## 3 Results
Thus, an appropriate combination of these parameters is \(P_{\text{CW}}=-12\) dBm and \(l_{bias}=90\) mA, for which \(EP=4.3\times 10^{-3}\), CW gain \(=12.05\) dB and \(AD_{1/0,max}=10.02\) dB. Figure 4: \(AD_{1/0,max}\) versus RSOA input power and bias current at (**a**) 3 Gb/s and (**b**) 3.5 Gb/s. Figure 3: RSOA continuous wave (CW) gain versus RSOA input power and bias current at (**a**) 3 Gb/s and (**b**) 3.5 Gb/s. Figure 2: Error probability at RSOA output versus RSOA (**a**) input power and (**b**) bias current. The horizontal dotted line denotes the FEC limit. The suitability of this choice is also confirmed by the resultant RSOA modulation bandwidth, which, as shown in Figure 5, is of the order of 3 GHz and hence consistent with the above findings. On the other hand, Figure 6 shows that the RSOA direct modulation capability is not affected by the injection current modulation amplitude and rise time since, despite their variation, the RSOA modulation bandwidth remains nearly the same. Physically this happens because the RSOA operating conditions set by the specified CW power and bias current are such that the negative effects of encoded signal extinction ratio degradation and amplitude clipping are appropriately balanced so that they become rather independent of the modulation current peak and rise time. Figure 7 compiles the encoded pulse waveforms, chirp and pseudo-eye diagrams (PEDs) for favorable (top) and adverse (bottom) RSOA direct modulation which occur at 3 Gb/s and 5 Gb/s, respectively. These opposing cases can be quantitatively compared not only against the \(EP\) but also against the maximum amplitude difference between marks, \(AD_{1,max}\), and \(AD_{1/0,max}\)[24], red chirp fluctuations (\(CF\)) [59] and PED eye opening (\(EO\)) [24]. Thus, \(EP=4.3\times 10^{-3}\) at 3 Gb/s vs. \(7.6\times 10^{-2}\) at 5 Gb/s, \(AD_{1,max}=0.22\) dB at 3 Gb/s vs. 1.03 dB at 5 Gb/s, \(AD_{1/0,max}=10.02\) dB at 3 Gb/s vs. 8.1 dB at 5 Gb/s, \(CF=2\%\) at 3 Gb/s vs. 15% at 5 Gb/s and \(EO=87\%\) at 3 Gb/s vs. 72% at 5 Gb/s. Figure 5: RSOA modulation response for input power \(-12\) dBm and bias current 90 mA. Figure 6: RSOA modulation response for different (a) peak modulation currents and (b) modulation current rise times. A passive single-bus MRR employed as notch filter can allow one to directly modulate the RSOA at enhanced data rate with acceptable performance. To this aim, we must properly select the MRR radius and detuning, and Figure 8 depicts the \(EP\) versus these parameters for three different RSOA direct modulation rates which are at least three times higher than the maximum data rate being possible for the RSOA alone. For acceptable performance, not only the \(EP\) should lie below the defined FEC limit, but also \(AD_{1,max}\) should be below 1 dB [60], \(AD_{1/0,max}\) over 10 dB [13], \(CF\) should tend to unity, or equivalently 0%, so that the phase variation per time increment of the encoded pulses is as balanced as possible [61], while the overshoot (\(OS\)) that inevitably manifests on the encoded pulses [22] should be kept within 25% [62]. The MRR key parameter specifications that efficiently account for, and compromise between, these conditions so that all metrics are acceptable up to 11 Gb/s are \(R\in\{8,12,16,20\}\)um and \(\Delta\lambda=0.67\) nm \(\pm\) 0.02 nm. These specifications have been derived by combining information from the results obtained both graphically (\(EP\)) and numerically (\(AD_{1,max}\), \(AD_{1/0,max}\), \(CF,OS\)). The existence of a specific permissible range of values for both MRR radius and detuning is physically attributed to the conditions that must be fulfilled for the MRR-based filter to efficiently mitigate the pattern-dependent impairments of the directly modulated RSOA. This requires the MRR to properly act upon the encoded pulses spectrum, which due to the RSOA direct modulation has been modified in accordance with the binary content and position of each pulse.