Measuring Laser Power Below the Noise Floor with a Lock-In Amplifier

Measuring optical signals in the femtowatt (10-15) to nanowatt (10-9) range can be very challenging.
This video explains how Lock-In Amplifiers can help make these measurements possible.

[@jan861]

Thanks a lot! I needed this and now I understood the basic concept! Could you say that a lock-in amplifier is the same as a preamplifier, just combined with an algorithm (the integration over the product of chopper signal and detector signal) ?

[@babotvoj]

The working principle of lock in is that you multiply it by the same frequency the signal is modulated with. It can be a square or sine or something else, so that the result contains a square of the sine the signal is modulated with. By lowpass filtering you get the signal divided by 2. Its basically the same principle of quadrature modulation/demodulation. Search in Google quadrat modulation/demodulation things will get clear.

[@raymondzhao9557]

interesting