Fiber lasers and amplifiers Zoltán Várallyay

Transcription

1 Fiber lasers and amplifiers Zoltán Várallyay Balaton Summer School in Physics

2 Overview FETI Ltd. ELI-ALPS Fiber Optics Basics Loss, dispersion and nonlinearity Nonlinear propagation in optical fibers What is an optical fiber laser What is an optical fiber amplifier The HR laser system at ELI-ALPS Brightest laser system in the World

3 Furukawa Electric Co. Ltd. Research Institutes Interesting facts: First undersea DC optical cable Superconductor magnets for LHC Automotive industry Foam development Optical design group Simulation group Common activities: Applied research for FEC Supervision of Trainees Supervision of MSc students Supervision of PhD students

4 Extreme Light Infrastructure Attosecond Light Pulse Source Jun, 2015 Laser ALPS-HR ALPS-SYLOS ALPS MIR ALPS-HF PW ALPS HF 100 Specs to be Specs by Dec 2023 delivered by kHz, 5 mj, <6fs 100kHz, 10 mj, 1030nm (CEP) 1 khz, 100 mj, < 5 fs 1 khz, 1J, <3 fs (CEP) (CEP) 19 Feb, khz, 150 µj, <60 10 khz, 10 mj, <60 fs 6-8 µm (CEP) 10 Hz, 34 J, 17 fs 10 Hz, 30J, <10fs 100 Hz, 0.5J, <10 fs

5 Fiber Optics Source: V SI < (single- mode condi.on) V SI = 2π λ a n2 h n l 2 M V SI 2 2 Z. Várallyay, PhD Thesis, available at sites.google.com/sites/zvarallyay

6 Theory: Fiber Optics Loss dp / dz = α P(z = 0) = P 0 P(z = L) = P T P T = P 0 exp( αl) α db = 10 L log " P % T $ ' = 4.343α Material loss # P 0 & Waveguide loss or confinement loss Bending loss Macrobending Microbending Source:

7 Dispersion of silica glass Ti:Sapphire Er Ho Yb Th R. Kitamura, L. Pilon, M. Jonasz, Optical constants of silica glass from, Appl. Opt. 46, 8118 (2007)

8 Theory: Fiber Optics Dispersion Material dispersion Sellmeier formula Schott formula Herzberger formula Waveguide dispersion Modal dispersion m n(λ) 2 K =1+ i λ 2 λ 2 i=1 L i n(λ) 2 = a 0 + a 1 λ 2 + a 2 λ 2 + a 3 λ 4 + a 4 λ 6 + a 5 λ 8 n(λ) 2 = A + BL + CL 2 + Dλ 2 + Eλ 4 + Fλ 6 L = 1 λ D(λ) = λ c d 2 n(λ) dλ 2

9 Photonic crystal fibers Large NA Large waveguide contribution to the dispersion Small-core area è high nonlinearity Large-core area è small nonlinearity ENDLESSLY SINGLE MODE! Z. Várallyay, Patent P ( ) Z. Várallyay and K. Saitoh, Photonic Crystal Fibre for Dispersion Controll, Frontiers in Guided Wave Optics and Optoelectronics, INTECH 2010, Z. Várallyay, and P. Kovács, All-silica, large mode area, single mode photonic bandgap fiber with Fabry-Perot resonant structures to be published in Journal of Optical Fiber Technology

10 Photonic crystal fibers Large NA Large waveguide contribution to the dispersion Small-core area è high nonlinearity Large-core area è small nonlinearity ENDLESSLY SINGLE MODE! Z. Várallyay, Patent P ( ) Z. Várallyay and K. Saitoh, Photonic Crystal Fibre for Dispersion Controll, Frontiers in Guided Wave Optics and Optoelectronics, INTECH 2010, Z. Várallyay, and P. Kovács, All-silica, large mode area, single mode photonic bandgap fiber with Fabry-Perot resonant structures to be published in Journal of Optical Fiber Technology

11 Tayloring the dispersion in hollowcore fibers Core expansion to detune the first period of holes Z. Várallyay, et al., Photonic bandgap fibers with resonant structures for tailoring the dispersion, Opt. Express 17, (2009)

