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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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NASA NTRS1989

Eigenpolarization theory of monolithic nonplanar ring oscillators

Diode-laser-pumped monolithic nonplanar ring oscillators (NPROs) in an applied magnetic field can operate as unidirectional traveling-wave lasers. The diode laser pumping, monolithic construction, and unidirectional oscillation lead to narrow linewidth radiation. Here, a comprehensive theory of the eigenpolarizations of a monolithic NPRO is presented. It is shown how the properties of the integral optical diode that forces unidirectional operation depend on the choice of the gain medium, the applied magnetic field, the output coupler, and the geometry of the nonplanar ring light path. Using optical equivalence theorems to gain insight into the polarization characteristics of the NPRO, a strategy for designing NPROs with low thresholds and large loss nonreciprocities is given. An analysis of the eigenpolarizations for one such NPRO is presented, alternative optimization approaches are considered, and the prospects for further reducing the linewidths of these lasers are briefly discussed.

Nilsson, Alan C.↗

NASA NTRS1990

Monolithic Unidirectional Nonplanar Ring Laser

New design has six reflecting surfaces. Differential loss required to achieve unidirectional oscillation 100 times as great as with four reflecting surfaces. Concept provides for narrow-linewidth operation in relatively inexpensive Nd:glass. Potential applications in metrology and spectroscopy.

Nilsson, Alan C.↗

NASA NTRS1990

Coherence properties of a doubly resonant monolithic optical parametric oscillator

A doubly resonant optical parametric oscillator (DRO) pumped with the second harmonic of a narrow-linewidth Nd:YAG laser is described. The linewidth of the DRO signal was less than 13 kHz, the DRO was shown to generate a phase-locked subharmonic of the pump at degeneracy, and the signal and the idler were shown to be mutually coherent with the pump and to be phase-anticorrelated with each other away from degeneracy. The signal-idler heterodyne linewidth was 500 Hz, and pump phase modulation was shown to transfer to the DRO phase at degeneracy.

Nabors, C. D.↗

NASA NTRS1991

Self-Heterodyne Laser-Spectrum Analyzer

Apparatus measures spectral widths of light emitted by GaAs or other narrow-linewidth diode lasers at wavelengths between 800 and 900 nm. Operating on quasi-homodyne of self-heterodyne principle, apparatus provides relatively high spectral resolution: measures spectral widths of order of 100 kHz. Fine spectral resolution of apparatus provides data relevant to use of AlGaAs diode lasers in such applications as coherent optical communication through free space and injection seeding of solid-state lasers. Also used to explore connections between lifetimes and spectral widths of diode lasers.

Saif, Babak↗

NASA NTRS1991

Molecular clouds in starburst galaxy nuclei

New CO observations of a complete sample of starburst galaxies are presented in order to determine the range of molecular gas masses and star-formation efficiencies that characterize these systems. The properties of the galaxies in the sample are given, and the H-alpha and R-band CCD images for six of the galaxies are shown. The novel feature of this study is that the galaxies in this study are infrared selected as opposed to optically selected. They exhibit levels of nuclear star formation activity which are comparable to the starburst galaxies M82 and NGC 253. A subset of galaxies with unusually narrow CO line widths, high star-formation efficiencies, and compact 10-micron distributions is revealed. It is proposed that, if gas distributions in these galaxies are also compact, as the narrow linewidths suggest, the high yields of massive stars per unit mass of molecular gas may be attributed to a star-formation mechanism which becomes more efficient as the gas density increases.

Young, Judith S.↗

NASA NTRS1991

Laser-based measurements of OH in high pressure CH4/air flames

Narrow-linewidth laser absorption measurements are reported from which mole fraction and temperature of OH are determined in high-pressure (1-10 atm), lean CH4/air flames. These measurements were made in a new high pressure combustion facility which incorporates a traversable flat flame burner, providing spatially and temporally uniform combustion gases at pressures up to 10 am. A commercially avialable CW ring dye laser was used with an intracavity doubling crystal to provide near-UV single mode output at approximately 306 nm. The UV beam was rapidly scanned over 120 GHz (0.1 sec scan duration) to resolve the absorption lineshape of the A-X (0,0) R1(7)/R1(11) doublet of the OH radical. From the doublet's absorption lineshape, the temperature was determined; and from peak absorption, Beer's Law was employed to find the mole fraction of OH. These data were obtained as a function of height above the flame at various pressures.

Battles, B. E.↗

NASA NTRS1991

Thermal design and test results for SUNLITE ultra-stable reference cavity

SUNLITE (Stanford University-NASA Laser In-Space Technology Experiment) is a space-based experiment which uses a reference cavity to provide a stable frequency reference for a terahertz laser oscillator. Thermal stability of the cavity is a key factor in attaining a stable narrow-linewidth laser beam. The mount which is used to support and align the cavity will provide thermal isolation from the environment. The baseline requirement for thermal stability of the cavity is 0.025 C/min, but the design is directed toward achieving stability well beyond this requirement to improve the science data gained. A prototype of the cavity mount was fabricated and tested to characterize the thermal performance. The thermal vacuum test involved stable high-resolution temperature measurements and stable baseplate temperature control over long durations. Based on test data, the cavity mount design satisfies the severe requirement for the cavity thermal stability.

