Optical Fiber Communications

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Optical Fiber Communications
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Fiber Optics

• Fiber optics uses light to send information
  (data).
• More formally, fiber optics is the branch of
  optical technology concerned with the
  transmission of radiant power (light energy)
  through fibers.
• Light frequencies used in fiber optic systems are
  100,000 to 400,000 GHz.

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Brief History of Fiber Optics

• In 1880, Alexander Graham Bell experimented with
  an apparatus he called a photophone.
• The photophone was a device constructed from
  mirrors and selenium detectors that transmitted
  sound waves over a beam of light.

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In 1930, John Logie Baird, an English scientist
and Clarence W. Hansell, an American scientist,
was granted patents for scanning and transmitting
television images through uncoated cables.
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In 1951, Abraham C.S. van Heel of Holland and
Harold H. Hopkins and Narinder S. Kapany of
England experimented with light transmission
through bundles of fibers. Their studies led to
the development of the flexible fiberscope, which
used extensively in the medical field.
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In 1956, Kapany coined the termed fiber optics.
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In 1958, Charles H. Townes, an American, and
Arthur L. Schawlow, a Canadian, wrote a paper
describing how it was possible to use stimulated
emission for amplifying light waves (laser) as
well as microwaves (maser).
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In 1960, Theodore H. Maiman, a scientist built
the first optical maser.
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In 1967, Charles K. Kao and George A. Bockham
proposed using cladded fiber cables.
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FIBER OPTIC DATA LINKS

• To convert an electrical input signal to an
  optical signal
• To send the optical signal over an optical fiber
• To convert the optical signal back to an
  electrical signal

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Fiber Optic Data Link
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Fiber Optic Cable

• The cable consists of one or more glass fibers,
  which act as waveguides for the optical signal.
  Fiber optic cable is similar to electrical cable
  in its construction, but provides special
  protection for the optical fiber within. For
  systems requiring transmission over distances of
  many kilometers, or where two or more fiber optic
  cables must be joined together, an optical splice
  is commonly used.

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The Optical Receiver

• The receiver converts the optical signal back
  into a replica of the original electrical signal.
  The detector of the optical signal is either a
  PIN-type photodiode or avalanche-type photodiode.

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The Optical Transmitter

• The transmitter converts an electrical analog or
  digital signal into a corresponding optical
  signal. The source of the optical signal can be
  either a light emitting diode, or a solid- state
  laser diode. The most popular wavelengths of
  operation for optical transmitters are 850, 1300,
  or 1550 nanometers

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Types of Optical Fiber

• Plastic core and cladding
• Glass core with plastic cladding (PCS)
• Glass core and glass cladding (SCS)

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Modes of Propagation

• Single mode there is only one path for light to
  take down the cable
• Multimode if there is more than one path

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Index Profiles
A graphical representation of the value of the
refractive index across the fiber

• Step-index fiber it has a central core with a
  uniform refractive index. The core is surrounded
  by an outside cladding with a uniform refractive
  index less than that of the central core
• Grade-index fiber has no cladding, and the
  refractive index of the core is nonuniform it is
  highest at the center and decreases gradually
  toward the outer edge

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Optical Fiber Configuration

• Single-Mode Step-Index Fiber has a central core
  that is sufficiently small so that there is
  essentially one path that light takes as it
  propagates down the cable
• Multimode Step-Index Fiber similar to the
  single-mode configuration except that the core is
  much larger. This type of fiber has a large
  light-to-fiber aperture, and consequently, allows
  more light to enter the cable.
• Multimode Graded-Index it is characterized by a
  central core that has a refractive index that is
  non-uniform. Light is propagated down this type
  of fiber through refraction.

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Single-Mode Step-Index Fiber

• Advantages
• There is minimum dispersion. Because all rays
  propagating down the fiber take approximately the
  same path, they take approximately the same
  amount of time to travel down the cable.
• Because of the high accuracy in reproducing
  transmitted pulses at the receive end, larger
  bandwidths and higher information transmission
  rates are possible with single- mode step-index
  fibers than with other types of fiber.
• Disadvantages
• Because the central core is very small, it is
  difficult to couple light into and out of this
  type of fiber. The source-to-fiber aperture is
  the smallest of all the fiber types.
• A highly directive light source such as laser is
  required.
• It is expensive and difficult to manufacture.

