Traveling wave modeling, simulation, and analysis of quantum-dot mode-locked semiconductor lasers

We present a new model for quantum-dot (QD) semiconductor lasers which includes the self-consistent treatment of electrical, optical, and thermal interactions. The approach is developed from the energy balance equation to incorporate the carrier temperature and a three-level rate equation to describe the carrier and photon dynamics within the QD active region operating simultaneously on two longitudinal modes: via ground-state (GS) and first-excited-state (ES) transitions. Using the presented model, the steady-state, small-signal modulation response and transient device characteristics are calculated and analyzed. Similar to the situation of mode competition of laser modes, it is found that the light output power of the GS mode enters the roll-off regime, while the power of the ES mode increases. The model also predicts the negative temperature dependency of the power threshold for the ES mode, which can be understood from the negative temperature-dependent behavior of incomplete clamping of the ES population above the GS threshold. The simulated results are in good agreement with experimental data from the devices reported earlier.

In this study we present a self-consistent numerical approach for quantum-dot (QD) semiconductor lasers that account for carrier heating during device operation. The self-consistent analysis incorporates the calculation of the carrier densities and their relevant temperatures inside the QDs and within the quantum-well (QW) that surrounds the QDs. Particularly, the model takes into account the contribution of carrier-heating and carrier-cooling rates on energy balance equations to describe carrier-temperature dynamics within the device active region.

Passively mode-locked two-section quantum-dot monolithic semiconductor lasers are promising optical pulse sources for nonlinear medical imaging and communication applications. Their advantages are their compactness, ease of operation and high efficiency compared to solid state lasers. Quantum-dots (QD) incorporated as the active medium in a diode laser offer peculiar advantages like broad gain bandwidth and fast recovery time of the QD absorber being advantageous for mode-locking operation and generation of ultrashort pulses. Implemented as an index guided tapered structure high pulse peak power of 2.25W with a pulse width of 360 fs can be generated. [1] Recently, a fully gain guided two-section tapered laser has been demonstrated with a peak power of 3.6 W and a pulse width of 3.2 ps with good beam quality. [2]

GaAs-based quantum dot (QD) semiconductor lasers at 1300 nm exhibit a number of benefits for use in metropolitan and local area networks. Low modulation-induced chirp attributed to small linewidth enhancement factor (LEF) in QDs overcomes some limitations in a directly-modulated laser compared to quantum well counterparts. Along with the temperature-insensitive low threshold current, these QD lasers demonstrate considerable prospects towards Peltier-free direct-modulation operation. In this work, we experimentally investigate the temperature-dependence of the LEF as a function of injection current and wavelength in state-of-the-art 1300 nm In(Ga)As/GaAs QD lasers grown by molecular beam epitaxy. These structures demonstrate a large state separation and a comparatively small inhomogeneous linewidth. Gain is measured using the Hakki-Paoli method and carrier related mode shift is extracted by correcting for thermal effects. Previous reports have described LEF values of 1-5 over smaller temperature ranges (20-85°C) than the GPON commercial specifications in 1300 nm In(Ga)As/GaAs QD structures. Here, at temperature range of -10 to 85°C extending beyond GPON commercial window, it is shown that even at higher temperatures where contribution from the excited state transition is expected to increase the LEF, the measured LEF remains essentially zero. This is ascribed to the large gain from the ground state transition. Such a low LEF at elevated temperatures bodes well for the use of such structures in applications where feedback insensitivity may be important. By analyzing individual modes as a function of their individual lasing thresholds, we also describe an interplay of mode position and lasing linewidth.

The present study includes a synchronization study on the modelling of rate equations and a study of the dynamics of a quantum dot semiconductor laser (QDSL). The dynamics of a quantum dot semiconductor laser depend on several factors including the delay time, as studied in this research via visual feedback, and by relying on practical results. A systematic study of communications for two quantum dots, the transmitter and receiver, and the possibility of synchronization between them, and a study of the relationship between the photon density, time, and the probability of filling the charge carriers over time. For different delay times ( 249 260 0.1)   to step and the pump current ratios , ( 1.5 ) T R th J J  and line width enhancement factor ( , ( 3.5) T R  are proven.

Monte Carlo markovian models of a dual-mode semiconductor laser with quantum well (QW) or quantum dot (QD) active regions are proposed. Accounting for carriers and photons as particles that may exchange energy in the course of time allows an ab initio description of laser dynamics such as the mode competition and intrinsic laser noise. We used these models to evaluate the stability of the dual-mode regime when laser characteristics are varied: mode gains and losses, non-radiative recombination rates, intraband relaxation time, capture time in QD, transfer of excitation between QD via the wetting layer... As a major result, a possible steady-state dual-mode regime is predicted for specially designed QD semiconductor lasers thereby acting as a CW microwave or terahertz-beating source whereas it does not occur for QW lasers.

Modeling of synchronization in quantum dot semiconductor lasers

We experimentally investigate the complex dynamics of a multi-mode quantum-dot semiconductor laser with time-delayed optical feedback. We examine a two-dimensional bifurcation diagram of the quantum-dot laser as a comprehensive dynamical map by changing the injection current and feedback strength. We found that the bifurcation diagram contains two different parameter regions of low-frequency fluctuations. The power-dropout dynamics of the low-frequency fluctuations are observed in the sub-GHz region, which is considerably faster than the conventional low-frequency fluctuations in the MHz region. Comparing the dynamics of quantum-dot laser with those of single- and multi-mode quantum-well semiconductor lasers reveals that the fast low-frequency fluctuation dynamics are unique characteristics of quantum-dot lasers with time-delayed optical feedback.

