SESAM modelocked solid-state lasers Lecture 2 SESAM! Semiconductor saturable absorber

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1 SESAM technology ultrafast lasers for industrial application SESAM modelocked solid-state lasers Lecture SESAM! Ursula Keller U. Keller et al. Opt. Lett. 7, 55, 99 IEEE JSTQE, 5, 996" Progress in Optics 6, -5,! Nature, 8, " Gain! SESAM solved Q-switching problem for diode-pumped solid-state lasers Output coupler! ETH Zurich, Physics Department, Switzerland Ultrafast Laser Physics Tutorial, Saturday. March, 6: 9: Ultrafast Optics Davos, Switzerland, March -8, ETH Zurich SESAM SEmiconductor Saturable Absorber Mirror " self-starting, stable, and " reliable modelocking of diode-pumped ultrafast " solid-state lasers Semiconductor saturable absorber SESAM Parameters: Saturation Fluence, Modulation Depth, Recovery Time... F sat, A Saturation fluence Reflectivity (%) 95 ΔR ns Non-saturable losses ΔR Modulation depth Pump-probe signal -fs excitation pulse -ps excitation pulse Incident pulse fluence F p ( µj/cm ) Pump-probe delay (ps) 5 Guidelines how to measure these parameters:" M. Haiml, R. Grange, U. Keller, Appl. Phys. B 79,, " and with improved accuracy: D. J. H. Maas, et al., Optics Express 6, 757, 8"

2 bandgap energy E g (ev) GaP AlGaAs GaAs-based SESAMs GaAs AlAs InP InGaAs. Lattice matched to GaAs" High refractive index contrast for DBR" InGaAs strained on GaAs" lattice constant a (Å) AlInGaAs 75 nm 6. µm. µm.5 µm InAs wavelength λ (µm) InP-based SESAMs Lattice matched to InP" Small refractive index contrast for DBR" GaInNAs-based SESAMs Thank you to Stanford, Bell Labs and ETH Zurich [In] = 6.5 % "! [N] = %" GaNAs" InGaAs! GaInNAs better lattice matched on GaAs" Opt. Lett. 7,, & Appl. Phys. Lett. 8,, " Stanford University Ph.D. student laser physics ultrafast measurement techniques microwave measurement tools Bell Labs, Holmdel MTS access to state-of-the-art semiconductor materials (MBE) ETH Zurich tenured Professor in Physics since 99 + resources to be fully empowered Enabled interdisciplinary approach with the combination of solid-state lasers, semiconductor physics, and microwave measurement techniques.

3 years of SESAM looking back years of SESAM Appl. Phys. B, 5-8, How did it all happen? This is the paper to read... years of ultrafast solid-state lasers: invited paper Why was it assumed that diode-pumped solid-state lasers cannot be passively modelocked? How was the SESAM invented? State-of-the-art performance and future outlook. Ultrashort pulse generation with modelocking First SESAM (first design called A-FPSA) April, 99 (submitted Nov. 6, 99) Active Modelocking A. J. De Maria, D. A. Stetser, H. Heynau Appl. Phys. Lett. 8, 7, 966! ns/div" 5 ns/div" Nd:glass first passively modelocked laser Q-switched modelocked! Q-switching problem in passively modelocked solid-state lasers: - active modelocking for solid-state lasers - dye lasers solved the problem " Year" Flashlamp-pumped solid-state lasers! 99 acousto-optic loss modulator needs RF power and water cooling

4 Ti+:Saphir Laser Innovation: before and after -Niveau-System Ti:Saphir Laser 98 How did I invent the SESAM? acousto-optic modelocker needs RF power and water cooling Peter Moulton MIT Lincoln Lab SESAM modelocker Kerr Lens Modelocking (KLM) Two experiments that should not have worked... Magic Modelocking CPM Ti:sapphire laser Postdeadline Paper CLEO 99 Nr. CPDP Postdeadline Paper Ultrafast Phenomena 99, Nr. PD modelocked Ti:sapphire laser without an absorber inside the cavity... should not work! Y. Ishida, N. Sarukura, H. Nakano Published result later in: D. E. Spence, P. N. Kean, W. Sibbett, Optics Lett. 6,, 99 Explained modelocking as a coupled cavity modelocking effect between two transverse modes used a dye saturable absorber inside the CPM Ti:sapphire laser... should not work! loss loss? gain Advantages of KLM! very fast thus shortest pulses! very broadband thus broader tunability gain Disadvantages of KLM! not self-starting! critical cavity adjustments (operated close to the stability limit)! saturable absorber coupled to cavity design (limited application) pulse time pulse time Both of them explained later by KLM by Keller et al. Optics Lett. 6,, 99 First Demonstration: D. E. Spence, P. N. Kean, W. Sibbett, Optics Lett. 6,, 99 Explanation: U. Keller et al., Optics Lett. 6,, 99 loss gain IEEE JQE 8,, 99 pulse time

