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NASA NTRS2000

Laser Brazing of High Temperature Braze Alloy

The Space Shuttle Main Engine (SSME) consists of 1080 conical tubes, which are furnace brazed themselves, manifolds, and surrounding structural jacket making almost four miles of braze joints. Subsequent furnace braze cycles are performed due to localized braze voids between the coolant tubes. SSME nozzle experiences extremely high heat flux (180 mW/sq m) during hot fire. Braze voids between coolant tubes may result in hot combustion gas escape causing jacket bulges. The nozzle can be disqualified for flight or result in mission failure if the braze voids exceed the limits. Localized braze processes were considered to eliminate braze voids, however, damage to the parent materials often prohibited use of such process. Being the only manned flight reusable rocket engine, it has stringent requirement on the braze process. Poor braze quality or damage to the parent materials limits the nozzle service life. The objective of this study was to develop a laser brazing process to provide quality, localized braze joints without adverse affect on the parent materials. Gold (Au-Cu-Ni-Pd-Mn) based high temperature braze alloys were used in both powder and wire form. Thin section iron base superalloy A286 tube was used as substrate materials. Different Laser Systems including CO2 (10.6 micrometers, 1kW), ND:YAG (1.06 micrometers, 4kW). and direct diode laser (808nm. 150W) were investigated for brazing process. The laser process variables including wavelength. laser power, travel speed and angle of inclination were optimized according to bead geometry and braze alloy wetting at minimum heat input level, The properties of laser brazing were compared to that of furnace brazing. Microhardness profiles were used for braze joint property comparison between laser and furnace brazing. The cooling rate of laser brazing was compared to furnace brazing based on secondary dendritic arm spacing, Both optical and Scanning Electron Microscope (SEM) were used to evaluate the microstructures of the braze materials and tube substrate. Metallography of the laser braze joint was compared to the furnace braze. SEM Energy Disperse X-Ray Spectra (EDX) and back scattered imaging were used to analyze braze alloy segregation. Although all of the laser systems, CO2, ND:YAG, and direct diode laser produced good braze joint, the direct diode laser was selected for its system simplicity, compactness and portability. Excellent laser and braze alloy coupling is observed with powder alloy compared to braze alloy wire. Good wetting is found with different gold based braze alloys. The laser brazing process can be optimized so that the adverse affect on the parent materials can be eliminated. Metallography of the laser braze joint has shown that quality braze joint was produced with laser brazing process. Penetration of the laser braze to the substrate is at neglectable level. Zero penetration is observed. Microstructure examinations shown that no observable changes of the microstructure (grain structure and precipitation) in the HAZ area between laser braze and furnace braze. Wide gaps can be laser brazed with single pass for up to 0.024 inches. Finer dendritic structure is observed in laser brazing compared with equiaxial and coarser grain of the furnace brazing microstructure. Greater segregation is also found in the furnace braze. Higher hardness of the laser braze joint comparing to furnace braze is observed due to the fast cooling rate and Finer microstructure in the laser brazing. Laser braze joint properties meet or exceed the furnace joint properties. Direct diode laser for thin section tube brazing with high temperature braze alloys have been successfully demonstrated. The laser's high energy density and precise control has shown significant advantages in reducing process heat input to the substrates and provide high quality braze joints comparing to other localized braze process such as torch, TIG, and MPTA processes. Significant cost savings can be realized particularly with localized braze comparing to a full furnace braze cycle.

Gao, Y. P.↗
NASA NTRS2010

Qualification and Selection of Flight Diode Lasers for Space Applications

The reliability and lifetime of laser diodes is critical to space missions. The Nuclear Spectroscopic Telescope Array (NuSTAR) mission includes a metrology system that is based upon laser diodes. An operational test facility has been developed to qualify and select, by mission standards, laser diodes that will survive the intended space environment and mission lifetime. The facility is situated in an electrostatic discharge (ESD) certified clean-room and consist of an enclosed temperature-controlled stage that can accommodate up to 20 laser diodes. The facility is designed to characterize a single laser diode, in addition to conducting laser lifetime testing on up to 20 laser diodes simultaneously. A standard laser current driver is used to drive a single laser diode. Laser diode current, voltage, power, and wavelength are measured for each laser diode, and a method of selecting the most adequate laser diodes for space deployment is implemented. The method consists of creating histograms of laser threshold currents, powers at a designated current, and wavelengths at designated power. From these histograms, the laser diodes that illustrate a performance that is outside the normal are rejected and the remaining lasers are considered spaceborne candidates. To perform laser lifetime testing, the facility is equipped with 20 custom laser drivers that were designed and built by California Institute of Technology specifically to drive NuSTAR metrology lasers. The laser drivers can be operated in constant-current mode or alternating-current mode. Situated inside the enclosure, in front of the laser diodes, are 20 power-meter heads to record laser power throughout the duration of lifetime testing. Prior to connecting a laser diode to the current source for characterization and lifetime testing, a background program is initiated to collect current, voltage, and resistance. This backstage data collection enables the operational test facility to have full laser diode traceablity.

