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In the search for a better understanding of the fundamental nature of the atom, the development of femtosecond pulsed laser sources has opened up for many applications, in particular in time-resolved physics. For many of those, like the generation of attosecond pulses through high-order harmonic generation, high intensity is needed and the quality of the beam is a critical factor. Most commonly, beam quality is quantified by the M^2-values, which relates the actual beam size to an ideal Gaussian beam. This procedure however is often very time consuming, the quality of the beam is only defined by one parameter and no information of the spatial phase or the focused intensity is obtained. This is what has prompted the development of a novel beam diagnostic tool, inspired by diffractive imaging. The diagnostic tool relies on the Fourier transform relation between the beam in two different planes. Through an iterative algorithm the spatial phase of the beam can be retrieved. With both the intensity profile and phase of the beam known, the beam profile through the focus as well as in the far-field, can be reconstructed. From which parameters like the M^2-value and the Strehl ratio, which relates the intensity to the theoretical best, can be derived. As the need for experimental set-ups with better resolution and with higher fidelity is evergrowing, we apply the developed tool to characterize and assess the attosecond pump-probe interferometric set-up that is being upgraded at the Atomic Physics Division at Lund University. The general set-up, together with improvements to its stability and flexibility are presented. Where the upgraded set-up shows promise for producing better resolved attosecond pump-probe experiments with a high fidelity. The beam diagnostic tool is very well-suited to evaluate the beam quality in different places in the set-up. Future prospects of improvements to the diagnostic tool, involves a full decomposition of the spatial phase into e.g. Zernike p, Vad är det minsta ni någonsin har mätt? Vad är den snabbaste händelsen ni någonsin observerat? Kanske har ni haft turen att studera naturen omkring er med hjälp av ett mikroskop, kanske har ni sett en "slow motion" film av explosioner som sker på endast en bråkdel av en sekund. Finns det någon begränsning på hur små objekt vi kan mäta, på hur snabba händelseförlopp vi kan observera? På samma sätt som man använder en linjal som en referens för att mäta hur långt någonting är, behövs en längd- och/eller en tidsskala som är kort nog för att fungera som en referens för det vi vill undersöka. Det har länge varit känt att man med hjälp av ljus kan studera naturen runt omkring sig på väldigt många olika sätt. Detta är möjligt eftersom våglängden på ljuset kan fungera som en längdreferens. För att undersöka fenomen på den atomära skalan, behöver man därför ljus som har våglängder i det extremultravioletta spektrat eller kortare. Samtidigt är problemet med att generera en tidsreferens som är kort nog, kvarstående. Det är nu fördelen med att studera naturen med hjälp av ljus blir uppenbar. Detta tack vare att ljus också kan genereras i form av korta pulser, som då även kan användas som en tidsreferens samtidigt. Med pulser som är på storleksordningen av några attosekunder (1 attosekund = 10^{-18} sekunder) kan man även studera de händelseförlopp som sker inuti en atom. Med väldigt välutvecklade tekniker, har forskare över hela världen skapat metoder för att framställa väldigt korta ljuspulser genom generering av harmoniska övertoner. Genom att fokusera ner den genererande ljuspulsen i gaser, resulterar detta i attosekundljuspulser med våglängder i det extremultravioletta spektrat som kan användas för att studera atomära fenomen i realtid. Attosekundsfysik kan, som beskrivet ovan, i sin absolut mest simpla form beskrivas som studerandet av molekylära och atomära fenomen, med hjälp av ultrakorta ljuspulser. Det kan vara svårt att få en känsla för hur kortvariga dessa pulserna ä

