Efficient fiber-to-waveguide coupling by a lens on the end of the optical fiber fabricated by focused ion beam milling

The purpose of this work is to demonstrate efficient optical coupling between a single, mode fiber (SMF) and a waveguide (LiNbO3-APE) using a micro-lens fabricated directly on the cleaved end of a fiber using a focused ion beam (FIB) milling process. The design, micro-fabrication and testing of diffractive optical elements (DOEs) with continuous relief fabricated on the tip of a single mode optical fiber are discussed in detail. A 30 keV focused Ga⁺ ion beam is used to mill a continuous relief microstructure; DOEs with diameters as small as 15 μm were fabricated. The design of the DOE-lens and the calculations related to the optical fiber-to-waveguide coupling were carried out using our own developed code. The profile of the fabricated lens was very well reproduced in ten levels each 100 nm thick. This fabricated DOE-lens was able to focus the Gaussian beam from the fiber, into a waveguide plane at a distance of 28 μm from the lens surface. The diameter of the beam leaving the fiber was of about 10.5 μm while the size of the focused waist was 5.2 μm. This led to efficient matching of the fundamental mode of the fiber to that of waveguide. We have also measured the coupling efficiency using a laser beam at 1550 nm wavelength. The optical coupling using the lens on the fiber end is 67% more efficient than with direct coupling between the fiber and the waveguide.