WSOF-2010 High Power Fiber lasers and Amplifiers: A tutorial overview William.Torruellas@JHUAPL.edu The views, opinions, and/or findings contained in this article/presentation are those of the author/presenter and should not be interpreted as representing the official views or policies, either expressed or implied, of the Defense Advanced Research Projects Agency or the Department of Defense. Distribution Statement "A" (Approved for Public Release, Distribution Unlimited) Acknowledgements: K. Tankala, B. Samson (Nufern) D. Brown, M. Dennis (JHU-APL) R. Berdine (AFRL) J. Mangano (DARPA-MTO)
Overview of the Presentation Optical Fibers: System Design Consideration Advantages of Optical Fiber Lasers Large Core Fibers: Single or Multi-Mode Fibers Gain in Fibers HP Fibers: Nonlinear Factory Nonlinear Limitations: SBS, SRS, Optical Damage Conclusions
Why are HP fiber lasers becoming attractive? Commercially achieved Fiber Laser Performances Fiber Output Power (kw) 120 100 80 60 40 20 0 0.9 m M2>15 M2=6 M2=1 20 kw/m 3 0.9 x 2.8 x 0.8 m DARPA 1kW Race 2000 2002 2004 2006 2008 2010 2012 2014 Year IPG-08 M 2 ~30 Projected Performance Today
Overall Advantages of Optical Fiber for HP
Fiber Basics: A Simple but Very Rich Geometry! An important design difference between fibers for telecom and fiber lasers is the low index coating applied to the fiber The primary coating in the case of doubleclad fibers for lasers is low index polymer (fluorinated) forming a high NA waveguide for the pump light to propagate Inner cladding (Silica) Outer cladding (polymer) Doped Core Pump light from diode
Brightness Enhancer Brightness: Etendue theorem: Etendue is Minimum Diode Laser Biggest Issue! Diodes: -Single Emitter Junction : 1x100µm (60 o x5 o ) -Bar : 19x(1x100)µm (60 o x5 o ) -Fiber Cladding : 200-400µm (20 o aperture NA=0.45) -Fiber Core : 10-30 µm (<5 o aperture NA=0.06)
Fiber Lasers emit in Atmosphere s Near-IR Transmission Windows: WAVELENGTH CONVERTERS 20km Transmission Atmospheric Visibility: 5,10, 23km YDFA EDFA/EYDFA HoDFA
Fiber Lidar : Delocalized hardware
High Efficiency > 25% WP Excellent Beam Quality M 2 1 Short Pulses <1ns, Eye Safety Collimator Polarizer & Focusing Lens Pump Diode Fiber Laser PPLN & Oven
Truly Opto-Electronics Isolators MZI Seed Diode Laser RF Amp Pump Diodes MZI Bias Controller Comm. Cable Power Cable
Compatible with Telecom-Like Architectures Intermediate Stage 1-7W of pump @ 976nm 60A diode driver Pump/Signal all DCF Combiner 1 st Power Amp 12-21W of pump @ 976nm 2 nd Power Amp 25W-60W of Pump at 976nm
HEL Yb 1 kw amplifier ~$150/Watt 10kW Chiller high power splice tray CPU Splice tray 1 Power Supply diodes
Enabling Incoherent/Coherent Beam Combining Spatial Beam Combining: reduces thermal blooming Adaptive Compensation: Improves optical fluence on target Modified Scaling Laws 40kW Free Propagation Single beam Tilt and Blooming 4x10kW Free Propagation Tilt and Blooming 4x10kW with Adaptive Compensation Tilt and Blooming are removed Intensity Patterns every 200m are shown in all 3 cases
Thermal Management: (uniformly distributed thermal load in cylindrical rod) T melt = 1982 o K
Thermal Performance
Comparison with Nd:YAG Thermal Adavantage = Geometry+Efficiency
Large Core Fibers Single or Multi-Mode Fibers?
