Fiber Optics Introduction
The principles of transmitting light by total internal reflection were first put forth in 1621. It wasn't until 1854, however, that the Englishman John Tyndall observed water spewing horizontally out of a rain barrel in the bright sunlight and noted that the reflected light seemed to be swept along with the water as it arced to the ground, rather than emerging from the water in a straight line. Tyndall reproduced this phenomenon in his laboratory and began to experiment with light transmission through total internal reflection. This was the basis of all fiber optic development.
One of the first practical uses of fiber optics was discovered in the 1950s when N. S. Kapany bundled glass fibers together into a coherent arrangement to make the first fiberscope. This was used to view internal body organs. The fiberscope has many industrial and aerospace uses.
Because of its high attenuation, the optical fiber was originally not considered for use in communication. However, in 1970 Donald Keck and Robert Mauer of Corning Glass developed a stepped index fiber with a loss of no more than 20 dB per kilometer. Today, new combinations of core and cladding material have provided fiber with a loss far below this “20 dB” fiber.
Fiber is made of two types of material that only differ by a slight amount. The inside silica material is called the “core” (n1) and the surrounding silica material is called the “cladding” (n2). To make the two materials different from each other, elements called “dopants” are diffused into the silica to change the index of refraction or optical density. The light in the fiber is mainly confined to the core due to the fact that the core index is doped such that it is only slightly larger than the index of the cladding. Typically, if the cladding index of refraction is 1.45 then the core index of refraction is around 1.46.
An outer coating protects the glass fiber (the core and the cladding). This coating is usually a polyamide or acrylate polymer. In a spaceflight cable, a loose tube buffer made of polyester elastomer surrounds the coated fiber.
The strength members surrounding the buffer are made of Kevlar in commercial cable but must be a Teflon/fiberglass material for spaceflight cable. Kevlar tends to hold moisture and can cause the actual silica structure to alter if it exposed to water for a long enough period of time. The outside jacketing is usually made of a Tefzel material.
Some key parameters to consider in fiber optic behavior are:
- Numeric Aperture
- Modes and V Number
Types of losses include:
- Fiber Cracks
- Fiber Mismatch
- Air Gaps
- End Face Obstructions
Fiber Optic Workmanship contains the following steps:
- Fiber Preparation
- Epoxy Mixing
- Connector Assembly
The overarching NASA standard for fiber optics is: FIBER OPTIC TERMINATIONS, CABLE ASSEMBLIES, AND INSTALLATION, NASA-STD 8739-5.