Fig. 1. (a) Light-current characteristics with temperature ranging from 15°C to 55°C. (b) Threshold current (burgundy) and external efficiency (jade) as a function of temperature.

Fig. 2. Optical spectra of the QD DFB laser from 15°C to 55°C (2×Ith).

Fig. 3. Temperature-dependent (a) DFB wavelength (burgundy), optical gain peak (jade), (b) optical wavelength detuning (gray), and side-mode suppression ratio (emerald).

Fig. 4. (a) Measured RIN spectra at several bias currents at 20°C. (b) Extracted damping factor γ as a function of the squared relaxation oscillation frequency fRO2 at 20°C (jade) and at 55°C (burgundy). (c) Tendency of K-factor versus the temperature; Tm is marked by the black dashed line.

Fig. 5. (a) Optical spectra around the DFB mode and the modulation sidebands of the DFB laser operating at 2×Ith under 55°C. The spectra obtained for four different optical delays are normalized to the main lobes. (b) Effective α factor in different operation conditions. Tm is marked by the black dashed line.

Fig. 6. Experimental setup used for the long-delay coherent external optical feedback measurement. ISO, optical isolator; PD, photodiode; PC, polarization controller; VOA, variable optical attenuator.

Fig. 7. RF spectra and optical spectra in different rext conditions, when the DFB laser operates at [(a) and (e)] 2×Ith and [(b) and (f)] 6×Ith at 25°C. Corresponding power mapping of the RF and optical spectra as a function of rext [(c) and (g)] at 2×Ith and [(d) and (h)] at 6×Ith.

Fig. 8. (a), (b) RF spectra and optical spectra in different rext conditions, when the DFB laser operates at 6×Ith at 55°C. (c), (d) Corresponding power mapping of the RF and optical spectra as a function of rext.

Fig. 9. Critical feedback level rcrit associated with the onset of coherence collapse (CC) operation under different operation conditions. Tm is marked by the black dashed line.