A viable beam control technique is critical for effective laser beam transmission through turbulent atmosphere. Most
of the established approaches require information on the impact of perturbations on wavefront propagated waves.
Such information can be acquired by measuring the characteristics of the target-scattered light arriving from a small,
preferably diffraction-limited, beacon. This paper discusses an innovative beam control approach that can support
formation of a tight laser beacon in deep turbulence conditions. The technique employs Brillouin enhanced fourwave
mixing (BEFWM) to generate a localized beacon spot on a remote image-resolved target. Formation of the
tight beacon doesn’t require a wavefront sensor, AO system, or predictive feedback algorithm. Unlike conventional
adaptive optics methods which allow wavefront conjugation, the proposed total field conjugation technique is
critical for beam control in the presence of strong turbulence and can be achieved by using this non-linear BEFWM
technique. The phase information retrieved from the established beacon beam can then be used in conjunction with
an AO system to propagate laser beams in deep turbulence.
This paper discusses a novel type of beam director for effective laser beacon formation in deep turbulence conditions. The concept of the proposed beam director is based on an innovative approach employing a Brillouin enhanced four-wave mixing (BEFWM) mechanism for generating a tight (small spot size) laser beacon on a remote image-resolved target. The BEFWM technique enables both amplification and total (phase and amplitude) conjugation of the beacon-forming beam without the need for wavefront sensors, deformable mirrors or predictive feedback algorithms. Total conjugation is critical for beam control in the presence of strong turbulence, whereas conventional adaptive optics methods do not have this capability. The phase information retrieved from the beacon beam can be used in conjunction with an AO system to propagate laser beams in deep turbulence.
Diffractive wavefront control with programmable optically efficient modulo gratings was previously demonstrated as a means of extending the capabilities of a telescope system by compensating large aberrations associated with variations in object range and field angle within the telescope's field of regard. Inherent in diffraction phenomenon, however, are wavelength dependences that impose limitations on operating spectral bandwidths. This paper builds on past work integrating a programmable diffractive element of large pixel count with a telescope system. We present a theoretical evaluation of the wavelength-dependent performance characteristics of a modulo diffractive aberration compensator and experimental demonstrations of wavelength-tunable and extended spectral bandwidth operation. Near-diffraction-limited-resolution imaging is demonstrated within the spectral operating limits imposed by diffraction theory.
Diffractive wavefront control with programmable optically efficient modulo gratings has been successfully demonstrated as a means of compensating large optical aberrations and steering an outgoing beam over small angles without mechanical motion. This work builds on past work to demonstrate the integrated performance of a programmable diffractive element of large pixel count with a telescope system. More specifically, a liquid-crystal-based spatial light modulator is used as a reconfigurable diffractive optical element in a telescope system to compensate for large aberrations associated with variations in object range and field angle within the telescope's field of regard. The telescope consists of a conventional primary mirror that gimbals relative to the other components of the system and a programmable diffractive optic that compensates for more than 40 waves of aberration associated with defocus and off-axis aberrations. In this setup, aberration compensation via the programmable diffractive optic allows the object range to be varied by 160 times the depth of focus and increases the diffraction-limited field of regard by a factor of 70 while maintaining near-diffraction-limited performance.
Remote-sensing technology for weathering assessment should be sensitive to both textural and compositional changes in a surface. The capabilities of active polarimetry in weathering assessment are investigated through measurements of the Mueller matrices of bare and painted metal and dielectric surfaces at visible laser wavelengths in the quasi-monostatic geometry. Weathering mechanisms investigated include particulate erosion and solar irradiation, which are found to alter certain off-diagonal Mueller elements by around 5% in measurements over illumination angle.
