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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS2017

Dynamic Control of Refractive Index During Pulsed-Laser-Deposited Waveguide Growth

Engineering the optical properties of waveguides is important for the optimization of their guided optical mode characteristics. Here, we document the dynamic control of the refractive index and composition of crystalline films, via the substrate temperature, during pulsed-laser-deposition growth of Er(1%)-doped yttrium gallium garnet on <100>-orientated single-crystal yttrium aluminium garnet. An increasing substrate temperature is observed to reduce the gallium content in the grown film, with a corresponding reduction of refractive index. We demonstrate the ability to accurately control the refractive index via this technique and use it to grow a complex multi-core crystal waveguide. Our results highlight the potential of using pulsed laser deposition to fabricate crystal films with bespoke optical properties and thus engineer passive and active waveguide devices in situ.

Temperature↗
NASA NTRS1995

Integrated back to back barrier-N-N(+) varactor diode tripler using a split-waveguide block

The back-to-back barrier-N-N(+) (bbBNN) varactor is a nonlinear device being developed for frequency multiplier applications above 100 GHz. Its symmetrical C-V characteristic, low series resistance, freedom from external bias and suitability to planarization make it an ideal choice for high frequency, low power, odd harmonic generation. In this paper, the performance of a 220-GHz waveguide tripler using, for the first time, a planar GaAs bbBNN device integrated on a quartz microstrip circuit is presented. A new split-waveguide block design has been employed to provide the proper embedding impedances to the device at the input and third harmonic output frequencies. A flange-to-flange tripling efficiency of 7% has been obtained at 220 GHz with an output power in excess of 700 mu W. This is believed to be the highest conversion efficiency yet reported for a tripler with an integrated device at this frequency. Theoretical calculations indicate that substantial improvement is possible with modest changes to the device and circuit parameters.

QUARTZ MICROSTRIP CIRCUITS↗
NASA NTRS2014

A Novel Multimode Waveguide Coupler for Accurate Power Measurement of Traveling Wave Tube Harmonic Frequencies

This paper presents the design, fabrication and test results for a novel waveguide multimode directional coupler (MDC). The coupler fabricated from two dissimilar waveguides is capable of isolating the power at the second harmonic frequency from the fundamental power at the output port of a traveling-wave tube (TWT). In addition to accurate power measurements at harmonic frequencies, a potential application of the MDC is in the design of a beacon source for atmospheric propagation studies at millimeter-wave frequencies.

harmonic frequencies↗
NASA NTRS1969

Optical frequency waveguide and ion transmission system

Electromagnetically generated, high-dielectric tube forms a waveguide which retains the electromagnetic energy within the beam, the trapped beam establishes an optical frequency waveguide appropriate for its own conduction with minimum diffraction loss.

Chiao, R. Y.↗
NASA NTRS1970

Quasi-optical equivalent of waveguide slide screw tuner

Tuner utilizes a metal plated dielectric grid inserted into the cross sectional plane of an oversized waveguide. It provides both variable susceptance and variable longitudinal position along the waveguide to provide a wide matching range.

Kurpis, G. P.↗
NASA NTRS1972

Waveguide switch protector

Device for detecting excessive operation of electric motors used to drive waveguide switches is described. Purpose of device is to prevent burnout of electric motor in event of waveguide stoppage at some point other than extreme limits of travel. Operation of equipment, components used to sense motor performance, and schematic diagram are included.

Kolbly, R. B.↗
NASA NTRS1972

Waveguide excited dielectric spheres as feeds.

The results of experiments with dielectric obstacles of various geometries placed directly upon the apertures of circular and rectangular waveguides are presented. It is found that dielectric spheres, and in some instances dielectric cubes, two to four wavelengths in dimension produce directive patterns with low sidelobe levels. For some cross sections, the measured gain of these antennas is greater (6 dB in some cases) than that produced by optimum horns having apertures of identical cross section. The linearity of the polarization of these antennas is essentially the same as that of an open-end waveguide antenna.

Chatterjee, J. S.↗
NASA NTRS1974

Analysis of finite-size phased arrays of circular waveguide elements

A derivation is presented for the calculation of the interelement mutual coupling in a finite-size planar array of waveguide-fed apertures covered by a multilayered dielectric and/or plasma. The general mutual admittance expression is evaluated for circular apertures and the mutual coupling calculations are verified experimentally for two transverse electric circular waveguide mode excited apertures. A parametric study of higher order mode aperture fields indicates that the only significant change in the circular aperture mutual coupling is due to the transverse magnetic mode, which introduces an additional phase shift. Qualitative agreement between calculations for a 183-element array of circular apertures and an infinite array establishes the validity of the finite-array theoretical model.

