# Optical Frequency Comb Expansion Using Mutually Injection-Locked Gain-Switched Lasers
## 3 Results and Discussion
### Comb Line Characterization  
#### 3.2.1 Phase Noise and Relative Intensity Noise  
The phase noise characterization of the MIL-GS OFC lines is performed by measuring the frequency-modulation (FM) noise spectrum using the modified delayed self-heterodyne method [41]. Three different comb lines (right edge (R), middle (M), left edge (L)), as marked by the arrow in Figure 4c (Lines filtered using an optical band pass filter (Yenista Optics XTM-50) with a bandwidth of 6 GHz) and Figure 5a (Lines filtered using a semiconductor laser based active demultiplexer [42]) are filtered for phase noise analysis and compared with the master. It is worth mentioning that the 3.125 GHz FSR comb lines are filtered using a semiconductor-based active demultiplexer, which operates on the principle of optical injection locking (OIL). The principle operation and detailed characterization of such active demultiplexers are investigated in [42]. The FM noise spectrum \(S_{f}(f)\), describes the power spectral density of the instantaneous frequency fluctuation. The Lorentzian-shaped optical linewidth (\(\delta f\)) can be retrieved from the flat (white) FM-noise component \(S_{o}\), using the relationship \(\delta f=S_{o}\cdot\frac{\pi}{2}\)[41]. The frequency range of the measurement is set to 1 GHz with a digital filter applied during the offline processing. The resultant FM noise spectrum of a 6.25 GHz comb line and the master laser are illustrated in Figure 6a. The optical linewidth of the master laser, and the middle (M), right (R) and left (L) comb lines (as marked with arrows in Figure 4c) are measured as \(\sim\)30, \(\sim\)40, \(\sim\)47, and \(\sim\)45 kHz, respectively. Furthermore, we measured optical linewidth of other comb lines (marked in numbers, Figure 4c) and found to be between 40 to 47 kHz across the entire comb, as illustrated in Figure 6c. The negligible fluctuation in the measured linewidth is due to the injection locking conditions [6]. Finally, linewidths of the OFC with the FSR of 3.125 GHz are measured to be \(\sim\)42 kHz (middle), \(\sim\)49 kHz (right), and \(\sim\)47 kHz (left), as shown in Figure 6b. The narrow optical linewidths in all the cases indicate the efficient transfer of phase noise from the master laser to the expanded OFC.  
Next, the random intensity fluctuations of the comb lines are evaluated by measuring the relative intensity noise (RIN) [43] of the same individual comb lines, as marked in Figures 4c and 5a. The RIN spectra are shown in Figure 7a,b for an FSR of 6.25 and 3.125 GHz, respectively. From the figure, one can observe that all the comb lines exhibit a similar RIN profile, which is comparable with that of the master laser. The averaged RIN (from DC to 6 GHz) for the 6.25 GHz and 3.125 GHz FSRs are measured to better than \(-\)152 dB/Hz for all cases, which is slightly worse than the \(-\)157.2 dB/Hz measured RIN of the master laser. The phase noise and RIN characterization results are presented in Table 4. The results demonstrate the excellent phase and intensity noise properties of the generated OFC, satisfying requirements of various applications.  
\begin{table}
\begin{tabular}{c c c} \hline
**MIL-GSL OFC** & **OCNR** & **Number of Lines** \\
**FSR** & **(Tone at -1550 nm)** & **(5 dB from Peak)** \\ \hline
6.25 GHz & 50 & 27 \\
3.125 GHz & 45 & 58 \\
1.5625 GHz & \(\sim\)42 & \(\sim\)113 \\
781.25 MHz & \(\sim\)38 & \(\sim\)214 (within 10 dB) \\
390.625 MHz & \(\sim\)33 & \(\sim\)453 (within 10 dB) \\ \hline \end{tabular}
\end{table}
Table 3: Summary of the MIL-GS OFC comb densification.