An optical fiber may be called a light-guide. This term has been borrowed from the radio and microwave field called waveguide.
In this figure, we show a strand of a fiber and its component parts. This figure is exaggerated and not to the scale. It is so to show some points.
A fiber strand or light-guide consists of an inner core surrounded by a cladding. Any additional coverings are protective. In this figure, this protective layer is a plastic coating.
The index of refraction of the core is assigned the notation n1, and the index of refraction of the cladding is assigned the notation n2. These two numbers are very important parameters. When a fiber is constructed such that n1 > n2, the structure (which includes the core and cladding) will act like a waveguide.
Silica glass (SiO2) is the base material for both the core and cladding. Dopants such as boron or germanium are used to adjust the refractive index. The index of refraction equals the speed of light in a vacuum divided by the speed of light in the medium of interest. The refractive index in a vacuum equals 1, by definition.
The practical propagation of light through an optical fiber is best explained using ray theory and Snell’s law.
Simply stated, we can say that when light passes from a medium of higher refractive index into a medium of lower refractive index, the refracted ray is bent away from the normal.
For instance, a ray travelling in water and passing into an air region is bent away from the normal to the interface between the two regions.
As the angle of incidence becomes more oblique, the refracted ray is bent more until finally the refracted ray emerges at an angle of 90° with respect to the normal and just grazes along the surface.
So the (a) figure shows an angle of incidence where the ray of light escapes entirely into free space. The (b) figure shows what is called the critical angle, where the refracted ray just grazes along the surface. The © figure shows the phenomenon of total reflection. This is when the angle of incidence exceeds the critical angle.
A glass fiber, when used for the transmission of light, requires total internal reflection.
Another property of the fiber for a given wavelength λ is the normalized frequency V as shown in this formula.
Here “a” is the core radius, n2 for unclad fiber is 1, and Δ is (n1-n2)/n1
The term square root (n12 – n22) is called the numerical aperture (NA). In essence, the numerical aperture is used to describe the light-gathering ability of a fiber. In fact, the amount of optical power accepted by a fiber varies as the square of its numerical aperture.
It is interesting to note that the numerical aperture is independent of any physical dimensions of the fiber.
To better understand numerical aperture (NA), let’s look at this figure. It shows the acceptance cone. As shown here, sin(θA) equals the numerical aperture NA. The concept of light-gathering capability of a fiber, expressed numerically by the NA, is graphically presented with the acceptance cone.