Example of Modes Profiles LP 01 LP 03 LP 02 LP 04
Confinement Factor: Overlap of the optical mode with the gain region LP 01 LP 03
Bend Losses and Mode Filtering Bend radius acts as a loss for the Optical field
Mode Filtering
Bend Losses are used for High Order Transverse Mode Filtering Final Filtering is Fundamental Transverse Mode Injection
Gain in Fibers
Energy Levels of Ytterbium: Quasi 3-level Almost ideal quantum defect
Cross Sections of Ytterbium in SiO 2 976nm 1030nm 915nm 1064nm
Typical Amplification Conditions
Example of Results SBS result in good agreement with reported result in the literature
HP Fibers : Nonlinear Factory Limits applications
Bulk Optical Damage for 6ns Pulses
Bulk Optical Damage for 6ns pulses S. Webster et al., in SPIE Proceedings of the 2006 Boulder Optical Damage conference
Surface Damage can limit how many milli- Joules can be transmitted or generated
Surface damage Limits the maximum Fluence For 1ns Pulses the Bulk Damage is > 300 J/cm 2 For 1ns Pulses the Surface damage is > 50 J/cm 2 Without EndCap For Aeff 2000 µm 2 Maximum Pulse energy is 1mJ With EndCap Aeff 5000µm 2 Maximum Pulse energy is 2.5mJ Reliable Operation is 1.5mJ/pulse @ 1ns
NLO performance : Critical Self-Focusing for the Fiber and End-Cap n 2 = 2.5 x 10-20 m 2 /W; P crit = 0.15 x λ 2 /(n o x n 2 )=4.5MW Critical Self-Focusing limits the propagation in the End-Cap 1ns operation => E p < 3mJ 0.5ns operation => Ep < 1.5mJ Z crit = 2.85mm for (25µm waist A eff = 2000 µm 2 ) Z o = 2.73mm for (25µm waist A eff = 2000 µm 2 ) E p (1ns) = 1.5mJ P av = 15W A eff (endcap) = 2.5x 2000 µm 2 = 5000 µm 2 E p (surface-damage) < 2.5mJ
Common Nonlinear Optical Interactions Nonlinear optical Interactions: SPM, XPM, 4WM: Ultrafast nonlinear perturbation of the electronic response of the optical polarizability. These interactions perturb the optical Phase. When the several optical fields are present energy exchange can occur periodically. SRS: Ultrafast (20fs) nonlinear interaction of the optical field with Optical- Phonons. The optical interaction weakly perturbs the atomic structure changing the refractive index but more importantly transfers power between optical fields. SBS: Slow (5-10ns) nonlinear interaction between the optical fields and acoustic-phonons. The optical field changes the atomic lattice and results in an acoustic wave reflecting energy in the counter-propagating direction.
SPM : Self-Phase-Modulation
How much SPM is acceptable? Rule of thumb: Large Aeff are needed for High peak power operation Spectral Broadening: For a Gaussian pulse and large SPM the resulting spectral broadening is almost equal to the Nonlinear Phase Shift
SPM @ 0.75ns/pulse and 12Kpps A eff =500µm 2 ; L eff = 1m
4WM : Other Four-Wave-Mixing Interactions XPM: Cross Phase Modulation The cross-phase modulation is twice as strong as the self-phase 4WM: Four Wave Mixing Results in energy exchange when conservation of photon energy and phase matching are met.
4WM example: ASE @1030nm 4WM images
4WM, XPM, SPM and SRS results in Super-Continuum Generation
SRS: Stimulated Raman Scattering SRS: Stimulated Raman Scattering The Nonlinearity is complex resulting in energy transfer between the pump and signal laser beams. The Pump and the signal are in resonance with a Raman Active Mode SRS is always Phase-Matched in Single Mode Fibers.
