the grating period and preferably also the grating shape are selected such that the radial or azimuthal polarization direction of the laser radiation is diffracted into higher diffraction orders and the phase difference between the radial and azimuthal polarization directions of the laser radiation is generated.
  • Generating a particularly large reflectance difference can also be used to generate the desired phase difference of the aforementioned GIRO effect.
  • the phase shifting device is designed as a polarizer device for generating radially or azimuthally polarized laser radiation.
  • the phase shift device in addition to the phase difference, also generates a sufficient degree of reflectance or transmittance difference between the radial and azimuthal polarization directions to form a radial or azimuthal polarization state of the laser radiation in the laser cavity.
  • a reflectance difference between the radial and the azimuthal polarization direction is generated, wherein the reflectance difference is preferably at least 5%, more preferably at least 15% and in particular at least 50%.
  • the invention also relates to a laser resonator with a polarizer device for generating radially or azimuthally polarized laser radiation, in which a phase shift device for generating a phase difference between the radial and the azimuthal polarization direction of the laser radiation is provided within the laser resonator, which is formed as described above so that the desired radial or azimuthal polarization state of the laser radiation in the laser resonator can be stabilized.
  • the laser resonator has at least one, preferably at least three, folding mirrors which fold the beam path of the laser radiation between an end mirror and an output mirror.
  • the folding mirrors allow a compact construction and thus a high mechanical stability of the laser resonator.
  • the laser resonator comprises a coupling-out mirror, which is designed as a phase-shifting device, and an end mirror, which is designed as a polarizer device.
  • the reflectance difference and the phase difference are generated at the two opposite ends of the laser resonator.
  • the laser resonator has an end mirror, which is designed as a phase shift device, and a Auskoppelapt, which is designed as a polarizer device.
  • end mirror which is designed as a phase shift device
  • Auskoppelspiegel which is designed as a polarizer device.
  • the functions of the end mirror and the Auskoppelspiegel are interchanged with respect to the case described above.
  • the laser resonator has a coupling-out mirror, which is designed as a phase-shifting device and as a polarizer device.
  • the combined generation and stabilization of the radially or azimuthally polarized laser radiation on a single optical element can save costs compared to the case where two or more optical elements are provided for this purpose.
  • the laser resonator has an end mirror, which is designed as a phase shifting device and as a polarizer device.
  • the end mirror of the laser resonator can be used for the combined generation and stabilization of the radially or azimuthally polarized laser radiation. Both in the end mirror and in the outcoupling mirror, it is advantageous in this case for the laser radiation to strike almost perpendicular to its surface.
  • the phase shifting device is embodied as a transmissive, in particular as a shape birefringent optical element, which is connected between the output mirror and the end mirror. is orders.
  • a birefringent element may optionally also be used, which is composed of a multiplicity of individual elements (facets) which each have different birefringent properties and, in their entirety, generate desired phase difference.
  • the phase shift device is arranged on the output mirror or the end mirror and is in direct contact with the output mirror or end mirror.
  • a phase-shifting device can serve, for example, a transmissive shape birefringent plate.
  • the phase shifting device can be fixed on the output mirror or the end mirror, e.g. by glued to the edge or stored in a common bracket.
  • the polarizer device is designed as Auskoppelapt or as an end mirror and is used to generate laser radiation with a radial or azimuthal polarization state by the difference in reflectance between these two polarization directions is chosen sufficiently large.
  • the polarizer device is designed as a transmissive, in particular as a form-birefringent optical element.
  • laser radiation with the unwanted state of polarization is not or only slightly transmitted and thus produces a difference in transmission between the radial and azimuthal direction of polarization.
  • the polarizer device is formed by the at least one folding mirror.
  • the radially or azimuthally polarized laser radiation is generated by the reflectance difference of the or the folding mirror between the perpendicular and parallel to the plane of incidence polarized components of the laser radiation, as shown in Fig. 3a.
