Micro-mechanical polarization-based modulator

Details Micro-mechanical polarization-based modulator Micro-mechanical polarization-based modulator

2003-01-27

2004-03-16

Schwartz, Jordan M. (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

C359S298000, C359S259000

Reexamination Certificate

active

06707595

ABSTRACT:

FIELD OF THE INVENTION
This invention generally relates to spatial light modulators and in particular relates to a polarization-based modulator comprising an array of micro-mechanical assemblies.
BACKGROUND OF THE INVENTION
Spatial light modulators have been adapted for use in a range of imaging applications, particularly in projection and printing apparatus. In operation, a spatial light modulator array provides a pattern of individual light modulators, each modulator typically corresponding to a pixel for representing a two-dimensional image. Light can be modulated by modifying the incident light according to selective absorption, reflection, polarization state change, beam steering, diffraction, wavelength separation, and coherence modification. Typically, the interaction of the light and modulator is enabled using electro-optic or acousto-optic materials, or a micro-mechanical structure, patterned with a series of addressing electrodes.
In particular, the liquid crystal display (LCD) is a widely used type of spatial light modulator, which operates by the modulation of the polarization state of incident light. LCDs are commonly used in laptop computer displays, pagers, and game displays, as well as in projection and printing systems. LCD spatial light modulators are available in a range of types and may use any of a number of underlying technologies, including devices using nematic, twisted nematic, cholesteric, smectic, and vertically aligned liquid crystal molecules. LCDs are described in numerous patents, including for example, U.S. Pat. No. 4,688,897 (Grinberg et al.); U.S. Pat. No. 5,039,185 (Uchida et al.); U.S. Pat. No. 5,652,667 (Kuragane); and U.S. Pat. No. 5,847,789 (Nakamura et al.). LCDs are also available in a wide range of sizes, from devices suited to micro-displays to devices used for direct view laptops. LCD performance characteristics, such as response time, angular acceptance, contrast, and control voltages, vary depending on the device.
Spatial light modulators that alter the polarization state of incident light have also been constructed using lead lanthanum zirconium titanate (PLZT), as described in U.S. Pat. No. 4,707,081 (Mir), U.S. Pat. No. 4,887,104 (Kitano et al.), and U.S. Pat. No. 5,402,154 (Shibaguchi et al.). While PLZT devices are robust relative to optical damage thresholds, these devices typically have modest modulation speeds (kHz range), require high drive voltages, and have electro-optic response curves with significant hysteresis.
LCD and PLZT devices are suitable for many applications, but have a number of inherent disadvantages, including relatively slow response times (typically a few ms) and significant optical response variations relative to the angle of incidence. Most LCD modulators are unable to provide both high modulation contrast and fast modulation speeds simultaneously. Modulation contrast not only varies with angle and wavelength, but can also be degraded by thermally induced stress birefringence when exposed to the large light loads common to projection applications. In demanding applications using LCDs, the systems are often enhanced through the use of carefully designed polarization compensators (for example see U.S. Pat. No. 4,701,028 (Clerc et al.) and U.S. Pat. No. 6,081,312 (Aminaka et al.), which boost contrast, but at the cost of additional optics to the system.
One approach to providing spatial light modulators with improved response time is to adapt micro-mechanical devices to this task. The digital micro-mirror device (DMD) from Texas Instruments, Dallas, Tex., as disclosed in U.S. Pat. No. 5,061,049 (Hornbeck), is one such device, which modulates by beam steering the incident light relative to the imaging optics. Micro-mechanical gratings, including the grating light valve (GLV), disclosed in U.S. Pat. No. 5,311,360 (Bloom), and the conformal grating modulator, disclosed in U.S. Pat. No. 6,307,663 (Kowarz), have been successfully developed. These gratings impart a phase pattern to the incident light, causing it to diffract when modulated. Both the micro-mirror and the grating modulators require the use of a Schlieren type optical system, with blocking apertures or angular separation, to distinguish between the modulated and un-modulated light. Alternately, a spatial light modulator with rolling micro-mechanical shutters is described in U.S. Pat. No. 5,233,459 (Bozler et al), which either blocks or transmits the incident light, according to the control signals. As compared to the electro-optical or acousto-optical devices, the micro-mechanical modulators typically provide a more uniform response, both within a device (from pixel to pixel) and relative to the properties of the incident light (angle of incidence, wavelength, etc.). The micro-mechanical optical modulators also typically provide faster response times (On to Off, and visa-versa) than do many of the electro-optical devices. While these devices have provided some improvements in performance, there is room for improvement. For example, DMD devices are capable of achieving higher speeds, but are presently limited in achieving high resolution, and limit the input light to a modest angular beam width (<10° or <F/3.0). By comparison, the GLV and related devices are generally limited to one dimensional structures, due to optical fill factor issues between adjacent rows.
While micro-mechanics have been applied to light modulation using beam steering, diffraction, and beam blocking mechanisms, there are further opportunities to bring the advantages of micro-mechanical (MEMS) structures to the area of optical modulation. In particular, an improved polarization modulator could be designed, with potentially faster response times and more uniform angular and wavelength responses as compared to some of the conventional electro-optical devices.
Micro-mechanical structures, which might be adaptable to the construction of a micro-mechanical polarization modulator, have been described, including motors, rotors, and mini-turbines. Exemplary structures and manufacturing processes for micro-motors are discussed in numerous prior art patents, including U.S. Pat. No. 5,252,881 (Muller et al.), U.S. Pat. Nos. 5,710,466 and 5,909,069 (both to Allen et al.), and U.S. Pat. No. 5,705,318 (Mehregany et al.). Micro-motors have been fabricated and tested on a scale as small as 60-100 &mgr;m diameter, which is of a size appropriate for building a pixilated spatial light modulator, although smaller motor diameters could be useful. U.S. Pat. No. 5,459,602 (Sampsell) and U.S. Pat. No. 5,552,925 (Worley) describe micro-motors that are adapted with revolving blade shutters. Alternately, U.S. Pat. No. 6,029,337 (Mehregany et al.) describes a micro-motor structured to facilitate the creation of a variety of devices, including a micro-polygon scanner and micro-grating optical scanner. In particular,
FIG. 4
of U.S. Pat. No. 6,029,337 illustrates the concept of a rotating diffraction grating (long pitch (p>>&lgr;)), mounted to a micro-motor, and used in an optical scanner. These devices, operating at rotational speeds up to 50,000 rpm (1.2 msec/rev.), can be used in optical systems for a variety of applications, including bar code scanners and laser printers.
However, U.S. Pat. No. 6,029,337 neither describes the design and construction of a micro-mechanical polarization spatial light modulator, nor anticipates the potential advantages of such a device and its application within a modulation optical system. In particular, such a device is necessarily fabricated with a surface structure that alters the polarization state of the incident light in accordance with it rotational position. Traditionally, optical polarizers have been constructed with bulk materials, such as crystal calcite, or as the Polaroid type dye sheets with stretched polymers, or as optical thin films within glass substrates (U.S. Pat. No. 2,403,731 (MacNielle)), or finally as aligned metallic needles embedded in a glass medium (U.S. Pat. No. 5,281,562 (Araujo et al.) and U.S. Pat. No. 5,517,356 (Araujo et al.)

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