Planar photonic bandgap structures for controlling radiation...

Optical waveguides – With optical coupler – Input/output coupler

Reexamination Certificate

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C385S129000, C385S131000, C385S132000

Reexamination Certificate

active

06684008

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed to robust planar photonic bandgap structures. In particular, the present invention is directed to robust planar photonic bandgap structures that, in connection with guiding radiation, control (for example, minimize, reduce or increase) radiation losses by employing layered dielectric structures. The present planar photonic bandgap structures can be used in connection with waveguides, waveguide bends, waveguide crossings, filters, switches and fiber-coupling structures.
BACKGROUND OF THE INVENTION
The propagation of electromagnetic radiation (that is, visible, infrared, ultraviolet, TeraHertz, millimeter wave and microwave radiation) can be dramatically altered in periodically patterned devices. Such devices may comprise discrete regions of materials where each region possesses a distinct index of refraction, or regions of materials in which the indices of refraction vary continuously throughout each region. If the index contrasts (for example, the differences between the indices of refraction for the discrete regions) are sufficiently large, an optically opaque material can be formed, even though the device is composed of transparent dielectric materials. The properties of such materials have attracted great scientific interest over the last ten to fifteen years, and are considered likely candidates for applications involving telecommunications, optical signal processing and integrated optical circuits. The dielectric properties of those materials are commonly analogized to the electronic properties of crystals, which are characterized by, among other things, what is known as the forbidden energy gap. That analogy has led to the term “photonic bandgap” (PBG). The PBG is a range of frequencies over which electromagnetic radiation is unable to propagate, and a PBG structure is a structure that exhibits a photonic bandgap. To achieve a photonic bandgap for electromagnetic radiation at a radial frequency &ohgr;, it is necessary to achieve a structure or material with a period on the order of the wavelength &lgr;, where &lgr;=2&pgr;c/&ohgr; and c is the speed of light in a vacuum. It is generally understood that a PBG structure or material is a structure or material through which electromagnetic radiation, at any frequency in the photonic bandgap, is unable to propagate in any direction and at any polarization. A photonic bandgap that exists irrespective of the direction or polarization of the electromagnetic radiation is known as a full photonic bandgap. Of course, it may be useful in practical applications to restrict the scope of the PBG (for example, to only one polarization), assuming that the material would be used only with light at a certain polarization.
Achieving a full photonic bandgap theoretically requires that the material be patterned in all three dimensions (that is, height, length and width). Such materials have been shown to function in the microwave region, and it has been suggested that they can function at optical frequencies. However, several challenges remain to be overcome: fabrication of uniform photonic bandgap material, patterning the material and adapting the material to applications such as waveguiding. Those challenges are at least initially attributable to the difficulty encountered in attempting to pattern materials in three dimensions on the scale of optical wavelengths of interest in telecommunications, which wavelengths are on the order of 1.5 &mgr;m.
One alternative to a full, three-dimensionally patterned PBG material is a patterned planar material. An example of such a patterned planar material is illustrated in FIG.
1
, which shows a uniform planar PBG structure. As shown in
FIG. 1
, a planar PBG is patterned, for example, by chemical or other etching, with a periodic array of holes. In such a patterned device, it is possible to achieve a bandgap for light propagating at any direction in the plane and for any polarization. Such a patterned device can be characterized as a restricted version of a full, three-dimensional PBG. In a planar PBG structure, confinement of light within the plane of the layers is normally required, and is achieved by suitable choices for the indices of refraction of the constituent layers. As used herein, the phrase “index of refraction profile” is a characterization of the relationship between indices of refraction and corresponding depths of the material(s) at issue. It is generally known in the field of planar photonic bandgap structures that confinement of light to patterned layers is normally achieved where the index of refraction profile within the patterned layers features indices of refraction that are all or substantially all higher than the indices of refraction in the index of refraction profiles for the substrate and superstrate. As used herein, the term “substrate” means an unpatterned layer or layers that are underneath the patterned region and that affect the propagation of the optical mode. Such an unpatterned layer is to be contrasted with a physical support, which provides structural stability, mounting and the like, and which plays no significant role in determining the optical propagation properties of electromagnetic radiation modes in the PBG structure. As used herein, the term “superstrate” means an unpatterned layer or layers that are above the patterned region and that affect the propagation of the optical mode.
For the structure of
FIG. 1
, for example, confinement in the vertical direction is achieved by having an index of refraction profile in the planar PBG layer that features indices of refraction that are all or substantially all higher than the indices of refraction in the index of refraction profiles for the air superstrate and the unpatterned substrate. The disadvantage of the planar PBG over the three-dimensional PBG is the potential for radiation losses in the planar configuration due to out-of-plane scattering.
In some cases, the substrate and/or superstrate comprise air. Where both the substrate and superstrate comprise air, a free-standing “membrane structure” is formed, as shown in FIG.
2
. As a practical matter, however, and as further shown in
FIG. 2
, a substrate comprising air necessarily has at least a second layer. Others have failed to recognize or appreciate the significance of that second layer(s) in controlling radiation loss. The structure of
FIG. 2
also has a number of mechanical disadvantages, primarily relating to fragility and poor heat conduction away from the active PBG layer.
Importantly, PBG structures do support the propagation of electromagnetic radiation at frequencies outside the photonic bandgap.
FIG. 1
illustrates light propagating in a PBG structure in such an allowed mode. Such allowed-mode propagation may be useful in applications such as beam collimation, prism-like refraction, and others. In such applications, it is important to control (for example, minimize, reduce or increase) radiation losses during propagation.
In addition to the above-mentioned applications, there are a number of other possible applications (for example, guiding light) for planar PBG structures that feature intentionally disturbed periodic structuring.
FIG. 3
, for example, shows such a defect waveguide based on the structure of
FIG. 1
, where a line of holes has been omitted during the patterning process. Analogous waveguide structures exist in the membrane geometry of
FIG. 2
, and are not illustrated here. In a waveguide such as that illustrated in
FIG. 3
, the surrounding undisturbed PBG regions are normally designed such that, at the desired frequency of operation, a photonic bandgap exists, thereby preventing light from entering the surrounding regions. In this way, light that is coupled into the waveguide (for example, from an external source) will remain in the waveguide. A key requirement for the suitability of such waveguides in practical applications is that they not suffer from large radiation losses.
The concept of guiding electromagnetic radiation (that is, light) in planar PBG structures

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