| Autor: |
Sokolovskii GS; Ioffe Institute, 26 Polytechnicheskaya str., St. Petersburg, 194021, Russia. gs@mail.ioffe.ru.; ITMO University, 49 Kronverksky pr., St. Petersburg, 197101, Russia. gs@mail.ioffe.ru., Melissinaki V; IESL-FORTH, N. Plastira 100, 70013, Heraklion, Greece.; Department of Physics, University of Crete, 71003, Heraklion, Greece., Fedorova KA; Aston Institute of Photonic Technologies, Aston University, Aston Triangle, Birmingham, B4 7ET, UK., Dudelev VV; Ioffe Institute, 26 Polytechnicheskaya str., St. Petersburg, 194021, Russia.; St. Petersburg State Electrotechnical University (LETI), 5 Prof. Popova str., St. Petersburg, 197022, Russia., Losev SN; Ioffe Institute, 26 Polytechnicheskaya str., St. Petersburg, 194021, Russia., Bougrov VE; ITMO University, 49 Kronverksky pr., St. Petersburg, 197101, Russia., Sibbett W; School of Physics and Astronomy, University of St. Andrews, St. Andrews, Fife, Scotland, KY16 9SS, UK., Farsari M; IESL-FORTH, N. Plastira 100, 70013, Heraklion, Greece., Rafailov EU; Aston Institute of Photonic Technologies, Aston University, Aston Triangle, Birmingham, B4 7ET, UK. |
| Abstrakt: |
Multimode high-power laser diodes suffer from inefficient beam focusing, leading to a focal spot 10-100 times greater than the diffraction limit. This inevitably restricts their wider use in 'direct-diode' applications in materials processing and biomedical photonics. We report here a 'super-focusing' characteristic for laser diodes, where the exploitation of self-interference of modes enables a significant reduction of the focal spot size. This is achieved by employing a conical microlens fabricated on the tip of a multimode optical fibre using 3D laser nano-printing (also known as multi-photon lithography). When refracted by the conical surface, the modes of the fibre-coupled laser beam self-interfere and form an elongated narrow focus, usually referred to as a 'needle' beam. The multiphoton lithography technique allows the realisation of almost any optical element on a fibre tip, thus providing the most suitable interface for free-space applications of multimode fibre-delivered laser beams. In addition, we demonstrate the optical trapping of microscopic objects with a super-focused multimode laser diode beam thus rising new opportunities within the applications sector where lab-on-chip configurations can be exploited. Most importantly, the demonstrated super-focusing approach opens up new avenues for the 'direct-diode' applications in material processing and 3D printing, where both high power and tight focusing is required. |
