## 2 Current CORNERSTONE Platforms
### Carrier Based Modulators  
For datacom applications, high speed transmission is essential, for which a modulator is required to encode the electronic signal onto an optical carrier from a continuous laser source. Most high speed modulators in Si photonics are carrier based, exploiting the plasma dispersion effect [62; 63]. A schematic of a typical Si carrier depletion modulator based on an MZI is illustrated in Figure 5. Due to the adverse effects of any \(pn\) junction misalignment [64], CORNERSTONE uses a self-alignment doping strategy [65] for the junction formation inside the Si rib waveguides. The \(n\)-type phosphorus implantation angle is set to 45\({}^{\circ}\) followed by \(-7^{\circ}\) to implant a small region of \(n\)-type Si in the waveguide rib region, which is initially \(p\)-type doped by boron implantation. Using such an approach, the junction position is defined by the implantation energy as opposed to the lithography alignment accuracy (which varies from chip-to-chip on the wafer) because the top of the Si rib is protected by the same hard mask during \(n\)-type implantation that is used to etch the waveguide. The doping concentration for the \(n\)-type region is in the range 1.12-1.65 \(\times\) 10\({}^{18}\) cm\({}^{-3}\) (region D and C in Figure 5a, respectively), and the \(p\)-type region has a doping concentration in the range 2.25-5.7 \(\times\) 10\({}^{17}\) cm\({}^{-3}\) (region B and A in Figure 5a, respectively). The \(\lambda\) = 1.55 \(\upmu\)m push-pull MZI modulator layout is shown in Figure 5b with a phase shifter length of 1.8 mm. A travelling wave electrode is used to ensure copropagation of the radio frequency (RF) and optical signals. The measured optical phase change efficiency changes from 1.35 to 1.9 V\(\cdot\)cm (see Figure 6) when the direct current (DC) reverse bias voltage varies from 1 to 9 V.  
Figure 4: Typical performance of thermal phase shifters for (**a**) strip waveguide configuration, (**b**) rib waveguide configuration and (**c**) switching speed for both waveguide configurations. Figure 5: A typical Si Mach-Zehnder interferometer (MZI) depletion modulator (**a**) cross-section schematic and (**b**) GDS-II layout. A single side RF signal with swing voltage 6 V was applied to a high-speed ground-signal-ground (GSG) probe, with a DC bias of \(-\) 8 V. The modulator is configured at the quadrature point resulting in a total optical insertion loss of approximately 5 dB. The travelling wave electrode pads were terminated with a 50 \(\Omega\) load. Figure 7 shows the measured optical eye diagram with a 3.9 dB extinction ratio and 40 Gb/s data rate for a 1.8 mm long device. The SOI platform is also compatible with a wavelength of 2 \(\upmu\)m, where hollow-core photonic bandgap fibers (HC-PBGFs) operate at their lowest predicted loss. This type of fiber exhibits substantially lower loss and nonlinearity than the best single mode fiber (SMF) [66]. Coincidentally, such a wavelength aligns well with the gain spectrum of thulium-doped fiber amplifiers (TDFAs). It therefore opens a new telecommunication window at 2 \(\upmu\)m wavelength [67]. Such a window, when used in conjunction with spatial division multiplexing (SDM), can offer a revolutionary capacity boost. SOI based detectors and modulators have both previously been demonstrated at \(\lambda\) = 2 \(\upmu\)m [68, 69]. Here, we further optimized the modulator design and fabricated the device within the CORNERSTONE MPW active device run. The device design is comparable to that shown in Figure 5, with the waveguide width and other passive components optimized for \(\lambda\) = 2 \(\upmu\)m.  
Dedicated 2 \(\upmu\)m equipment was used to characterize the devices, including: a Thorlabs TLK-L1950R tunable laser (1890 nm-2020 nm), an EOT ET-5000 high speed 2 \(\upmu\)m InGaAs detector (with rated bandwidth of \(>\) 12.5 GHz and peak responsivity of 1.3 A/W at 2 \(\upmu\)m); an amplified low speed 2 \(\upmu\)m InGaAs detector; and Thorlabs SM2000 silica fibers.