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Wilkerson, T. D.

Publications and source records attributed to Wilkerson, T. D..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Progress of research on water vapor lidar

Research is described on several aspects of stimulated Raman scattering (SRS) of 532 nm laser light in H2, D2, and CH4. The goals of this work are to develop a more thorough understanding of nonlinear processes involving the Raman effect and four-wave mixing, and to find the best way to generate radiation at several wavelengths simultaneously, for lidar applications. Issues addressed are conversion efficiency, optimization of operating conditions (gas pressure, confocal parameter, etc.) and the distribution of output pulse energy over three Stokes components, the first anti-Stokes component, and the zeroth order (pump) wavelength. The described research and results constitute another step in the development of SRS applications for NASA's atmospheric lidar program.

Wilkerson, T. D.↗
NASA NTRS1990

A self-seeded SRS system for the generation of 1.54-micron eye-safe radiation

A light source is described for the efficient generation of 1.54-micron for eye-safe aerosol lidar operation. The system is based upon an Nd:YAG laser at 1.06 micron which is then Raman-shifted in methane to produce light at the first Stokes wavelength of 1.54 micron. First Stokes light generated in the backward direction was retroreflected back into the Raman cell for amplification in the tail of the 10 ns pump beam. The energy conversion efficiency and the spatial beam quality of the amplified first Stokes were found to be adversely affected when operated at higher repetition rates due to a thermal gradient produced in the generation region. A Stokes energy of 25 mJ was obtained for a pumping energy of 140 mJ at a repetition rate of 10 Hz. The beam divergence of the amplified Stokes radiation was measured to be less than 1 mrad. The optimized results demonstrate the applicability of this radiation for eye-safe lidar measurements.

Chu, Z.↗
NASA NTRS1987

Raman-shifted dye laser for water vapor DIAL measurements

For improved DIAL measurements of water vapor in the upper troposphere or lower stratosphere, narrowband (about 0.03/cm) laser radiation at 720- and 940-nm wavelengths was generated by stimulated Raman scattering (SRS), using the narrow linewidth (about 0.02/cm) output of a Nd:YAG-pumped dye laser. For a hydrogen pressure of 350 psi, the first Stokes conversion efficiencies to 940 nm were 20 percent and 35 percent, when using a conventional and waveguide Raman cell, respectively. The linewidth of the first Stokes line at high cell pressures, and the inferred collisional broadening coefficients, agree well with those previously measured in spontaneous Raman scattering.

Grossmann, B. E.↗
NASA NTRS1986

Linewidth characteristics of Raman-shifted dye laser output at 720 and 940 nm

Raman conversion efficiency and line broadening are reported for Stokes operation at 720 and 940 nm, with hydrogen and deuterium as the Raman source, and using an Nd:YAG pumped Quanta-Ray PDL-2 dye laser. The dye laser linewidth is 0.2/cm (FWHM) with the grating alone as an intracavity element, and the conversion efficiency at 400 psi was found to be 40 and 20 percent for outputs of 720 and 940 nm, respectively. Pressure broadening coefficients of (9.2 + or - 0.9) x 10 to the -5th per cm/psi for hydrogen, and 7.7 x 10 to the -5th per cm/psi for deuterium, were obtained in good agreement with previous results. The linewidth at the first Stokes wavelength was shown to be determined by pressure broadening in the Raman medium.

Grossmann, B. E.↗
NASA NTRS1986

Strength, Width, and Pressure Shift Measurements of 54 Lines in the Oxygen A-Band

The absorption band of molecular oxygen, centered at 760]en1] nm, is the atmospheric absorber for the Differential Absorption Lidar (DIAL) systems used to measure atmospheric temperature, pressure, and density. To provide accurate line parameters for such systems, a careful spectroscopic study was made of the A-band, with measurements of line strengths, widths, pressure-induced frequency shifts, and collisional narrowing effects. The width and shift parameters were measured over a temperature range of -20 to 100 C so that the temperature dependence of these parameters can also be determined. To analyze the results, a least-squares fiting routine was written to fit standard line profiles to the observed profiles. These measurements, which include the first observations of pressure shifts and collisional narrowing in the band, are an important contribution to lidar system utilizing the A-band.

