High-power fibre Raman lasers at the University of Southampton Industry Day Southampton, April 2 2014 Johan Nilsson Optoelectronics Research Centre University of Southampton, England Also consultant to SPI Lasers, Southampton, England www.orc.soton.ac.uk/hpfl.html jn@orc.soton.ac.uk
New high power laser technology for New Wavelengths 2
Acknowledgements Co-workers Tianfu Yao Harish Achar Vasant Jayanta Sahu Morten Ibsen Christophe Codemard (SPI Lasers) Junhua Ji (now at ESI, Singapore) Carl Farrell (now at Heriot Watt university) Akira Shirakawa (now at University of Electro-Communications, Japan) Funding EPSRC AFOSR Industrial 3
All fibres made in Southampton (almost) Large core / large mode area Multi-core ribbon - Scalable!! Micro-structured holey fibre Air core Helical core fibre Circular birefringence Filtering Air clad High pump-na Photonic bandgap Delivery, pulse compression, gas-filled devices Birefringent (stress) Multi-core 4 53
What is a fibre Raman laser? It s a fibre laser! (Or amplifier) See one in P4009! Very Simple! Gain through stimulated Raman scattering (SRS) Inelastic scattering process Not stimulated emission No excited state No energy storage Instantaneous gain (no build-up time) Gain peaks ~13.2 THz or 440 cm 1 below pump frequency in silica 1030 nm pump 1080 nm signal Raman gain spectrum in silica fibres (R. H. Stolen and E. P. Ippen, Appl. Phys. Lett. 22, 276, 1973) SRS is relatively weak Still substantial at high power The higher power the better!!? 5
Why fibre Raman lasers? Brightness enhancement / beam cleanup Multimode pump single-mode output Power density on target determined by brightness rather than to power 6
Why fibre Raman lasers? Power scalable Pump flexibility -- not necessary to specify pump wavelength Pump with Yb-doped fibre lasers or any other laser Quantum defect < 5% with 1060 nm pumping Low thermal load Freedom in material (no need for rare earth doping) Better control of refractive index profile Very low loss materials Photo-darkening readily avoided 7
100 W cladding-pumped Raman fibre laser @ 1120 nm Output Power [W] 120 100 80 60 40 20 Laser Pump Through Fit =71% HR 85 m DCRF LF11 Flat Cleave 4% reflection 200 W ISO 6% loss Pump Dump POWER METER HT @ > 1090 nm HR @ <1090 nm 0 0 20 40 60 80 100 120 140 160 180 Launched Pump Power [W] Slope efficiency 71% Power conversion efficiency 62.5% M 2 = 1.6 Scalable to multi-kw-level!! 1064 nm Fibre source 200 W C. A. Codemard, J. Ji, J. K. Sahu, and J. Nilsson, 100 W CW cladding-pumped Raman fiber laser at 1120 nm, in Fiber lasers VII: technology, systems, and applications, Proc. SPIE vol. 7580, pp. 75801N (2010)
Why fibre Raman lasers? Instantaneous gain with no stored energy Not good for generating pulses Can be used to convert pulses Brightness enhancement Wavelength Temporal gain shaping Modulated gain at > 60 db Quasi-uni-directional gain in pulsed regime Eliminates need for isolator 9
Why fibre Raman lasers? Wavelength agile Stimulated Raman scattering is wavelength-agile New high-power wavelengths Limited only by transparency range of silica Yb Nd Silica Fiber Material Loss and some Rare-earth emission bands Nd Raman Er Tm Ho 20 15 10 5 Background Loss [db/km] Especially attractive to combine with wavelength-agile pump diode technology Emission wavelength determined by pump wavelength 0 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Wavelength [nm] Into the mid-infrared with non-silica glasses! 10
Near-diffraction-limited supercontinuum generation in a cladding-pumped nonlinear fibre converter J. Ji, C. A. Codemard, A. S. Webb, J. K. Sahu, and J. Nilsson Conf. On Lasers and Electro-Optics (CLEO 2010) San Jose, USA, May 16-21, 2010, paper CMMM5 Very simple high-power supercontinuum generation Scalable to very high powers (?)
