A Hybrid Refractive Diffractive Lens Applied In Extending The Focal Depth Of The Femtosecond Laser Beam

The femtosecond laser is widely used in surface micro-structuring, micro-optics and micro-structure manufacturing. It is an extremely important role in the micro- machining that the femtosecond laser plays. However, a focal depth and a focal spot of an output beam will affect the machining accuracy. In the conventional optics, the focal depth of the beam and the focal spot of the beam are regarded as two inter-constraint measures. When the focal depth is extended, the diameter of the focal spot will be dramatically expanded.In this dissertation, we consider a hybrid refractive diffractive lens that could extend the focal depth of the femtosecond laser beam while the lens prevents the diameter of the focal spot from expanding. The hybrid refractive diffractive lens is introduced in four aspects such as the theoretical analysis, the design simulation, the implementation and the experimental results.Firstly, according to the parameters of the femtosecond laser and the target of designing, the basic parameters of the hybrid refractive diffractive lens has been taken. MATLAB® was used to simulate an initial phase of diffractive surface with Fresnel diffraction theory, and then the fitting data that is in keeping with the formula of binary in Zemax® were gotten. An improved energy conservation law has been put forward and it was used to optimize the structure of the hybrid refractive diffractive lens, so that the final parameters of the hybrid refractive diffractive lens have been obtained. The lens material was BK7 glass. The diameter of the lens was 20.000 mm. The diameter of the diffraction surface was 11.611 mm. The central thickness of the lens was 2.004 mm. The convex curvature of the lens was 66.542 mm. The effective focal length of the lens was 95.257 mm. Parameters of the binary surface were-377.347, 335.116,-1174.462, 544.835, and 130.274. The focal depth of the femtosecond laser beam was about 1.5mm and the diameter of the spot was about 50μm, which was simulated by Zemax®.Secondly, the conventional overlay method was used to implement the diffractive surface of the hybrid refractive diffractive lens. According to the binary optics theory, the processing data was computed and the mask was drawn by the software L-edit®. The diffractive surface of the hybrid refractive diffractive lens had 8-steps and a plasma etching techniques was used to etch it. The refractive surface of the hybrid refractive diffractive lens was manufactured after the diffractive structure of the hybrid refractive diffractive lens has etched done.Finally, a microscope and three-dimensional morphology instrument was used to observe the morphology of the hybrid refractive diffractive lens. In order to test the focal depth of the femtosecond laser beam that passes though the hybrid refractive diffractive lens, the holes were punched on the PMMA material at the different plane and the diameter of these focal spots were measured. It has been found that the hybrid refractive diffractive lens could extend the focal depth of the femtosecond laser beam to about 1.5mm and the diameter of the micropores was to be about 100μm. Therefore, the method that this dissertation described can be used to design a hybrid refractive diffractive lens to extend the focal depth of the femtosecond laser beam.