Renewed interest in solid-state lasers operating in the mid-infrared region of the spectrum has been driven by several specific needs both within the government agencies and the commercial sector of the market. Among the needs are: probes for remote sensing through the atmosphere, wind-shear detection, communications, and medical applications. Reasons to use mid-infrared lasers rather than more conventional near-infrared solid-state lasers are associated primarily with wavelength. Mid-infrared lasers can be used to match a particular absorption, emission, or transmission feature. For example, by using wavelengths longer than ~1.5 μm, the transmission of the vitreous humor of the eye is very low. As such, these wavelengths cannot be focused on the retina and are considered to be eyesafe. By using these lasers, either by themselves or coupled with nonlinear optical frequency conversion techniques, many needs for tunable lasers in the 1.5 to 5.5 μm region of the spectrum can be met. In this presentation, progress on solid-state lasers in the mid-infrared will be highlighted.
Double heterostructures of InGaAsSb/AlGaAsSb quaternary alloys on (100) GaSb substrates were grown by the liquid phase epitaxy (LPE) technique. Room temperature operation near 2.1 μm wavelength has been achieved under pulsed conditions with pulsed threshold current density of 7kAcm-2.
Platinized tin oxide surfaces used for low-temperature CO oxidation in CO2 lasers have been characterized before and after reduction in CO at 125 and 250°C using ion scattering spectroscopy (ISS) and X-ray photoelectron spectroscopy (XPS). XPS indicates that the Pt is present initially as Pt02. Reduction at 125°C converts the Pt02 to Pt(OH)2 while reduction at 250°C converts the Pt02 to metallic Pt. ISS shows that the Pt in the outermost atomic layer of the catalyst is mostly covered by substrate species during the 250°C reduction. Both the ISS and XPS results are consistent with Pt/Sn alloy formation. The surface dehydration and migration of substrate species over surface Pt and Sn appear to explain why a CO pretreatment at 250°C produces inferior CO oxidation activities compared to a 125°C CO pretreatment.
For several years, tunable solid-state lasers have been used for atmospheric remote sensing. One approach is the Differential Absorption Lidar (DIAL) technique. With this technique, one atmospheric species, water vapor is measured by tuning one laser to the center of a water vapor line and by tuning another laser off of the line. The data are used by scientists to determine the vertical profiles of water vapor. One experiment, to make water vapor DIAL measurements from a down-looking airborne platform, is the NASA-LASE (Lidar Atmospheric Sensing Experiment) Project instrument to be flown on a NASA ER-2. The LASE wavemeter device, a classical Fabry-Perot interferometer approach, pro-vides wavelength control to form an autonomous system to calculate the wavelength centroid of each laser pulse. This real-time information is used to accurately tune the lasers to the required wavelengths of the atmospheric species. The difficulty in implementing this wavemeter is providing the performance requirements in the ER-2 environment. The temperature can vary from 10°C to 40°C and the pressure from 14.7 psi to 3.5 psi. To solve the thermal problem, the wavemeter optical components are enclosed in a housing that is controlled above the 40°C highest expected temperature at a tempera-ture of 43Ã‚Â°C. The interferometer is mounted in a thermal-vacuum chamber that is controlled to 45°C. The interferometer in vacuum provides an optical path with no refractive index variation. With the temperature control implemented, the next prob-lem was how to mount the interferometer in the thermal-vacuum chamber with little mechanical stress from the mounting fixture. After several configurations were investigated, a three fingered flexure provides a mounting to the stability of the laser source and of the electronic readout system used for the measurement. Fabry-Perot interferometers have been constructed of low thermal expansivity materials to form a three stage instrument. Two stages operating in a "bootstrap" mode provide the wavelength centroid measurement. The third stage provides data to retrieve the wavelength profile. Several classical wavelength reduction algorithms have been implemented that routinely exhibit an "end-to-end" system random noise of less than ±0.06 picometer.
