## 2 Polymer Waveguides
The attachment can be done passively and automatically by a pick-and-placer. No extra submount is needed for the backside illuminated PDs. Figure 1: Microscope images of the five-layer multi-core polymer waveguides, illuminated through the cross-section. (**a**) Broader view showing the total thickness of the polymer layers on silicon substrate and (**b**) detailed view showing the alignment of the waveguides on the 20- μm hexagonal lattice. Figure 2: Microscope images of the five-step 45 \({}^{\circ}\)mirrors. (**a**) Broader view and (**b**) detailed view showing the placement of the mirrors with backside illumination through the polymer, before the mirror surface metallization. (**c**) Detailed view illuminated from the top, revealing the metalized surface. The yellow arrows indicate the 10 \({}^{\circ}\) angle in which the metal vapor is applied. For testing of the mirrors, a tunable laser source is launched via a lensed fiber into the 1-cm long multicore waveguide chip and collected from the corresponding mirrors by a PD bar. The measurement setup is shown in Figure 3a. Index matching oil is used to fill the mirror groove to limit the beam divergence as well as to reduce the back reflection at the PD surface. The multi-step mirror losses are displayed in Figure 3b, ranging from 1.5 dB to 1.9 dB at 1550 nm. The bottom mirrors suffer gradually more losses, caused by the longer free-space beam propagation and associated larger beam divergence, scattering, and more material absorption in the oil filled region. The mirror loss can be further reduced by applying proper waveguide tapers to limit the beam divergence and by optimizing the dicing parameters for a smoother mirror surface. To increase the waveguide index contrast and bring down the device footprint, amorphous silicon nitride (SiN\({}_{x}\)) can be deposited on the polymer cladding by plasma enhanced chemical vapor deposition (PECVD) up to 300 \(\leavevmode\
obreak\ \leavevmode\
obreak\ \text{C}\)[7; 8; 9; 10]. The SiN\({}_{x}\) / polymer heterogeneous waveguides can then be created. The aspect ratio of the SiN\({}_{x}\) core can be adjusted to tailor the mode field diameter. For a thickness below 200 nm and a width of a few micrometers, the majority of the mode resides in the cladding and the waveguide loss depends primarily on the polymer material absorption. The minimal bending radius is 1 mm. The large aspect ratio often causes strong waveguide birefringence. Nevertheless, propagation loss below 1 dB/cm has been demonstrated for both polarizations [7]. The strong thermo-optic effect of the polymer material (coefficient ranging from \(-1\leavevmode\
obreak\ \times 10^{-4}\leavevmode\
obreak\ \text{K}^{-1}\) to \(-3\leavevmode\
obreak\ \times 10^{-4}\leavevmode\
obreak\ \text{K}^{-1}\)) can be utilized for such waveguides. Gratings can be created along the SiN\({}_{x}\) core, as shown in Figure 4a. Tunable Bragg reflectors and sampled grating filters have hence been demonstrated [7; 8; 9]. Further developed is a grating-assisted co-directional coupler consisting of two parallel asymmetric waveguides, one with SiN\({}_{x}\) core and the other with polymer core [10]. The SEM photo of the coupler is shown in Figure 4b. Long-period gratings are formed along the SiN\({}_{x}\) waveguide for phase matching and wavelength filtering. The coupler is differentially thermally tunable. The temperature gradient distribution, introduced by a pair of offset micro-heaters, can either red or blue-shift the filter curve, depending on which waveguide appears \"hotter\". A total tuning range of 82 nm is demonstrated experimentally. Figure 3: (**a**) Photograph of the 1-cm long multilayer waveguides and multi-step mirrors under test. Optical input (left): lensed fiber; electrical output (right): planar PD bar with probe contacts. (**b**) Mirror loss from each waveguide layer extracted from the PD responsivity. Such waveguides have the potential to substantially enhance the versatility of polymer-based waveguide platforms.