Ritter, K. J.↗
NASA NTRS1986

Bistatic LIDAR experiment proposed for the shuttle/tethered satellite system missions

A new experiment concept has been proposed for the shuttle/tethered satellite system missions, which can provide high resolution, global density mappings of certain ionospheric species. The technique utilizes bistatic LIDAR to take advantage of the unique dual platform configuration offered by these missions. A tuned, shuttle-based laser is used to excite a column of the atmosphere adjacent to the tethered satellite, while triangulating photometic detectors on the satellite are employed to measure the fluorescence from sections of the column. The fluorescent intensity at the detectors is increased about six decades over both ground-based and monostatic shuttle-based LIDAR sounding of the same region. In addition, the orbital motion of the Shuttle provides for quasi-global mapping unattainable with ground-based observations. Since this technique provides such vastly improved resolution on a synoptic scale, many important middle atmospheric studies, heretofore untenable, may soon be addressed.

Mccomas, D. J.↗
NASA NTRS1985

Lidar profiling of thermodynamic properties of the atmosphere

Principles and applications of pulsed laser methods for temperature, pressure, density, and humidity measurements in the altitude range 0-100 km are discussed. The differential absorption lidar (DIAL) method requires that at least one of the transmitted laser pulses be carefully tuned onto an absorption feature of the species, whereas Raman backscatter methods do not require tunability. OH and Na fluorescence require tuning of the excitation, and are the most suitable methods for high altitude where the atomic and molecular reemission is relatively unquenched by collisions.

Wilkerson, T. D.↗
NASA NTRS1982

Water absorption lines, 931-961 nm - Selected intensities, N2-collision-broadening coefficients, self-broadening coefficients, and pressure shifts in air

Intensities were measured for 97 lines of H2O vapor between 932 and 961 nm. The lines were selected for their potential usefulness for remote laser measurements of H2O vapor in the earth's atmosphere. The spectra were obtained with several different H2O vapor abundances and N2 broadening gas pressures; the spectral resolution was 0.046/cm FWHM. Measured H2O line intensities range from 7 x 10 to the -25th to 7 x 10 to the -22nd/cm per (molecules/sq cm). H2O self-broadening coefficients were measured for 13 of these strongest lines; the mean value was 0.5/cm per atm. N2-collision-broadening coefficients were measured for 73 lines, and the average was 0.11 cm per atm HWHM. Pressure shifts in air were determined for a sample of six lines between 948 and 950 nm; these lines shift to lower frequency by an amount comparable to 0.1 of the collision-broadened widths measured in air or N2. The measured intensities of many lines of 300-000 band are much larger than expected from prior computations, in some cases by over an order of magnitude. Coriolis interactions with the stronger 201-000 band appear to be the primary cause of the enhancement of these line intensities.

Giver, L. P.↗
NASA NTRS1981

Airborne differential absorption lidar system for water vapor investigations

Range-resolved water vapor measurements using the differential-absorption lidar (DIAL) technique is described in detail. The system uses two independently tunable optically pumped lasers operating in the near infrared with laser pulses of less than 100 microseconds separation, to minimize concentration errors caused by atmospheric scattering. Water vapor concentration profiles are calculated for each measurement by a minicomputer, in real time. The work is needed in the study of atmospheric motion and thermodynamics as well as in forestry and agriculture problems.

Browell, E. V.↗
NASA NTRS1981

Meteorological lidar developments

A research program on remote optical measurements of humidity and temperature is summarized, and new results are given of such measurements as well as new data on the near IR absorption spectrum of H2O. The basic atmospheric technique is differential absorption lidar (DIAL) using narrow band, tunable dye lasers. Approximate wavelengths are 720, 820, 940 nm (H2O) and 690, 760 nm (O2; temperature). The requisite absorption line strengths and widths are measured in a controlled environment by means of grating spectroscopy and tunable lasers. Prospects for routine lidar meteorology, even from airplane and satellite platforms, appear excellent in several important applications; the conditions and the optical properties of the atmosphere can be obtained remotely by these means.