g r = 0.5-1 10-13 m/w
SRS: Limits Power in Band Requires shorter fiber => Lowered efficiency
A eff as a fiber design parameter for SRS In some cases the limitation from standard step index fiber even at 30mm core diameter is still a problem Designs to further increase the mode field area have been established and demonstrated in Yb-doped amplifiers operating at very high peak power (>1MW) Normalized Signal Power 1 0.8 0.6 0.4 0.2 Step index fiber ~3X increase in MFA Normalized Signal Power 2 1.5 1 0.5 LFM fiber 0-30 -20-10 0 10 20 30 Radial Dimension µ m) ( 0-30 -20-10 0 10 20 30 Radial Dimension µ m) (
Solution to SRS limitation: Increasing A eff from 1300µm 2 to 2000µm 2 Increase in efficiency
How long can I make a fiber delivery fiber? P = 1kW gr = 1e-13 m/w Aeff = 150µm 2 Exponential Gain= G=exp(gr P/Aeff L) = exp(30) L < 45m This assumes no prior signal at the Raman Shifted Wavelength. For P= 10kW => L < 3m (Practical problem)
Stimulated Brillouin Scattering Single Frequency (<1MHz) High Power Fiber Amplifiers are limited by Stimulated Brillouin Scattering (SBS). Highest powers achieved to date: 200W from commercial vendors (Thermal Engineering) 500W Hero-laboratory results Large Core Double Clad-Fiber Amplifier ~ 10-20 m long I f I b V L On optical signal I f is amplified and interacts with backward propagating signal I b generated by spontaneous noise. The interaction creates a traveling acoustic grating due to electrostriction. The backward propagating signal is coherently amplified by the backscattering of the strong signal during amplification. SATURATION OF AMPLIFIED SIGNAL AND NOISY OUTPUT
SBS Gain In the unsaturated/low signal regime the backward propagation signal grows exponentially. The exponential gain depends on: - Material: Difficult to engineer - Polarization: The system requires linear polarization - The fiber length: Trade-off with thermal design and doping quenching - The ratio of the Field (Optical & Acoustic) Spatial Overlap functions and the Optical Mode Effective Area Fiber Core design
How Much Power Can I get in a Single Frequency with a Step Index Fiber? L = 10m g SBS = 2e-11 m/w A eff = 500µm 2 Exponential Gain= G=exp(g SBS P/A eff L) = exp(30) P < 75W This assumes Signal Less than 50MHz linewidth.
Acoustic Field & Optical Intensity for Large Core Step index Fiber NA=0.06
Single Core Double Clad Fibers NA=0.06 Modifying core dimension has limited impact on SBS frequency shift Modifying core dimension has no impact on ratio of overlap and Effective Area Core modification has little impact on SBS gain Leave it as is!
Acoustic Batman Profiles F OvLp (Step-Index)>4xF OvLp (Batman)
Potential Recipe Leave Core Unchanged Dope Second Core with GeO 2 and F Leaving refractive index unchanged Induce a large change in acoustic index
5900 m/s Low Order Mode 5950 m/s 11 th acoustic Order Mode 5850 m/s
Acoustic Mode Engineering: Three SBS Resonances
Acoustic Design: Moves ALL Acoustic modes outside of the core Three Parameters are investigated: g Dictated by fabrication
Predicted SBS Spectrum Baseline Step Index Fiber Based on Measured Coefficients Based on Literature Data For acoustic coefficients A Fiber design with a x5-10 improvement is theoretically achievable
Potential improvements with Fiber Core Designs No adjustable parameters: g SBS = 2e-11 m/w 7 x Suppression
Achieved Performance to date: 275W SBS Threshold Gen-II-b YDFA: 25/400; MFD=17µm 100mW Threshold Baseline SI 25/400 CW Exp. Pulsed Exp. APL Model Gen-II-b 5X improvement With Gen-II-c MFD=20µm 6-7X improvement
Impact of Nonlinearities SBS-SPM-Raman Field time dependent Model is needed g(z,t) is obtained from rate equations g b, g R and n 2 are for SiO 2
Impact of Pulse-width/Bandwidth on SBS Threshold for Amplitude Modulation Predicted SBS-SRS Threshold for repetitive Pulse operation 30/250 YDFA Dominated by Walk-Off Dominated by Phonon-Lifetime Dominated by Peak Power
Conclusions YDFA can be modeled accurately in the CW and repetitive pulse regimes 10kW is the practical limit for SM operation >50kW is commercially available only issue is cost of diodes. (50kW in 2008) In the CW regime SBS limits the maximum power for narrow linewidth sources (<100 MHz) In the short pulse regime SPM+4WM broaden the spectra SRS limits the energy in band for sub 1ns pulses Optical Damage limits the Peak Power to <1MW