Laser Science and Technology at LLE

Transcription

1 Laser Science and Technology at LLE Nd:glass High energy Electrical Yb:YAG High peak power Mechanical OPCPA High average power Eye injuries OPO Exotic wavelengths Fire J. Bromage Group Leader, Sr. Scientist University of Rochester Laboratory for Laser Energetics 12th Department of Energy Laser Safety Officer Workshop Rochester, NY 8 10 May

2 Summary Laser technology development at LLE requires careful consideration of safety LLE has long history of developing lasers for fusion and other scientific applications Diverse laser platforms present a range of safety issues eye safety thermal loads electrical and mechanical safety radiation Safety issues are complicated by having large facilities with multiple operators Development activities are often dynamic, in multi-user labs Mix of staff and students LLE develops lasers for other facilities, where laser expertise is varied G

3 LLE has a long history of developing and using laser technologies 1978: LHG-8 Nd:phosphate 1980: High-efficiency laser glass frequency tripling e 3~ ~ e Output 1:1 photon 3~ beam mix KDP type-ii KDP type-ii doubler o : THz 2-D SSD Intensity (arbitrary units) ~ 1985: Chirped-pulse amplification (CPA) o 2~ ~ Incident ~ beam 1985: OMEGA UV conversion 1989: Smoothing by spectral dispersion (SSD) 1981: 24-beam OMEGA National Laser Users Facility Radius (nm) 1999: Commercial spin-off of magnetorheological finishing (MRF) used for the National Ignition Facility (NIF) : OMEGA (EP) 1990: High-efficiency, uniform diffractive optics : 60-beam OMEGA 2000: High-fluence NIF optic coatings Target plane Random phase (Example phase) Grating phase Focusing lens : Dynamic Compression Sector (DCS) laser OPCPA pump laser F = 1.8 m 1995: Continuous distributed phase plate S599i 2005: Highly efficient OPCPA* Preamplifier LBO- LBO1A 1B 59 mj Power amplifier LBO-2 * OPCPA: optical parametric chirped-pulse amplification 3

4 The Omega Laser Facility comprises two multikiljoule Nd:glass laser systems OMEGA target chamber OMEGA EP target chamber Laser OMEGA Bay Compression chamber OMEGA Laser System Operating at LLE since 1995 Up to 1500 shots/year Fully instrumented 60 beams >30-kJ UV on target 1% to 2% irradiation nonuniformity Flexible pulse shaping Short shot cycle (1 h) More than half of OMEGA s shots are for external users. Main amplifiers 3 4 Beam 1 2 Booster amplifiers OMEGA EP Laser Bay OMEGA EP Laser System Construction completed 25 April 2008 Adds four NIF-like beamlines; 6.5-kJ UV (10 ns) G10425f 4

5 The largest amplifiers use Nd:glass pumped by water-cooled flash lamps High voltages: 10 to 13.5 kv Large stored energy in capacitor banks Many used in OMEGA and OMEGA EP also have one amplifier for our midscale facility, the Multi-Terawatt laser (MTW) OMEGA lamp assemblies OMEGA EP lamp assembly G

6 Closed-access procedures are required during laser shots for systems of this scale, as laid out in the Laser Facility Organization and Regulation Manual (LFORM) Access to the Laser Bays is carefully controlled using a combination of engineering controls and procedures Will hear more this afternoon from Jason Puth (Laser Facility Manager) G

7 The LLE laser inventory identifies almost 400 lasers Uses include beam characterization damage testing electro-optic sampling alignment optical quality control pulse shaping reflectometry laser development metrology nonlinear optics holography index measurement interferometry spectroscopy Most of these lasers have the potential to expose personnel to safety risks G

8 A variety of laser platforms are used for R&D at LLE, which pose a variety of risks Nd:glass High energy Electrical Yb:YAG High peak power Mechanical OPCPA* High average power Eye injuries OPO** Broad bandwidth Fire G11894 * OPCPA: optical parametric chirped-pulse amplification ** OPO: optical parametric oscillator 8

