## 5 Characterization
### CW Results and Discussion  
First, we measured the output optical power and spectrum of the three sources in Figure 8. Figure 9a plots the powers when the RSOA bias current is swept from 20 to 200 mA for a OBPF bandwidth of 2 nm. The blue and red curves show the measured powers for the ASE spectrum-sliced and the self-seeded RSOA-FCL sources, respectively. The RSOA-FCL delivers power, which is up to 9 dB higher than the power of the ASE spectrum-sliced source. At 100 mA bias current, the spectrum-sliced power and the RSOA-FCL powers are \(-13\) and \(-3.8\) dBm, respectively. The ASE spectrum-sliced source suffers from the high filtering loss of the OBPF. The green curves show the measured power for the externally seeded RSOA. At 100 mA bias current and seeding input powers of \(-25\) and \(-6\) dBm, the measured output powers are \(-0.9\) and \(4\) dBm, respectively. The corresponding 100 mA spectra for the three cases are plotted in Figure 9b. For comparison, the total RSOA ASE is depicted by the black curve. The ASE sliced spectrum (blue curve) is just a filtered version of the total ASE. However, the RSOA-FCL output shown by the red curve has a narrower spectrum with a stronger peak, as seen in the Figure 9b inset. The spectral shape qualitatively agrees with the simulated spectra in Figure 6c. The peaks are due to the RSOA ripple. The strong mode competition in the RSOA-FCL cavity results in a few modes with the least losses that dominate the spectrum. The green curve shows the single mode spectral output of the externally seeded RSOA. In the next step we have measured the RIN of the three sources. The measurement was done at 60, 100 and 140 mA RSOA bias currents. Figure 10a shows the RIN for the ASE spectrum-sliced source, and one can see that the RIN reduces with higher current. This is related to the increase in optical power with bias current, as shown in Figure 9a. The ASE-ASE beat noise results in a higher RIN at lower frequencies [36]. The RIN for the RSOA-FCL is plotted in Figure 10b. The RIN level is lower by more than 10 dB/Hz compared to that of the spectrum-sliced case, especially at lower frequencies. This is attributed to the reduction of the amplitude fluctuation noise of the seeding ASE by the saturated RSOA [37]. As the bias current is increased, the power in the RSOA-FCL cavity increases, thus strongly  
Figure 9: Experimental characterization: (**a**) shows measured optical power _versus_ bias current for the ASE spectrum-sliced, the self-seeded RSOA-FCL and the externally seeded RSOA colorless transmitters; (**b**) shows the corresponding spectrum at a bias current of 100 mA. The inset shows a zoomed-in plot for the three spectra. saturating the RSOA and improving the noise suppression efficiency. However, this noise suppression efficiency is not constant across the whole spectrum. Due to the slow RSOA gain recovery during amplitude fluctuation suppression, low frequency fluctuations are more efficiently suppressed. Another important point to note is that one can resolve the multiple longitudinal modes of the RSOA-FCL. Figure 10d shows the zoomed section of the RIN in Figure 10b, where modes spaced by free spectral range (FSR) of 6.4 MHz are visible. This relates to a \(L=15\) m (SMF cavity length) with \(\mathrm{FSR}=c/2 nL\), where \(c\) and \(n\) are the speed of light in a vacuum and the refractive index of SMF. Finally, the RIN of the externally seeded RSOA case is presented in Figure 10c. At 100 mA, its RIN is around 25 dB/Hz and 15 dB/Hz lower than the RIN of the ASE spectrum-sliced and the RSOA-FCL sources, respectively. Further, we investigated the impact of the cavity parameters on the RIN of the RSOA-FCL. First, the RIN is measured by adding 0, 2, and 4 dB more loss in the cavity and is plotted in Figure 11a. Initially, the total cavity loss due to the insertion losses of the different components is 7 dB. The RIN increases for higher losses. It is discussed in [37] that the RSOA needs to be in the saturation region to achieve efficient amplitude noise suppression and low RIN.