KEYWORDS: Heterodyning, Receivers, Signal to noise ratio, Absorption, Signal detection, Chemical detection, Oscillators, Transmitters, Bragg cells, Speckle
A high repetition rate, wavelength agile CO2 laser has been developed at the Air Force Research Laboratory for use as a local oscillator in a heterodyne detection receiver. Rapid wavelength selection is required for measurements of airborne chemical vapors using the differential absorption lidar (DIAL) technique. Acousto-optic modulators are used in the local oscillator to tune between different wavelengths at high speeds (greater than 100 Hz) without the need for moving mechanical parts. Other advantages obtained by the use of acousto-optic modulators are laser output power control per wavelength and rugged packaging for field applications. A series of experiments to simultaneously characterize the radiometric and chemical detection sensitivities of heterodyne and direct detection DIAL systems is being performed at Kirtland AFB, NM, and will be described. The wavelength agile local oscillator (WALO) has been incorporated into a heterodyne receiver, with the Laser Airborne Remote Sensing (LARS) system providing the laser transmitter and direct detection receiver. The experiment series is studying radiometric issues, spread spectrum operation, the effects of target-induced speckle, and the influence of atmospheric turbulence for both detection mechanisms. Measurements are being performed over a horizontal path at standoff ranges of 4 to 15 km, using both natural and man-made targets. Comparisons of the heterodyne and direct detection radiometric and chemometric results will be presented, and contrasted with predictions from simulations and models. The results will also be discussed in terms of the implications for fielding operational DIAL systems.
A high repetition rate, wavelength agile CO2 laser has been developed at the Air Force Research Laboratory for use as a local oscillator in a heterodyne detection receiver. Rapid wavelength selection is required for measurements of airborne chemical vapors using the differential absorption lidar (DIAL) technique. Acousto-optic modulators are used in the local oscillator to tune between different wavelengths at high speeds (greater than 100 Hz) without the need for moving mechanical parts. Other advantages obtained by the use of acousto-optic modulators are laser output power control per wavelength and rugged packaging for field applications.
A series of experiments to simultaneously characterize the radiometric and chemical detection sensitivities of heterodyne and direct detection DIAL systems is being performed at Kirtland AFB, NM, and will be described. The wvelength agile local oscillator (WALO) has been incorporated into a heterodyne receiver, with the Laser Airborne Remote Sensing (LARS) system providing the laser transmitter and direct detection receiver. The experiment series is studying radiometric issues, spread spectrum operation, the effects of target-induced speckle, and the influence of atmospheric turbulence for both detection mechanisms. Direct comparisons of the heterodyne and direct detection results will be presented, and the results will also be discussed in terms of the implications for fielding operational DIAL systems.
A high repetition rate, wavelength agile CO2 laser has been developed at the Air Force Research Laboratory for use as a local oscillator in a heterodyne detection receiver. Fats wavelength selection is required for measurements of airborne chemical vapors using the differential absorption lidar (DIAL) technique. Acousto-optic modulator are used to tune between different wavelengths at high speeds without the need for moving mechanical parts. Other advantages obtained by the use of acousto-optic modulators are laser output power control per wavelength and rugged packaging for field applications. The local oscillator design is described, and the results from laboratory DIAL measurements are presented. The coherent remote optical sensor system is an internal research project being conducted by the Air Force Research Laboratory Directed Energy Directorate, Active Remote Sensing Branch. The objective of the project is to develop a new long-range standoff spectral sensor that takes advantage of the enhanced performance capabilities coherent detection can provide. Emphasis of the development is on a low cost, compact, and rugged active sensor exclusively designed for heterodyne detection using the differential absorption lidar technique. State of the art technologies in waveguide laser construction and acousto- optics make feasible the next generation of lasers capable of supporting coherent lidar system requirements. Issues addressed as part of the development include optoelectronic engineering of a low cost rugged system, and fast data throughput for real time chemical concentration measurements. All hardware used in this sensor are off-the- shelf items, so only minor hardware modifications were required for the system as it stands. This paper describes a high-speed heterodyne detection CO2 DIAL system that employs a wavelength agile, acousto-optically tuned local oscillator in the receiver. Sample experimental data collected in a controlled environment are presented as well. Chemical detection using 12 wavelengths at 200 pulses per second has been demonstrated. Initial progress on experiments to make a direct, simultaneous comparison of heterodyne and direct detection DIAL systems will also be described.
