Abstract
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.
Highlights
Since their demonstration and development in 1970–80s1,2, optical tweezers have become one of the key laser-based assets in physics and biology for the contact-free manipulation of microscopic objects, through the harnessing of forces that originate from tightly-focused laser beams
Employing laser diodes for optical trapping and tweezing could be expected to have a profound impact on the further progress of this technology
By developing a solution to the beam-quality issue of diode lasers by employing appropriate beam shaping techniques, such as Bessel beam generation using an axicon, it can be envisaged that diode-laser-based optical manipulation could be rendered as a day-to-day and relatively cost-effective modality within medical biophotonics
Summary
It has been demonstrated that optical trapping is feasible with axicon-based Bessel beams generated from semiconductor lasers[21]. A series of experimental images were reproduced in Fig. 8 that demonstrated the two-dimensional optical trapping and manipulation of red blood cells with the super-focused beam from a high-M2 semiconductor laser. In this figure, red arrows indicate the tweezed object and its movement. Probably the most important applications of the demonstrated super-focusing approach seem to be within the direct applications of laser diodes (termed as ‘direct diode’) in materials processing and 3D printing, where both high power and tight focusing is required Nowadays most such applications rely on diode-pumped solid state lasers (including those with fibre and disc active media). Existing ones in soldering, welding, scribing, marking, engraving, paint stripping, powder sintering, synthesis, brazing and machining
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