Brian J. Thompson
Yes, indeed, we have made a few small changes to the journal Optical Engineering to launch the new year and
Volume 30. The arrival ofanew editor is always a signal to make a few adjustments just to mark the changing of the guard-but we hope that on this occasion the changes are worthwhile in their own right...
Special Section Guest Editorial
John A. Reagan, MEMBER SPIE
University of Arizona
Department of Electrical and Computer Engineering
Tucson, Arizona 85721
This special section on lidar provides a snapshot of a variety of ongoing and future lidar applications and developments. It is by no means all inclusive because the field oflidar has grown too large in the almost 30 years since its inception to be treated comprehensively in a single issue.
The University of Wisconsin High Spectral Resolution Lidar (UW HSRL) produces direct measurements of cloud and aerosol optical depth, extinction cross section, backscatter cross section, and backscatter phase
function. The HSRL uses a multietalon interferometer to separate the backsctter return into a component due to particle scattering and a component due to scattering from air molecules. The molecular backscatter component is affected by extinction but not by particle backscatter. Because the molecular backscatter cross section is determined by the known atmospheric density, the atmospheric extinction can be directly calculated from the measured decrease in molecular backscatter signal with range. The
separation of aerosol from molecular scattering is possible because the backscatter component from air is Doppler-broadened by the thermal yelocities
of the molecules, while the backscatter from more massive, slower moving particles remains spectrally unbroadened. Although the HSRL was originally designed for airborne nadir observation of boundary layer aerosol optical properties, increases in transmitted power, receiver improvements, and modified calibration techniques have allowed it to measure cirrus cloud optical properties. A continuously pumped, Q-switched, 4 kHz pulse repetition frequency, injection seeded, frequency doubled Nd:YAG laser, still under development, has recently been installed and has reduced cirrus cloud measurement averaging times by a factor of '-10.
We describe a new two-frequency lidar for measuring mesospheric Na temperature profiles that uses a stabilized cw single-mode dye laser oscillator (rms frequency jitter < 1 MHz) followed by a pulsed dye power amplifier (140 MHz FWHM linewidth) that is pumped by an injection-locked Nd:YAG laser. The laser oscillator is tuned to the two operating
frequencies by observing the Doppler-free structure of the Na D2 fluorescence spectrum in a vapor cell. The lidar technique and our initial observations
of the temperature profile between 82 and 102 km at Ft. Collins, CO (40.6°N, 105°W) are described. Absolute temperature accuracies at the Na layer peak of better than K with a vertical resolution of 1 km and an integration period of approximately 5 mm were achieved in this initial experiment. Finally, we discuss a multiple frequency technique for the simultaneous measurement of both temperature and Doppler wind profiles.
A lidar facility has been established at the Jet Propulsion Laboratory- Table Mountain Facility located at an altitude of 2300 m in the San Gabriel Mountains in Southern California. This facility is using the technique of differential absorption lidar to measure atmospheric ozone concentration profiles. Two separate systems are needed to obtain the profile from the ground up to an altitude of 45 to 50 km. A Nd:YAG-based system is described for measurements from the ground up to 1 5 to 20 km altitude, and an excimer-laser-based system for measurements from 15 km
to 45 to 50 km altitude. The systems were designed to make high-precision, long-term measurements to aid in the detection of changes in the atmospheric ozone abundance through participation in the Network of Detection of Stratospheric Change.
As a part of the international Network for the Detection of Stratospheric Change, Goddard Space Flight Center has developed a mobile differential absorption lidar capable of making precise and accurate measurements in the stratosphere between 20 and 45 km. We present in this paper a description of the instrument, a discussion of the data analysis,
and some results from an intercomparison held at JPL's Table Mountain Observatory in California during October and November 1988.
Differential absorption lidar and Raman lidar have been applied to the range-resolved measurements of water vapor density for more than 20 years. During this period, there have been considerable advances in laser and lidar technology, as well as in the understanding of the factors required to optimize both lidar techniques for water vapor measurements. Results have been obtained using both lidar techniques that have led to improved understanding of water vapor distributions in the atmosphere. This paper reviews the theory of the measurements, including the sources of systematic and random error; the progress in lidar technology and techniques during that period, including a brief look at some of the lidar systems in development or proposed; and the steps being taken to improve such lidar systems.
A short pulse (8 ns) coherent Nd:YAG lidar at 1.06 im has been developed for 1 m range-resolved lidar measurements of high velocity (>1 km/s) aerosol or distributed targets with a Doppler shift bandwidth of up to 1 GHz. This system, which also permitted simultaneous heterodyne and direct detection, has been utilized to make, for the first time, an experimental comparison of the average carrier-to-noise ratio (CNR), signal-to-noise ratio, and standard deviation of the lidar return signals from hard targets. Nearly equal CNRs were measured with heterodyne and direct detection at a relatively short range of 450 m near the ground due to the wide electrical bandwidth (1 GHz) ofthe system. The experimental results were in good agreement with theoretical predictions that included the effects of atmospheric turbulence, and indicate the importance of atmospheric turbulence in the optimal design of a coherent lidar receiver at 1 m.
This paper compares two detectors for visible laser radar: a 1-D detector that resolves a target in range and a 3-D detector that resolves a target in angle and range. For both, a short pulse laser illuminates the target. For both, the receiver is based on a streak camera, which detects reflected light from the illuminated target and resolves the light in time.
