Marsha J. Fox

Laser long-range remote-sensing program experimental results

A laser long range remote sensing (LRS) program is being conducted by the United States Air Force Phillips Laboratory (AF/PL). As part of this program, AF/PL is testing the feasibility of developing a long path CO2 laser-based DIAL system for remote sensing. In support of this program, the AF/PL has recently completed an experimental series using a 21 km slant- range path (3.05 km ASL transceiver height to 0.067 km ASL target height) at its Phillips Laboratory Air Force Maui Optical Station (AMOS) facility located on Maui, Hawaii. The dial system uses a 3-joule, 13C isotope laser coupled into a 0.6 m diameter telescope. The atmospheric optical characterization incorporates information from an infrared scintillometer co-aligned to the laser path, atmospheric profiles from weather balloons launched from the target site, and meteorological data from ground stations at AMOS and the target site. In this paper, we report a description of the experiment configuration, a summary of the results, a summary of the atmospheric conditions and their implications to the LRS program. The capability of such a system for long-range, low-angle, slant-path remote sensing is discussed. System performance issues relating to both coherent and incoherent detection methods, atmospheric limitations, as well as, the development of advanced models to predict performance of long range scenarios are presented.

Remote sensing measurements using a CO2 laser

The Phillips Laboratory is developing CO2 laser technology for making long range sensing measurements at multiple wavelengths in the 9 - 11 micron regime. A line selectable CO2 system that operates on both the P and R transitions at 9.6 and 10.6 microns is described. The device characteristics and laboratory calibration procedures designed to insure accurate measurements are discussed. The system is capable of making atmospheric gas measurements using either atmospheric backscatter or topographic reflection. Results of laboratory measurements using an SF6 absorption cell are presented. The techniques for data reduction and post processing are described. Included is the approach taken to perform the data reduction using multiple wavelengths for gas analysis and identification. Results will be used for design of a high power airborne system designed for a variety of military and environmental applications.

Design and performance simulations for an airborne DIAL system for long-range remote sensing applications

The U.S. Air Force Phillips Laboratory is evaluating the feasibility of long-standoff-range remote sensing of gaseous species present in trace amounts in the atmosphere. To date, the Phillips Laboratory program has been concerned with the preliminary design and performance analysis of a commercially available CO2 laser-based DIAL system operating from mountain-top-observatory and airborne platform and more recently with long-range ground testing using a 21.8 km slant path from 3.05 km ASL to sea level as the initial steps in the design and development of an airborne system capability. Straightforward scaling of the performance of a near-term technology direct-detection LIDAR system with propagation range to a topographic target and with the average atmospheric absorption coefficient along the path has been performed. Results indicate that useful airborne operation of such a system should be possible for slant path ranges between 20 km and 50 km, depending upon atmospheric transmission at the operating wavelengths of the 13C16O2 source. This paper describes the design of the airborne system which will be deployed on the Phillips Laboratory NC-135 research aircraft for DIAL system performance tests at slant ranges of 20 km to 50 km, scheduled for the near future. Performance simulations for the airborne tests will be presented and related to performance obtained during initial ground-based tests.

CO2 lidar measurements over a 20-km slant path

The Air Force Phillips Laboratory conducted a series of measurements in February, May and August 1995 at the Air Force Maui Optical Station (AMOS) facility on Maui, Hawaii, to determine system requirements for an airborne long path CO2 DIAL system. The lidar incorporates a cavity-matched mode-locked 3-J laser with the 60 cm diameter AMOS Beam Director Telescope. The one-way beam propagation path length was 21.3 km, originating at the AMOS facility on Haleakala at an altitude of 3.050 km ASL, and terminating at a target site near sea level. Both heterodyne and direct detection techniques are compared with respect to radiometric performance and signal statistics. Minimum detectable absorption levels for DIAL systems using both detection techniques and a variety of targets are estimated from long- range measurements with controlled absorbers. The signal correlation as a function of interpulse temporal separation was determined for long-range direct detection measurements. Radiometric models including system optical characteristics, beam propagation considerations, target reflectivity characteristics,a nd atmospheric effects have been developed and validated experimentally. A new receiver system is currently being fabricated and the laser transmitter is being upgraded for pulse-to-pulse wavelength agility, prior to incorporation into a C-135E airborne platform for future flight experiments.

