\begin{tikzpicture}
      \draw[very thick, -{latex}] (-4,0) -- (5,0);
      \pockelscell at (-3,0);
      \pockelscell[shift=-0.4] at (-1,0);
      \pockelscell[shift=-0.8] at (1,0);
      \pockelscell[shift=0.8] at (3,0);
\end{tikzpicture}