\chapter{Abandon the random phase approximation} \label{cha:rpa}

Recently we've made some measurements that seem to imply phase stability between the fs OPAs.  %
We've typically assumed that the OPAs have random phase on every shot, making coherent heterodyne
processes average to zero.  %
These measurements show that this is a very bad assumption.  %

I've taken the interferogram of OPA1 vs OPA2, Clearly the OPAs remain phase locked for many shots.  %
In 'over time' I show the spectral phase pattern (D = 500 fs) for 1000 single shot acquisitions
over 430 seconds in lab time.  %
The phase does drift, but it is certainly not quickly randomized.  %

I have more data showing:
How the spectral phase changes over the course of hours.
How the phase evolves as we scan the OPAs against each-other in color.
The reproducibility of phase as the OPA motors move away and then return to a given color.

I'll work this data up and send out another email with many more details and thoughts once I have
time.  %
This quick note is just to let the group know that we must abandon the 'random phase' assumption
when thinking about what heterodyne processes can happen as coherent artifacts.  %

\begin{figure}
  \includegraphics[width=\textwidth]{"opa_phase/cross interference"}
  \caption[CAPTION TODO]{
    CAPTION TODO
  }
\end{figure}

\begin{figure}
  \includegraphics[width=\textwidth]{"opa_phase/430 seconds"}
  \caption[CAPTION TODO]{
    CAPTION TODO
  }
\end{figure}