Faults, dimensions and other characteristics of a material or structure are sensed by a coherent beam's reflection from the material when probed with ultrasonic waves or when vibrating at high frequencies. The reflected beam acquires a dynamic phase change substantially different from its original phase and from the phase of a reference beam split from the common source beam. The reflected beam and the reference beam are superimposed by diffraction in an adaptive holographic beam-splitter, and the superimposed beams are detected by a photodetector capable of detecting small phase changes from ultrasonic surface displacements or perturbations. An apparatus and method are disclosed defining an improved crystal homodyne interferometer with an energy source coupled to the crystal and operative to maintain the crystal in a condition for emptying unwanted traps in the crystal.