Design of dispersion managed soliton systems

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Design of dispersion managed soliton systems

 

Author: Kwan, Yuk-ha
Title: Design of dispersion managed soliton systems
Degree: M.Phil.
Year: 2001
Subject: Optical communications
Optical fibers
Dispersion
Solitons
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Electronic and Information Engineering
Pages: xvi, 119 leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1566423
URI: http://theses.lib.polyu.edu.hk/handle/200/4902
Abstract: One of the factors that limits the maximum transmission rate of an optical fiber communication system is chromatic dispersion. Methods to curtail the pulse broadening effects of dispersion includes propagation at the zero dispersion wavelength, dispersion management, NRZ transmission format, and soliton transmission format. In 1999, a combination of dispersion management, NRZ format, specially made fibers, and wavelength division multiplexing pushed the aggregate bit rate past the 1 tera bits per second mark for error free transmission of 7,300 km [1]. Dispersion management is carried out by concatenation of fiber segments with different dispersion coefficients alternately. The local dispersion is chosen to be large to prevent coupling between the optical signals and the amplifier noises. The average dispersion is kept low to minimize the dispersion effects. The simplest configuration of a dispersion managed system is made of two types of fibers; one with constant anomalous dispersion coefficient and the other with constant normal dispersion coefficient. Soliton transmission format makes use of the intrinsic Kerr effect of an optical fiber to counter the dispersion effect. Distortionless propagation is possible for some specially shaped pulses if effects such as dissipation and higher order dispersion are neglected. Conventional wisdom has it that soliton transmission systems require fibers with constant anomalous dispersion. However, it was recently discovered that soliton propagation is possible even in dispersion managed systems. These so-called dispersion managed (DM) solitons have a number of advantages over the solitons of constant anomalous dispersion systems. One of the which is energy enhancement, i.e., the energy of a DM soliton is higher than that of a conventional soliton with the same average dispersion and pulse width. Energy enhancement depends on the pulse width as well as the fiber segment lengths and dispersion coefficients of the dispersion map. An empirical formula was obtained for two-step dispersion maps with equal length fibers. In this thesis, we study the effect of the proportion of normal dispersion fiber on energy enhancement in a two-step dispersion map. We find that if the average dispersion is anomalous and the map strength is less than 3, energy enhancement is not affected by the length of the normal dispersion segment. If the map strength is larger than 3, we find that a dispersion map that is made of normal dispersion fibers and a fiber Bragg grating has the maximum energy enhancement. On the other hand, a dispersion map that is made of anomalous dispersion fibers and a FBG has the minimum energy enhancement. The ratio of maximum to minimum energy enhancement can be as large as 2 as the map strength increases. Qualitatively similar results are obtained for zero average dispersion and normal average dispersion systems.

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