The high-energy airborne laser system must be able to effectively propagate a short- wavelength (near IR) laser beam over long path lengths at high altitude to be effective in the Theater Missile Defense mission. High-altitude atmospheric turbulence along these very long paths can have a potentially serious degrading effect on laser propagation. Adaptive optical systems design to compensate for the atmospherically induced aberrations are predicted to provide effective compensation to meet mission requirements. A program has been designed to characterize the relevant optical properties of the upper atmosphere and to conduct key experiments to confirm the feasibility of phase-only adaptive optics for this application

A series of experiments was conducted in January 1993 in which a pulsed laser beam was propagated from one aircraft to a receiver on board another aircraft. At the receiver, the resulting turbulence-induced scintillation patterns across an 80-cm aperture were recorded. The measured irradiance patterns were used to infer the fundamental performance limits imposed by turbulence-induced amplitude scintillation on imaging systems and laser transmitters employing adaptive optics to perform phase-only compensation. From the data, point and aperture-averaged scintillation statistics and amplitude correlation functions were also obtained. We describe the theoretical basis for the measurements. A theoretical analysis, applicable in the weak scintillation regime, is presented. Predictions based on wave optics computer simulations are also presented, and comparisons of the empirical results with the analysis and simulation predictions are shown. The methodology used to analyze the data recorded during the ABLEX tests is also detailed a description of the experiment and a discussion of the results obtained are presented in a companion paper.

ABLEX, or Airborne Laser Experiment, was a series of airborne experiments conducted in December 1992 and January 1993 in which a pulsed laser beam was propagated between two aircraft flying at high altitude. In these experiments, the scintillation patterns resulting from propagation through atmospheric turbulence were recorded. From these scintillation patterns, the fundamental performance limits of phase-only adaptive optics systems could be determined. Rather than field a highly complex and expensive airborne testbed equipped with a state-of-the-art adaptive optics system, the physics-limited performance was determined by a novel, and quite simple method that used a Fresnel lens collection system. The aircraft flew at separation ranging from 25 to 200 km. Data was obtained below, through, and above the tropopause. This paper describes those experiments, the experimental hardware, and the results obtained. Though there was a wide range of turbulence, as evidenced from scintillation statistics, the Strehl values inferred from these experiments provided direct evidence that atmospheric scintillation does not present a fundamental limitation to phase-only compensation.

Horizontal path temperature fluctuation measurements were made on board a KC-135 aircraft, at altitudes ranging from 30,000 to 40,000 feet, such that the altitudes below, in, and above the tropopause were sampled. In general, we find that there is a low background level of turbulence, and there appear to be superimposed o this background higher level turbulent 'patches.' These patches are a few km in extent, and the boundaries of these patches are abrupt with the transition to background taking place in distances of approximately 50 meters. These abrupt boundaries are consistent with the optical measurements taken at the same time. The measured structure functions of the form D(r) is approximate to rⁿ with 0.2<=n<=2 down to scale sized of 20 cm, which was the limiting resolution of the instrumentation. Each flight segment was conducted under constant conditions (i.e., speed, temperature, altitude). Contrary to expectations, the log slopes of either the power spectral densities or of the structure functions are often non-Kolmogorov, as characterized by a k^-5/3 power spectral density, or by a structure function of the form r^2/3. This statement must be tempered, however, by uncertainties in the frequency response of the temperature sensor. In addition, the data show regions in which the expected functional relation between the structure function and power spectral density does not hold. These characteristics of high-altitude turbulence suggest that past measurement techniques used to measure c_n² may be inapplicable in the high-altitude regime, and c_n² as a sole descriptor of turbulence may be incomplete.

Basing laser devices on aircraft leads to new problems in atmospheric transmission due to the long, nearly-horizontal propagation paths in the upper troposphere and stratosphere. This paper analyzed the molecular and aerosol extinction impact on near-IR aircraft-based laser performance. Calculations using standards atmospheric models are reviewed. Recent volcanic activity has required examination of the impact of stratospheric aerosols for propagation paths of interest. Calculations as a function of geometry are presented which use satellite limb viewing data to asses the impact of cirrus clouds and volcanic aerosols.

