# Optical Frequency Comb Expansion Using Mutually Injection-Locked Gain-Switched Lasers
## 2 Mutually Injection-Locked Gain-Switched Laser  
### Operational Principle  
The operational principle of the proposed scheme is as shown in Figure 0(a), and the line graph is illustrated in Figure 0(b)-e. It comprises two expansion stages: (i) expanded OFC generation via mutually injection-locked gain-switched (MIL-GS) lasers, (ii) further expansion and comb densification. Two semiconductor-based single-mode lasers are chosen as slave lasers and are gain-switched (GS) using a sinusoidal signal. The two GS slave lasers are then injection-locked by a semiconductor-based tunable laser, acting as a common master, as illustrated in Figure 0(b). The master laser provides the frequency and phase locking to both GS slave lasers, resulting in two OFCs emitting comb lines with a high degree of phase correlation between them. The two MIL-GS OFCs are then combined to generate a single broad OFC (Figure 0(c)). Subsequently, the combined MIL-GS OFCs are passed through a phase modulator (PM) for further expansion. Here, the PM can be operated in two modes: (a) expansion and (b) expansion and comb densification. Firstly, the MIL-GS OFC is expanded by driving the PM with a large voltage (multiples of \(V_{\pi}\)) RF signal at the gain-switching (fundamental) frequency (\(f_{s}\)), as depicted in Figure 0(d). Later, the PM is driven with multiples of fundamental FSR (\(n\ \times\ f_{s}\)), but at the same drive voltage as above. Secondly, in the comb densification mode (Figure 0(e)), the PM is driven with a sinusoidal signal that is at a sub-harmonic of the fundamental frequency \(\left(\frac{f_{s}}{m}\right)\). This results \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Ref.** & **Key Expansion** & **OFC** & **Expansion** & **Photonic** & **Demonstration/Comments** \\  & **Component** & **FSR** & **Factor** & **Integration** & **Demonstration/Comments** \\ \hline
[21] & \
\begin{tabular}{c} FP + phase \\ modulator \\ \end{tabular} & 10 GHz & \(\sim\) 2 & Yes &
\begin{tabular}{c} 40 nm wavelength tunability; \\ limited expansion \\ \end{tabular} \\ \hline
[20] & HNLF + DCF & 10.7 GHz & \(>\)3 & No &
\begin{tabular}{c} Wide comb generation; complexity \\ and stability issues \\ \end{tabular} \\ \hline
[35] & \
\begin{tabular}{c} Cascaded GS FP \\ lasers \\ \end{tabular} & 10 GHz & \(\sim\)2 & Yes &
\begin{tabular}{c} 20 nm wavelength tunability; limited \\ expansion; low OCNR \\ \end{tabular} \\ \hline
[36] & Dual-mode FP IL & 6.25 GHz & \(>\)2 & Yes &
\begin{tabular}{c} Simple; 30 nm wavelength tunability; \\ poor flatness; low OCNR \\ \end{tabular} \\ \hline
[38] & \
\begin{tabular}{c} VCSEL HNLF + \\ DCF + Modulator \\ \end{tabular} & 5.2 GHz & \(\sim\)3 & No &
\begin{tabular}{c} Wide comb generation; complexity; \\ stability issues; low OCNR \\ \end{tabular} \\ \hline
[34] & \
\begin{tabular}{c} Parallel cascaded \\ FP + SOA \\ \end{tabular} & 6.25 GHz & \(>\)3 & Yes &
\begin{tabular}{c} Wide comb generation; amplified \\ output power; 30 nm wavelength \\ tunability; complex \\ \end{tabular} \\ \hline
[37] & Dual-drive MZM & 9.5 GHz & \(>\)1.5 & Partially &
\begin{tabular}{c} Simple; low drive voltage; \\ limited expansion \\ \end{tabular} \\ \hline
[38] & \begin{tabular}{c} VCSEL + dual \\ mode injection \\ locking \\ \end{tabular} & 2.5 GHz & \(\sim\)2 & Yes & \begin{tabular}{c} Simple; limited expansion \\ \end{tabular} \\ \hline This work & \
\begin{tabular}{c} Mutual injection \\ locking + phase \\ modulator \\ \end{tabular} & 6.25 GHz & \(\sim\)3 & Yes &
\begin{tabular}{c} Wide comb generation; simple; FSR \\ tunability from 390 MHz to 6.25 GHz \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Survey summary of GSL expansion demonstrations. in the generation of a comb with an FSR that is _m_-times smaller than the fundamental one, thus realizing a simple comb densification. In other words, the FSR of the MIL-GS OFC can be tuned from GHz to MHz, without the need to re-optimize the GSL.