## 2 Amorphous Silicon Waveguides
### All Optical Wavelength Conversion of DPSK and QPSK Signals  
Benefitting from the nonlinear properties of the a-Si waveguides described is the previous section, wavelength conversion experiments at 20 Gb/s using either BPSK or QPSK signals were performed. The experimental set-up used for these experiments was first reported in [26] and is shown in Figure 4. One-millimeter-long waveguides were used for these experiments. The power of the CW pump inside the a-Si waveguide (estimated after taking into account the grating coupler losses) was set to 70 mW, well below the TPA threshold of the waveguide. An optical spectrum recorded at the waveguide output when a BPSK (Binary Phase Shift Keying) signals was used at its input is shown in Figure 5a. The measured FWM conversion efficiency was measured to be \(-26\) dB. Constellation diagrams for  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**\(\omega_{0}\) (dB/cm)** & **Re[\(\gamma\)] (Wm)\({}^{-1}\)** & **Im[\(\gamma\)] (Wm)\({}^{-1}\)** & **Free Carrier (FC) Recovery Time (ns)** & **\(\alpha_{\text{FC}}\) (cm\({}^{2}\))** \\ \hline
4.7 & \(800\pm 50\) & \(15.5\pm 2\) & \(1.87\) & \(0.8\times 10^{-17}\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: a-Si waveguide optical characteristics determined in [28].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reference** & **Re[\(\gamma\)] (Wm)\({}^{-1}\)** & **Im[\(\gamma\)] (Wm)\({}^{-1}\)** & **FOM (Figure Of Merit)** \\ \hline
[26] & 800 & 15 & 4.7 \\
[35] & 332 & 5.43 & 4.9 \\
[32] & 1200 & 12.5 & 7.6 \\
[36] & 770 & 19.6 & 3.12 \\
[45] & 2003 & 205 & 0.85 \\ \hline \hline \end{tabular}
\end{table}
Table 2: a-Si materials reported in the literature.  
both the original BSPK signal (back to back configuration) and DPSK idler are also shown in the figure. Figure 5b shows Bit Error Rate (BER) curves measured for the back-to-back (B2B) and the converted signal revealing that successful wavelength conversion was obtained with only 1 dB of power penalty at a BER = \(10^{-5}\).  
Wavelength conversion experiments using QPSK signals were also carried out. The input signal was a 10 Gbaud QPSK signal carrying a 20 Gb/s pseudorandom data sequence. Results obtained with this signal are outlined in Figure 6.  
Figure 6a,b shows constellation diagrams for both the B2B and the converted signal, whereas Figure 6c shows a comparison of the BER values obtained for these two signals. A penalty of 1 dB is observed at a BER = \(10^{-5}\) demonstrating that the waveguide can be used for the processing of fast, complex-modulation optical signals.  
Figure 4: Wavelength conversion experiment set-up. TF: Tunable filter. CW: Continuous Wave; TX: Transmitter [26].  
Figure 5: (a) Output DPSK (Differential Phase Shift Keying) spectra (resolution 0.05 nm) recorded at the waveguide output. Signal and Idler constellation diagrams are also shown as insets; (b) BER (Bit Error Rate) measurements for a 20 Gb/s BPSK signal. Input pump power was set to 70 mW [26].  
Figure 6: (a) QPSK (Quadrature Phase Shift Keying) B2B (back-to-back) and (b) converted constellation diagrams; (c) BER results [26].  
In this section, we showed the optical characteristics of the hydrogenated amorphous silicon platform [28] and briefly reviewed the performance of fabricated waveguides when used as all-optical wavelength converters for phase-encoded telecommunication signals [26]. Thanks to the absence of TPA in the C-band wavelength region, these devices can be considered as building blocks for applications in future all-optical networks. It is also worth noting that similar results, in terms of conversion efficiency and Optical Signal to Noise Ratio (OSNR) penalty, have been achieved in other platforms, such as crystalline silicon [18; 46], AlGaAs on SOI [47] or in semiconductor optical amplifiers (SOA)-based devices [48]. Please refer to Table 5 for a more detailed comparison between other nonlinear platforms and the a-Si pretended in here.