The index of refraction (IOR) is a way of measuring the speed of light in a material. Light travels fastest in a vacuum, such as outer space. The actual speed of light in a vacuum is 300,000 kilometers per second, or 186,000 miles per second.
Index of Refraction is calculated by dividing the speed of light in a vacuum by the speed of light in some other medium.
The Index of Refraction of a vacuum by definition has a value of 1.
The typical value for the cladding of an optical fiber is 1.46. The core value is 1.48. The larger the index of refraction, the more slowly light travels in that medium.
When a light ray traveling in one material hits a different material and reflects back into the original material without any loss of light, total internal reflection occurs.
Since the core and cladding are constructed from different compositions of glass, theoretically, light entering the core is confined to the boundaries of the core because it reflects back whenever it hits the cladding. For total internal reflection to occur, the index of refraction of the core must be higher than that of the cladding.
As mentioned earlier, electrical signals are converted to light signals before they enter an optical fiber. To ensure that the signals reflect and travel correctly through the core, the light must enter the core through an imaginary acceptance cone.
The size of this acceptance cone is a function of the refractive index difference between the core and the cladding.
In simpler terms, there is a maximum angle from the fiber axis at which light may enter the fiber so that it will propagate, or travel, in the core of the fiber. The sine of this maximum angle is the numerical aperture (NA) of the fiber. Fiber with a larger NA requires less precision to splice and work with than fiber with a smaller NA. Single-mode fiber has a small NA.
