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Stabilizing Lasers using Whispering Gallery Mode
Der Naturwissenschaftlichen Fakultät
der Friedrich-Alexander-Universität
Erlangung des Doktorgrades Dr. rer. nat.
vorgelegt von
Benjamin Sprenger
aus Frankfurt am MainAls 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 XiaoTo My ParentsdZusammenfassung
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-
8Wir produzieren Mikrokugeln mit 100 μm Durchmesser, und Güten von 10.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
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
−11wird die Allan Abweichung (10 mit 10 μs Mittelzeit) durch die “three-cornered-hat”
Methode bestimmt, indem mit zwei Referenzlasern gleichzeitig Schwebungen erzeugt
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
−13glichen. Die Präzision optischen Transfers beträgt 1.68× 10 bei 1 s Mittelung, und
−15verringert sich auf bis zu 10 bei Mittelung über eine halbe Stunde. Wie erwartet ist der
−10Radio Frequenz Transfer schlechter und beträgt 1.07× 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 100μm, are measured
8to have quality factors around 10 . 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
−11fiber delay line. Furthermore, the Allan deviation gives 10 at 10 μs averaging — cal-
culated using the three-cornered-hat method, by beating with two reference lasers simul-
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
−13optical transfer, the precision reaches 1.68× 10 after 1 s of averaging, and improves
−15down to 10 in half an hour of averaging. As expected, the radio frequency transfer is
−10worse, giving 1.07× 10 accuracy after 1 s.iv AbstractAcknowledgments
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. Gottfriedvi 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.