Converting an AM radio into a high-frequency lock-in amplifier in a stimulated Raman experiment

We describe an AM detection method and show that a normal radio has a nonlinear response for weak signals. Introducing an extra carrier and using a double modulation technique makes our detection unit sensitive to the phase of the rf signal. In this way, the radio combination becomes equivalent to a high-frequency lock-in amplifier and we are able to distinguish between the gain and depletion of the laser beams in a stimulated Raman experiment. Another important aspect is the retrieval of the linearity of the detection unit.