## 2 Polymer Waveguides  
Both ridge waveguides and channel waveguides can be fabricated on polymer platforms. The square-shaped core of a channel waveguide can be adopted to minimize the waveguide birefringence. The refractive index can be chosen from \(\sim\)1.4 to \(\sim\)1.7, depending on the chemical composition. The dimension of the core measures a few micrometers for single mode operation. Optical polymer layers can be formed by spin-coating and subsequent UV and temperature curing. Silicon wafers are usually taken as substrate, though the technology is also applicable on glass plates, ceramic and InP wafers. The structuring of the waveguide follows mainly three methods [20]: (1) standard semiconductor processing methods with photolithography and reactive ion etching; (2) direct UV lithographic patterning followed by wet-chemical rinsing; and (3) soft molding and imprinting. In this work, the first fabrication method has been used. The technology details have been published in [2; 5; 7]. Metals can be inserted virtually anywhere in the vicinity of the waveguide to function as heater electrodes for thermally tunable devices but also as RF electrodes to drive the electro-optic (EO) polymer-based modulator. The fabrication process can be repeated to stack up the polymer waveguide layers into a multi-core structure [5], as shown in Figure 1a,b. The waveguide features a square-core of 3 \(\upmu\)m \(\times\)3 \(\upmu\)m and a core/cladding index contrast of 1.48/1.45. A bending radius of 1.5 mm of such a waveguide will lead to a loss \(\sim\)0.12 dB for a 90 \({}^{\circ}\)bend. The five-layer polymer waveguides shown in Figure 1a add up to a total thickness of 108 \(\upmu\)m, within which the bottom cladding is 20 \(\upmu\)m directly above the silicon substrate, the inter-waveguide-layer distance is 17.32 \(\upmu\)m (4 times), and the upper cladding is 15.7 \(\upmu\)m below air. The waveguide loss among all five layers is below 1 dB/cm and the crosstalk between the waveguides at the 20-\(\upmu\)m hexagonal lattice is below \(-\)40 dB after a propagation distance of 3.8 cm. This technology can be exploited to fabricate three-dimensional photonic integrated circuits (PICs) with high vertical density. On a polymer platform, 45 \({}^{\circ}\)facets can be created by applying a fast-rotating angled dicing blade and sawing through the polymer chip [2]. The surface roughness can be minimized by the intrinsic \"polishing\" during sawing. The high concentration of small diamond grains on the blade tip, fast rotation speed and slow feeding speed can help generate a smooth surface. To improve the reflectivity, the 45 \({}^{\circ}\) surface should be coated with a thin layer of metal, typically Ti and Au, by directed metal evaporation. Apart from the design and processing simplicity, such 45 \({}^{\circ}\) deflection mirrors offer much broader coupling bandwidth, less polarization sensitivity, and less beam divergence, when compared to the vertical grating couplers. The 45 \({}^{\circ}\)groove is usually filled with index matching epoxy so that the back reflection on the straight facet can be suppressed and the beam divergence in the free space can be reduced. The 45 \({}^{\circ}\)mirrors can be arrayed by varying the dicing depths to tap out light from each of the multi-layer waveguide cores separately, as indicated in Figure 2. The depth of the mirrors must be calculated and well controlled so that the tip reaches, in the ideal case, the middle point between the two core layers. Light from each layer can then be subsequently extracted by the mirror array. To ensure that the metal coating reaches the very bottom of the deep mirror, while staying clear from the straight waveguide facet, thermal evaporation with well-defined deposition directivity must be used. The tilt angle must be carefully calculated. The waveguides can fan out with a larger in-plane pitch and the lateral distance between the mirrors can be adjusted to allow the attachment of either separate 1D planar PD bars or a common 2D PD array.