Optical waveguides – Optical fiber waveguide with cladding – Utilizing nonsolid core or cladding
Reexamination Certificate
2002-02-27
2003-10-28
Healy, Brian (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing nonsolid core or cladding
C065S393000, C065S406000, C065S409000, C065S408000
Reexamination Certificate
active
06640037
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a method of making a photonic crystal preform, having a defect, from which a photonic band-gap crystal waveguide fiber can be drawn, and particularly to a method of forming the defect of the photonic crystal preform.
2. Technical Background
Optical waveguides that guide light by the principle of total internal reflection have been in commercial use for more than two decades. Although optical waveguides of this design represent a quantum step forward in the field of telecommunication, work on alternative waveguide fiber designs continues. A particular drawback of the total internal reflection mechanism is that it acts to confine the light to a higher index portion, that is, the core, of the waveguide fiber. The higher index core is typically higher in density and so is characterized by higher attenuation due to Rayleigh scattering and by a higher non-linear coefficient. The deleterious non-linear effects can be mitigated by designing total internal reflection waveguides that have relatively high effective area. However, the complexity of the core refractive index profile usually increases for designs that provide larger effective area. This complexity usually translates into higher cost.
More recently, diffraction has been studied as a means to guide light in a material. In a light guiding protocol in which the confinement mechanism is diffraction, the material in which the light is guided, i.e., the core of the optical waveguide, can have a relatively low refractive index and thus a lower density. In fact, the use of a gas or a vacuum as a waveguide core becomes practical.
A particular structure well suited for use as a diffraction type optical waveguide is a photonic band-gap crystal. The photonic crystal itself is a regular lattice of features in which the spacing of the features is of the order of the light wavelength to be guided. The photonic crystal can be constructed of a first material having a first refractive index. Embedded in this first material, in the form of a regular lattice or array, is a second material having a second refractive index. This is the basic photonic crystal structure. Variations on this basic design can include more than two materials in the make up of the photonic band-gap crystal. The number of useful variations in the details of the lattice structure is also large.
In the basic photonic crystal structure, the “second material” can simply be pores or voids formed in the first material. That is, the voids serve as the second material in a photonic crystal. Depending upon the refractive index difference of the materials and the spatial arrangement and pitch (center to center distance between features) of the embedded features, the photonic crystal will not propagate light having a wavelength within a certain wavelength band. This is the “band-gap” of the photonic crystal and is the property of the photonic crystal that provides for light confinement. It is due to this property that the structure is given the name, photonic band-gap crystal.
To form an optical waveguide (or more generally, a structure that guides electromagnetic energy), a defect is formed in the photonic band-gap crystal. The defect is a discontinuity in the lattice structure and can be a change in pitch of the lattice, the replacement of a portion of the lattice by a material of different refractive index, or the removal of a portion of the photonic band-gap crystal material. The shape and size of the defect are selected to produce, that is, support, one or more modes of light propagation having respective wavelengths within the band-gap of the photonic crystal. The walls of the defect are thus made of a material, a photonic band-gap crystal, which will not propagate the mode produced by the defect.
In analogy with the total internal reflection optical waveguide, the defect acts as the waveguide core and the photonic band-gap crystal acts as the clad. However, the mechanism of the waveguide allows the core to have a very low refractive index thus realizing the benefits of low attenuation and small nonlinear coefficient.
Because of the potential benefits provided by a photonic band-gap crystal waveguide, there is a need to identify effective, low cost methods of making the photonic band-gap crystal waveguide that are tailored for use in a manufacturing operation.
One such method includes the steps of making a preform having a plurality of longitudinal passages, etching the passages to reach a desired preform geometry, and then drawing the preform into a photonic band-gap crystal waveguide fiber. Improving etching efficiency and performance of the waveguide made from the etched structure is, therefore, a primary enabling step presently under study.
SUMMARY OF THE INVENTION
One aspect of the present invention is a method of making a photonic crystal waveguide fiber preform using an assembly of a first and a second plurality of tubes. A first portion of the photonic crystal is made using a first plurality of tubes which have varying wall thickness along the circumference of the tube, where the circumference lies in a plane perpendicular to the length of the tube. It is understood that the wall thickness does not vary along the length of the tube. In assembling the tubes of this first portion of the photonic crystal, the thinner wall portions of the tubes abut one another. The second portion, i.e., the remaining portion, of the crystal is assembled using a second plurality of tubes which have a substantially constant wall thickness. The second plurality of tubes is configured to surround the first plurality of tubes. The photonic crystal so assembled is then etched until the portion of the photonic crystal where the thin walls abut is removed. Because the first plurality of tubes is assembled contiguously, etching away the wall portions where the thin walls abut has the effect of forming a defect in the photonic crystal. The first plurality of tubes is arranged so that the defect produces, i.e., supports or propagates, light modes having respective wavelengths that lie within the band gap of the photonic crystal.
In an embodiment in accord with the first aspect of the invention, the first plurality of tubes is configured so that the abutting thin walls are located relative to each other such that, if nearest neighbor abutting points are connected by a line, the line is continuous and closes upon itself. In the etching step, all of the tubes inside the continuous, closed line are removed from the photonic crystal. Advantageously the first plurality of tubes can be configured so that the line joining adjacent (nearest neighbor), thin-walled abutting points forms a symmetric geometrical shape such as a circle or a polygon.
In yet another embodiment, each member or certain selected members of the first plurality of tubes has an offset bore. That is, if one takes both the outer tube wall and the inner tube wall as each having respective symmetrical shapes, each having a center point, the two center points are spaced apart from each other. The respective inner and outer walls can be selected from a group consisting of a circle or a polygon. One advantageous choice is a hexagonal shape for the outer wall periphery and a circular shape for the inner wall periphery. The hexagonal shape for the outer wall periphery provides for a close-pack stacking of the tubes, i.e., a stacking with essentially no air gaps. The circular shape for the inner wall periphery is relatively easier to manufacture compared to a polygon shape, although this distinction is less appropriate when describing an extrusion method of making the tubes.
In yet a further embodiment in accord with the first aspect of the invention, the first plurality of tubes has a first composition different from that of the second plurality of tubes. The respective first and second compositions of the two tube types have different etch rates, the first etch rate being greater than the second etch rate. That is, the etching substance removes mat
Chervenak William J.
Corning Incorporated
Healy Brian
Wood Kevin S
LandOfFree
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