In the search for a better understanding of the fundamental nature of the atom, the development of femtosecond pulsed laser sources has opened up for many applications, in particular in time-resolved physics. For many of those, like the generation of attosecond pulses through high-order harmonic generation, high intensity is needed and the quality of the beam is a critical factor. Most commonly, beam quality is quantified by the M^2-values, which relates the actual beam size to an ideal Gaussian beam. This procedure however is often very time consuming, the quality of the beam is only defined by one parameter and no information of the spatial phase or the focused intensity is obtained. This is what has prompted the development of a novel beam diagnostic tool, inspired by diffractive imaging. The diagnostic tool relies on the Fourier transform relation between the beam in two different planes. Through an iterative algorithm the spatial phase of the beam can be retrieved. With both the intensity profile and phase of the beam known, the beam profile through the focus as well as in the far-field, can be reconstructed. From which parameters like the M^2-value and the Strehl ratio, which relates the intensity to the theoretical best, can be derived. As the need for experimental set-ups with better resolution and with higher fidelity is evergrowing, we apply the developed tool to characterize and assess the attosecond pump-probe interferometric set-up that is being upgraded at the Atomic Physics Division at Lund University. The general set-up, together with improvements to its stability and flexibility are presented. Where the upgraded set-up shows promise for producing better resolved attosecond pump-probe experiments with a high fidelity. The beam diagnostic tool is very well-suited to evaluate the beam quality in different places in the set-up. Future prospects of improvements to the diagnostic tool, involves a full decomposition of the spatial phase into e.g. Zernike p, Vad är det minsta ni någonsin har mätt? Vad är den snabbaste händelsen ni någonsin observerat? Kanske har ni haft turen att studera naturen omkring er med hjälp av ett mikroskop, kanske har ni sett en "slow motion" film av explosioner som sker på endast en bråkdel av en sekund. Finns det någon begränsning på hur små objekt vi kan mäta, på hur snabba händelseförlopp vi kan observera? På samma sätt som man använder en linjal som en referens för att mäta hur långt någonting är, behövs en längd- och/eller en tidsskala som är kort nog för att fungera som en referens för det vi vill undersöka. Det har länge varit känt att man med hjälp av ljus kan studera naturen runt omkring sig på väldigt många olika sätt. Detta är möjligt eftersom våglängden på ljuset kan fungera som en längdreferens. För att undersöka fenomen på den atomära skalan, behöver man därför ljus som har våglängder i det extremultravioletta spektrat eller kortare. Samtidigt är problemet med att generera en tidsreferens som är kort nog, kvarstående. Det är nu fördelen med att studera naturen med hjälp av ljus blir uppenbar. Detta tack vare att ljus också kan genereras i form av korta pulser, som då även kan användas som en tidsreferens samtidigt. Med pulser som är på storleksordningen av några attosekunder (1 attosekund = 10^{-18} sekunder) kan man även studera de händelseförlopp som sker inuti en atom. Med väldigt välutvecklade tekniker, har forskare över hela världen skapat metoder för att framställa väldigt korta ljuspulser genom generering av harmoniska övertoner. Genom att fokusera ner den genererande ljuspulsen i gaser, resulterar detta i attosekundljuspulser med våglängder i det extremultravioletta spektrat som kan användas för att studera atomära fenomen i realtid. Attosekundsfysik kan, som beskrivet ovan, i sin absolut mest simpla form beskrivas som studerandet av molekylära och atomära fenomen, med hjälp av ultrakorta ljuspulser. Det kan vara svårt att få en känsla för hur kortvariga dessa pulserna ä

