Quantum-dot mode-locked lasers with optical injection

Modal optical linewidths of a passively mode-locked and optical injection locked quantum dot laser are studied. For the free-running case the modal linewidth is in the order of tens of MHz and demonstrates a parabolic dependence on the mode optical frequency. The slope of the parabola, as was predicted theoretically, is proportional to the radio-frequency (RF) linewidth, which provides a direct measurement of the timing jitter. With optical injection the slave laser optical spectrum becomes narrowed and tunable via the master wavelength. Frequency resolved Mach-Zehnder gating measurements performed to characterize slave laser pulses showed significantly improved pulse time-bandwidth product with optical injection. Measurements of the modal optical linewidths of the injected laser have shown phase locking of the slave laser modes to the master laser in the vicinity of the injection wavelength. However, far from this wavelength modal linewidth of the slave laser increases to greater than that of the free running case, leading to increase of the RF linewidth and timing jitter with single-tone injection.