Observation of a cascaded process in intracavity terahertz optical parametric oscillators based on lithium niobate

Cascaded difference frequency generation has been observed in intracavity optical parametric oscillators based on bulk lithium niobate and producing nanosecond pulses of terahertz radiation. Two idler waves are generated, namely: the primary idler wave associated with the parametric down conversion process itself; and a secondary idler wave, due to difference frequency generation. Experimental investigations of the frequency, temporal evolution, propagation direction, intensity, phase matching and oscillation threshold of the generated down-converted waves are reported. The overall generation efficiency for the terahertz radiation is enhanced, thereby overcoming the Manley-Rowe limit. Advantages of the present approach over schemes based on periodically poled lithium niobate are identified.Publisher PDFPeer reviewe

Network ST radar and related measurements at Pennsylvania State University

Mesoscale meteorological measurements, analysis and prediction are some of the principal areas of research in the Department of Meteorology at Penn State. In anticipation of a staged turn-on of the three systems during the Summer and Fall of 1984, the nonconstruction-related efforts have focused on the software development necessary to allow essentially immediate use of network data. A 16-bit microcomputer has been programmed to serve as the network controller, communications interface and, at least for real-time purposes, the operational display system. Insofar as possible we have in this task built upon our substantial accumulated experience in working with the processing and display of Doppler sodar system signals. Once the radar-derived wind and turbulence profiles are communicated to the various interconnected Departmental computers they become just one component of a comprehensive data base which can be applied to a diverse set of ongoing basic and operational research programs

The influence of MRI scan position on patients with oropharyngeal cancer undergoing radical radiotherapy

<p>Background: The purpose of this study was to demonstrate how magnetic resonance imaging (MRI) patient position protocols influence registration quality in patients with oropharyngeal cancer undergoing radical radiotherapy and the consequences for gross tumour volume (GTV) definition and radiotherapy planning.</p> <p>Methods and materials: Twenty-two oropharyngeal patients underwent a computed tomography (CT), a diagnostic MRI (MRID) and an MRI in the radiotherapy position within an immobilization mask (MRIRT). Clinicians delineated the GTV on the CT viewing the MRID separately (GTVC); on the CT registered to MRID (GTVD) and on the CT registered to MRIRT (GTVRT). Planning target volumes (PTVs) were denoted similarly. Registration quality was assessed by measuring disparity between structures in the three set-ups. Volumetric modulated arc therapy (VMAT) radiotherapy planning was performed for PTVC, PTVD and PTVRT. To determine the dose received by the reference PTVRT, we optimized for PTVC and PTVD while calculating the dose to PTVRT. Statistical significance was determined using the two-tailed Mann–Whitney or two-tailed paired student t-tests.</p> <p>Results: A significant improvement in registration accuracy was found between CT and MRIRT versus the MRID measuring distances from the centre of structures (geometric mean error of 2.2 mm versus 6.6 mm). The mean GTVC (44.1 cm3) was significantly larger than GTVD (33.7 cm3, p value = 0.027) or GTVRT (30.5 cm3, p value = 0.014). When optimizing the VMAT plans for PTVC and investigating the mean dose to PTVRT neither the dose to 99% (58.8%) nor 95% of the PTV (84.7%) were found to meet the required clinical dose constraints of 90% and 95% respectively. Similarly, when optimizing for PTVD the mean dose to PTVRT did not meet clinical dose constraints for 99% (14.9%) nor 95% of the PTV (66.2%). Only by optimizing for PTVRT were all clinical dose constraints achieved.</p> <p>Conclusions: When oropharyngeal patients MRI scans are performed in the radiotherapy position there are significant improvements in CT-MR image registration, target definition and PTV dose coverage.</p&gt

Detection of a Moving Rigid Solid in a Perfect Fluid

In this paper, we consider a moving rigid solid immersed in a potential fluid. The fluid-solid system fills the whole two dimensional space and the fluid is assumed to be at rest at infinity. Our aim is to study the inverse problem, initially introduced in [3], that consists in recovering the position and the velocity of the solid assuming that the potential function is known at a given time. We show that this problem is in general ill-posed by providing counterexamples for which the same potential corresponds to different positions and velocities of a same solid. However, it is also possible to find solids having a specific shape, like ellipses for instance, for which the problem of detection admits a unique solution. Using complex analysis, we prove that the well-posedness of the inverse problem is equivalent to the solvability of an infinite set of nonlinear equations. This result allows us to show that when the solid enjoys some symmetry properties, it can be partially detected. Besides, for any solid, the velocity can always be recovered when both the potential function and the position are supposed to be known. Finally, we prove that by performing continuous measurements of the fluid potential over a time interval, we can always track the position of the solid.Comment: 19 pages, 14 figure

Wind measurement system

A system for remotely measuring vertical and horizontal winds present in discrete volumes of air at selected locations above the ground is described. A laser beam is optically focused in range by a telescope, and the output beam is conically scanned at an angle about a vertical axis. The backscatter, or reflected light, from the ambient particulates in a volume of air, the focal volume, is detected for shifts in wavelength, and from these, horizontal and vertical wind components are computed