Small models of light-weighted Si/SiC LIDAR mirrors have been fabricated via a scalable and cost effective chemical vapor deposition process. These mirrors are about 7.5-cm in diameter and consist of a Si cladded SiC faceplate and a light-weighted structure made of SiC. The light-weighted structure consists of an outer hexagonal cell with six (6) triangular inner cells. No bonding agent was used to attach SiC light-weighted structure to the faceplate. Flat and curved silicon carbide faceplates were replicated on graphite and then Si was cladded to this replicated faceplate in a CVD reactor. The mirrors were polished to a figure better than 1/5 of a wave and a finish of better than 5Å RMS. The scaling of the CVD process to yield Si/SiC light-weighted, f/1.6 mirrors, 40-cm in diameter is underway. The CVD route to fabrication of mirrors is fast and has the potential to yield several mirrors in a few weeks time from a single reactor.
We report on the initial results of a program aimed at developing low-cost diode laser arrays for use as solid-state laser pumps. Utilizing the Metal-Organic Chemical Vapor Deposition (MOCVD) technology, we have demonstrated excellent run-to-run reproducibility in emission wavelength, threshold current density and quantum effi-ciency. For four separate MOCVD growth runs involving a total of twenty 50 mm diameter wafers, we measured DH laser emission wave-lengths in the range of 811-813 nm. For this first experimental series, we achieved Jth values of approximately 1310 Amps/cm2 for broad area, unthinned devices from our growth runs. Differential quantum efficiencies of between 41% and 47% were measured on the non-facet-coated devices from all four runs. Single quantum well, separate confinement heterostructure lasers fabricated from wafers grown in the same MOCVD reactor exhibited near single-mode emission, with Jth values of approximately 300 Amps/cm2. Photolumines-cence data confirm quantum well widths of 80 Å and 150 Å for two different MOCVD growth runs.
The pumping dynamics of rare earth doped solid state laser materials are discussed here with an emphasis on the effects occurring in laser-pumped laser systems. A tunable alexandrite laser is used as the pump source for Nd3+-doped laser materials. It is found that the slope efficiency of the Nd laser operation depends strongly on the wavelength of the pump laser. For pump wavelengths resulting in low slope efficiencies strong fluorescence emission is observed from the sample in the blue-green spectral region. This is attributed to the excited state absorption of pump photons which occurs during radiationless relaxation from the pump band to the metastable state. It is shown to be an important loss mechanism in laser-pumped laser systems for specific pump wavelengths.
NASA has been deeply involved in the development of new LIDAR instruments to be used as atmospheric probes. One important application is for DIAL measurements of water vapor and oxygen content at different altitudes. LaserGenics Corp. has been involved in the development of a tunable Ti:Al203 laser oscillator for NASA Langley Research Center. The primary goals of our program are an oscillator with a linewidth of less than one picometer at each of three wavelengths, 724 nm , 760 nm , and 940 nm , with greater than 25 mJ of output energy per pulse at a repetition rate of 10 Hz. We will describe our oscillator design and how such problem areas as wavelength tuning, optical damage, astigmatism and conversion efficiency were overcome. Finally we will report the results achieved with the laser.
We report a simple theoretical model for the calculation of the dependence of filter quantum efficiency versus laser pump power in an atomic Rb vapor laser-excited optical filter. We present the calculations for a Rb filter transitions that can be used to detect the practial and important frequency-doubled Nd lasers. The results of these calculations show the filter's quantum efficiency versus the laser pump power. The laser pump powers required range from 2.4 to 60 mW per square centimeter of filter aperture.
The 2 - 5 micron (mid-IR) region is of interest for a number of applications. Efficient up conversion and down conversion techniques are being developed to obtain optical sources at mid-IR wavelengths. These techniques are reviewed and recent results using AgGaSe2 are reported. Gain in AgGaSe2 as high as 13 has been observed with a pump wavelength of 1.73 microns and a signar wavelength of 3.39 microns. Optical parametric oscillation in the 2.9 - 6.8 micron region has also been demonstrated.
