The ratio between the velocity of light in a vacuum to the velocity of light in a transmission medium, such as a given type of glass. Refraction refers to the phenomenon by which light changes velocity and changes direction (i.e., bends or refracts) as it exits one medium and enters another of different density.The IOR is mathematically expressed as n = c ÃƒÂ· v
where c is the velocity of light in a vacuum, and v is the speed of light in the given medium. So, IOR is the mathematical inverse of velocity of propagation (Vp). IOR is a convenient means for expressing the differences between the speed of light in different types of optical fiber, as well as between the core and cladding of a glass optical fiber (GOF).Table I-2 provides approximate IOR comparisons of various substances.
Table I-2: Index of Refraction (IOR)
|Medium||Signal Velocity (km/s)||Velocity of Propagation (Vp)||Index of Refraction (IOR)|
Not all glass is created equal, by the way.The raw material for all glass is quartz sand, a very pure sand comprising nearly 100 percent crystalline quartz silica. During the manufacturing process, the glass is purified to reduce the slight amount of iron oxide that might be present, various dopants (i.e., impurities) are added to alter brittleness and other characteristics, and heat treatments can be applied to produce tempered glass. Glass optical fiber is typically doped with some amount of germanium oxide (GeO2) or other compounds, which increase the IOR and variously impact other operating characteristics of the fiber at certain wavelengths. Erbium-doped fiber amplifiers (EDFAs) are used extensively in long haul fiber optic transmission systems (FOTS) in place of more traditional optical repeaters. It is extremely important to know the IOR of a given cable in order to calculate latency, i.e., the delay imposed on the signal, from end-to-end, by the medium. Latency has significant impact on the synchronization of transmitters, repeaters, multiplexers, and other active devices in a digital transmission system, and particularly in a high speed system such as a FOTS that runs at signaling speeds up to 40 Gbps. The IOR also has considerable impact on the physical construction of a glass optical fiber, which consists of an inner core surrounded by one or more layers of cladding.The optical signal is intended to travel through the inner core, and the cladding serves in various ways to ensure that happens. Step-index fiber is characterized by a sharp decrease in the IOR between the core and cladding, i.e., the cladding is sharply lower in IOR than the core material. This sharp step of approximately one percent in IOR at the core/cladding interface causes any errant light rays to reflect back into the core in a phenomenon known as total internal reflection. Graded-index fiber is characterized by a gradual decrease in the refractive index of the cladding through a great many layers of glass.The approach causes the errant light rays to gradually gain in velocity and bend back towards the core. Step-index construction is used largely in single-mode fiber (SMF) and graded-index in multimode fiber (MMF). See also core
, graded-index fiber
, step-index fiber
, and Vp