  • the phase shift means for generating a phase difference between the radial and the azimuthal Polarization direction of the laser radiation is designed in the laser resonator between 30 ° and 330 °, preferably between 60 ° and 300 °, more preferably between 65 ° and 90 °, most preferably between 70 ° and 80 °, in particular at 75 °.
  • FIGS. 1 a-c show the beam cross section of a laser resonator with linear (FIG. 1 a), azimuthal (FIG. 1 b) and radial (FIG. 1 c) polarized laser radiation;
  • Fig. 2 shows a folded laser resonator with a Auskoppelapt, a
  • FIGS. 3a, b the reflectivities R (FIG. 3a) and phase differences ⁇ (FIG. 3b) of the perpendicular and parallel polarized components of a laser beam as a function of the angle of incidence ⁇ of the laser beam for a dielectrically coated folding mirror;
  • FIGS. 4a-c a grid for generating radially polarized laser radiation in one
  • FIG. 4a An oblique view and a detail view according to III (FIG. 4a), a leaky mode grating applied to a dielectric substrate and multilayer system (FIG. 4b) and a GIRO effect grating applied to a dielectric substrate (FIG. 4c);
  • FIGS. 5a, b show embodiments of a laser resonator according to the invention with a coupling-out mirror as a phase shifting device and an end mirror as a polarizer device (FIG. 5a) and with the FIG Output mirror as a phase shift and polarizer device (Figure 5b);
  • FIGS. 6a-d show embodiments of a laser resonator according to the invention with a shape-birefringent plate as a phase-shifting device, which is mounted between end mirror and output mirror (FIGS. 6a, b), on the end mirror (FIG. 6c) or on the output mirror (FIG is;
  • FIGS. 7a, b Calculated reflectivities and reflectance differences (Figure 7a) and calculated phase differences (Figure 7b) of a leaky-mode grating
  • FIGS. 8a, b calculated reflectance and reflectance differences (FIG. 8a) and calculated phase differences (FIG. 7b) of a grid with GIRO
  • the Fign. 4a-c show known polarizer devices for generating radially or azimuthally polarized laser radiation, which can be arranged in a laser resonator.
  • Fig. 4a shows a grating mirror 10 with a grid 11, which has a ring structure with concentric grating grooves 12 and grid bars 13 and is produced on a surface 14 of a metallic substrate 15 by turning.
  • the laser radiation with GE tangentially to the grating grooves 12 extending polarization as TE polarized and designated perpendicular to the grating grooves 12 extending polarization as TM polarized.
  • the essential parameters by which the polarization is influenced are the grating period ⁇ , the depth d of the grating grooves 12 and the width b of the grating webs 13.
  • the filling factor f is the ratio of the width b of the grating webs 13 to the grating period ⁇ .
  • the TE component corresponds to the azimuthal polarization direction and the TM component corresponds to the radial polarization direction.
  • the grating 11 can be designed as described in DE 10 2004 042 748 A1, ie have a grating period ⁇ which is greater than the wavelength of the laser radiation in the laser resonator, or designed according to US Pat. No. 6,680,799 B1 or US Pat. No. 6,191,890 B1 be, ie have a grating period ⁇ , which is smaller than the laser wavelength.
  • FIG. 4 b shows a grating mirror 20 with a sub-wavelength grating 21 formed on a cover layer 24 of a multi-layer system 22, which is arranged on a dielectric substrate 23.
  • the sub-wavelength grating 21 may also be disposed within one or more layers of the multilayer system 22 or between the multilayer system 22 and the substrate 23 (not shown).
  • the grating period and the grating shape of the grating 21 are selected such that a polarization direction of the laser beam is coupled into at least one leaky mode of the multilayer system 22 and the substrate 23. This can be done as described in US 6,680,799 B1, wherein the spectral position of the at least one leaky mode is selected such that it is close enough to the laser wavelength ⁇ 0 .
  • the position of the reflectance difference ⁇ R between the axial and radial polarization directions can be influenced to be in the desired range.