Recent research and development work on AlGaInP visible laser diodes by SONY are reviewed. Performance and reliability of a gain-guided 670 nm laser with a tapered geometry are described. Efforts for achieving shorter wavelengths around 650 nm with using (111)B substrates, lower threshold current as well as higher power versions around 10 - 100 mW are also discussed.
The fluorescence spectra of plants excited with a pulsed nitrogen laser beam emitting at 337 nm were found to be related to plant type, as well as with changes in the physiology of the plant as the result of various kinds of environmental stress. The plant types which were studied included herbaceous dicots, monocots, hardwoods, and conifers. These plant types could be identified on the basis of differences in either the number of fluorescent bands, or the relative intensity of the bands. The dicots and monocots had fluorescent maxima at 440, 685, and 740 nm. The monocots could be distinguished from the dicots by virtue of having a much higher 440 nm/685 nm ratio. Hardwoods and conifers had an additional fluorescence band at 525 nm, but healthy conifers did not have a band at 685 nm. Differences in the fluorescent spectra which could be related to vigor status, were observed in conifers growing in an area where atmospheric deposition, i.e, acid rain and heavy metals, is known to be significant. Changes in the fluorescence spectra were also seen in plants grown under conditions of nutrient and drought stress.
The Tropical Atmospheric Lidar Observing System (TALOS) is proposed to be developed as a DIfferential Absorbtion Lidar (DIAL) system for flight aboard the Earth orbiting Space Station Freedom. TALOS will be capable of making high resolution vertical profile measurements of tropospheric water and tropospheric and stratospheric aerosols, clouds and temperature.
The prospect of obtaining directly measured winds on a global scale has raised auestions about the expected quality of the lidar wind measurements and the potential for biases due to sampling patterns and line-of-sight impediments. Extensive computer simulations are on-going to address these and other issues. One source of measurement bias is found in regions of the atmosphere where gradients in both lidar backscatter and the winds occur together. The potential biases that result are identified and their magnitudes estimated.
Pulsed CO2 lasers have many applications in aeronautics, space research, weather monitoring and other areas. Full exploitation of the potential of these lasers is hampered by the dissociation of CO2 that occurs during laser operation. The development of closed-cycle CO2 lasers requires active CO-02 recombination (CO oxidation) catalysts and design methods for implementation of catalysts inside lasers. This paper will discuss the performance criteria and constraints involved in the design of monolith catalyst configurations for use in a closed-cycle laser and will present a design study performed with a computerized design program that we have written. Trade-offs between catalyst activity and dimensions, flow channel dimensions, pressure drop, O2 conversion and other variables will be discussed.
A combined Raman-Rayleigh Lidar (Light Detection And Ranging) has recently been implemented at the Air Force Geophysics Laboratory's ground-based lidar station to measure neutral density from the lower stratosphere to the upper mesosphere. Rayleigh Lidar reliably measures relative densities in the region above 30 km. In this region, atmospheric extinction can be neglected and backscattering is primarily due to Rayleigh scattering. However, when density measurements are needed for the lower stratosphere two complications arise: the contribution of aerosol (Mie and Rayleigh) scattering to the Rayleigh signal and the effect of aerosol attenuation. Vibrational Raman scattering, being an elastic process for molecules only, can be used to resolve the first ambiguity. The second difficulty requires an inversion technique to help determine the attenuation profile from the Lidar signal and provide a transmission correction of this signal. For the lower stratosphere, the technique adopted in this laboratory is a three-step treatment of data. In step (1) Klett inversion is applied on the elastic scattering signal (Mie and Rayleigh) to obtain a transmission altitude profile. In step (2) the molecular signal from the Raman Lidar is corrected for atmospheric attenuation. In step (3), Raman data for below 25 km is spliced to Rayleigh data for above 25 km to give the entire profile of neutral density. Application of this analysis to experimental data will be shown and discussed.
