Fig. 1. (a) Cross-sectional TEM image of the active region. The inset shows the high-resolution bright-field scanning TEM image of a single dot. (b) Comparison of the room temperature PL spectra for samples grown under previous conditions and the optimized growth conditions employed in this work. (c) Schematic of the passive two-section MLL. (d) SEM image of the device showing the gap between the gain and SA.

Fig. 2. Two-section passive QD-MLL performance characterization at room temperature. (a) L−I characteristics for different SA reverse-bias voltages. (b) Fundamental RF peak SNR mapping. (c) RF spectrum in a 26.5 GHz span view (RBW: 1 MHz, VBW: 10 kHz). The inset shows the autocorrelation trace with Gaussian pulse fitting. (d) Pulse duration as a function of Igain with VSA=−3.5  V.

Fig. 3. (a) Typical CW L−I characteristics of the two-section QD MLL as a function of temperature when VSA=0  V. (b) Dependence of threshold current on reverse-bias voltage and temperature.

Fig. 4. Temperature-dependent characteristics of two-section passive QD-MLL with a constant VSA=−2  V and Igain of 49, 60, 64.7, 85, 148.5, and 210 mA at 20, 40, 60, 80, 100, and 120°C, respectively. (a) RF spectra (RBW: 1 MHz, VBW: 10 kHz). (b) Zoomed-in RF spectra, as shown in (a). (c) Pulse duration. (d) Color map depicting the regions of fundamental mode-locking (25 GHz) from 20°C to 120°C where the SNR>25  dB. (e) Optical spectra (resolution: 0.03 nm, VBW: 200 Hz). (f) 6 dB bandwidth and the corresponding number of comb teeth.

Fig. 5. (a) Optical comb under bias conditions of Igain=148.5  mA and VSA=−2  V at 100°C. (b) Average RIN for the whole optical comb shown in (a) from 0.5 to 10 GHz.