Liebe, Carl C.↗
NASA NTRS2009

2-Micron Laser Transmitter for Coherent CO2 DIAL Measurement

Carbon dioxide (CO2) has been recognized as one of the most important greenhouse gases. It is essential for the study of global warming to accurately measure the CO2 concentration in the atmosphere and continuously record its variation. A high repetition rate, highly efficient, Q-switched 2-micron laser system as the transmitter of a coherent differential absorption lidar for CO2 measurement has been developed in NASA Langley Research Center. This laser system is capable of making a vertical profiling of CO2 from ground and column measurement of CO2 from air and space-borne platform. The transmitter is a master-slave laser system. The master laser operates in a single frequency, either on-line or off-line of a selected CO2 absorption line. The slave laser is a Q-switched ring-cavity Ho:YLF laser which is pumped by a Tm:fiber laser. The repetition rate can be adjusted from a few hundred Hz to 10 kHz. The injection seeding success rate is from 99.4% to 99.95%. For 1 kHz operation, the output pulse energy is 5.5mJ with the pulse length of ~50 ns. The optical-to-optical efficiency is 39% when the pump power is 14.5W. A Ho:YLF laser operating in the range of 2.05 micrometers can be tuned over several characteristic lines of CO2 absorption. Experimentally, a diode pumped Ho:Tm:YLF laser has been successfully used as the transmitter of coherent differential absorption lidar for the measurement of CO2 with a repetition rate of 5 Hz and pulse energy of 75 mJ. For coherent detection, high repetition rate is required for speckle averaging to obtain highly precise measurements. However, a diode pumped Ho:Tm:YLF laser can not operate in high repetition rate due to the large heat loading and up-conversion. A Tm:fiber laser pumped Ho:YLF laser with low heat loading can operate in high repetition rate. A theoretical model has been established to simulate the performance of Tm:fiber laser pumped Ho:YLF lasers. For continuous wave (CW) operation, high pump intensity with small beam size is suitable for high efficiency. For Q-switched operation, the optimal energy extraction relies on the pump intensity, pump volume, and pump duration which is inversely proportion to the repetition rate. CW and Q-switched Ho:YLF lasers with different linear cavity configurations have been designed and demonstrated for a 30 W Tm:fiber pump laser. The CW Ho laser slope efficiency and optical-to-optical efficiencies reach 65% and 55%, respectively. The pulsed laser efficiency depends on the repetition rate. For 1 kHz operation, the optical-to-optical efficiency is 39% when the pump power is 14.5W. Currently, the injection seeding success rate is between 99.4% and 99.95%. After a ten thousand pulses, the standard deviation of the laser frequency jitter is about 3 MHz. It meets the requirements of highly precise CO2 concentration measurement. In conclusion, an injection seeded, high repetition rate, Q-switched Ho:YLF laser has been developed for a coherent CO2 differential absorption lidar. This master-slave laser system has high optical-to-optical efficiency and seeding success rate. It can potentially meet the requirements of the coherent detection of CO2 concentration by a differential absorption lidar technique.

Singh, Upendra N.↗
NASA NTRS1993

Multibeam Laser Altimeter for Planetary Topographic Mapping

Laser altimetry provides an active, high-resolution, high-accuracy method for measurement of planetary and asteroid surface topography. The basis of the measurement is the timing of the roundtrip propagation of short-duration pulses of laser radiation between a spacecraft and the surface. Vertical, or elevation, resolution of the altimetry measurement is determined primarily by laser pulse width, surface-induced spreading in time of the reflected pulse, and the timing precision of the altimeter electronics. With conventional gain-switched pulses from solid-state lasers and nanosecond resolution timing electronics, submeter vertical range resolution is possible anywhere from orbital altitudes of approximately 1 km to altitudes of several hundred kilometers. Horizontal resolution is a function of laser beam footprint size at the surface and the spacing between successive laser pulses. Laser divergence angle and altimeter platform height above the surface determine the laser footprint size at the surface, while laser pulse repetition rate, laser transmitter beam configuration, and altimeter platform velocity determine the spacing between successive laser pulses. Multiple laser transmitters in a single laser altimeter instrument that is orbiting above a planetary or asteroid surface could provide across-track as well as along-track coverage that can be used to construct a range image (i.e., topographic map) of the surface. We are developing a pushbroom laser altimeter instrument concept that utilizes a linear array of laser transmitters to provide contiguous across-track and along-track data. The laser technology is based on the emerging monolithic combination of individual, 1-sq cm diode-pumped Nd:YAG laser pulse emitters. Details of the multi-emitter laser transmitter technology, the instrument configuration, and performance calculations for a realistic Discovery-class mission will be presented.

