# Optical Frequency Comb Expansion Using Mutually Injection-Locked Gain-Switched Lasers
## 2 Mutually Injection-Locked Gain-Switched Laser
### Experimental Setup  
The experimental setup of the proposed MIL-GS OFC generation, expansion and comb densification scheme is shown in Figure 2. Here, the components with polarization maintaining fiber (PMF) and standard single mode fiber (SSMF) are indicated in brown and gray lines, respectively. Two commercially available discrete mode (DM) lasers, from Eblana Photonics [39] are chosen as the slave lasers. These are 250 \(\upmu\)m long FP laser diodes with a slotted ridge waveguide structure that constrains the lasing in a longitudinal mode, with a side mode suppression ratio (SMSR) larger than 60 dB, as shown in Figure 3a. The DM lasers are encased in TEC controlled 7 pin butterfly packages with a k-type RF connector, to enable high-speed modulation, and without an optical isolator to allow external light injection. They exhibit threshold currents (\(I_{th}\)) of 10 mA and a modulation bandwidth of ~12 GHz, when biased at ~53 mA. At the same bias, slave laser 1 and 2 emits an average power of 6 dBm and 7 dBm, respectively. Gain switching is achieved by applying an amplified sinusoidal signal (~24 dBm) in conjunction with the 53 mA DC bias to both lasers. Subsequently, the temperature of the lasers is tuned to achieve a partial overlap of their spectra (as shown in Figure 3b). Then, a narrow linewidth tunable laser (Purephotonics PCL550, \(\lambda_{injection}=1549.8\) nm, P = -3 dBm), acting as a common master laser, is tuned to the overlapping wavelength region, split using a 50:50 splitter and injected, via 3 port circulators (CIR 1 and 2) into the two GS lasers. It is worth noting that the components used in the MIL-GS OFC generation scheme are pigtailed with polarization maintaining fiber, which is aligned to the slow axis.  
The two-phase correlated OFCs generated this way are then combined using a 50:50 coupler and passed through a 20 GHz PM (EOspace). A polarization controller is used at the input of the PM to align the state of polarization of the input signal to that of PM. The modulator exhibits a \(V_{\pi}\) of ~4 V (at 6.25 GHz) and is driven at ~13.5 Vpp (corresponds to 3.3 \(\times\)\(V_{\pi}\)) by a second RF source (both RF sources are synchronized with 100 MHz reference signal). It is important to mention that the same RF source could be used for the OFC generation and the phase modulation. The expansion and spectral flattening (shaping) of the output comb are achieved by optimizing the synchronization between two arms using an optical delay line (ODL) for one arm, while RF phase shifter for another arm. In the expansion mode, the PM is driven at a frequency \(f=n\cdot f_{s}\), with \(n=1,2\) or \(3\), i.e., \(f=6.25\), \(12.5\), \(18.75\) GHz. Likewise, in the comb densification mode, the PM is driven at subharmonics of the fundamental frequency resulting in OFC FSRs of 3.125 \(\left(\frac{f_{s}}{2}\right)\),  
Figure 1: **(a)** Illustrative diagram of proposed MIL-GS OFC generation, expansion and comb densification scheme. **(b–e)** are the line graph of the operational principle.  
1.5625 GHz \(\left(\frac{f_{s}}{4}\right)\), 781.25 MHz \(\left(\frac{f_{s}}{8}\right)\), and 390.625 MHz \(\left(\frac{f_{s}}{16}\right)\). The optical spectrum is recorded using a high resolution (20 MHz) optical spectrum and analyzer (OSA).