How Solitons are used in optical communication?

It used to propagate the light in pulses at high debits into the fiber.

they propogate the light without changing the velocity of light

To understand why optical solitons are needed in optical fiber communication systems, we should consider the problems that limit the distance and/or capacity of optical data transmission. A fiber-optic transmission line consists of a transmitter and a receiver connected with each other by a transmission optical fiber. Optical fibers inevitably have chromatic dispersion, losses (attenuation of the signal), and nonlinearity. Dispersion and nonlinearity can lead to the distortion of the signal. Because the optical receiver has a finite sensitivity, the signal should have a high-enough level to achieve error-free performance of the system. On the other hand, by increasing the signal level, one also increases the nonlinear effects in the fiber. To compensate for the fiber losses in a long distance transmission, one has to periodically install optical amplifiers along the transmission line. By doing this, a new source of errors is introduced into the system—an amplifier spontaneous emission noise. (Note that even ideal optical amplifiers inevitably introduce spontaneous emission noise.) The amount of noise increases with the transmission distance (with the number of amplifiers). To keep the signal-to-noise ratio (SNR) high enough for the error-free system performance, one has to increase the signal level and hence the potential problems caused by the nonlinear effects. Note that the nonlinear effects are proportional to the product of the signal power, P and the transmission distance, L, and both of these multipliers increase with the distance. Summarizing, we can say that all the problems—dispersion, noise, and nonlinearity—grow with the transmission distance. The problems also increase when the transmission bit rate (speed) increases. It is important to emphasize that it is very difficult to deal with the signal distortions when the nonlinearity is involved, because the nonlinearity can couple all the detrimental effects together [nonlinearity, dispersion, noise, polarization mode dispersion (i.e., random birefringence of the fiber), polarization-dependent loss/gain, etc]. That happens when the nonlinear effects are out of control. The idea of soliton transmission is to guide the nonlinearity to the desired direction and use it for your benefit. When soliton pulses are used as an information carrier, the effects of dispersion and nonlin-earity balance (or compensate) each other and thus don’t degrade the signal quality with the propagation distance. In such a regime, the pulses propagate through the fiber without changing their spectral and temporal shapes. This mutual compensation of dispersion and nonlinear effects takes place continuously with the distance in the case of "classical" solitons and periodically with the so-called dispersion map length in the case of dispersion-managed solitons. In addition, because of the unique features of optical solitons, soliton transmission can help to solve other problems of data transmission, like polarization mode dispersion. Also, when used with frequency guiding filters (sliding guiding filters in particular), the soliton systems provide continuous all-optical regeneration of the signal suppressing the detrimental effects of the noise and reducing the penalties associated with wavelength-division multiplexed (WDM) transmission. Because the soliton data looks essentially the same at different distances along the transmission, the soliton type of transmission is especially attractive for all-optical data networking. Moreover, because of the high quality of the pulses and return-to-zero (RZ) nature of the data, the soliton data is suitable for all-optical processing.

Before moving to how Solitons are used in optical communication. 

Firstly most of them has to understand what is a solition?

"a quantum or quasiparticle propagated as a traveling nondissipative wave that is neither preceded nor followed  by another such disturbance." In mathematics & physics, a soliton is a self-reinforcing solitary wave, that maintains its shape while it propagates at a constant velocity.

now let us come back to question.

many researches are going on this but the conclusion which i came across from one of the research is below

" Soliton based optical fiber communication systems, using EDFA's, are more suitable for long haul communication because of their very high information carrying capacity and repeaters less transmission.

These systems are still to be developed for field applications. when transmission demand will increase and device technology will improve, they will be certainly employed in field. by using solition based optical switches multi GBPS data rate can be achieved for optical computation also. 

Hope you get an idea about it... Coz its a long topic can't keep going on writing essays on it... bro...