UHNA3 fiber with core etched by Xe focused ion beam and HF acid with deposited silica ball microlenses

The picture "Etched fibers before and after lens deposition.png" shows microscope images of the UHNA3 fiber facet before and after deposition of silica ball microlenses: (a) fiber etched with Xenon focused ion beam to a depth of 650 nm, (b) fiber shown in (a) with a 15 microns ball lens deposited onto the etched cavity, (c) fiber before HF etching, (d) fiber after etching in 1:1 solution of HF acid and distilled water for 2 minutes showing the etched core and area surrounding it with diameter of approximately 16 micrometers, (e) fiber shown in (d) with a 15 micron ball lens deposited onto the etched cavity. Scalebars are 20 microns. The picture "Microlens deposition setup.jpg" shows the setup used for the lenses deposition, consisting of a microscopic slide mounted onto a tilt stage with linear up-down moving stage, and rotatable fiber holder. For the lens deposition the whole setup is placed under microscope with camera view onto the fiber facet through the microscopic slide. Silica ball microlenses are sparsely distributed randomly onto the bottom surface of the microscopic slide. The glass plate is positioned over the fiber facet so that a lens of choice is close to the center of the etched cavity. The fiber is then gently pressed onto the glass and the microlens remains in the etched hole due to suction. The video of the microlens deposition is available here: https://doi.org/10.5281/zenodo.19691520. The deposition of the microlens results in increase of divergence of the output beam, as shown in picture "Measured beam divergence.png". The resulting numerical aperture calculated based on the linear fit of the far field is 0.35 for the fiber without lens and 0.6 for the fiber with lens. The dataset containing measured radii of the output beam as a function of distance from the fiber is included in "Measured beam divergence.txt".