Dynamics Of Vertical-cavity Surface-emitting Lasers Research Articles

Two-frequency emission and polarization dynamics at lasing threshold in vertical-cavity surface-emitting lasers

We investigate experimentally and theoretically the polarization dynamics in vertical-cavity surface-emitting lasers in the vicinity of the lasing threshold. If the net gain anisotropy between the two orthogonally polarized modes is close to zero, the laser operates in a partially polarized state. This state occurs at and slightly above the lasing threshold. It is characterized by anticorrelated dynamics of the two modes which result from completely anticorrelated dynamics at low frequencies and correlated dynamics on the time scale of the relaxation oscillations. If the net anisotropy is changed by a change of the detuning between the cavity resonance and the gain maximum, one of the two linear polarization states is selected depending on the sign of the net gain. For one sign the well-known phenomenon of polarization switching is recovered at higher currents. The experimental results show very good agreement with simulations based on a model that takes into account nonlinear spin dynamics and linear temperature-dependent detuning effects.

Description and analysis of low-frequency fluctuations in vertical-cavity surface-emitting lasers with isotropic optical feedback by a distant reflector

We analyze theoretically and experimentally the polarization dynamics of vertical-cavity surface-emitting lasers exposed to isotropic optical feedback. The theoretical investigations are based on a model that takes into account different inversion populations for charge carriers with opposite spin. In the deterministic case, we observe toruslike synchronized low-frequency fluctuations of one or both polarization modes in the vicinity of the solitary laser threshold, depending on the parameters and initial conditions. Both in experiment and in simulations including spontaneous emission noise, asymmetric low-frequency fluctuations are observed above and below the solitary laser threshold as well as a transition to coherence collapse further above threshold. The amount of excitation of polarization degrees of freedom and thus the occurrence of simultaneous low-frequency fluctuations of both polarization modes is shown to depend on the magnitude of the dichroism. Apart from the correlation properties on the external-cavity time scale, we find a good qualitative agreement between experiments and simulations.

Picosecond emission dynamics of vertical-cavity surface-emitting lasers: Spatial, spectral, and polarization-resolved characterization

We present two-dimensional (2-D) spatially and picosecond-resolved measurements of vertical-cavity surface-emitting lasers' emission, unveiling their rich dynamics, manifesting itself in temporal changes of the modal, polarization, spectral, and spatial characteristics. The measurements providing insight into the repetitive part of the dynamics were performed with a newly developed experimental technique. In addition, we present the first 2-D, spatially resolved, single-shot event measurements which provide a full complementary picture of the dynamical behavior. The experimental results are analyzed and discussed with respect to spatio-spectral interaction mechanisms of carriers and photons, explaining the observed behavior.

Low-frequency fluctuations and polarization dynamics in vertical-cavity surface-emitting lasers with isotropic feedback

We investigate experimentally the polarization dynamics in vertical-cavity surface-emitting lasers exposed to isotropic time-delayed optical feedback. The existence of low-frequency fluctuations with and without the simultaneous excitation of polarization degrees of freedom is demonstrated. The appearance of polarization dynamics is shown to depend on the magnitude of the intrinsic dichroism of the device under study. If there is no polarization dynamics, low-frequency fluctuations of a single mode are observed.

Transverse-mode dynamics in vertical-cavity surface-emitting lasers with optical feedback

We study the transverse-mode dynamics of vertical-cavity surface-emitting lasers with weak optical feedback. We use a model that takes into account the spatial dependence of the transverse modes and of two carrier density profiles, associated with confined carriers in the quantum well region of the laser and unconfined carriers in the barrier region. Optical feedback is included as in the Lang-Kobayashi model. We find that for adequate parameter values antiphase dynamics occurs. As the injection current varies, the antiphase dynamics is destroyed through a sequence of periodic mixed states leading to in-phase dynamics. In these mixed states there are time intervals in which the modes are in phase, followed by time intervals in which they are in antiphase. We study the origin of the antiphase dynamics, assessing the role of the different spatial profiles. We show that the competition between the different profiles leads to the observed antiphase behavior.

A comprehensive model for the modal dynamics of vertical-cavity surface-emitting lasers

We present a quasi-3-D dynamic model of vertical-cavity surface-emitting lasers (VCSELs). The interdependent processes of carrier transport, heat generation and dissipation, and optical fields are solved self-consistently for each point in time and space. An effective index model is adopted for the evaluation of the optical fields in the complex layer structure. The inclusion of a temperature- and carrier-density-dependent refractive index, and its time dependence, allows us to study the evolution of the transverse optical field distributions under dynamic conditions. The model is applied to a typical index-guided structure with a 7-/spl mu/m oxide aperture. A direct comparison is made using "cold" cavity modes, which is a normal technique when modeling the dynamics of VCSELs. Significant discrepancies are demonstrated both at smalland large-signal modulation, which indicates the need of a more sophisticated model for accurately predicting and understanding the geometry-dependent modal evolution.

Spatial investigation of transverse mode turn-on dynamics in VCSELs

Data transmission experiments with single-mode as well as multimode 850-nm vertical-cavity surface-emitting lasers (VCSELs) are carried out from a near-field point of view. Special attention is paid to important quantities like on/off-ratio and bit error rate (BER). A single-mode VCSEL oscillating on the fundamental LP/sub 01/ mode shows no change in eye opening, on/off-ratio, and BER at different lateral fiber coupling positions. In the case of a multimode VCSEL oscillating both on the LP/sub 01/ mode and LP/sub 11/ donut mode, we observe a significant lateral change in the on/off-ratio, which plays an important role in BER measurements.

