Optical system for increasing the contrast of pulsed laser radiation, laser system and method for increasing the contrast of pulsed laser radiation

Abstract

An optical system for increasing contrast of pulsed laser radiation includes a first polarization setting optical unit for setting an elliptical polarization state of the pulsed laser radiation, and a multipass cell having at least two opposing mirrors. The pulsed laser radiation passes the multipass cell with formation of a plurality of intermediate focus zones. The multipass cell is filled with a gas having an optical nonlinearity that causes an intensity-dependent rotation of an alignment of the elliptical polarization state of the pulsed laser radiation, such that the multipass cell outputs beam portions having differently aligned elliptical polarization states on account of the intensity-dependent rotation. The optical system further includes an optical beam splitting system for splitting the beam portions having differently aligned elliptical polarization states.

1 . An optical system for increasing contrast of pulsed laser radiation, the optical system comprising:
 a first polarization setting optical unit for setting an elliptical polarization state of the pulsed laser radiation,   a multipass cell having at least two opposing mirrors, wherein the pulsed laser radiation passes the multipass cell with formation of a plurality of intermediate focus zones, and wherein the multipass cell is filled with a gas having an optical nonlinearity that causes an intensity-dependent rotation of an alignment of the elliptical polarization state of the pulsed laser radiation, such that the multipass cell outputs beam portions having differently aligned elliptical polarization states on account of the intensity-dependent rotation, and   an optical beam splitting system for splitting the beam portions having differently aligned elliptical polarization states.   
     
     
         2 . The optical system as claimed in  claim 1 , wherein for setting a predetermined rotation angle of the alignment of one of the elliptical polarization states, at least one of the following parameters of the optical system is set or is settable:
 a gas pressure in the intermediate focus zones, wherein the optical system comprises a gas-filled cell and a pressure setting device for setting the gas pressure in the gas-filled cell,   a dispersion present in the multipass cell,   an ellipticity of the elliptical polarization state of the pulsed laser radiation,   a number of intermediate focus zones in the multipass cell,   focus diameters in the intermediate focus zones, and   Rayleigh lengths of the intermediate focus zones.   
     
     
         3 . The optical system as claimed in  claim 2 , wherein the predetermined rotation angle of the alignment of one of the elliptical polarization states is 90°. 
     
     
         4 . The optical system as claimed in  claim 1 , wherein the first polarization setting optical unit comprises a first waveplate. 
     
     
         5 . The optical system as claimed in  claim 4 , wherein the first waveplate comprises a λ/4 waveplate and/or a λ/2 waveplate. 
     
     
         6 . The optical system as claimed in  claim 1 , wherein the optical system furthermore comprises at least one of the following optical components:
 a pulse duration setting system for setting a pulse duration of primary laser pulses of the pulsed laser radiation,   a first optical telescope arrangement configured to image the pulsed laser radiation onto a predefined mode in the multipass cell, the first optical telescope arrangement being disposed downstream of the first polarization setting optical unit,   an input coupling mirror for coupling the pulsed laser radiation into the multipass cell,   an output coupling mirror for forwarding the pulsed laser radiation emerging from the multipass cell, and   a second optical telescope arrangement configured to collimate the pulsed laser radiation emerging from the multipass cell.   
     
     
         7 . The optical system as claimed in  claim 1 , wherein the multipass cell
 has a predetermined or settable number of intermediate focus zones, wherein the intermediate focus zones are produced with the opposing mirrors in a resonator set-up with identical radius of curvature of the opposing mirrors, and wherein the intermediate focus zones have substantially an identical diameter and an identical Rayleigh length,   wherein the intermediate focus zones are arranged next to one another or partly superposed on one another,   wherein the multipass cell is filled with a noble gas, and wherein a same gas pressure is present in each of the intermediate focus zones,   wherein the pulsed laser radiation passes through the intermediate focus zones with a substantially constant pulse duration and a substantially constant pulse energy, and/or   wherein the pulsed laser radiation passes through the intermediate focus zones with a stepwise nonlinear spectral broadening.   
     
     
         8 . The optical system as claimed in  claim 1 , wherein at least one of the opposing mirrors of the multipass cell
 comprises a convex mirror, wherein radii of curvature of the opposing mirrors match, and/or a distance between the opposing mirrors lies in a range of 95% to 105% of a sum of the radii of curvature of the opposing mirrors, and/or   wherein at least one of the opposing mirrors comprises a dispersive mirror with a dispersion contribution that compensates for a dispersive contribution of at least one pass of a primary laser pulse of the pulsed laser radiation through the multipass cell, and/or   wherein at least one of the opposing mirrors comprises at least one mirror segment, wherein the pulsed laser radiation impinges on the at least one mirror segment at least once during circulation of the pulsed laser radiation through the multipass cell.   
     
