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At least 91 records · Page 5

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NASA NTRS1990

A rate equation approach to gain saturation effects in laser mode calculations

Space exploration and research require large amounts of power. Solar pumped laser systems have been shown to have the potential for meeting the performance requirements necessary for power transmission in space. The successful design of laser systems for these applications will depend, in part, on having a clear understanding of the development of the dynamical processes in the laser cavity and on the effects that changes in physical and design parameters have on laser performance. In particular, it is necessary to know the amplitude and phase distributions of the laser beam at the output aperture when steady state operation is achieved in order to determine the far-field power distribution. The output from the laser will depend on the active medium, the optical environment of the active material and on the gain distribution in the active region as laser action builds up and reaches steady state. An important component in the design process will be a realistic model of the active laser cavity. A computer model of the laser cavity, based on Huygens' principle was developed. The code calculates the amplitude and phase of an optical wave reflected back and forth between the mirrors of a laser cavity. The original code assumes a gain distribution which does not change with the buildup of oscillations in the cavity. A step in the direction of realism is the inclusion of a saturable gain medium in the cavity. The objective is to incorporate saturation effects into the existing computer model.

Roberts, Lila F.↗

NASA NTRS1996

Semiconductor Laser Low Frequency Noise Characterization

This work summarizes the efforts in identifying the fundamental noise limit in semiconductor optical sources (lasers) to determine the source of 1/F noise and it's associated behavior. In addition, the study also addresses the effects of this 1/F noise on RF phased arrays. The study showed that the 1/F noise in semiconductor lasers has an ultimate physical limit based upon similar factors to fundamental noise generated in other semiconductor and solid state devices. The study also showed that both additive and multiplicative noise can be a significant detriment to the performance of RF phased arrays especially in regard to very low sidelobe performance and ultimate beam steering accuracy. The final result is that a noise power related term must be included in a complete analysis of the noise spectrum of any semiconductor device including semiconductor lasers.

SEMICONDUCTOR LASERS↗

NASA NTRS1973

Theory of a ring laser

Development of a systematic formulation of the theory of a ring laser which is based on first principles and uses a well-known model for laser operation. A simple physical derivation of the electromagnetic field equations for a noninertial reference frame in uniform rotation is presented, and an attempt is made to clarify the nature of the Fox-Li modes for an open polygonal resonator. The polarization of the active medium is obtained by using a Fourier-series method which permits the formulation of a strong-signal theory, and solutions are given in terms of continued fractions. It is shown that when such a continued fraction is expanded to third order in the fields, the familiar small-signal ring-laser theory is obtained.

Menegozzi, L. N.↗

NASA NTRS2000

Nonlinear Optical Properties and Applications of Polydiacetylene

Recently, we have demonstrated a picosecond all-optical switch, which also functions as a partial all-optical NAND logic gate using a novel polydiacetylene that is synthesized in our laboratory. The nonlinear optical properties of the polydiacetylene material are measured using the Z-scan technique. A theoretical model based on a three level system is investigated and the rate equations of the system are solved. The theoretical calculations are proven to match nicely with the experimental results. The absorption cross-sections for both the first and higher excited states are estimated. The analyses also show that the material suffers a photochemical change beyond a certain level of the laser power and its physical properties suffer radical changes. These changes are the cause for the partial NAND gate function and the switching mechanism.

Abdeldayem, Hossin↗

NASA NTRS2003

A novel two-step laser ranging technique for a precision test of the theory of gravity

All powered spacecraft experience residual systematic acceleration due to anisotropy of the thermal radiation pressure and fuel leakage. The residual acceleration limits the accuracy of any test of gravity that relies on the precise determination of the spacecraft trajectory. We describe a novel two-step laser ranging technique, which largely eliminates the effects of non-gravity acceleration sources and enables celestial mechanics checks with unprecedented precision.

ranging↗

NASA NTRS2008

Demonstration of Aerosol Property Profiling by Multi-wavelength Lidar Under Varying Relative Humidity Conditions

The feasibility of using a multi-wavelength Mie-Raman lidar based on a tripled Nd:YAG laser for profiling aerosol physical parameters in the planetary boundary layer (PBL) under varying conditions of relative humidity (RH) is studied. The lidar quantifies three aerosol backscattering and two extinction coefficients and from these optical data the particle parameters such as concentration, size and complex refractive index are retrieved through inversion with regularization. The column-integrated, lidar-derived parameters are compared with results from the AERONET sun photometer. The lidar and sun photometer agree well in the characterization of the fine mode parameters, however the lidar shows less sensitivity to coarse mode. The lidar results reveal a strong dependence of particle properties on RH. The height regions with enhanced RH are characterized by an increase of backscattering and extinction coefficient and a decrease in the Angstrom exponent coinciding with an increase in the particle size. We present data selection techniques useful for selecting cases that can support the calculation of hygroscopic growth parameters using lidar. Hygroscopic growth factors calculated using these techniques agree with expectations despite the lack of co-located radiosonde data. Despite this limitation, the results demonstrate the potential of multi-wavelength Raman lidar technique for study of aerosol humidification process.

