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. This is a higher resolution zoomed in image.
<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>