OPTICAL-FREQUENCY MIXERS IN PERIODICALLY POLED LITHIUM
NIOBATE: MATERIALS, MODELING AND CHARACTERIZATION
A DISSERTATION
SUBMITTED TO THE DEPARTMENT OF APPLIED PHYSICS
AND THE COMMITTEE ON GRADUATE STUDIES
OF STANFORD UNIVERSITY
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
Rostislav Vatchev Roussev
December 2006
c© Copyright by Rostislav Vatchev Roussev 2007
All Rights Reserved
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I certify that I have read this dissertation and that, in my opinion, it is fully
adequate in scope and quality as a dissertation for the degree of Doctor of
Philosophy.
Martin M. Fejer Principal Adviser
I certify that I have read this dissertation and that, in my opinion, it is fully
adequate in scope and quality as a dissertation for the degree of Doctor of
Philosophy.
Robert L. Byer
I certify that I have read this dissertation and that, in my opinion, it is fully
adequate in scope and quality as a dissertation for the degree of Doctor of
Philosophy.
Stephen E. Harris
Approved for the University Committee on Graduate Studies.
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Abstract
Efficient wavelength conversion is an attractive approach for obtaining coherent radiation in
regions of the spectrum where lasers are unavailable or impractical. Optical signal process-
ing in WDM networks, optical-CDMA communications, and quantum communication are
examples of applications that can utilize efficient nonlinear frequency conversion at low
power levels. Lithium niobate (LN) is a very promising material for the purpose, because it
has a mature crystal-growth process, wide transparency range, large second-order nonlinear
coefficient, and allows quasi-phasematching via periodic poling (PP). Waveguides enable
efficient conversion at low powers and can be formed via reverse proton-exchange. Precise
modeling of both the fabrication process and the properties of the resulting waveguides is
thus necessary for the demonstration of high-density optical integrated circuits.
This dissertation presents a complete fabrication model that accurately predicts the
nonlinear diffusion of protons in PPLN as well as the dispersion of the waveguides between
450 and 4000 nm. Using this model, waveguides are fabricated for two experiments: efficient
generation of 3–4-µm radiation for spectroscopy via difference frequency generation using
two near-IR lasers; and parametric amplification of 1.57-µm seed signal radiation for remote
wind sensing using a 1.064-µm pump laser.
The waveguides are fabricated in conventional congruent-composition LN. Photorefrac-
tive damage (PRD) and green-induced infrared absorption (GRIIRA) limit the generated
output power in these devices at room temperature due to the presence of high-intensity
visible light. Resistance to PRD and GRIIRA can be achieved by heavy doping with Mg2+,
or by using crystals with stoichiometric composition.
PRD-resistant, bulk near-stoichiometric lithium niobate (SLN) was fabricated by vapor-
transport equilibration (VTE) of originally congruent lithium niobate wafers with light MgO
(0.3–1 mol%) doping. Details of the poling process and the dependence of photorefractive
properties on crystal composition are presented. We obtained periodic poling down to a
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period of 7 µm and achieved 2 W at 532 nm via second harmonic generation in a 0.3 mol-
% VTE-MgO:LiNbO3 bulk crystal at room-temperature. These breakthroughs will enable
efficient tunable radiation from the visible to the mid-IR for a variety of applications.
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Acknowledgements
I have been very fortunate to pursue my advanced degree at the Stanford University depart-
ment of Applied Physics, where I had the opportunity to interact with many extraordinary
people. They have contributed significantly to my growth as a scientist and as a person. I
would like to acknowledge their support.
My research adviser, Prof. Martin Fejer, has helped me not only with difficulties in
research, but also in my personal life. Without his support, I would not have gotten this
far. I have been amazed at how he always found time to check if students had any difficulties,
and to offer very helpful advice. He has done that for me throughout my Stanford years,
and I am very grateful to him for that. I have learned a lot from him not only about science,
but also about responsibility and devotion. I enjoyed many technical discussions with my
adviser, which were both helpful and stimulating. I learned a large fraction of my technical
knowledge through direct interactions with him. I would also like to thank him for making
time to read and correct my dissertation according to my schedule.
Prof. Robert Byer has been very supportive of my collaborations with members of his
group. He has made me believe that a person can be great at many things and need not
worry about it. I would also like to thank Prof. Byer for the wonderful presentations about
beautiful places around the globe, for reading my dissertation during the only time interval
in which I could deliver it and for the encouraging final remarks.
I would like to thank Prof. Stephen Harris for the very stimulating and inspirational
presentations that he and his students have given at local symposia. In addition, I would
like to thank him for the great care taken in the teaching of the Nonlinear Optics class that
I took in my first year. Taking the class was a pleasant learning experience for me. Finally,
I would like to thank Prof. Harris for the great support during his reading of my thesis and
for the remarks of approval.
I would like to thank the other two members of my thesis defense committee - Prof.
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Robert Feigelson and Prof. James Harris, for accepting the responsibility on a short notice,
and for making my defense a memorable experience in a good way. I would also like to
thank Prof. Feigelson for helpful discussions about lithium niobate and vapor-transport
equilibration.
I would not be writing acknowledgements today if it were not for the support of my
reading committee and two of my student colleagues – Arun Sridharan and Carsten Lan-
grock. Carsten helped me a lot by proof-reading chapters of my dissertation and providing
helpful suggestions. I would like to also thank him for his perseverance throughout our
collaborations in the past, which helped us achieve remarkable results, as well as for being
a great friend. Arun has been a friend of mine since the beginning of my Ph.D. study. He
has helped me many times. We have had a significant amount of joint work and achieved
great results together. In addition, I would like to thank him for the superior technical and
logistical support related to the writing and submission of my dissertation over the past
several weeks.
I would like to thank Roger Route for getting me up to speed on the VTE process, as
well as for many thoughtful discussions about materials, processes, and furnace building.
In addition, I would like to thank Roger for quality advice about life.
I have been very fortunate to start my Ph.D. studies in the Fejer group under the direct
guidance of Krishnan Parameswaran. He spent a long time teaching me everything that
he knew about photolithography, periodic poling, waveguide fabrication, waveguide testing,
and testing equipment. In addition, he gave me a lot of helpful advice about living in the
Stanford area.
I would like to thank Andrew Schober for always showing interest in discussions about
physics and optics, and sharing his knowledge. I am also thankful to him for boosting my
self-confidence and optimism during hard times. I would like to thank Genady Imeshev for
providing me with a good background on short-pulse QPM theory, for our collaboration on
MgO-doped LiNbO3, and for helpful discussions about career paths after Stanford.
I am thankful to Jonathan Kurz for taking the initiative to pursue a more repeatable
fabrication process, and for the collaborative work during my early years at Stanford, when
we helped each other solve many technical problems. I would like to thank David Hum for
many helpful discussions about near-stoichiometric materials, periodic poling, and many
aspects of engineering in general. In addition, I am thankful to Xiuping Xie for developing
the code for two-dimensional modeling simulations, as well as for many discussions of the
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fabrication process. I would like to acknowledge the significant contributions of Joseph
Schaar to the automation of the short-pulse poling process. I am thankful to Paulina Kuo
for taking time to satisfy my curiosity about orientation-patterned GaAs, and to Jie Huang
for sharing his device fabrication and testing experience.
Besides Arun, other members of the Byer group with whom I have had significant col-
laboration were Karel Urbanek and Supriyo Sinha. Karel has contributed very significantly
in the OPA experiments in Chapter 3 which would not
