High peak power singlefrequency MOPFA for lidar applications L. Lombard, G. Canat, A. Durécu, J. Le Gouët, A. Dolfi- Bouteyre, M. Valla, B. Augère, D Goular, C. Besson.
Applications of wind lidars Wake vortices monitoring Lidar developments and tests on international airports (CREDOS, FIDELIO EU projects) Turbulence & windshear monitoring Turbulence & Wind mapping algorithms (UFO EU project) Courtesy from S. Wolf, IFALPA Airborne sensors Lidar developments for true air speed (AIM2 EU project) Windfarm optimization Use lidar information for wind mills optimization, feed-forward turbine control (Leosphere) Onera investigates most of those lidar applications - by developing the wind lidar - by developing the laser source Courtesy from Leosphere 2 216-6 CLRC, High peak power single-frequency MOPFA for lidar applications
Wind lidar principle LASER ν Δν ν + Δν AOM Amplifier V r V air λ = 1,55 µm 5/5 Coupler telescope ν + Δν + ν Doppler Local Oscillator ν detector ν + Δν + ν Doppler cos(δν + ν Doppler ) ν Doppler = 2.V r /λ = V r. 1.3 MHz/(ms -1 ) @1.55µm Signal processing Δν + ν Doppler =>Vr Pulse launch Detected beat signal Δν + ν Doppler Time distance 3 216-6 CLRC, High peak power single-frequency MOPFA for lidar applications
Wind lidar principle LASER λ = 1,55 µm ν Δν ν + Δν AOM Amplifier 5/5 Coupler Typical architecture ν + Δν MOPFA + ν Doppler Master oscillator power fiber amplifier telescope V r V air Local Oscillator ν detector Signal processing Δν + ν Doppler =>Vr ν + Δν + ν Doppler cos(δν + ν Doppler ) MO Why fiber lasers? High efficiency laser/amplifier sources, Compact lasers and amplifiers, No optical alignment for all-fiber systems, Low cost. +1MHz ν Doppler = 2.V r /λ AOM = V r. 1.3 MHz/(ms -1 ) @1.55µm Pulse shape Fiber amplifier(s) control MOPFA allows versatile pulsed sources: single-mode, singlefrequency, long-pulse (1ns 1µs) BUT: peak power limited by SBS for narrow linewidth 4 216-6 CLRC, High peak power single-frequency MOPFA for lidar applications
Wind lidar requirements => laser source requirements Example application Wind field monitoring around airport 1 km range; refresh rate: 1 mn for 36 CNR 2 E. PRF 1 + M 2 2 Source requirements: Narrow linewidth: Δv < 1 MHz High beam quality: M 2 < 1.3 Pulse duration: t pulse =.5 to 1 µs Maximum PRF = 1-15 khz Pulse energy E = 1 s µj => P peak = 1-1kW 5 216-6 CLRC, High peak power single-frequency MOPFA for lidar applications
Power (a.u.) Wind lidar requirements => laser source requirements Typical architecture MOPFA Master oscillator power fiber amplifier +1MHz Pump wave at n p MO AOM Pulse shape control Fiber amplifier(s) Stokes wave at n s =n p -n B Acoustic wave at n B Catastrophic when: P peak >P th Typical Brillouin gain spectrum at 155nm pump wave + Stokes wave acoustic wave Stokes wave pulse Stokes (SBS) Time (µs) P th Dn B ~ 3MHz n s g B ~ 2.1-11 m/w n B ~11GHz n p P th = 21 A eff g B L eff Aeff: effective core area Leff: effective fiber length gb: Brillouin gain 6 216-6 CLRC, High peak power single-frequency MOPFA for lidar applications
How to mitigate SBS in optical fiber? Sample SBS threshold: P th ~1 W in singlemode fibers P th ~3 W in commercial large mode area (LMA) fibers P th ~2 kw demonstrated in specialty fibers 1,2 Use of LMA with MFD > 3 µm Compatibility with good beam quality? P th = 21 A eff g B L eff Longitudinal variation of Brillouin frequency using temperature, fiber compositions, strain Compatibility with other requirements (beam quality )? Use of highly doped short fibers High efficiency? 7 216-6 CLRC, High peak power single-frequency MOPFA for lidar applications
Why is this not so simple to increase A eff? Fiber doping: Er and Er:Yb at 1,55µm Beam quality in large core fibers Increase A eff : 1. MFC fibers 2. PAS fibers: large core Alternative way: decrease L eff with strain Strain principle 3. Application on LMA fiber Coherent combining of 2 singlmode amplifiers of 7 cores of a multicore fibers 8 216-6 CLRC, High peak power single-frequency MOPFA for lidar applications
