Power Scaling of Tm:fiber Lasers to the kw Level Peter F. Moulton Q-Peak, Inc. CREOL Industrial Affiliates Day 2009 High Power Optical Sources for the 21st Century April 17, 2009
Outline Background Fundamentals of Tm:silica fiber lasers Fiber laser setup and results Support: HEL-JTO Contract Nos. FA9451-06-D-0009 and FA9451-08-D-0199 Technical work: Q-Peak: Glen Rines, Evgueni Slobodtchikov, Kevin Wall, Nufern: Gavin Frith, Bryce Samson, Adrian Carter
Relative eye safety is obtained for > 1400-nm wavelengths Retinal focusing can increase the power density by 10 5
Rare-earth laser transitions can provide eyesafe wavelengths in fibers Energy (wavenumber/10000) 1060 nm 930 nm 1550 nm 1950-2050 nm 1080 nm
Tm-ion cross relaxation allows excitation of two upper laser levels for one pump photon
Prior work with Tm:YAG lasers E.C. Honea, R.J. Beach, S.B. Sutton, J. A. Speth, S.C. Mitchell, J.A. Skidmore, M.A. Emanuel, and S.A. Payne, 115- W Tm:YAG Diode-Pumped Solid-State Laser, J. Quantum Electron. 33, 1592 (1997). K. S. Lai, P. B. Phua, R. F. Wu, Y. L. Lim, E. Lau, S. W. Toh, B. T. Toh, and A. Chng, "120-W continuous-wave diode-pumped Tm:YAG laser," Opt. Lett. 25, 1591-1593 (2000)
Recent advances in Tm-doped fiber-laser efficiencies show levels approaching Yb fibers
Fundamentals of Tm:silica fiber lasers
Absorption and emission cross sections for Tm:silica Cross section (cm2) 5.0E-21 4.5E-21 4.0E-21 3.5E-21 3.0E-21 2.5E-21 2.0E-21 1.5E-21 1.0E-21 5.0E-22 Absorption Emission Abs: Nufern Em: Walsh (NASA) 0.0E+00 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 Wavelength (nm)
Calculation of net gain in Tm:silica fiber laser We define the inversion fraction as: F = N 2 / (N 2 + N 1 ), where N 1 and N 2 are the inversion densities for the lower and upper Tm:silica laser levels. The net gain (or loss) cross section σ(λ) in the fiber as a function of wavelength, λ, is given by the relation: σ (λ) = F σ e (λ) (1-F) σ a (λ), where σ e (λ) and σ a (λ) are the emission and absorption cross sections. The gain or loss coefficient is σ (λ) multiplied by the concentration of active ions.
Plot of net gain cross section in Tm:silica vs. inversion fraction 5E-21 4E-21 Net gain cross section (cm2) 3E-21 2E-21 1E-21 0 1 0.9 0.8 0.7 1500 1600 1700 1800 1900 2000 2100 2200 0.6-1E-21 0.5 0.4-2E-21 0.3-3E-21 0.2-4E-21 0.1 0.05-5E-21 Wavelength (nm) Data on emission cross section from Walsh and absorption cross sections from Nufern
Tm:silica gain at low inversions 1.5E-22 1E-22 0.1 0.09 Net gain cross sections needed for 5-m fiber length, with gain of 25 Net gain cross section (cm2) 5E-23 0-5E-23-1E-22 0.08 0.07 0.06 0.05 0.04 0.03-1.5E-22 0.02 0.01-2E-22 1900 1920 1940 1960 1980 2000 2020 2040 2060 2080 2100 2120 2140 2160 2180 2200 Wavelength (nm)
Polished preforms and sample holder
Absorbance data from Lambda 9 measurements 0.3 0.25 0.2 Result: HI LO: 2.03-2.36 wt% (Tm LO 2 O 3 ) HI: 2.47-2.87 wt% Absorbance 0.15 0.1 0.05 0 600 650 700 750 800 850 Wavelength (nm)
The photodarkening issue has not appeared in pumping highly doped fibers at 790 nm Energy (cm-1) 40000 35000 30000 25000 20000 15000 3P2 3P1 3P0 1I6 1D2 1G4 3F2,3 10000 5000 790-nm pump 3H4 3H5 3F4 0 3H6 M.M. Broer, D.M. Krol and D.J. DiGiovanni, Highly nonlinear near-resonant photodarkening in a thulium-doped aluminosilicate glass fiber, Opt. Lett. 18, 799 (1993).
