Photograph of a <a href="https://en.wikipedia.org/wiki/Electro-optic_modulator" class="extiw" title="w:Electro-optic modulator">Electro-optic phase modulator</a>, which is basically a variable <a href="https://en.wikipedia.org/wiki/waveplate" class="extiw" title="w:waveplate">waveplate</a>. The metal case contains an optical crystal that changes its optical <a href="https://en.wikipedia.org/wiki/birefringence" class="extiw" title="w:birefringence">birefringence</a> according to an applied high voltage electric field. This allows to modulate the phase of a transmitted optical beam and quickly change its polarization.
<a href="//commons.wikimedia.org/wiki/User:Geek3" title="User:Geek3">Geek3</a>
CC BY-SA 4.0
<span class="int-own-work" lang="en">Own work</span>