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Multimode Step-Index Fiber

• Advantages
• Inexpensive and easy to manufacture.
• It is easy to couple light into and out they
  have a relatively high large source-to-fiber
  aperture.
•  
• Disadvantages
• Light rays take many different paths down the
  fiber, which results in large differences in
  their propagation times. Because of this, rays
  traveling down this type of fiber have a tendency
  to spread out.
• The bandwidth and rate of information transfer
  possible with this type of cable are less than
  the other types.

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Acceptance Angle Acceptance Cone

• The acceptance angle (or the acceptance cone half
  angle) defines the maximum angle in which
  external light rays may strike the air/fiber
  interface and still propagate down the fiber with
  a response that is no greater than 10 dB down
  from the peak value. Rotating the acceptance
  angle around the fiber axis describes the
  acceptance cone of the fiber input.

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Maximum Acceptance Angle
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Numerical Aperture

• For a step-index fiber NA Sin (Acceptance
  Angle)
• And NA
• For a Graded-Index NA sin (Critical Angle)
  
• The acceptance angle of a fiber is expressed in
  terms of numerical aperture. The numerical
  aperture (NA) is defined as the sine of one half
  of the acceptance angle of the fiber. It is a
  figure of merit that is used to describe the
  light-gathering or light-collecting ability of
  the optical fiber. The larger the magnitude of
  NA, the greater the amount of light accepted by
  the fiber from the external light source. Typical
  NA values are 0.1 to 0.4 which correspond to
  acceptance angles of 11 degrees to 46 degrees.
  Optical fibers will only transmit light that
  enters at an angle that is equal to or less than
  the acceptance angle for the particular fiber.

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Attenuation in Optical Fibers
L the length of fiber in kilometers Therefore
the unit of attenuation is expressed as dB/km
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Losses in the Optical Fiber

• Absorption Losses
• Material or Rayleigh Scattering Losses
• Chromatic or Wavelength Dispersion
• Radiation Losses
• Modal Dispersion
• Coupling Losses

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Absorption Losses

• Absorption loss in an optical fiber is analogous
  to power dissipation in copper cables impurities
  in the fiber absorb the light and convert it to
  heat.
• Absorption in optical fibers is explained by
  three factors
• Imperfections in the atomic structure of the
  fiber material
• The intrinsic or basic fiber-material properties
• The extrinsic (presence of impurities)
  fiber-material properties

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Absorption

• Essentially, there are three factors that
  contribute to the absorption losses in optical
  fibers
• ultraviolet absorption,
• infrared absorption,
• ion resonance absorption.

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Ultraviolet Absorption

• Is caused by valence electrons in the silica
  material from which fibers are manufactured.
• Light ionizes the valence electrons into
  conduction. The ionization is equivalent to a
  loss in the total light field and, consequently
  contributes to the transmission losses of the
  fiber.

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Infrared Absorption

• Is a result of photons of light that are absorbed
  by the atoms of the glass core molecules.
• The absorbed photons are converted to random
  mechanical vibrations typical of heating.

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Ion Resonance Absorption

• Is caused by OH- ions in the material.
• The source of the OH- ions is water molecules
  that have been trapped in the glass during the
  manufacturing process.
• Ion absorption is also caused by iron, copper,
  and chromium molecules.

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Material or Rayleigh Scattering Losses

• This type of losses in the fiber is caused by
  submicroscopic irregularities developed in the
  fiber during the manufacturing process.
• When light rays are propagating down a fiber
  strike one of these impurities, they are
  diffracted.
• Diffraction causes the light to disperse or
  spread out in many directions. Some of the
  diffracted light continues down the fiber and
  some of it escapes through the cladding.
• The light rays that escape represent a loss in
  the light power. This is called Rayleigh
  scattering loss.

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Chromatic or Wavelength Dispersion

• Chromatic dispersion is caused by light sources
  that emits light spontaneously such as the LED.
• Each wavelength within the composite light signal
  travels at a different velocity. Thus arriving at
  the receiver end at different times.
• This results in a distorted signal the
  distortion is called chromatic distortion.
• Chromatic distortion can be eliminated by using
  monochromatic light sources such as the injection
  laser diode (ILD).

