Add top-down mesh layout illustrations

chapter-qkd/chapter.tex

@@ -628,6 +628,12 @@ interlocking gear mesh.
 
 \subsection{Simple disc cover}
 
+\begin{figure}[h!]
+    \centering
+    \includegraphics[width=\textwidth,page=1]{shaft_countermeasures_b.pdf}
+    \caption[Coaxial disc mesh schema, top-down view]{Coaxial disc mesh schema, top-down view}
+\end{figure}
+
 In Chapter \todoplaceholder{Provide link to single-board IHSM chapter here}, we have shown how an IHSM that has been
 shrunk to a single, disc-shaped PCB is still useful because we can delegate key management functionality to the mesh
 monitoring circuit's microcontroller or a separate processor sitting next to it on the rotating mesh PCB, yielding a
@@ -648,8 +654,22 @@ the minimum bend radius specification that is common in telecom fiber optics.\to
 
 \todoplaceholder{Finish this part.}
 
+\subsection{Coaxial labyrinth meshes}
+
+\begin{figure}[h!]
+    \centering
+    \includegraphics[width=\textwidth,page=2]{shaft_countermeasures_b.pdf}
+    \caption[Coaxial labyrinth mesh schema, top-down view]{Coaxial labyrinth mesh schema, top-down view}
+\end{figure}
+
 \subsection{Offset labyrinth meshes}
 
+\begin{figure}[h!]
+    \centering
+    \includegraphics[width=\textwidth,page=3]{shaft_countermeasures_b.pdf}
+    \caption[Offset labyrinth mesh schema, top-down view]{Offset labyrinth mesh schema, top-down view}
+\end{figure}
+
 In QKD applications, the simple disc cover design shown above has two main limitations. First, the distance between the
 primary and secondary meshes must be large enough to allow for the fibers' minimum bend radius, resulting in more than
 \qty{10}{\milli\meter} of space available to an attacker. Second, the attacker only has to bend their tool twice to
@@ -663,10 +683,32 @@ Structural support is provided using a CNC machined or 3D printed part, which al
 connections from the shaft to the payload using Flexible Flat Cable (FFC). While the FFC can easily conform to the
 offset labyrinth's sharp corners, an optical fiber can not. Thus, instead of passing it straight through the labyrinth,
 the payload's fiber optic connections are passed through the labyrinth in a three-dimensional spiral shape, avoiding the
-meshes while simultaneously keeping the fibers' bend radii large.
+meshes while simultaneously maximizing the fibers' bend radii.
 
 \subsection{Interlocking gear meshes}
 
+\begin{figure}[h!]
+    \centering
+    \includegraphics[width=\textwidth,page=4]{shaft_countermeasures_b.pdf}
+    \caption[Offset gear labyrinth mesh schema, top-down view]{Offset gear labyrinth mesh schema, top-down view}
+\end{figure}
+
+The offset labyrinth design already achieves a high level of security through its complex passthrough shape, but its
+construction provides some challenges. First, with increasing offset, the step size of one mesh ring's diameter to the
+next increases as well. Even if we only use, say, four mesh rings, this results in a large outer diameter. Second, fiber
+passthrough in the plain offset configuration is possible, but the fiber must be wound in a spiral to pass the two
+meshes' rings alternating from one side to the other because the side with more space alternates from ring to ring.
+
+Both of these disadvantages can be worked around using a design where the two meshes interlock like gears. This does
+mean that the two meshes' rotation must be synchronized, but it allows for a tighter spacing even when using an offset
+design. Additionally, in a gear setup, the wide sides of the inter-mesh zones can be aligned to lie on the same side, so
+fiber passthrough can be realized more easily without the need to spiral the fiber around the axes of rotation.
+
+\subsection{Mesh synchronization}
+
+For geared meshes to work, both speed and phase of the rotation of the two meshes must be synchronized to a small error.
+In this setup, the mesh tabs act like gear teeth. Depending on the ratio between both meshes' tap counts, the two
+meshes do not have to rotate at the same rate of rotation. Instead, harmonic ratios are possible.
 
 \begin{figure}
     \centering