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New Yb3+-Doped Laser Materials and Their Application in Continuous-Wave and Mode-Locked Lasers

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New Yb3+-Doped Laser Materials and Their Application in Continuous-Wave and Mode-Locked Lasers New Yb3+-doped laser materials and their application in continuous-wave and mode-locked lasers D I S S E R T A T I O N zur Erlangung des akademischen Grades d o c t o r r e r u m n a t u r a l i u m (Dr. rer. nat.) im Fach Physik eingereicht an der Mathematisch-Naturwissenschaftlichen Fakultät I der Humboldt-Universität zu Berlin von Dipl. Phys. Peter Klopp geb. 14.12.1968, Wiesbaden Präsident der Humboldt-Universität zu Berlin Prof. Dr. Christoph Markschies Dekan der Mathematisch-Naturwissenschaftlichen Fakultät I Prof. Thomas Buckhout, PhD Gutachter: 1. Prof. Dr. Thomas Elsässer 2. Prof. Dr. Günther Huber 3. Prof. Dr. Achim Peters Tag der mündlichen Prüfung: 16.05.2006 Abstract Yb3+ laser media excel with high efficiency and relatively low heat load, especially in medium to high power laser oscillators and amplifiers. Mode-locking of Yb3+ laser systems can provide subpicosecond pulse durations at high average power. This work deals with two groups of the most promising novel Yb3+-activated laser crystals: Yb3+-activated monoclinic double tungstates, namely the isostructural crystals Yb:KGd(WO4)2 (Yb:KGW), Yb:KY(WO4)2 3+ (Yb:KYW), and KYb(WO4)2 (KYbW), and Yb -doped sesquioxides, represented by Yb:Sc2O3 (Yb:scandia). Spectroscopic data of KYbW were investigated as part of this thesis, finding an extremely short 1/e-absorption length of 13 micrometers at 981 nm. Continuous-wave (cw) and mode-locked laser performance of moderate-average-power lasers based on lowly Yb3+-doped tungstates were examined. Ultrashort pulse generation with Yb:KYW, Yb:KGW, and Yb:glass was compared in a passively mode-locked laser. A relatively high mode-locked laser efficiency was achieved due to a tapered diode pump laser with excellent beam quality. Quasi-cw and cw lasing of Yb3+ in highly doped and stoichiometric tungstate crystals were investigated. These materials are interesting for microchip and thin-disk lasers. Important issues were crystal quality and heat generation at high Yb3+ concentrations. For the first time, laser operation of tungstates with a Yb3+ concentration >>20% and finally, with KYbW, cw lasing of a stoichiometric Yb laser material was achieved. Furthermore, with KYbW, the smallest laser quantum defect ever for a laser crystal was demonstrated, 1.6%. Using a Yb:Sc2O3 laser medium, for the first time mode locking of an oscillator using a sesquioxide laser crystal was realized. Laser regimes with non-solitonlike and solitonlike pulse shaping were investigated, using a Ti:Sapphire laser and a tapered laser diode as pump sources. With a Ti:Sapphire-laser- pumped Yb:scandia laser the highest conversion efficiency with respect to absorbed pump power for any mode-locked Yb3+-based laser was achieved, 47%. Keywords: Yb3+ lasers, ytterbium-activated tungstates, ytterbium-doped sesquioxides, mode- locked lasers Abstract Yb3+-Lasermedien glänzen mit hoher Effizienz und relativ geringer thermischer Last, besonders in Laseroszillatoren und -verstärkern mittlerer bis hoher Leistung. Modenkopplung von Yb3+- Lasersystemen ermöglicht Subpikosekunden-Pulse bei hoher mittlerer