Preliminary experiments toward the implementation of Doppler spectral scanning differential absorption lidar (DSS DIAL) are described. In separate tests, CO2 laser pulses were reflected from either a ground-based retroreflector (36-km round-trip distance) or a retroreflector on the GEOS-3 satellite (approximately 2000-km round-trip distance). The returns were split into a reference channel and an absorptive gas-cell channel. The light was coherently detected with heterodyne receivers and analyzed. Results from the ground-based system produced data that matched expected values in one case but its repeatability remains to be determined. We are currently investigating the satellite-based system to assess the DSS DIAL technique.
Experimental chemical detection and radiometric results are presented from recent coherent differential absorption lidar (DIAL) ground tests conducted over four kilometer horizontal path at the Air Force Research Laboratory. Heterodyne data was collected simultaneously with the Laser Airborne Remote Sensing direct detection DIAL sensor for comparison. A brief system description of the bistatic coherent DIAL system is presented. These experiments illustrate some of the technological challenges encountered in the implementation of a wavelength-agile coherent chemical detection system.
KEYWORDS: Absorption, Receivers, Heterodyning, Carbon dioxide lasers, Sensors, Chemical analysis, LIDAR, Chemical detection, Carbon dioxide, Control systems
The Air Force Research Laboratory (AFRL) Active Remote Sensing Branch has developed the Laser Airborne Remote Sensing (LARS) system for long standoff range chemical detection using the differential absorption lidar (DIAL) technique. The system is based on a high-power CO2 laser which uses either the 12C16O2 or the 13C16O2 carbon dioxide isotopes as the lasing medium, and has output energies of up to 5 J on the stronger laser transitions. The lidar system is mounted on a flight-qualified optical breadboard designed for installation in the AFRL Argus C-135E optical testbed aircraft. This paper will present chemical detection results and issues arising from ground tests of the system performed from September to December 1998. Recent advances in implementing a frequency-agile heterodyne receiver to further increase the standoff range of the DIAL system will also be presented.
The Air Force Research Laboratory (AFRL) Active Remote Sensing Branch has developed the Laser Airborne Remote Sensing (LARS) system for chemical detection using the differential absorption lidar technique. The system is based on a high-power CO2 laser which can use either the standard 12C16O2 or the 13C16O2 carbon dioxide isotopes as the lasing medium, and has output energies of up to 5 J on the stronger laser transitions. The lidar system is mounted on a flight-qualified optical breadboard designed for installation into the AFRL Argus C- 135E optical testbed aircraft. The Phase I ground tests were conducted at Kirtland AFB in 1997, prior to the LARS flight tests performed in September 1997 at Kirtland AFB and the Idaho National Engineering and Environmental Laboratory. The Phase II ground tests were conducted in 1998 to determine the optimum performance of the LARS systems, after the incorporation of modifications and improvements suggested by the flight test results. This paper will present some of the chemical detection and radiometric results obtained during the Phase II ground tests. Following the presentation of the direct detection results, a summary of current work on a heterodyne DIAL system is given.
The Air Force Research Laboratory (AFRL) Active Remote Sensing Branch has developed the Laser Airborne Remote Sensing (LARS) system for chemical detection using the differential absorption lidar (DIAL) technique. The system is based on a high-power CO2 laser which can use either the standard 12C16O2 or the 13C16O2 carbon dioxide isotopes as the lasing medium, and has output energies of up to 5 J on the stronger laser transitions. The lidar system is mounted on a flight-qualified optical breadboard designed for installation into the AFRL Argus C- 135E optical testbed aircraft. The Phase I ground tests were conducted at Kirtland AFB in 1997, prior to the LARS flight tests performed in September 1997 at Kirtland AFB and the Idaho National Engineering and Environmental Laboratory (INEEL). The Phase II ground tests were conducted in 1998 to determine the optimum performance of the LARS system, after the incorporation of modification and improvements suggested by the flight test results. This paper will present some of the chemical detection and radiometric results obtained during the Phase II ground tests.
