Fiberoptic cable parameters Attenuation

The attenuation of a multimode fiber depends on the wavelength and the fiber construction, and ranges from around 3-8 dB/km at 850 nm and 1-3 dB/km at 1300 nm.

The attenuation of single-mode fiber ranges from around 0.4-0.6 dB/km at 1300 nm and 0.25-0.35 dB/km at 1550 nm.

Diameter

The fiber diameter is either 50 or 62.5 microns for multimode fiber or 8.5 microns for single mode.

• Multimode fibers (50 or 62.5 microns): In multimode fibers a beam of light has room to follow multiple paths, which are called modes, through the core. Multiple modes in a transmission produce signal distortion at the receiving end, due to the difference in arrival time between the fastest and slowest of the alternate light paths.

• Single-mode fibers (8.5 microns): In a single-mode fiber, the core is so narrow that the light can take only a single path through it. Single-mode fiber has the least signal attenuation, usually less than 0.5 dB/km. This type of cable is the most difficult to install, because it requires precise alignment of the system components and the light sources and detectors are very expensive. However, transmission speeds of 50 Gbps and higher are possible.

Wavelength

Fiber-optic systems today operate in one of three wavelength bands; 850, 1300 or 1550 nm. The shorter wavelengths have a greater attenuation than the longer wavelengths. Short-haul systems tend to use the 850 or 1300 nm wavelengths with multimode cable and light emitting diode (LED) light sources. The 1550 nm fibers are used almost exclusively with the long-distance systems using single-mode fiber and laser light sources.

Bandwidth

The bandwidth of a fiber is given as the range of frequencies across which the output power is maintained within 3 dB of the nominal output. It is quoted as the product of the frequencies of bandwidth multiplied by distance, for example 500 MHz-km. This means that 500 MHz of bandwidth is available over a distance of 1 km or 100 MHz of bandwidth over 5 km.

Dispersion

Modal dispersion is measured as nanoseconds of pulse spread per kilometer (ns/km). The value also imposes an upper limit on the bandwidth, since the duration of a signal must be larger than the nanoseconds of a tail value. With step-index fiber, expect between 15 and 30 ns/km. Note that a modal dispersion of 20 ns/km yields a bandwidth of less than 50 Mbps. There is no modal dispersion in single-mode fibers, because only one mode is involved.

Chromatic dispersion occurs in single mode cables and is measured as the spread of the pulses in picoseconds for each nanometer of spectral spread of the pulse and for each kilometer traveled. This is the only dispersion effect in single mode cables and typical values are in the order of 3.5 ps/nm-km at 1300 nm and 20 ps/nm-km at 1550 nm.

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