Ausgangsleistung. Diese Arbeit widmet sich zwei Gruppen der vielversprechendsten neuen Yb3+-aktivierten Laserkristalle: den strukturell analogen, monoklinen Doppelwolframaten Yb:KGd(WO4)2 3+ (Yb:KGW), Yb:KY(WO4)2 (Yb:KYW) und KYb(WO4)2 (KYbW) und den Yb -dotierten Sesquioxiden, vertreten durch Yb:Sc2O3 (Yb:Skandia). Die spektroskopischen Daten von KYbW, darunter eine extrem kurze 1/e-Absorptionslänge von 13 Mikrometern bei 981 nm, wurden im Rahmen der Dissertation vermessen. Die Lasereigenschaften niedrig Yb3+-dotierter Wolframate im Dauerstrich (cw)- und modengekoppelten Betrieb wurden in Lasern moderater Ausgangsleistung untersucht. Ultrakurzpuls-Erzeugung mit Yb:KYW, Yb:KGW und Yb:Glas wurde in einem passiv modengekoppelten Laser verglichen. Dabei wurde eine relativ hohe Lasereffizienz erreicht, Dank einer Trapezlaserdiode als Pumpquelle mit exzellenter Strahlqualität. Quasi-cw- und cw-Laserbetrieb von Yb3+ in hochdotierten und stöchiometrischen Wolframatkristallen wurden untersucht. Diese Materialien sind interessant für Mikrochip- und Scheibenlaser. Wichtige Fragestellungen waren die Kristallqualität und die Hitzeentwicklung bei hohen Yb3+-Konzentrationen. Erstmals wurde Lasertätigkeit von Wolframaten mit Yb3+- Konzentrationen >>20% und schließlich, mit KYbW, cw-Laserbetrieb eines stöchiometrischen Yb-Lasermaterials demonstriert. Weiterhin wurde mit KYbW der kleinste bisher für einen Laserkristall gemessene Laserquantendefekt erzielt, 1,6%. Unter Benutzung eines Yb:Sc2O3- Lasermediums wurde erstmals modengekoppelter Betrieb eines Oszillators mit Sequioxid- Laserkristall gezeigt. Betrieb mit nichtsolitonen- und solitonenartiger Pulsformung sowie mit Ti:Saphir-Laser oder Trapezlaserdiode als Pumplaser wurden untersucht. Mit einem Ti:Saphir- gepumpten Yb:Skandia-Laser wurde eine Konversionseffizienz von 47 % bezogen auf die absorbierte Pumpleistung erreicht, der bisher höchste Wert für einen modengekoppelten Yb3+- basierten Laser. Schlagwörter: Yb3+-Laser, Ytterbium-aktivierte Wolframate, Ytterbium-dotierte Sesquioxide, modengekoppelte Laser 1 New Yb3+-doped laser materials and their application in continuous-wave and mode-locked lasers 1. Introduction 3 2. Yb3+-activated laser materials 6 2.1. Yb3+ as a laser active ion 6 2.1.1. Yb3+ level scheme 6 2.1.2. Yb3+ versus Nd3+ 7 2.2. Crystals and glasses as hosts 9 2.2.1. Influence of the host on spectroscopic properties 9 2.2.2. Overview of hosts: spectroscopic properties 12 2.2.3. Thermo-mechanical and thermo-optical properties of the host 16 2.2.4. Overview of hosts: Thermo-mechanical and thermo-optical properties 19 2.2.5. Crystal quality and dopability 23 2.3. Stoichiometric materials 24 2.4. Yb3+-activated tungstates 28 2.5. Yb3+-activated cubic sesquioxides 36 3. Quasi-three- and quasi-four-level lasers 41 3.1. Definitions 41 3.2. Rate equations 42 3.3. Continuous-wave case 45 3.3.1. Inversion and saturation effects 45 3.3.2. Laser threshold estimations 51 3.3.3. Laser efficiency 62 4. Passive mode locking 67 4.1. Pulse shaping in passively mode-locked solid-state lasers 67 4.2. Pulse-shaping effects in detail 75 4.3. Stability of passive mode locking 93 5. Yb3+-activated tungstate laser experiments 96 5.1. Lowly Yb3+-doped KYW and KGW experiments 97 5.1.1. Continuous-wave operation of lowly doped KYW and KGW 97 5.1.2. Mode-locked operation of lowly doped KYW and KGW 100 5.1.2.1. Diode-pumped