Whiteman, D.N.↗

NASA NTRS2005

Learning for Autonomous Navigation

Robotic ground vehicles for outdoor applications have achieved some remarkable successes, notably in autonomous highway following (Dickmanns, 1987), planetary exploration (1), and off-road navigation on Earth (1). Nevertheless, major challenges remain to enable reliable, high-speed, autonomous navigation in a wide variety of complex, off-road terrain. 3-D perception of terrain geometry with imaging range sensors is the mainstay of off-road driving systems. However, the stopping distance at high speed exceeds the effective lookahead distance of existing range sensors. Prospects for extending the range of 3-D sensors is strongly limited by sensor physics, eye safety of lasers, and related issues. Range sensor limitations also allow vehicles to enter large cul-de-sacs even at low speed, leading to long detours. Moreover, sensing only terrain geometry fails to reveal mechanical properties of terrain that are critical to assessing its traversability, such as potential for slippage, sinkage, and the degree of compliance of potential obstacles. Rovers in the Mars Exploration Rover (MER) mission have got stuck in sand dunes and experienced significant downhill slippage in the vicinity of large rock hazards. Earth-based off-road robots today have very limited ability to discriminate traversable vegetation from non-traversable vegetation or rough ground. It is impossible today to preprogram a system with knowledge of these properties for all types of terrain and weather conditions that might be encountered.

mixture of Gaussians↗

NASA NTRS2013

Poisson-Spot Intensity Reduction with a Partially-Transparent Petal-Shaped Optical Mask

The presence of Poisson's spot, also known as the spot of Arago, formed along the optical axis in the geometrical shadow behind an obstruction, has been known since the 18th century. The presence of this spot can best be described as the consequence of constructive interference of light waves diffracted on the edge of the obstruction where its central position can··be determined by the symmetry of the object More recently, the elimination of this spot has received attention in the fields of particle physics, high-energy lasers, astronomy and lithography. In this paper, we introduce a novel, partially transparent petaled mask shape that suppresses the bright spot by up to 10 orders of magnitude in intensity, with powerful applications to many of the above fields. The optimization technique formulated in this design can identify mask shapes having partial transparency only near the petal tips.

Shiri, Shahram↗

NASA NTRS1976

The waveguide laser - A review

The present article reviews the fundamental physical principles essential to an understanding of waveguide gas and liquid lasers, and the current technological state of these devices. At the present time, waveguide laser transitions span the visible through submillimeter regions of the wavelength spectrum. The introduction discusses the many applications of waveguide lasers and the wide variety of laser configurations that are possible. Section 1 summarizes the properties of modes in hollow dielectric waveguides of circular, rectangular, and planar cross section. Section 2 considers various approaches to optical feedback including internal and external mirror Fabry-Perot type resonators, hollow waveguide distributed feedback structures, and ring-resonant configurations. Section 3 discusses those aspects of molecular kinetic and laser theory pertinent to the design and optimization of waveguide gas lasers.

Degnan, J. J.↗

NASA NTRS1986

Alexandrite laser source for atmospheric lidar measurements

During the past years, there has been a marked increase in interest in the applications of vibronic solid state lasers to meteorology and atmospheric physics. Two airborne lidar programs are now under development in France. The differential absorption lidar (DIAL) method with vibronic solid state lasers is very attractive for water vapor, temperature and pressure measurements. Alexandrite laser and titanium-sapphire are both suitable for these applications. However, only alexandrite rods are commercially available. The requirements on the laser source for airborne dial applications are two fold: (1) a restriction on laser linewidth and a requirement on stability and tunability with a good spectral purity; and (2) a requirement on the time separation between the two pulses. These constraints are summarized.

Pelon, J.↗

NASA NTRS1978

Atmospheric effects on CO2 laser propagation

An investigation was made of the losses encountered in the propagation of CO2 laser radiation through the atmosphere, particularly as it applies to the NASA/Marshall Space Flight Center Pulsed Laser Doppler System. As such it addresses three major areas associated with signal loss: molecular absorption, refractive index changes in a turbulent environment, and aerosol absorption and scattering. In particular, the molecular absorption coefficients of carbon dioxide, water vapor, and nitrous oxide are calculated for various laser lines in the region of 10.6 mu m as a function of various pressures and temperatures. The current status in the physics of low-energy laser propagation through a turbulent atmosphere is presented together with the analysis and evaluation of the associated heterodyne signal power loss. Finally, aerosol backscatter and extinction coefficients are calculated for various aerosol distributions and the results incorporated into the signal-to-noise ratio equation for the Marshall Space Flight Center system.

