Feature of Optical Soliton Sequence Propagation in Single-Mode Fiber

Estimates of information capacity of the optical system as product of maximum data rate in single soliton channel and long span distance are considered. The linear (fiber group velocity dispersion and optical losses) and nonlinear (self-phase modulation and Raman self-frequency shift) effects, the signal/noise ratio of pulse source are included in pulse evolution description. It was shown, that maximum optical system capacity can be achieved with NZDS-fiber.


Introduction
One important parameter of the fiber optic data system is the information capacity defined as the product of the bit rate B by the transmission distance z (the length of the span). Information capacity of a data transmission system directly depends on symbol pulse width -T o and their peak power P. Increase in speed of data transmission assumes reduction of duration of symbol pulse: the bit rate B = (Q* T o ) -1 , where Q-is on-off time ratio, that leads to broadening of a spectrum of symbol pulse that accelerates dispersive broadening of symbol pulse, on the one hand, and at the high power causes manifestation of nonlinear effects in the fiber light guide with another. To achieve bit rate 10 Gbit/s and more the symbol pulses of the picosecond and subpicosecond width must be used. Optical pulses of the subpicosecond width experience a strong dispersion broadening when propagated over the fiber-optics link. The dispersion compensation fibers (DCF) must be used at the end of the link to compensate accumulated dispersion. But DCF usually makes a big optical loss. To compensate the optical loss can be used optical amplifiers. But optical amplifiers make additional noise, so the signal-noise ratio gets worse. Therefore, dispersion compensation methods at the physical level are of interest for the high bit rate long haul fiberoptics systems. Such nonlinear effects as the self-phase modulation can be used to compensation the optical symbol pulse dispersion broadening.
Nonlinear effects of different nature have different effects on momentum evolution [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. The selfphase modulation can be used to compensate for the dispersion expansion of the pulse in the region of abnormal dispersion. However, other nonlinear effects may have a negative effect on bit pulse dynamics. Thus, at the subpicosecond width of the symbol pulse, Raman self-scattering can have a significant influence on its evolution. The features of propagation of optical pulses of subpicosecond width in the fiber considering both linear (fiber dispersion and optical losses) and nonlinear (self-phase modulation, Raman self-frequency shift) effects, as well as influence of noise of the source of these pulses are considered. It is shown that it is possible to optimize the parameters of the fiber-optics communication system in order to increase its information capacity.

The features of propagation of optical soliton of subpicosecond width
The solitons can be used as the symbol pulses in bit stream in order to overcome the dispersion limitation. The initial balance between the dispersion and nonlinearity represents the fundamental optical soliton when the initial power P o of the solitons:

Results
The information capacity defined as the product of the bit rate B by the transmission distance z for soliton system is presented in Figure 2. When using NZDS fibers with a symbol pulse width T o nearly 1 ps, the maximum information capacity 15 TBit/s • km is achieved. a b Figure 2. (a, b).The information capacity B*z in the fiber with biased dispersion with  2 = 2 ps 2 /km (a) and with biased dispersion with  2 = 18 ps 2 /km (b) as a function of initial pulse width T o .

Conclusion
It is shown that for fiber with predetermined parameters, it is possible to determine a range of initial symbol pulse durations T o at which it is possible to realize a maximum range z at a high bit pulse rate B such that the time jitter cased source amplitude fluctuation will be small.