Fig. 1. Optical spectrum of (a1) sole GS lasing and (a2) dual-state lasing of QD lasers. (b) Optical spectrum mapping with the increase of bias current for the dual-state QD laser. Dashed lines (1) and (2) in (b) mark the bias currents of (a1) and (a2), respectively.

Fig. 2. Experimental setup for investigating the feedback sensitivity of QD lasers. BKR, backreflector; PC, polarization controller; OSA, optical spectrum analyzer; PD, photodiode; ESA, electrical spectrum analyzer.

Fig. 3. Optical (column 1) and RF (column 2) spectrum mappings for QD laser operating at (a) 0.72×, (b) 1×, and (c) 1.25×IthES. Dashed lines mark the critical feedback levels.

Fig. 4. (a) Optical and (b) RF spectra of QD lasers operated at 1×IthES subject to high feedback strength of −9.9  dB (red) and low feedback strength of −29  dB (blue).

Fig. 5. Schematic representation of the electronic structure and carrier dynamics of QD lasers under optical feedback.

Fig. 6. GS threshold current, ES threshold current, and corresponding ES-GS threshold ratio with respect to ES-GS energy separation.

Fig. 7. Samples of the bifurcation diagrams (column 1), time series (column 2), and GS phase portraits (column 3). (a) ΔEGSES=65  meV, I/IthES=1.0, and fext=−12.0  dB; (b) ΔEGSES=80  meV, I/IthES=1.31, and fext=−11.0  dB; (c) ΔEGSES=110  meV, I/IthES=0.87, and fext=−13.0  dB. Green vertical dashed lines in the first column mark the fext taken in the second and third columns; rcrit extracted from the bifurcation diagrams are marked in the first column.

Fig. 8. Critical feedback levels as a function of normalized bias currents (I/IthES). Triangles, diamonds, and squares are numerically calculated for different ES-GS energy separations, while the dots are extracted from measurement results.

Fig. 9. Linewidth enhancement factor as a function of normalized bias currents (I/IthES) for GS and ES, respectively.

Fig. 10. Damping factor and relaxation oscillation frequency versus normalized bias currents (I/IthES).