# Simulation and Experimental Verification of the Thermal Behaviour of Self-Written Waveguides
## 4 Experimental Verification  
In order to verify the unexpected behaviour of the SWWs as function of temperature, i.e., the increase (instead of decrease) in the transmitted intensity with increasing temperature, an experimental setup was used based on a Peltier element (QC-32-0.6-1.2, Quick-Cool), and it was used to heat the sample. An applied thermal sensor (Heraeus Nexsensos M222) was employed to control the temperature. The SWW was created at room temperature in between two silica fibers on top of an aluminum plate, which was attached to the Peltier element. The writing and adjustment process is performed manually which may result in small differences in the performance for different experiments. The normal deviations between different performances are very small (negligible) if a trained user is running the experiment. An automation of this process based on image analysis and auto-alignment may also be possible and is likely to improve the repeatability. As UV-curable adhesive NOA68 (Norland Adhesive) was used. One of the fibers is connected to a multi-channel laser diode module (MCLS1, Thorlabs), which emits laser radiation at \(\lambda=406\) nm for the writing procedure and \(\lambda=638\) nm and \(\lambda=850\) nm for characterizing the structures. The required external curing was performed by using a germicidal lamp with an intensity of \(\approx\)1.8 mW cm\({}^{-2}\) at 254 nm (40-VL-208 G, Bio-Budget). The experimental setup is shown in Figure 8.  
The difference between the simulated setup and the experimental configuration shown in Figure 8 is the aluminum plate, which protects the heater surface against the monomer. This plate is not considered in the simulation. However, we experimentally measured the temperature on top of the aluminum plate, which is our reference layer for setting the temperature of the waveguide core. The measurement of the change of the optical transmission with the setup sketched in Figure 8 was achieved by using two different fiber-coupled light sources emitting laser radiation at \(\lambda=638\) nm and \(\lambda=850\) nm, respectively, and a fiber-coupled photodetector (S151C, Thorlabs) on the detection side. The used Peltier element enables us to measure \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Heating** & \\ \hline \(\mathbf{T_{Heater}}\)[\({}^{\circ}\)C] & \(\mathbf{T_{SWW}}\)[\({}^{\circ}\)C] & \(\mathbf{t}\)[\(s\)] \\ \hline
26.46 & 25 & 1.15 \\
31.42 & 30 & 0.83 \\
36.43 & 35 & 0.83 \\
41.4 & 40 & 0.81 \\
46.39 & 45 & 0.81 \\
51.37 & 50 & 0.93 \\
56.32 & 55 & 0.79 \\
61.28 & 60 & 0.8 \\ \hline \hline  & **Cooling** & \\ \hline \(\mathbf{T_{Heater}}\)[\({}^{\circ}\)C] & \(\mathbf{T_{SWW}}\)[\({}^{\circ}\)C] & \(\mathbf{t}\)[\(s\)] \\ \hline
61.28 & 60 & 0 (start) \\
56.32 & 55 & 1.76 \\
51.37 & 50 & 1.77 \\
46.39 & 45 & 1.78 \\
41.4 & 40 & 1.78 \\
36.43 & 35 & 1.78 \\
31.42 & 30 & 1.79 \\
26.46 & 25 & 1.79 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Values to be set for the heater to reach the desired temperature of the SWW in the simulation. the optical transmission within a temperature range of 25-60 \({}^{\circ}\)C. With this setup, it was also possible to study both expected behaviours of the SWW during the heating, which are the cases of decreasing and increasing optical transmissions, respectively, depending on the curing time applied during the waveguide fabrication. The achieved results of these measurements are sketched in Figure 10.  
As mentioned already, the reason for the opposite behaviour of the SWWs shown in Figure 10 is that different curing times of the external UV flood exposure were applied. Due to slightly different drop sizes of the resin, it was not possible to define a sharp threshold where the thermal response of the SWW switches from a decreasing (Figure 10 top) to an increasing (Figure 10 bottom) optical transmission as a function of temperature. An external curing time of more than 20 min usually results in complete curing of the droplet.