Suppression Of The Nonlinear Kerr Effect In Optical Fiber Communication Systems By Dispersion Management And Optical Phase Conjugation

Nonlinear Kerr effect is one of the most serious impairments of optical fiber communications. Dispersion management and optical phase conjugation (OPC) are two techniques of suppression of the nonlinear impairment. Both of them have pros and cons; however, their combination shows great advantages to combat the nonlinear impairments in optical fiber communication systems. In this dissertation, we discuss and compare the two techniques systematically. The efficiency of these techniques for countering the nonlinear effects is investigated, and analytical methods are proposed to design and optimize the systems. Moreover, the realization of the OPC of dispersed ultrashort pulse is investigated theoretically and experimentally.Dispersion management includes tuning the amounts of precompensation, residual dispersion per span (RDPS), and net residual dispersion (NRD). In order to suppress the nonlinear impairments mostly, the above three parameters need be optimized. For self-phase modulation (SPM)-limited systems, optimizing the NRD is necessary because it can improve the system performance greatly. Unfortunately, analytical model for optimizing NRD is lack. Two different analytical methods, i.e., minimizing the width and maximizing the center amplitude of the output pulse, are presented here to optimize NRD for SPM-limited dispersion-managed systems. An analytical expression of optimized NRD is obtained by the method of minimizing pulse width. This method can also be used to optimize all the three parameters mentioned above simultaneously. The method of maximizing center amplitude is simpler and more straightforward comparing to the first method. Addtionally, establishment database with two system parameters, nonlinear phase shift and product of dispersion parameter and square of bit rate, is proposed to estimate the nonlinear impairments of arbitrary single-channel dispersion-managed systems.The performance of dispersion managed systems is still limited by nonlinear penalty, even all the three parameters are optimized simultaneously. OPC is another promising technique, theoretically, through which the nonlinear impairments can be fully compensated. However, through systematical investigation, it is found that the effects of OPC on various nonlinearities are different in a practical system without power-symmetry. Some nonlinearities may even become worse if one of the nonlinearities is fully compensated.Extending the method of minimizing pulse width mentioned above, an analytical model is proposed for SPM-limited systems with nonlinearity compensated by OPC. Using this model, the single-pulse SPM-compensation efficiency of OPC can also be estimated conveniently for any systems.Furthermore, the OPC of dispersed ultrashort pulse is investigated theoretically and experimentally. The phase conjugator is realized by four-wave mixing (FWM) in a highly-nonlinear dispersion-shift fiber (HNL-DSF), and about -16 dB conversion efficiency and 20 nm 3dB conversion bandwidth is obtained experimentally. By use of this conjugator, transmission of a ~300 fs pulse along a 20-km long standard single mode fiber with midway OPC is inverstigated. Additionally, in the OPC experiment, the spectral distortion of phase conjugated-signal is observed. It is found theoretically that this spectral asymmetry FWM is induced by the nonlinear phase modulation from signal pulse and pump. Moreover, the amount of the nonlinear phase shift is found equal to the SPM-induced phase of signal transmitting through the HNL-DSF with 2.5 times length. The investigation of ultrashort pulse OPC is believed to benefit to the use of OPC in wideband, high speed transmission systems.