Imperial College London Physics Department Photonics Group Advanced Laser Sources for Industrial Processing and Remote Sensing Achaya Teppitaksak Thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy of Imperial College London January 2015 I declare that the work presented in this thesis is my own. Information derived from the work of others has been acknowledged in the text and a list of references is supplied at the end of the thesis. 2 Copyright The copyright of this thesis rests with the author and is made available under a Creative Commons Attribution Non-Commercial No Derivatives licence. Researchers are free to copy, distribute or transmit the thesis on the condition that they attribute it, that they do not use it for commercial purposes and that they do not alter, transform or build upon it. For any reuse or redistribution, researchers must make clear to others the licence terms of this work. 3 Acknowledgements I am heartily thankful to my supervisor, Prof. Mike Damzen for giving me the opportunity to work in his lab and for the encouragement, supervision and support from start of my PhD to the finish which enabled me to develop a deeper understanding of the subject. Furthermore, I would like to thank Ara, Emma, and Gabs for being such brilliant friends and for all their help in the lab. Special thanks goes to Gabs for sharing her cavity dumped Q-switching laser and Wenjun for designing and constructing a high temperature oven. I would also like to thank Simon, Martin, Judith and Marcia for being very helpful. I would like to thank all of my PhD friends in the Photonics Group for being lovely and making these four years such a great experience and to all my friends from Thailand for making my lunches so much fun. Special thanks go to Graham, Hugh and my family for being supportive over the last few months of writing up. Last but not least, the financial support of the Thai government is also gratefully acknowledged. 4 Abstract In the fifty years since their discovery, the use of laser oscillators and amplifiers has increased to cover a wide range of applications. This thesis develops diode-pumped solid-state (DPSS) lasers for two main applications: industrial processing and remote sensing. The first half of this thesis investigates the development of bounce geometry lasers that can be used to generate high power light sources suitable for industrial processing using diode-pumped Nd:YVO4 laser sources at both 1342nm and 1064nm transitions. The first of these investigations develops bounce geometry configuration Nd:YVO4 laser sources operating at 1342nm. For continuous wave (CW) operation at powers of 15.9W with 30% optical-to-optical efficiency were achieved. For pulsed operation, Q-switching based on an acousto-optic modulator and mode-locking based on nonlinear-mirror mode-locking were demonstrated. To suit a range of different industrial applications, a versatile gain switched laser diode source at a wavelength of 1064nm was developed to have independently adjustable pulse energies, pulse duration and repetition rates. To reach a commercially useful power level, a seed laser was amplified in a master oscillator power amplifier (MOPA) configuration using two ultrahigh-gain Nd:YVO4 bounce amplifiers in series. In a first amplifier (preamplifier), a small-signal gain of ~50dB with good TEM00 beam quality preservation was achieved with 24W pumping while a second power amplifier was used to achieve an average output power of up to ~14W using an input seed power of 188W. The second part of this thesis develops laser sources for remote sensing applications based on direct diode pumping of Alexandrite lasers in an end-pumping configuration. When compared to Q-switched Nd:YAG lasers, which are typically used for satellite based remote sensing, Alexandrite lasers have the potential to be more efficient and offer more flexible wavelength tunability. Following a broad 5 overview of Alexandrite lasers, this thesis investigates diode-pumped Alexandrite laser performance. To achieve a high average power, a compact laser cavity was built with output power as high as 26.2W and slope efficiency of 49%. This was more than an order of magnitude higher than previously reported from diode pumped Alexandrite lasers. To achieve TEM00 laser output, many extended cavity designs were investigated. Following this, to enhance the laser efficiency, an Alexandrite laser was developed utilizing the unique characteristics of temperature-dependent gain of Alexandrite and the performance from 20-150˚C was characterised. To demonstrate high pulse energies, suitable for remote sensing applications, for the first time, a direct diode pumping Q-switched Alexandrite was demonstrated. A Q-switched output pulse energy of >1mJ at 100Hz pulse repetition rate in TEM00 mode was achieved. 