Published on October 2, 2008
Latest developments in 790nm-pumped Tm-doped fiber laser systems for DIRCM applications. G. Frith, B. Samson, A. Carter, J. Farroni, K. Farley and K. Tankala, Nufern, CT, USA T. McComb CREOL, UCF, FL, USA W. Torruellas JHU-APL, MD, USA www.nufern.com
Application of Tm-doped fibers at 2µm • Tm-fiber lasers operating CW at 1908nm are an excellent pump source for Ho-YAG • Many groups are using this scheme to generate high pulse energies at 2.09µm for IRCM applications • Q-peak (ASSP 2005), BAe systems (Optics Letters, 2003) and DLR/IB LASER for example 2
Application of Tm-doped fibers at 2µm • In more recent publications (BAe systems, CLEO 2008) Tm-doped fibers have been used to amplify ~10nsec pulses at 2µm • Although fiber lasers are not likely to generate the pulse energies currently obtained from Ho:YAG MOPA systems generating 10~100mJ pulses, they could replace a Ho:YAG Q-switch oscillator by generating 0.1~1mJ pulses. 25W, 795nm diode 86W, 795nm Gain-switched Isolator Isolator Tm-doped fiber laser 25/250 Tm-doped 25/400 Tm-doped fiber fiber 50ns/div 23ns FWHM 3
Application of Tm-doped fibers at 2µm (BAe Systems, D. Creedon et al, Photonics West 2008) 35 Average Output Power (W) 30 • High average power 25 100kHz – 30.6W at 100kHz 20 90kHz – 18.6W at 50kHz 15 80kHz 10 70kHz • 36% slope at 100kHz 60kHz 5 50kHz 0 0 20 40 60 80 100 Pump Power (W) 35.00 30.00 • High peak-power Peak Power (kW) 25.00 • 15.3kW at 100kHz (20ns pulses) 20.00 • ~300µJ pulse energy 15.00 10.00 • 28.6kW at 50kHz (13ns pulses) 5.00 • ~400µJ pulse energy 0.00 • No nonlinear effects observed 0 20 40 60 80 100 Pump Power (W) 4
Some advantages of Tm-fibers at 2µm • Pulsed fiber systems could offer advantages over solid state lasers at higher rep rate/average power levels – Tm-doped fibers offer higher efficiency (important at high average powers) – Less cooling and often simpler cooling (air rather than water) – Intrinsic single mode beam quality • Monolithic all-fiber systems offer advantages for robustness & packaging over solid state lasers – Fiber lasers can enable flexible system designs with remote pumps and long delivery fibers Tx/Rx Electronics Laser Diodes for + Fiber Tray Power Amp Fiber Lidar with delocalized hardware Transport Fiber Transport Fiber Laser Diode for Tx/Rx @975nm Scanner @1550nm + Power Cables 5
Some advantages of Tm-fibers at 2µm • In addition to IRCM applications, high power lasers at “eye-safer” wavelengths are of interest in directed energy programs – see for example the recent RFI from JTO (RELI program) • Tm-fiber lasers are the highest efficiency, high power laser source operating at these “eye-safer” wavelengths • For example Er:Yb fibers at 1.5µm have very poor efficiency over 200W CW power 290W at 1576nm from an Er:Yb fiber laser 350 (ORC, Southampton University) 300 19% 4F 9/ Slope efficiency 4I 250 2 9/2 Laser power [W] 200 2F 4I 5 11/ /2 2I 4 150 40% 13/ 2 100 Measured Saturation curve fit 50 2F 4I 7 15/ /2 2 0 Yb3 Er3+ 0 200 400 600 800 1000 + Launched pump power [W] 6
Advantages of cladding pumping 790nm • At Nufern we have been focusing on direct pumping of Tm-silica fibers for high efficiency and power scaling of 2µm fiber lasers • Optimized Tm-doped fibers (using cross relaxation) operating with 790nm routinely deliver • >60% pump conversion efficiency • >100W power with single mode beam quality at 2µm • Offer a higher overall efficiency compared with the alterative scheme based on resonant pumping at 1560nm with a Er:Yb fiber laser 7
Advantages of cladding pumping 790nm • By optimizing the fiber composition, efficiencies of 790nm pumped Tm-fibers have steadily improved over the last 5 year • Including fibers from Southampton, ORC, OFTC (Australia, etc) 8
Power scaling at wavelengths <2000nm • Beyond ~2050nm Tm-doped silica is largely a 4-level system • Efficient operation at wavelengths less than 2000nm (using 790nm pumping) requires a fiber design with large core/clad area • For single mode operation, core diameters are limited to 10-25µm implying clad diameters must be 125 and 250µm at maximum (rather than 400µm clad fibers) • Therefore, power scaling at 1900-1950nm requires high power/brightness pumps 9
Results on power scaling: 1908nm MOPA. • 5W seed at 1908nm • 1.7m of Tm-doped LMA fiber counter-pumped with ~130W (2x65W pumps). • Fiber mounted on 90mm mandrel with helically cut U-shape channel for highly effective heat removal 1.7m length of LMA MO: 5W @ 1908nm Tm-doped fiber Mode stripper 795nm pump FBGs 2+1:1 combiner Cladding light stripper Fiber coupled 792nm pump modules (265W) 10
Results on power scaling: 1908nm MOPA • 70W output at 1908nm, pump power limited. Ideal source for pumping Ho:YAG • 53% slope efficiency - artificially low due to diodes shifting off wavelength (9dB absorption length at threshold to 6dB at full power). 1.0 Atmospheric transmittance 0.8 Laser spectrum (AU) Ho:YAG absorption 80 0.6 1908nm output power (W) Laser spectrum 0.4 Atmospheric transmittance 60 Ho:YAG absorption 0.2 40 0.0 1905 1906 1907 1908 1909 1910 Slope = 52.7% Wavelength (nm) 20 0 0 20 40 60 80 100 120 140 Launched pump power (W) 11
