## 3 Results
### Simple Waveguides
We observed a weaker effect of the writing velocity on the measured refractive index change between the polymer waveguides and the PDMS framework, as shown in Figure 4b, which is in good accordance with other works on photopolymerization [32]. At a constant laser intensity of \(\sim\)2.25 \(\times\) 10\({}^{12}\) W/cm\({}^{2}\) and 1.3 mm/s writing speed, we measured a refractive index change of \(\sim\)0.0075, while at higher speeds the refractive index contrast stabilizes around 0.005. The insets in both Figure 4a,b show the mean phase profiles recorded along the waveguides in our interferometric system. Figure 4: (**a**) Evolution of the refractive index as a function of increasing peak intensity (and constant writing speed v = 1.8 mm/s) is displayed. The laser peak intensity ranged between \(\sim\)2.10 \(\times\) 10\({}^{12}\) W/cm\({}^{2}\) and \(\sim\)2.74 \(\times\) 10\({}^{12}\) W/cm\({}^{2}\); (**b**) Refractive index contrast for waveguides recorded at \(\sim\)2.25 \(\times\) 10\({}^{12}\) W/cm\({}^{2}\) constant laser intensity and different writing speed, increasing from 1.3 to 2.5 mm/s. The insets display the measurement of the phase profile for different laser peak intensities and velocities, where each phase profile is the mean profile measured from three waveguides written with the same fabrication parameters; (**c**) Transmission loss in dB/cm as a function of wavelength, where colored filters have been inserted in the optical path to measure the output from three different simple waveguides written 500 \(\upmu\)m apart from each other; (**d**) Output intensity profile of three simple waveguides at an inter-waveguide distance of \(\sim\)18 \(\upmu\)m; light in the 535/43 nm spectral band has been coupled in the central waveguide. Waveguide outputs are displayed in the image inset. Error bars indicate the standard deviation from three waveguides fabricated with the same experimental parameters. Due to the elliptic cross-section, the V-number and thus the modal properties of the simple waveguides are birefringent. Based on a \(\Delta n\) of 0.005, the semi-major axis length of the elliptical core of about 1.5 \(\upmu\)m and the manufacturer-specified PDMS cladding refractive index of 1.406, we thus estimated that the upper limit of the V-number for the simple waveguides is 2.1, at the shorter end of the tested spectral band (535 nm), based on the V-number definition for elliptical waveguides reported in [33]. The V-number may be slightly higher (<0.5% as estimated from Sylgard-184 data) when material dispersion is considered. At the longer end (679 nm), the V-number is 1.6; therefore, the simple waveguides are single-mode. To show the crosstalk, we wrote simple waveguides with the same fabrication parameters (\(2.25\times 10^{12}\) W/cm\({}^{2}\) and 2 mm/s), and tested them at an inter-waveguide spacing of 20 \(\upmu\)m, which reduced to ~18 \(\upmu\)m (inset in Figure 4d) after the solvent process to remove the unreacted monomer and PL. At this inter-waveguide spacing and 1 cm sample length, the simple waveguides present crosstalk, as shown in Figure 4d where the intensity profile along the waveguides is shown. The image inset shows the simple waveguides output in the 535/43 nm spectral band; at ~18 \(\upmu\)m inter-waveguide spacing, the output light is clearly visible also from the two adjacent cores whereas only the central waveguide has been coupled.