## 3 Results  
### Simple Waveguides  
Finding the right combination of the various fabrication parameters is a crucial challenge in DLW photopolymerization, requiring a compromise between the irradiating laser intensity and writing speed in order to avoid material damage. We wrote waveguides over 5 cm long samples, isolated from each other by an inter-waveguide distance of about 500 \(\upmu\)m to eliminate any crosstalk. For each set of parameters, we fabricated five waveguides, from which three were interrogated for statistics. We investigated the finished samples of isolated single waveguides (simple waveguides) using a phase-contrast microscope (Olympus IX-71, Hamburg, Germany) to verify the formation of the poly-DVB waveguides embedded in the PDMS framework (Figure 3a,c). The cross-section of single-pass laser tracks (see the inset in Figure 3b for a typical example) was measured to be \(-0.6\)-\(1\upmu\)m wide and \(-1.5\)-\(3\upmu\)m high depending on the writing parameters, as shown in Figure 3b,d. The dimensions (the width and the height) of the focal volume during the fabrication procedure depend not only on the laser pulse energy and the numerical aperture of the writing objective but also on the writing speed. Figure 3a,b reveals the evolution of the waveguide geometry (width and height)as a function of increasing writing speed at a constant writing laser intensity (I \(\sim\) 2.25 \(\times\) 10\({}^{12}\) W/cm\({}^{2}\)). Figure 3c,d shows the waveguide geometry as a function of increasing laser intensity at a constant writing speed v = 1.8 mm/s. Overall, both the axial and lateral dimensions of the waveguide structure depend linearly on laser peak intensity and writing speed, showing an aspect ratio of about 1:3; nevertheless, experimental sample variations such as monomer/PI solution infiltration efficiency, and optical settings during the writing, strongly affect the waveguide fabrication where experimental conditions remain unchanged. This reflects in small geometrical fluctuations in the waveguide size throughout different samples fabricated with the same experimental parameters such as laser focal intensity and writing speed. This can be observed in the plot in Figure 3d. We measured the refractive index contrast as a function of focal intensity at a constant writing speed and as a function of writing speed at a constant focal intensity, as shown in Figure 4a,b, respectively. Our experiments suggest that the polymerization threshold for DVB is about \(\sim\)1.93 \(\times\) 10\({}^{12}\) W/cm\({}^{2}\) for simple waveguides based on visual determination with phase-contrast microscopy. As we show in Section 3.2, this is higher than the actual polymerization threshold, and it is possible to write waveguides at a slightly lower intensity, since phase-contrast microscopy cannot detect the presence of  
Figure 3: **(a)** Phase-contrast microscopy top view image of the recorded poly-DVB optical waveguides written at constant peak intensity I \(\sim\) 2.25 \(\times\) 10\({}^{12}\) W/cm\({}^{2}\) and varying fabrication speed, ranging from 1.3 to 2.5 mm/s; **(b)** Waveguide width and height evolution as a function of increasing writing speed where the error bar derives from the measurement of three waveguides; the inset shows the cross-sectional view of a single-track waveguide where the scale bar measures 5 \(\upmu\)m; **(c)** and **(b)** display the same as above as a function of increasing fabrication peak intensity from 1.93 to 2.74 \(\times\) 10\({}^{12}\) W/cm\({}^{2}\) and constant speed v = 1.8 mm/s. Error bars indicate the standard deviation derived from the measurement of five different waveguides geometry. Scale bars dimension in **(a)** and **(c)** is 50 \(\upmu\)m.  
low-concentration polymer in such a small volume with sufficient sensitivity. The refractive index change induced in the polymer material increases linearly from 0.005 to 0.012 with laser intensity in the 2.10-2.8 \(\times\) 10\({}^{12}\) W/cm\({}^{2}\) range (Figure 4a).