Basics of Industrial Fiber Optics

Fiber Optics Overview

Fiber Optics is the transmission of light signals through hair thin strands of glass or plastic. Historically, the standard medium for factory networking was copper cable. While the benefits of fiber optic technology were generally recognized, it was considered to be too expensive to install and difficult to terminate.

Technological improvements have led to a significant decrease in the expense of fiber optic systems, improvements in cable and components and refined installation and termination. Today's fiber optic cable is much smaller, lighter in weight, and more flexible than wire cable with similar information carrying capacity. Low-loss glass fiber optic cable also offers almost unlimited bandwidth, making it ideal for “smart” factories where considerable amounts of data must be transmitted.  Its numerous advantages 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.