Tommy Winther Berg

Ultrafast gain recovery and modulation limitations in self-assembled quantum-dot devices

Measurements of ultrafast gain recovery in self-assembled InAs quantum-dot (QD) amplifiers are explained by a comprehensive numerical model. The QD excited state carriers are found to act as a reservoir for the optically active ground state carriers resulting in an ultrafast gain recovery as long as the excited state is well populated. However, when pulses are injected into the device at high-repetition frequencies, the response of a QD amplifier is found to be limited by the wetting-layer dynamics.

InP based lasers and optical amplifiers with wire-/dot-like active regions

Long wavelength lasers and semiconductor optical amplifiers based on InAs quantum wire-/dot-like active regions were developed on InP substrates dedicated to cover the extended telecommunication wavelength range between 1.4 and 1.65 µm. In a brief overview different technological approaches will be discussed, while in the main part the current status and recent results of quantum-dash lasers are reported. This includes topics like dash formation and material growth, device performance of lasers and optical amplifiers, static and dynamic properties and fundamental material and device modelling. (Some figures in this article are in colour only in the electronic version)

Saturation and noise properties of quantum-dot optical amplifiers

Based on extensive numerical calculations, quantum-dot (QD) amplifiers are predicted to offer higher output power and lower noise figure compared to bulk as well as quantum well amplifiers. The underlying physical mechanisms are analyzed in detail, leading to the identification of a few key requirements that QD amplifiers should meet in order to achieve such superior linear characteristics. The existence of a highly inverted wetting layer or barrier region, acting as a carrier reservoir, is central to this performance enhancement. It is shown that amplified spontaneous emission acts to decrease the inversion of the wetting layer states, thus helping to quench the gain of these states, which might otherwise dominate.

Quantum dot amplifiers with high output power and low noise

Quantum dot semiconductor optical amplifiers have been theoretically investigated and are predicted to achieve high saturated output power, large gain, and low noise figure. We discuss the device dynamics and, in particular, show that the presence of highly inverted barrier states does not limit the performance of these devices.

Gain dynamics and saturation in semiconductor quantum dot amplifiers

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.

Polarization properties of honeycomb-structured photonic bandgap fibres

New polarization properties of photonic bandgap optical fibres are analysed. Exemplified by an analysis of honeycomb-structured photonic bandgap fibres, strong birefringence is found for modest non-uniformities (deviations from the 60° symmetry) in and around the core region. This suggests the application of photonic bandgap fibres as polarization-maintaining optical components.

Ultrafast signal processing in quantum dot amplifiers through effective spectral holeburning

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/.

Europeanist-programme roadmap for optical communications generated by theoptimist thematic network

On the basis of Europeanist project results in the field of photonics, theoptimist thematic network has produced a roadmap for optical communications. This work is described in the present article which provides scenarios for the evolution of the optical network in the coming 10 years. Predictable developments in optical components, sub-systems and systems and their consequences on the architecture and performances of the networks are analysed. Specific interest is focused on the various parts of the network structure namely the access, the metropolitan area, the wide area and the global networks. Technical documents elaborated by theoptimist consortium and minutes fromist workshops can be found on the website http://www.ist-optimist.org/.[1]. They constitute a main source for further information.RésuméÀ partir des résultats des projets européensist dans le domaine de la photonique, le réseau thématique «optimist» a généré une feuille de route pour les télécommunications optiques. L’article décrit les divers scénarios possibles pour les années à venir. Les développements prévisibles du domaine des composants et des sous-systèmes, et les progrès induits par ces devéloppements sur l’architecture et les performances des réseaux sont analysés. On se focalise sur les évolutions dans le réseau d’accès, le réseau métropolitain et le réseau-cœur. L’article est tiré des nombreux documents techniques élaborés par le consortium «optimist» et se trouvant sur le site web http://www.ist-optimist.org.

Ultrafast optical signal processing using semiconductor quantum dot amplifiers

The linear and nonlinear properties of quantum dot amplifiers are discussed on the basis of an extensive theoretical model. These devices show great potential for linear amplification as well as ultrafast signal processing.

Theoretical analysis of four wave mixing in quantum dot optical amplifiers

The four wave mixing properties of semiconductor quantum dot amplifiers have been investigated. The combination of strong nonequilibrium depletion of dot levels and a small linewidth enhancement factor results in efficient and symmetric four wave mixing.