Breitstreifenlaser haben eine breite Emissionsapertur, die es ermöglicht eine hohe Ausgangsleistung zu erreichen. Gleichzeitig führt sie jedoch zu einer Verringerung der lateralen Strahlqualität und zu ihrem nicht-stationären Verhalten. Forschung in diesem Gebiet ist anwendungsgetrieben und somit ist das Hauptziel eine Erhöhung der Brillanz, die sowohl die Ausgangsleistung als auch die laterale Strahlqualität beinhaltet. Um die zugrunde liegenden raumzeitlichen Phänomene zu verstehen und dieses Wissen zu nutzen, um die Kosten der Brillanz-Optimierung zu minimieren, ist ein selbst-konsistentes Simulationstool notwendig, welches die wichtigsten Prozesse beinhaltet. Zunächst wird in dieser Arbeit ein quasi-dreidimensionales elektro-optisch-thermisches Model präsentiert, welches wesentliche qualitative Eigenschaften von realen Bauteilen gut beschreibt. Zeitabhängige Wanderwellen-Gleichungen werden genutzt, um die inhärent nicht-stationären optischen Felder zu beschreiben, welche an eine Ratengleichung für die Überschussladungsträger in der aktiven Zone gekoppelt sind. Das Model wird in dieser Arbeit um eine Injektionsstromdichte erweitert, die laterale Stromspreizung und räumliches Lochbrennen korrekt beschreibt. Des Weiteren wird ein Temperaturmodel präsentiert, das kurzzeitige lokale Aufheizungen in der Nähe der aktiven Zone und die Formierung einer stationären Temperaturverteilung beinhalten. Im zweiten Teil wird das beschriebene Modell genutzt, um die Gründe von Brillanz-Degradierung, das heißt sowohl die Ursprünge der Leistungssättigung als auch des nicht diffraktionslimitierten Fernfeldes zu untersuchen. Abschließend werden im letzten Teil Laserentwürfe besprochen, welche die laterale Brillanz verbessern. Hierzu gehört ein neuartiges “Schachbrettlaser” Design, bei dem longitudinal-laterale Gewinn-Verlust-Modulation mit zusätzlicher Phasenanpassung ausgenutzt wird, um eine sehr geringe Fernfeld-Divergenz zu erhalten., Broad-area lasers are edge-emitting semiconductor lasers with a wide lateral emission aperture that enables high output powers, but also diminishes the lateral beam quality and results in their inherently non-stationary behavior. Research in the area is driven by application and the main objective is to increase the brightness which includes both the output power and lateral beam quality. To understand the underlying spatio-temporal phenomena and to apply this knowledge in order to reduce costs for brightness optimization, a self-consistent simulation tool taking into account all essential processes is vital. Firstly, in this work a quasi-three-dimensional opto-electronic and thermal model is presented, that describes well essential qualitative characteristics of real devices. Time-dependent traveling-wave equations are utilized to describe the inherently non-stationary optical fields, which are coupled to dynamic rate equations for the excess carriers in the active region. This model is extended by an injection current density model to accurately include lateral current spreading and spatial hole burning. Furthermore a temperature model is presented that includes short-time local heating near the active region as well as the formation of a stationary temperature profile. Secondly, the reasons of brightness degradation, i.e. the origins of power saturation and the spatially modulated field profile are investigated and lastly, designs that mitigate those effects that limit the lateral brightness under pulsed and continuous-wave operation are discussed. Amongst those designs a novel “chessboard laser” is presented that utilizes longitudinal-lateral gain-loss modulation and an additional phase tailoring to obtain a very low far-field divergence.

Breitstreifenlaser haben eine breite Emissionsapertur, die es ermöglicht eine hohe Ausgangsleistung zu erreichen. Gleichzeitig führt sie jedoch zu einer Verringerung der lateralen Strahlqualität und zu ihrem nicht-stationären Verhalten. Forschung in diesem Gebiet ist anwendungsgetrieben und somit ist das Hauptziel eine Erhöhung der Brillanz, die sowohl die Ausgangsleistung als auch die laterale Strahlqualität beinhaltet. Um die zugrunde liegenden raumzeitlichen Phänomene zu verstehen und dieses Wissen zu nutzen, um die Kosten der Brillanz-Optimierung zu minimieren, ist ein selbst-konsistentes Simulationstool notwendig, welches die wichtigsten Prozesse beinhaltet. Zunächst wird in dieser Arbeit ein quasi-dreidimensionales elektro-optisch-thermisches Model präsentiert, welches wesentliche qualitative Eigenschaften von realen Bauteilen gut beschreibt. Zeitabhängige Wanderwellen-Gleichungen werden genutzt, um die inhärent nicht-stationären optischen Felder zu beschreiben, welche an eine Ratengleichung für die Überschussladungsträger in der aktiven Zone gekoppelt sind. Das Model wird in dieser Arbeit um eine Injektionsstromdichte erweitert, die laterale Stromspreizung und räumliches Lochbrennen korrekt beschreibt. Des Weiteren wird ein Temperaturmodel präsentiert, das kurzzeitige lokale Aufheizungen in der Nähe der aktiven Zone und die Formierung einer stationären Temperaturverteilung beinhalten. Im zweiten Teil wird das beschriebene Modell genutzt, um die Gründe von Brillanz-Degradierung, das heißt sowohl die Ursprünge der Leistungssättigung als auch des nicht diffraktionslimitierten Fernfeldes zu untersuchen. Abschließend werden im letzten Teil Laserentwürfe besprochen, welche die laterale Brillanz verbessern. Hierzu gehört ein neuartiges “Schachbrettlaser” Design, bei dem longitudinal-laterale Gewinn-Verlust-Modulation mit zusätzlicher Phasenanpassung ausgenutzt wird, um eine sehr geringe Fernfeld-Divergenz zu erhalten., Broad-area lasers are edge-emitting semiconductor lasers with a wide lateral emission aperture that enables high output powers, but also diminishes the lateral beam quality and results in their inherently non-stationary behavior. Research in the area is driven by application and the main objective is to increase the brightness which includes both the output power and lateral beam quality. To understand the underlying spatio-temporal phenomena and to apply this knowledge in order to reduce costs for brightness optimization, a self-consistent simulation tool taking into account all essential processes is vital. Firstly, in this work a quasi-three-dimensional opto-electronic and thermal model is presented, that describes well essential qualitative characteristics of real devices. Time-dependent traveling-wave equations are utilized to describe the inherently non-stationary optical fields, which are coupled to dynamic rate equations for the excess carriers in the active region. This model is extended by an injection current density model to accurately include lateral current spreading and spatial hole burning. Furthermore a temperature model is presented that includes short-time local heating near the active region as well as the formation of a stationary temperature profile. Secondly, the reasons of brightness degradation, i.e. the origins of power saturation and the spatially modulated field profile are investigated and lastly, designs that mitigate those effects that limit the lateral brightness under pulsed and continuous-wave operation are discussed. Amongst those designs a novel “chessboard laser” is presented that utilizes longitudinal-lateral gain-loss modulation and an additional phase tailoring to obtain a very low far-field divergence.

