Optical waveguides – Optical fiber waveguide with cladding – Utilizing nonsolid core or cladding
Reexamination Certificate
2000-12-08
2003-03-25
Ullah, Akm E. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing nonsolid core or cladding
Reexamination Certificate
active
06539155
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a novel group of cladding designs, especially for use in optical fibres, wherein a larger photonic bandgap may be obtained to confine light in hollow cores.
BACKGROUND OF THE INVENTION
Optical fibres and integrated optical waveguides are today applied in a wide range of applications within areas such as optical communications, sensor technology, spectroscopy, and medicine. These waveguides normally operate by guiding the electromagnetic field (the light or the photons) through a physical effect, which is known as total internal reflection. By using this fundamental effect, the propagation (or loss) of optical power in directions perpendicular to the waveguide axis is reduced.
In order to obtain total internal reflection in these waveguides, which are often fabricated from dielectric materials (in optical fibres) or semiconductors (in integrated optics), it is necessary to use a higher refractive index of the core compared to the refractive index of the surrounding cladding.
Today the preferred signal transmission medium over long and medium distances is the optical fibre, and total internal reflection is, consequently, a physical property, which has been known and used in technological development for decades. During the past ten years, however, the development within the area of new materials has opened up the possibilities of localisation of light or control of electromagnetic fields in cavities or waveguides by applying a completely new physical property—the so-called photonic bandgap (PBG) effect.
The PBG effect may be introduced by providing a spatially periodic lattice structure, in which the lattice dimensions and applied materials are chosen in such a way that electromagnetic field propagation is inhibited in certain frequency intervals and in certain directions. These PBG materials have been described in one-, two-, and three-dimensional cases in the scientific literature and in several patents (see for instance U.S. Pat. Nos. 5,386,215, 5,335,240, 5,440,421, 5,600,483, 5,172,267, 5,559,825).
A specific class of components, which makes use of such periodic dielectric structures, are the optical fibres (or waveguides), in which the periodic variation appears in directions perpendicular to the waveguide axes, whereas the structures are invariant along the waveguide axes.
Within recent years, especially researchers from University of Bath, UK, (see e.g. Birks et al., Electronics Letters, Vol.31 (22), p. 1941, October 1995) have presented optical fibres realised by having a core surrounded by thin, parallel, and air-filled voids/holes in a silica-background material, and organising the air-filled voids in a periodic structure in the cladding region of the fibres.
Although the above-cited Birks et al reference discloses the idea of photonic bandgap guiding fibers, it has since then been realised that the requirement that the cladding structure exhibits photonic bandgap effect is not necessary for these so-called microstructured fibers to be able to guide light (see e.g. Knight et al., Journal of the Optical Society of America, A., Vol.15 (3), p.748, March 1998). The reason for this is that microstructured fibers, which have a core region with a higher refractive index than the effective refractive index of the cladding structure, are able to guide light by total internal reflection. In accordance with this, it has also been realised that a periodic arrangement of the air voids is not a requirement for the operation of high-index core microstructured fibers (see e.g. U.S. Pat. No. 5,802,236).
It is important to notice that all of the high-index core microstructured fibers, which have been demonstrated, have not had an operation based on photonic bandgap effects. But simply due to the higher refractive index of the core region compared to the cladding (see e.g. U.S. Pat. No. 5,802,236 for definition of the core and cladding indices), all high-index core fibers have a fundamental mode which is guided due to total internal reflection (also known as index guiding).
In contrast to the high-index core fibers, low-index core fibers (i.e. fibres having a core region with a lower refractive index than the cladding) are not able to guide light leakage-free in the core region through total internal reflection. However, by designing a periodic cladding structure correctly, this cladding structure is able to exhibit photonic bandgap effects, as described in the above-cited Birks et al. reference.
Designing the cladding structure correctly involves optimising the periodic arrangement of voids with respect to sizes, dimensions, and morphology. Cladding structure which are exhibiting photonic bandgap effects are able to reflect light of certain wavelength and incident angles. This means that the cladding structure is able to confine light, which satisfies the condition that the light falls within a photonic bandgap, to a spatial region surrounded by the cladding structure. This is even the case when the spatial region has effectively a lower refractive index than the cladding structure. This is the operational principle of PBG guiding optical fibres and other PBG waveguides (see e.g. Barkou et al., Optics Letters, Vol.24 (1), p. 46, January 1999).
Due to the radically different physical mechanism causing the waveguidance, microstructured fibers classify into (at least) two groups. Namely those that are operating by photonic bandgap effect, which we will call PBG fibres (we will also refer to them as bandgap fibres or low-index core fibers), and those operating by total internal reflection, which we will refer to as high-index core fibres or index-guiding fibres.
Waveguidance by photonic bandgap effects are of significant future interest, as it allows radically new designs of optical fibres and other types waveguides. In particular for optical fibres, the core is not required to have a higher refractive index than the cladding. Such low-index core optical fibers (e.g. hollow core fibers) may be exploited in numerous applications, e.g. in sensor systems or for use as an ultra-low loss transmission fibre in telecommunication systems.
Recently the first photonic bandgap guiding optical fibre was demonstrated (see Knight et al., Science, Vol.282 (5393), p. 1476, November 1998). The design of this fibre was based on a Honeycomb arrangement of air voids in a silica background material in the cladding, and a single periodicity-breaking low-index region formed the core. The advantages of using a Honeycomb-based cladding structure compared to e.g. a triangular structure are that the cladding structure exhibits photonic bandgap effects for smaller (and thereby more realistic) air void sizes.
It is a disadvantage that, due to the triangular cladding structure, the PBG of the structures described by Birks et al. are not optimised for guiding electromagnetic radiation using the PBG effect.
It is a further disadvantage that the light in the recently demonstrated Honeycomb based PBG fibre is distributed almost entirely in silica.
It is a still further disadvantage that the cladding structure in the recently demonstrated Honeycomb-based PBG fibre is not optimised for guiding light inside a hollow core.
It is a still further disadvantage that the honeycomb arrangement of air voids in the cladding structure in the recently demonstrated PBG fibre is not optimised for obtaining a large void filling fraction.
It may be a problem or disadvantage of the present realisation of optical fibres with periodic dielectric cladding regions that careful, close-packed stacking of either hexagonal rods and hexagonal glass tubes (with central voids) or direct stacking of circular rods and thin circular tubes is required. These tubes and rods have been arranged in a close-packed triangular structure in a preform, where after the preform has been drawn into an optical fibre. Although these fibres according to the reports in the international literature show quite interesting and new optical properties, one of the disadvantages has been that the close-packing of the tubes and
Barkou Stig Eigil
Bjarklev Anders Overgaard
Broeng Jes
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