## 4 SNR Characterization
### Performance Comparison of the Three Transmitter Schemes  
Here, we compared the SNR performance of the ASE spectrum-sliced with the self-seeded RSOA-FCL and the externally seeded RSOA sources. For this experiment, the ASE spectrum-sliced source was equipped with the 2 nm filter, similar to that of the RSOA-FCL. For the RSOA-FCL, we took the 15 m and 1 km cavities, which offered the best and the worst performance, respectively. Both  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**RSOA-FCL Cavity Length** & **Tx** & **Peak SNR (dB)** & **Usable SNR Bandwidth (GHz)** \\ \hline \multirow{2}{*}{15 m} & BtB & 19.45 & 6.7 \\  & 25 km NZDSF & 19.01 & 3.2 \\ \hline \multirow{2}{*}{115 m} & BtB & 19.5 & 6.4 \\  & 25 km NZDSF & 19.2 & 3.2 \\ \hline \multirow{2}{*}{1 km} & BtB & 17.7 & 4.5 \\  & 25 km NZDSF & 17.1 & 2.8 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Summary of results for the SNR performance of the RSOA-FCL with 15 m, 115 m and 1 km cavities for back-to-back (BtB) and 25 km non-zero dispersion shifted fiber (NZDSF) transmission.  
Figure 8: Measured SNR to compare the impact of transmission fiber on the performance of an RSOA-FCL: The solid and dotted lines show the measured SNR for BtB (back-to-back) and after 25 km non-zero dispersion shifted fiber (NZDSF) transmission, respectively. In both cases, the results are for 15 m, 115 m and 1 km fiber cavity lengths. The figure shows that the SNR bandwidth narrows by up to 50% after transmission over 25 km fiber.  
RSOA-FCLs were equipped with a 2 nm filter. The RSOA input and output power for the externally seeded scheme was \(-\)3 dBm, and \(\sim\)6 dBm, respectively. The bias current for all experiments was set to 100 mA. Figure 9a,b show the measured SNR values for BtB and transmission over 25 km NZDSF, respectively. It can be seen that the externally seeded RSOA and the self-seeded RSOA roughly offer a 10 dB and a 5 dB higher SNR over the ASE spectrum-sliced source across the whole spectral band. The self-seeded RSOA-FCL with the shorter cavity outperforms the longer cavity FCL by about 1-2 dB across the spectral band. And while the externally seeded RSOA exhibited the highest SNR bandwidth of \(\sim\)9.6 GHz for BtB, the 15 m and 1 km long RSOA-FCLs offer useable bandwidths of 6.7 and 4.5 GHz and the ASE spectrum-sliced scheme features a usable bandwidth of only 2.5 GHz.  
When transmitting the signals over 25 km NZDSF fiber, the usable SNR bandwidth reduced to \(\sim\)1.6 GHz for the ASE spectrum-sliced source. The RSOA-FCL declines to a usable bandwidths of 3 and 2.8 GHz for the 15 m and 1 km long cavities. And the externally seeded RSOA still offers a 6.7 GHz bandwidth with sufficient SNR for transmission. The source of the SNR degradation with transmission is the interplay of the chirp (induced by the direct modulation of the RSOA) and the dispersion of the transmission fiber. The narrower usable bandwidth recorded after transmission will influence the maximum achievable bit rate.  
We attribute the higher performance observed for the RSOA-FCL when compared to the ASE spectrum-sliced source to three main factors. First, the RSOA-FCL has a significantly narrower spectrum, which significantly reduces the impact of chromatic dispersion [17; 50]. Second, the RSOA-FCL has a larger output power. For example, at 100 mA, the RSOA-FCL has an output power of \(-\)3.4 dBm while the ASE spectrum sliced scheme emits \(-\)11.8 dBm. Third, the saturated RSOA in the cavity nonlinearly suppress the amplitude noise in the lasing cavity, which lowers the RIN. These three contribute to the improve SNR of the RSOA-FCL compared to the ASE spectrum sliced source [17].