## 4 SNR Characterization
### RSOA-FCL Performance  
Here, we perform three studies to analyze the performance of the RSOA-FCL: First, we investigate the impact of the filter bandwidth in the cavity. Second, we study the SNR dependence on cavity length and bias current. Last, we explore the transmission performance for different cavity lengths at optimized operation conditions. In all cases, we use the SNR over the modulation frequency as a measure for the link performance and for comparing parameters. The SNR over the modulation frequency is measured according to the following procedure. We transmit DMT signals and evaluate the SNR on each subcarrier at the receiver. Here, the modulation format and power of each subcarrier is identical. We generate a DMT signal having 256 subcarriers with QPSK as modulation format and equal electrical power for all subcarriers. A PRBS 11 is used as the transmitted bit sequence. The total symbol rate is 10 GBd. The IFFT size is 512 and a 2.5% cyclic prefix is added to each DMT symbol. The signal is generated offline with MATLAB. An AWG generates the analog signal with 0.5 Vpp (peak-to-peak voltage). The output of the RF amplifier was optimized to \(\sim\)3.5 Vpp. A bias tee combines the amplified signal with the specific bias current to drive the RSOA. On the receiver side, an RTO records the signal detected by a photodetector. Subsequently, the signal is demodulated in an offline  
Figure 5: Measured small signal modulation frequency response of an RSOA at 50–120 mA bias currents. The modulation bandwidth increases with bias current. At 100 mA, the RSOA has \(-\)3 and \(-\)6 dB bandwidths of 2.4 and 4.55 GHz. digital signal processing step. The measured EVM of the subcarriers is used to calculate the SNR of each subcarrier. This way, we can determine the frequency resolved SNR of the transmission link across the whole modulation bandwidth. This allows us to determine the usable bandwidth of the transmission link. The useable bandwidth is the frequency band, within which the SNR is high enough to still allow for a BER below 10\({}^{-3}\) with BPSK modulation. Theoretically, a minimum SNR of 6.7 dB is required. First, we investigated the impact of the WDM filter bandwidth in the RN. We compared OBPF bandwidths of 0.6 nm and 2 nm for three cavity lengths of 15 m, 115 m and 1 km standard single mode fiber (SSMF). The bias current was maintained at 100 mA. Figure 6 shows the SNR over the modulation frequency for both cases. The SNR is higher for the filter with wider bandwidth for all cavity lengths. For a 0.6 nm filter, the 15 and 115 m cavities have a peak SNRs of \(\sim\)14 dB. The maximum SNR obtained for the 1 km cavity with 0.6 nm is only \(\sim\)12 dB. The usable bandwidth of the 15, 115 m and 1 km cavities with 0.6 nm filter was \(\sim\)4.6, 4.5 and 2.7 GHz. When a 2 nm filter has been used in the receiver one finds SNRs that are 5 dB higher. The usable bandwidth then increases by \(\sim\)40% to \(\sim\)6.7, \(\sim\)6.4 and \(\sim\)4.5 GHz. The results are summarized in Table 1.  
One explanation for the SNR degradation with narrower filters would be that a narrower filter leads to higher losses [47]. For example, at 100 mA, the reflected power into the RSOA was \(--\)6 dBm and \(--\)2 dBm for the 0.6 and 2 nm filters, respectively. Thus, higher losses means that the RSOA is less saturated. Therefore, the RSOA will be unable to efficiently suppress the residual modulation and noise. As a consequence the RIN will be higher and the SNR is degraded [47]. The second explanation based on [48] is that in case of highly multimode sources, increasing the number of modes (in this case \begin{table}
\begin{tabular}{c c c c} \hline
**RSOA-FCL Cavity Length** & **OBPF \(-\)3 dB Bandwidth** & **Peak SNR (dB)** & **Usable SNR Bandwidth (GHz)** \\ \hline
15 m & 0.6 nm & 14.3 & 4.6 \\  & 2 nm & 19.45 & 6.7 \\ \hline  & 0.6 nm & 14.1 & 4.5 \\
115 m & 2 nm & 19.5 & 6.4 \\ \hline  & 0.6 nm & 11.9 & 2.7 \\
1 km & 2 nm & 17.7 & 4.5 \\ \hline \end{tabular}
\end{table}