One of the first practical uses of fiber optics was in the 1950’s when 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 found its way into many industrial and aerospace uses. Optical fiber was not considered for use in communication because of its high attenuation until 1970 when Keck and 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 Keck’s “20 dB” fiber.
Fiber is made of two types of material differing only 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 one different from the other, materials called “dopants” are diffused into the silica to change the index of refraction or optical density. The fact that the core index is doped such that it is only slightly larger than the index of the cladding is the mechanism that keeps light confined mainly to the core. Typically if the cladding index of refraction is 1.45 than the core index of refraction is around 1.46.
An outer coating protects the glass fiber (core and 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 exposed to water for a long enough period of time. The outside jacketing is usually made of a Tefzel material.
The entire Electro-magnetic spectrum spans from a wavelength of 100 km at 3 kHz frequency (audio) to a wavelength of 10-4 nm at 1010 Hz frequency (gamma rays).
Some key parameters to consider in fiber optic theory 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.