# Patent

## Title
Dispersion management system for soliton optical transmission system

## Abstract

A dispersion management system for soliton or soliton-like transmission systems comprises a length of optical fiber (L) in which a plurality of sections (I) made up of components (N,A) of opposite sign dispersions are concatenated together. The duration of the dispersion compensation phase is short in comparison with the propagation interval in the remainder of the system and that the path average dispersion is anomalous.

## Background
This is a continuation of PCT application No. PCT/GB96/02923, filed 27 Nov. 1996.


BACKGROUND
This invention relates to optical communications and, in particular, to optical communications systems in which information is transmitted by soliton or soliton-like pulses.
Laboratory demonstrations have recently been reported of soliton transmission in systems where the dispersion was not uniformly anomalous along the fibre, instead being periodically compensated by fibre of opposite (normal) sign dispersion. In this manner transmission was achieved at 20 Gb/s over 9000 km in a recirculating loop, and 8100 km in a straight line experiment. These figures are substantially in excess of what has previously been achieved without the use of soliton control techniques such as sliding filters and synchronous modulators. While it is thus clear that there are significant benefits to be gained from adopting dispersion management in soliton systems, to date there has been little conceptual explanation of the mechanisms behind this improvement.
BRIEF SUMMMARY OF THE INVENTION
The correct selection of dispersion is a critical issue in the design of amplified long haul optical communication systems. In the case of soliton formatted data, it is dictated by compromise between the desire to minimise timing jitter problems (implying low dispersion), and the need to maintain adequate energy per bit for successful detection. As the energy needed to form a soliton in a uniform fibre is proportional to the dispersion, the latter constraint places a lower limit on the permitted dispersion. Dispersion management is a technique in the context of non-return-to-zero (NRZ) formatted data in which fibres of opposite sign dispersions are concatenated together. This produces a high local dispersion at any given point, and yet a low path-average dispersion. We have found that, by adopting a suitable dispersion management scheme for soliton or soliton-like transmission, it is possible to increase the soliton energy substantially compared with the equivalent uniform fibre with equal path-average dispersion.
According to the present invention there is provided a dispersion management system for soliton or soliton-like transmission in which the duration of a dispersion compensation phase is short in comparison with the propagation interval in the remainder of the system.
Preferably the system excludes arrangements in which the dispersion map of one fibre is substantially closer to zero than that of its complementary fibre.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be particularly described with reference to the accompanying drawings in which:
FIG. 1 is a dispersion compensation map;
FIG. 2 shows the pulse profile at the beginning of each unit cell in a dispersion managed system. The dispersion map comprises alternating 100 km fibres with dispersions of −3 ps2/km and +2.8 ps2/km;
FIG. 3 shows the evolution of a over one period of the dispersion compensation cycle
FIG. 4 shows the energy required to launch a 20 ps FWHM soliton in dispersion managed system with alternating 100 km length fibres chosen such that the path-average dispersion remains at −0.1 ps2/km, and
FIG. 5 is an outline diagram of a dispersion management system in accordance with one aspect of the invention.
FIG. 6 is an outline diagram of a dispersion management system in accordance with one embodiment of the invention.



