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Conference on Lasers and Electro-Optics

Ultrafast signal processing in quantum dot amplifiers through effective spectral holeburning

T.W. Berg, Jesper Mørk, Alexander V. Uskov

Published Jun 25, 2003• 3 citations
Publisher

Abstract

Summary form only given. Significant progress has been obtained on quantum dot (QD) lasers, but the possible advantages of QD amplifiers are not yet clear. We show here that a relatively slow coupling between the optically active QD carrier states and the surrounding carrier reservoir can lead to efficient gain modulation suitable for ultrafast signal processing. The basis of this property is that the process of spectral hole burning (SHB) can become very effective. We consider a traveling wave optical amplifier consisting of the dot states, which interact with the optical signal (no inhomogeneous broadening included), and the wetting layer (WL), where current is injected. Time evolution is described by two coupled rate equations. Carrier capture from WL to dots is characterized by the capture time /spl tau//sub 0/.

DOI
10.1109/cleo.2002.1034452

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Summary form only given. Significant progress has been obtained on quantum dot (QD) lasers, but the possible advantages of QD amplifiers are not yet clear. We show here that a relatively slow coupling between the optically active QD carrier states and the surrounding carrier reservoir can lead to efficient gain modulation suitable for ultrafast signal processing. The basis of this property is that the process of spectral hole burning (SHB) can become very effective. We consider a traveling wave optical amplifier consisting of the dot states, which interact with the optical signal (no inhomogeneous broadening included), and the wetting layer (WL), where current is injected. Time evolution is described by two coupled rate equations. Carrier capture from WL to dots is characterized by the capture time /spl tau//sub 0/.
The theory of four-wave mixing (FWM) in the quantum dot (QD) semiconductor optical amplifiers (SOAs) is discussed by combining the QD rate equations system, the quantum-mechanical density-matrix theory, and the pulse propagation in QD SOAs including the three region of QD structure ground state (GS), excited state (ES), and wetting layer. Also, relations for differential gain, gain integral, and nonlinear susceptibility of both pump, probe, and signal pulses were discussed. Gain and differential gain have been calculated for QD structure. FWM efficiency and its components [spectral hole burning (SHB), carrier heating, and carrier density pulsation] are calculated. It is found that inclusion of ES in the formulas and in the calculations is essential since it works as a carrier reservoir for GS. It is found that QD SOA with enough capture time from ES to GS will reduce the SHB component, and so it is suitable for telecommunication applications that require symmetric conversion and independent detuning.
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.
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.
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.
The gain and phase recovery dynamics of quantum-dot (QD) semiconductor optical amplifiers are calculated, including all the optical transitions involved in successive carrier recovery processes. The carrier recovery dynamics of inhomogeneously broadened QDs is simulated by solving 1088 coupled rate equations. The respective contributions of QD states and quantum-well carrier reservoirs to the gain and phase changes are identified both temporally and spectrally. We show that the slow phase recovery component of the QD ground state is induced by the slow carrier dynamics of the carrier reservoir due to a slowly varying line shape function of the refractive index change.