Time-dependent wave-optics simulations of thermal blooming and turbulence effects on airborne laser beam propagation are described for a line-selected HF laser at approximately 2.93 micrometers wavelength. A near-horizontal propagation path is assumed together with the appropriate beam motion and slewing to correspond with a typical airborne laser theater missile defense engagement scenario. Results both with and without phase correction are presented for blooming, turbulence, and combined turbulence and blooming. The simulation results for uncorrected blooming and for uncorrected turbulence are in reasonable agreement with well-established scaling-model predictions. In the absence of turbulence, the blooming effects are negligible compared with the effects of turbulence. With correction of combined turbulence and blooming, the blooming effects are increased sufficiently to reduce the average on-axis Strehl by 40%.

A wave optics simulation has been used to compare the performance of adaptive optics systems in the presence of near-field and distributed turbulence. For a given total turbulence strength, the Greenwood frequency and the log-amplitude variance will degrade a phase-only adaptive optics system both because of the uncorrectable amplitude variations and because the scintillations will cause errors in the wavefront sensor and reconstructor.

It has been recognized for some time that full-order adaptive optics systems can provide considerable improvement in the performance of astronomical imaging systems. However, as 4-m-class telescopes become the standard for astronomical work it will become more difficult in terms of cost and complexity to build and operate full order AO systems, especially if one is interested in working in the visible. For the past several years, we have been developing simulation and analysis tools to study AO systems using laser guide stars. This analysis indicates that even low-order correction can provide improvements in image quality, especially when used in conjunction with computer post-processing algorithms.

An Atmospheric Turbulence Optical Model (ATOM) is presented that used cellular automata (CA) rules as the basis for modeling synthetic phase sheets. This method allows image fracture, scintillation and blur to be correctly models using the principle of convolution with a complex kernel derived from CA rules interaction. The model takes into account the changing distribution of turbules from micro-turbule domination at low altitudes to macro-domination at high altitudes. The wavelength of propagating images (such as a coherent laser beam) and the range are taken into account. The ATOM model is written in standard FORTRAN 77 and enables high-speed in-line calculation of atmospheric effects to be performed without resorting to computationally intensive solutions of Navier Stokes equations or C_n² profiles.

A ray optic model for the propagation of electromagnetic waves at optical frequencies through atmospheric channels with difference turbulence levels is presented. Using this model, the variation of the power at the receiver end, the rise times of the channel, and the amplitude fluctuations have been computed with respect to beam width, channel thickness, and degree of turbulence. It is expected that this mode will have a bearing on the design of ground-to- satellite and satellite-to-ground optical communication links.

This paper presents the description and verification of subassemblies as well as the evaluation of basic parameters of a heterodyne detection system performed in cooperation with the Institute of Quantum Electronics, Military University of Technology, Warsaw, Poland.

The effects of atmospheric turbulence on the imaging quality and resolution of an optical system, illuminated by the coherent and noncoherent light, are discussed in terms of the information content and the number N_d of degrees of freedom of an imaging system. lt is shown that, for coherent illumination system and the receiving radius R₂ is langer than the radius a of propagated beam ,the turbulent spread of the beam may lead a positive effect , and there is a potential superresolution, the resolution beyond the classical diffraction limit and for R₂ < a_t ,the N_d is independent on the atmospheric turbulence. For the particuar cases of incoherent illumination, N_d is found to be reduced from its turbulence--free value , the Shannon number , by the factor ((alpha)r₀/D₀ ) ², r₀ is the Fried's parmenter , D₀ is the receiving diameter and (alpha) is the function of the variable r₀/D₀. The information content of coherent incoherent illumination systems is decrease with the atmosphere--turbulence fluctuations increase.

This paper will briefly review techniques that allow phase errors that are inherent in multiple- aperture receivers or due to atmospheric turbulence in large single-aperture systems to be compensated and thus allow nearly diffraction-limited imaging at optical frequencies from such high aberrated structures. Attention will b given to the effects of using different types of arrays, the type of detection used, and the nature of the received signal. Emphasis will be place upon real-time operation but iterative post-image formation techniques such as maximum entropy and blind deconvolution will also be mentioned. A brief discussion of the hardware/technology required for optical phased arrays will be given.

