# High-Efficiency Quantum Dot Lasers as Comb Sources for DWDM Applications
## 3 Performance of CPMLLs
### Comb Bandwidth  
In order to investigate the performance of the CPMLLs as optical frequency comb sources, optical spectra from the devices in Figure 3 were collected for varying SA reverse biases and gain section drive currents. To prevent laser damage, the states in the gray region in the upper right hand corner of Figure 4a-i were not tested. These states exceeded the maximum allowed SA current, which was conservatively set at 16 mA total. The spectra collected were filtered by removing states with less than 15 dB SMSR. For all remaining states, the number of 60 GHz channels in a 6 dB bandwidth were tabulated, and the results are shown in Figure 4a-c. The maximum and average bandwidths of the three devices are listed in Table 1. The 10% device shows the largest bandwidth, the highest average  
Figure 3: Comparison of 3 rd harmonic quantum dot colliding pulse mode-locked lasers (CPMLLs). (**a**) Light–current and (**b**) single-sided wall plug efficiency curves with saturable absorber (SA), biased in parallel with the gain section (forward-biased), and reverse-biased at 2 V (circled curves). The three forward-biased curves in (**a,b**) are hard to distinguish, because the performance is similar. (**c**) The percent reduction in power from reverse biasing the SA. SA lengths are labeled on the curves as a total fraction of the cavity length.  
bandwidth, and the largest number of states with SMSR exceeding 15 dB. Figure 5a shows the comb with the largest achievable bandwidth of 42 comb lines, or 14.1 nm, from the 10% device at a gain current of 200 mA and an SA reverse bias of 3.3 V.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**SA Length** & **SMSR \(\geq\) 15 dB** & **Max. 6 dB BW** & **Ave. 6 dB BW** & **Max. PoLL** & **PoLL \(\geq-\)8 dBm** & **Max. Eff.** & **Ave. Eff.** \\ \hline
6\% & 252 & 30 & 20.1 & \(-\)6.1 & 27 & \(-\)29.2 & \(-\)32.3 \\
10\% & 390 & 42 & 24.0 & \(-\)5.1 & 117 & \(-\)28.9 & \(-\)32.3 \\
18\% & 304 & 35 & 18.0 & \(-\)5.2 & 51 & \(-\)29.3 & \(-\)35.3 \\ \hline \hline \end{tabular}
\end{table}
Table 1: List of important parameters from the devices tested. Saturable absorber length, as a total fraction of the cavity length, is listed as percentage in column one. The number of states measured with a side mode suppression ratio greater than 15 dB (SMSR \(\geq\) 15 dB) are listed in column two. The number of comb lines in the state with the largest 6 dB bandwidth is listed in column three. The average bandwidth of all states with SMSR \(\geq\) 15 dB is listed in column four. The power of the lowest line (PoLL) of a contiguous 20 line comb is listed in column five in dBm. The number of states with a PoLL \(\geq-\)8 dBm is listed in column six. The maximum wall plug efficiency of the lowest line (WPE20) is listed in column seven in dB. The average WPE20 of all states with SMSR \(\geq\) 15 dB is listed in column eight in dB.  
Figure 4: Color-coded plot of the 6 db optical bandwidth (**a**–**c**), measured in the number of 60 GHz spaced comb lines, power of the lowest line of a contiguous 20-line comb (**d**–**f**), and the wall plug efficiency of the lowest comb line (**g**–**i**) for total saturable absorb lengths of 6%, 10%, and 18% of the cavity length. Gray regions were not tested to avoid risk of device damage.