  • the design can also be designed so that several leaky modes are in the immediate vicinity of the laser wavelength ⁇ 0 . This is done by calculating the field distribution of the leaky modes for a given approach, ie, for a given choice of the parameters of the multilayer system 22, and identifying the layer (s) most in interaction with the leaky fashion stands (stand). The exact spectral position of this leaky mode can now be influenced by varying this layer (s). Numerical calculation methods (eg RCWA) can iteratively influence the desired spectral position of all leaky modes. It may be found that the chosen approach must be modified to vary the number of layers of the multi-layer system 22. With the spectral Placement of the leaky modes and matching grid design, which resonantly couple these leaky modes, can therefore be influenced on the spectral distribution of the reflectance difference ⁇ R of radial and azimuthal polarization direction.
  • the grating mirror 20 according to FIG. 4b can be designed as a highly reflective end mirror for a laser resonator, wherein the reflectance is set by the structure of the multilayer system 22. The indicated by an arrow, incident on the grating 21 laser radiation is reflected differently from this depending on the polarization direction.
  • the grating mirror 20 can also be designed such that the laser beam is incident from the side of the substrate 23 and the grating is arranged on the rear side of the end mirror (compare the arrangement in FIG. 4c).
  • FIG. 4c shows a further grating mirror 30 with a sub-wavelength grating 31 in which the grating period ⁇ is smaller than the laser wavelength ⁇ 0 (10.6 ⁇ m for CO 2 laser radiation).
  • the lattice parameters (grating period ⁇ , lattice shape) are chosen such that the grating 31 exhibits the so-called GIRO effect, which results in a high polarization selectivity. Details of the GIRO effect are described in the aforementioned US Pat. No. 6,191,890 B1.
  • first dielectric medium 32 eg gallium arsenide
  • second dielectric medium 33 eg air
  • the grating 31 formed at an interface between the two dielectric media 32, 33, which has a mean refractive index ⁇ IAV, which lies between the two refractive indices n 1 ( n 2)
  • the grating mirror 30 can be formed as a highly reflective end mirror. wherein the grating 31 is arranged on the rear side of the end mirror The incident laser beam indicated by an arrow traverses the first dielectric medium 32 and is reflected by the grating 31.
  • the Fign. 5a and 5b show two laser resonators 40, 40 'each having a highly reflective end mirror 41, 41' and a partially transmitting Auskoppelapt 42, 42 ', wherein in the beam path 43 between the end and Auskoppelapt two folding mirrors 44, 45 are arranged, which are at an angle of incidence of each 45 ° are operated.
  • the phase shift device is formed as a reflective optical element in the laser resonators 40, 40 'of Fig. 5a and 5b.
  • FIG. 5a shows a laser resonator 40, in which the output mirror 42 is designed as a phase shifting device and the end mirror 41 as a polarizer device.
  • the output mirror 42 is provided with a grating 46 which produces a phase difference ⁇ between the radial and azimuthal polarization direction of the laser radiation, and the end mirror 41 has a grating 47 which generates a reflectance difference ⁇ R between the radial and azimuthal polarization directions.
  • the end mirror 41 can also be designed as a phase shifting device and the coupling-out mirror 42 as a polarizer device (not shown).
  • Fig. 5b shows a laser resonator 40 ', in which the Auskoppelapt 42' is designed as a phase-shifting device and as a polarizer device.
  • the output mirror 42 ' is provided for this purpose with a grid 48 which produces a phase difference ⁇ and a reflectance difference ⁇ R between the radial and azimuthal polarization direction of the laser radiation, so that a radial polarization state is established in the laser resonator 40', as shown in FIG. 1c.
  • the intensity distribution of the radially polarized radiation generated in the laser resonator 40 ' is distributed annularly around the center of the beam cross section.
  • the grating 48 can be constructed as described in US Pat. No. 6,680,799 B1, DE 10 2004 042 748 A1, US Pat. No. 6,191, 890 B1 or the article by Goncharski et al. be executed described.
  • suitable parameters for the generation of the reflectance difference .DELTA.R reference is made to these documents, wherein the parameters of the grid 48 must be additionally selected such that adjusts the desired phase difference between radial and azimuthal polarization.
