The development of eye-safe, imaging, scannerless laser radar systems based on gated viewing with a range of some hundred meters is difficult due to the lack of fast switchable and sufficiently sensitive detectors for NIR wavelengths. Nevertheless, one basic approach is to gate an InGaAs-FPA-camera with an electro-optical modulator (EOM) in the range of about 30ns to achieve a sufficiently high resolution of depth. In this paper, we present field measurements of a system with a Nd:YAG-OPO laser of 1574nm wavelength, 7ns pulse length, 25Hz pulse frequency, and 80mJ pulse energy. Because of the special optical behavior of the EOM in conjunction with an adapted lens design and a resolution of a 128 by 128 pixel FPA the main interest is object detection, which leads to imaging with improved system performance. The experimental data will be compared with the theoretical performance model and a second system (at a wavelength of 532nm), concerning imaging under certain weather conditions. Based on the current system design we discuss problems of contrast modulation and speckle effects leading to an outlook for improvements of the system design.

The Army Research Laboratory is researching scannerless ladar systems for smart munition and reconnaissance applications. Here we report on progress attained over the past year related to the systems architectures, component development, and test results of the scannerless ladars. The imaging system architectures achieve ranging based on a frequency modulation/continuous wave technique implemented by directly amplitude modulation a near-IR diode laser transmitter with a radio frequency subcarrier that is linearly frequency modulated. The diode's output is collected and projected to from an illumination field in the downrange image area. The returned signal is focused onto an array of metal-semiconductor-metal (MSM) detectors where it is detected and mixed with a delayed replica of the laser modulation signal that modulates the responsivity of each detector. The output of each detector is an intermediate frequency signal whose frequency is proportional to the target range. This IF signal is continuously sampled over each period of the rf modulation. Following this, an N channel signal processor based-on field-programmable gate arrays calculates the discrete Fourier transform over the IF waveform in each pixel to establish the ranges to all the scatterers and their respective amplitudes. Over the past year, we have continued development of laser illuminators at .8 and 1.55 micrometers , built 1D self-mixing MSM detector arrays at .8 and 1.55 micrometers and built an N channel FPGA signal processor for high-speed formation of range gates. In this paper we report on the development and performance of these components and the results of system test conducted in the laboratory.

Laser radars have the unique capability to give intensity and full 3-D images of an object. Doppler lidars can give velocity and vibration characteristics of an objects. These systems have many civilian and military applications such as terrain modelling, depth sounding, object detection and classification as well as object positioning. In order to derive the signal waveform from the object one has to account for the laser pulse time characteristics, media effects such as the atmospheric attenuation and turbulence effects or scattering properties, the target shape and reflection (BRDF), speckle noise together with the receiver and background noise. Finally the type of waveform processing (peak detection, leading edge etc.) is needed to model the sensor output to be compared with observations. We have developed a computer model which models performance of a 3-D laser radar. We will give examples of signal waveforms generated from model different targets calculated by integrating the laser beam profile in space and time over the target including reflection characteristics during different speckle and turbulence conditions. The result will be of help when designing and using new laser radar systems. The importance of different type of signal processing of the waveform in order to fulfil performance goals will be shown.

We have developed an advanced angle-angle-range-intensity laser radar system designed for long-range imaging and target discrimination. Brassboard hardware has been built, and performance testing has demonstrated the capability of direct-detection imaging ladar systems to significantly improve probability of target discrimination over passive-only sensors. In this paper we discuss the most recent generation of short-pulse ladar hardware and the results of recent range testing of this hardware against accurate target models. Progress in the development of the next-generation short-pulse laser transmitter and GHz digitizing receiver is also described.

The progenitor for the Advanced Scientific Concepts, Inc. eye-safe 3D imaging system is a non-eye-safe, flash, 3D imaging ladar system, which was successfully tested by the authors in 9.98. This paper reviews the design of the original flash ladar system and presented 3D images taken with the system. In light of this original design, flash ladar eye-safe technologies are reviewed and a particular design is chosen as the most feasible for lightweight, low volume and low power applications. Data is presented from an advanced-design system.

This paper reviews the motivation, design and testing of a 3D imaging, sequential time-integration technology. Most recent test of a 10 by 10 and a 100 by 100 imaging chip are presented.

