We discuss a concept based on replacing a large-aperture monolithic adaptive optical transceiver telescope with an array of small phase-locked sub-apertures with adaptive correction of phase distortions incorporated directly into each sub-aperture, referred to here as adaptive photonics phase-locked elements (APPLE). In the APPLE system, the conventional high-resolution adaptive optics (AO) system is substituted by an array of low-resolution "local" AO sub-systems (distributed AO) operating in parallel.

BAE Systems reports on a concept utilizing a holographic approach to linear phase conjugation to compensate for atmospheric induced aberrations that severely limit laser performance. In an effort to improve beam quality, fine aimpoint control, and energy delivered to the target, BAE Systems has developed a novel aberration compensation technique. This technique, Holographic Adaptive Tracking (HAT), utilizes a Spatial Light Modulator as a dynamic wavefront reversing element to undo aberrations induced by the atmosphere, platform motion, or both. BAE Systems aberration compensation technique results in a high fidelity, near-diffraction limited laser beam delivered to the target.

Active systems, because they provide their own illumination, are capable of operating 24 hours a day and are not dependent upon the angle of the sun. Unlike passive systems, they can provide three-dimensional imaging. DARPA is currently developing systems, technologies, and signal processing to pioneer new or improve existing capabilities that employ active imaging capabilities. These involve both radar and ladar, ranging from a few MHz for foliage penetration to near-visible IR to achieve ultra-high resolution at long range. These capabilities would improve Battlefield Awareness (BA) and provide persistent Intelligence, Surveillance, and Reconnaissance (ISR) to perform target detection, recognition, and identification. This paper discusses two different approaches to active optical imaging. One is a coherent approach that uses synthetic aperture techniques with infrared laser radar, and another approach uses only the intensity of the speckle pattern in the aperture plane. Both are capable of producing ultra-high resolution at long range.

There is strong interest in developing adaptive optics solutions for extreme conditions, such as laser beam projection over long, horizontal paths. In most realistic operational scenarios there is no suitable beacon readily available for tracking and wave front sensing. In these situations it is necessary to create a bacon artificially. In this paper we explore two strategies for creating a beacon: (1) scattering an initially focused beam from a surface accomplished compensation for part of the path. In many cases of practical interest, beacons created by scattering of the light from a surface in the scene results in beacons which are anisoplanatic, and hence provide poor beam compensation results. Partial path compensation based on a Rayleigh beacon provides comparable performance in some cases.

We discuss the reliance on backscatter by a laser guide star to generate a propagative probe and we show that spatial reciprocity can be accomplished by compensating phase alone. An analytic plane to plane propagation framework is introduced in which the spatial reciprocity of Maxwell's equations Is utilized to demonstrate that the adaptive optics compensation can do no better than the beacon initial conditions (I.e. cannot correct for the beacon too.) It is shown analytically that use of point to point reciprocity reasoning fails. While the laser guide star itself may be compensated to optimize uplink spatial coherence at altitude, the backscattering process is completely incoherent and the backscattering volume constitutes a very bad mirror or diffuse source. While diffraction restores some coherency as described by the van Cittert-Zernike theorem[2,5], the consequences of the incoherency of the beacon, lead to problems for the adaptive optics system which do not affect natural guide stars. The main consequence for the laser guide star system is that the wave sensor of the adaptive optics cannot distinguish between the phase aberrations from the backscattering process and those phase aberrations induced by turbulence. The question of the beacon and propagative path being different is weighed within the context of correlated versus uncorrelated ensemble members of turbulence.

The symmetry operation associated with propagation reciprocity is complex conjugation and adaptive optics is used to physically carry out this symmetry operation. We use a plane-to-plane framework to describe the fundamental limits placed on implementing propagation reciprocity that arise due to diffraction. Compensation system performance is often analyzed using the ray optics limit (e.g. defining the isoplanatic angle). This limits the applicability of such results by ignoring the diffractive limits on the ability to sense the laser guide star phase and amplitude information. We describe how the diffractive limits of phase-only and full-field compensation arise in terms of this flow of information. The plane-to-plane framework also shows the role of the beacon initial conditions as the end result of complete spatial reciprocity.

The limits to the ability of adaptive optics to achieve spatial propagation reciprocity are determined by diffraction. The beacon is a prominent component in defining the diffractive limit, so diffraction plays a role in the optimal choice of beacon parameters. We show with an explicit example that a point-source beacon is not the optimal choice, and that a point-source beacon cannot be used to measure the diffractive limit of phase-only compensation. At the single scattering level, diffraction dictates the use of an extended coherent beacon. We also show with an explicit example that optical vortices are not branch points, thus a well-defined phase reconstruction from an initially coherent beacon propagated through strong or extensive turbulence will not be hindered by the presence of optical vortices.