12 Theory: Fiber Optics Nonlinearity P = ε ( 0 χ (1) E + χ (2) : EE + χ (3)!EEE +! ) n, α 2nd harmonic generation Sum frequency generation Nonlinear refraction Four-wave mixing Third-harmonic generation!n(ω, E 2 ) = n(ω)+ n 2 E 2 Helmholtz eigenvalue equation propagating modes Nonlinear Schrödinger equation spectral, temporal

13 Nonlinear pulse propagation Generalized Nonlinear Schrödinger equation: α 1 two photon absorption β m m th order dispersion γ 1 frequency dependent nonlinearity R(t) nonlinear response function γ = n 2ω ca eff # % $ A eff = 2π & E(x, y) 2 dx dy( ' E(x, y) 4 dx dy 2 Small core area Large core area L NL = 1 γp 0 Large nonlinearity Small nonlinearity Nonlinear length

14 Experimental arrangement for pulse broadening Z. Várallyay, et al., OSA Conference of ASSP, WD2 (2005) Vienna

15 Photograph on the experimental arrangement PCF output

16 Nonlinear pulse propagation, nj pulse compression Autocorrelation trace and spectral shape for a 12 fs pulse Z. Várallyay, et al., OSA Conference of ASSP, WD2 (2005) Vienna Z. Várallyay et al., Appl. Phys. B 86, (2007).

17 More on fiber optics Nonlinear propagation Numerical modeling Optical solitons Polarization effects Cross-phase modulation Stimulated Raman scattering Stimulated Brillouin scattering Four-wave mixing Highly nonlinear fibers Novel nonlinear phenomena Govind P. Agrawal, Nonlinear Fiber Optics, Academic Press 2007

18 Fiber lasers Ruby laser - Theodore Maiman in 1960 First fiber laser 1 Elias Snitzer Cladding pumping, 1988 Mirror (SA) Rear earth doped fiber 4% Fresnel reflection Laser output Pump diodes Pump diodes Advantages: high efficiency, low cost Power levels: 0.1 mw 10 mw (ps-oscillator, GHz repetition rate) [1] Charles J. Koester and Elias Snitzer, "Amplification in a Fiber Laser," Applied Optics, 3, , (1964).

19 Cladding pumped fiber lasers for higher power levels Hybrid end-pumped All-fiber end-pumped All-fiber intra-pumped M. N. Zervas, C. A. Codemard, High power fiber lasers: Review, IEEE J. Sel. Topics in Quantum Electron. 20, (2014).

20 Fiber laser in Furukawa A.Cserteg, et. al., Characterization of mode locking in an all fiber, all normal dispersion ytterbium based fiber oscillator, Proc. SPIE 9344, Fiber Lasers XII: Technology, Systems and Applications, 93442C (March 4, 2015)

21 Simulation of fiber lasers: Nonlinear dispersive propagation + gain Nonlinear Schrödinger equation (NLSE) 1 Rate equation (RE) + Power evaluation equation (PEE) Giles model 2 Pulse propagation of the signal: Fiber loss (gain?) Dispersion Nonlinearity Inversion level along the fiber Power levels along the fiber For signal (P s ) For forward pump (P fp ) For backward pump (P bp ) For forward ASE (P fase ) For backward ASE (P base ) Where are the two methods connected? 1 G. P. Agrawal, Nonlinear Fiber Optics, Fourth edition, Academic Press C. Giles, E. Desurvire, Modeling Erbium-Doped Fiber Amplifiers, J. Lightwave Technol. 9, (1994).

22 Connect the NLSE and the Giles model From Giles model we have the spectral power of the signal P s (ω, z) From NLSE we have the slowly varying, complex envelope function (A(T,z))!A(ω, z) 2 The absolute-square value of the Fourier transform of the complex envelope Gain for a small segment from PEE: Add to the NLSE as a negative loss A(z,T ) z " = ˆD + ˆN + # $ g(ω, z + Δz) = P s g(ω, z) 2 % & ' A(z,T ) ˆD ˆN (ω, z + Δz) P s (ω, z) is the dispersion terms is the nonlinear terms This is an iterative procedure because of the backward propagating signals

23 Fiber laser modeling The gain fiber is modeled the same way as the amplifier (NLSE + RE + PEE) 1 The solution procedure differs only in the type of iteration which here recalculates the oscillator until the stable pulse operation is found Other optical elements are modeled the appropriate way Passive fibers: NLSE Saturable absorber (if any) described by the power dependent absorption process Simulation process goes through all optical elements starting from ASE and do it several times (>1000) to build up the pulse operation. 1 Á. Szabó, Z. Várallyay, Numerical Study on the Saturable Absorber Parameter Selection in an Erbium Fiber Ring Oscillator, IEEE Photonics Technology Letters, 24, (2012).