     
         9 . The optical system as claimed in  claim 1 , wherein the multipass cell is configured so that a primary laser pulse of the pulsed laser radiation, for which a contrast is intended to be increased in the optical system, experiences substantially no change in a pulse duration and/or a pulse energy in the intermediate focus zones, and/or
 wherein the multipass cell is configured as a concentric or confocal resonator.   
     
     
         10 . The optical system as claimed in  claim 1 , wherein the beam portions having differently aligned elliptical polarization states comprise a useful beam portion with primary laser pulses and a residual beam portion with low-intensity laser radiation, wherein the residual beam portion has a radiation pedestal and/or low-intensity laser pulses preceding the high-intensity laser pulses and/or low-intensity laser pulses succeeding the high-intensity laser pulses. 
     
     
         11 . The optical system as claimed in  claim 1 , wherein the optical beam splitting system for splitting the beam portions having differently aligned elliptical polarization states comprises:
 a second polarization setting optical unit for returning each of the differently aligned elliptical polarization states to a linear polarization state, wherein the beam portions having differently aligned elliptical polarization states that are output by the multipass cell are converted into a useful beam portion and a residual beam portion having differently aligned linear polarization states, and   a beam splitter configured to output the useful beam portion and the residual beam portion on different beam paths.   
     
     
         12 . The optical system as claimed in  claim 11 , wherein the second polarization setting optical unit comprises a second waveplate, wherein the second waveplate comprises a λ/4 waveplate and/or a λ/2 waveplate. 
     
     
         13 . The optical system as claimed in  claim 1 , further comprising a control system configured to set at least one of the following parameters, for setting a predetermined rotation angle of the alignment of one of the elliptical polarization states:
 a pulse duration of primary laser pulses of the pulsed laser radiation,   a pulse energy of the primary laser pulses of the pulsed laser radiation,   an ellipticity of the elliptical polarization state of the pulsed laser radiation,   focus diameters in the intermediate focus zones,   Rayleigh lengths of the intermediate focus zones, and   a gas pressure of the gas in the intermediate focus zones.   
     
     
         14 . A laser system for emitting pulsed laser radiation, the laser system comprising
 a laser radiation source configured to output pulsed laser radiation, the pulsed laser radiation comprising primary laser pulses having pulse energies and pulse durations in a range of a few hundred femtoseconds or less, and   at least one optical system as claimed in  claim 1  for increasing a contrast of the pulsed laser radiation using a nonlinear elliptical polarization rotation in a plurality of intermediate focus zones of a multipass cell.   
     
     
         15 . The laser system as claimed in  claim 14  further comprising a pulse duration setting system for setting the pulse duration of the primary laser pulses. 
     
     
         16 . The laser system as claimed in  claim 14  further comprising an optical pulse duration compressor system for compensating for a dispersive contribution of the at least one optical system and/or for temporally compressing the primary laser pulses of the pulsed laser radiation as the primary laser pulses have experienced a nonlinear spectral broadening in at least one of the intermediate focus zones. 
     
     
         17 . A method for increasing the contrast of pulsed laser radiation, the method comprising the following steps:
 setting an elliptical polarization state of the pulsed laser radiation,   input coupling the pulsed laser radiation into a multipass cell having at least two mirrors, wherein the at least two mirrors are traversed with formation of a plurality of intermediate focus zones, wherein the multipass cell is filled with a gas having an optical nonlinearity that causes an intensity-dependent rotation of an alignment of the elliptical polarization state of the pulsed laser radiation in the intermediate focus zones, thereby beam portions having differently aligned elliptical polarization states are generated in the multipass cell,   output coupling the beam portions having differently aligned elliptical polarizations out of the multipass cell, and   separating the output-coupled beam portions into a useful beam portion with primary laser pulses and a residual beam portion with low-intensity laser radiation.   
     
     
         18 . The method as claimed in  claim 17 , furthermore comprising
 setting at least one of the following parameters for setting a predetermined rotation angle of the alignment of one of the elliptical polarization states in the multipass cell,   a gas pressure in the intermediate focus zones, r   a dispersion of the primary laser pulses that has accumulated in the multipass cella pulse duration of the primary laser pulses of the pulsed laser radiation,   a pulse energy of the primary laser pulses of the pulsed laser radiation,   an ellipticity of the elliptical polarization state of the pulsed laser radiation,   focus diameters in the intermediate focus zones, and   Rayleigh lengths of the intermediate focus zones.   
     
     
         19 . The method as claimed in  claim 18 , wherein the predetermined rotation angle of the alignment of one of the elliptical polarization states is 90°. 
     
     
         20 . The method as claimed in  claim 17 , wherein a pulse spectrum of the pulsed laser radiation assigned to the primary pulses broadens from intermediate focus zone to intermediate focus zone, on account of a nonlinear spectral broadening in the multipass cell, while a contrast is increased simultaneously.