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Radiation Losses

• Radiation losses are caused by small bends and
  kinks in the fiber.
• Essentially, there are two types of bends
• Microbends and constant-radius bends.
• Microbending occurs as a result of differences in
  the thermal contraction rates between the core
  and cladding material. A microbend represents a
  discontinuity in the fiber where Rayleigh
  scattering can occur.
• Constant-radius bends occur where fibers are bent
  during handling or installation.

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Modal Dispersion

• Modal dispersion or pulse spreading is caused by
  the difference in the propagation times of light
  rays that take different paths down a fiber.
• Obviously, modal dispersion can occur only in
  multimode fibers. It can be reduced considerably
  by using graded-index fibers and almost entirely
  eliminated by single-mode step-index fibers.

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Coupling Losses

• Coupling losses can occur in any of the following
  three types of optical junctions light
  source-to-fiber connections, fiber-to-fiber
  connections, and fiber-to-photodetector
  connections. Junction losses are most often
  caused by one of the following alignment
  problems lateral misalignment, gap misalignment,
  angular misalignment, and imperfect surface
  finishes.

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Coupling Losses
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Light Sources

• There are two devices commonly used to generate
  light for fiber optic communications systems
  light-emitting diodes (LEDs) and injection laser
  diodes (ILDs). Both devices have advantages and
  disadvantages and the selection of one device
  over the other is determined by system economic
  and performance requirements.

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Light-Emitting Diode (LED)

• Simply a P-N junction diode
• Made from a semiconductor material such as
  aluminum-gallium arsenide (AlGaAs) or
  gallium-arsenide-phosphide (GaAsP)
• Emits light by spontaneous emission light is
  emitted as a result of the recombination of
  electrons and holes

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Light-Emitting Diode (LED)

• The simplest LED structures are homojunction,
  epitaxially grown, or single-diffused devices.
• Epitaxially grown LEDs are generally constructed
  of silicon-doped gallium arsenide. A typical
  wavelength of light emitted is 940 nm, and a
  typical output power is approximately 3 mW at 100
  mA of forward current.
• Planar diffused (homojunction) LEDs output
  approximately 500 microwatts at a wavelength of
  900 nm.

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Light-Emitting Diode (LED)

• The primary disadvantage of homojunction LEDs is
  the nondirectionality of their light emission,
  which makes them a poor choice as a light source
  for fiber optic systems.
• The planar heterojunction LED is quite similar to
  the epitaxially grown LED except that the
  geometry is designed such that the forward
  current is concentrated to a very small area of
  the active layer.

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Light-Emitting Diode (LED)

• Advantages of heterojunction LED over the
  homojunction type
• The increase in current density generates a more
  brilliant light spot.
• The smaller emitting area makes it easier to
  couple its emitted light into a fiber.
• The small effective area has a smaller
  capacitance, which allows the planar
  heterojunction LED to be used at higher speeds.

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Light-Emitting Diode (LED)
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The Burrus etched-well LED

• For the more practical application such as
  telecommunications, data rates in excess of 100
  Mbps are required. The Burrus etched-well LED
  emits light in many directions. The etched well
  helps concentrate the emitted light to a very
  small area. These devices are more efficient than
  the standard surface emitters and they allow more
  power to be coupled into the optical fiber, but
  they are also more difficult to manufacture and
  more expensive.

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Edge-Emitting Diode

• These LEDs emit a more directional light pattern
  than do the surface-emitting LEDs. The light is
  emitted from an active stripe and forms an
  elliptical beam. Surface-emitting LEDs are more
  commonly used than edge emitters because they
  emit more light. However, the coupling losses
  with surface emitters are greater and they have
  narrower bandwidths.

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Injection Laser Diode (ILD)

• Advantages of ILDs
• Because ILDs have a more direct radiation
  pattern, it is easier to couple their light into
  an optical fiber. This reduces the coupling
  losses and allows smaller fibers to be used.
• The radiant output power from an ILD is greater
  than that for an LED. A typical output power for
  an ILD is 5 mW (7 dBm) and 0.5 mW (-3 dBm) for
  LEDs. This allows ILDs to provide a higher drive
  power and to be used for systems that operate
  over longer distances.
• ILDs can be used at higher bit rates than can
  LEDs.
• ILDs generate monochromatic light, which reduces
  chromatic or wavelength dispersion.

