Evgeny Viktorov

Model for mode locking in quantum dot lasers

We propose a model for passive mode locking in quantum dot lasers and report on specific dynamical properties of the regime which is characterized by a fast gain recovery. No Q-switching instability has been found accompanying the mode locking. Bistability can occur between the mode locking regime and the nonlasing state.

Optically injected quantum-dot lasers

The response of an optically injected quantum-dot semiconductor laser (SL) is studied both experimentally and theoretically. In particular, the nature of the locking boundaries is investigated, revealing features more commonly associated with Class A lasers rather than conventional Class B SLs. Experimentally, two features stand out; the first is an absence of instabilities resulting from relaxation oscillations, and the second is the observation of a region of bistability between two locked solutions. Using rate equations appropriate for quantum-dot lasers, we analytically determine the stability diagram in terms of the injection rate and frequency detuning. Of particular interest are the Hopf and saddle-node locking boundaries that explain how the experimentally observed phenomena appear.

Hybrid mode-locking in a 40 GHz monolithic quantum dot laser

Mode-locked semiconductor lasers are efficient sources of short optical pulses ideal for applications in high speed telecommunication systems. Especially promising for telecom applications is the new generation of quantum dot mode-locked lasers (QD-MLL) which demonstrate important advantages over the standard quantum well devices [1]. However, performance improvement is still an ongoing issue, in particular for the efficiency of hybrid mode-locking, — a commonly used technique to improve characteristics and synchronize the mode-locked pulses — namely the dependence of the locking regime on the frequency and power of the applied external signal. Based on our previous results on passive mode-locking in quantum dot lasers [2], we study experimentally and theoretically a monolithic two-section hybrid mode-locked quantum dot laser with periodically modulated reverse bias applied to the saturable absorber section.

The fast recovery dynamics of a quantum dot semiconductor optical amplifier

We consider a rate equation model of a quantum dot semiconductor optical amplifier that takes into account carrier capture, escape, and Pauli blocking processes. We evaluate possible differences between phonon-assisted or Auger processes being dominant for recovery. An analytical solution which corresponds to phonon-assisted interaction is then used to accurately fit experimental recovery curves and allows an estimation of both the carrier capture and escape rates.

Intradot dynamics of InAs quantum dot based electroabsorbers

The carrier relaxation and escape dynamics of InAs/GaAs quantum dot waveguide absorbers is studied using heterodyne pump-probe measurements. Under reverse bias conditions, we reveal differences in intradot relaxation dynamics, related to the initial population of the dots’ ground or excited states. These differences can be attributed to phonon-assisted or Auger processes being dominant for initially populated ground or excited states, respectively.

Stability of the mode-locked regime in quantum dot lasers

We report on experimental and theoretical studies of the stability regime of passive mode-locked quantum dot lasers, which is decisively larger than in quantum well lasers. A small range of Q-switched instability is observed at low gain currents. Transition to Q switching is inhibited due to fast damping of the relaxation oscillations. A double pulse mode-locking regime appears for longer cavities, and exhibits bistability and coupling to the fundamental mode-locking operation.

Analytical approach to modulation properties of quantum dot lasers

We analyze a microscopically based rate equation model for quantum dot lasers. The model separately treats the dynamics of electrons and holes, and the carrier-carrier scattering rates depend nonlinearly on the wetting layer carrier densities. Our objective is to determine analytical expressions for the relaxation oscillation frequency and damping rate. To this end, we consider the Class B limit of the five rate equations and apply asymptotic techniques. We consider two cases corresponding to either equivalent or drastically different decay rates for the electrons and holes. We show how they contribute to increase the relaxation oscillation damping rate compared to the damping rate of the conventional laser and that there exist optimal conditions on the control parameters in order to observe maximum damping.

Low-frequency fluctuations in two-state quantum dot lasers.

We study the feedback-induced instabilities in a quantum dot semiconductor laser emitting in both ground and excited states. Without optical feedback the device exhibits dynamics corresponding to antiphase fluctuations between ground and excited states, while the total output power remains constant. The introduction of feedback leads to power dropouts in the ground state and intensity bursts in the excited state, resulting in a practically constant total output power.

Recovery time scales in a reversed-biased quantum dot absorber

The nonlinear recovery of quantum dot based reverse-biased waveguide absorbers is investigated both experimentally and analytically. We show that the recovery dynamics consists of a fast initial layer followed by a relatively slow decay. The fast recovery stage is completely determined by the intradot properties, while the slow stage depends on the escape from the dot to the wetting layer.

Temperature dependence of pulse duration in a mode-locked quantum-dot laser

In this paper, we demonstrate, experimentally and theoretically, that in a mode-locked two-section quantum-dot laser, the pulsewidth decreases with temperature. The primary cause is the increase of carrier capture rate with temperature, leading to a faster absorption recovery