Making Optical Fiber

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    At the Core there are two main types of optical fiber: single-modeand multimode. Both types of fiber are composed ofonly two basic concentric glass structures: the core,which carries the light signals, and the cladding, whichtraps the light in the core

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    At the Core

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    At the Core Single-mode fiber has a smaller core -- only 9 microns in diameter -

    and only 6 times the wavelength of light it transmits. The small coresize limits the transmitted light in the fiber to only one principle mode,

    which minimizes dispersion of light pulses, increasing the distance thesignal can be sent.

    Multimode fiber has a large core diameter compared to the wavelengthof the transmitted light 50 or 62.5 microns. Therefore, multimode fiberpropagates more than one mode of light. With its relatively large core,multimode fiber suffers more dispersion than singlemode. Using agraded index core, where layers of light have lower index of refraction

    as you go further from the center of the core, minimizes dispersion butcomplicates the manufacturing process.

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    There are two main steps in the process ofconverting raw materials into optical fiber readyto be shipped:

    1. manufacturing of the pure glass preform and2. drawing of the preform

    Note:

    Preform -a piece of glass used to draw an optical fiber

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    Manufacturing The Preform The first step in manufacturing glass optical fibers is to make a solidglass rod, known as a preform. Ultra-pure chemicals -- primarily silicontetrachloride (SiCl4) and germanium tetrachloride (GeCl4) -- areconverted into glass during preform manufacturing. These chemicalsare used in varying proportions to fabricate the core regions for thedifferent types of preforms.

    The basic chemical reaction of manufacturing optical glass is:

    SiCl4 (gas) + O2 > SiO2 (solid) + 2Cl2 (in the presence of heat)

    GeCl4 (gas) + O2 > GeO2 (solid) + 2Cl2 (in the presence of heat)

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    Manufacturing The Preform The core composition of all standard communication fibers consists

    primarily of silica, with varying amounts of germania added to increasethe fiber's refractive index to the desired level. Single-mode fiberstypically have only small amounts of germania and have a uniformcomposition within the core. Multimode fibers typically have a muchhigher refractive index, and therefore much higher germania content.

    Also, the core composition and the refractive index of graded-indexmultimode fibers changes across the core of the fiber to give therefractive index a parabolic shape.

    There are several methods used to manufacture preforms. In theModified Chemical Vapor Deposition (MCVD) process, the highlycontrolled mixture of chemicals described above is passed through theinside of a rotating glass tube made of pure synthetic SiO2.

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    Manufacturing The Preform

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    Manufacturing The Preform The pure silica tube is mounted on a lathe

    equipped with a special heat torch. As the gassesflow inside the tube, they react to the heat by

    forming solid submicron particles, called "soot," inthe vicinity of the heat zone. Once the soot isformed, it is deposited on the inner wall of thetube. As the burner traverses over the depositedsoot, the heat transforms these solid white

    particles into pure, transparent glass, in a processcalled vitrification. The deposited material willform the core region of the optical fiber

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    Manufacturing The Preform

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    Manufacturing The Preform The process is repeated for many hours as each subsequent core layer is

    formed. For every sweep of the burner, the manufacturer can modifythe composition, viscosity and thickness of the deposited layer in orderto produce specific fiber designs. This, in conjunction with the abilityto change the speed at which the burner moves and the temperature ofthe flame, gives us the ability to manufacture a wide range of ultra-pureoptical fiber types.

    The precise delivery of chemicals is ensured through the design ofmanufacturing equipment that performs accurately and consistently.For example, mass f low controllers are used to meter chemicals for the

    reaction. All critical process parameters are monitored and controlledin real time by on-line computers that are also programmed to collectall pertinent process data.

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    Manufacturing The Preform After the desired amount of core material is deposited the chemical

    flow is eliminated, the speed of the torch is decreased and thetemperature of the flame is increased so that the tube collapses into asolid rod. At the end of this process, an operator separates the preformfrom the rest of the glass assembly and moves it to the next step. Theentire preform manufacturing process is highly automated withminimal human involvement.

    The refractive index of the manufactured preform is measured using apreform analyzer. This instrument measures critical core parametersand compares them to internal specifications. All pertinent parameters

    are plotted automatically on statistical control charts that the operatorsreview for every measurement.

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    Drawing The Fiber The next step in the process of producing optical fibers isto convert the manufactured preform into a hair-thin fiber.This is done in an operation called fiber draw. The tip of the

    preform is lowered into a high-purity graphite furnace.Pure gasses are injected into the furnace to provide a cleanand conductive atmosphere. In the furnace, tightlycontrolled temperatures approaching 1900C soften the tipof the preform. Once the softening point of the preform tip

    is reached, gravity takes over and allows a molten gob to"free fall" until it has been stretched into a thin strand.

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    DrawingThe Fiber

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    Drawing The Fiber

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    Drawing The Fiber The operator threads this strand of fiber through a

    series of coating dies, and the drawing process

    begins. The fiber is pulled by a tractor belt situatedat the bottom of the draw tower and then woundon winding drums. During the draw, the preform isheated at the optimum temperature to achieve an

    ideal drawing tension. Draw speeds of 10 - 20meters per second are not uncommon in theindustry.

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    Drawing The Fiber During the draw process the diameter of the drawn fiber is

    controlled to 125 microns within a tolerance of only 1micron. A laser-based diameter gauge is used to monitorthe diameter of the fiber. The gauge can sample the

    diameter of the fiber at rates in excess of 750 times persecond. The actual value of the diameter is compared to the125 micron target. Slight deviations from the target areconverted to changes in draw speeds and fed to the tractormechanism for correction. If the diameter of the fiber

    increases above its target, the drawing speed is increased; ifthe fiber diameter starts falling below the target, the drawspeed is decreased. Typically, 125 micron diameter controlis achieved within a six sigma interval of the 1 microntolerance.

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    Drawing The FiberA two layer protective coating is then applied to

    the fiber -- a soft inner coating and a hard outer

    coating. This two-part protective jacket providesmechanical protection for handling while alsoprotecting the pristine surface of the fiber fromharsh environments. These coatings are cured by

    ultraviolet lamps. The drawing process is wellautomated and requires virtually no operatorinteraction after the threading step.

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    Test and Measurement The drawn fiber is next

    tested, where alloptical and

    geometricalparameters arechecked to ensure thatthey meet stringent

    requirements.

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    Test and Measurement First, the tensile strength of the fiber is tested.

    Each spool of drawn fiber is wound through aseries of capstans and subjected to loads to ensure

    that the fiber has the minimal tensile strengthspecified. The fiber is then spooled onto shippingreels and cut to specified lengths.

    The fiber is tested for point defects with an OpticalTime Domain Reflectometer (OTDR), which usesscattered light to indicate the location of anyanomalies along the length of the fiber.

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    Test and Measurement The spooled fiber is automatically tested for transmission

    paramters including:

    attenuation: decrease in signal strength over distance

    bandwidth: information-carrying capacity; an importantmeasurement for multimode fiber numerical aperture: the measurement of the light acceptance angle

    of a fiber cut-off wavelength: in single-mode fiber the wavelength above

    which only a single mode propagates mode field diameter: in single-mode fiber the radial width of the

    light pulse in the fiber; impor