# Two Polarization Comb Dynamics in VCSELs Subject to Optical Injection
## 2 Experimental Polarization Dynamics
### Tailoring Comb Properties  
Figure 8 provides further insight in comb dynamics when varying injection parameters, the injection current in the VCSEL and the polarization of injected light. Figure 8a,b analyze the comb properties for an injected comb spacing of \(\Omega=2\) GHz and fixed detuning, \(\Delta\
u=1.6\) GHz. For the measurement of comb bandwidth, we consider an output comb line when its amplitude lies above \(-30\) dB from the maximum amplitude in the optical spectrum. When the spectrum total optical comb shows a big dip, the total comb bandwidth is estimated from the separate bandwidth of X-PM and Y-PM spectra. The polarization of the injected comb is parallel to that of the free-running VCSEL. When increasing injected power, the comb bandwidth for \(I=6\) mA and \(I=8\) mA current reaches a maximum of around \(P_{inj}=100\) uW and \(P_{inj}=200\) uW, respectively, as shown Figure 8a. The decrease in comb bandwidth is due to the bifurcation of the two polarization comb to a single polarization comb generation in X-PM, as shown in Figures 2i and 4f. Figure 8b shows that the best CNR is found for low injected power \(P_{inj}\). CNR decreases with injected power due to the increase in the noise pedestal, which destabilizes comb properties. Figure 8a,b also show that the comb bandwidth and CNR increase with the injection current for the same injected comb properties. We next highlight the impact of polarization of the injected comb lines on VCSEL output comb dynamics in Figure 8c,d. In Figure 8c,d, blue and red correspond to parallel and orthogonal optical injection cases, respectively. As discussed  
Figure 6: Bifurcation scenarios resulting in the excitation of the depressed polarization mode. These optical spectra are obtained for detuning \(\Delta\
u=-0.9\) GHz and injected comb spacing \(\Omega=4\) GHz. (**a,b**) Wave mixing at \(P_{inj}=3\) μW and \(P_{inj}=32\) μW, respectively, (**c,d**) single polarization comb at \(P_{inj}=48\) μW and \(P_{inj}=228\) μW, respectively, (**e,h,i**) two polarizations comb at \(P_{inj}=304\) μW, \(P_{inj}=560\) μW and \(P_{inj}=704\) μW, respectively, (**f**) two polarization harmonics comb at \(P_{inj}=376\) μW and (**g**) two polarization complex dynamics at \(P_{inj}=448\) μW. Figure 7: Polarization resolved-optical spectra corresponding to an example of two polarization comb dynamics in Figure 6e–i. (**a–c**) are obtained for detuning \(\Delta\
u=-0.9\) GHz and injected power \(P_{inj}=4328\) μW. (**a–c**) correspond to the optical spectra of Y-PM, X-PM and the superposition of Y-PM and X-PM, respectively. in [28], the two polarization comb dynamics are mainly observed when VCSEL is injected between the two linear polarizations. However, for parallel optical injection, the two polarizations combs are observed only when the central injected comb lines are close to the dominant polarization mode of the VCSEL (detuning close to zero) at low injection currents, as shown in Figure 8a,b. When the injection current increases, the two comb dynamics are observed in a large portion of the plane of the injection parameters. Figure 8c shows that comb bandwidth increases with the injected power in the parallel and orthogonal optical injection case, which is in agreement with our previous publication [26; 27; 28]. Interestingly, the comb bandwidth for parallel optical injection is better than orthogonal optical injection at high injection currents, as shown in Figure 8c. When comparing Figure 8a to [28], we can observe that the comb bandwidth at low injection current is better for orthogonal than parallel optical injection. We also checked that optical comb performances do not change with the increase in injection currents. In the case of parallel optical injection, the improvement in the comb performance with the increase in the injection current is due to polarization mode instability, i.e., the excitation of the depressed polarization mode in the VCSEL at a large injection current.