Shematic view of Laser Interferometrer Gravitational Wave Detector.
Interferometric detectors are regarded as promising devices for gravitational wave detection. Since the effects of gravitational waves appear as a fluctuation of optical path length, it is required to detect the phase differences between two orthogonal light paths of a Michelson type interferometer. To achieve a sensitive detection, a method called "RF Dark Fringe Locking" is generally used. Phase modulation applied by Electro-Optic(EO) modulators the interferometer move the measurement band to high frequencies. To realize a long optical path length in the interferometer, a multi-pass geometry called optical delay-line or Fabry-Perot cavity is used. One of the interfering output beams is detected by a photo detector, and its intensity is held to a minimum of interference(dark fringe) by a servo system.