Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1998-07-23
2000-10-03
Chan, Jason
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359161, H04B 1000, H04B 1016, H04B 1018
Patent
active
061281182
DESCRIPTION:
BRIEF SUMMARY
This invention relates to optical transmission systems and to methods of compensating for dispersion and non-linear effects in such links.
It is now widely accepted that optical fibre transmission systems represent the most likely future for telecommunications networks. Optical fibre is now in use in all sectors of telecommunications networks, for example in the TAT-12 trans-Atlantic cable and in core networks that interconnect cities. Optical fibre is even being deployed in the "local loop" as high bandwidth services such as ISDN and video on demand are being supplied to small businesses and residential customers. The primary advantage of optical fibre transmission systems over copper, coaxial, radio and satellite transmission systems is the combination of low attenuation (approximately 0.2 dBkm.sup.-1) and colossal bandwidth (theoretically hundreds of terahertz, with tens of gigahertz currently achievable). Unfortunately in standard telecommunications optical fibres the transmission window (centred on 1550 nm) that offers low attenuation has high dispersion, and the transmission window (centred on 1310 nm) that has zero dispersion also has relatively high attenuation. The combination of optical fibre transmission systems with optical amplifiers, e.g. erbium doped fibre amplifiers with the 1550 nm transmission window, has enabled the maximum span of optical fibre links to be greatly increased but has only replaced attenuation with dispersion as the limiting factor to the performance of 1550 nm optical fibre transmission systems.
When an optical pulse of nominal wavelength .lambda..sub.0 is launched into an optical fibre it will occupy a range of wavelengths defined by .lambda..sub.0 .+-..delta..lambda.. This can also be regarded as an optical signal with an angular frequency defined by the range .omega..sub.0 .+-..delta..omega.. As the optical signal propagates along the length of the fibre the optical pulse undergoes dispersion, leading to a broadening of the optical pulse. The pulse now occupies a range of wavelengths defined by .lambda..sub.0 .+-..DELTA..lambda.(or .omega..sub.0 .+-..DELTA..omega. in the frequency domain), which is greater than .lambda..sub.0 .+-..delta..lambda.. In optical fibre transmission systems adjacent bit pulses representing "ones" or "zeroes" may broaden until they begin to overlap, leading to inter symbol interference (ISI). As the level of ISI at a receiver increases, the bit error rate will increase accordingly due to the difficulty of interpreting the overlapping pulses.
The electrical field that constitutes the optical pulse can be described by constant along the z-axis, which runs parallel to the length of the fibre. The development of the optical pulse envelope is governed by Schrbdinger's non-linear equation ##EQU1## where .beta..sub.2 is the group velocity dispersion, which is dependent on the wavelength of the signal, .gamma. is the non-linear effect coefficient, .alpha. is the linear attenuation of the fibre and g(z) represents the amplification necessary to compensate for this attenuation.
The second term on the left-hand side of equation [2] determines the dispersion that affects the optical pulse. The first term on the right-hand side of equation [2] determines the extent of non-linear effects that will affect the transmission of the optical pulse, with the non-linear effect coefficient .gamma. being given by ##EQU2## where n.sub.2 is the non-linear index and A.sub.eff is the effective cross-section of the optical fibre. The second term on the right-hand side of equation [2] represents the absorption of the optical signal along the length of the fibre and the third term on the right-hand side of equation [2] represents the signal amplification necessary to compensate for this attenuation with g(z) given by ##EQU3## where z.sub.k is the position of the k-th amplifier and G.sub.k is its power gain. Thus, it can be seen from equation [2] that the loss of power in the optical fibre due to attenuation can be overcome by the suitable use of amplifiers, for example opti
REFERENCES:
patent: 5365362 (1994-11-01), Gnauck
Laming, Richard I. et al., "Transmission of 6 ps Linear Pulses Over 50 km of Standard . . . ", IEEE Journal of Quantum Electronics, vol. 30, No. 9, Sep. 1, 1994, pp. 2114-2119.
Marcenac, D.D. et al., "40Gbit/s Transmission Over 406km of NDSF Using Mid-Span . . . ", Electronics Letters, vol. 33, No. 10, May 8, 1997, pp. 879-880.
Royset, A. et al., "Linear and Nonlinear Dispersion Compensation . . . ", IEEE Photonics Technology Letters, vol. 8, No. 3, Mar. 1, 1996, pp. 449-451.
Bello Agustin
British Telecommunications
Chan Jason
LandOfFree
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