The interest in crystal effects from the point of view of lock-in amplifier technology is twofold. On one hand, these effects are the subject of experimental studies that use delicate instruments equipped with lock-in amplifiers to detect small amplitudes. On the other hand, they form the basis of devices that are used to modulate light (electro-optic modulators and photo-elastic modulators, for example) or of solid state actuators that are used to periodically move mirrors.
Magneto-optic, electromechanical, and electro-optic crystal effects are introduced on a phenomenological basis. Quite often these effects are so small that the use of lock-in detection is mandatory to measure them. In many cases, the optical part of the measurement instrument is an interferometer.