Garvin, J. B.↗
NASA NTRS2011

Receiver Signal to Noise Ratios for IPDA Lidars Using Sine-wave and Pulsed Laser Modulation and Direct Detections

Integrated path differential absorption (IPDA) lidar can be used to remotely measure the column density of gases in the path to a scattering target [1]. The total column gas molecular density can be derived from the ratio of the laser echo signal power with the laser wavelength on the gas absorption line (on-line) to that off the line (off-line). 80th coherent detection and direct detection IPDA lidar have been used successfully in the past in horizontal path and airborne remote sensing measurements. However, for space based measurements, the signal propagation losses are often orders of magnitude higher and it is important to use the most efficient laser modulation and detection technique to minimize the average laser power and the electrical power from the spacecraft. This paper gives an analysis the receiver signal to noise ratio (SNR) of several laser modulation and detection techniques versus the average received laser power under similar operation environments. Coherent detection [2] can give the best receiver performance when the local oscillator laser is relatively strong and the heterodyne mixing losses are negligible. Coherent detection has a high signal gain and a very narrow bandwidth for the background light and detector dark noise. However, coherent detection must maintain a high degree of coherence between the local oscillator laser and the received signal in both temporal and spatial modes. This often results in a high system complexity and low overall measurement efficiency. For measurements through atmosphere the coherence diameter of the received signal also limits the useful size of the receiver telescope. Direct detection IPDA lidars are simpler to build and have fewer constraints on the transmitter and receiver components. They can use much larger size 'photon-bucket' type telescopes to reduce the demands on the laser transmitter. Here we consider the two most widely used direct detection IPDA lidar techniques. The first technique uses two CW seeder lasers, one on-line and one offline that are intensity modulated by two different frequency sine-waves signals before being amplified by a common laser amplifier. The receiver uses narrowband amplitude demodulation, or lock-in, Signal processing at the given laser modulation frequencies [3,4]. The laser transmitter operates in a quasi CW mode with the peak power equal to twice the average power. The on-line and off-line lasers can be transmitted at the same time without interference. Another direct detection technique uses a low duty cycle pulsed laser modulation [5,6] with the laser wavelengths alternating between on-line and off-line on successive pulses. The receiver uses time resolved detection and can also provide simultaneous target range measurement. With a lower laser duty cycle it requires a much higher peak laser power for the same average power.

Sun, Xiaoli↗
NASA NTRS2003

Review of Laser Ablation Process for Single Wall Carbon Nanotube Production

Different types of lasers are now routinely used to prepare single wall carbon nanotubes (SWCNTs). The original method developed by researchers at Rice University utilized a "double pulse laser oven" process. A graphite target containing about 1 atomic percent of metal catalysts is ablated inside a 1473K oven using laser pulses (10 ns pulse width) in slow flowing argon. Two YAG lasers with a green pulse (532 nm) followed by an IR pulse (1064 nm) with a 50 ns delay are used for ablation. This set up produced single wall carbon nanotube material with about 70% purity having a diameter distribution peaked around 1.4 nm. The impurities consist of fullerenes, metal catalyst clusters (10 to 100 nm diameter) and amorphous carbon. The rate of production with the initial set up was about 60 mg per hour with 10Hz laser systems. Several researchers have used variations of the lasers to improve the rate, consistency and study effects of different process parameters on the quality and quantity of SWCNTs. These variations include one to three YAG laser systems (Green, Green and IR), different pulse widths (nano to microseconds as well as continuous) and different laser wavelengths (Alexandrite, CO, CO2, free electron lasers in the near to far infrared). It is noted that yield from the single laser (Green or IR) systems is only a fraction of the two laser systems. The yield seemed to scale up with the repetition rate of the laser systems (10 to 60 Hz) and depended on the beam uniformity and quality of the laser pulses. The shift to longer wavelength lasers (free electron, CO and CO2) did not improve the quality, but increased the rate of production because these lasers are either continuous (CW) or high repetition rate pulses (kHz to MHz). The average power and the peak power of the lasers seem to influence the yields. Very high peak powers (MegaWatts per square centimeter) are noted to increase ablation of bigger particles with reduced yields of SWCNTs. Increased average powers seem to help the conversion of the carbon from target into vapor phase to improve formation of nanotubes. The use of CW far infrared lasers reduced the need for the oven, at the expense of controlled ablation. Some of these variations are tried with different combinations and concentrations of metal catalysts (Nickel with Cobalt, Iron, Palladium and Platinum) different buffer gases (e.g. Helium); with different oven temperatures (Room temperature to 1473K); under different flow conditions (1 to 1000 kPa) and even different porosities of the graphite targets. It is to be noted that the original Cobalt and Nickel combination worked best, possibly because of improved carbonization with stable crystalline phases. The mean diameter and yield seemed to increase with increasing oven temperatures. Thermal conductivity of the buffer gas and flow conditions dictate the quality as well as quantity of the SWCNTs. Faster flows, lower pressures and heavier gases seem to increase the yields. This review will attempt to cover all these variations and their relative merits. Possible growth mechanisms under these different conditions will also be discussed.

