Understanding Fiber Optics


CHARACTERISTICS OF FIBER OPTIC LINKS A fiber optic interconnection system can generally be described by the following sketch:




Optical patchcord

It is made of the following elements : • emission system It contains a light source. To obtain a well functioning system, it is important to know : • the type of light source (LED or laser). • the operating wavelength (1300 nm for example). • the light source power. • the interface connector type at the light source output. • one or more optical patchcords The pachcords are defined by: • the fiber optic cable described by its length, the number of channels, the fiber type (singlemode, multimode 50/125 or 62,5/125). • the connectors terminated at both ends of the cable. • the whole patchcord is characterized by a loss named Insertion Loss, expressed in dB. This loss gives the quantity of lost light when introducing this patchcord in the transmission line. It includes the linear attenuation of the cable and the loss due to the connectors. • for singlemode patchcords, another important parameter, called Return Loss, represents the quantity of light which is reflected by the line and which returns in the direction of the source. The laser sources used in singlemode applications can be very sensitive to this phenomenon.

P in

P out

optical link

P back

The Insertion Loss of the link is: The Return Loss of the link is:

PI = 10 log (Pout / Pin) RL = 10 log (Pback / Pin)

• reception system It contains a receiver which has a photosensitive surface, and which converts the light into electric signal. It is important to know: • its sensitivity, i.e. the minimal quantity of detectable light. • the interface connector type at the receiver input.

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