Results on power scaling: 1920nm oscillator • Example of power scaling air cooled platform at short wavelength, >65W output power 1920nm, single mode laser • Full arbitrary modulation capability with <10µs rise / fall • ~19% WPE (neglecting cooling) – We are currently developing high- temp diodes to relax cooling requirements **Observed roll-off due to diode wavelength, not fiber 12
Results on scaling Tm-lasers towards kW • Output power from a single Tm-doped fiber is now approaching 1kW level • Change in slope is due to the diodes, latest results show efficiency >50% at >850W output power from a single Tm-fiber 13
Diode developments for efficient platforms • High brightness pump diodes at 793nm are critical for efficient operation at shorter wavelengths 1900-1950nm • High power/brightness pumps at 793nm delivering 20W into 105/125 0.22NA fiber are now becoming available • E-O efficiency ~43% (ex-fiber) 14
Pulsed 2µm system for non-linear conversion • Previous work (Jiang, Optics Lett. 2007) on gain switched Tm-doped fiber lasers used an amplified 1560nm diode as the source • Our oscillator uses a Q-switched Er:Yb fiber laser as the pulse sources, which has the advantage of producing higher power (no need for the amplifier stage) Er/Yb:Fiber Coil Fiber coupled Acousto-Optic Q-Switch 15
Latest generation PM-LMA Tm-doped fibers • After producing the gain switched pulses, we amplify them in a PM-LMA fiber to produce higher pulse energy, with linear polarized single mode beam quality • Linear polarized output from the amplifier increases the efficiency for non-linear conversion • These PM-LMA fibers require a raised refractive index pedestal to lower the effective core NA for robust single mode operation 14 Number of modes Stress member Pedestal 12 10 8 6 er 4 Outer 2 LMA Cladding Core 0 0.0 0.1 0.2 0.3 Fibre NA 16
Pulsed 2µm systems for non-linear conversion • Variable PRF (25-100kHz), up to ~5kW peak power and up to 6W ave. (PRF dependent), single polarisation. AOQ-switched 3.5m length of PM-TDF Er:Yb fiber laser 20cm PM-TDF Isolator 1908nm FBGs 2+1:1 combiner IRFS aplanatic Fiber coupled 790nm lenses pump modules (2 18W) 1.2 70 6 6 1.0 Maximum stable power (W) 5 5 60 0.8 Amplitude (AU) 4 4 Peak power (kW) Pulse width (ns) 50 0.6 3 3 29ns FWHM 0.4 40 2 2 0.2 30 0.0 1 1 -0.2 20 0 0 0 40 80 120 160 200 20 40 60 80 100 120 20 40 60 80 100 Time (ns) PRF (kHz) PRF (kHz) 17
Pulsed 2µm systems for non-linear conversion • System generates kW peak powers, polarized output, single mode beam quality • Ideal for frequency doubling • Using PPLN we frequency doubled 1908nm to 954nm • 60% conversion efficiency demonstrated • ~1W of average power generated at 954nm (full results submitted to Photonics West, 2009) 1000 PM amplifier 20W average power GHG Power @ 954.5nm (mW) 900 800 700 600 500 400 300 200 100 0 0 200 400 600 800 1000 1200 1400 Polarized Input Power (mW) 18
Example of long wavelength tuning in Tm- doped fibers to 2.125µm 25 20 Output power (W) Amplitude (AU) 15 1960 2000 2040 2080 2120 Wavelength (nm) 10 Slope = 41.4% 5 0 2123 2124 2125 2126 2127 0 20 40 60 Wavelength (nm) Launched pump power (W) • Lower efficiency attributed to cavity finesse • Onset of ASE seen at ~22W 19
Preliminary Lifetest of High Efficiency Tm-lasers • New Tm-fiber compositions have been designed to maximize cross- relaxation whilst minimizing energy transfer up-conversion. • Preliminary testing of 20W laser (500hrs) operating at 1950nm pumped at 792nm and 100W laser (25hrs) show stable operation • Life testing of high power 790nm diodes is also underway 25 110 20 100 Output power (W) 90 Power (W) 15 80 10 70 5 60 0 50 0 100 200 300 400 500 0 5 10 15 20 25 Time (hours) Time (hours) 20 20
Conclusions and Acknowledgements • Efficient pumping of optimized Tm-doped fibers at 790nm is becoming a mature technology, replacing resonant pumping of Tm-fibers at 1560nm – Slope efficiencies between 55-65% are typical in packaged monolithic systems • Concerns over degradation even at >100W output power using this scheme have not been observed – Including the demonstration of ~1kWatt level device that shows no saturation or roll-over • High brightness, high power diodes at 790nm are becoming available that enable air cooled fiber lasers designs at >100W – Diode E-O at ~45% enable high power 2µm sources with wall-plug efficiencies approaching 20% for the first time • These developments are enabling new lasers/amplifiers at wavelengths between 1850-2150nm for various DoD applications as well as commercial, sensing and medical • Thanks to Q-peak, BAe systems (Nashua), JH-APL, and the JTO for MRI funding. 21
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