Fiber optic

2.2.4 Fiber optics

Pulses of light sent down ultra-thin (as thin as a human hair or less), ultra-transparent fiber of glass, with total internal reflection (no loss in light as it bounces off side).
Pulse=1, Absence of light=0.
Electrical signal - to - light pulses - to electrical signal.
1-way system. 2-way comms uses 2 fibers, or 2 frequency bands on same fiber.

Sender - LED (low-spec) or Laser (high-spec) converts electrical signal to light pulses.
Receiver - Photodiode detector generates electrical pulse when light falls on it.

Fiber-Optic Chronology
Fiber optic laid on ocean floor. Atlantic cable laid 1988.
Compare with telegraph:
- The Victorian Internet, Tom Standage, 1998. - 19th cent. electric telegraph (also copper wires). First ever global communications network.
  1. First proper line: Paddington to Slough 1843.
  2. Washington to Baltimore 1844.
  3. First submarine cable: England linked to France 1845 (working properly from 1851).
  4. England linked to Ireland 1853.
  5. Telegraph used in Crimean War 1854.
  6. Atlantic cable laid 1858 (working properly from 1865). See here.
Fiber optic dominates Internet and telephone backbone now (except for wireless satellite links and wireless land links).
Fiber optic also used for LANs.
Less easy in the home - hard to replace millions of new local loops. Phone and cable TV (no install) and wireless broadband (easier install than fiber) still competing strongly.

Speeds

Fiber optic theoretical transmission limit 50 T bps.
Current practical limit around 50 G bps for 100 km without amplification - Problem is converting between electrical and optical fast enough.
At receiver, photodiode typically cannot convert faster than around 1 G bps.

Theoretical All-optical computers

Seems like computation speed will hit physical limits before communication speed does.
In future? - Computation is slow. Communication is fast. Redesign all systems to minimise computation, no matter how much bandwidth it wastes.

Comparison of fiber optic and copper

Fiber higher bandwidth.
Fiber needs less amplification/repeating. Fiber needs repeaters every 50 km. Copper every 5 km.
Fiber less affected by electromagnetic interference. But on downside, easily damaged if bent.
Fiber thinner and lighter: Makes big difference to telephone company with thousands of cables.
For new routes, fiber cheaper to install.
Massive installed base of copper.
Fiber more expensive, but clearly the future of all cables of more than a few metres is fiber.