Arepalli, Sivaram↗
NASA NTRS1992

Intense laser beams; Proceedings of the Meeting, Los Angeles, CA, Jan. 23, 24, 1992

Various papers on intense laser beams are presented. Individual topics addressed include: novel methods of copper vapor laser excitation, UCLA IR FEL, lasing characteristics of a large-bore copper vapor laser (CVL), copper density measurement of a large-bore CVL, high-power XeCl excimer laser, solid state direct-drive circuit for pumping gas lasers, united energy model for FELs, intensity and frequency instabilities in double-mode CO2 lasers, comparison of output power stabilities of CO and CO2 lasers, increasing efficiency of sealed-off CO lasers, thermal effects in singlet delta oxygen generation, optical extraction from the chemical oxygen-iodine laser medium, generation and laser diagnostic analysis of bismuth fluoride. Also discussed are: high-Q resonator design for an HF overtone chemical lasers, improved coatings for HF overtone lasers, scaled atmospheric blooming experiment, simulation on producing conjugate field using deformable mirrors, paraxial theory of amplitude correction, potential capabilities of adaptive optical systems in the atmosphere, power beaming research at NASA, system evaluations of laser power beaming options, performance projections for laser beam power to space, independent assessment of laser power beaming options, removal of atmospheric CFCs by lasers, efficiency of vaporization cutting by CVL.

Wade, Richard C.↗
NASA NTRS2013

High-Power Single-Mode 2.65-micron InGaAsSb/AlInGaAsSb Diode Lasers

Central to the advancement of both satellite and in-situ science are improvements in continuous-wave and pulsed infrared laser systems coupled with integrated miniaturized optics and electronics, allowing for the use of powerful, single-mode light sources aboard both satellite and unmanned aerial vehicle platforms. There is a technological gap in supplying adequate laser sources to address the mid-infrared spectral window for spectroscopic characterization of important atmospheric gases. For high-power applications between 2 to 3 micron, commercial laser technologies are unsuitable because of limitations in output power. For instance, existing InP-based laser systems developed for fiber-based telecommunications cannot be extended to wavelengths longer than 2 micron. For emission wavelengths shorter than 3 micron, intersubband devices, such as infrared quantum cascade lasers, become inefficient due to band-offset limitations. To date, successfully demonstrated singlemode GaSb-based laser diodes emitting between 2 and 3 micron have employed lossy metal Bragg gratings for distributed- feedback coupling, which limits output power due to optical absorption. By optimizing both the quantum well design and the grating fabrication process, index-coupled distributed-feedback 2.65-micron lasers capable of emitting in excess of 25 mW at room temperature have been demonstrated. Specifically, lasers at 3,777/cm (2.65 micron) have been realized to interact with strong absorption lines of HDO and other isotopologues of H2O. With minor modifications of the optical cavity and quantum well designs, lasers can be fabricated at any wavelength within the 2-to-3-micron spectral window with similar performance. At the time of this reporting, lasers with this output power and wavelength accuracy are not commercially available. Monolithic ridge-waveguide GaSb lasers were fabricated that utilize secondorder lateral Bragg gratings to generate single-mode emission from InGaAsSb/ AlInGaAsSb multi-quantum well structures. The device fabrication utilizes etched index-coupled gratings in the top AlGaAsSb cladding of the laser chip along the ridge waveguide, whereas commercial lasers that emit close to this wavelength include loss-coupled metal gratings that limit the output power of the laser. Semiconductor-laser-based spectrometers can be used to replace gas sensors currently used in industry and government. With the availability of high-power laser sources at mid-infrared wavelengths, sensors can target strong fundamental gas absorption lines to maximize instrument sensitivity.

Frez, Clifford F.↗
NASA NTRS2017

Spectral and Radiometric Calibration Using Tunable Lasers

A tunable laser system includes a tunable laser, an adjustable laser cavity for producing one or more modes of laser light emitted from the tunable laser, a first optical parametric oscillator positioned in a light path of the adjustable laser cavity, and a controller operable to simultaneously control parameters of at least the tunable laser, the first optical parametric oscillator, and the adjustable laser cavity to produce a range of wavelengths emitted from the tunable laser system. A method of operating a tunable laser system includes using a controller to simultaneously control parameters of a tunable laser, an adjustable laser cavity for producing one or more modes of laser light emitted from the tunable laser, and a first optical parametric oscillator positioned in a light path of the adjustable laser cavity, to produce a range of wavelengths emitted from the tunable laser system.

McCorkel, Joel↗
NASA NTRS1987

Diode pumped solid-state laser oscillators for spectroscopic applications

The rapid improvement in diode laser pump sources has led to the recent progress in diode laser pumped solid state lasers. To date, electrical efficiencies of greater than 10 percent were demonstrated. As diode laser costs decrease with increased production volume, diode laser and diode laser array pumped solid state lasers will replace the traditional flashlamp pumped Nd:YAG laser sources. The use of laser diode array pumping of slab geometry lasers will allow efficient, high peak and average power solid state laser sources to be developed. Perhaps the greatest impact of diode laser pumped solid state lasers will be in spectroscopic applications of miniature, monolithic devices. Single-stripe diode-pumped operation of a continuous-wave 946 nm Nd:YAG laser with less than 10 m/w threshold was demonstrated. A slope efficiency of 16 percent near threshold was shown with a projected slope efficiency well above a threshold of 34 percent based on results under Rhodamine 6G dye-laser pumping. Nonlinear crystals for second-harmonic generation of this source were evaluated. The KNbO3 and periodically poled LiNbO3 appear to be the most promising.