We introduce an analytical approach to the multi-state lasing phenomenon in <i>p</i>-doped and undoped InAs/InGaAs quantum dot lasers which were studied both theoretically and experimentally. It is shown that the asymmetry in charge carrier distribution in quantum dots as well as hole-to-electron capture rate ratio jointly determine laser's behavior in such a regime. If the ratio is lower than a certain critical value, the complete quenching of ground-state lasing takes place at sufficiently high injection currents; at higher values of the ratio, our model predicts saturation of the ground-state power. It was experimentally shown that the modulation <i>p</i>-doping of laser's active region results in increase of output power emitted via the ground-state optical transitions of quantum dots and in enhancement of the injection currents range in which multi-state lasing takes place. The maximum temperature at which multi-state lasing exists was increased by about 50&deg;C in the <i>p</i>-doped samples. These effects are qualitatively explained in the terms of the proposed model.

All-optical schemes for controlling complex delay-induced and spatio-temporal dynamics in multi-stripe and broad-area semiconductor lasers are presented. The underlying control-principle is based on continuous delayed optical feedback. Its influence on the general spatiotemporal dynamics of semiconductor lasers is discussed. The delayed feedback control principle allows successful control of delay-induced chaotic behavior as well as the dynamic optical filamentation in broad-area and multi-stripe lasers. The suppression of spatiotemporal instabilities in semiconductor laser arrays is achieved by selectively stabilizing relevant transverse optical modes. The relevant values of delay-times and delay-phase are obtained from a nonlinear eigenmode analysis based on a complex extension of the Karhunen-Loeve decomposition. Thereby a destructive interference condition in the higher-order transverse modes conveying the instabilities is achieved. The stabilization technique is demonstrated on multi-stripe laser arrays. In high-power broad-area lasers the spatiotemporal filamentation processes occur on both macroscopic and microscopic scales. Numerical simulations on the basis of Maxwell-Bloch equations for spatially extended semiconductor lasers reveal the internal spatial and temporal scales on which control consequently has to occur. It is demonstrated that the combined filamentation and propagation effects may be controlled by suitable spatially structured delayed optical feedback allowing, in particular, the control of coherent regimes in originally temporally and spatially chaotic states.

Dynamics of the emission spectra of semiconductor quantum dot (QD) lasers generating on two quantum states has been experimentally studied. Being pumped with 30-ns current pulses, a QD laser ceased to generate 2–5 ns after switch-on and exhibited a pause up to 10 ns or longer, depending on the pumping pulse amplitude. After the subsequent switch-on, the laser generated short (200–300 ps) pulses of emission from the excited state of QDs followed by minima of comparable duration (dark pulses) corresponding to the ground-state emission. This behavior is explained in terms of the laser Q-switching due to the charge carrier density redistribution between the excited and ground states.

Theoretical analysis of the gain and threshold current of a semiconductor quantum dot (QD) laser is given which takes account of the line broadening caused by fluctuations in quantum dot sizes. The following processes are taken into consideration together with the main process of radiative recombination of carriers in QDs: band-to-band radiative recombination of carriers in the waveguide region, carrier capture into QDs and thermally excited escape from QDs, photoexcitation of carriers from QDs to continuous-spectrum states. For an arbitrary QD size distribution, expressions for the threshold current density as a function of the root mean square of relative QD size fluctuations, total losses in the waveguide region, surface density of QDs and thickness of the waveguide region have been obtained in an explicit form. The minimum threshold current density and optimum parameters of the structure (surface density of QDs and thickness of the waveguide region) are calculated as universal functions of the main dimensionless parameter of the theory developed. This parameter is the ratio of the stimulated transition rate in QDs at the lasing threshold to the spontaneous transition rate in the waveguide region at the transparency threshold. Theoretical estimations presented in the paper confirm the possibility of a significant reduction of the threshold currents of QD lasers as compared with the conventional quantum well lasers.

The homogeneous broadening in semiconductor quantum dot (QD) lasers and optical amplifiers is studied theoretically. Based on a model for the electronic states of the coupled QD--wetting layer (WL) system, Coulomb interaction matrix elements are calculated, including both screening and exchange interaction. The homogeneous broadening due to various Auger processes, involving scattering of carriers between WL states and confined QD states, is calculated. The effects of the orthogonalization of WL states, QD confinement, QD density, and carrier density on the homogeneous broadening are studied systematically. We found that such WL-assisted Auger scattering is very efficient with subpicosecond dephasing times, and it is the dominant channel for the homogeneous broadening at high carrier density. Good agreement is achieved when comparing our theoretical results and recent experimental data.

When semiconductor lasers SLs are exposed to filtered optical feedback (FOB), they behave in two different ways. Under weak or strong feedback, the first is a singularly stable longitudinal mode with a small linewidth. The second is the so-called coherence breakdown condition, which also includes low frequency fluctuations (LFF) and chaotic oscillations. The latter has received a lot of attention recently due to the possible uses of chaotic lasers in chaotic lidar, covert communications, and chaotic correlation time-domain optical reflectometers, while the former has received a lot of research in recent years. We investigate the behavior of a quantum dot (QD) semiconductor laser subject to FOF from two separate external cavities both, numerically and experimentally. Our findings show that the second FOF allows for rich adjustment of the laser frequency. Our analysis of double-filtered optical feedback (DFOF) lasers is on fundamental solutions, sometimes referred to as continuous waves (CW) or external filtering modes (EFM), which result in a QD laser output with constant amplitude and frequency. The time delay suppression is dependent on the spectral width of the filter and how far it is from the solitary laser frequency, according to numerical calculations.

We demonstrate scaling in amplitude and time of the dynamics of quantum well and quantum dot semiconductor lasers subject to optical feedback from long delays, providing insights into the underlying physics of both devices.