## 3 Future CONERSTONE Platforms
### Germanium-On-Silicon
The optimized 1 \(\times\) 2 MMI widths (W), lengths (L) and center-to-center output port separation (S) were W\({}_{\text{MMI}}\) = 10 \(\upmu\)m, L\({}_{\text{MMI}}\) = 58.2 \(\upmu\)m, W\({}_{\text{taper}}\) = 4.75 \(\upmu\)m, L\({}_{\text{taper}}\) = 30 \(\upmu\)m and S = 4.75 \(\upmu\)m. The optimized 2 \(\times\) 2 MMI dimensions were identical except for the length of the multimode region, which was L\({}_{\text{MMI}}\) = 114 \(\upmu\)m. MMI loss was calculated from the difference in transmission between different numbers of successively linked MMIs. Loss was measured as 0.21 \(\pm\) 0.02 dB/MMI for 1 \(\times\) 2 MMIs and 0.37 \(\pm\) 0.07 dB/MMI for 2 \(\times\) 2 MMIs [141]. GOS long wavelength operation was demonstrated in the wavelength range \(\lambda\) = 7.5-8.5 \(\upmu\)m, with a minimum experimentally measured propagation loss of 2.5 dB/cm at \(\lambda\) = 7.575 \(\upmu\)m [144]. An input facet for end-fire coupling was fabricated using ductile machining instead of cleaving, allowing optical-quality end facets with rapid wafer-scale fabrication. A photonic method was used to measure loss instead of the cutback method. This used a splitting tree of 1 \(\times\) 2 MMIs to divide power equally between eight waveguides, which terminated in output  
Figure 15: SEM image of a ductile machined end facet of a germanium-on-silicon waveguide. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Device** & **Wavelength (\(\upmu\)m)** & **Parameter** & **Value** \\ \hline straight waveguide & 3.8 & propagation loss & 0.58 \(\pm\) 0.12 dB/cm \\
1 \(\times\) 2 MMI & 3.8 & insertion loss & 0.21 \(\pm\) 0.02 dB/MMI \\
2 \(\times\) 2 MMI & 3.8 & insertion loss & 0.37 \(\pm\) 0.07 dB/MMI \\ straight waveguide & 7.575 & propagation loss & 2.5 dB/cm \\
1 \(\times\) 2 MMI & 7.9 & insertion loss & 0.23 \(\pm\) 0.04 dB/MMI \\ racetrack resonator, 2 \(\upmu\)m GOS & 3.8 & insertion loss & 1 dB \\ racetrack resonator, 2 \(\upmu\)m GOS & 3.8 & extinction ratio & 17.46 dB \\ angled MMI, 2 \(\upmu\)m GOS & 3.8 & insertion loss & 3 dB \\ angled MMI, 2 \(\upmu\)m GOS & 3.8 & crosstalk & \(-\)10 dB \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Experimental performance of 3 \(\upmu\)m thick germanium-on-silicon (GOS) photonic components, which will be available in the CONERSTONE Process Design Kit (PDK). grating couplers. This allowed power to be quantified using a long wavelength infrared camera above the device instead of a conventional detector, thereby avoiding large coupling losses associated with output end-fire coupling. This improved reproducibility by avoiding both the need for consistent end facet preparation for all eight waveguides and the need to align to each waveguide separately. This also improved the tolerance to fabrication errors due to the symmetry of the MMI devices, which in theory have perfectly even splitting ratios. The future contains a large variety of potential applications that would benefit from the wide transparency and low loss operation of GOS devices. One obvious next step is to extend the wavelength range of the all-optical GOS modulator to demonstrate modulation at higher wavelengths, in order to take full advantage of the TPA window of Ge.