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FREE SPACE OPTICAL COMMUNICATIONS WITH HIGH INTENSITY LASER POWER BEAMING DANIEL EDWARD RAIBLE Bachelor of Science in Electrical Engineering Cleveland State University May 2006 Master of Science in Electrical Engineering Cleveland State University May 2008 submitted in partial fulfillment of requirements for the degree DOCTOR OF ENGINEERING at the CLEVELAND STATE UNIVERSITY June 2011 This dissertation has been approved for the Department of Electrical and Computer Engineering and the College of Graduate Studies by ________________________________________________ Dissertation Committee Chairperson, Taysir H. Nayfeh ________________________________ Department/Date ________________________________________________ Nigamanth Sridhar ________________________________ Department/Date ________________________________________________ Ana V. Stankovic ________________________________ Department/Date ________________________________________________ Petru S. Fodor ________________________________ Department/Date ________________________________________________ John F. Turner ________________________________ Department/Date This work is dedicated to the memory of my dearest friends, Michael Matthews and Jason Adams. It would have been great, and I miss you. ACKNOWLEDGEMENTS There are many people that I would like to express my gratitude towards, starting with my advisor Dr. Taysir Nayfeh, for affording me the opportunity to pursue my education through working at the Industrial Space Systems Laboratory, and for helping me develop my skills as a researcher. I would like to graciously thank my committee for their time and review starting with Dr. Nigamanth Sridhar for providing his background in wireless systems, Dr. Ana V. Stankovic for offering her knowledge in power electronics and systems, Dr. Petru S. Fodor and Dr. John F. Turner for supplying their expertise in optics and general physics, and finally Dr. Joseph A. Svestka for serving as a non-voting member and for sharing his expertise in systems engineering and mathematical programming. I am ever grateful for the technical guidance that I received from Bernie Sater of Photovolt, Inc. His lifelong pursuit of the VMJ technology is the enabler of this project. Special thanks to Ken Edwards and the Eglin AFRL for believing in and supporting this research. I am looking forward to what the future will bring with HILPB. Thanks to Hobson Lane, Bob Rice and NGST, as well as Colin Burke and LIMO for providing their facilities and invaluable expertise. Great thanks to Ray Beach, Fred Wolff and Jim Soeder of the NASA John H. Glenn Research Center for their support of the ISSL and our work over the years. I would also like to thank several graduate students and employees with whom I have had the honor of working with at the Industrial Space Systems Laboratory: Brian Fast, Dragos Dinca, Nick Tollis, Andrew Jalics, Sagar Gadkari, Scott Darpel, Maciej Zborowski, Harry Olar, Tom DePietro, Michael Wyban, Ishu Pradhan, Anita Wiederholt and David Avanesian. They made many great contributions to this research, and it has been a pleasure serving on the team with them. Thanks to Adrienne Fox and Jan Basch for all of their hard work behind the scenes, the rest of the Electrical and Computer Engineering Department, the Industrial and Manufacturing Engineering Department, and Pam Charity, Gregg Schoof and Joanne Hundt in the Dean‟s office for their support and guidance given to me while at the Fenn College of Engineering. Also, thanks to my NASA colleagues James Nessel, Alan Hylton and Robert Manning for lending a hand and giving me sanity checks during this work. I wish to particularly thank my wife, Jamie, for her eternal patience and love. I am grateful to my parents Elaine and Dennis, sister Janice, Uncle Richard, Uncle Fred and Aunt Anna Rae for their continual support and for encouraging me to always take the Giant Steps in life. FREE SPACE OPTICAL COMMUNICATIONS WITH HIGH INTENSITY LASER POWER BEAMING DANIEL EDWARD RAIBLE ABSTRACT This research demonstrates the feasibility of utilizing high intensity laser power beaming (HILPB) systems as a conduit for robust free-space optical communications over large distances and in challenging atmospheric conditions. The uniqueness of vertical multi-junction (VMJ) photovoltaic cells used in HILPB systems in their ability to receive and to convert at high efficiency, very high intensity laser light of over 200 W/cm2, presents a unique opportunity for the development of the robust free space optical communication system by modulating information signals onto the transmitted high intensity photonic energy. Experiments were conducted to investigate and validate several optical communications concepts. A laser modulator was implemented to exhibit the excellent transient response of the VMJ technology at very high illumination intensities, and thus show its applicability to optical communications. In addition, beam polarization optic stages were employed to demonstrate a secure multi-channel communications scheme. The off-axis response of the receiver and the beam profile were characterized in order to x evaluate the feasibility of developing acceptable pointing and tracking geometries. Finally, the impact of signal modulation on the total converted energy was evaluated and shown to have minimal effect on the overall power transmission efficiency. Other aspects of the proposed communication system are studied including: quantifying beamwidth and directivity, signal-to-noise-ratio, information bandwidth, privacy, modulation and detection schemes, transmission channel attenuation and disturbances (atmospheric turbulence, scintillation from index of refraction fluctuations, absorption and scattering from thermal and moisture variation) and beam acquisition tracking and pointing influence on the performance metrics of optical transmission technologies. The result of this research demonstrates the feasibility of, and serves as a comprehensive design guide for the implementation of a HILPB communication system. Such a system may be applied to mission architectures requiring generous amounts of link margin, critical privacy in battle field environment, and/or where the channel characteristics are dynamic and unknown. In addition, the developed mathematical models and empirical data support the ongoing wireless power transmission efforts by expanding the fundamental knowledge base of the HILPB technology. xi TABLE OF CONTENTS Page NOMENCLATURE ........................................................................................................ XV LIST OF TABLES ........................................................................................................... XX LIST OF FIGURES ....................................................................................................... XXI CHAPTER I: INTRODUCTION ........................................................................................ 1 1.1 A Brief History of Free Space Optical Communications ........................... 1 1.2 The HILPB System ................................................................................... 10 1.3 Future Potential for a High Intensity Laser Communications System ..... 16 1.4 Document Organization ............................................................................ 19 CHAPTER II: LITERATURE REVIEW OF LASER COMMUNICATIONS ............... 20 2.1 Advantages of Optical Communications .................................................. 20 2.2 Beam Polarization ..................................................................................... 27 2.3 Modulation and Demodulation Techniques .............................................. 35 2.3.1 Direct Detection Receiver ......................................................................... 36 2.3.2 Coherent Detection Receiver .................................................................... 42 2.4 Terrestrial and In-Space Issues ................................................................. 44 2.4.1 Terrestrial Links ........................................................................................ 45 2.4.2 Spatial Crosslink ....................................................................................... 52 xii 2.4.3 Waveguide Medium .................................................................................. 57 2.5 Beam Acquisition, Tracking and Pointing ................................................ 59 CHAPTER III: EXPERIMENT SETUP AND RESEARCH METHODOLOGY ........... 63 3.1 System Description – Optical Receivers ................................................... 63 3.2 System Description – Laser and Optics Bench ......................................... 69 3.3 System Description – Data Acquisition System ....................................... 72 3.4 Photovoltaic Array Cell Back-feeding ...................................................... 76 3.5 Comparison of Receiver Geometries ........................................................ 78 3.6 Optical Frequency Optimization ............................................................... 86 3.7 Beam Homogenization Optics .................................................................. 96 CHAPTER IV: EXPERIMENT PROCEDURE AND ANALYSIS .............................. 102 4.1 Beam Profile Characterization ................................................................ 102 4.2 Off-Axis Illumination ............................................................................. 108 4.3 Pulse Modulation ...................................................................................