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Specially Shaped Optical Fiber Probes: Understanding and Their Applications in Integrated Photonics, Sensing, and Microfluidics a Dissertation Specially Shaped Optical Fiber Probes: Understanding and Their Applications in Integrated Photonics, Sensing, and Microfluidics A Dissertation Submitted to the Faculty of the WORCESTER POLYTECHNIC INSTITUTE in partial fulfillment of the requirements for the Degree of Doctor of Philosophy in Mechanical Engineering by Yundong Ren June 2019 Advisory Committee: Assistant Professor Yuxiang Liu, Advisor (Mechanical Engineering Department, WPI), Professor Jamal Yagoobi (Head of Mechanical Engineering Department, WPI), Professor Douglas T. Petkie (Head of Physics Department, WPI), Associate Professor Songbai Ji (Biomedical Engineering Department, Mechanical Engineering Department, WPI) Professor Pawan Singh Takhar (Food Engineering, UIUC), Associate Professor Pratap Rao (Graduate Committee Representative, Mechanical Engineering Department, WPI) Abstract Thanks to their capability of transmitting light with low loss, optical fibers have found a wide range of applications in illumination, imaging, and telecommunication. However, since the light guided in a regular optical fiber is well confined in the core and effectively isolated from the environment, the fiber does not allow the interactions between the light and matters around it, which are critical for many sensing and actuation applications. Specially shaped optical fibers endow the guided light in optical fibers with the capability of interacting with the environment by modifying part of the fiber into a special shape, while still preserving the regular fiber’s benefit of low-loss light delivering. However, the existing specially shaped fibers have the following limitations: 1) limited light coupling efficiency between the regular optical fiber and the specially shaped optical fiber, 2) lack special shape designs that can facilitate the light-matter interactions, 3) inadequate material selections for different applications, 4) the existing fabrication setups for the specially shaped fibers have poor accessibility, repeatability, and controllability. The overall goal of this dissertation is to further the fundamental understanding of specially shaped fibers and to develop novel specially shaped fibers for different applications. In addition, the final part of this dissertation work proposed a microfluidic platform that can potentially improve the light-matter interactions of the specially shaped fibers in fluidic environments. The contributions of this dissertation work are summarized as follows: 1) An enhanced fiber tapering system for highly repeatable adiabatic tapered fiber fabrications. An enhanced fiber tapering system based on a novel heat source and an innovative monitoring method has been developed. The novel heat source is a low-cost ceramic housed electric furnace (CHEF). The innovative monitoring method is based on the frequency-domain optical transmission signal from the fiber that is being tapered. The enhanced fiber tapering system can allow highly repeatable fabrication of adiabatic tapered fibers. 2) A lossy mode resonance (LMR) sensor enabled by SnO2 coating on a novel specially shaped fiber design has been developed. The developed LMR sensor has a D-shape fiber tip with SnO2 coating. It has the capability of relative humidity and moisture sensing. The fiber-tip form factor can allow the sensor to be used like a probe and be inserted into/removed from a tight space. 3) Specially shaped tapered fibers with novel designs have been developed for integrated photonic and microfluidic applications. Two novel specially tapered fibers, the tapered fiber loop and the tapered fiber helix have been developed. The tapered fiber loop ii developed in this work has two superiority that differentiated itself from previous works: a) the mechanical stability of the tapered fiber loop in this work is significantly better. b) the tapered fiber loops in this work can achieve a diameter as small as 15 �m while still have a high intrinsic optical quality factor of 32,500. The tapered fiber helix developed in this work has a 3D structure that allows it to efficiently deliver light to locations out of the plane defined by its two regular fiber arms. Applications of the tapered fiber helices in both integrated photonic device characterizations and microparticle manipulations have been demonstrated. 