Lasers have historically been considered in the context of weapons, but recent progress has also permitted us to consider using lasers for more subtle applications such as designation, tracking, and discrimination. In this paper, we will review the state of the art of active tracking, including effects such as laser beam quality, diffraction, atmospheric turbulence, and other aspects of laser interactions with its propagation environment. We will present the theory for using lasers in these lower power applications.

A linear one-dimensional, 1x4096 pixel, zero-twist nematic liquid crystal spatial light modulator (SLM) was evaluated for laser beam steering and tracking applications. The commercially obtained SLM is designed to operate at, λ = 850 nm, allowing more than 2 π phase modulation. Due to voltage leakage the phase modulation experienced by the wave front differed from the ideal calculated phase patterns. This cross talk between pixels reduces the diffraction efficiency. Different methods developed to compensate for this effect are presented. The usable steering range of the SLM was extended to ± 2 degrees using improved phase patterns. A simple model was developed to simulate the optical effects of the voltage leakage. Preliminary tracking experiments were carried out in a laboratory set-up using a moving corner cube retro reflector. The beam steering SLM was implemented in a transceiver for free-space optical communication. Initial results using the transceiver up to 180 m range are presented.

Effective operation of a space control system requires extremely accurate acquisition, tracking, pointing and discrimination (ATPD) capabilities, and should provide the ability to disrupt or degrade an adversary space operations if needed. We present a laser based system concept that will offer an innovative solutions to satisfy many space control mission needs. A long-range adaptive laser tracking system (ALTS) described here will provide the required capabilities in target tracking and characterization. It is based on an approach that uses the target as one of the mirrors of the laser resonator. Then, due to specificity of laser, the parameters of its emission allow for deriving the complete information on spatial-angular position of the target, its range, velocity, and flight direction. In this paper we discuss the architecture and operational principles of the ALTS capable in performing the required ATPD function for a remote target. A double-cavity laser scheme with its resonators coupled through the phase-conjugate mirror (PCM) is at the heart of the system. Four-wave mixing mechanism is applied here to form the PCM. Such a scheme allows for automatic adaptive operation of the laser with movable mirror. Both, the results of the theoretical analysis and experimental studies of the proposed ALTS system will be presented, as well as the methods of detecting the spatial-temporal characteristics of the target (its position, range, velocity) through analysis of the received signal. In addition the perspectives of using the proposed ALTS for remote target imaging are also discussed.

There are a number of optical systems being considered for aircraft. These include communication systems, surveillance systems, and other pointing and tracking systems. The angular vibration of the vehicle is usually the primary disturbance that degrades the optical system performance. Boeing is involved in a project to measure and analyze the angular vibration of several different aircraft. This report will show a comparison of the angular vibration spectrums of large transport aircraft and smaller fighter aircraft over a range of flight conditions.

We examine the signal processing of both linear and sinusoidal frequency modulation (FM) coherent ladar returns from resolved and unresolved targets, which are spread in Doppler. The Doppler spread may be due to target spin, tumbling, or vibration as well as to the applied linear or sinusoidal-FM on the transmitted E-field. Monte Carlo realizations of the target surface random phasor reflector elements interact with the incident Efield producing laser speckle, and the speckled returns are analyzed in this study. The speckle signals are processed (1) using several spectrum (periodogram based) estimators, (2) the conventional “spectrogram” approach, and (3) ten joint time-frequency transforms (JTFT). We show that the Born-Jordan JTFT is superior to the other spectral estimators tested here in suppressing local oscillator laser noise and accurately estimating the target's spectrum for signal processing under speckle target return conditions pertaining to coherent laser radar. A new algorithm which sums particular pixels of the JTFT image is introduced and is shown to be much more robust in low CNR conditions than the JTFT maxima or JTFT centroid processing when utilizing the applied linear or sinusoidal-FM modulation waveform.

Using analytical calculations and wave-optic code simulations, we study the problem of shaping the spatial distribution of laser beam intensity in the far field by varying the near-field phase. We discuss possibilities of using adaptive-optic devices to form the near-field phase distribution to reach the desirable far-field intensity profile. As an example, we show how a doughnut-like far-field intensity could be achieved by introducing a specific phase aberration into the laser beam through a deformable mirror. Diffraction limitations on the far-field beam shaping are estimated.