5 Soliton Laser by Linn Mollenauer Why did the Sibbett group explain their results as a coupled modelocking effect? 98 Remember the mind set at that time was: Passive modelocking of solid-state laser does not work! Coupled cavity... but there was the soliton laser from Linn Mollenauer fiber with negative dispersion (GDD < ) soliton pulse compression in coupled cavity with shortens pulse in main cavity 989 Coupled Cavity Modelocking by Wilson Sibbett Coupled Cavity Modelocking by Blow & Wood 988 Coupled cavity Coupled cavity modelocking (CCM) fiber with positive dispersion (GDD > ) Dispersive pulse broadening in coupled cavity! How should that shorten pulse in main cavity? Explanation with APM (additive pulse modelocking): E. P. Ippen, H. A. Haus, L. Y. Liu, J. Opt. Soc. Am. B 6, 76, 989 Pulse is broadened yes,... but SPM makes an intensity dependent phase shift, which leads to constructive interference at the peak and destructive inteference in the wings with the pulse from the main cavity. This gives a pulse shortening effect of the coupled cavity. Coupled cavity any nonlinear element in the coupled cavity should shorten the pulse theoretical prediction with numerical modeling... physical insight was missing 5

6 Coupled Cavity Modelocking by Ursula Keller 99 Coupled Cavity Modelocking by Ursula Keller 99 τp Resonant passive modelocking (RPM) L Resonant passive modelocking (RPM) GaAs 95 Å GaAlxAs-x 75x 5 Å, x=. AlAs/GaAlAs 8 nm mirror semiconductor saturable absorber ( borrowed from my office neighbour Keith Goosen ) pulse compression in coupled cavity which shortens pulse in main cavity Coupled Cavity Modelocking by Ursula Keller L +δ L δl Surprise: no active cavity length stabilization was necessary modelocking was stable even with large cavity length detuning δ L δl U. Keller et al., Optics Lett. 5, 77, 99 Invention of the first SESAM device: A-FPSA Coupled cavity acts like a nonlinear Fabry Perot (FP) Antiresonant Fabry-Perot Saturable Absorber (A-FPSA) Example: Opt. Lett. 8, 7, 99" Rtop absorber RPM = amplitude (resonant) nonlinearity operated at maximum FP reflectivity requires no cavity length stabilization APM = phase nonlinearity operated not at maximum FP reflectivity requires cavity length stabilization Rbottom U. Keller et al., Optics Lett. 7, 55, 99 6

7 Scaling A-FPSA towards a single QW SESAM years of SESAM 995 Adjustable parameter: top reflector 996 Single 5 nm GaAs quantum well embedded inside a Bragg mirror L. R. Brovelli et al., Electron. Lett., vol., 87, 995 Introduction of the ackronym: SESAM Invited review article September 996, IEEE JSTQE Moving from Ti:sapphire to diode-pumped ss-laser Moving from Ti:sapphire to diode-pumped ss-laser was not trivial... why? Ti:sapphire 8 nm Opt. Lett. 5, 77, RPM modelocked Nd:YLF bandgap energy E g (ev) GaP AlGaAs 5.5 GaAs 5.6 AlAs 5.7 InGaAs 5.8 InP 5.9 AlInGaAs 75 nm 6. µm.. µm.5 µm InAs wavelength λ (µm) GaAs 95 Å 75x GaAl x As -x 5 Å, x=. AlAs/GaAlAs 8 nm mirror Nd:YLF µm IEEE JQE 8, 7, A-FPSA modelocked Nd:YLF Nd:YLF laser pumped with Ti:sapphire laser 99 mailed A-FPSA to California lattice constant a (Å) LT = low temperature MBE growth A-PFSA modelocked diode-pumped Nd:YLF and Nd:YAG laser 7

8 It also works to modelock fibers... LT grown SESAMs to reduce absorber recovery time 99 carried an A-FPSA to TU Vienna Adjustable parameter: absorber recovery time More on LT MBE growth and ion implantation to reduce recovery time of SESAMs: Appl. Phys. Lett., vol. 75, pp. 7-9, 999 Appl. Phys. Lett., vol. 7, pp , 999 Appl. Phys. Lett., vol. 7, -6, 999 Appl. Phys. Lett., vol. 7, pp. 69-7, 999 Physica B: Condensed Matter, vol. 7-7, pp. 7-76, 999 Stable pulses even with a long net gain window Stable pulses even with a long net gain window loss gain loss Why is the pulse stable with a long net gain window? gain R. Paschotta, U. Keller, Appl. Phys. B 7, 65, absorber delays pulse! loss Fully saturated absorber:" leading edge of pulse" negligible loss for" has significant loss from " trailing edge of pulse" the saturable absorber" pulse time Kerr lens modelocking (KLM) Fast saturable absorber D. E. Spence, P. N. Kean, W. Sibbett Opt. Lett. 6,, 99 pulse time Soliton modelocking not so fast saturable absorber F. X. Kärtner, U. Keller, Opt. Lett., 6, 995 time Dominant stabilization process with a relatively long net-gain window:! Picosecond domain: absorber delays pulse" The pulse is constantly moving backward and can swallow any noise growing behind itself." Femtosecond domain: dispersion in soliton modelocking" 8

