The branch of science devoted to the study and measurement of the interaction of waves, such as electromagnetic waves and acoustic waves. Note 1: The interaction of the waves can produce various spatial-, time-, and frequency-domain energy distribution patterns. Note 2: Interferometric techniques are used to measure refractive index profiles, e.g., those of the preforms from which optical fibers are drawn, and to sense and measure physical variables, such as displacement (distance,) temperature, pressure, and magnetic fields.