24 CW fiber laser modeling Problem: Fourier transformation relation between temporal and spectral space We have a finite bandwidth, temporal shape won t be a CW signal. The solution to avoid this issue is to use a phase diffusion model (Wiener process) 1 to determine the spectral behaviour (Next Slide details the flow chart of the algorithm) 1 First step: Giles-model to determine the approximate power levels Second step: Giles-model combined with the NLSE and an initial Lorentzian spectral shape is used obtained from the Wiener process Third step: Iterations until the boundary conditions are fulfilled and the initial spectral shape is converged. Arrangement of the linear oscillator HR high reflector OC output coupler TFB tapered fiber bundle YDF Ytterbium doped fiber 1 Á. Szabó, Z. Várallyay, A. Rosales-Garcia and C. Headley, A Fast Numerical Method to Predict Spectra of High Power CW Yb- Doped Fiber Lasers with Bidirectional Pumping, CLEO 2014 paper: JTu4A.62

25 Scheme of the model 1 Á. Szabó, Z. Várallyay, A. Rosales-Garcia and C. Headley, A Fast Numerical Method to Predict Spectra of High Power CW Yb- Doped Fiber Lasers with Bidirectional Pumping, CLEO 2014 paper: JTu4A.62

26 Fiber amplifiers Rear earth doped fiber Signal in Signal out splice Pump LD Pump LD For single mode amplifiers Typical power levels: 100 mw 1 W (single mode, ps-pulses, GHz rep.) Maximum pulse peak power: ~100 kw

27 Core-pump or cladding-pump Advantage of core pump: it supports fundamental mode operation Disadvantage: not so high power than in cladding-pump scheme Z. Várallyay, and J. C. Jasapara, Comparison of amplification in large area fibers using cladding-pump and fundamental-mode core-pump schemes, Optics Express 17, (2009).

28 Performance scaling of fibers High nonlinearity Laseroscillator Chirped-pulse amplification [1] Mode-area scaling [2] Stretcher Amplifier Compressor [1] D. Strickland and G. Mourou, Compression of amplified chirped optical pulses, Opt. Commun. 56, (1985). [2] J. Limpert et al., Yb-doped large-pitch fibres: effective single-mode operation based on higher-order [...], Light Sci. Appl. 1, 1 5 (2012).

29 Fibers for scaling the power PBG HOM CCC PCF LPF LCF MTF DMF PBG: G. Gu et al. "Ytterbium-doped large-mode-area all-solid photonic bandgap fiber lasers," Opt. Express 22, (2014) HOM: J. W. Nicholson et al. "Scaling the effective area of higher-order-mode erbium-doped fiber amplifiers," Opt. Express 20, (2012) CCC: X. Ma et al. "Single-mode chirally-coupled-core fibers with larger than 50µm diameter cores," Opt. Express 22, 9206 (2014) PCF: J. Limpert et al. Extended single-mode photonic crystal fiber lasers, Opt. Express 14(7), (2006) LCF: L. Dong et al. "Ytb-doped all glass leakage channel fibers with highly fluorine-doped silica pump cladding," Opt. Express 17, 8962 (2009) MTF: D. Jain et al. "Mode area scaling with multi-trench rod-type fibers.," Opt. Express 21, (2013) DMF: M. Laurila et al. "Distributed mode filtering rod fiber amplifier delivering 292W with improved mode stability," Opt. Express 20, 5742 (2012) LPF: J. Limpert et al. "Yb-doped large-pitch fibres: effective single-mode operation based on higher-order mode delocalisation, LSA 1, 1 5 (2012)

30 Power Scaling of single amplifier step-index fibers Numerical aperture defines guiding properties Scaling requires reduced numerical aperture V SI = 2π λ a NA = 2 n core n 2 core 2 n cladding < n cladding V SI (single- mode condi.on) Refractive index Scaling

31 Performance scaling Photonic-crystal fibers Delocalization of higher-order modes [1,2] 76% 26% 25% 16% 5% 5% Double-clad structure an integral part Favored excitation of fundamental mode by seed beam Favored amplification of fundamental mode by doping overlap 30x higher amplification for fundamental mode (assuming 30dB small-signal gain) Yb- doped region [1] F. Stutzki et al., High average power large-pitch fiber [...], Opt. Lett. 36, (2011). [2] J. Limpert et al., Yb-doped large-pitch fibres: [...], Light Sci. Appl. 1, 1 5 (2012).

