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Analytical and Numerical Analysis of Low Optical Overlap Mode Evanescent Wave Analytical and Numerical Analysis of Low Optical Overlap Mode Evanescent Wave Chemical Sensors A thesis presented to the faculty of the Russ College of Engineering and Technology of Ohio University In partial fulfillment of the requirements for the degree Master of Science Anupama Solam August 2009 © 2009 Anupama Solam. All Rights Reserved. 2 This thesis titled Analytical and Numerical Analysis of Low Optical Overlap Mode Evanescent Wave Chemical Sensors by ANUPAMA SOLAM has been approved for the School of Electrical Engineering and Computer Science and the Russ College of Engineering and Technology by Ralph D. Whaley, Jr. Assistant Professor of Electrical Engineering and Computer Science Dennis Irwin Dean, Russ College of Engineering and Technology 3 ABSTRACT SOLAM, ANUPAMA, M.S., August 2009, Electrical Engineering Analytical and Numerical Analysis of Low Optical Overlap Mode Evanescent wave Chemical Sensors (106 pp.) Director ofThesis: Ralph D. Whaley, Jr Demands for integrated optical (IO) sensors have tremendously increased over the years due to issues concerning environmental pollution and other biohazards. Thus, an integrated optical sensor with good detection scheme, sensitivity and low cost is needed. This thesis proposes a novel evanescent wave chemical sensing (EWCS) technique for ammonia (gaseous) and nitrite (aqueous) detection utilizing a low optical overlap mode (LOOM) waveguide structure. This design has the advantage of low modal fill factor and more field interaction with the sensing region compared to fiber sensors; hence eliminating the difficulty faced by traditional EWCS designs. Effective refractive index is a crucial parameter for analyzing the sensitivity, which is compared using the analytical and numerical methods such as finite element method (FEM) and semi-vectorial beam propagation method (BPM). The sensitivity of LOOM structures are calculated and compared analytically and numerically. Finally, these waveguide structures are analysed when integrated with a Mach-Zehnder Interferometer. Approved: ________________________________________________________ Ralph D. Whaley, Jr. Assistant Professor of Electrical Engineering and Computer Science 4 ACKNOWLEDGMENTS It’s a wonderful opportunity to thank my advisor, Dr. Ralph Whaley, Jr. PhD. for his consistent support and guidance in every step for the completion of my thesis. I highly respect and appreciate his ideas and inputs, and learned a lot of things besides academics such as being systematic, polite, humble and enthusiastic. Secondly, I would like to thank my committee members Dr.Savas Kaya, Dr.Avinash Kodi and Dr.Karen Coschigano for their extended support. I am grateful to Dr.Gines Lifante, who had provided me literature help and replies to various questions through email and also helping me with the results for Semi-vectorial BPM simulations. I also thank the Rsoft research group, mainly Mr. Matthew Frank, for troubleshooting various problems while working with the FEM software. My father Mr. Sharath Chander, my mother Mrs. Jhansi Rani and my sister Anusha have always been encouraging and showed tremendous interest towards my study and research, which contributed to my thesis to a great extent. I thank my friend Bhargav Kota, who has been motivating and always there for any kind of help and support. I would also like to thank my friends from India, Priyanka Kamath, Deepthi Chandu, Lakshmi Iyer and Kriti Reddy who stood by me always. I thank my seniors at Ohio University: Chandana Venkatayogi, Jyothsna Jakka, Vijaya Hari, Tanu Sharma, Sulalita Chaki and Mohor Chatterjee for their encouragement and understanding. Krishna Manoharan, one of our research members, helped a lot with his inputs and suggestions throughout the process. I thank him and Aarthi Srinivasan, for her warmth and help. I also thank my relatives Mr. P.Sri Kumar and Mrs. Shailaja for following me up all the while. Last, but not the least, I would like to thank our janitor, Mrs. Rose Mary. 5 TABLE OF CONTENTS Page ABSTRACT ........................................................................................................................ 3 ACKNOWLEDGMENTS .................................................................................................. 4 LIST OF TABLES .............................................................................................................. 9 LIST OF FIGURES .......................................................................................................... 10 CHAPTER 1: INTRODUCTION ..................................................................................... 12 1.1 Overview of Optical Sensors .................................................................................. 14 1.2 Bio-Chemical Sensor .............................................................................................. 16 1.3 Advantages of Planar Structures over Fibers for Optical Bio-sensing ................... 17 1.4 Importance and Phenomenon of Evanescent Wave Sensing .................................. 18 1.5 Types of Evanescent Wave Sensors ....................................................................... 21 1.6 Working Principle of Integrated Optic (IO) Sensors .............................................. 23 1.7 Single Mode Operation ........................................................................................... 25 1.8 LOOM Structure ..................................................................................................... 26 1.9 Analytes .................................................................................................................. 28 1.9.1 Gaseous Detection of Ammonia ...................................................................... 28 1.9.2 Aqueous Detection of Nitrites.......................................................................... 28 1.10 Material Used ........................................................................................................ 29 1.10.1 Indium Phosphide (InP) ................................................................................. 29 1.10.2 Amorphous Zinc Oxide (a-ZnO) ................................................................... 30 6 CHAPTER 2: ANALYTICAL AND NUMERICAL METHODS TO OBTAIN EFFECTIVE INDEX ........................................................................................................ 31 2.1 Characteristics of Optical Waveguides ................................................................... 31 2.2 Channel Waveguides .............................................................................................. 33 2.3 Analytical Solution: Effective Index Method (EIM) .............................................. 36 2.3.1 Theoretical Calculation of Effective Index for a Symmetric Planar Waveguide Structure ................................................................................................. 40 2.3.2 Wave Equations of a planar waveguide ........................................................... 42 2.3.3 Other Important Waveguide Parameters .......................................................... 43 2.3.4 Mode Parameters ............................................................................................. 44 2.3.5 Eigen Value Equation ...................................................................................... 45 2.4 Effective Index Calculations using Finite Element Method (FEM) ....................... 49 2.5 Effective Index Calculations Using Semi-Vectorial Beam Propagation Method (BPM) .............................................................................................................. 53 2.6 Vertical Cut Profile: Electric Field ......................................................................... 53 2.7 Confinement Factor ................................................................................................ 55 2.8 Modal Fill Factor .................................................................................................... 56 2.9 Gaseous and Aqueous Detection ............................................................................ 57 2.10 Modal Analysis of the Buried Structure ............................................................... 58 CHAPTER 3: SENSITIVITY ANALYSIS ...................................................................... 60 3.1 Analytical Method .................................................................................................. 60 3.2 Numerical Method: Sensitivity Analysis using FEM Approach ............................ 62 7 3.3 Sensitivity Analysis of Buried Waveguide Structure ............................................. 64 3.4 Surface Sensitivity of an Air LOOM Structure ...................................................... 66 CHAPTER 4: MACH ZEHNDER INTERFEROMETER ANALYSIS .......................... 67 4.1 Principle of Operation of an MZI ........................................................................... 68 4.2 Analysis of MZI ...................................................................................................... 69 CHAPTER 5: RESULTS AND DISCUSSION ................................................................ 70 5.1 Analytical and Numerical Results for Effective Index Calculations ...................... 70 5.2 Comparison of the Modal Fill Factor and Confinement Factor .............................. 72 5.3 Sensitivity Analysis ................................................................................................ 73 5.4 Mach-Zehnder Interferometer Output ...................................................................
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