  • the Fign. 6a, b show two further laser resonators 50, 50 'with a highly reflective end mirror 51 and a partially transmitting Auskoppelapt 52, in which in the beam path 53 between the end and the Auskoppelapt 51, 52nd two folding mirrors 54, 55 are arranged, which deflect a laser beam at an angle of incidence of 45 ° in each case.
  • the phase shift device is formed in both cases as a transmissive, shape-birefringent optical element 56, 56 'which is arranged between the end and the output coupling mirror 51, 52.
  • a birefringent element may optionally also be used, which is composed of a multiplicity of individual elements (facets) which each have different birefringent properties and, in their entirety, generate desired phase difference.
  • FIG. 6a shows a laser resonator 50, in which, in addition to the shape-birefringent element 56 (also referred to below as a birefringent plate), a further transmissive optical element 57 is provided as a polarizer device, which has a transmittance difference ⁇ T between the radial and azimuthal polarization directions the laser radiation generated.
  • a transmissive polarizer device 57 for generating radially or azimuthally polarized laser radiation
  • the outcoupling mirror 51 or the end mirror 52 can be embodied as polarizer device which generates a reflectance difference ⁇ R between radial and azimuthal polarization direction of the laser radiation (not shown).
  • the shape-birefringent plate 56 could also be disposed between the end mirror 51 and the first folding mirror 54 or between the coupling-out mirror 52 and the second folding mirror 55 (not shown).
  • FIG. 6b shows a laser resonator 50 ', in which the transmissive, birefringent plate 56' is designed as a phase shifting device and as a polarizer device.
  • the shape-birefringent plate 56 ' when traversed twice, produces the desired phase difference ⁇ and the desired transmittance.
  • Missionsgraddifferenz .DELTA.T between the radial and azimuthal polarization direction of the laser radiation in the laser resonator 50 '.
  • the Fign. 6c, d show laser resonators 60, 60 'with an end mirror 61 and a coupling-out mirror 62.
  • two folding mirrors 64, 65 are arranged, which deflect the laser beam by 90 ° in each case.
  • the function of the phase shifting device is as shown in Figs. 6a, b are filled by a shape-birefringent plate 66.
  • the output mirror 62 is formed, which generates a reflectance difference .DELTA.R between the radial and azimuthal polarization direction of the laser radiation.
  • the output mirror 62 a grid according to the Fign.
  • the shape-birefringent plate 66 is fixed on the end mirror 61 in the laser resonator 60 of FIG. 6c and on the output mirror 62 in the laser resonator 60 'of FIG. 6d.
  • the in Fign. 5a, b and 6a-d illustrated types of laser resonators 40, 40 '; 50, 50 '; 60, 60 ' have in common that they produce both a reflectance difference and a transmittance difference for producing a stable, axially symmetrical polarization state of the laser radiation and also locally influence the phase difference between the polarization directions of the laser radiation. It is understood that the skilled person can modify the arrangements shown above according to his needs, ie, for example, the number of folding mirrors or the wavelength of the laser radiation can vary.
  • polarizer or phase shift devices can also be arranged in a laser resonator, wherein these elements can also be embodied differently than described here and arranged at other locations in the laser resonator.
  • the folding mirrors 44, 45; 54, 55; 64, 65 optionally serve as polarizer means for generating radially or axially polarized laser radiation, so that it is possible to dispense with an additional, axially symmetrical polarizer device in the laser resonator.
  • the folding mirrors 44, 45; 54, 55; 64, 65 are also provided with grating structures which have the desired reflectance and phase differences and which are optimized for the typically occurring angles of incidence of 45 °. The Fign.
  • Figures 7a and 7b show the calculated reflectances R and reflectance differences ⁇ R ( Figure 7a) and the calculated phase differences ⁇ ( Figure 7b) for a grating formed as a phase shift and polarizer device and as an end mirror in a laser resonator.
  • the grating 21 is formed on the top layer 24 of germanium of the multilayer system 22 according to the structure shown in Fig. 4b, which is in turn deposited on the gallium arsenide dielectric substrate 23.
