Fiber Lasers at LLNL

Transcription

1 Fiber Lasers at LLNL J.W. Dawson, M.J. Messerly, J.E. Heebner, P.H. Pax, A.K. Sridharan, A. L. Bullington, R.E. Bonanno, R.J. Beach, C.W. Siders and C.P.J. Barty NIF & PS Directorate Lawrence Livermore National Laboratory This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344

2 People like fiber lasers because they are a simple-to-use, lowmaintenance, compact source of high-brightness, high-power laser light with wall plug efficiencies in excess of 30%

3 Double clad fiber amplifiers are conceptually simple devices Rare earth doped core absorbs pump light from cladding Light propagating in core stimulates emissions leading to brightness enhancement Optical fiber core defines output beam quality High surface area to volume ratio of core minimizes thermal effects High intensity over long lengths lead to highly efficiency process I>>Isat Yb 3+ fibers can achieve 85% optical to optical conversion efficiencies 3

4 Development of injection seed lasers has been a high priority at LLNL. The NIF laser is seeded by a fully rack-mounted state-of-the-art LLNL-engineered 1053-nm fiber laser system that operates 24/7 under full computer control. 11

5 The Advanced Radiographic Capability project will create a parallel short pulse front end for NIF 12

6 Photocathode Drive Laser LLNL s Mono-Energetic Gamma-Ray project is employing complementary fiber systems NIF s2.ppt Messerly PS&A Technical Review, May 11-12,

7 T-REX Seed Laser System World s first fiber-based photocathode driver Frequency and phase-locked to 3 ppm Foundation for future accelerators and FELs 1.8 mm-mrad rms state-ofthe-art emittance measured at 800 pc

8 Both systems are based upon a master oscillator power amplifier architecture MEGa-Ray Fiber Laser ARC Fiber Laser Due to time constraints I will focus on the ARC system 15

9 Challenges the systems face in order to meet their requirements Imperfections in the CFBGs Non-linear effects Pulse contrast Dispersion understanding and control 16

10 The mode locked oscillator is a self-similar design based on NPE* 17

11 Test data from the oscillator looks good 18

12 Spare oscillator was tested in NIF MOR for amplitude and synchronization stability NIF s2.ppt Phan Photon Science Technical Review, May 12,

13 We have accumulated over 1 year of data ARC Oscillator stability test in NIF MOR room from 01/07/09 to 04/02/10 Data acq down Air conditioning system down NIF s2.ppt Phan Photon Science Technical Review, May 12,

14 Amplified spontaneous emission (ASE) is the main challenge to high pre-pulse contrast 22

15 The requirement for 1-10nJ of clean stretched light complicates the system design 23

16 We investigated several pulse cleaning schemes and chose a resonant saturable absorber 24

17 Pulse contrast was measured with a photo-diode and attenuators before and after the cleaner 25

18 A high resolution FROG was taken of the output pulse at 49nJ 26

19 The clean pulse is stretched to 2.5ns in a chirped fiber Bragg grating (CFBG) 27

20 View of the CFBG/Pulse Cleaner chassis 28

21 After the CFBG, the pulse is amplified, split, sent through 120m of fiber and boosted to 100µJ 29

22 Set-up for recompressing pulses and measuring results 30

23 After passage through the full front end and amplification to 58µJ pulse contrast ~78dB Unsaturated peak voltage 0.272V OD used 3.75 ASE voltage 0.01V Scope impulse response Estimated 1ps contrast 400ps 61,200, or ~78dB

