FRINGE-COUNTING TECHNIQUE USED TO LOCK A SUSPENDED INTERFEROMETER

We implement a digital fringe-counting technique to measure in real ti me the relative mirror displacement of a suspended Michelson interfero meter with modulated optical path length for oscillations much larger than the laser wavelength (lambda). This provides the proper error sig nal for a servo mechanism that reduces the relative displacement withi n lambda/2. The implemented technique does not require extra optics or polarizers and thus can be used for interferometric gravitational wav e detectors as a starting procedure to get the system locked.