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Fiber optic cable is known for its very low attenuation (0.2dB/km) and wide bandwidth which allows for tremendous capacity enabled fiber links to span across oceans.Fiber optic cable is also known for the lowest latency which allows for super-fast data transmission required for real time video and data transmission. Whatever the requirement (speed, capacity or reach), the degradation that the signal experiences traversing through the fiber and different passive and active elements in-between before reaching the receiver needs to be meticulously calculated while planning an optical network.
The fundamental impairment, called the insertion loss is the loss in optical power that the signal experiences when an optical component (can also be simple splicing/ connectors) is introduced in the transmission line. The insertion loss is measured as the ratio of output optical power to input optical power with respect to the device under test and is expressed in [dB] as,
IL [dB] = 10 log10 (Pout / Pin).
Figure: Insertion loss arising from different elements added into a point-to-point link
An optical transceiver typically comes with the value for an optical power budget (PB). As the name implies, it defines the amount of optical power available for successfully transmitting signal over a distance of optical fiber. The transmission distance cannot yet be determined, as all the signal losses (insertion losses) at every component between the transmitter and receiver in the optical path reduces the available optical power budget. The most common optical components are connectors, splices, mux/demux, couplers, filters, and optical patch panels. The remainder of the optical power is calculated by subtracting the each of these loss terms from PB. Typically, a link loss margin of 3-5 dB is assumed to enclose the temporal degradation of the components. After subtracting the margin from the remainder, the maximum transmission distance can then finally be calculated by dividing the remainder by the known value for fiber attenuation.
Therefore it is very critical to know the insertion loss values of the components spread through the link to make a correct calculation. An OTDR (Optical Time Domain Reflectometry) can also be used to measure the exact loss profile of the link when the exact components or their insertion losses are unknown.