24 FROG data at 800nJ, B~1.4, 512X512, PC=42.3% 31

25 FROG data at 97µJ, B~6.2, 512X512, PC=24.0% 25

26 High energy, short pulse fiber lasers employing chirped pulse amplification (Summary) Impressive results in terms of pulse energy (~mj) and average power (>>100W) have been reported in the literature and at conferences To date, almost all high energy systems have reported around 1 ps pulse widths, but it is not clear the quality of those pulses was all that good Most systems have large temporal pedestals Frequency converting to the UV, trades pedestal for efficiency Large B-integrals (a measure of the amount of self phase modulation) is the key contributor to the temporal pedestal Understanding and mitigating B-integral issues needs to be a key R&D thrust for fiber lasers going forward A secondary, but still important area of concern is dispersion management of a system that is all glass This is needed to generate high quality CPA pulses below 1 ps R&D in compact stretchers with flexible dispersion is a second R&D thrust for short pulse fiber lasers going forward Fiber lasers are a promising future source of high average power short pulses due to gain media with broad band widths and their natural ability to achieve high average powers 26

27 Ytterbium doped silica optical fibers are reaching their physically-limited output powers 33

28 Eight physical phenomena govern CW fiber laser power scaling and all can be expressed as f(d,l) Bending SRS Rupture Pump Coupling Melting Damage Lensing d = core diameter L = fiber length SBS

29 Power limit contours SRS Contour lines units are kw 37 Pump Power Thermal Lens Ridge-like upper bound at 36.9 kw, regardless of fiber s diameter and length 35

30 Ridge at intersection of SRS and thermal limits SRS 37 Ridge power is independent of diameter and length P laser = 4π η k laser λ2 Γ 2 ln( G) 2 η heat dn dt g R L = 4a 2 ( ) 2 η heat dn dt Γ2 ln G η laser k λ 2 g R Pump Power Thermal Lens J.W. Dawson, M.J. Messerly, R.J. Beach, M.Y. Shverdin, E.A. Stappaerts, A.K. Sridharan, P.H. Pax, J.E. Heebner, C.W. Siders, C.P.J. Barty Analysis of the scalability of diffraction-limited fiber lasers and amplifiers to high average power, Optics Express, 2008, 16,

31 SBS limits narrowband lasers to lower powers and shorter lengths (< 4m) SBS 1.9 The laser power peak of this ridge is independent of core diameter and fiber length P laser = π η laser k λ 2 Γ 2 21 ln( G) 2 η heat dn dt g B ( Δν) L = a 2 ( ) η heat dn dt Γ2 42 ln G η laser k λ 2 g B ( Δν) Pump Power Thermal Lens SBS suppression fibers having acoustic anti-guides may improve this limit beyond 1.9kW 37

32 Bending perturbs a mode s size and shape n Bent Unbent r I Unbent Bent r Bend-induced distortion of effective area calculated via a BPM method, assuming step index fiber with NA =

33 Mode size limited to ~ 40µm, even for large bends Step Index Fiber R = R = 50cm R = 25cm R = 10cm Geometric effect, NOT material-dependent (Fibers shorter than ~1m do not need to be bent) 39

34 For 40µm core, achievable power ~ kw SRS 40 µm core diameter Pump Power Thermal Lens (Assuming diffraction limited beam) 40

35 Most physical constants are material-dependent Physical Constants R m = 2460 W/m T m = 1983 K k = 1.38 W/(m-K) dn/dt = 11.8 X 10-6 /K I damage = 35 W/µm 2 Physical Constants that might be improved with specialty fibers g R = m/w g B (0) = 5 X m/w State-of-the-Art Parameters B pump = 0.1 W/(µm 2 -steradian) Pump clad NA = 0.45 Α laser = 20 db α core = nm η laser = 0.85 η heat = 0.1 T c = 300K h = 10,000 W/(m 2 K) Γ = 0.8 G = 10 db λ = 1088 nm Red parameters are material independent. Our proceedings paper lists references for the following tabulated values. 41

36 Tm Silica Properties Units Tm Silica Er Silica Yb Phosphate YAG Yb Silica Rupture Modulus W/m Thermal Conductivity W/(m-K) Melt Temperature K dn/dt 1/K 1.18X X10-5 Raman Gain m/w Brillouin Gain m/w 5X X10-11 Damage Fluence W/µm Pump Brightness W/(µm 2 -Sr) Pump Absorption db/m Laser Efficiency Heat Fraction Pump Clad NA Laser Wavelength nm