Conventional adaptive optics methods for controlling the phase control element based on least-squares reconstructions of the measured residual phase error exhibit poor performance as scintillation becomes strong. This paper compares the performance of various closed-loop control methods for different phase sensor types (self-referencing interferometer, shearing, and Shack-Hartmann interferometers), and for both conventional and segmented piston-type deformable mirrors (DMs). Significant performance improvements are demonstrated using a weighted least-squares reconstructor that adaptively optimizes the weights at each frame based on the intensities associated with each phase difference measurement and their sums around closed loops. Although the reconstructors considered do not explicitly place branch-cuts in the reconstructed residual phase, branch-cut like features can appear in both the single frame reconstructions and the closed-loop actuator commands. It is also found that at higher Rytov numbers, segmented piston-type DMs outperform conventional deformable mirrors. It is believed that conventional DMs suffer a fitting error associated with branch cuts in the actuator commands that the piston-type DMs are immune to. Performance trends corresponding to self-referencing interferometers provides a useful benchmark since, unlike Shack-Hartmann and shearing interferometers, the phase measurements are not corrupted by scintillation effects.

A new approach, for estimating r₀ and the Greenwood frequency from wavefront sensor measurements, is introduced. Unlike other techniques, which use phase statistics calculated from reconstructed wavefront sensor measurements, this approach makes use of the slope discrepancy, the component of the measurement that is not reconstructed by a least squares reconstructor. It is shown that the r₀ estimator based on the slope discrepancy is a high degree-of-freedom estimator with a much smaller estimation variance than estimates based on the phase variance. An new temporal atmospheric sample rate, f₀(ε²), is introduced in this work. This quantity is calculated from the slope discrepancy structure function and sets the minimum required sampling rate to maintain the residual wavefront error below ε²/K for a system dependent gain, K. For Kolmogorov turbulence statistics it is shown that the Greenwood frequency is related to f₀ by f_G=f₀(K(2π/20)²)/40. In non-Kolmogorov turbulence, f₀ continues to be a legitimate specification of sampling requirements. These results are all illustrated with wave optics simulations.

This study reports the analysis of the long distance propagation characteristics of the annular beam and its application to lidar. In our analysis, the annular beam is formed by a couple of axicon prisms. The waveform of the annular beam is transformed in to the nearly nondiffractive beam through the propagation. The propagation characteristics can be easily controlled by the waveform of the outgoing annular beam. The center peak intensity, FWHM, and the intensity ratio of the center peak intensity of the transformed nondiffractive beam to the whole beam intensity were examined in the various viewpoints. We also considered the spread and focusing angle of the annular beam, and obtained the critical angle to transform the nondiffractive beam. We confirmed that the permissible error of the optical alignment has an enough margin with the desired beam divergence. The annular beam proves its merit on a co-linear type lidar because of utilization of a large reflecting telescope with high transmitting efficiency and near distance measurement with a narrow FOV.

Discussed will be the development of a model for both a line laser diode as well as an area array consisting of line laser diode elements. The need for this work comes from the difficult task of using a single strip laser diode with dimensions such as 1 centimeter in the y direction and 1 micron in the x direction while exhibiting substantially different divergences in each axis as well. The intention is to create a fairly uniform optical pump beam in both the x and the y axis through as much of the gain media as possible. The modeled, virtual diode, is telecentric only in the long or “slow” axis where the divergence is generally less than the diode's “fast” axis. The virtual diode model is used as a source in a lens design software program to predict performance of diodes and their accompanying optics as pump sources for micro-lasers. Corroboration between the model and actual systems have been experimentally verified and will be presented. The single, linear laser diode source is used as the basis for a laser diode array model is also presented.

A neutron transmutation doped (NTD) far-infrared p-Ge laser crystal and a melt-grown p-Ge laser are analyzed and compared. Though the doping level in the NTD active crystal is twice lower than optimal, the laser performance is comparable to that produced from high-quality melt-grown crystals because of superior dopant uniformity. Compensation was examined by comparing results of neutron activation analysis with majority carrier concentration. Study of impurity breakdown electric field reveals better crystal quality in NTD. The current saturation behavior confirms the expected higher doping uniformity over melt grown laser rods.

