Propagation Of Ultra-short Pulsed Laser And Its Diffraction Characteristics

Ultra-short laser pulse behaves some characteristics such as the short duration time，highenergy and ultrafast time resolution, so it has been applied in the studies about transientdynamics among physics, biology, chemistry, and so on. As one transmission medium of lightbeam in the optical communication, medical endoscope technology, induction technology,microscopy imaging, and etc, the dispersion and the loss of fiber are two importantparameters restricting optical signal transmission without distortion, so the smaller dispersionand loss of fiber can make the transmission of signal without any loss. The ultra-short pulsecontains rich frequency spectrum, any interference of the propagation medium can affect thewave envelope, and even there is no dispersion and loss of medium, the diffraction of apertureis unavoidable. The grating is one of the most important optical diffraction elements, whichhas been widely used in many areas including information storage and image encoded, thelight wave modulation and the high power laser pulses compression. Because of the spatialperiodicity of the grating, its self-image takes on in Fresnel diffraction region. This is theso-called Talbot effect of the grating. Usually, the diffraction distribution of gratingilluminated by the uniform monochromatic light can be easily analyzed by use of Fresneldiffraction formula. While the light source is partially coherent in time and space, such as theultra-short pulse laser, the partial coherence theory should be adopted to study the diffractionof grating. On basis of the studies about the ultra-short pulse, this paper studies thetransformation characteristic of ultra-short pulse in the fiber and its diffraction characteristicspassing through the grating, and the corresponding theoretical and experimental studies areperformed.This paper is mainly divided into the following five chapters.The first chapter introduces the formation of the ultra-short pulse, the development ofpulse technique and the methods of getting pulse. The structure of the fiber, the classificationand its transmission characteristic are introduced in detail. At the same time, the grating withdifferent structures and the Talbot effect are analyzed theoretically.The second chapter presents the experimental study about the visible and near infraredabsorption of fiber. With the polychromatic light illumination, the propagation loss of opticalfiber is measured according to the fiber truncation method. The absorption coefficient of fiberis obtained through the simple calculation. Several absorption peaks of fiber are consistent with those given in reference. The corresponding explanation for these absorption peaks isalso given.Chapter three studies the propagation of ultra-short laser pulse in the dispersion of theoptical fiber. The temporal signal is changed into the spectrum signal by the Fouriertransformation technique. The theoretic formula of the transformation signal of ultra-shortpulse is obtained. Then the reverse Fourier transform is done, and the temporal signal at anypropagation distance is given. The influence of the different disperse order on the pulseprofile is also discussed by the Taylor expansion of transmission constant. The calculationresults show that the second dispersion order spreads the pulse and the third dispersion orderdeforms the pulse.Chapter four studies the diffraction of high-density grating in deep Fresnel diffractionunder the illumination of monochromatic continuous light experimentally. The diffractionintensity of the grating in this region is detected in practical experiment by use of themicro-magnify technique. The experimental setup is contact and flexible to obtain themagnified diffraction patterns. The experimental results verify the exact Talbot image ofgrating can not appear at the exact Talbot distance. Oppositely, the quasi-Talbot imageappears at the nearer distance close to the exact Talbot distance. These experimental results fitwell with our theoretical results.Chapter five studies the Talbot effect of grating with the ultra-short pulse illumination.The theoretical formulas of the temporal and time-averaged diffraction distributions with theultra-short pulsed laser illumination are obtained on basis of the partially coherent theory. Thecorresponding diffraction distributions are simulated through numerical calculations, and theinfluence of the duration time of incident pulse and the chirp coefficient on Talbot effect ofgrating is discussed. The results show when the effective duration time of incident pulse islong enough, the temporal and time-average Talbot effect of grating appear at the Talbotdistance with respect to the central wavelength. Moreover, when the chirp coefficient issmaller, the effect of the pulsed laser on the Talbot effect of grating is light. At the largerpropagation distance, the diffraction distribution deviates severely from the structure ofgrating. Additionally, the comparison of the diffraction distributions of Gaussian, thesuper-Gaussian and the sech-shaped pulses shows how the diffraction of grating is influencedby the shape of pulse.