32 What else can we do to further increase the mode area? Amplifying Interferometer Artificial scaling of Front-End mode area Division Stage Amplifier 1 Amplifier 2 Amplifier N Combining Stage Compressor Use N amplifiers and combine the spatially separated pulses Best case: Improvement of the pulse energy and average power by a factor of N

33 Splitting and coherent combination E. Seise, A. Klenke, J. Limpert, A. Tünnermann, Coherent addition of fiber-amplified ultrashort laser pulses, Optics Express 18, (2010)

34 Splitting and coherent combination E. Seise, A. Klenke, J. Limpert, A. Tünnermann, Coherent addition of fiber-amplified ultrashort laser pulses, Optics Express 18, (2010)

35 HR laser. What did we request? Parameter name Mfv Min/Max Compressed pulse duration 1.5 cycles 2.5 cycles Output Energy stability 0.8% rms 1.8% rms Output beam quality (Strehl ratio) CEP stability (rms from 10 min shots) 100 mrad 200 mrad Duration of the project (from contract) 15 months 22 months Pulse energy >5mJ () 5mJ Repetation rate 100kHz 100kHz Further technical parameters Beam pointing instability (in the unit of diffraction limited beam divergence) Trouble-free uninterrupted operation (minimum) Warming up time (maximum) Supported period (from end of the project) < hours 60 minutes 12 months

36 Winning Laser Design (Jena Consortium) J. Limpert, et al. Light: Science & Applications, 1, 1-5 (2012). 8 channel, rod type, Ytterbium doped photonic crystal fiber amplifier

37 Used fiber

38 HR Laser Fiber CPA system IAP FSU Jena + Fraunhofer IAF + Active Fiber Systems GmbH Oscillator 1030 nm CEP stable External clock Pulse picking Streching & Preamplification 20 µj, 100 khz Wavelength 1030 nm Energy >1 mj Pulse duration <6.2 fs Rep.rate 100 khz CEP stability <100 mrad Energy stability 0.8% Strehl ratio 0.9 S P L I T I N G Main Amplifier 8 parallel channels Yb-doped rod type fiber amplifiers 8x60 W Compressor 2 Hollow core fiber, CM >1 mj, <6.2 fs C O M B I N I N G Chirped Mirror (CM) Compressor 3 mj, 200fs Compressor 1 Hollow core fiber, CM >1.5 mj, <30 fs FAT: May, 2016, SAT: July, 2016 PP for 5 mj, <5 fs -> in 2016

39 Hollow capillary compressors Gas Gas 2.5bar Argon CC- FCPA ~210fs 1mJ H 2 O Gas H 2 O Gas ~30fs 550µJ 8bar Neon <8 fs >350 µj H 2 O H 2 O [1] SiO 2 variable thickness [1] J. Rothhardt et al., 53 W average power few-cycle fiber laser system generating soft x rays up to the water [...], Opt. Lett. 39, 5224 (2014) [2] M. Nisoli et al. "Compression of high-energy laser pulses below 5 fs," Opt. Lett. 22, (1997)

40 Compression in capillary Test with 1 kw CW laser

41 Latest results Spectral changes and bandwidth as function of gas pressure Obtained spectrum and temporal shape after compression

42 Latest results Spectral changes and bandwidth as function of gas pressure Obtained spectrum and temporal shape after compression

43 Photographs

44 Photographs

45 Timing May, 2015 Start of the project (Kick-off meeting 11 th of May) Feb, 2016 Factory acceptance test Jul-Aug, 2016 Installation in Szeged (Site acceptance test) + Training Sep Oct, 2016 Trial period Sep, 2016 Sep, 2018 Warranty

46 THANK YOU FOR YOUR ATTENTION!