3D patterning method in femtosecond laser microprocessing using diffractive optical elements

We demonstrated a simple chromatic dispersion reduction method of 3-dimensional (3D) patterning of femtosecond pulses using a multi-level phase type diffractive optical element (DOE) and a focusing objective lens. Our method increases flexibility of femtosecond laser microprocessing. With appropriate focal length of the DOE and distance between the DOE and the focusing lens, large chromatic dispersion of the DOE resulting from spectral bandwidth of a femtosecond pulse can be reduced, and 3D focusing pattern of femtosecond pulse can be obtained not only controlled in focal plane but also in focal depth. The method was verified through optical and processing experiments with laser pulses of 400 fs duration and of 40 nm bandwidth. The focal length of the DOE and the objective lens was 1600 mm and 10 mm, respectively. Partially periodical structure of focusing points was formed at designed position and its focal depth were much smaller than that focused with only the DOE. By irradiating the constructed beam, microstructure was formed precisely inside SiO2 glass. The processed points are clearly separated each other with a separation of 5 mm and the spot sizes were almost same as those irradiated without the DOE.

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