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Injection Laser Diode (ILD)

• Disadvantages of ILDs
• ILDs are typically on the order of 10 times more
  expensive than LEDs.
• Because ILDs operate at higher powers, they
  typically have a much shorter lifetime than LEDs.
• ILDs are more temperature dependent than LEDs.

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Light Detectors

• There are two devices that are commonly used to
  detect light energy in fiber optic communications
  receivers PIN (p-type-intrinsic-n-type) diodes
  and APD (avalanche photodiodes).

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PIN Diode

• Assignment

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Avalanche Photodiode

• Assignment

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Basic Cable Design

• The two basic cable designs are the loose-tube
  cable and tight-buffered cable
• ( either a single fiber or a multi-fiber).
• Loose-tube cable, used in the majority of
  outside-plant installations in North America, and
  tight-buffered cable, primarily used inside
  buildings.

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Basic Cable Design

• The modular design of loose-tube cables typically
  holds up to 12 fibers per buffer tube with a
  maximum per cable fiber count of more than 200
  fibers. Loose-tube cables can be all-dielectric
  or optionally armored. The modular buffer-tube
  design permits easy drop-off of groups of fibers
  at intermediate points, without interfering with
  other protected buffer tubes being routed to
  other locations. The loose-tube design also helps
  in the identification and administration of
  fibers in the system.

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Basic Cable Design

• Single-fiber tight-buffered cables are used as
  pigtails, patch cords and jumpers to terminate
  loose-tube cables directly into optoelectronics
  transmitters, receivers and other active and
  passive components.
• Multi-fiber tight-buffered cables also are
  available and are used primarily for alternative
  routing and handling flexibility and ease within
  buildings.

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Loose-Tube Cable

• In a loose-tube cable design, color-coded plastic
  buffer tubes house and protect optical fibers. A
  gel filling compound impedes water penetration.
  Excess fiber length (relative to buffer tube
  length) insulates fibers from stresses of
  installation and environmental loading. Buffer
  tubes are stranded around a dielectric or steel
  central member, which serves as an anti-buckling
  element.
• The cable core, typically surrounded by aramid
  yarn, is the primary tensile strength member. The
  outer polyethylene jacket is extruded over the
  core. If armoring is required, a corrugated steel
  tape is formed around a single jacketed cable
  with an additional jacket extruded over the
  armor. Coated FiberOuter JacketSteel Tape Armor
  Inner Jacket Aramid Strength MemberBinderInterstit
  ial FillingCentral Member
• (Steel Wire or Dielectric) Interstitial
  FillingLoose Tube Cable
• Loose-tube cables typically are used for
  outside-plant installation in aerial, duct and
  direct-buried applications.

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Loose Tube Cable
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Tight-Buffered Cable

• With tight-buffered cable designs, the buffering
  material is in direct contact with the fiber.
  This design is suited for "jumper cables" which
  connect outside plant cables to terminal
  equipment, and also for linking various devices
  in a premises network.
• Multi-fiber, tight-buffered cables often are used
  for intra-building, risers, general building and
  plenum applications.
• The tight-buffered design provides a rugged cable
  structure to protect individual fibers during
  handling, routing and cable connection. Yarn
  strength members keep the tensile load away from
  the fiber.
• As with loose-tube cables, optical specifications
  for tight-buffered cables also should include the
  maximum performance of all fibers over the
  operating temperature range and life of the
  cable. Averages should not be acceptable.

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Tight-Buffered Cable
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Optical Fiber Connectors

• Optical connectors are the means by which fiber
  optic cable is usually connected to peripheral
  equipment and to other fibers. These connectors
  are similar to their electrical counterparts in
  function and outward appearance but are actually
  high precision devices. In operation, the
  connector centers the small fiber so that its
  light gathering core lies directly over and in
  line with the light source (or other fiber) to
  tolerances of a few ten thousandths of an inch.
  Since the core size of common 50 micron fiber is
  only 0.002 inches, the need for such extreme
  tolerances is obvious.
• There are many different types of optical
  connectors in use today. The SMA connector, which
  was first developed before the invention of
  single-mode fiber, was the most popular type of
  connector until recently.

