Adaptive Optics

We present simulation results for 192 Gb/s (16 ×12) OTDM transmission with different time delays over standard fiber G.652. The optical 192 Gb/s to 12 Gb/s demultiplexing (OTDM) is used and the results compared with 160 Gb/s to 10 Gb/s OTDM demultiplexing. The observations show the feasibility of an error free 192 Gb/s OTDM transmission with a minimum time delay of 1 ps which yields BER as low as 3.38E-89 while the minimum BER obtained at 1 ps time delay for 160 Gb/s OTDM is 1.21E-244, which is practically error free.

Continuous-facesheet and segmented Boston Micromachines Corporations' (BMC) Micro-Electrical Mechanical Systems (MEMS) Deformable Mirrors (DM) have been tested for their response to high-power visible-wavelength laser light. The deformable mirrors, coated with either protected silver or bare aluminum, were subjected to a maximum of 2 Watt laser-light at a wavelength of 532 nanometers. The laser light was incident on a ~ 3.5×3.5 cm area for time periods from minutes to 7 continuous hours. Spot heating from the laser-light is measured to induce a local bulge in the surface of each DM. For the aluminum-coated continuous facesheet DM, the induced spot heating changes the surface figure by 16 nm rms. The silver-coated continuous-facesheet and segmented (spatial light modulator) DMs experience a 6 and 8 nm surface rms change in surface quality with the laser at 2 Watts. For spatial frequencies less than the actuator spacing (300 mm), the laser induced surface bulge is shown to be rem...

In free-space optical communication, the propagation of a laser beam through the atmosphere causes wavefront distortions that decrease the coupling efficiency (CE) from free space to single-mode fiber. This tremendously degrades the performance of the communication channel even in the case of weak turbulence regime. In this Letter, we demonstrate that a multi-actuator adaptive lens working in closed loop with a wavefront sensor can strongly reduce the effect of turbulence while reducing the system complexity with respect to correction systems using deformable mirrors or liquid crystal spatial light modulators. We obtain a three-fold increase in the CE in weak turbulence regime.

During the last two decades, optical stellar interferometry has become an important tool in astronomical investigations requiring spatial resolution well beyond that of traditional telescopes. This book, first published in 2006, was the first to be written on the subject. The authors provide an extended introduction discussing basic physical and atmospheric optics, which establishes the framework necessary to present the ideas and practice of interferometry as applied to the astronomical scene. They follow with an overview of historical, operational and planned interferometric observatories, and a selection of important astrophysical discoveries made with them. Finally, they present some as-yet untested ideas for instruments both on the ground and in space which may allow us to image details of planetary systems beyond our own.

Free-space laser communication has been demonstrated with application potential in many areas such as line-of-sight communications, satellite communications and the last mile solution in a fiber optics networking. Both 0.8 and 1.5 micron wavelengths are currently used in state-of-the-art free space laser communication systems; unfortunately the system performance is imposed by atmospheric turbulence. To reduce the atmospheric effect in free-space laser communication systems, several techniques have been used, such as adaptive optics, aperture averaging and multiple transmitters; however, significant improvement has not been achieved. Theoretically, the seeing effect may be released using a longer wavelength. In this paper, we present a 3.5 micron free-space laser communication system model and its system performance evaluation. A 3.5 micron propagation model based on MODTRAN simulation results in different weather patterns is presented first, and a propagation link budget system model is described after that. The propagation channel performance evaluation results are presented by means of bit error rate versus various propagation distances.

We introduce adaptive optics as a technique to improve images taken by ground-based telescopes through a turbulent blurring atmosphere. Adaptive optics rapidly senses the wavefront distortion referenced to either a natural or laser guidestar, and then applies an equal but opposite profile to an adaptive mirror. In this paper, we summarize the application of neural networks in adaptive optics. First, we report previous work on employing multi-layer perceptron neural networks and back-propagation to learn how to sense and reconstruct the wavefront. Second, we show how neural networks can be used to predict the wavefront, and compare the neural networks’ predictive power in the presence of noise to that of linear networks also trained with back-propagation. In our simulations, we find that the linear network predictors train faster, they have lower residual phase variance, and they are much more tolerant to noise than the non-linear neural network predictors, though both offer improvement over no prediction. We conclude with comments on how neural networks may evolve over the next few years as adaptive optics becomes a more routine tool on the new large astronomical telescopes.

