How much power loss is normal for an optical power meter
A typical OPM is linear 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). Irrespective of power meter specifications, testing below about -50 dBm tends to be sensitive to stray ambient light leaking into fibers or connectors. To be able to judge whether a fiber optic cable plant is good, one does a insertion loss test with a light source and power meter and compares that to an estimate of what is a reasonable loss for that cable plant. This is not normally an issue, since the test wavelength is usually known, but has some drawbacks. Firstly, the user must set the meter to the correct test wavelength, and secondly, the presence of spurious wavelengths can result in wrong readings.
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