P.M.W. Skovgaard

Enhanced stability of MFA-MOPA semiconductor lasers using a nonlinear, trumpet-shaped flare

Monolithically integrated flared amplifier master oscillator power amplifier (MFA-MOPA) lasers are studied using a high-resolution computational model that resolves time as well as longitudinal and transverse space dependences and includes Lorentzian gain and dispersion dynamics. By altering the linear flare of the power amplifier into a nonlinear, trumpet-shaped flare to overlap the gain region to the expanding field, the instability threshold of the MOPA is increased by /spl sim/2 for single-longitudinal, single-transverse mode operation and /spl sim/3 for single-transverse mode operation. This enables the MOPA to maintain a stable, near-diffraction limited output beam for higher currents before the onset of transverse instabilities. Thus the trumpet-flared MOPA emits an output beam of significantly higher power and brightness. This increased stability is due to a large reduction in feedback from the output facet of the trumpet shaped MFA-MOPA.

Two-photon photoconductivity in semiconductor waveguide autocorrelators

We study experimentally and theoretically the mechanism of photocurrent generation in GaAs/AlGaAs waveguide heterostructure diodes by pulsed and CW radiation over wide ranges of average incident power and waveguide length. We confirm the importance of two-photon absorption in non-linear photocurrent generation and suggest that SHG mechanism is negligible. We determine the dynamic range in which the diode has a quadratic response to the incident optical peak power and hence is suitable for autocorrelation measurements of ultrashort optical pulses.

Subpicosecond heterodyne four-wave mixing experiments on InGaAsP semiconductor laser amplifiers

Abstract We give a detailed description of a recently proposed femtosecond heterodyne four-wave mixing experiment to investigate ultrafast coherent dynamics in waveguides and discuss similarities to femtosecond heterodyne pump-probe experiments. We demonstrate and discuss the potential and the limitations of the new technique on the basis of first experiments. Examples of measurements on an InGaAsP semiconductor amplifier are presented, that include transient four-wave mixing between pulses of the same and of orthogonal polarisations. Implications of our four-wave mixing results on the ultrafast dynamics in semiconductor amplifiers are discussed.

Toward filament-free semiconductor lasers

We outline physical models and simulations for suppression of self-focusing and filamentation in large aperture semiconductor lasers. The principal technical objective is to generate multi-watt CW or quasi-CW outputs with nearly diffraction limited beams, suitable for long distance free space transmission, focusing to small spots or coupling to single-mode optical fibers. The principal strategies are (1) optimization of facet damage thresholds, (2) reduction of the linewidth enhancement factor which acts as the principal nonlinear optical coefficient, and (3) design of laterally profiled propagation structures in lasers and amplifiers which suppress lateral reflections.

Room-temperature cw operation of InGaAsP/InP microdisk lasers

Microcavity semiconductor lasers are interesting for fundamentals of controlled spontaneous emission, thresholdless lasing, quantum physical interactions between photons, excitons and cavity polaritons. They also promise novel engineering applications such as low noise oscillators, filters and optoelectronic switches. Microdisk lasers which support whispering gallery modes are especially useful in that they provide 2D wavelength scale confinement. Novel InGaAsP/InP microdisks supported on glass substrates have recently been fabricated in our laboratories and have been pumped optically, resulting in the first CW room temperature lasing in these devices.

Inhomogeneous pumping of semiconductor lasers by contact profiling

We have demonstrated a simple yet versatile lithographic method to achieve any desired current density profile, both transversely and longitudinally. We have shown experimental results on broad area semiconductor lasers which confirm that the current density is significantly altered in the transverse direction and that the variation in the longitudinal direction is small. We emphasise that such a contact needs only a single voltage supply; all features are in electrical contact, and during operation the entire contact is maintained at the same electrical potential.

High brightness semiconductor lasers with reduced filamentation

We outline physical models and simulations for suppression of self-focusing and filamentation in large aperture semiconductor lasers. The principal technical objective is to generate multi-watt CW or quasi-CW outputs with nearly diffraction limited beams, suitable for long distance free space transmission, focusing to small spots or coupling to single mode optical fibres. The principal strategies are (a) optimization of facet damage thresholds, (b) reduction of the linewidth enhancement factor which acts as the principal nonlinear optical coefficient, and (c) design of laterally profiled propagation structures in lasers or amplifiers which suppress lateral reflections.

High brightness semiconductor lasers by inhomogeneous pumping

Summary form only given. High power semiconductor lasers have applications in spectroscopy, telecommunications, manufacturing and materials processing medicine, free space communication or energy transfer. Powers of /spl sim/1 W or more require large apertures /spl sim/100 pm which lead to high order transverse modes, filamentation and spatio-temporal instabilities, all of which degrade spatial coherence and therefore brightness. In this paper we describe recent theoretical and experimental research to develop laser structures and resonators to suppress filamentation and high order transverse mode oscillation. We have fabricated and analysed broad area lasers, unstable resonators, and flared lasers.

Improved near- and far-field distributions in broad area semiconductor lasers with enhanced current spreading

We have demonstrated a novel semiconductor laser design with enhanced lateral current spreading. This results in significantly improved near and far-field distributions when compared to a standard broad area laser with a similar FWHM near-field. The far-field at a total output power of 0.6 W is single lobed, symmetric, and less than 60 percent narrower than the standard device.

Microcavity Semiconductor Lasers

We review the theory and recent experimental developments in microcavity semiconductor lasers and their implications. Particular attention is paid to microdisk lasers which support whispering gallery modes. InGaAsP/InP microdisks have recently been fabricated in our laboratories and have been pumped optically, resulting in the first achievement of CW room temperature lasing in these devices.