We report the application of an actively stabilized CW mode-locked Nd:YLF laser to the problem of range-Doppler imaging. A laboratory demonstration of cm range and cross-range resolution is described. The effect of target speckle and glint on signal-to-noise ratio (SNR) and signal processing are discussed. Because of the wide range of mode-locked pulse and pulse train durations achievable with the solid state laser system, the single shot signal-to-noise ratio can be maximized. A Nd:YLF oscillator and Nd:glass amplifier laser system has distinct advantages over other high power laser systems in range-Doppler applications.

An analysis is presented that indicates the preferred optical arrangement for a stable laser oscillator is to use a hemispherical resonator. Qualitative observations suggest that laser frequency variation is independent of excitation position on the laser box structure. An excitation position that efficiently couples to the end plates on which mirrors are mounted produces substantially higher laser frequency variations.

This paper discusses several recent advances in CO2 laser catalysts including comparisons of the activity of Au/MnO2 to Pt/SnO2 catalysts with possible explanations for observed differences. The catalysts are compared for the effect of test gas composition, pretreatment temperature, isotopic integrity, long term activity, and gold loading effects on the Au/MnO2 catalyst activity. Tests conducted to date include both long-term tests of up to six months continuous operation and short-term tests of one week or more that include isotopic integrity testing.

The increasing power capability of diode lasers and methods for combining their outputs to create more powerful sources make them candidates for long range laser radar missions. Parametric analyses of search and ranging requirements indicate this is reasonable in above the horizon and night environments. Their electrical efficiency and modular packaging qualities make them suitable for lightweight sensors.

The requirements for a diode source for a laser radar system are presented. It is shown how microcollimation of incoherent diode laser arrays can produce a usable beam divergence. A unique diode source under development and the associated technologies required for a compact, efficient, reliable and low divergence source are described.

SAW chirp transformers perform real-time Fourier transforms by an analog realization of the chirp transform algorithm. They are capable of real-time bandwidths of more than 100 MHz, and more than 1000 points. A primary application is real-time Doppler analysis in airborne laser radar systems, where their small size, weight and power characteristics are dominant.

Modulators with a large instantaneous microwave bandwidth and high efficiency were developed for imposing frequency chirped waveforms on a high power CO2 laser output. These modulators may be cascaded to increase bandwidth and to increase efficiency. Details of the construction, together with the results, are presented.

The effects of optical and microwave heatings and thermally-induced birefringence in a CdTe modulator crystal on the performance of a linear FM CO2 laser radar are examined. This is conducted in terms of reductions in beam Strehl ratio and dynamic ranges of the Doppler shift and range for given optical and microwave powers. An analysis of the thermal lenses generated by these heatings is presented.

Electro-optic frequency shifters, or modulators, have been built with almost 100 percent efficiency. Conversions over 50 percent require the interaction of a circularly polarized high power microwave beam traveling synchronously with a circularly polarized laser beam in an electro-optic crystal. We describe the inhomogeneous quarter-wave transformer structures made for a CO2 laser beam modulator with a conversion efficiency in excess of 80 percent. Conventional impedance matching synthesis was adapted to include the requirements for maintaining circular polarization and handling high power. Measured results are presented.

Doppler lidars are prospective means for long-range measurements of wind velocity in the atmosphere, based on the registration of Doppler frequency shifts in the radiation backscattered by moving aerosol particles. The development of coherent lidar systems based on CW and TEA CO2 and on Nd:YAG lasers is presented. Test results from the application of these lidars to atmospheric wind velocity measurements are discussed.

Potentials of ground-based and of airborne CO2 Doppler lidars are discussed in relation to the problems of long range detection and parameters estimation of atmospheric turbulence. Our experience to use such systems working in CW and quasipulsed regimes is reviewed. A new method for the measurement of the C sub V-sq constant in turbulent atmosphere with a CW CO2 Doppler lidar is presented.

Solid state lasers are being actively investigated for use as efficient and long-lived optical sources in a wide variety of lidar/laser remote sensing systems. We have recently developed a short-pulse 1 micron Nd:YAG coherent Doppler lidar and a direct-detection 2.1 micron Ho lidar/DIAL in order to study the potential usage of such laser sources for atmospheric and hard target lidar measurements. In this paper, we review some of our recent work regarding these two lidar systems.

National attention has focused on the critical problem of detecting and avoiding windshear since the crash on August 2, 1985, of a Lockheed L-1011 at Dallas/Fort Worth International Airport. As part of The NASA/FAA National Integrated Windshear Program, the authors have defined a measurable windshear hazard index that can be remotely sensed from an aircraft, to give the pilot information about the wind conditions he will experience at some later time if he continues along the present flight path. The technology analysis and end- to-end performance simulation, which measures signal-to-noise ratios and resulting wind velocity errors for competing coherent lidar systems, shows that a Ho:YAG lidar at a wavelength of 2.1 micrometers and a CO2 lidar at 10.6 micrometers can give the pilot information about the line-of-sight component of a windshear threat in a region extending from his present position to 2 to 4 km in front of the aircraft. This constitutes a warning time of 20 to 40 s, even under conditions of moderately heavy precipitation. Using these results, a Coherent Lidar Airborne Shear Sensor (CLASS), using a Q-switched CO2 laser at 10.6 micrometers , is being designed and developed for flight evaluation in early 1992.

