William C. Edwards

Development of the Lidar Atmospheric Sensing Experiment (LASE)—An Advanced Airborne DIAL Instrument

The Lidar Atmospheric Sensing Experiment (LASE) Instrument is the first fully-engineered, autonomous Differential Absorption Lidar (DIAL) System for the measurement of water vapor in the troposphere (aerosol and cloud measurements are included). LASE uses a double-pulsed Ti:Sapphire laser for the transmitter with a 30 ns pulse length and 150 mJ/pulse. The laser beam is “seeded” to operate on a selected water vapor absorption line in the 815-nm region using a laser diode and an onboard absorption reference cell. A 40 cm diameter telescope collects the backscattered signals and directs them onto two detectors. LASE collects DIAL data at 5 Hz while onboard a NASA/Ames ER-2 aircraft flying at altitudes from 16–21 km. LASE was designed to operate autonomously within the environment and physical constraints of the ER-2 aircraft and to make water vapor profile measurements across the troposphere to better than 10% accuracy. LASE has flown 19 times during the development of the instrument and the validation of the science data. This paper describes the design, operation, and reliability of the LASE Instrument.

Injection seeding. II. Ti:Al/sub 2/O/sub 3/ experiments

Experiments have been designed and performed to determine the effects on the seeding efficiency of injection seed energy and power, alignment, and spatial mode matching between single longitudinal mode seed lasers and a power oscillator. The absorption features of H/sub 2/O have allowed the characterization of the seeding efficiency by selectively absorbing the laser output energy corresponding to the seed wavelength. This enabled the accurate measurement of the residual unseeded laser output energy. Both a pulsed and a continuous wave seed source were used for these experiments. This work compares the results of the experiments with a theory for injection seeding developed in the previous paper. >

Recent Progress in the Development of Neodymium-Doped Ceramic Yttria

Solid-state lasers play a significant role in providing the technology necessary for active remote sensing of the atmosphere. Neodymium-doped yttria (Nd:Y2O3) is considered to be an attractive material due to its possible lasing wavelengths of ~914 and ~946 nm for ozone profiling. These wavelengths, when frequency tripled, can generate ultraviolet (UV) light at ~305 and ~315 nm, which is particularly useful for ozone sensing using differential absorption light detection and ranging (LIDAR) technique. For practical realization of space-based UV transmitter technology, ceramic Nd:Y2O3 material is considered to possess a great potential. A plasma melting and quenching method has been developed to produce Nd3+-doped powders for consolidation into Nd: Y2O3 ceramic laser materials. This far-from-equilibrium processing methodology allows higher levels of rare earth doping than can be achieved by equilibrium methods. The method comprises two main steps: 1) plasma melting and quenching to generate dense, and homogeneous doped metastable powders and 2) pressure-assisted consolidation of these powders by hot isostatic pressing to make dense nanocomposite ceramics. Using this process, several in 1times1 ceramic cylinders have been produced. The infrared transmission of a 2-mm-thick undoped Y2O3 ceramic was as high as ~75% without antireflection coating. In the case of Nd:Y2O3, ceramics infrared transmission values of ~50% were achieved for a similar sample thickness. Furthermore, Nd:Y2O3 samples with dopant concentrations of up to ~2 at.% were prepared without significant emission quenching.

High-energy, efficient, 30-Hz ultraviolet laser sources for airborne ozone-lidar systems

Two compact, high-pulse-energy, injection-seeded, 30-Hz frequency-doubled Nd:YAG-laser-pumped Ti: sapphire lasers were developed and operated at infrared wavelengths of 867 and 900 nm. Beams with laser pulse energy >30 mJ at ultraviolet wavelengths of 289 and 300 nm were generated through a tripling of the frequencies of these Ti:sapphire lasers. This work is directed at the replacement of dye lasers for use in an airborne ozone differential absorption lidar system. The ultraviolet pulse energy at 289 and 300 nm had 27% and 31% absolute optical energy conversion efficiencies from input pulse energies at 867 and 900 nm, respectively.

Measurements from an Aerial Vehicle: A New Tool for Planetary Exploration

Aerial vehicles fill a unique planetary science measurement gap, that of regional-scale, near-surface observation, while providing a fresh perspective for potential discovery. Aerial vehicles used in planetary exploration bridge the scale and resolution measurement gaps between orbiters (global perspective with limited spatial resolution) and landers (local perspective with high spatial resolution) thus complementing and extending orbital and landed measurements. Planetary aerial vehicles can also survey scientifically interesting terrain that is inaccessible or hazardous to landed missions. The use of aerial assets for performing observations on Mars, Titan, or Venus will enable direct measurements and direct follow-ons to recent discoveries. Aerial vehicles can be used for remote sensing of the interior, surface and atmosphere of Mars, Venus and Titan. Types of aerial vehicles considered are airplane “heavier than air” and airships and balloons “lighter than air.” Interdependencies between the science measurements, science goals and objectives, and platform implementation illustrate how the proper balance of science, engineering, and cost, can be achieved to allow for a successful mission. Classification of measurement types along with how those measurements resolve science questions and how these instruments are accommodated within the mission context are discussed.

