Introduction to Optical Fibers

There are two types of optical fibers: multimode and single-mode (or monomode). These types define how light travels inside the fiber core.

Multimode: On multimode fibers the core diameter is greater than the core diameter of single-mode fibers, making the light to have several propagation modes, i.e. the light goes through the fiber core using several paths and not using a single path, like in single-mode fibers. Multimode fibers have a core diameter 50 to 100 microns (typical commercial values are 50, 62.5 and 100 microns) and a cladding diameter of 125 microns. Multimode fibers can be classified into graded-index and step-index, depeding on the refraction index between the core and the cladding - on graded-index there is a gradual change between the core and the cladding, while on step-index this change is abrupt, hence the name. Step-index fibers can transmit data up to 50 Mbps, while grade-index fibers can transmit data up to 1 Gbps. Multi-Mode fibers are also known as MMF and they are used by short-distance fibers.

Single-mode: single-mode fibers are used in long-distance cables, but they require connectors with better precision and expensive devices. On this kind of fiber the light has only one way of travelling inside the fiber core, hence its name. The core diameter is between 7 and 10 microns and its cladding diameter is around 125 microns, so both multi-mode and mono-mode cables have the same diameter, what makes the difference is the diameter of the core. There are three types of single-mode fibers: non dispersion-shifted fiber (NDSF), dispersion-shifted fiber (DSF) and non zero-dispersion-shifted fibers (NZ-DSF).

Optical Fiber

Figure 3: Difference between graded-index multimode, step-index multimode and single-mode optical fibers.

ITU released a series of standards in order to classify the properties of multimode and single-mode fibers:

ITU G.651: Graded-index multimode fibers with 50-micron core diameter and 125-micron cladding diameter.
ITU G.652: Non dispersion-shifted fiber (NDSF) single-mode fibers using 1,130-nm wavelenght supporting the following distances and transfer rates: 1,000 km (621 miles) at 2.5 Gbps, 60 km (37 miles) at 10 Gbps and 3 km (1.86 miles) at 40 Gbps.
ITU G.653: Dispersion-shifted (DSF) single-mode fibers.
ITU G.655: Non-zero dispersion-shifted (NZ-DSF) single-mode fibers. It supports the following distances and transfer rates: 6,000 km (3,730 miles) at 2.5 Gbps, 400 km (250 miles) at 10 Gbps and 25 km (15.5 miles) at 40 Gbps.

Important: Optical fibers trasmit light in a wave lenght invisible to the human eye. So, we can never look directly to the end of an optical fiber while it is connected to a system, because we can go blind looking at it.