Efficient beam splitting with continuous relief DOEs and microlens arrays

Abstract

Diffractive optical elements (DOEs) are of rising importance for many industrial laser applications, especially for laser beam shaping and laser beam splitting. Typically, such applications require high damage threshold of the diffractive optical elements as well as high diffraction efficiency. Usually DOEs with multilevel (step-like) phase profiles are made microlithographically and suffer from quantisation errors and scattering on profile derivative discontinuities. The steplike structure lowers the DOE damage threshold compared to the intrinsic material values. LIMO's microoptical technology is suitable for the production of high-precision free programmable continuous surface profiles in optical glasses, crystals and metals. It can be applied for manufacturing of microlens and micro-mirror arrays as well as for manufacturing of diffractive optics with continuous reliefs. Both the arrays and DOEs with continuous relief are suitable for high efficiency laser beam splitting. However, the design approaches to obtain a desirable solution for the corresponding continuous phase profiles are different. The results of the wave-optical simulations made by LIMO's own program and by VirtualLab software, and experimental studies for a 1 to 11 beam splitter with a continuous profile for the wavelength of 532 nm are presented. Continuous phase profiles for the DOEs were designed by a procedure based on the theory of beam splitting by a phase grating. Comparative theoretical and experimental studies were also done for splitting with a double-sided microlens array. For both types of beam splitting the efficiency can be very high (> 98%). The DOEs show especially high homogeneities of the resulting intensity distribution, however, they are much more sensitive to wavelength variations. The microlens arrays demonstrate even weaker ghost orders as the DOE splitters and their surface profiles are simpler. However, the efficiency and homogeneity suffer on interlens gaps.