Dynamics measurements of Tm:silica 800-nm emission
Decay data for 3 F 4 (upper laser) level shows two-lifetime dynamics 60000 Signal (arb. units) 50000 40000 30000 20000 LO data broadband Double exponential fit 633 usec lifetime 281 usec lifetime 10000 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Time (msec)
Initial portion of 3 F 4 signal shows feeding from pumped level 60000 50000 Signal (arb. units) 40000 30000 20000 10000 0 0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 Time (msec)
800-nm fluorescence provides data on cross-relaxation efficiency 60000 50000 LO sample HI sample Signal level (arb. units) 40000 30000 20000 7.9 usec to 1/e 5.6 usec to 1/e Assuming 45 usec lifetime for low Tm doping, efficiency of cross relaxation: 74% for LO 80% for HI 10000 Decay in tail: 24.3 usec (LO), 21.0 usec (HI) 0 0 10 20 30 40 50 60 70 80 90 100 Time (microseconds)
Fiber laser setup and results
Approach to scaling follows on work done by SOTON on Yb:fiber lasers HT @975 nm HR @~1.1 µm Signal output @~1.1 µm HT @975 nm HR @~1.1 µm Diode stack @975 nm, 0.6 kw Diode stack @975 nm, 1.2 kw HT @975 nm HR @~1.1 µm Double-clad Yb-doped fibre Signal power [kw] 1.4 1.2 1.0 0.8 0.6 0.4 0.2 Measured Linear fit Slope efficiency: 83% 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Launched pump powwer [kw]
Details of the 790-nm pump band (2 wt. % Tm) showing broad absorption 7 6 Attenuation (db/m) 5 4 3 2 1 0 750 760 770 780 790 800 810 820 830 840 850 Wavelength (nm)
350-W Laserline pump laser (1 of 2) 5-m delivery fiber Rack unit with diodes, power supply and cooler 1:1 lens focusing optics High-power connector
Pump laser wavelengths were 795 nm at full power Spectral emission data for pump lasers #1 and #2, respectively at a drive current of 55A, approximately 350 W of power output.
Q-Peak fiber-laser testbed power meter 2050 nm output Single-ended pump Active fiber coil clamp Dichroic mirror HR at 2050 nm HT at 790 nm clamp focusing head focusing head 793-nm pump 400-um, 0.2 NA fiber delivery Heat sink Meniscus 2.5-cm R concave surface HR at 2050 nm HT at 790 nm Pump Laser A Pump Laser B
Characteristics of Nufern-supplied fibers Fiber ID MM-TDF- 20/400-LO MM-TDF- 20/400-Hi LMA- 20/35/400-Hi Core diameter 17-23µm 17-23µm 17-23µm Clad diameter 385-415µm 385-415µm 385-415µm Core NA (effective) 0.2 0.2 0.1 Cladding NA 0.46 0.46 0.46 V value at 2µm >6 >6 <4 # of modes (2µm) 7 7 2 Cladding absorption ~2dB/m ~2.6dB/m ~2.6dB/m (795nm) Tm-concentration 2.7wt% 3.5wt% 3.5wt% Cladding Shape Octagon Octagon Octagon At 790.1 nm (2.5-nm linewidth) we measured 1.09 db/m for LO fiber (10-m length) and 1.54 db/m for HI fiber (7-m length)
250 Summary of highest-power LO and HI Tm:fiber lasers 225 LO fiber data 200 LO linear fit Output power (W) 175 150 125 100 75 HI fiber data ` 46.3% slope 50 25 0 0 50 100 150 200 250 300 350 400 450 500 550 Launched pump power (W)