In high power regimes, broad area semiconductor lasers usually suffer from poor beam quality due to their asymmetric beam divergence, large beam quality factor (M2) and from the absence of any intrinsic filtering mechanism that can be integrated inside the cavity. In this work, we present a compact photonic crystal spatial filter, fabricated by periodically modulating refractive index media on a glass substrate using tightly focused femtoseconds laser. This filter work by deflecting the higher angular field components in a given frequency range. We demonstrate the spatial filtering effect when placed in an extended cavity configuration for a single BAS emitter, with transverse width of 400-µm and cavity length 1500-µm. We report a decrease of the laser M2 value along the slow axis with the introduction of the photonic crystal inside the cavity, together with a brightness enhancement by a factor of 1.5 compared to that of an unfiltered case. These results were compared with those obtained in the far field domain, with a conventional spatial filter consisting of an intra-cavity slit., Peer Reviewed, Postprint (published version)

On-line image guidance using magnetic resonance (MR) imaging is expected to improve the targeting accuracy of proton therapy. However, to date no combined system exists. In this study, for the first time a low-field open MR scanner was integrated with a static proton research beam line to test the feasibility of simultaneous irradiation and imaging. The field-of-view of the MR scanner was aligned with the beam by taking into account the Lorentz force induced beam deflection. Various imaging sequences for extremities were performed on a healthy volunteer and on a patient with a soft-tissue sarcoma of the upper arm, both with the proton beam line switched off. T 1 -weighted spin echo images of a tissue-mimicking phantom were acquired without beam, with energised beam line magnets and during proton irradiation. Beam profiles were acquired for the MR scanner’s static magnetic field alone and in combination with the dynamic gradient fields during the acquisition of different imaging sequences. It was shown that MR imaging is feasible in the electromagnetically contaminated environment of a proton therapy facility. The anatomical MR images showed sufficient quality for target volume identification and positioning. The tissue-mimicking phantom showed no visible beam-induced image degradation. The beam profiles depicted no influence due to the dynamic gradient fields of the imaging sequences. This study proves that simultaneous irradiation and in-beam MR imaging is technically feasible with a low-field MR scanner integrated with a static proton research beam line.

We simulate and analyze how beam quality improves while being amplified in edge emitting broad area semiconductor amplifiers with a periodic structuring of the electrical contacts, in both longitudinal and lateral directions. A spatio-temporal traveling wave model is used for simulations of the dynamics and nonlinear interactions of the optical fields, induced polarizations and carrier density. In the case of small beam amplification, the optical field can be expanded into few Bloch modes, so that the system is described by a set of ODEs for the evolution of the mode amplitudes. The analysis of such model provides a deep understanding of the impact of the different parameters on amplification and on spatial (angular) filtering of the beam. It is shown that under realistic parameters the two-dimensional modulation of the current can lead not only to a significant reduction of the emission divergence, but also to an additional amplification of the emitted field, Peer Reviewed, Postprint (author's final draft)

We report the demonstration of high-power (840 mW) continuous-wave laser oscillation from Fe2+ ions in zinc selenide. The output spectrum of the Fe:ZnSe laser had a line-center near 4140 nm with a linewidth of 80 nm. The Beam quality was measured to be M2 /= 1.2 with a maximum slope efficiency of 47%. Small shifts observed in output wavelength with increased output power were attributed to thermal effects. No thermal roll-off of slope efficiency was observed at the maximum of output power., Sponsored in part by AFOSR. Published in Optics Letters, v37 n23 p5021-5023, 30 Nov 2012.

