# Two Polarization Comb Dynamics in VCSELs Subject to Optical Injection
## 2 Experimental Polarization Dynamics
### Influence of the Injection Current  
We next increase the injected comb spacing to \(\Omega=4\) GHz and the bias current to \(I=8\) mA, which is 2.67 times the threshold current. Such bias currents have been used in VCSELs to observe various nonlinear dynamics in the presence of single-mode injection with parallel polarization [37; 39]. Compared to the \(I=6\) mA bias current injection where nonlinear wave-mixing is accompanied by two polarization comb as a result of excitation of the normally depressed polarization mode, the VCSEL output bifurcates to selective amplification of the central injected lines in X-PM, as shown in Figure 6a-c. As observed in  
Figure 4: Bifurcation scenarios resulting in the excitation of the depressed polarization mode. These optical spectra are obtained for detuning \(\Delta\
u=1.6\) GHz and injected comb spacing \(\Omega=2\) GHz. (**a**) Stable output at \(P_{inj}=3\) μW, (**b**) wave mixing at \(P_{inj}=16\) μW, (**c**) two polarizations comb at \(P_{inj}=48\) μW, (**d**) two polarization harmonics comb at \(P_{inj}=112\) μW, (**e**) two polarization complex dynamics at \(P_{inj}=208\) μW and (**f**) X-polarization comb at \(P_{inj}=240\) μW.  
Figure 5: Polarization resolved-optical spectra corresponding to an example of two polarization comb dynamics in Figure 4c. (**a-c**) are obtained for detuning \(\Delta\
u=1.6\) GHz and injected power \(P_{inj}=20\) μW. (**a–c**) correspond to the optical spectra of Y-PM, X-PM and the superposition of Y-PM and X-PM, respectively.  
Figure 6c,d, selective amplification smoothly induces a broad optical comb characterized by the appearance of new frequency lines. When we increase the injected power, polarization mode competition results in the extension of the polarized comb to a much broader optical spectrum, as shown in Figure 6e. We checked through polarization resolved-optical spectra in Figure 7 that this overall comb corresponds to two combs based on the linear orthogonal polarization of the VCSEL. It is also possible to control the power in each polarization mode by fine-tuning the detuning. The two polarization combs and the polarized comb have the same repetition rate as the injected comb. By fine-tuning the injected power, the noise pedestal smoothly increases accompanied by harmonics comb line generation simultaneously in the two polarization combs as shown in Figure 6e. The harmonic comb significantly increases the number of resulting output lines and gives rise to a new comb with a low repetition rate ( \(\Omega=2\) GHz in Figure 6e). Most importantly, Figure 6e highlight a new possibility of broadband \(\Omega=2\) GHz comb generation using the same injection technique as in Figure 4c,d. When we keep increasing the injected power, the harmonics comb destabilizes to complex dynamics with remarkable frequency comb lines mainly around the dominant polarization mode (X-PM), as seen in Figure 6g. Interestingly, when further increasing injected power, VCSEL bifurcates again to the new broadband comb encompassing the two polarization modes with a significant increase in noise pedestal, as shown in Figure 6h,i. Similarly to Figure 6e, this overall comb is formed by two combs with the orthogonal polarization direction. The noise pedestal increases again with the injected power to deteriorate comb performance, such as CNR and comb bandwidths. We have shown that the properties (bandwidth and CNR) of the two polarization combs induced by parallel optical injection are limited by increasing the injected comb spacing for fixed bias current. However, the increase in bias current is, therefore, an alternative to improve polarization combs properties.  
Figure 6: _Cont._