OFC block diagram vs cable

An optical fibre communication (OFC) system converts an electrical message into light, sends it through a glass fibre, and converts it back to an electrical signal at the receiver.

Block diagram

 Message   +-----------+   +---------+   +--------+
 input --->| Encoder / |-->| Driver  |-->| Light  |
           | modulator |   | circuit |   | source |
           +-----------+   +---------+   +---+----+
                               LED/laser     |
                                             v
   ~~~~~~~~ optical fibre (splices, connectors, repeaters) ~~~~~~~~
                                             |
                                             v
 Message   +-----------+   +---------+   +---+----+
 output <--| Decoder / |<--| Amp. +  |<--| Photo- |
           | demod.    |   | regen.  |   |detector|
           +-----------+   +---------+   +--------+
                                          PIN / APD

1. Information source and encoder: the analog or digital message is coded (e.g. PCM) into an electrical signal.
2. Driver circuit: supplies the current that modulates the light source.
3. Light source: an LED or laser diode converts the electrical signal into light at 850, 1310 or 1550 nm.
4. Source-to-fibre coupler: launches the light into the fibre within its acceptance angle.
5. Optical fibre (channel): carries light by total internal reflection; splices, connectors and repeaters or optical amplifiers (EDFA) are added on long routes.
6. Photodetector: a PIN or avalanche photodiode converts light back to current.
7. Amplifier, regenerator and decoder: amplify, reshape and decode the signal to recover the message.

Advantages over cable (copper) communication

• Huge bandwidth (THz range), so very high data rates.
• Very low loss (0.2 dB/km), so long repeater spacing.
• Immune to electromagnetic interference, lightning and crosstalk.
• No electrical hazard or sparking; safe in explosive areas.
• Small size and light weight.
• Difficult to tap, so more secure.
• Made from silica (sand), a plentiful raw material.

Disadvantages

• Splicing and connectors need costly tools and skilled workers.
• Fibre is fragile; bending it below its minimum bend radius causes loss or breakage.
• Cannot carry electrical power to remote repeaters directly.
• Optical sources, detectors and test equipment are costly.
• Electrical–optical conversion is needed at each end.