Technology Basics
The discovery of the concept of solitons pre-dates fiber optics by more than 130 years. John Scott Russell discovered the concept of solitons in 1834 when he observed a wave propagate away from the bow of a canal barge without distortion. He followed that wave on horseback for several miles and found that it maintained it’s shape over that extraordinary distance.

In fiber optics, solitons are very narrow pulses of light that have a very specific shape. The shape is chosen so that as the soliton travels down the fiber and interacts with dispersion in the fiber and nonlinearities in the fiber, the initial pulse shape is maintained. A typical soliton pulse shape is shown in Figure 1.

This is fundamentally different from coding schemes such as NRZ. (Non-Return to Zero) As NRZ coded data encounters dispersion and nonlinearities in the fiber, the pulse shapes are steadily degraded until they eventually become unusable. The ability of soliton pulses to travel on the fiber and maintain it’s launch wave shape makes solitons a very attractive choice for very long distance, high data rate fiber optic transmission systems.
 

RZ (Return-to-Zero) pulses are close cousins to solitons. They are similar in that they are very narrow pulses of light, but they do not have the precise control of the pulse shape required with solitons, thus they are easier to generate. The downside is that RZ pulses will accumulate some distortion as they encounter dispersion and nonlinearity in the fiber. So while they may not be capable of as long distance transmission as solitons, they are much easier to create and perform close enough to solitons to be considered a viable alternative.

Figure 1 - NRZ (Non-Return to Zero) Format

Let’s take a close look at NRZ and RZ formats to better understand the distinction. Figure 2 and Figure 3 shows typical NRZ and RZ data patterns. The sequence shown is for a ten-bit data sequence (0100111010). In the NRZ data the level stays low for a zero and stay high for a one. In the RZ case, nothing happens for a zero and a very brief pulse appears for a one. The RZ case is much less cluttered and is better suited for very high data rates.
 

Dispersion Managed Soliton Transmission
Dispersion managed soliton transmission provides advantages over normal methods of dense wavelength division multiplexing (DWDM). The balanced use of nonlinearity and dispersion enables long haul signal propagation without distortion. Higher spectral efficiency and polarization mode dispersion (PMD) tolerance is achieved. Receiver sensitivity and dispersion tolerance increase with inherent compatibility with all-optical regeneration.

Dispersion management and solitons are essential technologies for anyone trying to develop a long-haul, DWDM fiber optic system operating at OC-192 rates (10 Gb/s) or higher. They become especially important when the target fiber is NDSF. NDSF fiber without dispersion management or soliton technology could only support the transmission of OC-192 to a distance of 4 km according to one source. When these technologies are optimally applied the distance can be stretched to more than 1,000 km.

Figure 2 - NRZ (Non-Return to Zero) Format

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Figure 3 - RZ (Return to Zero) Format

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