Measuring optical signals in the femtowatt (10-15) to nanowatt (10-9) range can be very challenging. 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.
Read full article by By Shimon Elstein, Senior Physicist at Ophir, here.