Fiber to rectangular waveguide optical coupling by means of diffractive elements

The present research work is devoted to the realization of an efficient fiber-waveguide 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. To transform the outcoming circular laser beam onto a rectangular, size adjusted, spot we have used a multilevel diffractive phase element fabricated directly on the top of the fiber by means of nanolitography. The diffractive phase element is calculated to focus and reshape the gaussian symmetric beam exiting a single-mode fiber into a desired asymmetric intensity distribution at the waveguide input plane. Phase modulation is obtained by multilevel profiling a polymeric material coated on the top of the fiber by means of a specific fabrication process including e-beam lithography and chemical etching. Experimental results obtained for fiber-waveguide coupling with a 20 microns diameter diffractive element are also presented.