Cross section of a <a class="external text" href="https://en.wikipedia.org/wiki/single-mode_optical_fiber">single-mode optical fiber</a> patch cord end, viewed through a <a class="external text" href="https://en.wikipedia.org/wiki/Fiberscope">fiberscope</a>, (i.e. an illuminated microscope for viewing the ends of fibers.) The large dark circle in the center is about 125 microns in diameter. Debris is also visible in the illuminated halo around the tip of the fiber, like a long strand.
<a href="//commons.wikimedia.org/wiki/User:Quantumavik" title="User:Quantumavik">Quantumavik</a>
CC BY 4.0
<span class="int-own-work" lang="en">Own work</span>