Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
1998-12-18
2001-01-30
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S291000, C359S572000, C359S573000
Reexamination Certificate
active
06181458
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of modulation of an incident light beam by the use of a mechanical grating device. More particularly, this invention discloses a mechanical grating device which has a significant improvement in the output of the diffracted light beam by the use of an optical coating. Furthermore the invention relates to a method of making a mechanical grating device with an optical coating.
BACKGROUND OF THE INVENTION
Mechanical or electromechanical spatial light modulators have been designed for a variety of applications, including image processing, display, optical computing, and printing. Optical beam processing for printing with deformable mirrors has been described by L. J. Hornbeck; see U.S. Pat. No. 4,596,992, issued Jun. 24, 1984, entitled “Linear Spatial Light Modulator and Printer”. A device for optical beam modulation using cantilever mechanical beams has also been disclosed; see U.S. Pat. No. 4,492,435, issued Jan. 8, 1985, to M. E. Banton entitled “Multiple Array Full Width Electro-mechanical Modulator,” and U.S. Pat. No. 5,661,593, issued Aug. 26, 1997 to C. D. Engle, entitled “Linear Electrostatic Modulator.” Other applications of electromechanical gratings include wavelength division multiplexing and spectrometers; see U.S. Pat. No. 5,757,536, issued May 26, 1998 to A. J. Ricco et al., entitled “Electrically Programmable Diffraction Grating,”.
Electro-mechanical gratings are well known in patent literature; see U.S. Pat. No. 4,011,009, issued Mar. 8, 1977 to W. L. Lama et al., entitled “Reflection Diffraction Grating Having a Controllable Blaze Angle,” and U.S. Pat. No. 5,115,344, issued May 19, 1992 to J. E. Jaskie, entitled “Tunable Diffraction Grating”. More recently, Bloom et al. described an apparatus and method of fabrication for a device for optical beam modulation, known to one skilled in the art as a grating-light valve (GLV); see U.S. Pat. No. 5,311,360, issued May 10, 1994, entitled “Method and Apparatus for Modulating a Light Beam”. This device was later described by Bloom et al. with changes in the structure that included: 1) patterned raised areas beneath the ribbons to minimize contact area to obviate section between the ribbon and substrate; 2) an alternative device design in which the spacing between ribbons was decreased and alternate ribbons were actuated to produce good contrast; 3) solid supports to fix alternate ribbons; and 4) an alternative device design that produced a blazed grating by rotation of suspended surfaces. See U.S. Pat. No. 5,459,610, issued Oct. 17, 1995, entitled “Deformable Grating Apparatus for Modulating a Light Beam and Including Means for Obviating Stiction Between Grating Elements and Underlying Substrate”, and U.S. Pat. No. 5,808,797, issued Sep. 15, 1998, entitled “Method and Apparatus for Modulating a Light Beam”. Bloom et al. also presented a method for fabricating the device; see U.S. Pat. No. 5,677,783, issued Oct. 14, 1997, entitled “Method of Making a Deformable Grating Apparatus for Modulating a Light Beam and Including Means for Obviating Stiction Between Grating Elements and Underlying Substrate”.
In all embodiments of the mechanical grating device or the Grating Light Valve device (GLV) in the aforementioned patent literature, a single metallic, reflective coating with a bare upper surface has been added to the top surface of the ribbons to apply the electrostatic force required for actuation, and also to increase the efficiency of diffraction of the device by increasing the reflectivity. In high-power applications, the reflective coating also results in longer lifetime of the GLV device. A high reflectivity is important to reduce damage of the top surface of the ribbons and avoid mechanical effects that might be attributed to a significant increase in the temperature of the device due to light absorption.
A method for fabricating another embodiment of the GLV device was presented by Bornstein et al.; see U.S. Pat. No. 5,661,592, issued Aug. 26, 1997, entitled “Method of Making and an Apparatus for a Flat Diffraction Grating Light Valve”. In this embodiment, a thin (<500 Å) dielectric layer is formed over the reflecting metallic layer on the top surface of the ribbons.
Dielectric multilayer coatings have been used on static metallic diffraction gratings to enhance diffraction efficiency and reduce absorption in the metal. D. Maystre et al. presented studies of two-layer and four-layer coatings on aluminum gratings in “Gratings for tunable lasers: using multi-dielectric coatings to improve their efficiency,” published in
Applied Optics,
vol. 19 (1980).
The prior art does not mention the problem of the effects of the optical coating on the mechanical properties of the ribbon elements.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a mechanical grating device which shows an increased the reflectivity at the wavelength of interest. Furthermore, the absorption of light is reduced, resulting in a longer lifetime of the mechanical grating device. It is also an object of the present invention to provide a mechanical grating device that diffracts light into the +1 and −1 orders in the unactuated state and reflects light into the 0 order in the actuated state.
The object is achieved by a mechanical grating device comprising:
a base having a surface;
a spacer layer provided above the base, said spacer layer defining an upper surface and a longitudinal channel defining a width is formed in said spacer layer;
a plurality of spaced apart deformable ribbon elements disposed parallel to each other and spanning the width of the channel, said deformable ribbon elements defining a top and a bottom surface;
a reflective layer added to the top surface of each of said ribbon elements; and
an optical coating provided on top of the reflective layer of every other ribbon element.
The above object is also achieved by a mechanical grating device comprising:
a base having a surface;
a spacer layer provided above the base, said spacer layer defining an upper surface and a longitudinal channel defining a width is formed in said spacer layer;
a plurality of spaced apart deformable ribbon elements disposed parallel to each other and spanning the width of the channel, said deformable ribbon elements defining a top and a bottom surface;
a reflective layer added to the top surface of each of said ribbon elements; and
an optical coating provided on top of the reflective layer of every ribbon element wherein the optical coating is formed as a stack of more than one transparent dielectric layers.
It is a further object to provide a method of making a mechanical grating device, which shows an increased reflectivity at the wavelength of interest; furthermore, the absorption of light is reduced, resulting in a longer lifetime of the mechanical grating device.
The above object is accomplished by a method comprising the steps of:
providing a spacer layer on top of a protective layer which covers a substrate;
etching a channel entirely through the spacer layer;
depositing a sacrificial layer at least as thick as the spacer layer;
rendering the deposited sacrificial layer optically coplanar by chemical mechanical polishing;
providing a tensile ribbon layer completely covering the area of the channel;
providing a reflective layer;
providing an optical coating;
patterning the optical coating, the reflective layer, and the ribbon layer in the form of a grating; and
removing entirely the sacrificial layer from the channel.
It is advantageous that the inventive mechanical grating device can be used for printing on photosensitive media. The efficiency is maximized to allow faster rates of printing while reducing the power requirements of the optical sources providing the incident illumination. For display and other applications, increased efficiency is also advantageous. Further, for application of the device to laser thermal printing, the absorption of light in the metal reflector layer is minimized.
In this invention, a plurality of layers comprising at least
Brazas, Jr. John C.
Kowarz Marek W.
Kruschwitz Brian E.
Eastman Kodak Company
Epps Georgia
Noval William F.
Shaw Stephen H.
Thompson Tim
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