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TEL AVIV UNIVERSITY THE IBY AND ALADAR FLIESCHMAN FACULTY OF ENGINEERING Department of Electrical Engineering – Physical Electronics GRADED INDEX SILVER HALIDE INFRARED WAVEGUIDES AND FIBERS – FABRICATION, PROPERTIES AND APPLICATIONS Thesis submitted for the degree “Doctor of Philosophy” by Dekel Ben-Zion Submitted to the senate of Tel-Aviv University May 2002 TEL AVIV UNIVERSITY THE IBY AND ALADAR FLIESCHMAN FACULTY OF ENGINEERING Department of Electrical Engineering – Physical Electronics GRADED INDEX SILVER HALIDE INFRARED WAVEGUIDES AND FIBERS – FABRICATION, PROPERTIES AND APPLICATIONS Thesis submitted for the degree “Doctor of Philosophy” by Dekel Ben-Zion This research work was carried out at Tel Aviv University under the supervision of Prof. Natan Croitoru from the Department of Electrical Engineering – Physical Electronics and Prof. Abraham Katzir from the School of Physics and Astronomy – Applied Physics Department. May 2002 This work was carried out under the supervision of Prof. Natan Croitoru Prof. Abraham Katzir To Amira, Ron, and Nadav In memory of my parents Ida and Aaron Acknowledgements I would like to express my deepest gratitude to my supervisors, Prof. Natan Croitoru and Prof. Abraham Katzir for their guidance and support throughout this work. My deepest thanks to Prof. Abraham Katzir who impressed me with his professionalism and creativity, as much as he inspired, supervised and helped me along on my scientific path. Special thanks to Arie Levite who is not only an excellent, professional and skillful assistant, but also a close friend, with whom I spent many interesting hours. I would like to thank all of my colleagues in the Applied Physics group, conducted by Prof. Katzir, for their help and support. In order not to omit anyone from the list of the many good people who helped me, I will not refer to any names, but I am grateful to each one of you. Many thanks to the members of the machine shop of the School of Physics and Astronomy managed by Yankale. Special thanks to Prof. Frank Moser for his effort in reading vast amounts of articles and for his helpful advise. Special Thanks to Dr. Zehava Barkay from the Wolfson Applied Materials Research Center. the group from the Clean Room, Mark Oksman, the colleagues in Prof. Croitoru’s group, and especially Dr. Sasha Inberg from the Electrical Engineering Department. Special gratitude to MLM (Israel Aircraft Industries) and Dr. Eli Oron who by reducing my workload enabled me to begin this research. Many thanks to the staff of the School of Engineering at Ruppin Institute for their patience and understanding. Last, but not least, I would like to thank my beloved wife Amira, for her everlasting patience, encouragement and support, and to my dear sons Ron and Nadav who have waited so long for this moment. Submission of this work is the fulfillment of a dream, which we all, as a family, shared. Abstract Optical planar waveguides in integrated optics and optical fibers in the visible have been used during the last two decades in a variety of applications and especially in optical communication. The advantage of using the visible spectrum is that materials and components for telecommunication, such as lasers, waveguides, modulators and detectors are common and their price is low. Communication in the mid-IR is far superior in principle over the visible spectrum, because the attenuation for materials in the mid-IR is lower by several orders of magnitude than in the visible. In spite of this fact, mid-IR communication is not used due to the unavailability of suitable materials and components. The objective of this work was to design and manufacture new optical elements for the mid IR in order to integrate them in integrated optics. The research was divided into three sections: • Design and manufacture different types of planar waveguides. • Design and manufacture graded index cylindrical fibers. • Applications of fibers and planar waveguides. The elements fabricated in this work are based on silver halide crystals (AgClBr). These materials exhibits wide transmission range, up to 20µm, they are non toxic, and can be prepared in any mixed halide composition. In addition, silver halides can be extruded to produce IR transmitting fibers. In the first part of the work, planar optical waveguides of two types were prepared: graded index and strip buried. It is known that for a mixed crystal the refractive index varies in a gradual behavior, such that for AgBr it is 2.16 and for AgCl - 1.98. The graded index waveguides were prepared by diffusion of Br into a substrate of AgCl. When Br ions diffuse a short distance into a AgCl crystal, a layer of higher index of refraction is formed. The diffused layer may serve as a waveguide for mid-IR radiation. A mid-IR laser beam that is focused into one end of such a waveguide will be transmitted to the other end by a series of total internal reflections. A few diffused waveguides were prepared with