A substrate inspection apparatus includes a laser light source configured to emit a laser beam, an optical splitter configured to split the laser beam into a first laser beam and a second laser beam, a delay stage configured to change a relative time delay of the second laser beam and optically connected to the optical splitter, a first modulator optically connected to the optical splitter and configured to change the first laser beam, and a feedback system configured to sense the second laser beam reflected from a substrate and configured to apply electrical feedback to the first modulator.