Optical waveguides – Planar optical waveguide
Reexamination Certificate
2002-06-11
2004-12-14
Lee, Diane I. (Department: 2876)
Optical waveguides
Planar optical waveguide
C385S014000
Reexamination Certificate
active
06832033
ABSTRACT:
BACKGROUND OF THE INVENTION
A. Field of the Invention
The present invention relates generally to photonic bandgap materials (also known as photonic crystals), and, more particularly to hetero-structure photonic bandgap materials.
B. Description of the Related Art
During the last decade photonic crystals have risen from an obscure technology to a prominent field of research. In large part this is due to their unique ability to control, or redirect, the propagation of light. E. Yablonovich, “Inhibited spontaneous emission in solid-state physics and electronics,”
Physical Review Letters
, vol. 58, pp. 2059-2062 (May 1987), and S. John, “Strong localization of photons in certain disordered dielectric superlattices,”
Physical Review Letters
, vol. 58, pp. 2486-2489 (June 1987), initially proposed the idea that a periodic dielectric structure can possess the property of a bandgap for certain frequencies in the electromagnetic spectra, in much the same way as an electronic bandgap exists in semiconductor materials. This property affords photonic crystals with a unique ability to guide and filter light as it propagates within it. Thus, photonic crystals have been used to improve the overall performance of many optoelectronic devices.
The concept of a photonic bandgap material is as follows. In direct conceptual analogy to an electronic bandgap in a semiconductor material, which excludes electrical carriers having stationary energy states within the bandgap, a photonic bandgap in a dielectric medium excludes stationary photonic energy states (i.e., electromagnetic radiation having some discrete wavelength or range of wavelengths) within that bandgap. In semiconductors, the electronic bandgap results as a consequence of having a periodic atomic structure upon which the quantum mechanical behavior of the electrons in the material must attain eigenstates. By analogy, the photonic bandgap results if one has a periodic structure of a dielectric material where the periodicity is of a distance suitable to interact periodically with electromagnetic waves of some characteristic wavelength that may appear in or be impressed upon the material, so as to attain quantum mechanical eigenstates.
A use of these materials that can be envisioned, is the optical analog to semiconductor behavior, in which a photonic bandgap material, or a plurality of such materials acting in concert, can be made to interact with and control light wave propagation in a manner analogous to the way that semiconductor materials can be made to interact with and control the flow of electrically charged particles, i.e., electricity, in both analog and digital applications. Photonic crystals have been used to improve the overall performance of many optoelectronic devices.
Optimizing the performance of a Photonic Integrated Circuit (PIC) in a single crystalline photonic crystal structure (unistructure) continues to be a challenge, due to the spatial constraint of the unistructure. This limitation, namely single crystalline structures, has a pronounced impact over the bandgap size, throughput efficiency, and back reflections that arise due to spatial mismatches at transitions between different Photonic crystal sections such as straight and angular waveguides.
Conventional two-dimensional photonic crystals have been formed from an evenly-spaced triangular lattice array of rods, each rod having a dielectric constant, and a background material surrounding the rods and having a dielectric constant different than the dielectric constant of the rods. Such a triangular lattice array photonic crystal is shown, for example, in FIG. 2 of U.S. Pat. No. 5,999,308. As shown in this patent, the triangular array consists of rows of rods, wherein adjacent rows of rods are offset from each other such that a rod from one row lies between two rods on an adjacent row in the z-direction.
A. Computational Method
The method initially used for theoretical analyses of photonic crystal structures is the plane-wave expansion method, which makes use of the fact that; eigenmodes in periodic structures can be expressed as a superposition of a set of plane waves. Using this approach, photon dispersion relations inside photonic crystal structures have been calculated. While this method can ensure an accurate solution for the dispersion properties of a photonic crystal structure, it is still limited due to the fact that transmission spectra, field distribution, and back reflections cannot be extracted, since it only considers propagating modes, whereas in a finite crystal there are also evanescent modes.
An alternative approach, which has been widely adopted in calculating both transmission spectra and field distribution, is based on numerical solutions of Maxwell equations using the finite-difference time-domain (FDTD) method. In particular, FDTD has been used to analyze multi-channel drop/add filters as well as other photonic crystal devices, to calculate transmission through sharp bends, and to study waveguiding mechanism through localized coupled cavities in three-dimensional photonic crystals.
To examine the FDTD method, a two-dimensional photonic crystal of a square lattice with a lattice constant &agr;=543 nm may be considered. The lattice may include dielectric rods with a dielectric constant of ∈
r
=11.56 and radius r=109 nm, in an air background. The transmission spectrum for this structure may be obtained using FDTD with periodic boundary conditions. The structure has a bandgap between &lgr;=1.234 &mgr;m and &lgr;=2.172 &mgr;m, for TM (transverse magnetic field) polarization. A beam splitter may be implemented in the structure to calculate its overall throughput efficiency.
B. Unistructure Beam Splitter
A common use for photonic crystals in integrated optical applications is that of an optical beam splitter and/or combiner. An optical beam splitter divides an optical beam into multiple signals for density routing. The split beam can then be recombined back into a single beam or further guided to another point with an optical beam combiner, depending upon the application. Presently, these operations have been hindered by the spatial constraints of the single crystalline structure, which in some cases limit their ability to efficiently perform their intended functions. Limitations such as back reflections, frequency selectivity, and/or bi-directionality of certain devices may have a pronounced impact on the performance and operation of some photonic integrated circuits. The hetero-structure photonic crystal devices of the present invention overcome these limitations, in addition to enhancing throughput efficiency as well as minimizing back reflections.
A conventional two-dimensional photonic crystal is the Cartesian lattice array
10
shown in FIG.
1
. The Cartesian lattice array
10
may be a square array or rectangular array and is formed from evenly-spaced columns and rows of rods
12
, each rod
12
having a dielectric constant, and background material
14
surrounding rods
12
and having a dielectric constant different than the dielectric constant of rods
12
. As shown in
FIG. 1
, the Cartesian lattice array
10
consists of rows and columns of rods
12
, wherein adjacent rows and columns of rods
12
are evenly spaced from each other in both the x-direction and the z-direction. As will be described more fully below, a line defect may be formed in Cartesian lattice array photonic crystal
10
by removing a portion of a column of dielectric rods
12
from the photonic crystal
12
. This creates a main waveguide
16
within the photonic crystal
10
.
The unistructure optical beam splitter
10
shown in
FIG. 1
consists of silicon posts
12
(having a dielectric constant ∈
r
=11.56) arranged on a rectangular lattice in air background
14
(having a dielectric constant ∈
b
=1.0). The photonic crystal structure
10
possesses a bandgap between &lgr;=1.234 &mgr;m and &lgr;=2.172 &mgr;m for TM polarization. An optical beam splitter (Y coupler)
10
consists of two basic el
Prather Dennis W.
Sharkawy Ahmed Samir
Shi Shouyun
Caputo Lisa M.
Connolly Bove & Lodge & Hutz LLP
Lee Diane I.
The University of Delaware
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