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At least 1,135 records · Page 63
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

X-ray spectroscopy with silicon pin and avalanche photo diodes

Results of an evaluation of silicon P-Intrinsic-N (PIN) photodiodes and Avalanche Photodiodes (APD) for the direct detection of soft x rays from 1 to 20 keV and for the detection of scintillation light output from CsI(TI) for higher x ray energies (30 to 1000 keV) are presented. About one keV resolution was achieved at room temperature for both the PIN and APD detectors for soft x rays (1 to 20 keV). Commercially available, low power (18 mV), low noise, hybrid preamplifiers, were used. These photodiodes were also coupled to CsI(TI) scintillator and obtained about 6 resolution at 662 keV. The photodiode frequency response matches well with the emission spectrum of the CsI(TI) scintillator providing good spectral resolution and a higher signal than NaI(TI) when viewed by conventional photomultipliers. A PIN-CsI(TI) combination provides a low energy threshold of around 60 keV while for the APD-CsI(TI) it is 15 keV.

Desai, U. D.↗
NASA NTRS1994

HO2 detection by near infrared absorption using tunable diode lasers

The strongest absorption lines are in the v sub 2 vibrational band around 1400/cm; several near-coincident line pairs in this region provide max linestrengths of 1.2 x 10 exp -20 sq. cm/(molecule*cm). High Frequency wavelength modulation coupled with sweep integration were used to minimize noise and maintain spectral discrimination. Reduced pressure sampling was performed using supersonic nozzle and an aerodynamic flow cell to minimize residence time and wall losses of radicals. The astigmatic mirror design provides pathlengths up to 200 m in volume of 3 liters with minimal optical interference fringes; longer pathlengths are possible. The detection limit depends on pathlength and minimum fractional absorptions.

Nelson, David↗
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 NTRS1994

Light emitting diodes as a plant lighting source

Electroluminescence in solid materials is defined as the generation of light by the passage of an electric current through a body of solid material under an applied electric field. A specific type of electroluminescence, first noted in 1923, involves the generation of photons when electrons are passed through a p-n junction of certain solid materials (junction of a n-type semiconductor, an electron donor, and a p-type semiconductor, an electron acceptor). The development of this light emitting semiconductor technology dates back less than 30 years. During this period of time, the LED has evolved from a rare and expensive light generating device to one of the most widely used electronic components. A number of LED characteristics are of considerable importance in selecting a light source for plant lighting in a controlled environment facility. Of particular importance is the characteristic that light is generated by an LED at a rate far greater than the corresponding thermal radiation predicted by the bulk temperature of the device as defined by Plank's radiation law. This is in sharp contrast to other light sources, such as an incandescent or high intensity discharge lamp. A plant lighting system for controlled environments must provide plants with an adequate flux of photosynthetically active radiation, plus providing photons in the spectral regions that are involved in the photomorphogenic and phototropic responses that result in normal plant growth and development. Use of light sources that emit photons over a broad spectral range generally meet these two lighting requirements. Since the LED's emit over specific spectral regions, they must be carefully selected so that the levels of photsynthetically active and photomorphogenic and phototropic radiation meet these plant requirements.

Bula, R. J.↗
NASA NTRS1995

Line-center/side-line diode laser seeding for DIAL measurements of the atmosphere

The ability of DIAL systems to probe deeply into the atmosphere is enhanced if the pulsed laser is capable of sequentially operating at different wavelengths, each wavelength corresponding to a different absorption cross-section of the gas of interest. Such wavelength versatility is particularly important for airborne or spaceborne DIAL measurements of H2O(v) due to its strong vertical gradient (several orders of magnitude). Strong H2O(v) cross-sections are necessary to retrieve H2O(v) data from high altitudes while the use of weaker cross sections enables penetration of the DIAL laser pulses to lower altitudes.

