The Design Of Wide-angle Beam Splitting DOE With Sub-wavelength Structure

Laser beam splitter is a kind of typical diffraction optical elements, which can split an incident beam into multiple uniform beams, so it has very important applications in many optical areas. Sometimes laser beam splitting DOE not only need to satisfy the requirements of high diffraction efficiency and high homogeneity, but also need to achieve the goal of wide-angle beam splitting. According to the diffraction optical theory, to increase the splitting angle it is necessary to reduce the period of DOE.However some problem will arise with the reduction of DOE period, firstly the approximation precision of scalar diffraction theory will be degraded, so it have to using vector diffraction theory to ensure the accuracy of design decisions; then the processing difficulty of DOE will increase, influenced by the machined error the real performance of DOE will be unsatisfactory.The one-dimensional laser beam splitting DOE have three structure forms:continuous type, binary type and multi-level type. There are some wide-angle laser beam splitting studies about continuous phase DOE and binary phase DOE, but both of them have respective shortcomings. For example the continuous phase DOE has very complicate surface structure, so it is really hard to design by vector method and the processing difficulty is huge too; the beam splitting performation of binary phase DOE is sensitive to error, these shortcomings limited their application. Multi-level phase DOE combined the advantage of them and overcomed the disadvantages, so it is the better choice for wide-angle laser beam splitting. Therefore this paper came up with the idea of using sub-wavelength multi-level DOE to achieve wide-angle beam splitting.Multi-level is a kind of periodic binary optical elements, each period is consist of multiple phase units, these phase units formed stepped structure. To modulate the phase of incident beam by these phase units, we can achieve laser beam shaping or splitting.This paper will detailedly analyzes the working mechanism of multi-level beam splitting DOE, and introduces its design principle, scalar diffraction theory, vectorial rigorous coupled-wave theory and relevant algorithms. We proposed using scalar diffraction theory and GS algorithm to design a normalized multi-level structure, this structure will be used as the initial value of Genetic Algorithm and then to do a vector optimization, finally we will acquire an sub-wavelength multi-level structure which can achieve wide-angle laser beam splitting. This paper will gives out some design examples of 1×8, 1×9 and 1×16 beam splitting sub-wavelength multi-level DOE, theirdiffraction efficiency reachs 90% and inhomogeneity under 5%, the angle between the two most marginal diffraction orders approximates to 30°. Whereafter, we put forward a corresponding processing scheme for these sub-wavelength multi-level beam splitting DOE. Finally we made error analysis and working wavelength bandwidth analysis for them, also the the applicability of scalar diffraction theory designing multi-level beam splitting DOE was discussed.