Byer, R. L.↗
NASA NTRS1990

Theoretical studies of solar pumped lasers

One concept for collecting solar energy is to use large solar collectors and then use lasers as energy converters whose output beams act as transmission lines to deliver the energy to a destination. The efficiency of the process would be improved if the conversion could be done directly using solar pumped lasers, and the possibility of making such lasers is studied. There are many applications for such lasers, and these are examined. By including the applications first, the requirements for the lasers will be more evident. They are especially applicable to the Space program, and include cases where no other methods of delivering power seem possible. Using the lasers for conveying information and surveillance is also discussed. Many difficulties confront the designer of an efficient system for power conversion. These involve the nature of the solar spectrum, the method of absorbing the energy, the transfer of power into laser beams, and finally, the far field patterns of the beams. The requirements of the lasers are discussed. Specific laser configurations are discussed. The thrust is into gas laser systems, because for space applications, the laser could be large, and also the medium would be uniform and not subject to thermal stresses. Dye and solid lasers are treated briefly. For gas lasers, a chart of the various possibilities is shown, and the various families of gas lasers divided according to the mechanisms of absorbing solar radiation and of lasing. Several specific models are analyzed and evaluated. Overall conclusions for the program are summarized, and the performances of the lasers related to the requirements of various applications.

Harries, Wynford L.↗
NASA NTRS1997

Elucidation of Free Radical and Optogalvanic Spectroscopy Associated with Microgravity Combustion via Conventional and Novel Laser Platforms

Combustion studies under both normal gravity and microgravity conditions depend a great deal on the availability and quality of the diagnostic systems used for such investigations. Microgravity phenomena are specially susceptible to even small perturbations and therefore non-intrusive diagnostic techniques are of paramount importance for successful understanding of reduced-gravity combustion phenomena. Several non-intrusive diagnostic techniques are available for probing and delineating normal as well as reduced gravity combustion processes, such as Rayleigh scattering, Raman scattering, Mie scattering, velocimetry, interferometric and Schlieren techniques, emission and laser-induced fluorescence (LIF) spectroscopy. Our approach is to use the LIF technique as a non-intrusive diagnostic tool for the study of combustion-associated free radicals and use the concomitant optogalvanic transitions to accomplish precise calibration of the laser wavelengths used for recording the excitation spectra of transient molecular species. In attempting to perform spectroscopic measurements on chemical intermediates, we have used conventional laser sources as well as new and novel platforms employing rare-earth doped solid-state lasers. Conventional (commercially available) sources of tunable UV laser radiation are extremely cumbersome and energy-consuming devices that are not very suitable for either in-space or in-flight (or microgravity drop tower) experiments. Traditional LIF sources of tunable UV laser radiation involve in addition to a pump laser (usually a Nd:YAG laser with an attached frequency-doubling stage), a tunable dye laser. In turn, the dye laser has to be provided with a dye circulation system and a subsequent stage for frequency-doubling of the dye laser radiation, together with a servo-tuning system (termed the 'Autotracker') to follow the wavelength changes and also an optical system (called the 'Frequency Separator') for separation of the emanating visible and UV beams. In contrast to this approach, we have devised an alternate arrangement for recording LIF excitation spectra of free radicals (following appropriate precursor fragmentation) that utilizes a tunable rare-earth doped solid state laser system with direct UV pumping. We have designed a compact and portable tunable UV laser system incorporating features necessary for both in-space and in-flight spectroscopy experiments. For the purpose of LIF excitation, we have developed an all-solid-state tunable UV laser that employs direct pumping of the solid-state UV-active medium employing UV harmonics from a Nd:YAG laser. An optical scheme with counterpropagating photolysis and excitation beams focused by suitable lenses into a reaction vacuum chamber was employed.

Misra, Prabhakar↗
NASA NTRS1997

Improvement in Suppression of Pulsed Nd:YAG Laser Light With Iodine Absorption Cells for Filtered Rayleigh Scattering Measurements

Filtered Rayleigh scattering using iodine absorption cells is an effective technique for obtaining density, temperature, and velocity measurements in high speed confined flows. By tuning a single frequency laser to a strong iodine absorption line, stray scattered laser light can be greatly suppressed. For example, the minimum transmission predicted by an iodine absorption model calculation is less than 10(exp -5) at the 18788.44/cm line using a 200 mm absorption cell containing iodine vapor at 0.46 T. Measurements obtained by other researches using a CW Nd:YAG laser agree with the model calculations. However, measurements made by us and by others using Q-switched, injection-seeded, frequency doubled Nd:YAG lasers only show minimum transmission of about 3 x 10(exp -3). This greatly reduces the applicability of the filtered Rayleigh scattering technique using these lasers in experiments having large amounts of stray scattered laser light. The purposes of the present study are to characterize the spectrum of the excess light transmitted by the iodine cell and to make changes to the laser to reduce the transmitted laser light. Transmission data as a function of laser frequency for the iodine absorption line at 18788.44/cm are presented. A planar mirror Fabry-Perot interferometer was used to characterize the frequency spectrum of the light passed through the cell. Measurements taken with the laser tuned to the center of the iodine absorption line show the light transmitted through the iodine cell to have a component with a bandwidth of about 40 GHz. This is probably caused by other modes in the laser that exist in spite of the single frequency injection beam. A second broadband component was also observed, possibly caused by the laser flash lamps or by fluorescence. An intracavity etalon was installed in the laser oscillator cavity to suppress the 40 GHz component. Measurements taken with the etalon tuned to the injection frequency showed a reduction in the transmitted laser light. This improvement allows the iodine cell to block significantly more of the stray laser light in filtered Rayleigh scattering experiments. Examples are given of filtered Rayleigh scattering measurements showing the effect of the etalon on measurements taken in a Mach 3 flow in the NASA Lewis 4 inch by 10 inch supersonic wind tunnel.

