X. Hachair

Cavity solitons in a driven VCSEL above threshold

We experimentally demonstrate the existence and the control of cavity solitons in externally driven vertical-cavity semiconductor lasers above threshold. A model including material polarization dynamics is used to predict and confirm the experimental findings.

Incoherent and coherent writing and erasure of cavity solitons in an optically pumped semiconductor amplifier

Cavity solitons in semiconductor amplifiers were, from the beginning of their study and observation, obtained either spontaneously or in a controlled manner by local coherent excitation. We describe two experiments that demonstrate coherent and, we believe for the first time, incoherent writing and erasure of cavity solitons in an optically pumped vertical-cavity semiconductor amplifier by short optical pulses.

Mode-switching in semiconductor lasers

In this paper, we experimentally analyze the modal dynamics of quantum-well semiconductor lasers. Modal switching is the dominant feature for semiconductor lasers that exhibit two or several active longitudinal modes in their time-averaged optical spectrum. In quantum-well lasers, these dynamics involve a periodic switching among several longitudinal modes, which follows a well-determined sequence from the bluest to the reddest mode in the optical spectrum. This feature is radically different from the well-known noise-driven mode-hopping occurring in bulk lasers which involves only two main modes. We analyze the differences in modal dynamics for these two kinds of laser by comparing the modal switching statistics and by studying the effects of noise and modulation in the pumping current.

Efficient Illumination for Microsecond Tracking Microscopy

The possibility to observe microsecond dynamics at the sub-micron scale, opened by recent technological advances in fast camera sensors, will affect many biophysical studies based on particle tracking in optical microscopy. A main limiting factor for further development of fast video microscopy remains the illumination of the sample, which must deliver sufficient light to the camera to allow microsecond exposure times. Here we systematically compare the main illumination systems employed in holographic tracking microscopy, and we show that a superluminescent diode and a modulated laser diode perform the best in terms of image quality and acquisition speed, respectively. In particular, we show that the simple and inexpensive laser illumination enables less than s camera exposure time at high magnification on a large field of view without coherence image artifacts, together with a good hologram quality that allows nm-tracking of microscopic beads to be performed. This comparison of sources can guide in choosing the most efficient illumination system with respect to the specific application.

Identification of the stimulated-emission threshold in high-{beta} nanoscale lasers through phase-space reconstruction

Nanoscale lasers sustain a few optical modes so that the fraction of spontaneous emission {beta} funnelled into the useful (lasing) mode is high (of the order of 10{sup -1}) and the threshold, which traditionally corresponds to an abrupt kink in the light-in-light-out curve, becomes ill defined. We propose an alternative definition of the threshold that is based on the dynamical response of the laser and is valid even for {beta}=1 lasers. The laser dynamics is analyzed through a reconstruction of its phase-space trajectory for pulsed excitations. Crossing the threshold, brings about a change in the shape of the trajectory and in the area contained in it. An unambiguous determination of the threshold in terms of this change is shown theoretically and illustrated experimentally in a photonic-crystal laser.

Transverse spatial structure of a high Fresnel number Vertical External Cavity Surface Emitting Laser

The transverse spatial structure of an optically-pumped, Vertical External Cavity Surface Emitting Laser is investigated experimentally. The Fresnel number of the laser cavity is controlled with an intracavity lens. We show how the emission profile changes when passing from a low to a high Fresnel number configuration and analyze the RF spectrum of the total laser intensity. Though the laser operates in a multi-longitudinal mode configuration, the transverse profile of the laser emission shows well organized patterns.

Optimization of the switch-on and switch-off transition in a commercial laser

The response of a Class B laser to a rapid change in one of its parameters is known to be accompanied by delay and ringing. It has been theoretically and numerically shown that the transition can be modified by using adequate functional shapes for the control parameter (e.g., the laser pump) in order to steer the laser from one point of operation to another. Here we experimentally show the implementation of these ideas in a commercial device: a semiconductor laser. We establish a procedure for optimizing a controlled switch-on and switch-off and obtain a clean, fast, and reliable square pulse, either in a single shot or in a repetitive sequence. The generality of this procedure promises a wide field of application for a variety of laser systems.

Cavity solitons in broad area VCSELs below threshold

Cavity solitons are stationary self-organized bright intensity peaks which form over a homogeneous background in the section of broad area radiation beams. They are generated by shining a writing/erasing laser pulse into a nonlinear optical cavity, driven by a holding beam. The ability to control their location and their motion by introducing phase or amplitude gradients in the holding beam makes them interesting as mobile pixels for all-optical processing units. We show the generation of a number of cavity solitons in broad area vertical cavity semiconductor microresonators electrically pumped above transparency but slightly below threshold. The observed spots can be written, erased and manipulated as independent objects. We analyze experimentally the cavity solitons domain of existence in the parameter space and how their characteristics are affected by inhomogeneities and impurities of the vertical cavity devices. A theoretical model, keeping into account the devices characteristics, reproduces numerically the experimental observations with good agreement.

Modal switching in quantum-well semiconductor lasers with weak optical feedback

We analyze theoretically and experimentally how the modal dynamics of quantum-well semiconductor lasers is affected by weak optical feedback. Without feedback, these lasers exhibit a regular switching among several longitudinal modes, following a well determined modal sequence and leaving the total intensity output constant. Using a multimode theoretical model we have identified the four wave mixing as the dominant mechanism at the origin of these intriguing dynamics, while the asymmetry of the susceptibility function of semiconductor materials allows to explain the modal sequence. In this manuscript we show that these dynamics, which is almost insensitive to current noise or modulation, is instead extremely sensitive to optical feedback. The experimental results are satisfactorily compared with the numerical predictions of the model, properly adapted for including weak optical feedback.

High-speed direct modulation of semiconductor lasers

Direct modulation of the injected current still represents an attractive and inexpensive technique for encoding information in the output of a semiconductor laser. The growing requirements in volume of information to be transmitted and their conflict with the progressive, and rapid, degradation of the signal at high speeds have made this simple technical solution less and less attractive. A brief analysis of the sources of the problem is offered. A viable and inexpensive way of improving DMs performance is discussed. High quality signals are predicted at data transmission speeds that exceed by over an order of magnitude those obtainable with DM.