37 Tm Silica is virtually identical to Yb Silica! SRS Limited SBS Limited Silica-based, so results not changed much. Biggest impact is pump laser brightness (not yet mature at 795 nm). Experiments suggest that heating in Tm is worse than expected (QE < 2:1), so plots may be optimistic. P laser = 4π η laser k λ 2 Γ 2 ln( G) 2 η heat dn dt g R Longer wavelength balances higher heat load, leading to no change 43

38 Er Silica Properties Units Tm Silica Er Silica Yb Phosphate YAG Yb Silica Rupture Modulus W/m Thermal Conductivity W/(m-K) Melt Temperature K dn/dt 1/K 1.18X X X10-5 Raman Gain m/w Brillouin Gain m/w 5X X X10-11 Damage Fluence W/µm Pump Brightness W/(µm 2 -Sr) Pump Absorption db/m Laser Efficiency Heat Fraction Pump Clad NA Laser Wavelength nm

39 Er Silica, SRS limited case SRS Limited: Pump Absorption Issue SRS Limited: 10X higher brightness Er:SiO 2 might reach 50% more power than Tm or Yb due to longer wavelength. Er:SiO 2 combines highest potential power and eye safety. However, work is needed to increase pump brightness ( nm) AND raise doping concentrations. 45

40 Er Silica, SBS limited case SBS Limited: Pump Absorption Issue SBS Limited: 10X higher brightness Er:SiO 2 might reach 50% more power than Tm or Yb due to longer wavelength. Er:SiO 2 combines highest potential power and eye safety. However, work is needed to increase pump brightness ( nm) AND raise doping concentrations. 46

41 Yb Phosphate Properties Units Tm Silica Er Silica Yb Phosphate YAG Yb Silica Rupture Modulus W/m Thermal Conductivity W/(m-K) Melt Temperature K dn/dt 1/K 1.18X X X X10-5 Raman Gain m/w X Brillouin Gain m/w 5X X X X10-11 Damage Fluence W/µm Pump Brightness W/(µm 2 -Sr) Pump Absorption db/m Laser Efficiency Heat Fraction Pump Clad NA Laser Wavelength nm

42 Yb Phosphate Properties Units Tm Silica Er Silica Yb Phosphate YAG Yb Silica Rupture Modulus W/m Thermal Conductivity W/(m-K) Melt Temperature K dn/dt 1/K 1.18X X X X10-5 Raman Gain m/w X Brillouin Gain m/w 5X X X X10-11 Damage Fluence W/µm Pump Brightness W/(µm 2 -Sr) Pump Absorption db/m Laser Efficiency Heat Fraction Pump Clad NA Laser Wavelength nm

43 Yb Phosphate SRS Limited (gr=20x10-13 m/w) 3 SRS Limited (gr=0.5x10-13 m/w) 19 Melt Damage Phosphates don t dissipate heat well, so melting and damage are issues. 49

44 Yb Phosphate SRS Limited (gr=20x10-13 m/w) 3 SRS Limited (gr=0.5x10-13 m/w) 19 Melt SBS Limited Damage 1 Phosphates don t dissipate heat well, so melting and damage are issues. High loss (~3dB/m*) is also a concern Melt * Lee, Digonnet, Sinha, Urbanek, Byer, IEEE J. Selected Topics in QE 15, (2009) 50

45 Losses lead to over-riding efficiency term, η fiber Allowed length vs. fiber loss phosphates silica (Assume gain = 10 db) η s Phosphates limit lengths to ~ 1 m For absorption losses, deposited heat must also be considered. 51

46 Thermal Yb Phosphate length ~ 1 m, due to losses (not SBS or SRS) SRS Melt 1m SRS Limited 0.5m SRS Limited At 3 db/m, loss limits power to <500 W At 1 db/m, loss limits power to <1 kw Limits are Damage, Melting and Thermal Lens 52