LMA HI2 fiber design used undoped, spliced ends 3 1 2 x 2 x 1 Fiber assembly: 5-m length of Tm-doped fiber (3), with two undoped, 3-m-long fibers (1) fusion-spliced (2) to the ends of the doped fiber Gain fiber: LMA HI2 Cores: 25 µm in diameter, NA: 0.08. Pump claddings: 400-µm in diameter, octagonal cross section Pump attenuation: 2.9 db/m
300 W from 25/400 Tm:fiber laser Output power (W) 325 300 275 250 225 200 175 150 125 100 75 50 25 0 Slope efficiency data, corrected for absorbed power, is 71.7% in good agreement with the value of 69.8% calculated by spectroscopy. The pump quantum efficiency is 1.84. 64.5% slope 61.8% slope LMA HI2 fiber data conduction cooled, new clamps Linear fit LMA HI2 fiber data conduction cooled Linear fit LMA HI2 fiber data water cooled Linear fit 0 50 100 150 200 250 300 350 400 450 500 550 600 Launched pump power (W) 59.1% slope 301 W
LMA HI2 fiber laser beam quality close to D.L. Beam width (µm) 1500 1000 500 Horiz. raw data Vert. raw data Horiz. processed data Vert. processed data Horiz. axis, M 2 x =1.21 Vert. axis, M 2 y =1.16 0 200 250 300 350 400 EP 7290 camera distance (mm)
Tuning of LMA HI2 laser limited on short-wavelength end by high gain 20 18 16 Power output (W) 14 12 10 8 6 4 2 0 1960 1980 2000 2020 2040 2060 2080 2100 Wavelength (nm) The laser was pumped from one end with 47 W, and had a 600g/mm Littrow grating as an end mirror
Next: Scale the Tm-doped fiber laser to 1 kw Signal output @~2 µm HT @790 nm HR @~2 µm Fiber coupled diode stacks 1000 W at 790 nm, 1000 um 0.22 NA HT @790 nm HR @~2 µm Double-clad Tm-doped fiber Cladding 625 um, 0.46 NA Core 35 um HT @790 nm HR @~2 µm
Rack of pump lasers, 1- kw Q-Peak pump data Hole for second 1 kw pump 1 kw pump Power Output (W) 1200 1000 800 600 400 200 I/O data from Laserlines 1-kW LDM (S/N 760420) Power through lens assembly Power through undoped fiber MM-GDF-625/35 0 1 1.5 2 2.5 3 3.5 4 Current Monitor (V) 350 W pumps 86% transmission through fiber (93% maximum with uncoated ends)
New world s record for Tm:fiber power 1000 900 885 W @ 50.7% efficiency 800 700 Output power (W) 600 500 400 300 51% slope Fiber 2 Fiber 1 200 100 0 0 250 500 750 1000 1250 1500 1750 2000 Launched pump power (W)
Cutback measurements on 35/625 fiber show absorption to be double exponential Relative transmission 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 T = A s exp (- α s L) + A l exp (- α l L) A s 0.493 α s (/m) 1.38 A l 0.395 α l (/m) 0.50 Source setpoint (V) and fit 1.2 1.3 1.4 Short fit Long fit Total fit 0.2 0.1 0 0 1 2 3 4 5 6 7 Length (m)
Cutback absorption measurements on 400-um cladding fibers - single exponential absorption 8 Power (W) 7 6 5 4 3 LO data LO fit LMA HI1 data LMA HI1 fit LMA HI2 data LMA HI2 fit 2 1 0 0 2 4 6 8 10 Length (m) Fiber Nominal doping (%) Cladding absorption coefficient (/m) Core absorption coefficient (/m) Area ratio Predicted cladding absorption coefficient (/m) Measured /predicted LO 2.7 0.292 158 0.00250 0.395 0.74 LMA-HI1 3.5 0.438 192 0.00250 0.480 0.91 LMA-HI2 4.5 0.664 247 0.00391 0.965 0.69
Thermal modeling for 200 W/m indicates cladding/buffer temperature well below 100 C 35/625/819 P critical = 382 W/m 25/400/550 P critical = 323 W/m The cladding/ buffer interface reaches 100 C sooner for the smaller diameter fiber even with a thinner buffer