Amundsen, Ruth M.↗

NASA NTRS1992

Thermal analysis and test of SUNLITE reference cavity for laser frequency stabilization

SUNLITE is a space-based experiment which uses a reference cavity to provide a stable frequency reference for a terahertz laser oscillator. Thermal stability of the cavity is a key factor in attaining a stable narrow-linewidth laser beam. This paper describes the thermal stability requirements on the cavity design and detailed thermal analysis performed, as well as thermal testing that was performed on a prototype. Analytical thermal models were correlated to the test data and additional modeling of the current design is presented. Suggestions for improving similar high-precision thermal tests are given.

Amundsen, R. M.↗

NASA NTRS1992

High-resolution OH LIF velocity measurement technique for high-speed reacting flows

A nonintrusive optical technique was developed for the quantitative study of velocity fields in steady, high-speed, reacting flows. A narrow-linewidth laser source was tuned through an isolated OH absorption line to measure the Doppler-shifted linecenter frequency relative to an iodine reference line. A counterpropagating beam approach was used to eliminate collisional impact shift effects. Pointwise measurements of velocity were made in a unique reacting underexpanded jet facility as an extensive calibration of the technique over a wide range of flow conditions. The extension of the technique to planar measurements is also discussed.

Klavuhn, K. G.↗

NASA NTRS1993

Rotational level-dependent collisional broadening and line shift of the A2Sigma(+)-X2Pi (1,0) band of OH in hydrogen-air combustion gases

Measurements of the collisional broadening and line shift of the (1,0) band of the A2Sigma(+)-X2Pi system of OH are reported in atmospheric pressure hydrogen-air combustion gases. The measurements were made using a single-mode, narrow linewidth, frequency-doubled ring dye laser operating near 283 nm. The OH was generated in the combustion gases of a flat flame H2-air burner. Collisional broadening parameters for equilibrium mixtures of H2, O2, H2O, and N2 were obtained spanning a range of fuel/air equivalence ratios from 0.6 to 1.6 and temperatures from 1500 to 2050 K. Measurements were obtained spanning rotational quantum numbers from 4.5 to 16.5. The collision induced frequency shift was determined to be 0.1 that of the collisional broadening.

Kessler, W. J.↗

NASA NTRS1993

Water-vapor absorption line measurements in the 940-nm band by using a Raman-shifted dye laser

We report water-vapor absorption line measurements that are made by using the first Stokes radiation (930-982 nm) with HWHM 0.015/cm generated by a narrow-linewidth, tunable dye laser. Forty-five absorption line strengths are measured with an uncertainty of 6 percent and among them are fourteen strong lines that are compared with previous measurements for the assessment of spectral purity of the light source. Thirty air-broadened linewidths are measured with 8 percent uncertainty at ambient atmospheric pressure with an average of 0.101/cm. The lines are selected for the purpose of temperature-sensitive or temperature-insensitive lidar measurements. Results for these line strengths and linewidths are corrected for broadband radiation and finite laser linewidth broadening effects and compared with the high-resolution transmission molecular absorption.

Chu, Zhiping↗

NASA NTRS1995

A high spectral resolution VLA search for H I absorption towards A496, A1795, and A2584

In this paper, we present the results of a Very Large Array (VLA) search for H I absorption with high spectral resolution (1.6 km/s) towards A496, A1795, A2584, and A2597. These observations are well matched to the properties of cold, optically thick H I clouds, where the line width is given by the width of an individual cloud rather than the dispersion in an ensemble of clouds. We do not detect any H I absorption with narrow linewidths in these clusters. Our limits mainly apply to clouds which are larger than a few tenths parsec-i.e., if the clouds are much smaller than the background radio source and have a low covering factor in velocity space, they could still escape detection. The estimated limits on column density (for clouds in this regime of parameter space) are 2-3 orders of magnitude less than the 10(exp 21)/sq cm required to explain the x-ray absorption seen in some cooling flow clusters. The combination of our high spectral resolution H I absorption searches with the existing lower spectral resolution H I absorption searches and the searches for H I emission makes it unlikely that atomic hydrogen is the dominant component of the cold x-ray absorbing gas in the inter-cloud medium (ICM).

O'Dea, Christopher P.↗

NASA NTRS1995

Improved Microwave Fiber-Optic Link

High power output and narrow linewidth of Nd:YAG laser and external modulator combination enable higher stability and higher dynamic range fiber-optic transmission of microwave signals over longer distances. System prototype to test concept of high fidelity transmission of received microwave signals over fiber-optic cables, without need to downconvert microwave signals for transmission. Useful in distribution of future, more stable, frequency reference signals, phased array radar systems, and aircraft landing systems using bistatic radar.

