There are certain practical issues in realizing solitonic communication.

•  Initial Pulse shape

In practice however, the optical pulses do not have this special shape. This is not a problem, since in presence of non-linearity, the pulse has self re-shaping capability. However, during reshaping of the pulse, the power gets distributed, and it is possible that after re-shaping the peak power may not be sufficient to give N=1. In that case the soliton will not be formed and the pulse will disperse for ever.

If initially the pulse power is adjusted to get N > 1, then after re-shaping the pulse will still have enough power and the fundamental soliton will be formed.

The solitons therefore can be easily observed inside a single mode optical fiber.

•  Fiber Loss

One of the major issue related to the solitonic communication is the fiber loss. In presence of loss, the pulse power reduces making the non-linearity weaker. The soliton then loses its shape quickly and the pulse starts dispersing.

There are various possibilities for the loss compensation.

•  The optical power can be increased at the beginning of the fiber link such that at the receiving end, the power is still enough to make N > 1. This however makes the initial power very and higher order soliton may get formed which has much distorted pulse shape.

•  Provide lumped periodic amplification along the fiber length. This is more feasible since the power does not increase to make N > 2, and the same time the power does not reduce to make N < 1. The soliton may change the shape during propagation since the non-linearity and the dispersion does not perfectly cancel each other at every point on the fiber, but overall the soliton may survive over a long length.

•  Provide distributed amplification so the pulse power is maintained more or less constant through out the propagation. Raman amplification is quite suitable for this purpose.

•  Soliton-Soliton Interaction

The solitons behave like charged particles. They show attraction and repulsion between them. If the solitons are close in time, they may get attracted to each other and colloid with each other. However, during the collision they do not lose their identity and they separate again as they travel. The solitons therefore show the pulsating behaviour over distance.

The interaction between the solitons can be avoided if the separation between the solitons is 7-8 times the soliton width.

For soolitonic communication, the data bits should be RZ and the duty cycle must be 10-15%. The data rate is then only 10-15% of the bandwidth used by the data.

•  Effect of Noise

In a long communication link with amplifiers placed periodically (as shown in Fig. ) to just compensate the fiber loss, the ASE accumulates.

The noise figure of the total system can be written as

The ASE superimposed on a soliton leads to a random change in the amplitude and phase of the soliton. The phase change alters the frequency of the soliton in random manner. The frequency change gets converted into the change in arrival time of the soliton due to dispersion. Therefore the ASE leads t a timing jitter of the soliton. This effect is called the Gordan-Haus effect.

The variance of the timing jitter is

Where is the population inversion factor of the amplifier, is the fiber loss, is the dispersion, is the link length, is the effective area of the fiber, and is the soliton width. The units for each parameters are mentioned in the brackets.

For a link 10000Km long, the maximum data rate for a dispersion shifted fiber and amplifier spacing of 50km, is about 10Gb/s.

he 10Gb/s seems to be a upper limit for the data transmission over long haul optical links.