A system for probing a DUT is provided, the system comprising a tunable or CW laser source, a modulator for modulating the output of the laser source, a beam optics designed to point a probing beam at a designated location on the DUT, optical detector for detecting the reflected beam, and collection and signal processing electronics. The system deciphers perturbations in the reflected beam by detecting beat frequency between operation frequency of the DUT and frequency of the modulation. In an alternative embodiment, the laser is CW and the modulation is applied to the optical detector.