A trailer-based lidar, named Humidity and Aerosol Lidar (HAL), is being built as a remote sensing tool to characterize atmospheric aerosol and water vapor in the line-of-sight. Water vapor and aerosol in the lower atmosphere are critical components affecting the propagation of high-energy laser beams and microwave. The sensor is developed to collect high temporal and vertical resolution data of atmospheric aerosols and water vapor. This ground-based system also serves as a demonstration and an engineering study of a flight-capable sensor for real-time diagnostic of the atmosphere. The lidar, operating on the principles of differential absorption, could measure water vapor to 10 km altitudes. It also measures aerosols and cloud backscatter at altitudes up to 18 km and ranges up to 90 km. Operating with a hemispherical scanner, the sensor could map the 3-dimensional field of aerosols and water vapor and provide vertical as well as horizontal structures. A unidirectional Alexandrite ring laser, operating in single mode near 727.49 nm, is the laser source. The sensor is designed to operate in day and night time. A description of the system, its wavelength calibration unit, the transmitter-receiver system and projected performance will be discussed. Results of the photo-acoustic calibration cell and wavelength selections will be presented. Preliminary results of water vapor and aerosols will be discussed.

Understanding the turbulence along a propagation path is required to evaluate new methods for tracking, pointing, and compensation of laser beams, studying image degradation, and interpreting remote sensing observations. This paper presents observations using a new balloon-ring platform equipped with multiple fine wire probes (1 μm diameter) at various separations for sensing both temperature and velocity fluctuations. These measurements provide profiles of the temperature structure parameter (C_T²), refractive index structure parameter (C_n²), eddy dissipation rate (ε), inner scale (l_o), and outer scale (L_o) values. Of particular interest is the path variability of C_n², ε, l_o, and L_o, and the relationship of these parameters to atmospheric stability. Since all “raw” data is archived, spectra are shown and discussed as to how often the atmosphere truly follows the “Kolmogorov-5/3,” or under what atmospheric conditions for which the structure function is represented by the r^2/3 law. Also the concerns of local isotropy and intermittency can be addressed. The interpretation of these results to propagation effects is discussed. Salient features of the new measuring system are presented as well as the rationale for its implementation, including the longstanding concern of wake contamination on conventional measuring systems tethered under a single balloon.

As known, the problem of compensation of turbulent distortions is an ill-posed one. Regularization methods are found to be inefficient. The problem can be solved by using local-linear super-resolution method. Its main feature consists in diminishing of problem dimensions. We use the terminology of multi sensors (rays) systems for this purpose and a new concept of resolving function R. Each linear device which forms image is characterized by two functions: PSF O and MTF M on observation area D. The function R resolves a function O, if their cyclical product (or convolution) is equal (or near) to d - delta function: R*O = O*R = d on D. The resolving image R*I on D can be presented as measured one by fine “registering system” with narrower PSF R*O that cannot be achieved physically. If area D is as small as one of O, then the corresponding values of M are relatively large ones and there is no problem for compensation of distortions. The special arrangement of local subareas of D is obtained by solving the next multi sensors (rays) problem: to separate the image I and the distorting PSF O on the observation area D so that at small quantity of data the resolution problem with R could be solved strictly and with the minimum border effect. Grid-generated turbulence in a shock tube was chosen as an example of homogeneous and isotropic turbulence. Statistical properties of this flow have been investigated experimentally. We found the correlation function and structure function for the fluctuations of refractive index. In our case of grid-generated turbulence the statistical properties are distinct from the Kolmogorov's two-thirds law. We modeled laser beam propagation through turbulent atmosphere and obtained the numerical results for the distortions of images. The distortion O (r) of PSF and the set of resolving functions R were found according to the structure function. The problem of compensation of distortions caused by turbulence was solved with the aid of a new local-linear super-resolution method. This method allows to resolve turbulent distortions of PSF at low signal-to-noise ratio.

