Physics

• Optical fibers are glass or plastic conduits as thin as a human hair, designed to guide light waves along their length
• Total internal reflection (TIR)


  The principle on which optical fibers work
• Advantages of optical fibers

  • Enormous bandwidth
  • Low transmission loss
  • Small size and less weight
  • Flexibility and mechanical strength
  • Immunity to interference and cross-talk
  • Signal security
  • Safe and non-hazardous
• Applications of optical fibers

  • Medical
  • Military
  • Industrial
  • Illumination
  • Communication
  • Sensors
• Core


  Innermost region of an optical fiber that guides light
• Cladding


  Middle region of an optical fiber that confines light within the core
• Sheath / Jacket / Buffer Coating


  Outermost region of an optical fiber that protects the core and cladding
• Total Internal Reflection


  When a light ray strikes the interface at an angle greater than the critical angle, the refracted ray is reflected back into the same medium
• Acceptance angle


  The maximum angle of launch that the light ray can have relative to the axis of the fiber in order to propagate down the fiber
• Fractional Refractive Index Change (Δ)


  The fractional difference between the refractive indices of the core and the cladding
• Numerical Aperture (NA)


  The light collecting ability of a fiber, expressed as the sine of the acceptance angle
• Modes of Propagation (Optical Paths)

  • Axial rays
  • Zig-zag rays
• Classification of Optical Fibers

  • Multi-Mode Fibers
  • Single Mode Fibers
  • Graded Index Fibers
  • Step Index Fibers
  • PCS Fibers
  • Glass Fibers
  • Plastic Fibers
• Constructive interference


  Light rays travelling along certain paths will be in phase
• Destructive interference


  Light rays travelling along certain other paths will be out of phase
• Modes of propagation


  The light ray paths along which the waves are in phase inside the fiber
• Classification of Optical Fibers

  • Multi-Mode Fibers
  • Single Mode Fibers
• Multi-Mode Fibers (MMF)


  • Can support more than one mode of propagation
  • Larger core diameter (d ≥ 50 μm)
• Single Mode Fibers (SMF)


  • Can support only one mode of propagation
  • Smaller core diameter (d ≈ 2 to 10 μm)
• Classification of Optical Fibers based on refractive index profile

  • Graded Index (GRIN) Fibers
  • Step Index (SI) Fibers
• Graded Index (GRIN) Fibers

  • Core of gradually decreasing refractive index (is maximum along axis)
  • Refractive index of core matches that of cladding at the interface
• Step Index (SI) Fibers

  • Core of uniform refractive index (n1)
  • Refractive index abruptly changes at core-cladding interface
• Classification of Optical Fibers based on material used

  • PCS Fibers
  • Glass Fibers
  • Plastic Fibers
• PCS Fibers


  • Core - Glass (silica – SiO2)
  • Cladding - Plastic
• Glass Fibers


  • Core - Glass (silica – SiO2)
  • Cladding - Glass (silica – SiO2)
• Plastic Fibers


  • Core - Plastic
  • Cladding - Plastic
• Calculating Numerical Aperture (NA) of an optical fiber in air

  1. Use formula: NA = √(n1^2 - n2^2)

  2. Where n1 is refractive index of core, n2 is refractive index of cladding
• Calculating Acceptance Angle (i(max)) of an optical fiber in water

  1. Use formula: i(max)(in water) = sin^-1(NAair/nwater)

  2. Where NAair is numerical aperture in air, nwater is refractive index of water
• 1. number


  A dimensionless parameter that determines the number of modes a fiber can support
• • If V < 2.405, the fiber can support only one mode (single mode fiber)
  • If V > 2.405, the fiber can support many modes (multi-mode fiber)
  • If V = 2.405 corresponds to cut off wavelength</b>
• Number of modes (N) supported by a fiber

  • For Step-Index (SI) fiber, N = V^2/2
  • For Graded Index (GRIN) fiber, N = V^2/4
• Basic Fiber Optic Communication System

  1. Transducer converts non-electrical message to electrical signal

  2. Electrical signal converted to optical signal by light source (LED/Laser)

  3. Optical signal transmitted through optical fiber

  4. Optical signal detected and converted back to electrical signal

  5. Electrical signal demodulated and converted to required form by transducer
• Fiber Optic Sensors


  • Temperature Sensor
  • Displacement Sensor
  • Force Sensor
• Types of Fiber Optic Sensors

  • Phase Modulated Sensor
  • Intensity Modulated Sensor
• Calculating Numerical Aperture (NA) of an optical fiber in water

  1. Use formula: NA = √(n1^2 - n2^2)/n0

  2. Where n1 is refractive index of core, n2 is refractive index of cladding, n0 is refractive index of water
• Calculating refractive index of core (n1) of an optical fiber

  1. Use formula: n1 = √[(n0 sin i(max))^2 + n2^2]

  2. Where n0 is refractive index of launching medium, i(max) is acceptance angle, n2 is refractive index of cladding
• Determining V-number and number of modes (N) for a step-index fiber

  1. Use formula: V = 2π(r/λ)√(n1^2 - n2^2)

  2. Where r is core radius, λ is wavelength, n1 is core refractive index, n2 is cladding refractive index

  3. For SI fiber, N = V^2/2
• If V = 4.409 and N = 8, the fiber is a step-index fiber
• Optical fiber


  Glass or plastic conduit (tube or channel) as thin as a human hair, designed to guide light waves along their length
• An optical fiber works on the principle of total internal reflection