Scott W. Teare

Liquid crystal technology for adaptive optics: an update

The idea of using liquid crystal devices as an adaptive optics component has been proposed by several authors. In recent years a vigorous research effort has been carried out, and it is still flourishing, in several countries. Mainly the research and experimental work has been concentrated in the USA, U.K. and Russia. There are several reasons why liquid crystals may represent a valid alternative to the traditional deformable mirror technology that has been used for the past two decades or so. The main attractiveness of LC resides in the cost. Current deformable mirror technology has a range of price going from

Properties of Liquid Crystals

2K to

Spatial Light Modulators

15K per channel. LC technology promises to be at least a couple of orders of magnitude cheaper. Other reasons are connected with reliability, low power consumption and with a huge technological momentum based on a wide variety of industrial applications. In this paper we present some preliminary characterizations of a new, large format device. Such devices have the potential for extremely high-resolution wave-front control due to the over 10,000 corrective elements. The characterization of the device, so far, consists of measurements of the overall optical quality and of the phase control relationship

Polarization and Birefringence in Liquid Crystal Devices

Understanding and working with liquid crystals requires a multidisciplinary set of skills that brings topics normally associated with physics, chemistry, and condensed matter physics to bear on a single problem. In this book our interest is to emphasize another discipline—that of optics—which, based on what we have discussed so far, has been a driving application for liquid crystals, as they are a primary component of display technologies. The unique properties of liquid crystals revolve around the position and orientation of their molecules, topics closely associated with condensed matter physics. The tools of condensed matter physics, which deals with the physics of the phases of condensed matter and attempts to explain the observed behavior drawing from thermodynamics and statistical mechanics, have evolved over time, in particular, with the acceptance of quantum mechanics. The early studies that led to the acceptance of quantum mechanics often involved studies of crystal structures, and the use of x-ray diffraction led to the understanding of the order structure of crystals being related to the orientation of atoms in a lattice. The positional variations of properties, anisotropies, are modeled to incorporate the uniqueness of each direction using tensor mathematics. Tensor descriptions of systems are not new in the area of condensed matter physics, and, clearly, liquid crystals deal with the more familiar condensed phases of solids and liquids and, in particular, crystals and liquids. The importance of position and orientation in describing the molecules of liquid crystals makes the descriptions common in condensed matter physics useful in modeling the anisotropies of liquid crystals and lead rapidly to the need to delve into the mathematical topic of vectors and tensors for describing liquid crystals.

Adaptive Optics with Liquid Crystals

The success of liquid crystals in visual display devices has assured their commercial viability and made them common in our everyday use. However, their uses and applications go well beyond the displays used on handheld devices. Projection systems, similar to displays, also make use of liquid crystals, but in the form of spatial light modulators. A spatial light modulator (SLM) is a device that imposes variations in the amplitude, phase, or both, of a light beam. These devices can be used to change the amplitude, polarization, and phase over the spatial extent of a light beam and have become the mainstay of presentation technology commonly used in business and education. Most likely, if you have been part of a group attending a recent presentation, the video display shown on the screen passed through a SLM. Liquid crystals used as displays or to modulate light for presentations just begins to touch on the uses of these materials. Spatial light modulators are being used in a wider range of applications than ever before. These novel technologies have greatly benefited from the research and technology development for their use in displays, but they are also of great interest in their own right.

Image-Stabilization Systems Simplified

Liquid crystals have revolutionized visual display systems and are the most common displays in the entertainment, computer, and mobile technology industries. The key to their use is their ability to affect and manipulate polarized light. Liquid crystals have the property of birefringence; i.e., the refractive index of the material depends on the propagation direction and the polarization of the light. In liquid crystals birefringence can be used to transform linearly polarized light to elliptically polarized light. This can be demonstrated where light blocked by passage through two crossed polarizers can be made to pass through crossed polarizers when a liquid crystal material is placed between the polarizers. In this chapter we will discuss polarization and birefringence in the context of the optical properties of liquid crystals, with a general introduction of optics.

Low-Order Wavefront Compensation

In the 17th century, Galileo Galilei pointed the first telescope to the Heavens. In doing so, not only did he provide humanity with an enhancement of the eye, but he completely changed our position in the universe, psychologically and philosophically. Since then, astronomy has continued to change our view of the universe and our relationship to it. The single most important technological change that has allowed the dramatic increase in astronomical knowledge has been the ability to continue building larger telescopes. The ability of a telescope to resolve two close stars is directly proportional to the wavelength of the light l used for the observations and inversely proportional to the diameter of the telescope D, often expressed as the ratio l∕D. This is why, in order to distinguish between ever closer pairs of stars, or to observe finer details on an astronomical object, the options are to increase the diameter of the telescope or to decrease the wavelength of light. The additional advantage of larger telescopes is that they collect more light on shorter-exposure time scales, improving the view of dimmer objects.

Use and characterization of very large devices for adaptive optics

Image-stabilization systems can take on a wide range of designs ranging from very simple to extremely complex in order to accommodate the needs of a specific application. While these systems can be very diverse, all of them have a great deal in common. Image-stabilization systems are built around a sensor, a wavefront compensator, and a control system that connects these two components. The major differences between systems are often limited to the components selected and the sophistication of the controller. In the previous chapters, the sensor and compensator components were introduced. This chapter explores the integration of these components and introduces a simple image-stabilization system. This model system can be constructed in almost any laboratory and provides a simplified example of a working tip-tilt system. Image-stabilization systems are comprised of three main components: the wavefront sensor, the compensation device, and a control computer, which includes the electrical interfaces to read the sensor and control the compensator. Many different sensors as well as compensators are available in the commercial market so many unique image-stabilization systems can be constructed. The choice of components for a specific system is based on the intended application and its specific properties. For the most part, this is based on the operating bandwidth required to stabilize the image to the desired level.

Characterization of Large Format LC devices for adaptive and active optics

A wavefront sensor is used to evaluate the optical-path-length differences or phase variations in an aberrated wavefront compared to a reference wavefront. Wavefront compensation requires an optical element that can change the effective optical path length over the wavefront. In general, wavefront sensors measure the wavefront variation with little regard for how the information is used; however, the optical element used for compensation needs to be well matched to the aberrated wavefront shape, or at least to the aberration being corrected. Image-stabilization systems compensate for low-order aberrations, typically tip and tilt and sometimes piston and defocus. To restore an aberrated wavefront to its pristine form, all the optical-path-length variations in the wavefront must be compensated; both high- and low-order modes. Wavefront compensation devices are often designed specifically for either high- or low-order correction, so the choice of the optical device defines the application of the particular system. Mirrors are the most common optical devices that can compensate for the tilt in a wavefront, or even change its direction for beam-steering applications. Some static mirrors can also be used to distort images, by placing on them a static shape or figure, and so providing a single phase change over the image. Such a mirror can be used to compensate for static aberrations by inducing a fixed correction. The shape of wavefronts passing through a turbulent medium will continue to evolve and change over time and distance. Low-order, slow varying aberrations are the most easily compensated; many optical devices can be used for this purpose. In general, these devices are mirrors that can be changed in angle and follow changes in the optical path length over the wavefront. The most challenging problems in optical compensation are related to wavefronts that undergo rapid changes, such as those introduced by a very turbulent atmosphere. These wavefronts can have wide variation in the shape of the wavefront over time on scales of a few milliseconds or less. The wavefront sensor and wavefront compensator must be able to operate at high speeds and accurately adjust to the changing wavefront shapes.

System and method of generating atmospheric turbulence for testing adaptive optical systems

The cost of adaptive optics technology is dominated by the cost of current deformable mirror technology which has a range of price going from