Seasholtz, Richard G.↗
NASA NTRS2000

The Geoscience Laser Altimeter System (GLAS) for the ICESAT Mission

The Laser In space Technology Experiment, Shuttle Laser Altimeter and the Mars Observer Laser Altimeter have demonstrated accurate measurements of atmospheric backscatter and Surface heights from space. The recent MOLA measurements of the Mars surface have 40 cm vertical resolution and have reduced the global uncertainty in Mars topography from a few km to about 5 m. The Geoscience Laser Altimeter System (GLAS) is a next generation lidar for Earth orbit being developed as part of NASA's Icesat Mission. The GLAS design combines a 10 cm precision surface lidar with a sensitive dual wavelength cloud and aerosol lidar. GLAS will precisely measure the heights of the Earth's polar ice sheets, establish a grid of accurate height profiles of the Earth's land topography, and profile the vertical backscatter of clouds and aerosols on a global scale. GLAS is being developed to fly on a small dedicated spacecraft in a polar orbit with a 590 630 km altitude at inclination of 94 degrees. GLAS is scheduled to launch in the summer 2001 and to operate continuously for a minimum of 3 years with a goal of 5 years. The primary mission for GLAS is to measure the seasonal and annual changes in the heights of the Greenland and Antarctic ice sheets. GLAS will continuously measure the vertical distance from orbit to the Earth's surface with 1064 nm pulses from a ND:YAG laser at a 40 Hz rate. Each 5 nsec wide laser pulse is used to produce a single range measurement, and the laser spots have 66 m diameter and about 170 m center-center spacings. When over land GLAS will profile the heights of the topography and vegetation. The GLAS receiver uses a 1 m diameter telescope and a Si APD detector. The detector signal is sampled by an all digital receiver which records each surface echo waveform with I nsec resolution and a stored echo record lengths of either 200, 400, or 600 samples. Analysis of the echo waveforms within the instrument permits discrimination between cloud and surface echoes. Ground based echo analysis permits precise ranging, determining the roughness or slopes of the surface as well as the vertical distributions of vegetation illuminated by the laser. Accurate knowledge of the laser beam's pointing angle is needed to prevent height biases when over sloped surfaces. For surfaces with 2 deg. slopes, knowledge of pointing angle of the beam's centroid to about 8 urad is needed to achieve 10 cm height accuracy. GLAS uses a stellar reference system (SRS) to determine the pointing angle of each laser firing relative to inertial space. The SRS uses a high precision star camera oriented toward local zenith and a gyroscope to determine the inertial orientation of the SRS optical bench. The far field pattern of each laser is measured pulse relative to the star camera with a laser reference system (LRS). Optically measuring each laser far field pattern relative to the orientation of the star camera and gyroscope permits the precise pointing angle of each laser pulse to be determined. GLAS will also determine the vertical distributions of clouds and aerosols by measuring the vertical profile of laser energy backscattered by the atmosphere at both 1064 and 532 nm. The 1064 nm measurements use the Si APD detector and profile the height and vertical structure of thicker clouds. The measurements at 532 nm use new highly sensitive photon counting, detectors, and measure the height distributions of very thin Clouds and aerosol layers. With averaging these can be used to determine the height of the planetary boundary layer. The instrument design and expected performance will be discussed.

Abshire, James B.↗
NASA NTRS2003

Evaluation and Characterization Study of Dual Pulse Laser-Induced Spark (DPLIS) for Rocket Engine Ignition System Application

