Cross section of a <a class="external text" href="https://en.wikipedia.org/wiki/Polarization-maintaining_optical_fiber">polarization-maintaining 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 small, eye-like circles around the central core are stress rods to create birefringence in the fiber and maintain polarization. The large dark circle surrounding them is about 125 microns in diameter.
<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>