From 23e201c598024d8acce18e451ada57a54d5e92eb Mon Sep 17 00:00:00 2001
From: Blaise Thompson <blaise@untzag.com>
Date: Sun, 15 Apr 2018 21:35:24 -0500
Subject: 2018-04-15 21:35

---
 opa/chapter.tex | 139 ++++++++++++++++++++++++++++++++++++++++++--------------
 1 file changed, 104 insertions(+), 35 deletions(-)

(limited to 'opa/chapter.tex')

diff --git a/opa/chapter.tex b/opa/chapter.tex
index 3d163e2..db88304 100644
--- a/opa/chapter.tex
+++ b/opa/chapter.tex
@@ -16,7 +16,8 @@
 \section{Introduction}  % =========================================================================
 
 In frequency-domain Multi-Resonant Coherent Multidimensional Spectroscopy (MR-CMDS), automated
-Optical Parametric Amplifiers (OPAs) are used to actively scan excitation color axes. [CITE]  %
+Optical Parametric Amplifiers (OPAs) are used to actively scan excitation color axes.
+\cite{CerulloGiulio2003a}  %
 To accomplish these experiments, exquisite OPA performance is required.  %
 During the experiment, motors inside the OPA move to pre-recorded positions to optimize output at
 the desired color.  %
@@ -84,6 +85,8 @@ propagated to all downstream stages.  %
 
 % BJT: consider putting an example curve figure
 
+% TODO: table of curve methods and attributes
+
 \section{TOPAS-C}  % ==============================================================================
 
 The TOPAS-C is a popular commercially available motorized OPA.  %
@@ -94,12 +97,40 @@ frequencies.  %
 It ranges from the mid infrared (accessible through difference frequency generation) to the
 ultraviolet (accessible through multiple second harmonic upconversion).  %
 
-% TODO: introduction to the internal design of the OPA
+\autoref{opa:fig:TOPAS-C} diagrams the internals of the TOPAS-C initial stage, where signal and
+idler are generated.  %
+Upon entering the OPA, roughly 98\% of pump light is split off immediately (BS1).  %
+The remaining 2\% goes on to be split again (BS2).  %
+After being attenuated further and passed through an aperture, part of this 800 nm light is sent
+into a sapphire plate to generate white light.  %
+This white light is then intentionally chirped, and mixed with the other small portion of the pump
+\python{non-collinearly} in NC1.  %
+The angle of the crystal is tuned, as is the relative arrival time of chirped white light and the
+small pump portion.  %
+These two degrees of freedom control the efficiency of conversion at a given color in C1, and
+together they make up the ``preamp''.
+I describe my strategy for preamp tuning in \autoref{opa:sec:preamp}.  %
+
+The signal portion from the preamp is picked off and meets the gigantic 98\% portion of pump split
+off at the very beginning in NC2.  %
+Again, the relative arrival time and crystal angle are motorized internally.  %
+Together these degrees of freedom make up the ``poweramp''.
+I describe my strategy for poweramp tuning in \autoref{opa:sec:poweramp}.  %
+
+After the poweramp, the output signal and idler can be sent through appropriate filters and,
+optionally, mixed further in three subsequent mixing stages to create all of the ranges seen in
+\autoref{opa:fig:ranges}.  %
+Each of these mixing stages has only crystal angle tunability.  %
+I describe my strategy for mixer tuning in \autoref{opa:sec:mixer}.  %
+
+It is important to realize that the total conversion efficiency for each output color varies wildly
+over all of the different mixing strategies.  %
+\autoref{opa:fig:powers} shows the empirical-best output energy achievable for each setpoint.  %
 
 \begin{figure}
   \includegraphics[width=\textwidth]{opa/OPA_ranges}
   \caption{
-    CAPTION TODO
+    TOPAS-C interaction ranges.
   }
   \label{opa:fig:ranges}
 \end{figure}
@@ -108,38 +139,52 @@ ultraviolet (accessible through multiple second harmonic upconversion).  %
   \includegraphics[width=\textwidth]{opa/TOPAS-C}
   \caption[TOPAS-C internal optics and beam path.]{
     TOPAS-C internal optics and beam path.  %
-    Image taken from manual, originally generated by Light Conversion [CITE].  %
+    Image taken from manual, originally generated by Light Conversion.  %
   }
   \label{opa:fig:TOPAS-C}
 \end{figure}
 
 \begin{figure}
   \includegraphics[width=\textwidth]{opa/OPA_powers}
-  \caption{
-    CAPTION TODO
+  \caption[TOPAS-C interaction range output powers.]{
+    TOPAS-C interaction range output powers.
   }
-  \label{opa:fig:preamp}
+  \label{opa:fig:powers}
 \end{figure}
 
