Jeffrey L. Ahl

Simultaneous estimation of aerosol cloud concentration and spectral backscatter from multiple-wavelength lidar data

We present a sequential algorithm for estimating both concentration dependence on range and time and backscatter coefficient spectral dependence of optically thin localized atmospheric aerosols using data from rapidly tuned lidar. The range dependence of the aerosol is modeled as an expansion of the concentration in an orthonormal basis set whose coefficients carry the time dependence. Two estimators are run in parallel: a Kalman filter for the concentration range and time dependence and a maximum-likelihood estimator for the aerosol backscatter wavelength and time dependence. These two estimators exchange information continuously over the data-processing stream. The state model parameters of the Kalman filter are also estimated sequentially together with the concentration and backscatter. Lidar data collected prior to the aerosol release are used to estimate the ambient lidar return. The approach is illustrated on atmospheric backscatter long-wave infrared (CO2) lidar data.

2-μm-pumped 8-12-μm OPO source for remote chemical sensing

A 100 Hz, optical parametric oscillator (OPO) lidar breadboard is designed, built and tested for remote chemical sensing in the 8 - 12 micrometers range. Continuous tuning is achieved by angle tuning a type II, silver gallium selenide (AgGaSe2) OPO crystal pumped in a single step by a 2.088-micrometers pump laser. The pump source for the OPO consists of a temperature stabilized, continuously pulsed, resonantly pumped Ho:YAG (2.088-micrometers ) laser, end-pumped by a diode- end-pumped Tm:YLF (1.9-micrometers ) laser. The 9 mm X 5 mm X 25 mm-long OPO crystal was mounted on a computer-controlled galvanometer scanner for rapid wavelength tuning (1.5 micrometers between shots). Continuous tunability was demonstrated from 7.9 to 12.6 micrometers with energies in the 50 - 400 (mu) J range. Quantum slope conversion efficiencies up to 40% were obtained. Far-field beam divergence measurement showed the output of the OPO to be 2.6 times diffraction limit. The improved OPO beam quality over previous studied tandem OPO systems is attributed to the reduced Fresnel number of the OPO cavity (idler resonating) and the better beam quality of the pump source. A LabWindows based data collection and analysis system is implemented. The effectiveness of the OPO as a source for chemical sensing is demonstrated by the collection of the absorption spectrum of ammonia.

Practical considerations for the design of CO(2) lidar systems.

A 10.6-microm single laser lidar system has been utilized to monitor the amplitude, standard deviation, and correlation of returns from foliage, hillside, and man-made targets as a function of the lidar system divergence and mode shape, the receiver field of view and receiver/transmitter alignment tolerance, the repetition rate, and the sampling time. Studies of the dependence of the system sensitivity on signal averaging and signal correlation demonstrate performance comparable with that achieved with reported dual laser lidar systems.

High speed tuning mechanism for CO2 lidar systems.

A method for rapidly tuning lasers is presented. The system utilizes a rotating eight-sided mirror and a fixed grating. It is demonstrated that the entire CO2 lasing spectrum can be tuned at effective rates of up to 400 Hz. It is shown that, although the pulse energy is diminished as the tuning rate is increased, the loss comes from the tail of the pulse, and the peak power is almost unchanged. In addition, the tuning method preserves the spatial beam profile while contributing a minimum of beam steering.

Estimation and Discrimination of Aerosols Using Multiple Wavelength LWIR Lidar

This paper presents an overview of recent work by the Edgewood Chemical Biological Center (ECBC) in algorithm development for parameter estimation and classification of localized atmospheric aerosols using data from rapidly tuned multiple-wavelength range-resolved LWIR lidar. The motivation for this work is the need to detect, locate, and discriminate biological threat aerosols in the atmosphere from interferent materials such as dust and smoke at safe standoff ranges using time-series data collected at a discrete set of CO2 laser wavelengths. The goals of the processing are to provide real-time aerosol detection, localization, and discrimination. Earlier work by the authors has produced an efficient Kalman filter-based algorithm for estimating the range-dependent aerosol concentration and wavelength-dependent backscatter signatures. The latter estimates are used as feature vectors for training support vector machines classifiers for performing the discrimination. Several years of field testing under the Joint Biological Standoff Detection System program at Dugway Proving Ground, UT, Eglin Air Force Base, FL, and other locations have produced data and backscatter estimates from a broad range of biological and interferent aerosol materials for the classifier development. The results of this work are summarized in our presentation.

Rapid-tuning device for CO2 heterodyne detection lidar

A device for rapid‐tuning cw, Q‐switched lasers for a CO2 heterodyne detection lidar is presented. It is shown that it is possible to utilize galvanometer‐driven mirrors to rapidly switch wavelengths over randomly selected lasing transitions in the 9–11 μm portion of the spectrum. Both a transmitter and a local oscillator are simultaneously switched between transitions while still achieving the frequency stability typically required for a coherent lidar system.

Evaluation of a galvanometric scanner for rapid tuning of CO2 lasers

Remote sensing of gaseous constituents in the atmosphere is possible with a single laser system operating at high (more than 100 Hz) pulse tuning rates so as to minimize the effects of atmospheric turbulence. This article investigates the possibility of using a galvanometric scanner as a rapid tuning device for both pulsed and continuous‐wave CO2 lasers. Preliminary cw experiments, performed in anticipation of future heterodyne lidar development, indicated that lines could be switched at rates up to 77 Hz over the entire spectrum, while much greater rates were achieved by restricting the angular displacement of the galvanometer. When used with a transversely excited atmospheric (TEA) laser, the galvanometer was able to switch lines as fast as 140 Hz. However, amplitude‐dependent position adjustments were necessary to compensate for settling time and/or thermal drift effects in order to obtain maximum energy output. A calibration curve was constructed and tested, but predictions of peak energy positions we...

Operation of a small high‐pressure laser using 13CO2

The operating characteristics of a tunable, high‐pressure laser operating with 13CO2 and 12CO2 are described. The laser was of a size comparable to those in operation with easily transportable rangefinders and lidars. It was operated at 2.4 atm and measurements were made of the spectral output, as well as the peak power and pulse duration.

Evaluation Of A Galvanometric Scanner For Rapid Tuning Of CO2 Lasers

Remote sensing of gaseous constituents in the atmosphere is possible with a single laser system operating at high (more than 100 Hz) pulse tuning rates so as to minimize the effects of atmospheric turbulence. This paper investigates the possibility of using a galvanometric scanner as a rapid tuning device for both pulsed and continuous wave CO2 lasers. When used with a TEA laser, the galvanometer was able to switch lines as fast as 140 Hz.

Evaluation Of A Galvanometric Scanner For Rapid Tuning Of CO 2 Lasers

Remote sensing of gaseous constituents in the atmosphere is possible with a single laser system operating at high (more than 100 Hz) pulse tuning rates so as to minimize the effects of atmospheric turbulence. This paper investigates the possibility of using a galvanometric scanner as a rapid tuning device for both pulsed and continuous wave CO2 lasers. When used with a TEA laser, the galvanometer was able to switch lines as fast as 140 Hz.