A lock-in amplifier (also known as a phase-sensitive detector) is a type of amplifier that can extract a signal with a known carrier wave from an extremely noisy environment (the signal-to-noise ratio can be -60 dB or even less). It is essentially a homodyne detector followed by a steep low pass filter, making it very narrow band. Practical lock-in amplifiers use mixing, through a frequency mixer, to convert the signal's phase and amplitude to a DC — actually a time-varying low-frequency — voltage signal.
The device is often used to measure phase shift, even when the signals are large and of high signal-to-noise ratio, and do not need further improvement.
Recovering signals at low signal-to-noise ratios requires a strong, clean reference signal the same frequency as the received signal. This is not the case in many experiments, so the instrument can recover signals buried in the noise only in a limited set of circumstances.
The lock-in amplifier was invented by Princeton University physicist Robert H. Dicke who founded the company Princeton Applied Research (PAR) to market the product.
Read more about Lock-in Amplifier: Basic Principles, Signal Measurement in Noisy Environments