Propagation Properties Of Modulated Beams Passing Through The Nonlinear Medium

The light field will be modulated by the defects of the optical elements, leading tothe output beam quality become worse; high-power laser beam passing through themedium may produce the nonlinear self-focusing, which reduces the load capacity ofhigh energy laser device. Therefore, it has become a research hotspot that nonlinearpropagation characteristics of the laser beams which are modulated by amplitude (orphase) type defects. The results of research have a certain reference value for the laserinduced damage, and the safe and stable operation of the optical element in thehigh-power laser engineering.In this academic dissertation, we investigate the light distributions of the highpower flat-topped Gaussion beam through thick the nonlinear medium with defectsand the free space in detail. Besides, the modulation of the defects on the Kerrmedium for the intensity distribution of Gaussian Schell-Model (GSM) beam is alsostudied. Moreover, the nonlinear propagation properties of broadband laser beam areparticularly analyzed. Theoretical analysis and numerical simulations are performedto compare the influence of corresponding parameters on the distribution of opticalintensity. The main content includes the following several aspects:1. Based on the nonlinear Schrodinger equations and split-step Fourier-transform,the intensity distribution of the high power flat-topped Gaussian beam propagating inthe nonlinear media and the free space is studied. The influence of the defects onmedium surface on the intensity evolution of flat-topped light beams in the mediumand the free space is scrutiny analyzed. The results show that the larger of the beamorder, the less of the beam self-focusing property, and the position related to themaximal intensity point is closer to the back surface of the medium. A stronger initialintensity, a thicker medium or a higher nonlinear coefficient, will lead to a strongerself-focusing property. The defect on surface of medium will produce a relativelylarge intensity, which is closer to the back surface of media. The effect of the phase modulated defect on the intensity evolution is greater than that of amplitudemodulated defect.2. Using Huygens Fresnel diffraction integral formula, the expression for thecross-spectral density of the Gaussian-Schell model (GSM) beam which is modulatedby the defect of the nonlinear Kerr medium surface is derived. The intensitydistribution of GSM beam through the thin Kerr medium is studied. Numericalsimulation shows that defect leads to emerge extreme intensity point near the rear ofthe nonlinear medium, which is more far away from the medium when the size ofdefect is increased. The partially coherent beam also can converge or diverge afterwhich passing through the medium with positive or negative nonlinear refractiveindex respectively. A longer the spatial coherence length, a greater the additionalphase shift in the medium or a bigger the size of defect, will lead to a more seriousmodulation for the light intensity.3. Based on the general nonlinear Schr dinger equation and the split-step Fouriermethod, the intensity distributions of a high-power broadband laser beam passingthrough a nonlinear optical medium with defects and then propagating in the free spaceare investigated. The influences of bandwidth of the laser beam, thickness of themedium, and the defects on the light intensity distribution are revealed. We find that thenonlinear optical effect can be suppressed and the uniformity of the beam can beimproved for the high-power broadband laser beam with a proper bandwidth. It is alsofound that under the same incident light intensity of broadband beam, a thicker mediumwill lead to a stronger self-focusing intensity, and that the influence of the defect ofoptical element on the intensity is stronger for the beam with narrowband than that withbroadband.