Yb(5%):KYW femtosecond laser 102 5.1.2.2. Diode-pumped Yb(5%):KGW femtosecond laser 104 5.1.2.3. Diode-pumped Yb:glass femtosecond laser and comparison 105 5.2. Highly Yb3+-doped KYW and stoichiometric KYbW experiments 109 2 5.2.1. Quasi-cw operation of highly doped KYW and stoichiometric KYbW 109 5.2.1.1. Yb(60%):KYW quasi-cw laser 110 5.2.1.2. Yb(80%):KYW quasi-cw laser 113 5.2.1.3. KYbW quasi-cw laser 115 5.2.1.4. Smallest laser quantum defect 125 5.2.2. Continuous-wave face-cooled Yb:KYW and KYbW lasers 128 5.2.2.1. Yb(20%):KYW continuous-wave laser 129 5.2.2.2. KYbW continuous-wave laser 131 6. Yb3+-doped scandia laser experiments 135 6.1. Continuous-wave operation of a Yb(0.7%):Sc2O3 laser 137 6.2. Mode-locked operation of Yb:Sc2O3 lasers 138 6.2.1. Ti:Sapphire-laser-pumped Yb(0.7%):Sc2O3 picosecond laser 138 6.2.2. Ti:Sapphire-laser-pumped Yb(0.7%):Sc2O3 femtosecond laser 140 6.2.3. Diode-pumped Yb(0.7%):Sc2O3 femtosecond laser 143 7. Summary 147 A. Appendix 149 List of references 173 List of symbols 187 List of abbreviations 192 Dank 193 Zusammenfassung 194 Publikationen, die im Zusammenhang mit dieser Dissertation erfolgten 197 Lebenslauf 199 Versicherung 200 3 1. Introduction Combining Yb3+ as the active ion and a suited glass or crystal host provides laser media with excellent optical and mechanical properties. For laser oscillators and amplifiers operating in the 1-μm-range, Yb3+-based laser materials have the potential to substitute many Nd3+-based materials, especially in medium to high power systems used for material processing. Here, Yb3+ laser media excel with high efficiency and relatively low heat load. Mode-locking of Yb3+ laser systems can provide subpicosecond pulse durations at high average power. Such short pulses can e.g. enhance the precision of drilling compared to Q-switched systems and permit the structuring of transparent materials [Chi96, Dra03, Min01, Pro95]. This work deals with two groups of the most promising novel Yb3+-activated laser crystals: 3+ Yb -activated monoclinic double tungstates, namely the isostructural crystals Yb:KGd(WO4)2 3+ (Yb:KGW), Yb:KY(WO4)2 (Yb:KYW), and KYb(WO4)2 (KYbW), and Yb -doped sesquioxides, represented by Yb:Sc2O3 (Yb:scandia). Highly efficient laser operation is demonstrated with lowly doped tungstate and scandia crystals in continuous-wave (cw) and mode-locked regimes. Highly doped KYW and stoichiometric KYbW are investigated. With KYbW, the first cw lasing of a stoichiometric Yb3+ laser material is achieved. Motivation A general advantage of Yb3+ lasers is that the conversion efficiency of pump light into laser output can be very high and heat production is relatively low, due to a small laser quantum defect (lqd) and the absence of excited state absorption, up- or downconversion mechanisms, which are a problem with many other laser active ions including Nd3+. All Yb3+-activated laser materials can be pumped by InGaAs diode lasers, which helps to build cheap, compact and reliable laser systems. A disadvantage is that Yb3+ laser systems operate in a quasi-three- or quasi-four-level scheme at 2 room temperature, since the lower laser level belongs to the ground state manifold F7/2 and is considerably thermally populated. This leads to reabsorption of the emitted laser light and therefore to higher lasing thresholds and reduced efficiency.
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