  • the grating 21 has a ring structure with concentric grating grooves and grid webs (see Fig. 4a) and is designed for operation with laser radiation at a laser wavelength A 0 of 10.6 microns.
  • the grating 21 further has a grating period ⁇ , which is less than the laser wavelength A 0 and 6.27 microns.
  • the depth d of the grating grooves is 3.42 ⁇ m and the width of the grating webs corresponds to half the grating period ⁇ of the grating 21, so that the filling factor f of the grating corresponds to 50%.
  • the parameters of the grating 21 have been optimized so that the unwanted azimuthal polarization component is coupled into leaky modes.
  • the grating equipped with these parameters makes it possible to generate a reflectance difference ⁇ R and a phase difference ⁇ , which enable the stable generation of radially polarized radiation in a laser resonator, as explained in more detail below.
  • FIG. 7 a shows the dependence of the reflectivities R of the TM and TE polarized laser radiation on the wavelength ⁇ of the incident laser radiation. It can be clearly seen that for certain values of the wavelength ⁇ , the TE component has minima of the reflectance R, which are referred to as leaky modes, the position of which depends on the nature of the multilayer system 22 and the substrate 23.
  • the wavelengths ⁇ i and A 2 belonging to the two leaky modes TE L i and TE L2 are arranged at such a small distance from the operating wavelength ⁇ 0 of the grating 21 that they are located within the spectral amplification bandwidth of an associated laser resonator.
  • Figures 8a and 8b show the calculated reflectances R and reflectance differences ⁇ R ( Figure 8a) and the calculated phase differences ⁇ ( Figure 8b) for a grating 31 formed as a phase shift and polarizer device and as an end mirror in a laser cavity.
  • the grating 31 is formed on a dielectric substrate 32 of gallium arsenide (GaAs) according to the structure shown in Fig. 4c, and has a ring structure with concentric grating grooves and grating lands (see Fig. 4a) and is included for operation with laser radiation a laser wavelength ⁇ o of 10.6 microns designed.
  • the grating period ⁇ is 6.27 ⁇ m
  • the depth d of the grating grooves is 3.42 ⁇ m
  • the calculated reflectance R of the grating 31 for TM and TE polarized laser radiation is shown in FIG. 8a as a function of the wavelength ⁇ , wherein the reflectance difference ⁇ R between the TM and TE components at the operating wavelength ⁇ 0 is 10.6 ⁇ m is maximum.
  • the reflectance R for the TM component is 99.2% and for the TE component 0.1%, ie, the reflectance difference ⁇ R is 99.1%.
  • the parameters of the grating 31 are chosen such that it exhibits the so-called GIRO effect, which results in an extraordinarily strong polarization selectivity.
  • the reflectance difference ⁇ R between the TM component and the TE component should be at least 5%, preferably at least 15%, particularly preferably at least 50%.
  • phase difference ⁇ can be achieved that the formed in the laser resonator, radially polarized Laser radiation even in the presence of phase-shifting optical elements, such as, for example, several folding mirrors (see Fig. 2) can be obtained.
  • the above-described radially symmetric gratings are designed such that the grating period ⁇ is constant over the entire field of the laser radiation. It is understood that for the generation and / or stabilization of the laser radiation, the grating period ⁇ or the grating shape can optionally also be variably selected in the radial direction, so that in particular the phase difference is dependent on the radial coordinate. In the case of the modulation of the grating period ⁇ , however, care must be taken that this is not chosen to be smaller in lattices of the type described in DE 10 2004 042 748 A1 than the laser wavelength ⁇ 0 or in gratings of the type described in US Pat. No. 6,680,799 B1 or US Pat. No. 6,191 , 890 B1 type does not exceed the laser wavelength ⁇ 0 .

