Wavelength Dependence of Backscatter by use of Aerosol Microphysics and Lidar Data Sets: Application to 2.1- mum Wavelength for Space-Based and Airborne Lidars.

An aerosol microphysics dataset was used to model backscatter in the 0.35-11-mum wavelength range, with the results validated by comparison with measured cw and pulsed lidar backscatter obtained during two NASA-sponsored airborne field experiments. Different atmospheric features were encountered, with aerosol backscatter ranging over 4 orders of magnitude. Modeled conversion functions were used to convert existing lidar backscatter datasets to 2.1 mum. Resulting statistical distribution shows the midtropospheric aerosol backscatter background mode of beta(2.1) to be between ~3.0 x 10(-10) and ~1.3 x 10(-9) m(-1) sr(-1), ~10-20 times higher than that for beta(9.1); and a beta(2.1) boundary layer mode of ~1.0 x 10(-7) to ~1.3 x 10(-6) m(-1) sr(-1), ~3-5 times higher than beta(9.1).

Spectral analysis of vibrational harmonic motion by use of a continuous-wave CO2 Doppler lidar.

Vibrational motion of a harmonic oscillator was investigated with a focused continuous-wave (cw) CO2 Doppler lidar at 9.1-microm wavelength. A continuum of frequencies along with many discrete, equally spaced, resonant frequency modes was observed. The frequency modes are similar in structure to the oscillatory longitudinal modes of a laser cavity and arise because of interference of the natural resonant frequency of the oscillator with specific frequencies within the continuum. Each consecutive resonant frequency mode occurred for a movement of the oscillator much less than the wavelength of incident lidar radiation. For vigorous vibration of the oscillator, the observed spectra may be indicating nonlinear motion.

Vertical variability of aerosol backscatter from an airborne-focused continuous-wave CO2 lidar at 9.1-microm wavelength.

Atmospheric aerosol backscatter measurements taken with a continuous-wave focused Doppler lidar at 9.1-microm wavelength were obtained over western North America and the Pacific Ocean from 13 to 26 September 1995 as part of a NASA airborne mission. Backscatter variability was measured for approximately 52 flight hours, covering an equivalent horizontal distance of approximately 30,000 km in the troposphere. Some quasi-vertical backscatter profiles were also obtained during various ascents and descents at altitudes that ranged from approximately 0.1 to 12 km. Similarities and differences for aerosol loading over land and ocean were observed. A midtropospheric aerosol backscatter background mode near 3 x 10(-11) to 1 x 10(-10) m(-1) sr(-1) was obtained, which is consistent with those of previous airborne and ground-based data sets.

Interference of backscatter from two droplets in a focused continuous-wave CO2 Doppler lidar beam.

With a focused continuous-wave CO(2) Doppler lidar at 9.1-microm wavelength, the superposition of backscatter from two appromately 14.12-microm-diameter silicone oil droplets in the lidar beam produced interference that resulted in a single backscatter pulse from the two droplets with a distinct periodic structure. This interference is caused by the phase difference in backscatter from the two droplets while they are traversing the lidar beam at different speeds, and thus the droplet separation is not constant. The complete cycle of interference, with periodicity 2pi, gives excellent agreement between measurements and lidar theory.

Comparison of Continuous-Wave CO(2) Lidar Calibration by Use of Earth-Surface Targets in Laboratory and Airborne Measurements.

Backscatter of several Earth surfaces was characterized in the laboratory as a function of incidence angle with a focused continuous-wave 9.1-mum CO(2) Doppler lidar for use as possible calibration targets. Some targets showed negligible angular dependence, while others showed a slight increase with decreasing angle. The Earth-surface signal measured over the complex Californian terrain during a 1995 NASA airborne mission compared well with laboratory data. Distributions of the Earth's surface signal shows that the lidar efficiency can be estimated with a fair degree of accuracy, preferably with uniform Earth-surface targets during flight for airborne or space-based lidar.

Signal processing and calibration of continuous-wave focused CO(2) Doppler lidars for atmospheric backscatter measurement.