This paper gives an overview of Lockheed Martin's effort as part the DARPA Three Dimensional Imaging Sensors Program. The overall system, the detector technology, the ROIC approaches, and the system model are discussed. The system approach has a laser transmitter that illuminates the entire sensor field-of-view (FOV) with a single laser pulse and collects the returned energy on a focal plane array (FPA) where each picture element (pixel) measures range. The detector is a short wavelength infrared HgCdTe APD array based on DRS Infrared Technology's well established architecture of HDVIP.. The ROICs implement 'on-chip' pixel level signal processing to generate 3-D imagery. The model is used to make predictions based on range, power, aperture, weather and FPA characteristics to help forecast expected device/readout/system performance.

Raytheon has recently been funded by DARPA to develop an FPA for single shot eyesafe ladar operation. The goal of the program is to develop new high speed imaging rays to rapidly acquire high resolution, 3D images of tactical targets at ranges as long as 7 to 10 kilometers. This would provide precision strike, target identification from rapidly moving platforms, such as air-to-ground seekers, which would enhance counter-counter measure performance and the ability to lock-on after launch. Also a goal is to demonstrate the acquisition of hidden, camouflaged and partially obscured targets. Raytheon's approach consists of using HgCdTe APD arrays which offer unique advantages for high performance eyesafe LADAR sensors. These include: eyesafe operation at room temperature, low excess noise, high gain to overcome thermal and preamp noise, Ghz bandwidth and high packing density. The detector array will be coupled with a Readout Integrated Circuit, that will capture all the information required for accurate range determination. The two components encompass a hybrid imaging array consisting of two IC circuit chips vertically integrated via an array of indium metal 'bumps'. The chip containing the PAD detector array and the silicon signal processing readout chip are independently optimized to provide the highest possible performance for each function.

We are developing a novel 2D focal plane array (FPA) with read-out integrated circuit (ROIC) on a single chip for 3D laser radar imaging. The ladar will provide high-resolution range and range-resolved intensity images for detection and identification of difficult targets. The initial full imaging-camera-on-a-chip system will be a 64 by 64 element, 100-micrometers pixel-size detector array that is directly bump bonded to a low-noise 64 by 64 array silicon CMOS-based ROIC. The architecture is scalable to 256 by 256 or higher arrays depending on the system application. The system will provide all the required electronic processing at pixel level and the smart FPA enables directly producing the 3D or 4D format data to be captured with a single laser pulse. The detector arrays are made of uncooled InGaAs PIN device for SWIR imaging at 1.5 micrometers wavelength and cooled HgCdTe PIN device for MWIR imaging at 3.8 micrometers wavelength. We are also investigating concepts using multi-color detector arrays for simultaneous imaging at multiple wavelengths that would provide additional spectral dimension capability for enhanced detection and identification of deep-hide targets. The system is suited for flash ladar imaging, for combat identification of ground targets from airborne platforms, flash-ladar imaging seekers, and autonomous robotic/automotive vehicle navigation and collision avoidance applications.

MIT Lincoln Laboratory is actively developing laser and detector technologies that make it possible to build a 3D laser radar with several attractive features, including capture of an entire 3D image on a single laser pulse, tens of thousands of pixels, few-centimeter range resolution, and small size, weight, and power requirements. The laser technology is base don diode-pumped solid-state microchip lasers that are passively Q-switched. The detector technology is based on Lincoln-built arrays of avalanche photodiodes operating in the Geiger mode, with integrated timing circuitry for each pixel. The advantage of these technologies is that they offer the potential for small, compact, rugged, high-performance systems which are critical for many applications.

A novel integration technology for the fabrication of active, or passive, focal plane array imagers has been developed. The integration scheme is based on the transfer of epitaxial layers to a surrogate substrate without critical alignment. Once the epitaxial layer is successfully transferred to the surrogate substrate, photodetector isolation, passivation, and fabrication are completed. To demonstrate the potential of the process, 320 x 256 arrays of InGaAs mesas were successfully transferred onto commercially-available focal plane array readout integrated circuits. Pitch and pixel resolution are limited by the available standard photolithography. InGaAs mesas transferred to silicon wafers with a pitch as small as 10 microns have been demonstrated. The process was optimized for the fabrication of high-performance vertical Schottky photodiodes. Dark-currents below 5 nA were observed with 44 mm diameter photodiodes. Responsivities of 0.55 A/W were obtained with a 1 micron InGaAs absorber. The new integration process can be used to easily achieve photodiodes with bandwidths higher than 20 GHz, without the use of an air-bridge.