Murty, S. S. R.↗

NASA NTRS1995

The Effects of Rapid Heating of Soot: Implications When Using Laser-Induced Incandescence for Soot Diagnostics

Recent experimental efforts have exploited the high temporal and spatial resolution of laser-induced incandescence (LII) as both a qualitative and quantitative measure of soot volume fraction. As a relatively new diagnostic technique, issues remain as to appropriate excitation laser intensities and the poential intrusive characteristics of LII. The high temperatures to which the soot is heated may accelerate heterogeneous reactions between the soot and flame gases. Vaporization of soot by high energy pulsed laser light has been theoretically modelled and experimentally observed. Potential physical and/or chemical changes in the laser-heated soot raises the question of how the LII signal depends upon these changes as well as the inferred soot volume fraction. Thus we investigated the effects of high energy pulsed laser light on the soot particles.

Vanderwal, Randy L.↗

NASA NTRS1988

Studies of blackbody-pumped lasers

Laser action was achieved in a large-scale pulsed system and meaningful experiments were carried out in the axial flow facility to understand the physics of solar-pumped lasers. Studies of the variation of laser mixture composition were done for mixtures of CO2 and N2O additions. Theoretical analysis and modeling of the system were also performed based on a model including radiative pumping, diffusion and wall deactivation, and vibrational energy transfer between two optically active species. Qualitative agreement between predictions in the parameter variation and the experimental results were obtained with CO and N2O mixtures. Several experimental studies were performed to quantify the properties of carbon monoxide as absorber and storage of energy in the blackbody pumped system.

Christiansen, W. H.↗

NASA NTRS1989

An investigation of a mathematical model of an optically pumped Ti(3+):Al2O3 laser system

During the last several years, solid state lasers were developed that have the potential for meeting rigorous performance requirements for space-based remote sensing of the atmosphere. In order to design a stable and efficient laser and to understand the effect on laser output of changes in the physical and design parameters, an understanding of the development of the dynamical processes of the laser is necessary. Typically, the dynamical processes in a laser system are investigated via rate equations describing the evolution of the occupancy in the electronic levels and of the photon density in the laser cavity. There are two approaches to this type of study. Most often, for the sake of simplicity, the spatial variations of the dynamic variables in the laser system are disregarded and the mathematical model consists of a system of first order nonlinear ordinary differential equations (ODE). The second approach is to take into account both spatial and temporal variations in the dynamic variables in the laser cavity. The resulting model consists of a first order semilinear system of partial differential equations (PDE). The model which was studied was studied was generic in the sense that it was a four-level laser system, but the parameters used in the numerical study were specific to Titanium-doped sapphire. For simplicity, a constant, spatially uniform pumping scheme was considered. In addition, a simplification of the model was made so that it treats a single lasing wavelength with a narrow bandwidth. The purpose was to investigate both versions of the mathematical model and to determine whether the numerical solutions are similar both qualitatively and quantitatively. The systems of ordinary differential equations were solved numerically using a Runge-Kutta-Fehlberg algorithm which was very efficient for typical values of the physical parameters. A numerical scheme, based on the Modified Euler method, for computing solutions to the system of partial differential equations was developed and implemented. The PDE model was solved numerically at the expense of greatly increased computer time.

Roberts, Lila F.↗

NASA NTRS1985

Laser Systems

Tunable diode lasers are employed as radiation sources in high resolution infrared spectroscopy to determine spectral characteristics of gaseous compounds. With other laser systems, they are produced by Spectra-Physics, and used to monitor chemical processes, monitor production of quantity halogen lamps, etc. The Laser Analytics Division of Spectra-Physics credits the system's reliability to a program funded by Langley in the 1970s. Company no longer U.S.-owned. 5/22/97

Source record↗

NASA NTRS1985

Cavity blackbody-pumpd lasers: Present research status

The basic concept for a blackbody cavity is given. A comparison between the direct solar-pumped laser efficiency and one which is indirectly pumped by a blackbody cavity shows that they are similar to a carnot cycle. The blackbody spectrum used to optically pump the lasant material is shown. Also shown is the physical principle behind the blackbody laser idea. An example of an infrared laser media is given. A system study was conducted to scale cavity blackbody pumped lasers to high output powers.

Christiansen, W.↗

NASA NTRS1998

Cloud and Radiation Mission with Active and Passive Sensing from the Space Station

A cloud and aerosol radiative forcing and physical process study involving active laser and radar profiling with a combination of passive radiometric sounders and imagers would use the space station as an observation platform. The objectives are to observe the full three dimensional cloud and aerosol structure and the associated physical parameters leading to a complete measurement of radiation forcing processes. The instruments would include specialized radar and lidar for cloud and aerosol profiling, visible, infrared and microwave imaging radiometers with comprehensive channels for cloud and aerosol observation and specialized sounders. The low altitude,. available power and servicing capability of the space station are significant advantages for the active sensors and multiple passive instruments.

Spinhirne, James D.↗

NASA NTRS1973

Three-dimensional gas turbulence measurement with a laser-Doppler velocimeter system

Laser-Doppler system records gas-velocity data over wide dynamic range in three-dimensional space without physical probe. System detects shift in laser beam scattered by flowing particles and uses this frequency to calculate particle velocities. Technique is based on principle that laser beam scattered by flowing particles is shifted in frequency by amount proportional to laser frequency.

Fuller, C. E.↗