Orbital debris in low Earth orbit (LEO) are now sufficiently dense that the use of LEO space is threatened by runaway collisional cascading. A problem predicted more than thirty years ago, the threat from debris larger than about 1 cm demands serious attention. A promising proposed solution uses a high power pulsed laser system on the Earth to make plasma jets on the objects, slowing them slightly, and causing them to re-enter and burn up in the atmosphere. In this paper, we reassess this approach in light of recent advances in low-cost, light-weight modular design for large mirrors, calculations of laser-induced orbit changes and in design of repetitive, multi-kilojoule lasers, that build on inertial fusion research. These advances now suggest that laser orbital debris removal (LODR) is the most cost-effective way to mitigate the debris problem. No other solutions have been proposed that address the whole problem of large and small debris. A LODR system will have multiple uses beyond debris removal. International cooperation will be essential for building and operating such a system.

Adaptive optics (AO) systems are used to increase the spatial resolution achieved by ground-based telescopes, which are limited by the atmospheric motion of air layers above them. Therefore, the real cut-off frequency is extended closer to the theoretical diffraction limit of the telescope thus allowing more high-frequency information from the object to be present in the image. Nevertheless, although the goal of image reconstruction and deconvolution algorithms is basically the same (i.e., to recover a “real” diffracted limit image, free of noise, from the object), and since the correction of AO is not complete (i.e., the effective cut-off frequency achieved by AO is still below the theoretical diffraction limit), the simultaneous use of such deconvolution algorithms over dataset acquired with AO is possible and desirable to further enhance their contrast. On the other hand, multiresolution tools like the wavelet transform (WT) have been historically introduced into multiple deconvo...

Conventional adaptive optics systems using a single wavefront corrector suffer from a limited field of view. Multi-conjugate adaptive optics use two or more corrector to improve off-axis correction. We describe an experimental system which simulates dual-layer turbulence, and present results using a single corrector showing anisoplanatic effects. Future experiments using a second corrector are also discussed.

CSP Ganged Heliostat Technologies
Investigations in a Tensile Based Non-imaging System
Abstract:
Background
Concentrating Solar Power (CSP) and specifically Power Tower or Beam Down type systems achieve high levels of solar concentration and efficiency. Collecting fields comprise a large fraction of the system installation and maintenance costs. Technological advances promise economically competitive solar power. Skysun, LLC proposes a ganged heliostat to significantly reduce these costs.
Method
Typically, a heliostat requires one each of the following: mirror module, support structure, dual axis drive, post/pedestal and foundation. Each of Skysun, LLC’s heliostats require: mirror module, reduced support structure and a single axis drive, eliminating the need for a pedestal and foundation for each heliostat. The ganged heliostat consists of two cables supporting a plurality of single-axis actuated heliostats. The cables act both as a supporting structure and as a translator of focusing motions to the many heliostats. The cables terminate to an actuated rotational member supported by a substantial post. Cable tension may be variable. This configuration reduces the ratio of posts and foundations to heliostats, and eliminates dual axis drive actuators, substituting single axis actuators instead. The ganged heliostat may be rotated to the vertical for ease of robotic cleaning and water reclamation. The ganged heliostat may also be inverted, with the reflective surface downward, to protect against weather events such as hail. Finally, the ganged heliostat may be secured to protect against high wind conditions.
The reflective surface, which can be deformed by cable and heliostat orientation, provides an efficient means to form a large concave collecting surface laying principally in the horizontal. Reflected incident rays, being non-normal, suffer from astigmatism. Novel deformations of the reflective surface eliminate astigmatic aberration. A toric - shaped deformation of the reflective surface reduces the size of a chosen astigmatic focus, yielding higher concentration. Latitudinal and longitudinal deformations maintain focus upon a fixed receiver. In the ideal, the astigmatic focus is reduced to a point.
Conclusion
The goal of this paper is to outline the relatively inexpensive methods utilized by Skysun’s ganged heliostat prototype and how the methodology may be scaled up. Skysun, LLC proposes a ganged heliostat to significantly reduce collecting field costs to $75/m2 installed.
Key words: solar concentrating CSP Power Tower Beam Down heliostat astigmatism deformation ganged