Chains of variable size spheres for focusing of multimodal beams in photonics applications

Using a chain of spheres with progressively decreasing diameters we designed optical microprobes capable of focusing multimodal optical beams delivered by hollow waveguides and multimode fibers. Such delivery systems are used in biomedical and photonics applications where significant power transmission properties are desirable or where single mode fibers are not available. A concrete example of such applications is represented by the ultraprecise laser surgery performed in mid infrared range. The design performed by numerical ray tracing is optimized for contact conditions during laser surgery involving short optical penetration depths of light in tissue. It is shown that chains with progressively decreasing sphere diameters can provide higher coupling efficiency compared to the chains of identical size spheres, while having better spatial resolution in comparison with a single sphere case. Potential applications of these microprobes include ultra-precise laser procedures in the eye and brain or piercing a cell, and coupling of multimodal beams into photonic microstructures.