Doping for 1.5µm laser amplification: Er vs Er:Yb erbium only (Er) erbium + ytterbium codoping (Er:Yb) 4I 9/2 4I 9/2 4I 11/2 4I 11/2 Er 3+ 4I 13/2 Yb 3+ Er 3+ 4I 13/2 4I 15/2 4I 15/2 Good beam quality direct pumping => no P doping, low core NA possible Low pump absorption Er doping <1 3 ppm => avoid inter-ions energy transfer Fiber/setup design/cost need for small cladding size and high brightness pump High pump absorption high Yb doping Parasitic emission at 1µm Power limited by Er-Yb back transfer + non participating Yb Challenging beam quality P doping => central dip in refractive index profile 9
Dn x 1 3-2 -15-1 -2-15 -1-5 5 1 15 2-2 -15-1 -5 5 1 15 2-5 5 1 15 2-2 -15-1 -5 5 1 15 2-2 -15-1 -5 5 1 15 2-2 -15-1 -5 5 1 15 2 Increase Aeff : Beam quality in large core fibers Standard step index profile LMA step index profile. Diam. 3µm / ON.1 Er:Yb doped fibers: large NA and central dip 12 1 8 6 4 2 Singlemode fiber Multimode fiber -2-8 -6-4 -2 2 4 6 8 Position (um) 1 216-6 CLRC, High peak power single-frequency MOPFA for lidar applications
Efficacité o-o (%) State of the art of narrow linewidth 1.5µm fiber sources 25 2 ONERA, 214 ONERA, 214 Fibertek, 211 Fibres phosphates (NP Photonics) Fibres phosphosilicates Fibres MFC Fibres aluminophosphosilicates Fibres aluminosilicates Qualité spatiale difficile à contrôler 15 1 5 Aculight 25 NP Photonics 211 FORC 214 Shangai 213, MTBF? ONERA 212 ONERA 215 ONERA 213 @ 158nm ONERA 29 ORC M 2 >2,1 NP Photonics 2 4 6 8 1 12 14 16 18 2 22 2 3 Puissance crête (W) 1 Difficile à fabriquer a compromise has to be made between efficiency and peak power strategies are required to maximize efficiency and peak power 11 216-6 CLRC, High peak power single-frequency MOPFA for lidar applications
Peak power (kw) 1. maintain uniform core profile Silica Phosphorous fibers: quality difficult to maintain Microstructured core fiber (IPHT/ONERA collaboration) Erbium Ytterbium large mode area fiber B-doped stress applying parts (for birefringence) F-doped Silica Er 3+ /Yb 3+ cores Mode field distribution Modeling Measurements, G. Canat, et al., Opt. Lett., 33, pp. 271-273 (28),,2,4,6,8 1, Time (µs) 2,5 2, 1,5 1,,5 P pump =28 W P pump =26 W 95 ns SBS-induced dip M 2 ~ 1.3 A eff ~ 8 µm 2 P th = 2 kw G. Canat, et al. CLEO 9, paper JTuB3 (29) 12 216-6 CLRC, High peak power single-frequency MOPFA for lidar applications
Refractive index difference (x1) Peak power (W) ASE fraction % Average signal power (W) 2. chose co-dopants (Al, P) Phospho-alumino-silicate (PAS) LMA 3µm (Er,Yb:AlP) DM PD/OSA BS DFB laser diode AOM EDFA 6+1->1 PM AWG 6x 1W LD @ 975nm PM 1 9 2 8 6 coeur Δn~1-3 8 7 6 15 4,5 4, 3,5 3, slope 26% 4 5 4 1 2,5 2, 2 M 2 ~1.35 3 2 1 5 1,5 1,,5-3 -2-1 1 2 3 Radial position (A.U.) 2 4 6 8 1 12 14 16 18 2 22 Launched pump power (W), 2 4 6 8 1 12 14 16 18 2 22 Launched pump power (W) Brillouin threshold P peak ~ 77W at 65ns pulse duration and 1kHz PRF Slope efficiency 26% Brillouin threshold P peak ~ 112W at 18ns pulse duration and 5kHz PRF Slope efficiency 19% W. Renard, et al. CLEO 215 13 13 216-6 CLRC, High peak power single-frequency MOPFA for lidar applications
Why is this not so simple to increase A eff? Fiber doping: Er and Er:Yb at 1,55µm Beam quality in large core fibers Increase A eff : 1. MFC fibers 2. PAS fibers: large core Alternative way: decrease L eff with strain Strain principle 3. Application on LMA fiber Coherent combining of 2 singlmode amplifiers of 7 cores of a multicore fibers 14 216-6 CLRC, High peak power single-frequency MOPFA for lidar applications
Strain δ Strain δ Power Frequency (GHz) Strain: principle (1) Absence of strain, (z)= everywhere Unstrained fiber With strain distribution, (z) Unstrained fiber 1% Strain 11.6 11.6 11.4 11.4 11.2 11.2 11 11 1.8 1.8 1.6 1.6 1 single peak Maximum Brillouin strength 1.5 Power 2 peaks in the spectrum, Brillouin threshold inscreases by x 2 1.5.1.2.3.4.5.6.7.8.9 1.2.4.6.8 1 Position 1% % position % position To gain more: continuous distribution 15 216-6 CLRC, High peak power single-frequency MOPFA for lidar applications
Strain δ Strain δ Power Frequency (GHz) Strain: principle (2) Absence of strain, (z)= everywhere Unstrained fiber With strain distribution, (z) Unstrained fiber 1% Strain 11.6 11.6 11.4 11.4 11.2 11.2 11 11 1.8 1.8 1.6 1.6 1 single peak Maximum Brillouin strength 1.5 Power 2 peaks in the spectrum, Brillouin threshold inscreases by x 2 1.5.1.2.3.4.5.6.7.8.9 1.2.4.6.8 1 Position 1% % position % position 16 216-6 CLRC, High peak power single-frequency MOPFA for lidar applications
