What frequency is used for multimode fiber

Multimode fiber operates primarily at 850 nm and 1300 nm wavelengths, with modal bandwidths ranging from 200 MHz·km to 4700 MHz·km depending on the fiber type.

Operational Wavelengths

Multimode fibers are designed to transmit light at 850 nm and 1300 nm, which are the standard wavelengths for short-distance communication within buildings or campuses. These wavelengths correspond to the emission of LEDs or VCSELs used in multimode systems, with 850 nm being the most common for high-speed data links and 1300 nm used for slightly longer distances or legacy systems .

Modal Bandwidth and Frequency

The frequency of multimode fiber is often expressed in terms of modal bandwidth, which defines the range of signal frequencies that can be transmitted with minimal distortion over a given distance. Modal bandwidth is measured in MHz·km, representing the product of the bandwidth and the distance over which it is measured . For example:

• OM1 fiber (62.5 µm core) has a modal bandwidth of approximately 200 MHz·km at 850 nm.
• OM2 fiber (50 µm core) offers around 500 MHz·km at 850 nm.
• OM3 fiber (laser-optimized 50 µm) provides 2000 MHz·km at 850 nm.
• OM4 fiber increases this to 4700 MHz·km at 850 nm.
• OM5 fiber supports wideband multimode transmission for multiple wavelengths, maintaining high modal bandwidth for advanced applications .

Factors Affecting Frequency

The achievable frequency in multimode fiber is limited by modal dispersion, which occurs because multiple light modes travel at slightly different speeds through the fiber core. Larger core diameters (50–62.5 µm) allow more modes, increasing modal dispersion and limiting the maximum frequency and distance for high-speed transmission . Using VCSELs and laser-optimized fibers (OM3–OM5) reduces modal dispersion, enabling higher data rates and longer distances at higher frequencies .

Practical Implications

• Multimode fiber is ideal for short-range, high-speed networks such as data centers, enterprise buildings, and campus backbones.
• The modal bandwidth determines the maximum data rate over a given distance; higher bandwidth fibers (OM3–OM5) support 10G, 25G, 40G, and 100G Ethernet over hundreds of meters.
• Frequency performance is also influenced by the launch conditions of the light source, fiber quality, and connector types .

In summary, the frequency capability of multimode fiber is defined by its modal bandwidth and operational wavelength, with modern OM3–OM5 fibers supporting high-frequency signals suitable for multi-gigabit short-distance communication.