Active photonic crystal waveguide device

Details Active photonic crystal waveguide device Active photonic crystal waveguide device

2001-08-15

2003-04-01

Ullah, Akm E. (Department: 2874)

Optical waveguides

Optical fiber waveguide with cladding

Utilizing nonsolid core or cladding

C385S004000, C385S008000, C385S040000, C385S129000, C385S142000

Reexamination Certificate

active

06542682

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an optical waveguide structure for an optical communication system, and particularly to a planar photonic crystal waveguide for implementing a variety of optical functions in an optical communication system.
2. Technical Background
Photonic crystals are periodic optical materials. The characteristic defining a photonic crystal structure is the periodic arrangement of dielectric or metallic elements along one or more axes. Thus, photonic crystals can be one-, two-, and three-dimensional. Most commonly, photonic crystals are formed from a periodic lattice of dielectric material. When the dielectric constants of the materials forming the lattice are different (and the materials absorb minimal light), the effects of scattering and Bragg diffraction at the lattice interfaces control the propagation of optical signals through the structure. These photonic crystals can be designed to prohibit optical signals of certain frequencies from propagating in certain directions within the crystal structure. The range of frequencies for which propagation is prohibited is known as the photonic band gap.
An exemplary two dimensional photonic crystal which is periodic in two directions and homogeneous in a third is shown in FIG.
1
. More specifically, the photonic crystal
10
is fabricated from a volume of bulk material
12
having a square lattice of cylindrical air-filled columns
14
extending through the bulk material in the z-axis direction and periodic in the x-axis and y-axis directions. For normal theoretical analysis and modeling, the photonic crystal
10
has conventionally been assumed to be homogeneous and infinite in the z-axis direction. In this exemplary figure, the plane of the two dimensional photonic crystal is the xy plane.
Another exemplary photonic crystal is shown in FIG.
2
. The photonic crystal
15
is similar to the photonic crystal
10
, but the cylindrical air-filled columns are disposed in a hexagonal array. A third exemplary two-dimensional photonic crystal is shown in FIG.
3
. The photonic crystal
16
is also similar to photonic crystal
10
, but consists of an array of dielectric columns
18
in an air background.
The propagation of optical signals in these structures is determined by a variety of parameters, including, for example, the radius of the columns, the pitch (center-to-center spacing of the columns) of the photonic crystal, the structural symmetry of the crystal (e.g. square, triangular, hexagonal, rectangular), and the lattice refractive indexes, (such as the index of the material of the columns and the index of the bulk material exterior to the columns).
FIG. 4
shows the photonic band diagram for a hexagonal array of air-filled columns in a dielectric bulk material. One skilled in the art will appreciate that there is a range of photon frequencies, known as the photonic band gap, for which propagation in the plane of the photonic crystal is prohibited. This photonic band gap, denoted by region
19
, is determined by the structure of the photonic crystal, especially by the parameters listed above.
A defect can be introduced into the crystalline structure for altering the propagation characteristics and localizing the allowed modes for an optical signal. For example,
FIG. 5
shows a two-dimensional photonic crystal
20
made from a dielectric bulk material with a square lattice of air-filled columns
22
and a linear defect
24
consisting of a row of missing air-filled columns. The band diagram for this photonic crystal structure is shown in FIG.
6
. The photonic band gap is denoted by the region
30
, while a band of allowed guided modes associated with the defect is denoted by the very thin region
32
. The exact position and shape of the region
32
on the graph of
FIG. 6
depends upon the photonic crystal parameters. Physically, this means that while optical signals of a given frequency are prohibited from propagating in the bulk photonic crystal
20
, they may propagate in the defect region
24
. An optical signal, whether a pulse or a continuous wave, traveling in the defect region
24
may not escape into the bulk photonic crystal
20
, and so is effectively wiaveguided in the defect region
24
. For a given wavevector, the region
32
only encompasses a narrow band of frequencies. Optical signals of a given wavevector must have frequencies within this narrow band in order to be guided in the defect
24
. In the theoretical case of the infinitely thick two-dimensional photonic crystal, light is not confined in the z-axis direction by the photonic crystal structure. While the defect in the above example is a constructed from a row of missing air-filled columns, other defect structures are possible. For example, a defect may consist of one or more columns of a different shape or size than those of the bulk photonic crystal.
Additionally, the crystal structure can be composed of several photonic crystal regions having different parameters, in which case the defect is located at the border between the two regions. Such a structure is shown in
FIG. 7
, in which the photonic crystal structure
40
has a first photonic crystal region
42
and a second photonic crystal region
44
. In the example of
FIG. 7
, in the first region
42
the cylindrical columns have radius R
1
and are arranged with a pitch P
1
. In the second region, the photonic crystal structure has different parameters, with a column radius of R
2
and a pitch of P
2
. This photonic crystal also has a photonic bandgap, with the possibility of a defect mode for allowing propagation of an optical signal. Because of this defect mode phenomenon and its dependence on the photonic crystal parameters, it is possible to control the propagation of an optical signal in a defect waveguide by controlling the parameters associated with the photonic crystal regions.
Since an optical signal propagating in a defect waveguide is prohibited from propagating in the bulk photonic crystal, it must follow the waveguide, regardless of the shape of the defect waveguide. An advantage of such a structure is that waveguides with a very small bend radius on the order of several wavelengths or even less are expected to have a very low bend loss, since an optical signal is prohibited from escaping the defect waveguide and propagating in the surrounding photonic crystal.
FIG. 8
shows the results of a simulation of propagation in a 2D photonic crystal wherein substantially all of the optical signal successfully navigates a 90° bend with a radius of curvature smaller than the wavelength of the optical signal. Likewise, waveguide splitters and combiners are expected to have low radiation losses.
FIG. 9
shows a 180° splitter in which nearly 100% transmission is achieved with the optical signal from the input guide
60
perfectly split into the two branches
62
and
64
. In this case, a pair of small columns was added in order to reduce the small fraction of light that was backreflected into the input guide
60
.
In-plane confinement by a photonic crystal defect waveguide can be combined with refractive confinement in the dimension normal to the photonic crystal to provide a defect channel waveguide. This is most commonly achieved by providing a thin slab of a two-dimensional photonic crystal (known as a planar photonic crystal) having a defect waveguide with lower refractive index materials above and below the photonic crystal waveguide. For example,
FIG. 10
shows the structure of a planar photonic crystal defect waveguide
70
with a core layer
71
, an underclad layer
72
, and an overclad layer
74
, all of which include a photonic crystal structure defining the defect channel waveguide. As used herein, an effective refractive index of a material is defined as the volume average refractive index of that material. In order to provide vertical confinement, the effective refractive index of the core layer
71
is higher than the effective refractive indices of the underclad layer
72
and the overclad layer

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