-\section{Preamp}  % ===============================================================================
+\section{Preamp} \label{opa:sec:preamp}  % ========================================================
 
 In TOPAS-C OPAs, a small portion of input light is used to generate a signal seed in a BBO crystal
 ``C1''.  %
 A motorized delay stage ``D1'' is used to temporally overlap a particular color in chirped white
 light with 800 nm pump.  %
 C1 angle is tuned to optimize phase matching.  %
-Measured seed intensity and color for all combinations of C1 and D1 position are shown in
-\autoref{fig:preamp}.  %
 
-Output color and intensity are not separable along the preamp motor axes.  %
-We therefore use a multidimensional fitting strategy to find the best preamp motor positions, as
-shown below.  %
-
-% TODO: procedure
+Measured seed intensity and color for all combinations of C1 and D1 position are shown in
+\autoref{opa:fig:preamp}.  %
+Crucially, output color and intensity are not separable along the preamp motor axes.  %
+We are obligated to use a multidimensional fitting strategy to find the best preamp motor positions
+at each setpoint.  %
+
+Luckily we have an InGaAs near-infrared array detector, so it is very quick to capture the entire
+output spectrum at each motor position.  %
+PyCMDS visits an entire series of (C1, D1) positions, scanning D1 about the prior best position for
+each C1 in the curve.  %
+
+\autoref{opa:fig:preamp_flowchart} diagrams the preamp processing procedure in its entirety.  %
+The original datset is three-dimensional in C1, D1, color.  %
+In the first step, the dimensionality is reduced by fitting each array slice to extract a center,
+amplitude and width.  %
+These fits are interpolated to find contours of constant output color.  %
+I then search along that contour in \emph{intensity} space to find the motor positions that give
+maximum intensity for that color.  %
+Finally I fit a smooth spline through those chosen values to generate the output curve.  %
 
 A representative preamp tune procedure output image is shown in \autoref{fig:autotune_preamp}.  %
+This is an automatically generated image from PyCMDS.  %
 The thick black line is the final output curve.  %
 The dark grey lines are the contours of constant color.  %
+Each contour of constant color is marked with the output color in nanometers.  %
 The colorbar shows the Delaunay-interpolated intensity values for each motor position.  %
 
 Preamp tuning takes less than 20 minutes, in large part due to a NIR array detector which collects
@@ -148,7 +193,7 @@ the full spectrum at each motor position.  %
 \begin{figure}
   \includegraphics[width=\textwidth]{opa/preamp}
   \caption{
-    CAPTION TODO
+    TOPAS-C preamp motortune.
   }
   \label{opa:fig:preamp}
 \end{figure}
@@ -156,26 +201,27 @@ the full spectrum at each motor position.  %
 \begin{figure}
   \includegraphics[width=\linewidth]{opa/preamp_flowchart}
   \caption{
-    CAPTION TODO
+    Preamp tune procedure flowchart.
   }
+  \label{opa:fig:preamp_flowchart}
 \end{figure}
 
 \begin{figure}
   \includegraphics[width=\linewidth]{opa/autotune_preamp}
   \caption{
-    CAPTION TODO
+    Preamp tuning output.
   }
   \label{opa:fig:autotune_preamp}
 \end{figure}
 
-\section{Poweramp}  % =============================================================================
+\section{Poweramp} \label{opa:sec:poweramp}  % ====================================================
 
 Once generated, the seed goes on to be amplified in a second BBO crystal ``C2'' with the rest of
 the 800 nm pump.  %
 Optimizing this amplification step is primarily a matter of setting C2 angle.  %
 A small delay correction ``D2'' is necessary to account for dispersion in the seed optics.  %
 To fully explore poweramp behavior, we need to tak a C2-D23 scan for each seed color.  %
-Measured output intensity and color in this 3D space is represented in \autoref{fig:poweramp}.  %
+Measured output intensity and color in this 3D space is represented in \autoref{opa:fig:poweramp}.  %
 Note that the motor axes are scans about the previously recorded tuning curve value.  %
 
