When there's loss in a fiber optic system, the measured power is less than the reference power, resulting in a negative logarithmic value and a negative dB reading on the meter. Despite the meter displaying a negative number, convention dictates referring to the loss as a positive. The measurement may be optical power from a test source, a transmitter or the input of receiver, measured in dBm, which is "absolute" power - absolute in that it refers to power calibrated to a national standard, so two people testing the same fiber output with different power meters calibrated to. An optical power meter (OPM) is a device used to measure the power in an optical signal. The term usually refers to a device used for measuring the average power in fiber optic systems. Other general purpose light power measuring devices are usually called radiometers, photometers, laser power. The power must be lower, of course, since we have loss, and 3dB is approximately a factor of 2, so the power the meter measured is 1mw divided by 2 = 1/2milliwatt or 0. Splitters, fusion splices, connectors and.
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