9 Laser pulse with pulse envelope E (t ) = A (t ) e Soliton modelocking: GDD negative, n > Pulse envelope A(t) Outputcoupler iω t Group Group Velocity Delay Dispersion Dispersion SelfPhaseModulation Gain Aout (T, t ) = e q(t ) Ain (T, t ) Electric field E(t) E (t ) = ~ E(ω) E (ω ) eiω t dω π Master equation: Opt. Lett, 6, 995 and IEEE JSTQE, 5, 996 Δω ω ~ A(Δω) Δω E (t ) = Slow Absorber ω TR A ( Δω ) = E (ω + Δω ) A (T, t ) = id iδ A (T, t ) A (T, t ) + g l + Dg q ( t ) A (T, t ) = T t t T t A (T, t ) = A sech exp iφ + continuum τ TR iω t E (ω + Δω ) ei (ω + Δω )t dδω = e A ( Δω ) eiδω t dδω π π Aout (T, t ) = q ( t ) Ain (T, t ) ΔA (T, t ) = q ( t ) A (T, t ) τ= D δ Fp,L Ultrashort pulse generation with modelocking A. J. De Maria, D. A. Stetser, H. Heynau Appl. Phys. Lett. 8, 7, 966! ns/div" 5 ns/div" Nd:glass first passively modelocked laser Q-switched modelocked! Q-switching problem in passively modelocked solid-state lasers: - active modelocking for solid-state lasers - dye lasers solved the problem " Year" Flashlamp-pumped solid-state lasers! 99 9

10 SESAM Parameters: Saturation Fluence, Modulation Depth, Loss... F sat, A Saturation fluence Reflectivity (%) 95 ΔR ns Non-saturable losses ΔR Modulation depth Q-switched mode locking is avoided if... C. Hönninger, R. Paschotta, F. Morier-Genoud, M. Moser, and U. Keller, JOSA B 6, 6 (999) E P > E sat,l E sat,a ΔR Q-switched mode locking QML = A eff, A F sat,a ΔR QML noise QML Incident pulse fluence F p ( µj/cm ) Guidelines how to measure these parameters:" M. Haiml, R. Grange, U. Keller, Appl. Phys. B 79,, " Time (multiples of round trip time) cw mode locking cw ML cw ML with improved accuracy: D. J. H. Maas, et al., Optics Express 6, 757, 8" Time (multiples of round trip time) Appl. Phys. B 58, 7, 99 SESAMs to reduce Q-switching instabilities SESAM design 5 ) SESAMs with low saturation fluence: antiresonant resonant E-SESAM Refractive index Refractive index Refractive index GaAs AlAs GaInNAs absorber 56 6 Standing wave pattern GDD and field enhanc. factor GaAs AlAs 56 6 GaAs AlAs Air 6 68 Air 6 68 SiO Distance from substrate (nm) Air Field enhancement E Field enhancement E n n Field enhancement E n Group delay dispersion (fs ) GDD (fs ) GDD (fs ) Wavelength (nm)..... Field enhancement factor ξ Field enhancement factor ξ Field enhancement factor ξ

11 Anti-resonant high-finesse SESAM (first SESAM design) Optics Lett., vol. 7, 55, 99 IEEE JSTQE, vol., 5, 996 Anti-resonant low-finesse SESAM Anti-resonant Fabry-Perot ϕ rt = ϕ b + ϕ t + knd = ( m + )π Anti-resonant Fabry-Perot ϕ rt = ϕ b + ϕ t + knd = ( m + )π ϕ b = π ϕ t = d = m λ n ϕ b = π ϕ t = d = m λ n, m = IEEE JSTQE, vol., 5, 996 Anti-resonant low-finesse SESAM High- versus low-finesse anti-resonant SESAM final high-finesse anti-resonant SESAM R top = 96% bottom Bragg mirror R bottom % Anti-resonant Fabry-Perot ϕ rt = ϕ b + ϕ t + knd = ( m + )π optical pulse spectrum SESAM without R top! AR-coated high-finesse SESAM design: AlGaAs/AlAs Al x Ga -x As SiO /TiO x =.9 saturable absorber ϕ b = ϕ t = d = λ n Absorber embedded into a Bragg reflector (quarter-wave stack of alternating high and low index material) Interferometric autocorrelation (IAC) SESAM modelocked Ti:sapphire laser I. D. Jung et al, Opt. Lett., 559, 995

