Ever growing importance of optical fiber technology in the fields of data transfer, sensors, imaging or advanced light generation pushes the effort of integration of functional microoptical elements in the fiber based systems . Here we present a femtosecond (fs) pulse based 3D laser lithography (3DLL) of free-form polymeric structures on tips of single mode optical fibers. Fabricated objects include both microoptical elements (for instance aspheric microlenses) and woodpile photonic crystals. Also, freeform 3D monoliths, combining several of these elements into one functional component, are integrated on the tip of the fiber [2, 3]. Methods allowing enhancement and large scale fabrication, such as advanced fiber holders or synchronization of linear stages and galvo-scanners, are discussed.
In contrast to standard lithographic techniques, fs 3DLL can be employed for processing a wide array of materials, including otherwise non-processable (non-photosensitized) polymers. It is relevant to the most of the fields where microoptics are applied, as photoinitiators introduce additional short wavelength (<400 nm) absorption to the material, resulting in information loss, lower optical damage threshold, parasitic fluorescence and, therefore, should be avoided if possible. One material that could be structured in this fashion is hybrid organic-inorganic zirconium containing SZ2080 which displays many favourable qualities such optical transparency and low shrinkage . Here we provide notes on peculiarities of 3DLL of a pure SZ2080. Experimental results show that compared to photosensitized counterpart photoinitiator-free polymer has comparable characteristics when it comes to fabrication throughput, optical properties (transmission spectra and surface roughness), mechanical strength (structure survival rate and adhesion to the substrate) as well as others. A physical phenomenon behind fs structuring of pure material such as interplay between multiphoton absorption and avalanche ionization  is highlighted.
Optical resiliency of microoptical elements created with this technology is investigated and their feasibility in applications reliant on high light intensities is shown. The most resistant material was found to be non-photosensitized SZ2080 as microlenses produced out of such material can withstand peak intensities of 515 nm 300 fs 200 kHz laser in range of GW/cm^2 within minutes to hours . Such results tie well with earlier findings showing that optical damage threshold of pure materials films are in fact higher than that of a photosensitized material . Additionally, CW 405 nm laser operating at the intensity of 8.66 GW/cm^2 cannot damage microoptical elements produced out of SZ2080 in pure and photosensitized form even after tens of hours of continuous exposure. Further comparison with the optical resiliency of microoptical elements fabricated out of other lithographic materials such as Ormocer and SU8 are made.
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Linas Jonušauskas, Titas Tičkūnas, Darius Gailevičius, Gedvinas Nemickas, Andrius Narmontas, Vytautas Purlys, Giedrė Grigalevičiūtė, and Roaldas Gadonas, "Femtosecond 3D laser lithography of photo initiator-free optically resilient microoptics on the tips of optical fibers (Conference Presentation)," Proc. SPIE 10681, Micro-Structured and Specialty Optical Fibres V, 106810D (Presented at SPIE Photonics Europe: April 25, 2018; Published: 23 May 2018); https://doi.org/10.1117/12.2307622.5788825133001.
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