An optical transmission system has three components: the light source, the transmission medium, and the detector.

Light source: LED (Light Emitting Diode) or Laser (Light Amplification by Simulated Emission of Radiation)

Transmission Media: ultra-thin fiber of glass

Detector: using light-electricity effect, generate an electrical pulse when light falls on it.

Single Mode:

  • glass cladding 125 microns dia.
  • glass core 5-8 microns
  • small core
  • less dispersion
  • suited for long distance applications


  • glass core 50 microns
  • glass cladding 125 microns dia.
  • larger core
  • greater dispersion, loss of signal
  • used for long distance, shorter than single mode

Fiber Optic Network (Ring):


The interface at each computer passes the light pulse stream through to the next link and also serves as a T junction to allow the computer to send and accept messages.Two types of interfaces are used.

  • Passive
  • Active

A passive interface consists of two taps fused onto the main fiber. One tap has an LED or laser diode at the end of it (for transmitting), and the other has a photodiode (for receiving). The tap itself is completely passive and is thus extremely reliable because a broken LED or photodiode does not break the ring. It just takes one computer off-line.

Active Interface: The incoming light is converted to an electrical signal, regenerated to full strength if it has been weakened, and retransmitted as light. The interface with the computer is an ordinary copper wire that comes into the signal regenerator.


Multiple analogue message signals or digital data streams are combined into one signal over a shared medium.

Types of Multiplexing

  1. Frequency-division multiple access (FDMA)
  2. Time-division multiple  access (TDMA)
  3. Code division multiple  access (CDMA)

Frequency-division multiplexing access (FDMA)

It allows several users to share different frequency channels by dividing the signal in a single time slot

Time-division multiplexing  access (TDMA)

It allows several users to share the same frequency channel by dividing the signal into different time slots

Code-division multiplexing access (CDMA)

CDMA does not assign a specific frequency to each user. Instead, every channel uses the full available spectrum. Individual conversations are encoded with a pseudo-random digital sequence