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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS2008

Dispersion-Enhanced Laser Gyroscope

We analyze the effect of a highly dispersive element placed inside a modulated optical cavity on the frequency and amplitude of the output modulation to determine the conditions for enhanced gyroscopic sensitivities. The element is treated as both a phase and amplitude filter, and the time-dependence of the cavity field is considered. Both atomic gases (two-level and multi-level) and optical resonators (single and coupled) are considered and compared as dispersive elements. We find that it is possible to simultaneously enhance the gyro scale factor sensitivity and suppress the dead band by using an element with anomalous dispersion that has greater loss at the carrier frequency than at the side-band frequencies, i.e., an element that simultaneously pushes and intensifies the perturbed cavity modes, e.g. a two-level absorber or an under-coupled optical resonator. The sensitivity enhancement is inversely proportional to the effective group index, becoming infinite at a group index of zero. However, the number of round trips required to reach a steady-state also becomes infinite when the group index is zero (or two). For even larger dispersions a steady-state cannot be achieved, and nonlinear dynamic effects such as bistability and periodic oscillations are predicted in the gyro response.

Smith, David D.↗
NASA NTRS2012

Theory of a Traveling Wave Feed for a Planar Slot Array Antenna

Planar arrays of waveguide-fed slots have been employed in many radar and remote sensing applications. Such arrays are designed in the standing wave configuration because of high efficiency. Traveling wave arrays can produce greater bandwidth at the expense of efficiency due to power loss in the load or loads. Traveling wave planar slot arrays may be designed with a long feed waveguide consisting of centered-inclined coupling slots. The feed waveguide is terminated in a matched load, and the element spacing in the feed waveguide is chosen to produce a beam squinted from the broadside. The traveling wave planar slot array consists of a long feed waveguide containing resonant-centered inclined coupling slots in the broad wall, coupling power into an array of stacked radiating waveguides orthogonal to it. The radiating waveguides consist of longitudinal offset radiating slots in a standing wave configuration. For the traveling wave feed of a planar slot array, one has to design the tilt angle and length of each coupling slot such that the amplitude and phase of excitation of each radiating waveguide are close to the desired values. The coupling slot spacing is chosen for an appropriate beam squint. Scattering matrix parameters of resonant coupling slots are used in the design process to produce appropriate excitations of radiating waveguides with constraints placed only on amplitudes. Since the radiating slots in each radiating waveguide are designed to produce a certain total admittance, the scattering (S) matrix of each coupling slot is reduced to a 2x2 matrix. Elements of each 2x2 S-matrix and the amount of coupling into the corresponding radiating waveguide are expressed in terms of the element S11. S matrices are converted into transmission (T) matrices, and the T matrices are multiplied to cascade the coupling slots and waveguide sections, starting from the load end and proceeding towards the source. While the use of non-resonant coupling slots may provide an additional degree of freedom in the design, resonant coupling slots simplify the design process. The amplitude of the wave going to the load is set at unity. The S11 parameter, r of the coupling slot closest to the load, is assigned an arbitrary value. A larger value of r will reduce the power dissipated in the load while increasing the reflection coefficient at the input port. It is now possible to obtain the excitation of the radiating waveguide closest to the load and the coefficients of the wave incident and reflected at the input port of this coupling slot. The next coupling slot parameter, r , is chosen to realize the excitation of that radiating waveguide. One continues this process moving towards the source, until all the coupling slot parameters r and hence the S11 parameter of the 4-port coupler, r, are known for each coupling slot. The goal is to produce the desired array aperture distribution in the feed direction. From an interpolation of the computed moment method data for the slot parameters, all the coupling slot tilt angles and lengths are obtained. From the excitations of the radiating waveguides computed from the coupling values, radiating slot parameters may be obtained so as to attain the desired total normalized slot admittances. This process yields the radiating slot parameters, offsets, and lengths. The design is repeated by choosing different values of r for the last coupling slot until the percentage of power dissipated in the load and the input reflection coefficient values are satisfactory. Numerical results computed for the radiation pattern, the tilt angles and lengths of coupling slots, and excitation phases of the radiating waveguides, are presented for an array with uniform amplitude excitation. The design process has been validated using computer simulations. This design procedure is valid for non-uniform amplitude excitations as well.

Rengarajan, Sembiam↗
NASA NTRS2003

Coherent Effects in Tiny Optics: Tunneling Through the Looking Glass

I will discuss two types of one-dimensional photonic bandgap (PBG) effects that can arise in systems of coupled spherical resonators: (1) nearly-free-photon Fabry-Perot photonic bands that arise in quarter-wave concentrically stratified spheres and, (2) tight- binding photonic bands that arise in weakly-coupled mutually-resonant spheres as a result of whispering-gallery mode splitting. These effects can be derived directly from Mie theory, in a more straightforward manner, by exploiting an analogy with stratified planar systems. For odd numbers of mutually-resonant lossless coupled ring resonators, the circulating intensity can increase exponentially with the number of resonators, which can potentially be exploited for the development of advanced sensors. For even numbers of resonators, mode splitting and classical destructive interference lead to a cancellation of absorption and slow light on-resonance, reminiscent of electromagnetic induced transparency. The analogy between these coherent photon trapping effects and population trapping in an atomic system will be explored.

Smith, David D.↗
NASA NTRS2006

Optical filter having coupled whispering-gallery-mode resonators

Optical filters having at least two coupled whispering-gallery-mode (WGM) optical resonators to produce a second order or higher order filter function with a desired spectral profile. At least one of the coupled WGM optical resonators may be tunable by a control signal to adjust the filtering function.

