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Fiber Optic Digital Link


To Study the fiber optic digital data transmitting and receiving through plastic fiber cable.

Apparatus Required





Fiber Optic Data Link

The fiber optic data link consists of a transmitter which converts an electrical signal to a light signal, an optical fiber to guide the light and a receiver which detects the light signal and converts it to an electrical signal. Light sources are either light emitting diodes (LED's) or laser diodes and detectors are phototransistors or Photodiodes.


The heart of the transmitter is a light source. The major function of a light source is to convert an information signal from its electrical form into light. Today's fiber-optic communications systems use, as a light source, either light-emitting diodes ( LEDs) or laser diodes (LDS). Both are miniature semiconductor devices that effectively convert electrical signals into light. They need power-supply connections and modulation circuitry. All these components are usually fabricated in one integrated package.

Fig: Transmitting LED HFBR 1251


This type LEDs needs external Driver circuits. Here we used IC 75451 for drive the LED.

Optical fiber

The transmission medium in fiber-optic communications systems is an optical fiber. The optical fiber is the transparent flexible filament that guides light from a transmitter to a receiver. An optical information signal entered at the transmitter end of a fiber - optic communications system is delivered to the receiver end by the optical fiber.

Model Diagram for Plastic Fiber cable

This is multi mode plastic fiber cable, in this fiber cable core diameter 1000 micron and fiber cladding index 1.402.



The key component of an optical receiver is its photo detector. The major function of a photo detector is to convert an optical information signal back into an electrical signal (Photocurrent). The photo detector in today's fiber - optic communications systems is a semiconductor photodiode (PD). This miniature device is usually fabricated together with its electrical circuitry to from an integrated package that provides power-supply connections and signal amplification.

Fig:Receiving Photo Diode HFBR 2521


In this type Photo Diode have internal driver circuits. So no need any external driver.


  • Connect +15V adapter to both transmitter and receiver module
  • Switch (sw1) ON the transmitter Module and CRO
  • Connect the CRO Probe, positive to P1 and negative to Ground P8.
  • Now we get a square wave generator output on CRO and vary the Freq pot meter min to max range.
  • Connect CRO positive to P2 test point now we get a TTL Output and vary the Freq pot meter for variable frequency.
  • Connect P2 and P9 using patch chord.
  • Connect the CRO positive to P12 test point and see the LED driver output.
  • Connect the 1 m Plastic fiber cable between transmitter Module LED to receiver module Photo Diode.
  • Switch (sw1) ON the receiver Module.
  • Connect the CRO probe positive to receiver module P3 test point and negative to P2 test point. Now we receive the TTL signal.
  • Connect P3 and P4 using patch chord.
  • Get a pure TTL output on P5 test point.
  • Keep CRO in dual mode, first channel connect to the transmitter Module P2 test point and second channel connect to the receiver module P5 test point.
  • Now vary the freq pot meter min to max and get Input (CH1), Output (CH2) TTL signal at variable frequency.


Refer below links for more experiments: