The fiber optic strands contain two concentric layers of silica glass, known as the core and the cladding, enclosed in a protective sheath. The core and the cladding are of different purities and so have different refractive indices, designed to ensure the light signals are transmitted along the core without escaping through the cladding that has a lower refractive index. The light itself is modulated according to the data into digital pulses through the use of a laser or LED (Light Emitting Diode). The lower refractive index of the cladding ensures the light signals are bent back into the core and this is known as TIR (Total Internal Reflection). Light will only escape through the cladding from the core if the angle exceeds what is known as the critical angle.
Fiber optic cable is usually categorised as Multimode or Single Mode, with multimode generally coming in main types:
Step-Index Multimode Fiber - where the core is typically in the range 50 - 100 microns in diameter. The light travels down the core in two ways, either directly through the core, or by zig zagging through the core through the action of Total Internal Reflection, where the light is reflected back into the core from the cladding. The different paths are referred to as modes, as they arrive at the destination separately in time due to the different length of the paths. The light pulse is an aggregate of the multiple modes, so the definition of the pulses can become more spread out with distance. For this reason multimode fiber is generally suited to short distance communications.
Graded-Index Multimode Fiber - has a core in which the refractive index decreases gradually from the centre of the core towards the outer extremities nearing the cladding. The fact that the refractive index at the centre of the core is higher, the light rays advance more slowly the nearer the centre of the core than nearer the cladding. Rather than zig zagging like Stepped-Index Multimode Fiber, the light traversing the core curves helically because of the graded index, thus reducing the travel distance. Most of the light therefore arrives at the destination around the same time, allowing the definition of the pulses to be maintained with less dispersion, often referred to as modal dispersion.
Single-Mode Fiber - is designed with a much narrower core of around 8 microns or less, with the difference between the refraction index of the core and cladding being less. The light rays tend to travel straight down the core in parallel, thus causing very little dispersion. For this reason, this type of fiber can be used over much longer distances when compared to Multimode fiber.
When determining which wavelengths of light to use for fiber optic communications, looking at the electromagnetic spectrum there are 3 principle windows of operation where the attenuation of signals is lower. This makes transmission at 3 particular wavelengths more efficient and provides the ability for a transmitter to efficiently generate the light and receiver to more easily detect the light pulses. The 3 windows are centered at 700nm, 1330nm and 1550nm. Multimode fibers tend to use 700nm with Light Emitting Diodes producing the light over relatively short distances. For ranges over 5Km, Single Mode fibres operate in the 1330 and 1550nm wavelengths. At 1550nm it is possible to have up to 240 separate wavelengths operating simultaneously, with each wavelength carrying 2.5Gbit/s of data. Although this is theoretically possible, practical systems are operational with 80 wavelengths on a single fiber optic pair. These systems come under the heading of DWDM (Dense Wavelength Division Multiplex).