2-D diffraction grating for substantially eliminating...

Details 2-D diffraction grating for substantially eliminating... 2-D diffraction grating for substantially eliminating...

2002-08-20

2004-10-05

Ben, Loha (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

C359S290000, C359S295000, C359S569000, C359S572000, C359S573000, C385S037000

Reexamination Certificate

active

06801354

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method of and an apparatus for modulation of a beam of light. More particularly, this invention is for a substantially flat reflective surface having selectively deformable portions for providing a diffraction grating.
BACKGROUND OF THE INVENTION
Designers and inventors have sought to develop a light modulator which can operate alone or together with other modulators. Such modulators should provide high resolution, high operating speeds (kHz frame rates), multiple gray scale levels, e.g., 100 levels or be compatible with the generation of color, a high contrast ratio or modulation depth, have optical flatness, be compatible with VLSI processing techniques, be easy to handle and be relatively low in cost. One such related system is found in U.S. Pat. No. 5,311,360.
According to the teachings of the '360 patent, a diffraction grating is formed of a multiple mirrored-ribbon structure such as shown in
FIG. 1. A
pattern of a plurality of deformable ribbon structures
100
are formed in a spaced relationship over a substrate
102
. The substrate
102
preferably includes a conductor
101
. Both the ribbons and the substrate between the ribbons are coated with a light reflective material
104
, such as an aluminum film. The height difference that is designed between the surface of the reflective material
104
on the ribbons
100
and those on the substrate
102
is &lgr;/2 when the ribbons are in a relaxed, up state. If light at a wavelength &lgr; impinges on this structure perpendicularly to the surface of the substrate
102
, the reflected light from the surface of the ribbons
100
will be in phase with the reflected light from the substrate
102
. This is because the light which strikes the substrate travels &lgr;/2 further than the light striking the ribbons and then returns &lgr;/2, for a total of one complete wavelength &lgr;. Thus, the structure appears as a flat mirror when a beam of light having a wavelength of &lgr; impinges thereon.
By applying appropriate voltages to the ribbons
100
and the conductor
101
, the ribbons
100
can be made to bend toward and contact the substrate
102
as shown in FIG.
2
. The thickness of the ribbons is designed to be &lgr;/4. If light at a wavelength &lgr; impinges on this structure perpendicularly to the surface of the substrate
102
, the reflected light from the surface of the ribbons
100
will be completely out of phase with the reflected light from the substrate
102
. This will cause interference between the light from the ribbons and light from the substrate and thus, the structure will diffract the light. Because of the diffraction, the reflected light will come from the surface of the structure at an angle &THgr; from perpendicular.
If a wavelength of other than &lgr; impinges thereon, there will only be partial reflectivity when the ribbons are in the “up”
0
state, since &THgr; is dependent on the wavelength &lgr;. Similarly, the light will only be partially diffracted to the angle &THgr; when the ribbons arc in the “down”
0
state. Thus, a dark pixel will display some light and a bright pixel will not display all the light if the wavelength of the light is not exactly at &lgr;. It is very expensive to utilize a light source that has only a single wavelength. Commercially viable light sources typically provide light over a range of wavelengths.
For the above described device to function within desired parameters requires that the heights and thickness of the ribbons and reflecting layers to provide structures are precisely &lgr;/2 when up and &lgr;/4 when down. Because of variances in manufacturing processing, the likelihood is small that the relative heights will be precisely &lgr;/2 when up and &lgr;/4 when down. Therefore, the expected parameters will be much poorer than theoretically possible.
Another difficulty with the above described structure results from an artifact of the physical construction. In particular, once in the down position, the ribbons tend to adhere to the substrate. Texturing the surface of the substrate aids in overcoming this adhesion. Unfortunately, the textured surface substantially degrades the reflective properties of the surface. This degrades the performance of the device.
The '360 patent teaches an alternate structure as shown in FIG.
3
. According to this conventional structure, a plurality of elongated elements are disposed over a substrate
200
. A first plurality of the elongated elements
202
are suspended by their respective ends (not shown) over an air gap
204
, as in the embodiment of
FIGS. 1 and 2
. A second plurality of the elongated elements
206
are mounted to the substrate
200
via a rigid support member
208
. The height of the support members
208
is designed to be &lgr;/4. A reflective material
210
is formed over the surface of all the elongated elements
202
and
206
.
In theory, the elongated elements
202
and
206
are designed to be at the same height when at rest. Thus, when all the elongated elements are up and at the same height there will be no diffraction. (In fact there may be some modest amount of diffraction due to the periodic discontinuities of the gaps between elongated elements. However, this period is half the period of the grating so that it diffracts at twice the angle of the desired diffracted light. Because the optics are configured to pick up diffracted light from only the desired angle, this unwanted diffraction is not captured and does not degrade the contrast ratio.)
In order to build a structure such as shown in
FIG. 3
, a layer must be formed of a first material having a predetermined susceptibility to a known etchant. Portions of that layer are removed through known techniques such as photolithography and etching. A second material is then formed in the voids of the removed material such as by deposition. This second material has a known susceptibility to the etchant which is different than the first material. The layer is formed of the elongated element material. This structure is etched to form ribbons of the elongated elements. Finally, the second material is removed by etching to form the suspended elongated elements
202
. A popular use for light modulators of the type described in the '360 patent is for use as a variable optical attenuator, VOA, for signals in a fiber-optic network.
FIGS. 4A and 4B
show how an articulated one-dimensional grating can be used to control the amount of light reflected into an optical fiber.
FIG. 4A
illustrates a reflective grating
320
in an undeformed state in which an incident light
310
from an optical fiber
305
impinges upon the reflective grating
320
. A numerical aperture (NA) of the optical fiber
305
determines an acceptance cone
315
in which the optical fiber
305
accepts light. In its undeformed state, the reflective grating
320
behaves much like a mirror; the incident light
310
is simply reflected back into the optical fiber
305
with no attenuation .
FIG. 4B
illustrates the reflective grating
320
in a deformed state in which the incident light
310
is diffracted at predominantly predetermined diffraction angles
325
. The diffraction angles
325
can be adjusted to be larger than the acceptance cone
315
of the optical fiber
305
thereby allowing attenuation of the incident light
310
. By controlling the deformation of the grating, the amount of light reflected back into the fiber can be controlled.
Unfortunately, when arbitrarily polarized light impinges on a linear one-dimensional (1D) grating, each polarization state interacts with the grating differently. Such a scenario is illustrated in
FIG. 5
in which an incident light beam
350
impinges upon a 1D grating
360
comprising a series of reflective ribbons placed in parallel. The incident light
350
includes a polarization state P and a polarization state S. Light polarized parallel to the ribbons (polarization state P) interacts with the 1D grating
360
differently than light polarized perpendicular to the ribbons (polarization st

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