Working In The Basement: 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). Further narrowing of the bandwidth by filtering or averaging will only provide a small additional reduction in the noise level.

In order to achieve significant improvements in noise rejection we need to turn to a lock-in amplifier. Lock-in amplifiers can improve noise rejection by 3 orders of magnitude or more. Furthermore, they can provide background signal rejection that is several orders of magnitude higher than the noise rejection.