## 2 Current CORNERSTONE Platforms
### Passive Devices
\begin{table}
\begin{tabular}{c c c c} \hline
**Structure** & **220 nm SOI Platform** & **340 nm SOI Platform** & **500 nm SOI Platform** \\ \hline Single mode rib waveguide & 3 dB/cm (D = 120 nm) & 0.6 dB/cm (D = 140 nm) & 2.6 dB/cm (D = 300 nm) \\  & & 2.8 dB/cm (D = 240 nm) & \\ Single mode strip waveguide & 4.0 dB/cm & 3.5 dB/cm & - \\ Rib waveguide 90\({}^{\circ}\) bend & 0.03 dB (R = 25 \(\mu\)m) & 0.02 dB (R = 25 \(\mu\)m) & 0.03 dB (R = 25 \(\mu\)m) \\ Strip waveguide 90\({}^{\circ}\) bend & 0.011 dB (R = 10 \(\mu\)m) & 0.014 dB (R = 10 \(\mu\)m) & - \\ Grating coupler (uniform) & 5.5 dB & 5.0 dB & 5.0 dB \\ Grating coupler (apodized) * & 3.0 dB & 1.0 dB & - \\ \(2\times 1\) MMI & 0.30 dB (strip) & 0.20 dB (strip) & 0.25 dB (rib) \\ \(2\times 2\) MMI & 0.30 dB (strip) & 0.30 dB (strip) & 0.25 dB (rib) \\ \hline \end{tabular}  
* The apodized grating coupler design is only available using e-beam lithography accessible by the multi-project-wafer (MPW)+ mechanism.  
\end{table} 
Table 2: Typical insertion loss of standard CORNERSTONE devices in various SOI platforms at a wavelength of 1550 nm (TE mode). Single mode (SM) waveguide width = 450 nm. R = bend radius. D = etch depth. MMI = multi-mode interferometer. Figure 1: Schematic and transverse electric (TE) mode profiles for a (**a**) single mode rib waveguide, and (**b**) single mode strip waveguide, at \(\lambda\) = 1.55 \(\mu\)m. The Si thickness is 220 nm in both cases, with the slab thickness equal to 100 nm for the rib waveguide. Waveguide width = 450 nm. Note that the \(x\)-axis and \(y\)-axis scales are not equivalent. The two types of waveguides (rib and strip) can be integrated together using a rib-to-strip taper, which is fabricated using a two-step self-aligned etching process, the schematic of which is shown in Figure 2. A typical insertion loss of this component is 0.07-0.12 dB when the taper length varies from 50 to 10 \(\upmu\)m, respectively. Optical coupling both from external laser sources to the PIC chips and from the PIC chips to external detectors can be based on grating couplers or edge couplers. Generally, edge couplers have much wider bandwidth, whilst grating couplers have the flexibility of positioning anywhere on the PIC. Typically, grating couplers have straight teeth with a fixed pitch and filling ratio with feature sizes \(>\) 250 nm to be compatible with DUV lithography. These grating couplers are fabricated with long tapers with a length of 350 \(\upmu\)m to taper from the coupler width of 10 \(\upmu\)m to the SM waveguide width. A coupler width of 10 \(\upmu\)m is used to closely match the mode profile of the optical beam exiting the fiber, which is used to input/output light. Alternatively, focusing grating couplers [38] have a much smaller footprint whilst maintaining compatibility with DUV lithography. If pursuing ultrahigh coupling efficacy, taking the advantage of e-beam lithography compatibility in CORNERSTONE, one can make subwavelength and apodized grating couplers [39; 40] (also see Section 2.4.1). The 1 dB bandwidth of both types of grating coupler design is typically about 35 nm. As for edge couplers, inverse taper couplers [41] are the most common, but these require accurate polishing of the waveguide facets for optimal coupling efficiency, yielding an approximate coupling efficiency of 2 dB per coupler with an inverse taper width of 200 nm measured in a wavelength range of 1265-1355 nm. Beam splitters/combiners are key components in a PIC, for example to form Mach-Zehnder interferometers (MZIs) for use in switching networks [42] for programmable photonics applications [43]. Three of the most commonly used beam splitters/combiners are multimode interferometers (MMIs), which are based on the self-imaging mechanism [44; 45], Y-splitters [46] and directional couplers (DCs) [47]. Generally, MMIs are more robust than both Y-splitters and DCs because they do not require accurately defined features.