The Geodynamics Laser Ranging System (GLRS) is a spaceborne laser ranging instrument being developed by NASA as a facility instrument for the Earth Observing System (Eos). GLRS is to be used to study regional and local scale crustal movements. As such, it is designed to make highly precise range measurements to retroreflector targets located in geophysically interesting sites. Using a two color ranging scheme, absolute range accuracies of several millimeters are expected. Simulations based on this accuracy and the Eos orbital parameters show that length of the intersite baseline between retroreflectors can be determined to several millimeters accuracy at distances from a few kilometers to several hundred kilometers with several passes of GLRS range data collected over a few day interval. Short arc techniques are used to minimize the effects of gravity field and other force model uncertainties. Relative heights can be determined to subcentimeter accuracy over comparable distances. The accuracy depends, in part, on the retroreflector locations relative to the orbital path, the number of laser shots used, and the viewing angles from the spacecraft. GLRS is also intended as a high precision laser altimeter with an intrinsic vertical accuracy of 10 cm and a horizontal resolution of about 70 meters. The altimetric function can be used to monitor the topography and roughness of ice sheets and land surfaces. With coarser vertical resolution it can also profile cloud-top heights. This paper describes the scientific objectives of the GLRS geodynamics measurements, and presents the results of recent simulations which assess the accuracy of the instrument in determining intersite baseline lengths and relative heights.
Laser studies dealing with various aspects of forest canopy measurement are reviewed. Aircraft LIDAR data have been used to estimate canopy density, tree heights, forest volume and biomass, to discriminate terrain units, and to add a height component to multispectral forest cover classifications. Linear comparisons between photogrammetrically-estimated canopy density and laser metrics indicate that the laser explains 45-65% of the variation noted in the photo estimates. Large variances are also associated with the relationship between laser canopy heights and the corresponding ground measurements. R-squared values on the order of 0.6 are typical; one study indicates that the laser height estimate will be within ±4 meters of the actual value 95% of the time. Estimates of woody biomass and volume are precise and repeatable between flightlines, however site-specific variation is high, limiting the use of the laser as a forest mapping tool. The laser metric-forest metric relationships are relatively coarse, due in part to 1) laser-ground misregistration errors; and 2) differences in the laser and ground sampling procedures. Integration of Global Positioning System data with the laser data stream, will, in the future, permit one to more accurately locate the laser transect on the ground, facilitating establishment of the necessary mathematical laser-ground relationships.
The Remote Active Spectrometer is a compact, lightweight sensor designed to demonstrate remote detection of chemical vapors. A prototype model was developed by Hughes Aircraft Company for the U.S. Army's Center For Night Vision and Electro-Optics, and the Chemical Research Development and Engineering Center. The Remote Active Spectrometer is comprised of four, frequency agile, CO2 laser transmitters (each operating at a rate of 10 hertz), optics for transmission, pointing, reception, and calibration, and detectors and electronics for information processing and recording. To provide a visual record of the scene observed a TV Sensor is integrated with the system. In this paper the Remote Active spectrometer is described, and its performance in the field discussed.
CO2 laser systems are particularly useful for remote measurement of atmospheric trace gases because of several factors: 1. many molecular species have absorption features in the 9-11 micron spectral region; 2. the atmosphere is relatively transparent; 3. CO2 lasers are discretely tunable in this spectral region, and are reasonably powerful or energetic. Indeed, CO2 laser systems have been applied to measuring a number of molecular species: water vapor, ethylene, ozone, ammonia, hydrazines , freons, methanol, and sulfur hexafluoride(SF6). While most of this work has been with direct detection, some work has been reported using heterodyne detection. In direct detection, a receiver acts as a "light bucket," detecting photons in a wide bandwidth, typically tens to hundreds of wavenumbers (cm-1 where 1 cm-1 = 30 GHz). In heterodyne detection, the detection region is narrowed to a few MHz by use of a local oscillator laser and RF-detection electronics so that the thermal background radiation is kept to a minimum. Thus, in direct detection, measurements are typically limited to ranges of 1-3 km, while in heterodyne detection, measurements can be made to ranges of 5-10 km. In the sections that follow, the hardware for the Mobile Atmospheric Pollutant Mapping (MAPM) System is described, along with measurement results using it, the absorption coefficients and measurement sensitivities for a number of molecular species, and the factors that limit measurement accuracy and range.
