Theoretical And Experimental Study On Photorefractive Spatial Solitons And Their Induced Waveguides

Photorefractive (PR) spatial solitons is the result of diffraction balanced the PR effect. For the merits of low optical power, long storage time, two dimensions stable and depend on wavelength, PR solitons are prospective in optical information processing and integrated optics. Focusing on the photorefractive spatial solitons and its waveguides, the author mainly completed the following works:1. The propagation properties of waveguides induced by dark and bright screening photovoltaic (PV) solitons are studied theoretically. Using the shooting method, we show that the number of guided modes in a waveguide induced by a bright screening PV soliton increases monotonically with the increasing intensity ratio of the soliton, which is the ratio between the peak intensity of the soliton and the dark irradiance. On the other hand, waveguides induced by dark screening PV solitons are always single mode for all intensity ratios and the confined energy near the center of a dark screening PV soliton enhances monotonically with increasing the intensity ratio. When the bulk photovoltaic effect is neglectable, these waveguides are those induced by screening solitons. When the external field is absent, these waveguides predict those induced by photovoltaic solitons.2. The propagation properties of waveguides induced by one dimensional grey screening solitons are studied. Solving the soliton equation numerically, we find that waveguides induced by grey screening solitons are always single mode for all intensity rations. On the other hand, our analysis indicates that the energy confined near the centre of the grey soliton and the propagation constant of the guided mode of the waveguide increase monotonically with intensity ratio increasing. When the soliton grayness increases, the energy confined near the centre of the grey soliton and the propagation constant of the guided mode of the waveguide decrease monotonically.3. Based on the beam incoherent character, one dimensional partially incoherent dark PV solitons and its waveguides are studied experimentally. The experimental results show that dark PV solitons can form with partial incoherent beam in noninstantaneous PR crystal LiNbO₃:Fe. The waveguides induced by this type of solitons can guide strong coherent beam. These results prove that the possibility of controlling and switching high-power laser beam with low-power incoherent sources.4. The temporal behavior of screening PV solitons, screening solitons and PV solitons is studied theoretically and experimentally. Based on time dependent band transport model, the evolutional equation of PR solitons is established. Using numerical method, we find that the formation time of screening PV solitons increases with increasing |J₀| for J₀≤0, while decreases with increasing amplitude for J₀>0. Moreover, we find that the formation time of screening solitons decreases with increasing amplitude, while the PV soliton's is relative to the intensity ratio. When the intensity ratio of PV soliton is smaller, the full width of half maximum (FWHM) of PV solitons decreases monotonically to a constant value; when the intensity ratio of the soliton is bigger, the FWHM of PV solitons first decreases to a minimum before it increases to a constant value. We further study the evolution of dark PV solitons in experiments. The results indicated that steady solitons can be observed at low optical power, while quasi-steady-state solitons can only be generated at higher optical power. Good agreement is found between theory and experiment.5. Bright PV solitons in LiNbO₃ crystal with negative refractive index change are studied theoretically and experimentally. For PV constant of PR crystal depending on wavelength, we choose different signal and background beam in theory analysis and experiment. We find that bright PV solitons can be obtained with background illumination forκ> 1, whereκis PV constant of background beam to that of signal beam. We also choose different coherent degree background beam in experiments and get soliton existence curve which has good agreement with theoretical prediction.