Attenuation
Because the core of a fiber is made of glass, impurities (such as iron, magnesium, or even water) and irregular structures can cause the light irradiance to decrease, a condition known as attenuation, as the light travels through kilometers of the core. The attenuation factor is well known for all the types of glass used in long-haul fiber-optic cables; you can find it in the fiber manufacturer’s catalog.
The unit decibel describes the ratio of the optical power input into the fiber to the optical power measured at the output of the fiber of some length; it helps gauge attenuation. Power, which is the rate at which the light carries energy, is a more-convenient, more easily measured quantity than irradiance for characterizing the behavior of light in a fiber, so most fiber systems characterize the light-carrying capabilities of fibers by noting the effect on the power.
The decibel is a log base 10 scale, so the number of decibels is equal to –10log(Pout/Pin), which means that for a power ratio of 1/10, the measure is 10 decibels; 1/100 is 20 decibels, 1/1,000 is 30 decibels, and so on.
The equation that tells you how much power you can get out of a fiber of a certain length is In this equation,
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