This paper analyzes the employability of laser weapons on a Navy Littoral Combat Ship (LCS) class ship to counter small and fast boat threats. A general model of laser weapons is established to identify the attributes that characterize the laser weapon system. Quantitative values of each attribute are identified and compared for four laser weapon systems that are currently under development by the Navy to determine their potential for employment on the LCS. Plausible operational scenarios of a suicide attack by multiple (up to three) small and fast motor boats equipped with Improvised Explosive Devices (IED) against LCS-class ships are described. These scenarios provide input parameters for computing the laser parameters that would be needed to neutralize such threats. Three types of laser technologies, the Solid-State Slab Laser (SSSL), Free Electron Laser (FEL), and Fiber Laser (FL), are assessed to determine whether they meet the laser weapon requirements that would be needed to counter the small boat threats described in the scenarios. The requirements involve power, beam size, ship infrastructure capacities, stabilization, response to multiple targets, and cost.

The objective of this work is to investigate the feasibility of intense laser-beam propagation through optical fibers for temperature and species concentration measurements in gas-phase reacting flows using coherent anti-Stokes Raman Scattering (CARS) spectroscopy. In particular, damage thresholds of fibers, nonlinear effects during beam propagation, and beam quality at the output of the fibers studied for the propagation of nanosecond (ns) and picosecond (ps) laser beams. It is observed that ps pulses are better suited for fiber-based nonlinear optical spectroscopic techniques, which generally depend on laser irradiance rather than fluence. A ps fiber-coupled CARS system using multimode step-index fibers is developed. Temperature measurements using this system are demonstrated in an atmospheric pressure, near-adiabatic laboratory flame. Proof-of-concept measurements show significant promise for fiber-based CARS spectroscopy in harsh combustion environments. Furthermore, since ps-CARS spectroscopy allows the suppression of non-resonant background, this technique could be utilized for improving the sensitivity and accuracy of CARS thermometry in high-pressure hydrocarbon- fueled combustors., Prepared in collaboration with Spectral Energies, LLC, Dayton, OH, Innovative Scientific Solutions, Inc., Dayton, OH, and Iowa State University, Ames, IA. Published online in Exp Fluids, Vol. 49, p969-984, September 1, 2010.

Directed Energy Applications for thin disk lasers demand improvements in materials, efficiency, thermal management, and most importantly beam quality. At the Air Force Research Laboratory ceramic Yb:YAG materials are being investigated along with various cooling techniques. 10-14mm diameter 0.2mm thick disks are mounted on silicon carbide (SiC), sapphire, and diamond submounts. From a larger platform, more than 6kW power is obtained from unmounted and submounted 35mm diameter disks. In conjunction with thermal modeling, we project a path towards high performance high power lasers., Presented at the SPIE Defense Conference held in Toulouse, France on 20-25 September 2010.

Beam quality optimization in mammography traditionally considers detection of a target obscured by quantum noise on a homogenous background. It can be argued that this scheme does not correspond well to the clinical imaging task because real mammographic images contain a complex superposition of anatomical structures, resulting in anatomical noise that may dominate over quantum noise. Using a newly developed spectral mammography system, we measured the correlation and magnitude of the anatomical noise in a set of mammograms. The results from these measurements were used as input to an observer-model optimization that included quantum noise as well as anatomical noise. We found that, within this framework, the detectability of tumors and microcalcifications behaved very differently with respect to beam quality and dose. The results for small microcalcifications were similar to what traditional optimization methods would yield, which is to be expected since quantum noise dominates over anatomical noise at high spatial frequencies. For larger tumors, however, low-frequency anatomical noise was the limiting factor. Because anatomical structure has similar energy dependence as tumor contrast, optimal x-ray energy was significantly higher and the useful energy region wider than traditional methods suggest. Measurements on a tissue phantom confirmed these theoretical results. Furthermore, since quantum noise constitutes only a small fraction of the noise, the dose could be reduced substantially without sacrificing tumor detectability. Exposure settings used clinically are therefore not necessarily optimal for this imaging task. The impact of these findings on the mammographic imaging task as a whole is, however, at this stage unclear., QC 20110927

