Optimization of broadband semiconductor chirped mirrors with genetic algorithm

Genetic algorithm was applied for optimization of dispersion properties in semiconductor Bragg reflectors for applications in femtosecond lasers. Broadband, large negative group-delay dispersion was achieved in the optimized design: The group-delay dispersion (GDD) as large as −3500 fs2 was theoretically obtained over a 10-nm bandwidth. The designed structure was manufactured and tested, providing GDD −3320 fs2 over a 7-nm bandwidth. The mirror performance was verified in semiconductor structures grown with molecular beam epitaxy. The mirror was tested in a passively mode-locked Yb:KYW laser

Prescribed Fire Alters Structure and Composition of a Mid-Atlantic Oak Forest up to Eight Years After Burning

Background Prescribed fire in Eastern deciduous forests has been understudied relative to other regions in the United States. In Pennsylvania, USA, prescribed fire use has increased more than five-fold since 2009, yet forest response has not been extensively studied. Due to variations in forest composition and the feedback between vegetation and fire, Pennsylvania deciduous forests may burn and respond differently than forests across the eastern US. We measured changes in forest structure and composition up to eight years after prescribed fire in a hardwood forest of the Ridge and Valley region of the Appalachian Mountains in central Pennsylvania. Results Within five years post fire, tree seedling density increased more than 72% while sapling density decreased by 90%, midstory density decreased by 46%, and overstory response varied. Following one burn in the mixed-oak unit, overstory tree density decreased by 12%. In the aspen–oak unit, where pre-fire harvesting and two burns occurred, overstory tree density increased by 25%. Not all tree species responded similarly and post-fire shifts in species relative abundance occurred in sapling and seedling size classes. Abundance of red maple and cherry species decreased, whereas abundance of sassafras, quaking aspen, black oak, and hickory species increased. Conclusions Forest composition plays a key role in the vegetation–fire relationship and localized studies are necessary to measure forest response to prescribed fire. Compositional shifts in tree species were most pronounced in the aspen–oak unit where pre-fire overstory thinning and two prescribed fires were applied and significant structural changes occurred in all stands after just one burn. Increases in fire-tolerant tree species combined with reductions in fire-intolerant species highlight the role of prescribed fire in meeting management objectives such as altering forest structure and composition to improve game habitat in mid-Atlantic hardwood forests

Optimal parameters of monolithic high-contrast grating mirrors

In this Letter a fully vectorial numerical model is used to search for the construction parameters of monolithic high-contrast grating (MHCG) mirrors providing maximal power reflectance. We determine the design parameters of highly reflecting MHCG mirrors where the etching depth of the stripes is less than two wavelengths in free space. We analyze MHCGs in a broad range of real refractive index values corresponding to most of the common optoelectronic materials in use today. Our results comprise a complete image of possible highly reflecting MHCG mirror constructions for potential use in optoelectronic devices and systems. We support the numerical analysis by experimental verification of the high reflectance via a GaAs MHCG designed for a wavelength of 980 nm

Multi-dimensional modeling and simulation of semiconductor nanophotonic devices

Self-consistent modeling and multi-dimensional simulation of semiconductor nanophotonic devices is an important tool in the development of future integrated light sources and quantum devices. Simulations can guide important technological decisions by revealing performance bottlenecks in new device concepts, contribute to their understanding and help to theoretically explore their optimization potential. The efficient implementation of multi-dimensional numerical simulations for computer-aided design tasks requires sophisticated numerical methods and modeling techniques. We review recent advances in device-scale modeling of quantum dot based single-photon sources and laser diodes by self-consistently coupling the optical Maxwell equations with semiclassical carrier transport models using semi-classical and fully quantum mechanical descriptions of the optically active region, respectively. For the simulation of realistic devices with complex, multi-dimensional geometries, we have developed a novel hp-adaptive finite element approach for the optical Maxwell equations, using mixed meshes adapted to the multi-scale properties of the photonic structures. For electrically driven devices, we introduced novel discretization and parameter-embedding techniques to solve the drift-diffusion system for strongly degenerate semiconductors at cryogenic temperature. Our methodical advances are demonstrated on various applications, including vertical-cavity surface-emitting lasers, grating couplers and single-photon sources

Projektowanie małych silników indukcyjnych z wykorzystaniem dedykowanego systemu ekspertowego

The paper describes the architecture of dedicated expert system building with using declarative and also algorithmic language programming. In first case was utilized pc-shell environment and in second - Delphi 2005.NET. This expert system is dedicated to designing of the small induction motors. In the proposed expert system, the database of knowledge was created. For the codification of knowledge simple rules, complex rules, frames and neural networks were used.W pracy przedstawiono strukturę systemu ekspertowego wspomagającego projektowanie małych silników indukcyjnych. Proponowany system ekspertowy został opracowany w środowisku programowania w logice pc-shell, natomiast interfejs użytkownika wykonano w środowisku Delphi 2005.NET. Proces projektowania silnika składa się z dwóch kroków. W pierwszym kroku na podstawie katalogowych danych znamionowych silnika, takich jak: moc znamionowa, napięcie znamionowe, układ połączeń uzwojenia stojana, sprawność, współczynnik mocy oraz wznios osi wału, wykonywane są obliczenia wstępne, w wyniku których wyznaczany jest zbiór danych *.dat zawierający główne wymiary obwodu magnetycznego silnika, liczbę oraz kształty i wymiary żłobków stojana i wirnika oraz parametry uzwojenia stojana (rodzaj uzwojenia, liczba zwojów szeregowych, gałęzi i drutów równoległych, rodzaj i średnica drutu nawojowego). Głównymi ograniczeniami na tym etapie projektowania są maksymalne wartości indukcji magnetycznej w szczelinie powietrznej oraz w poszczególnych elementach obwodu magnetycznego, a także maksymalne wartości gęstości prądu w uzwojeniach stojana i wirnika. Ponadto, w opracowywanych regułach brane są również pod uwagę ograniczenia technologiczne, np. minimalna, możliwa do wykonania szerokość zęba stojana lub wirnika, maksymalny współczynnik zapełnienia żłobka stojana, itp. Wygenerowany przez system ekspertowy zbiór *.dat staje się zbiorem danych wejściowych do programu obliczeń elektromagnetycznych małych silników indukcyjnych STAT v. 4, w wyniku których otrzymujemy zbiór wyników *.res. Zbiór ten jest następnie analizowany przez system ekspertowy pod kątem zgodności danych wynikowych z danymi wejściowymi i w przypadku nie spełnienia narzuconych tolerancji system ekspertowy dokonuje niezbędnych korekt, po czym obliczenia powtarzane są samoczynnie, aż do uzyskania zadawalających wyników. Wykorzystywana w systemie ekspertowym baza reguł i baza wiedzy korzysta z różnych metod kodyfikacji wiedzy, w postaci reguł prostych, reguł złożonych, ramek oraz wielowarstwowej sieci neuronowej