Roger R. Petrin

Wave optics simulation of atmospheric turbulence and reflective speckle effects in CO 2 lidar

Laser speckle can influence lidar measurements from a diffuse hard target. Atmospheric optical turbulence will also affect the lidar return signal. We present a numerical simulation that models the propagation of a lidar beam and accounts for both reflective speckle and atmospheric turbulence effects. Our simulation is based on implementing a Huygens-Fresnel approximation to laser propagation. A series of phase screens, with the appropriate atmospheric statistical characteristics, are used to simulate the effect of atmospheric turbulence. A single random phase screen is used to simulate scattering of the entire beam from a rough surface. We compare the output of our numerical model with separate CO(2) lidar measurements of atmospheric turbulence and reflective speckle. We also compare the output of our model with separate analytical predictions for atmospheric turbulence and reflective speckle. Good agreement was found between the model and the experimental data. Good agreement was also found with analytical predictions. Finally, we present results of a simulation of the combined effects on a finite-aperture lidar system that are qualitatively consistent with previous experimental observations of increasing rms noise with increasing turbulence level.

Quantitative chemical identification of four gases in remote infrared (9–11 µm) differential absorption lidar experiments

A combined experimental and computational approach utilizing tunable CO(2) lasers and chemometric analysis was employed to detect chemicals and their concentrations in the field under controlled release conditions. We collected absorption spectra for four organic gases in the laboratory by lasing 40 lines of the laser in the 9.3-10.8-mum range. The ability to predict properly the chemicals and their respective concentrations depends on the nature of the target, the atmospheric conditions, and the round-trip distance. In 39 of the 45 field experiments, the identities of the released chemicals were identified correctly without predictions of false positives or false negatives.

Remote mapping of vegetation and geological features by lidar in the 9–11-µm region

We report examples of the use of a scanning tunable CO(2) laser lidar system in the 9-11-mum region to construct images of vegetation and rocks at ranges as far as 5 km from the instrument. Range information is combined with horizontal and vertical distances to yield an image with three spatial dimensions simultaneous with the classification of target type. Object classification is based on reflectance spectra, which are sufficiently distinct to allow discrimination between several tree species, between trees and scrub vegetation, and between natural and artificial targets. Limitations imposed by laser speckle noise are discussed.

Materials characterization, optical spectroscopy, and laser damage studies of electrochromically and photochromically damaged KTiOPO4 (KTP)

The techniques utilized to study the surface and bulk properties of KTiOPO4 (KTP) were Rutherford backscattering (RBS), particle induced x-ray emission (PIXE), secondary ion mass spectrometry (SIMS), optical absorption and emission spectroscopy, and controlled laser damage. RBS and SIMS results provide strong evidence for potassium ion and titanium ion migration from the bulk to the electrode surface under an applied DC voltage. Optical measurements suggest the presence of Ti3+ ions in pristine, EC and PC damages KTP. Catastrophic damage was induced models will be presented to rationalize the RBS, PIXE and SIMS data for the impurities, and a damage mechanism consistent with the findings of the laser damage and optical absorption and emission experiments will be discussed.

Atmospheric effects on CO2 differential absorption lidar sensitivity

The ambient atmosphere between the laser transmitter and the target can affect CO2 differential absorption lidar (DIAL) measurement sensitivity through a number of different processes. In this work, we will address two of the sources of atmospheric interference with CO2 DIAL measurements: effects due to beam propagation through atmospheric turbulence and extinction due to absorption by atmospheric gases. Measurements of atmospheric extinction under different atmospheric conditions are presented and compared to a standard atmospheric transmission model (FASCODE). We have also investigated the effects of atmospheric turbulence on system performance. Measurements of the effective beam size after propagation are compared to model predictions using simultaneous measurements of atmospheric turbulence as input to the model. These results are also discussed in the context of the overall effect of beam propagation through atmospheric turbulence on the sensitivity of DIAL measurements.

