Fiber Optic
Overview
Fiber Optics is the transmission of light signals through hair
thin strands of glass or plastic. Historically the standard medium for factory
networking has been copper cable, in the past fiber optic technology was
expensive to install and difficult to terminate.
Technological improvements have led to a decrease in the expense of fiber
optic systems, improvements in cable and components, and refined installation
and termination. Today's fiber optic cable is usually much smaller, lighter in
weight, and more flexible then wire cable with similar information carrying
capacity. Today's low-loss glass fiber optic cable offers almost unlimited
bandwidth and advantages that set it apart from any transmission media
previously used.
Fiber Optic Cable Construction
Fiber optic cable is composed of:
- Core - The center of the fiber and is the vehicle for the transmission of
optical (light) signals and ranges in size from 5 to 200 microns in diameter.
- Cladding - The optical material which surrounds the core and causes the
light from the core that strikes it to be reflected or refracted keeping the
optical information strictly in the core. The cladding added to the core
increases the the diameter of the glass fiber to the range of 125 to 230
microns.
- Buffer Coating - The outer plastic coating protecting the fragile core and
cladding from damage.
- Jacket - This is sometimes used to provide additional protection to the
core and cladding. Strength members of various materials are added inside the
jacket to improve pull strength of the jacketed fiber.
System Basics
The basic point-to-point fiber optic transmission
system consists of 3 basic elements; the optical transmitter, fiber optic cable
and the optical receiver.
- Optical transmitter - Converts analog or digital input signals into
modulated light for transmission over an optical fiber. The most common
devices used as light sources in optical transmitters are Light Emitting
Diodes (LED's) and Laser Diodes (LD's). LED's have relatively large emitting
areas, they are more economical and generally used for short to moderate
transmission distances. LD's have very small light emitting surfaces and can
transmit many times more power to the fiber than an LED so it can be used for
long transmission distances. LD's are more expensive and may require
additional electronic circuitry to maintain system stability. The most popular
wavelengths of operation for optical transmitters are 850, 1300 or 1550
nanometers.
- Fiber optic cable - The transmission vehicle for the input signals. The
two commonly used fiber optic cables are multimode and singlemode.
Multimode Fiber - Has a larger core usually 62.5 microns and is used with
LED sources of 850 and 1300 nanometers.
Singlemode Fiber - has a much
smaller core usually about 9 microns and is used with laser sources at 1300
and 1550 nm.
- Optical receiver - converts the optical signal back into a replica of the
original electrical signal.
