Michael J. Kavaya

Target reflectance measurements for calibration of lidar atmospheric backscatter data.

Wavelength and angular dependence of reflectances and depolarization in the 9–11-μm region are reported for four standard targets: flowers of sulfur, flame-sprayed aluminum, 20-grit sandblasted aluminum, and 400-grit silicon carbide sandpaper. Measurements are presented and compared using a cw CO2 grating-tunable laser in a laboratory backscatter apparatus, an integrating sphere, and a coherent pulsed TEA-CO2 lidar system operating in the 9–11-μm region. Reflectance theory related to the use of hard targets to calibrate lidar atmospheric backscatter data is discussed.

Lidar aerosol backscatter measurements: systematic, modeling, and calibration error considerations

Sources of systematic, modeling, and calibration errors that affect the interpretation and calibration of lidar aerosol backscatter data are discussed. The treatment pertains primarily to ground-based pulsed CO2 lidars that probe the troposphere and are calibrated using hard calibration targets. However, a large part of the analysis is relevant to other types of lidar system such as lidars operating at other wavelengths; cw focused lidars; airborne or earth-orbiting lidars; lidars measuring other regions of the atmosphere; lidars measuring nonaerosol elastic or inelastic backscatter; and lidars employing other calibration techniques.

Atmospheric aerosol backscatter measurements using a tunable coherent CO2 lidar.

Measurements of atmospheric aerosol backscatter coefficients, using a coherent CO2 lidar at 9.25- and 10.6-μm wavelengths, are described. Vertical profiles of the volume backscatter coefficient β have been measured to a 10-km altitude over the Pasadena, Calif., region. These measurements indicate a wide range of variability in β both in and above the local boundary layer. Certain profiles also indicate a significant enhancement in β at the 9.25-μm wavelength compared with β at the 10.6-μm wavelength, which possibly indicates a major contribution to the volume backscatter from ammonium sulfate aerosol particles.

Evidence for speckle effects on pulsed CO2 lidar signal returns from remote targets

A pulsed CO2 lidar was used to study statistical properties of signal returns from various rough surfaces at distances near 2 km. These included natural in situ topographic materials as well as man-made hard targets. Three lidar configurations were used: heterodyne detection with single temporal mode transmitter pulses, and direct detection with single and multiple temporal mode pulses. The significant differences in signal return statistics, due largely to speckle effects, are discussed.

Optogalvanic stabilization and offset tuning of a carbon dioxide waveguide laser

A feedback loop employing the change in discharge impedance with output power is used to stabilize the frequency of a CO 2 waveguide laser. The relatively high operating pressure of the laser combined with a zero offset feature in the feedback loop allows continuous tuning of the stabilized frequency over a 300-400 MHz range.

Optoacoustic detection using Stark modulation.

Stark modulation of the absorbed laser radiation in an optoacoustic detector (or spectrophone) is reported. Measurements were made over a range of total pressure between 760 Torr and 50 Torr. Greatly enhanced molecular discrimination is suggested due to the tuning ability of the Stark-shifted absorption. The background signal obtained by operating in this mode is more than 500 times smaller than that obtained by operating the same optoacoustic detector in the conventional chopped radiation mode. The responsivity of the optoacoustic detector and the absorption coefficient of C(2)H(4) are presented as a function of total pressure.

Lidar telescope overlap function and effects of misalignment for unstable resonator transmitter and coherent receiver

Effects of lidar telescope overlap function modeling on the accuracy of aerosol backscatter measurements are discussed. The treatment pertains primarily to a lidar arrangement which combines high sensitivity and a large degree of geometrical signal compression but places stringent requirements on the accuracy of the overlap function modeling—coherent IR lidar with noncoaxial (side-by-side) transmit and receive telescopes. A model is presented which includes an unstable resonator transmitter and accommodates both coaxial and noncoaxial arrangements, and comparison is made to experimental results including effects of misalignment.

Pulse evolution and mode selection characteristics in a TEA-CO2 laser perturbed by injection of external radiation

Abstract A grating-tunable TEA-CO2 laser with an unstable resonator cavity, modified to allow injection of cw CO2 laser radiation at the resonant transition line by means of an intracavity NaCl window, has been used to study the coupling requirements for generation of single frequency pulses. The width and shape of the mode selection region, and the dependence of the gain-switched spike buildup time and the pulse shapes on the intensity and detuning frequency of the injected radiation are reported. Comparisons of the experimental results with previously reported mode selection behavior are discussed.

Tunable mode and line selection by injection in a TEA CO2 laser

Tunable mode selection by injection in pulsed CO2 lasers is examined, and both analytical and numerical models are used to compute the required injection power for a variety of experimental cases. These are treated in two categories: mode selection at a desired frequency displacement from the center frequency of a transition line in a dispersive cavity and mode (and line) selection at the center frequency of a selected transition line in a nondispersive cavity. The results point out the potential flexibility of pulsed injection in providing wavelength tunable high-energy single-frequency pulses.

Dependence of injection locking of a TEA CO 2 laser on intensity of injected radiation

We report the results of an experimental study to determine the minimum required injected power to control the output frequency of a TEA CO 2 laser. A CW CO 2 waveguide laser was used as the injection oscillator. Both the power and the frequency of the injected radiation were varied, while the TEA reasonator cavity length was adjusted to match the frequency of the injected signal. Single-longitudinal mode (SLM) TEA laser radiation was produced for injected power levels which are several orders of magnitude below those previously reported. The ratio of SLM output power to injection power exceeded 1012at the lowest levels of injected intensity.