Patrycja Śpiewak

Numerical model of capacitance in vertical-cavity surface-emitting lasers

In this paper we present a model of impedance and modulation time constants for vertical-cavity surface-emitting lasers (VCSELs) operating above threshold current. A 3D numerical model of potential distribution in the device under a constant bias is used to determine resistances and capacitances of an appropriate equivalent circuit. The model has been verified by comparing the theoretical and measured impedance as a function of frequency Z(f). The measured Z(f) is determined from S 11 small signal modulation experiments. The comparison has been performed for frequencies up to 40 GHz and a wide range of above threshold currents, for two oxide-confined VCSELs of different aperture diameters. We obtained a very good quantitative agreement for frequencies up to about 15 GHz and qualitative agreement over the entire range of currents and frequencies.

Impact of the top DBR in GaAs-based VCSELs on the threshold current, oxide-aperture diameter, and the cavity photon lifetime

This paper presents results of numerical simulations of a GaAs-based vertical-cavity surface-emitting laser, emitting at 980 nm. These simulations concern the influence of the number of top DBR pairs on the laser’s threshold parameters, as well as the optical loses in the cavity. Moreover, electrical parameters such as the device’s resistance and its capacitance-related temporal characteristics are analyzed as functions of the thickness of the top DBR. The simulations suggest that there is a possibility of a significant reduction in the number of pairs in the top DBR that can be beneficial in certain applications.

The influence of the VCSEL design on its electrical modulation properties

Here we investigate the influence of the p- and n-oxide-aperture radii in all-semiconductor GaAs-based verticalcavity surface-emitting lasers (VCSELs), designed for 980 nm, on the modulation time constant (τ). Our analysis shows that the minimum value of τ is obtained if the oxide layers on both sides of the junction have identical depths. The simulations of the number of oxide layers on both p- and n-type sides reveal that double p- and n-oxidations are the most effective in the reduction of the modulation time constant as compared to single oxide layers.

Metalized monolithic high-contrast grating as a mirror for GaN-based VCSELs

In this paper, we present a novel design of a nitride-based VCSEL emitting at 414 nm and perform numerical analysis of optical, electrical and thermal phenomena. The bottom mirror of the laser is a Al(In)N/GaN DBR (Distributed Bragg Reflector), whereas the top mirror is realized as a semiconductor-metal subwavelength-grating, etched in GaN with silver stripes deposited between the stripes of the semiconductor grating. In this monolithic structure simulations show a uniform active-region current density on the level of 5.5 kA/cm2 for the apertures as large as 10 μm. In the case of a broader apertures, e.g. 40 μm, we showed that, assuming a homogeneous current injection at the level of 5.5 kA/cm2 , the temperature inside the laser should not exceed 360 K, which gives promise to improve thermal management by uniformisation of the current injection.

Numerical study of VECSELs for generation of mid-infrared radiation

Two different approaches to developing new laser sources operating in the mid-infrared range based on vertical-external cavity surface-emitting lasers (VECSELs) are studied with the aid of numerical modelling. The first one consists in enhancing a maximal emission wavelength of currently available GaSb-based structures beyond 3 μm. The second approach consists in using dual-wavelength VECSEL (DW-VECSEL), emitting two coaxial laser beams of different wavelengths, to generate radiation from the 3-5 μm spectral range with the aid of difference frequency generation.

Intracavity and extracavity-contacted 980-nm oxide-confined VCSELs for optical interconnects and integration

Record-large modulation bandwidths of 30 GHz and larger have been achieved with state-of-the art directly and indirectly modulated VCSELs and VCSEL arrays. One next big challenge is to make VCSELs viable for integration onto silicon while maintaining large bandwidth values. Various integration schemes of VCSELs might require process variations potentially detrimental for large modulation bandwidths. We present and compare directly modulated oxide-confined top-emitting 980-nm VCSELs processed from one single epitaxial wafer design into four different extracavity and intracavity contact variations.

Modeling of optical and electrical confinements in nitride VCSELs

In this paper, we present numerical simulations of different types of nitride VCSELs. We analyzed structures with different DBR mirrors and electrical confinements. We compare threshold parameters, including threshold current, threshold temperature and optical field distribution for structures with an ITO contact and structures with tunnel junctions. Lasers emitting blue/violet and green radiation are analyzed from the point of view of their thermal properties.

Modelling of the modulation properties of arsenide and nitride VCSELs

In this paper, using our model of capacitance in vertical-cavity surface-emitting lasers (VCSELs), we analyze certain differences between an oxide-confined arsenide VCSEL emitting in the NIR region, and a nitride VCSEL emitting violet radiation. In the nitride laser its high differential resistance, caused partially by the low conductivity of p-type GaN material and the bottom contact configuration, is one of the main reasons why the nitride VCSEL has much worse modulation properties than the arsenide VCSEL. Using the complicated arsenide structure, we also analyze different possible ways of constructing the laser’s equivalent circuit.

Capacitance and modulation time constant in oxide-confined vertical-cavity surface-emitting lasers with different oxide layers

In this paper we analyze the influence of the thickness of two oxide layers in a typical oxide-confined vertical-cavity surface-emitting laser (VCSEL) on the capacitance of the device and its electrical modulation properties. An analysis of the distribution of the potential and the energy of the electric field in this device is presented, and the influence of these fields on the lasers capacitance is described. It is shown that the oxide layer closest to the active region contributes in a very different way to the lasers capacitance compared to the second slightly more distant oxide layer, and a quantitative analysis of the impact of the thicknesses of these layers on the modulation time constants is presented.

Impact of structure mounting of nitride laser bars on the emitted optical power

In this paper, an impact of mounting of structures of nitride laser bars their performance, emitted optical power in particular, is presented. The laser bars of nitride edge-emitting lasers of ridge-waveguide type the InGaN/GaN active areas have been considered. Laser performance has been analysed with the aid of an advanced self-consistent thermalelectrical model, calibrated using experimental data for a single diode laser. The simulated laser bars emit at 408 nm. An optimal number of laser emitters and their various arrangments have been considered. An appliation of Cu heat sinks of various dimensions as well as the p-side-up or the p-side-down laser configurations have been analysed. Moreover a possible application of a diamond heat spreader has been also taken into account.