Coherent instabilities in a semiconductor laser with fast gain recovery

We report the observation of a coherent multimode instability in quantum cascade lasers (QCLs), which is driven by the same fundamental mechanism of Rabi oscillations as the elusive Risken-Nummedal-Graham-Haken (RNGH) instability predicted 40 years ago for ring lasers. The threshold of the observed instability is significantly lower than in the original RNGH instability, which we attribute to saturable-absorption nonlinearity in the laser. Coherent effects, which cannot be reproduced by standard laser rate equations, can play therefore a key role in the multimode dynamics of QCLs, and in lasers with fast gain recovery in general.Comment: 5 pages, 4 figure

Determination of proton and neutron spectra in the LANSCE spallation irradiation facility

Materials samples were recently irradiated in the Los Alamos Radiation Effects Facility (LASREF) at the Los Alamos Neutron Science Center (LANSCE) to provide data for the Accelerator Production of Tritium (APT) project on the effect of irradiation on the mechanical and physical properties of materials. The targets were configured to expose samples to a variety of radiation environments including, high-energy protons, mixed protons and high-energy neutrons, and low-energy neutrons. The samples were irradiated for approximately six months during a ten month period using an 800 MeV proton beam with a circular Gaussian shape of approximately 2{sigma} = 3.0 cm. At the end of this period, the samples were extracted and tested. Activation foils were also extracted that had been placed in proximity to the materials samples. These were used to quantify the fluences in various locations

Comprehensive numerical model for cw vertical-cavity surface-emitting lasers

The authors present a comprehensive numerical model for vertical-cavity surface-emitting lasers that includes all major processes effecting cw operation of axisymmetric devices. In particular, the model includes a description of the 2D transport of electrons and holes through the cladding layers to the quantum well(s), diffusion and recombination processes of these carriers within the wells, the 2D transport of heat throughout the device, and a multi-lateral-mode effective index optical model. The optical gain acquired by photons traversing the quantum wells is computed including the effects of strained band structure and quantum confinement. They employ the model to predict the behavior of higher-order lateral modes in proton-implanted devices, and to provide an understanding of index-guiding in devices fabricated using selective oxidation

Determination of mixed proton/neutron fluences in the LANSCE irradiation environment

In support of the Accelerator Production of Tritium (APT) program, several materials were exposed to a high-energy proton and spallation neutron environments. Large differences in mechanical property changes in this environment are expected compared to the typical fusion or fission systems. To make proper dose correlations, it is important to accurately quantify the fluences. Activation foils consisting of a stack of disks of Co, Ni, Fe, Al, Nb and Cu were irradiated concurrent with mechanical testing samples in the Los Alamos Spallation Radiation Effects Facility (LASREF) at the Los Alamos Neutron Science Center (LANSCE) facility. The irradiation consisted of an 800 MeV, 1 mA proton beam and a W target in the beam provided a source of spallation neutrons. The maximum proton fluence was around 3 {times} 10{sup 21} p/cm{sup 2} and the maximum neutron fluence approximately 3 {times} 10{sup 20} n/cm{sup 2}. After irradiation, the foils were withdrawn and the radioactive isotopes analyzed using gamma spectroscopy. From initial estimates for the fluences and spectra derived from the Los Alamos High-Energy Transport (LAHET) Code System (LCS), comparisons to the measured levels of activation products were made. The Na-22 activation products in the Al foils were measured from different regions of the target in order to profile the spatial levels of the fluences. These tests gave empirical confirmation of the proton and neutron fluences of the irradiated samples throughout the target region

Spectral Unfolding of Mixed Proton/Neutron Fluences in the LANSCE Irradiation Environment

An analysis of activation foils from irradiations at the Los Alamos Neutron Science Center (LANSCE) was used to provide a basis for determination of combined proton and neutron fluxes. Materials samples were irradiated in the A-6 target area at LANSCE to explore the mechanical property changes in a high-energy proton/spallation neutron environment. Proper assessment of the effects of irradiation required quantifying the proton and neutron fluences. Foil stacks consisting of disks of Co, Ni, Fe, Al, Nb and Cu, were irradiated concurrently with mechanical property samples. The irradiation consisted of an 800 MeV, 1 mA proton beam, and a W target in the beam provided a source of spallation neutrons. The maximum proton fluence was 4×10^21 p/cm2 and the maximum neutron fluence was approximately 1×10^21 n/cm2. After irradiation, the foils were withdrawn and the radioactive isotopes sampled with gamma spectroscopy. Initial calculated estimates of the fluences we made with the Los Alamos High-Energy Transport (LAHET) Code System (LCS). The LCS code is a coupling of LAHET and MCNP (Monte Carlo N-Particle) to calculate transport of the high-energy protons, neutrons and photons. From the initial estimates and the measured activation products, STAYSL2 was used to produced a revised spectra of proton and neutron fluences. STAYSL2 is an extension of the STAY\u27SL code normally used for neutron activation analysis. STAYSL2 has been modified to include proton fluences in the analysis of activation to account for the mixed proton/neutron fluence in the target area. This methodology was applied to multiple foil stacks from different regions of the target in order to profile the spatial dependencies of the fluences