t. Thus, 10 dBm represents a signal 10 dB above or higher than 1 mW, while 20 dBm represents a signal 20 dB above 1 mW, and SO on. Since a 30-dBm signal is 30 dB or 1000 times larger than a 1-mW sig- nal, this means that 30 dBm is the same as 1 W. We can use this relation- ship to construct the watts-to-dBm conversion table that is contained in Table 2.2. In the wonderful world of transmission over fiber optics, the decibel- milliwatt (dBm) is used to reference optical power to 1 milliwatt. However, in addition to dBm you may periodically encounter the term dBu, SO let's turn our attention to that term. DECIBEL ABOVE 1 mW The term dB is used for optical power ref- erenced to 1 microwatt (wW). Thus, output power 1-uW input Power Budget Many times when we work with fiber optics we will encounter the term power budget without an explanation of its meaning. As we have just dis- cussed decibels we will briefly discuss power budget, although we will return to this metric when we cover fiber in the next chapter. TABLE 2.2 Power in watts Power in dBm, dBm Relationship between Watts and 0.1 mW Decibel-Milliwatts Team-Fly Understanding Light The term power budget is used to reference the difference in dB between the transmitted optical power, typically expressed in dBm, and the receiver sensitivity, also commonly expressed in dBm. dB is used, as the connection of circuits in tandem permits the power levels in deci- bels to be arithmetically added and subtracted. For example, if a known amount of optical power, in decibel-milliwatts, is inserted or launched into a fiber, and the losses, in decibels, for the various components such as lengths of fiber and connectors are known, the overall link loss can be easily computed with simple addition and subtraction. Because 1 dBu equals 0.001 dBm, 0 dBm is then equivalent to 30 dBu. Table 2.3 illus- trates the relationship between common values of decibel-milliwatts and decibel-microwatts. Because we noted the relationship between power in watts and deci- bel-milliwatts in Table 2.2