The Global Backscatter Experiment (GLOBE) objectives require intensive study of the global climatology of atmospheric aerosol backscatter at infrared wavelengths. The data obtained from GLOBE are of great importance for analysis of the global winds measurement application of coherent Doppler lidar. Ground-based and airborne backscatter lidars have been developed to measure atmospheric backscatter profiles at CO2 laser wavelengths. Descriptions of the calibration methodologies and selected measurement results are presented.

A novel, high PRF CO2 Doppler lidar designed to make range resolved velocity and DIAL measurements to ranges of 3 to 5 km is presented. The transmitter scheme selected is a master oscillator power amplifier combination. Details of the design are given and some preliminary test results are discussed.

The layered-sphere volumetric backscatter coefficient is examined for possibly enhancing the estimates of the aerosol component distributions and their physical parameters with examples utilizing a coherent Lidar at 10.6 microns. It is found that at this wavelength alumina particles coated with soot, water, or sulfuric acid may exhibit greatly reduced backscattering due to destructive interference of the reflected waves plus guided wave action in the absorbing layer. Also, the layered-sphere calculations easily reduce to solid-sphere calculations as needed.

To determine the local variability of the turbulence induced spread in wind velocity in a small and inaccessible region, an analysis of the expected performance of a CW, heterodyne detection, 10 microns wavelength laser radar is conducted. At this wavelength practically all of the backscatter signal power is due to scattering from aerosol particles. As these particles move with the wind velocity the Doppler shift they impart to the backscattered laser radiation will be indicative of the wind velocity.

Doppler lidar provides atmospheric wind velocity measurements at locations in the atmosphere that are remote from the lidar platform. Critical factors in lidar wind velocity measurement performance are (a) the received signal photons that are backscattered from atmospheric aerosols in the measurement region of interest, (b) the background photons that are scattered from aerosols within the lidar field of view and do not contribute to the signal but do introduce photon noise in the coherent detection receiver, and (c) the focussing of the lidar transmitter and coherent detection receiver to provide range resolution and to optimize velocity measurement performance when using a CW laser. Tbis paper models the interaction of these factors for a ground based lidar in a monostatic geometry for potential application to atmospheric research, atmospheric microburst detection for aircraft warning, and other areas. By analogy to established methods for characterizing the performance capability of other sensor systems, the lidar performance is characterized by the carrier-to-noise ratio (CNR) and by a noise equivalent scattering coefficient (NESC).

The edge technique is a powerful new method for the measurement of small frequency shifts which allow high accuracy measurement of atmospheric winds (0.2 to 1 m/sec) with high vertical resolution (10 m) using currently available technology.

This paper deals with the theory of centroid tracking of range-Doppler images created with a heterodyne-detection laser radar. A brief description of the system model is presented and the noise statistics of the image pixels are characterized. A center-of-mass centroid algorithm is used to determine the object's image location within the range-Doppler window. The effects of thresholding and frame averaging upon the performance of the centroid estimator are investigated. An analytical approach for setting the threshold is developed which is based on the criterion of making the estimator unbiased. A constant probability of detection algorithm in conjunction with an acceptable bias condition is described. Performance curves and simulation results are presented which support the theory. Center-of-mass centroid tracking performance, using simulated image data and data collected at the Firepond site in Westford, MA, is shown to be in agreement with the theoretical performance.

A hybrid analytic computer simulation approach is used to estimate centroid performance of laser radar images. This approach includes the statistical effects of shot-noise and speckle and provides the flexibility to model complex target geometry and surface scattering, image point spread response characteristics, and target glints. Results suggest that image edge effects have a pronounced impact on centroid accuracy. A bi-level threshold is shown to provide improved centroid stability over intensity and binary images.

The use of a range/passive-IR histogram as an approach to pixel-level fusion for cued target detection is discussed. Target detection algorithms for laser radar range imagery often use a number of computationally-intensive operations to locate targets in an image. These steps may include performing global geometric transforms, locating the ground plane, or applying size filters with associated rules. Each pixel in the image is processed multiple times, a time- consuming chore. An alternative to examining every pixel is to cue such detailed algorithms directly to an image location which is likely to contain a target. Cueing reduces the burden of searching or processing the entire image for regions of interest, greatly decreasing the number of computations needed for target detection. Cueing can be accomplished by combining registered laser radar range and passive-IR data. Since these data are taken simultaneously and are co-registered by Lincoln Laboratory's airborne laser radar, it is possible to combine them to form a powerful set of discriminants. There are two possible approaches to fusing the range and passive-IR data: (1) the domains can be processed for detections in two parallel streams and the resulting detection maps combined, or (2) the domains can be fused first and then processed as a single stream for detections. In the first method, sometimes called 'image-level' fusion, the processing algorithm for each domain can be optimized to obtain the best combination of detection and false alarm statistics. In the second method, the domains are combined at the pixel level, and the result is processed directly for target detection. The latter approach also reduces the number of computations since only data of interest to both domains is processed further. In this article, the multi-dimensional laser radar sensor and ongoing efforts to develop an automatic target recognition (ATR) system are described. The processing of pixel-registered laser radar range and passive-IR imagery for cued target detection using the range/passive-IR histogram is discussed. The authors present results for IR imagery with positive passive-IR target-to-background contrast to study the performance of the algorithm in real-world scenarios. These results are compared with a more typical detection method.