 The best position (zero displacement along both axes) is chosen to maximize output intensity while
@@ -187,8 +233,7 @@ dimensions (this is especially true at the edge output colors).  %
 In the poweramp, the increased dimensionaity makes it too expensive to do a full multidimensional
 tuning procedure.  %
 Instead we emply an iterative procedure as diagrammed below.  %
-
-% TODO: procedure
+\autoref{fig:opa:poweramp_flowchart} diagrams this iterative procedure.  %
 
 We always end the iteration(s) with C2 so that the OPA's color calibration is as good as
 possible.  %
@@ -209,7 +254,7 @@ curve (colored X's).  %
 \begin{figure}
   \includegraphics[width=\linewidth]{opa/poweramp}
   \caption{
-    CAPTION TODO
+    TOPAS-C poweramp motortune.
   }
   \label{opa:fig:poweramp}
 \end{figure}
@@ -217,14 +262,15 @@ curve (colored X's).  %
 \begin{figure}
   \includegraphics[width=\linewidth]{opa/poweramp_flowchart}
   \caption{
-    CAPTION TODO
+    Poweramp tune procedure flowchart.
   }
+  \label{opa:fig:poweramp_flowchart}
 \end{figure}
 
 \begin{figure}
   \includegraphics[width=\textwidth]{opa/d2}
   \caption{
-    CAPTION TODO
+    Poweramp D2 tuning output.
   }
   \label{opa:fig:d2}
 \end{figure}
@@ -232,24 +278,47 @@ curve (colored X's).  %
 \begin{figure}
   \includegraphics[width=\textwidth]{opa/c2}
   \caption{
-    CAPTION TODO
+    Poweramp C2 tuning output.
   }
   \label{opa:fig:c2}
 \end{figure}
 
-\section{Mixers}  % ===============================================================================
+\section{Mixers} \label{opa:sec:mixers}  % ========================================================
+
+Because mixers only have one degree of freedom each (crystal angle), there is really not that much
+ambiguity about what the ideal motor positions are.  %
+In fact, the best motor positions can be chosen simply by taking excursions relative to the old
+points (as in \autoref{opa:fig:d2}) and picking the points with the highest intensity.  %
+After choosing motor positions, a simple correction for actual output frequencies can be applied
+using the monochromator.  %
 
-[DESCRIPTION OF MIXERS]
+I have prepared two functions: \python{process_intensity} and \python{process_tune} which
+accomplish each of these goals.  %
+They are general, capable of being used for \emph{any} mixer or tune test.  %
+
+PyCMDS can also explicitly take a spectrum at each motor position.  %
+This information takes longer to collect, but less human intervention---so it is a valid strategy
+that is sometimes employed.  %
 
 \section{Generalizability}  % =====================================================================
 
+This chapter has considered the automated procedures used in tuning the TOPAS-C, just one of the
+four models of OPA owned by the Wright Group.  %
+Simply put, this is because the other three OPA models (all picosecond OPAs) are easy to tune.  %
+
+\autoref{opa:fig:ps_opa} displays the entire tuning space for generation of signal and idler in one
+of the picosecond OPAs.  %
+In contrast to the TOPAS behavior, where neither motor axis constrains the output very well,
+\emph{both} motors have very narrow features in this picosecond OPA.  %
+This means that it is at all times \emph{unambiguous} whether a given motor position is ideal.  %
+
+Much like the mixers, these OPAs can be readily tuned using a combination of the general functions
+\python{process_intensity} and \python{process_tune}.  %
+
 \begin{figure}
   \includegraphics[width=\textwidth]{"opa/signal_and_idler_motortune"}
-  \caption[CAPTION TODO]{
-    CAPTION TODO
+  \caption[Picosecond OPA motortune.]{
+    Motortune for picosecond OPA, monitored using a single pyroelectric detector.
   }
-\end{figure}
-
-\section{Future directions}  % ====================================================================
-
-% TODO: discuss Attune
\ No newline at end of file
+  \label{opa:fig:ps_opa}
+\end{figure}
\ No newline at end of file
-- 
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