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Everything You Ever Wanted to Know About dB But Were Afraid to Ask Telecommunications systems used electrical signals and copper wire to transmit voice messages long before lasers lightwaves and fiber optic cable To describe and measure power levels in electrical systems telecommunication engineers used the standard unit of decibels to express gain or loss and relative power levels Meanwhile scientists working with fiber optic signals were using units of milliwatts mW to determine the amount of light traveling down a fiber or the amount of light coupling from one fiber to another as would be expected with optical radiation As the telecommunications industry began to use fiber it did not adopt the milliwatts terminology Just the opposite happened fiber optics adopted the traditional telecommunications language of decibels or dBDecibel is defined as a unit used to express relative difference in power usually between acoustic or electric signals equal to negative ten times the common logarithm of the ratio of the two levels The main reason dB is used is because it makes power levels more manageable Thus its easier to add up power losses in a system For example a system with 40dB of fiber loss 25dB of connector loss30dB of splitter loss and05B of splice loss results in a 10dB system loss or the sum of each component loss Translating 10dB into a percentage based upon the formula given earlier results in a signal that is 10 of the original intensity Because dB can be used to describe both gain and loss it is important to carefully consider the given optical parameter A decibel expressing loss is a negative unit However in the fiber optics industry it is common practice to omit the negative sign and speak of a 3dB loss rather than-3dB For example a back reflection level of -40dB and 40dB are generally taken to mean the same thing reflected light 001 or light is reduced by 9999 and therefore one
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