Electronics Department

We compare the results of different optical verticalcavity surface-emitting laser models on the position-dependent effects of thin oxide apertures. Both scalar and vectorial models as well as hybrid models are considered. Physical quantities that are compared are resonance wavelength, threshold material gain, and modal stability. For large device diameters and low-order modes, the agreement between the different models is quite good. Larger differences occur when considering smaller devices and higher order modes. It is also observed that the spread in the resonance wavelengths is smaller than that for the threshold material gain.

Vertical cavity surface-emitting lasers (VCSELs) with a well-defined and predictable polarization of the emitted light are sought for a number of applications. In this paper, we show that one can define and stabilize the polarization of singleand multimode oxide-confined VCSELs with a monolithically integrated dielectric surface grating. In recent years, we have developed a three-dimensional, fully vectorial model for VCSELs, which proved to nicely reproduce the experimental results of quite complex structures, such as noncircular devices and phase-coupled VCSEL arrays. This software allows for the first time to analyze the effects of a dielectric grating in the output facet cap layer and its capability to fix the polarization of the emitted light. It is here employed as a design tool, yielding excellent agreement with the experimental data. Since the simulations predict the polarization behavior to be sensitively dependent on the grating parameters, hundreds of VCSELs with 99 different parameter sets, two grating orientations and active diameters of 4 and 7 m have been analyzed. Even VCSELs with eight or more coexisting modes turned out to be linearly polarized with an orthogonal polarization suppression ratio in excess of 15 dB. Theoretical and experimental emission far-fields are compared, and it is shown that diffraction side lobes can be prevented with properly chosen grating parameters which simultaneously ensure full polarization stability.

Vertical-cavity surface-emitting lasers (VCSELs) have become leading light sources in plenty of applications due to their good characteristics and low costs. There are, however, some features that need improvements; therefore, optimized or new designs ideas are necessary. To this aim, an electromagnetic simulation tool, which is fast and precise at the same time, is desirable; to cover all the possible requirements, it should be fully three-dimensional 3-D) and vectorial. A model with such features was first proposed by Bava et al. ("Three-Dimensional Model for Vectorial Fields in Vertical-Cavity Surface-Emitting Lasers," Phys. Rev. A, vol. 63, p. 23816, 2001), based on coupled-mode theory.

We present a comprehensive fully vectorial model for the cavity eigenmodes of oxide-cofined vertical-cavity surface-emitting lasers (VCSELs) with the details of their complex structure. It includes device-inherent symmetry-breaking mechanisms like noncircular geometries and material anisotropies related to the elasto-optic and electro-optic effect. The latter is accounted for in the model starting from the material and doping profiles. We compare these theoretical results with experimental findings of spectrally and polarization-resolved transverse mode nearfields of oxide-confined VCSELs with two different aperture diameters. Within a parametric study of the influence of aperture anisotropies we are able to calculate frequencies and gains of all transverse mode families, their polarization dependence and their spatial mode profiles which are in good agreement with the experimental findings.

We report the onset of wave chaos in a real-world vertical-cavity surface-emitting laser. In a joint experimental and modeling approach we demonstrate that a small deformation in one layer of the complicated laser structure changes the emission properties qualitatively. Based on the analysis of the spatial emission profiles and spectral eigenvalue spacing distributions, we attribute these changes to the transition from regular behavior to wave chaos, and justify the full analogy to two-dimensional billiards by model calculations. Hence, these lasers represent fascinating devices for wave chaos studies.

Polarization control in vertical-cavity surface-emitting lasers (VCSELs) has attracted a lot of interest and different techniques have been proposed to achieve it. Among them, one of the most attractive relies on slight modifications of existing and well-behaving devices, i.e., by introducing a noncircular transverse section layer somewhere in the device. Even though experimental verifications of this principle have already been carried out, a theoretical framework to better understand and possibly optimize such devices is still missing. This mainly originates from the need of a fully vectorial and three-dimensional approach. In this paper, we will undertake a joint experimental and theoretical effort: first, we give experimental evidence of polarization control by applying elliptical surface etching. Then, after having validated the vectorial electromagnetic model by comparing numerical and experimental results, we are able for the first time to explain and compare the polarization selection mechanisms and, consequently, to provide guidelines for optimized structures.

Vertical-cavity surface-emitting lasers (VCSELs) with single-mode, single-polarization emission at a wavelength of approximately 763 nm have become attractive for oxygen sensing. Up to now, VCSELs used for this application are single-mode because of a small active diameter which correspondingly leads to small optical output power. Employing the surface relief technique and in particular the surface grating relief technique, we have increased the single-mode output to more than 2.5 mW averaged over a large device quantity. To the best of our knowledge, this is the highest single-mode power ever reported for VCSELs in this wavelength range. Through the grating relief simultaneously we were able to stabilize the light polarization.

Monolithically integrated surface gratings have proven to control the polarization of single-mode and even multimode vertical-cavity surface-emitting lasers (VCSELs) very effectively. Unfortunately, up until now, the grating parameters have had to be known and realized very accurately for proper performance. The simulations and experimental results presented in this paper show in very good agreement that by changing the thickness of the cap layer of the VCSEL structure, the dependence of the polarization control on the grating parameters can be strongly reduced. With this modification, for multimode devices, we have achieved a stable polarization of all modes orthogonal to the grating grooves independent of the investigated grating period, the grating depth, and the orientation of the grating itself. The orthogonal polarization suppression ratio is, on average, 17.1 dB and exceeds 12 dB for 117 out of 120 highly multimode VCSELs. At the same time, the optimized layer design significantly reduces the diffraction in the far field, which occurs for grating periods larger than the emission wavelength of the laser.

