Novel Concepts for Versatile and High-Power Thin-Disk Laser Oscillators
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Research Collection Doctoral Thesis Novel Concepts for Versatile and High-Power Thin-Disk Laser Oscillators Author(s): Saltarelli, Francesco Publication Date: 2020 Permanent Link: https://doi.org/10.3929/ethz-b-000449261 Rights / License: In Copyright - Non-Commercial Use Permitted This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library DISS. ETH NO. 26982 NOVEL CONCEPTS FOR VERSATILE AND HIGH-POWER THIN-DISK LASER OSCILLATORS A thesis submitted to attain the degree of DOCTOR OF SCIENCES of ETH ZURICH (Dr. sc. ETH Zurich) presented by FRANCESCO SALTARELLI Laurea Magistrale in Fisica, Sapienza, Università di Roma born on 22.07.1992 citizen of Italy accepted on the recommendation of Prof. Dr. Dr. h. c. Ursula Keller, examiner Prof. Dr. Thomas Südmeyer, co-examiner 2020 “Don’t fake it till you make it. Fake it till you become it” — Amy Cuddy Contents Contents List of Figures ........................................................................................................... v List of Tables .......................................................................................................... vii List of Acronyms .................................................................................................... ix List of Symbols ........................................................................................................ xi Publications ............................................................................................................ xiii Abstract.................................................................................................................. xvii Sommario (abstract in Italian) ............................................................................. xxi 1 Introduction and motivation ......................................................................... 1 1.1 Ultrafast lasers ........................................................................................ 2 1.2 High-power laser oscillators ................................................................. 5 1.3 Motivation and outline of the thesis ................................................... 6 2 Framework of this thesis ............................................................................... 9 2.1 Thin-disk laser concept ......................................................................... 9 2.2 Cavity stability and gas-lens effect .................................................... 11 2.3 Active multi-pass cavities ................................................................... 14 2.4 SESAM and Kerr-lens modelocking ................................................ 15 2.5 Requirements for high-power modelocking .................................... 19 2.6 SESAM’s thermal lensing ................................................................... 19 2.7 Self-phase modulation cancellation .................................................. 21 i Contents 2.8 Nonlinear mirror modelocking .......................................................... 23 2.9 Power scaling of modelocked thin-disk lasers ................................ 25 3 Thermal effects in thin-disk lasers ............................................................. 29 3.1 Gas-lens effect in kW-class thin-disk lasers..................................... 30 3.1.1 Introduction ..................................................................................... 30 3.1.2 High-power single-transverse-mode TDL operation ................ 34 3.1.3 Thermal-lensing measurements .................................................... 35 3.1.4 Simulation of the gas-lens and gas-wedge effects ...................... 41 3.1.5 Conclusion........................................................................................ 45 4 Cascaded quadratic nonlinearities .............................................................. 47 4.1 Self-phase modulation cancellation in a high-power ultrafast thin-disk laser oscillator ...................................................................... 48 4.2 Supplementary material: Self-phase modulation cancellation in a high-power ultrafast thin-disk laser oscillator .............................. 58 4.2.1 Coupled wave equations ................................................................ 59 4.2.2 Pulsed second harmonic generation............................................. 60 4.2.3 Cavity design .................................................................................... 64 4.2.4 Thermal behavior of the nonlinear crystal .................................. 65 5 Nonlinear mirror modelocking ................................................................... 67 5.1 Modelocking of a thin-disk laser with the frequency-doubling nonlinear-mirror technique ................................................................ 69 5.1.1 Introduction .................................................................................. 69 5.1.2 NLM operating principles.............................................................. 73 5.1.3 Experiment ....................................................................................... 74 5.1.4 Numerical model of the NLM device .......................................... 81 5.1.5 Conclusion and outlook ................................................................. 85 5.2 Power-scaling of nonlinear-mirror modelocked thin-disk lasers . 87 ii Contents 5.2.1 Introduction ..................................................................................... 87 5.2.2 High-power NLM-modelocked oscillator ................................... 91 5.2.3 Study of the nonlinear-mirror modelocking regime .................. 97 5.2.4 Conclusion and outlook ............................................................... 105 6 Power scaling results .................................................................................. 109 6.1 Power scaling of ultrafast oscillators: 350-W average-power sub-picosecond thin-disk laser ......................................................... 111 6.1.1 Introduction ................................................................................... 111 6.1.2 Cavity design .................................................................................. 115 6.1.3 Laser performance in continuous wave and modelocked operation ........................................................................................ 119 6.1.4 Conclusion and outlook ............................................................... 123 6.2 Further power scaling to 430-W average power ........................... 125 6.2.1 Cavity design .................................................................................. 127 6.2.2 Modelocking and laser performance .......................................... 129 6.2.3 Conclusion and outlook ............................................................... 132 7 Conclusion and outlook ............................................................................. 133 Author contributions ........................................................................................... 137 Bibliography .......................................................................................................... 141 Curriculum Vitae .................................................................................................. 153 Acknowledgements .............................................................................................. 154 iii List of Figures List of Figures Fig. 1.1 Overview of state-of-the-art ultrafast lasers ..................................... 4 Fig. 2.1 Thin-disk concept ............................................................................... 10 Fig. 2.2 Cavity design and stability in thin-disk lasers ................................. 12 Fig. 2.3 Structure and saturation properties of a SESAM ........................... 18 Fig. 2.4 Cascaded quadratic nonlinear processes .......................................... 23 Fig. 2.5 Overview of state-of-the-art thin-disk laser oscillators ................. 27 Fig. 3.1 Beam quality for thin-disk laser operation in air vs vacuuum ...... 33 Fig. 3.2 Disk’s thermal lensing in thin-disk lasers ........................................ 38 Fig. 3.3 Beam quality in a cw thin-disk oscillator ......................................... 40 Fig. 3.4 Model to simulate the gas-lens and gas-wedge effects .................. 42 Fig. 3.5 Results of gas-lens and gas-wedge simulations ............................... 45 Fig. 4.1 Overview of intracavity GDD used in thin-disk lasers ................. 50 Fig. 4.2 Schematic of the laser cavity ............................................................. 51 Fig. 4.3 SPM cancellation in thin-disk lasers ................................................. 54 Fig. 4.4 Laser power slopes .............................................................................. 55 Fig. 4.5 Modelocking diagnostics at 210-W average output power ........... 57 Fig. 4.6 SHG efficiency in a SHG minimum, approximated formula ...... 63 Fig. 4.7 Evolution of the 1/e2 mode radius in the laser cavity .................. 66 Fig. 5.1 Overview of nonlinear-mirror modelocked lasers ......................... 72 Fig. 5.2 Operating principle of nonlinear-mirror modelocking ................. 74 Fig. 5.3 Schematic of the laser cavity and cavity mode ............................... 76 Fig. 5.4 Modelocking diagnostics at 21-W average output power ............. 77 Fig. 5.5 Numerical