Fabrication of diffractive optical elements for photonic applications by nanolithography

The present research work is devoted to the realization of an efficient optical coupling between single-mode fiber and rectangular waveguide. The outcoming laser beam exiting from the fiber has a gaussian transversal field distribution. On the contrary, the single-mode waveguide has an asymmetric transversal field distribution in X and Y-axis. The problem of transforming the outcoming circular laser beam onto a rectangular, size adjusted, spot can be solved realizing directly on the top of the fiber a diffractive optical element. This optic element is able to redistribute the diffractive electromagnetic field on a selected area by a suitable phase modulation of the light, in analogy with what happens for Fresnel zone plate lenses. Phase modulation is obtained by a multilevel profiling of a polymeric material coating the top of the fiber by means of e-beam lithography. The design of diffractive optical elements has been realized using our own algorithm and code. Focalization experiments have been made with the use of a 1550nm wavelength laser. Description of the lithography and fabrication processes are shown as well.