Polarization dynamics in vertical-cavity surface-emitting lasers with optical feedback: experiment and model

We study the polarization-resolved dynamics of a vertical-cavity surface-emitting laser with mode-matched and polarization-preserving optical feedback. The total power can display low-frequency fluctuations, which are associated with drops in power of the dominant linearly polarized mode and with bursts of power of the depressed mode. However, for most parameter values the vertical-cavity surface-emitting laser output does not display low-frequency fluctuations, although antiphase dynamics between the two linear polarization components has been found. A theoretical model that includes all reflections in the external cavity with a minimum computational cost is developed.

Transverse mode dynamics of VCSELs through space-time domain simulation

An approximation to the Maxwell-Semiconductor Bloch equations is used to model transverse mode dynamics of vertical-cavity surface-emitting lasers (VCSELs). The time-evolution of the spatial profiles of the laser field and carrier density is solved by a finite-difference algorithm. The algorithm is fairly general; it can handle devices of any shape, which are either gain or index guided or both. Also there is no a priori assumption about the type or number of modes. The physical modeling includes the nonlinear carrier dependence of the optical gain and refractive index and dispersion effects on the gain and the refractive index are also included.

Transient response of vertical-cavity surface-emitting lasers of different active-region diameters

The multimode dynamics of vertical-cavity surface-emitting lasers with different active-region diameters was measured under subnanosecond electrical excitation (800-ps pulse duration, 100-ps risetime). The dynamics is characterized by the delayed onset of higher order modes which have a turn-on delay that is dependent on the active-region diameter and the excitation parameters. A simple model that can be used to estimate this turn-on delay for large-area devices is presented. Polarization-resolved measurements show that, under this fast excitation condition, both orthogonal polarization states are isomorphic. The influence of the observed dynamics on the relative intensity noise of these devices is also discussed.

Polarization dynamics in vertical-cavity surface-emitting lasers with optical feedback through a quarter-wave plate

Square-wave switching of the intensities of the orthogonal linearly polarized components of the output of a vertical-cavity surface-emitting laser (VCSEL) found previously in experiments with polarization-changing optical feedback, is not found in rate equation models incorporating only birefringence and gain anisotropy, but is found in the model for VCSELs developed by San Miguel, Feng, and Moloney [M. San Miguel, Q. Feng, and J. V. Moloney, Phys. Rev. A 52, 1729 (1995)]. The dynamics is sensitive to both the feedback strength and the relaxation rate of the magnetization in the quantum well sublevels.

Beam shaping in vertical-cavity surface-emitting laser cavities

The transverse mode dynamics of vertical-cavity surface-emitting lasers is analyzed studying the passive cavity beam shaping mechanisms. Diffraction in the Bragg mirrors is found to favor the fundamental transverse mode compared to the higher order modes. Mode dependent effective complex reflection coefficients are computed. The effect of diffraction is compared to the effects of thermal lensing due to heating of the Bragg mirrors and of the plasma and active layer lattice.

Microscopic modeling of vertical-cavity surface-emitting lasers: many-body interaction, plasma heating, and transverse dynamics

A microscopic theory for the interaction between semiconductor quantum-well structures and laser fields based on the semiconductor Bloch equations is applied to vertical-cavity surface-emitting lasers (VCSELs) with the inclusion of plasma heating. The semiconductor Bloch equations are reduced to a set of equations for the first and second moments of the carrier distribution functions. Plasma heating and many-body effects are then studied by solving this set of equations in steady state under the approximation of a single transverse and longitudinal mode. The transverse- and longitudinal-mode dynamics of VCSELs is analyzed by solving the full space-time-dependent problem.

Effects of transverse-mode competition on the injection dynamics of vertical-cavity surface-emitting lasers

We investigate theoretically the static and dynamic characteristics of an optically injected vertical-cavity surface-emitting laser (VCSEL), under both single- and two-mode operation. Our model includes the spatial dependence of both the carrier density and the optical field. We find that within the locking range, a two-mode VCSEL can be forced to operate in a single transverse mode under appropriate conditions. Outside the locking range, the VCSEL shows a rich variety of nonlinear dynamics. Results show that the static and dynamic behaviours depend strongly on the frequency detuning, the injection power and the strength of the intermodal coupling induced by spatial hole burning. In particular, strong intermodal coupling can reduce the sensitivity of the VCSEL toward external injection, in terms of both static (locking) and dynamic (nonlocking) behaviours.

Polarization modulation dynamics of vertical-cavity surface-emitting lasers with an extended cavity

Vertical-cavity surface-emitting lasers (VCSELs) are known to exhibit a small birefringence and dichroism whose axes are directed along the crystal axes of the quantum wells and which fix the polarization of the oscillating light. In this paper, we consider the dynamics of the polarization of light in such a laser, extended by means of a quarter-wavelength plate in an external cavity. Periodic variations are experimentally observed. A theoretical analysis and numerical results are proposed, using a recent model which includes the carriers' spin interaction. A good match is found between experimental and numerical results.