Engineering topics

Ashley, Paul R.

Publications and source records attributed to Ashley, Paul R..

9 records

NASA NTRS2004

Modeling of Electro Optic Polymer Electrical Characteristics in a 3 layer Optical Waveguide Modulator

The electrical characteristics of electro optic polymer waveguide modulators are often described by the bulk reactance of the individual layers. However, the resistance and capacitance between the layers can significantly alter the electrical performance of a waveguide modulator. These interface characteristics are related to the boundary charge density and are strongly affected by the adhesion of the layers in the waveguide stack. An electrical reactance model has been derived to investigate this phenomenon at low frequencies. The model shows the waveguide stack frequency response has no limiting effects below the microwave range and that a true DC response requires a stable voltage for over 1000 hours. Thus, reactance of the layers is the key characteristic of optimizing the voltage across the core layer, even at very low frequencies (> 10(exp -6) Hz). The results of the model are compared with experimental data for two polymer systems and show quite good correlation.

Watson, Michael D.↗

NASA NTRS2004

Modeling of Optical Waveguide Poling and Thermally Stimulated Discharge (TSD) Charge and Current Densities for Guest/Host Electro Optic Polymers

A charge density and current density model of a waveguide system has been developed to explore the effects of electric field electrode poling. An optical waveguide may be modeled during poling by considering the dielectric charge distribution, polarization charge distribution, and conduction charge generated by the poling field. These charge distributions are the source of poling current densities. The model shows that boundary charge current density and polarization current density are the major source of currents measured during poling and thermally stimulated discharge These charge distributions provide insight into the poling mechanisms and are directly related to E(sub A), and, alpha(sub r). Initial comparisons with experimental data show excellent correlation to the model results.

Watson, Michael D.↗

NASA NTRS2004

Space Application Requirements for Organic Avionics

The NASA Marshall Space Flight Center is currently evaluating polymer based components for application in launch vehicle and propulsion system avionics systems. Organic polymers offer great advantages over inorganic corollaries. Unlike inorganics with crystalline structures defining their sensing characteristics, organic polymers can be engineered to provide varying degrees of sensitivity for various parameters including electro-optic response, second harmonic generation, and piezoelectric response. While great advantages in performance can be achieved with organic polymers, survivability in the operational environment is a key aspect for their practical application. The space environment in particular offers challenges that must be considered in the application of polymer based devices. These challenges include: long term thermal stability for long duration missions, extreme thermal cycling, space radiation tolerance, vacuum operation, low power operation, high operational reliability. Requirements for application of polymer based devices in space avionics systems will be presented and discussed in light of current polymer materials.

Watson, Michael D.↗

NASA NTRS2004

Fabrication and Testing of Binary-Phase Fourier Gratings for Nonuniform Array Generation

This effort describes the fabrication and testing of binary-phase Fourier gratings designed to generate an incoherent array of output source points with nonuniform user-defined intensities, symmetric about the zeroth order. Like Dammann fanout gratings, these binary-phase Fourier gratings employ only two phase levels to generate a defined output array. Unlike Dammann fanout gratings, these gratings generate an array of nonuniform, user-defined intensities when projected into the far-field regime. The paper describes the process of design, fabrication, and testing for two different version of the binary-phase grating; one designed for a 12 micron wavelength, referred to as the Long-Wavelength Infrared (LWIR) grating, and one designed for a 5 micron wavelength, referred to as the Mid-Wavelength Infrared Grating (MWIR).

Keys, Andrew S.↗

NASA NTRS2003

Binary-Phase Fourier Gratings for Nonuniform Array Generation

We describe a design method for a binary-phase Fourier grating that generates an array of spots with nonuniform, user-defined intensities symmetric about the zeroth order. Like the Dammann fanout grating approach, the binary-phase Fourier grating uses only two phase levels in its grating surface profile to generate the final spot array. Unlike the Dammann fanout grating approach, this method allows for the generation of nonuniform, user-defined intensities within the final fanout pattern. Restrictions governing the specification and realization of the array's individual spot intensities are discussed. Design methods used to realize the grating employ both simulated annealing and nonlinear optimization approaches to locate optimal solutions to the grating design problem. The end-use application driving this development operates in the near- to mid-infrared spectrum - allowing for higher resolution in grating specification and fabrication with respect to wavelength than may be available in visible spectrum applications. Fabrication of a grating generating a user-defined nine spot pattern is accomplished in GaAs for the near-infrared. Characterization of the grating is provided through the measurement of individual spot intensities, array uniformity, and overall efficiency. Final measurements are compared to calculated values with a discussion of the results.

