Displacement damage effects in proton irradiated vertical-cavity surface-emitting lasers

The displacement damage effects of vertical-cavity surface-emitting lasers (VCSELs) irradiated by 3 and 10 MeV protons in the range of Ф = 6.7×1012 p/cm2 to Ф = 1.6×1014 p/cm2 were investigated. The threshold current exhibited consistent degradation at the same displacement damage dose, as did the series resistance. Additionally, the external quantum efficiencies of 850 and 680 nm VCSELs were degraded by 2% and 21%, respectively. Further, the threshold current of the 850 nm VCSEL was restored by 14% after annealing at 20 mA, which is remarkably higher than that achieved by annealing only at high temperatures. These results support the applicability of VCSELs to both data communication and instrumentation applications in harsh radiation environments.

Highly reflective and conductive AlInN/GaN distributed Bragg reflectors realized by Ge-doping

We report on the realization of highly conductive and highly reflective n-type AlInN/GaN distributed Bragg reflectors (DBR) for use in vertical cavity surface emitters in a metalorganic vapor phase epitaxy process. While Ge-doping enables low-resistive n-type GaN/AlInN/GaN heterostructures, very high Ge doping levels compromise maximum optical reflectivities of DBRs. Simulations of the Bragg mirror's reflectivities together with structural analysis by X-ray diffraction reveal an increased absorption within the doped AlInN layers and interface roughening as major causes for the observed reduction of the optical reflectivity. By adjusting the Ge doping level in the AlInN layers, this structural degradation was minimized and highly conductive, 45-fold AlInN/GaN DBR structures with a maximum reflectivity of 99 % and vertical specific resistance of 5x10-4 Ωcm2 were realized.

Photonic implementation of spike timing dependent plasticity with weight-dependent learning window based on VCSOA

Based on the well-known Fabry–Pérot approach, after taking into account the variation of bias current of the vertical-cavity semiconductor optical amplifier (VCSOA) according to the present synapse weight, we implement the optical spike timing dependent plasticity (STDP) with weight-dependent learning window in a VCSOA with double optical spike injections, and numerically investigate the corresponding weight-dependent STDP characteristics. The simulation results show that, the bias current of VCSOA has significant effect on the optical STDP curve. After introducing an adaptive variation of the bias current according to the present synapse weight, the optical weight-dependent STDP based on VCSOA can be realized. Moreover, the weight training based on the optical weight-dependent STDP can be effectively controlled by adjusting some typical external or intrinsic parameters and the excessive adjusting of synaptic weight is avoided, which can be used to balance the stability and competition among synapses and pave a way for the future large-scale energy efficient optical spiking neural networks based on the weight-dependent STDP learning mechanism.

Electrical-stress driven oxidation in 940 nm oxide-confined VCSEL

In this work, accelerated stress tests have been performed on oxide confined vertical cavity surface emitting LASER arrays to study the formation of defects degrading the performance of the device. One such defect is an additional oxide volume forming at the oxide aperture edge, which is used for optical and electrical confinement. After producing an additional oxide volume the sample was investigated using transmission electron microscopy to estimate the oxidation speed. To produce further insights into the formation process, the temperature during such a stress test was estimated by experimentally measuring the thermal resistance, and by a thermodynamic transport simulation. Both methods produced very similar results showing a temperature increase of around 22 K for a dissipated power of 3.5 mW per emitter. However this temperature rise is very small when compared to oxidation models found in literature and should not be enough to promote the oxidation. This indicates the presence of a new enhanced oxidation mechanism, which could be connected to corrosion based failure mechanisms reported in literature.

Simulation and analysis of a Single Mode Indium Gallium Arsenide Phosphide Vertical Cavity Surface Emitting Laser for Optical Communication

This present work is about simulating and analysing a Vertical Cavity Surface Emitting Laser (VCSEL) structure used in optical fibre communication systems. In this paper a VCSEL structure made of seven Quantum Wells of Indium Gallium Arsenide Phosphide (InGaAsP) emitting at 1550 nm is simulated. The device is analysed looking at the following characteristics: Direct current current and voltage (IV) characteristics, light power against electrical bias, optical gain against electrical bias, light distribution over the structure, output power and threshold current. Specification of material characteristics, ordinary physical models settings, initial VCSEL biasing, mesh declarations, declaration of laser physical models, their optical and electrical parameters were defined using Atlas syntax. Mirror ratings and quantum wells are the two main parameters that were studied and analysed to come up with structure trends. By determining important device parameters such as proper selection of the emission wavelength and choice of material; a VCSEL with an output power of 9.5 mW was simulated and compared with other structures.