ISOLATOR-FREE DFB LASER FOR ANALOG CATV APPLICATIONS By Ayman Mokhtar,B.Sc., M.Sc., A Thesis Submitted to The Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Doctor of Philosophy Ottawa-Carleton Institute for Electrical and Computer Engineering Faculty of Engineering Department of Systems and Computer Engineering Carleton University Ottawa, Ontario, Canada January, 2007 © 2007 Ayman Mokhtar Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 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Abstract Fiber-pigtailed Distributed Feedback (DFB) laser diode modules often employ isolators to reduce the optical feedback induced intensity noise and distortion, and to meet the requirements imposed on the DFB lasers in analog optical transmission systems such as Cable Television (CATV). For some situations however, isolators could be avoided to reduce cost and simplify laser module manufacture while maintaining the desired DFB laser’s performance. In this thesis, Fabry Perot (FP) and DFB laser diode rate equations were augmented to include the effect of optical back-reflections. The proposed laser model was implemented in software and used as the basis for linearity and noise simulations in which the effect of the reflected optical power on the laser diode performance was studied. The model’s results were verified experimentally using an tunable induced back-reflection setup. Results suggest that it is beneficial to operate unisolated DFB laser diodes in feedback Regime V. This was achieved by attaching appropriate FBG to DFB laser. Two telecom­ munications wavelengths, 1310 nm and 1550 nm, were used to examine the performance of a DFB laser coupled to an Fiber Bragg Grating (FBG). A stable spectrum was showed and low Relative Intensity Noise (RIN) comparable to that of the isolated DFB laser was achieved for both wavelengths. A pre-distorter model including optical back-reflection effects was derived, implemented in software, and cascaded to the laser model to compensate for the laser nonlinearity. Simulation results showed that at 0.10 modulation index, average improvements of 45 dB in second order harmonic distortion, 55 dB in third order harmonic distortion, and 40 dB in different intermodulation distortions can be achieved using the proposed pre-distortion technique. Furthermore, at a modulation index of 0.04, the predistorter was found to iv Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. reduce the second order harmonic distortion and two-tone third order intermodulation distortion levels to less than -75 dB and -100 dB respectively. This hence renders the laser suitable for CATV applications. Our proposed pre-distorter constitutes a contribution to the state-of-the-art as it en­ hances the precision of currently used models by adding the effect of the optical feedback. v Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Acknowledgements I would like to express my deep appreciation and gratitude to Professor Samy Mahmoud, my supervisor and President of Carleton University, for his guidance and encouragement. I am greatly indebted to Professor Mahmoud for his dedication, talent, and generosity. I would also like to express my gratitude to Prof Leonard MacEachern, co-supervisor of this thesis, for his excellent advice and guidance during my doctoral research. His promptness and continual advice were highly appreciated. I would like to express my thanks to Dr. Samy Ghoniemy for his assistance and support. I am grateful to the Department of Systems and Computer Engineering for providing a suit­ able research environment, and for their kind support. I would also like to recognize Prof Jacques Albert and all members of Carleton Laboratory for Induced Photonic Structures for their kind assistance. I would like to express my appreciation to Mr. Nagui Mikhail for his highly qualified assistance. I deeply thank my best friend in Ottawa, Mohamed Abou El Saoud, for his efforts. I am grateful to my beloved country Egypt; this research would have not been possible without the financial support of the Egyptian Ministry of Defense. I would also like to express my deep thanks to my dearest mother, her prayers and encour­ agement facilitate my life. I deeply thank my brothers and sisters, especially my eldest brother Wael, for all the support and encouragement. I am indeed indebted to my dearest wife for all the sacrifices she made in order to facilitate a suitable environment for my doctoral work. I thank my daughters, MenatAllah, Yasmine, and Nour; completion of my doctorate would not have been possible without their love. Last but never the least, thanks to my great father who passed away before I could realize this dream. vi Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table of Contents Abstract iv Acknowledgements vi Table of Contents vii List of Tables xii List of Figures xiii 1 Introduction 1 1.1 Introduction ............................................................................................................. 1 1.2 Thesis Motivation .................................................................................................... 2 1.3 Objectives ................................................................................................................. 3 1.4 C ontributions .......................................................................................................... 3 1.5 Organization .......................................................................................................... 4 2 Background and Motivation 5 2.1 Laser Principles and Rate Equations ................................................................. 5 2.1.1 Laser Principles .......................................................................................... 5 2.1.2 Laser Diode Rate Equations C o n cep t .................................................... 6 2.1.3 FP vs DFB Laser Diode Rate E q u atio n s .............................................. 7 2.2 Conventional Laser Diode Rate Equations ...................................................... 8 2.3 Modified Laser Diode Rate E quations ................................................................. 8 2.4 Feedback Regimes .................................................................................................... 10 2.5 Conventional Laser Diode Rate Equations Including Optical Feedback effect 14 2.6 Analysis of Feedback Phenomenon .................................................................... 16 2.7 Changes to Laser Performance due to External Cavity ................................. 22 2.7.1 Emission Frequency Shift due to F eedback ........................................... 22 2.7.2 Single External Cavity Mode Condition ................................................. 23 2.7.3 Spectral Linewidth Change due to Optical Feedback .......................... 23 vii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2.7.4 Phase and Frequency Noise of Laser with Optical Back-reflections . 23 2.7.5 Laser Intensity Noise due to Optical Feedback .................................... 24 2.7.6 Dynamic Properties of Laser with External Cavity .............................. 25 2.8 CATV S y ste m s ......................................................................................................