Troy A. Rhoadarmer

Optimized multiemitter beams for free-space optical communications through turbulent atmosphere

Using laser beams with less than perfect spatial coherence is an effective way of reducing scintillations in free-space optical communication links. We report a proof-of-principle experiment that quantifies this concept for a particular type of a partially coherent beam. In our scaled model of a free-space optical communication link, the beam is composed of several partially overlapping fundamental Gaussian beams that are mutually incoherent. The turbulent atmosphere is simulated by a random phase screen imprinted with Kolmogorov turbulence. Our experiments show that for both weak-to-intermediate and strong turbulence an optimum separation between the constituent beams exists such that the scintillation index of the optical signal at the detector is minimized. At the minimum, the scintillation reduction factor compared with the case of a single Gaussian beam is substantial, and it is found to grow with the number of constituent beams. For weak-to-intermediate turbulence, our experimental results are in reasonable agreement with calculations based on the Rytov approximation.

Simple laboratory system for generating well-controlled atmospheric-like turbulence

We describe a simple optical system for generating atmospheric-like turbulence in the laboratory which allows for well-controlled testing of advanced adaptive-optical components and concepts. The system models a two-layer atmosphere using static phase plates and is capable of simulating a wide range of atmospheric conditions. The design of the hardware is presented along with results from the initial system modelling describing the theory of operation.

Development of a self-referencing interferometer wavefront sensor

The self-referencing interferometer (SRI) is an innovative wavefront sensor (WFS) developed specifically for applications requiring laser propagation in strong scintillation. The performance of conventional gradient sensors, like Shack-Hartmann WFSs or lateral shearing interferometers, are severely limited in these environments due to the presence of branch points in the wavefront phase. In comparison, the SRI WFS directly measures the wavefront field so its performance is not affected by the presence of branch points. Over the last two years the Starfire Optical Range has been developing a prototype SRI WFS to demonstrate its advantages in strong scintillation environments. This paper discusses some practical lessons learned in building and operating an SRI WFS and presents initial results from laboratory tests.

Real-time adaptive optimization of wavefront reconstruction algorithms for closed-loop adaptive optical systems

In recent years several methods have been presented for optimizing closed-loop adaptive-optical (AO) wave-front re- construction algorithms. These algorithms, which can significantly improve the performance of AO systems, compute the reconstruction matrix using measured atmospheric statistics. Since atmospheric conditions vary on time scales of minutes, it becomes necessary to constantly update the reconstruction so that it adjusts to the changing atmospheric statistics. This paper presents a method for adaptively optimizing the reconstructor of a closed-loop AO system in real time. The method relies on recursive least square techniques to track the temporal and spatial correlations of the turbulent wave-front. The performance of this method is examined for a sample scenario in which the AO control algorithm attempts to compensate for signal processing latency by reconstructing the future value of the wave-front from a combination of past and current wave-front sensor measurements. For this case, the adaptive reconstruction algorithm yields Strehl ratios within a few percent of those obtained by an optimal reconstructor derived from a priori knowledge of the strength of the turbulence and the velocity of the wind. This level of performance can be a dramatic improvement over the Strehls achievable with a conventional least squares reconstructor.

Low-cost, broadband static phase plate for generating atmosphericlike turbulence.

Over the past decade adaptive optics (AO) has proved its worth as AO systems have been used successfully on several telescopes to improve image resolution. As scientists and engineers push the technological state of the art in an effort to make bigger, faster, and better systems, it has become more and more important to test and verify the operation of these systems in a controlled laboratory setting. To perform full-system tests in the presence of atmospheric turbulence, some sort of turbulence generator is needed. We describe a simple, low-cost approach to making static phase plates that generate atmosphericlike wave-front aberrations. These plates have several advantages over traditional heated-air turbulence generators and, as such, are better suited for well-controlled, detailed testing of an AO system.

Noise analysis for complex field estimation using a self-referencing interferometer wave front sensor

A noise analysis is presented for complex field estimation using a self-referencing interferometer wave front sensor with an amplified reference. The wave front sensor is constructed from a phase-shifting, point diffraction interferometer. The reference field is created by coupling a part of the incident wave front into a single mode fiber where it is optically amplified. The noise characteristics of this wave front sensor are examined in terms of the field estimation Strehl. The effects of several system parameters are examined\nobreak—shot noise, read noise, quantization noise, spontaneous emission from the amplifier, the relative intensities of the signal and reference fields, and temporal phase shifting.