The rapid development of laser radar technology combined with the challenging needs for Army sensors to support the Future Combat System has led to a number of applications for 3D imaging systems. These systems applications include obstacle detection, target recognition, locating targets under foliage, UGV route planning and navigation and terrain mapping. Each application requires different system parameters such as pulse energy, repetition rate, and field of view. A number of issues must be addressed in the design of a system including atmospheric effects, laser energy requirements, wavelength selection and field of view.

Laser radar systems are required for various military applications including obstacle detection, target recognition, and terrain mapping. Each application requires different system parameters such as pulse energy, repetition rate, and field of view. This paper presents a review of a multifunction laser radar system developed and tested for the Cooperative Eyesafe Laser Radar Program (CELRAP) of the U.S. Army CECOM Night Vision and Electronic Sensors Directorate.

High-resolution (0.3-1 m) digital-elevation data is widely available from commercial sources. Whereas the production of two-dimensional (2D) mapping products from such data is standard practice, the visualization of such three-dimensional (3D) data has been problematic. The basis for this problem is the same as that for the large-model problem in computer graphics-- large amounts of geometry are difficult for current rendering algorithms and hardware. This paper describes a cost-effective solution to this problem that has two parts. First is the employment of the latest in cost-effective 3D chips and video boards that have recently emerged. The second part is the employment of quad-tree data structures for efficient data storage and retrieval during rendering. The result is the capability for real-time display of large (over tens of millions of samples) digital elevation models on modest PC-based systems. This paper shows several demonstrations of this approach using airborne lidar data. The implication of this work is a paradigm shift for geo-spatial information systems--3D data can now be as easy to use as 2D data.

We report the first demonstration of mid-IR coherent laser radar operation near 3.6 micrometers . In many low altitude environments, the wavelength region from 3.5 - 4 micrometers has advantages for laser beam propagation because the detrimental effects of scattering and turbulence are less severe than at shorter wavelengths. In addition, under conditions of high humidity, water vapor absorption in the mid-IR is also significantly lower compared to the long-IR region at 9-11 micrometers . The source in this work is a 100 mW, frequency stable cw-optical parametric oscillator (OPO) based on periodically poled lithium niobate. The frequency stability of the source is discussed and laboratory heterodyne experiments measuring small Doppler shifts from vibrating targets are described.

After a 5-year mission, a 4-year transit followed by a one-year mission orbiting the asteroid 433 Eros, the Near-Earth Asteroid Rendezvous-Shoemaker (NEAR) spacecraft made a controlled landing onto the asteroid's surface on 12 February 2001. Onboard the spacecraft, the NEAR Laser Rangefinder (NLR) facility instrument had gathered over 11 million measurements, providing a spatially dense, high-resolution, topographical map of Eros. This topographic data, combined with Doppler tracking data for the spacecraft, enabled the determination of the asteroid's shape, mass, and density thereby contributing to understanding the internal structure and collisional evolution of Eros. NLR data indicate that Eros is a consolidated body with a complex shape dominated by collisions. The offset between the asteroid's center of mass and center of figure indicates a small deviation from a homogeneous internal structure that is most simply explained by variations in mechanical structure. Regional-scale relief and slope distributions show evidence for control of some topography by a competent substrate. It was found that pulse dilation was the major source of uncertainty in single-shot range measurements from the NLR, and that this uncertainty remains consistent with the overall 6-m range measurement system accuracy for NEAR. Analysis of NLR data fully quantified the geodynamic nature of this planetesimal, ergo, illustrating the utility of laser altimetry for remote sensing.

