Why optical communications?

High speed electrical signals suffer from radiative losses and dispersion. Wires must be thick in order to carry a fast signal over large distance – that costs money and space.

The (bit-rate)(distance) product:

(from Agrawal 1998)

What do we mean technically by optical communication?

A generic system:

· GHz electronic-to-optical transducer

· transmission system (meters to thousand of km) – may include amplifiers

· GHz optical-to-electronic transducer

one optical fiber can carry thousands of voice messages

Limitations on optical fibers:

Dispersion: different wavelengths and/or modes travel at different velocities.

Attenuation: due to scattering and absorption of light.

Modal Dispersion: beams launched at different angles travel at different speeds

Solution: make single mode fibers

Absorption and material dispersion for SiO₂.

Material dispersion is important because a short pulse (of nominally monochromatic light) actually must contain a distribution of wavelengths (a Fourier transform effect)
A simple photodetector: converts an optical pulse into an electrical pulse.

small area decreases capacitance, increasing switching speed

Semiconductor lasers

A good site for a simple technical explanation:

http://britneyspears.ac/physics/vcsels/vcsels.htm

There are currently ~40 million kilometers of optical fiber installed and with about 5 million kilometers going in each year.

The next stages?

· Optical fibers from the router to your pc.

· Optical interconnects from processor chip to board or chip to chip

· Optical interconnects on the chip.