## 6 Data Transmission Comparison  
In this section, we discuss the data transmission performances of the ASE spectrum-sliced, the RSOA-FCL, and externally seeded RSOA schemes in the experimental setups of Figure 8.  
First, we are interested in the frequency response of the respective sources. Instead of measuring only the small signal modulation frequency response, we measure the frequency dependence of their SNR. The SNR frequency response is interesting, because it directly relates to the bit-error ratio (BER) and includes effects from both a degradation of the frequency response and also a frequency dependence of the RIN. The frequency-dependent SNR has been measured by transmitting a quadrature phase-shift keying (QPSK) encoded discrete multi-tone (DMT) signal with a pseudorandom binary sequence (PRBS) sequence of 2\({}^{11}\)-1 [13]. In our case, a 256 subcarrier DMT signal within a total bandwidth of 10 GHz has been generated by an arbitrary waveform generator (AWG) to drive the RSOA. The RSOA bias current has been optimized separately for each transmitter. In the case of the externally seeded RSOA scheme, the input seeding power was \(-4\) dBm. After direct detection, we measured the SNR of each subcarrier. The measured SNRs for the three schemes are plotted in Figure 13a. One can see that the RSOA-FCL sits between the spectrum-sliced and the externally seeded RSOA schemes. It has a \(\sim\)5 dB higher SNR than the former and a \(\sim\)5 dB lower SNR than the latter. For all cases, the SNR reduction at higher frequency is due to the bandwidth limitation of the RSOA. The small signal modulation frequencies of the RSOA for bias currents from 50 to 120 mA are plotted in Figure 13b. The RSOA has a 3 dB bandwidth of 2.4 GHz at 100 mA. Next, we transmitted 5 and 10 Gbit/s amplitude shift keying (ASK) non-return-to-zero (NRZ) signals with the three schemes. Figure 14 shows plots with measured quality factors \(Q^{2}\) (dB) for a BtB (back-to-back), a 8 km SMF and 25 km non-zero dispersion-shifted fiber (NZDSF) transmission experiment with 5 and 10 Gbit/s NRZ signals. Since the use of forward error correction (FEC) in MFH is a common option, we used the standard BER thresholds of \(10^{-2}\) and \(10^{-3}\) as a minimum  
Figure 13: Measured SNR and small signal frequency response: (**a**) Measured signal-to-noise ratio (SNR) _vs._ frequency for the RSOA ASE spectrum-sliced, self-seeded RSOA-FCL and externally seeded RSOA colorless transmitters driven at 100 mA; (**b**) Small signal modulation frequency response of an RSOA at different bias currents. requirement [38, 39]. Then we calculated the \(Q^{2}\) values for each transmitter scheme at various received input powers. We have been using \(Q^{2}\) values rather than BER as a quality metric, because our dominant noise source is the RSOA ASE noise that acts as an almost ideal white Gaussian noise source for not-too-high input powers [40]. The black dashed lines in Figure 14 indicate the \(Q^{2}\) values 7.3 and 9.8 dB which correspond to a BER of \(10^{-2}\) and \(10^{-3}\), respectively. In Figure 14a, it is shown how all three transmitter schemes can achieve \(Q^{2}\) values above 9.8 dB for a BtB transmission. Of the three schemes, the externally seeded RSOA delivers the highest \(Q^{2}\) with a value of up to 17 dB. The self-seeded RSOA-FCL follows with up a \(Q^{2}\) value of 15 dB. The RSOA ASE spectrum-sliced scheme has the lowest BtB \(Q^{2}\) with a value just above the 9.8 dB limit. For the 8 km SMF transmission, shown by the dashed lines, only the RSOA-FCL and the externally seeded RSOA can achieve a \(Q^{2}\) above 9.8 dB. Chromatic dispersion and chirp interplay reduce the performances in all three schemes. Of course, the penalty is far stronger for the spectrum-sliced source. We also transmitted the 5 Gbit/s signal over 25 km NZDSF with chromatic dispersion of 5 ps/nm\(\cdot\)km, depicted by dotted lines in Figure 14a. The transmission over NZDSF was carried out to reduce the impact of chromatic dispersion in order to extend the reach.