During the last five years, work has been underway at the Lawrence Livermore National Laboratory (LLNL) to develop a method for imaging gas clouds that are normally invisible to the human eye. The effort was initiated to provide an effective means of locating leaks of hazardous vapors. Although conventional point or line-of-sight detectors are well suited to the measurement of gas concentrations, their utility in identifying the origin and direction of travel of gas plumes is limited. To obtain spatial information from sensors that provide only zero- or one-dimensional readings, either sequential readings at many different locations from a single device, or multiplexed simultaneous measurements from a sensor array must be taken. The former approach is time consuming and, therefore, impractical in emergency situations where rapid action is required. The latter is useful only in cases where the probability of a hazardous release is high enough to warrant the prior installation of a sensor network. Either method demands high measuremental precision and sufficient discrimination against both interfering gases and interfering sources of the target gas. Backscatter Absorption Gas Imaging (BAGI) is a new technique that makes gas clouds and their surroundings "visible" in a real-time video image. It is superior to conventional sensors in characterizing the spatial properties of gas clouds because it provides data that are inherently two-dimensional. Less measuremental precision is required by the BAGI technique because it conveys information as contrasts between different areas in an image rather than as absolute concentration values. Furthermore, the pictorial display of this information allows it to be rapidly assimilated by emergency-response teams. The size and orientation of the plume are evident through comparison with familiar objects that also appear in the image. Subtler evaluations can be made as well, such as the distinction between innocous and hazardous sources of the target gas. For example, in using a conventional sensor to search for the source of a gas that is also present at low levels in automobile exhaust, one might be led astray near a highway. Gas imaging allows the searcher to recognize that the cars are producing the gas, and that they are not the objective of the search.
SRI International has developed two infrared differential absorption lidar (IR DIAL) systems to detect infrared-absorbing trace gases and environmental monitoring applications. The multiwavelength DIAL system operates at 10.6 μm and performs both path-integrated (column-content) and range-resolved measurements. This van-mounted system contains four grating-tuned Tachisto 555-G (custom) TEA CO2 lasers, with a nominal 1 J of transmitted energy at 10P(20). The lasers are typically fired with a 100 μs spacing to freeze the atmosphere. The IR receiver uses a 16-in. f/2.5 telescope and a liquid-nitrogen-cooled HgCdTe quadrant detector. The scanner allows pointing over a full hemisphere. The lidar data system features two DEC PDP-11/73 microcomputers and a computer automated measurement and control (CAMAC) data acquisition system for real-time data collection, signal averaging, data analysis, color graphics display, and storage on magnetic tape. The multiwavelength DIAL system has demonstrated path-integrated measurements to a range of 9 km and range-resolved measurements to 4 km. The triple CO2 DIAL system operates at 3.4 μm and utilizes three mini-TEA CO2 lasers and nonlinear crystals in a novel frequency-mixing technique to detect and discriminate selective hydrocarbons.
Feasibility studies have been carried out for a ground-based LIDAR experiment to measure spatially resolved rocket plume temperature profiles, based on a specific detection scheme proposed by workers at the University of South Florida. This experiment involves the Differential Absorption LIDAR (DIAL) measurement of temperature, using observations of the first vibrational overtone band of HC1. Results from plume flow field codes and spectral simulations have been used to test the feasibility of the above DIAL experiment, and additional preliminary ground based tests are proposed here, which would logically lead up to the more complex DIAL experiment.
