Study On The Key Technology Of Mosaic Grating Exposed By The System With Wave Aberration

Chirped pulse compression grating is the key element in the super fast laser system. When the high power laser system is to be upgraded, the chirped pulse compression system needs large-scale gratings. It is difficult to make large-scale grating by single exposure. So mosaic gratings produced by multiple exposures become an alternative method to obtain large diameter gratings. There is a strict requirement for the diffraction wave front aberration of the pulse compression gratings. So the stability of large aperture exposure system and its wave aberration are the major problems that should be solved.The background and research status of the large gratings are introduced in the article. The significance of the large grating is pointed out. Several methods of making mosaic gratings are introduced. Their advantages are analyzed as well as the disadvantages.The energy distribution of the far-field diffraction spot is the main baisis for evaluating mosaic grating. According to Kirchhoff diffraction theory, the diffraction wavefrontans and the energy-distribution of the far-field spot are simulated which are affected by the mosaic error. A large number of wavefronts with aberration are used to simulate the aligned-wavefronts, and their Peak to Vale values are obtained as well as the Root Mean Square values at the mosaic gap. The aligned-wavefronts and the far-field diffraction spots’characteristics are analyzed statistically, and the grating’s mosaic accuracy is pointed out. It provides the reference for experiments. A new method named parallel-mosaic wavefront is proposed in order to enhance the energy distribution of the far-field diffraction spot. The statistical PV value of the parallel-wavefronts is also analyzed as well as the far field spots’characteristics. Although it is difficult to confirm that parallel-wavefront is better than aligned-wavefront for some aberration. Statistically the peak to vale value of the parallel-wavefront is smaller and the far-field diffraction spot’s energy distribution is better than the aligned-wavefront.The mosaic-error and the stability of the large aperture exposure system are studied based on the latent-grating mosaic method. A three-dimensional fringe locking system is designed to solve the proboems. In addition three kinds of locking mode are put forward for the distoration of the interference fringe, which is affected by the wavefront aberration. It is pointed out that the period, phase and tilt errors between optical-grating and reference-grating change with the environment. Three-dimensional fringe locking system is tested by experiments, and the results show that the stability of holographic exposure is improved as well as the exposure contrast.In the condition of wave aberration, a mosaic method is presented to produce large sacle grating. The mosaic-error caused by wave aberration is analyzed. A mothod is proposed to prefabricated interference fringe in order to compensate the mosaic-error caused by wave aberration. In order to improve the energy distribution of far-field diffraction spot, two experiment plans are formulated which are named Aligned-Mosaic and Parallel-Mosaic. The mosaic gratings are produced by three-dimensional fringe locking system in the condition of different aberrations. The results show that the wave aberration has effect on mosaic-error. The prefabricated reference fringe can compensate the mosaic errors caused by wave aberration. The mosaic accuracy is less than 0.05γ. Finally, it is conformed that the energy distribution of far-field diffraction spot formed by Parallel-Mosaic grating may be better than the Aligned-Mosaic grating.