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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Minimized deterioration of ultrashort pulses in quantum dot optical amplifiers

Edeltraud Gehrig, Ortwin G. Hess

Published Sep 1, 2004• 1 citation
Publisher

Abstract

The dynamics of ultrashort pulses propagating in a quantum dot amplifier is determined by a complex nonlinear coupling and dynamic interplay of light fields and carriers in the spatially inhomogeneous quantum dot ensemble. Computational modeling shows that in spite of the large complexity the strong localization of the carrier inversion and the low amplitude phase coupling may allow the amplification and transmission of ultrahort light pulses with minimum deterioration of the pulse properties (e.g. pulse shape, duration). The theoretical description is based on spatially resolved Quantum Dot Maxwell-Bloch equations that describe the spatio-temporal light field and inter-/intra-level carrier dynamics in each quantum dot of a typical quantum dot ensemble. In particular, this includes spontaneous luminescence, counterpropagation of amplified spontaneous emission and induced recombination as well as carrier diffusion in the wetting layer of the laser. Intradot scattering via emission and absorption of phonons, as well as the scattering with the carriers and phonons of the surrounding wetting layer are dynamically included on a mesoscopic level. Spatial fluctuations in size and energy levels of the quantum dots and irregularities in the spatial distribution of the quantum dots in the active layer are simulated via statistical methods. Simulation results of the nonlinear pulse propagation in quantum dot optical amplifiers allow visualization and interpretation of fundamental nonlinear processes such as selective depletion and re-filling of quantum dot energy levels leading to a complex gain and index dynamics that affect the amplitude and phase of a propagating light pulse. Computational modelling thus may lay the foundation for an optimization and tayloring of pulse properties.

