Power Measuring Range
A typical OPM measures accurately under most conditions from about 0 dBm (1 milli Watt) to about -50 dBm (10 nano Watt), although the display range may be larger. Above 0 dBm is considered "high power", and specially adapted units may measure up to nearly + 30 dBm ( 1 Watt). Below -50 dBm is "low power", and specially adapted units may measure as low as -110 dBm. Irrespective of power meter specifications, testing below about -50 dBm tends to be sensitive to stray ambient light leaking into fibers or connectors. So when testing at "low power", some sort of test range / linearity verification (easily done with attenuators) is advisable. At low power levels, optical signal measurements tend to become noisy, so meters may become very slow due to use of a significant amount of signal averaging.
Read more about this topic: Optical Power Meter
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