Mission Impossible? Measuring Signals Below the Noise Floor with a Lock-In Amplifier

Measuring optical signals in the femtowatt (10^-15) to nanowatt (10^-9) range can be a daunting task. Signal levels this low are lost in typical detector noise levels and swamped by background light. The noise floor for photodiode detectors operated with a small bandwidth (~10 Hz) is on the order of 1 picowatt (10^-12). In order to achieve significant improvements in noise rejection we need to turn to a lock-in amplifier, which can improve noise rejection by 3 orders of magnitude or more.