Elements of optical communication system

An optical communication system sends information as modulated light through an optical fibre. Its main elements are the transmitter (electrical-to-optical conversion), the channel (fibre, with connectors, splices and repeaters) and the receiver (optical-to-electrical conversion).

Block diagram

 +--------+   +-----------+   +---------+
 | Info   |-->| Electrical|-->| Optical |
 | source |   | Tx (drive)|   | source  |
 +--------+   +-----------+   +----+----+
                                   |
            optical fibre cable    v
  ===[connector]====[splice]====[repeater]===
                                   |
 +--------+   +-----------+   +----v----+
 | Dest-  |<--| Electrical|<--| Photo-  |
 | ination|   | Rx (amp)  |   | detector|
 +--------+   +-----------+   +---------+

1. Information source

Provides the message: voice, video or data. Analog signals are usually digitised (PCM) and multiplexed (TDM) before transmission.

2. Electrical transmitter (drive circuit / modulator)

Encodes and shapes the signal and supplies the drive current that modulates the light source. Most systems use intensity modulation (on–off keying); coherent systems modulate phase as well.

3. Optical source

Converts the electrical signal into light at 850, 1310 or 1550 nm.

• LED: cheap, incoherent, wide spectrum; short multimode links.
• Laser diode: coherent, narrow spectrum, high power and speed; long-haul single-mode links.

4. Source-to-fibre coupler

A lens or direct butt coupling launches the light into the fibre core within its acceptance cone.

5. Optical fibre cable (transmission channel)

A glass core surrounded by a cladding of lower refractive index guides light by total internal reflection. Signal degradations are attenuation (absorption, scattering, bending; ~0.2 dB/km at 1550 nm) and dispersion (pulse spreading). The cable includes strength members and a protective jacket.

6. Connectors and splices

Join fibre lengths and equipment. Splices are permanent (fusion ~0.05 dB, mechanical ~0.2 dB); connectors are demountable (~0.3 dB).

7. Repeaters / optical amplifiers

On long routes the signal is boosted every 50–100 km. A regenerator detects, retimes and retransmits the signal; an optical amplifier (EDFA) amplifies light directly at 1550 nm without conversion.

8. Optical detector

Converts received light back into current. A PIN photodiode (low noise, cheap) or an avalanche photodiode (internal gain, more sensitive) is used.

9. Electrical receiver

A low-noise preamplifier and main amplifier, an equaliser, a filter, and a decision circuit with clock recovery restore the original digital pulses, which are then decoded and demultiplexed.

10. Destination

The user equipment (telephone, computer, TV) that receives the recovered message.