Coherent light generators – Particular resonant cavity – Plural cavities
Reexamination Certificate
2002-05-10
2004-05-11
Ip, Paul (Department: 2828)
Coherent light generators
Particular resonant cavity
Plural cavities
C372S092000, C372S039000, C372S043010
Reexamination Certificate
active
06735235
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates generally to the field of photonic crystals; and, more particularly, to a three-dimensional photonic crystal add-drop filter.
2. Description of Related Art
Photonic crystals (PC) are periodic dielectric structures that can prohibit the propagation of light in certain frequency ranges (see J. D. Joannopoulos, R. D. Meade and J. N. Winn,
Photonic Crystals
, Princeton University Press, Princeton, N.J., 1995). More particularly, photonic crystals are structures that have spatially periodic variations in refractive index; and with a sufficiently high refractive index contrast, photonic bandgaps can be opened in the structure's optical transmission characteristics. The term “photonic bandgap” as used herein and as is commonly used in the art is a frequency range in which propagation of light through the photonic crystal is prevented. In addition, the term “light” as used herein is intended to include radiation throughout the electromagnetic spectrum, and is not limited to visible light.
Two-dimensional photonic crystal slabs are known that comprise a two-dimensional periodic lattice incorporated within a slab body. In a two-dimensional photonic crystal slab, light propagating in the slab is confined in the direction perpendicular to the faces of the slab via total internal reflection. Light propagating in the slab in directions other than perpendicular to the slab faces, however, is controlled by the spatially periodic structure of the slab. In particular, the spatially periodic structure causes a photonic bandgap to be opened in the transmission characteristics of the structure within which the propagation of light through the slab is prevented. Specifically, light propagating in a two-dimensional photonic crystal slab in directions other than perpendicular to a slab face and having a frequency within a bandgap of the slab will not propagate through the slab; while light having frequencies outside the bandgap is transmitted through the slab unhindered.
It is known that the introduction of defects in the periodic lattice of a photonic crystal allows the existence of localized electromagnetic states that are trapped at the defect site, and that have resonant frequencies within the bandgap of the surrounding photonic crystal material. By arranging these defects in an appropriate manner, a waveguide can be created in the photonic crystal through which light having frequencies within the bandgap of the photonic crystal (and that would normally be prevented from propagating through the photonic crystal) is transmitted through the photonic crystal.
Three-dimensional photonic crystals that have spatial periodicity in three dimensions, and that can prevent the propagation of light having a frequency within the crystal's bandgap in all directions, are also known. For example, a known three-dimensional photonic crystal apparatus comprising dielectric elements stacked layer by layer is illustrated in
FIG. 1
(also see U.S. Pat. Nos. 5,335,240 and 5,406,573 and K. M. Ho, et al., “Solid State Commun.”, 89, 413, 1994).
The three-dimensional photonic crystal apparatus illustrated in
FIG. 1
is generally designated by reference number
10
and comprises a plurality of layers of elements arranged one on top of another. In
FIG. 1
, three-dimensional photonic crystal apparatus
10
comprises twelve layers
12
-
1
to
12
-
12
; however, twelve layers is intended to be exemplary only as the apparatus can comprise any desired plurality of layers.
Each layer
12
-
1
to
12
-
12
comprises a plurality of elements arranged to be parallel to and equally spaced from one another. In addition, the plurality of elements in each layer are arranged perpendicular to the elements in an adjacent layer.
In
FIG. 1
, the elements comprise rods, and layers
12
-
1
,
12
-
3
,
12
-
5
,
12
-
7
,
12
-
9
and
12
-
11
each comprise a plurality of rods
14
arranged in a direction parallel to the x-axis of the apparatus (as shown in FIG.
1
); and layers
12
-
2
,
12
-
4
,
12
-
6
,
12
-
8
,
12
-
10
and
12
-
12
each comprise a plurality of rods
16
arranged in a direction parallel to the y-axis of the apparatus. In addition, as shown in
FIG. 1
, in every other layer, the rods are laterally displaced with respect to one another by an amount equal to one-half the spacing between the rods in a layer. Specifically, in
FIG. 1
, the rods in layers
12
-
3
,
12
-
7
and
12
-
11
are aligned with respect to one another along the y-axis, but are laterally displaced, along the y-axis, from the plurality of rods in layers
12
-
1
,
12
-
5
and
12
-
9
. Also, the rods in layers
12
-
2
,
12
-
6
and
12
-
10
are aligned with respect to one another along the x-axis, but are laterally displaced, along the x-axis, from the plurality of rods in layers
12
-
4
,
12
-
8
and
12
-
12
.
The three-dimensional photonic crystal apparatus
10
of
FIG. 1
, can be described as comprising a photonic crystal having a three-dimensional array of unit cells therein in which a “unit cell” is defined as a cell having dimensions in the x and y directions equal to the spacing between the rods in the layers, i.e., the dimensions
41
and
42
in
FIG. 1
; and a dimension in the z-direction equal to the thickness of four layers, i.e., the dimension
44
in FIG.
1
.
In the three-dimensional photonic crystal apparatus illustrated in
FIG. 1
, rods
14
and
16
comprise dielectric rods of a material having a high dielectric constant, e.g., alumina, surrounded by a material having a low dielectric constant, e.g., air.
Wave division multiplexing is a process that permits the transmission capacity of an optical communications system to be increased. In particular, in a wave division multiplexer (WDM) system, information is transmitted using a plurality of optical carrier signals, each carrier signal having a different optical wavelength. By modulating each carrier signal with a different one of a plurality of information signals, the plurality of information signals can be simultaneously transmitted through a single waveguiding device such as a single optical fiber.
For a WDM system to function properly, the system must have the capability of extracting a carrier signal at a certain wavelength from one waveguide and adding the signal at that wavelength to another waveguide so as to redirect the path through which the extracted carrier signal travels.
FIG. 2
is a block diagram that schematically illustrates components of a WDM communications system. The system is generally designated by reference number
20
, and includes a signal source
22
that transmits a plurality of carrier signals at different optical wavelengths through an optical fiber or other waveguiding device
24
. The optical fiber
24
is connected to an extraction device
26
that is capable of extracting one or more of the carrier signals carried by the optical fiber
24
and redirecting the extracted signal or signals to another optical fiber or waveguiding device
28
. The remaining carrier signals carried by the optical fiber
24
are transmitted through the extraction device
26
to an optical fiber
30
or the like. The carrier signals carried by optical fibers
28
and
30
are then further processed by processing structure not illustrated in FIG.
2
.
Add-drop filters are commonly used in optical communications circuits to extract light of a particular wavelength from one waveguide and direct the extracted light to another waveguide. In effect, an add-drop filter allows light of one wavelength to be dropped from one path in an optical communications circuit and added to another path in the circuit.
Known add-drop filters, however, are not fully satisfactory for use as an extraction device in a WDM system. For example, in one known configuration, light propagates through conventional high dielectric waveguides and the cavities between the waveguides-are micro-rings (see B. E. Little and S. T. Chu,
Toward very large
-
scale integrated photonics
, Optics and Phot
Agilent Technologie,s Inc.
Al-Nazer Leith A
Ip Paul
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