We investigate a fundamental limit of atmospheric compensation for adaptive optical systems that use uncooperative beacons or laser guide stars. Laser guide stars are generated by backscattering processes that are naturally incoherent. The limit arises because the wavefront sensing component of the adaptive optics cannot differentiate between the incoherency introduced by atmospheric turbulence from that of the laser guide star generation process. The limit is significant and restrictive. Under absolutely ideal conditions with perfect compensation, the compensated Strehl ratio must be less than 0.3. Realistic conditions will reduce this by more than a factor of 2.

For some beam-control applications, we can rely on the cooperation of the target when gathering information about the target location and the state of the atmosphere between the target and the beam-control system. The typical example is a cooperative point-source beacon on the target. Light from such a beacon allows the beam-control system to track the target accurately, and, if higher-order adaptive optics is to be employed, to make wave-front measurements and apply appropriate corrections with a deformable mirror. In many applications, including directed-energy weapons, the target is not cooperative. In the absence of a cooperative beacon, we must find other ways to collect the relevant information. This can be accomplished with an active-illumination system. Typically, this means shining one or more lasers at the target and observing the reflected light. In this paper, we qualitatively explore a number of difficulties inherent to active illumination, and suggest some possible mitigation techniques.

The tracking algorithm is presented that reduces the influence of the camera motion on the tracking performance. The algorithm uses a change detector. The target motion is described by parameterized optical flow. The flow parameters are estimated using Kalman filtering. The algorithm allows us to estimate the target motion without any bias associated with the camera motion. The effects of thermal blooming on high-energy laser beacon for air-to-ground directed energy system are evaluated. The laser fluence at the target and power in the bucket are evaluated for various tactical engagement scenarios and different atmospheric conditions. The critical laser power that can be efficiently transmitted through the atmosphere is evaluated. Two techniques for mitigating the effects of thermal blooming including a method based on pointing of a high energy beam "downwind" to correct for the thermal blooming tilt and focusing a high energy beam beyond the target range are evaluated. We found that the power in the bucket at the target at the optical axis of a high energy beam for tactical directed energy system increases about one order of magnitude due to correction of the thermal blooming tilt.

Photon budget analysis for laser target tracking systems under atmospheric turbulence conditions is performed in the paper. This analysis includes evaluations of the effects of molecular and aerosol absorption/scattering at various propagation distances and tracking angles for the following laser tracking wavelengths: 0.53 micrometers, 1.06 micrometers and 1.55 micrometers and evaluations of tracking beam/target interaction (target light scattering, target-induced coherence degradation, influence of target shape) for targets with rough surfaces, retro-reflective tape, and a single retro-reflector.

In this paper we demonstrate that the amplitude and phase of the principle eigenfunction for the pupil-plane mutual intensity can be used to specify a beam that concentrates its intensity at the brightest point on the target in a target-in-the-loop system with a spatially incoherent reflected field. In addition, we discuss two methods for beam control: a method for which the beam amplitude and phase are determined as the principle eigenfunction for the measured (but incomplete) pupil-plane mutual intensity; and a method for which the beam amplitude and phase are determined to maximize a window-based image-plane sharpness measure. We demonstrate that the two methods are similar, and that both result in beams that correspond to the principle eigenfunction for an apodized mutual intensity function.

Real time wavefront control for adaptive laser communication and imaging system requires fast measurement of image quality. Statistical analysis of speckle field provides effective image quality criteria for adaptive correction of phase-distorted images. We propose an analog continuous time VLSI (very-large-scale-integration) spectrum analysis chip to provide such a real time image quality measurement. The chip takes the signal sensed by a photo detector which is located in the speckle field as analog input and computes its spectrum distribution continuously. Experiment and analysis on distorted laser beam was conducted with the analog spectrum analysis chip. Target-in-the-loop system is under development to demonstrate the capability of real time adaptive imaging

Laser pointing systems suffer from random errors and biases causing off-axis pointing and loss of signal at the target. Often sensors are not available at the target location to provide pointing related information. The present paper outlines a novel technique to adaptively control the beam pointing real-time based on boresight estimates obtained using the statistics of the reflected signal from the target, thus eliminating the need of beam position sensors at the target. A field realization of this technique is discussed. Also a laboratory arrangement that works successfully on this approach is demonstrated.

We report experimental results that demonstrate compensation of extended turbulence and thermal-blooming of high-energy lasers using target-in-the-loop techniques in a scaled laboratory environment. For these experiments the deformable mirror figure was controlled by an algorithm designed to maximize the target-plane intensity as measured by a camera at the transmitter. Results using this TIL configuration were compared under identical conditions to results obtained under control of a Hartmann wavefront sensor and least-squares reconstructor. Experiments were performed for a variety of propagation scenarios anticipated for tactical HEL applications and in all cases the TIL system was seen to outperform the conventional Hartmann-driven adaptive-optics system. We will discuss the details of the the target-in-the-loop algorithm, the laboratory configuration, and the experimental results.