Keys, Andrew S.↗

NASA NTRS1998

Measuring In-Plane Displacements with Variable Sensitivity Using Diffractive Optic Interferometry

This paper introduces a method called diffractive optic interferometry (DOI) which allows in-plane displacement components to be measured with variable sensitivity. DOI relies on binary optical elements fabricated as phase-type Dammann gratings which produce multiple diffraction orders of nearly equal intensity. Sensitivity is varied by combining the different wavefronts produced by a conjugate pair of these binary optical elements; a transmission element is used to produce several illumination beams while a reflective element, replicated on the surface of a specimen, provides the reference for the undeformed state. The steps taken to design and fabricate these binary optical elements are described. The specimen grating is characterized, and tested on a disk subjected to diametrical compression. Overall, the results are excellent, with experimental data agreeing to within a few percent of the theoretical predictions.

Shepherd, Robert L.↗

NASA NTRS1998

High Efficiency Binary Blazed Grating Waveguide Couplers

Binary blazed gratings are investigated as highly efficient waveguide couplers. Equations are derived for the design of efficient binary blazed grating waveguide couplers. The design approach relates waveguide blazed grating equations to Artificial Index Grating (AIG) equations to emulate a blazed grating. Using these relationships, binary blazed gratings can be accurately designed to output a single mode at a desired output angle. Binary blazed grating couplers can achieve single mode cladding output without substrate" radiation output modes. Much higher output angles can be achieved than with rectangular grating couplers. The use of the AIG grating structure simplifies fabrication approaches. Waveguide couplers were designed using these equations.

Watson, Michael D.↗

NASA NTRS1998

Optical Waveguide Output Couplers Fabricated in Polymers

Waveguide output couplers fabricated in Norland Optical Adhesive (NOA) #81 and AMOCO Ultradel 9020D polyimide are investigated. The output couplers are implemented using periodic relief gratings on a planar waveguide. Design theory of the couplers is based on the perturbation approach. Coupling of light from waveguide propagation modes to output radiation modes is described by coupled mode theory and the transmission line approximation of the perturbed area (grating structure). Using these concepts, gratings can be accurately designed to output a minimum number of modes at desired output angles. Waveguide couplers were designed using these concepts. These couplers were fabricated and analyzed for structural accuracy, output beam accuracy, and output efficiency. The results for the two different materials are compared.

Watson, Michael D.↗

NASA NTRS1996

Nonlinear Optical Properties of Organic and Polymeric Thin Film Materials of Potential for Microgravity Processing Studies

In this paper, we will take a closer look at the state of the art of polydiacetylene, and metal-free phthalocyanine films, in view of the microgravity impact on their optical properties, their nonlinear optical properties and their potential advantages for integrated optics. These materials have many attractive features with regard to their use in integrated optical circuits and optical switching. Thin films of these materials processed in microgravity environment show enhanced optical quality and better molecular alignment than those processed in unit gravity. Our studies of these materials indicate that microgravity can play a major role in integrated optics technology. Polydiacetylene films are produced by UV irradiation of monomer solution through an optical window. This novel technique of forming polydiacetylene thin films has been modified for constructing sophisticated micro-structure integrated optical patterns using a pre-programmed UV-Laser beam. Wave guiding through these thin films by the prism coupler technique has been demonstrated. The third order nonlinear parameters of these films have been evaluated. Metal-free phthalocyanine films of good optical quality are processed in our laboratories by vapor deposition technique. Initial studies on these films indicate that they have excellent chemical, laser, and environmental stability. They have large nonlinear optical parameters and show intrinsic optical bistability. This bistability is essential for optical logic gates and optical switching applications. Waveguiding and device making investigations of these materials are underway.

Abdeldayem, Hossin↗