4) Developed an acrylic-tape hybrid microfluidic platform that can allow function reconfiguration and optical fiber integration. A low-cost, versatile microfluidic platform based on reconfigurable acrylic-tape hybrid microfluidic devices has been developed. To the best of the author’s knowledge, this is the first time that the fabrication method of sealing the acrylic channel with a reconfigurable functional tape has been demonstrated. The tape-sealing method is compatible with specially shaped fiber integrations. iii Acknowledgements I would like to take this short page to express my sincere gratefulness to a large group of people who have all made great contribution to my Ph.D. study. First and foremost, I would like to express my deepest appreciation to my advisor Professor Yuxiang Liu, for his mentorship, support, and inspiration. Throughout my Ph.D., he has passionately taught me a great amount of exciting science and valuable skills. However, his influence on me goes far beyond professional development. Through his own words and deeds, I have learned a lot of valuable lessons about how to deal with choices and challenges in life. I cherish this past experience as his student, and I believe this experience will continue to lead me in my future adventures in life. I would like to thank my dissertation committee members, Professor Jamal Yagoobi, Professor Pratap Rao, Professor Yuxiang Liu, Professor Douglas Petkie, and Professor Pawan Singh Takhar for taking your valuable time to review my dissertation. Particularly, I want to thank Professor Rao for his generous help and valuable suggestions to my research. In the development of the lossy- mode-resonance sensor, his insights on material selection have played an important role in the success of the project. He has generously lent me many chemicals and chemistry related equipment for my research work. His student Tao Yan and Binod Giri have helped me with the SEM imaging. I would also like to thank Mr. Peter Hefti for his invaluable help in the machine shop. Without his help, many of my designs will be just a 2D schematic drawing. His optimistic loud laughter can always put smile on people’s face. I am deeply grateful to the all my lab mates and friends for their supports, they are (in no particular order), Chaoyang Ti, Yao Shen, Rui Zhang, Subhrodeep Ray, Mucheng Li, Mengqiao Yang, Haimi Tang, and Lite Zhou, without your company, my Ph.D. life will be much more arduous and less fun. Outside WPI, I would also like to thank Professor Misha Sumetsky and Dr. Yong Yang from Aston University in the UK. Thank you for hosting my stay in the UK for the summer research. Your passion in research has invigorated me to pursue a career in science. Finally, with all my heart, I want to thank my wife, Zeyan Huang, and my parents, Mr. Xuegong Ren and Mrs. Mitao Liao. Your unconditional support and encouragement are the sources of my strength. I’m deeply indebted to your sacrifice for me, this dissertation is dedicated to you. iv v Table of Contents CHAPTER 1 INTRODUCTION AND BACKGROUND .................................................... 1 1.1 PROBLEM OF INTEREST .................................................................................................... 1 1.2 REGULAR OPTICAL FIBERS ............................................................................................... 2 1.2.1 Basic information of regular glass optical fibers .................................................... 2 1.2.2 Optical modes of regular step-index optical fibers ................................................. 4 1.3 STATE OF THE ART OF SPECIALLY SHAPED FIBERS ............................................................ 6 1.3.1 Straight tapered optical fibers ................................................................................. 6 1.3.1.1 Basic information of the straight tapered optical fibers ..................................... 6 1.3.1.2 Existing fiber tapering methods .......................................................................... 8 1.3.1.3 Applications of the straight tapered fibers ........................................................ 16 1.3.2 Dimpled tapered fibers .......................................................................................... 22 1.3.2.1 Fabrication of dimpled tapered fiber ................................................................ 22 1.3.2.2 Application of the dimpled tapered fiber .......................................................... 23 1.3.2.3 Limitations of the dimpled tapered fiber ........................................................... 24 1.3.3 Tapered fiber ring resonators ................................................................................ 25 1.3.3.1 Fabrication of the tapered fiber ring resonators .............................................. 25 1.3.3.2 Sensing applications of the tapered fiber ring resonators ................................ 30 1.3.4 D-shape optical fibers ........................................................................................... 31 1.3.4.1 Fabrication of D-shape fibers ........................................................................... 31 1.3.4.2 Applications of the D-shape fibers ...................................................................
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