Diffractive optical element in free-space data transmission systems

摘要

Abstract: We demonstrate the design and performance of an array-type diffractive element, which is capable of modifying the beam of a matrix-type vertical-cavity-surface-emitting-laser (VCSEL). The diffractive element was designed for line-of-sight or non- line-of-sight multi-beam transmitter with a maximum illuminating angle of 50 degrees. To demonstrate wide-angle illumination, a single element providing the largest 50-degree illumination angle was designed and fabricated. The element was designed in the paraxial domain by the geometrical map- transformation method. Beam deflection to the desired angle was performed by Lohmann's detour-phase principle. The local diffraction efficiency of the binary element was analyzed by rigorous electromagnetic diffraction theory. The element was fabricated as a pixel-array, in which the pixel size was 200 nm $MUL 200 nm and the size of the element was 250 micrometer $MUL 250 micrometer. The performance of the element was characterized by measuring the irradiance at the observation plane by a CCD-camera, and by measuring the diffraction efficiencies of three diffraction orders with a separate detector. The measured diffraction efficiency and the irradiance distribution of the element for the $MIN@1st order was 27.5%, which was in fair agreement with the calculated 23.9% efficiency after taking the real properties of the VCSEL beam into consideration. !10