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Stabilizing Lasers Using Whispering Gallery Mode Resonators

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Stabilizing Lasers Using Whispering Gallery Mode Resonators Stabilizing Lasers using Whispering Gallery Mode Resonators Der Naturwissenschaftlichen Fakultät der Friedrich-Alexander-Universität Erlangen-Nürnberg zur Erlangung des Doktorgrades Dr. rer. nat. vorgelegt von Benjamin Sprenger aus Frankfurt am Main Als Dissertation genehmigt von der Naturwissenschaftlichen Fakultät der Friedrich-Alexander Universität Erlangen-Nürnberg Tag der mündlichen Prüfung: 2. 12. 2010 Vorsitzender der Promotionskommission: Prof. Dr. Rainer Fink Erstberichterstatter: Prof. Dr. Lijun Wang Zweitberichterstatter: Prof. Dr. Min Xiao To My Parents d Zusammenfassung Laser mit ultraschmalen Linienbreiten werden in hoch-präzisen Wissenschaften unent- behrlich. Flüstergaleriemoden-Resonatoren (whispering gallery mode, auch "WGM") mit höheren und höheren Güten werden ständig gefertigt, wie z.B. Mikrokugeln aus Silikat, und neuerdings auch diamantgedrehte kristalline Scheiben aus Kalziumfluorid. Desweit- eren muss die Übertragung von ultraschmalen Frequenzen verstanden und perfektioniert werden. Diese Arbeit konzentriert sich auf die Stabilisierung zweier Laser durch WGM Resonatoren, und die Verteilung stabiler optischer Frequenzen durch die turbulente At- mosphäre. Wir produzieren Mikrokugeln mit 100 μm Durchmesser, und Güten von 108.Wir verwenden diese als frequenzselektive Elemente in einem Faser-Ring-Laser, der eine Erbium-dotierte Faser als Gain-Medium enthält. Wir koppeln mit verjüngten optischen Fasern in die schmalen Resonanzen der Kavität ein, und mit Winkel-polierten Fasern wieder auf der gegenüberliegenden Seite aus. Die Linienbreite des Lasers verringert sich dadurch um fünf Größenordnungen — gemessen durch eine heterodyne Schwebung mit einem stabilen Referenz Laser — und kommt auf 170 kHz volle Breite bei halber Höhe. Außerdem wird eine Rotverschiebung von 16 pm/μW durch die Pump Leistung beobachtet. Ein verbesserter Aufbau verwendet Prismakopplung auf beiden Seiten einer robusten Kalziumfluorid WGM Disk (5 mm Durchmesser), und einen Halbleiter-basierten optis- chen Verstärker als Gain-Medium. Die Linienbreite beträgt 13 kHz, gemessen durch die selbst-heterodyne Schwebung mit einer 45 km Faser Verzögerungsstrecke. Desweiteren wird die Allan Abweichung (10−11 mit 10 μs Mittelzeit) durch die “three-cornered-hat” Methode bestimmt, indem mit zwei Referenzlasern gleichzeitig Schwebungen erzeugt werden. Mit einem Interferometer auf dem Dach des Instituts wird das optische Phasenrauschen über 100 m Freiraum bestimmt. Heterodyne Detektion wird verwendet, und optischer Frequenz Transfer wird mit einem Radio-Frequenz-Amplituden modulierten Laser ver- ii Zusammenfassung glichen. Die Präzision optischen Transfers beträgt 1.68 × 10−13 bei 1 s Mittelung, und verringert sich auf bis zu 10−15 bei Mittelung über eine halbe Stunde. Wie erwartet ist der Radio Frequenz Transfer schlechter und beträgt 1.07 × 10−10 gemittelt über 1 s. Abstract Ultra-narrow linewidth laser sources are becoming indispensable in high-precision sci- entific research. Whispering gallery mode (WGM) resonators are achieving higher and higher quality factors in fused silica microspheres, and, more recently, diamond-turned crystalline disks made of calcium fluoride are even superseding these. Furthermore, the dissemination of ultra-narrow frequencies must be understood and perfected. This work focuses on two lasers stabilized using WGM resonators, as well as the dissemination of stable optical frequencies through the turbulent atmosphere. The microspheres we fabricate, with diameters on the order of 100 μm, are measured to have quality factors around 108. We use such a microsphere as a frequency selective element in a fiber ring laser using erbium-doped fiber as a gain medium. We couple into the sharp modes of the microsphere by using tapered fiber coupling. Angle-polished fiber coupling is used on the other side, resulting in a narrow bandwidth filter. The laser’s linewidth is decreased by five orders of magnitude, as determined using the heterodyne beat technique with a stable reference laser, resulting in 170 kHz FWHM. Additionally, a red-shift of 16 pm/μW as a function of pump power is observed. An improved setup uses prism coupling on both sides of a rigid calcium fluoride WGM disk (5 mm in diameter), and a semiconductor optical amplifier as a gain medium. The linewidth is measured to be 13 kHz using the self-heterodyne beat technique with a 45 km fiber delay line. Furthermore, the Allan deviation gives 10−11 at 10 μs averaging — cal- culated using the three-cornered-hat method, by beating with two reference lasers simul- taneously. Using an interferometer set up on the roof of the institute, the optical phase noise induced over 100 m of free space is measured. Heterodyne detection is used, and optical frequency transfer is compared to a radio frequency amplitude modulated laser beam. In optical transfer, the