## 4 Results
This quantity is defined as \(G_{net}\) (dB) \(=G_{avg,\ RSOA}\) (dB) \(-G_{ar}\) (dB), where \(G_{avg,\ RSOA}\) is the average gain provided by the directly modulated RSOA and \(G_{ar}\) is the amplification reduction  
Figure 13: Optical spectrum at 4 Gb/s of (**a**) RSOA output, (**b**) BFL output at transparency, \(\Delta\lambda=0\) nm, (**c**) BFL output for detuning \(\Delta\lambda=-0.05\) nm. suffered by the encoded signal due to the BFL filtering action. Therefore, the calculation of \(G_{net}\) required the knowledge of \(G_{avg,\;RSOA}\) and \(G_{ar}\), which were found as follows. First, for NRZ encoding, \(G_{avg,\;RSOA}=G_{CW}\), where \(G_{CW}=8.99\) dB is the RSOA CW gain derived from Figure 5. Second, \(G_{ar}\) was extracted by measuring the mean value of the high level, \(V_{mean,\;1}\), and of the low level \(V_{mean,\;0}\), voltages, which were photodetected by the PD and recorded by the DCA, and taking their median, \(V_{med}=(V_{mean,\;1}+V_{mean,\;0})/2\). This value was then compared between the BFL outputs for the case that the BFL was detuned by \(\Delta\lambda=-0.05\) nm and for the case that the BFL was detuned at transparency, \(\Delta\lambda=0\) nm, which was equivalent to the RSOA output with BFL insertion losses compensated [32]. In this manner, we obtained \(G_{ar}\) (dB) = \(20\log|V_{med,\;BFL\;(\Delta\lambda=-0.05\;\text{nm})}/V_{med,\;BFL\;(\Delta \lambda=0\;\text{nm})}|=4.13\) dB. Therefore, \(G_{net}\) (dB) = \(8.99-4.13\) = 4.86 dB, which would permit propagation of the RSOA directly modulated 4 Gb/s data over approximately 20 Km of standard single-mode fiber (SSMF) without dispersion compensation. Based on this evidence, we placed an available spool of 21.35 Km SSMF after the booster EDFA, whose output power accounted for the fiber's losses. The results taken from the transmission experiment conducted under these settings showed that the form of the eye diagram was close to that when the BFL was not followed by the specific fiber spool. This is indeed verified by comparing Figure 14 to Figure 12f. Therefore the BFL could be employed in real fiber optic distribution and access fiber networks which exploit RSOAs as external modulators [1]. The previous results clearly demonstrated and confirmed that owing to the BFL it is possible to considerably increase the RSOA direct modulation speed. Given the analogous degree of extension of the nominal RSOA modulation bandwidth by other optical filtering schemes [20], we proceeded to investigate to what extent we could exploit this fact while ensuring an acceptable performance. For this purpose, we employed as evaluation criterion the \(EP\) that can be tolerated when forward error correction (FEC) is applied, \(EP_{FEC}\). This is the metric against which the performance of directly modulated RSOAs assisted by photonic filtering schemes is commonly assessed [17; 18; 19]. Figure 15 shows that the BFL allowed \(EP_{FEC}\) to be kept below the \(3.8\times 10^{-3}\) threshold [48] up to 11 Gb/s. Figure 14: Eye diagram for 4 Gb/s encoded signal transmission over 21.35 km standard single-mode fiber after the RSOA-BFL. Vertical scale: 20 mV/div. This corresponds to a higher optical filter's efficiency, which is defined as the ratio between the RSOA extended modulation speed to the RSOA modulation bandwidth, than previously reported [17; 18; 19]. Moreover, Figure 16 shows that the \(EP_{FEC}\) was minimized for an optimum detuning of \(\Delta\lambda=-0.05\) nm. This corroborates the observation and conclusion that in order to maximize the RSOA E/O bandwidth enhancement the CW signal should lie in the vicinity, and in the shorter wavelength side, of the BFL transfer function's nearest notch. Physically this helped suppress more efficiently (Figure 17c) the spectral components of the encoded signal, which were shifted to longer wavelengths (appearing as 'knee' in Figure 17b) with respect to the original CW signal's optical spectrum (Figure 17a) due to SPM inside the RSOA.