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Fiber Connectors
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Optical Splices

• While optical connectors can be used to connect
  fiber optic cables together, there are other
  methods that result in much lower loss splices.
  Two of the most common and popular are the
  mechanical splice and the fusion splice. Both are
  capable of splice losses in the range of 0.15 dB
  (3) to 0.1 dB (2).
• In a mechanical splice, the ends of two pieces
  of fiber are cleaned and stripped, then carefully
  butted together and aligned using a mechanical
  assembly. A gel is used at the point of contact
  to reduce light reflection and keep the splice
  loss at a minimum. The ends of the fiber are held
  together by friction or compression, and the
  splice assembly features a locking mechanism so
  that the fibers remained aligned.
• A fusion splice, by contrast, involves actually
  melting (fusing) together the ends of two pieces
  of fiber. The result is a continuous fiber
  without a break. Fusion splices require special
  expensive splicing equipment but can be performed
  very quickly, so the cost becomes reasonable if
  done in quantity. As fusion splices are fragile,
  mechanical devices are usually employed to
  protect them.

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Designing Optical Fiber Systems

• The following step-by-step procedure should be
  followed when designing any system.
• Determine the correct optical transmitter and
  receiver combination based upon the signal to be
  transmitted (Analog, Digital, Audio, Video,
  RS-232, RS-422, RS-485, etc.).
• Determine the operating power available (AC, DC,
  etc.).
• Determine the special modifications (if any)
  necessary (Impedances, Bandwidths, Special
  Connectors, Special Fiber Size, etc.).
• Calculate the total optical loss (in dB) in the
  system by adding the cable loss, splice loss, and
  connector loss. These parameters should be
  available from the manufacturer of the
  electronics and fiber.
• Compare the loss figure obtained with the
  allowable optical loss budget of the receiver. Be
  certain to add a safety margin factor of at least
  3 dB to the entire system.
• Check that the fiber bandwidth is adequate to
  pass the signal desired.

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BASIC TYPES OF OPTICAL FIBER CABLE

• Breakout Cable
• Interconnect Cable
• Loose Tube Cable
• Low Smoke Zero Halogen Cable
• LXE Light Guide Express Entry Cable
• Light Pack Cable
• Indoor/Outdoor Loose Tube Cable
• Tactical/Military Cable
• TEMPEST Cable Description

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Breakout Cable

• Breakout cables are designed with alldielectric
  construction to insure EMI immunity.
• These cables are obtainable in a wide range of
  fiber counts and can be used for routing within
  buildings, in riser shafts, and under computer
  room floors.
• The Breakout design enables the individual
  routing, or "fanning", of individual fibers for
  termination and maintenance.
• In addition to the standard duty 2.4 mm subunit
  design, a 2.9 mm heavy duty and a 2.0 mm light
  duty design are also available.

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Interconnect Cable

• Cable for interconnecting equipment is available
  in single-mode and multimode fiber sizes and its
  all dielectric construction provides EMI immunity
  .
• Available in one- and two-fiber designs, these
  cables are optimized for ease of connectorization
  and use as "jumpers" for intra-building
  distribution.
• Its small diameter and bend radius provide easy
  installation in constrained areas.
• This cable can be ordered for plenum or riser
  environments. Products include single fiber
  cable, twofiber Zipcord, and twofiber DIB
  Cable.
• Uncabled fiber, coated only with a thermoplastic
  buffer, is also available for pigtail
  applications with inside equipment.

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Loose Tube Cable

• Loose tube cables are for general purpose outdoor
  use.
• The loose tube design provides stable and highly
  reliable transmission parameters for a variety of
  applications.
• The design also permits significant improvements
  in the density of fibers contained in a given
  cable diameter while allowing flexibility to suit
  many system designs.
• These cables are suitable for outdoor duct,
  aerial, and direct buried installations, and for
  indoor use when installed in accordance with NEC
  Article 770.

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Features

• Different fiber types available within a cable
  (hybrid construction).
• Lowest losses at long distances, for use in duct
  aerial, and direct buried applications.
• Wide range of fiber counts (up to 216).
• Available with single-mode and multimode fiber
  types.
• All dielectric or steel central member.
• Loose Tube Cable is also available with armored
  construction for added protection.

