Beamforming using uniform coupled waveguide sections for efficient coupling to an optical fibre

Using rigorous numerical approaches, it is shown that the highly asymmetrical and divergent modal field profile of a photonic integrated circuit (PIC) could be modified to produce a near circular beam profile, which also could be expanded as required, to achieve efficient coupling to an optical fibre. This may be achieved by incorporating a uniform spot-size converter with two non-identical, but phase matched optical waveguides. For the secondary waveguide, using a more symmetrical cross-section allows the beam shape to be tailored to match the capture area of a particular optical fibre. Numerically simulated results, for a specific and typical structure, show that the design can be improved and the 61° and 26° vertical and horizontal beam angles of the original PIC can be significantly reduced to only 12° and 11.6°, respectively, by using an 8 μm×8 μm secondary core. The power coupling loss also can be significantly reduced from 12.3 dB to only 2.4 dB. On the other hand, by using a waveguide with a smaller core, which may be easier to fabricate, a nearly symmetrical beam shape can be produced, which can be much more readily coupled to a fibre, compared to the direct coupling of the original PIC.