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Multimode fiber-optic cable is a glass, plastic, or plastic-clad silica (PCS) optical core wrapped in non-absorptive cladding and used in the transmission of multiple wavelengths of light for short-distance digital communication. Multimode transmission varies the reflection angles of thousands of waveforms per second, carrying encoded digital information from transmitters to receiving decoders for conversion back to electronic signals. These waves can disperse in different manners over distance, which makes multimode fiber more suitable for use in applications of about 3 miles (five km) or less. Their cores, wider than single mode fibers, are about the width of a few human hairs, from about 60 to 900 microns (µm). They typically transmit infrared light from 850-1,300 nanometers (nm) from light-emitting diodes (LED).
Light wavelengths of about 850 nm serve the shorter distances of multimode fiber-optic cable, while wavelengths of 1,300 nm serve its longer ranges. These wavelengths traverse the fiber at critical angles, compelling them forward to converge as a single pulse at the destination point. The straighter low mode waves stay closer to the axis of the core. High mode waves bounce floor to ceiling off the cladding, losing some energy as heat and sometimes arriving later than the lower modes. This means that multimode fiber has more attenuation, or signal loss, and modal dispersion than the long-distance laser transmissions of single mode fiber.
In most applications of multimode fiber-optic cable, wave division multiplexing (WDM) is not used, so dual cores run the length of the fiber in order to increase transmission capacities. Typically, multimode fibers transmit data at rates of 10 megabits per second (Mb/s) to 10 gigabits per second (Gb/s). Multimode signal dispersions and attenuations worsen with distance, which can result in degraded or failed transmissions.
Numerous dispersion effects compound with distance, which can degrade signals along the waveguide. This is why more powerful single mode fibers are used for greater distances. In practical terms, optimizing transmission carrying capacities, distances, and supporting technologies means that the thousands of simultaneous telephone calls carried by copper networks can now exceed millions with the advent of optical-digital networks.
Light waves travel down the multimode fiber-optic cable in essentially two manners: step-index and graded-index propagation. Step-index mode resembles more of a zigzag pattern in cores of up to 100 µm in diameter. Transmission separates its waves to minimize signal overlap, which limits information carrying capacity. This mode is more suitable for short-length applications, as in handheld fiber-optic scopes, and should not be confused with single mode step index, in which parallel laser rays travel along a straight axis through a very narrow core.
Graded index mode carries helical waves. The high mode waves that bounce near the outer cladding move faster than the low mode waves near the axis. Higher modes ultimately travel a greater total distance, so they ideally arrive simultaneously with the lower mode waves in order to reduce dispersion and be read as a single pulse.
Typically made from glass, more plastic clad silica and plastic optical fiber (POF) materials have become available, further reducing costs. The least expensive and most common fiber type, multimode fiber-optic cable is widely used in local applications and infrastructures. Thin, nonflammable, and resistant to electrical and radio interferences, these durable, low-power digital networks are likely to find continued expansion into the domain of copper wire and beyond.