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Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/26—Optical coupling means
  • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
  • G02B6/2804—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers
  • G02B6/2821—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers using lateral coupling between contiguous fibres to split or combine optical signals
  • G02B6/2843—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers using lateral coupling between contiguous fibres to split or combine optical signals the couplers having polarisation maintaining or holding properties
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/245—Removing protective coverings of light guides before coupling
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/24—Coupling light guides
  • G02B6/26—Optical coupling means
  • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
  • G02B6/2804—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers
  • G02B6/2821—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers using lateral coupling between contiguous fibres to split or combine optical signals
  • G02B6/2835—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers using lateral coupling between contiguous fibres to split or combine optical signals formed or shaped by thermal treatment, e.g. couplers

Definitions

• the present invention relates generally to optical waveguide couplers and more specifically to optical couplers using single-mode optical fibers that are resistant to polarization changes.
• Fiber optic couplers are commonly used to transfer light from one optical waveguide to another for a variety of functions.
• fiber optic couplers are used in the communications field to rapidly convey .[.larger.]. .Iadd.large .Iaddend.amounts of data.
• Other applications include measurement systems in which high-precision measurements are made based upon an environmentally dependent shift in either the phase or wavelength of light traveling through an optical waveguide.
• Early couplers used for measurement applications had several disadvantages. For instance, the coupling ratio of early couplers typically varied with the polarization of the light passing through the coupler. Furthermore in some cases, the phase of the light passing through the coupler varied with polarization which was unsatisfactory for certain interferometric sensor .[.aplications.]. .Iadd.applications .Iaddend.such as the passive quadrature demodulator coupler matrix.
• Multimode optical fibers are fairly large fibers, having a core diameter on the order of 40 microns to 200 microns. Some multimode fibers may be as large as 4 millimeters. While multimode fibers can be used to produce couplers for transferring polarized light from one fiber to another, they are typically undesirable because polarized light transmitted in a multimode optical fiber has a tendency to become depolarized. Single-mode fibers, on the other hand, are more capable of transmitting polarized light with minimal depolarization of light during transmission over small distances.
• the core diameter of a single-mode fiber is approximately ten times smaller than the core diameter of a multimode fiber. This had led to serious difficulties in manufacturing couplers using single-mode fibers.
• the invention described hereinbelow represents an advancement in the art in that it teaches methods for coupling single-mode fibers and for producing polarization-insensitive optical couplers.
• a fiber comprises an inner core having a relatively high index of refraction, a cladding having a relatively low index of refraction, and a substrate having a relatively high index of refraction, but not necessarily equal to the index of refraction of the core.
• This type of fiber is commonly called "W-fiber” by those skilled in the art becamse the index profile of the fiber appears to resemble a "W”. It should be noted that other types of fibers do not have an outer substrate having a relatively high index of refraction.
• the subject development comprehends a polarization-insensitive optical device for coupling light which includes a first single-mode optical fiber having a first longitudinal segment having an inner core having a relatively high index and a cladding having a relatively low index of refraction.
• the optical device also includes a second single-mode optical fiber having a second longitudinal segment having an inner core having a relatively high index of refraction and a cladding having a relatively low index of refraction.
• a coupling region is included in the subject device having the first and second longitudinal portions fused together in parallel juxtaposition to one another.
• the coupler further includes a housing means for maintaining the first and second longitudinal portions substantially straight and for maintaining the first and second longitudinal portions in a stable environment.
• the invention further comprehends a method of making polarization-insensitive optical couplers comprising the steps of exposing first and second longitudinal cladding segments of first and second single-mode optical fibers; maintaining the optical fibers in linear parallel juxtaposition with one another along a portion of the exposed longitudinal segments; fusing the parallel juxtaposition segments of the fibers together to form a coupling region, and rigidly encapsulating the coupling region of the optical fibers while maintaining the linear parallel juxtaposition of the fibers.
• an object of the invention described herein is to provide a polarization-insensitive coupler which is simple to fabricate using single-mode optical fibers.
• FIG. 1 is a frontal view of one embodiment of the coupler described herein indicating the coupling region
• FIG. 2 is a frontal view of one embodiment of the coupler described herein illustrating a quartz housing in which the housing is broken away to provide a view of the coupling region;
• FIG. 3 is an illustration partially in phantom of one embodiment of the subject coupler after encapsulation.
• FIG. 4 is an isometric illustration of one embodiment of the equipment used to fabricate the couplers described herein.
