A detailed study of the pulse characteristics emitted from a monolithic Quantum Dot (QD) passively Mode-Locked Laser (MLL) has been performed using a state-of-the-art Frequency Resolved Optical Gating (FROG) pulse measurement system. While traditionally the time-domain pulse characteristics of semiconductor MLLs have been studied using digital sampling oscilloscope or intensity autocorrelation techniques, the FROG measurements allow for simultaneous characterization of time and frequency, which has been shown to be necessary and sufficient for true determination of mode-locked stability. In this paper, FROG pulse measurements are presented on a two-section QD MLL operating over wide temperature excursions. The FROG measurement allows for extraction of the temporal and spectral intensity and phase profiles from which the Group Delay Dispersion (GDD) can be determined. The magnitude of the GDD is found to decrease from 16.1 to 3.5 ps/nm when the temperature is increased from 20 to 50 oC, mirroring the trend of pulse width reduction at elevated temperature, which has been shown to correlate strongly with reduced unsaturated absorption. The possibility to further optimize pulse generation via intra-cavity dispersion compensation in a novel three-section MLL design is also examined, and shows strong potential toward providing valuable insight into the optimal cavity designs and operating parameters for QD MLLs.
In this paper, performance of monolithic quantum dot passively mode-locked lasers over broad temperature excursions is characterized. It is shown that there is a linear dependence between absorber to gain length ratio and the characteristic temperature that a device transitions from ground-state to excited-state lasing when the saturable absorber is grounded. The pulse shape and optical spectrum characteristics are examined in detail around these transition regimes. Experimental operational maps have also been constructed showing the range of biasing conditions that produce stable mode-locking across a wide range of temperatures. A comparison is made between regions of mode-locking stability for two devices having the same absorber to gain length ratio, with varying ridge waveguide widths. Finally, gain and absorption characteristics are derived from measurements of amplified spontaneous emission, and a correlation between reduced values of unsaturated absorption and reduced time-bandwidth product is shown. Key features in the experimental operational maps and their respective significance on the operation and design of future devices is discussed.
A nonlinear delay differential equation model for passive mode-locking in semiconductor lasers, seeded with parameters extracted from the gain and loss spectra of a quantum dot laser, is employed to simulate and study the dynamical regimes of mode-locked operation of the device. The model parameter ranges corresponding to these regimes are then mapped to externally-controllable parameters such as gain current and absorber bias voltage. Using this approach, a map indicating the approximate regions corresponding to fundamental and harmonically mode locked operation is constructed as a function of gain current and absorber bias voltage. This is shown to be a highly useful method of getting a sense of the highest repetition rates achievable in principle with a simple, two-section device, and provides a guideline toward achieving higher repetition rates by simply adjusting external biasing conditions instantaneously while the device is in operation, as opposed to re-engineering the device with additional passive or saturable absorber sections. The general approach could potentially aid the development of numerical modeling techniques aimed at providing a systematic guideline geared toward developing microwave and RF photonic sources for THz applications.