Beam quality optimization in mammography traditionally considers detection of a target obscured by quantum noise on a homogenous background. It can be argued that this scheme does not correspond well to the clinical imaging task because real mammographic images contain a complex superposition of anatomical structures, resulting in anatomical noise that may dominate over quantum noise. Using a newly developed spectral mammography system, we measured the correlation and magnitude of the anatomical noise in a set of mammograms. The results from these measurements were used as input to an observer-model optimization that included quantum noise as well as anatomical noise. We found that, within this framework, the detectability of tumors and microcalcifications behaved very differently with respect to beam quality and dose. The results for small microcalcifications were similar to what traditional optimization methods would yield, which is to be expected since quantum noise dominates over anatomical noise at high spatial frequencies. For larger tumors, however, low-frequency anatomical noise was the limiting factor. Because anatomical structure has similar energy dependence as tumor contrast, optimal x-ray energy was significantly higher and the useful energy region wider than traditional methods suggest. Measurements on a tissue phantom confirmed these theoretical results. Furthermore, since quantum noise constitutes only a small fraction of the noise, the dose could be reduced substantially without sacrificing tumor detectability. Exposure settings used clinically are therefore not necessarily optimal for this imaging task. The impact of these findings on the mammographic imaging task as a whole is, however, at this stage unclear., QC 20110927

Beam quality optimization in mammography traditionally considers detection of a target obscured by quantum noise on a homogenous background. It can be argued that this scheme does not correspond well to the clinical imaging task because real mammographic images contain a complex superposition of anatomical structures, resulting in anatomical noise that may dominate over quantum noise. Using a newly developed spectral mammography system, we measured the correlation and magnitude of the anatomical noise in a set of mammograms. The results from these measurements were used as input to an observer-model optimization that included quantum noise as well as anatomical noise. We found that, within this framework, the detectability of tumors and microcalcifications behaved very differently with respect to beam quality and dose. The results for small microcalcifications were similar to what traditional optimization methods would yield, which is to be expected since quantum noise dominates over anatomical noise at high spatial frequencies. For larger tumors, however, low-frequency anatomical noise was the limiting factor. Because anatomical structure has similar energy dependence as tumor contrast, optimal x-ray energy was significantly higher and the useful energy region wider than traditional methods suggest. Measurements on a tissue phantom confirmed these theoretical results. Furthermore, since quantum noise constitutes only a small fraction of the noise, the dose could be reduced substantially without sacrificing tumor detectability. Exposure settings used clinically are therefore not necessarily optimal for this imaging task. The impact of these findings on the mammographic imaging task as a whole is, however, at this stage unclear., QC 20110927

Beam quality optimization in mammography traditionally considers detection of a target obscured by quantum noise on a homogenous background. It can be argued that this scheme does not correspond well to the clinical imaging task because real mammographic images contain a complex superposition of anatomical structures, resulting in anatomical noise that may dominate over quantum noise. Using a newly developed spectral mammography system, we measured the correlation and magnitude of the anatomical noise in a set of mammograms. The results from these measurements were used as input to an observer-model optimization that included quantum noise as well as anatomical noise. We found that, within this framework, the detectability of tumors and microcalcifications behaved very differently with respect to beam quality and dose. The results for small microcalcifications were similar to what traditional optimization methods would yield, which is to be expected since quantum noise dominates over anatomical noise at high spatial frequencies. For larger tumors, however, low-frequency anatomical noise was the limiting factor. Because anatomical structure has similar energy dependence as tumor contrast, optimal x-ray energy was significantly higher and the useful energy region wider than traditional methods suggest. Measurements on a tissue phantom confirmed these theoretical results. Furthermore, since quantum noise constitutes only a small fraction of the noise, the dose could be reduced substantially without sacrificing tumor detectability. Exposure settings used clinically are therefore not necessarily optimal for this imaging task. The impact of these findings on the mammographic imaging task as a whole is, however, at this stage unclear., QC 20110927