A multi-dimensional laser radar sensor is developed to perform 3D imaging polarimetry. The imaging polarimeter is an extension of the existing, well-developed Streak Tube Imaging Lidar technology. With polarization optics added to the transceiver system, simultaneous capture of 3D high- resolution polarimetric and reflectance imagery is achieved over wide fields of view. Laboratory and field experiments exhibit the capability of the imaging polarimeter for enhancing clutter reduction, image segmentation, and target discrimination for low contrast and camouflaged targets.

We present a holographic lidar system, designed to give complete temperature profiles of the atmosphere. The lidar uses rotational Raman scattering (RRS) from 0-30km and Rayleigh scattering (RS) from 30-100km. The main feature of our lidar is a holographic optical element (HOE) which allows individual lines in the nitrogen rotational RRS to be extracted with high efficiency, along with the Rayleigh return. Due to the effectiveness of the holographic filters we have constructed, our lidar can achieve levels of performance far above existing systems using narrowband filters. The system requires no calibration to radiosondes and has nominal susceptibility to environmental fluctuations.

The Molecular Optical Air Data System (MOADS) is a compact optical instrument that can directly measure wind speed and direction, density, and temperature of the air surrounding an aircraft. From these measurements, a complete set of air data products can be determined. Single-axis wind tunnel testing of wind speed and density has just been completed for the current prototype. These wind tunnel measurements have shown that the current prototype meets wind speed accuracy predictions and initial results from density testing indicate a high level of correlation with absolute pressure transducer measurements. A preliminary design for the next generation instrument, the Joint Optical Air Data System (JOADS), has been completed and is intended to meet Joint Striker Fighter (JSF) requirements. Work is also underway to evaluate the application of MOADS to Unmanned Air Vehicles (UAVs), Reusable Launch Vehicles (RLVs), helicopters and weapon systems. Extensions of MOADS technology to wind shear, gust alleviation, and clear air turbulence detection for commercial aircraft are also being pursued. The basic instrument operation, preliminary ground testing (wind tunnel) results, comparison of these results to simulations, next generation instrument capabilities, and plans for a flight demonstration are discussed.

Traditional lidar provides little information on dense clouds beyond the range to their base (ceilometry), due to their extreme opacity. At most optical wavelengths, however, laser photons are not absorbed but merely scattered out of the beam, and thus eventually escape the cloud via multiple scattering, producing distinctive extended space- and time-dependent patterns which are, in essence, the cloud's radiative Green functions. These Green functions, essentially 'movies' of the time evolution of the spatial distribution of escaping light, are the primary data products of a new type of lidar: Wide Angle Imaging Lidar (WAIL). WAIL data can be used to infer both optical depth and physical thickness of clouds, and hence the cloud liquid water content. The instrumental challenge is to accommodate a radiance field varying over many orders of magnitude and changing over widely varying time-scales. Our implementation uses a high-speed microchannel plate/crossed delay line imaging detector system with a 60-degree full-angle field of view, and a 532 nm doubled Nd:YAG laser. Nighttime field experiments testing various solutions to this problem show excellent agreement with diffusion theory, and retrievals yield plausible values for the optical and geometrical parameters of the observed cloud decks.

A cw CO₂ Doppler lidar was modified for an airborne test of the principles of the true-airspeed sensor. During two flights in April 2000 the system was tested together with the wind sensor in the noise-boom of the research aircraft. The results are (i) there is an excellent correlation between the LOS components of the Doppler lidar and of the aircraft sensor; (ii) there is no dropout of dat also in very clean regions of the atmosphere; (iii) the Doppler sensor can also be used as an aerosol particle counter. A virtual instrument was used to design the sensor and was modified after the flight. It can be used to design a more compact fiber sensor.

The number of photons returning form a target in a given time interval is well described by a negative-binomial distributed random variable. A photomultipler tube (PMT) photon-counting detector is optimal for direct detection, and the number of detected-photon 'electron pulses' produced is also negative-binomially distributed per time bin, with a reduced mean due to the device quantum efficiency. These time distributed electron pulses are amplified and filtered by the preamplifier electronics prior to digitization and signal processing. The voltage output pulse per individual photo-electron event is known as the 'impulse-response- function' of the detector and preamplifier. In this study we employ a typical analog preamplifier filter response, modeled as a Butterworth lowpass filter of order two, which filters a 200 ps wideband PMT input voltage pulse. The random summation of these lowpass voltage impulse-responses, as created by the negative-binomial photon arrival times and random photo-electron creation, is the classical electronic 'shot-noise' random process. We derive numerically the voltage probability density function of this negative- binomial/impulse-response driven shot-noise random process following the stochastic process literature. We also show a technique to include PMT variations in gain, known as the 'pulse height distribution,' and to incorporate Gaussian baseline-noise voltage. Agreement with AMOR experiments is shown to be excellent. In addition, a Monte Carlo realization is presented, using the same impulse-response temporal shape, which also gives excellent agreement with AMOR data and with the analytical/numerical calculations.

