Optical waveguides – Directional optical modulation within an optical waveguide
Reexamination Certificate
2000-01-31
2003-02-18
Healy, Brian (Department: 2874)
Optical waveguides
Directional optical modulation within an optical waveguide
C385S012000, C385S008000, C385S010000, C385S014000, C385S015000, C385S017000, C385S037000, C385S040000, C385S130000, C385S131000, C385S016000, C385S901000, C436S518000, C436S006000
Reexamination Certificate
active
06522794
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to devices, particularly optical devices, for controlling propagation of energy, particularly optical beams, using electric field control. In particular, the invention relates to devices with poled structures, including periodically poled structures, and electrodes which permit controlled propagation of optical energy in the presence of controlled electric fields applied between electrodes. The invention relates to a fundamentally new class of flat panel optical displays.
The current technology for an EO switchable grating is shown in
FIG. 1
(Prior Art). In this structure, periodically patterned electrodes serve as the elements that define the grating. The underlying material does not have a patterned poled structure, as hereinafter explained. An input beam
12
is coupled into a electro-optically active material
2
which contains an electrically controllable permanent grating
6
. When the voltage source
10
to the grating electrodes is off, the input beam continues to propagate through the material to form the output beam
16
. When the grating-controlling voltage source is switched on, an index modulation grating is produced in the material, and a portion of the input beam is coupled into a reflected output beam
14
. The material has an electro-optically active poled region
4
with a single domain, with the same polarity throughout the poled structure. A first electrode
6
is interdigitated with a second electrode
7
on a common surface
18
of the substrate. When a voltage is applied between the electrodes, the vertical component of electric field along the path of the beam
12
alternately has opposite sign, creating alternate positive and negative index changes to form a grating. The strength of the grating is controlled by the voltage source connected between the two electrodes by two conductors
8
.
A second general problem with the existing art of EO and piezoelectric devices using uniform substrates and patterned electrodes is that the pattern of the excited electric field decays rapidly with distance away from the electrodes. The pattern is essentially washed out at a distance from the electrodes equal to the pattern feature size. This problem is aggravated in the case of a grating because of the very small feature size. Prior art gratings formed by interdigitated electrodes produce a modulated effect only in a shallow surface layer. EO structures interact weakly with waveguides whose dimension is larger than the feature size. While longer grating periods may be used in higher order interaction devices, the lack of sharp definition described above again seriously limits efficiency. The minimum grating period for efficient interaction with current technology is about 10 microns. What is needed is a way to maintain the efficiency of EO devices based on small structures, despite a high aspect ratio (i.e. the ratio of the width of the optical beam to the feature size). Switchable patterned structures are needed which persist throughout the width of waveguides and even large unguided beams.
There are several related technologies in the prior art that use light sources coupled with waveguide structures for display applications.
J. Viitanem and J. Lekkala (“Fiber optic liquid crystal displays,” SPIE Vol. 1976,
High
-
Definition Video,
pg. 293-302 (1993), and references therein) review the characteristics of flat panel displays that use the waveguide principle coupled with liquid crystal switching. A number of designs are discussed. All have the following common design principles. A modulated light source is mechanically scanned across a series of electro-optically active waveguides that form the row elements of the display. A series of parallel electrodes form the column locations for the display. Light is coupled out of the waveguides and scattered toward the viewer at a column spatial location using the electro-optic effect. Thus a two-dimensional array of pixels is formed.
In this prior art, light is confined in a waveguide which is composed of a core optical material that has an index of refraction that is larger than the surrounding cladding material. The light, normally confined primarily to the core, is forced to “leak” out of the core of the waveguide at a desired spatial location. The waveguiding effect is destroyed by electro-optically reducing the index difference between the core and the cladding along a certain distance. The electro-optically active material may, in principle, either reside in the core (to reduce the index) or in the cladding (to increase the index. In Viitanem et al., the cladding is active. The technique of destroying the waveguiding effect is called a “waveguide tap” in some of the prior art literature.
The “leaked” light propagates by free space diffraction to a scattering center where it is directed toward the viewer to form a pixel of the display. The light that “leaks” out of the destroyed waveguide is no longer spatially confined but expands in area according to standard diffraction theory as it propagates away from the destroyed waveguide segment. This two-dimensional expansion of the light causes three problems.
First, since the diffraction angle of the light previously confined to the waveguide is relatively small, a long interaction lengths results. (A significant fraction of the optical energy must leave the core region before it can be scattered toward the viewer.) This typically will limit the spacing of the scattering centers to be larger than 1 mm. This effect causes a low resolution display with a low pixel packing density.
Second, the two dimensional expansion of the beam makes it virtually impossible to collect a large fraction of the light on a scattering center and direct it toward the viewer. This causes the display to have a low electrical power efficiency.
Third, the two-dimensional expansion of the beam causes the scattering centers to be large, and hence the pixel size is large. This also degrades the display resolution.
A consequence of the large pixel spacing is that long waveguide lengths must be used to cover enough pixels for a display. The display must then operate in a region where the effects of waveguide loss are large, again reducing efficiency. Thus this prior art design suffers from low pixel packing density, a large pixel size, and a low electrical power efficiency.
What is needed to resolve these problems is the development of a short, efficient, low-loss electro-optic waveguide switch that routes the entire light beam out of the row waveguide and into a narrow solid-angle so that the switched light can be efficiently directed either towards a pixel scattering center or into another waveguide that leads to a scattering center. This will concentrate the light on the scattering center, maximizing both the efficiency of the display and the pixel packing density.
Another embodiment using the waveguide tap method is described in U.S. Pat. No. 5,106,181, April, 1992, and U.S. Pat. No. 5,009,483, April, 1991, Rockwell, III, “Optical Waveguide Display System”. Rockwell III discloses a display that uses waveguides to guide light in simultaneous rows. Light is coupled out of a waveguide into the cladding using the electro-optic effect in the cladding. Although the structure is different from that of Viitanem et al. above, this display suffers the same problems of low pixel density and inefficiency discussed above.
U.S. Pat. No. 5,045,847, September, 1991, Tarui et. al., “Flat Display Panel”, discloses another version of the “waveguide tap” method. A planar waveguide structure is used with a layered core material consisting of interspersed layers of a-SiN and a-Si. Light from a laser diode source is confined within the planar waveguide until a voltage is placed across the core of the waveguide. The voltage causes the index of the core to be reduced thus allowing light to escape the waveguide structure. This display suffers from all of the difficulties discussed above. This design has an additional efficiency penalty since all pixels are simultaneously i
Brinkman Michael J.
Deacon David A. G.
DeWath Edward J.
Dyer Mark J.
Field Simon J.
Gemfire Corporation
Haynes Beffel & Wolfeld LLP
Healy Brian
Wolfeld Warren S.
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