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Nelson, Thomas R.

Publications and source records attributed to Nelson, Thomas R..

10 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Global Futures: The Future of Nonrenewable Resource Security

Nonrenewable resources are distributed unevenly throughout the world. Throughout history, the ability to access nonrenewable resources has been a source of geopolitical tension. Modern civilization is increasingly dependent on unevenly distributed resources that are traded globally. This cooperation has helped to increase resource security by increasing affordable access to resources around the world. The continued accessibility of nonrenewable resources is the key uncertainty to security over the next 15 to 25 years. Increased global demands for nonrenewable resources, fragility to resource shocks, and geopolitical upheavals may threaten nonrenewable resource security. A reduction in resource security could exacerbate defense and economic vulnerabilities to resource shocks, and increased fears of resource insecurity could drive conflict over resources leading to geopolitical upheavals. National security organizations can help build resource security by making investments that create resilience to resource shocks. They can also promote international cooperation that builds trust. Together, increased resilience to shocks and increased global trust should help reduce fears of resource insecurity and expand global cooperation on resource issues, thereby bolstering resource security.

29 ENERGY PLANNING, POLICY, AND ECONOMY↗
DOE OSTI2017

Technology Empowerment: Security Challenges.

“Technology empowerment” means that innovation is increasingly accessible to ordinary people of limited means. As powerful technologies become more affordable and accessible, and as people are increasingly connected around the world, ordinary people are empowered to participate in the process of innovation and share the fruits of collaborative innovation. This annotated briefing describes technology empowerment and focuses on how empowerment may create challenges to U.S. national security. U.S. defense research as a share of global innovation has dwindled in recent years. With technology empowerment, the role of U.S. defense research is likely to shrink even further while technology empowerment will continue to increase the speed of innovation. To avoid falling too far behind potential technology threats to U.S. national security, U.S. national security institutions will need to adopt many of the tools of technology empowerment.

99 GENERAL AND MISCELLANEOUS↗
NASA NTRS2001

Characterization of a Multilayered Dielectric Transmissive Phase Modulator

We describe a multilayered dielectric stack configuration designed specifically for use as a transmissive phase modulator for broadband optical signals. Applications for this device range from full aperture wavefront correction to nonmechanical beam steering arrays for free space optical communication links. Our implementation employs alternating GaAs and AlAs layers of varying thickness on a GaAs substrate to create a bandpass region of high average transmission centered about the one micrometer wavelength. Within this transmission bandpass, the phase component of the complex transmission coefficient varies in a near-linear fashion with respect to wavelength. The transmission bandpass is designed to have a bandwidth of 21.0 nm (or 6.3THz frequency bandwidth) and to have an edge-to-edge phase change of greater than 47T radians. Modification of the stack materials' optical properties causes the transmission profile to shift spectrally, resulting in a phase modulation for bands of transmitted frequencies. Our broadband phase modulator imparts up to a full-cycle of phase modulation with low loss and low group velocity dispersion. We identify several methods for implementing the requisite modulation, including refractive index modulation through free carrier injection and optical path length modulation through variation in angle of incidence. At least one sample comprising 91 alternating layers has been fabricated to exhibit the bandpass properties required for optical signal phase modulation. We experimentally characterize the sample using an interferometer and spectrometer to measure the transmitted signal spectrum and relative phase modulation. We compare the experimental data to computational predictions and discuss the results.

Keys, Andrew S.↗
NASA NTRS1999

Resonant Transmissive Modulator Construction for Use in Beam Steering Array

An agile electronically configurable optical beam steering device for use in directional free-space transmission of optical signals is described. The device design employs an array of tunable resonant transmissive modulators constructed from multi-layered stacks of electro-optically active dielectric materials. Each modulator may be individually configured to transmit an optical signal with a specific amount of phase and group velocity modulation. The resulting diffractive interactions between modulator outputs provide a method for directional optical signal transmission. Of particular focus within this presentation is the optimized design of the individual modulator. It is widely known that periodic multi-layered stacks of dielectric materials exhibit reflective and transmissive resonances as described by the stack's transmission function. If an electric field is applied normal to the stack's layers, phenomena such as the electro-optic effect and/or the quantum-confined Stark effect may be employed to change the effective refractive index within the stack. This refractive index change alters the stack's transmission function and modulates the phase and group velocity of the transmitted optical signal. By varying the parameters that describe the stack's specific construction and operation, such as number of layers within the stack, refractive indexes of stack materials, layer thicknesses, and combinations of periodic verses non-periodic layer repetitions, custom transmittance functions may be generated. The transmitted optical signal carrier frequency and bandwidth must be carefully selected to maximize transmission and to minimize absorption. A computational optimization of the variables describing the stack's construction strives to maximize the amount of optical signal modulation verses applied voltage. Trade-offs between methods of increasing device performance verses the limitations of fabrication technologies are considered.

Keys, Andrew S.↗