电磁学应用讲义

6.013 Lecture 22:

Optical Communications

A. Overview

Optical communications is as old as smoke signals and mirrors reflecting sunlight. Today it is particularly important for long-haul and extremely wideband communications. Optical fibers now convey most intercontinental communications, although microwave satellites still serve geographically isolated users such as those on ships. Satellites also provide moveable capacity that can address transient shortfalls or failures across the globe; they simply point their antenna beams at the new users. Fibers have also been widely installed for both interstate and intrastate communications, and are beginning to migrate down into the local loop and to small businesses and homes. Extreme data rates are now also being conveyed between and within computers and even chips, although wires still have advantages of cost and simplicity for most ultra-short applications.

A significant niche market also exists for local line-of-sight optical links that provide extreme bandwidths for dedicated point-to-point communications. For example, companies can link laser beams between buildings and can quickly bypass inadequate or failed wire links connecting them to the global network, as happened after 9/11 in New York City.

Optical links also have great potential for very broadband inter-satellite or satellite-earth communications because small telescopes easily provide highly focused antenna beams. For example, beamwidths of telescopes with 5-inch apertures are typically one arc-second [1 arc-second is 1/60 arc-minutes, 1/602 degrees, 1/(57.3×602) radians, or 1/60 of the largest apparent diameters of Venus or Jupiter in the night sky].

The major components in fiber communications systems are the fibers themselves and the optoelectronic devices that manipulate the optical signals, such as detectors (discussed in the second recitation), amplifiers and sources (discussed in next lecture), modulators, mixers, switches (which can be MEMS-controlled mirrors, shutters, or gratings), filters, multiplexers, directional couplers, and others. These are assembled to create useful communications or computing systems. A typical wave-division multiplexed (WDM) amplified long fiber system is pictured in Figure 22-1.

Figure 22-1. Wave-division multiplexed amplifier