Wilkerson, T. D.↗
NASA NTRS1979

Lidar temperature profiling - Performance simulations of Mason's method

In Mason's method (1975) atmospheric temperatures are inferred from a measure of the Boltzmann distribution of rotational states in one of the vibrational bands of O2. Differential absorption is measured using three tunable, narrowband pulse lasers. The outputs of two are tuned to wavelengths at the centers of absorption lines at either end of a particular branch in the band; the third wavelength is in a region of no absorption. The temperature-altitude profile can be calculated from the ratio of the two line absorption coefficients plus a priori knowledge of the line parameters. In the present paper, computer simulations of various lidar configurations are made, using different line pairs in the atmospheric bands of O2 (approximately 630, 690, and 760 nm). Simulated results are presented for temperature profiles measured from a Space Shuttle lidar.

Schwemmer, G. K.↗
NASA NTRS1979

Water vapor differential absorption lidar development and evaluation

A ground-based differential absorption lidar (DIAL) system is described which has been developed for vertical range-resolved measurements of water vapor. The laser transmitter consists of a ruby-pumped dye laser, which is operated on a water vapor absorption line at 724.372 nm. Part of the ruby laser output is transmitted simultaneously with the dye laser output to determine atmospheric scattering and attenuation characteristics. The dye and ruby laser backscattered light is collected by a 0.5-m diam telescope, optically separated in the receiver package, and independently detected using photomultiplier tubes. Measurements of vertical water vapor concentration profiles using the DIAL system at night are discussed, and comparisons are made between the water vapor DIAL measurements and data obtained from locally launched rawinsondes. Agreement between these measurements was found to be within the uncertainty of the rawinsonde data to an altitude of 3 km. Theoretical simulations of this measurement were found to give reasonably accurate predictions of the random error of the DIAL measurements. Confidence in these calculations will permit the design of aircraft and Shuttle DIAL systems and experiments using simulation results as the basis for defining lidar system performance requirements

Browell, E. V.↗
NASA NTRS1979

Intensities and N2 collision-broadening coefficients measured for selected H2O absorption lines between 715 and 732 nm

Intensities and N2 collision-broadening coefficients are measured for 62 water vapor absorption lines between 715 and 732 nm potentially applicable to laser remote sensing of atmospheric water vapor. Absolute line strengths and widths were determined from spectra corrected for instrument resolution, air-path absorption and Lorentz and Doppler broadening for pure water vapor and water vapor-nitrogen mixtures in a multipass absorption cell with a base path length of 25 m (White cell). Line strengths are observed to range from 4 x 10 to the -25th to 4 x 10 to the -23rd kayser/molecule per sq cm, and collision broadening coefficients are found to be approximately equal to 0.1 kayser/atm.

Wilkerson, T. D.↗
NASA NTRS1979

Two-photon lidar technique for remote sensing of atomic oxygen

A technique is proposed for remote sensing of atomic oxygen by laser excitation of a two-photon transition. Excitation at 2256 A and subsequent fluorescence at 8447 A allow probing of the mesosphere and lower thermosphere without interference by absorption. The two-photon excitation rate is calculated, and its dependence on laser power, duration, and linewidth is discussed. Values are given for remote sensing from a spacecraft at 200 km ranging down to 80 km with 1-km range resolution. Accuracies of better than 10% are expected with a 1 J, a 20-psec excitation pulse and a 1-sq-m collecting telescope.

Mcllrath, T. J.↗
NASA NTRS1979

Lidar temperature profiling: Performance simulations of Masons method

Several methods of using lasers to measure atmospheric temperature profiles were described. Mason's suggestion was analyzed here to assess its capabilities for various lidar configurations. Temperatures were inferred from a measure of the Boltzmann distribution of rotational states in one of the vibrational bands of O2. Differential absorption was measured using three tunable, narrowband pulsed lasers. The outputs of two were tuned to wavelengths at the centers of absorption lines at either end of a particular branch in the band. The third wave-length was in a region of no absorption; its lidar return measured only the atmospheric backscatter, and therefore allowed calculations of the absorption coefficients at the other two wavelengths as a function of altitude. From the ratio of the two line absorption coefficients plus a priori knowledge of the line parameters, the temperature-altitude profile were calculated.

Schwemmer, G. K.↗