The use of a "Field Mapping" beam shaping technique for generating near field Gaussian intensity patterns from uniform intensity ("flat top") laser beams is described. The design objective was to simulate realistic High Energy Laser (HEL) far field intensity patterns for laser effects testing purposes, without having to propagate the large distances necessary to obtain the true far field intensity profile and beam size typical of realistic target engagement scenarios.
The field mapping approach presented uses a continuous surface mirror with a figure designed to redistribute the energy into a Gaussian distribution at the target plane. The use of a reflective system was desired to minimize cost and maximize wavelength diversity and laser damage threshold capability. Physical optics analyses are presented to illustrate the performance characteristics of a totally passive reflective field mapping beam shaper.
The results of an investigation into beam shaping techniques for generating near field Gaussian intensity patterns from uniform intensity ("flat top") laser beams are presented. The motivation for this study was the desire to produce realistic High Energy Laser (HEL) far field intensity profiles for laser effects testing, without propagating the large distances necessary to obtain the true far field pattern and beam size typical of HEL target engagement scenarios. To minimize cost, maximize wavelength diversity, and provide a high laser damage threshold capability, an all reflective optical system was preferred.
Though beam shaping systems are commonly used to convert Gaussian beams to flat tops beams, the reverse problem, that of converting flat tops to Gaussians, appears to be new territory. Most beam shaping approaches, particularly those that do not preserve phase, are not reversible. Two simple approaches that use segmented mirrors for converting flat tops to Gaussians are described here. While beam integrators, commonly used to convert Gaussians to flat tops do not work in reverse, the approaches presented use segmented mirrors resembling beam integrators, and have some similar benefits.
Geometric and physical optics analyses are presented to illustrate the performance characteristics of the different approaches at wavelengths of 1.315 and 3.8 microns. A simple method to reduce interference effects in the reshaped beam, that are present when a coherent source is used, are discussed.
An inexpensive large aperture (10 m class) receiver for optical wavelength imaging and remote sensing applications is discussed. The design was developed for active (laser illumination) imaging of remote objects using pupil plane measurement techniques, where relatively low optical quality collecting elements can be used. The approach is also well suited for conventional imaging at lower resolutions when light collection capability is of primary importance.
The approach relies on a large aperture heliostat consisting of an array of flat mirror segments, like those used in solar collector systems, to collect light from the region of interest. The heliostat segments are tilted in a manner to concentrate the light, by making the light from all segments overlap at a common point, resulting in a region of higher intensity about the size of a segment at the heliostat “focus”. A smaller secondary collector, consisting of a concave mirror located at the overlap point, further concentrates the light and forms a pupil image of the heliostat. Additional optics near the pupil image collimate the light for efficient transmission though a narrow band interference filter used to reduce sky background, and focus the light onto a PMT, or other sensor, for detection. Several design approaches for the collimating optics are discussed as well as system performance and limitations.
The Air Force Research Laboratory (AFRL) has initiated the LARRA (Laser Radar for Recognition and Assessment) program to investigate the use of laser radar signatures for identifying and determining the pose of satellites. The algorithms will use the 1-D range-amplitude information in the returns. To determine the identification and/or pose, the field returns will be correlated over a library of simulated returns. It is critical that the algorithms that produce the simulated returns do so accurately. AFRL has previously sponsored the development of TASAT (Time-Domain Analysis Simulation for Advanced Tracking), which has the capability to simulate the returns from satellites, to include the effects of atmospheric propagation. In this paper, we describe the modification and verification of TASAT using returns from mock satellites placed on the ground at a distance of 1 km from the laser. The laser is a mode-locked CO2 (10.6(mu) ) and the FWHM of the micropulses is 1.4 ns.
A field experiment to investigate an imaging technique based on reflection tomography has been performed at the Air Force Research Laboratory. The experiment, called HILT (Heterodyne Imaging Laser Testbed), involves the illumination of objects with short pulses of laser radiation, and the measurement of the temporal characteristics of return pulses scattered by the object. Return pulses, referred to as projections, provide range resolved information characteristic of the object surface shape and viewing angle. By obtaining multiple projections at different viewing angles, a tomographic reconstruction of the object's surface can be obtained. Recent testing has produced images of targets at a range of 990 meters using 1.4 ns pulses from a 10.6 (mu) CO2 laser. A heterodyne detection technique was utilized to record the weak return signals. The results obtained from this system are believed to be the first LADAR range resolved reflection tomographic images of diffuse objects in a field environment, and the first use of a heterodyne detection system for LADAR reflection tomography. A description of the system is provided and experimental results are presented.
The Air Force Research Laboratory is using a small-scale heterodyne laser radar (ladar) system for range-resolved imaging, among other applications. This system, called the Heterodyne Imaging Laser Testbed (HILT), is used for obtaining pulsed reflection returns from targets that are located on the ground at a distance of approximately 1 km. Over the past year, the resolution of the HILT's reflection tomographic images has improved from approximately 30 cm to approximately 10 cm. Presented in this paper are a description of HILT and tomographic image reconstructions of ground targets.
A micro-satellite, designed to aid ground-based laser imaging, ranging, and sensing systems as a calibration target, has been constructed and is scheduled to be launched in the fall of 2000. This low-earth orbit satellite carries a set of retro- reflectors (for visible and near-infrared wavelengths) that present a spatially extended target to sites on the ground. Several of the reflectors also impart a polarization signature to the reflected laser light. This paper discusses the specifications of the retro-reflectors, positioning of the reflectors on the satellite structure, passive control of the vehicle orientation, and ground-pattern characteristics of the reflected light.
A four channel imaging Stokes polarimeter has been designed and constructed at the Air Force Research Laboratories to measure the polarization properties of laser speckle patterns. An imaging polarimeter spatially measures the polarization state of light coming from an object, thereby producing a 'polarization image.' This provides a complete characterization, in terms of the 4 Stokes polarization parameters, of light received from different regions of an object. In addition to the intensity information obtained from conventional imagery, an imaging polarimeter also measures the two components of the electric field, and the phase between these components. This additional information has been shown in the laboratory to aid in the discrimination of otherwise similar looking materials. Due to the random phases present in unpolarized light, polarization images are most useful when the object being viewed is illuminated by a uniformly polarized light source, such as a simple laser illuminator. The system described was designed to image the pupil of an optical system containing speckle patterns created by illumination of diffuse objects with light from a pulsed 527 nm laser. Diagnostic techniques developed to measure path length differences between channels and various system calibration and characterization tests are described and results are presented.
An imaging polarimeter has been designed and constructed at the Air Force Phillips Laboratories to measure the polarization properties of laser speckle patterns. An imaging polarimeter spatially measures the polarization state of light coming from an object, thereby producing a `polarization image'. The system describes was designed to image the pupil of an optical system containing speckle patterns created by illumination of diffuse objects with light from a pulsed 1315 nm laser. Difficulties encountered due to demanding specifications required for polarimeter components and for coherent imaging systems in general are described and their solutions are discussed. Diagnostic techniques developed to measure path length differences and to identify the source of interference fringes are described. System calibration and characterization tests are described and results are presented.
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