12 High- versus low-finesse anti-resonant SESAM High- versus low-finesse anti-resonant SESAM final low-finesse anti-resonant SESAM bottom Bragg mirror R bottom % low-finesse SESAM design: AlGaAs/AlAs GaAs SA AlO d" nd = λ SESAM modelocked Ti:sapphire laser I. D. Jung et al, Opt. Lett., 559, 995 SESAM modelocked Ti:sapphire laser: I. D. Jung et al, Opt. Lett., 559, 995 Ultrabroadband SESAMs Semiconductor AlGaAs/AlAs Bragg mirror replaced by silver mirror: Ultrabroadband SESAM 5 pairs Al.7 Ga.8 As/AlAs pairs ZnTe/BaF pairs Al.8 Ga. As/CaF R. Fluck et al., Optics Lett., 7, 996 S. Schön et al., Appl. Phys. Lett. 77, 78,

13 Ultrabroadband SESAM SESAMs to reduce Q-switching instabilities n n o n e 5 ) SESAMs with low saturation fluence: ) SESAMs with inverse saturable absorption: S. Schön et al., Appl. Phys. Lett. 77, 78, Inverse saturable absorption (ISA) Consequences for the QML Threshold SESAM reflectivity for a pulse fluence F p the reflectivity decreases at higher pulse energies the roll-over = inverse saturable absorption R ISA (F p ) = R P (F p ) F is the inverse slope of the roll over F p F The smaller F, the stronger is the roll-over No inverse sat. absorption E P > E sat,l ΔR S C. Hönninger, R. Paschotta, F. Morier-Genoud, M. Moser, and U. Keller J. Opt. Soc. Am. B 6, 6 (999) E P Intracavity pulse energy E sat,l Gain saturation energy S = E P /E sat,a Saturation Parameter ΔR E P > E sat,l S S S R. Grange, M. Haiml, R. Paschotta, G. J. Spühler, L. Krainer, O. Ostinelli, M. Golling, U. Keller Appl. Phys. B 8, 5 (5) S = F ΔR F sat Less demands on the mode size in the gain medium More flexibility for cavity design Easier to suppress Q-switched mode locking With inverse sat. absorption Saturation parameter at maximum reflectivity

14 All-optical -GHz pulse generation at.5 µm Autocorrelation Optical spectrum SESAM modelocked Er-Yb:glass (ERGO) laser enabled world-record optical communication results Time-Bandwidth Products, Inc. makes such lasers commercially available ERGO laser, Photo of actual setup SESAM modelocked Er-Yb:glass laser Pulse repetition rate: GHz Max. output-power: 5 mw Optical bandwidth:.6 nm Pulse width:.6 ps (.7x TBP) A. E. H. Oehler et al., Opt. Express 6, 9, 8 Resonant SESAM: both SAM and negative GDD Resonant SESAM: both SAM and negative GDD Position of saturable absorber (SA): (a) close to a node (lower loss) (b) at peak of standing wave: strong absorption dip at resonance D. Kopf et al, Opt. Lett., 86, 996 D. Kopf et al, Opt. Lett., 86, 996

15 Resonant SESAM: both SAM and negative GDD SESAM without damage (even at > kw) Position of saturable absorber (SA): (a) close to a node (lower loss) (b) at peak of standing wave: strong absorption dip at resonance D. Kopf et al, Opt. Lett., 86, 996 High average power lasers: SESAM ML TDL TDL Moving from ss-lasers to semiconductor lasers Vertical external cavity surface emitting laser (VECSEL) Modelocked integrated external-cavity surface emitting laser (MIXSEL) Appl. Phys. B, vol. 88, Nr., pp. 9-97, 7 Modelocked VECSEL A. Giesen, et al., Appl. Phys. B 58, 65 (99) MIXSEL C. J. Saraceno et al., >7 W (!) Opt. Express, 55, First time > µj pulse energy from a SESAM modelocked Yb:YAG thin disk laser (TDL) Opt. Express 6, 697, 8 and CLEO Europe June 7 6 µj with a multipass gain cavity and larger output coupling of 7% (Trumpf/Konstanz) Opt. Express 6, 5, 8 Important Milestones:! MIXSEL with 6. W average output power: Opt. Express 8, 758, femtosecond VECSEL with > W average output power: Opt. Express 9, 88, 5

16 More info on the web... Web page of Prof. Ursula Keller at ETH Zurich: All papers are available to download as PDFs: SESAM milestones: Ultrafast solid-state laser: get started with the book chapter... VECSEL and MIXSEL: Frequency combs: Attoscience, Attoclock, Attoline: Viewgraphs to graduate lecture course: Ultrafast Laser Physics 6