M. B. Willemsen

Features of vectorial modes in phase-coupled VCSEL arrays: experiments and theory

We present a detailed study of oxide-confined, vertical-cavity surface-emitting lasers (VCSELs) where the reflectivity of the top mirror has been patterned by means of a metal grid, which at the same time acts also as an electrode. Owing to their features, these kind of devices are commonly referred to as phase-coupled VCSEL arrays. The analysis is based on a joint experimental and theoretical effort: the former is devoted to a complete characterization of the emission properties, while the latter is based on a comprehensive fully vectorial model for the structure eigenmodes with the details of their complex structure. The detected characteristics make them quite attractive for various applications and the comparison of their modal properties with the model is proved to be essential for a deep understanding of these lasers. In particular, we observed and explain, for the first time, a characteristic behavior of the lasing array, which displays spatially inhomogeneous polarization characteristics with symmetry properties with respect to the array diagonals. The good matching between theory and experiment opens new perspectives for an optimized device, where the typical four lobe far-field emission is converted in a narrow central lobe: two different proposals will be discussed.

Pinning of daisy modes in optically pumped vertical-cavity surface-emitting lasers

We have observed stable single-mode oscillation of optically pumped vertical-cavity surface-emitting lasers (i.e., without fabricated aperture) in several high-order daisy modes. The surprising stability of the nodal lines of a daisy mode is explained as being due to the combined action of many microscopic defects; when a daisy mode is moved across the wafer, the effective correlation length of the orientation of the pinning is of the order of 1 μm.

Polarization modal noise and dichroism in vertical-cavity semiconductor lasers

We show quantitatively how the limited polarization stability of vertical-cavity semiconductor lasers is related to the small dichroism and large quantum noise in these lasers. We introduce several techniques to measure this dichroism and noise. Suitable polarization projections are shown to yield either the heterodyne beat or the partition noise of the two polarization modes.

Self-pulsations in vertical-cavity semiconductor lasers

We report the observation of strong self-pulsations in oxide-confined vertical-cavity surface-emitting lasers. From optical spectra and intensity noise spectra, we deduce that the pulses are strongly chirped.

Characterizing and understanding VCSEL polarization noise

We review a relatively simple model for VCSEL polarization noise, which is based on the adiabatic elimination of the carrier spin dynamics, and show how this 2-dimensional model can often be further simplified to a 1-dimensional (Kramers) model. Experimental verification for the validity of these models comes from a series of experiments on proton-implanted and etched-post VCSELs. We quantify the amount of linear and nonlinear birefringence and dichroism, we demonstrate a polarization-type of four-wave mixing, and discuss the correlated modal intensity fluctuations in the two polarization modes. Finally, we discuss the jump statistics of a VCSEL with stochastic polarization switches, and present a time-resolved observation of the polarization evolution during such a switch.

Polarization-resolved linewidth-power product of a vertical-cavity semiconductor laser

We have studied the linewidth around threshold of the lasing and nonlasing polarization mode (x and y) in a TEM00 oxide-confined vertical-cavity semiconductor laser (VCSEL). We experimentally demonstrate: (i) that the nonlasing mode shows surprising behavior when the lasing mode passes through threshold, and (ii) that the polarization fluctuations in a VCSEL are limited by the same quantum noise that sets its finite Schawlow–Townes linewidth.

Spontaneous emission spectra of optically pumped vertical-cavity surface-emitting lasers

Abstract We study the spontaneous emission spectra of optically pumped vertical-cavity surface-emitting lasers (VCSELs) operating below threshold. By spatial and angular filtering of the spontaneous emission spectra we can relate the observed asymmetrical shape of these spectra to the subthreshold guiding properties of the VCSELs. We find a substantial difference between VCSELs with a thin gain medium, consisting of quantum wells, and VCSELs with a thick gain medium, consisting of bulk GaAs; this difference is explained by analyzing the relative weights of thermal index guiding and carrier-induced index anti-guiding.

Polarization stability of phase-locked arrays of vertical-cavity surface-emitting lasers

The polarization stability of the fundamental lasing super-mode obtained from coherently coupled arrays of vertical-cavity surface-emitting lasers has been investigated. Various devices have been analyzed and none showed the abrupt change of the polarization direction (flip) often observed in solitary devices. This polarization stability is due to a current independent dichroism of 0.5 GHz.

Frequency spectra and waveguiding of a family of daisy modes in vertical-cavity surface-emitting lasers

We have observed stable single-mode oscillation in a family of high-order daisy modes for a vertical-cavity surface-emitting laser that is optically-pumped with a doughnut-shaped beam. Measurements of the frequency separations and the radial profiles of the various modes show that the waveguiding has a near-quadratic nature.

Birefringence in TEM01 optically-pumped VCSELs

Abstract We study the polarization properties of optically-pumped vertical-cavity semiconductor lasers (VCSELs) pumped with a TEM01 beam and which also oscillate in this mode. When the two-lobed pump pattern is rotated, the birefringence of the laser changes periodically by ± 3.8 GHz accompanied by polarization rotation. We attribute these effects to stress-induced birefringence.