a variety of diffusion depths, and were characterized so that their transmission, scattering and atomic concentration were determined. The second type of planar waveguides was the strip buried one. These waveguides were constructed from a flattened fiber of AgCl0.4Br0.6 which was pressed into AgCl substrate. The manufactured waveguides were examined and compared to the diffused waveguides. The second part of the research includes the fabrication of graded index fibers in the mid IR. The main principle by which the fibers were prepared is the same as for the graded index waveguides. A rod made of AgBr was inserted into a cylindrical tube made from AgCl. The “rod in tube” preform was inserted into an oven for a diffusion to take place. The tube was removed and by an extrusion process, fibers were prepared. The poor contact between the rod and the tube resulted in an unpredicted diffusion depth. A few methods were developed in order to improve the contact between the crystals. All fibers from the various techniques are analyzed and compared to a core-only fiber as reference. The third part of the work includes three applications: (1) A laser marking system based on hollow waveguides and silver halide fibers (2). Thermal imaging based on hollow waveguides and silver halide fibers, as scanning elements. (3) IR spectroscopy using planar waveguides. The first application involved scanning of an exit tip of an optical fiber by magnetic means, in order to mark a surface. In this work a few marking examples were demonstrated. The second application, which is a simple thermal camera, used the same scanning technique except that in this case the focal plane of an IR scenario was scanned pixel by pixel, and the thermal image was constructed. A few thermal images were obtained in the work. The third application involved the detection of low percentage of alcohol and glucose in water based on ATR spectroscopy. The ATR spectroscopy is based on reducing the transmission through the waveguide because of the absorption of the evanescent wave in the sensed material (glucose or alcohol in our case). Minimum percentage of 1% was detected with this method. In summary, the preliminary planar waveguides and graded index fiber which were prepared, showed promising properties for future use in a verity of applications. By similar methods it would be possible to fabricate various types of infrared optical elements, which are similar to the ones use in integrated optics in the visible range. We believe that in the future these elements will pave the road for the fabrication of full integrated optical circuits in the mid IR and will have interesting applications in science, technology and medicine. TABLE OF CONTENTS 1. Chapter 1: Introduction 1.1 Motivation……………………………………………………………………… 1 1.2 Waveguides and fibers in the IR……………...……………………………. 3 1.2.1 Classification of infrared materials………………………………… 3 1.2.2 Classification of optical waveguides………………………………. 5 1.3 Step and graded index optical waveguides and optical fibers in the visible and near infrared………………………………………………………..7 1.3.1 Planar optical waveguides in the visible and near infrared...……….7 1.3.2 Optical fibers in the visible and near infrared…..………………….. 8 1.4 Step index and graded index waveguides and fibers in the IR…………….. 9 1.4.1 Planar waveguides in the mid-IR……………………...…………… 9 1.4.2 Fibers in the mid-IR…………..……………………………………. 9 1.5 The silver halide as an IR material for optical waveguides ………...……... 11 1.6 Objective and a layout of this work………………………………………... 12 1.7 References to chapter 1…………………………………………………….. 13 2. Chapter 2: Mathematical background for the IR waveguides 2.1 Introduction………………………………………………………………… 18 2.2 The strip planar optical waveguide – theory……………………………….. 19 2.2.1 The electromagnetic theory for planar uniform optical waveguides 19 2.3 The graded index optical waveguide – theory……………………………... 27 2.3.1 The electromagnetic theory for a graded index waveguide….…….. 27 2.3.2 Ray optics and the Eikonal equation for graded index waveguide… 35 2.3.3 Ray tracing in graded index planar waveguide using Monte Carlo method……..………………………………………………… 43 2.4 Mode and ray theory of the graded index optical fibers…………………… 44 2.5 References to chapter 2…………………………………………………….. 48 3. Chapter 3: Strip Buried Planar Waveguides in the IR 3.1 Introduction………………………………………………………………… 50 3.2 Strip buried IR waveguides - fabrication and characteristics……………… 50 3.2.1 Methods of fabrication strip buried waveguides in the visible range……………………………………………………...… 50 3.2.2 Fabrication of the AgClBr waveguides…………………………….. 51 3.3 Experiments made with strip buried IR waveguides………………………. 52 3.3.1 Experimental setup………………………………………………… 52 3.3.2 Waveguide thickness and index of refraction ……………………... 53 3.3.3 Waveguide propagation losses…………………………………….. 53 3.3.4 The maximal incident angle to the waveguide…………………….. 55 3.3.5 IR power propagation in a curved waveguide……………………..

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