This paper addresses the progress of technology development of a laser ignition system at NASA Marshall Space Flight Center (MSFC). Laser ignition has been used at MSFC in recent test series to successfully ignite RP1/GOX propellants in a subscale rocket chamber, and other past studies by NASA GRC have demonstrated the use of laser ignition for rocket engines. Despite the progress made in the study of this ignition method, the logistics of depositing laser sparks inside a rocket chamber have prohibited its use. However, recent advances in laser designs, the use of fiber optics, and studies of multi-pulse laser formats3 have renewed the interest of rocket designers in this state-of the-art technology which offers the potential elimination of torch igniter systems and their associated mechanical parts, as well as toxic hypergolic ignition systems. In support of this interest to develop an alternative ignition system that meets the risk-reduction demands of Next Generation Launch Technology (NGLT), characterization studies of a dual pulse laser format for laser-induced spark ignition are underway at MSFC. Results obtained at MSFC indicate that a dual pulse format can produce plasmas that absorb the laser energy as efficiently as a single pulse format, yet provide a longer plasma lifetime. In an experiments with lean H2/air propellants, the dual pulse laser format, containing the same total energy of a single laser pulse, produced a spark that was superior in its ability to provide sustained ignition of fuel-lean H2/air propellants. The results from these experiments are being used to optimize a dual pulse laser format for future subscale rocket chamber tests. Besides the ignition enhancement, the dual pulse technique provides a practical way to distribute and deliver laser light to the combustion chamber, an important consideration given the limitation of peak power that can be delivered through optical fibers. With this knowledge, scientists and engineers at Los Alamos National Laboratory and CFD Research Corporation have designed and fabricated a miniaturized, first-generation optical prototype of a laser ignition system that could be the basis for a laser ignition system for rocket applications. This prototype will be tested at MSFC in future subscale rocket ignition tests.

Osborne, Robin↗
NASA NTRS2007

Laser Sounder for Global Measurement of CO2 Concentrations in the Troposphere from Space

Measurements of tropospheric CO2 abundance with global-coverage, a few hundred km spatial and monthly temporal resolution are needed to quantify processes that regulate CO2 storage by the land and oceans. The Orbiting Carbon Observatory (OCO) is the first space mission focused on atmospheric CO2 for measuring total column CO, and O2 by detecting the spectral absorption in reflected sunlight. The OCO mission is an essential step, and will yield important new information about atmospheric CO2 distributions. However there are unavoidable limitations imposed by its measurement approach. These include best accuracy only during daytime at moderate to high sun angles, interference by cloud and aerosol scattering, and limited signal from CO2 variability in the lower tropospheric CO2 column. We have been developing a new laser-based technique for the remote measurement of the tropospheric CO2 concentrations from orbit. Our initial goal is to demonstrate a lidar technique and instrument technology that will permit measurements of the CO2 column abundance in the lower troposphere from aircraft. Our final goal is to develop a space instrument and mission approach for active measurements of the CO2 mixing ratio at the 1-2 ppmv level. Our technique is much less sensitive to cloud and atmospheric scattering conditions and would allow continuous measurements of CO2 mixing ratio in the lower troposphere from orbit over land and ocean surfaces during day and night. Our approach is to use the 1570nm CO2 band and a 3-channel laser absorption spectrometer (i.e. lidar used an altimeter mode), which continuously measures at nadir from a near polar circular orbit. The approach directs the narrow co-aligned laser beams from the instrument's lasers toward nadir, and measures the energy of the laser echoes reflected from land and water surfaces. It uses several tunable fiber laser transmitters which allowing measurement of the extinction from a single selected CO2 absorption line in the 1570 nm band. This band is free from interference from other gases and has temperature insensitive absorption lines. During the measurement the lasers are tuned on- and off- a selected CO2 line near 1572 nm and a selected O2 line near 768 nm in the Oxygen A band at kHz rates. The lasers use tunable diode seed lasers followed by fiber amplifiers, and have spectral widths much narrower than the gas absorption lines. The receiver uses a 1-m diameter telescope and photon counting detectors and measures the background light and energies of the laser echoes from the surface. The extinction and column densities for the CO2 and O2 gases are estimated from the ratio of the on and offline surface echo via the differential optical absorption technique. Our technique rapidly alternates between several on-line wavelengths set to the sides of the selected gas absorption lines. It exploits the atmospheric pressure broadening of the lines to weight the measurement sensitivity to the atmospheric column below 5 km. This maximizes sensitivity to CO2 in the boundary layer, where variations caused by surface sources and sinks are largest. Simultaneous measurements of O2 column will use an identical approach with an O2 line. Thee laser frequencies are tunable and have narrow (MHz) line widths. In combination with sensitive photon counting detectors these enables much higher spectral resolution and precision than is possible with passive spectrometer. 1aser backscatter profiles are also measured, which permits identifying measurements made to cloud tops and through aerosol layers. The measurement approach using lasers in common-nadir-zenith path allows retrieving CO2 column mixing ratios in the lower troposphere irrespective of sun angle. Pulsed laser signals, time gated receiver and a narrow receiver field-of-view are used to isolate the surface laser echo signals and to exclude photons scattered from clouds and aerosols. Nonetheless, the optical absorption change due to a change of a few ppO2 is small, <1 % which makes achieving the needed measurement sensitivities and stabilities quite challenging. Measurement SNRs and stabilities of >600:1 are needed to estimate CO2 mixing ratio at the 1-2 ppm level. We have calculated characteristics of the technique and have demonstrated aspects of the laser, detector and receiver approaches in th e laboratory We have also measured O2 in an absorption cell, and made C02 measurements over a 400 m long (one way) horizontal path using a sensor breadboard. We will describe these and more details of our approach in the paper.