Detection statistics for a coherent laser radar are substantially different from those of a direct detection laser radar. Direct detection ladar detection statistics vary depending upon the detection mode. Speckle noise also impacts the detection statistics. For a single-pixel single- frequency single-polarization coherent detection transceiver, speckle noise can only be suppressed through temporal averaging. Some degree of speckle averaging can also be achieved in coherent detection systems by using a multiple frequencies or dual polarizations. In addition to these, a direct detection receiver can exploit spatial diversity to suppress the effects of speckle. This paper develops example performance comparisons. We show that a photon-counting direct detection receiver can exploit spatial diversity to suppress the effects of speckle. This paper develops theory useful for describing the performance of these three receiver architectures against diffuse and glint targets and provides example performance comparisons. We show that a photon-counting direct detection receiver can, in principle, provide superior performance, however practical limitations of current detection technology particularly in the near IR spectral region reduces the performance margin and for many applications a coherent detection receiver provides superior performance.

Laser radar systems have found wide applications in the field of remote sensing. Reflectance as well as polarization features are used together for applications ranging from environmental monitoring to target classification. The Stokes parameters are ideal quantities for characterizing the above features because they provide useful information on both light intensity and polarization state. The University of Nebraska is currently refurbishing an airborne multi-wavelength laser radar system based on the NASA Goddard Space Flight Center (GSFC) developed Airborne Laser Polarimetric Sensor (ALPS). The system uses a Nd:YAG laser operating at wavelengths of 1064 nm and 532 nm, and contains four channels at each wavelength to measure the polarization states. This system was used to measure the Stokes parameters of backscattered laser light from different materials. These included canvas tarp, white paper, plywood, concrete, aluminum plate and anodized aluminum plate. The data provide an understanding of the polarized scattering properties of various materials, and are expected to be useful in developing target discrimination algorithms.

Mode-locked solid-state coherent laser radars operating continuously and at short wavelengths are capable of forming range-resolved Doppler and intensity images of distant spinning objects. The continuous mode-locked repetitive pulse operation allows demonstrated long laser coherence- lengths to achieve efficient heterodyne detection. The continuous repetitive pulsed operation also allows one to probe an object for arbitrarily long observation times resulting in precise center-of-mass-Doppler estimation and micro-Doppler vibration and spin measurements, as well as the temporal integration of the received signals to increase effective signal strength. We investigate this ladar waveform for imaging spinning cylinders at large distances, so that there is on the order of two detected photons approximately every 1,000 micro-pulses and applying simple estimators to the Doppler center-of-mass and Doppler-width for automatic target recognition purposes. A simple preliminary model for 'reflective' cylinders is also presented.

We present the concept of an optically pre-amplified intensity modulated lidar, where the modulation frequency is in the microwave domain (1-10 GHz). Such a system permits to combine directivity of laser beams with mature radar processing. As an intensity modulated or dual-frequency laser beam is directed on a target, the backscattered intensity is collected by an optical system, pass through an optical preamplifier, and is detected on a high speed photodiode in a direct detection scheme. A radar type processing permits then to extract range, speed and identification information. The association of spatially multimode amplifier and direct detection allows low sensitivity to atmospheric turbulence and large field of view. We demonstrated theoretically that optical pre-amplification can greatly enhance sensitivity, even in spatially multimode amplifiers, such as free-space amplifier or multimode doped fiber. Computed range estimates based on this concept are presented. Laboratory demonstrations using 1 to 3 GHz modulated laser sources and >20 dB gain in multimode amplifiers are detailed. Preliminary experimental results on range and speed measurements and possible use for large amplitude vibrometry will be presented.