Adaptive control and filtering for closed-loop adaptive-optical wavefront reconstruction

This paper discusses the application of adaptive control methods in the Atmospheric Simulation and Adaptiveoptics Laboratory Testbed at the Starfire Optical Range at the Air Force Research Laboratory, Kirtland AFB. Adaptive compensation is useful in adaptive optics applications where the wavefronts vary significantly from one frame to the next or where wind velocities and the strength of atmospheric turbulence change rapidly, rendering classical fixed-gain reconstruction algorithms far from optimal. The experimental results illustrate the capability of the adaptive control scheme to increase Strehl ratios and reduce jitter.

Performance of a woofer-tweeter deformable mirror control architecture for high-bandwidth high-spatial resolution adaptive optics

To compensate for large phase errors at high bandwidth, a dual deformable mirror (DM) architecture is introduced. One DM (the tweeter) handles the high spatial resolution errors with a small stroke, high bandwidth, capability while the other DM (the woofer) corrects the larger but more slowly varying phase errors with a larger stroke capability. An offload control architecture is shown to be very effective with the Kolmogorov turbulence spectrum. The architecture is also shown to be effective when used with a self-referencing interferomenter (SRI) wavefront sensor and an exponential control law for phase unwrapping. Performance is verified in simulation and in an advanced wavefront control testbed facility at the Air Force Research Laboratory.

Optimizing the performance of closed-loop adaptive-optics control systems on the basis of experimentally measured performance data

An experimental method is presented to optimize the control algorithm for a closed-loop adaptive-optics system employed with an astronomical telescope. The technique uses wave-front sensor measurements from an independent scoring sensor to calculate adjustments to the wave-front reconstruction algorithm and the bandwidth of the adaptive-optics control loop that will minimize the residual mean-square phase distortion as measured by this sensor. Specifying the range of possible adjustments defines the class of control algorithms over which system performance will be optimized. In particular, the technique can be used to compute an optimized wave-front reconstruction matrix for use with a prespecified adaptive-optics control-loop bandwidth, optimize the control-loop bandwidth for a given reconstruction matrix, optimize the individual modal control bandwidths for a fixed modal reconstructor, or simultaneously optimize both the wave-front modes and their associated control bandwidths for a fully optimized modal control algorithm. The method applies to closed-loop adaptive-optics systems that incorporate one or more natural or laser guide stars and one or more deformable mirrors that are optically conjugate to distinct ranges along the propagation path. Initial experimental results are reported for the case of a hybrid adaptive-optics system incorporating one natural guide star, one laser guide star, and one deformable mirror. These results represent what is to the authors’ knowledge the first stable closed-loop operation of an adaptive-optics system using multiple guide stars.

Construction and testing of the wavefront sensor camera for the new MMT adaptive optics system

This paper describes the construction and testing of the Shack-Hartmann wavefront sensor camera for the new MMT adaptive optics system. Construction and use of the sensor is greatly simplified by having the 12 X 12 lenslet array permanently glued to the detector array, obviating the need for any further realignment. The detector is a frame transfer CCD made by EEV with 80 by 80 pixels, each 24 microns square, and 4 output amplifiers operated simultaneously. 3 by 3 pixel binning is used to create in effect an array of quad-cells, each centered on a spot formed by a lenslet. Centration of the lenslet images is measured to have an accuracy of 1 micrometers rms. The maximum frame rate in the binned mode is 625 Hz, when the rms noise is 4.5-5 electrons. In use at the telescope, the guide star entering the wavefront sensor passes through a 2.4 arcsec squares field stop matched to the quall-cell size, and each lenslet samples a 54 cm square segment of the atmospherically aberrated wavefront to form a guide star image at a plate scale of 60 micrometers /arcsec. Charge diffusion between adjacent detector pixels is small: the signal modulation in 0.7 arcsec seeing is reduced by only 10 percent compared to an ideal quad-cell with perfectly sharp boundaries.