Broadening Of Mode Locking Pulses In Quantum Dot Semiconductor Lasers

In this monograph, the authors address the physics and engineering together with the latest achievements of efficient and compact ultrafast lasers based on novel quantum-dot structures and devices. Their approach encompasses a broad range of laser systems, while taking into consideration not only the physical and experimental aspects but also the much needed modeling tools, thus providing a holistic understanding of this hot topic.

In this dissertation, self-assembled InAs/InGaAs quantum dot Fabry-Pérot lasers and mode-locked lasers are investigated. The mode-locked lasers investigated include monolithic and curved two-section devices, and colliding pulse mode-locked diode lasers. Ridge waveguide semiconductor lasers have been designed and fabricated by wet etching processes. Electroluminescence of the quantum dot lasers is studied. Cavity length dependent lasing via ground state and/or excited state transitions is observed from quantum dot lasers and the optical gain from both transitions is measured. Stable optical pulse trains via ground and excited state transitions are generated using a grating coupled external cavity with a curved two-section device. Large differences in the applied reverse bias voltage on the saturable absorber are observed for stable mode-locking from the excited and ground state mode-locking regimes. The optical pulses from quantum dot mode-locked lasers are investigated in terms of chirp sign and linear chirp magnitude. Upchirped pulses with large linear chirp magnitude are observed from both ground and excited states. Externally compressed pulse widths from the ground and excited states are 1.2 ps and 970 fs, respectively. Ground state optical pulses from monolithic mode-locked lasers e.g., two-section devices and colliding pulse mode-locked lasers, are also studied. Transformed limited optical pulses (~4.5 ps) are generated from a colliding pulse mode-locked semiconductor laser. The above threshold linewidth enhancement factor of quantum dot Fabry-Pérot lasers is measured using the continuous wave injection locking method. A strong spectral dependence of the linewidth enhancement factor is observed around the gain peak. The measured linewidth enhancement factor is highest at the gain peak, but becomes lower 10 nm away from the gain peak. The lowest linewidth enhancement factor is observed on the anti-Stokes side. The spectral dependence of the pulse duration from quantum dot based mode-locked lasers is also observed. Shorter pulses and reduced linear chirp are observed on the anti-Stokes side and externally compressed 660 fs pulses are achieved in this spectral regime. A novel clock recovery technique using passively mode-locked quantum dot lasers is investigated. The clock signal (~4 GHz) is recovered by injecting an interband optical pulse train to the saturable absorber section. The excited state clock signal is recovered through the ground state transition and vice-versa. Asymmetry in the locking bandwidth is observed. The measured locking bandwidth is 10 times wider when the excited state clock signal is recovered from the ground state injection, as compared to recovering a ground state clock signal from excited state injection.

This thesis investigates the dynamics of passively mode-locked semiconductor lasers, with a focus on the influence of optical feedback on the noise characteristics. The results presented here are important for improving the performance of passively mode-locked semiconductor lasers and, at the same time, are relevant for understanding delay-systems in general. The semi-analytic results developed are applicable to a broad range of oscillatory systems with time-delayed feedback, making the thesis of relevance to various scientific communities. Passively mode-locked lasers can produce pulse trains and have applications in the contexts of optical clocking, microscopy and optical data communication, among others. Using a system of delay differential equations to model these devices, a combination of numerical and semi-analytic methods is developed and used to characterize this system.

The book addresses issues associated with physics and technology of injection lasers based on self-organized quantum dots. Fundamental and technological aspects of quantum dot edge-emitting lasers and VCSELs, their current status and future prospects are summarized and reviewed. Basic principles of QD formation using self-organization phenomena are reviewed. Structural and optical properties of self-organized QDs are considered with a number of examples in different material systems. Recent achievements in controlling the QD properties including the effects of vertical stacking, changing the matrix bandgap and the surface density of QDs are reviewed. The authors focus on the use of self-organized quantum dots in laser structures, fabrication and characterization of edge and surface emitting diode lasers, their properties and optimization with special attention paid to the relationship between structural and electronic properties of QDs and laser characteristics. The threshold and power characteristics of the state-of-the-art QD lasers are demonstrated. Issues related to the long-wavelength (1.3-mm) lasers on a GaAs substrate are also addressed and recent results on InGaAsN-based diode lasers presented for the purpose of comparison.

This thesis investigates passively mode-locked semiconductor lasers by numerical methods. The understanding and optimization of such devices is crucial to the advancement of technologies such as optical data communication and dual comb spectroscopy. The focus of the thesis is therefore on the development of efficient numerical models, which are able both to perform larger parameter studies and to provide quantitative predictions. Along with that, visualization and evaluation techniques for the rich spatio-temporal laser dynamics are developed; these facilitate the physical interpretation of the observed features. The investigations in this thesis revolve around two specific semiconductor devices, namely a monolithically integrated three-section tapered quantum-dot laser and a V-shaped external cavity laser. In both cases, the simulations closely tie in with experimental results, which have been obtained in collaboration with the TU Darmstadt and the ETH Zurich. Based on the successful numerical reproduction of the experimental findings, the emission dynamics of both lasers can be understood in terms of the cavity geometry and the active medium dynamics. The latter, in particular, highlights the value of the developed simulation tools, since the fast charge-carrier dynamics are generally not experimentally accessible during mode-locking operation. Lastly, the numerical models are used to perform laser design explorations and thus to derive recommendations for further optimizations.

1. Introduction to photonic quantum dot nanomaterials and devices. 1.1. Physical properties of quantum dots. 1.2. Active semiconductor gain media. 1.3. Quantum dot lasers. 1.4. Laser cavities -- 2. Theory of quantum dot light-matter dynamics. 2.1. Rate equations. 2.2. Maxwell-Bloch equations. 2.3. Quantum luminescence equations. 2.4. Quantum theoretical description -- 3. Light meets matter I: microscopic carrier effect. 3.1. Dynamics in the active charge carrier plasma. 3.2. Dynamic level hole burning. 3.3. Ultrashort nonlinear gain and index dynamics. 3.4. Conclusion -- 4. Light meets matter II: mesoscopic space-time dynamics. 4.1. Introduction: transverse and longitudinal mode dynamics. 4.2. Influence of the transverse degree of freedom and nano-structuring on nearfield dynamics and spectra. 4.3. Longitudinal modes. 4.4. Coupled space-time dynamics. 4.5. Conclusion -- 5. Performance and characterisation: properties on large time and length scales. 5.1. Introduction. 5.2. Spatial and spectral beam quality. 5.3. Dynamic amplitude phase coupling. 5.4. Conclusion -- 6. Nonlinear pulse propagation in semiconductor quantum dot lasers. 6.1. Dynamic shaping of short optical pulses. 6.2. Nonlinear femtosecond dynamics. 6.3. Conclusion -- 7. High-speed dynamics. 7.1. Mode-locking in multi-section quantum dot lasers. 7.2. Dependence of pulse duration on injection current, bias voltage and device geometry. 7.3. Radio frequency spectra of the emitted light. 7.4. Short-pulse optimisation. 7.5. Conclusion -- 8. Quantum dot random lasers. 8.1. Spatially inhomogeneous semiconductor quantum dot ensembles. 8.2. Coherence properties. 8.3. Random lasing in semiconductor quantum dot ensembles. 8.4. Conclusion -- 9. Coherence properties of quantum dot micro-cavity lasers. 9.1. Introduction. 9.2. Radial signal propagation and coherence trapping. 9.3. Influence of disorder. 9.4. Conclusions