STUDIES ON DISPERSION COMPENSATING FIBERS AND ERBIUM DOPED FIBER AMPLIFIERS PARTHASARATHI PALAI

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1 STUDIES ON DISPERSION COMPENSATING FIBERS AND ERBIUM DOPED FIBER AMPLIFIERS BY PARTHASARATHI PALAI Department of Physics Submitted in fulfillment; of the requirements of the degree of Doctor of Philosophy to the INDIAN INSTITUTE OF TECHNOLOGY, DELHI NOVEMBER, 1997

2 CERTIFICATE This is to certify that this thesis entitled STUDIES ON DISPERSION COM- PENSATING!TREKS AND ERBIUM DOPED FIBER AMPLIFIERS, being submitted by Parthasarathi Palai to the Indian Institute of Technology, Delhi, is a. record of bonafide research work carried out, by him. He has worked under our guidance and supervision and has fulfilled the requirements, which to our knowledge, have reached the requisite standard for the submission of this thesis. The results contained in this thesis have not been submitted in part or full to any other University or Institute for the award of any degree or diploma. I. : K. HYAGARAJAN Professor of Physics Professor of Physics Department of Physics Indian Institute of Technology, Delhi New Delhi-110 OK INDIA

3 ACKNOWLEDGEMENTS It. is my proud privilege that I have been a member of the Fiber Optics Group during the course of this work. I wish to express my deep gratitude and regard to Professor A.K. Ghatak who has been a constant source of inspiration and encouragement for my research endeavour. I am greatly indebted to my supervisors Professor K. Thyagarajan and Professor B.P. Pal for their invaluable guidance, keen interest, and deep involvement. during all stages of my research work. I am also grateful for the caw and support. they have extended to me. I am extremely grateful to Professor I.C. Goyal and Dr. R.K. Varshney, whose invaluable contributions and suggestions have helped me in achieving part of any research goal. I am thankful to Professor Arun Kumar, Professor Anurag Sharma, Dr. Ajit Kumar, Dr. B.D. Gupta, Dr. M.R. Shenoy, and Dr. Vishnu Priye for their considerations and suggestions throughout the course of this work. Finally, I would like to express my deep appreciation to my friends and family members, whose help, patience, and understanding have greatly contributed to 11w realization of this thesis. Parthasarathi Palai New Delhi November, 1997 iii

4 ABSTRACT In recent, years, upgradation of the already existing optical network consisting of 1310 mu optimized non-dispersion shifted fibers (NDSFs) for high speed communication is gaining importance. With the advent of erbium-doped fiber amplifiers (EDFAs) which operate in the 1550 nm band, loss in the communication systems is no longer a major issue. The limitations imposed by large chromatic dispersion in these systems, on the other hand, can be alleviated by dispersion compensating fibers ( DCFs). Many recent high speed optical communication systems have been demonstrated using components such as EDFAs and DCFs. In order to realize reliable communication systems these should be optimized for their efficient use. This is the main focus of this thesis. In the field applications, EDFAs are used as modules in which several tens of meters of doped fibers are bent to form a spool. We have analyzed the potential detrimental effects of bend such as bend induced loss and mode deformity on the gain efficiency of a 98() nm pumped EDFA. We have shown that, in the presence of bend, for certain range of the doped fibers parameters, the pump no longer r(iymains single nioded. The possible excitation of higher order mode which leads to inefficient. use of pump power can he avoided by choosing right parameters of the doped fiber. Further, we have shown that the effect of the mode deformity is almost negligible for any practical situation. The DCFs with a high negative dispersion coefficient are always attractive. We have proposed a novel DU design based on a highly a.sym metric directional

5 coupler structure.. The fundamental mode of the fiber has a very high negative dispersion coefficient at 1550 nm wavelength, which is almost 15 times higher than those achievable through conventional designs. We have shown that the proposed DCF has a. large mode field diameter and suffers from negligible bend inducedloss. We have also obtained the effects of the variation of fiber parameters on dispersion characteristics; these results should be useful from the manufacturing point of view. Further, we have also obtained a new set of parameters of the proposed design in order to achieve broadband dispersion compensation between 1530 to 1560 nm wavelength range over which the EDFAs are effective. The reliability of a communication system can also be improved by reducing the number of components. We have proposed and analyzed a novel scheme of simultaneous compensation of dispersion and loss by an erbium doped DCF. For an efficient use of such a fiber we have obtained various characteristics such as, the pump power requirement for a target gain, noise figure, variation of gain with pump power, etc. We have shown that with very low erbium ion concentration and nominal pump powers should indeed be possible to compensate dispersion and loss simultaneously. Nonlinear effects are gaining importance in optical fiber communica.tion systems which incorporate EDFAs for signal amplification. We have analyzed the effect of nonlinearity in the form of the self-phase modulation on signal transmission in a DCF-based dispersion compensated NDSF link. We have shown that in contrast to the linear regime of operation where both post-compensation (i.e., 1)(..!F followed by NDSF) and pre-compensation (NDSF followed by DC:11 vi

6 schemes behave in an identical manner, they behave very differently in the nonlinear regime. We have shown that a pre-compensation scheme with a slightly over compensation of dispersion can be an ideal choice for configuring a composite combination of DCF and NDSF to overcome large variations in the signal power in a dispersion compensated system. vii

7 Contents Certificate Acknowledgements Abstract iii V 1 Introduction 1 2 Modeling an Erbium-Doped Fiber: The Basic Equations Introduction Modeling Su in mary 30 3 Optimization of 980 nm Pumped Erbium Doped Fiber Amplifier in presence of Bend Introduction Theory Results and Discussions Summary 47 4 A Novel Design for a Dispersion Compensating Fiber Introduction Dispersion Compensating Schemes 4.3 Proposed Design of the Dispersion Compensating Fiber Numerical results and discussions 4.5 Summary 78 5 Simultaneous Compensation of Dispersion and Loss by an Erbium- Doped Dispersion-Compensating Fiber Introduction A Typical Design Example Theory Numerical Results and Discussions Summary 101 ix

8 6 Effect of Self-Phase Modulation on a Dispersion Compensated Link with Dispersion Compensating Fiber Introduction Modeling Numerical Results and Discussions Summary 1 25 Scope for Future Work 127 References 129 Author's Biography