Optical waveguides – Optical fiber waveguide with cladding – Utilizing nonsolid core or cladding
Reexamination Certificate
2000-06-09
2002-07-09
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing nonsolid core or cladding
C385S123000, C385S124000, C385S126000
Reexamination Certificate
active
06418258
ABSTRACT:
FIELD
This patent specifcaton relates to the field of optical fibers. More particularly, it relates to a microstructured optical fiber that allows for improved transmission efficiency and physical durability in fiber optic communication applications.
BACKGROUND
Advances in fiber optics technologies have made optical fiber communications the method of choice in the transmission of high bit-rate digital data over long distances. A conventional optical fiber is essentially an optical waveguide having an inner core and an outer cladding, the cladding having a lower index of refraction than the core. Because of the difference in refractive indices, the optical fiber is capable of confining light that is axially introduced into the core and transmitting that light over a substantial distance. Because they are able to guide light due to total internal reflection principles, conventional optical fibers are sometimes referred to as index-guiding fibers. Conventional optical fibers are made of fused silica, with the core region and the cladding region having different levels of dopants (introduced impurities) to result in the different indices of refraction. The cladding is usually doped to have a refractive index that ranges from 0.1% (single mode fibers) to 2% (multi-mode fibers) less than the refractive index of the core, which itself usually has a nominal refractive index of 1.47.
Conventional optical fibers have a solid cross-section. As light travels through the solid fused silica material, it is subject to several adverse effects that reduce the efficiency of information transfer and the practical distance over which information may be carried by the light. These effects include attenuation or loss (reduction in signal magnitude), dispersion (chromatic, waveguide and modal), miscellaneous nonlinearities (such as stimulated Raman scattering, stimulated Brillouin scattering, and optically induced birefringence), and other adverse effects.
Although the light is being transmitted through many meters or kilometers of solid material, relatively low losses can be experienced at certain wavelengths of light. Conventional fibers today, for example, have attenuations as low as 0.25 dB/km at 1550 nm—about 1% of the light entering the fiber still remains after 80 km. Using today's amplifier and detector technologies, this allows signals to go through more than 100 km of fiber without amplification, an important advantage in long distance communications. Nevertheless, it would be desirable to even further minimize the above mentioned adverse effects in optical fibers, for increasing the efficiency and reducing the cost of fiber optic communications.
Another problem that arises in conventional optical fibers relates to the physical durability and robustness of the optical fiber itself. A variety of outside influences can change the physical characteristics of optical fibers and affect how they guide light. As a first example, the bending of an optical fiber into tight loops or other tightly curved shapes may cause the unwanted propagation of microcracks in the fused silica structure. Upon such bending of the fiber, the microcracks can become larger and extend across a substantial portion of the cross-section of the optical fiber, rendering it inoperable. As a second example, external forces may squeeze or pinch the outside surface of an optical fiber, such as when a fiber is tightly pulled around a sharp corner. Upon such squeezing or pinching, the structure of the fused silica material inside the fiber may contort slightly. This can cause unwanted polarization effects in the light being transmitted through the fiber, also reducing the fiber's capacity or rendering it inoperable. Accordingly, it would be desirable to provide a fiber optic structure that is more robust to external bending, pinching, or squeezing of the fiber optic.
As described by Broeng et. al. in WO9964903, recent developments in optical fiber technology have been introduced by way of microstructured photonic bandgap (PBG) fibers. In contrast to conventional optical fibers in which a high-index core is surrounded by a low-index cladding, PBG fibers comprise a low-index (or even hollow) core surrounded by a higher-index cladding that contains carefully placed air voids. The air voids run longitudinally, parallel to the central axis of the fiber. When the air voids are placed in the cladding such that a cross-section of the fiber has a specific, predetermined, periodic pattern of air voids, a photonic bandgap (PBG) effect may be achieved. When the PBG effect is achieved, the cladding structure is capable of completely reflecting certain wavelengths of light at certain incident angles, and is thereby capable of confining the light to a region surrounded by the cladding structure for propagation down the length of the fiber. The PBG effect is achieved even though the refractive index within the region of confinement may be lower than that of the surrounding cladding structure.
PBG fibers, however, contain a crucial shortcoming in that proper operation is based on an interference effect. This is in contrast to conventional index-guiding fibers that guide due to total internal reflection. Because they depend on an interference effect, PBG fibers are extremely sensitive to even slight variations in the locations of the air voids in the cladding. Substantial deterioration in performance may take place if even one of the air voids is slightly misplaced. Even if properly manufactured to exacting tolerances, slight variations in the relative air void positions might be incurred due to external twisting, pinching, or squeezing that slightly deforms the fiber optic structure. This, in turn, may lead to drastic performance decreases. Accordingly, PBG fibers are not used today in practical fiber optic communication systems, although they continue to be the subject of laboratory research.
Another type of optical fiber having longitudinal air voids is presented in U.S. Pat. No. 5,802,236 to DiGiovanni et al., hereby incorporated by reference herein. The '236 patent discloses a microstructured optical fiber comprising a solid core region surrounded by a cladding region having a plurality of air voids. In contrast to PBG fiber in which careful periodic spacing of the air voids is required, the air voids of the '236 patent are not required to be periodic. The optical fiber of the '236 patent relies on index-guiding, and not on the PBG effect, to propagate the light down the fiber, the index-guiding effect being achieved when the effective refractive index of the cladding is less than that of the core. According to the '236 patent, because a portion of the cross-sectional area of the cladding is occupied by air voids, the effective index of refraction of the cladding region will be less than that of the core, and index-guiding will be achieved. Roughly speaking, the effective index of refraction of the cladding will be an average of the refractive index of air and the refractive index of the fused silica material, weighted according to the percentage of cross-sectional area occupied by each.
FIG. 1
illustrates a cross-sectional view of an optical fiber
100
similar to that disclosed in the '236 patent and having dimensions as described in the '236 patent. Optical fiber
100
comprises a core region
102
surrounded by a cladding region
104
. The core region
102
is solid glass material. The cladding region
104
is solid glass material surrounding a plurality of air-void cladding features, in particular, first cladding features
106
and second cladding features
108
. The first cladding features
106
are positioned such that the inscribed diameter of core region
102
is 1.017 &mgr;m. The first cladding features
106
each have a diameter of 0.833 &mgr;m, while the second cladding features
108
each have a diameter of 0.688 &mgr;m, the cladding features all having a center-to-center spacing of 0.925 &mgr;m.
Although it is less dependent on precise air void spacing, thereby resolving a problem presen
Cooper & Dunham LLP
Gazillion Bits Inc.
Palmer Phan T. H.
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