Abshire, James B.↗
NASA NTRS2011

Multiple-Zone Diffractive Optic Element for Laser Ranging Applications

A diffractive optic element (DOE) can be used as a beam splitter to generate multiple laser beams from a single input laser beam. This technology has been recently used in LRO s Lunar Orbiter Laser Altimeter (LOLA) instrument to generate five laser beams that measure the lunar topography from a 50-km nominal mapping orbit (see figure). An extension of this approach is to use a multiple-zone DOE to allow a laser altimeter instrument to operate over a wider range of distances. In particular, a multiple-zone DOE could be used for applications that require both mapping and landing on a planetary body. In this case, the laser altimeter operating range would need to extend from several hundred kilometers down to a few meters. The innovator was recently involved in an investigation how to modify the LOLA instrument for the OSIRIS asteroid mapping and sample return mission. One approach is to replace the DOE in the LOLA laser beam expander assembly with a multiple-zone DOE that would allow for the simultaneous illumination of the asteroid with mapping and landing laser beams. The proposed OSIRIS multiple-zone DOE would generate the same LOLA five-beam output pattern for high-altitude topographic mapping, but would simultaneously generate a wide divergence angle beam using a small portion of the total laser energy for the approach and landing portion of the mission. Only a few percent of the total laser energy is required for approach and landing operations as the return signal increases as the inverse square of the ranging height. A wide divergence beam could be implemented by making the center of the DOE a diffractive or refractive negative lens. The beam energy and beam divergence characteristics of a multiple-zone DOE could be easily tailored to meet the requirements of other missions that require laser ranging data. Current single-zone DOE lithographic manufacturing techniques could also be used to fabricate a multiple-zone DOE by masking the different DOE zones during the manufacturing process, and the same space-compatible DOE substrates (fused silica, sapphire) that are used on standard DOE s could be used for multiple- zone DOE s. DOEs are an elegant and cost-effective optical design option for spacebased laser altimeters that require multiple output laser beams. The use of multiple-zone DOEs would allow for the design and optimization of a laser altimeter instrument required to operate over a large range of target distances, such as those designed to both map and land on a planetary body. In addition to space-based laser altimeters, this technology could find applications in military or commercial unmanned aerial vehicles (UAVs) that fly at an altitude of several kilometers and need to land. It is also conceivable that variations of this approach could be used in land-based applications such as collision avoidance and robotic control of cars, trains, and ships.

Ramos-Izquierdo, Luis A.↗
NASA NTRS2005

High-Power Fiber Lasers Using Photonic Band Gap Materials

High-power fiber lasers (HPFLs) would be made from photonic band gap (PBG) materials, according to the proposal. Such lasers would be scalable in the sense that a large number of fiber lasers could be arranged in an array or bundle and then operated in phase-locked condition to generate a superposition and highly directed high-power laser beam. It has been estimated that an average power level as high as 1,000 W per fiber could be achieved in such an array. Examples of potential applications for the proposed single-fiber lasers include welding and laser surgery. Additionally, the bundled fibers have applications in beaming power through free space for autonomous vehicles, laser weapons, free-space communications, and inducing photochemical reactions in large-scale industrial processes. The proposal has been inspired in part by recent improvements in the capabilities of single-mode fiber amplifiers and lasers to produce continuous high-power radiation. In particular, it has been found that the average output power of a single strand of a fiber laser can be increased by suitably changing the doping profile of active ions in its gain medium to optimize the spatial overlap of the electromagnetic field with the distribution of active ions. Such optimization minimizes pump power losses and increases the gain in the fiber laser system. The proposal would expand the basic concept of this type of optimization to incorporate exploitation of the properties (including, in some cases, nonlinearities) of PBG materials to obtain power levels and efficiencies higher than are now possible. Another element of the proposal is to enable pumping by concentrated sunlight. Somewhat more specifically, the proposal calls for exploitation of the properties of PBG materials to overcome a number of stubborn adverse phenomena that have impeded prior efforts to perfect HPFLs. The most relevant of those phenomena is amplified spontaneous emission (ASE), which causes saturation of gain and power at undesirably low levels, and scattering of light from dopants. In designing a given fiber laser for reduced ASE, care must be taken to maintain a correct fiber structure for eventual scaling to an array of many such lasers such that the interactions among all the members of the array would cause them to operate in phase lock. Hence, the problems associated with improving a single-fiber laser are not entirely separate from the bundling problem, and some designs for individual fiber lasers may be better than others if the fibers are to be incorporated into bundles. Extensive calculations, expected to take about a year, must be performed in order to determine design parameters before construction of prototype individual and fiber lasers can begin. The design effort can be expected to include calculations to optimize overlaps between the electromagnetic modes and the gain media and calculations of responses of PBG materials to electromagnetic fields. Design alternatives and physical responses that may be considered include simple PBG fibers with no intensity-dependent responses, PBG fibers with intensity- dependent band-gap shifting (see figure), and broad-band pumping made possible by use of candidate broad-band pumping media in place of the air or vacuum gaps used in prior PBG fibers.

DiDomenico, Leo↗