Application of Spectral Density of Transitions in Semiconductor Laser Theory
In this paper the formulas for spectral speed of an emission, for gain, for spectral - volume energy density of spontaneous radiation, for modulation of spontaneous radiation output power spectrum near to a threshold, based on formula for spectral density of transitions, are derived. The contrast modulation index mk was defined; using of mk determines more split-hair accuracy of definition |thetas| from experimental data.
Nonlinear laser theory and natural linewidth in semiconductor lasers
Our approach to the problem is based on the following basic concepts: using a spatially-distributed laser model; using an approximate Kovalevski-Noppe (KN) expression for effective natural linewidth for semiconductor lasers with application of generalized parameters. Physical representations of secondary spontaneous fluctuations have been formulated. These allow deriving formulas for generalized parameters in the KN expression for linewidth, on the basis of which the gain, foundation of the nonlinear system of equations, is derived. The effective natural linewidth is calculated after solving the nonlinear system of equations. Thus, the theory of natural linewidth is an integral part of the nonlinear theory on the basis of which the output power, linewidth, and threshold current for Fabry-Perot semiconductor lasers are calculated. The calculations made allow explaining some experimental measurements of natural linewidth for the Fabry-Perot semiconductor lasers.
Monte Carlo Study of the Effect of Backscatter Materail Thickness on 99 mTc Source Response in Single Photon Emission Computed Tomography
Introduction SPECT projections are contaminated by scatter radiation, resulting in reduced image contrast and quantitative errors. Backscatter constitutes a major part of the scatter contamination in lower energy windows. The current study is an evaluation of the effect of backscatter material on FWHM and image quality investigated by Monte Carlo simulation. Materials and Methods SIMIND program was used for simulation of a Siemen’s dual-head variable angle scintillation gamma camera. Planar and SPECT scanning of a 99m Tc source and a Jaszczak phantom for varying thicknesses of Perspex slabs, as a backscatter media, were analyzed using the photopeak and scatter windows. Simulated planar images and reconstructed tomographic images were evaluated qualitatively, by two nuclear medicine specialists, and quantitatively, by Structural Similarity (SSIM) Index. Results In the 99m Tc photopeak window, no significant change in total counts due to backscatter material was measured. In the scatter windows, scattering was overestimated compared with a simulated backscatter free SPECT system. For instance, at a thickness of 10 cm, total counts of a 99m Tc source detected in the 72 keV windows eventually doubled with increasing backscatter material, compared with the situation without backscatter material. The backscatter contribution plateaued when more than 7 cm of scatter material was placed but there were optimized results for a backscatter thickness of 4.5 cm. Better image quality for the thickness was confirmed by the results of eye interpretation and also by SSIM algorithm. Conclusion Backscatter should be taken into account, particularly in model-based scatter correction methods in SPECT for an accurate simulation system optimization.