Bailey, M. C.↗
NASA NTRS1974

Diffused optical waveguides

Diffused optical waveguides are studied using the modal and the ray tracing approach, with an emphasis on the case of a hollow waveguide with diffuse cladding. The properties of the guided modes are investigated in detail. The applications of these structures to optical circuits, integrated optics lasers and capillary gas lasers are discussed.

Elachi, C.↗
NASA NTRS1974

Hollow-cylinder waveguide isolators for use at millimeter wavelengths

A semiconductor waveguide isolator consisting of a hollow column of a semiconductor mounted coaxially is considered in a circular waveguide in a longitudinal dc magnetic field. An elementary and physical analysis based on the excitation of plane waves in the guide and a more rigorous mode matching analysis are presented. These theoretical predictions are compared with experimental results for an InSb isolator at 94GHz and 75 K.

Kanda, M.↗
NASA NTRS1975

Design and analysis of optically pumped submillimeter waveguide maser amplifiers and oscillators

The design and experimental measurements are described of an optically pumped far-infrared (FIR) waveguide maser; preliminary measurements on a FIR waveguide amplifier are presented. The FIR maser was found to operate satisfactorily in a chopped CW mode using either methanol (CH3OH) or acetonitrile (CH3CN) as the active molecule. Two other gases, difluoroethane and difluoroethylene, produced an unstable output with high threshold and low output power when operated in the chopped CW mode. Experimental measurements include FIR output versus cavity length, output beam pattern, output power versus pressure, and input power. The FIR output was the input to an amplifier which was constructed similar to the oscillator. An increase of 10% in output power was noted on the 118.8 microns line of methanol.

Galantowicz, T. A.↗
NASA NTRS1974

Hollow-cylinder waveguide isolators for use at millimeter wavelengths

The device considered in this study is a semiconductor waveguide isolator consisting of a hollow column of a semiconductor mounted coaxially in a circular waveguide in a longitudinal dc magnetic field. An elementary and physical analysis based on the excitation of plane waves in the guide and a more rigorous mode-matching analysis (MMA) are presented. These theoretical predictions are compared with experimental results for an InSb isolator at 94 GHz and 75 K.

Kanda, M.↗
NASA NTRS1975

Radiation from a homogeneous sphere mounted on a waveguide aperture

The results of an experimental and theoretical study of the effects of placing a dielectric sphere over the aperture of a circular waveguide are presented. A dielectric sphere excited by a Huygens' source is used as the theoretical model to interpret the experimental results. Calculations based on this model show good overall agreement with experimental patterns and directivity measurements. It is concluded that dielectric loaded antennas such as the dielectric sphere excited by a waveguide can produce directivities in excess of that obtained by a uniformly illuminated aperture of the same cross section, particularly for dielectric objects with dimensions of two wavelengths or less. Measurements of the coupling between these antennas show that they, in general, are not as closely coupled as plain apertures without dielectric object loading.

Croswell, W. F.↗
NASA NTRS1976

Diffused waveguiding capillary tube with distributed feedback for a gas laser

For use in a waveguide gas laser, a capillary tube of glass or ceramic has an inner surface defining a longitudinal capillary opening through which the laser gas flows. At least a portion of the inner surface is corrugated with corrugations or channels with a periodicity Lambda where Lambda = 1/2 Lambda, Lambda being the laser gas wavelength. The tube includes a diffused region extending outwardly from the opening. The diffused region of a depth d on the order of 1 Lambda to 3 Lambda acts as a waveguide for the waves, with the corrugations producing distributed feedback. The evanescent component of the waves traveling in the diffused region interact with the laser gas in the opening, gaining energy, and thereby amplifying the waves travelling in the diffused region, which exit the diffused region, surrounding the opening, as a beam of wavelength Lambda.

Elachi, C.↗
NASA NTRS1976

Waveguide CO2 laser gain - Dependence on gas kinetic and discharge properties

A simple rate-equation approach is used to examine the gas-kinetic and discharge properties of waveguide CO2 lasers. The dependence of the population inversion and laser small-signal gain on gas pressure, gas mixture, pumping rate (discharge current), tube bore diameter, and wall temperature is calculated along with the dependence of laser tunability on the gas-kinetic properties and cavity losses. It is found that for low-loss cavities, the laser tunability may substantially exceed the molecular full width at half-maximum. Furthermore, the more helium-rich gas mixtures give greater tunability when cavity losses are small, and less tunability when cavity losses are large. By contrast with conventional lasers, the waveguide-laser transition is homogeneously broadened. Thus, the dependence of gain on gas pressure and other kinetic properties differs substantially from that predicted by scaling results from conventional low-pressure lasers.

Cohen, S. C.↗