Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1996-01-29
1999-01-19
Negash, Kinfe-Michael
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359134, 359160, 359173, 359179, H04B 1000
Patent
active
058619704
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods and apparatus for compensating for dispersion in optical communications systems, and in particular to methods and apparatus employing optical phase conjugation.
2. Related Art
In order to have a high transmission capacity, an optical communications system must have low dispersion, this means that pulses of light travelling along the waveguide, generally an optical fibre, of the optical communications system should not suffer significant distortion. This distortion may arise from a number of sources. If the optical communications system employs multi-mode fibre, each of the different modes will have a different group velocity, thus modulated signals, i.e. pulses of light passing down the multi-mode optical fibre, which are made up of a number of different modes of the waveguide will experience a different group delay from each of their modes. This causes a pulse formed from more than one mode to spread out as it propagates, and is called intermodal dispersion. Once consecutive pulses have spread out so that they are no longer distinguishable, one from the other, the information transmission limit of the optical communications system has been reached. This limit is expressed as a bandwidth distance product since it will be reached at a higher bit rate for a shorter optical communications link. Intermodal dispersion between the modes of multi-mode fibres is one of the reasons why modern optical communications systems have moved to the use of single mode optical fibre which, since it only supports one optical mode, does not suffer from intermodal dispersion.
However single mode optical communications systems do suffer from pulse spreading due to the small, but finite bandwidth of the optical source employed. This type of pulse spreading is called chromatic dispersion, and is due to two effects. Firstly, material dispersion is present because the refractive index of a dispersive medium, such as silica from which optical fibres are typically made, depends on wavelength. Secondly, waveguide dispersion, since the propagation characteristics of the single mode supported by a single mode fibre also depend on wavelength. Since the material dispersion of silica is positive at most wavelengths of interest for optical communications systems, and the waveguide dispersion for single mode fibres is negative, these two effects can be carefully balanced in a well designed optical fibre so as to Vive zero total, chromatic dispersion at the operating wavelength of the optical communications system.
The vast majority of optical communications systems which have been installed worldwide contain single mode optical fibre which has been designed for use in the 1.3 .mu.m low loss window, and as such has low chromatic dispersion at this wavelength. In recent years the rapid development of erbium doped fibre amplifiers (EDFA) has meant that fibre loss, and thus the power budget of optical communications systems, is no longer the fundamental limit to achievable transmission distance. However these EDFAs are only operable in the 1.55 .mu.m optical transmission window so that if existing optical communication links are to be upgraded, for example to operate at higher bit rates, these systems must operate in the 1.55 .mu.m window, over optical fibre designed for use at 1.3 .mu.m. Thus the fundamental bandwidth distance product transmission limit when upgrading an existing optical communications system is that imposed by dispersion. Furthermore, even for systems having fibre designed for use at 1.55 .mu.m, as very high bit rates are approached, unless very narrow linewidth, externally modulated lasers are employed, dispersion again is the fundamental limit to transmission capacity.
A number of methods of compensating for dispersion are known. In one such technique the optical signal, at the transmitter end of the optical communications system, is deliberately distorted before being launched into the optical fibre. The distortion imposed upon th
REFERENCES:
patent: 5365362 (1994-11-01), Gnauck et al.
Yazaki et al, "Chirping Compensation Using a Two-Section Semiconductor Laser Amplifier", Journal of Lightwave Technology, vol. 10, No. 9, Sep. 1992, New York, US, pp. 1247-1254.
Patent Abstracts of Japan, vol. 16, No. 301 (P-1379) 3 Jul. 1992 & JP A 04 081 724 (NEC).
Sherlock Gerard
Tatham Martin Christopher
British Telecommunications public limited company
Negash Kinfe-Michael
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