Optical Fibre System By Mohd Nasir bin Said Telecommunications Department Advance Technology Training Centre Kulim Kedah Darul Aman

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Optical Fibre SystemByMohd Nasir bin
SaidTelecommunications DepartmentAdvance
Technology Training Centre Kulim Kedah Darul Aman
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What is "Fiber Optics"?

• It's the communications technology that works by
  sending signals down hair thin strands of glass
  fiber

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History

• It began about 30 years ago in the RD labs
  (Corning, Bell Labs, ITT UK, etc.) and was first
  installed in Chicago, IL, USA in 1976. By the
  early 1980s, fiber networks connected the major
  cities on each coast.

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• By the mid-80s, fiber was replacing all the telco
  copper, microwave and satellite links.
• In the 90s, CATV discovered fiber and used it
  first to enhance the reliability of their
  networks, a big problem. Along the way, the
  discovered they could offer phone and Internet
  service on that same fiber and greatly enlarged
  their markets.

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• Computers and LANs started using fiber about the
  same time as the telcos
• Other applications developed too aircraft, ship
  and automobile data busses, CCTV for security,
  even links for consumer digital stereo!

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Light

• What is light?
• Energy of from electromagnetic wave and particle.
• What is photon?
• Particle from of light
• Photonics to lights is like Electronics to
  current

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The electromagnetic Spectrum

• The light used in optical fiber network is one
  type of electromagnetic energy.
• When an electric change moves back and forth, or
  accelerates, a type of energy called
  electromagnetic energy is produced.
• This energy is the form of waves can travel
  through a vacuum, the air, and through some
  materials like glass.
• An important property of energy wave is the
  wavelength
• Radio, microwave,radar, visible light,x-rays are
  all types of electromagnetic energy.

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The electromagnetic spectrum
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• Wavelenghts are not visible to the human eye are
  used to transmit data over optical fiber.
• These wavelenghts are slightly longer than red
  light and are called infrared light.
• These wavelenghts were selected because they
  travel through optical fiber better than other
  wavelenghts.

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Ray model of light

• When electromagnetic waves travel out from a
  source, they travel in straight lines
• These straight lines pointing out from the source
  are called rays.
• In the vacuum of empty space, light travels
  continuously in a straight line at 300,000km per
  second.
• However,light travel at different, slower speed
  other through other materials like air,water and
  glass.
• When a light ray called the incident ray,crosses
  the boundry from one material to another , some
  of the light energy in the ray will be reflected
  back.
• This is why you can see yourself in window glass.
• The light that is reflected back is called
  reflected ray.

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Ray model of light

• The light energy in the incident ray that is not
  reflected will enter the glass
• Refracted ray-The entering ray will be bent at an
  angle from its original path.
• How much the incident light ray is bent depends
  on the angle at which the incident ray strikes
  the surface of the glass and the different rates
  of speed at which light travels through the two
  substance.
• The optical density of the glass determines how
  much the rays of light in the glass.
• Optical density refers to how much a light ray
  slows down when it passes through a substance.
• The greater the optical density of a material,
  the more it slows light down from its speed in a
  vacuum.
• The ratio of the speed of light in a material to
  the speed of light in a vacuum is called the
  Index of Refraction

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• When light traveling in a transparent material
  meets the surface of another transparent material
  two things happen-
• a) some of the light is reflected reflection
• b) some of the light is transmitted into the
• second transparent material -refraction

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• The bending of light is called refraction and it
  depends upon the fact that light travels at one
  speed in one material and at a different speed in
  a different material.
• As a result each material has its own
  Refractive Index which we use to help us
  calculate the amount of bending which takes
  place. Refractive index is defined as
• n C
  ?
• where
• n is the refractive index
• C is the speed of light in a vacuum
• ? is the speed of light in the material

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• The indexes of refraction of several common
  materials are given above
• Vacuum -1.0
• Air 1.0003
• Water-1.33
• Ethyl Alcohol -1.36
• Silicon -3.4
• Index of refraction is based on a wavelength of
  light emitted from a sodium flame (5890 Å)

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Snell Law

• How a light ray reacts when it meets the
  interface of two transmissive materials that have
  different indexes of refraction can be explained
  with Snells law.

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• Snells law simply states
• n1 sin ?1 n2 sin ?2
• where
• n1 refractive index of material 1 (unit less)
• n2 refractive index of material 2 (unit less)
• ?1 angle of incidence (degrees)
• ?2 angle of refraction (degrees)

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Critical Angle

• The critical angle is defined as the minimum
  angle of incidence at which a light ray may
  strike the interface of two media and result in
  an angle of refraction of 90? or greater
• This definition pertains only when the light ray
  is traveling from a more dense medium into a less
  dense medium. The critical angle can be derived
  from Snells law as follows
• n1 sin ?1 n2 sin ?2
• sin ?1 n2 sin ?2
• n1

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TIR

• The transmitted ray now tries to travel in both
  materials simultaneously for various reasons this
  is physically impossible so there is no
  transmitted ray and all the light energy is
  reflected. This is true for any value of ?1, the
  angle of incidence is equal to or greater than ?

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• We can define the two conditions necessary for
  TIR to occur
• 1. The refractive index of the first medium is
• greater than the refractive index of the
• second one.
• 2. The angle of incidence, ?1, is greater than
  or equal to
• the critical angle, ?c
• The phenomenon of TIR causes 100 reflection. In
  no other situation in nature, where light is
  reflected, does 100 reflection occur. So TIR is
  unique and very useful.

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Numerical Aperture

• The numerical aperture of a core is the range of
  angles of incident light rays entering the fiber
  that will be completely reflected.
• Modes- The paths which a light ray can follow
  when travelling down a fiber.
• By controlling both conditions, the fiber run
  will have total internal reflection. This give a
  light wave that can be used for data
  communications.

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Fiber Construction And Geometry
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• Te core is the light transmission element at the
  center of the optical fiber.
• Cladding is also made of silica but with a lower
  index of refraction than the core.Light rays
  travelling through the fiber core reflect off
  this core to cladding interface as they move
  through the fiber by TIR
• Surrounding the cladding is a buffer material
  that is usually plastic. The buffer material
  helps sheid the core and cladding from damage.
• The strenght material surrounds the
  buffer,preventing the fiber cable from being
  strecthed when installer pull it.The material
  used is often Kevlar, the same material used to
  produce bulletproof vest.
• The outer jacket surrounds the cable to protect
  the fiber against abrasion,solvents and other
  contaminations.

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Type of Fiber andMode of Propogation

• Single Mode
• Multimode Step index
• - Graded Index

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• If the diameter of the core of the fiber is large
  enough so that there are many paths light can
  take through the fiber, the fiber is called
  multimode fiber.
• Single mode fiber has a much smaller core that
  only allows light rays to travel along one mode
  inside the fiber.

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Transmission mode
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Single Mode

• High Bw applications- 4 Ghz
• Low losses 0.3-0.5 db/km
• Small Core area 8-10 micron
• Tx at 1300nm-1550nm wavelength
• Higher cost.

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Multimode Step Index

• BW of 10Mhz/km
• Loss of 5-20db/km
• Large core 200-1000micron
• Cladding 1035 micron
• Limited transmission distance
• Tx at 660-1060

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Multimode Graded index

• BW up to 600Mhz/km
• Losses of 2 to 10 db/km
• Cores of 50/62.5/85/100 micron
• Cladding of 125 and 140 micron
• Effective with laser or LED sources

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TQ
Q A ?