Svend Bischoff

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.

All-optical wavelength conversion and signal regeneration using an electroabsorption modulator

All-optical wavelength conversion and signal regeneration based on cross-absorption modulation in an InGaAsP quantum well electroabsorption modulator (EAM) is studied at different bit rates. We present theoretical results showing wavelength conversion efficiency in agreement with existing experimental results, and the signal regeneration capability of the device is investigated. In particular, we demonstrate the dependence of the extinction ratio of both the converted signal and the control signal on the device length and on the power level of the control signal. We also show how the sweep-out dynamics influences the results.

Analytical expression for the bit error rate of cascaded all-optical regenerators

We derive an approximate analytical expression for the bit error rate of cascaded fiber links containing all-optical 2R-regenerators. A general analysis of the interplay between noise due to amplification and the degree of reshaping (nonlinearity) of the regenerator is performed.

Modulational instability of electromagnetic waves in media with varying nonlinearity

The process of modulational instability of nonlinear plane waves is investigated in media with periodically and randomly varying nonlinearity. The wave behavior is analyzed in the parametric instability region under the joint action of periodic modulation and the self-steepening effect, and a recurrence phenomena is observed for the nonlinear wave in the parametric resonance region. Stochastic parametric resonance is predicted in the case of a random media. The increment value of stochastic parametric resonance obtained numerically is in good agreement with the theoretical estimate.

Monolithic colliding pulse mode-locked semiconductor lasers

In this paper we investigate the temporal and spectral properties of monolithic colliding pulse mode-locked (CPM) quantum-well lasers. A large-signal model for the CPM laser is derived, which explains the mode-locking behaviour of both passively and hybridly mode-locked CPM lasers. The pulse-shaping and wavelength chirp of the CPM pulses are found to be the result of a complicated interplay between the gain and saturable absorber dynamics. The experimentally measured pulse width dependence upon variation of gain current and applied reverse bias on the saturable absorber is explained by fast gain dynamics. The experimentally observed broadening of the optical power spectrum with gain current is found mainly to be due to the self-phase modulation in the gain sections.

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.

One- and two-phonon capture processes in quantum dots

Multiphonon capture processes are investigated theoretically and found to contribute efficiently to the carrier injection into quantum dots. It is shown that two-phonon capture contributes where single-phonon capture is energetically inhibited and can lead to electron capture times of a few picoseconds at room temperature and carrier densities of 1017 cm−3 in the barrier.

Comparison of all-optical co- and counter-propagating high-speed signal processing in SOA-based Mach–Zehnder interferometers

The all-optical signal processing performance of a Mach–Zehnder interferometer (MZI) is investigated. Calculated switching windows are used to investigate and understand the physical mechanisms limiting the high speed performance. Especially, the co- and counter-propagating operation of the MZI is discussed and important differences in the performance for the two schemes are addressed. The non-regenerative all-optical clear and drop functionality is investigated for a 2, 4 and 8 × 40 Gbit/s signal, showing good performance in the co-propagating case. Regenerative simultaneous clear and drop functionality in a single MZI is demonstrated experimentally and compared to the large signal model predictions.

Bidirectional four-wave mixing in semiconductor optical amplifiers: theory and experiment

Bidirectional four wave mixing (FWM) is investigated in a bulk semiconductor optical amplifier (SOA) for dispersion compensation and for the clear/drop functionality in optical time division multiplexed (OTDM) systems. Good performance for bidirectional midspan spectral inversion (MSSI) is theoretically predicted for bit rates of 10, 20, and 40 Gb/s and is shown to be in agreement with measurements performed at 10 and 20 Gb/s. Measurements of the clear/drop functionality using the bidirectional technique show excellent performance for a 4/spl times/10 Gb/s signal and is again in good agreement with simulations. The clear/drop functionality is also simulated for 4/spl times/20 Gb/s and 4/spl times/40 Gb/s signals.

BER estimation for all-optical regenerators influenced by pattern effects

An efficient method is presented for the estimation of the bit-error rate (BER) of a system employing all-optical regenerators influenced by pattern effects. We theoretically study noise accumulation and noise redistribution in long distance transmission systems employing a delayed interference signal wavelength converter for all-optical regeneration. The BER is studied for return-to-zero signals at bit rates of 2.5 Gb/s (no patterning) up to 40 Gb/s (strong patterning). The calculation of the BER is based on pattern dependent transfer functions, which may be obtained numerically or measured.