The time resolution is 250 ps, so the target is resolved into 4 cm range cells. The 1-D detector focuses the reflected light to a point. The output
is the 1 -D, range-resolved projection of the target. The 3-D detector images the target on a focal plane, which is dissected by a fiber optic image converter attached to the streak camera. The output is a 3-D image of the target. For both detectors, we show data from two simple targets. The paper also compares two methods of remote sensing using these detectors:
2-D range tomography using data from the 1-D detector and angle-angle-range imagery using the 3-D data.
Clear-air atmospheric motions can be visualized for evaluating atmospheric transport and diffusion parameters using tracer techniques in which a specific gas or particulate material is released into the atmosphere and the distribution of the tracers is evaluated by photographic or in situ sampling. However, photographic methods apply only to relatively dense tracer concentrations, and three-dimensional structure is difficult to evaluate. In situ sampling normally does not provide required spatial resolution over both horizontal and vertical directions; in addition, the platform supporting the in situ sensor may modify tracer concentration
distributions. Lidar provides a method of mapping the three-dimensional distribution of tracer material released into the atmosphere with high spatial resolution from remote distances. A lidar operated from an airborne platform in a downward viewing direction can map tracer concentration
distributions over long downwind distances and can also profile the earth's surface elevation so that atmospheric behavior in complex terrain can be
effectively analyzed. This paper presents examples of airborne lidar used to observe the atmospheric distribution of tracer material by means of
elastic scattering from smoke plumes generated within the near-surface mixed layer, fluorescent scattering from dye particles released above the mixed layer by means of a cropduster aircraft, and differential absorption lidar measurements of SF6 tracer gas released at the surface level.
A lidar system is described that measures laser pulse time-offlight and the distortion of the pulse waveform for reflection from Earth surface terrain features. This instrument system is mounted on a highaltitude aircraft platform and operated in a repetitively pulsed mode for measurements of surface elevation profiles. The laser transmitter makes
use of recently developed short-pulse diode-pumped solid-state laser technology. Aircraft position in three dimensions is measured to submeter accuracy by use of differential Global Positioning System receivers. Instrument construction and performance are detailed.
A lidar system based on the 100 in. optical collimator at Wright- Patterson Air Force Base has been developed for middle atmosphere studies. The system has been demonstrated by recording Rayleigh backscatter returns from mesospheric air molecules at altitudes up to 90 km. These returns were then used to develop atmospheric density profiles. The design
of the system provided several unique engineering challenges due to the long focal length and size of the collimator used as the receiver telescope. Careful optical engineering in the receiver and an innovative, modular approach led to a design that eliminates potential problems due to defocus, detector nonuniformity, and detector saturation.
The Lidar In-Space Technology Experiment (LITE) is being developed by NASA/Langley Research Center for flight on the Space Shuttle. The system will detect stratospheric and tropospheric aerosols, probe the planetary boundary layer, measure cloud top heights, and measure atmospheric temperature and density in the range of 10 to 40 km. The system consists of a nominal 1 m diameter telescope receiver, a three-color neodymium: YAG laser transmitter, and the system electronics. The instrument makes extensive use of Space Shuttle resources for electrical power,
thermal control, and command and data handling. The instrument will fly on the Space Shuttle in mid-1993. This paper presents the engineering aspects of the design, fabrication, integration, and operation of the instrument. A companion paper by members of the LITE Science Steering Group that details the science aspects of LITE is in preparation and will be published at a later time.
This paper describes approaches for using the strong return signals from ground/sea reflections to improve upon the information that can be retrieved from spaceborne lidar observations. Relations are presented for computing lidar ground/sea returns, and examples are given for representative lidar parameters and surface characteristics. Various
methods are outlined for obtaining information about atmospheric transmittance, surface reflectance, and aerosol/cloud features from lidar ground/sea returns. Techniques for retrieving aerosol extinction profiles from atmospheric lidar returns are also presented, including retrieval examples.
A new aplanatic front lens system for microscope immersion objectives is proposed. This system consists of a uniform-index spherical outer shell suitably completed by an immersion liquid of the same index and an inner bead glass with spherically symmetric refractive index distribution. Such a doublet is theoretically capable of the stigmatic and real imaging between two concentric spheres, the one confined within the outer shell area (immersion) and the other external to the system. The refractive index profiles of the inner bead glass that are required for perfect imaging have been calculated on the basis of rigorous formalism of geometrical
optics. The derived index variations turn out to be quite moderate and seem to be technologically available.
Temperature and melting conditions during flash fusing are studied. As a first step to clarify the microscopic behavior of toner, the temperature changes, considering thermal nonlinearity and the local melt viscosity dependence on the temperature changes, are examined. Consideration of the nonlinearity is performed by modification of the thermophysical properties employed in the temperature calculations, on the bases of the qualitative results of experiments. Suitable modification methods are a thermal conduction constant model and increment of the
heat capacity model. The modification is necessary for the region from the start to the time the peak temperature appears on the toner surface. The local melt viscosity is analyzed by superposition of the calculated temperature changes and measured melt viscosity of the bulk toners. The melt viscosity varies a little with the kind of toner for the mid region in the toner layer. The melt viscosity calculated for the mid-region is one requirement determining the fixing strength. This means that to get good fixing strength, not only is it necessary to lower the melt viscosity, but also the thermophysical properties of the toner must be raised.
A thermal imaging apparatus is described for the nondestructive detection of subsurface defects in materials that would not usually lend themselves to thermal imaging because of their low emissivity and high susceptibility to background reflection noise. This is accomplished by transferring the thermal image produced by surface temperature perturbation
of the workpiece material to a high emissivity material with which it is continuously brought in contact. The transferred thermal image may
be observed by a suitable infrared device, resulting in a high radiance image with minimum reflectivity or variable emissivity noise. Numerical simulations, as well as experimental results, are presented.