• first and second single-mode optical fibers 12, 14 are provided in the preferred embodiment of the subject development.
• Each optical fiber 12, 14 has a respective substrate 16, 18 and core and cladding 20, 22.
• the diameter of the core of each single-mode fiber is on the order of 5 microns whereas the diameter of the cladding of each fiber is on the order of 75-125 microns.
• each fiber 12, 14 has a longitudinal segment having a core and cladding diameter 20, 22 of approximately 15-50 microns. It should be obvious to one skilled in the art that the length of the longitudinal segment can be varied greatly without deviating from the intent of the subject invention. In the preferred embodiment, the exposed core length is approximately one centimeter in order to ensure complete coupling between the first optical fiber 12 and the second optical fiber 14.
• the longitudinal segments 20, 22 are fused together in coupling region 24. The invention described herein comprehends that throughout the coupling region, the first and second fibers 12 and 14 have cores which are maintained in linear parallel juxtaposition with one another.
• the fused fiber 12, 14 of FIG. 1 are illustrated in a rigid housing 26.
• the method of placing the fused fibers within the housing will be discussed in greater detail hereinbelow. It is important to note, at this point however, that the housing provides a rigid support to fused fibers 12 and 14 throughout coupling region 24.
• the entire assembly is encapsulated in a rubber-like material 28.
• Encapsulation of the assembly provides a further means for stabilizing the subject device.
• the encapsulation material may provide strain relief to the ends of the fibers which extend beyond the housing because the housing may be encapsulated by a dipping technique in which the ends of the fibers are also coated with the encapsulation material.
• two single-mode fibers are prepared.
• a variety of single-mode fibers may be used, depending on the wavelength of operation of the coupler.
• Each coupler would be designed to be laser specific, that is, the fiber used for one wavelength of light may be made from a different material, and have a different core diameter and index of refraction than a coupler designed for another wavelength of operation.
• Important characteristics of any fiber chosen include: good concentricity, excellent core uniformity, and an appropriate refractive index profile. While various lengths of optical fiber may be used, the present procedure in the laboratory is to use two lengths of optical fiber that are 50 centimeters long.
• Each fiber has a plastic jacket and an RTV (room temperature vulcanizing .[.silicone.]. .Iadd.silicon.Iaddend.) coating.
• the plastic jacket is cut away and any RTV is removed with a hydrofluoric acid rinse.
• each fiber 12 and 14 is placed in respective groves 30-33 of clamping devices 36, 38.
• the fibers are then cleaned with a hydrofluoric acid/water/alcohol rinse according to techniques known to those skilled in the art.
• etching of the fibers is initiated. It is the object of the etching portion of the procedure to remove the substrate 16 and 18 of each fiber 12 and 14 (FIG. 1) to expose core and cladding 20 and 22 along a coupling region 24 of each of the fibers so as to provide an appropriate index profile.
• An appropriate index profile is one in which the outer surface, or cladding, of each fiber has a lower index of refraction than the core of each fiber.
• etching techniques may be used, in view of the particular fiber used in the preferred embodiment, it is desirable to etch the subject fibers by a heated etching technique.
• the fibers are placed in close proximity to an etching station which is heated by a thermoelectric module.
• a drop of etchant is placd on top of the etching station to etch a longitudinal portion of the fiber.
• the fibers are then rinsed with water to prevent further etching.
• the very fragile etched longitudinal segments 20, 22 are then brought in parallel juxtaposition with one another by wrapping opposite ends of each exposed core length to one another with a thread or other wrapping material 42, 44.
• a thin plastic sheet is desirable to prevent breakage of the fragile cores 20, 22.
• a small drop of adhesive material is deposited on each end 46, 48 of the exposed core length adjacent to threads 42, 44. The glue is allowed to cure so that the cores 20, 22 remain in parallel juxtaposition with one another throughout coupling region 24 when threads 42 and 44 are removed from the exposed cores 20, 22.
• the coupling region of the fibers 12, 14 are then heated.
• a lighted torch is simply passed along the coupling region 24 while the longitudinal segments are in axial tension until the segments 20, 22 are fused together throughout the length of the coupling region.
• a quartz tube 26 is then measured and cut to the desired length to surround the exposed cores of the subject coupler.
• a slot extending the length of the quartz tube 26 may be provided to allow insertion of the tube about fibers 12, 14.
• the tube may be cut in half longitudinally to allow each half to be simply placed about the fibers.
• the ends of the fibers 12, 14 are then glued to the ends 81, 83 of the quartz tube to suspend the etched and fused segments of the fibers containing the coupling region 24 in the approximate center of the tube 26.
• the glued assembly is then allowed to cure as necessary.
• the quartz tube is then dipped in a material to provide an elastic covering over tube 26.
• the invention described herein is insensitive to changes in polarization due to its unique design.
• birefringence resulting from twists, bends, stresses, or strains imposed on the fibers will cause changes in polarity of light transmitted through the fibers.

Abstract

Description

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Cited By (21)

Citations (37)

• 1988
  • 1988-07-22 US US07/222,908 patent/USRE33296E/en not_active Expired - Lifetime

Patent Citations (42)

Non-Patent Citations (50)

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