We survey radiative Green function theory (1) in linear transport theory where numerical procedures are required to obtain specific results and (2) in the photon diffusion limit (large optical depths) where it is analytically tractable, at least for homogeneous plane-parallel media. We then describe two recent applications of Green function theory to passive cloud remote sensing in the presence of strong three-dimensional transport effects. Finally, we describe recent instrumental breakthroughs in 'off-beam' cloud lidar which is based on direct measurements of radiative Green functions with special attention to the data collected during the Shuttle-based Lidar In-space Technology Experiment (LITE) mission.

We have experimentally validated the concept of a differential image motion (DIM) lidar for measuring vertical profiles of the refractive index structure characteristic C by building a hard-target analog of the DIM lidar and testing it against a conventional scintillometer on a 300 m horizontal path, throughout a range of turbulent conditions. The test results supported the concept and confirmed that the structure characteristic C can be accurately measured with this method. Analysis of the effect of scintillation on DIM lidar has been performed. It is shown that the lidar has a scintillation resistant capability. Turbulence and lidar calculations were performed. These calculations confirmed that the DIM lidar is practical.

We have experimentally validated the concept of a differential image motion (DIM) lidar for measuring vertical profiles of the refractive index structure characteristic C_n² by building a hard-target analog of the DIM lidar and testing it against a conventional scintillometer on a 300 m horizontal path, throughout a range of turbulent conditions. The test results supported the concept and confirmed that the structure characteristic C_n² can be accurately measured with this method. Analysis of the effect of scintillation on DIM lidar has been performed. It is shown that the lidar has scintillation resistant capability. Turbulence and lidar calculations were performed. These calculations confirmed that the DIM lidar is practical.

An imaging lidar system is developed for capturing laser- induced fluorescence imagery. With minor changes to the transmitter and receiver optics, the system operates in a number of different modes including 3D multispectral, hyperspectral, multi-excitation hyperspectral, and fluorescence-lifetime hyperspectral. All of these sensor functions provide discriminating capabilities for targets exhibiting spectral fluorescence signatures.

The paper consists of two parts. In the first part the main characteristics of heavy sea are discussed that are essential for radar observation at grazing angles, the sea surface spike characteristics are analyzed in the framework of the statistical theory of random process surges above some boundary, the sea surface shadowing function is considered. In the second part the radar spike characteristics of sea backscattering at frequencies of 10.0, 35.0, 75.0 and 140.0 GHz are analyzed including the polarization and polarization-spectral characteristics of rough sea backscattered signals. During the spikes the depolarization coefficients, spectrum central frequency and spectrum width increasing is marked and the second maximum appearance in the scattered signal spectra is observed. The probable mechanisms are analyzed, the one of these is the backscattering from spray-drop fraction formed by breaking of sea wave.

The International Space Station (ISS) is an extremely large and flexible structure that requires validated structural models for control and operation. We have developed a 5-lb, 150 in3 laser radar to remotely measure vibration of the ISS structure and determine the structural mode frequencies and amplitudes. The Laser Dynamic Range Imager (LDRI) specifications include a 40-degree field of view, range resolution of 0.1 inches, images of 640 by 480 pixels, and a 7.5 Hz update rate. The sensor flew on the Space Shuttle in December of 2000 and provided range video of the newly installed P6 truss and solar array panels during thruster firing. Post flight analysis constructed motion time histories from selected structures. The measured vibration spectra captured the desired mode frequencies and amplitudes with a resolution of 0.02 to 0.1 inches. Additional measurements of curvature in the solar array panels demonstrated the potential for on-orbit characterization or inspection of structures.

We present data on a novel short-pulse eyesafe lidar transceiver for utilization in high-resolution heterodyne detection Doppler wind sensing. Operating at 20 Hz, the transmitter is a 1.3 micrometers pumped solid state Raman laser running at 1.556 micrometers , and is injection seeded using a direct diode master oscillator. This system is coupled to a hemispherical scanner to measure atmospheric winds, with the data validated against a commercially-available 2 micrometers lidar system. We typically measured atmospheric returns from greater than 2 km, with range resolution less than 6 m.