Bistability and hysteresis in an optically injected two-section semiconductor laser

The effect of coherent single frequency injection on two-section semiconductor lasers is studied numerically using a model based on a set of delay differential equations. The existence of bistability between different continuous-wave and nonstationary regimes of operation is demonstrated in the case of sufficiently large linewidth enhancement factors

Nonstationary generation of electromagnetic radiation in nonequilibrium mirror-confined plasma

We demonstrate the use of a laboratory setup based on a magnetic mirror trap with plasma sustained by a gyrotron radiation under the electron cyclotron resonance conditions aimed at identifying the role of the background plasma as a trigger of the electron cyclotron instability. New regime of instability has been revealed during the plasma decay after the gyrotron switch-off when the plasma density becomes low enough, so that the electron plasma frequency is much less than the electron gyrofrequency. At this stage, we observe the excitation of electromagnetic waves which propagate nearly perpendicular to the magnetic field and cause precipitation of energetic electrons to the trap ends. The instability is detected as series of quasi-periodic broadband pulses of electromagnetic radiation (25…27 GHz frequency, typical pulse duration of 1…10 microseconds) and related precipitation of energetic (>10 keV) electrons. These emissions of the fast electrons can be attributed to the development of the kinetic instability of the extraordinary wave propagating quasiperpendicular to the magnetic field near the fundamental cyclotron harmonic.Исследованы временные и частотные характеристики квазипериодических импульсов электромагнитного излучения в распадающейся плазме импульсного ЭЦР-разряда в прямой аксиально-симметричной магнитной ловушке. Зарегистрированы серии квазипериодических импульсов электромагнитного излучения на частоте 25...27 ГГц с типичной длительностью импульса 1...10 мкс и связанные с ними высыпания энергичных (> 10 кэВ) электронов. Наблюдаемое излучение интерпретировано как результат резонансного взаимодействия горячих электронов с быстрой необыкновенной волной, распространяющейся в разреженной плазме поперек внешнего магнитного поля.Досліджено часові та частотні характеристики квазіперіодичних імпульсів електромагнітного випромінювання в плазмі, що розпадається, імпульсного ЕЦР-розряду в прямій аксіально-симетричній магнітній пастці. Зареєстровані серії квазіперіодичних імпульсів електромагнітного випромінювання на частоті 25...27 ГГц з типовою тривалістю імпульсу 1...10 мкс та пов'язані з ними висипання енергійних (> 10 кеВ) електронів. Випромінювання, що спостерігається, інтерпретовано як результат резонансної взаємодії гарячих електронів з швидкою незвичайною хвилею, що розповсюджується в розрідженій плазмі поперек зовнішнього магнітного поля

Generation of wideband electromagnetic radiation on a decay stage of a mirror-confined plasma produced by ECR discharge

A specific nonlinear regime of electron cyclotron instability is discussed aimed at explanation of complex temporal patterns of stimulated electromagnetic radiation from a mirror trap with non-equilibrium plasma typical of ECR discharge. This regime is characterized by self-modulation of a plasma cyclotron maser due to coherent interference of two counter-propagating unstable waves with degenerate frequencies. The proposed simple theoretical model allows reproducing multi-scale behavior of quasi-periodic pulses of electromagnetic radiation and precipitation of energetic electrons detected at a laboratory setup based on a mirror trap with plasma sustained by mm-wave gyrotron radiation.Обсуждается новый нелинейный режим электронно-циклотронной неустойчивости, объясняющий сложную временную динамику импульсов электромагнитного излучения сильнонеравновесной плазмы ЭЦР-разряда в прямой магнитной ловушке. Режим реализуется при самомодуляции циклотронного мазера полем биений двух встречных неустойчивых волн с вырожденными частотами. Предложенная простая теоретическая модель позволяет воспроизвести многомасштабную структуру импульсов электромагнитного излучения и высыпания энергичных электронов, зарегистрированных в лабораторном эксперименте с использованием прямой ловушки с плазмой, поддерживаемой излучением гиротрона миллимeтрового диапазона.Обговорюється новий нелінійний режим електронно-циклотронної нестійкості, що пояснює складну часову динаміку імпульсів електромагнітного випромінювання сильнонеравномірної плазми ЕЦР-розряду в прямій магнітній пастці. Режим реалізується при самомодуляціі циклотронного мазера полем биття двох зустрічних нестійких хвиль з виродженими частотами. Запропонована проста теоретична модель дозволяє відтворити багатомасштабну структуру імпульсів електромагнітного випромінювання та висипання енергійних електронів, зареєстрованих у лабораторному експерименті з використанням прямої пастки з плазмою, що підтримується випромінюванням гіротрона миллиметрового діапазону