Status report of an airborne CO2 transceiver for remote sensing employing direct and coherent detection

A multi-joule, wavelength agile, CO2 transceiver is being assembled in support of a two phase, airborne chemical sensing demonstration employing both direct (Phase I) and coherent (Phase II) detection methods. The Phase II, coherent detection transceiver concept design, and performance are described below.

Atmospheric characterization studies supporting the development of a long-range CO2 laser-based DIAL system

The Air Force Phillips Laboratory is testing the feasibility of developing a long-path, CO2 laser-based DIAL system for remote sensing applications from an airborne platform. The validity of DIAL system performance simulations for long slant-range paths is being established by means of well-characterized field experiments in which the contributions of atmospheric transmission and atmospheric-turbulence-induced beam spreading and scintillation are being independently measured concurrently with DIAL system radiometric performance. Initial measurements were performed with both diffuse and specular targets using a 3.2 km path located at the Phillips Laboratory Starfire Optical Range. Measurements reported herein were performed using a slant-range path of 21.3 km originating at the Phillips Laboratory AMOS facility on Maui, Hawaii. The latter location offers a slant-range propagation path from 3.04 km above sea level (ASL) to near sea level. The DIAL system under test utilized a 4-joule class laser coupled to 61 cm aperture beam director telescope. Measurements were performed with the laser operating on the C13 isotope in order to increase the atmospheric transmission with respect to a laser operating at C12O216 wavelengths. Concurrent atmospheric optical characterization measurements were performed with an infrared scintillometer operating over the same path and at the same wavelength as the DIAL system. Results of atmospheric propagation characterization measurements are described in this paper and results of DIAL system performance and comparisons to simulations are described in accompanying papers.

Long-path CO2 lidar measurements

The Air Force Phillips Laboratory is conducting a series of measurements at the Air Force Maui Optical Station (AMOS) facility on Maui, Hawaii, to determine system requirements for an airborne long path CO2 DIAL system. The lidar incorporates a cavity-matched 3-J laser with the 60 cm diameter AMOS laser beam director telescope. The beam propagation path is approximately 21 km, originating at the AMOS facility on Haleakala at an altitude of 3 km ASL, and terminating at a target site near sea level. Both heterodyne and direct detection techniques are being compared with respect to radiometric performance and signal statistics. Radiometric models including system optical characteristics, beam propagation considerations, target reflectivity characteristics, and atmospheric effects have been developed and validated experimentally. Predictions and results are presented, compared, and discussed.

Design, analysis, and testing of a wavelength-agile differential absorption light detection and ranging (DIAL) system for long-standoff range operation

The U.S. Air Force Phillips Laboratory is developing a differential absorption light detection and ranging (DIAL) system for use in long-range remote sensing of trace atmospheric species. A wavelength-agile (WAL) transverse-electric-atmospheric (TEA) carbon dioxide laser operating on P- and R-branch transitions in the 9.4 micrometers bans is used as the pulsed radiation source for the system. A master oscillator-power amplifier (MOPA) WAL configuration is planned for the future to achieve the necessary increase in pulse energy required for extended range operation. The key system components, including the WAL source and power amplifier, transmitter, and receiver optical systems, and data collection equipment are described. The results of preliminary tests using and SF6 absorption cell in a 50-m laboratory path and filed measurements using a 3.2 km path at the Phillips Laboratory Starfire Optical Range are presented. Performance predictions for operation using the WAL source alone and for the MOPA configuration for extended ranges are presented and discussed.

Image reconstruction from an undersampled focal plane using a sparse telescope array

A technique for storing high spatial frequency information in low frequency zero regions of an image spectrum has been implemented and evaluated. This technique can be used for sparse arrays of small telescopes acting to synthesize a larger pupil. The number of detectors required in the two-dimensional image focal plane is reduced by a factor of four.