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.

In a conventional WDM transmission system such as is shown in Figure 1 a transmitter 10 is arranged to provide on the trunk fibre 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 fibre (DSF) with the channels located in the negative dispersion regime (with the channel wavelengths shorter than the wavelength of minimum dispersion, λ ₀ , of the fibre) . 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 fibre (NDSF) 16 with a λ 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 λ 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 Figures la and lb whilst the effect of compensation is illustrated by Figure lc.

This invention seeks to provide a compensation system and method which provides improved compensation.

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 each providing an add/drop channel, wherein in the branch there is provided means for pre-dispersing the wavelength of the add channel, prior to routing to the trunk, with a dispersion characteristic of opposite sign to the dispersion occurring in the trunk, thereby to compensate for dispersion of that wavelength occurring along the trunk.

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 pre-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: -

Figure 1 illustrates dispersion compensation in the trunk by a method known to us and previously described, Figure 2 illustrates dispersement compensation in a branch in accordance with the invention,

Figure 3 illustrates one compensation arrangement in accordance with the invention,

Figure 4 illustrates an alternative compensation arrangement in accordance with the invention,

Figure 5 illustrates yet another alternative compensation arrangement in accordance with the invention.

The same reference numerals will be employed for similar components throughout the description. Referring now to Figure 2 there is shown schematically a WDM system similar to that illustrated in Figure 1 and having optional NDSF 16. The Figure shows a branching unit 18 in the form of a wavelength add/drop multiplexer (W-ADM) which is arranged to route a signal carried by a specific wavelength from the trunk onto a drop fibre 20 for onward transmission by a receiver at the end of the branch and to introduce to the trunk a signal carried by the same or a different wavelength from a transmitter at the end of the

branch which signal wavelength is provided on the add fibre 22. For such branched WDM systems that include wavelength multiplex branching units along the trunk (main) cable, the present invention provides means to independently equalise the wavelength channel that is add/dropped at the branching units. Figure 2 illustrates the principle and Figures 2a, 2b, 2c show chromatic dispersion that occurs in the trunk and in the drop and add fibres respectively whilst Figure 2d illustrates the compensation which has been introduced in the drop/add channel. To facilitate understanding, a special case is shown where the channels have been equalised immediately before the branching unit 18. The compensation introduced is arranged so that the differential dispersion which occurs at the end of the system is minimised. The branch wavelength channels that are dropped are preferably from the edge of the spectrum, on both the long and short wavelength ends. This is useful to reduce the total amount of system pre-emphasis (if used) as these channels are further from the gain peak of the amplifiers and so by travelling a shorter distance, the required channel power is reduced. If the system is equalised for the centre of the gain spectrum (middle channels) then the other channels are the ones which suffer the maximum differential dispersion. Consequently, if we pre-disperse the channel when it is in the spur (when it is just a single wavelength rather than part of a multiplex) we can tailor the differential dispersion to any value we like at the end of the system.

The channels undergo differential dispersion, as

before, in the first part of the system and are equalised, on the centre wavelength, just before the branching unit. As a result the dispersion is centred around zero but shows the same accumulated differential dispersion as before. The shortest wavelength channel, in this case, is dropped out of the spectrum in the trunk and detected in the spur. The add channel, at the same wavelength as the drop, is pre- dispersed such that the cumulative dispersion at the output of the branching unit 18 (for the add channel) is the same magnitude but opposite sign of dispersion when compared to the drop channel immediately before the branching unit 18. After transmission through an equal length of line (with the same dispersion characteristic) the shortest wavelength channel is now dispersed the same amount as the centre channel and so will be perfectly compensated at the receiver. This is best illustrated by looking at some example numbers. If we assume the wave lengths are,

λ _s = 1554 nm (shortest wavelength) λ _c = 1558 nm (centre wavelength) λ _L = 1554 nm (longest wavelength)

λ ₀ = 1580 nm (wavelength of dispersion zero of transmission fibre) slope = 0.07 ps/nm ² .km and we have a system of 2000 km, with a branching unit in the middle of the system, which add/drops the shortest wavelength and dispersion compensation (at the centre wavelength) immediately before the branching unit 18. If we also assume that the dispersion of the spur is negligible

when compared to the trunk fibre, we can then compare the case with and without spur equalisation.

Table 1 - Dispersion (no spur equalisation) in ps/nm

As can be seen this is preferable to the case with no add/drop as the add channel is reentered with zero dispersion, so reducing the differential dispersion. It should be noted however, that if the equalisation does not take place immediately before the branching unit, this can also make things very much worse. If we pre-disperse the add channel to have the same magnitude, but the opposite sign then we have,

Table 2 - Dispersion (with spur equalisation) in ps/nm

In a system with more than one W-ADM, we would then add/drop the longest wavelength channel and repeat the procedure to minimise the differential dispersion.

There are a number of ways in which compensating dispersion can be introduced in the spur branch and some possible arrangements will now be described with reference to Figures 3 to 5 which show part of the system of Figure 2 with detail of the spur branch.

In Figure 3 a spur branch terminal has a transmitter 24 and a receiver 26 with a coupler 28 which in the illustration is shown as a three port circulator. The terminal is coupled via a fibre 29 incorporating a dispersion compensating fibre element 30 (of positive or negative dispersion as necessary) to the drop and add lines 20, 22 via a coupler 32 which in the illustration is shown as a three port coupler. The compensating fibre element is used bi-directionally so the same compensation is given to the add channel as to the drop. This technique produces a compensated channel at the receiver in the spur. This system works best with dispersion compensation immediately before the branching unit 18. Instead of using a compensating fibre element the fibre 29 between the couplers 28 & 32 may be chosen to have dispersion characteristics which compensate directly. Figure 4 uses two dispersion compensating fibres 34,

36 one in the receive (drop) fibre 20 and one in the transmit (add) fibre 22. This allows different compensation in the drop path than in the add path if required. Note

that this (or Figure 1) could be used to compensate for any dispersion in the spur fibre (e.g. if NDSF was used ⁾ if desired.

Figure 5 employs spur add and/or drop fibres 22, 20 which are chosen to have a dispersion characteristic which compensates directly.

This may be undesirable for other transmission reasons, but is still possible.

The schemes described so far assume that there has been equalisation immediately before the branching unit 18. Whilst this is probably the easiest to understand, by using arbitrary dispersion at the transmit spur (add) we can set the characteristic independent of where the equalisation is performed. This is demonstrated in the table below which uses the same example system, but with the trunk dispersion equalisation immediately after the W-ADM, to show that this system is not dependant upon the equalisation before the W- ADM.

Dispersion Start of Before After After End of End of of system W-ADM W-ADM W-ADM system system

(no EgM (no EgM (with EoM (no EgM (with Eq".) λ_ 0 -1260 -1260 +280 -980 +560 λc 0 -1540 -1540 0 -1540 0 λ ₅ 0 -1820 -1260' + 280 -1540 0

Differential 0 560 280 280 560 560 dispersion

^• - Add channel dispersed by -1260 ps/nm after modulation. Table 3 - Arbitrary dispersion compensation (dispersion in ps/nm)

So with arbitrary equalisation at the spur, we can achieve the same improvement in the differential dispersion of the channels .

Summary A simple scheme using pre-dispersion is proposed for

WDM systems where the maximum differential dispersion is controlled. This may be of particular importance for high bit rate systems where the limitations of chromatic dispersion are more important due to the smaller pulse widths, but finite dispersion is required to control non¬ linear effects, in particular four wave mixing.