Study On The Mosaic Errors Judgment Of Holographic Grating Mask Based On Measurement Of The Diffraction Wavefront

Large-aperture diffraction gratings play a vital role in many applications,where the aperture of the pulse compression gratings should reach meter level for satisfying the requirement of high-power chirped-pulse-amplified laser systems.However,it is quite difficult and financially impractical to directly manufacture large-aperture gratings by using single holographic exposure.Alternatively,the difficulty and cost can be significantly reduced by using holographic exposure mosaic technology to obtain the large-aperture grating.In order to avoid the problems existing in the mosaic grating method of latent image,the mosaic grating method of developing region by region is proposed.Nevertheless,the differences of the groove shapes will cause the dislocation between the reflected diffraction wavefronts at gap of the mosaic grating in this method,which will seriously interfere with the judgment of lateral displacement error when experimentally detecting the mosaic error.Therefore,in this paper,the influence of phase differences of diffraction wavefront introduced by the groove differences at the gap of the mosaic grating on the judgment of lateral displacement error is deeply explored and a method for judging the grating mosaic error based on measurement of the diffraction wavefronts is proposed.This paper mainly has the following aspects:1.The research background and status of large-aperture diffraction gratings are introduced,and the existing exposure mosaic methods are briefly described.However,aiming at the problems of the existing methods,the mosaic technology and alignment method of developing region by region are introduced.Because the differences of groove shapes will affect the judgment of lateral displacement error in mosaic grating,it is necessary to calculate the phase difference of diffraction wavefronts caused by groovedifferences.2.According to the rigorous coupled wave theory,the formula for calculating the initial phase of the diffraction wavefront is deduced and the phase calculation program for grating diffraction is compiled.The correctness of the phase calculation program is illustrated by comparing the calculation results of compiled program with those of commercial software as well as experimental results.3.According to the phase shift theorem in grating diffraction,a method for judging lateral displacement error in mosaic grating is proposed based on 0-order diffraction wavefront.The phase difference of the diffraction wavefronts at the gap of grating is firstly obtained by measuring the 0-order diffraction wavefronts with single wavelength and multiple angles or double wavelengths.Subsequently,the groove parameters are searched by the library matching method and the lateral displacement error of the mosaic grating is further deduced.In order to verify the validity of the proposed method above,two mosaic gratings with groove differences are experimentally fabricated and measured,and the results show that it is feasible to deduce the lateral displacement error of the mosaic grating by measuring the diffraction wavefronts with single wavelength and multiple angles.4.The influence of groove differences on multi-layer dielectric grating mask is discussed in detail.The theoretical calculation results demonstrate that the proposed judgment method is also applicable to multi-layer dielectric gratings even though there is a certain film thickness error.By setting the diffraction wavefront and diffraction efficiency as the evaluation criteria,the applicable range of groove shape of the etched multi-layer dielectric grating is calculated.The applicability of the mosaic error judgment method for multi-layer dielectric gratings is experimentally verified.