Sergio R. Restaino

Light Propagation through Anisotropic Turbulence

A wealth of experimental data has shown that atmospheric turbulence can be anisotropic; in this case, a Kolmogorov spectrum does not describe well the atmospheric turbulence statistics. In this paper, we show a quantitative analysis of anisotropic turbulence by using a non-Kolmogorov power spectrum with an anisotropic coefficient. The spectrum we use does not include the inner and outer scales, it is valid only inside the inertial subrange, and it has a power-law slope that can be different from a Kolmogorov one. Using this power spectrum, in the weak turbulence condition, we analyze the impact of the power-law variations α on the long-term beam spread and scintillation index for several anisotropic coefficient values ς. We consider only horizontal propagation across the turbulence cells, assuming circular symmetry is maintained on the orthogonal plane to the propagation direction. We conclude that the anisotropic coefficient influences both the long-term beam spread and the scintillation index by the factor ς(2-α).

Foveated, wide field-of-view imaging system using a liquid crystal spatial light modulator.

The field-of-view (FOV) of a simple imaging system can be dramatically improved using a liquid crystal spatial light modulator (SLM). A SLM can be used to correct the off-axis aberrations that often limit the useful FOV of an imaging system giving near diffraction-limited performance at much larger field angles than would otherwise be possible. Foveated imaging refers to the variation in spatial resolution across the image caused by using the SLM in this application, and it is useful in reducing bandwidth requirements for data transmission.

On the use of dual frequency nematic material for adaptive optics systems: first results of a closed -loop experiment.

The use of liquid crystal devices for wavefront control has been suggested and implemented by several authors. In this paper we report some preliminary results on the use of Nematic based liquid crystal devices. Several experimental efforts have been carried out in the past few months. One of the main aims was to characterize a new device that uses dual frequency nematic material in a closed loop arrangement.

Foveated imaging demonstration

A wide field-of-view (FOV), theoretically diffraction-limited imaging system is demonstrated using a single positive lens (a singlet), a reflective liquid crystal spatial light modulator (SLM), a turning mirror and a CCD camera. The SLM is used to correct the off-axis aberrations that would otherwise limit the useful FOV of our system. Foveated imaging refers to the variation in spatial resolution across the image caused by using the SLM in this manner.

Demonstration of new technology MEMS and liquid crystal adaptive optics on bright astronomical objects and satellites

We present here results using two novel adaptive optic elements, an electro-static membrane mirror, and a dual frequency nematic liquid crystal. These devices have the advantage of low cost, low power consumption, and compact size. Possible applications of the devices are astronomical adaptive optics, laser beam control, laser cavity mode control, and real time holography. Field experiments were performed on the Air Force Research Laboratory, Directed Energy Directorates 3.67 meter AMOS telescope on Maui, Hawaii.

Characterization and control of a multielement dual-frequency liquid-crystal device for high-speed adaptive optical wave-front correction

Multielement nematic liquid-crystal devices have been used by others and ourselves for closed-loop adaptive control of optical wave-front distortions. Until recently the phase retardance of available devices could be controlled rapidly in only one direction. The phase retardance of the dual-frequency device can be controlled rapidly in both directions. Understanding the dynamics of the phase retardance change is critical to the development of a high-speed control algorithm. We describe measurements and experiments leading to the closed-loop control of a multielement dual-frequency liquid-crystal adaptive optic.

Liquid crystal based active optics

Liquid crystal spatial light modulators, lenses, and bandpass filters are becoming increasingly capable as material and electronics development continues to improve device performance and reduce fabrication costs. These devices are being utilized in a number of imaging applications in order to improve the performance and flexibility of the system while simultaneously reducing the size and weight compared to a conventional lens. We will present recent progress at Sandia National Laboratories in developing foveated imaging, active optical (aka nonmechanical) zoom, and enhanced multi-spectral imaging systems using liquid crystal devices.

Gaussian beam propagation in maritime atmospheric turbulence: long term beam spread and beam wander analysis

Laser beam propagation in maritime environment is particularly challenging, not only for scattering and absorption due to high humidity, but also for a different behavior of atmospheric turbulence with respect to terrestrial propagation. Recently, a new power spectrum for the fluctuations of the refractive index in the Earths atmosphere has been introduced to describe maritime atmospheric turbulence. This maritime power spectral model shows a characteristic bump, similar to Hills bump, that appears when the product between the wavenumber and the inner scale is around unity, κ • l0 ~1. In this paper, under weak turbulence conditions, we use the mentioned maritime power spectrum to analyze long term beam spread, beam wander and Strehl ratio of a Gaussian beam wave propagating through maritime atmospheric turbulence.

Non-mechanical zoom system

In order to optically vary the magnification of an imaging system, mechanical zoom lenses, such as those found on 35mm cameras, require multiple optical elements and use cams or gears to adjust the spacing between individual or groups of lenses. By incorporating active elements in the optical design, we can eliminate the need to change lens separations and create an imaging system with variable optical magnification that has no macroscopic moving parts.

Laboratory and field demonstration of a low cost membrane mirror adaptive optics system

Abstract We present here results of laboratory and field experiments using the OKO technologies membrane mirror as an adaptive optics device. The device can be operated at high temporal bandwidths from a low cost PC based control system. We have constructed a complete adaptive optics system costing less than twenty thousand dollars. In this paper we present results from this system used both in laboratory tests and on a one meter telescope located at Apache Point, New Mexico.