Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
2001-05-10
2003-04-15
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S037000, C385S054000, C385S124000, C385S128000, C385S134000, C427S059000, C427S059000
Reexamination Certificate
active
06549712
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for removing a polymeric covering from the surface of an optical fiber followed by applying a protective recoating composition after modifying the structure of the optical fiber. More particularly the present invention provides a process including physical and chemical steps to remove coating from a portion of a coated optical fiber while shaping transition regions of coating adjacent to the boundaries of a stripped portion of the optical fiber.
BACKGROUND OF THE INVENTION
Interconnection of fiber optic networks requires high precision devices in the form of optical connectors that join optical fibers to peripheral equipment and other optical fibers while maintaining adequate signal strength. In operation an optical connector centers the small fiber so that the light gathering core lies directly over and in alignment with a light transmitting source or another fiber. Sections of optical fiber may also be spliced together using mechanical splicing or fusion splicing techniques.
Special features may be built into selected, relatively short lengths of optical fibers to be spliced into fiber optic networks. An optical fiber Bragg grating represents a light-modifying feature that may be introduced or written into an optical fiber by exposure to ultraviolet light. The ability to write such gratings leads to a variety of devices. Bragg gratings may be applied in telecommunications systems, for example, to control the wavelength of laser light, and to introduce dispersion compensation. Fiber optic applications of fiber Bragg gratings, outside of telecommunications, include spectroscopy and remote sensing.
The process of introducing special features such as Bragg gratings into an optical fiber may include a number of steps requiring handling of relatively short lengths of optical fiber during a series of manufacturing operations. An optical fiber typically requires removal of protective coatings before changing the physical characteristics of the fiber to include a Bragg grating. One manufacturing process requires the removal of protective buffers and coatings to reveal the bare surface of an optical fiber. Several processes are known for removing protective layers, such as buffers and coatings, from the surface of optical fibers. They include mechanical stripping, chemical stripping and thermal stripping.
Mechanical stripping of optical fibers and related coated filaments requires careful positioning of sharp tempered metal blades to expose a bare surface portion of a fiber without cutting or scratching or otherwise physically damaging the fiber surface. Known methods of mechanical stripping relate to cutting blade design and how a coating may be removed from the surface of a fiber. The predominant use of mechanical stripping involves the removal of protective layers from the ends of optical fibers, insulated wires and related filaments, prior to joining the filament ends together. U.S. Pat. No. 4,434,554 describes an optical fiber stripping device including a flat base having a number of fiber receiving channels of suitable depth to ensure only removal of a buffer coating from each fiber, when a blade penetrates the coating. The blade moves parallel to the axis of a fiber or group of fibers using a paring action to remove protective material. Channel size, based upon fiber diameter determines the selection of a flat base to provide a device that strips a fiber end without damaging the fiber itself.
One way to avoid damage to the bare surface of an optical fiber requires the use of blades designed to penetrate the protective buffer or fiber coating without reaching the fiber surface. Suitable blades have a substantially semicircular sharpened edge of a radius slightly larger than the radius of the bare optical fiber. Two opposing blades, penetrating the protective layer around the fiber, interfere with each other before the cutting edges reach the fiber surface. After penetrating a protective layer, close to the end of a fiber, movement of the blades parallel to the fiber axis displaces a section of the layer to provide a bare fiber end untouched by the blades.
Japanese patent JP 875930 uses a mechanical stripping process to remove coating from a section of optical fiber. Initially, an angled cutting blade rotates about two separated points to form notches in the circumference of the buffer coating over the optical fiber. A separate straightedge blade then moves parallel to the fiber axis to remove coating material from between the sharply angled notches.
U.S. Pat. Nos. 4,630,406, 5,269,206, 5,481,638, 5,684,910, and 5,819,602 describe the manufacture and design of blades for cutting insulation from e.g. insulated electrical wires and optical fibers. Successful mechanical stripping using such blades may require additional treatments, including softening the protective layer as in U.S. Pat. No. 5,481,638 requiring a chemical filled chamber first to soften an encapsulating layer then to clean plastic material from the blades after stripping. U.S. Pat. No. 5,684,910 teaches an optical fiber having improved mechanical strippability. The improvement includes the use of a frangible boundary layer between a fiber coating and a buffer to facilitate separation from the bare fiber. Previous teachings include initial blade movement perpendicular to a filament axis, to penetrate a coating, followed by movement parallel to the filament axis to expose bare filament ends by displacement of protective layers.
Chemical stripping may be used as an alternative to mechanical stripping for preparing bare fiber ends. U.S. Pat. Nos. 4,865,411 and 4,976,596 deal with controlled removal of coating, by gradual withdrawal of a coated fiber from a chemical bath, to produce a contoured shallow taper adjacent to the bare glass fiber surface. A fixture, according to U.S. Pat. No. 5,451,294 provides support while dipping the end of a coated optical fiber into a chemical bath to dissolve coating from the end. Organic solvents and related softening agents may be used to remove coatings from optical fibers as described in U.S. Pat. Nos. 5,567,219, 5,681,417, 5,714,196, and 5,896,787. Chemical stripping methods include common problems related to the handling of chemicals especially, when the chemical strippers involve corrosive liquids.
Stripping by rapid heating may be used instead of mechanical or chemical stripping. One example of this process, described in U.S. Pat. No. 6,123,801, uses a hot inert gas to melt buffer coating and blow it from the surface of an optical fiber. The process requires high pressure gas streams and temperatures in the region of 800° C. to strip coating from the fiber. U.S. Pat. No. 5,939,136 describes a process for preparing optical fiber devices including thermal removal of a coating from an optical fiber, preferably performed using a heated gaseous stream. U.S. Pat. Nos. 5,964,957 and 5,968,283 further describe the use of heat to remove coatings from optical fibers.
A reason for removing protective buffers and related coatings from an inner section of optical fibers is the need to change the characteristics of the fiber such as by writing of a refractive index grating, also known as a Bragg grating, in the core of an optical fiber. Refractive index changes occur during exposure of a bare fiber to radiation from an ultraviolet laser or similar exposure device. The majority of protective coatings for optical fibers absorb the fiber modifying radiation. This explains the need to remove the coatings before writing a refractive index grating. Fibers coated to a thickness exceeding 400 &mgr;m and those having silicone containing coatings respond poorly to mechanical stripping and chemical stripping as methods for removing optical fiber coatings.
Chemical stripping using hot concentrated sulfuric acid does not always displace optical fiber coatings as expected. Thick, silicone-containing coatings, in particular, may react poorly in the presence of hot sulfuric acid. Some coatings may not dissolve cleanly, or may tend to form gelatinous st
Abe Shinichi
Cowher John T.
Dower William V.
Gatica Anthony W.
Kordecki Jason D.
3M Innovative Properties Company
Ball Alan
Sanghavi Hemang
Wong Eric K.
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