DETAILED DESCRIPTION
Our work is based upon numerical integration of the Nonlinear Schrödinger Equation (NLS), using the dispersion map shown in FIG. 1. The arrangement of a typical system is shown in FIG. 5 and comprises a transmitter T and Receiver R lined by a length L of fibre. This fibre is divided into elements 1 comprising separate sections of fibre N with normal dispersion and fibre A with anomalous dispersion. In a specific embodiment these are of equal length, although the unit cell is defined to start and end at the mid point of one of the fibres. In the specific examples presented, each of the fibres will be 100 km long, and the path average dispersion −0.1 ps2/kM. The nonlinear coefficient was taken to be 2.65 rad/W/km. To simplify the problem we have chosen to neglect loss and high order dispersion throughout.
We have confirmed the existence of quasi-stable soliton or solitary wave solutions to this dispersion map. FIGS. 2 and 3 show the observed behaviour when the dispersion values alternated between −3.0 ps2/km and +2.8 pS2/km, and a 20 ps FWHM Gaussian pulse of peak power 650 μW was launched into the fibre. FIG. 2 shows the intensity profiles at the start of each unit cell; it can be seen that the pulse profile at these points remains unchanged over successive cycles of the dispersion map. The evolution within one unit cell is shown in FIG. 3, the pulse alternately compressing and dispersing as the sign of the dispersion is switched. The power spectrum remains essentially unchanged within the unit cell.
There are three constraints which must be satisfied to obtain stable solutions to the periodic dispersion map. Firstly, the path average dispersion must be anomalous, in order that the Kerr induced spectral broadening can be compensated. Secondly, the period of the dispersion compensation cycle must be short compared to the nonlinear length of the system. For a 1000 Km fibre, the dispersion compensation length is preferably 100 Km or less. Finally, dispersion maps in which one of the fibres is much closer to zero dispersion than the other should be avoided. as otherwise energy is rapidly coupled out of the pulse into dispersive waves.
The advantages conferred by a dispersion management scheme on soliton communications stems from the fact that more energy is required to launch a stable pulse than in the equivalent uniform system with equal path average dispersion. This is demonstrated in FIG. 4, which plots the pulse energy of the stable solution as a function of the difference between the dispersion values of the two individual fibres. Preferably, the difference between fibre dispersions is less than 12.0 ps2/Km and, ideally, less than 4.0 ps2/Km. Under optimum conditions it will be 0.1 ps2/Km or less. Greater differences between the two fibres results in more energy being required to form a stable pulse; we have also found that lengthening the unit cell's period (with a given pair of dispersion values) increases the required energy. The mechanism behind this increased energy requirement can be understood from the intensity profile within the unit cell, FIG. 3. Due to the cycle of dispersive broadening and compression, the peak power of the pulse is generally lower than the initial launch power. Therefore the rate of self phase modulation (SPM) is reduced compared to the equivalent uniform fibre, and so more energy is required to balance the path-average dispersion. In the frequency domain, the process could be construed as a reduction in the efficiency of four wave mixing. of which SPM is a special case.
Another highly novel feature of these solitary waves is that their pulse shapes are not the hyperbolic secants of regular optical fibre solitons. The example pulse profile which we have displayed is almost exactly Gaussian in nature, however this is only a special case for that particular dispersion map. As the dispersion variation is increased there is a transition from the uniform fibre hyperbolic secant soliton (time-bandwidth-product 0.32) to Gaussian (0.44) form, and then to pulse shapes with higher still time-bandwidth-products. An interesting connection can be made at this point with the “stretched pulse” design of mode-locked fibre laser. These incorporate cavities with two opposite signs of dispersion and also produce Gaussian shaped pulses.
In cases of soliton or soliton-like transmission in dispersion compensated fibres employing a configuration with zero path average dispersion, undistorted pulse propagation was obtained in this situation due to the presence of optical filters in the recirculating loop. The stable pulses then arose from balancing SPM against filtering, rather than SPM against the path-average dispersion.
CONCLUSION
The technique of dispersion management has the potential to make a significant impact of the realisation of soliton communication systems. It provides major performance benefits, and has the distinct advantage of requiring only passive components. While, in a preferred embodiment, we have used equal lengths of two different fibres, alternative embodiments may use discrete dispersion compensators fabricated from highly dispersive materials as shown for example in FIG. 6. The adoption of dispersion management represents a convergence between the techniques used in soliton and NRZ formatted transmission.

## Claims

1. An optical communication system for transmitting a soliton or substantially soliton pulse, comprising
a plurality of dispersion elements, each dispersion element from the plurality of dispersion elements including at least a fiber length and a discrete dispersion compensator, the fiber length and discrete compensator having different dispersions, wherein the path average dispersion of the plurality of dispersion elements is anomalous.

2. The optical communication system of claim 1, wherein the discrete dispersion compensator is fabricated from a highly dispersive material.

3. The optical communication system of claim 1, wherein at least one of the discrete dispersion compensators is fabricated from a highly dispersive material.

4. The optical communication system of claim 1, wherein the soliton or substantially soliton pulse has a time-bandwidth product greater than the time-bandwidth product of a Gaussian-shaped pulse.

5. The optical communication system of claim 1, wherein the soliton or substantially soliton pulse has a time-bandwidth product greater than the time-bandwidth product of a Gaussian-shaped pulse.

6. An optical communication system comprising
a plurality of sections, each section including at least two dispersion elements that have dispersions of opposite sign, wherein the plurality of sections permits propagation of a stable or quasi-stable optical pulse, and wherein the optical pulse has a time-bandwidth product greater than a time-bandwidth product of an optical pulse that is Gaussian in shape.

7. The optical communication system of claim 6, wherein the optical pulse alternately expands and compresses as it propagates through the sections.

8. The optical communication system of claim 6, wherein the path average dispersion of the plurality of sections is zero or anomalous.

9. The optical communication system of claim 6, wherein the difference between the dispersion magnitudes of the two dispersion elements is less than 12 ps2/Km.

10. The optical communication system of claim 9, wherein the difference between the dispersion magnitudes of the two dispersion elements is less than 4 ps2/Km.

11. The optical communication system of claim 10, wherein the difference between the dispersion magnitudes of the two dispersion elements is less than 0.1 ps2/Km.

12. The optical communication system of claim 6, wherein the two dispersion elements of a section comprise an optical fiber length and a discrete dispersion compensator.

13. A method of optical communication comprising:
generating a plurality of optical pulses; and
launching the plurality of optical pulses through an optical communication system comprising a plurality of dispersion elements, each dispersion element from the plurality of dispersion elements including at least a fiber length and a discrete dispersion compensator, the fiber length and the discrete dispersion compensator having different dispersions, wherein the path average dispersion of the plurality of dispersion elements is zero or anomalous, such that the optical pulses are transmitted as soliton or substantially soliton pulses.

14. A method of optical communication comprising:
generating a plurality of optical pulses; and
launching the plurality of optical pulses through an optical communication system comprising a plurality of sections, each section including at least two dispersion elements that have dispersions of opposite sign, wherein the plurality of sections permits propagation of corresponding stable or quasi-stable optical pulses, and wherein the stable or quasi-stable optical pulses have a time-bandwidth product greater than a time-bandwidth product of optical pulses that are Gaussian in shape.