3. Use of strain distribution to increase the SBS threshold Backscattered power (A.U.) Peak power (W) Experiment: 1579nm high peak power single frequency MOPFA Strain distribution 1,,9,8,7,6,5,4,3,2,1, Spectral broadening unbstrained fiber strainde fiber 1,8 11, 11,2 11,4 11,6 Frequency (GHz) 4 MHz 225 MHz 2 18 16 14 12 1 8 6 4 2 G.Canat et al., CLEO Europe 213, G. Canat et al, ICSO 214 SBS threshold increase SBS limit strainded amplifier 55W 1.7 kw 26µJ SBS limit standard amplifier 23% efficiency 1 15 2 25 3 35 4 Launched pump power (mw) 17 216-6 CLRC, High peak power single-frequency MOPFA for lidar applications
Why is this not so simple to increase A eff? Fiber doping: Er and Er:Yb at 1,55µm Beam quality in large core fibers Increase A eff : 1. MFC fibers 2. PAS fibers: large core Alternative way: decrease L eff with strain Strain principle 3. Application on LMA fiber Coherent combining of 2 singlmode amplifiers of 7 cores of a multicore fibers 18 216-6 CLRC, High peak power single-frequency MOPFA for lidar applications
Coherent combining principle 1 Master oscillator (pulsed or continuous, narrow linewidth) N amplifiers with brightness: B ampli = P/(M²)² (P power, M² beam quality) Phase controller compensates for phase variations Sum of power of N lasers with same spatial, spectral, temporal characteristics Final brightness B final = N*B ampli. 19 216-6 CLRC, High peak power single-frequency MOPFA for lidar applications
4. Peak power improvement by Pulse Coherent Combining Pulse energy (µj) 15 1 5 Controller disabled Controller enabled Amp1 and Amp2 amplify the common MO Amp1 and Amp2 outputs overlap and interfere on a 5/5 beam splitter A CW multi-dithering phase controller is used (LOCSET) to minimize O 2 output ( maximize O 1 output) The setup is adapted to pulse operation using a leak between pulses. 2 4 6 8 1 Time (s) No beam degradation Overall beam combining efficiency: 95% Lombard et al., Opt. Lett. 36, 523-525 (211) 2 216-6 CLRC, High peak power single-frequency MOPFA for lidar applications
Pulse Coherent Combining in a Wind Lidar 3 Configurations to check CBC impact on instrument: - Single pulse amplifier (3W) - Coherently combined pulse amplifiers (3W) - Coherently combined pulse amplifiers (96W) Amp 1 Amp 2 Output CBC A 49 W 29.5 W OFF B 18.5 W 1.6 W 29 W ON C 49 W 51 W 96 W ON Procedure: 1 min in each configuration Comparison of measurements: - Carrier to Noise Ratio (CNR) (quality of the signal) - Estimated frequency (wind speed) - Instrument noise floor 21 216-6 CLRC, High peak power single-frequency MOPFA for lidar applications
Lidar performance comparison CNR and estimated frequency Results: CNR is equivalent for CBC ON and CBC OFF CNR @ 96W is +5dB compared to CNR @ 29W (expected 5.2dB) conf. A (1 lasers, 29.5W) 1 min conf. B (2 lasers, 29W) 1 point = 1 shots.1s conf. C (2 lasers, 96W) Noise floor is equivalent for CBC ON and CBC OFF.21 m/s.19 m/s.8 m/s First demonstration of wind lidar based on coherent combining! 22 216-6 CLRC, High peak power single-frequency MOPFA for lidar applications Lombard et al., Opt. Lett. 4, 13-133 (215)
5. Coherent combination of a multicore fiber Several Fibers Multicore Fiber Easily increase the number of channel One pump and one phase modulator per channel Common pump and phase modulator Common environment 23 216-6 CLRC, High peak power single-frequency MOPFA for lidar applications
4. Coherent Beam Combining : some experimental results 3.7 meters Residual phase fluctuations ~ λ/27 DOE zero order Combining efficiency 63% 24 216-6 CLRC, High peak power single-frequency MOPFA for lidar applications
Sum up Onera has investigated various strategies to mitigate SBS Generating uniform core by microstructuration Adjusting fiber composition in PAS fibers Applying strain on the fiber Coherent combining Based on new specialty fibers and new architecture, high peak power single frequency MOPFA are available at 1.55 µm for wind lidar applications. 25 216-6 CLRC, High peak power single-frequency MOPFA for lidar applications
Long range wind speed measurements Horizontal measurement on a typical day. Wind speed ~ 4 km/h Horizontal measurement during «Hermann» storm. Wind speed up to 11 km/h 26