## 3 Experiment  
In order to run the experiments in a rational and self-consistent manner, we specified first the RSOA (CIP, model SOA-RL-OEC-1550, cf. photo in Figure 2a) driving conditions defined by the amount of the optical power and DC current, which is launched into, and biases, respectively, the RSOA. For this purpose, we utilized as decision criterion the RSOA electrical modulation bandwidth on which we investigated the impact of these two parameters. The setup that we built to conduct the relevant measurement is depicted in Figure 3.  
Figure 1: (**a**) Block diagram of directly modulated RSOA followed by a serially connected BFL and (**b**) schematic illustration of RSOA response, \(|H_{RSOA}|\), (black line), transfer function of equalizing filter, \(|H_{filter}|\), (green line), and combined magnitude response (red line). Figure 2: Photograph of (**a**) RSOA and (**b**) BFL modules used in the experiments. The core module was an auto-calibrated vector network analyzer (VNA). The VNA provided an RF signal that was superimposed, via a bias-tee, on the RSOA DC bias, which was thus modulated within a frequency range of 10 MHz-12 GHz for fully capturing the RSOA electro-optical (E/O) response. The variations of the RSOA gain induced due to the modulation of the injection current were mapped on a continuous wave (CW) optical signal, which was emitted from a software-controlled laser source tuned around \(\lambda=1560\) nm and entered the RSOA through an optical circulator (OC). The RSOA encoded output was coupled back into the OC and passed through an optical bandpass filter (OBPF) to eliminate unwanted RSOA noise components. It was subsequently detected by a photodiode embedded inside the VNA and was converted again into electrical form, hence closing the circuit path established between the VNA electrical output and the VNA optical input port. In this manner, the VNA calculated the complex scattering parameter, S21, and produced from its squared modulus the RSOA E/O transfer function which, as shown in Figure 4a, had a low-pass characteristic. By controlling the RSOA DC bias, \(I_{bias}\), via a current source, and the CW optical power launched into the RSOA, \(P_{CW}\), with a variable optical attenuator (VOA), we further obtained the normalized  
Figure 4: RSOA E/O response measured for (**a**) constant and (**b**) varying CW input power and DC bias. The dashed line indicates the 3-dB electrical modulation bandwidth. Figure 3: Experimental setup for measuring RSOA E/O response. RSOA: reflective semiconductor optical amplifier, VNA: vector network analyzer, OC: optical circulator, OBPF: optical bandpass filter, VOA: variable optical attenuator, PC: polarization controller, PD: photodiode. Solid and dashed lines show optical and electrical paths, respectively. E/O response for different values of these parameters, as shown in Figure 4b. From these curves we observed that increasing one parameter while keeping the other constant, and vice versa, was beneficial for the 3-dB electrical modulation bandwidth, \(B_{mod}\), which in either case was increased because of the reduction of the RSOA effective carrier lifetime [30,31]. According to this evidence, choosing \(P_{CW}=-5\) dBm and \(I_{bias}=70\) mA results in \(B_{mod,\ RSOA}=0.89\) GHz. The first choice caused the RSOA gain to drop by 5.4 dB from its unsaturated value, as shown in Figure 5a. Thus the RSOA was directly modulated under dynamically stressful conditions [32]. With the CW power going into the RSOA fixed to its set value, the second choice lied in the linear region of the gain-current curve shown in Figure 5b and hence made the RSOA amenable to direct modulation [33]. Having defined the RSOA operating point, the setup of Figure 3 was modified by directly modulating the RSOA with a \(2^{10}-1\) bit-long non return-to-zero (NRZ) data from the pulse pattern generator (PPG) of a bit error rate tester (BERT) (Figure 6). When this signal was applied to the 50 \(\Omega\) RSOA impedance, it offset the RSOA DC current by \(\pm 10\) mA.