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. 2018 Mar 13;8:4467. doi: 10.1038/s41598-018-22790-z

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© The Author(s) 2018

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Characterization of light coupling efficiency and output power density for NA = 0.66 TFs. (a) Light coupling across the NA = 0.66 angular acceptance. The input power (red line) was measured at the entrance of the tapered region. The output power (black line) was measures in close proximity to the taper tip, as shown in the inset. A drop in power output was observed for low input angles. (b) Power coupling efficiency, calculated as the ratio between output and input power, versus light input angle. The coupling efficiency is approximately constant for θ_in > 10°. (c) Scanning electron microscope images of a NA 0.66 TF. While the left panel shows a homogeneous portion of the taper region, scale bar 400 µm, the close-up in the right panel shows degradation of the taper surface in proximity of the tip, scale bar is 20 µm. (d) The three panels show the distribution of power density on the taper surface when 12 mW/mm² are emitted for three input angles, namely θ_in = 15°, 24°, 35°. As light is out-coupled from a wider region when the input angle increases, the input power has to be varied accordingly to maintain a constant output power density.