# Impact of Chirp in High-Capacity Optical Metro Networks Employing Directly-Modulated VCSELs
## 4 Discussion and Conclusions  
We propose the use of directly-modulated long-wavelength VCSELs to implement S-BVT for capacity transmission higher than 50 Gb/s per wavelength in metropolitan area systems characterized by reduced cost, power consumption, and footprint. The most significative drawback of the direct modulation of VCSELs is frequency chirp, in particular when associated to optical filtering as in the network nodes. Thus, we analyzed its impact in case of dense WDM (25-GHz) WSS filtering in a DMT-based transmission system with direct modulation of the optical source and coherent detection at the receiver end to compensate for the cumulated chromatic dispersion. A performance evaluation by suitable simulations has been undertaken both in case of DSB and SSB transmissions. Since an optical bandwidth of 25 GHz can be filled up, a 10-GHz DSB and a 20-GHz SSB signals have been used. In order to obtain the chirp parameters to be used in reliable simulations for the transmission performance evaluation, preliminary measurements on InP VCSELs with two designs (a high-bandwidth device based on a short-cavity technology and a widely-tunable laser with MEMS top mirror) have been performed. A third case including the exploitation of an EML was added for reference; in order to focus on the chirp impact only, the remaining simulation parameters have been maintained unchanged for all the considered cases; in particular, the modulation bandwidth was set to 18 GHz. The maximum transmitted capacity for both DSB and SSB DMT modulation were evaluated as a function of the number of crossed nodes in a mesh metro network and of the received OSNR. The results indicate that, especially for many WSS crossings, in cases of DSB modulation, lower chirp parameters allow higher capacities, whereas in cases of SSB modulation and lower OSNR, the presence of a limited amount of chirp supports better performance, in particular for lower OSNR values (close to 30 dB). The presence of frequency chirp associated with direct modulation broadens the spectral occupancy in the optical domain. In cases of DSB transmission, the optical filter is perfectly centered on the VCSEL carrier, and the best results can be achieved by a chirp-free optical signal, which shows an optical spectrum with minimum bandwidth. On the other hand, in cases of SSB transmission, the frequency chirp superimposed on the signal permits to reduce the CSPR with respect to the \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **OSNR** & **40 dB** & **35 dB** & **30 dB** \\ \hline \multirow{2}{*}{Reach} & 35-km span & 70 km & 210 km & 735 km \\  & 65-km span & — & 65 km & 260 km \\ \hline \multirow{2}{*}{Short-cavity} & Capacity w 1 WSS & 78.6 Gb/s & 72.6 Gb/s & 57.3 Gb/s \\  & Capacity w 3 WSS & 68.6 Gb/s & 63.1 Gb/s & 54.5 Gb/s \\ \multirow{2}{*}{VCSEL} & Capacity w 5 WSS & 61 Gb/s & 58.4 Gb/s & 50.5 Gb/s \\ \hline \multirow{2}{*}{Tunable} & Capacity w 1 WSS & 76.2 Gb/s & 70 Gb/s & 56.2 Gb/s \\  & Capacity w 3 WSS & 73.3 Gb/s & 67 Gb/s & 55 Gb/s \\ \multirow{2}{*}{VCSEL} & Capacity w 5 WSS & 70.4 Gb/s & 63.8 Gb/s & 54 Gb/s \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Transmitted capacities and SSMF reach for SSB DMT as a function of: span length, VCSEL type/chirp, and number of crossed WSS. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **OSNR** & **40 dB** & **35 dB** & **30 dB** \\ \hline \multirow{2}{*}{Reach} & 35-km span & 70 km & 210 km & 735 km \\  & 65-km span & — & 65 km & 260 km \\ \hline \multirow{2}{*}{Short-cavity} & Capacity w 1 WSS & 45.5 Gb/s & 30.9 Gb/s & 22.8 Gb/s \\  & Capacity w 3 WSS & 35.3 Gb/s & 27.5 Gb/s & 19.9 Gb/s \\ \multirow{2}{*}{VCSEL} & Capacity w 5 WSS & 31.1 Gb/s & 25.2 Gb/s & 19.2 Gb/s \\ \hline \multirow{2}{*}{Tunable} & Capacity w 1 WSS & 61.2 Gb/s & 51 Gb/s & 34 Gb/s \\  & Capacity w 3 WSS & 52.6 Gb/s & 47.8 Gb/s & 32.4 Gb/s \\ \multirow{2}{*}{VCSEL} & Capacity w 5 WSS & 48 Gb/s & 43.75 Gb/s & 30 Gb/s \\ \hline \hline \end{tabular}
\end{table}