Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
1999-03-17
2001-09-11
Ben, Loha (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S291000, C359S295000, C359S298000, C359S290000
Reexamination Certificate
active
06288824
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPEMENT
Not Applicable.
REFERENCE TO A MICROFICHE APPENDIX
Not Applicable.
BACKGROUND OF THE INVENTION
a. Field of the Invention
The present invention relates to the optical shutter and more particularly, to the flat panel electrostatic display device having a twodimensional array of these shutters; each shutter contains electrically actuated membrane supported by a resilient beam which allows the membrane to move over fixed electrodes on the substrate and thereby modulate transmission and reflection of the incoming light.
b. State of the Prior Art
The electrostatic optical modulator employing a resilient electrode moving over a static electrode has been a subject of many patents and publications. The difference between them originates essentially from the shape of electrodes involved and the direction of movement of the flexible electrode relative to the static one. There are several patents on the electrostatic modulators and displays in which the flexible electrode moves to a curved static electrode thus providing an opening for the light transmission, see e.g. U.S. Pat. No. 4,229,075. Other approaches use an originally curved membrane film which then flattens as the membrane electrostatically moves toward the fixed electrode and thus modulates the light, see e.g. U.S. Pat. Nos. 4,208,103 and 4,786,149.
Various examples of the electrostatic optical shutters are based on the field induced bending of a cantilever membrane which is moved toward a flat static electrode thereby changing the optical state, see U.S. Pat. Nos. 3,553,364, 3,600,798, 4,229,732, 4,731,670 and 5,781,331. Another approach relies on an electrostatically induced twisting of the membrane mirror from its initial flat position to vary the optical reflection, see U.S. Pat. Nos. 3,746,911 and 4,710,732. This electrostatic modulator, known as the Deformable Mirror Device, has been reduced to practice as a commercially available projection display, see e.g. L. J. Hombeck “128×128 deformable mirror devices”, IEEE Trans.on Electr. Dev., ED-30, p.539,1983, and D. R. Pape and L. J. Hombeck “Characteristics of the deformable mirror device for optical information processing”, SPIE 388, p.65, 1983.
Of background interest are also the optical modulators in which electrically actuated membrane displacement changes the conditions of the optical resonance existing between the membrane and the substrate, thus varying the state of optical reflection, see K. W. Goossen et al. “Silicon modulator based on mechanically active antireflection layer with 1 Mbit/sec capability for fiber-on-the-loop applications”, IEEE Phot.Techn. Lett. 6,1119,1994; and O. Solgaard et al., “Deformable grating optical modulator”, Optics Letters, 17, p.688, 1992.
OBJECTS AND SUMMARY OF THE INVENTION
It is a general object of the present invention to introduce a new, fast and cost effective electrically actuated electromechanical optical shutter.
Another object of the present invention is to provide a flat panel display in which these shutters compose an electrically controlled two dimensional array of pixels.
Yet another object of the present invention is to provide a detailed description of the operation, fabrication and performance of the proposed flat panel display.
According to the present invention, the optical shutter (or the display pixel) consists of a membrane disposed over and attached to an optically transparent and electrically insulating substrate through one or two long and narrow tether beams. The membrane, separated from the substrate by an air gap, is made as a film of electrically insulating and optically transparent material. Both the membrane and the substrate are identically coated with an opaque and electrically conductive film in the shape of parallel and periodically repeating stripes, occupying the entire optically active area of the shutter. The stripes on each element of the shutter are connected at one or both ends, thereby providing two separate continuous electrodes facing each other over a narrow air gap. The opaque stripe in each period of the grating structure is wider than the opening, while the grating structure in the membrane is shifted by a half a period relative to that in the substrate. This implies that when no voltage is applied, the combined grating structure of both electrodes completely blocks the light (“off”-state).
Upon application of the voltage between the electrodes, the membrane is attracted to the substrate. The key factor is that the membrane moves to its most energetically favorable position in which the conducting opaque stripes spatially coincide on both electrodes. In this case, the combined stripe structure of two electrodes contains periodically repeating openings and therefore passes the light (“on”-state). In the shutter designed for the display application, the total length of the membrane movement, both laterally (half a period) and toward the substrate, can be made as small as a few microns. This provides a high modulation speed at a moderate voltage amplitude. To minimize the air drag, the membrane film is etched away within the openings. As shown below, for realistic dimensions of the membrane and the stripe structure, the “off”-to-“on”switching time can be made in the range of ~1 &mgr;s for a suitable for the display operation voltage amplitude of 10V.
According to the present invention, two-dimensional array of the shutters comprises the display capable of operating in both transmissive and reflective optical modes. Simple passive matrix architecture with rowby-row pixel addressing can be utilized. To prevent inter-pixel cross talk, inevitably existing in the passive matrix scheme, a resistor is connected in parallel to the shutter, to lower the pixel impedance, while the shutter-resistance circuit is connected in series with a diode. Such a combination prevents formation of parasitic conductive loops over the display matrix. Both the resistor and the diode are made as thin films. The amorphous Si as a material for these films is preferable.
Two different pixel designs are considered according to the present invention. The first and simple case relates to a twoterminal pixel configuration operating with only one controlling voltage. The second design employs a three-terminal approach operating with two controlling voltages to provide a bistability for the membrane position relative to the substrate. In the first configuration, the pixel transmits or reflects light only during a short period of time, defined by the row addressing time t
r
, plus some time t
m
needed for the membrane to return to its original “off” state position under elastic force of the tether beam as the voltage is reduced to zero. The time tr is controlled by the frame time tf and the number of the rows N: t
r
=t
f
/N. In the second, three-terminal case, due to bistability effect, the membrane remains in the “on”-state during the entire frame time (per color), which increases the pixel brightness.
In both approaches considered, the color can be implemented by subdividing the frame time into three periods for three main colors. For each color, light emitting diodes (LED) can be used for pulsed illumination. Short pixel switching time allows realization of the gray levels through repetition of the “on”-“off” switching procedures within the row addressing time for the two-terminal pixel structure and within the entire frame time for the three-terminal configuration.
REFERENCES:
patent: 4385798 (1983-05-01), Yevick
patent: 4753517 (1988-06-01), Samek
patent: 5260719 (1993-11-01), Maloney
patent: 5629794 (1997-05-01), Magel et al.
patent: 5745281 (1998-04-01), Yi et al.
Ben Loha
Brodsky Charles I.
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