Optical waveguides – With optical coupler – Plural
Reexamination Certificate
1999-01-15
2001-05-08
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S044000, C385S046000
Reexamination Certificate
active
06229935
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of Invention
This invention relates to dispersion slope compensation in optical transmission systems and more particularly to compensation in wavelength division multiplex systems (WDM) having one or more branches.
2. Description of Related Art
In a conventional WDM transmission system such as is shown in
FIG. 1
a transmitter
10
is arranged to provide on the trunk fiber
12
different traffic signals each on a different wavelength which wavelength is intended for receipt by a specific receiver which may be located at the end of a branch from the trunk. Such systems normally employ optical amplifiers/repeaters
14
at spaced locations along the trunk to compensate for signal attenuation with distance along the trunk. Such systems employ dispersion shifted optical fiber (DSF) with the channels located in the negative dispersion regime (with the channel wavelengths shorter than the wavelength of minimum dispersion, &lgr;
0
, of the fiber). One method, known to us, of compensating for dispersion occurring on the trunk is for the net dispersion to be periodically equalised using non-dispersion shifted fiber (NDSF)
16
with a &lgr;
0
of around 1300 nm (positive dispersion regime). The system can only be equalised to a net dispersion zero at one particular wavelength, without splitting the channels and individually equalising them (very complicated). Consequently the other channels will accumulate additional dispersion depending upon the wavelength offset from the net &lgr;
0
and the dispersion slope of the transmission fibre. This differential dispersion is not reset by the equalisation procedure. The effect of dispersion on the longest and shortest wavelengths at two locations along the trunk are illustrated by
FIGS. 1
a
and
1
b
whilst the effect of compensation is illustrated by
FIG. 1
c.
U.S. Pat. No. 5,224,183 describes a WDM signal compensation system in which dispersion is under compensated and the remaining compensation is carried out via separate means per channel either at the transmitter or receiver end. This adds considerably to the complexity of the system.
BRIEF SUMMARY OF INVENTION
This invention seeks to provide a compensation system and method which provides improved compensation in a simple manner.
According to one aspect of the invention there is provided a WDM optical transmission system having an optical fibre trunk with one or more branching units characterised in the provision of an add/drop channel, having means for pre-dispersing the wavelength of the add channel with a dispersion characteristic of opposite sign to the dispersion characteristic which occurs in the trunk. By employing a dispersion characteristic of opposite sign to the dispersion occurring in the trunk, compensation for dispersion of that wavelength occurring along the trunk is achieved.
There may be provided in the branch drop channel means for dispersing the drop wavelength with a dispersion characteristic of opposite sign to dispersion of that wavelength occurring in the trunk.
One possible implementation of the system is that the means for pre-dispersing the wavelength of the add channel, or the add and drop channels, is the add fibre, or add and drop fibre itself, which is/are chosen to have a required dispersion characteristic.
In an alternative implementation of the system the means for dispersing the wavelength of the add channel, or add and drop channel comprises a dispersion compensation fibre element of opposite dispersion characteristic coupled in line in the add channel fibre or a compensation fibre element in each of the add and drop channel fibres.
In yet another alternative implementation of the system the means for pre-dispersing the wavelength of the add channel comprises a common fibre path which provides bi-directional compensation. The fibre of the common fibre path may itself be chosen to have the required dispersion characteristic to compensate for dispersion of the wavelength of the add channel in the trunk or the fibre of the common fibre path may include a dispersion compensation element of opposite dispersion characteristic coupled in line in the common fibre to compensate for dispersion of the wavelength of the add channel in the trunk. The common fibre path may be coupled with the drop and add channels via a three port circulator. The common fibre path may be coupled with a transmitter and a receiver of a branch terminal via a three port circulator.
The system may include a chromatic dispersion compensator provided in the trunk prior to the branching unit and the dispersion compensator may be arranged to compensate the intermediate channel wavelength with the drop channel wavelength being chosen to be an upper or lower wavelength.
According to another aspect of the invention there is provided a method of compensating for dispersion occurring in the trunk of a WDM optical transmission system comprising the step of applying to a wavelength to be added to the trunk from a branch a dispersion of opposite sign to the dispersion of that wavelength occurring on the trunk.
In order that the invention and its various other preferred features may be understood more easily, some embodiments thereof will now be described, by way of example only, with reference to the drawings, in which:
REFERENCES:
patent: 5224183 (1993-06-01), Dugan
patent: 5978131 (1999-11-01), Lauzon et al.
patent: 195 16 439 A1 (1995-11-01), None
patent: 0 732 819 A2 (1995-03-01), None
patent: 0 658 988 A1 (1995-06-01), None
CLEO '95, Conference on Lasters and Electro-Optics, Opt. Soc. America, May 1995, Baltimore, MD, pp. 92-93, XP000616828, D. Simeonidou et al.: “Effect of Local Fiber Dispersion on the Performance of Long Distance Transmission Systems With Erbium-Doped Fiber Amplifiers”, see p. 92, column 2.
Patent Abstracts of Japan, vol. 96, No. 002, JP 08 054525 A (Furukawa Electric Co Ltd., Feb. 27, 1996.
Jones Kevan Peter
Simeonidou Dimitra
Alcatel
Palmer Phan T. H.
Ware Fressola Van der Sluys & Adolphson LLP
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