Design and fabrication of diffractive optical element-microlens with continuous relief fabricated on-top of optical fibre by focused ion beam for fibre-to-waveguide coupling

The purpose of this work is to demonstrate efficient optical coupling between a single mode fibre and a waveguide (LiNbO₃-APE) using a diffractive optical elements (DOEs)-microlens with continuous relief fabricated directly on the top of a cleaved fibre using a focused ion beam (FIB) milling process. DOE's with diameters as small as 15 urn were fabricated by means of FIB milling process. A focused Ga + ion beam was used to mill a continuous relief microstructure at a 30 kV acceleration voltage. The design of the DOE-microlens and the calculations related to the optical fibre-to-waveguide coupling was carried with our own developed code. The profile of the fabricated DOE-microlens was very well reproduced in nine annulus each of 100nm thick. The focusing performance of DOE-microlens with nine annulus was investigated. This fabricated DOE-microlens was able to focalize the Gaussian beam leaving the fibre, into a wave guide plane at a distance of 28 μm from the DOE-microlens surface. The fundamental mode size of the beam leaving the fibre was of about 10.5 μm while the size of the focused waist was 5.2 μm. This has led to an efficient matching the fundamental mode of the fibre vis-à-vis that of waveguide, Coupling efficiency test were carried using a laser beam λ = 1550-nm wavelength. The optical coupling using the DOE-microlens on-top-of the fibre-to-waveguide is 67% more efficient than the direct coupling between the fibre and the waveguide. This shows that the design meets the application's requirement for fibre-waveguide coupling, and that DOE-microlens fabricated by FIB technology are practicable.