Two continuous-wave (CW) focused CO(2) Doppler lidars (9.1 and 10.6 µm) were developed for airborne in situ aerosol backscatter measurements. The complex path of reliably calibrating these systems, with different signal processors, for accurate derivation of atmospheric backscatter coefficients is documented. Lidar calibration for absolute backscatter measurement for both lidars is based on range response over the lidar sample volume, not solely at focus. Both lidars were calibrated with a new technique using well-characterized aerosols as radiometric standard targets and related to conventional hard-target calibration. A digital signal processor (DSP), a surface acoustic wave spectrum analyzer, and manually tuned spectrum analyzer signal analyzers were used. The DSP signals were analyzed with an innovative method of correcting for systematic noise fluctuation; the noise statistics exhibit the chi-square distribution predicted by theory. System parametric studies and detailed calibration improved the accuracy of conversion from the measured signal-to-noise ratio to absolute backscatter. The minimum backscatter sensitivity is ~3 × 10(-12) m(-1) sr(-1) at 9.1 µm and ~9 × 10(-12) m(-1) sr(-1) at 10.6 µm. Sample measurements are shown for a flight over the remote Pacific Ocean in 1990 as part of the NASA Global Backscatter Experiment (GLOBE) survey missions, the first time to our knowledge that 9.1-10.6-µm lidar intercomparisons were made. Measurements at 9.1 µm, a potential wavelength for space-based lidar remote-sensing applications, are to our knowledge the first based on the rare isotope (12)C (18)O(2) gas.

Lidar calibration technique using laboratory-generated aerosols.

A new calibration technique for continuous-wave Doppler lidars that uses an aerosol scattering target has been developed. Calibrations with both single- and many-particle scattering were performed at the same lidar operating conditions as in atmospheric measurements. The calibrating targets, simulating atmospheric aerosols, were laboratory-generated spherical silicone oil droplets with known complex refractive indices and sizes, hence with known single-particle backscatter cross sections as obtained from Mie theory. Measurements of lidar efficiency with the conventional hard target calibration method were consistently higher by a factor of ~2 than measurements with the aerosol calibration technique. This result may have important implications for lidar backscatter estimates both for aerosol modeling efforts and for optimal design of future lidar systems. The aerosol calibration method provides a validation of basic lidar theory for particle scattering for coherent detection.

Comparison of calculated aerosol backscatter at 9.1- and 2.1-microm wavelengths.

Calculated aerosol backscatter for three common atmospheric aerosol compositions is higher at 2.1 microm than at 9.1 microm for low backscatter conditions and almost comparable for high backscatter conditions.

Laser-induced stimulated Raman scattering in the forward direction of a droplet: comparison of Mie theory with geometrical optics.

Comparison of Mie theory calculations of the internal electromagnetic source function for a 120-microm-diameter water droplet with geometrical optics suggests that the field enhancement located at the critical ring region encircling the axis in the forward direction of the droplet can support stimulated Raman scattering as found experimentally.

Pressure dependence of the laser-induced breakdown thresholds of gases and droplets.

Laser-induced breakdown threshold intensities for helium, argon, xenon and clean air were measured as a function of pressure (p < 900 Torr) at wavelength lambda = 0.532 microm using the Nd:YAG laser with 6.5-ns pulse duration. Pressure dependence of the breakdown of a 50-microm diam water droplet in these gases was also investigated. For pure gases, different free electron generation processes and electron loss processes dominate in different pressure regions. The water droplets decrease the breakdown thresholds up to 3 orders of magnitude depending on the pressure of the particular gas surrounding the droplet. For the droplet in He, Ar, and clean air for p < 800 Torr, the breakdown at the threshold intensity occurs inside the droplet and is independent of pressure. For the droplet in Xe, the breakdown occurs inside the droplet for p < 140 Torr; however, for p < 140 Torr, the breakdown occurs outside the droplet and is dependent on pressure. Transition from the breakdown inside to outside the droplet takes place in the pressure region where the breakdown thresholds of the bulk liquid and the pure gas are approximately equal.

Electromagnetic field enhancement in small liquid droplets using geometric optics.

The EM field enhancement in the forward direction of a dielectric sphere was derived leading to high-energy density in a critical ring region where, experimentally, the nonlinear processes a re observedf or small liquid droplets.