precision reaches 1.68 × 10−13 after 1 s of averaging, and improves down to 10−15 in half an hour of averaging. As expected, the radio frequency transfer is worse, giving 1.07 × 10−10 accuracy after 1 s. iv Abstract Acknowledgments I want to express my deep gratitude to all the people that helped in making this thesis a success. Without their significant contributions this work would have been impossible. I would like to take this time to thank them. First of all, I am greatly indebted to my PhD advisor, Prof. Lijun Wang. He always inspired me, and managed to motivate me with his ingenious suggestions. I thank him for letting me follow my interests, and for providing insights and guidance when I required it. It was always my dream to make original contributions to science, and I am grateful that I had the possibility to do this in Prof. Wang’s group. I owe my eternal thanks to Dr. Harald G. L. Schwefel, whom I have been working with for two years now. We have been office mates for even longer, and he has been a friend, and a source of inspiration to me the entire time. Whenever I needed help, be it work-related or not, he would always take the time and encourage me with his positive attitude and ubiquitous knowledge. I also thank him for proof-reading my thesis. Many co-workers in Prof. Wang’s group have aided me in the understanding of physics, and influenced my life in a positive way. Furthermore, I have made many friends that I will never forget. I want to thank everyone for making the atmosphere in the group as pleasant as it was. Dr. Zehuang Lu always lent a helping hand, and took as much time as necessary to explain physical concepts to me and help me in publishing my first pa- pers. I also thank him for proof-reading the atmospheric transfer chapter of the thesis. Dr. Jie Zhang was never too busy to answer my questions and motivate me to keep going with her benevolent attitude. My office mate Dr. Sergiy Svitlov and I always exchanged stories, and I thank him for helping in the zero-crossings calculations he performed. Dr. Vladimir Elman took a lot of time to aid us in the design of the external cavity diode laser. Dr. Rachit Sharma and Dr. Jan Schäfer were always there to encourage me, but also to distract me and talk about Life, the Universe and Everything when it was necessary. All of the group members contributed to this thesis in some way, and I really appreciate it. All my thanks to Sebastian Bauerschmidt, Dr. Marian Florentin Ciobanu, Prof. Dr. Gottfried vi Acknowledgments Döhler, Simon Grams, Simon Heugel, Dr. Hua Hu, Dr. Tau Liu, Dr. Stefan Malzer, Dr. Jessica Mondia, Dr. Mingying Peng, Dr. Felix Müller, Dr. Sascha Preu, Dr. Christian Rothleitner, Dr. Alois Stejskal, Max Tillmann, Dr. Bo Wang, Dr. Jinxiong Wang, Dr. Jianwei Zhang, Dr. Quanzhong Zhao, and Dr. Yanning Zhao. I want to acknowledge and thank the supporting people in the Max Planck Institute. Kirsten Oliva, Lisa Spann, Dr. Sabine König, Nadine Danders, Anja Deckert, Caroline Edenharter, and Hildegard Porsch have all helped me significantly throughout the years. I always felt welcome, no matter whom I had to speak to. I also thank Dr. Carsten Schür and Heike Auer, the organizers of the International Max Planck Research School, who helped make my stay a pleasant one. Additionally, I want to thank the mechanical work- shop, specifically Bernhard Thomann, Robert Gall, and Thomas Spona, for constructing various holders and adapters, and for giving me important advice in my own construc- tions. Adam Käppel and Lothar Meier were always available for any electronics related questions that I had, and always designed and built electronic circuits when necessary — I really appreciate it. Thank you to Namvar Jahanmehr and Daniel Ploß for their help with the FIB and the SEM. Also, I am grateful for the IT support from Michael Zeller, Kristin Gregorius, and Benjamin Klier. I am also indebted to Florian Sedlmeir, Josef Fürst, Dr. Christoph Marquardt, and Dr. Dmitry Strekalov (NASA Jet Propulsion Laboratory, CA) for the fruitful discussions we had about whispering gallery modes. I also thank Dr. Markus Schmidt and Dr. Holger Hundertmark for their expert advice relating to fibers and other optics related questions. Thank you also to Sebastian Stark, who helped me with random questions about optics. Finally, I want to give a huge thanks and a hug to my parents, Ruth and Volker, for giving me the incredible opportunity to pursue a career in science. I don’t know how I can fully show my appreciation. My brother Thorsten, and my sister Rabea were always there for me, and influenced my life greatly. I have always looked up to them, and I always will. My family has always given me the support and motivation I needed. Without their appraisal I would never have made it this far. I am also greatly indebted to my girlfriend Daniela. I thank her for always being there for me when I was frustrated that things were not working, and moreover for being there to celebrate when things went just as I wanted them to.
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