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Low Smoke Zero Halogen Cable

• HalexRTM is a low smoke, zero halogen fiber
  optic cable, designed to replace standard
  polyethylene jacketed fiber optic cables in
  environments where public safety is of great
  concern.
• In addition to having low smoke properties,
  HalexR cable meets the NEC requirements for
  risers, passes all U.S. flame requirements for UL
  1666 and UL 1581, and is OFNR listed up to 156
  fibers.

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LXE Light Guide Express Entry Cable

• The LXE (Lightguide Express Entry) sheath system
  is designed with the loop distribution market in
  mind, where express entry (accessing fibers in
  the middle of a cable span) is a common practice.
  
• The LXE sheath system achieves a 600 pound (2670
  N) tensile rating through the use of linearly
  applied strength members placed 180 degrees
  opposite each other.
• High density polyethylene (HDPE) is used for the
  cable jacket to provide both faster installation,
  through a lower coefficient of friction, and
  optimum cable core protection in hostile
  environments.

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Features

• Strength members in cable sheath (not in cable
  core).
• Nonmetallic cable core.

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Light Pack Cable

• Lightpack Cable consists of fiber "bundles" held
  together with color coded yarn binders.
• Cable can hold up to 144 fibers and still
  maintain a large clearance in the core tube.
• A waterblocking compound, specifically designed
  for LIGHTPACK Cable, adds extra flexibility,
  protects the fiber and virtually eliminates
  microbending losses.
• Lightpack cable is compact size, rugged design,
  contains a high density polyethylene sheath and
  has a high strengthtoweight ratio.

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Indoor/Outdoor Loose Tube Cable

• The RLT Series of loose tube fiber optic cables
  is designed for installation both outdoors and
  indoors in areas required by the (NEC) to be
  riser rated Type OFNR. They meet or exceed
  Article 770 of the NEC and UL Subject 1666 (Type
  OFNR). They also meet CSA C22.2 No. 232M1988
  Type OFNFT4.
• All of the RLT products utilize a proprietary
  ChromaTek 3 jacketing system that is designed for
  resistance to moisture, sunlight and flame for
  use both indoors and outdoors. These cables are
  loose tube, gelfilled constructions for
  excellent resistance to moisture. They are
  available with single-mode or multimode fibers
  with up to a maximum of 72 fibers.

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Indoor/Outdoor Loose Tube Cable

• Because these outdoor cables are riser rated,
  they eliminate the need for a separate point of
  demarcation, i.e., splicing to a riser rated
  cable within 50 feet of the point where the
  outdoor cable enters the building as required by
  the NEC. These cables may be run through risers
  directly to a convenient network hub or splicing
  closet for interconnection to the electro-optical
  hardware or other horizontal distribution cables
  as desired.
• No extra splice or termination hardware is
  required at the entrance to the facility, and
  cable management is made easier by the use of
  just one cable. This installation ease is
  especially useful in Campus type installations
  where buildings are interconnected with outdoor
  fiber optic cables.

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Tactical/Military Cable

• Tactical cable utilizes a tight buffer
  configuration in an all dielectric construction.
• The tight buffer design offers increased
  ruggedness, ease of handling and
  connectorization.
• The absence of metallic components decreases the
  possibility of detection and minimizes system
  problems associated with electromagnetic
  interference.

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Features

• Proven compatibility with existing ruggedized
  connectors.
• Lightweight and flexible no anti-buckling
  elements required.
• Available in connectorized cable assemblies.
• Available with 50, 62.5 and 100 micron multimode
  fibers, as well as single-mode and
  radiation-hardened fibers.

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TEMPEST Cable Description

• For use where secure communications are a major
  consideration, and Tempest requirements must be
  met. The Tempest rated cable is available in a
  variety of cable constructions.
• Tempest relates to government requirements for
  shielding communications equipment and
  environments.
• One common application is the use of fiber optic
  cable in conjunction with RF shielded enclosures.
  These enclosures have been specially constructed
  to suppress the emission of RF signals, and must
  meet the Transient Electro-magnet, Pulse
  Emanation Standard (TEMPEST).

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Cont.

• For a system to be TEMPEST qualified, it must be
  tested in accordance with MIL-STD285, and it
  must also meet the requirements stated in NSA
  656. All elements of the system, individually
  and combined, must meet the TEMPEST standard.
• In the case of fiber optics, the "system"
  consists of the cable (which is dielectric and
  nonconductive), and the tube through which the
  cable passes.