Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2003-05-05
2004-11-16
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S292000, C359S572000
Reexamination Certificate
active
06819469
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to display devices and their components. More particularly, the present invention relates to spatial light modulators for use in holographic display systems, optical information processing systems, direct-write lithography systems and adaptive optics.
2. Description of the Related Art
A spatial light modulator (SLM) is an optical device that allows modulation of incoming incident light with desired amplitude or phase pattern. Spatial light modulators are commonly used in projection television and display systems. Two common types of spatial light modulators used presently are digital micro-mirror device (DMD) and liquid crystal devices (LCD). Various types of DMDs are disclosed for example in U.S. Pat. No. 4,956,619, and U.S. Pat. No. 5,083,857. A digital micro-mirror device generally comprises a matrix of micro-mirrors suspended above a substrate of the device. A voltage is applied between the micro-mirror and electrode which allows individual adjustment of the light reflection angles of the micro-mirror, and thus control of the light reflection of the array is achieved.
Liquid crystal devices (LCD) typically comprise electrodes on the opposite sides of a layer of liquid crystal that changes polarization of incident light in response to an applied voltage. When no electric field is applied, light does not changes polarization and passes through the polarization filter; when an electric field is applied, the polarization changes and the polarization filter absorbs light.
These two types of spatial light modulators, DMD and LCD, are suitable and widely utilized for projection television and computer display systems. However, these spatial light modulators have technical and physical limitations that limit their usage in holographic systems and other modern technical applications that require high resolution, i.e. small size of pixels. The DMDs and LCDs typically have a geometric size of pixel elements on the order of dozens of microns. For example, a typical micro-mirror element has size of 10-20 microns (&mgr;m). Another disadvantage of existing spatial light modulators is that they have a long time of modification of the optical elements state, i.e. changing the image pattern, on the order of milliseconds.
There are other types of spatial light modulators used for specific applications. One such type is known as multiple quantum-well device that is comprised of multiple layers of semiconductor material sandwiched between transparent electrodes. The voltage applied to electrodes alters the absorption of light in the wavelength near the band gap of the material, which is typically GaAs or AlGaAs. An example of a multiple quantum well spatial light modulator, specifically for optical pattern recognition, is disclosed in U.S. Pat. No. 5,844,709. That device is designed to provide a combination of optically and electrically addressed spatial light modulators for the specific task of high performance pattern recognition. However, multiple quantum well devices are not well suited for other tasks of spatial light modulators because those devices can operate only in a limited spectrum of light near the band gap of the material, normally in the infrared spectrum. Further, the quantum-well modulator is difficult, and therefore costly, to fabricate with a large size of image area. Another type of spatial light modulator uses a magneto-optic material that changes polarization of incident light depending on the magnetization of pixel elements.
A further known type of light modulators is based on electro-optic materials that change their refractive index in response to an electric field. A first-order (linear) electro-optic effect, known as the Pockels effect, and a second-order (quadratic) electro-optic effect, known as the Kerr effect, occur in response to the electric field. Electro-optical devices typically utilize optical materials such as LiNiO
3
or lead zirconate titanate ceramic (PLZT). Such electro-optic light modulators are typically used for time-modulation of a light beam. But such devices are usually not designed and not suitable for spatial modulation of light specified by a 2-dimensional high-resolution pattern. Further, the size of a typical electro-optical modulator is quite big, so it is insufficient to serve as an element of a high-resolution spatial light modulator.
A classical electro-optical modulator is comprised of a wafer or volume of electro-optic material with electrodes attached to opposite sides of the material. Such device is capable of time-modulation of light beam. There have been attempts to make a compound optical system by combining multiple electro-optical modulators on one wafer. For example, U.S. Pat. No. 4,746,942, suggests a wafer of electro-optic material with a number of surface-mounted electrodes to form multiple independently operated electro-optic modulators. However, this type of device suffers from interference of the electric fields between electrodes, which limits how many modulators can be on one die. One solution to this problem is disclosed in U.S. Pat. No. 6,486,996, which suggests an optical system comprising a plurality of discrete protrusions of electro-optic material. Each discrete protrusion is electrically and optically isolated from each other. Such device permits multiple electro-optical modulators placed closely on one die. Nevertheless, that device is designed for time-modulation of a fixed number of light beams, such as used in printers and telecommunication. That device is not designed and not capable to perform at high-resolutions because the size of the optical elements (protrusions in electro-optic material) is far too large for displaying a high-resolution image pattern.
Holography is the application of optical technology best known for reproducing three-dimensional images. In addition to the simple production of 3-dimensional images, modern holography is used in broad range of applications, which includes, for example, holographic interferometers, optical memory systems, optical information processing systems, adaptive optics and may other technical areas. Typical holographic devices utilize film for recording the interference pattern of two beams of light, one reflected from the object and one reference beam. The two beams are created from a laser passed through beam expander optics and through a beam splitter that separates incident laser light into two beams. One fraction of the laser light is reflected from the beam splitter and directed toward the object, and a portion of light is reflected from the object and incident to a recording media (which is a film or glass plate covered with photographic emulsion). A second portion of laser light passes through the beam splitter and is reflected by a mirror toward the recording media such that the two laser light beams generate an interference pattern that is recorded on recording media. The photographic emulsion on the recording media is then developed by known photographic development process. The recorded hologram picture can be reconstructed by illuminating the developed film with the light beam at the appropriate incident angle, typically equal to the incident angle of the original reference light beam. The resulting reflected or transmitted light creates virtual or real image of the original object, depending on the setup of light beams during the recording. Some types of holograms may be observed (reconstructed) using conventional (non-laser) white light source. Other types of hologram may be observed only using laser light source.
However, the above process of creating hologram is only suitable for making a hologram of a static object, requiring an elaborate optical system well protected from vibration and external light. It also requires time for developing photographic emulsion on the recording media. It is therefore not suitable for recording images of moving objects.
Computers have been used to calculate interference patterns for holograms. However, constructing an operationa
Arnall Golden & Gregory LLP
Epps Georgia
Hasan M.
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