KEYWORDS: Data acquisition, Control systems, Computing systems, LIDAR, Chemical analysis, Absorption, Control systems design, Pulsed laser operation, Data processing, Chemometrics
The Air Force Research Laboratory (AFRL) Laser Remote Optical Sensing (LROS) program has developed the Laser Airborne Remote Sensing (LARS) system for chemical detection using the differential absorption lidar (DIAL) technique. Airborne tests during the last year resulted in chemical detection at a slant range of 30 km. As the next step in the development process, concepts for a compact, semi-autonomous DIAL system are being considered. This paper describes the conceptual design and external interfaces of the acquisition, processing, and control system computers required to operate a semi-autonomous DIAL system. The conceptual design of the VME-based real-time computer system uses three CPUs: (1) a data acquisition and control CPU which synchronizes experiment timing and pulsed CO2 laser operation while controlling lidar subsystem components such as pointing and tracking, wavelength sequencing, and optical alignment; (2) a data reduction CPU which serves as the semi-autonomous controller and performs real-time data reduction; and (3) a data analysis CPU which performs chemometric analysis including chemical identification and concentration. The triple-CPU and multi-layered software decouple time-critical and non-critical tasks allowing great flexibility in flight-time display and processing.
A wavelength agile coherent LIDAR system is under development at the Air Force Research Laboratory (AFRL). Coherent lidar has the potential of longer-range sensitivity than conventional direct detection systems. AFRL is conducting risk reduction experiments to address issues involved with coherent LIDAR. Issues include speckle noise inherent to heterodyne detection, robust signal analysis algorithm development, and integration of a wavelength agile laser as the local oscillator to the receiver. The transmitter laser, developed for the LARS program, is a high energy TE CO2 laser hardened for airborne operations. Performance of the wavelength agile local oscillator laser is presented, and preliminary data from risk reduction experiments.
The Air Force Research Laboratory has developed and operated an airborne CO2 DIAL system for chemical detection of trace gases in the atmosphere'. This system, designated Laser Atmospheric Remote Sensing (LARS), is used for chemical detection of trace gases in the column content, topographical backscatter mode wherein detection of trace chemicals is performed by ratioing the backscattered signal strengths of combinations of transmitted CO2 laser lines absorbed by the trace chemical(s) to the backscattered signal produced by non-absorbed laser lines. Identification and quantification of trace chemical signatures sampled at multiple discrete CO2 laser frequencies is dependent upon isolation of the chemical signature from the absorption spectrum of the multi-kilometer atmospheric slant path over which measurements are made. Ambient atmospheric concentrations of C02, H20, and 03 contribute discrete line absorptions in the 9 im —11 tm spectral region in which the LARS system operates. The detailed form of the atmospheric absorption spectrum depends upon the concentration of each absorber and its variation with altitude along the slant measurement path. In addition to discrete atmospheric line absorption that must be accounted for in the DIAL measurements, a weaker continuum (smooth, slowly wavelength-varying) absorption due to water vapor must also be taken into account.
The Air Force Research Laboratory has developed and tested an airborne CO2 differential absorption lidar system for the remote detection of chemicals. The Laser Airborne Remote Sensing DIAL system uses topographic backscatter to provide a long-range measurement of the column-content absorption of chemical plumes in the path of the laser beam. A high-power CO2 laser, capable of operation on multiple isotopes, and a Mersenne telescope constitute the major transceiver components. In addition to the laser, telescope, and transceiver optics, several onboard diagnostic instruments were mounted on the flight bench to monitor and optimize the system performance during airborne operation. The flight bench, electronics racks, and data acquisition and experiment control stations were designed to be integrated onto the AFRL C-135E research aircraft, and to utilize the existing pointing and tracking system on the aircraft.
The Phillips Laboratory Remote Optical Sensors (ROS) program is developing the Laser Airborne Remote Sensing (LARS) system for chemical detection using the differential absorption lidar (DIAL) technique. The system is based upon a high-power CO2 laser which can use either the standard 12C16O2 or the 13C16O2 carbon dioxide isotopes as the lasing medium, and has output energies in excess of 4 J on the stronger laser transitions. The laser, transmitter optics, receiver telescope and optics, and monitoring equipment are mounted on a flight-qualified optical breadboard designed to mount in the Argus C-135E optical testbed aircraft operated by Phillips Laboratory. The LARS system is being prepared for initial flight experiments at Kirtland AFB, NM, in August 1997, and for chemical detection flight experiments at the Idaho National Engineering Laboratory (INEL) in September 1997. This paper briefly describes the system characterization, and presents some results from the pre- flight ground testing.
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.
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.
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.
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.
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.
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.
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.
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