Low-Threshold AlGaN-based UVB VCSELs enabled by post-growth cavity detuning

The performance of vertical-cavity surface-emitting lasers (VCSELs) is strongly dependent on the spectral detuning between the gain peak and the resonance wavelength. Here, we use angle-resolved photoluminescence spectroscopy to investigate the emission properties of AlGaN-based VCSELs emitting in the ultraviolet-B spectral range with different detuning between the photoluminescence peak of the quantum-wells and the resonance wavelength. Accurate setting of the cavity length, and thereby the resonance wavelength, is accomplished by using doping-selective electrochemical etching of AlGaN sacrificial layers for substrate removal combined with deposition of dielectric spacer layers. By matching the resonance wavelength to the quantum-wells photoluminescence peak, a threshold power density of 0.4 MW/cm2 was achieved, and this was possible only for smooth etched surfaces with a root mean square roughness below 2 nm. These results demonstrate the importance of accurate cavity length control and surface smoothness to achieve low-Threshold AlGaN-based ultraviolet VCSELs

A 310 nm Optically Pumped AlGaN Vertical-Cavity Surface-Emitting Laser

Ultraviolet light is essential for disinfection, fluorescence excitation, curing, and medical treatment. An ultraviolet light source with the small footprint and excellent optical characteristics of vertical-cavity surface-emitting lasers (VCSELs) may enable new applications in all these areas. Until now, there have only been a few demonstrations of ultraviolet-emitting VCSELs, mainly optically pumped, and all with low Al-content AlGaN cavities and emission near the bandgap of GaN (360 nm). Here, we demonstrate an optically pumped VCSEL emitting in the UVB spectrum (280-320 nm) at room temperature, having an Al0.60Ga0.40N cavity between two dielectric distributed Bragg reflectors. The double dielectric distributed Bragg reflector design was realized by substrate removal using electrochemical etching. Our method is further extendable to even shorter wavelengths, which would establish a technology that enables VCSEL emission from UVA (320-400 nm) to UVC (<280 nm)

Transformation of conventional transformers for enhanced DC mitigation in AC power networks with advanced grid support

The flow of quasi-DC or DC in AC networks may trigger half-cycle saturation of electrical grid transformers that could lead to their internal heating or grid collapse. Apart from the requirement of DC mitigation measures, the conversion of conventional power grid into an intricate network demands the need for power equipment with dynamic control capability. The transformers provide the most strategic point in the grid for the introduction of DC protection and grid-support features. This paper proposes a fractionally rated power-electronics module coupled to neutral and ground terminals of conventional transformers that delivers efficient power network protection against quasi-DC or DC flow with different advanced grid-support features injected on the transformer primary. The proposed concept is validated on an experimental hardware prototype employing power hardware-in-the-loop (P-HIL) configuration of Typhoon HIL-402 and compared with the simulation results in this paper. Also, a transformer and associated module protection approach employing a hybrid bypass switch has been suggested and experimentally validated in this work. The experimental results validate the capability of the module to counter DC injection, perform harmonics mitigation, voltage control or unbalance compensation, impedance matching and power flow control at the same injection point

DC-side harmonic compensation in DC traction applications

A problem associated with dc traction substations, is the non-robustness of the passive filters used for the filtering of the harmonics induced by the 12-pulse rectifiers. These harmonics cause problems with the communications and the signaling on the dc railway lines. Interference with the railway signaling system can have serious and sometimes fatal effects. Therefore, a method is being devised by which the problematic harmonics can be compensated for by means of an Active Power Filter (APF) on the dc-side of the 12-pulse rectifiers.Conference Pape

Operation of the unified power controller as harmonic isolator

The Unified Power Controller (UPC) is a tool in the implementation of Flexible AC Transmission Systems (FACTS). It provides for the equivalent of static VAr compensation and series injection using back-to-back force commutated converters. This paper proposes a control strategy to extend UPC operation to allow for the isolation of harmonics due to non-linear loads. Simulation results based on the Electromagnetic Transients Program (EMTP) are used to illustrate device performance in a power system environment. Experimental results based on a single phase laboratory implementation verify the proposed control algorithm.Conference Pape

Control options for a bi-directional multilevel traction chopper

A high power bi-directional multilevel chopper is developed as a design prototype for an inverting dc traction substation with Active Power Filtering incorporated. The 'Imbricated Cells Multilevel Chopper' topology is put into practice, where capacitors are used as floating voltage sources. Control options for this chopper are discussed and simulation results are compared. Differences between analog and digital control are also highlighted as a DSP-implemented control strategy is considered for duty-cycle control in the final system.Conference Pape