Nonlinear Effects Of Partially Coherent Light In Silica Fibers

High power fiber amplifier systemshave widespread applications in areas such as industrial processing,medicine and deep space optical communication,etc.Compared with coherent light,partially coherent light with phase randomly distributed in the frequencydomainhas advantages of speckle-noise-free imaging and smooth illumination,thus is preferable for the applications of low coherence interferometry,optical coherence tomography(OCT)and optical gyroscopes,and is a promising candidate for the inertial fusion driver.For partially coherent light in fibers,thenonlinear effects such as self-focusing,self-phase modulation(SPM),stimulated Raman scattering(SRS)and stimulated Brillouin scattering(SBS)are still lack of explorations.In this dissertation,thesenonlinear effects for partially coherent light in fibers are studied.Coherent property of light is taken into consideration by utilizing Wigner transform method to solve self-focusing threshold of partially coherent light in large-mode-area(LMA)silica fibers.The formula for self-focusing threshold applicable for both coherent and partially coherent light in LMA silica fibers is derived,which only depends on the beam quality factor M2 and the bandwidth(35)?.It shows that thedecrease of light coherence contributes to the increase of the self-focusing power threshold.This formula can be conveniently used to estimate the self-focusing threshold in experiments and help design partially coherent high power fiber amplifiers.Aphenomenological model for SPM-induced spectral broadening of partially coherent light in silica fibers is presented.The partially coherent light is approximated to a cosine power-modulated light with the modulation period depending on the coherent time and the modulation depth depending on the dispersion factor in fibers.A simple and practical spectral broadening formula is given.Experiments including spectral broadenings of narrowband and broadband incoherent light in silica fibers are performed,which validates our spectral broadening model quantitatively.Based on Wigner transform method to solve the coupled nonlinear Schrodinger equations including Raman effects,SRS evolution of partially coherent light in silica fibers is numerically investigated.It is found that the SRS threshold for partially coherent light is the same as that for the coherent light.We further report the experimental verification that the SRS threshold for partially coherent light can bepredicted by the classical SRS threshold formula.In addition,possible applications based on the SRS effects are put forward,e.g.the elimination of pulse peaks by SRS effects,and the reshaping of pulsesby the interaction of forwardand backward SRS.Since self-phase modulation plays a key role to increasethe SBS critical power and the SRS threshold may be reached before SBS takes place,the classical SBS threshold is revised by considering the spectral broadening and SRS effects.It is found that there is a cut-off bandwidth for the SBS effect.That means when the input bandwidth is larger than the cut-off bandwidth,the SBS effect will be suppressed due to the spectral broadening and the show up of SRSeffect.This phenomenon is observed in the experiment.