Extended Sensitivity Meters
A class of laboratory power meters has an extended sensitivity, of the order of -110 dBm. This is achieved by using a very small detector and lens combination, and also a mechanical light chopper at typically 270 Hz, so the meter actually measures AC light. This eliminates unavoidable dc electrical drift effects. If the light chopping is synchronized with an appropriate synchronous (or "lock-in") amplifier, further sensitivity gains are achieved. In practice, such instruments usually achieve lower absolute accuracy due to the small detector diode, and for the same reason, may only be accurate when coupled with singlemode fiber. Occasionally such an instrument may have a cooled detector, though with the modern abandonment of Germanium sensors, and the introduction of InGaAs sensors, this is now increasingly uncommon.
Read more about this topic: Optical Power Meter
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