Superfocusing of high-M2 semiconductor laser beams:experimental demonstration

The focusing of multimode laser diode beams is probably the most significant problem that hinders the expansion of the high-power semiconductor lasers in many spatially-demanding applications. Generally, the 'quality' of laser beams is characterized by so-called 'beam propagation parameter' M2, which is defined as the ratio of the divergence of the laser beam to that of a diffraction-limited counterpart. Therefore, M2 determines the ratio of the beam focal-spot size to that of the 'ideal' Gaussian beam focused by the same optical system. Typically, M2 takes the value of 20-50 for high-power broad-stripe laser diodes thus making the focal-spot 1-2 orders of magnitude larger than the diffraction limit. The idea of 'superfocusing' for high-M2 beams relies on a technique developed for the generation of Bessel beams from laser diodes using a cone-shaped lens (axicon). With traditional focusing of multimode radiation, different curvatures of the wavefronts of the various constituent modes lead to a shift of their focal points along the optical axis that in turn implies larger focal-spot sizes with correspondingly increased values of M2. In contrast, the generation of a Bessel-type beam with an axicon relies on 'self-interference' of each mode thus eliminating the underlying reason for an increase in the focal-spot size. For an experimental demonstration of the proposed technique, we used a fiber-coupled laser diode with M2 below 20 and an emission wavelength in ~1μm range. Utilization of the axicons with apex angle of 140deg, made by direct laser writing on a fiber tip, enabled the demonstration of an order of magnitude decrease of the focal-spot size compared to that achievable using an 'ideal' lens of unity numerical aperture

Impact of the carrier relaxation paths on two-state operation in quantum dot lasers

We study InGaAs QD laser operating simultaneously at ground (GS) and excited (ES) states under 30ns pulsed-pumping and distinguish three regimes of operation depending on the pump current and the carrier relaxation pathways. An increased current leads to an increase in ES intensity and to a decrease in GS intensity (or saturation) for low pump range, as typical for the cascade-like pathway. Both the GS and ES intensities are steadily increased for high current ranges, which prove the dominance of the direct capture pathway. The relaxation oscillations are not pronounced for these ranges. For the mediate currents, the interplay between the both pathways leads to the damped large amplitude relaxation oscillations with significant deviation of the relaxation oscillation frequency from the initial value during the pulse

The effect of slow passage in the pulse-pumped quantum dot laser

In recent years, quantum-dot (QD) semiconductor lasers attract significant interest in many practical applications due to their advantages such as high-power pulse generation because to the high gain efficiency. In this work, the pulse shape of an electrically pumped QD-laser under high current is analyzed. We find that the slow rise time of the pulsed pump may significantly affect the high intensity output pulse. It results in sharp power dropouts and deformation of the pulse profile. We address the effect to dynamical change of the phase-amplitude coupling in the proximity of the excited state (ES) threshold. Under 30ns pulse pumping, the output pulse shape strongly depends on pumping amplitude. At lower currents, which correspond to lasing in the ground state (GS), the pulse shape mimics that of the pump pulse. However, at higher currents the pulse shape becomes progressively unstable. The instability is greatest when in proximity to the secondary threshold which corresponds to the beginning of the ES lasing. After the slow rise stage, the output power sharply drops out. It is followed by a long-time power-off stage and large-scale amplitude fluctuations. We explain these observations by the dynamical change of the alpha-factor in the QD-laser and reveal the role of the slowly rising pumping processes in the pulse shaping and power dropouts at higher currents. The modeling is in very good agreement with the experimental observations

Slow passage through thresholds in quantum dot lasers

A turn on of a quantum dot (QD) semiconductor laser simultaneously operating at the ground state (GS) and excited state (ES) is investigated both experimentally and theoretically. We find experimentally that the slow passage through the two successive laser thresholds may lead to significant delays in the GS and ES turn ons. The difference between the turn-on times is measured as a function of the pump rate of change and reveals no clear power law. This has motivated a detailed analysis of rate equations appropriate for two-state lasing QD lasers. We find that the effective time of the GS turn on follows an -1/2 power law provided that the rate of change is not too small. The effective time of the ES transition follows an -1 power law, but its first order correction in ln is numerically significant. The two turn ons result from different physical mechanisms. The delay of the GS transition strongly depends on the slow growth of the dot population, whereas the ES transition only depends on the time needed to leave a repellent steady state

Dropout dynamics in pulsed quantum dot lasers due to mode jumping

We examine the response of a pulse pumped quantum dot laser both experimentally and numerically. As the maximum of the pump pulse comes closer to the excited-state threshold, the output pulse shape becomes unstable and leads to dropouts. We conjecture that these instabilities result from an increase of the linewidth enhancement factor α as the pump parameter comes close to the excitated state threshold. In order to analyze the dynamical mechanism of the dropout, we consider two cases for which the laser exhibits either a jump to a different single mode or a jump to fast intensity oscillations. The origin of these two instabilities is clarified by a combined analytical and numerical bifurcation diagram of the steady state intensity modes

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

We analyze the dynamics of a mode-locked quantum-dot edge-emitting semiconductor laser consisting of reversely biased saturable absorber and forward biased amplifying sections. To describe spatial non-uniformity of laser parameters, optical fields and carrier distributions we use the traveling wave model, which takes into account carrier exchange processes between wetting layer and quantum dots. A comprehensive parameter study and an optical mode analysis of operation regimes are presented