Nonlinear pulse propagation and saturation gain grating phenomena in optical fiber devices

Abstract: Nonlinear effects are known to influence the propagation ofoptical waves in fibers already at modest power levels. Thisprovides us with a unique tool to observe nonlinear phenomenaunder comparatively simple experimental conditions and alsoopens the door to numerous important applications.One of the most interesting nonlinear phenomena is theformation of different types of optical solitons - opticalpulses that can propagate in a dispersive medium withoutbroadening. Solitons play an important role in the developmentof ultra-high speed optical communications and in thegeneration and control of ultrashort optical pulses. Thistheses deals with different theoretical aspects of nonlinearpulse propagation and generation of optical solitons.When sub-picosecond optical solitons propagate in a fiber,they experience a dynamic nonlinear response that gives rise tointrapulse Raman scattering. We analytically derived theself-frequency shift of optical solitons in such a medium andshowed that solitons continuously radiate a part of theirenergy into dispersive waves, which gives rise todelocalization of the optical wave.Considerable attention in this thesis is paid to the theoryof soliton generation and propagation in nonlinear amplifyingmedia. The stability of the pulse generation in passivelymode-locked lasers with fast saturable absorption wasanalytically analyzed by using the complex Ginzburg-Landauequation. The same equation was employed to model modulationalinstability fiber lasers with intracavity Fabry-Perot filter,which generate a train of optical solitons at a high repetitionrate. By taking into account the effect of dynamical gainsaturation on the ultrashort pulse generation in lasers, wediscovered a new type of self-accelerating dissipative solitonsthat have a continuous frequency drift, but a stationaryintensity profile in broad band gain media. We also showed thatthe existence of self-accelerating dissipative solitons maylead to peculiar transient processes in the pulse propagation,such as self-tuning of optical solitons to the zero-dispersionpoint and periodic interconversion of optical solitons.The fiber nonlinearity may also induce detrimental effects,for instance pulse-shape distortions that usually rise duringcompression of chirped pulses. We proposed a new method basedon time frame transformation for efficient simulation of strongcompression of chirped pulses in optical fibers.Another interesting nonlinear phenomenon is the formation ofsaturation gain gratings in amplifying media. In the last partof the original research work in this thesis, we developedmodels for RE-doped DFB fiber lasers that include the effect ofsaturation gain gratings and pump depletion and showed thatthese effects significantly influence the laser performance. Wealso derived an analytical expression for saturation gaingratings in amplifying media with arbitrary diffusion rate ofexcited states. This solution constitutes a "bridge" betweenthe two known limiting cases of week and strong diffusion, andprovides a useful tool for correct analysis of single-modelasers.Keywords:Optical fibers, modulational instability,optical solitons, intrapulse Raman scattering, self-frequencyshift, complex Ginzburg-Landau equation, mode-locking,distributed-feedback lasers, saturation gain gratings.