| The content of this paper is divided into two parts, the first part is design and experiments of diffractive optical elements(DOE) in beam shaping, the second part is analysis of sub-wavelength diffractive grating in super resolution.The first part of the thesis worked on diffractive optical elements in beam shaping. Diffractive optical elements are widely used in the engineering application, they are in different system environment, the aim of our reserch is to optimize the design of diffractive optical elements to accommodate different system, so that the beam shaping effect can meet the engineering need certainly. Due to the subelements of the DOE we studied are in the hundreds or several tens of microns order of magnitude, so the design method is based on the scalar diffraction theory. The main contents and results are as follows:A, Influence and solutions of reflection of the surface of DOE in high power laser system.In inertial confinement fusion (ICF) systems, the high power laser system made a special request to the optical element, the intensity modulation problems caused by surface reflection of the DOE must catch our attention, because severe intensity modulation of the laser beam can deteriorate the light beam, even damage other optical components. We calculated the reflected light field distribution of the 320mm diameter DOE with the Fresnel diffraction theory, due to the reflected field was modulated by the phase of the DOE, there would be some larger intensity extremum points, the energy of these points can’t be ignored. The usual solutions are surface antireflection on optical element and small angle tilted, but there is no appropriate process of surface antireflection to multi step surface DOE at present. So we made small angle inclined of the DOE to avoid the reflected light to reach to upper level element directly. Under the DOE tilted circumstances, the change of phase was analyzed and the distribution of the focal spot light field inclined was calculated, we found the smoothing of the focal spot would be damaged to some extentand since stronger highlights appears in the center of the focal spot. In order to verify the simulation results, the existing 70mm diameter test piece was tested. The test results show that energy gathered in the center of the focal spot, mainly consistenting with the simulation result. Subsequently, the phase compensation method was presented, in this method the phase was adjusted and made be equal to the phase previous. The simulation results show that the beam shaping effect of the tilted DOE was consistent with the original design results by using the phase compensation method, the focal spot quality can be retained. B, Design and experiment of beam shaping elements in dual lens system.In different systems, DOE will be in different front-end environment, sometimes there is a single lens, sometimes there is lens group. For example, in laser direct writing device, there are dual lens in front of DOE. Lens group has its own advantages design, on the one hand, it increased the desigh freedom, on the other hand it could reduce energy loss, because one more sampling window was calculated, the relationship between the input sampling window and output sampling window would be decided by the distance between the two lens, so that the proportion of focal spot for the output window would be reduced, the higher order diffraction spots could be suppressed.In the dual lens system, the transmission of light is not a Fraunhofer diffraction so that can’t be expressed by a Fourier transform, but can divided into two Fresnel diffraction. Based on the beam propagation process, we improved the―I-O‖iterative algorithm, which adapted dual lens system design, and the iterative process was presented. The DOE was fabricated, then we used several test methods to evaluate the effect. First, we used photographic paper to accept the focal spot, sampling results showed the clear contour shape; in order to accurately know the shaping result, we used the continuous weak light source with CCD receiving shaping after the focal spot, tests showed focal spot has a sharp edge, which is consistent with the laser direct writing device technical requirement; finally we measured the diffraction efficiency with energy meter, the measurement results is about 70-80%, based on previous experience in the production of DOE, this result is normal, and it can meet the demand of a certain project. The second part of the thesis is about the sub-wavelength metal gratings used in super resolution study. Since the concept of―perfect lens‖has been proposed, related research has become a hot area. The resolution of the far-field superlens (FSL) in experiment reach to tens of nanometers level, its imaging process consists of two main parts : Firstly, enhanced the evanescent field by surface plasmon resonance effect using silver thin film; then, converted enhanced evanescent field to propagation field by the sub-wavelength grating. In the second part, design of appropriate sub-wavelength grating to meet the -1 order transmittance is far greater than 0 transmittance is one of the key problems. At present, FSL only realized one-dimensional imaging in the experimental, because ordinary sub-wavelength gratings can only broaden the spectrum of light field on one direction. If we can design appropriate concentric circle grating which diffraction characteristics can meet the request of converting evanescent field into the propagation field, the two dimension FSL imaging may be more feasible. It is the motivation of the research.We need use the vector diffraction theory to analyse the diffraction characteristic of the sub-wavelength DOE. To the grating, such periodic structure, rigorous coupled wave analysis (RCWA) method was introduced firstly, and then RCWA method in a cylindrical coordinate system was deduced;finally,the diffraction characteristics of concentric circle grating was analysed. Analysis results show that the circular gratings can be in a certain range of incident angle to meet FSL requirements... |
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