Beam quality optimization in mammography traditionally considers detection of a target obscured by quantum noise on a homogenous background. It can be argued that this scheme does not correspond well to the clinical imaging task because real mammographic images contain a complex superposition of anatomical structures, resulting in anatomical noise that may dominate over quantum noise. Using a newly developed spectral mammography system, we measured the correlation and magnitude of the anatomical noise in a set of mammograms. The results from these measurements were used as input to an observer-model optimization that included quantum noise as well as anatomical noise. We found that, within this framework, the detectability of tumors and microcalcifications behaved very differently with respect to beam quality and dose. The results for small microcalcifications were similar to what traditional optimization methods would yield, which is to be expected since quantum noise dominates over anatomical noise at high spatial frequencies. For larger tumors, however, low-frequency anatomical noise was the limiting factor. Because anatomical structure has similar energy dependence as tumor contrast, optimal x-ray energy was significantly higher and the useful energy region wider than traditional methods suggest. Measurements on a tissue phantom confirmed these theoretical results. Furthermore, since quantum noise constitutes only a small fraction of the noise, the dose could be reduced substantially without sacrificing tumor detectability. Exposure settings used clinically are therefore not necessarily optimal for this imaging task. The impact of these findings on the mammographic imaging task as a whole is, however, at this stage unclear., QC 20110927

This paper discusses methods for characterization of high power lasers. Specifically, these methods have been developed for the High Energy Laser Joint Technology Office and used for independent, government-sponsored testing in the 25 and 100 kW phases of the Joint High Power Solid State Laser program. Primarily this paper addresses measurement of power and beam quality.

This paper discusses methods for characterization of high power lasers. Specifically, these methods have been developed for the High Energy Laser Joint Technology Office and used for independent, government-sponsored testing in the 25 and 100 kW phases of the Joint High Power Solid State Laser program. Primarily this paper addresses measurement of power and beam quality.

The aim of this work was to develop, implement and evaluate an independent system with which to calculate the absorbed dose, delivered by high-energy X-ray beams, to the prescription point and the depth of dose maximum. The introduction of such a system in the clinical routine may help ensure high-quality treatment and avoidance of errors which may jeopardise the clinical outcome of the treatment (i.e. under- or overdose). A set of equations for calculating the absorbed dose to the prescription point was compiled in a software application (“HandCalc”), which is completely independent of the treatment planning system (TPS). For instance, HandCalc includes models to calculate the absorbed dose from photons scattered in the patient, the transmission of the primary kerma in the patient, the variation of the primary kerma in air with collimator setting (i.e. head scatter), and corrections for heterogeneities in the patient. A new expression for the transmission of the primary kerma in the patient was derived in which the coefficients are strictly defined (and given a physical interpretation) by the first two moments of the spectral distribution of the incident beam. Further investigations also revealed that these moments can be used to determine water-to-air stopping power ratios more accurately than other beam quality indices. In practice, the moments are derived from “in-air equivalent”, narrow-beam measurements using a mini-phantom. The degree of in-air equivalence was investigated with Monte Carlo simulations, which showed that the optimum measurement depth in a mini-phantom is somewhat below the depth of dose maximum. Based upon comparisons with measurements and the TPS, a clinical action level of +/- 4% was chosen for HandCalc. Deviations greater than this are, with all probability, due to erroneous handling of the patient dataset during the preparation phase. An “entrance dose factor” was added in order to correct the dose calculations at the depth of dose maximum

A phase-space inequality is derived for beams of arbitrary state of spatial coherence. It applies to the product of a factor which expresses the effective coherence area of the source that generates the beam and the effective angular spread of the beam; and, by analogy with coherent beams, it may be regarded as a measure of the beam quality. It is found that the factor attains a minimum for the entire class of Gaussian Schell-model beams (which include the Hermite Gaussian laser mode)., QC 20100525

A number of laser propagation codes have been assessed as to their suitability for modeling Army High Energy Laser (HEL) weapons used in an anti- sensor mode. We identify a number of areas in which systems analysis HEL codes are deficient. Most notably, available HEL scaling law codes model the laser aperture as circular, possibly with a fixed (e.g. 10%) obscuration. However, most HELs have rectangular apertures with up to 30% obscuration. We present a beam-quality/aperture shape scaling relation which can be useful when applying these codes to realistic designs for HELs. Originator supplied keywords: High energy lasers; Laser propagation; Beam quality; Diffraction; Laser aperture; Pupil function; Thermal blooming, US Army Tradoc, Operations Research Activity, http://archive.org/details/laserpropagation00rock, MLPR# TRASANA 5029, NA

