Question

Explain with neat sketch block diagram of optical fiber communication system and list out sources and detectors suitable for it.

Answer 1

OR

1. The light source is normally an infrared LED or LASER device which is driven by the current value from the V to I convertor. It emits light which is proportional to the drive current. Thus light which is proportional to the input voltage value is generated and given as input to fiber

2. A source to fiber interface is used for coupling the light source to the fiber optic cable.

The light emitted from the source is inserted into the fiber such that maximum light emitted from it is coupled to the fiber.

Optical Splice:

1. For creating long haul communication link, it is necessary to join one fiber to other fibers permanently. For this purpose, optical splicing techniques are used to join different fibers.

Optical Coupler/ Beam splitter:

1. Optical couplers are used to couple the light output from the fiber end to the device which can be receiver or regenerator.

2. Beam splitters are used to split the light beam which can be given to other equipment.

Regenerator/ Repeater:

1. After an optical signal is launched in to a fiber, it will become progressively attenuated and distorted with increasing distance because of scattering, absorption and dispersion mechanisms in the glass material.

2. Therefore repeaters are placed in between to reconstruct the original signal and again retransmit it.

3. The signal is processed in electronics domain and hence optical to electrical conversion and electrical to optical conversions are performed in the repeater.

Optical Amplifier:

1. After an optical signal has travelled a certain distance along a fiber, it becomes greatly weakened due to power loss along the fiber.

2. Therefore, when setting up an optical link, engineers formulate a power loss budget and add amplifiers or repeaters when the path loss exceeds the available power margin.

3. The periodically placed amplifiers merely give the optical signal a power boost, whereas a repeater attempts to restore the signal to its original shape.

Receiver: 

1. At the destination of an optical fiber transmission line there is a coupling device (connector) which couples the light signal to the detector.

2. Inside the receiver is a photodiode that detects the weakened and distorted optical signal emerging from the end of an optical fiber and converts it to an electrical signal. (Referred to as photo current).

3. I to V convertor produce an output voltage proportional to the current generated by the light detector. Thus, we obtain output value which was given to the system as data input.

Optical source: Light source at the transmitter end which converts electrical current into optical signal. Light sources which are used are LED and LASER.

Detector: light detector at the receiver end which converts optical energy electrical signal.

The light detectors which are used as PIN photodiode and avalanche photo diode.