Accurate Download Gratis
Elly Garnand
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Micro-optics are widely used in numerous applications, such as beam shaping, collimation, focusing and imaging. We use femtosecond 3D printing to manufacture free-form micro-optical elements. Our method gives sub-micrometre accuracy so that direct manufacturing even on single-mode fibres is possible. We demonstrate the potential of our method by writing different collimation optics, toric lenses, free-form surfaces with polynomials of up to 10th order for intensity beam shaping, as well as chiral photonic crystals for circular polarization filtering, all aligned onto the core of the single-mode fibres. We determine the accuracy of our optics by analysing the output patterns as well as interferometrically characterizing the surfaces. We find excellent agreement with numerical calculations. 3D printing of microoptics can achieve sufficient performance that will allow for rapid prototyping and production of beam-shaping and imaging devices.
For the combination of micro-optical elements with fibre optics, different approaches have recently been proposed using fabrication techniques such as focused ion-beam milling17,18, interference lithography19,20,21, nano-imprint technology22,23, photolithography24, polishing techniques25, through-fibre patterning26,27, ultraviolet curing of surface-tension-shaped droplets in combination with coreless silica fibres28,29, inject printing of photocurable polymers30,31,32, laser micromachining33,34 and two-photon polymerization35,36,37,38,39,40,41,42,43 in order to fabricate dielectric elements directly onto the end facet of an optical fibre. However, they suffer from drawbacks such as low resolution, small fabrication volumes, the difficulty to achieve necessary large optical heights because of the required glass slides or complex fabrication setups. The additional restrictions that are required by designs of optical elements result in the fact that, to our knowledge, until now no combination of the specific design, fabrication and verification of sub-micrometre accurate free-form optics has been reported. Also, free-form optics on single-mode fibres with specific design targets cannot be fabricated by most of the mentioned methods.
To validate the complete value chain from design, manufacturing and performance assessment, dip-in laser lithography with different photoresists in a multiple step process is used44,45,46. Using this technique, we demonstrate different applications of sub-micrometre optical components for beam collimation, generation or correction of astigmatism, beam shaping and polarization control. This allows, for example, the specific shaping of the intensity distribution directly at the output of an optical fibre by a combination of diffractive and refractive elements.
Figure 1 shows 3D printed optical elements where in the first case (Fig. 1a) a collimation lens has been printed onto a single-mode fibre. In addition, the photo depicts that the lens appears as transparent and glassy as the fibre. The second image (Fig. 1b) shows a free-form optical surface with a saddle-type shape aligned on a single-mode fibre. Below is a chiral three-dimensional photonic crystal structure for polarization filtering, and on top a diffractive polymer structure for beam shaping is imprinted. The three-dimensional aspect of the helical photonic bandgap material plays a crucial role in the working principle of the circular polarization filter, as will be discussed further below47,48,49,50,51. We demonstrate with this image the powerful diversity of capabilities when using 3D femtosecond printing in the microoptical realm. The third image (Fig. 1c) displays the possibility to include air into the design process in order to manufacture compound lenses with sophisticated aberration correction capabilities. This is something that would be completely beyond the abilities of other fabrication methods such as inkjet-type 3D printing, as used by companies such as Luxexcel52.
Backside illumination of the fibre is used to accurately centre the fibre core with respect to the writing beam by observing the end facet of the fibre with a CCD (charge-coupled device) camera. This method can also be used for hybrid systems consisting of two different photoresists (Fig. 1b)53. In a two-step process, we first fabricate a chiral photonic crystal structure as circular polarization filter on the end facet of an optical fibre using IP-Dip (Nanoscribe GmbH), and in a second step, we use the photoresist IP-S (Nanoscribe GmbH) to create the saddle-shaped free-form lens with a Fresnel zone plate on top (Fig. 1b). The different photoresists are chosen according to their writing characteristics. Namely, we use a photoresist with a high resolution for the lower part, whereas the upper part is fabricated by a photoresist with lower resolution, such that smoother surfaces can be achieved (Fig. 1b).
We have demonstrated various sub-micrometre optical elements, enabling intensity distribution shaping and polarization control with surface deviations between design and manufactured element of less than one wavelength. Our approach of using three-dimensional dip-in laser lithography is not limited to any kind of special optics. Furthermore, it is a fabrication platform for numerous optical elements based on diffraction, reflection and refraction in the sub-micrometre region, where among other things it is suitable for fibre technology, but similarly it is as well versatile for the combination with other substrates such as light-emitting diodes, vertical-external-cavity surface-emitting-lasers (VECSEL) and other light-emitting structures in the sub-micrometre range. Therefore, highly compact functional hybrid optical elements can be manufactured. In addition, our lenses are not limited to one refractive surface, allowing for the fabrication of optical components with two or even more compounded free-form or diffractive surfaces. Also, from the performance, we deduce that internal refractive index variations due to polymerization inhomogeneities are not detrimental to the optical performance. Our method extends additive optical manufacturing into the micro- and nanooptics realm and opens a whole new field in endoscopy and micro-imaging and -illumination.
In particular, for the fabrication process, a droplet of photoresist is deposited onto the objective lens and subsequently, the fibre is dipped into the photoresist. After aligning the fibre, the two-photon direct laser writing process is accomplished. To develop the structure, the unexposed photoresist has to be removed by a development bath. Using this method, the structure is perfectly aligned with respect to the core of the optical fibre. An illustration of the fabrication process can be found in Supplementary Fig. 1.
The structures shown in Figs 2 and 3 are fabricated by moving the fibre layer-by-layer with ultra-precise piezo actuators. The structures in Figs 1, 4 and 5 are manufactured by scanning the laser focus lateral by galvanometric mirrors. The vertical movement is as well carried out by piezo actuators. This method allows for fast and reproducible fabrication of sub-micrometre optical elements. In Supplementary Fig. 2, the high reproducibility of our method is demonstrated by fabricating two similar lenses with nominally identical fabrication parameters. The optical performance of the two lenses is in good agreement, which demonstrates the high reproducibility of the fabrication method.
We gratefully acknowledge financial support by DFG (SPP1391, FOR730 and GI 269/11-1), Bundesministerium fr Bildung und Forschung (13N9048, 13N10146 and PRINTOPTICS), Baden-Wrttemberg Stiftung (Internationale Spitzenforschung II and Intelligente Optische Sensorik), ERC Advanced Grant (COMPLEXPLAS), the Ministerium fr Wissenschaft, Forschung und Kunst Baden-Wrttemberg (Az: 7533-7-11.6-8). We thank ZygoLOT GmbH, Jan Niehues and Peter Lehmann from the University of Kassel for white light interferometer measurements, Michael Thiel and Andr Radke from Nanoscribe GmbH for help with lens fabrications and stimulating discussions, as well as Bettina Frank for performing atomic force microscope measurements. We would like to thank Angelika Hoffmann and Stefan Kaiser from OEC AG for stimulating discussions.
T.G. and H.G. conceived the concept. T.G. performed numerical simulations, structural designs, direct laser writing, experimental characterization and wrote the manuscript. S.T. and A.H. performed numerical calculations. H.G. participated in planning the experiments and supervised the project. All authors participated in discussions and contributed to editing of the manuscript.
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