Optical waveguides – Accessories – Attenuator
Reexamination Certificate
2001-03-07
2004-09-21
Kim, Ellen (Department: 2874)
Optical waveguides
Accessories
Attenuator
C385S100000
Reexamination Certificate
active
06795635
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention is directed to an optical waveguide preform or fiber having a structure that varies in the axial direction. In particular, the novel preform or waveguide exhibits a clad layer refractive index that varies along the waveguide length, the variation due to change in the clad layer porosity or composition. The invention includes methods for making the novel waveguide preform and fiber.
Optical waveguide fibers having a periodically structured clad layer have been discussed. As an example, the periodic structure of the clad layer may be a photonic crystal as described by Knight et al., “All Silica Single Mode Optical Fiber with Photonic Crystal Cladding”,
Optics Letters
, V. 21, No. 19, 1 Oct. 1996, and by Birks, et al., “Endlessly Single Mode Photonic Crystal Fiber”,
Optics Letters. V.
22, No. 13, 1 Jul. 1997. In these two articles, a single mode fiber having a silica core and a porous silica cladding is described. The pores or voids in the silica clad layer are elongated and extend from end to end of the clad layer. The pores are arranged in a periodic hexagonal pattern to form the clad layer into a photonic crystal. The waveguide fiber so configured can be a single mode fiber at any wavelength.
Further work with waveguide fibers having a porous or pore filled clad layer is described in European patent publication EP 0 810 453 A1. In this publication, the clad layer contains elongated pores which serve to lower the average clad layer refractive index. The elongated pores are not arranged in a periodic pattern so the light guiding mechanism in this waveguide is refraction at the core-clad boundary.
The essentially limitless range of cut off wavelength, or, alternatively, the potential absence of any cut off wavelength, available in a photonic crystal clad layer is an advantage in single mode waveguide design. Also useful, in terms of offering an additional design variable, is the relative refractive index difference, &Dgr;, due to a clad layer containing a particular volume of non-periodic pores. This volume is controlled by controlling the air filling fraction in the fiber as is described below.
However, neither of these designs provide for axial changes in relative refractive index. Such axial changes are advantageous in single mode fiber designs intended to provide for management of dispersion. In addition, because the axial changes in relative index are due to changes in the clad layer, a new set of parameters, such as, pore volume, pore cross section, and pore pattern, are available to alter mode power distribution in the waveguide and thus alter key waveguide fiber properties. Combinations of axial changes in clad structure with the numerous core index profile designs are contemplated which will provide unique waveguide fiber properties. Clad layers which incorporate both photonic crystal light guiding and refractive light guiding are contemplated as advantageous in waveguide designs for dispersion management. In addition the present invention incorporates clad layer structures which contain an array of features, periodic or randomly distributed, comprising a material in place of the pores, which adds still further flexibility in waveguide fiber design.
SUMMARY OF THE INVENTION
The present novel waveguide preform and fiber and method of making the waveguide preform and fiber provide extra waveguide design variables and are advantageous in the making of dispersion compensating or dispersion controlled waveguides.
A first aspect of the invention is an optical waveguide fiber preform comprising a core glass region and a clad glass layer disposed upon the core glass. For convenience of description, the clad glass layer is said to be divided into segments that lie along the preform axis. The density of the clad glass changes in a direction, which is called the preform axis, parallel to the core region such that the clad glass density changes from segment to segment from a higher to a lower or from a lower to higher value. That is, the respective adjacent segment densities are not a monotone function of axial position.
The preform clad layer density can be made to alternate from high to low and low to high in adjacent segments by changing the porosity of the clad layer. In particular, respective adjacent segments along the preform axis could alternate between a condition in which the clad layer contains pores and a condition in which the clad layer is essentially free of pores. In an embodiment of the novel preform, the pores are elongated and arranged in a periodic array which can have pitch, i.e., a spacing between corresponding points in the pores. The pitch may be selected to lie in a number of different ranges. For use at optical telecommunication wavelengths the preform pitch is advantageously selected such that in the fiber drawn from the preform the pitch is in the range of about 0.4 &mgr;m to 20 &mgr;m. A typical outside diameter of the glass fiber is about 125 &mgr;m. The low end of this range provides a pitch in the drawn fiber effective to form a photonic crystal in the range of telecommunications signal wavelengths. However, applicants have verified that spacing or pitch in the range of tens of microns can advantageously be used in the making of a waveguide having an axially varying clad. Although an upper limit of 20 &mgr;m is set forth here, applicants contemplate the usefulness of still larger clad layer feature pitch. The upper limit of feature spacing or pitch is in fact a practical limit determined from the clad layer thickness.
Applicant has found that the diameter of the elongated pores as well as their pitch is important in determining the properties of the waveguide fiber drawn from the preform. In a particular embodiment, the ratio of the pore diameter to the pitch of the array of elongated pores is in the range of about 0.1 to 0.9.
The core glass of the preform may have a wide range of refractive index profiles. A refractive index profile of a region is the value of refractive index, or relative refractive index, &Dgr;, as a function of radial position across the region. The definitions of refractive index profile, segmented profile, &Dgr;, and &agr;-profile are known in the art and may be found in U.S. Pat. No. 5,553,185, Antos et al. or U.S. Pat. No. 5,748,824, Smith, which are incorporated herein by reference. Thus the core region of the preform may have a step shape, a trapezoidal shape, either of which may be rounded at sharp changes in slope, or an &agr;-profile shape. Further, the core region may be segmented into two or more portions and each of the portions may take on the alternative profiles set forth above. The design of this core region in conjunction with clad layer modulation determines the dispersion properties and other performance characteristics of the waveguide fiber.
The refractive index of a base glass material, such as silica, can be changed by incorporating dopants such as germania, alumina, phosphorus, titania, boron, fluorine and the like. Rare earth dopants such as erbium, ytterbium, neodymium, thulium, or praseodymium may also be added to provide a preform, which can be drawn into an optical amplifier waveguide fiber.
In another embodiment of the novel preform, the clad density toggles between two values from segment to segment along the preform axis. This toggling, together with the pre-selected core structure, determines the dispersion management characteristics of the fiber, as set forth above. Here again, the density may be controlled by controlling porosity volume in the clad layer segments. As an alternative, the density may be controlled by controlling the volume of a dopant glass added to the base clad layer glass. The dopant glass can appear as elongated filaments in the base glass of the clad. These filaments may be arranged in a periodic array in analogy to the arrangement of the elongated pores discussed above. One may speak of the filaments as being filled elongated pores, although it is to be understood that the filaments may be formed using several processes known in t
Fajardo James Conrad
Granger Gary Paul
Chervenak William J.
Corning Incorporated
Kim Ellen
Short Svetlana Z.
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