In this section, we investigate and compare performance of the EO comb lines when used as carriers for superchannel transmission, both before and after distillation with the MRR.
In order to compare the noise reduction in the carrier lines achieved via comb distillation, we first measured the performance of the comb lines without distillation. Here, the comb lines tested in the experiment are marked by the red arrow as shown in Fig. 4
Fig. 5 shows the Q2, BER and GMI plotted against comb line OSNR for the comb lines indicated in Fig. 4, without employing any comb distillation. In this case, the comb lines were used as optical carriers for modulation. As seen in Fig. 5 (a), the quality factor, Q2, increased linearly and eventually saturated around Q2 = ~ 20 dB at an OSNR of > 25 dB, suggesting that the OSNRs above 25 dB were sufficient for the optical carriers in our system. To benchmark the performance of the EO comb lines without distillation, we measured the required OSNR at Q2 = 19.3 dB, corresponding to 0.6 b/symbol or a 5% reduction in GMI for dual polarization 64QAM. At this indicative limit of Q2 = 19.3 dB as illustrated with a yellow dashed line, we found that the required OSNR for all tested lines was in a range of 22-25 dB. When investigating the effect of distillation on BER (Fig. 5 (b)), we see that the BER mirrored the trend in Q2 - dropping and saturating at a BER below 10-2 when the OSNR was > 25 dB. The threshold of Q2 = 19.3 dB in the Q2 plot translates into a BER of 1.3 × 10-2, where we see that the required OSNR was similar to that shown in the Q2 plot. This indicates that the noise could still be considered a Gaussian distributed noise field contribution after distillation. For the GMI plot in Fig. 5 (c), the GMI exhibited a similar trend to Q2, levelling off at ~11.6 b/symbol. With the reference for Q2 and BER as given in Fig. 5 (a), (b), this relates to the GMI limit of 11.4 b/symbol regarded as a 5% reduction of the ideal achievable information rates of 64QAM at 12 b/symbol. Again, the required OSNR to achieve this limit also coincided with that needed for the Q2 and BER.
From the results presented in Fig. 5, we notice that if the OSNR of the tested lines is above 25 dB, the performance can exceed our benchmark of a 5% reduction in GMI. Therefore, we could regard OSNR = 25 dB as the lowest OSNR for our EO comb to achieve the benchmark without narrowband filtering for lines, which can be used to inform us of power requirements for optical frequency combs when used as carriers in optical communication systems.
B. Results with distillation
Figure 6 (a)-(c) shows that the performance of the system using the EO comb lines that were distilled (filtered) by the MRR improves significantly, as compared to the un-distilled case. Figure 6 (a)-(b) shows that all traces saturated at Q2 values of almost 21 dB and a BER < 9 x 10− 3 when the OSNR was > 20 dB. Moreover, all lines required just ~ 13–15 dB of OSNR to reach the limits. For the GMI plot in Fig. 6 (c), we see that GMI also penetrated the limit of 11.4 b/symbol at OSNR ~ 13–15 dB and began to flatten at a GMI of 11.6 once the OSNR was adjusted to > 17 dB.
The plots (Figs. 5 and 6) of these metrics clearly show that the improvement in performance realised when distilling comb lines with the MRR is significant, achieving up to 7 dB in Q2 when comparing Figs. 5 and 6 for OSNR below 25 dB, i.e., where the optical noise dominates. However, once OSNR increases, the improvement is less noticeable since the optical noise plunges into the same level as transceiver noise which is the upper limit of the system. Moreover, one can also conclude that comb distillation can extend the transceiver noise dominated performance ~ 10 dB of OSNR as indicated in the difference of required OSNR at the 5% information reduction limits for both cases, (i.e. required OSNR ~ 22–25 dB for the bypassed case whereas 13–15 dB for the distilled case).
To gain further insight into the performance of the system in terms of OSNR, for Q2 = 19.3 dB, we interpolated the Q2 values from each comb line with and without distillation and solved the fitting equations for the required OSNR. Figure 7 shows that required OSNR is generally flat, with small fluctuations, for all frequencies. The average required OSNR at the limits for carriers with and without distillilation are ~ 24 dB and ~ 15 dB, respectively, both varying by ~ +/- 0.5 dB. This implies a reduction of the required power per line by 9 dB, suggesting that one could either introduce more comb lines to support a larger overall optical bandwidth of the superchannel transmission, or lower the power of the seeding laser for the EO comb generation to reduce power consumption.
Using the average required OSNR of 15 dB for the distilled carriers as a benchmark, we fit the curve and interpolated GMI values at this OSNR for the two cases, to estimate the increase in achievable bit rate achievable by comb distillation. Figure 8 shows that the values for the narrowband filtered lines lie around the corresponding GMI limit of 11.4 b/symbol with a small fluctuation of ≤ 0.1 b/symbol, while for the un-distilled lines GMI values lie around GMI = 9.5 b/symbol with a larger variation of ~ 0.5 b/symbol. This indicates that distillation can improve GMI by about 2 bits/symbol for the 64QAM signals we investigated, showing that comb distillation can provide a real increase in achievable information rates for combs amplified from a low power seed.
In order to qualitatively visualize the impact of comb distillation, Fig. 9 shows the signal constellations for both distilled and un-distilled carriers, at an OSNR of 15 dB. Here, we see that the signal constellation for the distilled carriers exhibits clearly distinguished points with a small distribution arising from Gaussian distributed noise in the central part, with a noticeable increase in variance for the outer lying points. This contrast of variation with constellation point amplitude is more severe than the outer points in the case of un-distilled lines. This can be explained by the fact that all symbol points have the same level of OSNR, hence the further out from the origin the symbols are, the higher is noise level.