Savchenkov, Anatoliy↗
NASA NTRS1988

Design of a simple active controller to suppress helicopter air resonance

A coupled rotor/fuselage helicopter analysis with the important effects of blade torsional flexibility, unsteady aerodynamics, and forward flight is presented. Using this mathematical model, a nominal configuration is selected that experiences an air resonance instability throughout most of its flight envelope. A simple multivariable compensator using conventional swashplate inputs and a single body roll rate measurement is then designed. The controller design is based on a linear estimator in conjunction with optimal feedback gains, and the design is done in the frequency domain using the Loop Transfer Recovery method. The controller is shown to suppress the air resonance instability throughout wide range helicopter loading conditions and forward flight speeds.

Takahashi, M. D.↗
NASA NTRS1982

Development and validation of preliminary analytical models for aircraft interior noise prediction

Predictions are made of the transmission of sound into an unpressurized and unstiffened cylinder under random and harmonic excitations, in order to validate the preliminary version of an airplane interior noise prediction model which is based on an analysis of the power flow type. The predictions are compared to experimental results, and statistically significant differences between predictions and measurements are found to be primarily due to input data deficiencies. Because the model requires only the value of a simple integral for determining interior spatial coupling, and resonance frequencies to determine frequency coupling, the known dynamics of the fuselage can be utilized without difficulty.

Pope, L. D.↗
NASA NTRS2009

Critical Coupling Between Optical Fibers and WGM Resonators

Two recipes for ensuring critical coupling between a single-mode optical fiber and a whispering-gallery-mode (WGM) optical resonator have been devised. The recipes provide for phase matching and aperture matching, both of which are necessary for efficient coupling. There is also a provision for suppressing intermodal coupling, which is detrimental because it drains energy from desired modes into undesired ones. According to one recipe, the tip of the single-mode optical fiber is either tapered in diameter or tapered in effective diameter by virtue of being cleaved at an oblique angle. The effective index of refraction and the phase velocity at a given position along the taper depend on the diameter (or effective diameter) and the index of refraction of the bulk fiber material. As the diameter (or effective diameter) decreases with decreasing distance from the tip, the effective index of refraction also decreases. Critical coupling and phase matching can be achieved by placing the optical fiber and the resonator in contact at the proper point along the taper. This recipe is subject to the limitation that the attainable effective index of refraction lies between the indices of refraction of the bulk fiber material and the atmosphere or vacuum to which the resonator and fiber are exposed. The other recipe involves a refinement of the previously developed technique of prism coupling, in which the light beam from the optical fiber is collimated and focused onto one surface of a prism that has an index of refraction greater than that of the resonator. Another surface of the prism is placed in contact with the resonator. The various components are arranged so that the collimated beam is focused at the prism/resonator contact spot. The recipe includes the following additional provisions:

Matsko, Andrey↗
NASA NTRS1993

Lumped elements characterize Q in dielectric resonators

It has been earlier observed (Podcameni et al., 1981) that, as the coupling factor between a microstrip-coupled dielectric resonator and the line becomes much larger than unity, the unloaded quality factor (Q) of the resonator decreases. In this paper it is shown that this effect can be explained using lumped-element models of the coupling line, when the dielectric resonator is either overcoupled or undercoupled to the line.

Hearn, Chase P.↗
NASA NTRS1972

Theory of two coupled lasers.

A transmitting window is used to weakly couple two resonant optical cavities. When active medium is placed in one cavity, the system describes a laser coupled to a resonant cavity, while active medium in both cavities describes two coupled lasers. Equations of motion for the fields in the two cavities are found. These equations are solved in the steady state when the cavities are sufficiently closely tuned that the lasers in the two cavities oscillate with the same frequency. In the locked situation, the second cavity tends to stabilize the frequency of operation against changes in length of the first cavity. When there is no active medium in the second cavity, and that cavity becomes very long, the laser acts as if it were looking into free space.

Spencer, M. B.↗
NASA NTRS1979

Efficiency enhancement of coupled-cavity TWT's through cavity resonance tapering

The paper examines efficiency enhancement of coupled-cavity traveling-wave tube (TWT) through cavity resonance tapering. Beam-wave resynchronization through circuit velocity reduction is used for TWT efficiency enhancement, with circuit velocity reduction in coupled cavity TWT's accomplished through period tapering. However, the amount of the latter is limited by the stability considerations, so that beyond a critical value of velocity reduction, the tube may be subject to zero drive oscillations originating in the velocity taper region. The coupled-cavity resonance tapering allows the velocity reduction to continue beyond the limit of stable period tapering, and it is accomplished by a gradual reduction in the cavity resonance frequency, with the period and the circuit bandwidth unchanged. The advantages of cavity resonance tapering vs period tapering are discussed, and test data are presented with the results of large-signal computer calculations. It is shown that cavity resonance tapering can produce efficiencies as period tapering without incurring the same risk of lower band-edge oscillations.

Connolly, D. J.↗
NASA NTRS1995

Optimization of Coherent Lidar Performance Using Graded- Reflectance Transmitter Resonator Optics in the Low Equivalent Fresnel Number Region

Using a diffractive eigenmode treatment to model the laser output, we show that graded-reflectance resonator optics offer significant efficiency benefits over conventional hard-edge coupled unstable resonators in the context of coherent detection lidar applications. Extending previous work pertinent to the high equivalent Fresnel number regime, we have modelled the optimum performance of a notional super-Gaussian coupled cavity as a function of the key resonator parameters in the low equivalent Fresnel number regime.

lidar graded-reflectance transmitter resonator↗