Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2001-04-18
2002-12-17
Ullah, Akm E. (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Reexamination Certificate
active
06496632
ABSTRACT:
This application claims the benefit of U.S. provisional application No. 60/094,609, filed Jul. 30, 1998.
BACKGROUND OF THE INVENTION
The invention relates to a method of making photonic crystals and passive components comprising photonic crystals. In particular, the method includes one or more extrusion steps to produce a cellular or channeled object followed by a step of viscously sintering the object. The sintered, channeled object is heated and drawn to a final diameter.
A photonic crystal is a structure having a periodic variation in dielectric constant. The periodic structure may be 1, 2 or 3 dimensional. The photonic crystal allows passage of certain light wavelengths and prevents passage of certain other light wavelengths. Thus the photonic crystals are said to have allowed light wavelength bands and band gaps which define the wavelength bands which are excluded from the crystal.
At present, the wavelengths of interest for telecommunication applications are in the range of about 800 nm to 1800 nm. Of particular interest is the wavelength band in the range of about 1300 nm to 1600 nm.
Light having a wavelength in the band gap may not pass through the photonic crystal. Light having a wavelength in bands above and below the band gap may propagate through the crystal. A photonic crystal exhibits a set of band gaps which are analogous to the solutions of the Bragg scattering equation. The band gaps are determined by the pattern and period of the variation in dielectric constant. Thus the periodic array of variation in dielectric constant acts as a Bragg scatterer of light of certain wavelengths in analogy with the Bragg scattering of x-rays wavelengths by atoms in a lattice.
Introducing defects into the periodic variation of the photonic crystal dielectric constant can alter allowed or non-allowed light wavelengths which can propagate in the crystal. Light which cannot propagate in the photonic crystal but can propagate in the defect region will be trapped in the defect region. Thus, a point defect within the crystal can serve as a localized “light cavity”. Analogously, a line defect in the photonic crystal can act as a waveguide for a mode having a wavelength in the band gap, the crystal lattice serving to confine the guided light to the defect line in the crystal. A particular line defect in a three dimensional photonic crystal would act as a waveguide channel, for light wavelengths in the band gap. A review of the structure and function of photonic crystals is found in, “Photonic Crystals: putting a new twist on light”,
Nature,
vol. 386, Mar. 13, 1997, pp. 143-149, Joannopoulos et al.
A first order band gap phenomenon is observed when the period of the variation in dielectric constant is of the order of the light wavelength which is to undergo Bragg scattering. Thus, for the wavelengths of interest, i.e., in the range of about 1300 nm to 1600 nm, as set forth above, a first order band gap is achieved when the period of the variation is about 500 nm. However, photonic crystal effects can occur in crystals having dielectric periodicity in the range of about 0.1 &mgr;m to 5 &mgr;m. A two or three dimensional photonic crystal having even this larger spacial periodicity is difficult to fabricate.
In U.S. Pat. No. 5,774,779, Tuchinskiy, a method of making multi-channeled structures is described. Rods are bundled together and reduced in diameter by extrusion. The step of bundling and extrusion may be repeated using rods which have already been extruded one or more times. However, no step of drawing is disclosed, so that channel density, expressed as number of channels per unit area, is not large enough to produce a photonic crystal.
There is a need for a method of making photonic crystals of two or three dimensions which is repeatable, versatile, and potentially adaptable to a manufacturing environment, as compared to that of a laboratory.
SUMMARY OF THE INVENTION
The primary object of the invention is to combine extrusion technology, including the technology of powder extrusion, with glass drawing technology to address the problem of fabricating photonic crystals of all types. The term drawing describes a process in which a viscous body of material is stretched along a pre-selected dimension. To stretch the viscous body without causing tears in the body, the viscosity of the body and drawing tension applied to the body are properly adjusted. The viscosity of the body may be controlled by controlling the temperature of the body. A first aspect of the invention is a method of making a photonic crystal having a band gap. A material comprising at least one glass powder and a binder is extruded through a die to form a body having a first and a second face spaced apart from each other, each face having a plurality of openings. The respective openings in each face are the ends of channels, which extend along the dimension between the two faces.
Suitable glass powders for making the crystal include Pyrex™ and substantially pure silica powder. The extruded body is then heated to drive off the binder at a first temperature and further heated to a higher second temperature to viscously sinter the particulate of the glass powder to form a sintered, extruded glass body. This sintered glass body is further heated and drawn, along the dimension between the two faces, to reduce the diameter of the channels extending between the two channels. The drawn body is referred to as a glass rod or glass fiber having a plurality of channels which extend along the long axis of the fiber or rod. The drawing temperature is typically higher than the sintering temperature, although for certain glass compositions and drawing tensions the drawing temperature may be lower than the sintering temperature.
An optional series of steps may be used if, after extrusion, the body is too large to be accommodated in a drawing furnace. That is, the cross sectional area, taken perpendicular to the dimension between the two faces, of the body and thus the size of the plurality of channels may be reduced by:
filling the channels with a pliable material;
passing the body, in a direction along the channels, through one or a series of reducing dies; and,
removing the pliable material.
This pliable material, which may be a micro-crystalline wax as set forth in Provisional Application No. 60/068230, serves to maintain the channels as the body is passed through one or a series of reducing dies. A reducing die may take the form of a funnel with an entrance opening of dimension commensurate with the cross sectional dimension of the body and an exit opening reduced in size by a factor of 2 or more relative to the entrance opening. After the reducing step, the pliable material is removed
In order for the channeled glass fiber to function as a photonic crystal, the array of channel openings is distributed periodically across the faces of the fiber. For the wavelengths of particular interest at this time in telecommunications, the period of the array of the final drawn fiber or rod is in the range of about 0.4 &mgr;m to 5 &mgr;m. The novel method disclosed and described herein can produce arrays having periods less than 40 &mgr;m, preferably less than 5 &mgr;m and most preferably less than 1 &mgr;m.
Also, the dielectric constant of the channels must be different from that of the material forming the walls of the channels by a factor of about 3 to provide a useful band gap. For example the channels may be filled with air or evacuated to provide the requisite difference in dielectric constant. As an alternative the channels could be filled with essentially any solid or fluid having the appropriate dielectric constant as compared to that of the glass body.
The required dimensions of a photonic crystal depend upon the intended use thereof. Of particular importance is the crystal area which will be illuminated by a beam of light incident upon the crystal which will propagate through the crystal or a defect in the crystal. The area of the beam may be characterized, for example, by the mode field diameter of the beam. For wavelengths that a
Borrelli Nicholas Francis
Wight, Jr. John Forrest
Wusirika Raja Rao
Chervenak William J.
Corning Incorporated
Suggs James V.
Ullah Akm E.
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