Range-resolved co-pointing multiple wavelength lidar backscatter from aerosols is analyzed for a summer day in the northeast United States. Lidar backscatter wavelengths are 355 nm, 532 nm, and 1064 nm and were measured at a vertical range gate of 60 meters. The altitude range of lidar measurement is from the surface to 4 km above ground level and the measurement period spanned five hours from late afternoon through several hours after sunset. Vertical profiles of temperature, relative humidity, and wind velocity, and surface visibility, were also measured to characterize the prevailing air mass. Lidar aerosol backscatter was significant through 3 km and diminished rapidly above. Several aerosol models selected on an a priori basis are used to compute backscatter ratios for wavelength pairs using scattering theory. These are compared with the profiles of measured backscatter ratios in an attempt to infer the type of aerosol present in the lower atmosphere and estimate multiple wavelength extinction. Measured backscatter ratios agreed with the ratios for soot, water-soluble, and haze aerosol models at the lowest altitudes with little agreement above 1 km for any model. Extinction estimates derived from lidar backscatter at 300 m were significantly higher than the corresponding values deduced from surface observations.

An experimental validation of the differential image motion (DIM) lidar concept for measuring C_n² is reviewed. The field validation was performed 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 turbulence conditions. The test results supported the concept and confirmed that the structure characteristic C_n² can be accurately measured with this method. A practical method is described for extending the validation technique to vertical profiles of C_n².

The agility and speed with which directed energy can be retargeted and delivered to the target makes a laser weapon highly desirable in tactical battlefield environments. A directed energy system can effectively damage and possibly destroy relatively soft targets on the ground. In order to accurately point a high-energy beam at the target, the directed energy system must be able to acquire and track targets of interest in highly cluttered environments, under different weather, smoke, and camouflage conditions and in the presence of turbulence and thermal blooming. To meet these requirements, we proposed a concept of a multi spectral tracker, which integrates three sensors: SAR radar, a passive MWIR optical tracker, and a range-gated laser illuminated tracker. In this paper we evaluated the feasibility of the integrated optical tracker and arrived to the following conclusions: a) the contrast enhancement by mapping the original pixel distribution to the desired one enhances the target identification capability, b) a reduction of the divergence of the illuminating beam reduces rms pointing error of a laser tracker, c) a clutter removal algorithm based on active contours is capable of capturing targets in highly cluttered environments, d) the daytime rms pointing error caused by anisoplanatism of the track point to the aim point is comparable to the diffraction-limited beam spot size, f) the peak intensity shift from the optical axis caused by thermal blooming at 5 km range for the air-to-ground engagement scenario is on the order of 8 μrad, and it is 10 μrad at 10 km range, and e) the thermal blooming reduces the peak average power in a 2 cm bucket at 5 km range by a factor of 8, and it reduces the peak average power in the bucket at 10 km range by a factor of 22.

We investigated the effect of strong turbulence on wavefront sensing and compensation by using an adaptive optics (AO) system for non-cooperative targets. We found that for a non-cooperative target in high scintillation environments beacon anisoplanatism degrades the AO system performance, in addition to the scintillation and branch points. In deep turbulence, a beacon wave from an extended beacon samples different turbulent inhomogeneities and acquires different phase aberrations than a beacon wave from a point reference source. This corrupts the optical field of the beacon wave and degrades the AO performance. To mitigate the effects of beacon anisoplanatism we introduced two approaches. The first method is based on an optimization of the aperture diameter of the illuminating beam to reduce the spot size of the beacon. We found that this approach provides a modest Strehl ratio gain for some range of turbulent conditions. The second method includes an estimate of the Green's function for propagation through turbulence from the phase measurements made with an extended beacon, and the use of the Green's function estimate in the beam control system. We plan to evaluate this method in the future work.

Having shown the feasibility of passive daytime observation on board of a 155mm gyrostabilized artillery shell, the French-German Research Institute of Saint-Louis (ISL) has started working on projectile boarded night vision systems. Laser illumination was preferred over the use of passive IR imaging mainly for reasons of detector performance and cost. The active imaging system designed for artillery shells will have to undergo accelerations of the order of 15000 g. It will begin the observation of the target area about 1000 m before impact and transmit the images to the ground station before its destruction. A laser illuminator with a peak power of 1 kW before beam shaping and a pulse duration of 50 μs with a repetition rate of 25Hz has been developed for this application. It is based on a laser diode stack emitting at 800nm. A special atten-tion has been given to the beam shaping operation. The beam divergence closely matches the field of view of the imaging optic and has a constant and homogeneous intensity profile over the target. Other fundamental criteria have also been taken into account, such as a compact size, simplicity and low cost, without losing the efficiency of the collimator. A prototype of the active imaging system with a field of view of 10° has been built, tested and validated on the ground. Integration of the illuminator into a 155 mm shell has begun.