Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1995-09-07
1997-11-04
Chin, Wellington
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359161, 359179, H04B 1012
Patent
active
056846150
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to optical systems and in particular optical systems within which optical solitons are propagated.
BACKGROUND OF THE INVENTION
The word soliton was first coined by Kabusky and Kruskal to describe the particle-like behaviour of the solitary wave solutions of the numerically treated Korteweg-deVries equation. Now, more than 100 different non-linear partial differential equations exhibit soliton-like solutions. Optical solitons belong to the class of envelope solitons and can be described by the non-linear Schroding (NLS) equation.
Hasegawa and Tappert in 1973 were the first to show theoretically that, in an optical fibre, solinary waves were readily generated and that the NLS equation description of the combined effects of dispersion and the non-linearity self-phase modulation, gave rise to envelope solitons. It was not until seven years later, in 1980 that Mollenauer and co-workers first described the experimental realization of the optical soliton, the delay primarily being due to the time required for technology to permit the development of low loss single-mode fibres. Since 1980 there has been tremendous research effort in the field of optical solitons, the research being driven by the promise of massively increased bit rates, through the application of ultra short soliton pulses in long-distance optical communication networks. In addition, solitons are attractive for use in ultra fast optical switching and processing, primarily based on interferometric techniques.
Unfortunately, additional non-linearities which had not been considered originally have put limits on potential systems for long distance optical communication. Nevertheless, it is likely that optical soliton systems for long distance optical communication, for example transatlantic systems, will be installed within five years or so. Nevertheless, the raising of the limits on transmission distance remains a focus of much research. Similarly, the related problem of stability in switching and storage systems which use optical solitons is a major focus of much current research.
One of the main constraints on the design of optical fibre soliton communication systems is the interaction force which exists between adjacent pulses in the bit stream, see J P Gordon, Optics Letters, 8, (1983), page 596; K J Blow, N J Doran, Electronics Letters, 19, (1983), 429-430; P L Chu, C Desem, Electronics Letters, 19, (1983), 956-957. It has been shown that bandwidth filtering or temporal modulation are effective means of combatting the other source of timing errors in such systems, namely Gordon-Haus jitter. However, spectral filtering has relatively little effect on the interaction forces when proper account is made of the evolved pulse parameters, see M Nakazawa, H Kubota, Electronics Letters, 20, 28, (1992), 958-960. Wabnitz has shown, Optics Letters, 18, (1993), 601-603, in the context of a ring cavity, that the interactions may be controlled by the injection of a low amplitude cw wave; however this technique is less suited for use in transmission systems. Recently Francois Georges examined, Optics Letters, 18, (1993) 583-585, the use of a combined phase and amplitude modulation cycle to suppress the interactions. In that work neighbouring pulses were placed at opposite extrema of the phase modulation component, thus breaking the coherence between them. The modulation function in, question was however, somewhat unphysical. Additionally the present inventors have shown elsewhere that one extremum of a phase modulator's cycle enhances the Gordon-Haus jitter, see N J Smith, K J Blow, D J Firth, K Smith, Optics Communications, Vol. 102, 324-238, 1993.
In our above-mentioned paper, we disclose that a positive phase modulation may be used to reduce Gordon-Haus jitter. However, as indicated above in relation to bandwidth filtering and temporal modulation, while techniques have been devised for controlling Gordon-Haus jitter, the control of soliton-soliton interactions has proved to be much more difficult.
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Blow Keith J.
Smith Kevin
Bacares Rafael
British Telecommunications public limited company
Chin Wellington
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