We investigate performance of a tiled array of adaptive photonics phase-locked elements (APPLE) or fiber collimators for (real-time) compensation of atmospheric phase aberrations. The compensation technique is based on a decoupled stochastic parallel gradient descent (D-SPGD) optimization of performance metrics. We further compare the APPLE system and the monolithic aperture adaptive optics system based on the SPGD optimization algorithm and demonstrate that the tiled fiber array is more efficient in phase aberration compensation over the large range of atmospheric turbulence strengths and under both static and dynamic turbulence conditions.

We present the laboratory experiments of phase locking of a multi-channel tiled fiber array using a stochastic parallel gradient descent (SPGD) feedback controller demonstrating the compensation effect of the simulating phase-induced distortions based on the model-free optimization of the received signal strength. An all-polarization-maintaining (PM)-fiber optical configuration is used to simplify the free-space transceiver system. The atmospheric aberrations are simulated by a multi-channel integrated optical phase modulator which obtains input control voltages from an array of multi-channel independent sinusoidal signal generators. A similar multi-channel phase modulator which obtains input control voltages from a computer-based SPGD controller is used to compensate the simulating phase distortions. The experimental results show that the constructive interference state is reached through phase locking of the multi-channel tiled fiber array for phase distortions up to 180 hertz for each channel. The update rate of the computer-based SPGD controller is ~16,000 iterations per second. The average compensation bandwidth is about 310 Hz

Over the past decade, a number of electrostatically-actuated MEMS deformable mirror devices have been used for adaptive control in beam-forming and imaging applications. One architecture that has been widely used is the silicon device developed by Boston University, consisting of a continuous or segmented mirror supported by post attachments to an array of parallel plate electrostatic actuators. MEMS deformable mirrors and segmented mirrors with up to 1024 of these actuators have been used in open loop and closed loop control systems to control wavefront errors. Frame rates as high as 11kHz have been demonstrated. Mechanically, the actuators used in this device exhibit a first-mode resonant frequency that is in the range of many tens of kilohertz up to a few hundred kilohertz. Viscous air damping has been found to limit operation at such high frequencies in air at standard pressure. Some applications in high-speed tracking and beam-forming could benefit from increased speed. In this paper, several approaches to achieving critically-damped performance with such MEMS DMs are detailed, and theoretical and experimental results are presented. One approach is to seal the MEMS DM in a full or partial vacuum environment, thereby affecting air damping. After vacuum sealing the device's predicted resonant behavior at tens of kilohertz was observed. In vacuum, the actuator's intrinsic material damping is quite small, resulting in considerable oscillation in step response. To alleviate this problem, a two-step actuation algorithm was employed. Precise control of a single actuator frequencies up to 100kHz without overshoot was demonstrated using this approach. Another approach to increasing actuation speed was to design actuators that reduce air damping effects. This is also demonstrated in the paper.

The proper collimating of the laser beam between the fiber optics transmitter and receiver is among the most important requirements to free space telecommunications. The targeting of the receiver over near-horizontal propagation in kilometers long distance requires fast control of the tip/tilt deviation of the laser beam emitting from the fiber optic transmitter because of beam direction fluctuations due to atmospheric turbulences. This deviation can be accomplished with fast deviation of the fiber optics emitting tip placed into focus of the collimating lens. Similar control could allow one to keep the input fiber optics tip in focus of the receiver's input lens. In this report we describe the design of the compact adaptive fiber optics collimator based on the X-Y positioner of the fiber optics tip utilizing the piezoelectric bimorph actuators for X-Y displacement of the fiber tip placed in the focal planes of the transceiver.

Electrochromic (EC) materials are used in "smart" windows that can be darkened by applying a voltage across an EC stack on the window. The associated change in refractive index (n) in the EC materials might allow their use in tunable or temperature-insensitive Fabry-Perot filters and transmissive-spatial-light-modulators (SLMs). The authors are conducting a preliminary evaluation of these materials in many applications, including target-in-the-loop systems. Data on tungsten oxide, WO₃, the workhorse EC material, indicate that it's possible to achieve modest changes in n with only slight increases in absorption between the visible and ~10 μm. This might enable construction of a tunable Fabry-Perot filter consisting of an active EC layer (e.g. WO₃) and a proton conductor (e.g.Ta₂O₅) sandwiched between two gold electrodes. A SLM might be produced by replacing the gold with a transparent conductor (e.g. ITO). This SLM would allow broad-band operation like a micromirror array. Since it's a transmission element, simple optical designs like those in liquid-crystal systems would be possible. Our team has fabricated EC stacks and characterized their switching speed and optical properties (n, k). We plan to study the interplay between process parameters, film properties, and performance characteristics associated with the FP-filter and then extend what we learn to SLMs. Our goals are to understand whether the changes in absorption associated with changes in n are acceptable, and whether it's possible to design an EC-stack that's fast enough to be interesting. We'll present our preliminary findings regarding the potential viability of EC materials for target-in-the-loop applications.

This paper illustrates continuing theoretical and simulation work examining the use of different methods to control a deformable mirror in an adaptive optics system without a wavefront sensor. Two independent techniques, stochastic optimization, or SPGD, and a newly developed functional approximation method, are discussed. Specific results from simulation work performed at SAIC are presented.