Logan, Ronald T.↗

NASA NTRS1995

Development of Filtered Rayleigh Scattering for Accurate Measurement of Gas Velocity

The overall goals of this research were to develop new diagnostic tools capable of capturing unsteady and/or time-evolving, high-speed flow phenomena. The program centers around the development of Filtered Rayleigh Scattering (FRS) for velocity, temperature, and density measurement, and the construction of narrow linewidth laser sources which will be capable of producing an order MHz repetition rate 'burst' of high power pulses.

Miles, Richard B.↗

NASA NTRS1994

Spectral control of an alexandrite laser for an airborne water-vapor differential absorption lidar system

A narrow-linewidth pulsed alexandrite laser has been greatly modified for improved spectral stability in an aircraft environment, and its operation has been evaluated in the laboratory for making water-vapor differential absorption lidar measurements. An alignment technique is described to achieve the optimum free spectral range ratio for the two etalons inserted in the alexandrite laser cavity, and the sensitivity of this ratio is analyzed. This technique drastically decreases the occurrence of mode hopping, which is commonly observed in a tunable, two-intracavity-etalon laser system. High spectral purity (greater than 99.85%) at 730 nm is demonstrated by the use of a water-vapor absorption line as a notch filter. The effective cross sections of 760-nm oxygen and 730-nm water-vapor absorption lines are measured at different pressures by using this laser, which has a finite linewidth of 0.02 cm(exp -1) (FWHM). It is found that for water-vapor absorption linewidths greater than 0.04 cm(exp -1) (HWHM), or for altitudes below 10 km, the laser line can be considered monochromatic because the measured effective absorption cross section is within 1% of the calculated monochromatic cross section. An analysis of the environmental sensitivity of the two intracavity etalons is presented, and a closed-loop computer control for active stabilization of the two intracavity etalons in the alexandrite laser is described. Using a water-vapor absorption line as a wavelength reference, we measure a long-term frequency drift (approximately 1.5 h) of less than 0.7 pm in the laboratory.

Ponsardin, Patrick↗

NASA NTRS1998

Single Frequency, Pulsed Laser Diode Transmitter for Dial Water Vapor Measurements at 935nm

We report a tunable, single frequency, narrow linewidth, pulsed laser diode transmitter at 935.68nm for remote sensing of atmospheric water vapor. The transmitter consists of a CW, tunable, external cavity diode laser whose output is amplified 2OdB using a tapered diode amplifier. The output is pulsed for range resolved DIAL lidar by pulsing the drive current to the diode amplifier at 4kHz with a .5% duty cycle. The output from the transmitter is 36OnJ/pulse and is single spatial mode. It maintains a linewidth of less than 25MHz as its wavelength is tuned across the water vapor absorption line at 935.68nm. The transmitter design and its use in a water vapor measurement will be discussed.

Switzer, Gregg W.↗

NASA NTRS1999

High Efficiency End-Pumped Ho:Tm:YLF Disk Amplifier

Space based coherent lidar for global wind measurement requires an all solid state laser system with high energy, high efficiency and narrow linewidth that operates in the eye safe region. A Q-switched, diode pumped Ho:Tm:YLF 2 micrometer laser with output energy of as much as 125 mJ at 6 Hz with an optical-to-optical efficiency of 3% has been reported. Single frequency operation of the laser was achieved by injection seeding. The design of this laser is being incorporated into NASA's SPARCLE (SPAce Readiness Coherent Lidar Experiment) wind lidar mission. Laser output energy ranging from 500 mJ to 2 J is required for an operational space coherent lidar. We previously developed a high energy Ho:Tm:YLF master oscillator and side pumped power amplifier system and demonstrated a 600-mJ single frequency pulse at a repetition rate of 10 Hz. Although the output energy is high, the optical-to-optical efficiency is only about 2%. Designing a high energy, highly efficient, conductively cooled 2-micrometer laser remains a challenge. In this paper, the preliminary result of an end-pumped amplifier that has a potential to provide a factor 3 of improvement in the system efficiency is reported.

Yu, Jirong↗

NASA NTRS2000

Coupling Light from a High-Q Microsphere Resonator Using a UV-induced Surface Grating

High-Q microspheres with whispering-gallery modes have very narrow resonances that can be used for fiber-optic filters, ultra-compact narrow-linewidth lasers and optical/microwave oscillators. Whispering-gallery modes were previously excited in microspheres using evanescent optical fields. The necessary phase synchronism was obtained by adjusting the incident angle of input light beam (prism coupler) or adjustment of the waveguide propagation constant (fiber taper coupler). For many applications, however, bulky near-field couplers are undesirable. They compromise the symmetry and generate stray fields. Also, the control of coupling is crucial for the performance of microsphere resonators: in analogy with radio frequency circuits, the loading Q-factor should be less than the intrinsic Q-factor, Q(sub L) less than or equal to Q(sub O). Ideally one should combine a stable coupling element and a resonator into a single microsphere component.

Ilchenko, V. S.↗