Agile beam steering of optical radiation using phased arrays offers significant advantages, such as weight, stability, speed and power requirements, over conventional beam steering systems based on large optics, telescopes, and gimbals. Phased arrays incorporating programmable diffractive optics systems based on MEMS or liquid crystal spatial light modulators are being investigated for a number of applications including large aberration compensation, near-diffraction-limited imaging and agile beam steering. A prototype system uses discrete phase steps to approximate modulo-2π phase profiles and operates with 307,200 independently addressable elements, 100% fill factor and total optical efficiencies of up to 93%. This paper presents analysis of an agile beam steering phased array system incorporating physical parameters such as fill factor, 2π reset fidelity and influence function. Diffractive wavefront control with non-2π resets is shown to produce continual beam steering. Expressions and modeling of the far-field beam pattern and off-axis beam steering efficiency are presented. Measured diffraction efficiencies show close comparison with calculated values.
Wide-dynamic-range wavefront control is demonstrated with a programmable 2-D high-resolution liquid-crystal-based phase modulator system. This system operates with 307,200 independently addressable phase modulating elements, a phase modulation range that exceeds 2, and phase resolution that is better than one-tenth of an optical wavelength at 532-nm wavelength. A demonstration of large aberration compensation via modulo-2 phase profiles shows high-fidelity wavefront compensation with less than one-quarter-wave peak-to-peak residual aberration. The results of a numerical model of diffraction efficiency via discrete phase-step approximations to modulo-2 tilt functions are presented. The model treats the effects of a variable fill factor and an interpixel influence function associated with this technology. Optimization of the diffraction efficiency includes computational compensation for nonlinearities in the system phase response. Measured diffraction efficiencies are compared to calculated values, and show operation near the theoretical limit for this system.
Programmable diffractive optics utilizing a high-resolution liquid-crystal phase modulator is demonstrated as a technique for large-range, two-dimensional aberration control. A high-resolution phase modulator system introduces modulo-2π phase compensation via discrete-step phase modulation and operates with 307,200 independently addressable elements, and total optical efficiencies of up to 93%. Near-diffraction-limited imaging and beam directing is demonstrated in a telescope system with large off-axis aberrations. Wavelength-agile and extended spectral bandwidth operation are also demonstrated.
Programmable diffractive optics utilizing a high-resolution liquid-crystal phase modulator is demonstrated as a technique for large-range, two-dimensional aberration control. A high-resolution phase modulator system introduces modulo-2π phase compensation via discrete-step phase modulation and operates with 307,200 independently addressable elements, and total optical efficiencies of up to 93%. Near-diffraction-limited imaging and beam directing is demonstrated in a telescope system with large off-axis aberrations. Wavelength-agile and extended spectral bandwidth operation are also demonstrated.
Conventional beam steering for optical sensors is mechanically complex, consisting of a large number of optical and mechanical components. Steering the field of view requires large and heavy gimbal components. One approach to non-mechanical beam steering consists of a phase-modulated optical fiber array. Agile beam steering of optical radiation using fiber arrays offer significant advantages, such as weight, stability, speed and power requirements, over conventional beam steering systems based on large optics telescopes and gimbals. This paper represents an analysis of an agile beam steering system incorporating a fiber array and phase modulator to continually track a moving object. Expressions and modeling of the far-field beam pattern and off-axis beam steering efficiency are presented.
Surface properties and optical properties of several deformable mirror arrays (DMA) without actuators were characterized. The mirror arrays are micro-electronic- mechanical system (MEMS) devices which were fabricated by Boston University for wavefront correction in adaptive optics. The surface properties measured for the samples agree with the properties specified for the BU-MEMS-DMA structures. Scattering and diffraction by the mirror arrays were measured at a wavelength of 632.8nm. The DMA with the etching pattern generates a diffraction pattern full of special structures. The broadening is serious for a rough sample while it is negligible for a smooth continuous membrane DMA. The diffraction pattern demonstrates that the DMA with an RMS roughness of 300nm is not suitable for the adaptive optics to correct for wavefront error. The continuous membrane DMA with roughness less than 10 nm are useful for adaptive optics.
