A fiber-optic cable provides a pipeline that can carry large amounts of information. Copper wires or copper coaxial cable carry modulated electrical signals but only a limited amount of information, due to the inherent characteristics of copper cable.
Free-space transmission, such as radio and TV signals, provides information transmission to many people, but this transmissions scheme cannot offer private channels. Also, the free-space spectrum is becoming a costly commodity with access governed by the FCC. Fiber-optic transmission offers high bandwidth and data rates, but it does not add to the crowded free-space spectrum.
Information Modulation Schemes
The modulation scheme is the manner in which the information to be transported is encoded. Encoding information can improve the integrity of the transmission, allow more information to be sent per unit time, and in some cases, take advantage of some strength of the communication medium or overcome some weakness.
Three basic techniques exist for transmitting information such as video signals over fiber optics:
- Amplitude modulation (AM) includes baseband AM, radio frequency (RF) carrier AM, and vestigial sideband AM.
- Frequency modulation (FM) includes sine wave FM, square wave FM, pulse FM, and FM-encoded vestigial sideband.
- Digital modulation of the optical light source with the ones and zeros of a digital data stream. A sim- plified explanation is that the light or laser source is off for a digital zero and on for a digital one. In actual practice, the light source never completely shuts off. The light source modulates darker and lighter for digital zero and one information.