diff options
Diffstat (limited to 'active_correction')
| -rw-r--r-- | active_correction/chapter.tex | 70 |
1 files changed, 50 insertions, 20 deletions
diff --git a/active_correction/chapter.tex b/active_correction/chapter.tex index 4f7003d..7ecbe2f 100644 --- a/active_correction/chapter.tex +++ b/active_correction/chapter.tex @@ -19,13 +19,13 @@ MR-CMDS is subject to a number of possible artifacts, many of them stemming from nature of the frequency-tunable light sources we currently have. %
It is self-evidently desirable to correct these artifacts, when possible. %
Indeed many of these artifacts, such as OPA power, phase mismatch and absorption effects have
-regularly been corrected for. % TODO: link to examples in applications section
+regularly been corrected for. % TODO: link to examples in applications section, cite
These corrections are applied after measurement, typically including information from other sources
(such as absorption spectra, in the case of absorption effect corrections).
A more interesting class of corrections are ``active'' corrections---that is, corrections that must
be actively applied during acquisition and cannot be applied in post processing. %
-These corrections are more insidious, and they are often neglected because the hardware and/or
+These corrections are more insidious, as they are often neglected because the hardware and/or
software does not allow for them. %
In this chapter I explore some of these active correction strategies that are useful in the context
@@ -34,17 +34,19 @@ Some of these strategies have already been implemented, others are partially imp others are still just ideas. %
I hope to show that active correction is a particularly useful strategy in MR-CMDS. %
-[SPECTRAL DELAY PARAGRAPH]
+Section ... addresses spectral delay correction, where automated delay stages are used to
+explicitly correct for small changes in optical path length at different pulse frequencies. %
-[POYNTING CORRECTION PARAGRAPH]
+Section ... addresses poynting correction, where mirrors with motorized pitch and yaw control are
+used to actively correct for small changes in OPA output poynting. %
-[EXCITATION POWER CORRECTION PARAGRAPH]
+Section ... addresses (dual) chopping, used to actively subtract artifacts such as scatter and
+unwanted nonlinear outputs. %
+Chopping can only account for intensity level (additive) artifacts. %
+Fibrillation is the opposite of chopping, as it can only account for amplitude level
+\emph{iterference} effects. %
+Section ... addresses fibrillation. %
-[CHOPPING PARAGRAPH]
-
-[FIBRILLATION]
-
-\clearpage
\section{Spectral delay correction} % ============================================================
As a frequency domain technique, MR-CMDS requires automated tuning of multiple OPAs. %
@@ -53,13 +55,14 @@ Crucially, the relative arrival time of each pulse must be carefully controlled the MR-CMDS experiment. %
Unfortunately, changing the output frequency also changes the optical path length, meaning that
there is some unavoidable coupling between delay and frequency axes. %
-Because we have full control over delay through delay stages, we can correct for this phenomenon by
-choosing a different zero delay \emph{offset} for each OPA output color. %
+Because we have full control over delay with our automated stages, we can correct for this
+phenomenon by choosing a different zero delay \emph{offset} for each OPA output color. %
This strategy has been dubbed ``spectral delay correction''. %
-Spectral delay correction (SDC) is certainly the longest running active correction strategy employed
+Spectral delay correction (SDC) is certainly the oldest active correction strategy employed
within the Wright Group. %
-SDC was first implemented by Schuyler Kain within his COLORS acquisition software. [CITE] %
+SDC was first implemented by Schuyler Kain within his COLORS acquisition software.
+\cite{KainSchuyler2017a} %
COLORS' implementation was hardcoded for one particular OPA / delay configuration---it wasn't until
PyCMDS that fully arbitrary SDC became possible through the autonomic system (see section ...). %
Erin Boyle ``backported'' similar functionality into to ps\_control, although her implementation
@@ -74,7 +77,13 @@ A special method of \python{Data}, \python{Data.offset} is designed to do the ne interpolation for \emph{post hoc} SDC. %
In many experiments spectral delay must be actively corrected for. %
-Fully coherent experiments do
+Fully coherent experiments are typically performed by scanning OPA frequencies while attempting to
+keep delays constant. %
+In such experiments, the dataset does not in-and-of-itself contain the information needed to
+offset in post processing. %
+Indeed it can easily become time-prohibitive to collect the full response. %
+For a three-beam experiment, an entire two dimensional delay-delay collection would be required at
+each pixel to allow for post-correction. %
It has been found that SDC is necessary for each individual scanned OPA, and for each separate path
when pulses from a single OPA are split. %
@@ -82,7 +91,7 @@ The difference between different paths is typically small, but enough to move pu amount relative to each-other. %
For this reason, SDC for split OPAs is a multidimensional problem, which in principle requires a
multi-dimensional acquisition to fully record. %
-In practice, however, these corrections are recorded iteravely. %
+In practice, however, these corrections are typically recorded iteratively. %
White light sources are also interesting to consider in the context of spectral delay
correction. %
@@ -101,7 +110,28 @@ using a delay stage. % COLORS' has taken this idea to it's logical conclusion, with support for ``OPAs'' that are actually
controlled by delay stages, although the idea has not yet been realized in practice. %
-[DESCRIPTION OF FIGURE]
+\autoref{act:fig:sdc} contains two plots that were automatically generated by PyCMDS in the context
+of an experiment. %
+In this case, the user used a sapphire plate as a nonresonant medium to record the spectral delay
+dependence. %
+It was a three beam $\omega_1$, $\omega_2$, $\omega_{2^\prime}$ experiment, so three corrections
+were necessary: D2 vs OPA1, D1 vs OPA2, and D2 vs OPA2. %
+Here we focus only on D2 ($\tau_{21}$) vs OPA1, the simplest of the corrections. %
+
+In the left-hand subplot of \autoref{act:fig:sdc} we see the original experiment. %
+Without corrections applied, the user scanned OPA1 vs D2. %
+The curvature in the plot is due entirely to SDC, as sapphire is entirely nonresonant (driven). %
+Using WrightTools, PyCMDS fits each slice to find the delay that gives maximum signal. %
+It then passes those separate fits through a spline to guess the ultimate SDC dependence. %
+PyCMDS makes a best guess in regions where there is not enough signal to determine the appropriate
+delay, like in at 800 nm in the left hand plot. %
+The magnitude of the corrections are roughly 30 fs in this particular experiment: not large, but
+enough to change signal levels by roughly a factor of 2. %
+In other cases SDC is as much as 200 fs. %
+
+In the right-hand subplot the user has taken the same scan again, this time after corrections were
+applied. %
+The delay traces (horizontal) peaks at the same value for every OPA1 position (vertical). %
\begin{figure}
\includegraphics[width=0.45\textwidth]{"active_correction/sdc_before"}
@@ -109,6 +139,7 @@ controlled by delay stages, although the idea has not yet been realized in pract \caption[CAPTION TODO]{
CAPTION TODO: SPECTRAL DELAY CORRECTION FIGURE
}
+ \label{act:fig:sdc}
\end{figure}
\section{Poynting correction} % ==================================================================
@@ -130,9 +161,6 @@ controlled by delay stages, although the idea has not yet been realized in pract }
\end{figure}
-\section{Excitation power correction} % ==========================================================
-
-\clearpage
\section{Chopping} % =============================================================================
\subsection{Scatter} % ---------------------------------------------------------------------------
@@ -374,3 +402,5 @@ This is a well known strategy for removing unwanted interference terms \cite{Spe McClainBrianL2004a}. %
\section{Conclusions} % ==========================================================================
+
+In the future I'd like to do excitation power correction. %
\ No newline at end of file |