9 The large Nd:glass amplifier has a number of safety systems related to the high-voltage flash lamps 15-cm Nd:glass disk amplifier (four disks) Disks and flash lamps G11895 Power conditioning unit Grounding points LLE Electrical Safety Officer: Scott Householder 9

10 Ultrafast pulses require broad bandwidths Time-bandwidth product sets the minimum bandwidth for a given pulse width Bandwidth required for ultrafast pulses D x. Do = constant 2 D m = Do. m c The constant depends on the pulse shape Gaussian pulse: 0.44 sech 2 pulse: 0.31 Dm, bandwidth (FWHM) (nm) Minimum bandwidth for Gaussian pulse, centered at 800 nm (Fourier transform limited) ,000 10,000 Dx, pulse width (FWHM*) (ps) G11896 * FWHM: full width at half maximum 10

11 Noncollinear optical parametric amplifiers (NOPA s) support large bandwidths Nonlinear process in a crystal Energy conservation Signal Nonlinear crystal (2) Idler Provides great flexibility because the upper state is virtual, so it is not constrained by material spectroscopy (like traditional laser materials) Pump Amplified signal Virtual upper state Momentum conservation ( phase matching ) Noncollinear angle, a k S k P Signal bandwidth k I Energy Pump Idler Signal Signal bandwidth Real ground state G

12 Optical parametric chirped-pulse amplification (OPCPA) provides a path to multipetawatt lasers producing femtosecond-kilojoule pulses Ultra-broadband front end Pulse stretcher Optical parametric amplifiers Pulse compressor Pump lasers Chirped-pulse amplification* is key for producing intense pulses. G10918d * D. Strickland and G. Mourou, Opt. Commun. 56, 219 (1985). 12

13 At LLE, 200-nm systems are seeded using white-light continuum (WLC) generation to produce sub-13-fs pulses WLC generation was first observed in glass (1970) Widely used to produce stable, coherent source of broadband radiation from a relatively narrowband, ultrafast source 250 fs, 0.8 nj,1046 nm peak power >1.5 MW YAG (undoped) 4 mm Blue-shifted shock wave forms at back of pulse 100 nm Filament forms as a result of the balance between self-focusing (n 2 ) and plasma defocusing (multiphoton ionization) G11910 * M. Bradler, P. Baum, and E. Riedle, Appl. Phys. B 97, 561 (2009). 13

14 At LLE, 200-nm systems are seeded using white-light continuum (WLC) generation to produce sub-13-fs pulses WLC generation was first observed in glass (1970) Widely used to produce stable, coherent source of broadband radiation from a relatively narrowband, ultrafast source WLC beam G11910a 250 fs, 0.8 nj,1046 nm peak power >1.5 MW 100 nm YAG (undoped) 4 mm Blue-shifted shock wave forms at back of pulse Filament forms as a result of the balance between self-focusing (n 2 ) and plasma defocusing (multiphoton ionization) Spectrum (normalized) Spectrum (normalized) NOPA gain Short-pass filter blocks pump Wavelength (nm) * M. Bradler, P. Baum, and E. Riedle, Appl. Phys. B 97, 561 (2009). 14

15 At LLE, 200-nm systems are seeded using white-light continuum (WLC) generation to produce sub-13-fs pulses WLC generation was first observed in glass (1970) Widely used to produce stable, coherent source of broadband radiation from a relatively narrowband, ultrafast source 250 fs, 0.8 nj,1046 nm peak power >1.5 MW 100 nm YAG (undoped) 4 mm Blue-shifted shock wave forms at back of pulse Filament forms as a result of the balance between self-focusing (n 2 ) and plasma defocusing (multiphoton ionization) Spectrum (normalized) WLC beam Spectrum (normalized) NOPA gain Short-pass filter blocks pump Wavelength (nm) Spectral phase (rad) Intensity (arbitrary units) NOPA1 spectrum and phase* 0.6-nJ pulses S(~) Wavelength (nm) NOPA1 pulse after prism compressor Measured (12.8 fs) Time (fs) G11910b * M. Bradler, P. Baum, and E. Riedle, Appl. Phys. B 97, 561 (2009). 15

