Krassimir Panajotov

Polarization switching in VCSEL's due to thermal lensing

We demonstrate a new mechanism for polarization switching in slightly anisotropic vertical-cavity surface-emitting lasers (VCSELs) based on index guiding due to the effect of thermal lensing. This polarization switching takes place when the laser operates on the short-wavelength side of the gain maximum. The two orthogonal linear polarizations correspond to slightly different wavelengths and switching occurs from shorter toward longer laser wavelength as the drive current increases. Polarization modulation has been observed up to 50 MHz with a polarization contrast ratio of 20:1.

PlaneWave Admittance Method- a novel approach for determining the electromagnetic modes in photonic structures.

In this article we present a novel approach for determining the electromagnetic modes of photonic multilayer structures. We combine the plane wave expansion method with the method of lines resulting in a fast and accurate computational technique which we named the plane wave admittance method. In addition, we incorporate perfectly matched layers at the boundaries parallel to the multilayer surfaces which allow for easy determination of leaky modes. The convergence of the method is verified for the case of photonic crystal slab showing very good agreement with the results obtained with full three-dimensional plane wave expansion method while the numerical effort is largely reduced. The numerical implementation of the method will be soon available on the web.

Optical feedback induces polarization mode hopping in vertical-cavity surface-emitting lasers.

Vertical-cavity surface-emitting lasers subjected to weak polarization-insensitive optical feedback are studied experimentally and theoretically. We find that the feedback induces random anticorrelated hopping between the two orthogonal linearly polarized modes. This polarization mode hopping is accompanied by rapid anticorrelated oscillations in the linearly polarized intensities at the external-cavity frequency. The study of a simple stochastic delay differential equation suggests that these oscillations generated by the delay are typical of any hopping phenomenon between states.

Effect of photon-energy-dependent loss and gain mechanisms on polarization switching in vertical-cavity surface-emitting lasers

We have analyzed the effect of the photon energy and temperature dependence of both the gain and the total losses inside the cavity to understand the polarization behavior of vertical-cavity surface-emitting lasers. The assumption that the losses are dominated by free-carrier absorption in the p-doped mirror is made. Developing a new theoretical approach, we are able to predict different polarization switching regimes in which switching occurs from the high- to the low-frequency mode, from the low- to the high-frequency mode, or both consecutively. All these predictions have been experimentally verified by our measurements on GaAs/AlGaAs proton-implanted vertical-cavity surface-emitting lasers.

Mapping of the dynamics induced by orthogonal optical injection in vertical-cavity surface-emitting lasers

In this paper, the output characteristics of an oxide-confined AlGaAs-As quantum well vertical-cavity surface-emitting laser (VCSEL) under orthogonal optical injection are mapped as a function of the strength of the optical injection and the detuning between the injection frequency and the free-running frequency of the solitary laser, for a very large range of frequency detuning (from -82 to 89 GHz). The injection light is polarized orthogonally with respect to the solitary VCSEL output light. As the injection strength increases the VCSEL switches to the master laser polarization. Polarization switching is accompanied by a rich nonlinear dynamics, including limit cycle, wave mixing, subharmonic resonance and period doubling route to chaos. Polarization switching is found with but also without injection locking. Injection locking occurs with the slave fundamental transverse mode or, for large positive detunings, with the first-order transverse mode.

Deterministic polarization chaos from a laser diode

Chaotic behaviour is observed in the polarization of the output from a vertical-cavity surface emitting laser without the need for any external stimulus or feedback. The origin is nonlinear coupling between two elliptically polarized modes within the device.

Impact of in-plane anisotropic strain on the polarization behavior of vertical-cavity surface-emitting lasers

We experimentally demonstrate that the presence of switching between two fundamental modes with orthogonal linear polarization in vertical-cavity surface-emitting lasers and the current at which this phenomenon occurs depend on both the magnitude and the orientation of an externally induced in-plane anisotropic strain. We interpret this behavior by considering the anisotropy in gain and in refractive index, both induced by the in-plane strain, and by accounting for a redshift of the two gain curves as a result of current-induced heating.

Minimal rate equations describing polarization switching in vertical-cavity surface-emitting lasers

Intensity rate equations for two nearly degenerate modes are derived for current driven polarization switching (PS) in vertical-cavity surface-emitting lasers (VCSELs). We show that the introduction of a current-dependent gain that saturates with increasing optical power leads to a minimal description of a PS experiment. The main advantage of these rate equations compared to more sophisticated model equations is the possibility to describe analytically the PS dynamical phenomenon. We obtain a simple expression for the switching time which agrees with experimental observations.

Nonlinear dynamics accompanying polarization switching in vertical-cavity surface-emitting lasers with orthogonal optical injection

We present an experimental investigation of a vertical-cavity surface-emitting laser (VCSEL) submitted to orthogonal optical injection, i.e., the injected light is linearly polarized and orthogonal to that emitted by the solitary VCSEL. Bifurcation boundaries of qualitatively different dynamics are mapped out in the frequency detuning-injection strength plane. We unveil rich and complex dynamics including injection locking, limit cycle, wave mixing, and period doubling route to chaos.

Transverse Mode Switching and Locking in Vertical-Cavity Surface-Emitting Lasers Subject to Orthogonal Optical Injection

In this paper, we report on theoretical and experimental investigation on polarization and transverse mode behavior of vertical-cavity surface-emitting lasers (VCSELs) under orthogonal optical injection as a function of the injection strength and of the detuning between the injection frequency and the free-running frequency of the solitary laser. As the injection strength increases the VCSEL switches to the master laser polarization. We find that the injection power necessary to obtain such polarization switching is minimum at two different values of the frequency detuning: the first one corresponds to the frequency splitting between the two linearly polarized fundamental transverse modes, and the second one appears at a larger positive frequency detuning, close to the frequency difference between the first-order and the fundamental transverse modes of the solitary VCSEL. We show theoretically that both the depth and the frequency corresponding to the second minimum increase when the relative losses between the two transverse modes decrease. Bistability of the polarization switching is obtained for the whole frequency detuning range. Such a bistability is found for the fundamental mode only or for both transverse modes, depending on the value of the detuning. The theoretical and experimental optical spectra are in good agreement showing that the first-order transverse mode appears locked to the external injection