Passively mode-locked semiconductor lasers are inexpensive sources of short optical pulses with high repetition rates. They find applications in high-precision metrology and high-capacity optical interconnects, where pulse trains with low amplitude and timing jitter are required. Hybrid mode-locking  and dual-cavity optical feedback  allow for the reduction of the timing jitter, but add further electronics and optics and thereby also additional costs. New concepts of monolithic mode-locked semiconductor laser geometries for improved pulse stability has therefore become of major interest.
Quantum dot (QD) based mode-locked lasers receive quite some attention as they exhibit reduced spontaneous emission . However, the internal QD carrier dynamics act as a filter on the pulse generation, which can lead to strong pulse asymmetries with trailing edge plateaus or pulses . This effect becomes more pronounced at greater optical gain and thereby makes the generation of stable, high power pulses challenging.
In this work, we characterize the performance of a monolithic multi-section QD mode-locked laser with tapered geometry at the pulse emission facet. The laser output is analyzed using optical and radio-frequency spectra, auto-correlation traces and optical power measurements, from which the pulse peak power, pulse width, amplitude jitter and pulse-to-pulse timing jitter are calculated. For different laser biasing conditions, we report ultra short, high power pulses with outstanding stability.
Combining a traveling-wave model for the electric field propagation with microscopically based quantum-dot (QD) charge-carrier rate equations, we investigate the performance and dynamics of the laser for different biasing conditions. To allow for numerical efficiency, the traveling-wave equation is transformed by an integration along its characteristic curve to a set of coupled delay-equations . The excitonic charge-carrier dynamics are described by microscopic scattering processes that consider Pauli-blocking and a detailed balance condition. The light-matter interaction includes a charge-carrier dependent amplitude-phase coupling and uses a time-domain filter function to describe the gain spectrum of the QD ensemble.
Pulse characteristics and statistics are extracted from time-series obtained by direct integration of the delay-differential equations. Our simulations nicely reproduce experimental results. We map dynamic regimes and pulse train characteristics for varying absorber lengths, absorber placements and taper angles and show, that trailing edge pulses can be suppressed for suitable configurations, leading to excellent pulse train stabilities at high output power. We report an optimum taper angle, where the range of injection currents, that yield stable fundamental mode-locking, is maximized.
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Stefan Meinecke, Lukas Drzewietzki, Christoph Weber, Benjamin Lingnau, Stefan Breuer, and Kathy Lüdge, "Performance and dynamics of mode-locked lasers with complex geometry (Conference Presentation)," Proc. SPIE 10682, Semiconductor Lasers and Laser Dynamics VIII, 106820D (Presented at SPIE Photonics Europe: April 24, 2018; Published: 23 May 2018); https://doi.org/10.1117/12.2307592.5788825459001.
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