Sachse, Glen↗
NASA NTRS1994

Diode-pumped Ho:Tm:YLF laser pumping and AgGaSe2 parametric oscillator

Tuning of both the pump laser, a Ho:Tm:YLF laser operating on the 5 I 7 - 5 I 8 transition, and an AgGaSe2 parametric oscillator has been demonstrated. Tuning of the Ho:Tm:YLF laser is complicated but not frustrated by the existence of both CO2 and H2O lines in the vicinity of the laser transition. Tuning of the parametric oscillator was achieved by tuning of the pump laser. Injection seeding of the parametric oscillator on the nonresonant signal was also demonstrated. In addition, the measured efficiencies of the parametric oscillator were compared by two different methods, measuring the parametric-oscillator output energy and measuring the energy depleted from the pump. By comparison of these measurements, the intrinsic efficiency of the parametric oscillator cab be determined

Barnes, Norman P.↗
NASA NTRS1994

Measurement of n-type Dry Thermally Oxidized 6H-SiC Metal-oxide Semiconductor Diodes by Quasistatic and High-Frequency Capacitance Versus Voltage and Capacitance Transient Techniques

Dry-oxidized n-type 6H-SiC metal-oxide-semiconductor capacitors are investigated using quasistatic capacitance versus voltage (C-V), high-frequency C-V, and pulsed high-frequency capacitance transient (C-t) analysis over the temperature range from 297 to 573 K. The quasistatic C - V characteristics presented are the first reported for 6H-SiC MOS capacitors, and exhibit startling nonidealities due to nonequilibrium conditions that arise from the fact that the recombination/generation process in 6H-SiC is extraordinarily slow even at the highest measurement temperature employed. The high-frequency dark C-V characteristics all showed deep depletion with no observable hysteresis. The recovery of the high-frequency capacitance from deep depletion to inversion was used to characterize the minority-carrier generation process as a function of temperature. Zerbst analysis conducted on the resulting C-t transients, which were longer than 1000 s at 573 K, showed a generation lifetime thermal activation energy of 0.49 eV.

Neudeck, P.↗
NASA NTRS1997

Investigation of Fiber Optics Based Phased Locked Diode Lasers

Optical power beaming requires a high intensity source and a system to address beam phase and location. A synthetic aperture array of phased locked sources can provide the necessary power levels as well as a means to correct for phase errors. A fiber optic phase modulator with a master oscillator and power amplifier (MOPA) using an injection-locking semiconductor optical amplifier has proven to be effective in correcting phase errors as large as 4pi in an interferometer system. Phase corrections with the piezoelectric fiber stretcher were made from 0 - 10 kHz, with most application oriented corrections requiring only 1 kHz. The amplifier did not lose locked power output while the phase was changed, however its performance was below expectation. Results of this investigation indicate fiber stretchers and amplifiers can be incorporated into a MOPA system to achieve successful earth based power beaming.

Burke, Paul D.↗
NASA NTRS1998

Development of Advanced Laser Diode Sources

The design and operation of InGaAs-GaAs-AlGaAs asymmetric cladding ridge waveguide distributed Bragg reflector lasers is presented. Targeted for the remote sensing of water vapor with absorption lines in the lambda approximately 930 nm region, these devices operate CW with threshold currents as low as 11 MA and slope efficiencies as high as 0.37 W/A. Tbey also operate with over 30-dB side-mode suppression, and the typical CW characteristic temperature, T(sub o), is 95 K.

