Studies On Diffraction Efficiency Of Multilayer Diffractive Optical Elements

The technology of multilayer diffractive optical elements (MLDOEs) is one of the frontier subjects in the field of diffractive optics. MLDOEs can achieve very high diffraction efficiency within a wide spectrum, which can solve the problem of the traditional single layer diffractive optical elements with the diffraction efficiency decreases significantly when the wavelength deviates from the design wavelength. In this paper, based on the scalar diffraction theory and the phase grating equation, the diffraction efficiency of diffractive optical elements is analyzed. By discussing the relevant parameters such as the microstructure height, the choise of the substrate materials and so on, the diffraction efficiency of single layer diffractive optical elements with oblique incidence is analyzed. The achromatic and athermal properties of diffractive optical elements, the narcissus intensity evaluation for diffractive optical elements are analyzed. The third-order aberration theory of diffractive optical elements is discussed.The design method of harmonic diffractive optical elements is analyzed. The special dispersion property of harmonic diffractive optical elements and the diffraction efficiency of harmonic diffractive optical elements which is different from tradional single layer diffractive optical elements are discussed. Based on the diffraction efficiency with oblique incidence of single layer diffractive optical elements, the relationship and the mathematics model of the diffraction efficiency and incidence angle for MLDOEs are built up. The relationship of diffraction efficiency with different incident direction is also analyzed. The diffraction efficiency of MLDOEs, not only the doulble layer diffractive optical elements, but also the three layer diffractive optical elements is systematically studied with different incident directions, different substrate materials and different selection of the third materials with oblique incidence angle.The manufacturing errors of depth-scaling, periodic width and so on have a direct influence on diffraction efficiency of diffractive optical elements. Based on the analysis of single layer diffractive optical elements, the effect of the manufacturing errors on diffraction efficiency for MLDOEs is analyzed. The relationship of diffraction efficiency and depth-scaling errors are presented for two different cases, with the two relative depth-scaling errors change in the same sign and in the opposite sign. As the depth-scaling errors change consistently, the corresponding diffraction efficiency declines relatively slow. This conclusion can be used to guide the manufacturing of diffractive optical elements, realize the maximization of diffraction efficiency with the effect of manufacturing errors.The experiment method of measuring diffraction efficiency of diffractive optical element is studied. Based on the definition of the diffraction efficiency of diffractive optical elements, the measuring principle of diffraction efficiency is discussed, and the double path measurement device of diffraction efficiency is set up. The diffraction light of the main order can be obtained by the reasonable choice of the pinhole aperture of the measuring device, and the subprime diffraction light will be filtered out, ensuring the accuracy of the measuring results. The diffraction efficiency of the single layer diffractive optical elements in the designed hybrid refractive-diffractive optical system is measured at three laser wavelengths over the visible waveband. By comparing the measuring results with the theoretical calculating results, the double path measurement device can be used to measure the diffraction efficiency of diffractive optical elements with high accuracy.In order to verify the relationship of the diffraction efficiency and the incidence angle for MLDOEs, a MLDOEs and an optical system contained this MLDOEs are designed and manufactured. This optical system is used to measure the diffraction efficiency within the incidence angles in the ranges of0°to30.6°, and the measured diffraction efficiency of MLDOEs under different incidence angles are obtained which can be used to verify the theoretical diffraction efficiency. The relationship of the measured diffraction efficiency and the incidence angle is in correspondence with the relationship of the theoretical diffraction efficiency and incidence angle.