DOI
10.1117/12.544633

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The dynamics of ultrashort pulses propagating in a quantum dot amplifier is determined by a complex nonlinear coupling and dynamic interplay of light fields and carriers in the spatially inhomogeneous quantum dot ensemble. Computational modeling shows that in spite of the large complexity the strong localization of the carrier inversion and the low amplitude phase coupling may allow the amplification and transmission of ultrahort light pulses with minimum deterioration of the pulse properties (e.g. pulse shape, duration). The theoretical description is based on spatially resolved Quantum Dot Maxwell-Bloch equations that describe the spatio-temporal light field and inter-/intra-level carrier dynamics in each quantum dot of a typical quantum dot ensemble. In particular, this includes spontaneous luminescence, counterpropagation of amplified spontaneous emission and induced recombination as well as carrier diffusion in the wetting layer of the laser. Intradot scattering via emission and absorption of phonons, as well as the scattering with the carriers and phonons of the surrounding wetting layer are dynamically included on a mesoscopic level. Spatial fluctuations in size and energy levels of the quantum dots and irregularities in the spatial distribution of the quantum dots in the active layer are simulated via statistical methods. Simulation results of the nonlinear pulse propagation in quantum dot optical amplifiers allow visualization and interpretation of fundamental nonlinear processes such as selective depletion and re-filling of quantum dot energy levels leading to a complex gain and index dynamics that affect the amplitude and phase of a propagating light pulse. Computational modelling thus may lay the foundation for an optimization and tayloring of pulse properties.
We present theoretical and experimental results on the propagation of ultrashort pulses in quantum-dot (QD) laser amplifiers. The propagation time of the light pulses is controlled by the pulse itself (self-induced speed control) or by injection of a second pump pulse (external speed control). Our simulations on the basis of spatially and temporally resolved QD Maxwell-Bloch equations reveal that the excitation and relaxation dynamics induced by the propagating pulse or a pump pulse within the active charge carrier system leads to a complex gain and index dynamics that may either speed up or slow down the propagating light pulse. The physical effects allowing for the dynamic speed control could be ascribed to complex (coherent and incoherent) level dynamics leading to dynamic gain saturation and index dispersion. The dependence of the propagation time on injection current density and pulse energy is discussed. The numerical results of pulse reshaping and propagation times in the gain and absorptions regime are compared to experimental results
We present a theoretical model for the description of carrier and light dynamics in a quantum dot semiconductor optical amplifier that includes the inhomogeneous broadening of the quantum dots (QDs) via a spatially resolved statistical approach. The model is based on Maxwell-Bloch equations and takes into account the scattering of charge carriers between 2-D wetting layer states and bound quantum dot states, the amplification, and the wave-guiding of the light fields in the optical cavity. Simulations allow the analysis of the occupation probability of the quantum dot levels at steady state and during optical excitation by a femtosecond pulse. The influence of homogeneously and inhomogeneously broadened quantum dot media on spatial and spectral hole burning is revealed and discussed. It is shown that spatially varying dot properties lead to the reshaping of the optical pulse in the active medium.
In this work, the ultrafast gain dynamics of a quantum dot (QD)-based semiconductor optical amplifier (SOA) is modeled on the basis of semiconductor Bloch equations that include microscopically calculated nonlinear scattering rates between QD carriers and the surrounding carrier reservoir. This enables us to separately study the dynamics of electrons and holes inside the device as well as the coherent effects related to the fast polarization dynamics. We show that the optical pulse power and the dephasing time of the polarization mainly affect the gain depletion inside the active region, while the electric injection current density and thus the internal carrier dynamics influence the gain recovery. We observe that carrier–carrier scattering is the source of desynchronized behavior of electrons and holes in the recovery dynamics of QD-based SOAs. The amplification of pulse trains in the SOA predicted by our model agrees well with experimental data.
Quantum dot (QD)-based semiconductor optical amplifiers offer unique properties compared with conventional devices based on bulk or quantum well material. Due to the bandfilling properties of QDs and the existence of a nearby reservoir of carriers in the form of a wetting layer, QD semiconductor optical amplifiers may be operated in regimes of high linearity, i.e. with a high saturation power, but can also show strong and fast nonlinearities by breaking the equilibrium between discrete dot states and the continuum of wetting layer states. In this paper, we analyse the interplay of these two carrier populations in terms of a simple rate equation model. Based on the steady-state and small-signal properties of the model, we analyse and discuss the optical modulation response and the four-wave mixing properties of QD semiconductor optical amplifiers, in particular emphasizing the role of ultrafast gain dynamics.
Commercial devices for optical coherence tomography greatly benefit from the exceptional features of broadband light-emitting quantum dot superluminescent diodes (QDSLDs). Here, light generation occurs at the transition from spontaneous to stimulated emission, the regime of the amplified spontaneous emission. In this context, initially spontaneously emitted photons are amplified by stimulated emission processes when traversing through the QDSLD, which leads to strong light amplification. The suitable choice of the waveguide geometry and the gain medium formed by quantum dots, enables large spectral widths of some terahertz combined with a rather higher degree of spatial coherence. Modern measurement methods based on two-photon absorption processes provide a temporal resolution of some femtoseconds and thus allow correlation studies of the emitted QDSLD light. Also from a theoretical point of view, the characterization of the amplified spontaneous emission generated by QDSLDs and their associated photon statistics represents an interesting and challenging research topic. Especially in a particular temperature regime these devices exhibit uncommon properties with regard to the temporal field and intensity correlations: While the field correlation reflects the rather highly incoherent nature of light emitted by QDSLDs due to its spectral width of several THz, a reduction of the second order correlation from 2 to 1.33 at a temperature of T=190K was observed in the Semiconductor Optics group of Prof. W. Elsäßer at the Technical University of Darmstadt in 2011. The understanding of the occurrence of these hybrid coherent light states, which are simultaneously incoherent in first and coherent in second order correlation function is the subject of this thesis. In a first step we find the quantum mechanical light state associated with the QDSLD to be well described by a multimode phase-randomized Gaussian state by comparison with experimental results. In the second step we present a microscopic theory of the amplified spontaneous emission, which allows an explanation of the temperature-dependent noise suppression of broadband QDSLDs. For this purpose we consider distinguishable quantum dots, which are embedded in a strongly absorptive bulk material that defines a waveguide. Tilted and anti-reflection coated output facets, leading to a suppression of longitudinal modes, are modeled by beam splitters that couple the internal field to the surroundings. Regarding the spectral properties of QDSLDs, the broadband light generated inside the diode is described by a multimode electric field. This multimode quantum field theory yields rate equations for the optical power densities and the level occupation of the inhomogeneous ensemble of quantum dots within the diode. With the help of the input-output formalism, we determine the optical power spectrum. As a main result, we find the broadband external power spectrum to be a convolution of the intra-diode photon spectrum with a Lorentzian response. This finding corresponds with experimentally available spectra. Furthermore, based on the quantum theory of QDSLDs we determine the central second-order degree of coherence. It reveals a reduction within a special detuning regime and therefore allows the interpretation of the hybrid coherent light phenomenon from a quantum optical point of view.