Spectroscopic analysis of infrared DIAL measurements

A combined experimental and computational approach utilizing CO2 infrared gas lasers and chemometric multivariate analysis was employed to detect chemicals and their concentrations in the open atmosphere under controlled release conditions. Absorption spectra of four organic gases were collected in the laboratory by lasing 40 lines of a Synrad 15 W CO2 laser in the 9.3 to 10.8 micron range. Several chemometric calibration models were constructed based on this IR data using the Partial Least Squares computational technique. The chemometric models were used to analyze in near real time the field DIAL data acquired over this exact wavelength range at round trip distances of 7 and 13 km. It will be shown that the ability to predict the chemicals and their respective concentrations depends on a variety of factors. In 39 of the 45 experiments, the identities of the released chemicals were correctly identified without predictions of false positives or false negatives. Under the best field conditions, we achieved predictions of absolute concentrations within 30% of the actual values.

Target characterization in 3D using infrared lidar

We report examples of the use of a scanning tunable CO2 laser lidar system in the 9-11 micrometers region to construct images of vegetation and rocks at ranges of up to 5 km from the instrument. Range information is combined with horizontal and vertical distances to yield an image with three spatial dimensions simultaneous with the classification of target type. Object classification is made possible by the distinct spectral signatures of both natural and man-made objects. Several multivariate statistical methods are used to illustrate the degree of discrimination possible among the natural variability of objects in both spectral shape and amplitude.

Using active TIR imaging for ground cover identification and mapping

The authors report examples of the use of a scanning tunable CO/sub 2/ laser lidar system in the 9-11 ym region to construct images of vegetation and rocks at ranges of up to 5 km from the instrument. Range information is combined with horizontal and vertical distances to yield an image with three spatial dimensions simultaneous with the classification of target type. Reflectance spectra in this region are sufficiently distinct to discriminate between several tree species, between trees and scrub vegetation, and between natural and artificial targets. Lidar sensing of vegetation offers some complementary characteristics to passive remote sensing. In the thermal infrared (8-12 /spl mu/m), lidar interrogation of natural targets is essentially a pure reflectance measurement, unaffected by topographical shading and spatial variations in temperature. Differential measurements using CO/sub 2/ lasers have been known for some time to be useful in discriminating between vegetation types, tree species, and rock types. The authors have investigated the utility of a scanning CO/sub 2/ DIAL system in constructing vegetation maps for broad areas.

Atmospheric effects on CO/sub 2/ differential absorption lidar performance

CO/sub 2/ differential absorption lidar (DIAL) performance can be adversely affected by the ambient atmosphere between the laser transmitter and the target through a number of different processes. This work addresses two sources of atmospheric interference with multi-spectral CO/sub 2/ DIAL measurements: effects due to beam propagation through atmospheric turbulence and extinction due to absorption by atmospheric gases. The authors compare the effective beam size after propagation to predictions from a beam propagation model that includes turbulence effects such as beam steering and beam spreading. They also compare the experimental measurements of atmospheric extinction to those predicted by both a standard atmospheric transmission model (FASCODE) and a chemometric analysis.

Advanced laser sensing receiver concepts based on FPA technology

The ultimate performance of any remote sensor is ideally governed by the hardware signal-to-noise capability and allowed signal-averaging time. In real-world scenarios, this may not be realizable and the limiting factors may suggest the need for more advanced capabilities. Moving from passive to active remote sensors offers the advantage of control over the illumination source, the laser. Added capabilities may include polarization discrimination, instantaneous imaging, range resolution, simultaneous multi-spectral measurement, or coherent detection. However, most advanced detection technology has been engineered heavily towards the straightforward passive sensor requirements, measuring an integrated photon flux. The need for focal plane array technology designed specifically for laser sensing has been recognized for some time, but advances have only recently made the engineering possible. This paper will present a few concepts for laser sensing receiver architectures, the driving specifications behind those concepts, and test/modeling results of such designs.