Simple setup DM: dichroic mirror. HR@1064 nm, HT@ >1116 nm. DM Supercontinuum Lens 100 m DC fibre Residual pump DM Lens Isolator DM Lens Raman seed: ensures beam quality shortens fibre length Raman pump @ 1064 nm Raman seed @ 1112 nm Nanosecond pump: cheap, easily built, long dispersion length Raman pump Raman seed DC fibre 1064 nm Pulsed 30 ns / 150 khz Multimode M 2 ~3.2 Launching efficiency >89% Power varied Diffraction Launching 1112 nm cw 88 mw limited efficiency >92% 20 mm diameter, 0.06 NA core / 50 mm diameter, 0.2 NA inner cladding ~ 0.24 db background loss over 100 m at 1 mm
Simple fibre n (x10-4 ) 30 25 20 15 10 5 Fabricated in house Core diameter 20 mm, NA ~0.06 - Compatible with kw of output power Cut-off wavelength ~1567 nm Inner cladding diameter 50 mm, NA ~0.2 0-5 -20-10 0 10 20 Radius [ m] 13
Supercontinuum spectrum Power [mw/nm] 10 7 10 6 8.05 kw, 100 m 10 5 8.05 kw, 700 m 10 4 10 3 10 2 10 1 10 0 10-1 10-2 10-3 10-4 10-5 800 1000 1200 1400 1600 Wavelength [nm] Power density: 40 mw/nm for the 100 m long fibre around 1400 nm For the 700 m long fibre, the spectrum is very flat; power density is between 0.1 mw/nm and 10 mw/nm over large range Significant anti-stokes for 700 m fibre With highest pump peak power and 100 m and 700 m long fibres -100 1750 1800 1850 1900 1950 2000 Wavelength [nm] 8.05 kw Background noise -70-80 -90 Power [dbm] OSA uncalibrated in this region
Supercontinuum power Average output power vs. launched average pump power (100 m long fibre) Ouput power [W] 25 20 15 10 5 0 Output power Output power excluding residual pump 0 5 10 15 20 25 30 Launched pump power [W] Highest conversion efficiency is 75% at 20.4 W of launched pump power Pump launch efficiency 89% Remaining pump ~ 2 W in core & inner cladding
Raman amplifier with ultra-broad reconfigurable gain spectrum through cascaded Raman conversion Raman amplifier pumped by diode-seeded Yb-doped fibre MOPA Rectangular pulses of variable height allows for broad and reconfigurable Raman gain spectrum Cascaded Raman conversion Gain across 3 Stokes orders 14 Flat gain possible! 13 1st 2nd 3rd On/off gain (db) 12 11 10 9 8 7 6 5 4 3 2 1 0 10601080 1100 11201140 1160 11801200 1220 12401260 1280 1300 Wavelength (nm) 0% 30% 60% 90% 12.6 db on/off gain at 1116 nm 13 db on/off gain at 1172 nm Carl Farrell, Christophe A. Codemard, and Johan Nilsson, "Spectral gain control using shaped pump pulses in a counterpumped cascaded fiber Raman amplifier," Opt. Express 18, 24126-24139 (2010) 11.6 db on/off gain at 1235 nm
High power, high efficiency fibre Raman laser pumped by multimode laser diodes at 975 nm Advanced Solid State Lasers, Paris, Oct 29 2013, ATu1A.4 Tianfu Yao and Johan Nilsson Optoelectronics Research Centre, University of Southampton, England Two fibres: GRIN (MM) Double-clad jn@orc.soton.ac.uk www.orc.soton.ac.uk/hpfl.html
Direct diode pumping Very simple 18
Directly diode-pumped Raman laser cavity setup f= 20 mm HR Pump @ 975 nm DM 1 f= 11 mm f= 8 mm DM 2 f= 20 mm VBG Graded-index multimode fiber Laser output @ 1019 nm Diagnostics Power meter & optical spectrum analyzer Outcoupling end: feedback from 4% reflecting perpendicular cleave Far end of fibre: DM 2: HR for both pump & Stokes (but not second Stokes) 80% launch back efficiency Total signal roundtrip loss: 19.5 db for 3 km fibre; 17.3 db for 1.5 km fibre 19