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  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
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  • Optics & Photonics (AREA)
  • Lasers (AREA)

Abstract

La présente invention concerne un dispositif à déphasage (42') destiné à la stabilisation d'un rayonnement laser à polarisation radiale ou azimutale à l'intérieur d'une cavité laser (40'). Cette cavité laser (40') comporte un dispositif à déphasage (42') servant à la production d'un rayonnement laser à polarisation radiale ou azimutale. Le dispositif à déphasage (42') est constitué de façon à obtenir pour chaque cycle dans la cavité laser (40') un déphasage (Δφ) se situant entre environ 30° et 330°, de préférence entre environ 70° et 290°, de façon plus préférentielle entre environ 160° et 200° et en particulier d'environ 180° entre les sens de polarisation radial et azimutal du rayonnement laser, avec une symétrie axiale. L'invention concerne également une cavité laser (40') comportant un tel dispositif à déphasage (42').
PCT/DE2008/001136 2007-07-19 2008-07-14 Dispositif à déphasage et cavité laser pour la production d'un rayonnement laser à polarisation radiale ou azimutale Ceased WO2009010043A2 (fr)

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DE102007033567A DE102007033567A1 (de) 2007-07-19 2007-07-19 Phasenschiebe-Einrichtung und Laserresonator zur Erzeugung radial oder azimutal polarisierter Laserstrahlung

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JP2011204942A (ja) * 2010-03-26 2011-10-13 Mitsubishi Electric Corp レーザ発振器
EP2913137A1 (fr) 2014-02-26 2015-09-02 Bystronic Laser AG Dispositif de traitement au laser et procédé
CN105006728A (zh) * 2014-04-17 2015-10-28 大族激光科技产业集团股份有限公司 径向偏振激光系统
CN105024263A (zh) * 2014-04-17 2015-11-04 大族激光科技产业集团股份有限公司 激光器谐振腔
US10429584B2 (en) 2016-11-22 2019-10-01 Lumentum Operations Llc Rotary optical beam generator
US10656334B2 (en) 2016-11-22 2020-05-19 Lumentum Operations Llc Rotary optical beam generator
US10690855B2 (en) 2016-11-22 2020-06-23 Lumentum Operations Llc Tapered non-concentric core fibers
US11347069B2 (en) 2016-11-22 2022-05-31 Lumentum Operations Llc Rotary optical beam generator

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DE102008030374B4 (de) 2008-06-26 2014-09-11 Trumpf Laser- Und Systemtechnik Gmbh Verfahren zum Laserschneiden und CO2-Laserschneidmaschine
EP3362839B1 (fr) 2015-10-14 2021-01-27 TRUMPF Lasersystems for Semiconductor Manufacturing GmbH Système de polarisation, dispositif de génération de rayonnement euv associé et procédé de polarisation linéaire d'un faisceau laser

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US5627847A (en) * 1995-05-04 1997-05-06 Regents Of The University Of Minnesota Distortion-compensated phase grating and mode-selecting mirror for a laser
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JP2011204942A (ja) * 2010-03-26 2011-10-13 Mitsubishi Electric Corp レーザ発振器
EP2913137A1 (fr) 2014-02-26 2015-09-02 Bystronic Laser AG Dispositif de traitement au laser et procédé
WO2015128833A1 (fr) 2014-02-26 2015-09-03 Bystronic Laser Ag Dispositif d'usinage laser et procédé
CN105006728A (zh) * 2014-04-17 2015-10-28 大族激光科技产业集团股份有限公司 径向偏振激光系统
CN105024263A (zh) * 2014-04-17 2015-11-04 大族激光科技产业集团股份有限公司 激光器谐振腔
US10429584B2 (en) 2016-11-22 2019-10-01 Lumentum Operations Llc Rotary optical beam generator
US10656334B2 (en) 2016-11-22 2020-05-19 Lumentum Operations Llc Rotary optical beam generator
US10690854B2 (en) 2016-11-22 2020-06-23 Lumentum Operations Llc Rotary optical beam generator
US10690855B2 (en) 2016-11-22 2020-06-23 Lumentum Operations Llc Tapered non-concentric core fibers
US11347069B2 (en) 2016-11-22 2022-05-31 Lumentum Operations Llc Rotary optical beam generator

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