47 Yb YAG (single crystal & ceramic) Properties Units Tm Silica Er Silica Yb Phosphate YAG Yb Silica Rupture Modulus W/m Thermal Conductivity W/(m-K) Melt Temperature K dn/dt 1/K 1.18X X X X X10-5 Raman Gain m/w X Brillouin Gain m/w 5X X X X X10-11 Damage Fluence W/µm Pump Brightness W/(µm 2 -Sr) Pump Absorption db/m Laser Efficiency Heat Fraction Pump Clad NA Laser Wavelength nm

48 Yb YAG (single crystal & ceramic) Properties Units Tm Silica Er Silica Yb Phosphate YAG Yb Silica Rupture Modulus W/m Thermal Conductivity W/(m-K) Melt Temperature K dn/dt 1/K 1.18X X X X X10-5 Raman Gain m/w X Brillouin Gain m/w 5X X X X X10-11 Damage Fluence W/µm Pump Brightness W/(µm 2 -Sr) Pump Absorption db/m Laser Efficiency Heat Fraction Pump Clad NA Laser Wavelength nm

49 Yb YAG (1 of 2) SRS Limited (1 nm) SRS Limited (Broadened to 10 nm) Damage Damage SRS is more pronounced in YAG than SiO 2, though bandwidth broadening may overcome this. Loss and fiber flexibility are issues, leading to length restrictions which may ultimately limit power to 10 s of kw. 55

50 Yb YAG (2 of 2) SBS Limited (Upper limit SBS gain [5*10-12 m/w]) SBS Limited (At SRS Bandwidth broadened gain [10-13 m/w]) Damage SBS limit in YAG may be high, but if it exceeds the SRS limit, the latter dominates. On the right, we only consider SBS values that match the SRS limit (~ m/w) Loss and fiber flexibility are issues, leading to length restrictions which may ultimately limit power to 10 s of kw. 56

51 Lineouts of YAG based contour plots at 1m 1m SRS Limited 1m SBS Limited Damage SBS Thermal Lens If losses are < 1 db/m, then SRS-limited and SBS-limited lasers are both limited to 10 s of kw. For SBS-limited (narrowband) lasers, this is a significant improvement. 57

52 Pulse energy limits in fibers appear to have been reached Bending Damage Extractable Energy SRS We believe that any further scaling of fiber laser average power and pulse energy will require fundamental changes in the fiber itself. This will require the ability to make new fibers. Kerr Lens Self Focusing 4MW hard limit 58

53 We have the technical expertise to draw fibers and the capability can be obtained for a modest cost Tower features Tower height: 8.2 m Preforms up to 1 m long and 50 mm dia. Fiber from 80 to 500 µm UV single acrylate coating Tractor for pulling rods and canes from 0.5 to 2.0 mm with automated cane cutter Pressure control system with 2 control points capable of vacuum to +100 kpa 2nd pyrometer enabling tower to draw soft glass fibers Tower Cost: $600K Facility Cost: ~$600K Will provide the ability to make new waveguide designs in a 1-2 week timeframe depending upon the complexity of the design Draw tower 59

54 Quartz rods and tubes and even Yb 3+ -doped starting glass is easily obtainable commercially Fused Silica Fluorinated Fused Silica Yb 3+ -Doped Fused Silica There are a number sources of raw materials including Momentive, Schott and Kigre

55 Photonic crystal fibers will be made by the stack and draw technique We intend to employ this capability to investigate waveguide designs with the potential to scale power and pulse energies beyond the limits we have computed for simple cases

56 Future directions Short pulse lasers Plan to deploy ARC system on NIF Developing improved systems for MEGa-Ray and LBNL Power and energy scaling Have internally funded R&D program to investigate new waveguide designs Have externally funded program to look at beam combining of pulsed sources Long term goal: Increase fiber laser power and pulse energy while shortening output pulse width in order to enable sources capable of addressing new science applications such as X-ray, EUV seed sources and laser based particle acceleration

57 J. Dawson, April 6,