Publication from BAA1
V = 2π a λ o NA a is core radius, λ is wavelength Scaling issues for Tm-doped fibers compared to Yb-doped fibers V < 2.405 for single-mode fiber Optical damage fluence (dielectric breakdown): scales as λ Raman gain: scales as 1/ λ Brillouin gain: scales as 1/ (λ) 2 x 1/linewidth Thus, for the same V parameter, compared to Yb-doped fibers, Tm-doped fibers have: 8 X higher fiber end-facet damage threshold 8X higher stimulated Raman scattering threshold TBD higher stimulated Brillouin scattering threshold
High-power single-frequency results from NGAS Gregory D. Goodno, Lewis D. Book, and Joshua E. Rothenberg SPIE Photonics West January 27, 2009
Applications of high-power Tm-doped fiber lasers Directed energy IRCM Remote Sensing for CBW detection Remote Sensing of Global Winds Coherent laser radar Driver for laser ultrasonics NDT for aircraft parts Pump source for mid-ir ultrafast systems
Ho:YLF MOPA chain produces record for hybrid system with Tm:fiber pumps Tm-pump #1 ~120 W at 1940 nm Osc/ Amp #1 Ho-stage/ Regime CW 100 Hz 500 Hz Tm-pump #2 ~120 W at 1940 nm Tm-pump #3 ~120 W at 1940 nm Amp #2 Amp #3 Osc/Amp #1 39 W 55 mj 50 mj Amp#2 76 W 110 mj 95 mj Amp#3 115 W 170 mj 140 mj Tm-fiber laser TLR-100-1940 IPG Photonics www.ipgphotonics.com Operation regime Beam Profile Output power Wavelength Polarization Linewidth CW TEMoo 120 W 1940 nm Random 2 nm
ZGP OPO/OPA layout for 3200-nm generation Ho-MOPA BS Ch. 1 <60 mj HR Ch. 2 <100 mj OPA output DM3 DM4 OPA ZGP 10 mm DM1 DM2 Ristra OPO ZGP 10 mm ZGP obtained from Inrad HR
ZGP OPA - various repetition rates 35 30 500 Hz 25 Signal seed ~10 mj OPA output (mj) 20 15 Signal seed ~9 mj Signal seed ~9 mj 100 Hz 300 Hz 500 Hz 10 5 0 0 20 40 60 80 100 120 Ho-pump energy (mj) OPA pump beam size remains optimized for max pulse energy of ~100 mj (100 Hz)
Cr:ZnSe CPA system based on Tm:fiber pumps 1.9-2.0-µm 5-W P M T m:fiber C W pump laser 1.94-µm 50-W T m:fiber C W pump laser 2.05-µm Ho:Y L F Q-s witched pump laser (17 mj, 1 khz) 2.5-µm CEP-s tabilized C r:z ns e femtosecond laser (100 fs, 1 nj, 100 MHz) P uls e s tretcher 100 fs 100 ps 2.5-µm C r:z ns e regenerative amplifier (4.5 mj, 1 khz) CEPs tabiliz ation s etup Output: 2.5 µm, 100 fs 3 mj, 1 khz P uls e compressor
CW Cr:ZnSe laser generates 130 fs pulses at 2530 nm Tm:fiber las er C r:z ns e CaF 2 pris ms 2% O.C. SESAM R OC = 100 mm R OC = 150 mm 1 0.8 Amplitude [au] 0.6 0.4 0.2 0-0.2-200 -100 0 100 200 Time [fs]
Conclusions Tm:silica fiber lasers may provide power levels and efficiencies approaching that of Yb:silica fibers We have measured some fundamental properties of Tm:silica to better understand laser operation With a 25/35/400 Tm:silica fiber laser, we generated 301 W, with 60% conversion of launched pump power to laser output The laser slope efficiency indicates that each pump photon generates 1.84 laser photons With a 35/625 Tm:silica laser we have generated 885 W of power, a new record for this technology.