Improving efficiency of CO 2 Laser System for LPP Sn EUV Source K.Nowak*, T.Suganuma*, T.Yokotsuka*, K.Fujitaka*, M.Moriya*, T.Ohta*, A.Kurosu*, A.Sumitani** and J.Fujimoto*** * KOMATSU ** KOMATSU/EUVA *** Gigaphoton 2010 EUVL Workshop 23 June, 2010
OUTLINE Introduction Current status of multi-kw CO2 system Topics Sources of laser power limitations Energy storage in medium Impact of power level on optics performance Multi-line amplification for higher efficiency Effective pre-amplification High-quality 20kW CO2 laser system Summary 23 June 2010, EUVL Workshop P2
High power laser system current achievements Laser System 60W Oscillator Wave length: 10.6um Rep. rate :100kHz Pulse width :20 ns (FWHM) Laser Power : 13 kw @ 30% duty Pulse Width : 20 ns Repetition Rate : 100 khz Beam quality : M2 <1.2 Wall plug eff. : <5% 3 kw 13 kw EUV100 W at I/F equivalent Pre-Amplifier RF-excited CO2 laser Main-Amplifier RF-excited CO2 laser 23 June 2010, EUVL Workshop P3
Laser beam profile and pulse shape Temporal pulse shape Laser beam profile Pulse duration : 20 ns (FWHM) Pedestal component : <10% M 2 < 1.3 23 June 2010, EUVL Workshop P4
Energy level diagram of CO2-N2 system 23 June 2010, EUVL Workshop P5
Key timescales in carbon dioxide medium Vibrational relaxation time τ v = 0.5 μs Pressure: 100 [torr] Rotational relaxation time R.L.Abrams, Appl. Phys. Lett. 25, pp.609, 1974 Φ: partial ratio P: Pressure [torr] T: Temperature [K] τ r = 1.9 ns (520 MHz) Typical Gas Parameter -CO 2 :N 2 :He=1:1:8 - Pressure: 100 [torr] - Temperature: 450 [K] Total power gain of short pulse amplification is decreased compared to CW amplification 23 June 2010, EUVL Workshop P6
Multi-line amplification for short pulse timescale Numerical Calculation Result of Amplification with Multi-Line Oscillator This work was preformed by Research Institute for Laser Physics, St. Petersburg, Russia [V.E. Sherstobitov et al] -X- is the amplification ratio between the (P16-P22) spectrum and the P20 line Estimated extracted power of 15 kw amplifier module in case of input power with 5 kw Multi-line : > 6.5 kw (5 kw 1.3) Single-line : > 5 kw Extracted efficiency from pumping power is estimated Multi-line : > 7.2 % Single-line : > 5.5 % 23 June 2010, EUVL Workshop P7
Multi-line Master Oscillator recent results Multi-wavelength seeded oscillator Seeder λ1=p18 λ2=p20 (more planned) Multi-pass amplifier High output beam quality M 2 <1.3 (Far-field beam profile shown below) High pulse stability (3.5Mpulses shown below) 23 June 2010, EUVL Workshop P8
Other sources of efficiency and laser energy limitations Optimum input: 5-7kW power Present input: 60W power (ZnSe) Moderate thermal lensing (ZnSe) Severe thermal lensing Minor mirror distortion (Diamond) Moderate thermal lensing Significant mirror distortion Pulsed Optical Damage (1J cm -2 ) 23 June 2010, EUVL Workshop P9
Computer modeling of the laser system Computer model of the system was developed and verified experimentally No Gaussian beam approximations Beam propagation code implements diffraction integrals and modal propagation Medium model pulsed (Frantz-Nodvik) and CW operation Optical components modeled: apertures, windows, lenses, mirrors Tilts, offsets of beam and components accounted for Optical characteristics of optical components accounted for (transmission, reflectivity, absorption, phase aberrations ) Heat-induced phase distortions in all windows, lenses and mirrors calculated for the individual incident beam profiles based on 3D steady-state heat flow code Realistic calculation of the beam propagation in the laser system Actual geometry and layout of each amplifier internal structure coded in detail Actual interconnect optics coded Understanding of the key aspects of system performance obtained Impact of average power level on beam quality Effect of input beam parameters Effect of optical misalignments 23 June 2010, EUVL Workshop P10
Impact of power level on beam quality ZnSe window example Typical ZnSe window Measurement plane 2m distance 10mm diameter collimated Gaussian beam Incident on the window WRoC Wavefront Radius of Curvature Negative converging Positive - diverging 25mm At multi-kw power levels: Phase aberration control becomes necessary Appropriate optical materials have to be used 23 June 2010, EUVL Workshop P11
Efficient pre-amplification simulation results Multi-kW pre-amplifier Input beam Gaussian (M 2 =1), 16mm 1/e2 diameter >2kW output achieved so far at 100W input power (at 20% duty factor) Good beam quality M 2 < 2 at multi-kw level Compact size Improved efficiency 23 June 2010, EUVL Workshop P12
Main amplifier characteristics - simulation Good performance at 20kW average power predicted Beam tilts and offset typical for good alignment 5.3 kw 9.7 kw 15.7 kw 21.2 kw 23 June 2010, EUVL Workshop P13
20kW average power system simulation results 100% duty operation modelled Output beam profile Osc. Pre- Amplifier Main amplifier system 20kW operation at 100% duty High beam quality maintained thanks to phase distortion compensation by adaptive optics Improved overall efficiency thanks to efficient preamplification Reduced footprint Single-lobe high quality spot at focal point (far-field) 23 June 2010, EUVL Workshop P14
Summary High Power CO2 laser MOPA system has been achieved with: 13 kw output power at 100 khz, 20 ns(fwhm) duty: 30% (on 30msec, off 100msec) Realistic computer model developed Efficient amplification with RF-excited CO2 laser effective pre-amplification + multi-line Multi-line oscillator under development >40% extraction efficiency of pre-amplifier predicted Efficiency of Multi-line amplification prediction of 1.3 times higher than Single-line 20kW system technically feasible No showstoppers at 20kW power level! (as predicted by numerical modeling) Acknowledgments This work was supported by the New Energy and Industrial Technology Development Organization -NEDO- Japan. 23 June 2010, EUVL Workshop P15
Mahalo!