When selecting components for a fiber optic system, there are two optical fiber factors that affect transmission performance: bandwidth and attenuation.

Bandwidth

Bandwidth is the measure of the data carrying capacity of the fiber. The greater the bandwidth, the greater the information capacity.

Bandwidth is expressed in a frequency distance form (MHz-km). Example: A 200 MHz-km fiber can move 200 MHz of data up to one kilometer or 100 MHz of data as far as two kilometers.

Attenuation

In addition to physical changes to the light pulse which result from frequency or bandwidth limitations, there are also reductions in the level of optical power as the light pulse travels to and through the fiber.

This optical power loss. or attenuation, is expressed in dB/km (decibels per kilometer) at a specified wavelength.

Intrinsic Optical Fiber Loss

Light is an electromagnetic wave of vibrating nature. Short wavelengths are in the ultraviolet spectrum.

Microwaves, radar, television and radio operate in the longest wavelength areas. In between the ultraviolet and the microwave spectrums, we have fiber optic wavelengths, which are in the infrared spectrum.

Just as the speed of light slows when traveling in transparent materials, each infrared wavelength is transmitted differently within the fiber. Therefore, attenuation, or optical power loss, must be measured in specific wavelengths for each fiber type.

Wavelengths are measured in nanometers (nm) - billionths of meters - which represent the distance between two cycles of the same wave.

Losses of optical power at the different wavelengths occur in the fiber due to absorption, reflection and scattering. These occur over distance depending on the specific fiber, its size, purity and refraction indexes.

The amount of optical power loss due to absorption and scattering of optical radiation at a specified wavelength is expressed as an attenuation rate in decibels of optical power per kilometers (dB/km).

Fibers are optimized for operation at certain wavelengths. For example, less than 1 dB/km loss is attainable in 50/125µm multimode fiber operating at 1300nm, and less than 3 dB/km (50% loss) is attainable for the same fiber operating at 850nm.

These two wavelength regions, 850 or 1300nms, are the areas most often specified for fiber optic transmission today. These wavelengths are commercially usable with today's transmitters and receivers. Optical fibers have also been optimized at the 1550 nm region for single mode transmission systems.

Microbending Loss

Without protection, an optical fiber is subject to losses of optical power caused by microbending. Microbends are minute fiber deviations caused by lateral forces which cause optical power loss from the core.

Different types of protection for the fiber are available to minimize microbending.

Step index fibers are relatively more resistant to microbending losses than graded index.