A lidar facility has been established at the JPL - Table Mountain Observatory at an altitude of 2300 m in the San Gabriel Mountains N.E. of Los Angeles. This facility uses the laser remote sensing technique of differential absorption lidar (DIAL) to derive atmospheric ozone concentration profiles. Two separate systems are used to obtain the ozone profile; a Nd:YAG based troposphere system will measure from the ground to 20 km and an excimer based system measures from 15 km to 50 km. The systems are designed to make long-term measurements so that small changes in the ozone abundance can be detected. This paper will describe the stratospheric ozone lidar system and present some of the results which have been obtained since it began routine operation in January 1988.
The principle of operation of a space based Doppler lidar wind measuring system is discussed along with laser wavelength selection considerations. Differences in accommodating the Laser Atmospheric Wind Sounder (LAWS) on the Earth Observing System (EOS) polar platform and the Manned Space Station are presented. The impact of the LAWS instrument support subsystems are specifically discussed.
Three different examples of atmospheric density currents are observed by the NOAA Doppler lidar. The lidar has Droved to be an extremely useful sensor to study the mesoscale dynamics of these clear-air phenomena.
At Colorado State University, a small compact CO2 Doppler lidar is being built from commercial components. This paper describes the problems associated with use of commercial off-the--shelf components and discusses the frequency stability of such a system.
An algorithm is presented for estimation of the logpower difference needed in DIAL observations for a system assumed to use only a single laser that is tuned to different wavelengths sequentially. Account is taken of the nonlinear measurement equation, missing observations and signal fluctuations by use of adaptive Kalman filter techniques, and filter performance is demonstrated with simulated data.
Three potential techniques for making wind measurements from space are discussed, and coherent detection Doppler lidar chosen for further consideration. The impact of different laser transmitter wavelengths on the design of a Doppler lidar are considered. Specifically covered in the paper are coherent detection systems based on the 2.1μm Tm:Ho:YAG laser and the 9.1μm 12C18O2 laser. Both systems engineering aspects and subsystem impacts are discussed.
Pulsed CO2 lasers have many remote sensing applications from space, airborne, and ground platforms. The NASA LAWS system will be designed to measure wind velocities from polar earth orbit for a period of up to three years. Accordingly, this application requires a closed-cycle pulsed CO2 laser which necessitates the use of an efficient CO-02 recombination catalyst for these dissociation products which otherwise would degrade the laser operation. The catalyst must not only operate at low temperatures but also must operate efficiently for a three year period. To minimize atmospheric absorption and enhance aerosol scatter of laser radiation, the LAWS system will operate at 9.1 micrometers with an oxygen-18 isotope CO2 lasing medium. Consequently the catalyst must preserve the isotopic integrity of the rare-isotope composition in the recombination mode. The research effort at NASA LaRC has centered around development and testing of CO oxidation catalysts for closed-cycle pulsed and common and rare-isotope CO2 lasers. We have examined available commercial catalysts both in a laser and under simulated closed-cycle laser conditions with our efforts aimed toward a thorough understanding of the fundamental catalytic reaction and have utilized these data to design experimental techniques to both better understand the mechanism and to design and synthesize new catalyst compositions to better meet the catalyst requirements for closed-cycle pulsed CO2 lasers. Catalysts have been tested continuously in excess of three months; however the long term requirements of the LAWS necessitates that life-times be projected with confidence. Our research 'demonstrates that a long-term decay of catalytic efficiency persists in currently available catalysts. In this paper we report results whereby the decay in catalytic efficiency for a recently developed catalyst was found to be significantly lower than previously available catalysts while at the same time operating quite well under ambient temperature conditions. Synthesis and mechanistic details in the development of the new catalyst composition are discussed.
The global winds measurement application of coherent Doppler lidar is discussed, emphasizing the importance of optimizing overall system efficiency. Major loss categories are briefly assessed, and the implications of potential improvements in receiver efficiency are highlighted.