Bandwidth

The optical losses and usable bandwidth of a fiberoptic system have to be taken into account. As mentioned
previously, multimode fibers have greater losses and less bandwidth compared to single mode.
Single mode has lower losses and very high bandwidth than does multimode.

Most manufacturers of multimode fiber-optic cable do not specify dispersion. They will provide a figure
of merit known as the bandwidth-length product or just bandwidth with units of MHz-kilometer. For
example, 500 MHz-km translates to a 500 MHz signal that can be transported 1 km. The product of the
required bandwidth and transmission distance cannot exceed 500:

BW × L ≤ 500

A lower bandwidth signal can be sent a longer distance.
A 100 MHz signal can be sent

L = BW – product/BW
= 500 MHz-km/100 MHz
= 5 km

Single-mode fiber typically has a dispersion specification provided by the manufacturer. The dispersion
is specified in picoseconds per kilometer per nanometer of light source spectral width or ps/km/nm. This
loosely translates to the wider the spectral bandwidth of the laser light source, the more dispersion. The analysis of dispersion of a single-mode fiber is very complex. An approximate calculation can be made with
the following formula:

BW = 0.187/(disp × SW × L),

where:

disp is the dispersion of the fiber at the operating wavelength with units seconds per nanometer per
kilometer.

SW is the spectral width (rms) of the light source in
nanometers.

L is the length of fiber cable in kilometers.

For example, with a dispersion equal to 4 ps/nm/km, spectral width of 3 nm, and a transmission length
of 20 km, then:

BW = 0.187/(4 × 10–12 s/nm/km) × (3 nm) × (20 km)
BW = 779,166,667 Hz or about 800 MHz.

If the spectral width of the laser light source is doubled to 6 nm the bandwidth will drop to about 390
MHz. This shows how significant the spectral width of the laser source is on the usable bandwidth of a fiber.
If a laser light source with a narrow optical spectral width is used, or a fiber with a lower dispersion figure,
the bandwidth and transmission distance will increase.

In single-mode fiber communications, there are two basic types of laser light sources. The first type is the
less expensive laser that uses Fabre-Perot laser diode (FP-LD) technology. The FP-LD is an inexpensive
choice for digital fiber-optic communication. With a spectral width of typically 4 nm or more, it is primarily
used for lower bandwidth or short-distance applications. The second is the distributed feedback
laser diode (DFB-LD) technology. These light sources are more expensive and are widely used for longdistance fiber-optic communications. The typical spectral width for a DFB laser is about 1 nm. When a DBF laser is used in combination with a low dispersion fiber, the transmission bandwidth and distance can be significantly higher.

Table 6-10-2 Typical Optical Fiber Loss

Table 6-10-4 Typical Fiber Optic Bandwidth

See Table 6.10-2, which shows the typical fiber-optic cable losses, and Table 6.10-4, which shows the bandwidth for different types of fiber cable.

Applications, Bandwidth of the Fiber Optic Medium, NAB Engineering Handbook, Tech Talk with Jim Jachetta bandwidth, data rate, fiber, multimode, optic, singlemode