The present invention relates to optical fibers, and in particular to a hollow core photonic bandgap optical fiber.
Optical fibers are long, thin waveguide strands. They are used in a variety of applications involving the reception, transmission and detection of radiation, ranging from optical telecommunications to scientific instrumentation to ornamental displays. Conventional optical fibers have a high-index core surrounded by a low-index cladding, a geometry that results in the total internal reflection of radiation entering the fiber over a select range of angles. Other forms of conventional optical fibers are referred to as xe2x80x9clight tunnelsxe2x80x9d and use reflection from reflective walls rather than total internal reflection. Conventional optical fibers can have a variety of core geometries, including elliptical cores, double cores, polarization-maintaining cores, and hollow cores.
Hollow core optical fibers are desirable where transmission losses need to be minimized, since in conventional optical fibers most of the radiation loss in an optical fiber occurs from absorption in the solid core. However, conventional optical fibers that depend on total internal reflection for waveguiding and confinement need to have a core with a higher dielectric constant than that of the cladding. Such a structure is at odds with an optical fiber having a hollow central core, since air has a dielectric constant lower than all known practical cladding materials. Thus, hollow core optical fibers generally require specialized design considerations.
There are a number of different types of hollow core optical waveguides that operate on the either principle of total internal reflection or the principles of conventional reflection. For example, hollow optical waveguides through silicon wafers with highly reflective coatings on the side of the hollow core are disclosed in U.S. Pat. No. 6,090,636 to Geusic et al. An optical fiber interconnect through a silicon wafer with a hole filled with two different dielectric materials is disclosed in U.S. Pat. No. 6,150,188 to Geusic et al. A hollow optical fiber or hollow core waveguide consisting of a dielectric material coated with a highly reflective exterior coating is disclosed in U.S. Pat. No. 5,815,627 to Harrington, wherein the hollow core can contain one guided wave and the annular ring another guided wave. A hollow core optical waveguide having a highly reflective coating on the inside of a glass tube is disclosed in U.S. Pat. No. 6,141,476 to Matsuura.
As mentioned above, a desirable property for an optical fiber is minimal transmission loss. Accordingly, there has been significant effort to form optical fibers from materials that have high transmission and high reflection. Some of this effort has been directed to forming optical fibers from quasi-two-dimensional photonic crystals. A photonic crystal is a substrate within which is formed an array of period structures through which radiation of a particular wavelength or energy is forbidden to propagate. The result is a material with a very high reflectivity. Two-dimensional photonic bandgap crystal structures have been reported not only at optical wavelengths but at acoustic wavelengths as well.
It is well known in the semiconductor industry that a series of cylindrical holes judiciously formed in a solid semiconductor material can be used to form a quasi-two-dimensional photonic bandgap crystal. FIG. 1 is a plan view of a conventional two-dimensional photonic crystal formed from a substrate 110 patterned with an triangular array of cylindrical holes 120.
An example prior art quasi-two-dimensional photonic bandgap optical fiber 210 is illustrated in FIGS. 2A and 2B. The optical fiber 210 includes an array of holes 220 formed within a cylindrical substrate 226. The fiber includes an outer cladding 228. A solid core 232, referred to as a xe2x80x9ccore defect,xe2x80x9d is at the center of the substrate. Radiation 242 is reflected within the bandgap of the photonic crystal produced by the periodic array of holes surrounding the core and is confined to the core. The radiation travels down the length of the solid core by total internal reflection made possible by the low average index of refraction of the cladding as compared to that of solid core 232. Though optical fiber 210 is photonic crystal based, it utilizes total internal reflection like a conventional optical fiber and is relatively lossy because the solid core absorbs radiation.
Accordingly, what is needed is a photonic crystal optical fiber with a hollow core that allows for low-loss radiation propagation down the fiber by virtue of highly reflectivity walls as a result of the photonic bandgap of the crystal.
A hollow core photonic crystal optical fiber formed from several smaller hollow core optical fibers layered around a hollow core is shown. The fiber works on the principle of quasi-two-dimensional photonic crystals to confine radiation in a guided wave. The fiber has a true photonic bandgap in which radiation of a particular frequency (or equivalently, energy or wavelength) is totally forbidden, thereby providing a very high reflection coefficient to radiation incident the walls of the hollow core.
The present invention includes an array of hollow core optical fibers arranged longitudinally about an axis to define a central hollow core about the axis. The array is formed so as to act as a quasi-two-dimensional photonic crystal with a photonic bandgap that allows radiation of a select frequency range to propagate down the central hollow core.
The present invention further includes a plurality of hollow core optical fibers arranged longitudinally in an array having a two-dimensional triangular lattice structure so as to form a quasi-two-dimensional photonic crystal having a photonic bandgap, and a central hollow core sized to accept radiation corresponding to the photonic bandgap. The hollow core optical fibers are, for example, conventional silica fibers.
The present invention also includes a system that includes a hollow core photonic bandgap optical fiber having an input end and an output end. The photonic bandgap optical fiber is made up of an array of hollow core optical fibers arranged longitudinally to form a two-dimensional photonic crystal lattice having a photonic bandgap. The array of optical fibers has at least one omitted optical fiber so as to leave a longitudinal central aperture that forms the hollow core in the array. The hollow core in the array is capable of guiding light having a frequency within the photonic bandgap of the array. The system further includes a radiation source optically coupled to the input end, and a photodetector optically coupled to the output end.
The present invention also includes a method of guiding radiation. The method includes forming a two-dimensional array of hollow core optical fibers to form a quasi-two-dimensional photonic crystal. The crystal has a photonic bandgap. In forming the array of fibers, at least one hollow core optical fiber is omitted. This creates a central hollow core in the array through which light of a select frequency that falls within the photonic bandgap is guided. The method further includes introducing radiation into the central hollow core having a frequency within the photonic bandgap.
These and other embodiments, aspects, advantages and features of the present invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art by reference to the following description of the invention and referenced drawings or by practice of the invention. The aspects, advantages, and features of the invention are realized and attained by means of the instrumentalities, procedures, and combinations particularly pointed out in the appended claims.