Coleman, J. J.↗
NASA NTRS1998

development of a medium repetition rate (10 Hz - 500 Hz) diode pumped laser transmitter for airborne scanning altimetry

Since the late 1980's, NASA has developed several small, all-solid state lasers of low repetition rates for use as transmitters in prototype LIDAR and raster scanned altimetry retrieval systems. Our early laser transmitters were developed for high resolution airborne altimetry which employed cavity dumping techniques to produce a pulse shape with a 1 ns rise time. The first such laser was the SUMR (Sub-millimeter resolution) transmitter which used a side pumped, D-shaped half-rod of Nd:YAG for the oscillator active media and produced approximately 3 ns pulses of 100 micro-J energy at a 40 Hz repetition rate. (Coyle and Blair, 1993; Coyle et al., 1995) After several upgrades to improve rep rate and pulse energy, the final version produced 1.2 mJ pulses at 120 Hz with a 3.7 ns pulse width. The laser has become known as SPLT (Sharp Pulsed Laser Transmitter), and has flown successfully on a variety of airborne altimetry missions. (Coyle and Blair, 1995; Blair et al., 1994) From building these systems, we have accrued valuable experience in delivering field-deployable lasers and have become aware of the advantages and disadvantages of employing new technologies. For example, even though the laser's main operating environment is in a "cold" aircraft during flight, the laser must still operate in very warm temperatures. This is important if the mission is based in the desert or a tropical climate since ground calibration data from stationary targets must be gathered before and after each data flight. Because conductive cooling is much more convenient than closed loop water flow, achieving the highest possible laser efficiency is becoming a high priority when designing a flight laser. This is especially true for lasers with higher pulse energies and repetition rates which are needed for high altitude scanning altimeters and LIDARs.

Coyle, D. Barry↗
NASA NTRS1996

Influence of Hot Carrier Transport on the Transient Response of an InGaAs/InAlAs Metal-Semiconductor Schottky Diode Structure

The calculated transient characteristic of a heterostructure, rectifying contact is theoretically examined. It is found that hot carrier transport drastically affects the output terminal characteristics of the heterostructure Schottky contact and, hence, the working of a blocking contact. This is of importance to the understanding of InGaAs MSM devices in particular, as well as any structure which contains a blocking contact in general.

Salem, Ali F.↗
NASA NTRS2001

Predictions of Chemical Species via Diode Laser Spectroscopy

A technique to predict temperature and chemical species in flames from absorbance measurement of one chemical species is presented. Predicted temperature and mole fractions of methane and water agreed well with measured and published results.

Chen, Shin-Juh↗
NASA NTRS1996

Light-Emitting Diodes (LED) for Primary Animal Habitat Lighting in Highly Controlled Environments

Significant alterations in Biological Clock responses have been reported following sidereal time changes (e.g., Jet-lag), and exposure to microgravity (e.g., daytime sleepiness). Additionally, light reduces circulating melatonin (spectral specificity greatest between 450-500 nm). It was hypothesized that LEDs can replace the current light sources used in zero gravity and terrestrial research laboratories because of their small size, low mass, low energy consumption and long functional life. This report evaluates the capacity of LEDs to entrain the circadian system of rats as judged by measurement of overt behavioral circadian rhythms (activity, feeding, drinking). These data were collected in highly controlled environments similar to the shuttle Animal Enclosure Modules. Two groups were compared: control - animals exposed to standard cool-white fluorescent lights, and test - animals exposed to LEDs with a spectral power distribution matching the fluorescent lights. Gross locomotor activity, feeding and drinking frequencies were continuously monitored and stored at 10 minute intervals. Animals were exposed to the following photoperiods: 28 days of 12L:12D, 19 days of 24L:0D and 16 days of 12L:12D. Light intensities tested varied between 0.1 to 100 lux. Rats received food and water ad libitum, and temperature and humidity were controlled throughout the study. The general health status of all rats was acceptable for each day of this study. No incidents of aggressive behavior were observed. Growth, locomotor activity, food and water consumption were comparable for all groups of animals, i.e, the circadian characteristics of the animals under these conditions were comparable. These results indicate that LED arrays are as effective in maintaining circadian rhythm stability as the commonly used cool-white fluorescent light sources. LEDs with their flexible spectrum, low energy requirements and minimal heat production have advantages for some chronopharmacology studies and for microgravity animal habitats.