Total power (W) Results, 3 & 1.5 km GRIN MM Relative power (dbm) 975 nm Diode pumped Raman output 2D Graph 1 25-30 1.5 km Resolution: 2 nm Total output (W) 20 15 10 Relative power (dbm) -45-60 -75 5 0 0 10 20 30 40 50 Launched pump power (W) Launched pump power (W) Results: Signal wavelength 1019 nm, QD ~ 4.3% Slope efficiency increases for shorter fibre length: 80% Slightly higher threshold: 19 W Record output power: 20 W M 2 = 5-90 900 950 1000 1050 1100 Wavelength (nm) Wavelength (nm) Col 1 vs Col 2 20
2D Graph 3 GRIN MM fibre Raman gain fibre Normalised refractive index, a.u Normalised index step Delta n 1.2 DCRF (preform RIP) L10284 - Preform Refractive Index Profile 0.04 0.9 0.6 0.3 0.0-80 -40 0 40 80 Radius (um) (µm) fibre Col 1 vs Col 2 1. OFS OM-4 GRIN MM Core (cladding) (Ge-doped silica) 62.5 µm diameter 0.275 NA 2. ORC DCRF 14.6 (38) µm diameter 0.1 (0.3) NA 0-7 -5-3 -1 1 3 5-0.005 Preform Radius (mm) fibre length 0.035 0.03 0.025 0.02 0.015 0.01 0.005 Background loss 3 km/ 1.5 km 1.7 db/km @ 975 nm 1.5 db/km @ 1019 nm 0.65 km 7.5 db/km @975 nm 6.9 db/km @975 nm 21
Output power (W) Relative power (dbm) Laser output (W) 6 5 4 3 2 1 Results, 0.65 km DCRF Fibre pump launch end laser power f = 0.19*(x-33) 0 20 30 40 50 60 70 Launched pump Pump power (W) (W) Results: M 2 = 1.9 Record output power: 6 W Total launched pump power (W) vs Laser output 1(W) Relative power level (dbm) -10-20 -30-40 -50-60 -70 Pump launch end spectrum -80 920 970 1020 1070 1120 Wavelength(nm) Col 5 vs Col 6 Slope efficiency 19%; Threshold: 33 W First fibre Raman laser cladding-pumped directly by diodes Efficiency limited by high background loss Resolution: 2 nm 22
Operation down towards 800 nm possible 835 nm fibre Raman laser pulse-pumped by a MM laser diode -30 Relative power level (dbm) -40-50 -60-70 -80 800 820 840 860 Wavelength (nm) Tianfu Yao and Johan Nilsson, Short-wavelength fiber Raman laser pulsepumped by multimode laser diode at 806 nm, SOF Topical Meeting Colorado USA 17-21 June 2012. 23
More to come! Power scaling kw-level with direct diode-pumping Multi-kW with solid-state laser pumping ( tandem-pumping ) New wavelengths From 800 nm to beyond 2000 nm Control of competing nonlinearities 24
Signal power (mw) 60 SBS suppression in Raman amplifier dither : 300 mv, Pump Modulation 260 MHz dither : 300 mv, No pump Modulation 260 MHz 40 20 0 200 300 400 500 600 700 800 Enables sub-ghz linewidth Input pump (mw) Proof-of-principle investigations at low power 25
Summary Fibre Raman lasers will allow Record-breaking power levels Multi-kW New wavelengths New operating regimes with new types of control Direct diode-pumping particularly attractive Efficiency Simplicity Wavelength coverage Improvements in diodes and fibres This field will move forward rapidly Lots of opportunities! 26
Pump sources for high-power laser work Well over 10 kw of diode pump power available across ORC Mostly 9xx nm, ~ 1 kw @ 793 nm for Tm-pumping Up to 5 kw @ 975 nm can be deployed in single setup We also have a 4 kw, 1030 nm Yb:YAG disk laser (Trumpf) 27
4 kw TRUMPF disk laser facility 1030 nm, M 2 ~ 10 Laser pumping Raman, Yb... Wavelength conversion Materials processing Power handling / stress tests Looking for collaboration with users 28
Backscattered power [a.u.] 1.7 kw quasi-single-frequency Yb-doped fibre MOPA For coherent beam combination with deep target-in-loop capability 6 5 4 Fibre: T0173 Modulation frequency: 150 MHz 300 MHz SBS onset 3 2 1 JTO Contract FA9451-06-D-0014 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Signal power [kw] Power limited by available pump power 2.4 kw from two diode sources 1.7 kw @ 300 MHz (M 2 = 1.6) Record power for high-gain MOPA In total 3.5 kw of 9xx nm diode pump power available in this lab