First of all, let us understand what an optical fiber is. Optical fiber is a fiber constructed of glass or plastic of a specific nature. It does not contain any metal material and thus avoids Electro-Magnetic Interference (EMI) and distortion of information along with the distance. This results in high accuracy of data along the transmission cable. The objective of fibre optic cable is to carry light waves along its length. The core technology for optical transmission is total internal reflection. Total internal reflection to ensures that the light is always along the body inner reflection material, so always keep a fiber in the body. As a result, the loss of signal is minimized, and hence the distance that the message can be carried without losing the signal is maximized. The light waves carrying the information along the channel have a path of propagation.

Multimode fiber optic cable can support the transmission of multiple paths of propagation with a higher amount of information at the same time. On the other hand, a single mode cable can only support a propagation path with a smaller bandwith. Because multimode cables carry higher amount of information at the same time, they have a higher diameter of the core used to carry the message compared to the single mode ones. Also, the higher diameter can make the multimode cables with higher transmission power. Usually multimode cables are used for short distance communications for usually a distance less than 500 meters, while single mode cables are popular for long distance communications.

Multi mode fiber optic cable has a large diametral core that allows multiple modes of light to propagate. Because of this, the number of light reflections created as the light passes through the core increases, creating the ability for more data to pass through at a given time. Because of the high dispersion and attenuation rate with this type of fiber, the quality of the signal is reduced over long distances. This application is typically used for short distance, data and audio/video applications in Local Area Networks (LANs). However, RF broadband signals cannot be transmitted over multimode fiber.

Multimode fiber cable is usually 50/125 and 62.5/125 in construction. This means that the core to cladding diameter ratio is 50 microns to 125 microns and 62.5 microns to 125 microns (shown as the figure). The transition between the core and cladding can be sharp, which is called a step-index profile, or a gradual transition. The two types have different dispersion characteristics and thus different effective propagation distance. Multimode fibers may be constructed with either graded or step-index profile, and those with graded index fiber is better in accuracy and performance. The numbers 50 µm and 62.5 µm refer to the diameters of the glass or plastic core, the part of the fiber that carries the light which encodes your data. The dimensions are sometimes specified as 50/125 μm and 62.5/125 μm, to include the diameter of the cladding. (The cladding confines the light to the core because it has a lower index of refraction.) Cable construction is shown in the following diagram, indicating the cable core, cladding, and outer jacket diameters.

Multimode fibers are identified by the OM ("optical mode") designation as outlined in the ISO/IEC 11801 standard.

• OM1 multimode fiber, for fiber with 200/500 MHz*km overfilled launch (OFL) bandwidth at 850/1300nm (typically 62.5/125um fiber)
• OM2 multimode fiber, for fiber with 500/500 MHz*km OFL bandwidth at 850/1300nm (typically 50/125um fiber)
• OM3 multimode fiber, for laser-optimized 50um fiber having 2000 MHz*km effective modal bandwidth (EMB, also known as laser bandwidth), designed for 10 Gb/s transmission
• OM4 multimode fiber, for laser-optimized 50um fiber having 4700 MHz*km EMB bandwidth designed for 10 Gb/s, 40 Gb/s, and 100 Gb/s transmission

Article Source: Multimode Fiber Optic Cables Tutorial