Design of high power tapered waveguide semiconductor lasers

An advanced beam propagation model was developed to show that the far field narrows with good suppression of higher order modes for an appropriate temperature rise, without significant power penalty. To verify the accuracy of the model, the dependence of far field pattern on bias conditions were assessed both experimentally and theoretically, initially under pulsed conditions to reduce thermal effects. The results highlight the optimum taper angle and the role of local heating effects.

High power laser with integrated lens using focused ion beam etching

In this study, a collimating lens is introduced at the output facet of a tapered waveguide laser to compensate for the divergence of the optical mode. The collimating lens is shown to enhance the laser efficiency while simultaneously reducing the far field divergence.

High-power and picosecond pulse generation from a passively Q-switched tapered InGaAs/GaAs laser

Tapered waveguides have been used for enhancing pulse powers in Q-switched AlGaAs and InGaAsP lasers. This paper reports on passively Q-switched pulses with 1.53 W peak power and 41-ps FWHM from an InGaAs/GasAs (970 nm) double-contact tapered semiconductor laser in a well defined single-lobed far-field.

Plume attenuation under high power Nd:yttritium-aluminum-garnet laser welding

During high-power continuous wave (cw) Nd:yttritium-aluminum-garnet (YAG) laser welding a vapor plume is formed containing vaporized material ejected from the keyhole. The gas used as a plume control mechanism affects the plume shape but not its temperature, which has been found to be less than 3000 K, independent of the atmosphere and plume control gases. In this study high-power (up to 8 kW) cw Nd:YAG laser welding has been performed under He, Ar, and N2 gas atmospheres, extending the power range previously studied. The plume was found to contain very small evaporated particles of diameter less than 50 nm. Rayleigh and Mie scattering theories were used to calculate the attenuation coefficient of the incident laser power by these small particles. In addition the attenuation of a 9 W Nd:YAG probe laser beam, horizontally incident across the plume generated by the high-power Nd:YAG laser, was measured at various positions with respect to the beam-material interaction point. Up to 40% attenuation of the probe laser power was measured at positions corresponding to zones of high concentration of vapor plume, shown by high-speed video measurements. These zones interact with the high-power Nd:YAG laser beam path and, can result in significant laser power attenuation. © 2004 Laser Institute of America.

Laser-vapour interaction in high-power cw Nd:YAG laser welding

During high-power cw Nd:YAG laser welding a vapour plume is formed containing vaporised material ejected from the keyhole. Spectroscopic studies of the vapour emission have demonstrated that the vapour can be considered as thermally excited gas with a stable temperature (less than 3000K), not as partially ionised plasma. In this paper, a review of temperatures in the vapour plume is presented. The difficulties in the analysis of the plume spectroscopic results are reviewed and explained. It is shown that particles present in the vapour interact with the laser beam, attenuating it. The attenuation can be calculated with Mie scattering theory, however, vaporisation and particle formation also both play a major role in this process. The laser beam is also defocused due to the scattering part of the attenuation mechanism, changing the energy density in the laser beam. Methods for mitigating the effects of the laser beam-vapour interaction, using control gases, are presented together with their advantages and disadvantages. This 'plume control' has two complementary roles: firstly, the gas must divert the vapour plume from out of the laser beam path, preventing the attenuation. Secondly, the gas has to stabilise the front wall of the keyhole, to prevent porosity formation.

On bandlimited fading channels at high SNR

This paper provides an overview of results on the capacity of noncoherent, multiple-input multiple-output (MIMO) flat-fading channels with a bandlimited power spectral density. The focus is on results that concern the capacity at high signal-to-noise ratio (SNR). In particular, the capacity pre-log, defined as the limiting ratio of the capacity to the logarithm of the SNR as the SNR tends to infinity, is studied. It is observed that the capacity pre-log is a function of the number of antennas as well as of the bandwidth of the fading channel's power spectral density. It is further observed that the capacity pre-log can be achieved with a simple communication system where the data detection and the channel estimation are performed separately. © 2011 ACM.

Trench insulated gate bipolar transistor - a high power switching device

This paper presents a comprehensive theoretical study of the Trench Insulated Gate Bipolar Transistors (TIGBT). Specific physical and geometrical effects, such as the accumulation layer injection, increased channel density, increased channel charge and transversal electric field modulation are discussed. The potential advantages of the Trench IGBT over its conventional planar variant are highlighted. It is concluded that the Trench IGBT is one of the most promising structures in the area of high voltage MOS-controllable switching devices.