Despite the meter displaying a negative number, convention dictates referring to the loss as a positive value. For example, a meter reading of "-3. 0 dB" signifies a loss of 3. The signal has traveled a long distance and lost strength. You're reading the signal at the far end of a fibre run, where most of the light has already been used or lost. Typical power levels measured by an optical power meter: Telecom transmitters: 0 to +10 dBm (1 to 10 milliwatts), Receivers: -30 dBm (1 microwatt) DWDM systems with fiber amplifiers: +10 to +20 dBm (10 to 100 milliwatts), Receivers: -20 to -30 dBm (1-10 microwatt) Data links and LANs: 0 to -10 dBm. The one thing most important thing to understand with optical power meter is knowing how to read the numbers on it. Thus, a source. Likewise, -10 dBm is 0.
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