6 Publications Journal Papers 1. A Teppitaksak, G. M. Thomas, and M. J. Damzen, “Investigation of a versatile pulsed laser source based on a diode seed and ultra-high gain bounce geometry amplifiers.,” Opt. Express, vol. 23, no. 9, pp. 12328–36, May 2015. 2. A. Teppitaksak, A. Minassian, G. M. Thomas, and M. J. Damzen, “High efficiency >26 W diode end-pumped Alexandrite laser.,” Opt. Express, vol. 22, no. 13, pp. 16386–92, 2014. Conference Submissions 1. M. J. Damzen, G. M. Thomas, A. Teppitaksak, E. Arbabzadah, W. Kerridge-Johns, and A. Minassian,. Diode-Pumped Alexandrite Laser—a new prospect for Remote Sensing. In Conference on Lasers and Electro-Optics/Pacific Rim (p. 25B3_2). Optical Society of America, 2015 2. G. M. Thomas, A. Minassian, A. Teppitaksak, and M. J. Damzen, "High Energy Q-switching and Cavity Dumped Q-switching of a Diode-pumped Alexandrite Laser," in Advanced Solid State Lasers, (Optical Society of America, talk, paper ATu3A.7. 2015 3. A. Teppitaksak, G. M. Thomas, and M. J. Damzen, "Gain-switched diode laser seeding of ultra-high-gain Nd:YVO4 bounce amplifier system as a versatile pulsed laser source," in 2015 European Conference on Lasers and Electro-Optics - European Quantum Electronics Conference, (Optical Society of America, talk , paper CA_3_4, 2015 4. M. Damzen, A. Teppitaksak, G.M. Thomas, and A. Minassian, Progress in diode-pumped Alexandrite lasers as a new resource for future space Lidar missions. In International Conference on Space Optics, Vol. 7, p. 10, OCT 2014 5. A. Teppitaksak, G. M. Thomas, and M. J. Damzen, High efficiency diode-pumped Alexandrite laser for remote sensing”, In Photon14 (p.5) Poster, 2014 6. M. J. Damzen, A. Teppitaksak, G. M. Thomas, A. Minassian, Highest power and Q-switched diode end-pumped Alexandrite laser, 6th EPS-QEOD Europhoton Talk ThD-T1-O-06, 2014 7. A. Teppitaksak, G. M.Thomas, and M. J. Damzen (2013, May). Versatile Pulsed Source using a pulsed diode seed and ultrahigh gain bounce geometry amplifier. In Lasers and Electro-Optics Europe (CLEO EUROPE/IQEC), Poster, 2013 7 Contents 1 Introduction ..................................................................................................... 11 1.1 Background ......................................................................................................... 11 1.2 Diode Pump Solid-State Laser ............................................................................ 12 1.2.1 Laser Diode .................................................................................................... 13 1.2.2 Solid-State Material ........................................................................................ 15 1.3 Diode Pumped Solid-State Laser Geometries .................................................... 18 1.3.1 Rod ................................................................................................................. 19 1.3.2 Slab ................................................................................................................. 20 1.3.3 Thin Disk ........................................................................................................ 21 1.4 Thermal Effects .................................................................................................. 22 1.4.1 The Generation of Heat .................................................................................. 22 1.4.2 Impact of Heating on Laser Operation ........................................................... 24 1.4.3 Cavity Stability ............................................................................................... 25 1.5 Pulse Operation ................................................................................................... 29 1.6 Q-Switching ........................................................................................................ 29 1.6.1 Method of Q-Switching .................................................................................. 30 1.7 Mode-locking ...................................................................................................... 34 1.7.1 Methods of Mode-locking .............................................................................. 36 1.8 Gain Switching ................................................................................................... 41 1.9 Thesis Outline ..................................................................................................... 42 2 Bounce Geometry Laser ................................................................................. 45 2.1 Introduction ........................................................................................................ 45 2.2 Bounce