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Hall, D. R.

Publications and source records attributed to Hall, D. R..

5 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Implementation of nonreflective boundary condition at the outflow boundary

Numerical techniques for the solution of free surface flows about a disturbance on or near a free surface were developed. Any disturbance moving on or near a liquid surface generates surface waves that propagate away from the disturbance. The potential flow problems in which the evolution of free surface waves is the principal phenomenon of interest are considered. The distinct feature of these problems is that the position of the surface is also unknown and is to be determined as part of the solution. Two boundary conditions are required at the free surface since there are two unknown functions there, the free surface height and the potential function. The required boundary conditions are satisfied by the kinematic condition which states that no flow can cross the surface and the dynamic condition which matches the fluid pressure to the pressure acting on the surface through Bernoulli's equation.

Hall, D. R.↗
NASA NTRS1973

Finite-aperture waveguide-laser resonators.

A general theory of finite-aperture waveguide-laser resonators is developed which represents the external reflectors by matrices that couple linearly polarized waveguide modes having the same azimuthal symmetry. The theory allows the determination of resonator efficiency, resonator frequencies, and laser near- and far-field patterns. Computations of the coupling loss for the fundamental waveguide mode as a function of mirror curvature, separation, and aperture are in good agreement with recent infinite-aperture calculations in the limit of large apertures and indicate three low-loss configurations: (1) large radius of curvature mirrors close to the guide; (2) large radius of curvature mirrors centered at the guide entrance; and (3) generally smaller curvature mirrors separated by half their curvature from the guide entrance.

Degnan, J. J.↗
NASA NTRS1973

Finite aperture waveguide laser resonators

A general theory of finite aperture waveguide laser resonators is developed which represents the external reflectors by matrices which couple linearly polarized waveguide modes having the same azimuthal symmetry. The theory allows the determination of resonator efficiency, resonator frequencies, and laser near and far field patterns. Computations of the coupling loss for the fundamental waveguide mode as a function of mirror curvature, separation, and aperture are in agreement with recent infinite aperture calculations in the limit of large apertures and indicate three low-loss configurations: large radius of curvature mirrors close to the guide; large radius of curvature mirrors centered at the guide entrance; and generally smaller curvature mirrors separated by half their curvature from the guide entrance. Design guidelines for construction of high efficiency CO2, CO and He-Ne waveguide laser resonators are summarized in tabular form.

Degnan, J. J.↗
NASA NTRS1972

Multichannel television coupling modulation experiments using a CO2 laser.

Experiments are described in which a carbon dioxide laser oscillating at 28.4 THz has been modulated simultaneously by three commercial television signals with carriers at 54, 66, and 82 MHz. Signal-to-noise degradation in the system was measured to be less than 1 dB, corresponding to modulator drive power of 1 W. This combination of wide bandwidth and low power consumption was made possible by the application of coupling modulation. This technique employs an intracavity electrooptic crystal to which the modulating fields are applied. Despite the fact that the crystal is positioned inside the laser cavity, the maximum bandwidth attainable is independent of both the cavity Q and the laser transition linewidth, and is limited only by modulator characteristics and optical transit time. Rigrod's (1965) method has been adapted to obtain an expression for the power coupled out of the laser. Modulator frequency response and drive power requirements are also summarized. It is seen that the noise bandwidth performance of the system would allow simultaneous modulation by 17 color television channels, or equivalently, more than a 300-megabit/sec capacity.

Hall, D. R.↗