Compact high-pulse-energy ultraviolet laser source for ozone lidar measurements

An all solid-state Ti:sapphire laser differential absorption lidar transmitter was developed. This all-solid-state laser provides a compact, robust, and highly reliable laser transmitter for potential application in differential absorption lidar measurements of atmospheric ozone. Two compact, high-energy-pulsed, and injection-seeded Ti:sapphire lasers operating at a pulse repetition frequency of 30 Hz and wavelengths of 867 and 900 nm, with M2 of 1.3, have been experimentally demonstrated and their properties compared with model results. The output pulse energy was 115 mJ at 867 nm and 105 mJ at 900 nm, with a slope efficiency of 40% and 32%, respectively. At these energies, the beam quality was good enough so that we were able to achieve 30 mJ of ultraviolet laser output at 289 and 300 nm after frequency tripling with two lithium triborate nonlinear crystals.

Solid state UV laser development for the remote sensing of ozone from space

Development of a UV laser transmitter capable of operating from a space platform is a critical step in enabling global earth observations of aerosols and ozone at resolutions greater than current passive instrument capabilities. Tropospheric chemistry is well recognized as the next frontier for global atmospheric measurement. Moreover, global measurement of tropospheric ozone with high vertical resolution (2.5 km) from space were endorsed for the EX-1 Mission by NASAs Post2002 Mission Planning Workshop. At this workshop, held in Easton, Maryland, in August 1998, it was recognized that a space-based UV Differential Absorption Lidar (DIAL) system was necessary in order to obtain this high- resolution capability for measurements of ozone and aerosols. The results of this workshop can be found at http:llwww.hq.nasa.gov/office/ese/nra/RFldodgelPanelrev.html. For the EX-1 Mission, the UV DIAL measurement would be complemented with passive measurements of ozone precursor gases and pollutant tracer species. Langley Research Center (LaRC) and the Canadian Space Agency (CSA) have jointly studied the requirements for a satellite based, global ozone monitoring instrument. The study, called Ozone Research using Advanced Cooperative Lidar Experiment (ORACLE) has defined the DIAL instrument performance, weight and power, and configuration requirements for a space based measurement. In order to achieve the measurement resolution and acceptable signal-to-noise from lidar returns, 500mJ/pulse (10 Watts average power) is required at both, 308nm and 320nm wavelengths. These are consecutive pulses, in a 10 Hz, double-pulsed format. The two wavelengths are used as the on- and off-lines for the ozone DIAL measurement.1 5 NASA Langley is currently developing technology for a UV laser transmitter capable of meeting the ORACLE requirements; this development effort is focused on improving the efficiency of converting 1im laser energy to the 308 and 320nm energies needed for the DIAL measurement. Our approach includes making maximum use of existing, space-qualified optical components to reduce risk, cost and development time. Our experimental efforts to date have shown that our UV generation scheme is viable, and that energies greater than lOOmJ/pulse are possible. Future work will focus on improving efficiency and on addressing reliability, size and scalability issues. Our goal is to improve the optical conversion efficiency from the current state of the art, currently at 5%, to a minimum of 12%. We will accomplish this by using OPO/OPA and sum frequency mixing technology to generate the required UV wavelengths. The technology being developed has undergone an extensive peer review and down-select process from 20 possible UV generation schemes through in-house and industry trade studies and by experimental investigations. By using the selected technique and a diode pumped laser, a wall plug efficiency (electrical to optical) of greater than 2% is expected. In this paper, we will briefly discuss the study effort to date, the overall system design, and the down select process for the proposed laser design. We will describe UV laser technology that minimizes the total number of optical components (for enhanced reliability) as well as the number of UV coated optics required to transmit the light from the laser (for enhanced optical damage resistance). While the goal is to develop a laser that will produce 500 mJ of energy, we will describe an optional design that will produce output energies between 100-200mJ/unit and techniques for combining multiple laser modules in order to transmit a minimum of 500mJ of UV energy in each pulse of the on- and off-line pulse pairs. This modular laser approach provides redundancy and significantly reduces development time, risk and cost when compared to the development of a single, 500mJ double-pulsed laser subsystem. Finally, we will describe the common source for seeding the OPOs such that the absolute wavelength and linewidth of each transmitter module will be controlled and summarize the laser development effort to date, including results that include the highest known UV energy ever produced by a solid-state laser operating in this wavelength region.

Laser Technology Development at NASA Langley Research Center for Space based Applications

In this paper, salient features of various laser based developmental efforts carried out at NASA Langley Research Center for coherent and direct detection LIDAR systems for land, airborne and space based platforms will be discussed.

NASA DIAL/LIDAR Laser Technology Development Program

This paper gives an overview of the Laser Systems Branch solid-state laser technology development program. Emphasis is placed on the unique advanced laser developments needed to meet measurement requirements for important future NASA remote sensing missions.

High-pulse-energy 300-nm Ti:sapphire laser for airborne ozone lidar measurements

A compact high pulse energy Ti:Sapphire laser with its third harmonic has been developed for airborne ozone differential absorption lidar to study the distribution and concentration of ozone throughout the troposphere. The Ti:Sapphire laser, pumped by a commercial frequency-doubled Nd:YAG laser with a pulse repetition frequency of 20 Hz is seeded by a single mode 900 nm diode laser. More than 130 mJ/pulse was achieved at the fundamental wavelength of 900 nm. Two nonlinear, Lithium Triborate crystals were used for the Third Harmonic Generation resulting in output pulse energy of more than 39mJ at 300nm, which is used as the off-line wavelength of an airborne ozone DIAL system. The energy conversion efficiency from 900 nm to 300 nm was 30 percent as compared to the theoretical value of 36 percent.