Bjarne Tromborg

Bistability and low-frequency fluctuations in semiconductor lasers with optical feedback: a theoretical analysis

Near-threshold operation of a semiconductor laser exposed to moderate optical feedback may lead to low-frequency fluctuations. In the same region, a kink is observed in the light-current characteristic. Here it is demonstrated that these nonlinear phenomena are predicted by a noise driven multimode traveling-wave model. The dynamics of the low-frequency fluctuations are explained qualitatively in terms of bistability through an iterative description. >

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)

Traveling wave analysis of semiconductor lasers: modulation responses, mode stability and quantum mechanical treatment of noise spectra

We present a traveling wave analysis of a general class of semiconductor lasers, which includes multisection DFB/DBR lasers and gain-coupled DFB lasers. The analysis leads to new semianalytic expressions for the small-signal IM and FM modulation responses, the intensity and FM noise spectra, and the linewidth. The expressions are given in terms of solutions to four coupled linear homogeneous differential equations and can easily be evaluated numerically. We also derive a stability parameter /spl sigma/, for which /spl sigma/ >

Nonlinear injection locking dynamics and the onset of coherence collapse in external cavity lasers

A theoretical analysis is presented of the experimentally observed collapse of coherence in semiconductor lasers exposed to moderate optical feedback. A main point in the analysis is, that coherence collapse in the delayed-feedback (DFB) system shows up as bistability in a much simpler model which corresponds to an injection locking system. Based on this, the authors derive a simple analytical expression, valid for DFB as well as Fabry-Perot lasers, for the critical feedback level at which coherence collapse sets in. Simulations of the nonlinear injection locking model reveal the presence of complicated nonlinear dynamics with period-doubling bifurcations and coexisting attractors even inside what is normally denoted the stable locking range. >

Numerical investigation of electromagnetically induced transparency in a quantum dot structure.

.A numerical investigation of pulse propagation in a quantum dot structure in the regime of electromagnetically induced transparency is reported. The quantum dot is described as a cone on top of a wetting layer and the calculated energy levels and dipole moments are used in an effective three-level model. Pulse propagation characteristics such as degree of slowdown, absorption, and pulse distortion are investigated with respect to their dependence on the dephasing rates and pulse width. It is seen how Rabi oscillations can seriously distort the pulse when the spectral width of the pulse becomes too large compared to the width of the EIT window.

Noise spectra of semiconductor optical amplifiers: relation between semiclassical and quantum descriptions

The paper presents a comparison between a semiclassical and a quantum description of the output noise spectra of semiconductor optical amplifiers. The noise sources are represented by Langevin noise functions or operators and by the vacuum field operator, and the analysis takes into account the effects of increasing gain saturation along the amplifier waveguide. It is shown that the difference between the semiclassical and quantum descriptions sums up to a shot noise term and to a correction to the semiclassical autocorrelation relation for the carrier noise. A discussion is given of the shapes of the noise spectra, the relative importance of various noise contributions, and their dependence on input power.

Dephasing times in quantum dots due to elastic LO phonon-carrier collisions

Interpretation of experiments on quantum dot (QD) lasers presents a challenge: the phonon bottleneck, which should strongly suppress relaxation and dephasing of the discrete energy states, often seems to be inoperative. We suggest and develop a theory for an intrinsic mechanism for dephasing in QDs: second-order elastic interaction between quantum dot charge carriers and LO-phonons. The calculated dephasing times are of the order of 200 fs at room temperature, consistent with experiments. The phonon bottleneck thus does not prevent significant room temperature dephasing.

Evolution of chirped pulses in nonlinear single-mode fibers

Computer studies of the evolution of pulses in nonlinear single-mode fibers are presented. In the case of anomalous dispersion, the time-bandwidth product of the initial pulse is varied by imposing a linear frequency chirp. The pulse broadening resulting from chirp greatly exceeds the broadening caused by the loss. In the case of normal dispersion the evolution of an initially unchirped pulse is shown to depend critically on the loss. This fact is explained by the position of the chirps maxima and minima relative to the pulse-amplitude distribution.

Noise and regeneration in semiconductor waveguides with saturable gain and absorption

We have modeled the noise properties of a novel waveguide device with regenerative properties. The device consists of alternating sections of saturable gain and absorption, which give a nonlinear power transfer function. We investigate the relative intensity noise spectra and signal-to-noise ratio after the device by both a small-signal analysis and large-signal simulation, and we show that the gain saturation gives noise redistribution at the mark level. We also examine the influence of the nonlinearity on the noise probability density function and show that the standard approximations of Gaussian and noncentral /spl chi//sup 2/ distributions do not give a satisfying description. The strength and weaknesses of the limiting cases of static transfer functions and linear noise transfer as well as the small-signal analysis are examined in the case of bit-error-rate estimation in a cascade of regenerators. The interplay between increased nonlinearity and noise is investigated and we show that the increased nonlinearity achieved by additional device sections can improve the cascadability although more amplified spontaneous emission noise is added.

Line broadening caused by Coulomb carrier–carrier correlations and dynamics of carrier capture and emission in quantum dots

Mechanisms of pure dephasing in quantum dots due to Coulomb correlations and the dynamics of carrier capture and emission are suggested, and a phenomenological model for the dephasing is developed. It is shown that, if the rates of these capture and emission processes are sufficiently high, significant homogeneous line broadening of the order of several meV can result.