Modeling of grating compensated dispersion managed soliton systems

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Modeling of grating compensated dispersion managed soliton systems


Author: Kwan, Yuk-ha
Title: Modeling of grating compensated dispersion managed soliton systems
Degree: Ph.D.
Year: 2006
Subject: Hong Kong Polytechnic University -- Dissertations
Optical communications
Optical fibers
Department: Dept. of Electronic and Information Engineering
Pages: xxix, 177 p. : ill. ; 30 cm
Language: English
InnoPac Record:
Abstract: The transmission rate and propagation distance of optical fiber communication systems are limited by channel impairments such as chromatic dispersion, intrinsic fiber nonlin-earity, polarization mode dispersion, amplifier noise, etc. We focus on the methods to curtail the chromatic dispersion which results in pulse broadening. The most effective solution is dispersion management which can be applied to NRZ and RZ (includes soliton) transmission formats as well as on-off and DPSK modulation schemes. In 2004, through a combination of dispersion management, RZ transmission format, DPSK modulation scheme, Raman amplification, and wavelength division multiplexing techniques, the aggregate bit rate has reached the 6 tera bits per second mark for error free transmission of 6,120 km [I]. Dispersion management is carried out by concatenation of fiber segments with different dispersion coefficients alternately such that the average dispersion is small. Another method to counter the dispersion effect is to use soliton transmission format that makes use of the intrinsic Kerr effect of optical fibers. It was discovered that soliton propagation is possible even in dispersion managed (DM) systems; they are called DM solitons. Chirped fiber gratings (CFGs) are very attractive as dispersion compensators because of its compact size and their ability to compensate higher-order dispersion. The main drawback of using CFGs for dispersion compensation is their intrinsic group delay ripples (GDR). The GDR is formed during grating fabrication. Group delay ripple causes inter-symbol interference (ISI) through the overlapping of the side peaks, generated by GDR, in the temporal pulse profiles. As a result, the transmission performance drops. In NRZ transmission, the amplitudes of the side peaks increase linearly with the number of CFGs along the transmission line. In this thesis, we find that DM solitons exist in the DM fiber systems compensated by CFGs with GDR. The use of solitons suppresses the growth of the amplitudes of the side peaks. We found that the GDR could modify the grating dispersion. The modification of dispersion is minimum when the ripple period is much shorter than the signal bandwidth that is the general cases in practice. Then, we developed two analytical methods for designing grating-compensated DM soliton systems using CFGs with no GDR. Currently, time consuming numerical simulations are used to obtain the design parameters for a given bit rate in DM transmission systems. We are now providing an explicit analytical formula to obtain either the initial pulse parameters or the dispersion map parameters for a given transmission rate (or pulse width) up to 160 Gb/s. We verified that the analytical results agreed with the results obtained by direct numerical simulations. The current work also includes a novel method of using nonlinear optical loop mirror (NOLM) to reduce the ISI induced by the GDR in CFG. Conventional methods focus on improving the grating quality. The transmission record of grating compensated systems using the best CFGs is 500 km at 40 Gb/s in single channel system. Using NOLMs, however, we can achieve transoceanic propagation at the same transmission speed even in the presence of amplifier noise and random variation of GDR parameters in CFGs along the transmission line. The novel application of NOLMs can be applied to any systems involving CFGs to remove the effects of GDR.

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