Power cable reflectivities have been measured for both dry and wet conditions using a 10.6 micrometers coherent CO2 laser radar. Measurements were made at a range of 620 m using two types of cable: 1-in.-diameter Grosbeak and 3/8-in. Grouse, at +/- 60 degree(s) aspect angles relative to the normal in increments of 10 degree(s). The cables were both artificially sprayed, in an effort to simulate realistic rainfall rates, as well as being exposed to light rain. Results show that the drop in reflectivity of wet cables compared to dry cables is a function of aspect angle with a mean drop for the Grosbeak and Grouse cables of -5 dB and -1.3 dB, respectively. Measurements of the drying rates of the cables showed a return to the dry state reflectivity in approximately 100 sec after being liberally doused.

Various reconstruction algorithms are discussed in the context of a recently proposed self-reference holographic imaging technique. These algorithms are applied to obtain high resolution images of laser illuminated objects with a strong specular backscattered component. Examples of linear and nonlinear image reconstruction algorithms are presented with performance approaching optimal bounds. Expressions are obtained allowing determination of the dependence of the quality of reconstructed image on target reflectivities and the selection of illuminating wavefronts. A critical evaluation of results indicates the need for more complete modeling to derive system requirements and limitations.

Several new algorithms for reconstructing an object image from its Fourier transform holograms are presented. All of the image reconstruction algorithms are classified into two types: those that require multiple, different illumination wavefronts and a new algorithm that requires only a single illumination wavefront. The advantages and disadvantages of the two types are discussed.

We investigate the way laser-speckle noise and limited signal power affect the quality of images reconstructed from diffraction field data obtained with a pupil-plane array of optical heterodyne detectors. Modeling the detected signal from each detector sub-element as a circular-complex Gaussian random variable and taking into account the random amplitudes of the detected signals arising from laser-speckle effects, we compute the SNR of the Fourier spectrum of the coherent intensity image formed from the array of heterodyne field measurements. The resulting SNR expression is compared to that obtained earlier, P.S. Idell (1988), for the same quantity estimated from photocount-limited, focal plane detector measurements. This comparison shows that the spatial frequency SNR performance of both systems is identical when the systems are operated at high signal-level operating conditions (number of signal photocounts per speckle much greater than one). When signal levels drop significantly below one signal photocount/speckle, we find that focal plane imaging performs somewhat better at estimating all but the very highest spatial frequencies.

A straight-forward computer simulation technique is presented for determining the spatial intensity distribution that can be expected when a rough object is illuminated with partially coherent light. The technique is useful for problems in which the illumination source is a laser whose spectral content can be described as a sum of delta function components. Laboratory and computer simulation results that illustrate illumination coherence effects on imagery obtained with the method known as imaging correlography are presented.

Several lidar applications are reviewed including short range solid target measurements with extremely high velocity resolution, air velocity measurements, altimetry and wind tunnel measurements of air velocity utilizing the dual beam scatter method and carefully sized artificial aerosols. The use of optical fibers and diode lasers permits construction of devices that are relatively inexpensive and rugged with high sensitivity. The signal processing system is based on digital Fourier transform techniques and is implemented on a DOS-based computer utilizing a data translation array processor board.

This paper discusses the scenario of an autonomous landing like that required for the Mars Rover Sample Return Mission. An application of laser radar for conducting autonomous hazard detection and avoidance is discussed. A trade-study is performed to identify operational and implementation constraints as well as the state of the art in component technology.

A laser radar using an array of heterodyne detectors offers the possibility of fine resolution angle-angle imaging. The heterodyne measurements, however, are subject to phase errors due to atmospheric turbulence and mechanical misalignment. A method is described that employs digital shearing of the heterodyne measurements as a means to remove phase errors. By this method large phase errors can be corrected without requiring a beacon or a glint. This digital shearing laser interferometry method was investigated theoretically and demonstrated via computer simulations which included photon noise and various types of phase errors. The method was also successfully applied to data collected in a simple laboratory experiment.

A sophisticated 3D laser radar sensor simulation, developed and applied to the task of autonomous hazard detection and avoidance, is presented. This simulation includes a backward ray trace to sensor subpixels, incoherent subpixel integration, range dependent noise, sensor point spread function effects, digitization noise, and AM-CW modulation. Specific sensor parameters, spacecraft lander trajectory, and terrain type have been selected to generate simulated sensor data.