Density gradients, which refract laser light within the cavity, degrade beam quality. In addition to wall influences and viscous effects which cause density gradients, there is another mechanism. This mechanism, which is due to wakes and compression waves from heat (vibration energy to translation and rotation) addition in a supersonic stream, appears to have been overlooked. This appropriate equation is stated and discussed. A semigraphical solution procedure is outlined. Contours of constant density have been calculated for circular and rectangular cavities. Graphs of the isodensity contours are given. (Author), http://archive.org/details/densityinhomogen00fuhs, NA

Thesis (Ph. D.)--University of Rochester. Institute of Optics, 2015., The performance of high-power broad-area diode lasers is inhibited by beam filamentation induced by free-carrier-based self-focusing. The resulting beam degradation limits their usage in high-brightness, high-power applications such as pumping fiber lasers, and laser cutting, welding, or marking. Finite-difference propagation method simulations via RSoft’s BeamPROP commercial simulation suite and a custom-built MATLAB code were used for the study and design of laser cavities that suppress or avoid filamentation. BeamPROP was used to design a tapered, passive, multi-mode interference cavity for the creation of a self-phase-locking laser array, which is comprised of many single-mode gain elements coupled to a wide output coupler to avoid damage from local high optical intensities. MATLAB simulations were used to study the effects of longitudinal and lateral cavity confinement on lateral beam quality in conventional broad-area lasers. This simulation was expanded to design a laser with lateral gain and index prescription that is predicted to operate at or above state-of-the-art powers while being efficiently coupled to conventional telecom single-mode optical fibers. Experimentally, a commercial broad-area laser was coupled in the far-field to a single-mode fiber Bragg grating to provide grating-stabilized single-mode laser feedback resulting in measured spectral narrowing for efficient pump absorption. Additionally a 19 GHz-span, spatially resolved, self-heterodyne measurement was made of a broad-area laser to study the evolution/devolution of the mode content of the emitted laser beam with increasing power levels.

A model for calculating the index of refraction of atomic species present in the cavity of a Xenon Fluoride Laser is applied to Neon and Neon*(3s) . The model considers the variation from unity to be a function of the absorption cross section. Below the ionization threshold, the cross section is a set of discrete transitions between the various energy levels of the specie of concern. Above the 5p level, the discrete transitions become indistinct and the continuum cross section function is extended into this region. The continuum cross section function is a polynomial function fitted to independent research results. The results for Neon I agree with previous theoretical and experimental results. The model is extended to Neon*(3s). Comparison with the results of independent research of the polarizability and cavity phase shift derived from the calculation indicate accuracy is within ten percent., Defense Advanced Research Projects Agency, ARPA Order Number 3747, N0001479WR90174, Proj Elem 62301E Program Code 9E20, http://archive.org/details/asimplemodelforc1094518865, Defense Advanced Research Projects Agency, ARPA Order Number 3747, N0001479WR90174, Proj Elem 62301E Program Code 9E20, Captain, United States Army, Approved for public release; distribution is unlimited.

The main motivation behind this work was to answer the following question: what is the impact of unintentional external optical feedback on laser performance. During the course of this PhD, however, two more questions were raised: how does the spectral external feedback (i.e. intentional feedback from a grating) enhances the spectral performance of the laser? And why does the beam quality of Distributed Bragg Reflector (DBR) tapered lasers degrade at >2.5 times the threshold current? To answer these two questions, new simulations were performed and more results were obtained. The impact of external feedback can be positive or negative. High-brightness lasers are mostly characterised under stand-alone conditions. Nevertheless, high-brightness lasers are almost always operated in external cavity configurations. The external cavity will produce reflections, whether they are intentional reflections off mirrors or gratings, or unintentional reflections off the optics elements. In order to have an accurate prediction of how the high-brightness laser will perform in an external optical system, a modelling tool, which is capable of self-consistently modelling the laser cavity and the fields propagating in the external optical system, is needed. In this work, an external cavity laser simulation tool was developed. This tool consists of an in-house 2.5D laser modelling tool, Speclase; a commercial coherent ray-tracing software, OpticStudio; and an interface software to bidirectionally couple the two different light modelling algorithms used by these tools: finite difference beam propagation (FD-BPM) method and coherent ray-tracing. The author was responsible for: developing the interface software – using Matlab - between Speclase and OpticStudio. Also, the author cooperated with Dr. Kaunga-Nyirenda to modify Speclase so that the coupling between Speclase and OpticStudio modelled the physics correctly. This way the software was able to produce external cavity designs to study