Link Loss Calculation

                 Losses occur at many points in a fibre optic system. We have to ensure that the light source launches enough power into the fibre to provide enough power at the receiver. The receiver has limited sensitivity.

Transmitter output - Receiver input = Losses + Margin (All calculations are done in dB)

        Types of Loss

For single mode fiber cable with two most commonly used wavelengths— 1310 nm and 1550 nm—The attenuation measurement will vary depending upon which wavelength is in use. Attenuation is measured in dB and is quoted as attenuation in dB/km.

         Fiber Loss Variables

• Attenuation:

All fiber has losses from absorption and back reflection of the light caused by impurities in the glass. Attenuation is a function of wavelength and needs to be specified or measured at the wavelength in use.

• Modal Dispersion:

The higher the data rate, the shorter the distance the signal can travel before modal dispersion creates an inability to accurately detect the signal (i.e. a "1" from a "0").

• Dispersive Losses:

Another dispersion effect, which causes pulse spreading, and limits distance is chromatic dispersion, where the broader spectrum of light can result in varying travel times for different parts of a light pulse.

• Splices:

Although small and often insignificant, there is no perfect loss-less splice. Many errors in loss calculations are made due to a failure to include splices. Average splice loss is usually less than 0.1 dB.

• Connectors:

Like splices, there is no perfect loss-less connector. It is important to note that even the highest quality connectors can get dirty. Dirt and dust can completely obscure a fiber light wave and create huge losses. A 0.5 dB loss per connector is commonly the worst case scenario assuming a cleaned and polished connector is used. There will always be a minimum of two connectors per fiber segment, so remember to multiply connector loss by two.

• Safety Buffer:

It is common to add a loss as a design margin. Allowing 2 - 3 dB of loss can take fiber aging, poor splices, temperature and humidity, etc., into account and ensure a solid system.