16 Broadband, ultrashort lasers require broadband eye protection Optical density (OD) > 7 is needed over much of the visible spectrum pump lasers broadband amplifiers Low visible-light transmission (e.g., 22%), so extra task lights are needed OD T (%) Wavelength (nm) G11898 * LLE Laser Safety Officer: Eugene Kowaluk 16

17 Multi-user labs have interlock systems and beacons to support a range of sources Switches to enable interlocks Connected to door indicators Light on beacon in lab G

18 In some cases, eyewear with pairs of filters must be used to obtain sufficient coverage 0.2 m M 1.5 M MP 1.5 M 3 HR HR BiBO M 3 OC M 2 Pump laser HR M 3 M 3 M 2 M 2 HR 2-mm OPO synchronously pumped by a 45-W Yb:YAG laser (1.0 nm) Normalized amplitude (arbitrary units) Autocorrelation fs Delay (ps) 2 Normalized amplitude (arbitrary units) Spectrum 30 nm Wavelength (nm) OPO produced ultrafast pulses (<500 fs) at 2 nm, with a 200-nm tuning range, and energies 3 higher than previously produced with an OPO. G

19 In some cases, eyewear with pairs of filters must be used to obtain sufficient coverage 0.2 m M1.5 M1.5 MP M3 HR HR BiBO M2 HR OC Autocorrelation 700 fs Delay (ps) 2 Normalized amplitude (arbitrary units) Normalized amplitude (arbitrary units) M M3 M3 M mm OPO synchronously pumped by a 45-W Yb:YAG laser (1.0 nm) M2 Pump laser Spectrum HR Filter for coverage in visible and near IR (<0.53 nm, 0.8 to 1.1 nm) nm Wavelength (nm) OPO produced ultrafast pulses (<500 fs) at 2 nm, with a 200-nm tuning range, and energies 3 higher than previously produced with an OPO. Filter for coverage in short-wave IR (1 to 3 nm) G11900a 19

20 High-average-power (HAP) lasers require cooled beam dumps and thermal barriers Laser parameters Energy Power Repetition rate Pulse width Time bandwidth product 6.1 nj 45 W 7.1 MHz 1.0 ps 0.32 SESAM* Intensity (arbitrary units) M11 Polarizer GTI7 Quarterwave plate GTI2 Active multipass cavity M10 Autocorrelation 0 Delay (ps) 3.5 M8 M9 Spectrum (arbitrary units) GTI5 GTI4 M1 1.0 GTI6** GTI3 Spectrum 1.5 ps nm Thin-disk laser head Wavelength (nm) Enclosures rated for HAP Water-cooled beam dump E24536b * SESAM: semiconductor saturable absorbing mirror **GTI: Gires Tournois interferometer 20

21 Thermal imaging cameras provide invaluable information Safety locate sources of heat Development identify cause of component failure used in conjunction with system models to evaluate laser performance G

22 Large vacuum chambers require safety systems to protect operators and contents Compressing short pulses must be done in vacuum to avoid air breakdown and nonlinearities There is a large amount of stored energy, so a window must not fail because of a laser pulse MTW grating compressor chamber Window for 75-mm 2 beam G11902 *LLE Mech. Safety Officer: Milt Shoup 22

23 At high intensities, target shots require shielding from x rays and gamma rays MTW target chamber with moveable lead shield wall Dosimeter Wall mount G11903 LLE Radiation Safety Officer: Walter Shmayda, Ph.D. 23

24 Summary/Conclusions Laser technology development at LLE requires careful consideration of safety LLE has long history of developing lasers for fusion and other scientific applications Diverse laser platforms present a range of safety issues eye safety thermal loads electrical and mechanical safety radiation Safety issues are complicated by having large facilities with multiple operators Development activities are often dynamic, in multi-user labs Mix of staff and students LLE develops lasers for other facilities, where laser expertise is varied G