Optical fields propagation through the Earth's turbulent atmosphere are subject to time varying phase distortions. These phase distortions place severe limitations on electro- optical systems, such as imaging, tracking and vibrometry sensors. The generation of artificial beacons by laser backscatter (guide stars) for wavefront sensing may not be acceptable and natural beacons for wavefront sensing may not be available. Diversity techniques, however, allow passive wavefront measurement of the aberrated wavefront. This paper reports a phase diversity method based upon the transport- of-intensity equation for recovering phase from irradiance measurements. Digitized intensity data over a rectangular array of data is expanded into Zernike polynomials and a matrix method used to solve the transport-of-intensity equation. The phase diversity technique is evaluated in the pupil as well as the focal plane. Simulation and experimental data is provided.
The Microwave-to-Optical Transformation (MOT) method, reported previously, makes use of an optical thin film filter design took known as the Optical Admittance Diagram, the characteristic matrix and the quarter wave rule. Stripline elements are re-configured and modeled as sequential quarter wave optically thin film layers. The previous work was qualitative and mainly focused on the intuitively useful analysis that the MOT method offers the microwave engineer. A more rigorous formalism shall be presented in this paper which allows the computation of E- Field, both amplitude and phase. In addition, new graphical techniques that provide the designer further insight will accompany the E-field analysis. This is a novel extension of the previous work. Microwave and optical computing circuits and components that span a vast wavelength range may be adaptable to this technique depending on their specific utilization. This paper will first develop an extension of the MOT method for electric field strength characterization. Secondly, this technique will be applied to the Wilkinson Power Divider used in the previous work. Use of this already MOT-characterized stripline component will validate the utility of this technique by also analyzing the electric field through the same simple microwave circuit. Finally, this technique will be applied to the design of a co-planar waveguide transition component used for launching power to a Field Emitter Array.
We apply an iterative solution approach, the method of ordered multiple interactions (MOMI), to solve the magnetic and electric field integral equations which are pertinent to scattering by a thin dielectric film bounded by a semi- infinite dielectric substrate and by a semi-infinite dielectric superstrate. The thin film boundaries are randomly rough, which is numerically generated. We consider two- dimensional geometry (invariance in the y-direction) and investigate convergence for a TM polarized incident Gaussian tapered beam.
We present a summary of recent imagery and tracking measurements made with the ROBS laser radar system operating at White Sands Missile Range. The ROBS instrument utilizes a 0.5 meter aperture optical system, 3 - 5 micron imaging cameras, and a coherent CO2 laser radar for range and Doppler measurements. The optical system is based on a roving fovea design, which enables signal target tracking over large angles at high track update rate, and rapid retargeting between multiple targets. Results of several field measurements are presented.
This paper addresses the potential augmentation of a quasi-stationary Unmanned Aerial Vehicle with a highly agile beam steering optical system. In addition to the primary application of relaying laser power from a ground station to low earth orbit satellites, applications include (1) precision tracking and ranging at distances of a few hundred kilometers, (2) covert communications to distances of 80 km utilizing only a modulable corner cube at the receiving end and (3) pollution detection and control and (4) continuous meteorological analysis of high altitude wind, CO2 content, liquid water content, ice particle effective radius, effective drop size, optical depth and density, turbulence structure and emissivity profile.
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.
Currently there is considerable interest in agile beam-steering technologies for laser radar application that require the ability to point rapidly to a large number of widely spaced objects. Examples of such applications of agile pointing include imaging sensors for tracking and discrimination of military targets, sensors for surveillance and tracking of space objects in peacetime, and optical communication systems. In general, pointing agility allows a laser radar system to operate more efficiently by allowing each system platform to address a large number of separate targets. Pointing agility may be realized in a number of ways; for example, by employing steering mirrors, electro-acoustic and electro-optic devices, electro- optic phased arrays, and micro lens arrays. This paper will review agile beam-steering technologies. In particular, signal-processing techniques and experimental verification of imaging with coherent optical arrays will be addressed.
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