Raman compensation techniques for wavelength-division-multiplexed soliton-based optical fiber communications

The discovery of bright soliton propagation in single-mode optical fiber has brought out the possibility of using solitons for high-capacity, long-distance repeaterless fiber communication. In this report we study the loss compensation technique for wavelength-division-multiplexed multichannel soliton-based optical fiber communications. The loss compensation in fibers can be obtained by amplifying the decaying signals via the effect of stimulated Raman scattering. For a soliton-based communication system, the amplification can be obtained by periodically coupling pump waves with frequencies higher than those of channels into the fiber. It is expected that by properly choosing the pump wavelengths and coupled pump intensities, the soliton energy can be recovered at the end of each amplification period where the pump waves are coupled into the fibers. In this situation, the pulse shapes of the signals can still be soliton-like after long-distance propagation. When multiple channels and multiple pump waves are considered, we have to consider not only the stimulated Raman scattering between the pump waves and channels but also those among the channels and among the pump waves. This report provides a systematic discussion of the loss compensation techniques with various pump wave numbers. Because the pumping parameters are dependent on the choice of channel wavelengths, it is impossible to design a set of pumping parameters valid for a variety of multichannel systems. A two-channel and a three-channel system are considered for demonstrative purposes. It is found that many parameters, such as channel wavelengths, pump wavelengths, pump intensities and soliton widths, must be properly chosen when the pump wave number is lower than the channel number. This complicated choice and other difficulties make such a compensation technique not attractive. However, when the pump wave number is equal to or higher than the channel number, the compensation technique is quite promising. For any given channel wavelengths, the pump wavelengths and intensities can be easily chosen for the compensation for all channels. Several numerical examples are demonstrated. Numerical simulations are also given to check the computed pumping parameters. The beam-propagation method is used for these numerical simulations.