Modern telecommunications systems rely on fiber optic cables for transferring optical data signals over significant distances with low loss and minimal attenuation. Conventional fiber optic cables include one or more optical fibers surrounded radially by a protective buffer, a strengthening layer, and an outer sheath or jacket. Each optical fiber consists of a cylindrical core covered by an annular cladding. The core is the light carrying element or waveguide of the optical fiber that transports the optical data signals as light pulses from a light source to a receiving device. The core typically comprises a strand of a high-purity silica glass doped to provide a relatively high index of refraction. The cladding likewise consists of high-purity silica having a relatively low index of refraction, which promotes total internal reflection of light at the cylindrical interface with the core. The buffer, formed of an acrylate, a polyamide or a like polymer, is a protective layer that encases the cladding. Surrounding the buffer is an annular layer of strengthening material, which prevents elongation when a tensile force is applied to the fiber optic cable. The outer jacket protects the inner layers against abrasion and the infiltration of solvents and other contaminants.
Stripped optical fibers are used in various applications including hermetic sealing, pigtailing laser diodes, fiber arrays, fiber Bragg gratings, and amplifier seeding. Fiber Bragg gratings, for example, are widely used in the fabrication of various functional devices for wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM). A fiber Bragg grating is created inside the core, for example, using ultraviolet radiation to inscribe, write or project the lines of the grating. Before forming the fiber Bragg grating, all coatings must be stripped from the fiber optic cable so that the outer surface of the cladding or core is exposed over a length on the order of a few centimeters. New generations of devices will scale the length of the stripped region downward to sub-centimeter proportions and, eventually, to sub-millimeter proportions.
Although the outer jacket and strengthening layer are relatively simple to excise from the buffer, the buffer cannot be easily removed from the cladding. Mechanical stripping, chemical stripping, thermo-mechanical stripping and laser stripping are among the conventional methods used to remove the buffer from the cladding of the optical fiber and are individually described below. These conventional buffer removal methods are generally not effective and, at the least, are not efficient for stripping the cladding from the outer surface of the core after the buffer is removed.
Various deficiencies of mechanical stripping limit its usefulness for stripping optical fibers. Mechanical stripping is a manual procedure that restricts productivity because the optical fibers must be processed individually, not in batches. Mechanical stripping cannot taper the cladding thickness at the peripheries of the stripped region or transition zones. Moreover, mechanical stripping from latent defect and reliability issues in that the glass of the optical fiber may be scratched or nicked, which reduces fiber strength and splice strength. Because mechanical stripping is performed manually, the stripping is not reproducible between optical fibers in a single batch and among various batches of optical fibers. In addition, mechanical stripping is best suited for end stripping and is not effective for mid-span stripping.
Chemical stripping removes the buffer using an aggressive etchant such as hot sulfuric acid. The fiber optic cable is bent into a loop and dipped into the etchant. Chemical stripping fails in many regards in its ability to strip the buffer from optical fibers effectively and efficiently. First, the stripped length of the buffer cannot be precisely controlled during the etching process. Second, the minimum bend radius, about 15 millimeters, of the optical fiber controls the minimum length of the buffer than can be removed by immersion in an etchant bath. Third, the thickness of the buffer cannot be tapered at the transition zones of the stripped span. Fourth, the etchant may wick between the buffer and cladding at the peripheries. As a result, the optical fiber itself may be attacked by the residual etchant after the buffer is removed from the etchant bath, which results in a reduced tensile strength. Finally, the end point of the wet chemical process may be difficult to detect because the optical fiber is immersed in the etchant bath.
Thermo-mechanical stripping heats the fiber optic cable to soften the buffer and uses a blade to scrape the buffer from the exterior of the cladding. However, stripping fiber optical cables thermomechanically has several significant drawbacks. Although mid-span stripping is possible, thermo-mechanical stripping cannot taper portions of the buffer in the transition zones and may actually damage those portions. Moreover, polyamide buffers are especially difficult to remove by thermo-mechanical stripping. Finally, the manual process used to perform thermo-mechanical stripping lacks reproducibility.
Finally, laser stripping uses an ultraviolet laser to strip the buffer from the optical fiber. A primary deficiency of laser stripping is that the transition zones of the buffer are not tapered for mid-span stripping. Moreover, laser stripping is a relatively slow process that makes large-scale optical fiber stripping operations both time consuming and commercially impractical.
To remedy these deficiencies of conventional stripping, the present invention provides an apparatus and method for efficiently and effectively removing the buffer and, optionally, the cladding from a fiber optic cable.