Francis Pascal Romstad

Measurement of pulse amplitude and phase distortion in a semiconductor optical amplifier: from pulse compression to breakup

We have performed extensive measurements of the propagation of ultrashort pulses in a semiconductor bulk amplifier using an ultrasensitive cross frequency-resolved optical gating technique. Pulses of 175-fs duration with energies from below 1 fJ to above 100 pJ are measured both in amplitude and phase after propagation through the device. While only moderate reshaping effects occur at pulse energies of below 1 pJ, strong amplitude distortion together with nonlinear chirp is found for input energies of 5-100 pJ. This leads to a pulse narrowing by more than a factor of two when the amplifier is biased for material transparency or absorption and to a pronounced pulse breakup in the gain regime.

Heterodyne technique for measuring the amplitude and phase transfer functions of an optical modulator

We propose a technique based on heterodyne detection for accurately and simultaneously measuring the amplitude and phase transfer functions of an optical modulator. The technique is used to characterize an InGaAsP multiple quantum-well electroabsorption modulator. From the measurements we derive the small-signal /spl alpha/-parameter and the time-dependent chirp for different operation conditions.

Separation of coherent and incoherent nonlinearities in a heterodyne pump-probe experiment

We demonstrate that the transient coherent nonlinearity (coherent artifact) affecting the pump-probe response of semiconductor optical amplifiers can be experimentally separated from the incoherent transient. The technique is based on measuring the mirror component of the coherent artifact which is a background-free four--wave mixing signal at a different frequency with respect to the transmitted probe in a heterodyne detection scheme. Measurements on amplifiers of different length reveal strong deviations from the commonly expected symmetric shape of the coherent artifact in case of long waveguides.

All-optical demultiplexing and wavelength conversion in an electroabsorption modulator

Cross-absorption modulation in an all electroabsorption modulator is utilised to perform 80/10 Gb/s all-optical demultiplexing. An improvement in receiver sensitivity at 10 Gb/s is demonstrated when wavelength converting.

Absorption recovery in strongly saturated quantum-well electroabsorption modulators

We observe experimentally that a quantum-well electroabsorption modulator, when strongly saturated by a highly energetic optical pulse, may exhibit an absorption recovery time much longer than for excitation with a low-energy pulse. Using a comprehensive drift-diffusion type model, we are able to explain this effect theoretically. The prolongation of the absorption recovery is induced by carrier distribution effects, not by field-induced changes in the dynamical transport parameters such as the time carriers take to escape from the wells.

Large-area single-mode photonic bandgap VCSELs

We demonstrate that the photonic bandgap effect can be used to control the modes of large area vertical cavity surface emitting lasers. We obtain more than 20 dB side mode suppression ratios in a 10-micron area device.

Measurement of the amplitude and phase transfer functions of an optical modulator using a heterodyne technique

We present a new technique that measures the full amplitude and phase transfer curves of the modulator as a function of the applied bias, from which the small signal /spl alpha/-parameter can be calculated. The technique measures the amplitude and phase transfer functions simultaneously and directly, compared to techniques where a time-consuming data analysis is necessary to calculate the a-parameter and an additional measurement is necessary to estimate the phase. Additionally, the chirp profile for all operation points can be calculated.

Ultrashort pulse-propagation effects in a semiconductor optical amplifier: microscopic theory and experiment

We present microscopic modeling and experimental measurements of femtosecond-pulse interactions in a semiconductor optical amplifier. Two novel nonlinear propagation effects are demonstrated: pulse break-up in the gain regime and pulse compression in the transparency regime. These propagation phenomena highlight the microscopic origin and important role of adiabatic following in semiconductor optical amplifiers. Fundamental light-matter interactions are discussed in detail and possible applications are highlighted.

2.5 Gbit/s modulation of 1300 nm single-mode photonic crystal VCSELs

Long wavelength single fundamental mode photonic crystal VCSELs have been fabricated and demonstrated to deliver 1 mW of output power up to 90degC. 2.5 Gbit/s modulation experiments show clear and open eyes

Characterisation of a MQW electroabsorption modulator as an all-optical demultiplexer

A detailed experimental investigation of the all-optical switching properties of an InGaAsP MQW electroabsorption modulator has been performed. Using high pump pulse energies and high reverse bias settings, switching windows were demonstrated with extinction ratios up to 25 dB and widths down to 10 ps, possibly facilitating 100/10 Gb/s demultiplexing.