## 3 Experiment
The RSOA output was connected to a BFL (cf. photo in Figure 2b), which was implemented using a 3 dB (50:50) polarization maintaining (PM) fused coupler, an intraloop polarization controller (PC) and a segment of polarization maintaining fiber (PMF) with birefringence \(B\approx 3.3\times 10^{-4}\) in the C-band. The total PMF length, \(L\approx 8.5\) m, resulted in the periodic comb-like transfer function shown in Figure 7a, which comprised of maxima (peaks) and minima (notches) repeated every \(\approx\)0.87 nm  
Figure 5: RSOA gain vs. (**a**) CW input power and (**b**) DC bias for \(I_{bias}=70\) mA and \(P_{CW}=-5\) dBm, respectively. Figure 6: Experimental setup of RSOA electrical modulation. RSOA: reflective semiconductor optical amplifier, BFL: birefringent fiber loop, PPG: pulse pattern generator, OC: optical circulator, OBPF: optical bandpass filter, VOA: variable optical attenuator, PM 50:50: 3 dB polarization maintaining coupler, PMF: polarization maintaining fiber, PC: polarization controller, EDFA: erbium doped fiber amplifier, PD: photodiode, DCA: digital communications analyzer, OSA: optical spectrum analyzer. Solid and dashed lines show optical and electrical paths, respectively. (Figure 7b), equal to the BFL free spectral range (\(FSR=\lambda^{2}/BL\)). Setting the PC to generate a rotation of 90\({}^{\circ}\) to beams coming from both loop directions [34] optimized the amplitude difference between peaks and notches, or the peak-to-notch contrast ratio (PNCR), to 22 dB (Figure 7c). To enhance stability against environmental perturbations by affordable means, the whole construction was enclosed inside a box, which was placed on a marbled base. In this manner, the peaks and notches could retain their spectral position without drifting for a duration that conveniently allowed taking the target measurements. In order to quantitatively assess this possibility, we observed with the help of the OSA the change in the BFL response notches' position with time. This change can be incurred due to random environmental perturbations, such as daily temperature variations or air currents from laboratory staff movements. In fact, according to a calculation conducted for the BFL and input signal parameters values in our experiment and for a typical temperature-dependent birefringence coefficient of \(-7\times 10^{-8}\) /K, a drift in temperature of more than \(\sim\)1.3 \({}^{\circ}\)C only can cause a notch shift beyond half the BFL FSR [35], which is the range within a notch is allowed to lie to mitigate the pattern effects induced in the RSOA [27]. Thus we found that the notches' displacement due to reasons like the aforementioned ones was of the order of \(+0.1\) nm for \(\sim\)12 min, as it can be observed in Figure 8a. This is better perceived using the experimental data derived for the notch shift in time and displaying with simulation assistance the BFL transfer function at different time instants (Figure 8b). Figure 7: (a) BFL measured transmission response, with free spectral range (\(FSR\)) of 0.87 nm (b) and peak-to-notch contrast ratio (PNCR) of 22 dB (c) indicated by vertical and horizontal cursors, respectively. Consequently, this interval provided enough margin to perform the experiment and obtain valid and repeatable results while scanning the relative spectral offset between carrier and notches with sufficient degree of resolution. After the permissible window closed, it was necessary to compensate for the inevitable notches' shift, and the more practical way of doing this was by fine adjusting the carrier position against the notch being closer to its longer sideband. As in our case a notch shifts with temperature at a rate of \(\sim\)0.33 nm/\({}^{\circ}\)C, confining this shift to the bounds specified above would require to keep the temperature deviations within \(\pm\)0.3 \({}^{\circ}\)C, which in turn could be achieved with a thermoelectric cooler (TEC) [36].