## 4 SNR Characterization
### RSOA-FCL Performance

Table 1: Summary of results to compare the peak signal-to-noise ratio (SNR) and usable SNR bandwidth for the RSOA-FCL (reflective semiconductor optical amplifier fiber cavity laser) with 0.6 nm or 2 nm filter bandwidths. For both filters, the fiber cavity length varied between 15 m, 115 m and 1 km. OBPF: optical band-pass filter. Figure 6: Measured SNR comparison of self-seeded RSOA-FCL transmitter with WDM filter bandwidths of 0.6 and 2 nm in RN. The dotted and solid lines show the measured SNR for the 0.6 and 2 nm filters, respectively. For both cases, three different cavity lengths, 15 m, 115 m and 1 km were used. 0.6 nm filter cavity has peak SNR which is \(\sim\)5 dB lower than that of the 2 nm one. -wider filter) will distribute the noise over the multiple modes thus lowering the noise spectral density and RIN. Subsequently, we continue the investigation with the 2 nm filter only. Next, we further investigated the impact of the distribution fiber cavity length and the bias current on the SNR of the self-seeded RSOA-FCL. We considered cavity lengths with 15 m, 115 m and 1 km SSMF to mimic WDM-PONs with distribution fibers in the range of tens of meters up to a few kilometers, for example in FTTA [49]. The RSOA bias current was set at 80, 100, 110 and 120 mA. Figure 7a,b show similar performance for cavities with 15 m and 115 m length. The insets in Figure 7a,b which are the transmitted (black curve) and the received (red curve) DMT electrical spectrums also show similar characteristics. The similar performance originates from negligible chromatic dispersion for the two lengths. In both cases, the frequency dependent SNR increases with the bias current, which is related to the improved RSOA frequency response for higher bias currents as shown in Figure 5. The SNR degradation for higher modulation frequencies is due to the bandwidth limitations of the RSOA as discussed in Figure 5. At 110 mA, both achieve maximum SNR of \(\sim\)19 dB. The usable bandwidth where the SNR is still sufficiently high to receive a BPSK signal is \(\sim\)6.7 and \(\sim\)6.4 GHz for cavity lengths of 15 m and 115 m, respectively. In Figure 7c we have plotted the SNR for the longer 1 km SSMF cavity. It is way lower than those in Figure 7a,b. This can be understood by the fact that total accumulated chromatic dispersion is larger. Since the RSOA-FCL is highly multi-mode, chromatic dispersion leads to considerable mode partition noise. This noise increases the RIN [50; 51], causing a SNR degradation. A bit higher loss of the 1 km fiber cavity makes only a minor contribution to the SNR increase. At 110 mA, the maximum SNR is \(\sim\)17 dB only and the usable bandwidth is now \(\sim\)4.5 GHz. In contrast to the RSOA-FCL with shorter cavity lengths, the SNR in the 1 km cavity is not increasing with bias current. We expect that this  
Figure 7: Measured SNR versus subcarrier frequency of self-seeded RSOA-FCLs operated at 80, 100, 110 and 120 mA bias currents for cavity lengths of (**a**) 15 m, (**b**) 115 m and (**c**) 1 km. The plots indicate that the SNR bandwidth generally increases with the bias current. The 1 km cavity shown in (**c**) has a reduced SNR peak and bandwidth when compared to the shorter cavities. The dashed line indicates the minimum SNR of 6.7 dB required to transmit BPSK, thus, it determines the usable bandwidth. In each plot, the insets show a sample electrical domain DMT spectra. The black and the red curves show the transmitted and received DMT electrical spectrum, respectively, at 110 mA. The received DMT spectrum for the 1 km cavity RSOA-FCL shown in plot (**c**) inset has a higher noise level compared to that of the 15 and 115 m cavities spectra shown in insets of (**a**,**b**). originates from the strong chirp during direct modulation at higher bias currents. The chirp causes spectral broadening which increases chromatic dispersion penalty. This induces an additional SNR degradation for higher bias currents [52].