It has been proven experimentally that the singlemode output power of vertical-cavity surface-emitting lasers (VCSELs) can be increased significantly with an inverted surface relief and that a reliable polarization control can be achieved by a monolithically integrated surface grating for even highly multimode VCSELs. In this paper, for the first time we report on a combination of these two techniques, namely, an inverted grating relief to realize VCSELs with a stable polarization and an increased single-mode output power. A statistically relevant number of monolithic, oxide-confined 850-nm VCSELs is investigated. All devices with a grating relief have a stable polarization with its orientation defined by the grating grooves. On the other hand, 64% of the reference devices exhibit polarization switches. Concurrently, the maximum single-mode output power is enhanced by more than a factor of three. Within the entire range of implemented grating parameters, the worst grating relief device still delivers higher single-mode output power than the best reference device.

The effective interplay of simulation and experimental results for analysis and optimization of microelectromechanical system (MEMS)-tunable vertical-cavity surface-emitting lasers (VCSELs) operating at wavelength around 1.55 m is presented. The VCSEL combines a MEMS with concave Al-GaAs-GaAs mirror membrane and an InP-based active cavity with tunnel junction aperture in a hybrid two-chip assembly. Using electrothermal MEMS actuation the included air-gap can be expanded and the cavity resonance can be tuned to longer wavelengths. The experimental results are compared with the theoretical results provided by VELM (VCSEL ELectroMagnetic), the efficient code based on the coupled mode model and adapted for the first time to handle curved-mirror geometries. The vectorial code is found to be able to fully reproduce the experimental results, such as device tuning range, modal frequency splitting, threshold gains and modal selectivity.

Monolithically integrated full-aperture surface gratings are shown to control the polarization of all modes of even highly multi-mode 850 nm oxide-confined standard industrial vertical-cavity surface-emitting lasers (VCSELs). An orthogonal polarization suppression ratio (OPSR) of more than 11 dB up to thermal rollover is achieved for an output power of 23 mW. For devices with 8 mW output power, an OPSR of more than 20 dB at thermal rollover is observed, without major drawbacks to the overall laser performance. † Pierluigi Debernardi is with the IEIIT-CNR c/o Politecnico di Torino,

In this paper we present a complete model to evaluate the electro-optic response of a semiconductor quantum well structure. Heavy and light hole mixing in the valence band is included by using a variational technique to determine the basis states. The excitonic effects, which are clearly evident at room temperature in quantum well structures, arise from coulomb interaction between the charged particles; the corresponding many body problem is treated in the framework of the second quantization approach, by writing the total hamiltonian of the interacting electron-hole plasma. The electro-optic responses, both for TE and TM polarized light, are computed by summing up the contributions of all the different transitions; they show a good agreement with experimental results.

Based on design guidelines from a three-dimensional, fully vectorial model, we have fabricated vertical-cavity surface-emitting lasers (VCSELs) with a monolithically integrated dielectric surface grating for polarization control. For VCSELs with emission wavelengths of 850 and 980 nm we have achieved orthogonal polarization suppression ratios (OPSRs) above 15 dB for all modes up to thermal rollover, which very well agrees with theory. It is shown both theoretically and experimentally that the grating has no influence on the emission far-field. The surface grating has also been combined with a surface relief to stabilize the polarization and to increase the fundamental mode output power at the same time.

The polarization competition in vertical-cavity surface-emitting lasers with two nearly degenerate modes is analyzed with the aim of studying their noise properties and, in particular, amplitude fluctuations and spectral linewidth. The coupling between the two modes is attributed to carriers, different spin populations, and structural anisotropies, including the consequences of the electro-optic effect that results from current injection. The model is based on quantum equations of motion with Langevin noise sources. Results for the noise spectra in the regions where mode competition is stronger are reported and discussed.

Within a joint collaboration between modeling, technology and experiments, we investigate the polarization-resolved spatial emission and turn-on dynamics of oxide-confined vertical-cavity surface-emitting lasers with an integrated surface-relief grating. By applying a time-resolved imaging technique we demonstrate that the introduced high dichroism also maintains its influence dynamically. This leads to highly polarization-stable spatially fundamental Gaussian mode emission on 100-ps timescale. Finally, the achieved progress, but also the limits of this promising stabilization scheme are discussed.

Maximization of the catalyst efficiency in automotive fuel-injection engines requires the design of accurate control systems to keep the air-to-fuel ratio at the optimal stoichiometric value AF . Unfortunately, this task is complex since the air-to-fuel ratio is very sensitive to small perturbations of the engine parameters. Some mechanisms ruling the engine and the combustion process are in fact unknown and/or show hard nonlinearities. These difficulties limit the effectiveness of traditional control approaches. In this paper, we suggest a neural based solution to the air-to-fuel ratio control in fuel injection systems. An indirect control approach has been considered which requires a preliminary modeling of the engine dynamics. The model for the engine and the final controller are based on recurrent neural networks with external feedbacks.

Maximization of the catalyst efficiency in automotive fuel-injection engines requires the design of accurate control systems to keep the air-to-fuel ratio at the optimal stoichiometric value AF. Unfortunately, this task is complex since the air-to-fuel ratio is very sensitive to small perturbations of the engine parameters. Some mechanisms ruling the engine and the combustion process are in fact unknown and/or show hard nonlinearities. These difficulties limit the effectiveness of traditional control approaches. In this paper, we suggest a neural based solution to the air-to-fuel ratio control in fuel injection systems. An indirect control approach has been considered which requires a preliminary modeling of the engine dynamics. The model for the engine and the final controller are based on recurrent neural networks with external feedbacks. Requirements for feasible control actions and the static precision of control have been integrated in the controller design to guide learning toward an effective control solution.