The product of bandwidth and length is the decisive parameter of transmission media for determining frequency and length restrictions. In general, this value can be determined for metallic and optical transmission media. In practice, however, the bandwidth-length product is specified exclusively for optical waveguides, since these are used for longer distances.
The product of bandwidth and length (bandwidth in MHz, length in km) depends on the fiber type and the wavelength of the injected light and is determined by mode and material dispersion. It is noticeable that the bandwidth length product increases from the step fiber via the gradient fiber to the single mode fiber by about one power of ten in each case, i.e. by a factor of 10.
The bandwidth length product is the reciprocal of the mode dispersion. For example, if the mode dispersion is 2.5 ns/km in the case of a gradient fiber, then the bandwidth length product is 400 MHz x km. This means that you can transmit 400 MHz over 1 km, 800 MHz over 500 m, or 1 GHz over 400 m. After these distances, the signal must be amplified again.
Typical values for the bandwidth length product for multimode fibers are 200 MHz x km for a light source with 850 nm wavelength and 500 MHz x km for a light source with 1,300 nm wavelength. In the context of Ethernet technologies such as Gigabit Ethernet and 10 Gigabit Ethernet, new, much purer gradient fibers have been developed, the New Fiber, which have bandwidth length products of up to 10 GHz x km at 850 nm.