The spatial correlation algorithm (SCA) corrects for severe array phase distortion without requiring finding a source of opportunity within the target area for possible phase conjugation. The algorithm predicts and corrects aperture distortion using the spatial correlation properties of range-resolved target or backscatter returns.Also, target rotation may be exploited. The SCA is capable of correcting arbitrarily large phase errors that can be totally independent from one array element to another. We extended the algorithm to 2D optical array geometries. The resulting weighted lease squares 2D SCA is optimal. Combined with a new phase-unwrapping technique, the solution is direct, in the sense that it works all differential phase measurements simultaneously. Results of an extensive statistical analysis characterizing the gain of the mainlobe of the resulting point spread function are also presented.

In this paper we consider some of the implications of steering broad spectral band radiation using dynamic gratings. The similarities and differences in dispersion between dynamic gratings formed with electro-optic phase arrays and with microlens arrays are highlighted. Optical design considerations are presented that limit the dispersion in the image. System designs that use refractive, diffractive, and combinations of refractive and diffractive components to partially correct for the dispersion are analyzed.

In this paper we highlight some of the advantages of lightweight beam steerers that can be employed at or near the entrance aperture of the system. One advantage of such an implementation is reduced field aberrations. The effect of the choice of beam steerer location on image quality is quantified. A specific example of the use of agile beam steerers in a laser radar system is considered. Configurations are described that allow continuous angular coverage. The effect of steering efficiency is discussed.

In this paper we describe a novel liquid crystal device for rapidly steering laser beams for high-power and large-aperture applications. The device consists of an array of optical phase modulators that contain a thin, active, liquid crystal layer sandwiched between two substrates. The unique aspect of the device is that each phase modulator can produce a linear (blazed) phase gradient, rather than the constant phase profile typical of other liquid crystal beam- steering devices. It is designed for use over a wide range of wavelengths and is particularly well suited for the deflection of short-wavelength laser beam. In this paper, we will describe device design, theoretical performance (diffraction efficiency and time response), and present experimental results of a device built to deflect a 1.064-micrometers laser beam.

The sensitivity of space debris detection using a satellite-borne, low-power laser as a reference for terrestrial adaptive optics is considered. It is shown that there is sufficient sensitivity to permit detection of sunlit particles of debris a few cm in diameter.

The Rapid Optical Beam Steering (ROBS) sensor suite is being developed under the Ballistic Missile Defense Organization (BMDO) to permit precision tracking of long-range missile interceptor events. The sensor suite consists of a very wide field-of-view (FOV) MWIR staring array (28 degrees) for target detection, a narrower FOV (3.5 mrad) MWIR array for acquisition, tracking, and scene viewing, as well as a CO₂ laser radar for range and Doppler tracking. The sensor suite is mounted on a ROBS telescope that has high angular agility. Without the highly agile ROBS telescope, the use of IR sensors such as laser radar and staring arrays in smart munitions testing has been considered impractical. Unfortunately, the alternative, which is using highly sophisticated range radar, has not been very successful due to the large resolution cells caused by its long wavelength. The purpose of this paper is to examine the potential performance of the very high resolutions (in angle, range, and Doppler) ROBS sensor suite to satisfy the difficult tracking requirements associated with smart munitions testing.

The interferometric scanning laser beam position control system based on Ronchi interferometer is investigated. The new technical decision, increasing the accuracy and ranges of measurements are considered. The basic ratios and the results of experimental research are adduced.

We have developed a 2D multibeam binary-optic-based scanner for transmission/receiver function for LADAR and other applications under a Small Business Innovation Research (SBIR) contract from Eglin Air Force Base. Multibeam scan provides many unique advantages including increased data rate for pulsed laser, increased scan coverage, and programmable broadcasting for optical interconnect applications.

The feasibility of phase telescope arrays for coherent optical space communications is demonstrated by a proof-of-concept laboratory experiment. The incident optical power is collected by four subtelescopes and coherently combined into a single monomode output fiber. The implemented optical receive array antenna is self-phasing, i.e., the optical subfield pistons are automatically adapted with respect to the direction of the incident wavefront. The telescope array is completely independent of any subsequent receiver and of the data modulation format employed. Our experimental setup operates at a wavelength of 1064 nm. With an optical input power of 1 nW per subaperture, the system efficiently combines the optical input subwaves and responds to a step-shaped change of the input wavefront direction within 1 ms.