Winget, C. M.↗
NASA NTRS2001

Experimental Investigation of Local Oscillator Chains with GaAs Planar Diodes at Cryogenic Temperatures

This paper will describe a robust test-bed that has been built to measure multiplier performance over a wide range of temperatures and frequencies. In a 182-212 GRz designed balanced doubler the peak efficiency at 201 GHz improves from 22% to 28% upon cooling from 300 K to 120 K. This stage is then used to pump a 362-424 GRz balanced planar doubler. The peak chain efficiency increases from 3.4% to 6% when the two cascaded doublers are cooled from 300 K to 120 K. This enables the production of 10 mW of peak output power at 377 GHz, which ought to be sufficient for driving the next stage multiplier.

Maestrini, A.↗
NASA NTRS1997

Comparison of Light Emitting Diodes (LED) and Fluorescent Light on Suppression of Pineal Melatonin in the Rat

To validate a novel LED array for use in animal habitat lighting by comparing its effectiveness to cool-white fluorescent (CWF) lighting in suppressing pineal gland melatonin. Male Sprague-Dawley rats, 175-200 g, were maintained under control conditions for 2 weeks (food and water ad lib, 12L: 12D CWF, 18 uW/square cm). Dark adapted animals (animals before lights on) were exposed to 5 min of LED or CWF light of similar spectral power distribution. Two groups of rats (LED vs. CWF) were compared at 5 light intensities (100, 40, 1, 1.0, and 0. 1 lux). A control group was placed into the exposure apparatus but not exposed to light. After exposure, pineal glands were rapidly removed and assayed for melatonin by RIA. Results. The dark-exposed control groups matched with the 5 intensity groups (100, 40, 10, 1.0, and 0.1 lux) showed mean + SEM pineal melatonin values of 1167 +/- 136, 1569 +/- 126, 353 +/- 34, 650 +/- 124, and 464 +/- 85, pg/ml respectively. The corresponding CWF exposure data were 393 1 41, 365 +34, 257 +/- 13, 218 +/- 42, and 239 +/- 71 pg/ml, respectively. Corresponding LED exposure data were 439 +/- 25, 462 +/- 50, 231 +/- 6, 164 +/- 12, and 158 +/- 12 pg/ml, respectively. Rats exposed to both experimental light conditions at all illuminances studied showed significant melatonin suppression (p less than 0.01, ANOVA). In no case was the melatonin suppression induced by LED illuminance significantly different from the melatonin suppression elicited by the same intensity of CWF light. The results show that a novel LED light source can suppress pineal melatonin equal to that of a conventional CWF light source.

Winget, Charles M.↗
NASA NTRS2002

The Use of NASA Light-Emitting Diode Near-Infrared Technology for Biostimulation

Studies on cells exposed to microgravity and hypergravity indicate that human cells need gravity to stimulate growth. As the gravitational force increases or decreases, the cell function responds in a linear fashion. This poses significant health risks for astronauts in long-term spaceflight. The application of light therapy with the use of NASA LEDs will significantly improve the medical care that is available to astronauts on long-term space missions. NASA LEDs stimulate the basic energy processes in the mitochondria (energy compartments) of each cell, particularly when near-infrared light is used to activate the color sensitive chemicals (chromophores, cytochrome systems) inside. Optimal LED wavelengths include 680, 730 and 880 nm and our laboratory has improved the healing of wounds in laboratory animals by using both NASA LED light and hyperbaric oxygen. Furthermore, DNA synthesis in fibroblasts and muscle cells has been quintupled using NASA LED light alone, in a single application combining 680, 730 and 880 nm each at 4 Joules per centimeter squared. Muscle and bone atrophy are well documented in astronauts, and various minor injuries occurring in space have been reported not to heal until landing on Earth. An LED blanket device may be used for the prevention of bone and muscle atrophy in astronauts. The depth of near-infrared light penetration into human tissue has been measured spectroscopically.

Whelan, Harry T.↗